Ambient Seawater Research Articles

A New Negative Carbon Isotope Interval Caused by Manganese Redox Cycling After the Shuram Excursion

AbstractSeveral negative C isotope excursions (CIEs) occurred at the end of the Neoproterozoic era which have been generally attributed to the oxidation of organic carbon using sulfate as the terminal electron acceptor and the subsequent release of 13C‐depleted dissolved inorganic C (DIC). Based on new analyses from the Doushantuo Formation in South China, we observe a negative C isotope excursion right after the well‐known Shuram excursion. This excursion is equivalent to the ∼550 Ma negative CIE which is globally expressed within several continental margins. However, the origins of this CIE in the termination of Ediacaran remain unresolved. Here, we hypothesize that this post‐Shuram negative CIE was caused by a localized manganese cycling that began with the oxidation of hydrothermal Mn(II) in a water column to insoluble Mn(IV)‐oxide, followed by accumulation of Mn(IV)‐oxide to the seafloor and its subsequent dissolution via Mn(IV) reduction leading to the release of dissolved Mn(II) and 13C‐depleted DIC into ambient seawaters. This ultimately led to the precipitation of particulate Mn(II)‐carbonate characterized by low δ13Ccarb values ranging from −11.1‰ to −2.8‰. The presence of microbial fabrics in association with the Mn(II)‐carbonate further suggests that Mn(II)‐carbonate precipitation took place at the seafloor in shallow sun‐lit waters rather than in the deeper sediment pile, which archived ambient seawater C isotopic signal. Although most Ediacaran negative CIEs were generally attributed to sulfate reduction, our findings suggest that at a local level, Mn cycling can also lead to negative CIE in the Neoproterozoic, and potentially at other times in Earth's history.

RADIOCARBON AND URANIUM PROFILES IN MARINE GASTROPODS AROUND THE JAPANESE ARCHIPELAGO

ABSTRACT In this study, we investigate the distribution of radiocarbon and uranium in the calcified opercula of Turbo sp. collected from Ryukyu region and Chiba, Japan, to explore the potential of U/Th dating using mollusks collected from the Japanese archipelago. We acquired high-resolution radiocarbon and uranium concentration measurements using single-stage accelerator mass spectrometry and laser ablation−inductively coupled plasma−mass spectrometry. Our results show that uranium in the opercula of modern Turbo sp. is unevenly distributed at concentrations 1000 times less than those in coral skeletons. Radiocarbon found in the calcified opercula samples record ambient seawater radiocarbon values as well as coral skeletons. Uranium in the calcified opercula of Holocene Turbo marmoratus were also unevenly distributed and concentrated within the opercula in a different manner than observed in modern samples, suggesting uranium exchange after death. Our results suggest variable uptake of uranium isotopes into mollusk shells and highlights the need for rigorous sample selection criteria when choosing mollusks species for U/Th dating around Japan.

Distinct microbial communities in an ascidian-crustacean symbiosis.

Ascidians are marine invertebrates known to occasionally host symbiotic crustaceans. Although the microbiomes of both ascidians and free-living crustaceans have been characterized, there is no documentation of microbial communities in an ascidian-crustacean symbiosis. Samples of the solitary ascidian Ascidia sydneiensis and ambient seawater were collected in Belize. Four symbiotic amphipod crustaceans were retrieved from the branchial sac of the animal, and their microbiomes were compared with those from their ascidian host (tunic and branchial sac compartments) and seawater. Microbiome richness and diversity differed significantly between sample types, with amphipod microbiomes exhibiting significantly lower diversity than tunic and ambient seawater samples. Microbiome composition also differed significantly between sample types and among all pairwise comparisons, except for branchial sac and amphipod microbiomes. Differential operational taxonomic unit (OTU) analyses revealed that only 3 out of 2553 OTUs had significantly different relative abundances in amphipods compared with ascidian branchial sacs, whereas 72 OTUs differed between amphipod and tunic and 315 between amphipod and seawater samples. Thus, different body compartments of A. sydneiensis hosted distinct microbiomes, and symbiotic amphipods contained microbiomes resembling the region they inhabit (i.e., the branchial sac), suggesting that environmental filtering and co-evolutionary processes are determinants of microbiome composition within ascidian-crustacean symbioses.

