Oxygen Minimum Zone Research Articles

Reconstruction of the Paleo‐Ocean Environment Using Mineralogical and Geochemical Analyses of Mixed‐Type Ferromanganese Nodules From the Tabletop of Western Pacific Magellan Seamount

AbstractVariations in the geochemistry and mineralogical compositions of ferromanganese (Fe‐Mn) nodules are closely related to variations in environmental parameters. Therefore, analysis of Fe‐Mn nodules can reconstruct the paleo‐ocean environment. Here, three differently shaped Fe‐Mn nodules were collected from the open seamount 9‐1 of the western Pacific Magellan Seamount. Geochemical and mineralogical analyses were conducted using micro X‐ray fluorescence (μ‐XRF) and Synchrotron X‐ray powder diffraction to reconstruct the paleo‐ocean environment. We found that the shape of the three nodules was different; however, the variations in their geochemical and mineralogical properties were similar. For all three nodules, the μ‐XRF elemental distribution revealed a distinct alternation between the Mn‐rich and Fe‐rich layers. The Mn‐rich and Fe‐rich layers surrounded the nuclei and appeared as concentric circles. Fe‐rich and Mn‐rich layers exhibited low Mn/Fe ratios (<2.5) and were dominated by vernadite, whereas layers with high Mn/Fe ratios (>2.5) were dominated by todorokite, respectively. These findings suggest an early diagenesis process at tabletop of the Magellan Seamount. The high contents of Ca and P and the existence of carbonate fluorapatite in the center of Fe‐Mn nodules indicate that the formation of these nodules began during the Miocene phosphatization event (19–16 Ma). In the early stages of nodule formation, a diagenetic process drove Fe‐Mn nodule formation. Because of global cooling from 9 Ma, early diagenesis was weakened, after which the formation of hydrogenetic Fe‐Mn nodules became dominant. As a result, variations in oxygen minimum zones and oxic‐suboxic fronts were recorded in Fe‐Mn nodules from tabletop of the seamount.

Coastal N2 Fixation Rates Coincide Spatially With Nitrogen Loss in the Humboldt Upwelling System off Peru

AbstractMarine nitrogen (N2) fixation supports significant primary productivity in the global ocean. However, in one of the most productive regions of the world ocean, the northern Humboldt Upwelling System (HUS), the magnitude and spatial distribution of this process remain poorly characterized. This study presents a spatially resolved data set of N2 fixation rates across six coastal transects of the northern HUS off Peru (8°S–16°S) during austral summer. N2 fixation rates were detected throughout the waters column including within the Oxygen Minimum Zone (OMZ) between 12°S and 16°S. N2 fixation rates were highest where the subsurface OMZ (O2 < 20 μmol L−1) was most intense and estimated nitrogen (N) loss was highest. There, rates were measured throughout the water column. Hence, the vertical and spatial distribution of rates indicates a colocation of N2 fixation with N loss in the coastal productive waters of the northern HUS. Despite high phosphate and total dissolvable iron (TdFe) concentrations throughout the study area, N2 fixation was still generally low (1.19 ± 3.81 nmol L−1 d−1) and its distribution could not be directly explained by these two factors. Our results suggest that the distribution was likely influenced by a complex interplay of environmental factors including phytoplankton biomass and organic matter availability, and potentially iron, or other trace metal (co)‐limitation of both N2 fixation and primary production. In general, our results support previous conclusions that N2 fixation in the northern HUS plays a minor role as a source of new N and to replenish the regional N loss.

Gene loss and symbiont switching during adaptation to the deep sea in a globally distributed symbiosis

