Kiseong Hyeong

Coral reefs at 34°N, Japan: Exploring the end of environmental gradients

We describe the architecture and development of the highest latitude coral reefs currently known on Earth, located in Japan at 34°N. The reefs are distributed within turbid inner bays and undergo winter sea-surface temperatures (SSTs) that fall to 13 °C, well below the generally accepted lower limit (18 °C in winter) of tropical coral reef formation. Despite low SSTs and high turbidity, coring indicates reefs ranging to 555 cm in thickness since ca. 4.3 k.y. ago. The reefs exhibit high adaptability to this extremely marginal environment. Variability of the Tsushima Warm Current and the Asian monsoon could have affected the evolution of these reefs. The corals are dominantly faviids, in contrast to Acropora and Porites that tend to dominate low-latitude, tropical-subtropical reefs. Defining the end of the distributional range, the reefs serve as baselines for understanding coral reef distribution along SST and turbidity gradients.

Sr‐Nd isotope composition and clay mineral assemblages in eolian dust from the central Philippine Sea over the last 600 kyr: Implications for the transport mechanism of Asian dust

Dust transport to the tropical/subtropical northwestern Pacific over the past 600 kyr was investigated using radiogenic isotopes (87Sr/86Sr and eNd), together with the clay mineral composition, of eolian dust preserved in a sediment core obtained from the Philippine Sea (12°30′N, 134°60′E). These data revealed the influence of two prevailing dust sources, namely, the Asian deserts and nearby volcanic arcs (e.g., the Luzon Arc), with average contributions of around 70% and 30%, respectively, from each. The clay mineral composition of the core resembled dust from the central Asian deserts (CADs; e.g., the Taklimakan Desert) as in the north-central Pacific, but published aerosol data collected near the study site during winter/spring have the mineralogical signature of dust originating from the East Asian deserts (EADs). These data indicate that the relative contribution of EAD dust increases with the northeasterly surface winds associated with the East Asian Winter Monsoon (EAWM) during winter/spring, but the Prevailing Westerlies and Trade Winds that carry dust from the CADs is the dominant transport agent in the overall dust budget of the study site. The results of this study contradict the prevailing view that direct dust transport by the EAWM winds in spring dominates the annual flux of eolian dust in the northwest Pacific.

Cyclic sedimentation across a Middle Ordovician carbonate ramp (Duwibong Formation), Korea

SummaryThe Middle Ordovician Duwibong Formation (about 100 m thick), Korea, comprises various lithotypes deposited across a carbonate ramp. Their stacking patterns constitute several kinds of meter-scale, shallowing-upward carbonate cycles. Lithofacies associations are grouped into four depositional facies: deep- to mid-ramp, shoal-complex, lagoonal, and tidal-flat facies. These facies are composed of distinctive depositional cycles: deep subtidal, shallow subtidal, restricted marine, and peritidal cycles, respectively. The subtidal cycles are capped by subtidal lithofacies and indicate incomplete shallowing to the peritidal zone. The restricted marine and peritidal cycles are capped by tidal flat lithofacies and show evidence of subaerial exposure. These cycles were formed by higher frequency sea-level fluctuations with durations of 120 ky (fifth order), which were superimposed on the longer term sea-level events, and by sediment redistribution by storm-induced currents and waves. The stratigraphic succession of the Duwibong Formation represents a general regressive trend. The vertical facies change records the transition from a deep- to mid-ramp to shoal, to lagoon, into a peritidal zone. The depositional system of the Duwibong Formation was influenced by frequent storms, especially on the deep ramp to mid-ramp seaward of ooid shoals. The storm deposits comprise about 20% of the Duwibong sequence.