Chronic low salinity stress rescued masculinization effect in farmed Cynoglossus semilaevis population

Salinity, being an indispensable abiotic factor crucial for the survival of marine organisms, has demonstrated diverse alterations globally in response to the current trend of global warming. In this study, the effect of chronic low salinity stress on teleosts' sex differentiation was investigated using Cynoglossus semilaevis, an economically important fish with both genetic and environmental sex determination system. The cultivation experiment was conducted employing artificially simulated seawater of 20 ppt and ambient sea water of 30 ppt to rear juveniles C. semilaevis. Throughout the experiment, the growth performance was assessed and the histology of gonadal development was examined, a significantly lower masculinization rate was observed in LS group. To gain further insights, transcriptome analysis was conducted using raw reads obtained from 53 libraries derived from gonads of 55 days post fertilization (dpf) and 100 dpf juveniles in both LS and CT groups. GO/KEGG enrichment were further proceeded, Terms and pathways involved in reproduction ability, germ cell proliferation, immune function, steroid metabolism etc., were illuminated and a possible crosstalk between HPI and HPG axis was proposed. WGCNA was conducted and two hub genes, hspb8-like and Histone H2A.V were exhibited to be of great significance in the changes of masculinization rate. Our findings provided solid reference for sex differentiation study of GSD + ESD species in a constantly changing ocean environment, as well as practice guiding significance for the environmental management for the culture of C. semilaevis.

Technical note: An autonomous flow-through salinity and temperature perturbation mesocosm system for multi-stressor experiments

Abstract. The rapid environmental changes in aquatic systems as a result of anthropogenic forcings are creating a multitude of challenging conditions for organisms and communities. The need to better understand the interaction of environmental stressors now, and in the future, is fundamental to determining the response of ecosystems to these perturbations. This work describes an automated ex situ mesocosm perturbation system that can manipulate several variables of aquatic media in a controlled setting. This perturbation system was deployed in Kongsfjorden (Svalbard); within this system, ambient water from the fjord was heated and mixed with freshwater in a multifactorial design to investigate the response of mixed-kelp communities in mesocosms to projected future Arctic conditions. The system employed an automated dynamic offset scenario in which a nominal temperature increase was programmed as a set value above real-time ambient conditions in order to simulate future warming. A freshening component was applied in a similar manner: a decrease in salinity was coupled to track the temperature offset based on a temperature–salinity relationship in the fjord. The system functioned as an automated mixing manifold that adjusted flow rates of warmed and chilled ambient seawater, with unmanipulated ambient seawater and freshwater delivered as a single source of mixed media to individual mesocosms. These conditions were maintained via continuously measured temperature and salinity in 12 mesocosms (1 control and 3 treatments, all in triplicate) for 54 d. System regulation was robust, as median deviations from nominal conditions were < 0.15 for both temperature (∘C) and salinity across the three replicates per treatment. Regulation further improved during a second deployment that mimicked three marine heat wave scenarios in which a dynamic temperature regulation held median deviations to < 0.036 ∘C from the nominal value for all treatment conditions and replicates. This perturbation system has the potential to be implemented across a wide range of conditions to test single or multi-stressor drivers (e.g., increased temperature, freshening, and high CO2) while maintaining natural variability. The automated and independent control for each experimental unit (if desired) provides a large breadth of versatility with respect to experimental design.

Inshore juvenile lobsters threatened by warming waters and migratory fish predators in southern New England

The southern New England stock of the American lobster Homarus americanus has declined since its peak in the 1990s. While the decline of lobsters has been directly attributed to warming sea temperatures, indirect effects such as migratory fish species shifting north may also be important. Two sets of experiments and a survey of gut contents collected from wild-caught scup Stenotomus chrysops and black sea bass Centropristis striata during the summer were performed to assess thermal and predatory stressors on juvenile lobsters. A temperature experiment compared the growth and survival of young-of-year (YOY) and 1 yr old lobsters in ambient seawater and seawater warmed or cooled by 3°C. Two predation experiments compared the survival of juvenile lobsters in treatments with scup, black sea bass, and Asian shore crabs Hemigrapsus sanguineus. Increased water temperature did not increase mortality in juvenile lobsters, but it did tend to have a negative impact on growth and weight gain of YOY lobsters and weight gain of 1 yr old lobsters. In the predation experiments, fewer lobsters survived in mesocosms with black sea bass and scup than in the control treatment. Fish gut contents contained few lobsters, but black sea bass stomachs contained many juvenile cancer crabs, signifying that the fish prey in cobble habitat that is preferred by both juvenile cancer crabs and lobsters. While juvenile lobsters can survive current temperatures, they may face compounding challenges as warmer water slows their growth and leaves them more susceptible to predation by increasing numbers of migratory scup and black sea bass.

