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The TEX paleothermometer on the base of isoprenoid glycerol dialkyl glycerol tetraethers (iGDGTs) has been widely applied to various marine settings to reconstruct past sea surface temperatures (SSTs). However, it remains uncertain how well this proxy reconstructs SSTs in marginal seas. In this study, we analyze the environmental factors governing distribution of iGDGTs in surface sediments to assess the applicability of TEX paleothermometer in the South China Sea (SCS). Individual iGDGT concentrations increase gradually eastwards. Redundancy analysis based on the relative abundance of an individual iGDGT compound and environmental parameters suggests that water depth is the most influential factor to the distribution of iGDGTs, because thaumarchaeota communities are water-depth dependent. Interestingly, the SST difference (Δ T) between TEX derived temperature and remote-sensing SST is less than 1°C in sediments with water depth>200 m, indicating that TEX was the robust proxy to trace the paleo-SST in the region if water depth is greater than 200 m. [ABSTRACT FROM AUTHOR]

This study analyzed the variations of soil organic carbon (SOC), total nitrogen (TN), and total phosphorus (TP) with depth in coastal wetlands of Laizhou Bay, Bohai Sea. Generally, average contents of SOC and TN were higher at tidal flooding Guangrao wetland (GRA) than those at Changyi (CY) wetland where the tidal can rarely reached. Among the five profiles, higher variations of SOC and TN were observed in CYB1 and CYB2, where the TP contents showed lower variations. SOC showed a rapid decrease with depth in CYB1 and CYB2, whereas they tended to increase with depth in GRA1 and GRA2. Vertical distributions of TN were significantly correlated with SOC in profile GRA1, CYB1, and CYB2. However, no clear trend was found on the vertical variations of TP. Additionally, C, N, and P ecological stoichiometry in these two coastal wetlands had high complexities within wetland. The C/N ratios were generally higher, while the C/P ratios and the N/P ratios were comparatively low, indicating the coastal wetland biogeochemical processes were more limited by N and P, with the former may being the primary factor. Our results also indicated that both vegetation coverage and tidal flooding were important in the biogeochemical processes of these coastal wetlands. [ABSTRACT FROM AUTHOR]

Glycerol dibiphytanyl glycerol tetraethers (GDGTs) are important archaeal membrane lipids that widely occur in marine water columns and sediments. Based on its sensitivity to temperature, the paleotemperature index TEX 86 was developed and extensively studied. However, GDGTs in sediments are not only affected by their external sources, but also by internal biochemical processes. The effect of bottom water redox conditions on GDGT composition and TEX 86 in the surface sediments remains unclear. In this study, field investigations in the eastern China marginal seas were conducted to trace the responses of sedimentary GDGTs to bottom dissolved oxygen (DO). Our results showed that the TEX 86 was independent of the DO level. However, the decrease in DO was accompanied by an increase in GDGT Ring Index (RI) and relative abundance of crenarchaeol (Cren) in contrast to the decrease pattern for GDGT-0, suggesting that DO has important effects on the composition of GDGTs but shows limited impact on TEX 86 in the study area. These changes were probably driven by archaeal adaptation, shift of community structure and degradation of GDGTs. In addition, strong relationships between DO and RI and between DO and GDGT abundances (i.e., %GDGT-0, GDGT-0/Cren) indicate that these parameters can be used as potential redox proxies for bottom water, especially in relatively stable stratified waters. Further studies coupled with archaeal community are needed to cover more areas to constrain their applicability and driving mechanisms. • The decrease in DO was accompanied by an increase in %Cren but a decrease in %GDGT-0. • Bottom DO has limited impact on sedimentary TEX 86. • RI, %GDGT-0 and GDGT-0/Cren can be used as potential proxies for bottom DO. [ABSTRACT FROM AUTHOR]

Dissolved inorganic tin (DISn) and its spatial variation were examined in Bohai Bay seawaters to understand the DISn behavior and pollution in this area. DISn concentration gradually increased with the distance from the coast and showed a slight decrease with the increasing depth from surface water, suggesting the scavenged behavior of tin with an atmospheric input to surface water. Besides, the higher DISn values also were found near the Haihe Estuary inferring that the riverine input was a source of DISn. Based on the data in this study, a preliminary estimate of the tin budget via riverine input and atmospheric deposition has been established. According to our estimate, about 2 × 10 and 8.47 × 10 g/year of tin reach Bohai Bay via rivers and atmosphere. Environmental factors such as suspended particulate material, salinity, total organic matter, pH, nutrients, and phytoplankton had the important influences on DISn distribution. Among them, the negative correlation between DISn and phytoplankton at most stations might indicate the biological uptake of tin. [ABSTRACT FROM AUTHOR]

