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Home , Cryogenian, Gaskiers glaciation, Late Paleozoic icehouse, Snowball Earth, Sturtian glaciation

’s Outgassing and Climatic Transitions: The Slow Burn Towards Environmental “Catastrophes”?

N. Ryan McKenzie1 and Hehe Jiang2

1811-5209/19/0015-0325$2.50 DOI: 10.2138/gselements.15.5.325

n multimillion- timescales, outgassing from the Earth’s interior Solid Earth carbon degassing occurs at mid-ocean ridges, rifts, provides the principal source of CO2 to the ocean– , volcanic arcs in subduction zones, Owhich plays a fundamental role in shaping the Earth’s baseline . orogenic belts, and large igneous Fluctuations in global outgassing have been linked to icehouse–greenhouse provinces (Fig. 1). The global transitions, although uncertainties surround paleo-outgassing fluxes. Here, we volcanic flux of CO2 probably varied significantly throughout discuss how volcanic outgassing and the have evolved in concert geologic time. Changes in subduc- with changes in and biotic . We describe hypotheses tion zone length, in the volume of driving mechanisms for the icehouse–greenhouse and of a mantle plume, and/or in the explore how climatic transitions may have influenced past biotic crises and, global oceanic production rate can result in at least a two-fold in particular, how variable outgassing rates established the backdrop for increase in volcanic CO2 inputs carbon cycle perturbations to instigate prominent mass extinction events. (Lee et al. 2013). Variation in the CO2 input would, therefore, play Keywords: carbon cycle, outgassing, , climate, biodiversity, a fundamental role in changing mass extinction long-term baseline climate states. These tectonic/magmatic processes LONG-TERM CARBON CYCLE also contribute to modifying the Earth has transitioned between two major baseline climate climate– weathering feedback strength, which is states throughout its history: icehouse (or Age) intervals influenced by the exhumed supply of weatherable minerals, that consist of generally cool climates with well-developed dissolution kinetics, and hydrologic conditions (Hartmann polar ice sheets, and warm greenhouse intervals devoid et al. 2014; Maher and Chamberlain 2014; Jiang and Lee of ice caps. Transitions between icehouse and greenhouse 2019). The long-term carbon cycle stabilizes planetary intervals are largely influenced by the partial pressure climate due to the temperature sensitivity of silicate weath- of atmospheric (pCO2). The transfer of ering—at high temperatures weathering rates increase and carbon between the solid Earth (endogenic system) and rapidly draw CO2 out of the atmosphere, so cooling the the ocean–atmosphere (exogenic system) on million-year climate. Because surface temperature is, in part, controlled timescales is governed by the long-term carbon cycle. The by pCO2, the relation between CO2 inputs and the weath- pCO2 is a result of the balance between the rate of CO2 ering feedback can be represented as the curve in Figure 2. inputs through magmatic/metamorphic degassing and the In general, we expect a positive correlation between the rates of carbon removal via silicate weathering and organic input or output with steady state pCO2, with the slope of carbon burial (Berner 2004) (Fig. 1). Due to the small size the curve representing the feedback strength. With linear of the surface carbon reservoir relative to the inputs from the solid Earth, a silicate weathering between silicate weathering burial carbonate weathering Volcanic, metamorphic and temperature is required to Corg burial Corg weathering CO2 degassing stabilize atmospheric CO2 and climate on timescales of longer than 105 (Walker et al. 1981). Thus, steady-state pCO2 levels are determined by the CO degassing input and the relative strength of 2 the silicate weathering feedback MOR mechanisms (Berner 2004).

