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

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. Key Points We 14C date highly detrital sediment from the Antarctic margin containing very little autochthonous carbon We employ composite and isotope dilution Ramped PyrOx 14C dating, considering blank contamination for each Both approaches sacrifice analytical precision for gains in accuracy; application to other detrital systems depends on uncertainty limits