Kyung Sik Woo

Mid-latitude interhemispheric hydrologic seesaw over the past 550,000 years

An interhemispheric hydrologic seesaw—in which latitudinal migrations of the Intertropical Convergence Zone (ITCZ) produce simultaneous wetting (increased precipitation) in one hemisphere and drying in the other—has been discovered in some tropical and subtropical regions. For instance, Chinese and Brazilian subtropical speleothem (cave formations such as stalactites and stalagmites) records show opposite trends in time series of oxygen isotopes (a proxy for precipitation variability) at millennial to orbital timescales, suggesting that hydrologic cycles were antiphased in the northerly versus southerly subtropics. This tropical to subtropical hydrologic phenomenon is likely to be an initial and important climatic response to orbital forcing. The impacts of such an interhemispheric hydrologic seesaw on higher-latitude regions and the global climate system, however, are unknown. Here we show that the antiphasing seen in the tropical records is also present in both hemispheres of the mid-latitude western Pacific Ocean. Our results are based on a new 550,000-year record of the growth frequency of speleothems from the Korean peninsula, which we compare to Southern Hemisphere equivalents. The Korean data are discontinuous and derived from 24 separate speleothems, but still allow the identification of periods of peak speleothem growth and, thus, precipitation. The clear hemispheric antiphasing indicates that the sphere of influence of the interhemispheric hydrologic seesaw over the past 550,000 years extended at least to the mid-latitudes, such as northeast Asia, and that orbital-timescale ITCZ shifts can have serious effects on temperate climate systems. Furthermore, our result implies that insolation-driven ITCZ dynamics may provoke water vapour and vegetation feedbacks in northern mid-latitude regions and could have regulated global climate conditions throughout the late Quaternary ice age cycles.

Cyclic tidal successions of the Middle Ordovician Maggol Formation in the Taebaeg area, Kangwondo, Korea

Sedimentologic investigation of the Ordovician Maggol Formation in the Hyeoldong and Gumunso sections, Taebaeg area, has been carried out to delineate the depositional environment, lithostratigraphy, and cyclicity as well as to infer the paleoclimatic conditions under which the formation was deposited.Based on lithology, sedimentary structure, and fossil contents, the Maggol Formation can be divided into nine lithofacies and their corresponding depositional environments. These are 1) bioclastic/intraclastic/peloidal/oolitic packstone to grainstone (lower intertidal to subtidal); 2) biotubated limestone (subtidal); 3) lime mudstone with bioturbation and microbial lamination (lower intertidal); 4) ribbon rock (intertidal); 5) lime mudstone with microbial lamination (upper intertidal to supratidal); 6) stromatolitic limestone (shallow subtidal to lower intertidal); 7) calcareous shale (upper intertidal to supratidal); 8) fine laminated dolomite (upper intertidal to supratidal); and 9) flat pebble conglomerate (subtidal to supratidal).The lithofacies suggests that the Maggol Formation was deposited in a tidal flat environment, which shows repetitive cyclical, tidal successions. Based on the successive nature of nine lithofacies, 28 facies associations can be observed. All of the tidal successions in this study show shallowing-upward sequences, and at least 20 successions from the Gumunso area and 71 successions from the Hyeoldong area can be recognized in the outcrop.Stratigraphically, the Maggol Formation can be divided into three members based on lithology and lithofacies in this study, and this can be applied at least in the southern limb of the Baegunsan Syncline. The lower member is characterized by calcareous shale, bioturbated and/or laminated limestone, ribbon rock, and flat pebble conglomerate. The middle member is composed of about 30 m-thick, grey massive dolomite. The upper member contains bioclastic/peloidal packstone to grainstone, biotubated and/or laminated limestone, fine laminated dolomite, and solution-collapse breccia.The presence of evaporite mineral casts as well as related sedimentary structures suggests that the upper member might have been deposited in a sabkha-type tidal flat under arid climate whereas the lower member in a Bahamian-type tidal flat under humid climate.

Stable isotope profiles of bivalve shells: seasonal temperature variations, latitudinal temperature gradients and biological carbon cycling along the east coast of Korea

Abstract The stable oxygen ( δ 18 O ) and carbon ( δ 13 C ) isotopes of the living bivalve shells collected from four localities (Keojin, Jumunjin, Pohang and Kampo) in the shallow waters along the east coast of Korea (Sea of Japan) were measured at close intervals along the maximum growth line of bivalve shells to obtain the isotope profiles. High-resolution δ 18 O profiles exhibit distinct and annual cycles from which the seasonal temperature variations of each locality can be compared. The amplitude of δ 18 O profiles as well as δ 18 O max and δ 18 O min values demonstrates that each bivalve clearly represents the seasonal temperature variations at the collection site and the latitudinal temperature gradients can be deduced. The δ 13 C profiles show less distinct cyclicity, in contrast to the δ 18 O profiles, which may reflect the stability of available carbon isotope in the water column throughout the collection sites. The small range of variation of the δ 13 C values is likely to be attributed to the shallow water depth above the thermocline where the seasonal change of carbon isotope may not be well recorded. The insignificant level of seasonal blooming of primary production is likely to result in the steady biological cycling of carbon isotope associated with consistent δ 13 C values. However, the northward decrease of mean δ 13 C values may imply either the gradual increase in light carbon ( 12 C ) due to the organic matter oxidation or the different level of primary production activity on the carbon isotope.

