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Sediment Transport Drives Tidewater Glacier Periodicity

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Sediment Transport Drives Tidewater Glacier Periodicity ARTICLE DOI: 10.1038/s41467-017-00095-5 OPEN Sediment transport drives tidewater glacier periodicity Douglas Brinkerhoff1, Martin Truffer1 & Andy Aschwanden 1 Most of Earth’s glaciers are retreating, but some tidewater glaciers are advancing despite increasing temperatures and contrary to their neighbors. This can be explained by the coupling of ice and sediment dynamics: a shoal forms at the glacier terminus, reducing ice discharge and causing advance towards an unstable configuration followed by abrupt retreat, in a process known as the tidewater glacier cycle. Here we use a numerical model calibrated with observations to show that interactions between ice flow, glacial erosion, and sediment transport drive these cycles, which occur independent of climate variations. Water availability controls cycle period and amplitude, and enhanced melt from future warming could trigger advance even in glaciers that are steady or retreating, complicating interpretations of glacier response to climate change. The resulting shifts in sediment and meltwater delivery from changes in glacier configuration may impact interpretations of marine sediments, fjord geochemistry, and marine ecosystems. 1 Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK 99775, USA. Correspondence and requests for materials should be addressed to D.B. (email: [email protected]) NATURE COMMUNICATIONS | 8: 90 | DOI: 10.1038/s41467-017-00095-5 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/s41467-017-00095-5 espite a globally consistent trend of glacier mass loss1, primary ways. First, we consider sediment transport due to D~1/3 of Alaska’s tidewater glaciers are advancing2. glaciofluvial rather than deformational processes15, and we This trend shows little spatial consistency, suggesting include a physically consistent representation of both erosion and a dynamical rather than climate mechanism is responsible. depositional processes using mass conservation13.Wefind that Tidewater glaciers differ from their terrestrial terminating cousins that this coupled model, when calibrated with observations, can because mass loss occurs not only by surface melt, but also by produce all phases of the TGC without externally driven changes calving and ablation at the oceanic boundary. The mass budget of in climate and with a period and amplitude in general agreement such glaciers is therefore sensitively dependent on conditions at with observations from the geologic record. These temporal the marine terminus such as water temperature, motion, and and spatial scales are strongly influenced by the availability of depth. One example of this dependence, first observed at meltwater, which drives the rate of sediment transport and Columbia Glacier3, is that the presence of a terminal shoal can subsequently controling the rate of glacier advance. This lead to a considerable terminus advance relative to a position in dependence is sufficiently strong that a change in meltwater absence of sedimentation. Such glaciers also tend to be unstable: availability due to climate warming may (perhaps counter-intui- ice flux from calving increases with terminus depth, and on a tively) trigger advance in tidewater glaciers that are currently retrograde slope (such as the upstream side of a moraine) this stable or retreating. Although necessarily a simplification of positive feedback can lead to catastrophic retreat4, 5. The coupling the complete physical processes governing tidewater glacier of nonlinear ice dynamics and sediment transport leads to a dynamics, our results provide a basis for assessing how such continuum of dynamical behavior known as the tidewater glacier natural variability in sedimentation and meltwater regimes could cycle (TGC), which can be described in four archetypal phases6. impact fjord ecosystems, interpretation of marine sedimentary In the advancing stage, development and advection of a shoal records, and predictions of ice volume change. at the front reduces calving flux, causing glacier thickening and advance. The shoal may be subaqueous (e.g., Hubbard Glacier, advancing 35 m per year7) or subaerial (e.g., Taku Glacier, Results advancing 10 m per year8, Fig. 1). Eventually, the glacier enters an Calibration. Our model’s physical configuration is inspired by extended phase, in which the balance of accumulation and Taku Glacier, with a mean length of ~50 km, and a maximum ablation halts advance (e.g., Brady Glacier9). A glacier enters elevation of 2200 m. Figure 2 illustrates the geometry, climate, the retreating phase when the glacier can no longer maintain and sediment transport rate 100 years after the grounding line sufficient thickness to remain grounded