Laboratory and Field Culture of Larvae of The Slipper Limpet, Crepidula fornicata.

The calyptraeid gastropod mollusk, Crepidula fornicata, has been widely used for studies of larval developmental biology, physiology, and ecology. Brooded veliger larvae of this species were collected by siphoning onto a sieve after natural release by adults, distributed into the culture at a density of 200/L, and fed with Isochrysis galbana (strain T-ISO) at 1 x 105 cells/mL. Shell growth and acquisition of competence for metamorphosis were documented for sibling larvae reared in ventilated 800 mL cultures designed for equilibration to ambient air or to defined atmospheric gas mixtures. Contrasting with these laboratory culture conditions; growth and competence data were also collected for larvae reared in a 15 L flow-through ambient seawater mesocosm located in a field population of reproductive adults. Growth rates and timing of metamorphic competence in the laboratory cultures were similar to those reported in previously published studies. Larvae reared in the field mesocosm grew much faster and metamorphosed sooner than reported for any laboratory studies. Together, these methods are suited for exploring larval development under predetermined controlled conditions in the laboratory as well as under naturally occurring conditions in the field.

Effects of Increased Temperature on Brain and Sensory Development in the Port Jackson Shark (Heterodontus portusjacksoni)

Morphological differences in the peripheral (sensory) and central (brain) nervous system may confer sensory and/or behavioral variation in elasmobranchs, both across taxa and throughout ontogeny. Over the last century, sea surface temperatures have increased over 0.5 °C and are predicted to rise 1–4 °C by the year 2100, potentially affecting species’ physiological performance negatively. As the nervous system of fishes grows continually throughout their lives, it may be highly plastic in response to environmental changes. This study examined the effects of increased rearing temperature on nervous system development in Port Jackson sharks (Heterodontus portusjacksoni). Egg cases (n = 21) were collected from Gulf St. Vincent (Adelaide, SA) and placed into either ambient (17.6 °C) or 3 °C above ambient seawater conditions through hatching and reared for up to five months post-hatch. Relative volumes of the eyes and nose (olfactory rosette) were quantified using magnetic resonance imaging, and relative brain size and size of major brain regions were compared between the two treatment groups. The size of the olfactory bulbs and tegmentum varied significantly between the treatment groups, which suggest differences in primary, secondary, or tertiary sensory processing and/or motor functions at elevated temperatures. While studies on acute responses to environmental conditions cannot inform true adaptation across broad timescales, understanding the effects of increased temperature on the brain phenotype can aid in predicting how elasmobranchs may fare in response to changing ocean conditions.

Increased coral biomineralization due to enhanced symbiotic activity upon volcanic ash exposure

Coral reefs, which are among the most productive ecosystems on earth, are in global decline due to rapid climate change. Volcanic activity also results in extreme environmental changes at local to global scales, and may have significant impacts on coral reefs compared to other natural disturbances. During explosive eruptions, large amounts of volcanic ash are generated, significantly disrupting ecosystems close to a volcano, and depositing ash over distal areas (10s - 1000s of km depending on i.a. eruption size and wind direction). Once volcanic ash interacts with seawater, the dissolution of metals leads to a rapid change in the geochemical properties of the seawater column. Here, we report the first known effects of volcanic ash on the physiology and elemental cycling of a symbiotic scleractinian coral under laboratory conditions. Nubbins of the branching coral Stylophora pistillata were reared in aquaria under controlled conditions (insolation, temperature, and pH), while environmental parameters, effective quantum yield, and skeletal growth rate were monitored. Half the aquaria were exposed to volcanic ash every other day for 6 weeks (250 mg L−1 week−1), which induced significant changes in the fluorescence-derived photochemical parameters (ΦPSII, Fv/Fm, NPQ, rETR), directly enhanced the efficiency of symbiont photosynthesis (Pg, Pn), and lead to increased biomineralization rates. Enhancement of symbiont photosynthesis is induced by the supply of essential metals (Fe and Mn), derived from volcanic ash leaching in ambient seawater or within the organism following ingestion. The beneficial role of volcanic ash as an important micronutrient source is supported by the fact that neither photophysiological stress nor signs of lipid peroxidation were detected. Subaerial volcanism affects micronutrient cycling in the coral ecosystem, but the implication for coral ecophysiology on a reef scale remains to be tested. Nevertheless, exposure to volcanic ash can improve coral health and thus influence resilience to external stressors.

