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

Projections of Antarctica\u27s 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 \u3e1 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

Seasonal SST reconstruction by the UK'37 proxy for the Younger Dryas-Holocene transition in sediments of the Cariaco Basin, sediment core MD03-2621

The UK'37 sea surface temperature (SST) proxy, based on the distribution of long-chain akenones, was measured via mass spectrometry imaging (MSI) at 100-µm resolution in a 60-cm section of the well-dated core MD03-2621 from the Cariaco Basin (10.6782, -64.9715). This section spans an age of ~ 11.9 to 11.2 kyr b2k and thus includes the Younger Dryas-Holocene transition. Proxy data from each micrometer-sized spot were assigned to the upwelling and non-upwelling season based on the color of the sediment in that spot. As a result, separate records for the upwelling and non-upwelling seasons could be obtained. In order to translate UK'37 values into SST, a correction factor to account for differences between MSI-based data and conventional UK'37 data was applied. SST was calculated with the BAYSPLINE model

Mean annual SST reconstruction by the UK'37 proxy for the Younger Dryas-Holocene transition in sediments of the Cariaco Basin, sediment core MD03-2621

The UK'37 sea surface temperature (SST) proxy, based on the distribution of long-chain akenones, was measured via mass spectrometry imaging (MSI) at 100-µm resolution in a 60-cm section of the well-dated core MD03-2621 from the Cariaco Basin (10.6782, -64.9715). This section spans an age of ~ 11.9 to 11.2 kyr b2k and thus includes the Younger Dryas-Holocene transition. The resulting record provides insights into interannual variability during this most recent glacial to interglacial transition. In order to translate UK'37 values into SST, a correction factor to account for differences between MSI-based data and conventional UK'37 data was applied. SST was calculated with the BAYSPLINE model

Deglacial increase of seasonal temperature variability in the tropical ocean

The relatively stable Holocene climate was preceded by a pronounced event of abrupt warming in the Northern Hemisphere, the termination of the Younger Dryas (YD) cold period1,2. Although this transition has been intensively studied, its imprint on low-latitude ocean temperature is still controversial and its effects on sub-annual to decadal climate variability remain poorly understood1,3,4. Sea surface temperature (SST) variability at these timescales in the tropical Atlantic is expected to intensify under current and future global warming and has considerable consequences for environmental conditions in Africa and South America, and for tropical Pacific climate5–8. Here we present a 100-µm-resolution record obtained by mass spectrometry imaging (MSI) of long-chain alkenones in sediments from the Cariaco Basin9–11 and find that annually averaged SST remained stable during the transition into the Holocene. However, seasonality increased more than twofold and approached modern values of 1.6 °C, probably driven by the position and/or annual range of the Intertropical Convergence Zone (ITCZ). We further observe that interannual variability intensified during the early Holocene. Our results demonstrate that sub-decadal-scale SST variability in the tropical Atlantic is sensitive to abrupt changes in climate background, such as those witnessed during the most recent glacial to interglacial transition

Deglacial increase of temperature variability in the tropical ocean

The warm and relatively stable Holocene climate was preceded by a pronounced event of abrupt warming in the Northern Hemisphere, the termination of the Younger Dryas cold period1,2. While this transition has been intensively studied, its imprint on low latitude ocean temperature is still controversial and its effects on sub-annual to decadal climate variability remain poorly understood1,3,4. We applied the extraordinary resolution provided by mass spectrometry imaging of long-chain alkenones5,6 to sediments from the tropical Cariaco Basin7, and reveal that the seasonal amplitude of reconstructed sea surface temperature increased more than twofold during the transition into the Holocene, while average temperature was not altered. We further observe modulations in interannual sea surface temperature variability that we attribute to a muting of the El Niño-Southern Oscillation at the end of the Younger Dryas, and a subsequent intensification during the early Holocene. These dynamics are consistent with the modeled interplay of meltwater and ice sheet forcing and suggest that climate recovery in the Pacific preceded the North Atlantic Younger Dryas-Holocene transition. Our results demonstrate that the abrupt changes that completed the most recent glacial to interglacial transition had pronounced effects on sub-and interannual climate variability in the Tropical North Atlantic

Multi-molecular ¹⁴C evidence for mineral control on terrestrial carbon storage and export

Compound- and compound class-specific radiocarbon analysis of source-diagnostic 'biomarker' molecules has emerged as a powerful tool to gain insights into terrestrial carbon cycling. While most studies thus far have focused on higher plant biomarkers (i.e. plant leaf-wax n-alkanoic acids and n-alkanes, lignin-derived phenols), tracing paedogenic carbon is crucial given the pivotal role of soils in modulating ecosystem carbon turnover and organic carbon (OC) export. Here, we determine the radiocarbon (¹⁴C) ages of glycerol dialkyl glycerol tetraethers (GDGTs) in riverine sediments and compare them to those of higher plant biomarkers as well as markers of pyrogenic (fire-derived) carbon (benzene polycarboxylic acids, BPCAs) to assess their potential as tracers of soil turnover and export. GDGT Δ¹⁴C follows similar relationships with basin properties as vegetation-derived lignin phenols and leaf-wax n-alkanoic acids, suggesting that the radiocarbon ages of these compounds are significantly impacted by intermittent soil storage. Systematic radiocarbon age offsets are observable between the studied biomarkers, which are likely caused by different mobilization pathways and/or stabilization by mineral association. This article is part of the Theo Murphy meeting issue 'Radiocarbon in the Anthropocene'.ISSN:1364-503XISSN:1471-296