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Under the steadily intensifying human activities in coastal areas, it is crucial to distinguish the sources of organic matter (OM) in sediments for better understanding of the environmental change. In this study, 25 surface sediments collected along the harbor and creeks areas off Karachi coast were used for total organic carbon (TOC), total nitrogen (TN), stable carbon (δ13C) and nitrogen (δ15N) isotopic compositions, phytoplankton biomarkers (brassicasterol, dinosterol and cholesterol) and glycerol dialkyl glycerol tetraethers (GDGTs) analyses, to trace the distribution of OM and identify its sources. TOC, TN, δ13C, and δ15N are in the range of 0.04% to 5.28%, 0.01% to 0.57%, − 17.55‰ to − 26.31‰, and 2.83‰ to 6.04‰, respectively. C/N values range from 4.24 to 21.04. The δ13C, δ15N, C/N, BIT (Branched Isoprenoid Tetraether) index together with phytoplankton biomarkers show both marine and terrestrial OM input to the creeks and Harbor stations. In contrast, the stations from Western Coast indicate a marine source predominance. A clearly impact of human activities is seen from sediments near river mouths. [ABSTRACT FROM AUTHOR]

In the context of global warming and rapid environment change in the Arctic, the supply of organic matter (OM) has increased significantly and a large amount of OM are buried on the Arctic shelf. Studying the fate of OM in Arctic shelf sediments is crucial to understanding the global carbon sink. As a marginal sea of the Arctic Ocean, the Chukchi Sea is one of the most critical areas where OM is buried. Based on the surface sediment samples collected during the sixth Chinese National Arctic Research Expedition in the summer of 2014 and the Sino-Russian joint Arctic Research Expedition in the summer of 2016, this study takes amino acids (AAs) as the primary tool to explore the source and degradation of OM in the surface sediments of the Chukchi Sea. This study shows that total hydrolyzable amino acid (THAA) concentrations (dry weight) are high, with a mean value of (32.7 ± 15.8) µmol/g. Their spatial distribution is related to primary productivity, hydrodynamic conditions, sediment properties and other factors. The source of OM in the surface sediments of the Chukchi Sea is dominated by diatom-dominated marine productivity, with some input from terrestrial sources. Bacteria, as the main source of the D-enantiomer of AA (D-AA), not only have transforming effect on OM, but their cell walls and remnants likewise supply the OM pool. Based on a series of diagenetic indicators, we conclude that the OM in the surface sediments of the Chukchi Sea has undergone extensive degradation [DI (degradation index) = −0.59 ± 0.44], and the degradation degree in the slope is higher than that in the shelf. This study uses AA to explore the sources and degradation of OM in the sediments of the Chukchi Sea, which facilitates our understanding of OM transport and transformation on the Arctic shelf. [ABSTRACT FROM AUTHOR]

During the 3rd Chinese National Arctic Research Expedition cruise in the summer of 2008, nutrients (NO3, NO2, SiO32, and PO43) and dissolved oxygen were measured in the western Arctic Ocean, to derive the vertical distribution of nutrient tracers and its relationship to water structure and biogeochemical processes. The nutrient data show that surface waters had the lowest NO3/PO43 (mean of 0.5) and SiO32/PO3 (mean of 2.8) values in the water column, suggesting an excess of phosphate. Winter Bering Shelf water (wBSW) had high Si* (16.7 µmol/L; Si*=[Si(OH)4]-[NO3]) with negative N* (−11.7 µmol/L; N*=[PO43]-16[PO43]+3.5 µmol/L) in the water column, indicating nitrate deficiency. The warm Atlantic layer had positive N* (0.8 µmol/L) and negative Si* (−5.4 µmol/L) compared with Pacific source water. The vertical distribution of nutrients indicates that wBSW can be characterized by N* minimum and Si* maximum. In contrast, minima of Si* and SiO32/PO43 below 200 m indicate the distribution of Atlantic warm water. [ABSTRACT FROM AUTHOR]

