North Pacific Intermediate Water Research Articles

Assessing transient changes in the ocean carbon cycle during the last deglaciation through carbon isotope modeling

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.

Methane Distribution, Production, and Emission in the Western North Pacific

AbstractThe 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

In the North Pacific Subtropical Gyre, which is one of the largest oligotrophic regions, there is a lack of information regarding the sources and transport of trace metals through water mass mixing. Rare earth elements (REEs) are essential for tracing lithogenic sources and water mass transport. In this study, we present dissolved REE concentrations and the factors controlling their distributions in the northwest Pacific during a GEOTRACES cruise (GP09). In the surface water along 11°N, we observed input signals from the Philippine and the Hawaiian Islands, characterized by positive Eu anomalies and slightly elevated REE concentrations. By incorporating our data and published REE data from the northwest Pacific to the southeast Pacific (40°N–40°S), we demonstrated that the REE concentrations and Yb/Nd ratios can distinguish North Pacific Intermediate Water (NPIW), Antarctic Intermediate Water (AAIW), and modified AAIW. By estimating the ratio of water mass mixing, we suggest that heavy REEs are predominantly contributed by water mass mixing (e.g., 93 % ± 4% for Yb) and can be used as semi-conservative tracers to quantify the mixing of NPIW and modified AAIW at the potential density of 27.2 kg/m3. At ∼800 m depth at stations K12, K13, and K14, weak negative Ce anomalies (>0.1) were observed, indicating the lateral transport of water masses imprinted with sediment signals from the Philippine Islands. In the deep waters (>1500 m) at the Luzon Strait (station K1), we propose that the weak Ce negative anomalies (>0.08) and low Yb/Nd ratios (<5), combined with the low beam transmission, are the result of particle resuspension and release. In deep water (>2000 m), combining high-resolution REE measurements with water mass analysis, our research reveals that dissolved REEs (except Ce) are dominantly controlled by water mass mixing (e.g., 80 %∼100 % for Yb and 70 %∼100 % for Nd). For the non-conservative behavior, which is not explained by water mass mixing, the residual fraction of heavy REEs (<20 % for Yb) originates from release of sinking particles (organic matter and siliceous particles), while the remaining concentrations of light REEs (<30 % for Nd) may be influenced by a combination of particle release and scavenging processes. These findings highlight valuable information about lithogenic sources and the proportions of REE distributions that are controlled by physical and biogeochemical processes. Moreover, it emphasizes the applicability of heavy REEs as effective tracers for understanding basin-scale water mass mixing in the northwest Pacific.

Surface and subsurface dispersal of radioactive materials from Fukushima by subpolar gyre and intermediate waters in the North Pacific

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.

Enriched Regions of 228Ra Along the U.S. GEOTRACES Pacific Meridional Transect (GP15)

AbstractThe half‐life of 228Ra (5.7 years) aligns well with near‐surface and near‐bottom ocean mixing timescales. Because 228Ra is sourced from sediments, regions of enhanced activity represent water that has recently interacted with sediments on the continental margin or seabed. The GP15 meridional transect from Alaska to Tahiti along152°W encountered several regions in the upper ocean where 228Ra was enriched. These enrichments follow surface and subsurface ocean current patterns and pair with earlier measurements of 228Ra and transient radionuclides to reveal the origins of these enriched regions. An enriched region at Alaska margin stations 1–3 was sourced locally but did not extend to the Alaskan trench at station 4. A large shallow region between 47° and 32°N. was sourced from the west by the North Pacific Current; another shallow enriched region between 11° and 5° N was also sourced from the west by the North Equatorial Countercurrent. Subsurface enrichments (100–400 m) between 18 and 47°N were associated with Central Mode Water and North Pacific Intermediate Water. The 228Ra activities in the upper Pacific were six times lower than activities in the Atlantic. In deep waters the primary enrichment was 27°–47°N. Two stations (32° and 37°N) were especially enriched, having near‐bottom inventories several times greater than other stations. With these two exceptions the remaining Pacific stations exhibited averaged inventories lower than those in the Atlantic. There was one region of enriched 223Ra (half‐life = 11 days) above the Puna Ridge near Hawaii.

