Modern Ocean Research Articles

Formation of micro-spherulitic barite in association with organic matter within sulfidized stromatolites of the 3.48 billion-year-old Dresser Formation, Pilbara Craton.

The shallow marine and subaerial sedimentary and hydrothermal rocks of the ~3.48 billion-year-old Dresser Formation are host to some of Earth's oldest stromatolites and microbial remains. This study reports on texturally distinctive, spherulitic barite micro-mineralization that occur in association with primary, autochthonous organic matter within exceptionally preserved, strongly sulfidized stromatolite samples obtained from drill cores. Spherulitic barite micro-mineralization within the sulfidized stromatolites generally forms submicron-scale aggregates that show gradations from hollow to densely crystallized, irregular to partially radiating crystalline interiors. Several barite micro-spherulites show thin outer shells. Within stromatolites, barite micro-spherulites are intimately associated with petrographically earliest dolomite and nano-porous pyrite enriched in organic matter, the latter of which is a possible biosignature assemblage that hosts microbial remains. Barite spherulites are also observed within layered barite in proximity to stromatolite layers, where they are overgrown by compositionally distinct (Sr-rich), coarsely crystalline barite that may have been sourced from hydrothermal veins at depth. Micro-spherulitic barite, such as reported here, is not known from hydrothermal systems that exceed the upper temperature limit for life. Rather, barite with near-identical morphology and micro-texture is known from zones of high bio-productivity under low-temperature conditions in the modern oceans, where microbial activity and/or organic matter of degrading biomass controls the formation of spherulitic aggregates. Hence, the presence of micro-spherulitic barite in the organic matter-bearing Dresser Formation sulfidized stromatolites lend further support for a biogenic origin of these unusual, exceptionally well-preserved, and very ancient microbialites.

Chamberlainium pentagonum (Conti) n.comb. et Spongites fruticulosus (Corallinales, Rhodophyta) dans les calcaires miocènes de Méditerranée occidentale

Molecular phylogenetic analyses discovered an unexpected biodiversity of coralline algae in modern oceans. This species richness is unlikely to be an exclusive characteristic of recent ecosystems. With the aim to investigate fossil coralline biodiversity, a large dataset of western Mediterranean Miocene specimens, previously identified as Spongites fruticulosus (and synonymized taxa), have been revised based on the current taxonomic framework. The analysis recognized two distinct groups. The first group includes the specimens fitting within the current description of S. fruticulosus. The second group consists of Chamberlainium pentagonum comb. nov., the first fossil representative of the genus. This species has been separated from S. fruticulosus on the basis of its smaller conceptacles and its thinner conceptacle roof. The very same characters have been highlighted by modern molecular phylogenetic analyses for separating Chamberlainium from Spongites. Chamberlainium pentagonum, similarly to the fossil specimens of Spongites fruticulosus, occurs in most of the investigated area and the two species coexist in several localities, indicating a similar and broad ecological tolerance for both taxa. These results suggest that Miocene coralline algal biodiversity is probably underestimated and prove the convenience of using large datasets for the study of fossil coralline algae.

Rare earth elements and Nd isotopes as tracers of modern ocean circulation in the central Mediterranean Sea

