Constraints on the Timing and Extent of Deglacial Grounding Line Retreat in West Antarctica

Abstract

Projections of Antarctica's contribution to future sea level rise are associated with significant uncertainty, in part because the observational record is too short to capture long‐term processes necessary to estimate ice mass changes over societally relevant timescales. Records of grounding line retreat from the geologic past offer an opportunity to extend our observations of these processes beyond the modern record and to gain a more comprehensive understanding of ice‐sheet change. Here, we present constraints on the timing and inland extent of deglacial grounding line retreat in the southern Ross Sea, Antarctica, obtained via direct sampling of a subglacial lake located 150 km inland from the modern grounding line and beneath >1 km of ice. Isotopic measurements of water and sediment from the lake enabled us to evaluate how the subglacial microbial community accessed radiocarbon‐bearing organic carbon for energy, as well as where it transferred carbon metabolically. Using radiocarbon as a natural tracer, we found that sedimentary organic carbon was microbially translocated to dissolved carbon pools in the subglacial hydrologic system during the 4.5‐year period of water accumulation prior to our sampling. This finding indicates that the grounding line along the Siple Coast of West Antarctica retreated more than 250 km inland during the mid‐Holocene (6.3 ± 1.0 ka), prior to re‐advancing to its modern position.

A method for successful collection of multicores and gravity cores from Antarctic subglacial lakes

“…Preliminary data from the gravity cores indicate that over 2 m of core section is present in our cores once correlated. Careful planning, ability to overcome challenges in the field, and serendipity with good weather all contributed to a unique sediment record (Davis et al, 2023;Siegfried et al, 2023;Venturelli et al, 2023) that will potentially elucidate many subglacial geophysical processes, biogeochemical cycles from an unexplored environment, and regional and global climate phenomena. We anticipate that our success can be replicated and exceeded in future endeavors.…”

Sediment oxygen consumption in Antarctic subglacial environments

“…Conversely, Mercer Subglacial Lake sedimentary oxygen consumption rates are comparable to those measured in Lake Hazen, which is a high Arctic lake fed by runoff from glaciers (St. Pierre et al 2019). Total organic carbon content of Mercer Subglacial Lake sediments is 0.3–0.5% in the upper sedimentary unit and < 0.2% in the lower sedimentary units (Davis et al 2023; Venturelli et al 2023), compared to > 1% in Toolik Lake and Lake Superior (Cornwell and Kipphut 1992; Li et al 2012). Total organic carbon is a strong driver of sedimentary oxygen consumption and overall benthic microbial activity (Jørgensen et al 2022).…”

“…Total organic carbon is a strong driver of sedimentary oxygen consumption and overall benthic microbial activity (Jørgensen et al 2022). However, the reactivity of the organic carbon also plays a role (Middelburg 1989) and, in the case of Mercer Subglacial Lake, the organic carbon may be sourced from previous retreats of the Antarctic Ice Sheet, rendering it older and likely less reactive (Venturelli et al 2023). The low TOC content and likely low reactivity of TOC in Mercer Subglacial Lake sediments would lead to the comparatively lower rates of sedimentary oxygen consumption we measured (Table 1; Fig.…”

“…Aquatic sediments beneath Antarctic ice masses are biogeochemically active ecosystems (Michaud et al 2017; Vick‐Majors et al 2020; Davis et al 2023; Venturelli et al 2023). These subglacial ecosystems are poised for changes as ice shelves disintegrate and ice sheet grounding lines retreat in response to warming of the Southern Ocean and its interactions with Antarctic ice (Rignot et al 2013; Ingels et al 2021).…”

“…The objective of our study was to make oxygen microprofiles from sediment cores collected from beneath the Antarctic ice sheet and Ross ice shelf and quantify sedimentary oxygen consumption as a proxy for overall state of biological activity. Based on the paleo‐history and microbiology of these systems (e.g., Michaud et al 2017; Davis et al 2023; Venturelli et al 2023), we hypothesized that sedimentary oxygen consumption would be measurable and similar to other limnetic and marine benthic habitats with low inputs of photosynthetically derived organic carbon. Oxygen microsensors were used to make the first high‐resolution, ex‐situ sedimentary profiles of dissolved oxygen and model these profiles to estimate sedimentary oxygen consumption and carbon oxidation rates beneath Antarctic ice.…”