Abstract Biological productivity, shaped by climate and environmental factors, is critical to climate feedback mechanisms and the global carbon cycle. This study investigates the measurement of six phytosterols (β‐sitosterol, stigmasterol, campesterol, dinosterol, epi‐brassicasterol, and 24‐methylene cholesterol) from core MD01‐2414 in the central Okhotsk Sea, covering the past 1.5 million years (Ma). These sterols serve as proxies for terrestrial and marine productivity in the central Okhotsk Sea and northeast Siberia. Sterol concentrations reflect global glacial/interglacial cycles between 1.2 and 0.6 Ma, with higher and lower values during interglacial and glacials intervals, respectively. X‐ray fluorescence (Ba/Ti) and total organic carbon/total nitrogen (TOC/TN) ratios indicate shifts in marine and terrestrial sources, confirming biological productivity as a key driver of sterol deposition. Sterol fluxes, combined with sea surface temperature records from the northwest Pacific and sea ice proxies from the Bering Sea, reveal an extreme cold interglacial (Marine Isotope Stage, MIS 23) and prolonged glacial conditions during MIS 22, which stressed both terrestrial and marine ecosystems. Increased sea ice expansion during this period likely fostered North Pacific Intermediate Water formation, reducing upwelling and CO 2 exchange between bottom waters and the atmosphere. Integration of sterol data with regional records, including pollen, temperature, and sedimentary facies from Lake El'gygytgyn, highlights a warming event at the onset of MIS 32, peaking in late MIS 32. This warming precedes the “super‐interglacial” MIS 31 and coincides with maxima in boreal and austral summer insolation, underscoring its significance in regional climate evolution.

Did the North Pacific Intermediate Water influence the South China sea during the last glacial?

The vertical distribution of small microplastics (SMPs; 10-300 μm in size) and its relation to water masses were investigated through seawater sampling and hydrographic surveys from the sea surface to 1000 m in the North Pacific Ocean. The average ± standard deviation of SMP concentrations in 12 layers at four stations was 6910 ± 2620 particles m-3. Concentrations were high in isopycnal layers between potential densities of 23 and 25σθ (100-300 m depths). Elevated concentrations were also frequently detected below the North Pacific Intermediate Water (NPIW), characterized by a salinity minimum around the 26.6-27.0σθ (approximately 600 m depth) isopycnal layers. A simple modeling approach to reproduce the observed SMP distribution suggested two pathways for SMPs floating in surface convergence zones. One pathway is the weak settling of SMPs of which the density becomes close to neutral, causing the along-isopycnal subduction from isopycnal layers outcropping at the sea surface to subsurface layers above the NPIW. Therefore, the global inventory of weakly settling near-neutral SMPs is expected to be high in the subsurface layers. Meanwhile, the strong settling via biological processes causes the other pathway from the surface euphotic layer to deep layers that never outcrop at the sea surface.

In marine ecosystems, Vibrio species are as they facilitate nutrient cycling and impact the condition of marine life. To understand their ecological dynamics and how they adapt to various environmental situations, this study examined the vertical distribution pattern and assembly processes of Vibrio species across a depth gradient (5–6000 m) within the Kuroshio Extension in the Northwest Pacific Ocean. Through quantitative PCR and high-throughput sequencing based on 16S rRNA genes, the abundance of Vibrio spp. showed a strong vertical stratification. Vibrio community compositions varied significantly among the ocean surface mixed layer (5-105 m, UL), the pycnocline and North Pacific Intermediate Water layer (155-700 m, ML), and bathypelagic layer (>1000 m, BL), which was reflected by a strong vertical depth decay pattern. In the UL, Vibrio sagamiensis, Paraphotobacterium marinum, V. caribbeanicus, V. campbellii and Photobacterium phosphoreum were the dominated species. V. pomeroyi was the most abundant species in ML and BL, and V. sagamiensis, P. marinum and P. phosphoreum usually persisted in deeper water layers, reflecting their potential adaptations to deep ocean conditions. Both deterministic factors (e.g., temperature, salinity, dissolved oxygen, , and ) and stochastic processes shaped Vibrio community assembly mechanism, with stochasticity dominating community structure in UL and heterogeneous selection playing a key role in ML and BL. Our findings highlight the complex interplay between environmental gradients and stochasticity in shaping Vibrio communities along the depth in the water column, contributing to a deeper understanding of their dynamics in the open ocean.