Burial dolomitization and dedolomitization of the late Cambrian Wagok Formation, Yeongweol, Korea

The Cambrian Wagok Formation in Yongweol, Korea, involved two stages of dolomitization, separated by an intermediate phase of dissolution, dolomite cementation, and dedolomitization. Textural, isotopic, and chemical investigations of the dolomites and calcite indicate that they all formed or recrystallized in a burial diagenetic environment. Cathodoluminescence petrography shows two stages of dolomitization (Types I & II). Type I dolomite is non- to dully luminescent with a heavily corroded outre rim, comprising more than 90% of the whole sequence, whereas Type II dolomite is brightly luminescent and overgrew mostly on the corroded Type I dolomite. Replacement of Type I dolomite by calcite before the Type II dolomitization is supported by the following textural evidences: 1) the tip of Type I dolomite is heavily corroded; 2) relic crystal boundaries as well as ghosts of Type I dolomite can be observed within calcite; 3) calcite contains relic crystals of Type I dolomite; and 4) Type II dolomites always show sharp crystal surfaces, and cross-cut into calcite. Carbon isotopic compositions of Type I dolomite are in the range of −0.6 to +0.8‰ (PDB), which suggests that carbon isotopes were buffered by preexisting marine carbonates. Lighter oxygen isotopic values (−9.3 to −8.2‰, PDB) as well as xenotopic texture indicate that Type I dolomites formed in a burial diagenetic environment. Type I & II dolomites and dedolomite are Fe-, Mn-, Na-, and Sr-poor, all less than 300 ppm. Thus, similar trace and minor elemental contents of the three phases suggest that the diagenetic fluids responsible for all the phases were chemically similar in terms of trace and minor elements. The higher initial87Sr/86Sr ratio of Type I dolomite (0.7102–0.7105) than that of Cambrian seawater (0.7088–0.7092) suggests that the dolomitizing fluids were basin-derived.

Cretaceous lacustrine stromatolites in the Gyeongsang Basin (Korea): Records of cyclic change in paleohydrological condition

Textural observation and stable isotope analyses of middle Cretaceous lacustrine stromatolites of the Sinyangdong Formation, Gyeongsang Supergroup (Korea) reveal that the alternating laminations preserve the cyclic change of paleohydrological conditions influenced by local paleoclimate. These alternating laminations are repetitive, consisting of a lower fibrous calcite and an upper microbial micrite layer forming a couplet, which is individually about 2–5 mm thick. The lower-to-upper boundaries are always gradational, but upper-to-lower boundaries are sharp and abrupt, reflecting not only cessation of growth but also subaerial dissolution. A δ18O enrichment from fibrous calcite to microbial micrite is consistent, indicating lowering of paleolake level mainly due to strong evaporation during the dry period when the stromatolite grew. Simultaneous and coeval depletion in δ13C from fibrous calcite to microbial micrite suggests that more available are12C atoms provided by the decay of organic matter due to the decreasing paleoproductivity and lowering lake level. These systematic trends of texture and isotopic composition may be intrinsically linked to local paleoclimate change.

A reconnaissance on the use of the speleothems in Korean limestone caves to retrospective study on the regional climate change for the recent and geologic past

More than 1,000 limestone caves are extensively developed in Gangwon-do and Chungcheongbuk-do, Korea. A variety of numerous speleothems are actively growing in these caves today. A potential to use speleothems as regional paleoclimate proxy was explored by analyzing soda straw, stalactite and stalagmite. These proxy recorders that have grown during the past several decades were investigated using the presence of excess210Pb. Most specimens collected from six limestone caves were found to have an excess210Pb, indicating that they are less than about 100 yrs old. This excess210Pb was employed to estimate the growth rate for a tubular “soda straw” that yielded a longitudinal growth rate of 1.7 mm/yr past 60 years. Coeval decrease in δ13C values of the soda straw, spanning the time period of about 1930–1995 AD, may reflect the carbon isotope ratio of about 1930–1995 AD, may reflect the carbon isotope ratio of atmospheric CO2 for the same period. Studies on fine scale variations in the isotopic composition of recent speleothems along with age control using210Pb excess method from the speleothems in Korean limestone caves promise a great potential for the reconstruction of climate and environmental changes during the past hundred years or so.

Enhancing the role of geoconservation in protected area management and nature conservation

Acknowledgement by the International Union for the Conservation of Nature (IUCN) that geodiversity is part of natural diversity and geoheritage is part of natural heritage should help to strengthen the position and delivery of geoconservation through engagement with the wider nature conservation agenda. In particular, we identify six key areas offering opportunities to enhance the standing and mainstreaming of geoconservation: (1) integrating geoconservation principles in protected area management, including the promotion of geoheritage conservation across the full range of IUCN Protected Area Management Categories; (2) supporting biodiversity conservation and adaptation to climate change through the nature-based solutions approach and ‘conserving nature’s stage’; (3) contributing to natural capital and ecosystem services valuation; (4) contributing to conservation in the marine environment; (5) enhancing the connections between people, place, and nature and contributing to human well-being; and (6) promoting ecosystem stewardship and contributing to the achievement of the UN Sustainable Development Goals. Adoption of a more outward-looking approach should help to progress the integration of geoconservation within nature conservation, protected area planning and management, and broader environmental strategies and policies.

