Ocean Margin Research Articles

You Shall Not Pass: The Pacific Oxygen Minimum Zone Creates a Boundary to Shortfin Mako Shark Distribution in the Eastern North Pacific Ocean

ABSTRACTAimShoaling of large oxygen minimum zones (OMZs) that form along eastern margins of the world's oceans can reduce habitat availability for some pelagic fishes. Our aim was to test the hypothesis that habitat compression caused by shoaling of the Pacific OMZ in tropical regions creates a boundary to the southern distribution of shortfin mako sharks (Isurus oxyrinchus) in the Eastern North Pacific Ocean.LocationEastern North Pacific and Western North Atlantic oceans.MethodsWe compared environmental conditions between areas used by satellite‐tagged mako sharks in the Eastern North Pacific, encompassing the world's largest OMZ, to those used in the Western North Atlantic where no OMZ is present. In the Pacific we quantified the effects of temperature and dissolved oxygen (DO) on depth use and tested if sharks spent less time in areas with strong habitat compression over the OMZ than expected by chance.ResultsThe southern distribution of sharks in the Pacific corresponded with the apex of OMZ shoaling in the North Equatorial Current. Sharks in the Atlantic occupied areas with warm surface temperatures (≥ 26°C) more often than the Pacific, and waters with these temperatures in the Atlantic had greater DO at depth. Sharks in the Pacific reduced time near the surface in warm temperatures and consistently avoided depths with low DO and spent less time in areas with strong habitat compression than expected by chance.Main ConclusionsThe combination of warm surface temperatures and shoaling of the OMZ creates a soft boundary to mako shark movements in the Eastern North Pacific Ocean. The expected expansion of OMZs due to climate change could have considerable impact on future distribution of mako sharks and other pelagic fish. As such, development of species distribution models to predict the effects of climate change on pelagic fish distributions should incorporate oxygen availability.

Spatiotemporal evolution of the early Permian multi-source-to-sink system in the northwestern margin of the North China Block: Implications for the paleogeographic reconstruction along the southern margin of the Paleo-Asian Ocean

The northwestern margin of the North China Block, situated at the intersection of multiple tectonic units, presents a complex geological setting that challenges paleogeographic reconstructions. The subduction and closure of the Paleo-Asian Ocean have further complicated the understanding of sediment provenance during the Early Permian. In this study, we integrated sedimentological, geochemical, and UPb dating methods to delineate the Early Permian source-to-sink systems along the northwestern margin of the North China Block. The identification of four key source-to-sink systems—the Alxa, Langshan, Serteng–Wula, and Jining–Daqing—reveals a pronounced east–west provenance differentiation, indicative of the regional tectonic and sedimentary responses to subduction of the Paleo-Asian Ocean.During the Shanxi Formation deposition, the western regions were dominated by the Inner Mongolian continental island arc as a principal sediment source, fostering distal meandering river deltas. Conversely, the eastern regions were predominantly influenced by the North China Block basement, leading to the deposition of proximal, shallow meandering river deltas. As the Shihezi Formation deposition ensued, the western source-to-sink systems continued to receive sediments from the uplifted Inner Mongolian Arc, transitioning to distal braided river deltas amidst an orogenic setting. Meanwhile, the eastern systems, particularly Jining–Daqing, showed increased contributions from the North China Block basement, forming mixed-source, shallow braided river deltas.This research provides a refined understanding of the spatiotemporal evolution of source-to-sink systems and their responses to the tectonic setting, offering valuable insights into the paleogeographic evolution along the southern margin of the Paleo-Asian Ocean.

