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The Southern Ocean plays an important role in modulating Pleistocene atmospheric CO2 concentrations, but the underlying mechanisms are not yet fully understood. Here, we report the laser grain-size distribution and Mn geochemical data of a 523 kyr-long sediment record (core ANT30/P1-02 off Prydz Bay; East Antarctica) to trace past physical changes in the deep Southern Ocean. The core sediments are predominantly composed of clay and silt-sized material. Three grain size end-members (EM) as well as three sensitive grain size classes (SC) were discerned, interpreted as Ice Rafted Debris (EM1 and SC1), and coarse (EM2 and SC2) and fine (EM3, SC3) materials deposited from bottom nepheloid layers, respectively. Ratios of EM2/(EM2+EM3) and SC2/SC3 reveal changes in the local bottom current strength, which is related to the deep ocean diapycnal mixing rate, showing higher values during interglacial periods and lower values during glacial periods. MnO was enriched at each glacial termination, probably caused by abrupt elevations in Antarctic bottom water (AABW) formation rate. Lower AABW formation rate and reduced deep diapycnal mixing during glacial periods enhanced deep Southern Ocean stratification, contributing to glacial atmospheric CO2 drawdown. The elevated AABW formation and enhanced deep diapycnal mixing during glacial terminations alleviated such deep stratification, promoting deeply sequestered CO2 to outgas.

Sediment color reflectance contains important information on paleoenvironmental changes. Such data are routinely measured on sediment cores from the Southern Ocean. Their usefulness, however, is undervalued. Here for the first time, color reflectance of an ~520-kyr sediment core retrieved off Prydz Bay (East Antarctica) was intensively investigated to correlate sediment color changes to changes in sediment compositions. Total carbon, total organic carbon, X-ray fluorescence-derived Fe/Ti and Br/Ti ratios, water content and mineral composition of sediment from this core, and color reflectance of another two sediment cores retrieved nearby were analyzed in this study. The raw reflectance data were transformed into their first-order derivatives and subjected to factor analysis algorithms. A four-factor model was selected to unmix the data set, explaining >80% of the total data variance. Its robustness is supported by similar factor analysis results from the other two cores. The four factors represent (from Factors 1–4) pulses of sedimentary Mn enrichment and Fe3+/Fe2+ changes in clay minerals, relative content changes of colorless components, hematite and goethite, and water, respectively. Scores of Factor 1 show high spikes at glacial terminations, recording enhanced deglacial ventilation in the abyssal Southern Ocean. Scores of Factor 2 mainly reflect biogenic components and chlorite but are further complicated by refractory terrigenous organic carbon and Mn-oxides/-hydroxides. Higher Scores of Factor 3 occur mainly at peak interglacial intervals, associated with enhanced transport of sediment of Antarctic origin by oceanic currents. Water content (Factor 4) in the core sediment was significantly influenced by opal content and sediment grain size compositions.