Oxygen Minimum Zone Research Articles

Seasonal intensification of oxygen minimum zone: linking Godavari River discharge to fall hypoxia in the Bay of Bengal

IntroductionThis study investigates the biogeochemical impact of Godavari River discharge (GRD) on the Bay of Bengal (BoB), focusing on the formation of an intense and shallow oxygen minimum zone (OMZ) near the river mouth during the fall season. Unlike the BoB’s typical intermediate-depth OMZ, this subsurface (~40-200 m) phenomenon is attributed to the interplay of GRD-driven nutrient enrichment, coastal upwelling, enhanced productivity, and subsequent organic matter decomposition.Data and MethodsOur analysis using the Biogeochemical-Argo floats and World Ocean Atlas 2018 data reveals that a clear shoaling and intensification of the OMZ in the fall season. Further, a comparative analysis at two geographically distinct locations highlighted the pivotal role of GRD.Results, Discussion, and ImplicationsThe location directly influenced by GRD exhibited significantly higher chlorophyll-a blooms, net primary production during the southwest monsoon, and pronounced oxygen consumption during the fall compared to the other. Our analysis suggests that GRD fuels primary productivity, leading to organic matter abundance and intense oxygen depletion in the subsurface layers, driving the observed shallow OMZ. Understanding the complex interplay between GRD, stratification, upwelling, and biogeochemical processes is crucial for predicting the impact of altered riverine inputs on coastal ecosystems, greenhouse gas emissions, and the overall health of the coastal BoB.

Deep-ocean macrofaunal assemblages on ferromanganese and phosphorite-rich substrates in the Southern California Borderland.

Mineral-rich hardgrounds, such as ferromanganese (FeMn) crusts and phosphorites, occur on seamounts and continental margins, gaining attention for their resource potential due to their enrichment in valuable metals in some regions. This study focuses on the Southern California Borderland (SCB), an area characterized by uneven and heterogeneous topography featuring FeMn crusts, phosphorites, basalt, and sedimentary rocks that occur at varying depths and are exposed to a range of oxygen concentrations. Due to its heterogeneity, this region serves as an optimal setting for investigating the relationship between mineral-rich hardgrounds and benthic fauna. This study characterizes the density, diversity, and community composition of macrofauna (>300 μm) on hardgrounds as a function of substrate type and environment (depth and oxygen ranges). Rocks and their macrofauna were sampled quantitatively using remotely operated vehicles (ROVs) during expeditions in 2020 and 2021 at depths above, within, and below the oxygen minimum zone (OMZ). A total of 3,555 macrofauna individuals were counted and 416 different morphospecies (excluding encrusting bryozoans and hydrozoans) were identified from 82 rocks at depths between 231 and 2,688 m. Average density for SCB macrofauna was 11.08 ± 0.87 ind. 200 cm-2 and mean Shannon-Wiener diversity per rock (H'[loge]) was 2.22 ± 0.07. A relationship was found between substrate type and macrofaunal communities. Phosphorite rocks had the highest H' of the four substrates compared on a per-rock basis. However, when samples were pooled by substrate, FeMn crusts had the highest H' and rarefaction diversity. Of all the environmental variables examined, water depth explained the largest variance in macrofaunal community composition. Macrofaunal density and diversity values were similar at sites within and outside the OMZ. This study is the first to analyze the macrofaunal communities of mineral-rich hardgrounds in the SCB, which support deep-ocean biodiversity by acting as specialized substrates for macrofaunal communities. Understanding the intricate relationships between macrofaunal assemblages and mineral-rich substrates may inform effects from environmental disruptions associated with deep-seabed mining or climate change. The findings contribute baseline information useful for effective conservation and management of the SCB and will support scientists in monitoring changes in these communities due to environmental disturbance or human impact in the future.

