Atlanta Geological Society Newsletter

ODDS AND ENDS

Dear AGS members, Greetings! I know you have heard me go on and March Meeting on about basic geological concepts; deep time, Join us Tuesday, March 26, 2019 at uniformitarianism, etc. Well, I have further proof that even more mundane events, like problems the Fernbank Museum of Natural drilling, are the same world‐wide and even History, 760 Clifton Road NE, Atlanta extending to other planets. We, the US Space GA. The social starts at 6:30 pm and program, currently have a probe exploring Mars, the meeting starts at approximately InSight. On board is the Heat Flow and Physical 7:00 p.m. Properties Package, HP3. This is designed to advance a temperature probe up to 5m into the This month out presentation is Martian soil and take temperature “Investigating Rare Biomineralization measurements in various modes. This probe is Structures in Trilobites” advanced with essential a hammer drill direct push rig with sensors. presented by Ms. Raya Greenberger. So, it is with no small amusement that I read that Please find more information about the the probe got to a depth of about 3m and could presentation and Ms. Greenberger’s not advance any further. There are consultations bio on the next page. going on about the operations, software glitches or maybe it just encountered a rock in the Please come out, enjoy a bite to subsurface. Who among us has not experience eat, the camaraderie, an interesting refusal on a DPT rig? It appears that not only presentation and perhaps some have the grander concepts of stratigraphy and discussion on the importance of sedimentology made the inter‐planetary jump to Mars but so have drilling problems. biomineralization of fossils. After an unforeseen delay, I am looking forward to Ms. Raya Greenberger’s presentation on www.atlantageologicalsociety.org biomineralization structures in trilobites. In reading the abstract, I’m impressed that current facebook.com/Atlanta-Geological- students are pushing much deeper into paleo that we ever did. Hope to see you on the 26th. Society Ben Bentkowski, President

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A Natural Experiment Reveals the Impact of Hydroelectric Dams on the Estuaries of Tropical Rivers

INTRODUCTION During the last decades, many voices have expressed concern about the environmental impact of large dams. Although hydroelectric power is renewable and can reduce CO2 emissions derived from the burning of fossil fuels for the generation of thermoelectricity, large hydroelectric dams may also have important adverse consequences on the environment because they alter river hydrology, nutrients concentration and amounts, and the lifecycles of species that depend on freshwater habitats. Reductions in water pulses downstream can increase substrate salinity, lower the groundwater table, and make water unusable for drinking and irrigation. Decomposition of organic matter drowned in the dam’s reservoir can emit large amounts of methane and can also promote the leaching of toxic metals from the flooded minerals and rocks in the reservoir. Sediments that are crucial for natural cycles are trapped in the dam’s reservoir affecting the normal functioning of downstream ecosystems, including wetlands and coastal lagoons. Thirty-three of the largest deltas in the world are now rapidly receding and sinking, with serious consequences for regional agriculture and livelihoods. Although alterations of the water flow by damming and diversions are not the only cause of deltaic degradation (aquifer water extraction and oil drilling, for example, seem to be playing a role too), they are the most important common cause globally.

While in many developed nations strong public opinion has gradually developed against dams and the era of big dam building is considered to be over, many developing countries still see the construction of dams as a strong incentive for development, capable of mobilizing investment, activating the demand for industrial supplies such as steel and concrete, offering employment, and providing renewable energy. However, the environmental costs in terms of loss of land, alterations to the water regime, habitat disruption for riverine species, sediment trapping in the reservoir, and the degradation of water quality are often not weighed appropriately against the purported benefits. Strong debates persist around this issue throughout the developing world, and many studies have tried to tackle the complex issue of economic tradeoffs between hydropower generation and the associated environmental impacts in an attempt to achieve a goal of generating electricity while at the same time avoiding many of the environmental costs of large dams.

Tropical rivers normally carry large amounts of suspended sediments that feed sandbars, deltas, and accretional coastlines. Most of these sediments become trapped in the body of large reservoirs, changing the coastal dynamics in the estuaries of dammed rivers. The impact of large dams in coastlines, estuaries, deltas, and lagoons is commonly understudied in the environmental studies of hydroelectric projects. Furthermore, when evaluated, it has historically been approached by analyzing time series documenting the state of the estuary and the lower basin before and after the damming of the river. This approach, however, is based on repeated observations along the same river and not based on true statistical replicates. Long-term change can be the result of many other factors that have varied with time and not necessarily of the dam itself. A comparative study between dammed and undammed rivers may provide critically needed evidence.

The Pacific coast of Mexico in the States of Nayarit and Sinaloa provides an ideal setting for this approach. Four rivers run roughly parallel to each other down the Sierra Madre from the Mexican Plateau and reach the Pacific coast relatively near to each other. Two of them have been dammed for hydroelectricity, while the other two still flow free into the coastal estuaries. The southernmost one, the Santiago River, harbors four dams (Aguamilpa, finished in 1994; El Cajón, 2007; La Yesca, 2012; and Santa Rosa, 1964), and the northernmost one, the Fuerte River, was dammed in 1956 by the El Mahone dam. The remaining two rivers, the San Pedro and the Acaponeta, still flow free onto the coastal plains (they both have some small impoundments for irrigation but are still largely

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A Natural Experiment Reveals the Impact of Hydroelectric Dams on the Estuaries of Tropical Rivers (Continued) since the 1960s by Curray et al. The ridges are formed by coastal accretion derived from the continental input of sediments brought down from the Sierra Madre into the coastal plains by three main rivers: Acaponeta, San Pedro, and Santiago. After post-Pleistocene sea-level rise stabilized, ca. 5000 years before the present, the ridges started to form at a mean rate of approximately 1 m every 12 years. The ridges are made up of a mixture of sediments derived from the longshore transportation of the Santiago and San Pedro rivers and the onshore transportation of reworked sediment from the now drowned paleo-river delta constructed on the continental shelf during the previous sea-level low stand in the Pleistocene and early Holocene. The current boundary between the large inner lagoons and the ridge system marks the ancient coastline from where the ridges grew. Curray’s seminal studies allow calculating the rate at which the Marismas coastline expanded and grew into the ocean shelf, as well as the amount of sediment brought in the past by the rivers that gave origin to this unique system. The dynamics of sediment deposition along the coastline, and particularly of the coastal ridge systems, is at the core of current concerns about the environmental impact of hydroelectric dams. Reductions in sediment loads from sediment trapping in reservoirs may reverse the historic accretion of the lagoon systems, opening the way to the erosion of sandbars and coastal ridges and, potentially, to the destruction of the coastal wetland ecosystems.

Figure. 1 The Marismas Nacionales lagoon system.

(A) Satellite image of Marismas Nacionales showing the inland system of Pleistocene and early Holocene lagoons and the coastal system of late Holocene beach ridges separating the lagoons from the sea (photo credit: Google Earth). (B) Aerial view of the parallel beach ridges derived from successive events of coastal accretion.

RESULTS Striking differences in the coastal dynamics of the four river systems were found between the four rivers in their coastal dynamics as evaluated through an analysis of both Landsat and Google Earth images, encompassing both decadal and shorter-term time scales.

Multidecadal coastal dynamics

The coast around the estuary of the Fuerte River, dammed in 1956, receded during the two study periods, losing on average 7.9 ha year−1 during 1975–1990 and 11.5 ha year−1 during 1990–2010 (Figure. 2). The coast around the estuary of the Santiago River, first dammed in 1994, grew between 1975 and 1990 at a mean rate of 4.5 ha year−1, before the first dam was built, but started to rapidly recede during the 1990–2010 period at a rate of 21.5 ha year−1 (Figure. 3A). In contrast, the coast around the Acaponeta River stayed largely stable, gaining on average 0.2 ha year−1 during 1975–1990 and losing 0.4 ha year−1 during 1990–2010. Last, the coast of the San Pedro River showed fast accretion rates during both periods, gaining 7.4 ha year−1 during 1975–1990 and 5.2 ha year−1during 1990–2010.

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A Natural Experiment Reveals the Impact of Hydroelectric Dams on the Estuaries of Tropical Rivers (Continued)

Figure. 2 Multidecadal coastal change.

Landsat images of the coast adjacent to the estuaries of the Fuerte, Acaponeta, San Pedro, and Santiago rivers for years 1975, 1990, and 2010 (photo credit: changematters.esri.com/compare). The images at the right show changes in the coastline during the periods indicated in the figures; blue color indicates coastal accretion, and red indicates coastal erosion.

Figure. 3 Coastal accretion rates.

(A) Areal accretion or erosion rates (ha year−1) for the Santiago, San Pedro, Acaponeta, and Fuerte coastlines for the 1975–1990 period (hatched bars) and the 1990–2010 period (open bars) derived from Landsat images. Free-flowing rivers are coded in green, and dammed rivers are in red (note that the Santiago River was free flowing in the 1975–1990 period and dammed in 1990–2010 period). (B) Linear advance or retreat rates (m year−1) during the 2003–2015 period derived from Google Earth images for the Santiago, San Pedro, Acaponeta, and Fuerte sandbars (see Figure. 4 for details on the image dates). Free rivers are coded in green, and dammed rivers are in red.

Recent coastal dynamics An analysis of the Google Earth images for the 2003–2015 interval confirmed the general Landsat trends (Figure. 4): During the last decade, the coastline in the sandbar of the Santiago River retreated at a rate of −48.5 m year−1, while the sandbar of the San Pedro River grew at a rate of 2.8 m year−1, the Acaponeta River stayed largely stable at a rate of −0.3 m year−1, and the Fuerte River retreated at a rate of −14.3 m year−1 (Figure. 3B). Comparing the mean rates by means of a t test, we found that the rate in the Acaponeta River did not differ significantly from zero (P = 0.31), while the sandbar of the San Pedro River is significantly advancing (P = 0.0002), and those of the Santiago and Fuerte rivers are significantly retreating (P < 0.0001 and P = 0.015, respectively).

Ground surveys The vegetation surveys confirmed the trends observed in the satellite images (Figure. 5): The two nondammed rivers (San Pedro and Acaponeta) showed defined successional accreting coasts along their sandbars, with pioneer species in the seaward front, followed by fixed dune perennials such as coastal mesquite (Prosopis juliflora), then by a mature tropical dry forest, and lastly by an inland lagoon mangrove forest with black mangrove (Avicennia germinans) in the mudflats and red mangrove (Rhizophora mangle) along the lagoon fringe. The two dammed rivers (Santiago and Fuerte) showed signs of intense erosion along their seaward fringe, with

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A Natural Experiment Reveals the Impact of Hydroelectric Dams on the Estuaries of Tropical Rivers (Continued) the accretional sandbars gone, inland lagoon mangrove forests being exposed directly to the erosive action of the waves, and dense masses of dead trunks and stumps being washed into the sea. The diversity of terrestrial vegetation in the sandbars of the four rivers differed drastically. While in the sandbars of the San Pedro and the Acaponeta we identified 33 and 26 different species of terrestrial plants, respectively, we counted eight and four species only in the Fuerte and the Santiago sandbars. Overall, these species richness values differ significantly (χ2 = 32.9, df = 3; P < 0.0001): The two nondammed rivers shelter significantly higher species richness than the dammed rivers, and the richness of the four sandbars is positively correlated with the rate of coastal accretion and erosion (Figure. 6A). Furthermore, many of the species present in the coastal dunes and tropical dry forests of the first two rivers are regional or local endemics of high conservation value (see Supplementary Materials).

