Young-Suk Bak

Late Holocene cyclic glaciomarine sedimentation in a subpolar fjord of the South Shetland Islands, Antarctica, and its paleoceanographic significance: Sedimentological, geochemical, and paleontological evidence

The glaciomarine sedimentary record of the fjord head (Collins Harbor) in Maxwell Bay, South Shetland Islands (West Antarctica), a large marine calving embayment, contains repeating couplets of organic-rich massive diamicton and organic-poor stratified diamicton. The massive diamicton is characterized by high total organic carbon (TOC) content and carbon to nitrogen (C/N) ratios and was deposited in a cold climate regime by iceberg-rafted sedimentation from coastal fast ice in which algal plants, as well as gravels, were entrained. The stratified diamicton is characterized by low TOC content and C/N ratios and was formed in a warmer climate regime when the flux of icebergs was suppressed, but turbid meltwater discharge continued to produce lamination. When the meltwater discharge decreased in cold climatic conditions, and resultant phytoplankton productivity was reduced due to the increased sea-ice coverage, ice rafting from shorefast sea ice might have played a major role in entraining benthic algae, as well as loads of sand and gravel, along the coastal area, resulting in an increased C/N ratio and gravel content in the massive diamicton. Accelerator mass spectrometry (AMS) radiocarbon analyses conducted on well-preserved calcite shells were used to construct a chronology for the past 3000 years. Fluctuations in TOC are recorded (approximately four cycles over this time period), with the average duration of a cooling cycle being ∼500 years. These cycles may be correlative with the high-frequency (550 yr) variability in reduced Circumpolar Deep Water (CDW) on the West Antarctic Peninsula shelf, because a decrease in CDW may be related to reduced deep water production in the North Atlantic during colder periods, as demonstrated for glacial intervals throughout the Pleistocene.

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.

Climate changes in the South Orkney Plateau during the last 8600 years

Climatic and oceanographic changes in the South Orkney Plateau, western Antarctica, during the last 8600 years are reconstructed from a 525 cm long gravity core based on sedimentological, geochemical and diatom analyses. The core sediments are composed mostly of light greyish olive diatomaceous silt and mud with a few diatom ooze laminae in the basal part. The core can be divided at 350 cm into two units (4800 cal. yr BP): the lower unit is characterized by variable total organic carbon (TOC) content and higher CaCO3 content, and the upper unit is characterized by higher TOC and lower CaCO3. The content of biogenic silica varies similar to TOC content in the lower unit but does not increase in the upper unit despite increased TOC. The variations in the organic matter composition and the amount of carbonate suggest that Scotia Sea water had been dominant in the study area prior to 4800 cal. yr BP. Warmer conditions during the middle Holocene are also supported by the fewer sea-ice diatom taxa and a more sub-polar form of Eucampia antarctica in the lower unit. The increased sea ice and decreased influence of Scotia Sea water in the upper unit reflect climate cooling occurred at 4800 cal. yr BP.

Late Quaternary Paleoclimatic Change in the Ulleung Basin, East Sea, Korea

Paleoclimatic changes in the late Quaternary sediments deposited in the East Sea were analyzed by studying diatoms. A total of 95 species belonging to 47 genera were identified from the Core02GHP-02 of the Ulleung Basin in the East Sea, Korea. In the Core 02GHP-02, U-Oki (169–181 cm; 9.3 ka), AT (464–465.5 cm; 22 ka) and U–Ym (556–559 cm; 33 ka) tephra layers were recognized. The chronological divisions of 02GHP-02 may be correlated with the climatic changes from the glacial interval (730–620 cm; MIS3), to interstadial (620–500 cm; MIS3), to the last glacial maximum (500–390 cm; MIS 2), to the deglaciation (390–290 cm; MIS 2), to the late glacial (290–190 cm; MIS 1), and to the Holocene (190–10 cm; MIS 1). It is speculated that diatoms were rarely found during the glacial interval when the 02GHP-02 core was deposited; during the interstadial (MIS 3) and deglaciation, a mixture of warm-water and cold-water species were found. In particular, Fragilariopsis doliolus seems to have appeared in the East Sea after 8 kaBP. In the lower layers of the Holocene deposits, cold-water species such as Neodenticula seminae were frequently found, while in the upper layers, warm-water species such as Hemidiscus cuneiformis were found in relatively large abundance. Therefore, the findings indicate that the climate became warmer during the transition from the lower layer to the upper layer of the Holocene deposits.

