https://doi.org/10.1130/G46916.1 Manuscript received 26 August 2019 Revised manuscript received 23 November 2019 Manuscript accepted 26 November 2019 © 2019 Geological Society of America. For permission to copy, contact [email protected]. A signifcant acceleration of ice volume discharge preceded a major retreat of a West Antarctic paleo–ice stream Philip J. Bart1 and Slawek Tulaczyk2 1 Department of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana 70803, USA 2 Department of Earth and Planetary Sciences, University of California Santa Cruz, Santa Cruz, California 95064, USA ABSTRACT SEDIMENT AND ICE DISCHARGE For the period between 14.7 and 11.5 cal. (calibrated) kyr B.P, the sediment fux of Bind- FROM THE PALEO–BINDSCHADLER schadler Ice Stream (BIS; West Antarctica) averaged 1.7 × 108 m3 a−1. This implies that BIS ICE STREAM velocity averaged 500 ± 120 m a−1. At a fner resolution, the data suggest two stages of ice Radiocarbon ages from benthic foramin- stream fow. During the frst 2400 ± 400 years of a grounding-zone stillstand, ice stream fow ifera (Bart et al., 2018) (Table 1) indicate that averaged 200 ± 90 m a−1. Following ice-shelf breakup at 12.3 ± 0.2 cal. kyr B.P., fow acceler- the paleo-BIS grounding line had retreated ated to 1350 ± 580 m a−1. The estimated ice volume discharge after breakup exceeds the bal- 70 km from its maximum (LGM) position by ance velocity by a factor of two and implies ice mass imbalance of −40 Gt a−1 just before the 14.7 ± 0.4 cal. (calibrated) kyr B.P. and stabi- grounding zone retreated >200 km. We interpret that the paleo-BIS maintained sustainable lized at the Whales Deep Basin GZW with a discharge throughout the grounding-zone stillstand frst due to the buttressing effect of its fringing ice shelf in front of it. Shortly after fringing ice shelf and then later (i.e., after ice-shelf breakup) due to the stabilizing effects of 12.3 ± 0.6 cal. kyr B.P., this ice shelf collapsed, grounding-zone wedge aggradation. Major paleo–ice stream retreat, shortly after the ice-shelf but the grounding line maintained its position breakup that triggered the inferred ice fow acceleration, substantiates the current concerns until 11.5 ± 0.3 cal. kyr B.P. Hence, grounding- about rapid, near-future retreat of major glaciers in the Amundsen Sea sector where Pine line stillstand at the Whales Deep Basin GZW Island and Thwaites Glaciers are already experiencing ice-shelf instability and grounding- lasted for 3.2 ± 0.7 k.y., with the ice-shelf break- zone retreat that have triggered upstream-propagating thinning and ice acceleration. up dividing it into two distinct phases. The constraints on the GZW sediment vol- INTRODUCTION 2014; Prothro et al., 2018). Detailed analyses of ume and its depositional time frame (Bart et al., Analyses of modern ice stream systems have the now ice-free areas, where grounding-zone 2017a, 2017b, 2018; McGlannan et al., 2017) provided much information about their behavior wedges (GZWs) are exposed on the seafoor, provide an opportunity to reconstruct ice dis- (Alley et al., 1986; Rignot et al., 2004; Vieli provide records of the dynamic retreat history of charge history from the paleo-BIS at the time et al., 2007; Scambos et al., 2009). These stud- marine-based ice streams. These retreat records of GZW deposition (Table 1). We defne the av- ies have shown that ice streams accelerate when are important because they provide insight into erage sediment fux rate, q, as the ratio of the buttressing is reduced by either ice shelf thin- the processes and rates needed to predict future total volume of deposited sediments, V, and ning and/or collapse. The fow acceleration is of ice retreat in West Antarctica (e.g., Bingham the duration of deposition, ΔT. We express the concern because models suggest that it may trig- et al., 2017). sediment fux rate across the grounding line as ger a tipping-point response in which the extent Here we use published data from the Whales a product of the ice stream width, W, the effec- of grounded ice abruptly contracts (Alley et al., Deep Basin (eastern Ross Sea) to quantify tive sediment thickness, h, as well as the aver- 2015; Pattyn, 2018). On the other hand, sedi- the evolution of ice and sediment fuxes that age sediment transport velocity, us, which itself ment transport by ice streams may also result preceded a >200 km retreat of the paleo– is expressed as a fraction, f, of the ice stream in