Abstract This study seeks to identify drivers of differences in ocean acidification (OA) rates throughout the upper ocean at Station ALOHA in the North Pacific Subtropical Gyre (NPSG). OA is intensified in the subsurface due to increases in natural and anthropogenic carbon pools, and their interactions. Enhanced subsurface trends are found for all OA indicators. This includes parameters that have previously been identified to exhibit nonlinear interactions between anthropogenic and natural carbon ([H+], pCO2, pH, Revelle Factor), as well as parameters that are not subject to this response (aragonite saturation state (ΩAr)). Different parameters have trend maxima in each of the water masses in the upper 500 m, driven by different mechanisms. Enhanced acidification is noted in the North Pacific Tropical Water (NPTW) from 2015 until 2020. This is due to the interplay of a circulation slowdown during a prolonged negative phase of the North Pacific Gyre Oscillation (NPGO) with other anomalous atmospheric forcing that altered source water chemistry, including large‐scale freshening. Sustained increased acidification is also associated with freshening and cooling in the Subsurface Salinity Minimum (SSM) over the whole time‐series, with considerable oxygen loss and nutrient increases. In the North Pacific Intermediate Water (NPIW), a well‐documented circulation slowdown has led to enhanced CO2 ingrowth from remineralization, buffered by increasing carbonate dissolution. Local changes seem to play a smaller role. In the SSM and NPTW, enhanced acidification is associated with cooling and freshening, providing new insights on how OA can accelerate beyond the well‐documented warming and souring of the ocean.

Abstract The interplay between the northward‐flowing surface oceanic currents and the southward‐flowing intermediate water masses regulated ocean stratification and mixing in the Northwest Pacific. Radiolaria, a biogenic proxy, was used to reconstruct changes in ocean stratification in the central Okinawa Trough (OT) paleoceanography since 20 ka. Our results revealed that changes in the shallow water (0–500 m), upper‐intermediate water (500–1,000 m), and lower‐intermediate water (1,000–1,600 m) are triggered by variations in the Kuroshio Current (KC), the North Pacific Intermediate Water (NPIW), and Pacific Central Water (PCW), respectively. The KC influence was stronger during the Holocene rather than during the last deglaciation, increased during the ∼11.7–8.2 ka, weakened during the 8.2–4 ka, declined at 4–2 ka, and rapidly increased after 2 ka. In contrast, the influence of PCW increased during the last deglaciation, particularly during Heinrich Stadial 1 (HS1) and Younger Dryas (YD), but remained weak throughout the Holocene. This pattern appears to be closely linked to the Atlantic/Antarctic provenance events associated with Atlantic Meridional Overturning Circulation variability. Meanwhile, the influence of NPIW was strong during the YD but gradually weakened throughout the Holocene. Notably, a rapid decline in NPIW influence occurred during the Neoglaciation (∼4–2 ka), similar to the KC, suggesting a high‒low latitude linkage driven by atmosphere‒ocean feedbacks mechanisms. This study provides robust data and a new perspective for understanding the connection to deglacial and Holocene variability in North Pacific upper and intermediate ocean circulation.

