Holocene sedimentation in the Hupo Trough of the southwestern East Sea (Japan Sea) and development of the East Korea Warm Current

Various sediment properties, such as mean grain size, total organic carbon, total nitrogen, C/N ratio, CaCO3, and biogenic opal content, were analyzed for a box core (BC02; 45 cm long) and a gravity core (GC02; 628 cm long), which were collected from the western margin of the Hupo Trough located off the eastern coast of Korea. The study area has been affected by the East Korea Warm Current (EKWC), a branch of the Tsushima Warm Current (TWC). The analytical results obtained for BC02 and the upper part of GC02 were in agreement, affirming the core-top preservation of GC02. Based on the corrected calibrated AMS 14C dates, the sedimentation rate of GC02 changed abruptly at ~8.2 ka from ~4.0–10.2 cm/kyr in the lower part to ~56.6–91.0 cm/kyr in the middle to upper part. This corresponds to the lithologic change from sandy mud to mud sediments showing the mean grain size change from 6.9 to 46.0 μm. Diverse paleoceanographic proxies representing the surface water condition exhibited varying degree of change at ~8.2 ka, after which all the properties remain almost unchanged, implying stable and continuous depositional conditions following the complete development of the EKWC. Furthermore, it indicated that the sediment depositional conditions in the Hupo Trough in response to the EKWC might have stabilized at ~8.2 ka since the opening of the Korea Strait during the Holocene sea level rise. Moreover, microfossil data from previous studies on the establishment of the TWC in the East Sea (Japan Sea) support our interpretation that the sediment properties revealed the Holocene development of the EKWC in the Hupo Trough.

Symmetric Instability and Ageostrophic Secondary Circulation in the East Korea Warm Current

<p>Submesoscale dynamics and ocean-atmosphere exchange process in frontal regions play an important role in regulating ocean overturning circulation and cycles of materials (including carbon) and energy, yet our understanding on the dynamics is limited primarily due to lack of relevant observation. To investigate frontal processes such as symmetric instability (SI) and ageostrophic secondary circulation (ASC), multiple comprehensive hydrographic and current observations were made with marine meteorological measurements across a sharp front of the East Korea Warm Current (EKWC) over spring 2017, summer 2017 and fall 2018. Submesoscale features were identified from the observations, estimating diagnostic variables that are the Ertel’s potential vorticity (f<sub>q</sub>), balanced Richardson number angle ( ), and Ekman buoyancy flux (EBF). The results with f<sub>q</sub> < 0 along the front,  corresponding to SI regime, and enhanced EBF along the surface of front support that submesoscale overturning circulation induced by down-front wind is due to the SI and ASC. The ASCs with ageostrophic current estimated using the Omega equations further provide vertical motions in the vicinity of the front. Our results suggest that the western boundary currents like EKWC within the North Pacific marginal sea strongly interact with local wind to impact submesoscale overturning circulation and (re-)distribution of materials via SI and ASC.</p>