Figure. 4 Short-term coastal change. Comparative historic images of the sandbars adjacent to the mouths of the Fuerte, Acaponeta, San Pedro, and Santiago rivers for the dates indicated in the image (photo credit: Google Earth). The images at the right show changes in the coastline during the two periods: blue indicates coastal accretion, and red indicates coastal erosion.

Figure. 6 Impact on vegetation and fisheries.

(A) Log-linear relationship between the accretion rate of the coastline (negative values indicate the coast is receding) and the plant species richness of the sandbar (r2 = 0.98, χ2deviance of the model = 67.0, df = 1; P < 0.0001). (B) Log-linear relationship between the accretion rate of the coastline and the number of boats fishing in the estuary (r2 = 0.95, χ2deviance of the model = 97.3, df = 1; P < 0.0001). (C) Total number of fishing boats in the lower basin of each of the three rivers. The number of boats counted on the free rivers was significantly higher than the number of boats in the dammed rivers (χ2 = 1907.3, df = 3; P < 0.0001). In all plots, free-flowing rivers and damned rivers are indicated by green and red markers, respectively.

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A Natural Experiment Reveals the Impact of Hydroelectric Dams on the Estuaries of Tropical Rivers (Continued)

Impact on fisheries The number of boats operating in each estuary was also positively correlated with the rate of coastal accretion (Figure. 6B), linearly decreasing from 163 boats in the San Pedro estuary with the highest accretion rate to 47 boats in the Santiago estuary with the highest erosional rate. These findings were consistent with the 2006 boat survey in the whole floodplain wetlands of each river: In the 2006 counts, the entire lower basins of the San Pedro and the Acaponeta rivers had, on average, 1083 and 396 fishing boats, respectively, while the Fuerte and the Santiago rivers only harbored 19 and 47 boats, respectively (Figure. 6C). The difference in the number of fishing boats is mirrored in total landings: According to fisheries data presented in the environmental impact statement (EIA), the San Pedro River (Camichín, 634 ha) yielded in the 2007–2008 fishing season 658 tons of shrimp, oysters, and fish, while the concession in the mouth of the Santiago River (Villa Juárez, 3146 ha) yielded only 35 tons in the same period. The same study reports that, as a whole, the pooled concessions in the wetland plains of the San Pedro River (78,157 ha) yielded in 2007–2008 a total of 2352 tons of fisheries.

DISCUSSION Our analyses show that the damming of tropical rivers, with the subsequent reduction of sediment load reaching the coasts, has highly destructive effects on the stability and productivity of the coastline and lower estuaries. The two rivers dammed for hydroelectricity in their lower basin are experiencing rapid coastal recession in what should otherwise be an accretional coastline. Over 1 million tons of sediment are trapped in the dams along the Fuerte and Santiago rivers every year (see section S2 for details), and as a result, the coastline around the estuary of the Santiago River is losing more than 20 ha of coastal tropical forests and mangroves every year. The coastline around the estuary of the Fuerte River, which was dammed over half a century ago, is still receding rapidly losing every year approximately 10 ha of mangrove forests, now exposed to the direct erosive action of the waves. The economic consequences of this destructive coastal erosion induced by the damming of the rivers are multiple.

Loss of open sea fisheries services It has been shown that mangroves in general (and fringe red mangrove forests in particular) provide critical habitat and food for the growth and survival of many important open sea fisheries during their juvenile stages. On the basis of their contribution to fisheries, the value of fringe mangroves in the Gulf of California has been estimated at approximately 650,000 US$/ha. If we assume, based on our field sampling, that of the total area of forests lost every year near the mouth of the Santiago River, some 2 ha (ca. 10%) correspond to fringe red mangroves, then it follows that approximately US$ 1.3 million are lost every year from Mexico’s natural capital in fishery services. The estimation of the fishing effort in the Gulf of California suggests that the fishing effort is highest close to highly populated human settlements on the coast and/or fishing camps where the number of fishing vessels is high. The river mouths analyzed in this research are located along the eastern side of the Gulf of California, where the highest fishing effort values were estimated. Although El Fuerte river mouth is located between Guaymas (in the north) and Mazatlán (in the south) along the most populated coastline in the region, the fishing effort is significantly less than the fishing effort in the San Pedro Mezquital river mouth, supporting our results that the degradation of the estuarine ecosystems by dam impacts have significant negative effects in the fishing sector independently of the distribution of the fishing effort in the region.

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A Natural Experiment Reveals the Impact of Hydroelectric Dams on the Estuaries of Tropical Rivers (Continued) Loss of coastal protection and other mangrove services Apart from their role as providers of habitat for fisheries, mangroves and accretional sandbars provide a whole set of additional services including coastal protection from hurricanes and tropical storms, wildlife conservation services, and recreational habitat. On the basis of the provision of these services, Costanza et al. valued mangrove ecosystems and coastal wetlands at 194,000 US$/ha. It follows then that the annual loss of more than 20 ha of mangrove and coastal forests observed in the coast around the Santiago River represents an annual loss of US$ 3.9 million from Mexico’s natural capital based on the provision of other environmental services different from fisheries.

Release of previously immobilized carbon Mangrove forests are one of the most carbon-rich ecosystems on earth, harboring, on average, more than 1000 tons of belowground carbon per hectare. Mangroves at Marismas Nacionales, however, are not as rich in carbon as those in other lagoons, possibly as a result of the rapid dynamics of the coastal ridge accretional system. Our own measures have estimated some 300 tons/ha of total carbon in these forests. Thus, the annual loss of ca. 20 ha implies the release of some 6000 tons of carbon eroded into the ocean and presumably later decomposed and released to the atmosphere in the form of CO2 or methane. Using the standard estimate of 21 US$/tons of carbon used in carbon trading valuations, it follows that ca. 130,000 US$ is lost every year into the form of de- immobilized carbon.

Loss of biodiversity services The banding of coastal vegetation along the coastal ridges is a major factor in the observed biodiversity collapse. The dunes and dry tropical forests are much more diverse in plant species than the mangrove forests and, being the two ecosystems closest to the sea, are the first to go as the coast recedes due to river damming. The tropical dry deciduous forests of the Pacific coast of Mexico rank among the most endangered ecosystems in the country. They have been put under threat by the expansion of agriculture, ranching, and urbanization. For this reason, the stretches of forest that survive in the coastal sandbars of this region play not only an important role in shoreline protection and buffering against hurricanes but also in the conservation of Mexican biodiversity and wildlife habitat.

Degradation of estuarine livelihoods It is a well-established fact that catadromous migration, in which some fish species migrate from the ocean to feed in river estuaries, dominates in tropical coasts where the productivity of estuarine and lagoon normally exceeds that of the ocean. This elevated productivity in tropical rivers depends chiefly on the input of nutrients brought into the estuaries by the rivers’ suspended sediments. The process of estuarine fertilization by the continuous input of continental sediments explains why the number of fishing boats in the dammed rivers is only 2 to 5% of the number of boats in the free-flowing San Pedro River and why the total landings reported for the mouth of the Santiago River is only 5% in mass of those reported for the San Pedro River. In economic terms, this is very important for the region. The lower basin of the San Pedro River yields every year more than 1000 tons of shrimp, some 600 tons of oysters, and some 620 tons of different fish species that, at ex-vessel prices, generate a regional income of US$ 5.8 million distributed among 3800 fishers and for the benefit of 26,000 community members (see section S3). All four rivers have fishing markets nearby, and the two rivers where fisheries have collapsed still show evidence of large, now abandoned, fishing camps in their estuaries, a fact that suggests that fishing was intense in these estuaries in the past. Thus, based on the comparative data of

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A Natural Experiment Reveals the Impact of Hydroelectric Dams on the Estuaries of Tropical Rivers (Continued) fishing boats and landings for the four rivers, it becomes clear that the damming of the Santiago and the Fuerte rivers for hydroelectricity may have reduced this regional source of income in approximately 95% or more in each estuary.

CONCLUSION The most important environmental argument frequently brought forth in favor of hydroelectric dams is that of emission reductions. These reductions, however, are often partially offset in tropical ecosystems by the emission of methane (CH4) and CO2 from decomposing organic matter from the forests submerged in the reservoir but can be, and often are, calculated in environmental studies around new hydroelectric projects.

However, the damages a hydroelectric project can cause in the coast and the lower part of tropical basins, in terms of loss of mangrove services and estuarine productivity, may add a significant amount to the environmental costs of a dam and are rarely calculated. These costs should be estimated and added to the many other well-known impacts of hydroelectric dams in both the reservoir itself and in the upper and mid-basin. The building of a dam on free rivers such as the San Pedro or the Acaponeta can imperil the livelihoods of fishing and rural communities in the floodable coastal plains, a social impact that should be added to the number of villagers that often lose their land and their sacred sites under the flooded reservoir. Last, although not easily quantifiable in economic terms, large dam projects imply the loss of important coastal biodiversity and imperil the continued formation and dynamics of accretional coastal landscapes.

METHODS Experimental design We surveyed four of five largest rivers of the Pacific coast of Mexico: Two of them (the Fuerte and the Santiago rivers) are dammed, while the other two (the Acaponeta and the San Pedro) run free. The fifth and largest river of the Mexican Pacific coast, the Balsas River, was not surveyed because the estuary and the neighboring oceanic coasts have been walled to stop the accelerated coastal recession that started after the building of the Infiernillo dam in the lower basin. By comparing the dammed against the undammed rivers, we analyzed the way sediment trapping has affected the estuaries. We analyzed (i) long-term multidecadal change using Landsat images, (ii) short-term (<10 years) change using Google Earth Pro images, (iii) current vegetation condition through ground surveys, and (iv) estuarine productivity by counting fishing boats and using official statistics on estuarine fisheries production.

Multidecadal coastal dynamics Using Landsat satellite images (pixel size of 30 m × 30 m) from the website changematters.com, we compared coastal sectors spanning a latitudinal range of 5.5 min (ca. 10 km of coastline), centered around the estuaries of the four rivers. For each coastal sector, we selected images for three dates: 1975 (earliest image set available), 1990, and 2010 (most recent set available). Using the color signature identifier of a digital image editor (Adobe Photoshop), we selected in each image all the pixels that corresponded to seawater and coastal unvegetated strands and simplified each image into a binary representation of terrestrial and marine pixels. We then superimposed the binary images and calculated the area of coastal vegetation that was lost or gained during the 1975–1990 and the 1990–2010 periods. Because areal calculations are derived from pixel counts, which are frequency data, we obtained standard errors for each areal estimate using the mean-to-variance properties of the Poisson distribution. To make the more recent satellite images comparable with those of 1975, we used in all cases false-color infrared imagery in which terrestrial vegetation shows up in red, and we took in all images the seaward limit of terrestrial vegetation as the boundary of terrestrial ecosystems.