Sedimentary organic matter signature hints at the phytoplankton-driven biological carbon pump in the central Arabian Sea

Abstract. The central Arabian Sea, a unique tropical basin, is profoundly impacted by monsoon wind reversal affecting its surface circulation and biogeochemistry. Phytoplankton blooms associated with high biological productivity and particle flux occur in the northern part of the central Arabian Sea due to summer-monsoon-induced open-ocean upwelling and winter convection. The core oxygen minimum zone (OMZ) at intermediate water depths is another important feature of the northern central Arabian Sea and fades southward. In this study, we attempt to interlink how these factors collectively impact phytodetrital export to the sediment. Short sediment core-top (1 cm) samples representing the recent particle flux signatures were analysed from five locations (21 to 11° N; 64° E) in the central Arabian Sea. Previously, we used core-top (0–0.5 cm) samples and observed a trend between diatom frustule abundance and diversity with bulk sedimentary parameters indicating a spatial variability in phytodetrital export to the sediment. To verify this observation further, lipid biomarkers of key phytoplankton groups and a sea surface temperature (SST) proxy have been analysed in addition to diatom frustules. The C37 alkenone-based SST proxy indicated cooler SST (27.6 ± 0.25 °C) in the north (21–15° N) mostly due to upwelling (summer) and convective mixing (winter). Warmer SSTs (+0.4 °C) are measured in the south, which usually remains nutrient-poor. This trend was consistent with satellite-derived average SST values (2017–2020). Lipid biomarker analysis suggests that dinoflagellates were likely to be the highest contributor, as indicated by dinosterol and its degradative product dinostanol, followed by brassicasterol and C37 alkenone, likely representing diatoms and coccolithophores, respectively. The north, which largely experiences periodic phytoplankton blooms and is influenced by the thick OMZ, revealed the highest contents of organic matter, diatom frustules (diversity and abundance), dominated by large, thickly silicified cells (e.g. Coscinodiscus and Rhizosolenia) and phytoplankton lipid biomarkers, as well as lower contents of zooplankton biomarkers (cholesterol and cholestanol). In contrast, relatively smaller chain-forming centric (e.g. Thalassiosira) and pennate (e.g. Pseudo-nitzschia, Nitzschia, Thalassionema) diatom frustules along with lower phytoplankton lipid biomarker contents were found in the south, where zooplankton biomarkers and silicious radiolarians were more abundant. The possible impacts of the OMZ on particle flux related to the phytoplankton community, including zooplankton grazing and other factors, have been discussed.

The formation and ventilation of an oxygen minimum zone in a simple model for latitudinally alternating zonal jets

Abstract. An advection–diffusion model coupled to a simple dynamical ocean model is used to illustrate the formation and ventilation of an oxygen minimum zone. The advection–diffusion model carries a passive tracer with a source at the western boundary and a Newtonian damping term to mimic oxygen consumption. The dynamical model is a non-linear one-and-a-half-layer reduced-gravity model. The latter is forced by an annually oscillating mass flux confined to the near-equatorial band that, in turn, leads to the generation of mesoscale eddies and latitudinally alternating zonal jets at higher latitudes. The model uses North Atlantic geometry and develops a tracer minimum zone remarkably similar in location to the observed oxygen minimum zone in the eastern tropical North Atlantic (ETNA). This is despite the absence of wind forcing for a subtropical gyre and the shadow zone predicted by the ventilated thermocline theory. Although the model is forced only at the annual period, the model nevertheless exhibits decadal and multidecadal variability in its spun-up state. The associated trends are comparable to observed trends in oxygen within the ETNA oxygen minimum zone. Notable exceptions are the multi-decadal decrease in oxygen in the lower-oxygen minimum zone and the sharp decrease in oxygen in the upper-oxygen minimum zone between 2006 and 2013.

Coastal <i>Synechococcus</i> strains can exploit low oxygen habitats

We found that PhycoErythrin-rich <i>Synechococcus</i> achieved faster growth rates (µ), across the spectral bandwidths from 405 – 730 nm, under 2.5 µM [O<sub>2</sub>], characteristic of Oxygen Minimum Zones (OMZs), than under 250 µM [O<sub>2</sub>], whereas PhycoCyanin-rich strain showed generally similar µ under 2.5 and 250 µM [O<sub>2</sub>]. For PhycoCyanin- and PhycoErythrin-rich <i>Synechococcus</i>, µ showed also positive linear responses to both Phycobiliproteins:Chlorophyll <i>a</i>, and to cumulative diel PSII electron flux, although the relations vary across strain and [O<sub>2</sub>]. Electron transport downstream of Photosystem II was generally higher for both PhycoCyanin- and PhycoErythrin-rich strains under 250 µM [O<sub>2</sub>], since cyanobacteria show strong capacity for electron flow away from PSII to O<sub>2</sub>, particularly under excess excitation. Even though electron transport was faster under 250 µM [O<sub>2</sub>], the PhycoErythrin-rich strain showed a higher growth yield of electron transport under 2.5 µM [O<sub>2</sub>]. PhycoErythrin-rich <i>Synechococcus</i> are currently typically found at greater depths, and lower light, than are PhycoCyanin-rich strains, but we suggest that the PhycoErythrin-rich strains are actually limited to lower light by an interaction between light and full air-saturated [O<sub>2</sub>]. In expanding Oxygen Minimum Zones PhycoErythrin-rich strains will likely exploit higher light niches, across a wider spectral range.