Organic carbon burial and their implication on sea surface primary productivity in the middle Okinawa Trough over the past 200 ka

The long-term burial of organic carbon in marginal seas plays a critical role in Earth’s carbon cycle and climate change. However, the mechanism of organic carbon (OC) burial in the Okinawa Trough (OT) during glacial-interglacial timescales remains unclear. In this study, we analyzed the foraminiferal carbon isotopes, total organic carbon (TOC), and δ13C-TOC over the past 200 ka in core Z1 collected in the central OT. We aimed to reveal the history of OC burial in the middle Okinawa Trough during the past 200 ka, and we combined our findings with relevant paleoenvironmental indices to reveal underlying mechanisms. We found reduced surface primary productivity during MIS 6, which may indicate changes in the pathways of the Kuroshio Current (KC). Furthermore, we observed decoupling between high TOC flux and low OC burial during glacial periods. We proposed that the dilution effect caused by the high sedimentation rate and poor OC preservation during the glacial period resulted in the low TOC content. Ventilation of the North Pacific Intermediate Water (NPIW) regulated the redox conditions of the intermediate water in the Okinawa Trough. Additionally, the intensified Kuroshio Current during interglacial phases led to water column stratification, creating reducing conditions in the bottom water and facilitating improved OC preservation. Subsequently, the enhanced water column oxygenation resulting from the oxygen carried by the intensified glacial NPIW weakened the burial of OC. This study sheds new light on our understanding of the carbon cycle in marginal seas on a glacial-interglacial timescale.

Roles of changes in land weathering intensity in the Nd cycle of the South China Sea during the past 30 kyr as inferred from neodymium isotopic composition in foraminifera

Neodymium isotopic compositions (εNd) in the marginal sea is influenced by the ocean circulations as well as the lithogenic input from continents, but their relative contributions and the impact of detrital sediments are not well constrained. In this study, Nd isotopic compositions of mixed planktonic foraminifera species and carbonate-free fraction of sediment from two cores collected at 1250–1350 m water depth in the southwestern (core MD01–2393) and northeastern (core MD18–3569) South China Sea (SCS) have been investigated in order to constrain the relative contributions of lithogenic Nd inputs from large Asian rivers and hydrological variations at intermediate depth in the SCS. The foraminiferal εNd values of both cores (−7.3 ± 0.2 to −5.8 ± 0.2 for core MD18–3569 and − 8.1 ± 0.2 to −7.2 ± 0.2 for core MD01–2393) indicate strong modifications of the initial εNd of the North Pacific Intermediate Water (NPIW) (εNd of −4) flowing into the SCS by unradiogenic sediments (−13 to −11) from large Asian rivers. The foraminiferal εNd record of core MD01–2393 displays significantly unradiogenic values from 18 to 8 cal kyr BP which are associated with intensification of monsoon rainfall and river input of detrital material characterized by strongly altered minerals (high illite chemical index and kaolinite/illite ratio) deriving from tropical plain soils of the Mekong River basin. We suggest here that pedogenetic minerals from tropical plain soils are more efficient in terms of Nd exchange with the seawater than primary minerals produced by intense physical erosion during the glacial period. In contrast, the foraminiferal εNd record for the northern core MD18–3569 is characterized by radiogenic εNd values from 18 to 12 cal kyr BP; these values have been linked to an increased intrusion of radiogenic glacial NPIW to the northern SCS and greater ventilation of water masses (deduced from Δ14C) in the northern deep basin of the SCS compared to the southern one. This time interval is associated with newly formed NPIW in the subarctic Pacific Ocean beginning at about 18 cal kyr BP. Overall, our results indicate that the state of chemical weathering of sediment delivered by rivers is the main factor controlling the detrital modification of past seawater εNd distributions in the marginal sea of the SCS.