Comparison of oxygen isotope profile of a fossil bivale with the modern hydrographic condition: case study of the Seoguipo Formation (Korea)

The early to middle Pleistocene Seoguipo Formation, cropping out along the southern coast of Cheju Island (Korea), consists of shallow marine deposits containing numerous macro-and micro-fossils with prolific trace fossils. Oxygen isotopic compositions of a well-preserved marine bivalve (Mizuhopecten tokyoensis hokrikuensis) from the formation were measured through a high-resolution sampling along the maximum shell growth to track the paleoceanographic conditions during the lifespan of the bivalve. The oxygen isotope profile shows two distinct cyclic patterns with seasonal contrast. The minimum (i.e., summer) and maximum (i.e., winter) δ18O values denote about 2.9 to 3.4‰ (PDB) of the seasonal variation, which can be converted to approximately 13 to 15°C in seasonal temperature variation. Using the paleotemperature equation for calcite, the bimonthly measurements of temperature and salinity at water depths of 10 m and 30 m surveyed by the National Fisheries Research and Development Agency (Korea) between 1991 and 1997 enable us to construct the theoretical oxygen isotope profile that represents the present-day environmental condition. The constant δ18O values of seawater are assumed in the equation, although the salinity varies seasonally to some extent. Direct comparison between the fossil mollusk profile (ancient condition) and the predicted isotope profile (present-day condition) explains that the summer and winter paleotemperatures during fossil shell growth were quite lower than the present. It is also possible that the bivalve might grow in the18O-enriched condition different from the present-day seawater.

The origin of cold-water authigenic carbonates from deep-water, muddy sediments in the Ulleung Basin, East Sea of Korea

Carbonate concretions associated with recently discovered gas hydrates from the East Sea (Sea of Japan), Korea, were investigated to delineate their origin based on textural, mineralogical and stable isotopic data. Authigenic carbonates were sampled from the 8 m-long piston core recovered from a water depth of 2072 m during a gas hydrate exploration cruise in June 2007. The carbonate concretions occur at three intervals from a depth of 220 to 280 cm from the top of core within hemipelagic muds above massive and vein-filling gas hydrate layers from depth of 329 to 655 cm. Semi-lithified carbonates occur as concretions with knobby, rounded and irregular morphologies. Textural observation of the concretions reveals porous surfaces with a spongy texture and distinctive internal layering. These concretions are mostly composed of aragonite with relatively minor contribution of calcite, with transition from calcite- to aragonite-dominating phase towards the periphery. Highly negative δ13C values (δ13C=−43 to − 27‰ PDB) suggest that methane was the main carbon source for their formation. Oxygen isotope values (δ18O = 1.1 to 3.1‰ PDB) are slightly enriched due to the relatively stable cold temperature of the East Sea bottom water (0–4 °C). Distinctively separate and narrow ranges of stable isotope compositions from each concretion and the enrichment of carbon isotopes with depth indicate that the concretions at three intervals formed independently during different periods, but under similar anoxic diagenetic conditions by anaerobic oxidation of methane. In addition, distinctive geochemical clusters of each concretion with depleting oxygen isotope trend toward the periphery strongly suggest that the concretions in this study were formed separately by pore water interactions with adjacent surrounding pelagic muds.

Sedimentation and seafloor mound formation in the southern slope of the Ulleung Basin, East Sea, Korea, since the Last Glacial Maximum

Detailed analysis of core sediments and acoustic characteristics of Chirp sub-bottom profiles from a submarine ridge in the southern slope of the Ulleung Basin, East Sea, Korea, reveals changes in sedimentary processes and deformation of sedimentary sequence induced by seafloor mound formation since the Last Glacial Maximum (LGM). The core sediments consist generally of four sedimentary facies which represent hemipelagic sedimentation under a poorly-oxygenated bottom-water condition during the LGM (crudely laminated mud), shoreface erosion and seaward transport off the shelf break during the post-glacial transgression (bioturbated sandy mud or muddy sand), hemipelagic sedimentation under generally well-oxygenated bottom-water condition after the sea-level rise (bioturbated mud), and episodic precipitation of authigenic carbonates by methane venting since the LGM (authigenic carbonate-bearing mud), respectively. The formation of authigenic carbonate-bearing mud facies and associated accumulation of near-seafloor gas hydrates might have led to up-warping of the sediment cover since the LGM that resulted in the seafloor mounds protruding along the crest line of the ridge. The effects of up-warping by expansion of the subsurface gas hydrates are firstly manifested in this paper by the abrupt lateral changes in the sedimentary facies distribution and uplifted acoustic reflectors between the mounds and the background seafloor.