Sub‐ice shelf sediment geochronology utilizing novel radiocarbon methodology for highly detrital sediments

Abstract

Sub‐ice shelf sediments near Larsen C ice shelf (LIS‐C) show fine‐scale rhythmic laminations that could provide a near‐continuous seasonal‐resolution record of regional ice mass changes. Despite the great potential of these sediments, a dependable Late Quaternary chronology is difficult to generate, rendering the record incomplete. As with many marginal Antarctic sediments, in the absence of preserved carbonate microfossils, the reliability of radiocarbon chronologies depends on presence of high proportions of autochthonous organic carbon with minimized detrital organic carbon. Consequently, acid insoluble organic (AIO) 14C dating works best where high productivity drives high sediment accumulation rates, but can be problematic in condensed sequences with high proportions of detrital organic carbon. Ramped PyrOx 14C dating has progressively been shown to improve upon AIO 14C dates, to the point of matching foraminiferal carbonate 14C dates, through differential thermochemical degradation of organic components within samples. But in highly detrital sediments, proportions of contemporaneously deposited material are too low to fully separate autochthonous organic carbon from detrital carbon in samples large enough to 14C date. We introduce two modifications of the Ramped PyrOx 14C approach applied to highly detrital sediments near LIS‐C to maximize accuracy by utilizing ultra‐small fractions of the highly detrital AIO material. With minimization of the uncertainty cost, these techniques allow us to generate chronologies for cores that would otherwise go undated, pushing the limits of radiocarbon dating to regions and facies with high proportions of pre‐aged detritus. Wider use of these techniques will enable more coordinated a priori coring efforts to constrain regional glacial responses to rapid warming where sediments had previously been thought too difficult to date.

The marine geological imprint of Antarctic ice shelves

“…16,23,118 ; number in parenthesis is distance from GL. Absolute diatom abundance (ADA; millions of valves/gram sediment) 17,22,63,64,96 . Percentage total organic carbon (TOC)/Nitrogen (N) and total carbon (TC)/N data 16,63 together with unpublished data from George VI and Larsen C sub-ice-shelf cores.…”

“…4 and 5) 20,24,54,63 . With increasing distance from the GL, the gravel and sand rich sediments are eventually replaced by sometimes thick sequences of laminated to massive muds and sandy muds deposited by sediment-laden meltwater plumes 16,25,26,64 , observed to extend 250 km from the GL 25 . The overall thickness of this unit is primarily a function of time, but also the competing influence of sediment delivery, and strength and longevity of ocean currents (which can be tidally generated) 16,23,64 .…”

“…With increasing distance from the GL, the gravel and sand rich sediments are eventually replaced by sometimes thick sequences of laminated to massive muds and sandy muds deposited by sediment-laden meltwater plumes 16,25,26,64 , observed to extend 250 km from the GL 25 . The overall thickness of this unit is primarily a function of time, but also the competing influence of sediment delivery, and strength and longevity of ocean currents (which can be tidally generated) 16,23,64 . On large ice shelves, where the GL and calving line can be separated by >100 km, the laminated muds grade into massive muds with progressive distance from the GL before eventually reaching a theoretical zone where sedimentation is almost zero (‘null zone’ in Figs.…”

“…124 ), but it is often viewed as a last resort owing to the potential for large error caused by “contamination” with reworked fossil carbon 125 . Recent advances in Ramped Pyrolysis (RP) 14 C dating minimises the effect of contamination by combusting sediments at gradually increasing temperatures to thermally break down the bulk organic matter 64,126,127 . The gradual temperature increase allows for the separation of the more thermochemically reactive younger constituents, which are preferably combusted under lower temperatures, from the reworked more stable older constituents, so that separate components can be dated independently 127 .…”

Compound‐Specific Radiocarbon Analysis of (Sub‐)Antarctic Coastal Marine Sediments—Potential and Challenges for Chronologies

“…The main input pathway of ancient OC likely is erosion and re-working by the glaciers terminating to the southeast of the coring location ( Figure 1). In Antarctica, high bulk OC 14 C ages were reported from shelf sediments deposited in front of ice shelf calving fronts, for example, around the Antarctic Peninsula (e.g., Rosenheim et al, 2008Rosenheim et al, , 2013Subt et al, 2017) and in the Ross Sea (e.g., Andrews et al, 1999;Ohkouchi et al, 2003), reflecting relatively high and variable petrogenic OC contributions in surface sediments. In the Ross Sea the high spatial variation of petrogenic OC (ranging from 13% to 90%) is mainly controlled by the proximity to the Ross Ice Shelf and the lithology of the catchments (e.g., coal bearing strata in the catchment; Ohkouchi & Eglinton, 2006).…”