Chemosynthetic symbioses between bacteria and invertebrates occur worldwide from coastal sediments to the deep sea. Most host groups are restricted to either shallow or deep waters. In contrast, Lucinidae, the most species-rich family of chemosymbiotic invertebrates, has both shallow- and deep-sea representatives. Multiple lucinid species have independently colonized the deep sea, which provides a unique framework for understanding the role microbial symbionts play in evolutionary transitions between shallow and deep waters. Lucinids acquire their symbionts from their surroundings during early development, which may allow them to flexibly acquire symbionts that are adapted to local environments. Via metagenomic analyses of museum and other samples collected over decades, we investigated the biodiversity and metabolic capabilities of the symbionts of 22 mostly deep-water lucinid species. We aimed to test the theory that the symbiont played a role in adaptation to life in deep-sea habitats. We identified 16 symbiont species, mostly within the previously described genus Ca. Thiodiazotropha. Most genomic functions were shared by both shallow-water and deep-sea Ca. Thiodiazotropha, though nitrogen fixation was exclusive to shallow-water species. We discovered multiple cases of symbiont switching near deep-sea hydrothermal vents and cold seeps, where distantly related hosts convergently acquired novel symbionts from a different bacterial order. Finally, analyses of selection revealed consistently stronger purifying selection on symbiont genomes in two extreme habitats - hydrothermal vents and an oxygen-minimum zone. Our findings reveal that shifts in symbiont metabolic capability and, in some cases, acquisition of a novel symbiont accompanied adaptation of lucinids to challenging deep-sea habitats.

The effects of seawater thermodynamic parameters on the oxygen minimum zone (OMZ) in the tropical western Pacific Ocean

The continuous expansion of the oxygen minimum zone (OMZ) is a microcosm of marine hypoxia problem. Based on a survey in M4 seamount area of Tropical Western Pacific Ocean, the effects of thermodynamic parameters on OMZ were discussed. The study showed thermodynamic parameters mainly affect the upper oxycline of OMZ. The increase in temperature aggravates seawater stratification, which not only shallows oxycline but also increases the strength of DO stratification, promoting the expansion of OMZ. Based on relationships between thermodynamic parameters, water mass and DO, OMZ in this area is defined as follows: the water layer with low DO between the lower boundary of high-salt area and 1000 m. Moreover, the study showed that though there is no “seamount effect” on a scale of 3000 m, low-value areas of DO form at the bottom of seamount. This study will provide an evidence for expansion of OMZ exacerbated by global warming.

Deposition of cenomanian – Turonian organic-rich units on the mid-Norwegian margin: Controlling factors and regional implications

On the mid-Norwegian margin, extensive rifting and subsequent deposition of thick Cretaceous and Cenozoic sediments have buried the traditional Upper Jurassic organic-rich shales too deep. Consequently, these organic-rich shales are overmature and spent in the deep basins on the mid-Norwegian margin. The absence of well-control, variable seismic quality and in particularly, the great burial depth, makes it difficult to identify alternative Upper Jurassic and Lower Cretaceous organic-rich units. By combining high-resolution 2D seismic data, well logs, and Rock-Eval data, this study documents the presence of alternative organic-rich units in the Cretaceous succession on the Halten Terrace and the Vøring Basin. Multiple seismic horizons which correspond to regional flooding surfaces and define a series of seismic sequences have been mapped across the study area. The regionally extensive upper Cenomanian horizon is associated with wireline log signals and Rock-Eval parameters which imply the presence of a potential source rock unit. Source rock evaluation indicate that this unit contains mainly kerogen Type III on the Halten Terrace, suggesting an organofacies with significant contribution from terrestrial sources. In the Vøring Basin, the unit is sparsely drilled but appears to be mature, thus displaying a relatively limited potential. One well from the Vigrid Syncline demonstrate somewhat higher potential, with Rock-Eval data indicating a kerogen Type II composition. As such, more prolific units seems to exist in the Vøring Basin, albeit exhibiting a patchy distribution. We speculate that the deposition and preservation of this upper Cenomanian organic-rich unit record the development of an extended oxygen minimum zone attributable to increased primary production and sluggish water circulation, linked to the global Oceanic Anoxic Event 2 (OAE 2). However, local physiographic conditions, such as high sedimentation rates, erosion by gravity flows and periodically oxygenated conditions hindered preservation of a significant quantities of organic matter, thus limiting the thickness and quality of the upper Cenomanian organic-rich unit on the mid-Norwegian margin.

