Hyesu Yun

Diatom evidence for Holocene paleoclimatic change in the South Scotia Sea, West Antarctica

Diatom data of the core sediment from a deep basin in the South Scotia Sea, West Antarctica provide high-resolution information on changes in oceanographic processes and paleoclimate during the late Quaternary. Three main climatic changes can be distinguished in diatom assemblages: Last Glacial Maximu (LGM), mid-Holocene climatic optimum and Neoglacial cold event. Diatom assemblages have been deposited in a variable sea ice condition over the last 25,000 yr in response to the climate change. During the LGM to early-Holocene (23,370–8,300 yr BP), the core site might be influenced by increased dense sea-ice cover, which could reduce biogenic flux from the surface water, depositing relatively increased amount of sea ice-related diatoms (Actinocyclas actinochilus, Eucampia antarctica, Fragilariopsis curta, andFragilariopsis cylindrus). During these periods, it is likely that the increased sea ice cover between the Weddell and Scotia seas would have severely preventedChaetoceros resting spores in the Weddell Sea ice margin from being laterally advected to the Scotia Sea, resulting in the reduction ofC. resting spore abundance in the sediment. Afterward, a warm period followed from 8,300 to 2,400 yr BP in the mid Holocene when open water assemblages (Rhizosolenia styliformis andThalassiosira antarctica (warn)) were deposited. Significant dilution of the number ofFragilariopsis kerguelensis indicates the opening of communication between the Weddell and Scotia seas allowing lateral advection ofC. resting spores from the Weddell Sea to the Scotia Sea. A colder condition (Neoglacial cooling) then resumed since <2,400 yr BP in the late Holocene supported by other paleoclimatic records in the Antarctic Peninsula. The assemblage is characterized either by the increase of sea ice-related diatoms (A. actinochilus andF. cylindrus) or by the decrease of open water taxa (R. styliformis andT. antarctica (warm)) compared to that in the mid-Holocene optimum. The loose sea-ice assemblage was, however, different from dense sea-ice assemblage deposited in the LGM. Similarity of abundance ofChaetoceros between the Neoglacial and the mid-Holocene implies that the loose sea ice condition was not able to sufficiently restrict the lateral advection ofC. resting spore to the Scotia Sea from the Weddell Sea.

Radiolarians from the Pohang Basin, Southeast Korea and Paleoceanographic Implications

: We identified a total of 101 species and two subspecies of radiolarians belonging to 56 genera from 95 samples collected from the Hagjeon and Duho Formations in the Pohang Basin of the southeastern Korean Peninsula. On the basis of the biostratigraphic range of Cyrtocapsella cornuta and Theocorys redondoensis, the depositional period of the upper Hagjeon and lowest Duho Formations was determined to be early to late Middle Miocene. The occurrence of deep-dwelling radiolarians indicates that the paleobathymetry seems to become gradually progressing toward an upper bathyal environment in the middle part of the Hagjeon Formation. However, we prefer to accept another interpretation for the occurrence of deep-sea indicators in the Hagjeon and the lowest part of the Duho Formations, and consider the presence of a region of upwelling cold water that might have simulated a deep-water environment in relatively shallow water. This interpretation is based on the present upwelling of a cold-water mass off the southeast coast of Korea, the occurrence of upwelling microfossils from the Pohang Basin, and the effect of the closing of the Korea Strait approximately 15 Ma. We also considered that the uppermost part of the studied section represents a shallow-water environment.

Stratigraphy of the BP-1 well from Sora Sub-basin

This study carried out palynological analysis and seismic interpretation to establish a stratigraphic and environmental reconstruction mainly based on fossil palynomorphs and seismic reflection data correlated with the oil exploation well (BP-1) located in the Sora Sub-basin. There were frequent environmental and floral changes due to sea level change in the Sora Sub-basin. The palynomorph assemblages found in the well sediments enabled paleoecological zonation of the well sediment sequence resulting in 4 zones: Ecozone III, Ecozone IV, Ecozone V, Ecozone VI. Index fossils among palynomorphs indicate geological ages of the units within the well ranging from Eocene to Pleistocene, and paleoenvironment varies from freshwater to inner neritic marine. Previous studies suggest that the marine deposits were slightly different in stratigraphic range from well to well. It is considered the difference is credited to geomorphological setting. This study also shows stratigraphic correlation between existing wells and BP-1 well to establishes a standard stratigraphy of the Domi Basin.