Basin-specific pollution and impoundment effects on greenhouse gas distributions in three rivers and estuaries.

Large uncertainties exist regarding the combined effects of pollution and impoundment on riverine greenhouse gas (GHG) emissions. It has also been debated whether river eutrophication can transform downstream estuaries into carbon sinks. To assess human impacts on the riverine and estuarine distributions of CO2, CH4, and N2O, two source-to-estuary surveys along three impounded rivers in Korea were combined with multiple samplings at five or six estuarine sites. The basin-wide surveys revealed predominant pollution effects generating localized hotspots of riverine GHGs along metropolitan areas. The localized pollution effect was pronounced in the lower Han River and estuary adjacent to Seoul, while the highest GHG levels in the upper Yeongsan traversing Gwangju were not carried over into the faraway estuary. CH4 levels were elevated across the eutrophic middle Nakdong reaches regulated by eight cascade weirs in contrast to undersaturated CO2 indicating enhanced phytoplankton production. The levels of all three GHGs tended to be higher in the Han estuary across seasons. Higher summer-time δ13C-CH4 values at some Nakdong and Yeongsan estuarine sites implied that temperature-enhanced CH4 production may have been dampened by increased CH4 oxidation. Our results suggest that the location and magnitude of pollution sources and impoundments control basin-specific longitudinal GHG distributions and estuarine carryover effects, warning against simple generalizations of eutrophic rivers and estuaries as carbon sinks.

Increased terrigenous input from North America to the northern Mendeleev Ridge (western Arctic Ocean) since the mid-Brunhes Event.

Mid-Brunhes Event (MBE) occurred at approximately 420 ka between Marine Isotope Stage 11 and 12, and is considered the most pronounced climatic shift during the last ~ 800 kyrs. On the other hand, it is unclear if the MBE was global, despite being observed in the high-latitude Northern Hemispheric cryosphere in terms of climate systems. A 5.35-m long gravity core ARC5-MA01 was obtained from the northern Mendeleev Ridge in the western Arctic Ocean to track the paleoenvironmental changes in terms of the terrigenous sedimentation in response to the glacial-interglacial climate changes across the MBE. Geochemical proxies (biogenic opal, total organic carbon, C/N ratio, carbon isotope of organic matter, and calcium carbonate) of MA01 suggest that the terrigenous input was generally higher during the interglacial periods. Based on a mineralogical examination, most of the terrigenous input was attributed to the abundance of dolomite and the increased kaolinite content from North America. In particular, most paleoceanographic proxies showed that the terrigenous input from North America was enhanced distinctly during the post-MBE interglacial periods. These results suggest that the MBE in the western Arctic Ocean was a global climatic shift closely linked to cryospheric development in North America during the middle Pleistocene.

High Spatial-Resolution Red Tide Detection in the Southern Coast of Korea Using U-Net from PlanetScope Imagery.

Red tides caused by Margalefidinium polykrikoides occur continuously along the southern coast of Korea, where there are many aquaculture cages, and therefore, prompt monitoring of bloom water is required to prevent considerable damage. Satellite-based ocean-color sensors are widely used for detecting red tide blooms, but their low spatial resolution restricts coastal observations. Contrarily, terrestrial sensors with a high spatial resolution are good candidate sensors, despite the lack of spectral resolution and bands for red tide detection. In this study, we developed a U-Net deep learning model for detecting M. polykrikoides blooms along the southern coast of Korea from PlanetScope imagery with a high spatial resolution of 3 m. The U-Net model was trained with four different datasets that were constructed with randomly or non-randomly chosen patches consisting of different ratios of red tide and non-red tide pixels. The qualitative and quantitative assessments of the conventional red tide index (RTI) and four U-Net models suggest that the U-Net model, which was trained with a dataset of non-randomly chosen patches including non-red tide patches, outperformed RTI in terms of sensitivity, precision, and F-measure level, accounting for an increase of 19.84%, 44.84%, and 28.52%, respectively. The M. polykrikoides map derived from U-Net provides the most reasonable red tide patterns in all water areas. Combining high spatial resolution images and deep learning approaches represents a good solution for the monitoring of red tides over coastal regions.

Orbital-scale denitrification changes in the Eastern Arabian Sea during the last 800 kyrs.

Denitrification in the Arabian Sea is closely related to the monsoon-induced upwelling and subsequent phytoplankton production in the surface water. The δ15N values of bulk sediments collected at Site U1456 of the International Ocean Discovery Program (IODP) Expedition 355 reveal the orbital-scale denitrification history in response to the Indian Monsoon. Age reconstruction based on the correlation of planktonic foraminifera (Globigerinoides ruber) δ18O values with the LR04 stack together with the shipboard biostratigraphic and paleomagnetic data assigns the study interval to be 1.2 Ma. Comparison of δ15N values during the last 800 kyrs between Site U1456 (Eastern Arabian Sea) and Site 722B (Western Arabian Sea) showed that δ15N values were high during interglacial periods, indicating intensified denitrification, while the opposite was observed during glacial periods. Taking 6‰ as the empirical threshold of denitrification, the Eastern Arabian Sea has experienced a persistent oxygen minimum zone (OMZ) to maintain strong denitrification whereas the Western Arabian Sea has undergone OMZ breakdown during some glacial periods. The results of this study also suggests that five principal oceanographic conditions were changed in response to the Indian Monsoon following the interglacial and glacial cycles, which controls the degree of denitrification in the Arabian Sea.

First evidence of denitrification vis-à-vis monsoon in the Arabian Sea since Late Miocene.

In the Arabian Sea, South Asian monsoon (SAM)-induced high surface water productivity coupled with poor ventilation of intermediate water results in strong denitrification within the oxygen minimum zone (OMZ). Despite the significance of denitrification in the Arabian Sea, we have no long-term record of its evolution spanning the past several million years. Here, we present the first record of denitrification evolution since Late Miocene (~10.2 Ma) in the Eastern Arabian Sea, where the SAM generates moderate surface water productivity, based on the samples retrieved during the International Ocean Discovery Program (IODP) Expedition 355. We find that (i) the SAM was persistently weaker from ~10.2 to 3.1 Ma; it did not intensify at ~8 Ma in contrast to a few previous studies, (ii) on tectonic timescale, both the SAM and the East Asian Monsoon (EAM) varied synchronously, (iii) the first evidence of denitrification and productivity/SAM intensification was at ~3.2-2.8 Ma that coincided with Mid-Pliocene Warm Period (MPWP), and (iv) the modern strength of the OMZ where denitrification is a permanent feature was attained at ~1.0 Ma.