The First Report of Late Valanginian–Early Aptian (Early Cretaceous) Ammonites and Chemostratigraphy of Eastern Anatolian, Olur‐Erzurum, Türkiye

ABSTRACTThis study is based on stable isotope analysis of a rich ammonite assemblage from the Sogukcam Formation, which is widely exposed in Yesilbaglar (Olur‐Erzurum, NE Türkiye). The presence of ammonid, planktic, and benthic foraminifers in the marine sediments suggests that they are from the early Cretaceous period. The late Valanginian‐early Aptian‐rich ammonite assemblage includes the following genera and species: Acrioceras sp., Barremites difficilis, Barremites sp., Crioceratites duvalii, Crioceratites sp., Deshayesites aff. dechyi, Deshayesites sp.1, Deshayesites sp.2, Deshayesites sp.3, Deshayesites sp., Dufrenoyia cf. dufrenoyi, Dufrenoyia cf. furcata, Dufrenoyia sp., Hemihoplites sp., Heteroceras sp., Neocomites sp., Phyllopachyceras infundibulum, Protetragonites cf. quadrisulcatus, Protetragonites sp., Ptychoceras sp., Turkmeniceras cf. geokerense, and Turkmeniceras sp. This assemblage suggests that the Sogukcam Formation's deeper facies formed in the late Valanginian to early Aptian period. Fossils and δ18O data show normal paleosalinity levels. During the late Valanginian‐early Aptian period, δ18O values ranged from −3.67‰ to −2.24‰, with paleotemperatures ranging from 21.3°C to 27.9°C. δ13C positive values range from +0.65‰ to +2.86‰. δ13C isotope data show the oxygen minimum zone (OMZ) and changes in sea level and productivity. The presence of planktonic foraminifera and the ammonite assemblage indicates that the formation was deposited in warm subtropical waters ranging from the outer shelf to the open sea.

Iron redox shuttling and uptake by silicate minerals on the Namibian mud belt

Anoxic marine sediments represent an important source of bioavailable iron (Fe) to the ocean. The highest sedimentary Fe fluxes are observed in open-marine oxygen minimum zones where anoxic bottom waters are in contact with ferruginous surface sediments. Here, sedimentary Fe release, transport and re-deposition (i.e., Fe shuttling) may generate a lateral pattern of sedimentary Fe enrichment and depletion, which can be used to trace redox-related Fe mobility in the paleo-record. However, depending on the balance between terrigenous and authigenic (i.e., shuttle-related) Fe flux, the stability of bottom water redox conditions as well as post-depositional processes of mineral alteration, the sedimentary fingerprint of an Fe redox shuttle may be obscured and difficult to identify.We investigated sedimentary Fe cycling along two transects across the Namibian mud belt with variable terrigenous sedimentation (23°S < 25°S) and during two seasons with opposing bottom water redox conditions (oxic in austral winter versus anoxic to sulfidic in austral summer). On both transects, substantial benthic Fe fluxes up to −50 µmol m−2 d−1 were inferred based on pore water profiles. The magnitude of these fluxes is comparable to those reported for other open-marine oxygen minimum zones. On the transect at 23°S, Fe source areas with ferruginous surface sediments were clearly separated from Fe sink areas with highly sulfidic surface sediments. Therefore, Fe redox shuttling was reflected by a lateral pattern of reactive Fe depletion and enrichment relative to the terrigenous background sedimentation. By contrast, on the transect at 25°S, benthic Fe fluxes were temporally and spatially more variable and surface sediments were ferruginous or only weakly sulfidic. Therefore, sedimentary Fe depletion and enrichment was less pronounced at 25°S. In the Fe sink area at 23°S, hydrogen sulfide was present at the sediment surface during both sampling campaigns and solid phase data suggest that Fe sulfide minerals represented the main burial phase of reactive Fe. By contrast, at 25°S excess Fe was associated with potassium (K) rather than reduced sulfur. While a differing sediment provenance cannot be ruled out entirely, combined evidence from pore water silica profiles, K to Fe stoichiometric relationships and electron microprobe images suggest that laterally derived excess Fe was incorporated into pre-existing and/or authigenic clay minerals during early diagenesis. Iron uptake by clay minerals may be supported by frequent redox oscillations and sediment mixing preventing preservation of Fe sulfide minerals and promoting Fe and K fixation in clay minerals. The burial fluxes of excess Fe associated with sulfide minerals at 23°S and silicate minerals at 25°S were similar. Our findings thus underscore that neoformation or alteration of silicate minerals can be important processes within the low-temperature marine Fe cycle.