1 Department of Earth Sciences Schematic of tectonic controls on the long-term Figure 1 University of Hong Kong carbon cycle. Oceanic crust is in light grey; Pokfulam, Hong Kong sediments on the oceanic crust and that form the accretionary E-mail: [email protected] prism are in light green; (unmetamorphosed and metamorphosed) in blue; granite bodies in red. Abbreviations: 2 Department of Earth Sciences C = organic carbon; MOR = mid-ocean ridge. University of Toronto org Toronto, ON, Canada E-mail: [email protected]

Elements, Vol. 15, pp. 325–330 325 October 2019 kinetically supply ICEHOUSE–GREENHOUSE CONDITIONS stable climate limited limited IN Since the Period (716–635 Ma), Earth has shifted between numerous icehouse–greenhouse states upper threshold (Fig. 3). The Cryogenian was one of the most extensive icehouses, being characterized by two events when ice sheets reached paleo-shorelines in equato- rial regions. The first of these Snowball Earth events, input known as the , occurred ~716 Ma, and or the second was the that terminated ~635 Ma. Subsequent relatively short-lived, yet expansive, output glaciations included the (~580 Ma) and the glaciation at the end of the Period (~440 Ma). The lower threshold extended from the to the late , and our current icehouse climate was initiated runaway runaway icehouse pCO2 greenhouse Cz Paleozoic Schematic illustration of climate states having varying N Pg K J Tr P C D S O Є Ediac. Cryogenian Figure 2 weathering feedback and outgassing conditions. Dispersal Pangea Assembly Dispersal Rodina Continental Low outgassing rates combined with kinetically limited weathering Breakup Breakup Configuration regimes allows Earth to reach a state where extensive icehouse (or snowball) conditions can be achieved. Conversely, elevated outgas- Collision

“Big 5” Mass Rate Extinction sing combined with supply-limited weathering (i.e., a negligible 2.0 silicate weathering feedback) can push Earth into a runaway green- Extinctions Alpine–Himalayan house. Adapted from Lee et al. (2019).

1.0 weathering feedback, the system remains regulated and runaway processes cannot happen. However, global or regional weathering processes can be either “rate-limited”, 0.0 wherein an excess of weatherable minerals are available and Proportion Genera the weathering rate is limited by kinetic rate, or “supply- 0.6

limited”, which occurs when all weatherable substrates Extinction are exhausted and the weathering rate is limited by the 0.4 supply of fresh materials. As such, the negative feedback mechanism may not scale linearly with exogenic CO2 0.2 content. For example, in a low pCO2 scenario, tempera- ture could drop below some threshold where weathering kinetics become negligible because surface temperatures 0.0 are, in part, controlled by pCO2. As a result, the system p (hypercali ers) reaches a threshold of atmospheric pCO2 such that weath- 0.6 ering rates are insensitive to CO2, and further decreases in carbon input combined with ice effects could 0.4 result in runaway icehouse conditions. Similarly, silicate weathering may become supply-limited even with high 0.2 “reef gap” “reef Cumulative Frequency temperature and high pCO2, where the system reaches a Young Zircon Detrital 0.3 threshold above which increased CO2 does not result in increased weathering. This can result in runaway green- house conditions (Fig. 2). Therefore, “thresholds” in the silicate weathering feedback 0.2 exist, above or below which weathering rates become Increasing Young Young Increasing Zircon Proportion Proportion Zircon insensitive to pCO2, leading to failure of silicate weath- ering feedback in climate regulation (Foley 2015; Kump 2018). Crossing these thresholds would move the planet 0.1 to a new steady state with different feedback parameters. For example, in a low pCO2 scenario, temperature could drop below the water freezing point where weathering kinetics become sluggish, or even cease completely. If the Icehouse Icehouse Icehouse Cenozoic Cenozoic system maintains low CO2 input, the planet would remain Mesozoic- Greenhouse Greenhouse Cryogenian Gaskiers glaciation Sturtian glaciation Late Paleozoic Late Hirnantian glaciation Early Cenozoic Early Cenozoic Marinoan glaciation ice-covered and be dominated by runaway ice-albedo Early Paleozoic feedback. In a globally supply-limited scenario, the system 0 100 200 300 400 500 600 700 800 reaches a threshold above which increased CO2 does not Age (Ma) result in increased weathering, thus rapid emission of large Composite of climate states over last ~720 million quantity of CO2 may exhaust the capacity of the long- Figure 3 years, including the proportion of young detrital term climate regulator associated with silicate weathering, zircons, the proportion of benthic hypercalcifiers, the proportion of resulting in runaway greenhouse conditions (Foley 2015; genera extinctions, the extinction rates, and the various continental Kump 2018) (Fig. 2). configurations. Red = greenhouse conditions; blue = icehouse conditions. Ediac. = Ediacaran. Modified after McKenzie et al. (2014, 2016), Alroy (2008), and Bambach et al. (2004).