Microstratigraphic evidence of oceanographic and tectonic controls on hydrogenetic ferromanganese crusts in the NW Pacific seamounts

Selected hydrogenetic ferromanganese crusts were characterized at a microstratigraphic scale across various NW Pacific seamounts. The crusts encompassed over 100 sites, and were mostly found on old rock outcrops. The crusts also exhibit spatial compositional variation and are largely enriched in Co and Ni. However, the parameters controlling this compositional variation remain unclear. To address this issue, we characterized the microstructure, mineralogical and chemical compositions of seven carefully selected samples from topographic highs in the mid-latitudes on the Philippine Sea and Pacific plates and revealed the following trends: (1) the lower phosphatized layer formed in the middle Miocene or earlier and only on the Pacific Plate, (2) an increasing Co/Mn ratio with younger age from the substrate to the surface, and generally higher in the Pacific Plate than the Philippine Sea Plate, and (3) an increase of detrital quartz and plagioclase since approximately 5 Ma, with high Al/Mn ratios near the continental margin and to a lesser extent in pelagic areas. These results are partly congruent with those of other sediment cores from the other NW Pacific basins. Furthermore, the secular increase of Co and detrital minerals in the crust samples suggests a strong influence of oceanographic and tectonic conditions, especially the expansion of the oxygen minimum zone (OMZ) for the former and a significant input of detrital materials from the continents for the latter.

Taphonomy of bivalve skeletal remains as a means of detecting changes in oxygen depletion and recognizing ancient upwelling environments

Abstract Bivalve shells from shelf and slope environments of the Benguela upwelling system were examined for their state of preservation for the first time, and were used to test whether their preservation is a proxy for spatially variable seafloor taphonomic conditions that lead to different degrees of time averaging. Distinctive shell damage-profiles show increasing shell damage further from the centre of the upwelling cells affected by anoxic conditions (opal and organic-rich) and towards the carbonate- and phosphate-rich facies (with slower sedimentation rates at the margins of the cells. Discrete taphofacies range from a very low damage profile (opal and organic-rich, facies 1) to very high damage (phosphate, facies 3), with elevated numbers of rounded, fragmented and bored shells as well as very little associated sedimentary matrix. This gradient in taphofacies is associated with a gradient from census assemblages with low damage to (within-habitat time-averaged) assemblages in carbonate and (environmentally-condensed) phosphate facies of moderate and higher species richness that are associated with increasing time averaging. This pattern follows the upwelling facies gradient of high opal and organic matter in areas of most intense upwelling cells and outward to aerated but food-abundant carbonate-rich seafloors and spatially coexisting phosphorite-rich concentrations. Thus, the taphonomic gradient ranges from stressful, oxygen-depleted seafloor environment under upwelling regimes that limit metazoan predators and borers (and thus minimize destruction), to well-oxygenated seafloors with increased predators and micro-boring animals that all foster post-mortem shell destruction. The rarity of encrusters is a conspicuous feature of these facies across the whole gradient that may reflect temporary recurrence of oxygen-depleting conditions even at the carbonatic and phosphatic facies. The state of preservation of these shells can serve as a modern analogue for identifying similar settings in the geological record but also for Anthropocene settings affected by eutrophication and expanding oxygen minimum zones. Molluscan taphofacies with unusually low levels of skeletal damage from borers and encrusters can thus represent sensitive proxies of conditions affected by Anthropocene deoxygenation and eutrophication.

Evaluating future climate change exposure of marine habitat in the South East Pacific based on metabolic constraints

IntroductionOn-going climate change is now recognized to yield physiological stresses on marine species, with potentially detrimental effects on ecosystems. Here, we evaluate the prospect of using climate velocities (CV) of the metabolic index (Φ) for assessing changes in habitat in the South East Pacific.MethodsOur approach is based on a species with mean ecophysiotype (i.e. model species) and the use of a global Earth System Model simulation (CESM-LE) under RCP 8.5 scenario. The SEP is chosen as a case study as it hosts an Oxygen Minimum Zone and seamounts systems sustaining local communities through artisanal fisheries.Results and DiscussionOur results indicate that CVΦ pattern is mainly constrained by the oxygen distribution and that its sign is affected by contrasting oxygen trends (including a re-oxygenation in the upper OMZ) and warming. We further show that CVΦ is weakly dependent on physiological traits composing Φ, which conveys to this metrics some value for inferring the projected mean displacement and potential changes in viability of metabolic habitat in a region where physiological data are scarce. Based on sensitivity experiments to physiological traits and natural variability, we propose a general method for inferring broad areas of climate change exposure regardless of species-specific Φ. We show in particular that for the model used here, the upper OMZ region can be considered a “safe” area for the species with ecophysiotype close to that of 71 species used to derive the model species. Limitations of the approach and perspectives of this work are also discussed.

