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We assessed the applicability of the nutrient diol index (NDI) as a proxy for sea surface nutrients by analyzing sediments from a box core (ES14-BC03) collected in the southwestern continental slope of the East Sea, or the Japan Sea (hereafter the East Sea). The estimated sedimentation rate based on the 210Pb chronology was 0.15 cm yr−1 over the last 240 yrs. The NDI-derived phosphate (0.69 μmol L−1) and nitrate (8.63 μmol L−1) concentrations for the core-top sediment were within the average phosphate (0.26 ± 0.08 μmol L−1) and nitrate (3.38 ± 2.32 μmol L−1) concentration ranges for the last ∼20 years, accounting for both estimation error and variation in observational data. The distributions of the NDI-diols varied slightly through time, grouping into two distinct clusters (Cluster 1 and Cluster 2) in both principle component analysis (PCA) and hierarchical clustering of principal components (HCPC). Cluster 2 represented a time period with higher relative abundances of the C28 and C30 1,14-diols, resulting in higher NDI-derived nutrient concentrations than those of other periods. Interestingly, during this period, the U 37 K ' -derived sea surface temperatures (SSTs) also decreased. These results indicate that higher surface nutrient conditions and colder SSTs occurred between 1884 CE and 1911 CE, which might be associated with stronger upwelling intensity at the study site. This study is the first application of the NDI to a down-core, demonstrating that the NDI can be a useful proxy that provides helpful information about past sea surface nutrient conditions.

Long chain alkyl diols (LCDs) have previously been used to develop several diol indices as a proxy for upwelling or stratification conditions and sea surface temperatures. However, there is still a lack of data on the distribution of LCDs in the Pacific Ocean, which calls into question to what extent these diol indices can serve as a proxy in the Pacific Ocean. Hence, we investigated the distribution of LCDs in suspended particulate matter (SPM) samples collected in July 2015 along a south-north transect ranging from the East Sea of Korea to the Bering Sea in the northwestern Pacific region. Our results showed that both saturated and unsaturated C28 and C30 1,14-diols were dominant in most SPM samples investigated. The principal component analysis (PCA) results of the SPM data revealed that the C28:1 and C30:1 1,14-diols were positively associated with nutrient concentrations. However, only the C28:1 1,14-diol in addition to the C28 1,14-diol was positively associated with nutrient concentrations in the global surface sediment data set previously published, while the C30:1 1,14-diol was related to sea surface temperature (SST). Based on these observations, we defined a new diol index, i.e., the nutrient diol index (NDI), which included the C28 1,14-diol as well as the C28:1 1,14-diol in the numerator and all the 1,13-, 1,14-, and 1,15-diols, excluding the C32 1,15-diol, in the denominator. The NDI was positively associated with the ocean atlas values of surface water phosphate and nitrate concentrations for the SPM and surface sediment data sets. Accordingly, the results of our study showed that the NDI has potential as a quantitative nutrient proxy for estimating sea surface phosphate (or nitrate) changes. Further studies should be undertaken in order to assess the applicability of the NDI as a paleo-nutrient proxy for confidently reconstructing surface water nutrient variations in the past, by analyzing more SPM, surface sediments, and sediment cores collected from various oceanic settings.

Drastic sea ice retreats in the Antarctic Peninsula, and the consequent environmental changes have brought about the consequences of biological adaptation and food competition. The isotopic niche is reflected by the ecological position and functional role of a species, which can be altered depending on these environmental changes. We assessed the isotopic niche and trophic positions (TP) of 3 seabirds on the Barton Peninsula, King George Island, Antarctica, to understand ecological interaction among the species. The average TP of Antarctic krill (Euphausia superba) (hereafter “krill”) (2.6 ± 0.1) was estimated by using the compiled published data of the compound-specific nitrogen isotope ratio of glutamic acid and phenylalanine in the Southern Ocean, and TP values of the seabirds based on that of krill to be reliable according to ecological knowledge. Our results on the overlap of the isotopic niches of seabirds suggests potential diet competition among the consumers, such as the brown skua (Stercorarius antarcticus) and south polar skua (Stercorarius maccormicki), whereas the distinct and broad isotopic niche width of kelp gulls (Larus dominicanus) suggesting that their adaptation through generalization was related to their extended habitat and various food sources. Our research indicates that isotopic niche of seabirds reflects their survival strategy for food competition. Consequently, these seabirds can be easily influenced by alternative food sources, including terrestrial and human-derived sources, by rapid environmental changes, indicating that they are valuable as key environmental species and require long-term monitoring in the Antarctic Peninsula.

We investigated the source, composition and reactivity of sedimentary organic carbon (OC) in a river-dominated continental marginal sea, the eastern Yellow Sea. A multi-proxy approach was applied to 9 riverbank sediments and 69 marine surface sediments, combining bulk and molecular organic parameters. The riverbank sediments (n=9) have on average low C/N ratio (4.8±0.5) and enriched δ13CTOC values (–21.5±0.6%) while the BIT index is on average 0.27. The sedimentary OC in the marine surface sediments appears to have a predominantly marine origin (on average C/N ratio=7.0±0.6 and δ13CTOC=–21.9±0.5%, n=69) with minor contribution of continental (i.e. soil- and lake/river-derived) OC (on average BIT index=0.00±0.01, n=69). However, the Δ14C values were depleted (on average –227±53%, n=8). Accordingly, our results highlight that fossil OC, potentially derived from erosion of sedimentary bedrocks in the catchment areas and/or human activities is being contributed to the sedimentary OC pool in the eastern Yellow Sea. More work is needed to better constrain the source, composition, and age of the organic material supplied to the eastern Yellow Sea, given the lack of biogeochemical data from the Korean rivers.

Paleohydrological and paleoenvironmental changes were reconstructed using sedimentary records from the northern East China Sea, representing the past 15,000 years (from the last glacial period to the Holocene). The n -alkane distributions show many factors, from the changing origins (plant types) of organic matter input to marine sediments. Two climatic conditions were distinguished (warm/humid and cold/dry) by comparing the organic carbon isotope ratio (δ 13 C org ) with the reported planktonic foraminiferal oxygen isotope ratio (δ 18 O) of Globigerinoides ruber . A shift in the n -alkane-specific δ 13 C values reveals that the hydrological changes in the northern East China Sea were related to the carbon sources for aquatic macrophytes. The alkenone sea surface temperatures [SSTs (U K′ 37 )] record of the 07YSPC12 shows a range of 20.3 °C–26.5 °C (average 22.9 °C). Lower temperature SSTs were recorded during the Younger Dryas, with an average of 21.7 °C. The variation in the alkenone SSTs is interpreted to indicate the environmental conditions of the growth of alkenone synthesized organisms.