New Heat Flow Measurements Offshore Montserrat: Advective Heat Flow Detected via MeBo Borehole Temperature Logging

AbstractNew heat flow measurements collected at the Lesser Antilles Arc using a Hybrid Lister‐Outrigger probe and a new logging‐while‐tripping MeBo70 drilling approach provide the first high‐resolution (meter‐to‐cm‐scale) temperature‐depth measurements across the Lesser Antilles volcanic arc and offer new insight into heat and fluid transfer at a convergent oceanic margins. At multiple sites where logging‐while‐tripping MeBo temperature measurements were made, temperature increases linearly with depth in shallowly buried hemipelagic sediment but is isothermal or significantly hotter in deeper, courser‐grained sediments associated with mass flows. We interpret these isothermal zones as regions where advective heat flow—perhaps caused by convection or pressure‐driven advection—dominates. The implication is that apparently conductive heat flow regimes observed in the shallowest upper 5–10 m of hemipelagic sediment across the Lesser Antilles Arc measured using standard lister‐type probes may often unknowingly be influenced by deeper, advective flow along buried mass transport deposits at this site. Since mass transport deposits are ubiquitous on convergent margins, higher permeability mass transport deposits may play a fundamental and previously unrecognized role in fluid and heat transport.

Kinematics of rift linkage between the Eastern and Ethiopian rifts in the Turkana Depression, Africa

AbstractRift initiation within cold, thick, strong lithosphere and the evolving linkage to form a contiguous plate boundary remains debated in part owing to the lack of time–space constraints on kinematics of basement‐involved faults. Different rift sectors initiate diachronously and may eventually link to produce a jigsaw spatial pattern, as in the East African rift, and along the Atlantic Ocean margins. The space–time distribution of earthquakes illuminates the geometry and kinematics of fault zones within the crystalline crust, as well as areas with pressurized magma bodies. We use seismicity and Global Navigation System Satellites (GNSS) data from the Turkana Rift Array Investigating Lithospheric Structure (TRAILS) project in East Africa and a new digital compilation of faults and eruptive centres to evaluate models for the kinematic linkage of two initially separate rift sectors: the Main Ethiopian Rift (MER) and the Eastern rift (ER). The ca. 300 km wide zone of linkage includes failed basins and linkage zones; seismicity outlines active structures. Models of GNSS data indicate that the ca. 250 km‐wide zone of seismically active en echelon basins north of the Turkana Depression is a zone, or block, of distributed strain with small counterclockwise rotation that serves to connect the Main Ethiopian and Eastern rifts. Its western boundary is poorly defined owing to data gaps in South Sudan. Strain across the northern and southern boundaries of this block, and an ca. 50 km‐wide kink in the southern Turkana rift is accommodated by en echelon normal faults linked by short strike‐slip faults in crystalline basement, and relay ramps at the surface. Short segments of obliquely oriented basement structures facilitate across‐rift linkage of faults, but basement shear zones and Mesozoic rift faults are not actively straining. This configuration has existed for at least 2–5 My without the development of localized shear zones or transform faults, documenting the importance of distributed deformation in continental rift tectonics.

Effects of the Emeishan large igneous province (ELIP) and coastal upwelling on paleoenvironment and organic matter accumulation during the Middle Permian (Capitanian), South China

The late Middle Permian (Capitanian) was characterized by many global geological, biotic and environmental events, along with the widespread deposition of organic-rich strata. Elucidating the relationships between organic matter (OM) accumulation and critical geological events is of significance for Earth system science and petroleum resource geology, but the mechanism remains unclear. To fill the knowledge gap, this study investigated the Gufeng Formation in the middle Yangtze region of South China to examine the links between OM accumulation and geological events, e.g., the Emeishan large igneous province (ELIP) and coastal upwelling. Our results show that the Gufeng Formation exhibits significant OM enrichment, with an average total organic carbon (TOC) content of 8.34 wt%. The paleoceanographic conditions went through three stages during the Capitanian: the early–middle stage of high primary productivity and anoxic–sulfidic conditions; the middle stage of moderate productivity and anoxic conditions; and the late stage of low productivity and dysoxic conditions due to weakened upwelling and a global sea-level fall. OM accumulation was influenced mainly by upwelling-induced high productivity and anoxic conditions related to the oxygen minimum zone (OMZ), with a secondary control of carbonate dilution. The ELIP possibly had a limited contribution to high TOC contents. In South China, the Gufeng and uppermost Maokou formations were marked by high OM contents (average TOC > 3 wt%; maximum = 40 wt%), resulting primarily from widespread coastal upwelling along the eastern margin of the Paleo-Tethys Ocean during the Middle Permian. ELIP volcanism mainly triggered rapid climate warming during the late Capitanian and had a regional and variable effect on OM enrichment for different regions in South China.