Trace metals as a redox proxy in Arabian Sea sediments in and below the oxygen minimum zone

Sedimentary trace metals are widely used to reconstruct past bottom water redox conditions. The calibration of trace metals as a redox proxy for oxygen minimum zones (OMZs) can still be improved. Here, we combine pore water and solid phase Mo, U, Re and V profiles with Fe and Mn data for ten sites along a bottom water oxygen (2 to 83 μmol L−1 O2) and water depth gradient (885 to 3010 m) in and below the perennial OMZ in the northern Arabian Sea (Murray Ridge). Trends in sedimentary Mo, U and Re contents generally follow ambient bottom water redox conditions, with the highest enrichments in OMZ sediments, supporting the validity of these trace metals as redox proxies. Vanadium and Fe are exclusively enriched in the sediments of the most anoxic OMZ site and do not capture further redox changes in and below the OMZ. We attribute the absence of a redox trend in sedimentary Fe content to the mildly reducing conditions in the sediments, with little FeS formation and benthic release of Fe. Manganese, in contrast, is depleted in the OMZ sediments and enriched in sediments below the OMZ, in accordance with loss from OMZ sediments and transfer of Mn to deeper sites (“Mn shuttling”). Manganese oxides are likely a key carrier of Mo and V to the sediments, especially below the OMZ, while diffusion across the sediment-water interface supplies U. Sedimentary Mo, U and V contents in the present-day Arabian Sea OMZ are generally lower than observed in other perennial OMZs. This may be related to a lower input of organic matter in this part of the Arabian Sea when compared to other OMZs, and hence, less anaerobic degradation of organic matter and less authigenic fixation of metals, even at the same bottom water oxygen concentrations. Our results have implications for the detection of OMZs in the geological record, implying that thresholds in trace metal concentrations for perennial OMZs may be lower than previously considered. Comparison of our Mo, U, Re and V data to trace metal records from 15 to 200 ka for the same Arabian Sea region suggests that the OMZ was periodically wider and more reducing in the past.

Intermediate water circulation drives distribution of Pliocene Oxygen Minimum Zones

Oxygen minimum zones (OMZs) play a critical role in global biogeochemical cycling and act as barriers to dispersal for marine organisms. OMZs are currently expanding and intensifying with climate change, however past distributions of OMZs are relatively unknown. Here we present evidence for widespread pelagic OMZs during the Pliocene (5.3-2.6 Ma), the most recent epoch with atmospheric CO2 analogous to modern (~400-450 ppm). The global distribution of OMZ-affiliated planktic foraminifer, Globorotaloides hexagonus, and Earth System and Species Distribution Models show that the Indian Ocean, Eastern Equatorial Pacific, eastern South Pacific, and eastern North Atlantic all supported OMZs in the Pliocene, as today. By contrast, low-oxygen waters were reduced in the North Pacific and expanded in the North Atlantic in the Pliocene. This spatially explicit perspective reveals that a warmer world can support both regionally expanded and contracted OMZs, with intermediate water circulation as a key driver.

Influence of the ITCZ and OMZ on the isotopic composition of suspended particulate matter in the western tropical North Atlantic