Mathematical model of oxygen minimum zones in the vertical distribution of oxygen in the ocean.

Processes determining the amount and spatial distribution of dissolved oxygen in the ocean have been a focus of intense research over the last two decades. Anomalies known as Oxygen Minimum Zones (OMZs) have been attracting growing attention, in particular because their growth is believed to be a result of the global environmental change. Comprehensive understanding of factors contributing to and/or controlling the emergence and evolution of OMZs is still lacking though. OMZs are usually thought to result from an interplay between the oxygen transport through the water column from the ocean surface and variable oxygen solubility at different water temperature. In this paper, we suggest a different, novel mechanism of the OMZ formation relating it to the oxygen production in phytoplankton photosynthesis in a stratified ocean. We consider a simple, conceptual model of the coupled phytoplankton-oxygen dynamics and show that the model predictions are in qualitative agreement with some relevant field observations.

Studies of Hydrogen Sulfide Contamination of the Deep-Water Basin of the Middle Caspian Sea During Cruise of the R/V “Issledovatel Kaspiya” September 2022

Results of the comprehensive studies of hydrological and chemical structure of the Middle and Northern Caspian Sea carried out aboard r/v “Issledovatel Kaspiya” in September 2022 are presented. It is shown, despite the continuing decrease in sea level, there is no aeration of the deep layers, the hydrogen sulfide layer persists with a tendency to increase the concentration of hydrogen sulfide in the bottom layer. There is an increase of the oxygen minimum zone. The changes of the Caspian Sea ecosystem have led to a weakening of the removal of nutrients from the photic layer into the bottom layers and, as a consequence, a decrease in the concentration of silicon in the bottom layer. The results obtained will allow to evaluate the trends of ongoing changes.

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.

Potential effects of the Emeishan large igneous province on Capitanian marine anoxia in the Upper Yangtze region

The widespread Capitanian (late Guadalupian) marine anoxia/euxinia has long been regarded as a key driver of the end-Guadalupian (middle Permian) biotic crisis. However, the cause of this marine anoxia is debated, particularly regarding the influence of the Emeishan large igneous province (ELIP). To investigate the contribution of the ELIP to marine anoxia and the possible causal mechanisms, we undertook a conodont biostratigraphic and geochemical study of the middle Permian Maokou Formation in a platform-to-trough transect in the Upper Yangtze region, South China. Our results show that the depositional facies of the Maokou Formation changed from a carbonate ramp to an intra-platform trough within the Jinogondolella (J.) altudaensis zone in the northwestern Yangtze region, which can be attributed to the initial activity of the ELIP. Mantle-derived Sr inputs in the initial and main stages of the ELIP led to two decreases in 87Sr/86Sr during the Capitanian, in the J. shannoni–J. altudaensis and J. prexuanhanensis–J. xuanhanensis zones. The elevated 87Sr/86Sr values during the late Capitanian may have been due to enhanced continental weathering caused by rapid climate warming in response to subaerial eruptions of the ELIP. The deep-water anoxia–euxinia expanded during the middle Capitanian, as indicated by increased MoEF/UEF and V/(V + Ni) values, along with the disappearance of burrows and appearance of small pyrite framboids in the J. altudaensis zone in the Cheng 20 well. However, shallow-water anoxia occurred during the late Capitanian (i.e., J. prexuanhanensis–J. xuanhanensis zone), as evidenced by positive Ce anomalies and losses of aerobic benthic species in the Erya section. Importantly, marine anoxia and negative δ13Ccarb excursions occurred synchronously, but earlier in deep water than in shallow water, potentially indicating an expansion of the oxygen minimum zone (OMZ). The deep-water anoxia corresponded to a decrease in 87Sr/86Sr ratios and the appearance of an intra-platform trough in the J. altudaensis zone, whereas the shallow-water anoxia in the late Capitanian coincided with elevated 87Sr/86Sr ratios. This suggests that the initial activity of the ELIP promoted the development of the OMZ in deep waters during the middle Capitanian, while the subaerial eruptions of the ELIP drove climate warming that led to the expansion of the OMZ into shallow-water platforms during the late Capitanian.