Hydrothermal amorphous silica, barite and orpiment from the crater area of seamount (SM-13) off Nicobar island, Andaman sea: Indications for the development of a new hydrothermal field

The Andaman sea characterized by trench-arc-backarc elements (Andaman-Nicobar island-arc, the Andaman back-arc spreading center, the seamount complex, and the back-arc basin) is a complex active basin and is known for hydrothermal activity in some of the seamounts (subaerial and submarine) of the volcanic arc. In the present study, 13 submarine seamounts (SM1- SM13) in the southern Andaman volcanic arc, extending from 6°47′N: 94°43′E to 7° 56′ N: 94° 2.47′E, were surveyed onboard RV Sindhu Sadhana (November 2021) to find the evidences for the hydrothermal activity. Out of the 13 seamounts, at one crater seamount (SM-13) we have observed evidence for hydrothermal activity characterized by plumes rich in dissolved gases (mainly CO2) in the water column, live chemosymbiotic organisms (Bathymodiolus species), and neoforming minerals such as amorphous silica, barite and orpiment. We observed two plumes rich in dissolved gases (mainly CO2), called GP1 and GP2, in the water column at two different water depths i.e. 400 and 380 mbsl respectively. Geochemical analysis of the water samples shows enhanced TCO2 concentrations within the plumes rich in dissolved gases (mainly CO2) observed in sub-bottom profiler data, however, the dissolved methane concentrations do not show any enrichment when compared with ambient seawater concentrations. Apart from the enhancement in the TCO2 concentrations, pH and total alkalinity data also show fluctuations within the same depth zone which may be attributed to the hydrothermal plume exhausting from the crater seamount. The occurrence of live chemosymbiotic organism Bathymodiolus species indicate an active and continuous supply of H2S. The precipitation of neoforming hydrothermal minerals such as amorphous silica, tabular barite, and orpiment on the surface of the rock samples further supports the hydrothermal activity. Previous studies carried out in 2007 have reported hydrothermal ferromanganese oxides precipitated from low-temperature hydrothermal fluids (Surya prakash et al., 2012) and inferred SM-13 as a dormant submarine volcano (Kamesh Raju et al., 2012). However, after 14 years, we observe volatile emissions to the water column, hydrothermal mineral precipitation and live chemosymbiotic organisms from the same seamount. These observations of the present study infer development of a new hydrothermal field in the least explored Andaman Sea. This study adds up a new active site to the global hydrothermal inventory which may allow better estimation of global hydrothermal flux and its role in oceanic elemental cycle.

Effects of ocean acidification on growth and photophysiology of two tropical reef macroalgae.

Macroalgae can modify coral reef community structure and ecosystem function through a variety of mechanisms, including mediation of biogeochemistry through photosynthesis and the associated production of dissolved organic carbon (DOC). Ocean acidification has the potential to fuel macroalgal growth and photosynthesis and alter DOC production, but responses across taxa and regions are widely varied and difficult to predict. Focusing on algal taxa from two different functional groups on Caribbean coral reefs, we exposed fleshy (Dictyota spp.) and calcifying (Halimeda tuna) macroalgae to ambient and low seawater pH for 25 days in an outdoor experimental system in the Florida Keys. We quantified algal growth, calcification, photophysiology, and DOC production across pH treatments. We observed no significant differences in the growth or photophysiology of either species between treatments, except for lower chlorophyll b concentrations in Dictyota spp. in response to low pH. We were unable to quantify changes in DOC production. The tolerance of Dictyota and Halimeda to near-future seawater carbonate chemistry and stability of photophysiology, suggests that acidification alone is unlikely to change biogeochemical processes associated with algal photosynthesis in these species. Additional research is needed to fully understand how taxa from these functional groups sourced from a wide range of environmental conditions regulate photosynthesis (via carbon uptake strategies) and how this impacts their DOC production. Understanding these species-specific responses to future acidification will allow us to more accurately model and predict the indirect impacts of macroalgae on coral health and reef ecosystem processes.