The tropical North Atlantic Ocean is a complex region where the intertropical convergence zone (ITCZ) and the oxygen minimum zone (OMZ) play important roles in the control of the physical and biogeochemical processes and affect the efficiency of the carbon pump. Changes in the composition of the particulate organic matter (POM) have been extensively studied in the eastern tropical North Atlantic and recent findings suggested less effective remineralization in the OMZ by comparison with well‑oxygenated waters. However, dynamics of the POM in the western tropical North Atlantic, including the effect of the hypoxic condition on the isotopic signature of the POM, has been poorly understood. This study uses data collected in October 2018 during the PIRATA XVIII cruise (38°W) to describe the biogeochemical variability and isotopic composition of suspended POM in the water column, with emphasis on the effect of the ITCZ and the OMZ on the POM. The ITCZ, located between 5°N and 10°N, was responsible for the low sea surface salinity and local upwelling, thus favoring the high biological productivity found in this band. Although a clear relation between δ13CPOC values and the ITCZ position was not found, low δ15NTPN values (∼2‰) and high silicate concentration observed on the surface between 4°N-6°N suggested that dust deposition and its associated enhancement of the diazotrophic activity may be important in the southernmost part of the ITCZ. Preferential remineralization of fractions rich in 13C (proteins, carbohydrates) and nitrogen compounds resulted in low δ13CPOC and high C:N values observed at depth. However, these alterations were attenuated in the OMZ, suggesting that reduced remineralization expected in waters with low oxygen concentrations may also affect the isotopic POM signature. Additionally, the minimum δ15NTPN value (∼1‰) observed in the core of the OMZ confirms that, although microbial activity may be important in this region, nitrogen-loss processes are not predominant in the western tropical Atlantic.

Dissolved trace metals (Fe, Mn, Pb, Cd, Cu, and Zn) in the eastern Indian Ocean

The distribution of trace metal micronutrients, which are crucial in many biotic enzymatic reactions, in the eastern Indian Ocean is not well understood. Here, we measured the concentrations of dissolved iron (dFe), manganese (dMn), lead (dPb), cadmium (dCd), copper (dCu), and zinc (dZn) at full depth in the water column of the eastern Indian Ocean (from the Bay of Bengal to the south of the equator). Throughout, dFe was replete in the surface mixed layer (SML). Moreover, the supply of dFe from below the SML and atmospheric deposition met the phytoplankton demand. In the thermocline, the subsurface maxima of dFe coincided with the dissolved oxygen (DO) minima. In addition, the distribution of Fe* in the Bay of Bengal indicated an additional input of dFe into the water column, apart from the biogenic organic particle regeneration. Unlike dFe, other trace metals were not enriched in the oxygen minimum zone, indicating that high Fe* value was caused by the release of dFe from the estuary or continental shelf under oxic conditions. In deep water, dFe, dMn, and dPb levels were low, whereas dCd, dCu, and dZn concentrations increased with depth. However, dFe and dMn levels were elevated in coastal bottom waters, where Fe* values were also high, indicating the supply of these metals from external sources. Exploring the relationship between trace metals and phosphate at the thermocline illustrated how oxygen-deficient conditions in the Bay of Bengal affect trace metal (dCd in particular) and phosphate ratios, compared with the global trend.

Diversity and Quantitative Detection of Clade I Type nosZ Denitrifiers in the Arabian Sea Oxygen Minimum Zone.

A significant amount of nitrous oxide (N2O) is effluxed into the atmosphere as a result of marine denitrification in the Arabian Sea (AS) oxygen minimum zone (OMZ). An assessment of temporal variations in the diversity and abundance of nosZ denitrifiers was performed to establish the relative importance of these bacteria in denitrification. Sampling was conducted at the Arabian Sea Time Series (ASTS) location and a quantitative PCR (qPCR) ana-lysis was performed. We detected a high abundance of the nosZ gene at core OMZ depths (250‍ ‍m and 500 m), indicating the occurrence of denitrification in the AS-OMZ. The maximum abundance of the nosZ gene was observed during the Spring Intermonsoon (SIM) at 250‍ ‍m (1.32×106 copies L-1) and 500‍ ‍m (1.50×106 copies L-1). Sequencing ana-lysis showed that nosZ denitrifiers belonged to the classes Alpha-, Beta-, and Gammaproteobacteria. Taxonomic ana-lysis revealed that most OTUs were affiliated with Pseudomonas, Rhodopseudomonas, and Bradyrhizobium. Diversity indices and richness estimators confirmed a higher diversity of nosZ denitrifiers at 250‍ ‍m than at 500‍ ‍m during all three seasons. The present results also indicated that dissolved oxygen (DO) and total organic carbon (TOC) are critical factors influencing the diversity and abundance of the nosZ-denitrifying bacterial community.