Elements 326 October 2019 during the later (~33 Ma). Prominent green- assumed criteria for a prominent low- weathering house intervals spanned the Ediacaran through the early system, yet the Himalaya modifies the climate system such Paleozoic, the middle Paleozoic, and the Mesozoic to early that it can play a major role in global weathering. Cenozoic. Interestingly, the record is sparse The relationship between long-term climate transitions with regard to icehouse intervals. Aside from a few putative and low-latitude emplacement of mafic bodies in deep glaciogenic deposits in sedimentary sequences, time is not clear (McKenzie et al. 2016; Mills et al. 2017). the ~2.3 Ga Huronian glaciations represent the only well- While some LIP and arc-terrane emplacement events may documented icehouses prior to the Cryogenian, i.e., there have been coincident with cooling events, many were not. is an ~1.6 billion year interval without any known evidence Multiple LIPs have erupted in equatorial locations without of icehouse climates. Understanding what processes drive any notable cooling effects, e.g., during the and deviations from one steady-state climate to a new one the . Similarly, extensive collisional belts existed remains a topic of high interest, with ongoing debates around equatorial regions throughout the Cambrian, focused on whether climatic transitions are controlled by which remained a greenhouse interval (e.g., McKenzie et changes in the CO2 inputs or changes in the efficiency of al. 2014; Cao et al. 2017; Mills et al. 2017). On the other the weathering sink. hand, changes of the volcanic CO2 flux into the surface Weathering-driven models for seek system appear to be directly associated with major climatic out mechanisms that can enhance global weathering transitions. Multiple studies focusing on paleogeographic efficiency and CO2 consumption. Weathering rates are volcanic arc distributions have shown that changes in arc dependent on temperature, precipitation, and exposed length relate to icehouse–greenhouse transitions at various reactive surface area of the material being weathered. A intervals in Earth history (e.g., Lee et al. 2013; Mills et al. leading hypothesis postulates that the concentration of 2017). Analysis of detrital zircons in global sedimentary continents in low- (i.e., the tropics) causes intense deposits that span the past ~720 million years showed a weathering due to high precipitation and hot and humid direct relationship between zircon production—a proxy climates, which causes cooling and favors icehouse condi- for regional continental arc magmatism—and all major tions (Marshall et al. 1988). This hypothesis has led to an climatic transitions throughout this interval (McKenzie et increased focus on concentrations of mafic crustal rocks in al. 2016) (Fig. 2). Continental arc systems are of particular low-latitudes because mafic minerals weather more rapidly, interest because, via decarbonation reactions, they have the and at lower temperatures, than felsic minerals, thereby ability to liberate carbon from preserved in the enhancing the “weatherability” of the crust. Increasing upper plate (Lee et al. 2013; Mason et al. 2017). crustal mafic rocks in the tropics can be accomplished While outgassing may generally determine baseline climate through the eruption of large igneous provinces (LIPs) or states, pCO2 represents a balance between both sources and by accretion of island-arc terranes in low-latitudes. Cooling sinks. Quantifying outgassing rates is a difficult task in during the Cryogenian and Cenozoic has been attributed deep time due to the uncertainties in estimating modern to low-latitude LIP emplacement (e.g., Goddéris et al. 2003; CO2 fluxes and the potential additive input from crustal Kent and Muttoni 2013), whereas tropical arc-terrane accre- carbon sources, which may be more complicated for LIPs. tion might be responsible for discrete cooling events during Furthermore, the development of continental arcs often the and Cenozoic (Jagoutz et al. 2016) and for involves compressional mountain building events that the end-Ordovician Hirnantian icehouse (Swanson-Hysell enhance weatherability, thereby dampening the outgas- and Macdonald 2017). Mafic LIPs emplaced on stable sing contribution (Jiang and Lee 2017). As noted above, may have a large initial reactive surface area, but, weathering rates are dependent on multiple variables that without extensive uplift and physical erosion, that surface are difficult to estimate in Earth’s history. Therefore, our could be rapidly leached, leaving the greater volume of understanding of ancient climatic transitions relies on our the unexposed blocked from the weathering zone—a ability to interpret the rock record. For example, the lack of process termed “soil-shielding” (e.g., Hartmann et al. 2014). a consistent relationship between the sporadic occurrence This makes the arc-terrane collisions a more appealing of low-latitude mafic bodies and global cooling does not mechanism by which to change weatherability and to mean their occurrences did not influence climate change: drive cooling, because accretionary processes induce uplift, their contributions may have depended on baseline condi- erosion, and exhumation of the bedrock, vastly increasing tions during their emplacement. its reactive surface area and weathering potential. The inter- action between mineral supply and reaction kinetics makes CLIMATE STEADY STATES, THRESHOLDS, orogenic systems preferential sites for enhanced weath- AND “CATASTROPHES” ering. At present, tectonically active regions are thought to account for substantial CO2 consumption (Hartmann et al. At some periods during Earth’s history, long-term 2014). Still, the role of tectonics on weathering can be quite endogenic processes might have “pre-set” the climate complicated and numerous factors need to be considered baseline close to a threshold that could have been easily when assessing deep-time climatic influences. crossed from shorter-termed events, such as LIPs, bolide impacts, or seafloor release. The combination of Take the Himalayas as an example. The Himalayan foreland long-term fluctuations in CO2 emissions and short-term (the Indo-Gangetic plains) yields weathering fluxes compa- events could have played an important role in biologic rable to basaltic provinces (West et al. 2002). While the evolution. Perturbations of Earth’s carbon cycle can lead to Himalaya lie outside of the tropics, its extensive east– mass extinctions if they exceed either a critical rate at long west strike and mountain heights interferes with the timescales or a critical size at short timescales (Rothman atmospheric circulation, which ultimately drives the South 2018). The influences of rapid CO2 fluctuations from LIP Asian Monsoon (Boos and Kuang 2009). The combination events on mass extinctions are already discussed elsewhere of the hot and humid monsoon with the continuous supply in this issue of Elements; thus, we will speculate on the of fresh weatherable silicate minerals into the foreland relative importance of changes in the baseline pCO2 associ- basin via uplift and erosion of the orogenic belt is why ated with variations in outgassing on carbon cycle pertur- the Himalayan system yields such high weathering fluxes. bations and on Earth system “catastrophes”. From a paleo-perspective, the Himalaya would not meet the

