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.

Microbial interaction with minerals are significantly linked with depositional conditions during glacial and interglacial periods, providing a unique redox condition in the sedimentary process. Abiotic geophysical and geochemical properties, including sedimentary facies, magnetic susceptibility, grain size, clay mineralogy, and distribution of elemental compositions in the sediments, have been widely used to understand paleo-depositional environments. In this study, microbial abundance and diversity in the core sediments (6.7 m long) from the northeastern slope of Dokdo Island were adapted to characterize the conventionally defined sedimentary depositional units and conditions in light of microbial habitats. The units of interglacial (Unit 1, 14.5 ka) were distinctively identified in the core, showing a sharp boundary marked by the laminated Mn-carbonate (CaM) mud between bioturbated (Unit 1 and 2) and laminated mud (Unit 3). Based on the marker beds and the occurrence of sedimentary facies, core sediments were divided into three units, Unit 1 (14.5 ka, glacial), in descending order. The sedimentation rate (0.073 cm/year), which was three times higher than the average value for the East Sea (Sea of Japan) was measured in the late glacial period (Unit 2), indicating the settlement of suspended sediments from volcanic clay in the East Sea (Sea of Japan), including Doldo Island. The Fe and Mg-rich smectite groups in Unit 2 can be transported from volcanic sediments, such as from the volcanic island in the East Sea or the east side of Korea, while the significant appearance of the Al-rich smectite group in Unit 1 was likely transported from East China by the Tsushima Warm Current (TWC). The appearance of CaM indicates a redox condition in the sedimentary process because the formation of CaM is associated with an oxidation of Mn²⁺ forming Mn-oxide in the ocean, and a subsequent reduction of Mn-oxide occurred, likely due to Mn-reducing bacteria resulting in the local supersaturation of Mn²⁺ and the precipitation of CaM. The low sea level (−120 m) in the glacial period (Unit 3) may restrict water circulation, causing anoxic conditions compared to the late glacial period (Unit 2), inducing favorable redox conditions for the formation of CaM in the boundary of the two units. Indeed, Planctomycetaceae, including anaerobic ammonium oxidation (ANAMMOX) bacteria capable of oxidizing ammonium coupled with Mn-reduction, was identified in the CaM layer by Next Generation Sequencing (NGS). Furthermore, the appearance of aerobic bacteria, such as Alphaproteobacteria, Gammaproteobacteria, and Methylophaga, tightly coupled with the abundance of phytoplankton was significantly identified in Unit 1, suggesting open marine condition in the interglacial period. Bacterial species for each unit displayed a unique grouping in the phylogenetic tree, indicating the different paleo-depositional environments favorable for the microbial habitats during the glacial and interglacial periods.