Comprehensive surveys were conducted in the Kocebu deep seamount and the M4 shallow seamount in the Western Pacific Ocean in March 2018 and May 2019, respectively. The distributions of nutrients in euphotic zone of the two seamount-areas were revealed, and the causative controlling factors were analyzed. Results show that the vertical distribution of nutrients in the two seamount-areas accorded with the general law of the oligotrophic ocean. The concentrations of NO3-N, PO4-P, and SiO3-Si generally increased gradually with the increase of water depth, and they were extremely low in water layers within 100 m. The area of high NO2-N concentration well agreed with the Deep Chlorophyll Maximum Layer. On the other hand, the distribution of water masses and phytoplankton and hydrological conditions in the two seamount-areas were different, resulting in lower concentrations of NO3-N, PO4-P, and SiO3-Si in the water layers below 100 m in the Kocebu seamount area than those in the M4 seamount area. In addition, NO2-N was affected by the distribution of phytoplankton, and distributed mainly in the water layers of 150 and 100 m. There was upwelling in the euphotic zone of M4 seamount area, causing accumulations of nutrients and phytoplankton around the seamount, forming a "seamount effect"; however, no such an effect was found in Kocebu seamount area. Affected by the composition of biological community and the "seamount effect", the nitrogen limitation in the M4 seamount area was not significant, and the dissolved inorganic nitrogen (DIN):PO4-P and SiO3-Si:DIN were closer to the Redfield ratios. [ABSTRACT FROM AUTHOR]

The response of marine ammonia-oxidizing archaea (AOA) to environmental stress is reflected in changes in their membrane lipid composition, particularly the unique isoprenoid glycerol dialkyl glycerol tetraethers (iGDGTs). However, the influence of dissolved oxygen (DO) on the composition of iGDGTs in the ocean remains unclear. This study aims to investigate the link between DO levels and the fractional abundances of iGDGTs in the East China Sea (ECS) to establish a redox proxy. Results suggested that the absolute abundances of iGDGTs were influenced by Thaumarchaeota biomass when DO concentrations exceeded 2 mg/L. Vertical distributions of iGDGTs through the water column suggested their transport from bottom waters to sediments. Increasing proportions of iGDGT-0 and the sum of iGDGT-1, iGDGT-2 and iGDGT-3 from suspended particulate matters to surface sediments indicated their preferential preservation. DO concentrations (2–6 mg/L) in the water column showed a significant positive correlation with the relative abundance of crenarchaeol (cren%) but a negative correlation with iGDGT-0%, suggesting insufficient DO levels to promote AOA cyclization. However, bottom DO concentrations exhibited significant negative correlations with both cren% and iGDGT-0% in surface sediments, attributed to enhanced cyclization of iGDGTs and Euryarchaeota abundance within more reducing sediments, respectively. The consistent relationship between iGDGT-0% and DO in both water column and sediments enabled iGDGT-0% to be a potential redox proxy. Temporal variations in iGDGT-0% in the ECS in recent decades aligned well with in-situ DO monitoring data, further validating iGDGT-0% as a promising redox proxy. • Oxygen impact on iGDGTs in the East China Sea was studied to develop a redox proxy. • Thaumarchaeota determined the abundance of iGDGTs when oxygen was available. • Sediment iGDGTs showed preferential preservation of iGDGT-0, −1, −2, −3. • Correlations between iGDGT-0% and DO in water/sediments enabled it as a redox proxy. • iGDGT-0% was used to reconstruct DO changes in the East China Sea in past decades. [ABSTRACT FROM AUTHOR]