Metallogenetic process of Xunmei hydrothermal field (26°S), South Mid-Atlantic Ridge: Constraints from in-situ sulfur isotope and trace elements of sulfides

Seafloor hydrothermal sulfides at slow-spreading mid-ocean ridges (MORs) are of economic and scientific significance. The formation of newly discovered submarine hydrothermal sulfides on the South Mid-Atlantic Ridge (SMAR), particularly with respect to the fluid evolution, remains poorly understood. The Xunmei hydrothermal field is a typical volcanic dome-type hydrothermal field at 26°S on the SMAR. Based on the mineralogical zonation and assemblages, six distinct ore-forming stages were identified from low- to high-temperature stages with a seafloor weathering stage. The crystallinity of pyrite/marcasite and grain size of chalcopyrite increase gradually from the outer rim to the inner conduit of a sulfide chimney. The positive δ34SV-CDT values of pyrite/marcasite (2.1‰–8.2‰), chalcopyrite (2.5‰–5.6‰), and sphalerite (2.9‰–7.0‰) suggest 62%–91% of S was derived from the leaching of basement basalts, 9%–38% from reduction of seawater sulfate. In situ trace element data for sulfides show enrichments in Mn, Ag, Tl, and Pb in outer zones A and B, while enrichments in Se, In, and Sn in inner zones C and D, indicating an increase in precipitation temperature during chimney growth. Low pyrite Co contents, low Tl/Pb, low Sb/Pb, low Bi/Pb, high As, Ag, Cu, and Pb concentrations, suggesting that the pyrites precipitated from fluids that may have undergone supercritical phase separation during their ascent. High-temperature, Cl-depleted, vapor-rich hydrothermal fluids discharged into ambient cold seawater over multiple stages and probably evolved from low (<240 °C) to medium (∼ 263 °C) and high (∼ 317 °C) temperatures, reached a highest temperature of ∼335 °C, and then evolved to medium to low temperatures (∼ 270 °C to <240 °C) during the wanning of hydrothermal venting. The sulfur fugacity (fS2) likely evolved from a relatively low to high condition within the scope of intermediate sulfidation, and then decreased to low condition. The redox states (fO2) probably were strongly affected by seawater influx and evolved from relatively oxidized to original reduced conditions. The salinity decreased during chimney growth. Sulfide precipitation in the Xunmei hydrothermal field probably was the result of a combination of fluid-seawater mixing and phase separation. We propose a chimney growth and hydrothermal evolution models for the Xunmei hydrothermal field.

Rare earth element and yttrium (REY) geochemistry of 3.46–2.45 Ga greenalite-bearing banded iron formations: New insights into iron deposition and ancient ocean chemistry

Finely laminated cherts enclosing nanoparticles of greenalite and apatite are ubiquitous in Archean-Paleoproterozoic (3.46–2.45 Ga) ferruginous cherts, jaspilites and Banded Iron Formations (BIFs) and the greenalite and apatite are considered to be primary deposits. The cherts and BIFs are chemical sedimentary rocks interpreted to have been precipitated in marine settings prior to the first permanent rise in atmospheric oxygen at the Great Oxidation Event (GOE) ca. 2.45–2.32 M.y. ago. As chemical sediments, they are potential archives of the solutions from which they precipitated, incorporating signals from hydrothermal fluids and ambient seawater. Previous studies of rare earth elements and Y (REY) in pre-GOE BIFs have mostly found an “Archean seawater signature” with positive Eu anomalies, attributed to the influence of high-temperature hydrothermal processes, and positive anomalies for La, Gd and Y, ascribed to seawater. REY abundances determined by in situ LA-ICP-MS are presented for well-preserved, laminated greenalite-bearing cherts from ten formations of pre-GOE ferruginous cherts and BIFs from Western Australia. The samples come from a wide range of depositional environments, e.g., submarine proximal volcanic environments, basin floor, slope and deep marine shelf, and are between 3.46 Ga to 2.45 Ga in age. Five groups with different REY patterns are identified, namely (i) mafic-volcanic-influenced vent-proximal chert from the Marble Bar Chert Member of the Duffer Formation of the Warrawoona Group; (ii) felsic volcanic- and sediment-associated chert from the Kangaroo Caves Formation of the Sulphur Springs Group and the Wilgie Mia Formation of the Murchison Supergroup, (iii) ferruginous cherts in shelf sediments from the Bee Gorge Member of the Wittenoom Formation of the Hamersley Group, (iv) BIFs from the Nammuldi Member of the Marra Mamba Iron Formation and Joffre Member of the Brockman Iron Formation, Hamersley Group, and (v) BIFs from the Dales Gorge Member of the Brockman Iron Formation. Of these, only the Dales Gorge Member BIF has a typical Archean seawater signature while the others have REY patterns likely reflecting differing source fluids and environments of deposition but not necessarily global ocean chemistry. Analyses of chert containing sub-micron-sized particles of greenalite and apatite indicate that the likely hosts of the REEs are apatite, siderite and possibly greenalite. The REY patterns of the greenalite-bearing cherts may differ from those of bulk samples of the same formations, perhaps reflecting a diagenetic overprint in the bulk samples, whereas the greenalite-bearing cherts likely preserve their depositional compositions, locked in by early silicification.