Evolution of ocean circulation and water masses in the Guaymas Basin (Gulf of California) during the last 31,000 years revealed by radiolarians and silicoflagellates in IODP expedition 385 sediment cores

The high-resolution analysis of radiolarians and silicoflagellates in sediments from Holes U1545A and U1549A drilled during IODP Expedition 385 in the Guaymas Basin, in the Gulf of California provides detailed insights into the evolution of ocean circulation and water masses, and its relation to Eastern Tropical Pacific Ocean climate conditions, over the past 31,000 cal years BP (based on AMS radiocarbon dates). In the pre-Last Glacial Maximum, the Guaymas Basin experienced alternating circulation patterns of California Current Water (CCW) and Gulf of California Water (GCW), with an extended presence of the Pacific Intermediate Water (PIW) owing to: amplified jet streams; southern movement of the California Current System (CCS) and the incursion of CCW into the gulf; and increased North Pacific Intermediate Water (NPIW) formation. The Last Glacial Maximum witnessed the incursion of CCW due to the stronger CCS. The dominance of the PIW indicates the expansion and formation of NPIW. The Heinrich-I event as manifested in the core record, displays two distinct patterns, one suggesting GCW-like dominance and the other, the occurrence of CCW. The Bølling-Ållerød interstadial featured the entry of Tropical Surface Water (TSW), GCW, and CCW, linked with the northward migration of the Intertropical Convergence Zone. In the Younger Dryas, CCW dominated, transitioning to GCW as colder climatic conditions and more intense CCS. The Holocene displayed alternating periods of TSW and GCW, with a modern monsoon regime from 7,600 to 1,000 cal years BP. From 1,000 cal years BP to the present the ITCZ shifted to the south.

Remote and local controls on dissolved oxygen in the Western Tropical Pacific thermocline during the last 700 kyr

The dissolved oxygen level of the low-latitude Pacific thermocline is a sensitive tracer of global ocean circulation, especially at high latitudes. However, its variability covering glacial–interglacial cycles has not been well documented. Based on the thermocline-dwelling foraminiferal population extracted from Calypso Core MD06–3047, we derived a record of the Western Tropical Pacific (WTP) thermocline oxygenation over the last 700 kyr. We found that the glacial thermocline oxygenation gradually enhanced to dome-like peaks during glacial periods, with significant relative abundance of thermocline-dwelling species minimum events. We suggest that the remote physical process of lateral O2 supply (dominated by Southern Ocean (SO) ventilation and Antarctic Intermediate Water advection, with a minor contribution of North Pacific Intermediate Water (NPIW)) and the local biological process of O2 consumption (by organic biological productivity, determined by iron supply and NPIW-driven nutrient supply) may together control the WTP thermocline oxygenation on glacial–interglacial cycles. The surprisingly synchronous co-variation indicates that SO ventilation and Antarctic Intermediate Water advection have the potential to be the primary controlling factors. Furthermore, the consistent and simultaneous dramatic increments in Marine Isotope Stages 13–11 transition and each deglacial period provide robust support for a close coupling between the global (mid-Brunhes) climatic shift scenario of SO ventilation and the WTP thermocline oxygenation.

