Mid-Holocene Retreat and Late Holocene Re-advance of the Grounding Line in the Ross Sea Sector

Knowledge of past ice sheet configurations is vital because it can inform predictions of future ice sheet dynamics and be utilized in calibrating ice sheet models. The topology of grounding line retreat in the Ross Sea Sector of Antarctica has been much debated (Conway et al., 1999; Lee et al., 2017; Lowry et al., 2019; McKay et al., 2016) . Until recently, it was assumed that the current outline of the West Antarctic Ice Sheet (WAIS) was the smallest extent since the Last Glacial Maximum (LGM). However, new evidence suggests that the WAIS grounding line retreated beyond its current location earlier in the Holocene and subsequently re-advanced to reach its modern position (Kingslake et al., 2018) . Here, we further constrain post-LGM grounding line behavior in the Ross Sea Sector using a two-phase model of radiocarbon input and decay in subglacial sediments from six sampling locations. In addition, we reinterpret steep basal temperature gradients, measured previously at three sites in this region, which we explain as resulting from recent ice shelf re-grounding accompanying grounding line re-advance. At one location, Subglacial Lake Whillans (SLW), where sediment porewater chemistry profile is known, we constrain the grounding line re-advance by simulating ionic diffusion. Our results indicate that the grounding line retreated over our field sites within the past ca. 8900 years: 6200 - 1300 years ago, 8900 - 3000 years ago, 3100 - 750 years ago, and 3100 - 1150 years ago for our sites at SLW, the Whillans Ice Stream (WIS), the Kamb Ice Stream (KIS), and the Bindschadler Ice Stream (BIS) respectively. Re-advance over the sites was recent (1700 - 300 years ago for SLW, 2100 - 500 years ago for WIS, 1100 - 250 years ago for KIS, and 700 - 550 years ago for BIS). Our modeled chronological constraints of grounding line retreat and re-advance are consistent with low carbon-to-nitrogen ratios measured in our subglacial sediment samples and with the lack of grounding zone wedges in front of the modern grounding line.