Elements 327 October 2019 Most major mass extinctions have occurred during green- an extensive “reef gap” when calcifying were of house climates. This might be due to the fact that the Earth markedly low abundance (e.g., McKenzie et al. 2014 and has spent more time in a greenhouse state (Fig. 3). However, refs therein) (Figs. 2 and 3). Three major mass extinction a link between some extinctions and environmental condi- events occur around the transition from the Paleozoic tions has been recognized that involves the carbon cycle. to the Mesozoic: the mid-Permian “” (~260 In particular, changes in carbonate saturation state have Ma), the end-Permian (~251 Ma), and the end-Triassic (201 been postulated as key drivers of extinctions (Knoll and Ma) extinctions, all of which were coincident with LIP Fischer 2011). The early Paleozoic and the middle Permian eruptions and carbon cycle perturbations. These extinc- through to the end-Triassic encapsulate the highest marine tions are also noted for selection against calcifying animals extinction rates (Bambach et al. 2004; Alroy et (Clapham and Payne 2011; Knoll and Fischer 2011). Much al. 2008) (Fig. 3). The Cambrian “dead interval” of the like the Cambrian, the Mesozoic recorded numerous ocean early Paleozoic is characterized by generally low biodiver- anoxic events (OAEs) that are believed to have been the sity with multiple mass extinction events associated with result of episodic CO2 perturbations (Jenkyns et al. 2010). ocean anoxia and carbon cycle perturbations, as well as The end- mass extinction also exhibited selection­