Graphical Abstract Highlight Research Three distinct water layers were identified: surface (0-50 m), thermocline (50-180 m), and deep (>180 m). The thermocline layer is the most stable, with high Brunt-Väisälä frequency and low Thorpe displacement values. The highest energy dissipation rates were observed in the thermocline layer. Vertical diffusivity values were highest in the thermocline layer and decreased with depth. Abstract The Flores Sea is on the western ITF trajectory connecting the Pacific and Indian oceans. Identification and quantification of turbulent mixing of water masses in the Flores Sea are essential for analyzing large-scale ocean circulation processes, including the circulation of the Indonesian ocean interior. However, direct estimations of turbulent mixing in the Flores Sea as a part of the ITF are underestimated. This research aims to determine water conditions, stratification, and water mass structures. This research used data obtained from the CTD instrument applying a Yoyo casting method deployed in March − April 2023. On the other hand, the Thorpe method was used to estimate turbulent vertical mixing based on the values ​​of energy dissipation and vertical diffusivity. The waters are stratified into three layers, mixed layer (1−50 m), thermocline layer (50−180 m), and deep layer (180−500 m). The CTD data showed the presence of a stable thermocline layer dominated by ITF water masses carrying water masses from the Pacific Ocean (North Pacific Intermediate Water (NPIW) and North Pacific Subtropical Water (NPSW)) from the western ITF path. The energy dissipation value obtained at the study site was about 3.36E-07 W Kg-1 and the vertical diffusivity value was approximately 5.25E-05 m2s-1. The thermocline layer showed a large energy dissipation value which was strongly associated with the friction of the ITF, suggesting that turbulent mixing in this region is primarily driven by the interaction of ITF water masses with the surrounding environment.

Abstract Below the aragonite saturation horizon (ASH), the aragonitic skeletons of deep‐sea reef building corals are more susceptible to dissolution. Ocean acidification is causing the ASH to shallow worldwide, threatening the health and future of deep‐sea coral reefs. Deep‐sea reefs in the North Pacific already exist at or below the ASH, making them particularly vulnerable to future ocean acidification. Here we analyze multiple years (2014–2019) of seawater chemistry data from the Hawaiian‐Emperor Seamount Chain (HESC), focusing particularly on intermediate depths (300–800 m) where deep‐sea reefs have been found. Intermediate water masses were identified across the HESC based on characteristic temperature, salinity, and density ranges. We then characterize the corresponding carbonate chemistry of each water mass. North Pacific Intermediate Water (NPIW) dominates at intermediate depths for most of our sites. However, the influence of Pacific Subpolar Intermediate Water (PSIW) increases north of 29°N. PSIW has a shallower ASH and lower oxygen conditions than NPIW. The increasing influence of PSIW may thus play a role in restricting reef development, partially explaining why deep‐sea reefs have not been found on seamounts north of Koko (34.8°N) in this region. In addition, topographic induced upwelling and temporal variability (seasonal, annual) have the potential to shift the ASH by >100 m depth. As ocean acidification progresses, chronic exposure to corrosive waters may negatively affect reef development and persistence. Characterizing the current carbonate chemistry conditions and variability is critical for informed decision making and management efforts to preserve these valuable ecosystems under future climate change.

Spatial variability of temperature and salinity in the cold intermediate layer and water circulation in the Okhotsk Sea are considered for typical cold and warm conditions of the spring season (May-June) determined by separate averaging of all available data of oceanographic observations and diagnostic modeling. The «cold» and «warm» years were separated following to the typization proposed by V.A. Luchin and V.I. Matveev [2016]. The total area with water temperature below 0 oC is evaluated for both types. The difference of water temperature between the types reaches 1 oС on the depths of 30 and 90 m in certain parts of the sea, but decreases to 0.5 oС at the lower boundary of the cold layer. The northward (West Kamchatka Current) and northwestward (Middle Okhotsk Sea flow) transport of the warm and salty transformed North Pacific Intermediate water prevails for the «warm» type, as could be seen from spatial distribution of temperature and salinity in the above cold subsurface layer and the isolines patterns. Water density in the cold layer generally increases toward the northern and northwestern coasts of the sea where it is significantly higher in springs of «cold» years than in «warm» years. The density increases in the southern part of the sea, too, but without such difference between the types because of advective nature of this pattern. The major water gyres vary in size and intensity between the types, in particular, the anticyclonic gyre over the deep-water Kuril Basin is more intense in cold springs. The North Okhotsk Current, flowing westward along the northern shore, turns to the south at 144o E in springs of the cold type. Besides, another alongshore current appears under cold conditions that flows from the Shantar Islands to northern Sakhalin Island and blocks the Amur River outflow to the Okhotsk Sea. On the contrary, in springs of the warm type, the outflow of freshened water from the Amur to the north reaches the latitude of 56.5o N, then turns to the south and replenishes the East Sakhalin Current.