Relationship between polymetallic nodule genesis and sediment distribution in the KODOS (Korea Deep Ocean Study) Area, Northeastern Pacific

Polymetallic nodule and sediment characteristics were investigated for two blocks (KR2 and KR5) in the Korea Deep Ocean Study (KODOS) area in order to better understand nodule distribution and the potential effects of sediments on nodule genesis. The northern block (KR2) is dominated by hydrogenetic nodules, whereas the southern block (KR5) is dominated by diagenetic nodules. Sediments in the study area are assigned to three major lithologic units which are distinctive in color and texture. The northern block is characterized by a thick, metalpoor Unit 1 sediment, which is thin in the southern block, where metal-rich Units 2b and 3 occur close to the surface. The distribution of different nodule genetic types in the northern and southern blocks can be attributed to topographic variations (topographic high near seamounts in KR2 and abyssal plain in KR5) and different sedimentation rates (0.1 and 0.32 mm/kyr in blocks KR2 and KR5, respectively). The southern block has a geologic setting more conducive to diagenetic nodule formation, such as flat topography and sediment composition. Nodule distribution in the studied blocks might also be explained by the distribution of the sediment units of different metal contents. The northern block, in which Unit 1 is thicker, has more abundant hydrogenetic nodules, possibly because Unit 1 prevents metals that are remobilized from the underlying sediments from reaching the seabed where the nodules are forming.

Textural and geochemical characteristics of Fe−Mn crusts from four seamounts near the Marshall Islands, western Pacific

Textural and geochemical properties of ferromanganese crust (Fe−Mn crust) samples from four adjacent seamounts near the Marshall Islands were investigated to delineate the paleoceanographic condition on their growth history. The Fe−Mn crust samples of this study show four distinct layers (layers 1 to 4 from top to bottom). The uppermost layer 1 is massive and black, and is enriched in hydrogenetic elements such as Mn, Co, Ni, and Mo. The next layer 2 is porous and filled with sediment. Detrital (Al, Rb, and Ti) and biogenic (Cu, Zn, and Ba) elements are enriched in layer 2. The layers 3 and 4 are phosphatized layers which are impregnated with carbonate fluorapatite (CFA), and therefore their primary mineralogy and geochemistry were not preserved. The property of layer 2 suggested that this layer had grown under the condition of high biogenic and detrital flux. Such a condition may be met in the Inter-tropical Convergence Zone (ITCZ) where the northeast and southeast trade winds meet. Considering the present location and paleotracking of seamounts, layer 2 appears to have formed when these seamounts were beneath the ITCZ. On the other hand, layer 1 may have started to grow after the seamounts moved out of the ITCZ with the northwestward movement of the Pacific plate. Our study indicates that the Fe−Mn crusts can be used to trace the paleolocation of ITCZ when precise age determination and information on the plate movement are provided.

Evaluation of Resuspended Sediments to Sinking Particles by Benthic Disturbance in the Clarion-Clipperton Nodule Fields

The geochemical properties of sinking particles and sediments in the Clarion-Clipperton Zone were examined to develop a quantitative indicator with which to evaluate the contribution of sediment resuspended by nodule mining activity to sinking particles. The ratio of lithogenic material to organic carbon varies from ∼3 in sinking particles to ∼211 in sediments. This ratio is easily measured and is not easily affected by degradation and/or dissolution in the water column. A mixing model indicates that the ratio may be used as a potential proxy for estimating the contribution of resuspended sediment derived from mining operations to sinking particles.

Movement of the Intertropical Convergence Zone during the mid‐pleistocene transition and the response of atmospheric and surface ocean circulations in the central equatorial Pacific

This paper investigates the causes of a brief, but prominent, cooling episode (1.1–0.8 Ma) that occurred in the equatorial upwelling region of the Atlantic and Pacific during the mid-Pleistocene transition (MPT) using temporal changes in dust provenance, regional hydrology, and surface productivity recorded in a deep-sea sediment core from the central equatorial Pacific. The 87Sr/86Sr and ɛNd values of the inorganic silicate fraction indicate deposition of dust from Australia and Central/South America before 0.8 Ma, but a gradual increase in Asian dust deposition after 0.8 Ma. The change in dust provenance was accompanied by an increased dust flux and a decrease in surface productivity and salinity. These changes can be explained by the southward movement of the Intertropical Convergence Zone (ITCZ) and North Equatorial Counter Current (NECC) and the direct influence of these features on the site after 0.8 Ma. Our results, together with previously published Atlantic data, suggest the northward position of the ITCZ between 1.1 and 0.9 Ma, and the southward position thereafter. The meridional movement of the ITCZ is in phase with the cooling and warming trend in upwelling regions in the equatorial Pacific and Atlantic, which suggests strengthening of southeast trades relative to its northern counterpart between 1.1 and 0.9 Ma as a plausible cause of this brief cooling event. The southward movement of the ITCZ from 0.9 to 0.8 Ma indicates more significant cooling in the Northern Hemisphere (NH) than in the Southern Hemisphere, which is supportive of the interpretation that the NH ice sheet expanded significantly and stabilized after 0.9 Ma.