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A Natural Experiment Reveals the Impact of Hydroelectric Dams on the Estuaries of Tropical Rivers (Continued) Recent coastal dynamics We used Google Earth Pro historic images of the coastal sandbars immediately adjacent to the mouth of the four rivers to compare coastal change around the estuaries of the four rivers (Santiago, November 2010–April 2013; San Pedro, August 2010–March 2013; Acaponeta, October 2003–May 2013; and Fuerte, February 2004–March 2014). The time span of these images is variable because it depends on the availability of historic imagery but is always much shorter than that of the Landsat images, encompassing in all cases less than a decade and never reaching earlier than 2004. The pixel resolution, however, is higher (1.3 m × 1.3 m for all images available for the study sites) and allows for a more detailed assessment of coastal change. In all four rivers, we selected images at the same scale, encompassing roughly 1 km of ocean coast. Using the same procedure as with the Landsat images, we transformed each image into a binary representation of marine versus terrestrial environments, selecting in each image the marine pixels formed by ocean surface and unvegetated beach strands, and separating them from the remaining pixels, classified as terrestrial environments. By subtracting both images, we could identify areas that corresponded to terrestrial environments in the first date and had been eroded into the ocean later or, conversely, areas that were occupied by seawater or unvegetated beaches in the first image and had accreted into a vegetated terrestrial environment later. The area of lost/gained terrestrial pixels was divided by the length of the coast to obtain a linear measure of coastal advancement or retreat. Last, by numerically resampling the image with 10 random transects perpendicular to the coastline, we got an estimate of the standard error of the linear rate.

Ground surveys We visited each of the coastal sites identified in the Google Earth images and made in each one a rapid vegetation survey to assess the status of the ecological communities and confirm or reject the hypotheses developed using the remote sensing images. The detailed methods for our vegetation survey are given as section S1. Our main goal in this ground-truthing survey was to establish whether the coastal plant communities showed the effects of accretion or recession. In accreting (growing) coasts, we expected to find a gradient with young pioneering dune plants in the seaward fringe gradually transitioning onto fixed dunes, followed by a mature coastal bar forest, and then onto the inland lagoon communities of mangroves. In receding (eroding) coasts, we expected to find older trees, typical of more inland communities, in the seaward fringe being exposed directly to the erosive action of the waves. The relationship between plant diversity, expressed as species richness in 1 ha of coastal sandbar, and coastal accretion/erosion rates was evaluated using log-linear regression.

Fishing activity and catches To test whether estuarine fishing activities were affected by the large dams, we used Google Earth images to count the number of fishing boats in each estuary, up to 2 km upstream from the mouth of the river. In each image, we counted the total number of fishing boats that were visible, including both open-water skiffs (known as pangas) and river canoes (cayucos). The relationship between coastal accretion/erosion rates and the number of fishing boats was also evaluated using log-linear regression. We also obtained from a database developed by the World Wildlife Fund’s Mexico Program a detailed tally of all the boats visible in the lower river plains for the four rivers, counted by analyzing Google Earth images in spring and fall (May and December) of year 2006. To get an estimate of year- long fishing pressure in each basin, we averaged the May and December counts. Last, we obtained fisheries data from the EIA of the Las Cruces Hydroelectric Project (i) to compare fisheries landings in the estuaries of the San Pedro and the Santiago rivers and (ii) to estimate the total landings that local fishers obtain from the lower San Pedro plains in the entire Marismas Nacionales wetlands.

Read more about this article at: http://advances.sciencemag.org/content/5/3/eaau9875

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Atmospheric Forcing of Rapid Marine-terminating Glacier Retreat in the Canadian Arctic Archipelago INTRODUCTION In the northern high latitudes, one of the regions thought to be highly vulnerable to environmental change is the Canadian Arctic Archipelago (CAA), which contains the greatest area of global land ice outside the ice sheets in Greenland and Antarctica. With a glaciated area of ~146,000 km2, the CAA has the potential to contribute 199 ± 30 mm to global mean sea level rise. Rates of ice mass loss from the CAA have increased sharply during the 21st century, from the Queen Elizabeth Islands (QEI) in the north [31 ± 8 gigatons (Gt) year−1 in 2004–2006, increasing to 92 ± 12 Gt year−1 in 2007–2009] to Baffin and Bylot Islands (BBI) further south (from 11.1 ± 3.4 Gt year−1 in 1963–2006 to 23.8 ± 6.12 Gt year−1 in 2003–2011). Although total ice mass loss is known, changes in individual glaciers have not been analyzed regionally.

There is increasing evidence in both polar regions that rates of frontal ablation of ice flowing into the ocean are strongly controlled by subsurface ocean temperatures. Unprecedented warming of the Arctic Ocean and surrounding water bodies therefore makes pan-Arctic marine-terminating glaciers vulnerable to rapid retreat. Oceanic forcing has already been recognized as a key control on the fluctuations of outlet glaciers in western Greenland, where rates of submarine melting are up to two orders of magnitude greater than surface melt rates. Warming of subpolar North Atlantic water entering Baffin Bay is considered to be triggering an increase in the rate of submarine melting of Greenland’s outlet glaciers. However, on the Canadian side of Baffin Bay, the role of oceanic forcing on marine-terminating glaciers has not been directly investigated and is largely unknown, although it has been hypothesized that ocean temperatures might be influencing glacier retreat.

With over 300 marine-terminating glaciers in the CAA, dynamic feedbacks associated with glacier frontal changes have clear potential to influence decadal-scale ice losses from the region. Recent studies have shown that increasing ice discharge to the ocean has contributed to recent mass loss from the CAA in some locations. For example, ~62% of current ice discharge (i.e., via calving) from the QEI is channeled through Trinity and Wykeham glaciers, southeast Ellesmere Island, where flow speed has markedly accelerated over the past 10 to 15 years. In this area, at least half of the observed thinning rate at the glacier termini has been attributed to changes in ice dynamics rather than to surface mass balance (SMB) changes. Changes taking place at the terminus of marine- terminating glaciers form a crucial component of mass balance and have a major influence on coastal ecosystems, and yet, long-term regional patterns and trends in the frontal changes of glaciers in the CAA have never been examined. Here, we present a comprehensive analysis of decadal changes in the frontal positions of 300 marine- terminating glaciers in the region. We analyze ocean temperature measurements, together with downscaled 1-km- resolution regional climate model output, to evaluate the relative importance of oceanographic and atmospheric influences on the observed changes in glacier frontal positions since the late 1950s.

RESULTS Marine-terminating glacier frontal changes We determined ~7 frontal positions for each glacier by digitizing images from a range of sources collected at approximately decadal intervals (see Materials and Methods, fig. S1, and table S1). Our comparison of historical aerial photography from 1958/1959 with Landsat satellite imagery in 2015 indicates that >94% of the 300 marine- terminating glaciers investigated have, overall, retreated during this period (Figure. 1). The mean frontal change rate was −9.3 m year−1 (±1.4 m year−1 SE) for the 211 glaciers in the QEI and −6.9 m year−1 (±0.7 m year−1 SE) for the 89 glaciers in BBI. Total area loss by frontal retreat since 1958 was 387 km2 in the QEI and 22.5 km2 in BBI. In both regions, the absolute area change associated with glacier retreat is related to both the width and length of the glacier (in absolute terms, larger glaciers retreat greater distances than smaller ones), so we normalized area changes according to the length of glacier basins in ~2000. Mean relative length changes since the late 1950s were

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Atmospheric Forcing of Rapid Marine-terminating Glacier Retreat in the Canadian Arctic Archipelago (Continued) −4.2% (±0.3% SE) in the QEI and −4.1% (±0.3% SE) in BBI (Figure. 1). Of the 211 marine-terminating glaciers in the QEI, 23 have previously been identified as surge type, so we excluded these from further analysis because their terminus behavior can be independent of climate forcing. In addition, our study does not include glaciers situated along the northern coast of Ellesmere Island, which are strongly influenced by the buttressing effect of multiyear sea ice and interactions with floating ice shelves and have been described elsewhere.

Figure. 1 Spatial patterns of marine-terminating glacier front change from 1958/1959 to 2015.

In the QEI, 197 glaciers have retreated, and 12 have advanced since 1959. Five of those that have advanced are surge type. In the BBI region, 87 glaciers have retreated, and 2 have advanced since 1958. Named features are those referred to in text. GF, Grise Fiord weather station; PoW Icefield, Prince of Wales Icefield; Manson IF, Manson Icefield; T-W glaciers, Trinity and Wykeham glaciers.

Similar decadal patterns in frontal variability occurred throughout the study region: Glaciers showed slow retreat in the earlier years (1958 to late 1990s), and retreat rates increased considerably after ~2000 (Figures. 2, A and B). The mean frontal change rates in the QEI and BBI during the 1960s were −7.0 and −0.6 m year−1, respectively, increasing to −29.1 and −22.9 m year−1since 2009/2010 (table S2). This trend is evident in both absolute and relative length changes and across all glacier types (e.g., different sizes, lengths, and frontal widths). The greatest absolute retreat rates occurred in the QEI, where mean glacier lengths and terminus widths are greater than those of glaciers further south. One notable anomaly in the BBI region is a period of increased retreat in the early 1980s (Figure. 2B). Another period of note is the earliest interval in the QEI region (1959–1975), when retreat rates were greater than in the two subsequent periods (Figure. 2A).

Figure. 2 Temporal trends in glacier length, surface ablation and runoff.

Marine-terminating glacier length changes (percentage per year) by time interval for (A) QEI and (B) BBI. Trends in mean runoff (millimeter w.e. year–1) and mean ablation area (as the percentage of glacier basin area) calculated from RACMO2.3 statistically downscaled to 1 km for (C) QEI and (D) BBI. Surge-type glaciers are excluded. Thick horizontal lines are mean values, and outer horizontal lines are ±1 SE.

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Atmospheric Forcing of Rapid Marine-terminating Glacier Retreat in the Canadian Arctic Archipelago (Continued) Similar trends are observed across different latitudes, showing that there are no clear spatial variations in the temporal pattern of accelerating retreat (Figure. S2). At all latitudes in the QEI, the lowest retreat rates occurred in the earlier periods, and rates were highest in the two periods since 2002. Although mean relative retreat rates are lower in the two most northerly latitudinal bands, they exhibit the same temporal trend. In the BBI region, the pattern is broadly similar, but during the period 1980–1985, there was a notable increase in retreat in each band north of 70°N. The highest regional mean retreat rate (−0.33% year−1) occurred in the most recent period, 2009– 2015, in southeast Ellesmere Island (77°N to 78°N). The greatest absolute retreat rates in the CAA were observed on the coalescent Trinity and Wykeham glaciers (78.0°N, 78.6°W), where they reached −338 m year−1 between 1976 and 1984 and − 255 m year−1 between 2009 and 2015, with a total retreat of 4.8 km since 1959.

Ocean temperatures and regional frontal change The coherency in glacier retreat rates across the region suggests a common external driver. Subsurface ocean temperatures on the eastern side of Baffin Bay have been identified as a significant control on frontal change rates of glaciers in western Greenland, and so, we first consider the possible influence of ocean temperatures on glacier fluctuations on the Canadian side of Baffin Bay. Although ocean temperature measurements from locations close to glacier fronts in the CAA are generally sparse, there are sufficient data sources for regional-scale analyses. TOPAZ4 reanalysis data (gridded at 12.5 km from 1991 to 2015) are available from the Copernicus Marine Environment Monitoring Service (CMEMS) and show spatial patterns in mean temperatures at standard depths (Figure. 3). Strong differences in temperature between the east and west sides of Baffin Bay are evident: Off southwest Greenland, warm waters (>1°C) are present at all depths, whereas waters at depths of 200 m and shallower surrounding the CAA are typically cold (<−1°C). This pattern is explained by the cyclonic circulation in northern Baffin Bay from the West Greenland Current System, which advects warm, saline Atlantic waters northward along western Greenland and carries fresh and cold Arctic waters southward along eastern Baffin Island. Additional cold water masses occur off northern Baffin Island as a result of wintertime freezing within the polynyas of northern Baffin Bay (in Smith, Jones, and Lancaster Sounds; locations marked on Figure. 1). Cold waters in Smith Sound originate from water from the Pacific that enters via the Arctic Ocean and Nares Strait. In recent decades, however, changes have occurred in Baffin Bay, including warming in areas affected by the Atlantic inflow.