Carbonate-hosted manganese deposits and ocean anoxia

Late Devonian (ca. 360 Ma), Early Carboniferous (ca. 330 Ma), and Early Triassic (ca. 250 Ma) manganese deposits in the South China Block support an emerging view that some Mn carbonates form through direct synsedimentary (authigenic) precipitation. These Mn carbonates accumulated on distal shelves and are interbedded with lime mudstone and heterozoan carbonates that accumulated in coastal upwelling environments. Lithofacies, Ce anomalies combined with vanadium, uranium, and molybdenum enrichments indicate that the Mn carbonates were primarily precipitated under anoxic conditions. We provide a robust dataset to clarify the precipitation pathway of these Mn carbonates and explore the implications of using them as proxies of regional shallow water and deep-ocean anoxia. In the studied deposits, Mn calcite encases dolomite rhombs (<10 μm) or aggregates, indicating heterogenous nucleation, which is supported by similar lattice parameters of both minerals with a small lattice mismatch (<2.1%). The presence of micro-sized euhedral rhombs and subhedral aggregates of rhombs, the non-stoichiometric compositions, and characteristic organic matter Raman spectroscopy peaks near the D and G bands in the dolomite nuclei suggest an organogenic origin. Authigenic organogenic dolomite precipitation is interpreted to have preconditioned the sediment with nucleation sites for the subsequent precipitation of Mn carbonates in the shallow seafloor. This is consistent with δ13C values (−2.61‰) of Mn carbonate bulk samples that are slightly lower than coeval seawater (0.31‰, from host rocks), suggesting incorporation of 12C during microbial respiration of sedimentary organic matter during the precipitation of organogenic dolomite and hydrothermal sourced inorganic carbon. The absence of Mn in shallow facies suggests terrestrial input of Mn was minimal. Geochemical and sedimentologic evidence suggests that coastal upwelling delivered manganous anoxic deep waters to the shelves. Eu/Eu⁎ ratios (<1.5) and decoupling of Fe and Mn imply long-range transport of hydrothermal Mn, during which hydrothermal fingerprints were muted through mixing with ambient seawater. The transport of hydrothermal Mn to shelves indicates that the regional deep oceans at the time must have been anoxic, as Mn2+ would be easily oxidized and become insoluble under oxygen-rich conditions, preventing its long-distance transport. This precipitation pathway and depositional model implies that Mn carbonates are a largely unrecognized proxy for tracking ancient oceanic redox fluctuations, particularly during times of well-oxygenated global oceans. Hence, these Mn deposits record at least transient regional deep-ocean oxygen depletion in the generally well-oxygenated Late Paleozoic and Early Triassic oceans. The recognition that this periodic anoxia was broadly coeval with the anoxia-related Hangenberg, Serpukhovian, and Permian-Triassic biotic crises suggests a potential relationship between Mn carbonate accumulation and extinction events.

Calibration of Sr/Ca Ratio and In Situ Temperature Using Hawaiian Corals

AbstractThe Sr/Ca ratio of modern coral skeletons can record local seawater temperature (T) and is an important tool for reconstructing past environments. However, site‐specific calibrations are required to ensure accurate temperature reconstructions. Here, we examine three modern coral skeletons collected at contrasting sites on the island of Oahu, Hawaii to establish the first accurate calibrations for this region and investigate site specific influences on the calibration process. Satellite T data, which is used for many calibrations, may not be able to derive an accurate thermometer. For our shallow lagoonal sites, satellite T had smaller seasonal T ranges, which resulted in significantly higher slopes of Sr/Ca‐T compared to using in situ T. The traditional age model based on aligning only min/max values can lead to errors in the Sr/Ca‐T calibration due to variable growth rates. An enhanced age model which adds midpoint alignments between the min/max peak values can account for seasonal changes in growth rate and reduce the error. On the same island, site‐ and time period specific conditions can cause notable differences in the Sr/Ca‐T calibrations. The coral from an estuarine embayment showed a high Sr/Ca offset, likely due to high Sr/Ca in ambient seawater. For corals which experienced thermal stress, lower slopes were observed probably due to elevated Sr/Ca values during the period of thermal stress.