Climatological distribution of ocean acidification variables along the North American ocean margins

Abstract. Climatologies, which depict mean fields of oceanographic variables on a regular geographic grid, and atlases, which provide graphical depictions of specific areas, play pivotal roles in comprehending the societal vulnerabilities linked to ocean acidification (OA). This significance is particularly pronounced in coastal regions where most economic activities, such as commercial and recreational fisheries and aquaculture industries, occur. In this paper, we unveil a comprehensive data product featuring coastal ocean acidification climatologies and atlases, encompassing the fugacity of carbon dioxide, pH on the total scale, total hydrogen ion content, free hydrogen ion content, carbonate ion content, aragonite saturation state, calcite saturation state, Revelle factor, total dissolved inorganic carbon content, and total alkalinity content. These variables are provided on 1° × 1° spatial grids at 14 standardized depth levels, ranging from the surface to a depth of 500 m, along the North American ocean margins, defined as the region between the coastline and a distance of 200 nautical miles (∼370 km) offshore. The climatologies and atlases were developed using the World Ocean Atlas (WOA) gridding methods of the NOAA National Centers for Environmental Information (NCEI) based on the recently released Coastal Ocean Data Analysis Product in North America (CODAP-NA), along with the 2021 update to the Global Ocean Data Analysis Project version 2 (GLODAPv2.2021) data product. The relevant variables were adjusted to the index year of 2010. The data product is available in NetCDF (https://doi.org/10.25921/g8pb-zy76, Jiang et al., 2022b) on the NOAA Ocean Carbon and Acidification Data System: https://www.ncei.noaa.gov/data/oceans/ncei/ocads/metadata/0270962.html (last access: 15 July 2024). It is recommended to use the objectively analyzed mean fields (with “_an” suffix) for each variable. The atlases can be accessed at https://www.ncei.noaa.gov/access/ocean-carbon-acidification-data-system/synthesis/nacoastal.html (last access: 15 July 2024).

Salinity and nutrient condition as key factors drive the assembly of sediment prokaryotic communities

Despite extensive research on the geographical patterns of microbial communities, our comprehension of the mechanisms underlying their spatial distribution is still limited. Natural ecosystems provide opportunities to investigate the structure, connectivity, and assembly processes of prokaryotic communities. Saline lakes, mangroves, ocean margins, cold seeps, and open oceans as five distinct natural ecosystems exhibit varied levels of salinity and nutrient condition (carbon sources, electron donors, and electron acceptors). Based on the analysis of 197 sets of published 16S rRNA gene amplicon sequencing data on sediment samples of the five habitats, differences in salinity and nutrient conditions were identified to play a critical role in governing the composition, connectivity, and assembly process of prokaryotic communities. Specifically, unique prokaryotic community patterns were observed in these habitats, e.g., mangrove sediment communities were shown to have the highest alpha diversity and the lowest community-level ribosomal RNA gene operon (rrn) copy numbers, compared to the open ocean sediment communities. Positive correlation predominated connections (>80% of total connections) of the prokaryotic microbial networks in the five habitats. Communities within nutrient-rich saline lake and cold seep sediments exhibit the strongest and closest connections. Using the dissimilarity-overlap curve and null model, differences in composition, connectivity, and assembly process were found to be predominantly governed by deterministic forces. These findings enhance our understanding of microbial ecology in typical saline environments and enable us to investigate intricate ecosystems.