North Atlantic temperature control on deoxygenation in the northern tropical Pacific

Ocean oxygen content is decreasing with global change. A major challenge for modelling future declines in oxygen concentration is our lack of knowledge of the natural variability associated with marine oxygen inventory on interannual and multidecadal timescales. Here, we present 10 annually resolved 200 year-long records of denitrification, a marker of deoxygenation, from a varved sedimentary archive in the North Pacific oxygen minimum zone covering key periods over the last glacial–interglacial cycle. Spectral analyses on these records reveal strong signals at periodicities typical of today’s Atlantic multidecadal oscillation. Modern subsurface circulation reanalyses regressed on the positive Atlantic and Pacific Climatic Oscillation indices further confirm that North Atlantic temperature patterns are the main control on the subsurface zonal circulation and therefore the most likely dominant driver of oxygen variability in the tropical Pacific. With currently increasing temperatures in the Northern Hemisphere high latitudes and North Atlantic, we suggest deoxygenation will intensify in the region.

Size fractionation informs microbial community composition and interactions in the eastern tropical North Pacific Ocean

Abstract Marine microorganisms are drivers of biogeochemical cycles in the world's oceans, including oxygen minimum zones (OMZs). Using a metabarcoding survey of the 16S rRNA gene, we investigated prokaryotic communities, as well as their potential interactions with fungi, at the coastal, offshore, and peripheral OMZ of the eastern tropical North Pacific. Water samples were collected along a vertical oxygen gradient, and large volumes were filtered through three size fractions, 0.22 μm, 2 μm, and 22 μm. The changes in community composition along the oxygen gradient were driven by Planctomycetota, Bacteroidota, Verrucomicrobiota, and Gammaproteobacteria; most are known degraders of marine polysaccharides and usually associated with the large particle-associated community. The relative abundance of Nitrososphaerota, Alphaproteobacteria, Actinomycetota, and Nitrospinota were high in free-living and small particle-associated communities. Network analyses identified putative interactions between fungi and prokaryotes in the particle-associated fractions, which have been largely overlooked in the ocean. In the small particle-associated network analysis, fungal ASVs had exclusively negative connections with SAR11 nodes. In the large particle-associated network analysis, fungal ASVs displayed both negative and positive connections with Pseudomonadota, SAR324, and Thermoplasmatota. Our findings demonstrate the utility of three-stage size-fractioned filtration in providing novel insights into marine microbial ecology.

Arctic Alaska deepwater organic carbon burial and environmental changes during the late Albian–early Campanian (103–82 Ma)