Coastal marine settings are important in terms of geochemical cycles and biological productivity. Climate change is predicted to affect coastal environment via hypoxia and Submarine Groundwater Discharge (SGD). Appropriate proxies could help to better understand oxygenation history and the role of SGD in regulating hypoxia. This would also benefit prediction of potential outcomes of future environmental changes. The sensitivity of benthic foraminiferal shell chemistry to environmental conditions opens the possibility to use them as proxies of coastal hypoxia and SGD. We report here that the average Mn/Ca ratios in tests of living benthic foraminiferal shells in the Changjiang River Estuary (CJE) is 2.3 times higher during hypoxia periods than under well-mixed conditions. In addition, Ba/Ca ratios in living benthic foraminiferal shells co-varied well with radon-inferred SGD signals. Fluctuations of Mn/Ca and Ba/Ca ratios in tests of a single foraminiferal shell along successive chambers corresponds well with seasonal-scale variations of hypoxia and SGD. We suggest that Mn/Ca and Ba/Ca ratios within intra-tests of benthic foraminifer can provide a reliable proxy for past hypoxia and SGD trends. • This work investigated proxies to record hypoxia and SGD on shells of same foraminiferal chambers. • Synchronous tendency of Mn/Ca and Ba/Ca ratios in tests of a single foraminiferal shell along successive chambers reflects seasonal-scale variations of hypoxia and SGD. • This work will provide a novel proxy for reconstructing the contribution of SGD to hypoxia over a long-term history. [ABSTRACT FROM AUTHOR]

The particulate amino sugars (PAS), organic carbon (POC) and nitrogen (PN), the chlorophyll a (Chl- a), and the heterotrophic bacterial cell count (HBC) in the Yangtze River Estuary (YRE) were determined to study the biological source, bioavailability and bacterial contribution of particulate organic matter (POM) under the influence of algal bloom during the spring and autumn of 2017. The contents of PAS were 2.6–69.6 nmol/(mg dw) and made up 5.29 ± 2.55% of POC and 5.39 ± 2.77% of PN. The strong positive correlations between Chl- a and POC, PN and PAS and their similar distributions indicated that phytoplankton was a major source of POM and PAS in the YRE. When algal blooms occurred, the high ratios of glucosamine to galactosamine (GlcN/GalN) (~10.9) and the high carbon and nitrogen normalized yields of PAS (%PAS-POC and %PAS-PN, respectively) clearly indicated the planktonic (especially zooplankton) source of POM, although the high muramic acid (Mur) contents implying high contents of fresh bacterial organic matter (OM) were also found. Additionally, the high proportion of fresh plankton- and bacterial-derived OM indicated that the POM was more labile when algal bloom occurred. In contrast, when the primary production was low, the lower %PAS-POC and GlcN/GalN ratios (< 3) in the POM demonstrated the decreased contribution of planktonic OM and the relatively elevated contribution of bacterial OM. In addition, the higher %PAS-PN and the lower DO concentrations in seawater indicated that the POM experienced some degree of degradation but was still at an early stage in the nearshore site in autumn. In spring, the Changjiang diluted water (CDW) carried considerable plankton-derived POM into the estuary and influenced the distribution of POM. In autumn, the terrestrial OM might become more bacterial-derived and had less influence on the bulk POM of the estuary. The bacterial OM estimated by the Mur accounted for an average of 10.8 ± 3.7% and 40.5 ± 19.1% of the POC in spring and autumn, respectively, which was approximately two times higher than the results based on the heterotrophic bacterial cell count (HBC), indicating that most of the bacterial OM in the POM was detritus. In summary, the content, source and bioavailability of POM were significantly affected by algal bloom and the CDW, and the amino sugars represent a reliable tool to indicate the source and bioavailability of POM. • Source and bioavailability of Particulate organic matter (POM) significantly affected by algal bloom. • More labile and plankton origin of POM when algal bloom occurred • More degraded and bacterial derived POM when low primary production • Bacterial OM contributed 11% and 41% of POC in spring and autumn, respectively. [ABSTRACT FROM AUTHOR]

The continuous uptake of anthropogenic CO2 by the ocean leads to ocean acidification, which is an ongoing threat to the marine ecosystem. The ocean acidification rate was globally documented in the surface ocean but limited below the surface. Here, we present a monthly four-dimensional 1°×1° gridded product of global seawater pH, derived from a machine learning algorithm trained on pH observations at total scale and in-situ temperature from the Global Ocean Data Analysis Project (GLODAP). The constructed pH product covers the years 1992–2020 and depths from the surface to 2 km on 41 levels. Three types of machine learning algorithms were used in the pH product construction, including self-organizing map neural networks for region dividing, a stepwise algorithm for predictor selection, and feed-forward neural networks (FFNN) for non-linear relationship regression. The performance of the machine learning algorithm was validated using real observations by a cross validation method, where four repeating iterations were carried out with 25 % varied observations for each evaluation and 75 % for training. The constructed pH product is evaluated through comparisons to time series observations and the GLODAP pH climatology. The overall root mean square error between the FFNN constructed pH and the GLODAP measurements is 0.028, ranging from 0.044 in the surface to 0.013 at 2000 m. The pH product is distributed through the data repository of the Marine Science Data Center of the Chinese Academy of Sciences at http://dx.doi.org/10.12157/IOCAS.20230720.001 (Zhong et al., 2023). [ABSTRACT FROM AUTHOR]