Eddy-Induced Subsurface Spiciness Anomalies in the Kuroshio Extension Region

Abstract While mesoscale eddy-induced temperature and salinity (T and S) variations at depth levels were widely reported, those on isopycnal surfaces have been largely unexplored so far. This study investigates temperature and salinity anomalies (T′ and S′; dubbed “spiciness anomalies”) on isopycnal surfaces induced by mesoscale eddies in the Kuroshio Extension (KET) region, with a focus on the North Pacific Intermediate Water (NPIW) layer of 26.3–26.7σθ. Cyclonic eddies (CEs) and anticyclonic eddies (AEs) tend to cluster on the northern and southern flanks of the KET jet, respectively. These eddies are characterized by a large radius (CEs: 61.94 km; AEs: 68.05 km), limited zonal movement, and a tendency of meridional movement (CEs: 0.35 cm s−1 southward; AEs: 0.66 cm s−1 northward). The average eddy-induced T′ and S′ are −0.25°C (0.06°C) and −0.05 psu (0.01 psu) for CEs (AEs) in the 26.3–26.7σθ layer. We propose two mechanisms for the generation of subsurface spiciness anomalies, respectively, for moving eddies that travel over long distances with trapped waters and quasi-stationary meander eddies that are generated by the meanders of the KET front. The T′ and S′ induced by moving eddies cumulatively drive cross-front water exchanges. Meander eddies shift the position of the front and induce redistribution of properties. However, these anomalies do not contribute to heat and salt exchanges between water masses. This work provides a useful benchmark for model simulations of mesoscale isopycnal variability in subsurface waters.

Evolution of Oceanography of the Central Northwest Pacific Over the Past 10 Million Years With Focus on Late Miocene Global Cooling

AbstractSince the middle Miocene climatic transition, the Earth's climate has steadily cooled. The late Miocene global cooling (LMGC) and the Northern Hemisphere Glaciation (NHG) were two key cooling events occurring during this time. To better understand the mechanisms underlying these cooling events, changes in radiolarian microfossil assemblages were examined in this study, aiming at the reconstructing of oceanographic changes that have occurred at Ocean Drilling Program site 1208 during the last 10 million years. Sea surface temperatures (SSTs) were reconstructed based on radiolarian species that were extant 0–10 million years ago. Reconstructed SSTs were then compared with previously published alkenone‐based SSTs at site 1208, and it was found that overall, using SSTs based only on extant radiolarian species yielded a correct record for the last 10 million years. However, large discrepancies were observed between radiolarian‐ and alkenone‐based SSTs during LMGC and the NHG. These discrepancies were attributed to the sustained influence of subsurface water (at depths from ∼50 to 100 m) on assemblages of radiolarians during extreme cooling events. Relative abundances of other radiolarian groups indicated that during LMGC, there was a reorganization of the regional oceanography that probably weakened the Pacific meridional overturning circulation, increased the meridional temperature gradient, and caused a southward migration of the subtropical front. Probably, the North Pacific intermediate water expanded southeastward during NHG.

Genetic divergence and migration patterns of a galatheoid squat lobster highlight the need for deep-sea conservation.

Information on genetic divergence and migration patterns of vent- and seep-endemic macrobenthos can help delimit biogeographical provinces and provide scientific guidelines for deep-sea conservation under the growing threats of anthropogenic disturbances. Nevertheless, related studies are still scarce, impeding the informed conservation of these hotspots of deep-sea biodiversity. To bridge this knowledge gap, we conducted a population connectivity study on the galatheoid squat lobster Shinkaia crosnieri - a deep-sea foundation species widely distributed in vent and seep ecosystems in the Northwest Pacific. With the application of an interdisciplinary methodology involving population genomics and oceanographic approaches, we unveiled two semi-isolated lineages of S. crosnieri with limited and asymmetrical gene flow potentially shaped by the geographic settings, habitat types, and ocean currents - one comprising vent populations in the Okinawa Trough, with those inhabiting the southern trough area likely serving as the source; the other being the Jiaolong (JR) seep population in the South China Sea. The latter might have recently experienced a pronounced demographic contraction and exhibited genetic introgression from the Okinawa Trough lineage, potentially mediated by the intrusion of the North Pacific Intermediate Water. We then compared the biogeographic patterns between S. crosnieri and two other representative and co-occurring vent- and seep-endemic species using published data. Based on their biogeographical subdivisions and source-sink dynamics, we highlighted the southern Okinawa Trough vents and the JR seep warrant imperative conservation efforts to sustain the deep-sea biodiversity in the Northwest Pacific.