Mass Extinctions End-Triassic End-PermianGuadalupian A 10,000 CO Onset of 2 (ppm) Deglaciation

1,000

Modeled CO2 14 100 B 6,000

10 CO 2 (ppm)

6 3,000 Glacial Frequency

2

Proxies for CO2 0 0 C Detrital zircon age data Triassic 0.03

0.02

Frequency 0.01

0 D Detrital zircon Permian 0.07 age data 0.06

0.05

0.04 Icehouse Late Paleozoic Paleozoic Late 0.03

Frequency 0.02

0.01 0 500 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 420 440 460 480 Age (Ma)

The Paleozoic–Mesozoic transition. (A) Modeled a marked increase in outgassing from volcanic activity beginning at Figure 4 paleoconcentrations of CO2. (B) Paleoconcentrations ~290 Ma that is coincident with the onset of deglaciation. The of CO2 obtained from proxies. (C) Frequency of detrital “young” proxy and modeled CO2 increase are decoupled by ~50 My from zircon ages from Triassic strata. (D) Frequency of detrital “young” the onset of deglaciation. Late Paleozoic Icehouse duration from zircon ages from Permian strata. Timing of mass extinctions are Montañez and Paulsen (2009); CO2 proxy data from Zeebe (2012); indicated by vertical red dashed lines. Glacial frequency is shown in modeled CO2 curve from Royer et al. (2014) and zircon frequencies data blue with the onset of deglaciation from the Late Paleozoic from McKenzie et al. (2016). icehouse beginning at ~290 Ma. The zircon data indicate there was

Elements 328 October 2019

Mass Extinctions Botomian MarjumianSteptoeanSunwaptan Hirnantian ANT AUS IND Duration of tectonic activity Taconic Arc Accretion AFR on Gondwanan constituents NCB SAM

4‰ carb

C 0‰ 13 δ -4‰

0.7092 Sr 86 0.7086 Sr/ 87 0.7080

40°

O apatite) 32° 16 Temperature (from δ (from 24°

1,600 Modern Fauna 1,200 Great Ordovician

Biodiversi cation Event 800 Cambrian ‘Dead Interval’ Paleozoic Fauna 400

Marine Genera Marine Invertebrate Cambrian Fauna 2 Series 3 Tremado. Dap. Darriw. Sand. -Hirnan. Cambrian Ordovician

510 500 490 480 470 460 450 440 Age (Ma)