Seawater rare earth element (REE) concentrations and Nd isotopic composition (εNd) are increasingly applied as valuable tracers of oceanographic processes such as water mass mixing and lithogenic inputs to seawater. However, their measurements are basically lacking in the Mediterranean Sea water column. This study analyzes 9 seawater stations around the central Mediterranean Sea to clarify the relative importance of external sources, vertical (biogeochemical) processes and lateral water mass transport in controlling REE and εNd distributions. Concentrations of REE do not show nutrient-like profiles with depth, likely indicative of relatively young waters with limited accumulation of remineralized REE. Light REE (LREE) present a non-conservative behavior, which largely peak at surface waters and rapidly decrease with depth. The negative correlation of surface LREE enrichment with offshore distance highlights the influence of continental input from the western Italian coast to the Tyrrhenian surface waters. In contrast to other regions with reported boundary exchange, this process does not modify the εNd values here. On the other side, distributions of dissolved heavy REE (HREE) and εNd display a conservative behavior that can be explained by mixing of western- (MAW and WMDW) and eastern- (LIW and EMDW) originated waters. We test this hypothesis with an Optimum Multi-Parameter Analysis (OMPA) including HREE and εNd parameters. Even though the limited data set, consistent results of water mass fractions are obtained for the four main water masses although with some particularities. While LIW takes on major importance when considering HREE in the model, EMDW fractions are preferentially detected with εNd. This latter finding implies a noticeable deep water flux across the Sicily Strait into the Western Mediterranean that was not clearly evidenced before.

High resolution osmium data record three distinct pulses of magmatic activity during cretaceous Oceanic Anoxic Event 2 (OAE-2)

Oceanic Anoxic Event 2 (OAE-2) occurred at the Cenomanian-Turonian boundary (∼94.1 Ma) and was a time of profound global changes in ocean chemistry and the carbon cycle. This event was characterized by a positive carbon isotope excursion (CIE) caused by massive organic carbon burial, global greenhouse temperatures, ocean deoxygenation, and changes in ocean life driven by large igneous province (LIP) activity. LIPs throughout the Phanerozoic have had dynamic magma flux, with episodes of major eruptions interspersed with periods of relatively less intense eruptions. A possible trigger for LIP activity throughout the Phanerozoic has been attributed to extraterrestrial impacts because there are multiple contemporaneous occurrences of large craters, LIP activity, and mass extinctions in the geologic record. At the Cenomanian-Turonian boundary, there is a 25 km diameter (rim-to-rim) complex crater in NW Alberta, Canada known as the Steen River impact structure dated at 91 ± 7 Ma (Carrigy and Short, 1968). An alternative explanation for those craters found contemporaneous with LIP activity and mass extinctions is that they were created by large explosive events related to LIP activity. Explosive events associated with mantle plume incubation beneath cratonic lithosphere have been suggested to create geologic features commonly attributed to impacts (e.g., shocked quartz, microspherules, etc.). Currently, the trigger for LIP activity during OAE-2, as well as the duration of LIP activity and the temporal variation and magnitude of eruption rates are not well constrained.To address the issue of LIP eruption rates and the trigger for LIP activity, we examined osmium (Os) abundances and isotopes as well as highly siderophile element (HSE; for this study: Re, Ru, Pd, Os, Ir, Pt) abundance data from a continuous sedimentary section spanning OAE-2. The section is from the Eagle Ford Group in the Iona-1 core, deposited in the Cretaceous Western Interior Seaway (KWIS). We found three high Os concentration intervals with mantle-like initial Osi isotope (initial 187Os/188Os) values of ∼0.16. These intervals are interpreted to reflect high-flux LIP magmatic pulses. Between the pulses, lower Os abundances with more radiogenic Osi values of ∼0.20 are observed, which we interpret as low-flux LIP activity between the high-flux periods. This trend of high-and-low flux Os concentration pulses with mantle-like Osi values during the high flux periods is found in another KWIS core (USGS Portland #1) deposited to the north of Iona-1, and in core Deep Sea Drilling Project (DSDP) Site 530 Hole A (hereafter DSDP 530A; drilled off-shore Namibia in the Angola Basin) deposited in the Southern Hemisphere. Before and throughout the Iona-1 core OAE-2 interval, HSE abundance patterns indicate a mantle source for the unradiogenic Os, and are not consistent with an extraterrestrial impact trigger or contribution to LIP activity during OAE-2.This evidence for multiple high-flux pulses of LIP activity driving ocean deoxygenation has implications for the modern ocean, which is currently experiencing deoxygenation. These results provide new constraints on subsequent high-flux periods that extended the event. The first high-flux period started ∼60 Kyr after our selection of the onset of the CIE. The second and third high-flux periods started ∼270 and ∼400 Kyr after the onset of the CIE, respectively. After the third high-flux period, δ13Corg and Os isotope ratios shifted back to pre-excursion values over ∼585 kyr. In the Iona-1 core, OAE-2 lasted for 1.04 ± 0.12 Myr based on our selection of the CIE.