The TetraEther indeX of 86 carbon atoms (TEX 86) is widely used as a proxy to reconstruct past sea surface temperatures. Most current applications of TEX 86 are primarily based on analyzing the composition of isoprenoid glycerol dialkyl glycerol tetraethers (isoGDGTs) that comprise TEX 86 in sediments, with the assumption that the sedimentary isoGDGTs are mainly derived from the surface mixed layer. Here we report on the variations in the isoGDGT distribution, archaeal abundance and community through the water column of the Western Pacific Ocean, directly testing the export depth of isoGDGTs and constraining the temperature records of TEX 86. Our data show that maximum isoGDGT concentrations occurred in subsurface waters (150–200 m) with maximum archaeal abundances. The ratio between isoGDGTs bearing 2 vs. 3 cyclopentane moieties, i.e. [2/3] ratio, increased with depth, which is likely related to the shift of the archaeal community from Ca. Nitrosopelagicus-dominance to norank_f__Nitrosopumilaceae -dominance. Models based on the [2/3] ratios in the water column predicted an average export depth of isoGDGTs to sediments of around 150–200 m, consistent with the robust relationship between the compiled sedimentary TEX 86 and the annual mean subsurface temperature. Taken together, our findings support that TEX 86 records subsurface rather than surface temperatures in the open ocean. [ABSTRACT FROM AUTHOR]

Compound‐specific isotope analysis has opened up a new realm for resolving the sources and transformation processes of marine organic matter. However, the stable carbon isotope patterns of amino sugars remain unknown. We examined δ13C of amino sugars in marine phytoplankton and heterotrophic bacteria, and the variations in amino sugar δ13C during 66‐d planktonic organic matter degradation experiments, to investigate the metabolic sources and transformations of amino sugars by bacterial reworking. The δ13C values of glucosamine (GlcN) and galactosamine (GalN) were comparable in heterotrophic bacteria (difference Δδ13CGlcN–GalN = 0.4–4.0‰) but pronouncedly different in phytoplankton (Δδ13CGlcN–GalN = 4.3–16.6‰), suggesting similar synthesis pathways of GlcN and GalN in bacteria that differed from phytoplankton. Compared to GlcN and GalN, bacteria preferentially use isotopically light organic compounds for muramic acid (MurA) synthesis. During simulated microbial degradation of organic matter, the δ13C difference between GlcN and GalN decreased from 5.8‰ on the initial day to 1‰ at a late stage in the experiment, but the difference between GlcN and MurA remained at 5.3‰. This difference is consistent with the pattern in cultured phytoplankton (average Δδ13CGlcN–GalN = 5.9‰ ± 1.4‰) and heterotrophic bacteria (average Δδ13CGlcN–MurA = 4.6‰ ± 3.4‰), indicating enhanced bacterial resynthesis as degradation proceeded. Based on the difference in δ13C among GlcN, GalN, and MurA, we propose a novel index of variation in amino sugar δ13C, representing amino sugar resynthesis, to describe the diagenetic state of organic matter. Together, these findings suggest that amino sugar δ13C can be used as a new tool to track heterotrophic processes of marine organic matter. [ABSTRACT FROM AUTHOR]

A marine survey was conducted from 18 May to 13 June 2014 in the East China Sea (ECS) and its adjacent Kuroshio Current to examine the spatial distribution and biogeochemical characteristics of dissolved oxygen (DO) in spring. Waters were sampled at 10‒25 m intervals within 100 m depth, and at 25‒500 m beyond 100 m. The depth, temperature, salinity, and density (sigma-t) were measured in situ with a conductivity-temperature-depth (CTD) sensor. DO concentrations were determined on board using traditional Winkler titration method. The results show that in the Kuroshio Current, DO content was the highest in the euphotic layer, then decreased sharply with depth to about 1 000 m, and increased with depth gradually thereafter. While in the ECS continental shelf area, DO content had high values in the coastal surface water and low values in the near-bottom water. In addition, a low-DO zone off the Changjiang (Yangtze) River estuary was found in spring 2014, and it was formed under the combined influence of many factors, including water stratification, high primary productivity in the euphotic layers, high accumulation/ sedimentation of organic matter below the euphotic layers, and mixing/transport of oceanic current waters on the shelf. Most notable among these is the Kuroshio intruded water, an oceanic current water which carried rich dissolved oxygen onto the continental shelf and alleviated the oxygen deficit phenomenon in the ECS, could impact the position, range, and intensity, thus the formation/destruction of the ECS Hypoxia Zone. [ABSTRACT FROM AUTHOR]

