Branched glycerol dialkyl glycerol tetraethers (brGDGTs) with lower (sparsely-branched; SB-) and higher (overly-branched; OB-) numbers of methylated branches relative to the “regular” brGDGTs (B-GDGTs) are abundant in anoxic waters in the Black Sea. Observed changes in abundances and numbers of methylated branches of the entire series OB-GDGTs, B-GDGTs, and SB-GDGTs relative to dissolved oxygen (DO) levels in anoxic waters suggest that these compounds can potentially track changes in oceanic DO levels through time. To explore this, we determine the entire brGDGT series in surface or near-surface sediments from sites with different DO distributions in marine waters and sediments, extending the limited core-top collection of these lipids. We propose a modified methylation index based on only OB-GDGTs, called MOB, to avoid the potential impacts of terrestrial-derived B-GDGTs. Interestingly, MOB values in our extended core-top collection are strongly related to changes in bottom-water DO concentrations rather than the site-specific minimum DO values, i.e. usually within mid-depth oxygen minimum zones (OMZs). This suggests that sedimentary lipids are likely derived from heterotrophic bacteria living at the sediment-water boundary in sediments while lipids produced within mid-depth OMZs are not effectively exported to deep oceans. Analysis of MOB values in ancient sediments in the East Equatorial Pacific shows a gradual decline in bottom water DO, correlating with the progressive increase in global export productivity, organic carbon burial, and elevated level of deep-water nutrient contents since the middle Miocene. These findings highlight the potential of MOB as a tool for reconstructing past oceanic (de)oxygenation events.

Insight into the causes of the West Antarctic Ice Sheet (WAIS) stability over middle Pleistocene glacial/interglacial (G/IG) cycles is fundamental to our understanding of the response of the climate system to the cryosphere. Here, to clarify the mechanism of WAIS stability during the late Quaternary period, we provide iceberg-rafted debris (IRD) contents, clay mineral, and Sr-Nd isotopic analyses of the piston core ANT34/A2-10. The core was recovered from the seasonal sea ice region in the Antarctic Zone of the Amundsen Sea with a ~773 ka BP chronology. The endmember analysis of clay minerals shows marked differences in sediment provenance at site ANT34/A2-10 between IRD peak interval and low IRD content interval in G/IG cycles. And the Sr-Nd isotopic endmember analysis in IRD peak intervals restricts the sediment provenance in the Victoria Land. We suggest that shifts in the sediment provenance resulted from the variations in iceberg trajectories, which connected to the significant shifts in the atmospheric system at the IRD peak intervals. Moreover, a contemporaneous strengthened ocean-driven positive feedback occurred between the increased wind-driven upwelling of warm, well-ventilated Circumpolar Deep Water and the intense ice mass loss process (including iceberg calving and basal melting process) with the instability of the WAIS. Furthermore, our results reveal that the variation of WAIS stability is sensitive to the local summer insolation forcing. These pieces of evidence recorded in the pelagic South Pacific Southern Ocean may strongly reflect the significant variations in ocean-driven and orbital forcing on WAIS stability on the orbital scale.