The Brazilian Santos basin underwater soundscape monitoring project (PMPAS-BS)

This paper describes the Santos Basin Underwater Soundscape Monitoring Project (PMPAS-BS), a Brazilian ocean soundscape monitoring initiative. The main objective of the project is to quantify and assess hydroacoustic noise of anthropogenic origin in a large sedimentary basin extending from 23° S to 28° S on the southeastern Brazilian continental margin of the South Atlantic Ocean. Noise associated with oil and gas (O&G) exploration and production activities is the primary target, but this oceanic region also has busy shipping lanes for commercial, military, and fishing vessels. The two main hubs of Brazil’s export and import of goods by sea are located in this region: Santos and Rio de Janeiro ports. The project has three measurement components: mobile monitoring based on gliders and drifting acoustic profilers, fixed shallow-water monitoring based on acoustic measurements at coastal stations near shipping lanes associated with exploration and production activities in the Santos Basin, and fixed oceanic monitoring based on deep-water mooring lines equipped with passive autonomous acoustic recorders near production units, shipping lanes, and areas with lower intensity of O&G activities (pristine or reference sites). Numerical modeling of anthropogenic underwater acoustic noise has also been included as a fourth project component. The PMPAS-BS covers an area of more than 251,000 km2 and uses several instruments with different methods and sensors for acoustic measurements. Its results provide current sound levels over a very large region of the western South Atlantic, both in areas more and less affected by anthropogenic activities.

Global geologic expert system and database for predicting carbonate reservoirs from seismic: Application to the Upper Jurassic

Carbonate systems are influenced by a great variety of physical and biological controlling factors that operate from global to local scales. The resulting intrinsic complexity of carbonate platforms makes them difficult to predict, especially when data are limited. Predicting geologic geometries and properties based on limited sampling or uncalibrated seismic data generally relies on a priori knowledge and equivocal interpretations that are marked by geologist perception and personal experience. To overcome these uncertain interpretations of such a complex natural system, which can become critical in frontier exploration, we developed an expert system that relies on a process-based method and a standardized data set using normalized information and parameters. The main innovation relies on the realization of knowledge- and process-based synthetic carbonate stratigraphic architectures that support seismo-stratigraphic interpretations. The workflow consists of four steps: (1) bibliographic compilation of a geologic database for each case study supported by quantitative parameters (e.g., sedimentation duration and thickness) and qualitative parameters (geodynamic context, seismic architecture, and facies model); (2) statistical analyses to establish consistent geologic classes and spatiotemporal trends; (3) process-based modeling to simulate stratigraphic architectures associated with carbonate sedimentation processes in a physically constrained numerical environment and testing different geologic hypotheses; and (4) realization of a predictive palaeogeographic map representing the global distribution of carbonate stratigraphic architectures, and estimation of controlling parameters for unconstrained case studies. The expert system is based on 77 case studies of Upper Jurassic carbonate platforms, which reveal the resemblance of these carbonate systems, in response to uniform global palaeoclimatic conditions and sea level. Significant local differences in stratigraphic architectures are related to specific geodynamic contexts and subsidence trends. The thickest carbonate platforms are developed in extensive/passive geodynamic settings such as the Central Atlantic Ocean margins, while thinner platforms form in intra- and peri-cratonic settings such as those of the Arabian region.

Molecular and radiocarbon constraints on the fate of sedimentary organic carbon in a human-impacted river-dominated ocean margin