Sedimentation rate and biogenic elements of a sediment core KET-1 recovered from Kuroshio Mainstream, east of Taiwan, were determined to investigate variation of organic carbon burial and its response to climate fluctuations. The results showed that the organic carbon burial flux was mainly depended on the sedimentation rate and the input of river particles had a dilution effect on total organic carbon (TOC) content of the sediment. Based on the organic carbon burial flux, the entire depositional process could be divided into three stages, which exactly corresponded to three specific climate periods over the past millennium. During the Medieval Warm Period (MWP, AD 1150-1250), the strong East Asian summer monsoon (EASM) caused more river input to the sea, resulting in high sedimentation rate and high organic carbon burial flux, meanwhile the dilution effect of river particles as well as low marine primary productivity resulted in relatively low proportion of marine organic matter (MOM) in the sediment, which was reflected by low TOC content. During the Little Ice Age (LIA, AD 1400-1835), lower organic burial flux and higher marine primary productivity as well as higher TOC content in the sediment were accompanied by weakened EASM. In the Modern Period (MP, AD 1835-present), however, the correlation between organic carbon burial and climate fluctuations that was found to be significant during the MWP and LIA had been greatly changed, and anthropogenic forcing was thought to be the primary driving factor. Overall, the climate variability and anthropogenic activities over the past millennium dominated the organic carbon burial in the sediment along the Kuroshio mainstream. [ABSTRACT FROM AUTHOR]

Rare earth elements (REEs) can be used to trace source materials and identify their provenances, because of significant conservation and immobility during chemical alteration processes after erosion of materials from the provenance. This study focused on the temporal variation of REEs for columnar sediments from the mouth of Jiaozhou Bay in North China to understand the potential controls for the geochemical variations of sediments. Through extraction experiments, we identified that the residual fraction is the main host for REEs compared with other fractions (i.e., exchangeable and carbonate fraction, easily reducible oxides fraction, reducible oxides fraction, magnetite fraction). REE ratios (e.g., LaN/SmN and LaN/YbN; N: normalized by chondrite) lack correlations with grain size or the chemical index of alteration (CIA), which is correlated with major elements. All these indicate that these REE variations reflect the varying contribution of source materials from different provenances instead of grain size or chemical weathering effects. REE ratios (e.g., LaN/SmN and LaN/YbN) remain relatively constant until the depth of roughly 40 cm (equivalent to the year 1995), and show obvious changes beyond this depth. Compared REE characteristics of Jiaozhou Bay with those of neighboring rivers and bedrocks, the relative contributions of Dagu River-Jiaolai River, and Licun River may have been increased during the sedimentary processes, which could be caused by the construction of reservoir and related change of aquaculture (e.g., rapid accumulation of organic materials). [ABSTRACT FROM AUTHOR]