Influence of methane seep activities and sea-level changes on elemental and isotopic compositions and abundance of carbonates in sediments of the Okinawa Trough since the last glacial period

The elemental and isotopic compositions and abundance of carbonates in marine sediments are used to reconstruct ancient climate and ocean conditions. These carbonates usually include biogenic and authigenic carbonates in origin, which are controlled by sedimentary and diagenetic processes, respectively. However, the specific mechanisms involved in controlling the origin of biogenic and authigenic carbonates are still unclear. To address this knowledge gap, we analyzed core CSHC-4 sediment from the Okinawa Trough to examine the elemental and isotopic compositions of carbonates and their relationship to methane seepage and sea-level changes. Our results indicate that high-Mg calcite is the dominant authigenic carbonate, indicating moderate methane seep intensity. Negative carbon isotope values as low as −4.06‰ V-PDB of the bulk sediments deposited in the glacial period suggest a connection between the formation of authigenic carbonates and methane seepage. Negative oxygen isotope values as low as −2.6‰ V-PDB may be attributed to the input of 18O-depleted fluids influenced by gas hydrate formation. Biogenic carbonates contribute the majority (96.3%) of the total carbonate contents in sediments. Biogenic carbonate contents are lower during glacial periods and higher during interglacial periods. The reduced carbonate content during glacial periods may be due to lower export productivity influenced by the deepening of North Pacific Intermediate Water, as well as terrestrial inputs and anoxic conditions during low sea level periods. Our findings highlight the importance of considering both biogenic and authigenic carbonates when reconstructing paleoclimate and paleoceanography based on their geochemical characteristics.

Water mass mixing contributes dynamics of dissolved organic matter in Kuroshio-Oyashio confluence region

The Kuroshio-Oyashio confluence region (K/O region) plays a significant role in the global carbon cycle and climate change. In this study, we examined the concentration and composition of dissolved organic matter (DOM) in water samples collected from depths of 0 to 1000 meters between 2019 and 2021. The concentration of dissolved organic carbon (DOC) ranged from 30.54 to 113.21 μmol L-1, with higher concentrations observed in surface waters. The biological index (BIX) decreases with increasing depth, indicating decreasing in-situ production of plankton downwards. Additionally, the humification index (HIX) increased with depth, suggesting the transformation of labile to refractory DOC by bacteria. Using parallel factor analysis, we identified two humic-like (C1, C3) and one protein-like (C2) fluorescent components in 467 water samples. Water samples collected in 2020 exhibited significantly higher specific ultraviolet absorbance at 254 nm (SUVA254) (0.5 ± 0.2 L mg-1 m-1) and higher intensities of C1 (8.2 ± 2.69 *10-3 R.U.) and C3 (9.22 ± 4.39 *10-3 R.U.) compared to samples from 2019 and 2021. We found that water temperature had a positive correlation with C2 intensity (r = 0.33; p&amp;lt; 0.01), but negative correlations with C1 (r = −0.72; p&amp;lt; 0.01) and C3 intensity (r = −0.55; p&amp;lt; 0.01). Comparison of measured and theoretical values using a conservative physical mixing model revealed active biogeochemical processes of DOM during water mixing of the Kuroshio, Oyashio, and North Pacific Intermediate Water that change concentration and composition of DOM. Since the hydrology in the K/O region is sensitive for the ongoing climate warming, further investigation is needed to understand water mass mixing and associated DOM dynamics.