Climate warming has a significant impact on the sea ice and ecosystem of the Arctic Ocean. Under the increasing numbers of melt ponds in Arctic sea ice, the phytoplankton communities associated with the ice system are changing. During the 7th Chinese National Arctic Research Expedition cruise in summer 2016, photosynthesis pigments and nutrients were analyzed, revealing differences in phytoplankton communities between melt ponds and open water in the central Arctic. Photo synthetic pigment analysis suggested that Fuco (5-91 μg m-3 and Diadino (4-21 μg m-3) were the main pigments in the open water. However, the melt ponds had high concentrations of Viola (7-30 μg m-3), Lut (4-59 μg m-3) and Chl b (11-38 μg m-3), suggesting that green algae dominated phytoplankton communities in the melt ponds. The significant differences in phytoplankton communities between melt ponds and open water might be due to the salinity difference. Moreover, green algae may play a more important role in Arctic sea ice ecosystems with the expected growing number of melt ponds in the central Arctic Ocean. [ABSTRACT FROM AUTHOR]

The organic matter (OM) preserved in Arctic Ocean sediments is of great importance to the global carbon budget. However, works that apply multiple proxies to determine the distribution and concentration of organic carbon (OC) in the surface sediments of the northern Bering and Chukchi Seas remain limited. Here a multiproxy approach based on bulk OM parameters and the branched vs. isoprenoid tetraether (BIT) index was used to investigate the distribution and sources of OM in the surface sediments of the northern Bering and Chukchi Seas. Binary and ternary mixing models were applied to trace the contribution of different OC sources to the total OC in the study area. The δ13C values of the sediments provided by the binary model showed that the proportion of terrestrial OC fell in the range of 27.4%−79.8% (46.2% on average). The BIT index returned the lowest fraction (4.8%−27.3%, 12.0% on average). The ternary mixing model was employed to determine the plant-, soil-, and marine-derived fractions of the total OM. The ternary model showed that 11.5%±6.3%, 31.4%±9.5%, and 57.1%±12.4% of OM in the sediment of the study area was derived from soil, plants, and marine sources, respectively. The differences in OM composition between the west and east sides of the Chukchi Sea were controlled by OM inputs from key water masses (i.e., Anadyr Water and Alaska Coastal Water), river discharge, and the nutrient supply from the Pacific inflow that supports marine productivity. [ABSTRACT FROM AUTHOR]

Seasonal meltwater input creates a thin freshen layer in surface seawater under ice, which largely shifts the algae assemblages. Our recent observation of photosynthetic pigments in the high Arctic showed that ice bottom and 5 m of seawater under ice contained relatively high concentration of fucoxanthin, while chlorophyll b and lutein were the major diagnostic pigments in ice-water interface and 0 m of seawater under ice. Additionally, a notable change of dominant phytoplankton occurred in the top 5 m of seawater under ice, from chlorophytes-dominated at surface to diatoms-dominated at 5 m depth, which might attribute to the sharp salinity gradient (salinity from 12.5 to 28.1) in the surface seawater under ice. Our results imply that phytoplankton community in surface layer under ice would become more chlorophytes in the future warming Arctic Ocean. [ABSTRACT FROM AUTHOR]

The content of organic carbon (OC) normalized to the specific surface area (SSA) of sediment is widely used to trace variations in OC loading (OC/SSA). This study presents observations of OC/SSA of surface sediments collected in the Chukchi Sea, a typical Arctic marginal sea. Shelf sediments exhibit much higher OC/SSA values than slope sediments in the study area. Compared with OC/SSA values reported from the East Siberian Shelf and Mackenzie River, the slope sediments possess lower OC loading. This abrupt decrease in OC/SSA is mostly related to the lower primary production on slope as well as possible oxidization processes. The results of linear regression analysis between OC and SSA indicate a sedimentary source rock for the OC in the Chukchi Sea sediments. Moreover, shelf sediments with low SSA possess a larger rock OC fraction than slope sediments do. The dataset of the present study enables a more thorough understanding of regional OC cycling in the Chukchi Sea. [ABSTRACT FROM AUTHOR]