Bacterial metabolism largely drives the sequestration of refractory organic matter in the ocean. However, a lack of understanding exists regarding the abundance and reactivity of bacterial particulate organic matter (POM). Here we report the bacterial contributions to suspended POM collected in the oligotrophic Western Pacific Warm Pool (WPWP). Around 27% of particulate organic carbon (POC) and ∼39% of particulate nitrogen (PN) in the surface ocean were derived from bacteria. Most of the bacterial POM (∼87%) was labile or semi‐labile, and ∼85% of bacterial POM was removed between depths of ∼100–300 m. Bacterial POM constituted only ∼8% and ∼13% of refractory POC and PN, respectively. The rapid cycling of bacterial POM in upper waters was likely related to oligotrophic conditions and facilitated by higher temperatures in the WPWP. Taken together, these observations indicate that bacterial POM plays a crucial role in supplying energy for bacterial respiration. Plain Language Summary: Bacteria transform a large fraction of the organic matter made by primary producers and contribute to long‐term oceanic carbon sequestration with bacterial detritus, thereby contributing to ocean uptake of atmospheric carbon dioxide. The abundance and reactivity of bacterial organic matter are critical to consider in this carbon storage pathway. Here we collected suspended particulate organic matter (POM) in the Western Pacific Warm Pool (WPWP) and assessed the bacterial POM contribution. A substantial proportion of bacterial POM was found in the surface ocean, but most (∼85%) was rapidly removed between depths of 100–300 m as particles sink. The high temperature and oligotrophic features of WPWP facilitate the use of bacterial POM as an energy source for bacterial respiration. This rapid turnover of bacterial POM in the upper ocean results in a contribution of only ∼8% to long‐term carbon sequestration. Our results suggest that bacterial POM exhibits high reactivity and only a small fraction can lead to long‐term carbon sequestration. Key Points: We assess bacterial contributions to suspended particulate organic matter (POM) in the Western Pacific Warm Pool on the basis of D‐amino acid biomarkersBacterial organics constitute 27% of surface ocean particulate organic carbon (POC) and 39% of particulate nitrogen (PN), but majority (∼87%) is labile or semi‐labileRapid cycling of bacterial POM in the upper ocean results in a contribution of only ∼8% to refractory POC and ∼13% to PN [ABSTRACT FROM AUTHOR]

Phosphorus (P) is an important macronutrient that can limit primary productivity in coastal marine ecosystems. In this study, four sediment cores were collected in the Jiaozhou Bay to study the phosphorus forms and their bioavailability, including exchangeable or loosely sorbed P (Ex-P), iron-bound P (Fe-P), authigenic P (Ca-P), detrital P (De-P) and organic P (OP), which were separated and quantified using a sequential extraction method (SEDEX). The results showed that the concentration of total P (TP) in core sediments ranged from 6.23 to 10.46 μmol/g, and inorganic P (IP) was the dominated P form. Fe-P and De-P were the main chemical forms of IP in core sediments. The profile variation of OP presented the most significant among the phosphorus forms. Whereas the concentrations of Ex-P, Fe-P, and Ca-P varied slightly with depth, indicating that the transformation of Ex-P, Fe-P, Ca-P, and OP could occur during sedimentary P burial. Moreover, the distribution of P species was influenced by many factors, including terrigenous input, biological processes, organic matter degradation and increasing human activities. High total organic carbon (TOC)/OP ratio occurred in the Jiaozhou Bay, ranging from 73 to 472 (average, 180 ± 81) in core sediments, which was caused by the increasing terrestrial organic matter. The ratio of TOC/P reactive ranged from 24 to 101 (average 46 ± 15) in core sediments (lower than the Redfield ratio), implying a surplus of sedimentary reactive P compared with TOC. Potential bioavailable P (BAP) accounted for about 28.2–60.8% (average, 47.1 ± 7.4%) of TP in core sediments, and presented an increasing trend since 1980s, which might be responsible for the shift of phytoplankton community composition during these decades. [ABSTRACT FROM AUTHOR]

Concentrations of particulate organic carbon (POC) and total hydrolyzable amino sugars (THAS) were measured along a transect of the dynamic South Yellow Sea (SYS) to investigate the bioreactivity and bacterial reworking of particulate organic matter (POM). Results showed that POM bioavailability was linked with primary production, as revealed by the significant correlation between chlorophyll- a concentrations and the diagenetic indicator glucosamine/galactosamine (GlcN/GalN). Production of bioavailable POM could rapidly stimulate microbial activity, generating hot spots of heterotrophic alteration. Lower GlcN/GalN ratios (<3) observed in the entire SYS indicate that POM underwent extensive microbial alteration. In particular, extremely low GlcN/GalN ratios (∼0.7) were found in the Yellow Sea Cold Water Mass, reflecting high bacterial alteration of POM. Estimates based on the bacterial biomarker muramic acid showed that on average ∼13% of POM in the SYS was of bacterial origin. Elevated bacterial contributions were found in both nearshore and offshore areas. Strong mixing in the nearshore and the presence of cyclonic eddies in offshore waters may increase the residence time of POM in the water column and thus promote bacterial transformation of POM. Overall, our findings indicate that bacterial reworking of POM varies with productivity and that the extensive bacterial transformation of the remaining POM observed in the water column probably enhances long-term carbon sequestration. [ABSTRACT FROM AUTHOR]