on the shoal, and the reached its minimum following model initialization. Mean associated reduction in basal drag leads to retreat into progres- bedrock erosion rates were tuned to match the tectonic uplift rate, sively deeper water, triggering the instability described above as well as observations from coastal Alaska16. Fluvial erosion (e.g., Columbia Glacier, which began such a retreat in 1985 that rates were calibrated with repeat radar measurements from Taku continues presently6, Fig. 1). Ungrounding results either from the Glacier17. Fluvial deposition rates were derived from theoretical ice thinning or from the bed lowering due to erosion or mass settling velocities of sediment samples taken from boreholes at wasting. Retreat ends when the glacier approaches the terminus position it would assume in the absence of sedimentation, and the abTaku Columbia terminus effectively re-grounds on bedrock. This retracted state is usually many kilometers shorter than the advanced terminus position. A sediment shoal may then rebuild and the cycle begin again. The time scales of the TGC have been inferred from a combination of direct observation, radiocarbon dating, and tree-ring analysis at several glaciers in Alaska, and cycle periods 10 11 range from a few hundred years to a few thousand . 1933 1985 Zero-dimensional modeling efforts have shown that sediment 2016 2016 deposited at the glacier front can initiate advance through the – calving reduction mechanism defined above12 14, even when fi highly simpli ed models of ice dynamics (e.g., volume-area 5 km scaling) and sediment transport are used. Such models are also 19 km capable of simulating the retreat phase, although the degree of hysteresis between the time scales of advance and retreat is not explicitly captured. Models were inconsistent about whether an external perturbation is required to initiate retreat. Models designed to simulate the TGC in temperate glaciers have invoked Fig. 1 Advancing versus retreating glaciers in coastal Alaska. a Taku Glacier, either glaciofluvial erosion13 or sediment deformation12 as a which began advancing around 1850, is in the advance stage of the mechanism for transporting the terminal shoal. More physically tidewater glacier cycle (TGC) (note the image obliquity). Image shows the complete models have been applied to the problem of coupled ice terminus in 1933 (yellow line), when the glacier still possessed a vigorous flow and sediment deformation, the primary mechanism of calving front, versus 2016, where the glacier now terminates subaerially on sediment transport for the relatively dry marine ice sheets15. glaciofluvial sediment (blue line). Red dots are the same location in each Interestingly, such simulations were shown capable of producing image. b Columbia Glacier, which began retreating in the mid 1980s, is in unforced distinctly asymmetrical oscillations in ice extent with the dynamic retreat phase of the TGC. The yellow line is the terminus obvious analogies to the TGC, albeit over hundred thousand position in August 1985, near its maximum, whereas the blue line is the rather than 100 year time scales. terminus position as of October 2016. Photo Credits: top left: U.S. Navy, Here we test the hypothesis that TGCs can be explained solely 1929. Taku Glacier: From the Glacier Photograph Collection. Boulder, due to the coupling of ice dynamics, glacial erosion, and fluvial Colorado USA: National Snow and Ice Data Center. Digital media. sediment transport by running a series of numerical experiments Bottom left: M. Truffer. Top right: Landsat 7, data available from the U.S. with a new model that explicitly simulates both ice and sediment Geological Survey. Bottom right: Landsat 8, Data available from the U.S. dynamics. Our approach builds upon previous work in two Geological Survey 2 NATURE COMMUNICATIONS | 8: 90 | DOI: 10.1038/s41467-017-00095-5 | www.nature.com/naturecommunications NATURE COMMUNICATIONS | DOI: 10.1038/s41467-017-00095-5 ARTICLE 2,400 2.5 a 0.0 ) –1 1,600 (m yr 800 –8.0 Surf. mass bal. Elevation (m) 0 ) –1 200 b Surface 150 100 Bed 50 Speed (m yr 0 101 c –15 10–1 ) ) –1 –30 –1 0 –45 (m yr (mm yr –10–1 –60 1 Bedrock elev. change –10 Rate of sed. thick. chg. –40 –30 –20 –100 10 x (km) Fig. 2 Model state during the advance phase of the TGC. a Geometric state of the glacier and surface mass balance rate (blue hashed line) typical of glaciers in coastal Alaska. Black box corresponds to extent shown in Fig. 3. b Surface and basal velocities. c. Sediment thickness rate of change due to fluvial transport (orange) and bedrock elevation rate of change due to basal sliding (magenta line), both of which were calibrated to roughly match observations from Taku Glacier in Southeast Alaska Taku Glacier’s terminus. The conclusions presented here are between thinning and ice flux4. Catastrophic
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