Highly diverse diazotrophs drive high N2 fixation rates in a shallow submarine hydrothermal system

Dinitrogen (N2) fixation provides nitrogen sources supporting active chemoautotrophic processes in submarine hydrothermal systems. Here, the diazotrophic phylogenetic diversity and N2 fixation profile were investigated in depth along a sharp biogeochemical gradient from the hydrothermal vents to ambient seawater in a shallow submarine hydrothermal system (SHS) off Kueishantao Islet, Taiwan. Diazotrophic community compositions in the SHS covered all four known nifH clusters (I, II, III, and IV) and contained both characteristics of diazotrophic communities in deep-sea hydrothermal systems and terrestrial geothermal systems. The seawater around the vents was primarily dominated by nifH phylotypes affiliated with chemolithotrophic sulfur-oxidizing and iron-reducing bacteria. In contrast, in the seawater directly above the hydrothermal vents, the nifH community markedly contained diverse heterotrophic bacteria. The analysis, which focused on amino acid composition, protein spatial structure, and the possible co-occurrence of alternative nitrogenases in the presence of molybdenum-nitrogenase, further revealed that diazotrophic nitrogenase found around the hydrothermal vents exhibits potential molecular adaptations to high temperature, low pH, and sulfur-rich conditions. The relatively low N2 fixation rates (NFRs) were observed around the vents, where high concentrations of inorganic nitrogen compounds were present. However, notably, high NFRs were detected in seawater directly above the hydrothermal vents, which were 2–5 times higher than those at the reference site. The abundant iron, along with the depletion of nitrogen nutrients potentially resulting from nitrogen assimilation and loss processes, may have led to a high iron:nitrogen ratio, inducing the observed high NFRs. The co-occurrence of diverse nifH-containing heterotrophic bacteria and high NFRs suggests that these heterotrophs may play a major role in N2 fixation in this environment. This study suggests that the SHSs may be potential hotspots for marine N2 fixation, contributing significantly to the global ocean's N2 fixation budget and warranting further investigation.

Oyster reefs' control of carbonate chemistry-Implications for oyster reef restoration in estuaries subject to coastal ocean acidification.

Globally, oyster reef restoration is one of the most widely applied coastal restoration interventions. While reefs are focal points of processes tightly linked to the carbonate system such as shell formation and respiration, how these processes alter reef carbonate chemistry relative to the surrounding seawater is unclear. Moreover, coastal systems are increasingly impacted by coastal acidification, which may affect reef carbonate chemistry. Here, we characterized the growth of multiple constructed reefs as well as summer variations in pH and carbonate chemistry of reef-influenced seawater (in the middle of reefs) and ambient seawater (at locations ~50 m outside of reefs) to determine how reef chemistry was altered by the reef community and, in turn, impacts resident oysters. High frequency monitoring across three subtidal constructed reefs revealed reductions of daily mean and minimum pH (by 0.05-0.07 and 0.07-0.12 units, respectively) in seawater overlying reefs relative to ambient seawater (p < .0001). The proportion of pH measurements below 7.5, a threshold shown to negatively impact post-larval oysters, were 1.8×-5.2× higher in reef seawater relative to ambient seawater. Most reef seawater samples (83%) were reduced in total alkalinity relative to ambient seawater samples, suggesting community calcification was a key driver of modified carbonate chemistry. The net metabolic influence of the reef community resulted in reductions of CaCO3 saturation state in 78% of discrete samples, and juvenile oysters placed on reefs exhibited slower shell growth (p < .05) compared to oysters placed outside of reefs. While differences in survival were not detected, reef oysters may benefit from enhanced survival or recruitment at the cost of slowed growth rates. Nevertheless, subtidal restored reef communities modified seawater carbonate chemistry in ways that likely increased oyster vulnerability to acidification, suggesting that carbonate chemistry dynamics warrant consideration when determining site suitability for oyster restoration, particularly under continued climate change.