The Influx of the Tsushima Current into the Central Ulleung Basin of East Sea (Sea of Japan), Korea

A total of 85 samples, collected from the UBGH1–9 core taken from the Ulleung Basin, East Sea, Korea, were analyzed using diatom assemblages. 111 diatom species belonging to 46 genera were identified, and three diatom assemblage zones were established on the basis of occurrence and distribution pattern of diatoms. Diatom assemblage zone I (134.10–174 m) is characterized by a relatively high abundance of marine species, while the increased number of the brackish species is recorded in diatom assemblage zone II (75–125 m). The assemblage zones IIIa became drastic drop of valve abundances and brackish planktons, whereas it became increase during the IIIb. High Td values which indicate an influence of warm current are recorded both in diatom assemblage zone I and III, and low Td values in diatom assemblage zone II. Analysis of diatom assemblages indicating that the depositional condition moved from oceanic to littoral-neritic environments and that paleotemperature underwent a shift from warm to cold condition at the middle interval, and from cold to warm condition in the upper interval of the UBGH1–9 core. This suggests that the lower (130–162 m) and upper intervals (0–20 m) of the UBGH1–9 core were deposited in the warm current condition (Tsushima Warm Current).

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.

Diatom Succession Representing the Paleoclimatic Change from Laminated Sediments around Antarctica

This study investigated the paleoclimatic change using diatoms that were extracted from the high-resolution laminated layers of diatom ooze sediment cores GC08-EB01 in the eastern basin of the Bransfield Strait, Antarctica. The range of diatom valves per gram of dry sediment was from in quantitative diatom assemblage analysis. Laminations are classified using visually dominant diatom species and terrigenous content. Biogenic diatom ooze laminae characterised by bloom of Corethron crilophilum, Eucampia antarctica, Fragilariopsis curta, F. kerguelensis, Odontella weissflogii, Proboscia inermis, R. styliformis, Thalassiosira antarctica, and Chaetoceros resting spores. Terrigenous laminae characterised by mixed diatom assemblage. The ratio of (Fragilariopsis curta+F. cylindrus)/Thalassiosira antarctica increase in horizons, suggesting increased sea-ice cover in the study area during the late Holocene (cold events). As a result, five cold events are identified on the basis of frequency of the critical taxa throughout the section.

Revision of the conodont zonation of the uppermost Hwajeol Formation (Furongian), Taebaeksan Basin, Korea

The uppermost interval of the Hwajeol Formation, Taebaeksan Basin, has been assigned to the Fryxellodontus inornatus-Monocostodus sevierensis-Semiacontiodus lavadamensis Zone. The main species in the zone are Fryxellodontus inornatus Miller, Hirsutodontus hirsutus Miller, Monocostodus sevierensis (Miller), Semiacontiodus lavadamensis (Miller), S. nogamii Miller, Utahconus utahensis (Miller), and Cordylodus proavus Müller. These species were re-examined to evaluate the biostratigraphy of the Bangteogol and Maesangol sections. Data were added from eleven sections around the Taebaeksan Basin, which were studied previously by the first author. Conodont occurrences above the Cambrooistodus minutus Zone are characterized by a lack of vertical and lateral continuity in sections. Conodont recovery is relatively poor but is sufficient for biostratigraphic assignment. This trend of conodont occurrence seems to be related to sedimentary process, including debris slides and eustatic sea-level fluctuation. Nevertheless, detailed study of the characteristic species of the zone leads to the conclusion that retention of the zone is reasonable. However, it is renamed the M. sevierensis-S. lavadamensis Zone because Fryellodontus inornatus occurs stratigraphically lower than the other two nominate species. The M. sevierensis-S. lavadamensis Zone is correlated with equivalent zones of western USA, Canada, North China, and Australia.

Late quaternary climate changes around the elephant islands, Antarctic Peninsula

Sixty-seven species of diatoms of 28 genera were identified in Core GC03-C2 acquired from the north slope of Elephant Island, Antarctic Peninsula. The number of diatom valves per gram of dry sediment ranged from 0.2∼17.3×107g−1, and these were dominated by Fragilariopsis kerguelensis (65.8%). Diatom assemblage analysis reconstructed the Quaternary paleoclimatic change the Elephant Islands. Four diatom assemblage zones were identified according to the frequency of critical taxa as follows: zone I, from 830 to 710 cm (Antarctic Cold Reversal); zone II, from 700 to 550 cm (Deglaciation zone); zone III, from 540 to 260 cm (warm period; Holocene); and zone IV, from 250 to 0 cm (cool period; Holocene). The high abundance of reworked species includes Actinocyclus ingens, Denticulopsis hustedtii, D. praedimorpha, and D. dimorpha appeared in Zone I by turbidity currents and ice rafting in the area during the glaciations-deglaciation event.