CHEMTAX analysis of high-performance liquid chromatography (HPLC) pigment was conducted to study phytoplankton community structure in the northern Bering Sea shelf, where a seasonal subsurface cold pool emerges. The results showed that fucoxanthin (Fuco) and chlorophyll a (Chl a) were the most abundant diagnostic pigments, with the integrated water column values ranging from 141 to 2 160 μg/m and 477 to 5 535 μg/m, respectively. Moreover, a diatom bloom was identified at Sta. BB06 with the standing stock of Fuco up to 9 214 μg/m. The results of CHEMTAX suggested that the phytoplankton community in the northern Bering Sea shelf was dominated by diatoms and chrysophytes with an average relative contribution to Chl a of 80% and 12%, respectively, followed by chlorophytes, dinoflagellates, and cryptophytes. Diatoms were the absolutely dominant algae in the subsurface cold pool with a relative contribution exceeding 90%, while the contribution of chrysophytes was generally higher in oligotrophic upper water. Additionally, the presence of a cold pool would tend to favor accumulation of diatom biomass and a bloom that occurred beneath the halocline would be beneficial to organic matter sinks, which suggests that a large part of the phytoplankton biomass would settle to the seabed and support a rich benthic biomass. [ABSTRACT FROM AUTHOR]

Total organic carbon and organic carbon stable isotope, biogenic silica, chlorin were measured from a high resolution sediment core to indicate the variation in paleoproductivity and the environmental implications over the past decades (1942 to 1997) in the Changjiang Estuary. Based on these biomarkers, the shift in the phytoplankton community structure in the past decades was discussed in response to the long-term variations in nutrient concentrations and nutrient ratios. The results showed that the δ13C values varied from −26.15‰ to −19.5‰, suggesting the combined organic carbon sources of riverine and marine organisms. Based on the biogenic proxies, paleoproductivity changes were categorized into three stages: low production before 1950; an increase in production with the dominance of diatoms during 1950 to 1980, consistent with increasing of nutrient concentrations; a decrease in production after 1980 when the diatom production decreased while the production of non-silicious phytoplankton increased due to high nutrient inputs, and high N/P and P/Si ratios in the Changjiang Estuary. The sedimentation records also indicated that the riverine organic carbon increased since 1980. [ABSTRACT FROM AUTHOR]

The nutrient-rich Pacific Ocean seawater that flows through the Bering Strait into the Chukchi Sea is generally considered to be the most important source of nutrients to the Arctic euphotic zone. The inflow is characterized by nitrogen deficit and low nitrate/phosphate (N/P) ratios; this is ascribed to sedimentary denitrification on the Chukchi shelf by preoccupant opinions. However, the Chukchi Sea also has high primary production, which raises the question of whether the biological pump may also significantly modulate nutrient properties of the throughflow. Here, we show that nitrate concentrations of the Pacific inflow gradually decrease northward in association with notable biological utilization. The phytoplankton N/P uptake ratio was 8.8±2.27, higher than the N/P ratio of Pacific inflow water (5–6). This uptake ratio, in combination with efficient vertical nitrogen export, serves to preferentially remove nitrogen (relative to phosphorus) from upper waters, thereby further intensifying the Arctic nitrogen deficit. Accordingly, as large as about 111.7×109 mol N yr−1 of nitrate was extra consumed, according to the real N/P uptake ratio rather than the ratio of the Pacific inflow, which may be as great as half the nitrogen loss ascribed to sedimentary denitrification. Our findings suggest that besides sedimentary denitrification, biological disproportionate utilization of nutrients in the Chukchi Sea upper water is another important contributor to the nitrogen limitation and excess phosphorus in the upper Arctic Ocean. In the rapid Arctic change era, the predicted reinforced biological carbon pump could further impact the nutrient dynamics and biogeochemical process of the Arctic Ocean. [ABSTRACT FROM AUTHOR]