Figure. 3 Mean annual ocean temperatures at standard depths of 50, 100, 200, and 400 m.

Data are from CMEMS TOPAZ4 Arctic Ocean Physics reanalysis at 12.5-km grid spacing, modeled from all available satellite and in situ observations between 1991 and 2015. The black line is the 100-km buffer offshore from the CAA coastline.

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Atmospheric Forcing of Rapid Marine-terminating Glacier Retreat in the Canadian Arctic Archipelago (Continued)

To assess ocean temperatures close to marine-terminating glaciers in the CAA, we use in situ temperature measurements (1972–2015) obtained from the World Ocean Database 2013 (WOD13) for standard depths between 50 and 500 m. These depths represent the range of bathymetry close to the coast, as calculated from the National Oceanic and Atmospheric Administration (NOAA) International Bathymetric Chart of the Arctic Ocean (see Materials and Methods). Although only 19 glaciers have ocean temperature measurements within 5 km of their fronts, these show comparable mean temperatures to those with measurements up to 20 km away (figs. S3 and S4). In total, 117 glaciers have ≥10 conductivity-temperature-depth (CTD) measurements within 20 km of their fronts, which provides a sample that is more than sufficient for statistical analysis. First, following methods outlined in, we compare mean overall glacier frontal changes (1958/1959 to 2015) with their adjacent mean ocean temperatures (1972 to 2015). From this, we find no statistically significant correlation at any depth (Figure. 4 and table S4). The mean ocean temperatures within 20 km of glacier fronts are below 0°C at all depths (Figure. S4), and although these are above the freezing temperature of sea water (calculated from pressure and mean salinity), the melt induced by water at such low mean temperatures would be insufficient to drive retreat at the scale observed. For example, a study of glaciers in Svalbard found that the relationship between frontal ablation rate and ambient subsurface ocean temperatures was strong and linear above 0°C, but little or no retreat occurred when ocean temperatures were below 0°C.

Figure. 4 Mean overall (1958–2015) glacier front changes (as the percentage of glacier length in 2000) by mean in situ ocean temperatures (1971–2015) at specific depths. Results are for all glaciers with 10 or more ocean temperature measurements within 20 km of their fronts (glacier counts are in table S3, and correlation coefficients are in table S4). No significant correlation is observed between magnitude of frontal retreat and temperatures at any depth.

Spatial and temporal analysis of in situ temperature measurements shows that, north of 75°N, mean ocean temperatures up to 100 km from the coast have been below 0°C almost universally since the earliest records (Figure. 5 and table S5). Although heat content is low at these northern latitudes, a statistically significant increase in temperature occurred between the 1970s and 2000s at all depths (where data are available). Another study has shown that, on a local scale, there are variations to the regional trend, such as to the east of Devon Island and southeast Ellesmere Island, where temperatures at 200-m depth were lower in 2000–2010 relative to 1995–1999. At latitudes south of 75°N, mean temperatures have been warmer than 0°C at 400- and 500-m depths since records began, and warming has occurred at all depths, except for a temperature decrease between 72°N and 74°N at 500- m depth (table S5). Although ocean warming has occurred throughout much of the region, the temperatures have been substantially below those seen to be driving melt in western Greenland and elsewhere. For example, the rapidly retreating Jakobshavn Isbrae is adjacent to the warm (mean, ~1.8°C) subsurface water of Disko Bay.

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Atmospheric Forcing of Rapid Marine-terminating Glacier Retreat in the Canadian Arctic Archipelago (Continued)

Figure. 5 Temporal trends from World Ocean Database (WOD13) data (up to 100 km offshore).

Mean ocean temperatures are separated by standard depths and degrees latitude for (A) QEI and (B) BBI. Error bars are ±1 SE.

The ice thickness and water depth at the grounding line also influence the degree of oceanic control on glacier retreat. Results from a recent study show that the mean ice thickness at the grounding line of 35 tidewater glaciers in the QEI is estimated to be ~230 m and that only eight glaciers have a grounding line thickness that exceeds 300 m. In the BBI region, the mean ice thickness at the grounding lines is estimated to be even smaller (~100 m). Therefore, the warmer water present at ocean depths deeper than 400 m will have little impact on the glaciers. By comparison, many outlet glaciers in Greenland extend into fjords to depths of 600 m or more, where they are exposed to deep warm Atlantic water. Thus, while some large glaciers in the CAA may be vulnerable to melt in specific locations where they sit in deeper and warmer water, we find no evidence that ocean temperatures have driven the long-term regional patterns in the retreat of marine-terminating glaciers in the CAA.

Atmospheric temperatures and regional frontal change Atmospheric temperatures in the CAA have increased rapidly in the 21st century, but the relationship between rising temperatures and the retreat rates of marine-terminating glaciers has not previously been investigated. To quantify this, we evaluate modeled climate data from the Regional Atmospheric Climate Model 2.3 (RACMO2.3), statistically downscaled from 11- to 1-km resolution. The downscaled data provide a detailed reconstruction of glacier SMB components for the period 1958–2015, corrected for biases in elevation and ice albedo. The 1-km product corresponds well with altimetry records and shows a close correlation between surface meltwater production and measured mean annual near-surface temperatures. From the downscaled version of RACMO2.3, we calculate mean ablation rates [millimeter water equivalent (w.e.) per year], mean ablation area ratio (as the percentage of basin size), and mean runoff (millimeter w.e. per year) for the full drainage basins of all 300 marine-terminating glaciers (see Materials and Methods and Figure. S5).

Correlations between overall relative changes in glacier length (1958–2015) and the mean surface ablation components over the same period are all statistically significant [Spearman’s rank (rs), two-tailed, P < 0.01; table S6]. Glaciers that have retreated more are found to drain basins that have (i) higher mean ablation rates, (ii) larger ablation areas, and (iii) greater runoff, compared to those that have retreated less (Figure. 6). The strongest correlation (rs = −0.506, P < 0.01) is between overall frontal change and mean runoff. Overall, retreat rates are

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Atmospheric Forcing of Rapid Marine-terminating Glacier Retreat in the Canadian Arctic Archipelago (Continued) lowest north of 78°N in Nares Strait (Figure. 1 and Figure. S2A), where runoff is the least (Figure. S6), near- surface air temperatures are the lowest, and sea ice concentration remains high. There has been an increase in runoff from marine-terminating glaciers throughout the CAA, with the highest runoff occurring most recently (Figures. S6 and S7 and table S7). The mean runoff at all latitudes during this latter period is over 40% greater than the mean before 2001. Few weather stations are situated in the region, but available records all show a trend toward more positive anomalies in mean summer near-surface air temperature (Figure. S8).

The temporal trends in runoff and ablation area extent between 1958 and 2015 correspond closely with the trends in frontal retreat rates in both the QEI and BBI regions (Figure. 2 and table S7). During concurrent periods, retreat rates are lower during times of lower runoff and smaller ablation areas and greater when runoff and ablation areas are larger. The only period for which this is not the case is 2002–2009 in the QEI, when the runoff and ablation areas decreased slightly when frontal retreat rates were increasing (although the direction of change is still consistent; Figures. 2, A and C). In BBI, the correlation persists across all six time periods, including the anomalous period of greater glacier retreat in 1980–1985, when there was a distinct increase in runoff/ablation (Figures. 2, B and D). A recent study of surface mass loss from 1-km downscaled RACMO2.3 output shows that ice masses in the CAA experienced significant near-surface atmospheric warming (+1.1°C above the 1958–1995 mean) in the mid- 1990s, which caused increases in meltwater runoff across the region. The region-wide acceleration in retreat rates since ~2000 corresponds well with the modeled and measured increase in mean annual near-surface atmospheric temperatures. No lag effects are observed between the surface ablation rates and the frontal changes at the scale of the multiyear periods used in this study. This indicates a rapid and near-synchronous response of marine- terminating glacier fronts to atmospheric temperature changes throughout this region.

Figure. 6 Mean overall (1958–2015) glacier front changes (as the percentage of glacier length in 2000) by mean basin runoff and relative ablation area over the same time period. Glacier counts are in table S3. Error bars are ±1 SE. Statistically significant (P < 0.01) correlations are present between retreat rate and the surface ablation variables (table S6).

DISCUSSION AND CONCLUSIONS Our analysis indicates a rapid increase in the rate of glacier retreat in the CAA since the start of the 21st century. The evidence does not support ocean temperatures as the driver of the regional trends in glacier retreat along the western side of either Baffin Bay or Nares Strait, despite ocean forcing being recognized as a key driver of glacier frontal retreat on the eastern side of Baffin Bay in west Greenland. Both ocean temperature measurements and reanalysis data show predominantly cold waters, both on a broad regional scale (up to 100 km from the coast) and closer to glacier fronts (where data are available within 20 and 5 km). In contrast to west Greenland, CAA glaciers are relatively thin at their grounding lines and tend to occupy shallower water. They are, therefore, less vulnerable to deep ocean temperature changes, and the mean ocean temperatures at shallower depths have been too low to have driven the observed region-wide patterns of retreat.

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Atmospheric Forcing of Rapid Marine-terminating Glacier Retreat in the Canadian Arctic Archipelago (Continued) Results from the 1-km downscaled RACMO2.3, however, demonstrate a significant correlation between marine- terminating glacier front retreat rates and SMB components (mean ablation rate, ablation area ratio, and mean runoff) over the period 1958–2015. The relationship is quantitatively robust and occurs across the region. Moreover, temporal variations in retreat rates and surface runoff rates are largely synchronous, suggesting that there is a rapid response of CAA marine-terminating glacier fronts to near-surface atmospheric warming. External forcings on the fluctuations of marine terminating glaciers have been noted in varying degrees in other locations on Earth, but the regional dominance of atmospheric temperatures in driving marine-terminating glacier retreat has not been observed elsewhere.

Glacier frontal retreat rates have followed a similar trend to the rates of surface mass loss in the CAA, which have accelerated since ~2000. We suggest that the mechanisms by which increased surface air temperatures drive rapid retreat of marine-terminating glaciers in this region are likely to be much the same as those that drive retreat of the land-terminating glaciers. That is, increased melt and subsequent meltwater runoff reduces mass delivery to glacier termini through reductions in glacier flow, with the consequence that glaciers thin and retreat in their ablation areas. Other mechanisms observed on marine-terminating glaciers in Greenland, such as increased surface melt amplifying the delivery of ocean heat to the terminus via the creation of an upwelling plume at the grounding line, are less likely to be a primary mechanism behind regional glacier retreat in the CAA. This is because most of CAA glaciers terminate in shallow water, making them less vulnerable to intrusion of warmer deep water. However, this mechanism could be important on the local scale for the few CAA glaciers that terminate in deep water, although further research is required to examine this.

It is now widely acknowledged that ocean temperature increase has been the dominant driver of glacier retreat in other polar regions in recent years, particularly along the western Antarctic Peninsula, around the West Antarctic Ice Sheet, and in western Greenland. In contrast, we show that, in the CAA, the substantial rise in atmospheric temperature in the 21st century has outweighed any regional impact of changing ocean temperature on marine- terminating glacier frontal behavior. It follows that ocean temperature cannot be assumed to be the primary driver of marine-terminating glacier retreat in all polar regions and that studies of local processes are needed to understand the impacts of climate change on glacier behavior.