Pop-up archival tags reveal environmental influences on the vertical movements of silvertip sharks Carcharhinus albimarginatus

Vertical space use informs the ecology and management of marine species, but studies of reef-associated sharks often focus on horizontal movements. We analysed the vertical movements of silvertip sharks Carcharhinus albimarginatus using pop-up archival tags deployed on 7 individuals in the Chagos Archipelago, central Indian Ocean. The sharks changed depth predictably with water column thermal structure, moving deeper with seasonal increases in mixed layer depth while occupying a narrow ambient water temperature range around ~27°C. At shorter timescales, higher resolution data from 5 tags showed that silvertip shark depth varied cyclically with surface light levels, increasing during daylight and on nights around the full moon. This matches the diel vertical migrations of many fish species, suggesting that the sharks’ light-driven depth changes might relate to foraging. While most vertical movements (>98%) were within the mixed layer, deeper dives to 200-800 m occurred approximately every 3 d. High-resolution data from one recovered tag showed the shark ascending deep (>200 m) dives in 2 sharply defined phases, initially fast then slow. Analysis of dive profiles against dissolved oxygen (DO) data suggested that the shark may have ascended rapidly to escape low DO levels at depth, then reduced its ascent rate by 50-80% once DO levels increased. While a small sample, the electronic tags deployed in this study revealed the silvertip sharks’ predictable use of mixed layer waters, narrow thermal range and apparent intolerance of hypoxic conditions. These characteristics may exacerbate the species’ vulnerability as oceanic warming and shoaling oxygen minimum zones modify vertical habitat availability.

Sedimentary cycling of zinc and nickel and their isotopes on an upwelling margin: Implications for oceanic budgets and paleoenvironment proxies

Zinc and Ni are essential micronutrients whose stable isotope systematics in marine sediments represent promising, but still developing, tracers of past ocean chemistry and biology. Sediments from upwelling continental margins have been identified as important sinks for Zn and Ni, driven by high productivity, incorporation of the metals into cells and organic carbon burial. At present, however, our understanding of how Zn and Ni isotope composition of upwelling sediments reflects specific processes in the water column is incomplete. Here, we present coupled Zn and Ni abundance and isotope data for both solid phase and porewater in a series of sediment cores collected on a shelf-to-slope transect across an oxygen minimum zone (OMZ) from the southwest African margin off Namibia.Zinc/Al and Ni/Al ratios in Namibian margin sediments are elevated relative to the lithogenic background, by factors of 1.4–2.4 and 1.7–5.7, respectively. Systematic relationships between solid phase Zn and Ni concentrations and organic carbon accumulation corroborate the view that authigenic Zn and Ni are mainly delivered to these sediments via export production from the photic zone. Corrections for detrital Ni are sometimes substantial, but authigenic δ60Ni values are within uncertainty of the modern deep ocean, at 1.38 ± 0.18 ‰ (mean and 1SD, n = 42). Authigenic δ66Zn is more variable, at −0.09 to 0.50 ‰, at or below the value for the deep ocean average. This latter observation can be attributed to Zn isotope fractionation during sequestration into authigenic ZnS phases in the sediments, either quantitatively, or partially.Porewater systematics are more complex and the profiles do not necessarily reflect the redox state of the sediment-porewater system at the time of sampling, but rather characteristics inherited from more reducing conditions in a temporally heterogeneous system. Porewater zinc concentrations (30–369 nM) are much higher than the deep ocean and increase with depth beneath the sediment–water interface. Despite this prominent increase in Zn concentrations, δ66Zn is rather constant and lighter than the deep ocean average, at 0.18 ± 0.06 ‰ (mean and 1SD, n = 33). These observations are most consistent with the buffering of the small porewater pool by the dissolution of a previously formed sulfide phase. Porewater Ni concentrations are closer to the deep ocean, mostly at 5–15 nM. The lightest porewater isotope compositions (δ60Ni as low as 0.15 ‰) are consistent with a Mn-oxide source of Ni, whereas the heaviest isotope compositions (up to 1.76 ‰) clearly reflect uptake of light Ni into sulfide at depth within the sediment.Our data, for the largest upwelling site in the modern ocean, add to sparse data from the eastern Pacific, and confirm that upwelling margin sediments bury Ni that is close to the deep ocean in isotope composition, but Zn that is generally lighter. The data thus confirm the importance of such margins in setting the heavy isotope composition of oceanic Zn. Sediments from this kind of setting hold significant potential for the construction of records of past oceanic Ni isotopes.