Geophysical and Geological Exploration of Cobalt-rich Ferromanganese Crusts on a Seamount in the Western Pacific

Co-rich ferromanganese crusts (Fe-Mn crusts) distributed on the seamounts in the western Pacific are potential economic resources for cobalt, nickel, platinum, and other rare metals in the future. Regulations for prospecting and exploration of Fe-Mn crusts in the Area, which enables the process to obtain an exclusive exploration right for blocks of the fixed size, were enacted recently by the International Seabed Authority, which led to public attention on its potential for commercial development. Evaluation and selection of a mining site can be established based on abundance and grade of Fe-Mn crusts in the site as well as topography that should be smooth enough for mining efficiency. Therefore, acquisition of shipboard echo-sounding and acoustic backscatter data are prerequisite to select potential mine sites in addition to visual and sampling operations. Acoustic backscatter data can be used to locate crust-covered areas in a regional scale with the understanding of acoustic properties of crust through its correlation with visual and sampling data. KIOST had collected the topographic and geologic data to assess the resources potential for Fe-Mn crusts in the west Pacific region from 1994 to 2001. However, they could not obtain acoustic backscatter data that is crucial for the selection of prospective mining sites. Therefore, additional exploration surveys are required to carry out side scan sonar mapping combined with seafloor observation and sampling to decide the blocks for application of an exclusive exploration right.

Hydrology of the Gulf intra-coastal waterway in the San Bernard Brazos river estuaries, Texas, USA: Oxygen isotopic ratio and salinity

A hydrological study using oxygen isotopic ratios and salinities was carried out in the San Bernard-Brazos River Estuaries, Brazoria County, Texas, USA, to understand the influence of 4 fresh water sources draining into the area, Brazos River, San Bernard River, Oyster Creek, and Jones Creek, on various parts of the Gulf Intra-Coastal Waterway (GIW) that transects these rivers half to 1 mile inland of the Gulf of Mexico. The measured oxygen isotopic ratios and salinities of GIW waters are explained with simple mixing between one of the 4 fresh water sources with marine water and made it possible to identify 3 subareas of the GIW, each of which is influenced by different fresh-water source. These sub-areas locate on the western part of the Brazos River, between the Brazos and Old Brazos River, and on the eastern part of the Old Brazos River, and have their freshwater source from the San Bernard River, Brazos River, and Oyster Creek, respectively. An interesting result was found in the GIW between the San Bernard and Brazos Rivers, where fresh water is mainly sourced from the San Bernard River although its discharge rate is 40 to 100 times less than that of the Brazos River. This westerly flow of the San Bernard River toward the larger Brazos River is due to shallowing of the San Bernard River at the river mouth, which restricts free discharge of river water to open sea and diverts the flow toward the GIW on both sides of the river. The influence of the other two creeks, Oyster and Jones, was minor and limited near the confluence with the GIW due to minor influxes. The generalized flow pattern derived in this study is essential for the construction of contingency plan against toxic contaminations, which will minimize environmental impact on numerous organisms living in the GIW. The results of this study are also useful for tracing of contaminants in the studied GIW.

Zooxanthellate Scleractinian Corals of Jeju Island, Republic of Korea

We report on seven zooxanthellate scleractinian corals (hermatypic or reef-building corals) found at the Jeju Island, off the southern coast of Korea. The Jeju Island, located at one of the northern limits of coral distribution in the East China Sea, has experienced significant sea surface temperature rises in the past century. In this chapter, we describe the systematic characteristics and identification keys of these seven species. This study thus provides a baseline not only for understanding coral community structure at the latitudinal limits of distribution, but also for examining ecosystem modification resulting from global warming.