MATERIALS AND METHODS Glacier frontal positions data sources The baseline image on which our study is based is the Natural Resources Canada (NRCan) image mosaic of the CAA, compiled from Landsat 7 scenes between 1999 and 2002, and orthorectified using ground control and conjugate points. The seamless mosaic is considered accurate to ~20 m, or less than one image pixel (30 m). We downloaded the image tiles for our region of interest from the Canadian Open Government portal (https://open.canada.ca/data/en/dataset). The glacier basin outlines were downloaded from the Global Land Ice Measurements from Space (GLIMS) database, with manual checking and correction of gross errors (e.g., coregistration of the basin outlines to the NRCan Landsat 7 mosaic). The glacier frontal positions for the period 1999–2002 were digitized from the mosaic and joined to the glacier basin outlines to produce closed polygons, with relevant attributes (such as glacier length, type, and ID) from the GLIMS database. Marine-terminating glaciers were identified as those that terminated in the ocean at the time of the Landsat 7 mosaic (1999–2002). In many cases, it was not possible to identify from the images the extent to which the glacier descended below sea level, and so, we used the general term “marine-terminating” rather than “tidewater” glacier. In total, we mapped all 300 marine-terminating glaciers in the CAA with basin size greater than 2 km2 (in 1999–2002), excluding those on the northern coast of Ellesmere Island (Figure. 1).

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Atmospheric Forcing of Rapid Marine-terminating Glacier Retreat in the Canadian Arctic Archipelago (Continued) Glacier frontal positions were manually digitized, using Esri ArcGIS10.5 software from a range of sources (table S1). Aerial photographs, from survey missions between 1956 and 1961, were the first observations that covered the whole region. These were accessed from the NRCan National Air Photo Library. The glacier frontal positions were mapped from the aerial photographs by on-screen digitizing from scans of the photographs georeferenced to the Landsat 7 mosaic base image. Where original photographs were not available, the glacier fronts were digitized from scans of National Topographic System map sheets, which were compiled in the 1960s from the aerial photographs. An average of seven frontal positions, at approximately decadal intervals, were mapped per glacier. Post-1960, most glacier frontal positions were digitized from Landsat satellite images. These were accessed and downloaded using the U.S. Geological Survey Earth Explorer portal (https://earthexplorer.usgs.gov/). Image resolutions ranged from 5 m (aerial photographs) to 60 m (Landsat Multispectral Scanner), with all post-2000 images (Landsat Enhanced Thematic Mapper Plus and Landsat 8) at 15-m resolution. High relative positional accuracy was maintained by ensuring that the frontal positions were georegistered to the geospatially accurate Landsat 7 mosaic. The digitization methodology follows standard procedures when mapping glacier changes in previous work. Uncertainties in the glacier change results stem from the reliability of the source material, measurement error, and methods used to calculate change, as documented in Cook et al. Similarly, in this study, it is estimated that the digitization uncertainties are less than the largest image pixel size (60 m). Uncertainty in the data analysis methods is described in the following section.

All glacier frontal data were assigned the relevant source material attributes including date, year, and source ID. In total, 2039 frontal positions were digitized for 211 glaciers in the QEI and 89 in BBI (Figure. S1).

Glacier frontal change analysis Change rate calculations. All frontal positions digitized for each basin were joined to the glacier basin outline, resulting in individual area polygons for each time period. Glacier area measurements take into account uneven changes along the ice front and offer a more precise assessment of change than can be obtained using along-flow sample lines that intersect the glacier fronts. Differencing of polygon areas from one epoch to another provides values of absolute area change. We used the established “box method”, with some minor modifications. Using ArcMap, a box polygon was digitized for each glacier, intersecting the glacier front at its maximum width and extending upflow to an undefined distance, ensuring that all decadal positions were contained within the box (Figure. S1). Each frontal position was then clipped to the box and projected into equal-area projection (WGS84 Lambert Azimuthal Equal Area). The area of each clipped polygon (in square kilometers) enabled area change calculations between time periods, and the mean changes in length were calculated by dividing the area changes by the width of the glacier box. The rates of change (in meters per year) were then calculated by dividing the changes in length by the number of years that elapsed between the frontal positions. The magnitude of absolute glacier retreat is related to glacier size (larger glaciers tend to change more in absolute terms), so to enable comparisons between basins, we normalized changes according to glacier length. The relative length changes (as percentage per year) were calculated relative to the glacier length in 1999–2002 (using the length attribute reported in GLIMS).

Statistical analyses of regional and temporal glacier changes include mean absolute (meters per year) and relative (percentage per year) rates of frontal change for glaciers in the QEI and BBI over each time interval (Figure. 2 and table S2) and relative rates of frontal changes over each time interval at 1° latitudes (Figure. S2). For all statistical analyses, mean values are considered a suitable measure, with SE bars to show the uncertainty around the estimate of the mean, considered the most valuable measure based on the large sample size. The CAA were split into the two broad regions of QEI and BBI for the following reasons: (i) clarity in data analysis, (ii) the notably different glacier basin sizes and frontal widths between the two regions, and (iii) the differences in image availability, which

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Atmospheric Forcing of Rapid Marine-terminating Glacier Retreat in the Canadian Arctic Archipelago (Continued) results in differing start/end years for the time periods. Grounding line ice thickness. Ice thickness at the grounding lines of tidewater glaciers in the QEI was calculated by Van Wychen et al. using data from a range of sources. Ice thickness values for the glaciers in the present study were obtained from Van Wychen et al.

Ocean temperature data and methods Ocean temperature data. 1) TOPAZ4 Arctic Ocean reanalysis data (http://marine.copernicus.eu/). The TOPAZ4 Arctic Ocean Physics reanalysis data provide physical ocean variables for the time period 1991–2015. These data are available from the CMEMS (product identifier: ARCTIC_REANALYSIS_PHYS_002_003). The TOPAZ reanalysis system assimilates all available satellite and in situ temperature-salinity profile observations, resulting in modeled data as a 12.5-km resolution grid. We downloaded the mean monthly temperature and salinity NetCDF files at the subsurface standard depths available: 50, 100, 200, 400, and 700 m. Mean annual temperatures were calculated from the grid values at each depth. The quality information document (http://cmems- resources.cls.fr/documents/QUID/CMEMS-ARC-QUID-002-001a.pdf) gives the estimated accuracy for modeled temperatures as statistical values for bias and root mean square (RMS) differences. The forecast biases are −0.35°C at 5- to 100-m depth, −0.2°C at 100- to 300-m depth, and 0.5°C at 300- to 800-m depth. Forecast RMS differences are 0.6°C at 5- to 100-m depth, 0.8°C at 100- to 300-m depth, and 1.0°C at 300- to 800-m depth. These accuracy estimates are for the full domain, but there is variability in the system accuracy, e.g., in some regions with few temperature profile data, the reanalysis Root Mean Square error (RMSE) can be up to 1.0°C. Our statistical analysis does not include TOPAZ4 reanalysis data, and therefore, these errors do not affect our results. Rather, the data are used to illustrate the regional overview of mean annual ocean temperature at each depth (Figure. 3).

2) World Ocean Database in situ measurements (www.nodc.noaa.gov/OC5/WOD13). The World Ocean Database (WOD13) is a comprehensive set of historical ocean profile data collected between 1972 and 2015. Data are provided by the NOAA National Centers for Environmental Information. We selected in situ data from CTD profiles for the region surrounding the CAA. We downloaded temperatures at the following standard depths available: 50, 100, 150, 200, 300, 400, and 500 m. WOD13 water properties at standard depths were measured and reported either directly or by interpolation, as set out by commonly used criteria. The data variables include the measurement station ID, latitude/longitude, date, and year, in addition to temperature and salinity values. To download the data for our region of interest, we used the Ocean Data View software provided by NOAA. Error estimates described in the WOD13 report state that the accuracy of CTD measurements varies with instrument design, typically from 0.005° to 0.001°C for temperature. The overall quality of CTD measurements are also affected by calibration drift (i.e., prolonged use of the CTD instrument in the sea), data streaming problems, varying speeds of the CTD, and rapid changes in the ocean environment. The measurements are assigned a flag in quality control checks and measurements that are considered to fail quality control are excluded from WOD13. Other flags range from good quality (1), to potentially bad quality but based on visual inspection found no reason not to be used in scientific research (4). For our study, we included all measurements available in WOD13, and our representation of error in the statistical analysis methods is SE, considered the most valuable owing to the large sample size.

Bathymetric data (www.ngdc.noaa.gov/mgg/bathymetry/arctic/arctic.html). The standard depths at which the ocean temperature measurements were chosen were based on the range of depths close to the glacier fronts. These were established from the NOAA International Bathymetric Chart of the Arctic Ocean v3, 500-m grid (Figure. S3). The grid is compiled from all available multibeam, dense single beam, and land data and built using a gridding algorithm at 500-m resolution. It is currently recognized as the best Arctic Ocean Digital Bathymetric

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Atmospheric Forcing of Rapid Marine-terminating Glacier Retreat in the Canadian Arctic Archipelago (Continued) Model available. We downloaded and clipped the dataset to our region of interest and removed land data before undertaking statistical analysis of depths close to glacier fronts.

Buffers extending from the marine-terminating glacier fronts (at their positions ca. 2000) were created at 5- and 20- km radii (Figure. S3). The mean bathymetry within the 20-km buffer for all 300 glaciers is 166 m of which 40 glaciers have mean bathymetry deeper than 400 m. Within 5 km of all glacier fronts, the mean bathymetry is 90 m, and 11 glaciers have a mean bathymetry deeper than 400 m within 5 km of their fronts. Of the 117 glaciers with ≥10 ocean temperature measurements within 20 km, 36 have a mean depth deeper than 400 m (mean, 477 m). When all glaciers are separated by 1° latitudes, the mean depths within 20 km are shallower than 400 m in all latitudinal degree bands, and within 5 km, mean depths are shallower than 200 m. Ocean temperature statistics. Regional modeled mean ocean temperatures were mapped in ArcMap using TOPAZ4 gridded data for the region surrounding the CAA and west Greenland (Figure. 3). Ocean temperature analysis close to glacier fronts was carried out using in situ data from the WOD13 within the buffers at 20- and 5-km radii from each glacier front (Figure. S3). In total, 117 glaciers have ≥10 CTD measurements within 20 km of their fronts, and 19 glaciers have ≥5 CTD measurements within 5 km of their fronts. Measurements within both radii have similar mean temperatures at each depth (Figure. S4). Glaciers with ocean measurements within 20 km provided a wide-ranging sample for statistical analysis. Our investigations of glacier front change and ocean temperature correlations include glacier relative change value bins and mean ocean temperatures at each depth (glacier counts are shown in table S3). Spatiotemporal trends are based on WOD13 ocean temperature data within the 100-km buffer zone (Figure. 5). The region size was broken down by 1° latitudinal bands for regional mean ocean temperature analysis (Figure. S3).

Atmospheric temperature data and methods RACMO2.3 downscaled to 1-km data source. A detailed reconstruction of SMB between 1958 and 2015 is available from the RACMO2.3 statistically downscaled to 1 km. The model is a downscaled version of the 11-km RACMO2.3, corrected for biases in elevation and bare ice albedo, and provides regional SMB data at a scale suitable for measuring mean values within individual glacier basins in the CAA. The SMB output is in millimeter w.e. year−1.