Early Paleozoic tectonics, climate, and biodiversity. indicate high radiogenic 87Sr fluxes from chemical weathering of Figure 5 Note that the reduction in Gondwanan tectonism old continental crust. Carbonate δ13C tracks carbon cycle variation coincides with a global cooling trend, consistent with the zircon and perturbations. Abbreviations on tectonic duration activity: record in Figure 3. Taconic arc exhumation had initiated by ANT = ; AUS = ; IND = ; AFR = Africa; NCB = ~465 Ma. The timing of five mass extinction events (Botomian, North China Block; SAM = South America. Modified from McKenzie Majumian, Steptoean, Sunwaptan, and Hirnantian) are indicated et al. (2014). Taconic arc-accretion range from Swanson-Hysel and with vertical dashed red lines. 87Sr/86Sr relative highs Macdonald (2017). against calcifying animals (Knoll and Fischer 2011). The with ages close to the depositional ages of the strata relative Cretaceous–Paleogene extinction—last of the “big 5”— to the total populations (McKenzie et al. 2016) (Fig. 3), corresponded with both the Deccan (India) LIP and a closer evaluation of the data shows that the relatively bolide impact at Chicxulub (Mexico), but lacks notable young grains present form a distinct young population evidence for marked selectivity against calcifying animals. that increase in abundance ~290 Ma, which is coincident However, this extinction followed the diversifi- with the onset of deglaciation (Fig. 4). These young <290 cation of planktonic calcifying organisms that produced Ma zircons are seen on multiple geographic regions (North a new deep-ocean carbonate reservoir that could buffer America, South America, China, Africa, Southeast Asia, against sustained acidification, thus changing carbon cycle Australia) and represent a rapid, nearly synchronous global dynamics (e.g., Zeebe 2012). increase in the volcanic CO2 flux at this time (a magmatic The collective similarities of environmental conditions “flare up”) that may have caused warming (Fig. 4). This and extinction dynamics during the early Paleozoic, mid change in surface environment could have amplified the Paleozoic, and late Permian–Triassic greenhouses imply that effects of the perturbations caused by LIPs during this time. background CO2 fluxes and greenhouse baseline conditions There is a final point for consideration. It has been can increase the sensitivity of the surface environment to shown that major cooling events, such as the Cryogenian short-term sizeable CO2 injections. Whereas the Cambrian “Snowball Earth" glaciations and the end-Ordovician corresponded to a protracted interval of widespread volca- Hirnantian glaciations, correspond with marked reductions nism, the Permian–Triassic witnessed a dramatic rise in in global magmatism (McKenzie et al. 2016; Mills et al. pCO2 (Zeebe 2012; Royer et al. 2014) (Figs. 2 and 4). The 2017). The Hirnantian glaciation follows an interval that transition out of the late Paleozoic icehouse was associated had a progressive reduction in and global cooling with an increase in pCO2 (Montañez and Poulsen 2008). and that was punctuated with a rapid drop in temperature Although detrital zircon data from Permian and Triassic coincident with the end-Ordovician Hirnantian extinc- strata may show low-abundances of “young” zircon dates tion (Fig. 5). Ordovician cooling may have been driven by

Elements 329 October 2019 increased weathering due to low-latitude arc-terrane colli- intervals may have been more amenable to mass extinc- sions of the Taconic orogenic system (Swanson-Hysell et al. tions. Conversely, reduced outgassing may allow tempera- 2017). Given a baseline climate state with reduced global tures to drop to the low levels that inhibit background outgassing and lower background pCO2, the exhumation of ­weathering rates, where the tropical emplacement of large a large mafic terrane in the tropics may have been sufficient mafic terranes can change weathering kinetics to initiate to have changed global weathering kinetics and to have short-lived “catastrophic” drops in temperature leading caused the drop in temperature needed to induce a rapid to Snowball Earth events or extinction-driving icehouses. extinction-linked glaciation. Accordingly, the slow exchanges of carbon between the In summary, the Earth has experienced many carbon cycle endogenic and exogenic systems profoundly influence perturbations, although their influences on the climate Earth’s climatic state and its biospheric stability. system and biosphere have been disparate. Whereas pCO2 always represents a balance between carbon sources and ACKNOWLEDGMENTS sinks, changes in global outgassing appear responsible We thank the Deep Carbon Observatory (DCO) for for driving multimillion-year swings in baseline climate organizing this issue and including us. Joshua West, Cin-Ty state. Extended intervals of high volcanic outgassing may Lee, and Noah Planavsky provided helpful discussions. not sustain ocean acidification, but shifts in baseline Constructive critiques and comments from Editors Jodi conditions can influence the carbon cycle and make it Rosso and Patrick Roycroft, and two anonymous reviewers more sensitive to perturbation. Effectively, greenhouse greatly enhanced the manuscript.

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