Organic carbon (OC) burial in river-dominated ocean margins plays a pivotal role in the global carbon cycle, impacting atmospheric CO2 levels over the long term. Despite its significance, uncertainties persist regarding the influence of external environmental factors and intrinsic properties on sedimentary OC. In this study, we conducted a comprehensive analysis of surface sediments from the East China Sea, examining geochemical properties (including total OC content [TOC], Δ14C, δ13C, and C/N ratio), terrestrial biomarkers (n-alkanes), and mineral properties (such as specific surface area, Al/Si ratio, and mineral composition). Our aim was to shed light on the fate of sedimentary OC.The surface sediment's Δ14C values displayed significant spatial heterogeneity, delineating four distinct sub-regions. Strong positive correlations (all p < 0.01) were found between the ∆14C values and fine-grained sediments, specific surface area, and clay minerals, suggesting the potentially pivotal role of mineral protection in shaping the fate of sedimentary OC. The proportion of terrestrial OC gradually decreased towards the south, while marine OC proportion increased, corresponding to the enrichment of Δ14C. The co-variation of Δ14C values, mineral properties, and OC source proportions suggests that terrestrial OC may undergo progressive replacement by marine OC during southward transport. Temporal variations in ∆14C values indicated that seabed erosion led to a significant increase in ∆14C values (p < 0.01) in the coastal mud belt, a phenomenon likely common in river-dominated ocean margins globally due to the new sediment cycle during the Anthropocene.

Nitrogen Fixation at the Mid‐Atlantic Bight Shelfbreak and Transport of Newly Fixed Nitrogen to the Slope Sea

AbstractContinental shelves contribute a large fraction of the ocean's new nitrogen (N) via N2 fixation; yet, we know little about how physical processes at the ocean's margins shape diazotroph biogeography and activity. Here, we test the hypothesis that frontal mixing favors N2 fixation at the Mid‐Atlantic Bight shelfbreak. Using the 15N2 bubble release method, we measured N2 fixation rates on repeat cross‐frontal transects in July 2019. N2 fixation rates in shelf waters (median = 5.42 nmol N L−1 d−1) were higher than offshore (2.48 nmol N L−1 d−1) but did not significantly differ front frontal waters (8.42 nmol N L−1 d−1). However, specific N2 uptake rates, indicative of the relative contribution of diazotroph‐derived N to particulate N turnover, were significantly higher in frontal waters, suggesting that diazotroph‐derived N is of greater importance in supporting productivity there. This study furthered captured an ephemeral shelf‐water streamer, which resulted from the impingement of a warm core ring on the shelf. The streamer transported shelf‐water diazotrophs (including UCYN‐A and Richelia spp., as assessed by qPCR) offshore with sustained high N2 fixation rates. This feature injected &gt;50 metric tons d−1 of newly fixed N to the Slope Sea—a rate equivalent to ∼4% of the total N flux estimated for the entire Mid‐Atlantic Bight. As intrusions of Gulf Stream meanders and eddies onto the shelf are increasing in frequency due to climate change, episodic lateral fluxes of new N into the Slope Sea may become increasingly important to regional budgets and ecosystem productivity.

Black Sea Morphotectonic Nature

Abstract The presence of oceanic crust on the Black Sea bottom indicates that this sea basin was a section - a Pontic Ocean gulf at the northern margin of the Phanerozoic Tethys Ocean. The late Phanerozoic subduction of the Tethys oceanic crust under the European continental massif in this area took place in the subduction zone, whose modern traces are found today south of the Crimean Peninsula. On the surface of the subducted oceanic crust were numerous fragments of the destroyed continental crust on the northern margin of the Gondwana continent. These relatively lighter than oceanic crust crustal fragments (or a significant part of them) were not subducted. They accumulate in the immediate vicinity of the subduction zone on the unsub ducted remnants of oceanic crust. Such is the case with the Black Sea. It is not an imposed subcontinental basin, but a last modern relict of the northern margin of the Late Phanerozoic Tethys Ocean.

