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.

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Variations in clay mineral assemblages have been widely used to understand changes in sediment provenance during glacial and interglacial periods. Smectite clay minerals, however, have a range of various elemental compositions that possibly originated from multiple different sources. Therefore, it might be crucial to distinguish the various types of smectites by analyzing their elemental composition in order to verify the sediment provenances with certainty. This hypothesis was tested for the clay mineral characteristics in a marine sediment core from the southern Drake Passage (GC05-DP02). Rare earth elements and ε Nd data had previously indicated that fine grained detritus was supplied from the Weddell Sea to the core site during interglacial periods, when the sediments contained more Al-rich smectite (montmorillonite). Indeed, marine sediments collected close to the Larsen Ice Shelf on the eastern Antarctic Peninsula continental shelf, western Weddell Sea embayment, show more Al-rich smectite components as compared with other possible West Antarctic sources, such as the Ross Sea embayment or King George Island, South Shetland Islands. Furthermore, two types of smectite (Al-rich and Al-poor) were identified in core GC360 from the Bellingshausen Sea shelf, suggesting that during glacial periods some sediment is derived from subglacial erosion of underlying pre-Oligocene sedimentary strata containing predominantly Al-rich montmorillonite. This finding reveals different sources for smectites in sediments deposited at site GC360 during the last glacial period and during the present interglacial that show only minor differences in smectite contents. For the interglacial period, two groups of smectite with a wide range of Al-rich and Mg-Fe-rich were identified, which indicate delivery from two different sources: (1) the detritus with high contents of Mg-Fe-rich smectite supplied from Beethoven Peninsula, southwestern Alexander island and (2) the detritus with higher contents of Al-rich smectite (montmorillonite) possibly derived from the subglacial reworking of pre-Oligocene sedimentary strata. These results demonstrate that the elemental compositions of smectites can be used to differentiate the sources of smectites in marine sediments, which is an important tool to define sediment provenance in detail, when down-core changes observed in clay mineral assemblages are interpreted.

Variations in grain size, clay mineral composition, and stable isotopes (δ13C and δ15N) are closely linked to the sedimentary facies that reflect mineralogical and geochemical modification during the retreat and advance of the Larsen ice shelf. A whole round core of marine sediment (EAP13-GC17, 236 cm below the sea floor) was collected on the northwestern Larsen B embayment of the Antarctic Peninsula during a marine geological expedition (the ARA13 Cruise Expedition by the Korea Polar Research Institute, 2013). Four sedimentary facies (U1–U4) were clearly distinguishable: bioturbated sandy mud (open marine, U1), laminated sandy mud (sub–floating ice shelf, U2), sandy clay aggregates (deglacial, U3), and muddy diamictons (sub-glacial, U4), as well as interbedded silty. Clay minerals, including smectite, chlorite, illite, and kaolinite, were detected throughout the core. An increase in the clay mineral ratio of smectite/(illite + chlorite) was clearly observed in the open marine condition, which was strongly indicated by both a heavier isotopic composition of δ13C and δ15N (−24.4‰ and 4.3‰, respectively), and an abrupt increase in 10Be concentration (~30 times). An increase in the average values of the crystal packet thickness of illite (~1.5 times) in U1 also indicated sediments transported in open marine conditions. Based on the clay mineral composition in U1, the sediments are likely to have been transported from the Weddell Sea. The clay mineralogical assessments conducted in this region have significant implications for our understanding of paleodepositional environments.

Abstract. During the Late Pleistocene–Holocene, the Ross Sea Ice Shelf exhibited strong spatial variability in relation to the atmospheric and oceanographic climatic variations. Despite being thoroughly investigated, the timing of the ice sheet retreat from the outer continental shelf since the Last Glacial Maximum (LGM) still remains controversial, mainly due to a lack of sediment cores with a robust chronostratigraphy. For this reason, the recent recovery of sediments containing a continuous occurrence of calcareous foraminifera provides the important opportunity to create a reliable age model and document the early deglacial phase in particular. Here we present a multiproxy study from a sediment core collected at the Hallett Ridge (1800 m of depth), where significant occurrences of calcareous planktonic and benthic foraminifera allow us to document the first evidence of the deglaciation after the LGM at about 20.2 ka. Our results suggest that the co-occurrence of large Neogloboquadrina pachyderma tests and abundant juvenile forms reflects the beginning of open-water conditions and coverage of seasonal sea ice. Our multiproxy approach based on diatoms, silicoflagellates, carbon and oxygen stable isotopes on N. pachyderma, sediment texture, and geochemistry indicates that abrupt warming occurred at approximately 17.8 ka, followed by a period of increasing biological productivity. During the Holocene, the exclusive dominance of agglutinated benthic foraminifera suggests that dissolution was the main controlling factor on calcareous test accumulation and preservation. Diatoms and silicoflagellates show that ocean conditions were variable during the middle Holocene and the beginning of the Neoglacial period at around 4 ka. In the Neoglacial, an increase in sand content testifies to a strengthening of bottom-water currents, supported by an increase in the abundance of the tycopelagic fossil diatom Paralia sulcata transported from the coastal regions, while an increase in ice-rafted debris suggests more glacial transport by icebergs.