The methods used in producing the model, along with accuracy assessments based on in situ mass balance measurements, are reported in Noël et al. These are summarized as follows. The SMB values from both the 11- and 1-km versions of RACMO2.3 were assessed against 4356 stake measurements within the CAA. Correcting for elevation and bare ice albedo bias during the downscaling process significantly improved the accuracy. Assessment with observational data showed that, relative to the 11-km model, the 1-km downscaled version of RACMO2.3 has a much improved mean SMB bias (40 mm w.e.), RMSE (380 mm w.e.), and R2 (0.74). The product uncertainty for SMB was estimated at 11.0 and 4.5 Gt year−1 for Northern CAA (NCAA = QEI) and Southern CAA (SCAA = BBI), respectively. To obtain these estimates, the 4356 in situ SMB measurements from 1961 to 2016, e.g., from Sverdrup Glacier on Devon Island (yellow dots in Figure. S5), were binned in 250 mm w.e. intervals for which the mean bias was estimated (i.e., modeled minus measured SMB), which was then applied to total ablation and accumulation areas across the region. The RACMO2.3 product was further evaluated using independently derived mass changes from ICESat and CryoSat-2 altimetry measurements. The downscaled dataset shows good agreement with the altimetry records, and both winter accumulation and summer ablation are accurately resolved. In addition, the SMB product falls well within ICESat/CryoSat-2 measurement uncertainty. Ice mass loss estimates from the downscaled SMB product are similar to estimates from other mass loss studies. Mass loss in 2004–2009 was estimated at 33.8 ± 11.5 Gt year−1 for the NCAA and 24 ± 4.5 Gt year−1 for the SCAA. For comparison, a study by Gardner et al. estimated mass loss from GRACE (Gravity Recovery and Climate Experiment) to be 39 ± 9 and 24 ±

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Atmospheric Forcing of Rapid Marine-terminating Glacier Retreat in the Canadian Arctic Archipelago (Continued) 7 Gt year−1 for NCAA and SCAA, respectively.

SMB calculations per glacier basin. For our study, the SMB calculations per glacier basin were made from the downscaled RACMO2.3 by converting the gridded values to points for analysis in ArcGIS. The 1-km resolution data enabled statistical analysis of ablation components within each glacier basin (Figure. S5). The mean ablation rates per glacier basin were calculated from negative (i.e., <0) SMB values, in mm w.e. year−1. Net ablation occurs in the region of the glacier basin where ice loss through surface melt, sublimation, and runoff is greater than ice gain through precipitation and riming. The ablation area ratio (in percentage) was therefore calculated as the area where SMB < 0 mm w.e. year−1, divided by the glacier basin area. This is calculated from the gridded data, i.e., the number of negative SMB points divided by the total number of grid points that constitute each glacier basin. The ablation area ratio enables analysis of patterns of change across all glaciers by normalization of basin areas. Mean annual runoff (in mm w.e. year−1) represents basin-wide nonrefrozen rain and meltwater induced by the surface energy balance, and we calculated this from the sum of the runoff component values divided by the number of grid points per basin. It should be noted that summer runoff consist partly of rainfall, which does not contribute to mass loss, but the flux is small.

We calculated mean values of these components across the whole study period (1958–2015) and, for each time period, used in the glacier front change study. Mean runoff patterns show similar temporal trends for marine- terminating glaciers at all degrees latitude, with a statistically significant increase in recent time periods (Figure. S6). This is reflected in the spatiotemporal patterns of runoff from ice masses throughout the CAA (Figure. S7). Automatic weather station data. Near-surface (2 m) air temperature data are available from weather stations operated by Environment Canada from 1948 to the present (http://climate.weather.gc.ca/). There are six weather stations located throughout the CAA (locations shown in Figure. 1), and although some are situated at a considerable distance from the marine-terminating glaciers in this study, the records provide a useful indication of long-term temperature trends in the wider region. Temperature measurements are available at the following weather stations in the QEI: Eureka (1947–2014), Resolute (1947–2014), and Grise Fiord (1984–2007); and in BBI: Pond Inlet (1975–2007), Cape Hooper (1957–2007), and Cape Dyer (1959–1993). Mean summer air temperatures, for the months of June to August (JJA), were calculated from monthly mean temperatures derived from daily averages. Annual anomalies in mean summer temperatures were calculated relative to the mean temperature across the complete time period available for each weather station (Figure. S8).

Read more about this article at: http://advances.sciencemag.org/content/5/3/eaau8507 Waterfall-chasing Scientists Uncover Rare, Self-forming Cascades

Most waterfalls tell a clear story about their origins: Yosemite Falls in California cascades over a sheer granite cliff, the remains of ice age glaciers that once carved out the valley’s steep walls. Others are the result of major earthquakes or sudden changes in rock type. But some, like the Seven Teacups in California’s Sierra Nevada mountains (above), have no obvious external cause. Now, scientists have a new potential explanation for these mysterious waterfalls. What makes the Seven Teacups so inscrutable is the unbroken granite slope they’re etched into. There’s no sudden cliff to suggest that past earthquakes or retreating glaciers created the falls, and the composition of the granite is roughly the same throughout, meaning a change from harder to softer rock wasn’t responsible for the falls’ creation.

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Waterfall-chasing Scientists Uncover Rare, Self-forming Cascades (Continued)

To test whether such a waterfall could form without external causes, researchers built a 7.3-meter artificial river, or flume, with a 20% downhill slope. For 20 minutes at a time, researchers sent a steady flow of gravel-laden water along a flat riverbed made of soft foam. Then, the team used a laser scanner to measure any changes in the surface of the foam riverbed.

Over the course of 11 trials, the gravel and water wore away tiny undulations in the foam. Two of those depressions got deeper and deeper until they became miniature, selfmade waterfalls, the researchers report today in Nature. Feedback from the water, the gravel, and the erosion of the artificial riverbed interacted to make the cascades, suggesting the same mechanism could be responsible for waterfalls like the Seven Teacups.

To determine how widespread this phenomenon is, the researchers say they’ll need to chase more waterfalls—and develop some way to identify these selfmade cascades. The findings could clarify the origins of such waterfalls on Earth and even Mars, where astronomers have spotted dried-up riverbeds—and what may be the remains of martian waterfalls. If waterfalls can arise without external causes, scientists will need to be cautious in using them to reverse engineer the Red Planet’s history.

Read more about this article at: http://www.sciencemag.org/news/2019/03/waterfall-chasing-scientists-uncover-rare-self-forming-cascades

A Homespun Canadian Telescope Could Explain Mysterious Radio Signals From the Distant Universe

PENTICTON, CANADA—Reporting from the Dominion Radio Astrophysical Observatory here requires old- school techniques: pad and pen. Upon arrival, I must turn off my digital recorder and cellphone and stash them in a shielded room with a Faraday cage—a metal mesh that prevents stray electromagnetic signals from escaping. The point is to keep any interference away from the observatory's newest radio telescope, the Canadian Hydrogen Intensity Mapping Experiment (CHIME).

On a clear, cold day in January, Nikola Milutinovic stands on the vertiginous gantry that runs along the focus of one of CHIME's four 100-meter-long, trough-shaped dishes. Milutinovic, a scientific engineer at the University of British Columbia (UBC) in Vancouver, scans their reflective surfaces for snow, which generally sifts through the metallic mesh but sometimes sticks and freezes. Snow-covered hills surround him, shielding CHIME from the cellphone towers, TV transmitters, and even microwave ovens of nearby towns. "If you switched on a cellphone on Mars, CHIME could detect it," he says.

CHIME's quarry is neither so faint nor so close. The telescope is smaller and cheaper than other leading radio observatories. But by luck as much as design, its capabilities are just right for probing what may be the most

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A Homespun Canadian Telescope Could Explain Mysterious Radio Signals From the Distant Universe compelling new mystery in astronomy: signals from the distant universe called fast radio bursts (FRBs). Discovered in 2007, FRBs are so bright that they stick out in the data like a peak in the nearby Canadian Rockies—so long as a telescope is watching and its electronics are fast enough to pick out the pulses, which last only a few thousandths of a second.

Just days before I visit, CHIME—still in its shakedown phase—had made global headlines for bagging 13 new FRBs, bringing the total known to more than 60. Nearly that many theories exist for explaining them. One of the few things researchers know for sure, from the nature of the pulses, is that they come from far beyond our Milky Way. But in an instant, each event is over, leaving no afterglow for astronomers to study and frustrating efforts to get a fix on their origin.

Whatever generates FRBs must be compact to produce such short pulses, astronomers believe, and extremely powerful to be seen at such great distances. Think neutron stars or black holes or something even more exotic. FRBs can repeat—although strangely, only two of the dozens known appear to do so. The repetition could rule out explosions, mergers, or other one-time cataclysmic events. Or repeating and solitary FRBs could be different animals with different sources—theorists just don't know.

What they need are numbers: more events and, most important, more repeaters, which can be traced to a particular environment in a home galaxy. CHIME will deliver that by surveying the sky at high sensitivity. Its troughs don't move, but they observe a swath of sky half a degree wide, stretching from one horizon to the other. As Earth turns, CHIME sweeps across the entire northern sky. Sarah Burke-Spolaor, an astrophysicist at West Virginia University in Morgantown, says its sensitivity and wide field of view will enable it to survey a volume of the universe 500 times bigger than the one surveyed by the Parkes radio telescope in Australia, which discovered the first FRB and 21 others. "CHIME just has access to that all day, every day," she says.

Once CHIME's commissioning phase is over later this year, scientists think it could find as many as two dozen FRBs per day. "Within a year, it will be the dominant discoverer of FRBs," says Harvard University astrophysicist Edo Berger.

The strange-looking telescope has been a labor of love for the small team behind it—labor being the operative word. A contractor assembled the dishes, lining the troughs with a radio-reflective steel wire mesh. But everything else was painstakingly assembled by researchers from UBC, the University of Toronto, and McGill University in Montreal. That includes 1000 antennas fixed beneath the gantry at each trough's focus, 100 kilometers of cabling, and more than 1000 computer processors that sit inside radiation-shielded shipping containers next to the dishes.

"Everyone has put their hands on the telescope," says Milutinovic, who puts in shifts monitoring it and its computer systems. It's not just a desk job. Although he left alone two baby ospreys that nested on a tall pole near the telescope, he has called in conservationists to remove other birds that set up house in the telescope's structure, along with the occasional rattlesnake. When a humidity sensor in one of the computer containers goes off at night, Milutinovic makes the 25-minute drive to the deserted observatory to check it out. He worries about other nocturnal visitors. "I've seen the tracks of coyote, and there's a bear that hangs around here."

In a field in which front-rank telescopes cost billions, the CA$20 million CHIME looks set to have an impact out of all proportion to its price tag. "CHIME shows you can build a telescope that makes the world news pretty cheaply," Milutinovic says.

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A Homespun Canadian Telescope Could Explain Mysterious Radio Signals From the Distant Universe

Hydrogen hunt None of that was part of CHIME's original job description. Back in 2007, a group of cosmologists in Canada had the idea of building a cheap telescope to measure the 3D distribution across the universe of hydrogen gas clouds, which glow faintly at radio frequencies. The aim, says Keith Vanderlinde of the University of Toronto, was to map ripples in the density of matter created soon after the big bang and chart their expansion over cosmic history. A change in the expansion rate would tell researchers something about dark energy, the mysterious force thought to be accelerating the universe's growth. "Any handle we can get on it would be a huge boon to physics," Vanderlinde says.