Influence of Hydrodynamics on the Composition and Reactivity of Particulate Organic Matter in a Large River Influenced Ocean Margin

AbstractMarginal seas influenced by large rivers are characterized by complex hydrodynamic and organic matter cycling processes. However, the impacts of hydrodynamics on the composition and reactivity of particulate organic matter (POM) remain unclear. Here we conducted a comprehensive study on the bulk, molecular and biological properties of suspended POM in the Changjiang Estuary and adjacent area subjected to strong currents, eddies as well as typhoons during spring and autumn. D/L‐enantiomers of particulate amino acids (PAA) were analyzed to evaluate the bioreactivity of POM and quantify bacterial‐derived organic carbon. We found that POM bioavailability as indicated by carbon‐normalized yields of PAA (PAA‐C%) reflected the ecosystem productivity. Relatively high PAA‐C% values (20−35%) were observed in productive areas influenced by Changjiang River plume, cyclonic eddies and typhoons, likely related to the enhanced nutrient availability arising from hydrodynamic processes. In contrast, the oligotrophic Taiwan Warm Current‐influenced regions featured relatively low POM bioavailability (PAA‐C% &lt; 10%) despite typhoons facilitating water mixing. The PAA‐C% values showed a significant positive correlation with extracellular enzyme activity, indicating that bioavailable POM can rapidly stimulate heterotrophic transformation. Hot spots of elevated bioavailable POM showed high contributions of bacterial organic carbon. A large portion (∼2/3) of bacterial organic carbon was present in the form of bacterial detritus, suggesting that patches of these biological hot spots represent important sites of carbon sequestration. Together, our findings indicate that fresh POM production is largely controlled by nutrient supply driven by hydrodynamic processes, with important implications for carbon sequestration in the dynamic ocean margins.

Depositional and circulation changes at the Chukchi margin, Arctic Ocean, during the last two glacial cycles

The shallow Chukchi-East Siberian margin of the Arctic Ocean was repeatedly impacted by Pleistocene glaciations and related changes in sea level and circulation. The depositional history across the last two Arctic glacial cycles is investigated in sediment core ARC6-C15 from the foot of the Chukchi Rise, an extension of the Chukchi Sea shelf that was impacted by the East Siberian Ice Sheet (ESIS). Our proxy data indicate a large extent of the ESIS, likely blocking the westward sediment transport from the Laurentide Ice Sheet (LIS) during the penultimate glaciation estimated to have occurred within an age span from Marine Isotope Stage (MIS) 4 to 6. The deglacial environments are characterized by enhanced sediment inputs from the East Siberian and Chukchi shelves, while overlying interstadial/interglacial sediments have a signature of the Chukchi Sea and the Beaufort Gyre. During the last glaciation, the ESIS had a smaller impact on the Chukchi margin, and the deglaciation was dominated by the LIS sediment sources. Glacial discharge from the Mackenzie area is identified in the Bølling/Allerød interval, while later deglacial pulses can be tracked to the Canadian Arctic Archipelago. In addition to glacigenic sediment inputs, both deglacial intervals contain FeMn micronodules, possibly formed under the influence of meltwater pulses during sea level rise. An increase in the chlorite content that likely marks the flooding of the Bering Strait is identified at the start of the Holocene. A pronounced oxygen and carbon isotope excursion in the early Holocene may indicate hydrographic changes in the Arctic Ocean related to the 8.2 ka meltwater discharge event.

Transient fertilization of a post-Sturtian Snowball ocean margin with dissolved phosphate by clay minerals

Marine sedimentary rocks deposited across the Neoproterozoic Cryogenian Snowball interval, ~720-635 million years ago, suggest that post-Snowball fertilization of shallow continental margin seawater with phosphorus accelerated marine primary productivity, ocean-atmosphere oxygenation, and ultimately the rise of animals. However, the mechanisms that sourced and delivered bioavailable phosphate from land to the ocean are not fully understood. Here we demonstrate a causal relationship between clay mineral production by the melting Sturtian Snowball ice sheets and a short-lived increase in seawater phosphate bioavailability by at least 20-fold and oxygenation of an immediate post-Sturtian Snowball ocean margin. Bulk primary sediment inputs and inferred dissolved seawater phosphate dynamics point to a relatively low marine phosphate inventory that limited marine primary productivity and seawater oxygenation before the Sturtian glaciation, and again in the later stages of the succeeding interglacial greenhouse interval.