The acidification of the marginal seawater was a more intricate process than the ocean. Although some studies have been done on seasonal acidification in the bottom water of Chinese marginal seas, research on surface water acidification has still been insufficient. We analyzed the acidification properties and controlling factors in the Bohai Sea (BS) and Yellow Sea (YS) surface water during April 2023. The observation showed that the average surface water pH of the BS, North Yellow Sea (NYS), and South Yellow Sea (SYS) were 8.09 ± 0.06, 8.13 ± 0.05, and 8.15 ± 0.05. Phytoplankton significantly impacted pH and Ω arag , while riverine inputs and biological activity played a vital role in controlling DIC and TA. The Yellow River significantly impacted the BS. The North Yellow Sea Cold Water Mass had a limited impact on acidification, while the South Yellow Sea Cold Water Mass significantly affected the SYS. Regarding seasonal fluctuations, Ω arag was significantly higher in summer than in other seasons. DIC and TA showed different patterns in both the BS and YS, with a minimal fluctuation in pH. Over the last two decades, the pH in the BS showed a slight annual decline, and the rate of change was (−1.45 ± 2.19) × 10−5 yr−1. In contrast, the NYS and SYS have slightly risen, with rates of change of (2.39 ± 1.24) × 10−5 and (1.23 ± 0.76) × 10−5 yr−1. We believed that surface water acidification in the BS and YS did not follow the expected trend of significant acidification observed in open oceanic regions. Instead, the acidification process in these marginal seas was dominated by local factors such as riverine inputs, biological activity, and cold water masses, resulting in minimal pH changes over the last two decades. [Display omitted] • River inputs and phytoplankton influenced BS and YS surface seawater acidification. • Summarizing data of past 20 years on surface water acidification in BS and YS. • The BS and YS surface water may not be acidified. [ABSTRACT FROM AUTHOR]

The oxygen minimum zone (OMZ) is an important representative of marine hypoxia in the open ocean, and it is developing rapidly under the context of global warming. However, the research on OMZ in the Western Pacific is still deficient. This study focused on its basic characteristics and impact on the degradation of particulate matters in the M4 seamount of Western Pacific. The results showed that the OMZ is located at 290–1100 m, just below the high-salinity area and thermocline. The M4 seamount has a weak impact on the OMZ, and only the bottom waters contacting with the seamount have a weak decrease in dissolved oxygen (DO). With the increase of water depth, particulate nitrogen and phosphorus decrease first above and in the OMZ and then increase below the OMZ, while particulate organic carbon (POC) gradually decreases. The low-DO environment in the OMZ is not conducive to the degradation of particulate matters, which promotes the transport of particulate matters to the deep sea, and most particulate matters have the lowest degradation rate here. The waters above the OMZ have the fastest change rate of particulate matters, in which particulate organic phosphorus (POP) and particulate inorganic phosphorus (PIP) are preferentially degraded, and the degradation rate of them is significantly higher than particulate organic nitrogen (PON) and particulate inorganic nitrogen (PIN). The particulate nitrogen and phosphorus in the waters below the OMZ continue to increase, while PON/total particulate nitrogen (TPN) and POP/total particulate phosphorus (TPP) increase significantly, and the increase rate of PIN and PIP is far lower than PON and POP, indicating that the increase of organic matters in particulate matters is more significant. It is speculated that this phenomenon might be related to the input of Antarctic Bottom Water or the in-situ production by microorganisms. This study revealed the relationship between OMZ and different particulate matters, which may provide a valuable pathway for the biogeochemical effects of OMZ in the Western Pacific. [Display omitted] • Different kinds of particulate matters show different fates in the OMZ. • The OMZ in the M4 seamount is located at 290–1100 m with a threshold of 3.2 mg/L. • The OMZ slows down the degradation of particulate matters. [ABSTRACT FROM AUTHOR]