Extremely depleted radiocarbon impact on estimation of Glacial North Pacific Intermediate Water ventilation

Abstract Mesoscale eddies play important roles in ocean biogeochemical cycles by enhancing lateral and vertical transport of materials and energy. We investigated the nutrient biogeochemistry, including the nitrogen cycle, in the northern South China Sea (NSCS) under the influence of an anticyclonic mesoscale eddy detached from the Kuroshio loop current (KLC). The detached eddy transported substantial amounts of water and nutrients that originated from the Kuroshio to the southwest hinterland of the NSCS. Using a nitrogen mass and isotope balance model, we estimated that the enhanced particulate nitrogen flux exported from the euphotic zone at the eddy center area was 2.4 ± 1.7 times that in background area. In addition, the nitrogen fixation and atmospheric depositions (NFAD) nitrogen flux was estimated to be 0.18 ± 0.15 mmol m−2 d−1. The average proportion of new production sustained by NFAD nitrogen relative to dissolved inorganic nitrogen (DIN) supplied from below was 12%, and the turnover time of DIN in the euphotic zone within the eddy was estimated to be ∼9 days. The eddy‐trapped Kuroshio water influenced the distribution of nutrients and nitrate isotope composition above 400‐m depth in the NSCS. The remineralization of newly fixed nitrogen originating from NFAD and the subsequent nitrification regulated nitrate isotope composition above 800 m, whereas assimilation also played a role within the euphotic zone. Deep‐water communication with the North Pacific and the input of the North Pacific Intermediate Water introduced the denitrification signal that originated from the eastern tropical North Pacific into the NSCS.

Abstract. Iron (Fe) and macronutrient supplies and their ratios are major factors determining phytoplankton abundance and community composition in the North Pacific. Previous studies have indicated that Okhotsk Sea Intermediate Water and North Pacific Intermediate Water (NPIW) transport sedimentary Fe to the western subarctic Pacific. Although the supply of Fe and macronutrients from subsurface waters is critical for surface phytoplankton productivity, return paths from NPIW to the subsurface and their impact on the abundance and community composition of the organisms have not been fully understood. In this study, Fe and macronutrient turbulent fluxes, as well as the flux ratios from NPIW to surface waters, were calculated based on a chemical dataset, which included Fe and macronutrient concentrations, with turbulent mixing parameters obtained from the same cruise and same station along the 155° E transect in summer. Additionally, vertical flux divergence was calculated from the estimated vertical fluxes. Surface and subsurface phytoplankton community composition was evaluated in the CHEMTAX program based on algal pigment measurements. The results show that diatom abundance is significantly correlated with the vertical fluxes of Fe and macronutrients, especially with Fe and silicate (Si) fluxes, and with the Fe / N flux ratio along the section line. These results suggest that diatom abundance was controlled by Fe supply from subsurface waters in summer. The computed turbulent flux divergence in the subarctic gyre and Kuroshio–Oyashio transition area suggests that enhanced concentrations of Fe and Si in the subsurface layer were supplied from NPIW.