Sulfate distribution and sulfate reduction in global marine sediments

The quantification of anaerobic oxidation of organic matter in the global seabed is to a large extent based on transport-reaction modeling of pore water ions involved in the mineralization processes. As the predominant of these processes, sulfate reduction can be modeled from the depth distribution of sulfate and other relevant solutes. An active organoclastic sulfate reduction is typically characterized by sulfate profiles with a concave-down shape. We use here a comprehensive database to show that half of all competent sulfate profiles have an opposite, concave-up shape, which may appear inconsistent with this concept. We also show that there is a strong discrepancy between sulfate profiles obtained by gravity coring and by IODP coring, with generally much higher sulfate flux and shallower sulfate penetration depth in the gravity cores (the term IODP here covers the international ocean drilling programs, DSDP, ODP and IODP). The discrepancy is correlated with a selectivity for coring sites in the published studies, by which gravity coring is focused towards the ocean margins and continental shelf and towards high surface water productivity and high sedimentation rate. In contrast, IODP coring is more frequently done in low sedimentation regions and is largely absent from the continental shelf. As a result, estimates of sulfate reduction in the global seabed may be biased by a selection of only IODP core data for the modeling. Furthermore, the modeling of pore water profiles shows net rates of sulfate reduction, while 35S-radiotracer experiments show the higher gross rates of the process. Estimates from 35S-radiotracer measurements suggest that dissimilatory sulfate reduction oxidizes 77 Tmol yr−1 of organic carbon globally, which is 3- to 4-fold higher than recent data based on modeling of IODP data. The potential biases by global estimates of sulfate reduction in marine sediments are discussed.

The ocean carbon pool: a vital component of the global carbon cycle

The global carbon cycle is an integral part of the Earth System. Of the land, atmosphere, and ocean components of the global carbon cycle that exchange carbon on the timescales of decades to centuries, the ocean contains more than 90% of carbon. The ocean carbon pool represents a critical component of the Earth's carbon cycle, playing a pivotal role in regulating atmospheric carbon dioxide (CO2) levels and influencing climate dynamics. The exponential increase of total anthropogenic CO2 emissions in the industrial era implies the ocean's uptake has increased exponentially, reaching 2.5 ± 0.6 Pg C yr-1 for 2009-2018. Without the ocean and land sinks, atmospheric CO2 levels would be close to 600 ppm. The ocean carbon pool comprises dissolved inorganic carbon (DIC), organic carbon, and particulate organic matter, collectively responsible for the sequestration and release of carbon into the atmosphere. Phytoplankton, the microscopic marine plants, play a fundamental role in the oceanic carbon cycle by photosynthesizing and fixing atmospheric CO2 into organic matter. This organic matter can be transferred to the deep ocean through the biological pump, further contributing to the storage of carbon in the form of sinking particles. The bulk of the global ocean margin represents a carbon sink of ~0.1-0.2 Pg C. Oceanic processes, such as ocean circulation and upwelling, help redistribute carbon from surface waters to the deep ocean. The solubility pump, which is driven by changes in temperature and salinity, also affects the solubility of CO2 in seawater. These natural processes work to mitigate the increase in atmospheric CO2 concentrations and help regulate global temperatures.

Physical mechanisms affecting phytoplankton variability along the Chilean coast

Chile has high phytoplankton production due to being a classic example of an Eastern Boundary Upwelling System. Monthly averaged chlorophyll-α (Chl) and physical parameters (sea surface temperature, precipitation rate, southerly and westerly winds) were studied off the Chilean coast from 2002 to 2018, in order to understand the primary production along this important ocean margin. The coastal margin was split into three zones and ten sub-sections. The Northern Zone had a low phytoplankton production with small seasonal variability, except in its north. This pattern is due to a narrow shelf, weak winds, lack of precipitation and relatively stable weather conditions driven by the Southeast Pacific Subtropical Anticyclone (SPSA). The Central Zone presented a seasonally varying production, with a high Chl concentration in summer and early spring. This is linked to the SPSA movement and sunlight reduction during the winter. A high Chl activity is seen in the Southern Zone despite this Zone being at the limits of the SPSA effect, leading to weak longshore winds only during the warm season. Overall, this study has demonstrated the importance of shelf width and the upwelling driven by the presence or absence of the SPSA for ocean primary production. Thus, the most productive region is from 35°S to 45°S owing to both variables being present.