Characteristics of North Pacific water masses in the Bali Sea

The Bali Sea is the trajectory of Indonesian Throughflow (ITF)’s water mass which originates from the North Pacific (NP) Ocean before exiting through Lombok Strait. Therefore, the NP water masses have an impact on the characteristics of the water mass in this area. The study aims to describe the stratification and structure of water masses and calculate the contribution of North Pacific Subtropical Water (NPSW) and North Pacific Intermediate Water (NPIW) using Optimum Multiparameter (OMP) analysis. Stratification of the Bali Sea consists of mixed, thermocline, and deep layers. There were found remnants of NPSW and NPIW inside the thermocline layer, deeper than their origin in the NP Ocean. The core layer of NPSW has an S-max value of 34,6 psu at 120-170m depth, while NPIW has an S-min value of 34,3 psu at the deeper layer, 260-280m depth. The NPIW more dominant presence in the Bali Sea with 80% contribution at a depth of 200-215 meters, while NPSW contribution is 70% at a depth of 110-130 meters. NPSW’s contribution decreases from east to west and vice versa the NPIW.

North Pacific Water Mass Contribution in the Northern Sea of Lembata Island, East Nusa Tenggara

As the Indonesian Throughflow (ITF) passes through the Flores Sea, we expect it will affect the water mass character in the northern sea of Lembata Island. Therefore, we examined the contribution of North Pacific (NP) water mass, i.e., the North Pacific Subtropical Water (NPSW) and the North Pacific Intermediate Water (NPIW) in the study area. We used 30 CTD casts from OTEC Geological Survey Center Expedition 2017 and ARGO float data, analyzed through the TS diagram and Optimum Multiparameter (OMP). The remnant of the NPSW (NPIW) core layer was found at 140-190 (220-330) m depth, deeper than it occurs in the NP source regime, and of previous pathway seas i.e. Makassar Strait. The OMP results show NPSW (NPIW) contribution is about 20-75 (50-70)%. Contrary to the ITF direction from the western pathway, the value of NP water mass contribution is larger in the eastern than in the western part of the northern sea of Lembata Island. We speculated the ITF flows away from the northern sea of Lembata Island, blocked by the headland in the eastern part of Flores Island. Then a small part of ITF is deflected back to the west, but unfortunately, no detailed current is available in the study area.

Impacts of nepheloid layers and mineralogical compositions of oceanic margin sediments on REE concentrations and Nd isotopic compositions of seawater

Nepheloid layers and turbidity currents transport large quantities of detrital sediments to deep-sea basins. Locally they may control seawater – detrital particle interactions and modify dissolved REE concentrations and εNd distributions of the oceanic margin. However, to date, such processes have been poorly documented, especially in oceanic margins characterized by contrasted mineralogical compositions. Dissolved REEs and εNd from three water stations collected in the northern South China Sea (SCS) close to the Taiwanese deep-sea canyons have been analyzed, with the aim of determining the influence of nepheloid layers and mineralogical composition of lithogenic inputs on dissolved REE and εNd distributions in the ocean. Results indicate that the most radiogenic εNd value (−2.5 ± 0.2), observed at about 400 m, corresponds to the inflow of North Pacific Intermediate Water (NPIW) into the SCS. When compared with previously published Nd isotope, our current study suggests that the εNd value of NPIW which enters the SCS (from 300 to 800 m water depth) is not modified by unradiogenic sediments off the southern margin of Taiwan Island, whereas they become less radiogenic along the margin of eastern China due to Nd exchange with unradiogenic sediments (between −10.2 and −12.6) from the tropical soils of Chinese rivers basins. We have proposed that pedogenetic minerals from Chinese tropical soils modify the Nd isotopic compositions of seawater more efficiently than fresh detrital minerals resulting from very high rates of physical erosion on Taiwan Island. The MREE/MREE*, La/Sm and La/Yb ratios in nepheloid layers indicate that lateral transportation of lithogenic material within the water column can preferentially adsorb LREE compared to MREE and HREE through the scavenging process. However, this process has a negligible impact on seawater εNd of the northern SCS. At the bottom of several water stations, a negative excursion of εNd values, associated with an increase in MREE/MREE* ratios and a decrease in La/Sm ratios, suggests a remineralization flux resulting from the dissolution of Mn oxides during early diagenesis of deep-sea sediments.