rapid sedimentation at the grounding zone. Bindschadler Ice Stream (paleo-BIS; West fow velocity, U: Subglacial and grounding-zone sedimentation Antarctica) from the outer continental shelf. V aggradation may act as a negative feedback that The Whales Deep Basin contains a large-volume q ≡=uWs hf= UWh. (1) counters dynamic thinning of the ice stream and overlapping stack of GZWs, which have been ∆T stabilizes the ice-stream grounding zones (e.g., mapped with seismic and subbottom profles and Here we assume that till-bedded ice streams Alley et al., 2007). multibeam bathymetric data (Bart et al., 2017a, have constant velocity throughout ice thickness The understanding of modern ice-stream 2017b) and individually cored (McGlannan and width (e.g., Tulaczyk et al., 2000). For basal sediment fux (e.g., Alley et al., 1986) has been et al., 2017) (Figs. 1A–1C). This stratigraphic debris transport in ice, f is equal to 1 because highly instrumental in interpreting the glacial framework is constrained by radiocarbon ages sediments travel with the same velocity as ice landforms and deposits on the outer continen- (Bart et al., 2018) and provides evidence indi- itself, and we take h to be the equivalent thick- tal shelves that formed after the Last Glacial cating that rapid GZW construction delayed the ness of a till layer that would form if sediments Maximum (LGM) (Anderson, 1999; Ó Cofaigh retreat of grounded ice for hundreds of years in the debris-laden basal ice would form a till et al., 2002; Jakobsson et al., 2012; Klages et al. after the breakup of its fringing ice shelf. layer with porosity of 40% (e.g., 3.5 ± 0.6 m CITATION: Bart, P.J., and Tulaczyk, S., 2020, A signifcant acceleration of ice volume discharge preceded a major retreat of a West Antarctic paleo–ice stream: Geology, v. 48, p. XXX–XXX, https://doi.org/10.1130/G46916.1 Geological Society of America | GEOLOGY | Volume XX | Number XX | www.gsapubs.org 1 Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/doi/10.1130/G46916.1/4913426/g46916.pdf by University of California Santa Cruz user on 11 February 2020 600 1500 A 180° B 1000 150°E 150°W Whales Deep Basin shelf edge LEGEND embayed paleo– grounding line 2000 Whales Deep Basin 2500 3000 600 600 140 500 + bathymetry 120 continental shelf edge Ross Sea 100 76°S B ess (m) contour CF 76°S kn 80 + 120°E D West Antarctica 300 seismic 60 + + 400 40 ++ 400 400 core NBP9902 + + H + Ross Sea drainage 20 500 GZW thic 500 350 RIS E divides 200 core NBP1502B + 0 a 500 y 400 GL Ma e 300 400 s 600 paleo-BIS 600 5005000 H Byd drainage + + B o Glomar Challenger B B B area B + u K t K a 500 Basin z 400 W n M ice streams 700 k 600 90°E B and shelves 600 0 a 55055 n 8 5° C k + Little America c a l v i r o n t Basin n g f Antarctic 500 78°S Peninsula 78°S East Antarctica 700 550 7 700 5° RFIS 700 LIS 500 Roosevelt 60°E 60°W 600 Ross Ice Shelf Island 200 6 5° Atlantic Ocean 800 600600 0 100 km 0 750750 30°E 30°W 0100 km 0 5 5505 180°8 170°W 160°W6 C S N 600 450 Bathymetric saddle 488 7 525 700 LGM unconformity 6 Shelf LGM 562 7 4 5 3 edge 7 5 unconformity 800 600 2 1 avel time (ms) 637 tr Water depth (m) 900 675 o-way Tw 0510203040 50 km 712 VE=150 1000 750 D E supraglacial melt water lakes 0 0 GZW4 ice shelf 500 GZW7 500 U = 1350 m a ter depth (m) Ub = 200 m a ter depth (m) b basal crevasses 600 600 Wa Wa 0510 20 30 40 50 km Figure 1. (A) Map of Antarctica showing ice streams and buttressing ice shelves. Dashed lines in continental interiors demarcate drainage areas of the East and West Antarctic Ice Sheets that converge into the Ross Sea. Gray shading shows the paleo–Bindschadler Ice Stream (paleo- BIS) drainage area during the Last Glacial Maximum (LGM). B—Bindschadler Ice Stream; D—David Glacier; Byd—Byrd Glacier; M—Mercer Ice Stream; W—Whillans Ice Stream; K—Kamb Ice Stream; Ma—MacAyeal Ice Stream; E—Echelmeyer Ice Stream; CF—calving front; RIS—Ross Ice Shelf; GL—grounding line; RFIS—Ronne-Filchner Ice Shelf; LIS—Larsen Ice Shelf. Modern-day shelf edge position is shown as short-dash line. (B) Bathymetry of the eastern Ross Sea continental shelf, from Davey and Nitsche (2005). Paleo-BIS was confned to Whales Deep Basin between the Hayes and Houtz Banks. Light-gray rectilinear lines are locations of seismic data. Gray squares are cores acquired by Mosola and Anderson (2006), and crosses are those described by McGlannan et al. (2017). NBP9902 and NBP1502B refer to R/V Nathaniel B.