Distribution and stability of Mn complexes in the ocean: Influence of hydrothermal plumes and weather events

AbstractWe measured the speciation of dissolved Mn from the surface to just above the hydrothermal vents at 9°50′N East Pacific Rise in the open ocean of the Pacific over a 3‐week period. Total dissolved Mn concentrations ranged from 2.2 to 135 nmol L−1 with a significant contribution of dissolved Mn(III) bound to humic acid in one third of our samples representing up to 64% of the total dissolved Mn. These humic complexes were mostly detected in the hydrothermal vent plume and at the redox boundaries of the oxygen minimum zone in the water column. In the hydrothermal plume, the Mn(III)‐humic acid stabilized the manganese in solution up to a ~ 10,000‐fold dilution of the venting water. In the upper water column, Mn(III)‐humic acid was only detected after a squall and rain event, which indicates that it is a transient species, persistent over days to weeks. This temporal variability highlights the importance of non‐steady‐state processes in the open ocean, which may help to explain previous observations of a dissolved Mn maximum within oceanic oxygen minimum zones.

Relationship and stratification of multiple marine ecological indicators: A case study in the M2 seamount area of the Western Pacific Ocean

Seawater stratification is an important ecological indicator and typical feature in the marine environment, which is of great significance to the material cycle and distribution of organisms. In March 2016, a survey was conducted in the M2 seamount of the Western Pacific Ocean, seawater stratification of temperature, salinity, dissolved oxygen (DO), nutrients, bacteria, and the impact of the M2 seamount were discussed. It is found that the seawater stratification of each parameter is obvious in this area. At the depth of 0–75 m, there is a mixed layer, and these parameters remain relatively stable; at 75–500 m, there are obvious thermocline, gradient layer of bacteria, and nutricline; at the depth deeper than 500 m, these parameters remain relatively stable. In addition, at 75–150 m, there is a high-salt zone dominated by the North Pacific Tropic Water, while at 270–1370 m, there is an oxygen minimum zone with low DO. There is a close relationship in seawater stratification of different parameters. At 75–500 m, the density gradient dominated by the thermocline limits the water exchange, then directly controls the gradient layer of DO, nutrients, and bacteria. Additionally, the double actions of the high-salt zone and thermocline affect the stratification of other parameters. At 0–75 m, due to the abundant DO, sufficient carbon source, and low nutrients, the bacteria could grow largely, while at 75–500 m, bacteria could consume a large amount of DO to decompose organic matter, leading to a decrease in DO and an increase of nutrients. Although the M2 seamount is a shallow seamount, there is no upwelling around it, which weakly influences the seawater stratification of different parameters. This study will provide a new perspective on seawater stratification and its ecological effects.

Nitrous Oxide Distributions in the Oxygenated Water Column of the Sargasso Sea

ABSTRACT This study presents dissolved nitrous oxide (N2O) concentrations in the water column at the Bermuda Atlantic Time-series Study (BATS) station and uses a subset of these measurements to estimate air-to-sea flux for four specific time points between September 2018 and June 2019. N2O concentrations at BATS were in the range of 4.0 nmol L−1–16.9 nmol L−1, with vertical profiles which were the mirror inverse of dissolved oxygen. Regardless of season, N2O concentration maxima were found within the oxygen minimum zone (OMZ). The highest maximum N2O values were observed in November and lowest in October. As the water column at BATS remains consistently at dissolved oxygen concentrations greater than 140 µmol L−1, and therefore aerobic, we assume that the bulk of N2O production occurs through nitrification. A nitrification source is supported by a correlation between excess N2O (ΔN2O) below the mixed layer, apparent oxygen utilization (AOU) and nitrate concentrations. We estimate a pooled average yield of 0.027% to 0.038% N2O from nitrification at BATS. Finally, estimates of air–sea exchange of N2O using regional average monthly wind speeds indicated that this region acts as a weak source or a sink of atmospheric N2O, and varies between months.