Decoupled neodymium and hafnium isotopes in seawater as insights into the Late Neoarchean weathering process

We here present trace element and coupled Sm-Nd and Lu-Hf isotope data for both pure and detritus-rich BIF samples collected from the ∼2.51 Ga Zhalanzhangzi BIF, North China Craton to investigate sources of BIF-forming materials to the ambient seawater, and to evaluate factors controlling the decoupling of Hf and Nd isotope systems in Late Neoarchean seawater. Both pure and detritus-rich BIFs exhibit positive Eu anomalies, positive correlation between εNd(2.51Ga) values and Fe2O3 contents, and moderate negative to positive εNd(2.51Ga) values, revealing a combined submarine hydrothermal and terrigenous sources for REY and iron to ambient seawater. Instead of interpreting the correlation between Ge/Si ratio and iron (or silica) content of BIFs as the mixing of hydrothermal and terrigenous sources of silica, we preferred a terrigenous source for silica to Neoarchean seawater. Moreover, by compiling published Ge/Si data, we argue that although hydrothermal fluids might be the dominant source of silica to early Archean oceans, a continent-derived riverine flux was expected to be the main source of silica to ambient seawater, following the substantial increase in land mass during Neoarchean. The εNd(2.51Ga) and εHf(2.51Ga) values of pure BIFs reveal decoupling of Nd and Hf isotopes in ambient seawater, and suggest that incongruent terrestrial weathering had significant effects on oceanic geochemistry at the end of Archean eon. In addition, based on the observation that Neoarchean seawater was characterized by more radiogenic εHf values when compared to its modern counterpart, we posit a weak to moderate chemical weathering conditions for Neoarchean continents.

Study on the critical sediment concentration determining the optimal transport capability of submarine sediment flows with different particle size composition

Submarine sediment flows is one of the main means for transporting sediment to the deep sea, often traveling long-distance and transporting significant volumes of sediment for tens or even hundreds of kilometers. Its strong destructive force often causes serious damage to submarine utilities on its course of movement. The sediment concentration of the sediment flow determines its density difference with the ambient seawater, and this density difference determines the flow ability of the sediment flow, and thus affects the final deposition locations of the transported sediment. In this paper, sediment flows of different sediment concentration with various silt and clay weight ratios (referred to as silt/clay ratio) are studied using flume tests. Our test results indicate that there is a critical sediment concentration at which sediment flows travel the fastest for a specific sediment composition. The critical sediment concentrations and their corresponding maximum velocities for each of the four silt/clay ratios are obtained. The results further indicate that the clay content is linearly negatively correlated with the critical sediment concentration. As the sediment concentration increases, the flow behaviors of sediment flows transform from the flow state to the collapsed state, and the fluid properties of the two turbid suspensions with changing flow behaviors are both Bingham fluids. Additionally, this paper also provides a microscopic explanation of the above-mentioned results by analyzing the arrangement of particles within the sediment flow.

Isotopic disequilibrium in brachiopods disentangled with dual clumped isotope thermometry

The stable oxygen and clumped isotope composition of brachiopod calcite are important proxies for the reconstruction of Phanerozoic seawater temperatures and δ18O values. The utility of brachiopods as a temperature archive is nonetheless challenged by indications that their shells precipitate out of isotopic equilibrium with ambient seawater, though the origin of disequilibrium effects has remained elusive. Here, we apply the recently developed dual clumped isotope thermometer (i.e., the simultaneous measurement of Δ47 and Δ48 values in CO2 derived from the acid digestion of carbonates) to modern and fossil brachiopods to investigate disequilibrium signatures recorded in brachiopod calcite. We present evidence that disequilibrium signatures are derived from the combination of two rate-limiting processes: the hydration/hydroxylation of CO2, and, potentially, the diffusion of HCO3−/CO32− in water. An empirical correction for clumped isotope disequilibrium is presented, based on correlation between measured disequilibrium offsets in the Δ47 and Δ48 values of modern brachiopods (Δ47disequilibrium = –0.88 × Δ48disequilibrium + 0.02). Dual clumped isotope thermometry of Eocene age brachiopods from Seymour Island (∼65 °S) yields temperatures in agreement with previously reported Δ47 derived temperatures from coeval bivalves. In contrast, uncorrected Δ47 derived temperatures from the same brachiopods are 6–11 °C colder. Measurement of dual clumped isotope composition alongside δ18O values of brachiopod carbonate should also enable more accurate reconstruction of seawater-δ18O. In comparison with group-specific Δ47-temperature calibrations, which correct for an average kinetic bias in Δ47, dual clumped isotope thermometry accounts for the magnitude of disequilibrium inherent to an individual sample, facilitating more reliable paleotemperature and seawater-δ18O reconstructions.