The middle Cretaceous greenhouse period experienced profound environmental change including episodes of enhanced global burial of organic carbon marked by carbon isotopic excursions (CIEs). However, the role and response of polar regions like the newly formed, partially enclosed Arctic Ocean Basin during middle Cretaceous carbon burial remains enigmatic. We present the first Arctic deepwater CIE record that characterizes conditions offshore of the Alaska margin north of 75°N paleolatitude. Organic carbon isotopes (δ13Corg) and 103–82 Ma ash zircon U-Pb dates from the distal Hue Shale record multiple Albian–Campanian CIEs during slow ∼3–15 m/Myr sediment accumulation rates. Average total organic carbon (TOC) increased substantially during large 2–3 ‰ CIEs of the ∼101 Ma Albian-Cenomanian boundary event (from 7 to 18 % TOC) and ∼94 Ma Cenomanian-Turonian boundary event (5 to 10 % TOC). Turonian TOC remained elevated (8–13 %) during high global sea levels and temperatures of the Cretaceous Thermal Maximum, followed by an increase from 7 to 11 % TOC during the ∼90 Ma late Turonian event 1.5 ‰ CIE. Average TOC subsequently decreased in the Coniacian–Campanian, but relative maxima occurred during subtle 0.5–1 ‰ CIEs interpreted as the ∼87 Ma late Coniacian event (increase from 4 to 7 % TOC), ∼85 Ma Horseshoe Bay event (3.5 to 4.5 % TOC), and ∼84 Ma Santonian-Campanian boundary event (3.5 to 5 % TOC). Increases in hydrogen index and productivity proxies (P, Ba, Nd) that accompanied each CIE episode with enhanced TOC suggest a strong link between marine productivity and organic carbon burial at short-term CIE timescales. However, long-term (>5–8 Myr) changes in trace metal redox (Mo, Fe, V) and salinity (B/Ga) proxies suggest shifts in prevailing environmental conditions at timescales longer than the CIEs. Late Albian–middle Turonian marine salinity occurred during euxinic (103–98 Ma) and suboxic (98–90 Ma) conditions with deposition interpreted to have occurred within and beneath an oxygen minimum zone, respectively. In contrast, late Turonian–early Campanian (90–82 Ma) freshening and restricted euxinic basin conditions may signal the start of widespread restriction known to characterize the Paleogene Arctic. Overall, these results highlight that middle Cretaceous Arctic deepwater remained a productive marine carbon sink coupled to the global carbon cycle despite evolving Arctic greenhouse conditions.

Geographic distribution of modern dinoflagellate cysts in surface sediments of the Arabian Sea: significance of taxa and morphometry as potential ecological indicators.

Studies on dinoflagellate cysts in the Arabian Sea (AS) are limited to the coastal waters, but no information from the deeper depths. The dinoflagellate cyst assemblages in surface sediment samples (0-2cm) from the deeper depths (up to ~ 4500m) of central (oxygen minimum zone (OMZ)) and southeastern (oligotrophic) AS revealed that the relatively good numbers of cyst concentrations reach deeper depths of OMZ (3505m) and oligotrophic (4368m) regions, but the former harbored more cyst concentrations than the latter. The cyst concentration and species count (including HAB species) recorded here are lower compared to the eastern (EAS) and western (WAS) AS, but the autotrophic cyst dominance (74-83%) at deeper depths is in contrast with the heterotrophic dominance in coastal AS. Of the recorded 41 cyst species (belonging to 18 genera), four species (cyst of Cladopyxis sp., Operculodinium janduchenei, Stelladinium bifurcatum, and Protoperidinium monospinum) from the deepest part of oligotrophic AS form the first report. In contrast, Spiniferites and Lingulodinium cysts were common occurrences. Taxonomic comparison with literature revealed (i) the prevalence of more cosmopolitan species (32 species) which could be due to the prevalence of large and small-scale lateral transport of cysts in oligotrophic regions followed by OMZ and coastal regions, respectively, and (ii) very few region-specific species, i.e., cyst of Protoperidinium latissimum, Lejeunecysta sabrina, cyst of Protoperidinium denticulatum in EAS and Impagidinium patulum, and I. strialatum, in WAS. Interestingly, variability in the morphometry was evident between the coastal and open oceans in some cosmopolitan cysts, e.g., Operculodinium centrocarpum and Lingulodinium machaerophorum. These findings from the less studied pelagic regions will contribute to the growing knowledge of dinoflagellate cyst distribution patterns and highlight the significance of cyst taxa and morphology as potential ecological indicators for AS.

Metagenomic insights into bacterial dynamics and niche partitioning in response to varying oxygen gradients in the Arabian Sea oxygen minimum zone (OMZ)

This study investigates the impact of oxygen gradients on bacterial diversity and community composition within the Arabian Sea oxygen minimum zone (OMZ), providing insight into their ecology in this unique environment. Vertical profiling of physicochemical parameters and bacterial community composition was conducted at two stations within the OMZ using Illumina sequencing of the V3-V4 regions of the 16S rRNA gene. The oxygen levels in the water column of the study locations varied from oxic to hypoxic and Station-I exhibited intense anoxic conditions. Bacterial diversity analyses revealed a rich assemblage of 24 bacterial phyla dominated by Proteobacteria, with significant presence of Actinobacteria and Planctomycetes. SIMPER (Similarity percentage) analysis highlights taxa such as Synechococcophycideae, Flavobacteriia in oxic water and Phycisphaerae as significant contributors to community dissimilarity in oxygen-deficient water, emphasizing their ecological roles in nutrient cycling and the impact of oxygen availability on marine biodiversity. Canonical Correspondence Analysis (CCA) further elucidates the relationships between bacterial communities and environmental variables, with oxygen, temperature, and nutrient concentrations playing crucial roles in shaping bacterial community distributions. This research enhances our understanding of microbial ecology in the Arabian Sea OMZ by revealing how microbial communities respond to changing oxygen conditions, providing insights into the ecological dynamics of the region. Furthermore, these findings underscore the need for continued research to predict and mitigate the effects of environmental changes on marine ecosystems, particularly in the context of expanding OMZs and global biogeochemical cycles.