CHIME would also be an excellent machine for studying pulsars. Pulsars are neutron stars, dense cinders of collapsed giant stars, that shoot electromagnetic beams out of their poles while rotating like a celestial lighthouse, sometimes thousands of times per second. Astronomers on Earth detect the beams as metronomic pulses of radio waves. CHIME will monitor 10 pulsars at a time, 24 hours a day, for hiccups in their perfect timekeeping that could result when passing gravitational waves stretch intervening space.

When CHIME was conceived, few people were thinking about FRBs because the first, found in 2007 in archival Parkes telescope data, was such an enigma. It had a high dispersion measure, meaning the pulse was smeared across frequencies because free electrons in intergalactic space had slowed the burst's low-frequency radio waves disproportionately. The high dispersion measure suggested the burst came from billions of light-years away, far beyond our local group of galaxies.

The pulse was still bright, implying the source's energy was a billion times that of a pulsar pulse. Yet its short duration meant the source could be no bigger than 3000 kilometers across because signals could not cross a larger object fast enough for it to act in unison and produce a single, short pulse. A citysize pulsar could fit in that space. But how could a pulsar detonate so powerfully?

Astronomers were tempted to dismiss that first burst as a mirage. But it was no anomaly: Another pulse was uncovered in Parkes archival data in 2012. Then, after an upgrade with new digital instruments, Parkes detected four more in 2013, all with high dispersion measures, suggesting cosmically distant origins. That paper "made me a believer," says McGill astronomer Victoria Kaspi, who was working to integrate pulsar monitoring into CHIME.

The paper also sparked a realization: CHIME could be adapted to look for FRBs, too. "Vicky called me up and said, ‘You know, this would also make a good FRB machine,’" recalls Ingrid Stairs, a collaborator of Kaspi's at UBC.

Unlikely partners The upgrade was not easy. Catching FRBs requires finer time and frequency resolution than mapping hydrogen. CHIME's data would have to be logged every millisecond across 16,000 frequency channels, Kaspi says. To do that meant tinkering with the correlator, the fearsomely parallel computer that chomps through the 13 terabits of data streaming every second from CHIME's 1024 antennas—comparable to global cellphone traffic.

The time-critical astrophysicists needed a different output from the sensitivity-is-everything cosmologists. The cosmologists, eager to map the cosmic clouds, could get by without the extra resolution. At the end of each day, they could download data onto a hard disk and ship it to UBC for leisurely processing. But that wasn't an option for the FRB hunters, who needed high-resolution data that would quickly overwhelm a hard drive. Kaspi and her

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A Homespun Canadian Telescope Could Explain Mysterious Radio Signals From the Distant Universe colleagues devised algorithms to scan in real time just a few minutes of high-resolution data stored in a buffer. If an event is detected, the key 20 seconds of data around it are saved. If there's nothing, they're dumped. Searching for FRBs is "smash and grab science," says team member Paul Scholz of the Dominion Radio Astrophysical Observatory in Okanagan Falls.

As test observations began in 2017, the team got twitchy about how many FRBs CHIME would see. CHIME was observing at frequencies of 400 to 800 megahertz (MHz), lower than the 1.4-gigahertz frequency used to detect most FRBs. A 300-MHz survey at a different telescope had found nothing, and another survey at 700 to 800 MHz saw just a single burst. "It was worrying, especially in the lower part of the band," Stairs says.

Those worries evaporated in July and August 2018, when the team struck gold with the 13 new FRBs, even though sections of the telescope were sporadically taken offline for adjustments. The haul, published in Nature in January, included one repeater—only the second yet discovered. Kaspi declined to provide an update on the number of FRB discoveries since last summer, citing two unpublished papers in the works. But she says CHIME is "fulfilling expectations." "It's a bit like drinking from a firehose, but in a good way," she says.

Theories abound Theorists want all that CHIME will deliver, and then some. A poverty of information is allowing ideas to run riot. "Almost every aspect of FRBs is in play for theorists," Berger says. An online catalog of FRB origin theories had 48 entries at the time of writing. Many theorists initially put forward models based on the violent collapse or merger of compact objects, including white dwarfs, neutron stars, pulsars, and black holes. But the discovery of repeaters shifted speculation to sources that would not be destroyed in the act of generating a burst.

Active galactic nuclei, the supermassive black holes at the centers of galaxies, spew winds and radiation that might trigger a burst by striking nearby objects—a gas cloud, a small black hole, or a hypothetical quark star. Or the bursts might come from more speculative phenomena, such as lightning strikes in the atmospheres of neutron stars or the interaction of hypothetical dark matter particles called axions with black holes or neutron stars. Amanda Weltman, a theorist at the University of Cape Town in South Africa, does not discount even more fanciful ideas such as cosmic strings, hypothetical threadlike defects in the vacuum of space leftover from the moments after the big bang. They "could be releasing fast radio bursts in a number of ways," she says.

But as the number of detected FRBs moved from single digits into dozens, astronomers realized the bursts could be downright common, detectable by the thousands every day if the right telescopes were watching. "That's a serious problem for a lot of models," Berger says.

FRB 121102, the first repeating event detected, may be the most revealing FRB so far. The Arecibo telescope in Puerto Rico saw its first burst in 2012, but since then dozens more have been seen coming from that spot on the sky. In 2017, the 27-dish Karl G. Jansky Very Large Array in New Mexico revealed the FRB resides in the outskirts of a distant dwarf galaxy and that the location coincides with a weak but persistent radio source. That dim radio glow may emanate from a supernova remnant—an expanding ball of gas from a stellar explosion, which could have formed a black hole or neutron star that powers the FRB. In another clue, the polarization of the FRB's radio waves rotates rapidly, suggesting they emanate from a strong magnetic environment.

Brian Metzger, a theorist at Columbia University, believes a young magnetar—a highly magnetized neutron star— resides at the center of the cloud and powers the bursts. In a scenario developed with his colleagues, its magnetic fi ld fi t f th t i ll fl bl ti t h ll f l t d i t l th

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A Homespun Canadian Telescope Could Explain Mysterious Radio Signals From the Distant Universe field serves as a fizzy store of energy that occasionally flares, blasting out a shell of electrons and ions at nearly the speed of light—an outburst resembling a coronal mass ejection from our sun, but on steroids. When the flare hits ion clouds leftover from previous flares, the resulting shock wave boosts the strength of the clouds' magnetic field lines and causes electrons to spiral around them in concert. Just as synchrotrons on Earth whip electrons around racetracks to emit useful x-rays, those gyrations spawn a coherent pulse of radio waves.

Magnetars are often invoked to explain such energetic events, Metzger says. "They're a catch-all for anything we don't understand. But here it's kind of warranted." CHIME team member Shriharsh Tendulkar of McGill wonders whether objects such as magnetars could explain both repeaters and single-burst FRBs. Single-burst FRBs might "start out regular as repeaters, then slow as [the source's] magnetic field weakens," he says.

But according to Weltman, it's too early to declare the mystery solved. "There are so many clues here, but they do not yet point to a single conclusive theoretical explanation," she says.

As Earth turns, CHIME's troughs sweep the sky for fleeting radio bursts, focusing signals to antennas in an overhead gantry.

Knowledge in numbers As observers amass new FRBs, different classes of events may emerge, perhaps offering clues about what triggers them. FRBs may also turn out to come from specific types of galaxies—or regions within galaxies— which could allow theorists to distinguish between active galactic nuclei and other compact objects as the sources. "We need statistics and we need context," Metzger says.

In the coming years, other FRB spotters will come online, including the Hydrogen Intensity and Realtime Analysis eXperiment in South Africa and the Deep Synoptic Array in California. With their widely spaced arrays of dishes, both facilities will precisely locate FRBs on the sky—something CHIME can't do for now. "They're all going for localization because they know CHIME will clean up on statistics," Scholz says.

The CHIME team, not to be outdone, is drawing up a proposal to add outriggers, smaller troughs at distances of hundreds of kilometers, which will record the same events from a different angle and so help researchers pinpoint them. "With all these new efforts, there'll be substantial progress in the next few years," Metzger says.

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A Homespun Canadian Telescope Could Explain Mysterious Radio Signals From the Distant Universe

For now, as CHIME's commissioning phase winds down, Milutinovic's job is to ensure that it keeps doing its job. "You want it to be boring," he says. "It's the weather that gives us most issues"—snow on the troughs, summer heat waves that tax the cooling system for the electronics. Then there's the grass, a wildfire risk. Every summer, the observatory invites ranchers to graze their cattle on-site—not only to be neighborly, but also because cows emit less radio frequency interference than a lawn mower. But they can't graze right around CHIME because they might chew on cables. So Milutinovic relies on diesel-powered mowers, which, lacking spark plugs, pose less of an interference problem.

But he longs for an even better high-resolution grass-cutting tool. "We thought of having a CHIME goat."

Read more about this article at: http://www.sciencemag.org/news/2019/03/homespun-canadian-telescope-could-explain-mysterious- radio-signals-distant-universe

New Fuel Cell Could Help Fix the Renewable Energy Storage Problem

If we want a shot at transitioning to renewable energy, we’ll need one crucial thing: technologies that can convert electricity from wind and sun into a chemical fuel for storage and vice versa. Commercial devices that do this exist, but most are costly and perform only half of the equation. Now, researchers have created lab-scale gadgets that do both jobs. If larger versions work as well, they would help make it possible—or at least more affordable— to run the world on renewables.

The market for such technologies has grown along with renewables: In 2007, solar and wind provided just 0.8% of all power in the United States; in 2017, that number was 8%, according to the U.S. Energy Information Administration. But the demand for electricity often doesn’t match the supply from solar and wind. In sunny California, for example, solar panels regularly produce more power than needed in the middle of the day, but none at night, after most workers and students return home.

Some utilities are beginning to install massive banks of batteries in hopes of storing excess energy and evening out the balance sheet. But batteries are costly and store only enough energy to back up the grid for a few hours at most. Another option is to store the energy by converting it into hydrogen fuel. Devices called electrolyzers do this by using electricity—ideally from solar and wind power—to split water into oxygen and hydrogen gas, a carbon-free fuel. A second set of devices called fuel cells can then convert that hydrogen back to electricity to power cars, trucks, and buses, or to feed it to the grid.

But commercial electrolyzers and fuel cells use different catalysts to speed up the two reactions, meaning a single device can’t do both jobs. To get around this, researchers have been experimenting with a newer type of fuel cell, called a proton conducting fuel cell (PCFC), which can make fuel or convert it back into electricity using just one set of catalysts.

PCFCs consist of two electrodes separated by a membrane that allows protons across. At the first electrode, known as the air electrode, steam and electricity are fed into a ceramic catalyst, which splits the steam’s water molecules into positively charged hydrogen ions (protons), electrons, and oxygen molecules. The electrons travel through an external wire to the second electrode—the fuel electrode—where they meet up with the protons that crossed through the membrane. There, a nickel-based catalyst stitches them together to make hydrogen gas (H2). In previous PCFCs, the nickel catalysts performed well, but the ceramic catalysts were inefficient, using less than

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New Fuel Cell Could Help Fix the Renewable Energy Storage Problem (Continued)

Novel fuel cells can help store electricity from renewables, such as wind farms, by converting it into a chemical fuel for long-term storage and then changing it back to electricity when needed.

70% of the electricity to split the water molecules. Much of the energy was lost as heat.