Oxygen isotope constraints on the ventilation of the modern and glacial Pacific

Changes in Pacific tracer reservoirs and transports are thought to be central to the regulation of atmospheric CO2 on glacial–interglacial timescales. However, there are currently two contrasting views of the circulation of the modern Pacific; the classical view sees southern sourced abyssal waters upwelling to about 1.5 km depth before flowing southward, whereas the bathymetrically constrained view sees the mid-depths (1–2.5 km) largely isolated from the global overturning circulation and predominantly ventilated by diffusion. Furthermore, changes in the circulation of the Pacific under differing climate states remain poorly understood. Through both a modern and a Last Glacial Maximum (LGM) analysis focusing on oxygen isotopes in seawater and benthic foraminifera as conservative tracers, we show that isopycnal diffusion strongly influences the mid-depths of the Pacific. Diapycnal diffusion is most prominent in the subarctic Pacific, where an important return path of abyssal tracers to the surface is identified in the modern state. At the LGM we infer an expansion of North Pacific Intermediate Water, as well as increased layering of the deeper North Pacific which would weaken the return path of abyssal tracers. These proposed changes imply a likely increase in ocean carbon storage within the deep Pacific during the LGM relative to the Holocene.

Radiation and energetics of M2 internal tides in the Okhotsk Sea

Tidal mixing around the Kuril Island Chain (KIC) significantly modulates the transport of Okhotsk Sea Mode Water into the Pacific Ocean and, hence, the formation of North Pacific Intermediate Water. Although previous studies have revealed the dominant role of trapped K1 internal tides in driving mixing, the energetic M2 internal tides that have been observed by altimetry are not sufficiently clarified. In this study, we investigated the generation, radiation, and dissipation of semidiurnal M2 internal tides in the Okhotsk Sea using a high-resolution model. The results showed that KIC is the main generation site for M2 internal tides, with an integrated energy conversion of 2.07 GW. The generated M2 internal tides propagated over a long distance into the Okhotsk Sea and Northern Pacific Ocean and exhibited asymmetry on the two sides of the KIC. In addition, the generation of M2 internal tides on the Kashevarov Bank in the northwestern Okhotsk Sea made a minor contribution. Although the integrated dissipation of the M2 internal tides at KIC was lower than that of the K1 internal tides, the M2 internal tidal energy dissipation rates exceeded 10−8 W/kg near the topography. More importantly, energy dissipation in the far field was dominated by radiating M2 internal tides, which induced higher diapycnal diffusivity than K1 internal tides. These results revealed the important role of M2 internal tides in driving mixing at KIC and surrounding regions, which was previously overlooked.

Interactions Between Depositional Regime and Climate Proxies in the Northern South China Sea Since the Last Glacial Maximum

AbstractSedimentary deposits from the northern South China Sea (SCS) can provide important constraints on past changes in ocean currents and the East Asian summer monsoon (EASM) in this region. However, the interpretation of such records spanning the last deglaciation is complicated because sea‐level change may also have influenced the depositional processes and patterns. Here, we present new records of grain size, clay mineralogy, and magnetic mineralogy spanning the past 24 kyr from both shallow and deep‐water sediment cores in the northern SCS. Our multi‐proxy comparison among multiple cores helps constrain the influence of sea‐level change, providing confidence in interpreting the regional climate‐forced signals. After accounting for the influence of sea‐level change, we find that these multi‐proxy records reflect a combination of changes in (a) the strength of the North Pacific Intermediate Water inflow, (b) the EASM strength, and (c) the Kuroshio Current extent. Overall, this study provides new insights into the roles of varying terrestrial weathering and oceanographic processes in controlling the depositional record on the northern SCS margin in response to climate and sea‐level fluctuations.