Episodic intensification of marine phosphorus burial over the last 80 million years

Marine phosphatization events cause episodic carbonate fluorapatite (CFA) precipitation on seamounts, and are commonly linked to growth hiatuses in ferromanganese (Fe-Mn) crusts. However, the complete record of these events and their paleoenvironmental significance remains poorly understood, in large part due to poor age constraints. Here, we apply U-Pb dating to CFA in Fe-Mn crusts from Western Pacific seamounts. These data exhibit good alignment with Sr isotope ages, revealing six potential phosphatization events. This established CFA chronology tightens the timespan of phosphatization events and refines the age framework of Fe-Mn crusts. We subsequently utilize a multiproxy approach to demonstrate that the phosphatization events occurred coeval with the expansion of oceanic oxygen minimum zones. The Western Pacific Fe-Mn crusts thus document major perturbations in global oceanic phosphorus cycling, which appear to have been driven by climate-induced increases in primary productivity linked to changes in global ocean circulation.

Distribution patterns of micronektonic crustaceans (Decapoda, Euphausiacea, and Lophogastrida) in the tropical and subtropical Atlantic Ocean

Large pelagic crustaceans are a main component of the micronekton community in the deep-sea having an important role in the food webs and the biological carbon pump. However, they are scarcely studied in comparison to other groups such as mesopelagic fish. Here, we analyse day/night and bathymetric variability in taxonomic composition, abundance, and biomass across a latitudinal transect in the Atlantic Ocean from off Brazil (15°S) to the Canary Islands (25°N). A total of 95 species were identified belonging to 9 different families, of which Euphausiidae was the most abundant family and Acanthephyridae the family contributing the most to the total biomass. We found distinct assemblages associated with Atlantic ecoregions related to the environmental variables. Diel vertical migrations were detected along the entire transect, even crossing the oxygen minimum zone, likely due to the metabolic adaptations of these organisms.

Nitrite-oxidizing bacteria adapted to low-oxygen conditions dominate nitrite oxidation in marine oxygen minimum zones.

Nitrite is a central molecule in the nitrogen cycle because nitrite oxidation to nitrate (an aerobic process) retains fixed nitrogen in a system and its reduction to dinitrogen gas (anaerobic) reduces the fixed nitrogen inventory. Despite its acknowledged requirement for oxygen, nitrite oxidation is observed in oxygen-depleted layers of the ocean's oxygen minimum zones (OMZs), challenging the current understanding of OMZ nitrogen cycling. Previous attempts to determine whether nitrite-oxidizing bacteria in the anoxic layer differ from known nitrite oxidizers in the open ocean were limited by cultivation difficulties and sequencing depth. Here, we construct 31 draft genomes of nitrite-oxidizing bacteria from global OMZs. The distribution of nitrite oxidation rates, abundance and expression of nitrite oxidoreductase genes, and relative abundance of nitrite-oxidizing bacterial draft genomes from the same samples all show peaks in the core of the oxygen-depleted zone (ODZ) and are all highly correlated in depth profiles within the major ocean oxygen minimum zones. The ODZ nitrite oxidizers are not found in the Tara Oceans global dataset (the most complete oxic ocean dataset), and the major nitrite oxidizers found in the oxygenated ocean do not occur in ODZ waters. A pangenomic analysis shows the ODZ nitrite oxidizers have distinct gene clusters compared to oxic nitrite oxidizers and are microaerophilic. These findings all indicate the existence of nitrite oxidizers whose niche is oxygen-deficient seawater. Thus, specialist nitrite-oxidizing bacteria are responsible for fixed nitrogen retention in marine oxygen minimum zones, with implications for control of the ocean's fixed nitrogen inventory.

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.