Now, two research teams have made key strides in improving this efficiency. They both focused on making improvements to the air electrode, because the nickel-based fuel electrode did a good enough job. In January, researchers led by chemist Sossina Haile at Northwestern University in Evanston, Illinois, reported in Energy & Environmental Science that they came up with a fuel electrode made from a ceramic alloy containing six elements that harnessed 76% of its electricityto split water molecules. And in today’s issue of Nature Energy, Ryan O’Hayre, a chemist at the Colorado School of Mines in Golden, reports that his team has done one better. Their ceramic alloy electrode, made up of five elements, harnesses as much as 98% of the energy it’s fed to split water.

When both teams run their setups in reverse, the fuel electrode splits H2 molecules into protons and electrons. The electrons travel through an external wire to the air electrode—providing electricity to power devices. When they reach the electrode, they combine with oxygen from the air and protons that crossed back over the membrane to produce water.

The O’Hayre group’s latest work is “impressive,” Haile says. “The electricity you are putting in is making H2 and not heating up your system. They did a really good job with that.” Still, she cautions, both her new device and the one from the O’Hayre lab are small laboratory demonstrations. For the technology to have a societal impact, researchers will need to scale up the button-size devices, a process that typically reduces performance. If engineers can make that happen, the cost of storing renewable energy could drop precipitously, helping utilities do away with their dependence on fossil fuels.

Read more about this article at: http://www.sciencemag.org/news/2019/03/new-fuel-cell-could-help-fix-renewable-energy-storage- problem

Ancient Switch to Soft Food Gave us an Overbite—and the Ability to Pronounce ‘F’s and ‘V’s

Don't like the F-word? Blame farmers and soft food. When humans switched to processed foods after the spread of agriculture, they put less wear and tear on their teeth. That changed the growth of their jaws, giving adults the overbites normal in children. Within a few thousand years, those slight overbites made it easy for people in farming cultures to fire off sounds like "f" and "v," opening a world of new words.

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Ancient Switch to Soft Food Gave us an Overbite—and the Ability to Pronounce ‘F’s and ‘V’s (Continued)

The newly favored consonants, known as labiodentals, helped spur the diversification of languages in Europe and Asia at least 4000 years ago; they led to such changes as the replacement of the Proto-Indo-European patēr to Old English faeder about 1500 years ago, according to linguist and senior author Balthasar Bickel at the University of Zurich in Switzerland. The paper shows "that a cultural shift can change our biology in such a way that it affects our language," says evolutionary morphologist Noreen Von Cramon-Taubadel of the University at Buffalo, part of the State University of New York system, who was not part of the study.

Postdocs Damián Blasi and Steven Moran in Bickel's lab set out to test an idea proposed by the late American linguist Charles Hockett. He noted in 1985 that the languages of hunter-gatherers lacked labiodentals, and conjectured that their diet was partly responsible: Chewing gritty, fibrous foods puts force on the growing jaw bone and wears down molars. In response, the lower jaw grows larger, and the molars erupt farther and drift forward on the protruding lower jaw, so that the upper and lower teeth align. That edge-to-edge bite makes it harder to push the upper jaw forward to touch the lower lip, which is required to pronounce labiodentals. But other linguists rejected the idea, and Blasi says he, Moran, and their colleagues "expected to prove Hockett wrong."

First, the six researchers used computer modeling to show that with an overbite, producing labiodentals takes 29% less effort than with an edge-to-edge bite. Then, they scrutinized the world's languages and found that hunter- gatherer languages have only about one-fourth as many labiodentals as languages from farming societies. Finally, they looked at the relationships among languages, and found that labiodentals can spread quickly, so that the sounds could go from being rare to common in the 8000 years since the widespread adoption of agriculture and new food processing methods such as grinding grain into flour.

An ancient woman from Romania shows an edgeto-edge bite (left). A Bronze Age man from Austria had a slight overbite (right).

Bickel suggests that as more adults developed overbites, they accidentally began to use "f" and "v" more. In ancient India and Rome, labiodentals may have been a mark of status, signaling a softer diet and wealth, he says. Those consonants also spread through other language groups; today, they appear in 76% of Indo-European languages.

Linguist Nicholas Evans of Australian National University in Canberra finds the study's "multimethod approach to the problem" convincing. Ian Maddieson, an emeritus linguist at the University of New Mexico in Albuquerque, isn't sure researchers tallied the labiodentals correctly but agrees that the study shows external factors like diet can alter the sounds of speech. The findings also suggest our facility with f-words comes at a cost. As we lost our ancestral edge-to-edge bite, "we got new sounds but maybe it wasn't so great for us," Moran says. "Our lower jaws are shorter, we have impacted wisdom teeth, more crowding—and cavities."

Read more about this article at: https://www.sciencemag.org/news/2019/03/ancient-switch-soft-food-gave-us-overbite-and-ability-pronounce-f-s-and-v-s

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March 2019 Atlanta Geological Society PG Candidate Workshop

Date: Saturday, March 30, 2019 Time: 10:00 am to 12:00 pm Venue: Fernbank Science Center (check-in with the receptionist for the specific classroom location) 156 Heaton Park Drive, N.E.

Atlanta, GA 30307 678-874-7102 http://fsc.fernbank.edu/

Speaker: Ken Bechely, PG

Subject: Glaciology and Glacial Geomorphology

Ken has a BS and MS in Earth Science from Northern Illinois University and has been a PG in Georgia since 1993. Ken retired from AECOM in 2016. He has been a consultant for 28 years, and was with EPA in Illinois for 13 years prior to moving to Georgia in 1986. He has first hand experience with glacial features. Ken will cover the processes and terminology of glaciers as well as the landforms remaining after their recession.

Check with the Fernbank Science Center receptionist when you arrive for the classroom we will be using. The Center is approximately 1-mile north off East Ponce deLeon Avenue from the Fernbank Museum Of Natural History where the AGS monthly meetings are held.

Also, please forward this message to anyone interested in becoming a Georgia Registered Professional Geologist, or anyone who might be interested in the topic. The classes are open to all. Membership in the AGS is not required, however, $25 per year ($10 for students) is quite a bargain for one the most active geological groups in the Southeast. For more information on becoming a member, visit www.atlantageologicalsociety.org. Contact us at the addresses below if you have questions about the workshop or the exams.

Atlanta Geological Society Professional Registration/Career Development Committee Ken Simonton, P. G., [email protected] Ginny Mauldin-Kenney, [email protected]

AGS March 2019 Page 31

Fernbank Events & Activities

Double Feature: Pterosaurs & Jurassic STEM Day Park Saturday March 16, 2019 Friday March 15, 2019 Join us as we experiment with For one night only, see Jurassic Park breakthroughs in science, technology, as it’s meant to be seen—blazing engineering and math. across a 4-story giant screen. Learn more Learn more

Fernbank Forest Bird Walk Native Plants & Wildflowers Tour Sunday March 17, 2019 Thursday March 21, 2019 Discover more about the feathered Explore the diversity of native inhabitants of Fernbank Forest, both wildflowers and learn to identify them permanent residents and visitors. during a guided tour.

Learn more Learn more

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A New Way to Museum Take a walk on the wild side as you explore 75 acres of new outdoor nature adventures. WildWoods and Fernbank Forest combine to highlight the natural world through immersive trails, educational programming, hands-on exhibits and beautiful scenery.

New for Summer! Experience the wonders of nature on Fernbank’s giant 4-story screen with Backyard Wilderness 2D, and enjoy hands-on nature adventures outside in WildWoods.

Great Barrier Reef 3D On view until May 2, 2019

Narrated by acclaimed Australian actor Eric Bana, Great Barrier Reef 3D captures the natural beauty and exquisite strangeness of the world’s largest living wonder and introduces us to the visionaries and citizen scientists who are helping us better understand and protect this awesome, bizarre, and vibrant living world. Meet reef native Jemma Craig on an expedition to document efforts to preserve the reef and swim with giant mantas, sea turtles, sharks and Minke whales as you explore this awe-inspiring natural wonderland on the world’s largest screens!

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Fernbank Museum of Natural History

(All programs require reservations, including free programs)

Now showing in the Fernbank IMAX movie theater:

Pterosaur: Flight in the Age of the Dinosaurs Showing February 9 through May 5, 2019 Neither dinosaur nor bird, pterosaurs flew with their fingers, walked on their wings, and ruled the skies. Journey through the Mesozoic Era with the largest flying animals that ever lived. This immersive exhibit includes remarkable rare fossils and casts, hands-on and digital interactives, enormous life-sized models, stunning dioramas and iPad stations with a custom app allowing guests to personalize their experience.

Earthflight 3D Showing August 3 through December 13, 2018 Just as dinosaurs began their domination of Earth, pterosaurs ruled the prehistoric skies. Some with wingspans as long as a modern jet plane, these flying reptiles were as spectacular in appearance as they were amazing in flight. Join world-renowned naturalist and documentary filmmaker David Attenborough as he recounts the fascinating story of how we humans first discovered that these creatures were real and how they were even able to get off the ground and indeed soar...until their sudden disappearance from Earth.

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AGS Officers AGS Committees

President: Ben Bentkowski AGS Publications: Open [email protected] Phone (770) 296‐2529 Career Networking/Advertising: Todd Roach Phone (770) 242‐9040, Fax (770) 242‐8388 Vice‐President: Steven Stokowski [email protected] [email protected]

Continuing Education: Open

Secretary: Rob White Fernbank Liaison: Miranda Gore Shealy Phone (770) 891‐0519 Phone (404) 929‐6341 [email protected] [email protected] Doug John Treasurer: John Salvino, P.G. Phone (404) 929‐6342 Phone: 678‐237‐7329 [email protected] [email protected]

Georgia PG Registration: Ken Simonton Past President Phone: 404‐825‐3439 Shannon Star George [email protected] [email protected] Ginny Mauldin‐Kenney, ginny.mauldin@gmailcom

Teacher Grants: Bill Waggener

Phone (404)354‐8752 AGS 2019 Meeting Dates [email protected] Listed below are the scheduled meeting dates for 2019. Please mark your calendar Hospitality: John Salvino, P.G. [email protected] and make plans to attend.

Membership: Burton Dixon

2019 Meeting Schedule [email protected] April 30

May 28 Social Media Coordinator: Carina O’Bara June 25 [email protected] July 30

August 27 Newsletter Editor: James Ferreira Phone 508‐878‐0980

[email protected] PG Study Group meetings

April 27 Web Master: Ken Simonton May 25 [email protected] June 29 July 27 www.atlantageologicalsociety.org

August 31

AGS March 2019 Page 35 ATLANTA GEOLOGICAL SOCIETY www.atlantageologicalsociety.org

ANNUAL MEMBERSHIP FORM Please print the required details and check the appropriate membership box.

DATE:______

NAME:______

ORGANIZATION:______

TELEPHONE (1): TELEPHONE (2):

EMAIL (1): EMAIL (2):

STUDENT $10

PROFESSIONAL MEMBERSHIP $25

CORPORATE MEMBERSHIP $100 (Includes 4 professional members, please list names and emails below)

NAME: EMAIL:

NAME: EMAIL:

NAME: EMAIL:

NAME: EMAIL:

For further details, contact the AGS Treasurer: John Salvino [email protected]

Please make checks payable to the “Atlanta Geological Society” and bring them to the next meeting or remit with the completed form to: Atlanta Geological Society, Attn: John Salvino 3073 Lexington Avenue Woodstock, Georgia 30189

To pay electronically; click https://squareup.com/store/atlanta‐geological‐society

CASH CHECK (CHECK NUMBER:______)