Abstract The Kuroshio Current (KC) has witnessed rapid surface warming during the past half-century, impacting the marine ecosystems in surrounding regions. However, the vertical structure of the warming KC remains unclear. This study utilizes historical hydrographic observational data and ocean model experiments to investigate temperature changes of the KC in the East China Sea since 1970. The KC at the Pollution Nagasaki (PN) section has shown rapid warming in the upper 350 m and insignificant warming or cooling trends in the subsurface layer of 350–800 m. Our diagnosis suggests that the rapid upper-layer warming results from the downward displacement of isopycnal surfaces, whereas the subsurface cooling arises from lateral advection along isopycnal surfaces. In addition to surface heating, surface wind changes over the subtropical North Pacific-induced by the phase shift of the Pacific Decadal Oscillation (PDO)-also enhance the upper-layer warming of the KC by driving downwelling Rossby waves. The subsurface cooling reflects property changes of the North Pacific Intermediate Water (NPIW) which can be traced to buoyancy fluxes in the subpolar northwestern Pacific. By linking the regional changes observed in the ECS to basin-scale processes over the North Pacific, this work contributes to the understanding of the response of the KC to climate change.

Water mass stability and mixing in the Banda Sea derived from Global Data Repository and the Jalacitra II Expedition

Molybdenum isotopic evidence for linked changes in North Pacific Intermediate Water and subtropical Northwest Pacific redox conditions over the last 200 k.y

Abstract Millennial‐scale nitrogen (N) cycling processes in marginal seas and their response to climate change have not been well understood. Here, we present high‐resolution (ca. 110 years) organic nitrogen isotope (δ15Norg) data since the last deglaciation (16.1 ka) derived from a highly resolved sediment core in the northern South China Sea, aiming to explore millennial‐scale N cycling processes in this area. Unlike most bulk nitrogen isotope (δ15Nbulk) records from the South China Sea, the δ15Norg records show a clear response to well‐defined climatic episodes during the last deglaciation and early Holocene (EH, ∼11.7 to 9 ka), but exhibit a gradually decreasing trend in mid‐to‐late Holocene (since ca. 9 ka). During the last deglaciation and EH, the upper water column N dynamics are controlled by the lateral transport of surface nitrate from eastern tropical Pacific (ETP) presumably via the North Pacific Intermediate Water (NPIW) and to some extent, influenced by the altered input of terrigenous matter driven by sea level change. The significant decrease in δ15Norg since the mid‐Holocene (at ca. 9 ka) can be best explained by the increase in local N2 fixation forced by enhanced El Niño. This mechanism is consistent with modern observations. Overall, our results may reflect the main controlling factors of surface ocean N dynamics have shifted from zonal transport of nitrate from the ETP to El Niño since the mid‐Holocene.

Abstract. Atmospheric carbon dioxide concentration (pCO2) has increased by approximately 80 ppm from the Last Glacial Maximum (LGM) to the early Holocene. The change in this atmospheric greenhouse gas is recognized as a climate system response to gradual change in insolation. Previous modeling studies suggested that the deglacial increase in atmospheric pCO2 is primarily attributed to the release of CO2 from the ocean. Additionally, it has been suggested that abrupt change in the Atlantic meridional overturning circulation (AMOC) and associated interhemispheric climate changes are involved in the release of CO2. However, understanding remains limited regarding oceanic circulation changes and the factors responsible for changes in chemical tracers in the ocean during the last deglaciation and their impact on atmospheric pCO2. In this study, we investigate the evolution of the ocean carbon cycle during the last deglaciation (21 to 11 ka BP) using three-dimensional ocean fields from the transient simulation of the MIROC 4m climate model, which exhibits abrupt AMOC changes similar to those observed in reconstructions. We investigate the reliability of simulated changes in the ocean carbon cycle by comparing the simulated carbon isotope ratios with sediment core data, and we examine potential biases and overlooked or underestimated processes in the model. Qualitatively, the modeled changes in atmospheric pCO2 are consistent with ice core records. For example, during Heinrich Stadial 1 (HS1), atmospheric pCO2 increases by 10.2 ppm, followed by a reduction of 7.0 ppm during the Bølling–Allerød (BA) period and then by an increase of 6.8 ppm during the Younger Dryas (YD) period. However, the model underestimates the changes in atmospheric pCO2 during these events compared to values derived from ice core data. Radiocarbon and stable isotope signatures (Δ14C and δ13C) indicate that the model underestimates both the activated deep-ocean ventilation and reduced efficiency of biological carbon export in the Southern Ocean and the active ventilation in the North Pacific Intermediate Water (NPIW) during HS1. The relatively small changes in simulated atmospheric pCO2 during HS1 might be attributable to these underestimations of ocean circulation variation. The changes in Δ14C associated with strengthening and weakening of the AMOC during the BA and YD periods are generally consistent with values derived from sediment core records. However, although the data indicate continuous increase in δ13C in the deep ocean throughout the YD period, the model shows the opposite trend. It suggests that the model either simulates excessive weakening of the AMOC during the YD period or has limited representation of geochemical processes, including marine ecosystem response and terrestrial carbon storage. Decomposing the factors behind the changes in ocean pCO2 reveals that variations in temperature and alkalinity have the greatest impact on change in atmospheric pCO2. Compensation for the effects of temperature and alkalinity suggests that the AMOC changes and the associated bipolar climate changes contribute to the decrease in atmospheric pCO2 during the BA and the increase in atmospheric pCO2 during the YD period.