The partial pressure of CO2 (pCO2) and air-sea CO2 exchange flux (FCO2) in the southern Yellow Sea (SYS, 120–125°E, 31.5–37°N) were investigated basing on the field surveys conducted in April of 2011, and June, July, October of 2012. With significant spatial variations, surface pCO2 ranged from 243 to 574μatm, 206 to 620μatm, 102 to 655μatm and 328 to 557μatm in April, June, July and October, respectively. Nearshore area of Shandong Peninsula and Jiangsu Shallow (depth<50m) were pCO2-supersaturated (pCO2=400–600μatm), as the result of intensive water mixing which brought the bottom CO2-rich water to the surface layer. Conversely, offshore area of SYS center (depth>50m) was pCO2-undersaturated (pCO2<390μatm) in April, June and October, but supersaturated in July. Phytoplankton production sustained by abundant nutrient and suitable hydrodynamic conditions was of great importance for this undersaturated pCO2. Moreover, extreme low pCO2 (pCO2<300μatm) was observed in the Changjiang plume (32.5–33.5°N, 123–125°E) in July, which was also related with the biological uptake of CO2. Average air-sea CO2 exchange flux of the SYS in April, June, July and October was −3.16±0.40mmolm−2 d−1,−4.56±0.34mmolm−2 d−1, −0.36±0.51mmolm−2 d−1, and 6.67±0.57mmolm−2 d−1, respectively. As a whole, the SYS behaved as a weak CO2 sink during April to October, with an average flux for about −0.35mmolm−2 d−1. As for the controlling factors for pCO2 variation, temperature played the dominant role in October, whereas the non-temperature factors, such as vertical mixing, Changjiang plume and biological activity, were considered as the primary controlling factors in June and July. Spatially, the control of temperature on pCO2 was predominant in the offshore SYS; the non-temperature factors were predominant in the shallow nearshore area, especially in coast of Shandong Peninsula and the Jiangsu Shallow. [ABSTRACT FROM AUTHOR]

Alkaline phosphatase activity (APA) is always used to evaluate the phosphorus (P) status of phytoplankton community and considered as a proxy for phosphate (DIP) stress or deficiency. In this study, we presented seasonal distributions of APA [including free APA (F-APA) and particulate APA (P-APA)] and enzyme kinetics and compared to the distribution of phosphorus concentrations, planktonic biomass and other environmental variables reflecting community P status in moderate DIP-deficient Jiaozhou Bay, North China. Low potential APA (V max) were detected in winter (mean 51.3 nM h-1), which was probably due to low water temperature, while higher APA corresponded well to high phosphate requirement and high biological productivity in other three seasons. Normalized total APA (specified by chlorophyll a and bacteria biomass) showed an inverse hyperbolic relationship with DIP and the threshold phosphate concentration for the transition from high to low APA was around 0.2 μM in Jiaozhou Bay. Combined with results of APA, P concentrations (including DIP and DOP) and half saturation constant (K m), our findings suggested that phytoplankton and bacteria in Jiaozhou Bay may be experiencing a relief from P-limitation in winter; while in other three seasons, these organisms were mainly under persistent P stress. Moreover, in order to compensate for phosphate deficiency, we hypothesize that plankton not only by means of promoting enzyme production as indicated by increase of V max , but also by an improved enzyme affinity as represented by a decrease of the Michaelis constant (K m). Consequently, higher APA and lower DOP stocks in summer were probably a result of greater utilization of the DOP pool to maintain high primary production. [Display omitted] • Alkaline phosphatase (AP) was mainly produced by phytoplankton. • APA levels reflected seasonal phosphorus demand of phytoplankton. • Phytoplankton may compensate P stress by increasing potential APA and lowering K m. [ABSTRACT FROM AUTHOR]

As a serious consequence of ocean warming and increased stratification, a rapid decrease in dissolved oxygen (DO) content of the world's oceans has attracted more and more attention recently. In open oceans, the decline of DO is characterized by the expansion of oxygen minimum zones (OMZs) in the ocean interior. Vast OMZs exist within the mesopelagic zones of the Tropical Western Pacific (TWP), but have gained very little attention. In this study, we focus on characteristics of OMZs in three typical seamounts areas (named Y3, M2, and Kocebu, respectively) of the TWP. Based on distributions of DO, the OMZs of the three seamounts areas are very different in scope, thickness, and the minimum oxygen content. The significantly different characteristics of OMZs at the seamounts are mainly because they are located in regions affected by different ventilation and consumption characteristic. To quantitatively describe the intensity of OMZs, a parameter, IOMZ, is firstly proposed. According to this quantitative parameter, the intensity order of OMZs for the three seamounts areas is Kocebu>M2>Y3. Potential biogeochemical effects of OMZs in the three seamounts areas are discussed using IOMZ. With higher IOMZ, the degradation particulate organic carbon (POC) tends to be lower. Yet because of the limited data, their relationship still need more research to prove. However, if this relationship holds in global oceans, the presence of seamounts would—under climate warming with expanding OMZs—promote vertical transport of POC resulting in an enhanced biological pump. Our study provides a new way to quantitatively study the impact of OMZs on the efficiency of biological pump. [ABSTRACT FROM AUTHOR]