Satellite-estimated air-sea CO2 fluxes in the Bohai Sea, Yellow Sea, and East China Sea: Patterns and variations during 2003–2019

The Bohai Sea (BS), Yellow Sea (YS), and East China Sea (ECS) together form one of the largest marginal sea systems in the world, including enclosed and semi-enclosed ocean margins and a wide continental shelf influenced by the Changjiang River and the strong western boundary current (Kuroshio). Based on in situ seawater pCO2 data collected on 51 cruises/legs over the past two decades, a satellite retrieval algorithm for seawater pCO2 was developed by combining the semi-mechanistic algorithm and machine learning method (MeSAA-ML-ECS). MeSAA-ML-ECS introduced semi-analytical parameters, including the temperature-dependent seawater pCO2 (pCO2,therm) and upwelling index (UISST), to characterise the combined effect of atmospheric CO2 forcing, thermodynamic effects, and multiple mixing processes on seawater pCO2. The best-selected machine learning algorithm is XGBoost. The satellite-derived pCO2 achieved excellent performance in this complicated marginal sea, with low root mean square error (RMSE = 20 μatm) and mean absolute percentage deviation (APD = 4.12 %) for independent in situ validation dataset. During 2003–2019, the annual average CO2 sinks in the BS, YS, ECS, and entire study area were 0.16 ± 0.26, 3.85 ± 0.68, 14.80 ± 3.09, and 18.81 ± 3.81 Tg C/yr, respectively. Under continuously increasing atmospheric CO2 concentration, the BS changed from a weak source to a weak sink, the YS experienced interannual fluctuations but did not show significant trend, while the ECS acted as a strong sink with CO2 absorption increased from ∼10 Tg C in 2003 to ∼19 Tg C in 2019. In total, CO2 uptake in the entire study area increased by 85 % in 17 years. For the first time, we present the most refined variation in the satellite-derived pCO2 and air-sea CO2 flux dataset. These complete ocean carbon sink statistics and new insights will benefit further research on carbon fixation and its potential capacity.

Spatial and seasonal dynamics of biogenic silica in a eutrophic marginal sea, the East China Sea

Biogenic silica (BSi) is a key component of the marine silicon cycle, which is mainly driven by diatom metabolism notably contributing to primary production and export of particulate organic carbon (POC), particularly in ocean margins. Based on a large data set collected in the East China Sea (ECS), including BSi, fucoxanthin (Fuco), POC, and total suspended matter (TSM), we systematically explored the distribution and control of BSi in a typical eutrophic marginal sea. Spatially, BSi concentrations generally decreased from the shelf to the slope during all seasons, because the former is largely fed by river plumes and/or coastal currents enriched in nutrients favoring diatom growth. Abundant BSi was also observed in nearshore bottom waters probably influenced by sediment resuspension indicated by high TSM concentrations. Seasonally, BSi concentrations were on average higher in summer and autumn than in spring and winter, which reflects elevated diatom productivity during warm seasons. The BSi standing stock in the shallow water column of the ECS was significantly correlated with that of Fuco demonstrating diatoms’ dominant control on BSi dynamics. In addition, significant relationships between BSi and POC standing stocks were observed in summer and autumn, indicating the major role of diatoms in C fixation. By comparing with the northern South China Sea (NSCS), we suggested relatively small seasonal variability of BSi on the ECS shelf but significant decrease during cold seasons in the ECS slope. In the latter case, diatoms are ecologically and biogeochemically more important and more sensitive to the changing physical and/or chemical conditions than in the oligotrophic NSCS slope.