Abstract The ocean is a source of atmospheric methane (CH4), but there are still large uncertainties in the estimations of global oceanic CH4 emission due to sparse data coverage. In this study, we investigated the spatial distribution and influencing factors of CH4 in the Western North Pacific (WNP) during May–June 2021. High‐resolution continuous underway measurements showed that surface CH4 concentrations indicated an obvious spatial gradient with an increase from the south to the north due to the influence of water mixing between Kuroshio Extension (KE) and Oyashio. Surface water was generally oversaturated with respect to the atmospheric CH4, and high CH4 fluxes occurred in the Kuroshio‐Oyashio transition region due to high productivity and intensive air‐sea interaction, emphasizing the importance of the Kuroshio‐Oyashio transition region in global oceanic CH4 emission. Vertically, subsurface CH4 maximums were observed around 50–300 m due to in situ production through multiple pathways, and their distributions in the water column were affected by subduction of North Pacific Intermediate Water (NPIW) and advective transport. Methylphosphonate (MPn) enrichment experiment and 16S rRNA gene sequencing showed that in subtropical region and Kuroshio‐Oyashio transition region, Vibrio spp. might produce CH4 by degrading MPn. Although this process was inhibited by inorganic phosphorus and regulated by iron stress, it might be a potential source of CH4 in the oxygenated water in the WNP. Our results contribute to better constrain the global oceanic CH4 emission, and help understanding the role of biological and physical processes in regulating CH4 emission in the WNP.

Dissolved rare earth elements in the North Pacific Subtropical Gyre: Lithogenic sources and water mass mixing control

Radioactive materials were released into the ocean following the Fukushima Daiichi Nuclear Power Plant accident in 2011. Six years after the accident, the radioactive material concentration was markedly increased in the Okhotsk Intermediate Water (OIW) of the Sea of Okhotsk. This material may have been subjected to southward subsurface dispersal by the North Pacific Intermediate Water (NPIW), which originates from the OIW. The spatiotemporal limitations of available methods have made it challenging to track the dispersal paths of radioactive materials in the North Pacific Subpolar region. Here, we performed a tracer experiment using a three-dimensional numerical model to determine the path of 137Cs from Fukushima to the Sea of Okhotsk via surface subpolar gyre currents and subsurface dispersion by OIW and NPIW. The results showed that the 137Cs concentration in the Sea of Okhotsk increased via the surface current and moved progressively southward via OIW six years after the accident and eastward via OIW and NPIW nine years after the accident, indicating that 137Cs transported by NPIW entered the subtropical region. Based on experiments, this temporal change was mainly caused by ocean currents. Thus, subsurface recirculation of radioactive material via the OIW and NPIW should be considered based on the predicted path and travel time of additional materials released from the power plant.