The deposition of type II-S Jordan oil shale in the context of Late Cretaceous source rock formation in the Eastern Mediterranean realm. Insights from organic and inorganic geochemistry and petrography

This study provides a detailed geochemical and petrographic investigation of 22 Upper Cretaceous Jordan Oil Shale (JOS) samples obtained from various wells distributed across Jordan. The JOS is generally characterized by high amounts of organic matter with total organic carbon (TOC) contents of 10–29 wt%, high carbonate contents of 15–75 wt%, and variable biogenic SiO2 contents of 1–50 wt%. The organic matter is oil-prone, reflected by a clear predominance of liptinite macerals and Rock-Eval Hydrogen Index (HI) values of 608–1078 mg HC/g TOC. In terms of elemental composition, bulk and molecular organic geochemistry and petrology, the JOS is similar to other age-equivalent organic-rich rocks from the Eastern Mediterranean. Primary bioproductivity was probably controlled by a Late Cretaceous upwelling event that affected the whole region. Low Pr/Ph ratios <0.5 indicate that organic matter preservation was not only controlled by the formation of an upwelling-related oxygen minimum zone but also by further oxygen depletion in isolated sub-basins. This setting led to vulcanization of kerogen; proven by a positive correlation between calculated organic sulfur contents and the amount of thiophenes generated during Curie-point pyrolysis. Although the thermal maturity is low (Tmax values < 430°C), significant amounts of hydrocarbons (up to 30 kg HC per ton of rock) are present within these rocks. The present bitumen fraction shows high concentrations of asphaltenes (10–65%) and NSO compounds (30–85%) with higher contributions in samples with the lowest thermal maturities. The relative contributions of aliphatic and aromatic compounds are correspondingly low, but increase with higher thermal maturities. Kinetic parameters suggest that isothermal pyrolysis at 350°C might be already enough to artificially convert 80% of the reactive kerogen within about one day, making the JOS a potential target for oil shale retorting.

A ∼ 50 kyr record of carbonate (pteropods) preservation from the Laccadive Sea, Northern Indian Ocean

To evaluate the preservation pattern of pteropods and their relationship with climatic and oceanographic history in the Laccadive Sea, temporal variation analysis of pteropod abundance was carried out. We employed preservation indices from calcite (Globigerina bulloides%, Globorotalia menardii abundance), as well as aragonite (e.g., total pteropod abundance, Limacina Dissolution Index (LDX), fragmentation ratio) as a proxy. To determine if pteropod shells have been preserved over time, we used estimated pteropod abundance. The pteropod preservation record displays excellent preservation during cold stadials, evidenced by the lower values of aragonite dissolution proxies than during the interglacials/interstadials, similar to the preservation records from other northern Indian Ocean cores. The shallow aragonite compensation depth (ACD), weaker oxygen minimum zone (OMZ), and the lower southwest monsoon (SWM)-induced productivity is thought to be the cause of the basin-wide pteropod preservation events during the cold stadials (ACD). Additionally, during an intense northeast monsoon (NEM), the advection of cold, low-saline waters from the Bay of Bengal to the Laccadive Sea, as well as the intrusion of southern-sourced intermediate water ventilation, may have caused a deep vertical mixing of oxygen-rich surface waters, raised the pH of thermocline waters and deepened the ACD. However, the local fluctuations in the water mass properties, such as the increased productivity maxima, the intense OMZ, and shallow ACD, as well as changes in the aragonite, are responsible for the poor pteropod abundance, poor preservation and strong dissolution during the Holocene, Bølling-Allerød (B/A) and interstadial periods.The calcification proxy indicates that the aragonite undersaturation and reduced calcification occurred during 19–16.5 kyr, preferably due to the depletion in the oceanic alkalinity caused by enhanced upwelling-induced carbonate ion exchange between the intermediate and deep water. In contrast, the preferential dissolution of smaller shells in the sediments (marked by increased average shell size and higher values of Limacina dissolution index (LDX)) corresponds to strengthened OMZ and shallower ACD, pointing towards the post-depositional dissolution of aragonite shells. Therefore, the overall decrease of pteropod content of the deposits in the stadial/interstadials suggests a combination of monsoon-associated changes in water column properties, variability in aragonite saturation, intermediate water ventilation and sediment rate.