Equilibrium fractionation and isotope exchange kinetics between aqueous Se(IV) and Se(VI)

The selenium (Se) isotope system has been proposed as a redox proxy in environmental and paleoceanographic studies. However, Se isotope exchange among various Se species can potentially interfere with redox-related isotope signatures, and is still poorly understood. In this work, we investigated Se isotope exchange kinetics and equilibrium fractionations between aqueous Se(IV) and Se(VI) under various experimental conditions. At pH = 7, low-Se concentration experiments (0.026 mM Se(IV) and 0.026 mM Se(VI)) at 25 °C, 38 °C and 60 °C were conducted for 900 days, while high-Se concentrations (0.13 mM Se (IV) and 0.14 mM Se(VI); 1.3 mM Se(IV) and 1.4 mM Se(VI)) at 60 °C were conducted for 1547 days. All experiments did not reach isotopic equilibrium, with observed Se isotope fractionations <0.20‰. Adding an electronic shuttle (Anthraquinone-2, 6-disulfonate) did not increase the isotope exchange rate. These results show that under the experimental conditions examined, the isotope exchange reaction between aqueous Se(IV) and Se(VI) is extremely slow.The exchange kinetics between Se(IV) and Se(VI) were also investigated using a 82Se tracer. The exchange rates (R) at 0.13 mM Se(IV) and 0.13 mM Se(VI) at 25 °C, 38 °C and 60 °C were determined to be ≤6.34 × 10−10 M day−1, ≤1.12 × 10−09 M day−1 and ≤1.17 × 10−09 M day−1, respectively. Using the upper bound for the isotope exchange rate at 25 °C and theoretically calculated equilibrium fractionations, and assuming a first order isotope exchange reaction between Se(IV) and Se(VI) by analogy to the sulfur system, the timescale of isotope exchange between aqueous Se (IV) and Se (VI) in a natural lake (Sweitzer Lake, Colorado, USA) was estimated. The minimum half-time (t1/2, time to reach 50% isotopic equilibrium) and the minimum time for detectable isotope exchange (tmin) are ≥440,000 and ≥18,000 years, respectively. In the modern oceans, t1/2 and tmin are ≥51 million and ≥3.6 million years, respectively. These timescales are much longer than the residence time of Se in Sweizer Lake (2.4 years) and the modern ocean (26,000 years). Therefore, when using Se isotopes to trace the biogeochemical cycle of Se in lakes and oceans, the effect caused by isotope exchange between aqueous Se(IV) -Se(VI) systems is insignificant.

Ice-rafted debris as a source of non-conservative behaviour for the \u03b5Nd palaeotracer: insights from a simple model

Neodymium isotopic composition (εNd) has enjoyed widespread use as a palaeotracer, principally because it behaves quasi-conservatively in the modern ocean. However, recent bottom water εNd reconstructions from the eastern North Atlantic are difficult to interpret under assumptions of conservative behaviour. The observation that this apparent departure from conservative behaviour increases with enhanced ice-rafted debris (IRD) fluxes has resulted in the suggestion that IRD leads to the overprinting of bottom water εNd through reversible scavenging. In this study, a simple water column model successfully reproduces εNd reconstructions from the eastern North Atlantic at the Last Glacial Maximum and Heinrich Stadial 1, and demonstrates that the changes in scavenging intensity required for good model-data fit is in good agreement with changes in the observed IRD flux. Although uncertainties in model parameters preclude a more definitive conclusion, the results indicate that the suggestion of IRD as a source of non-conservative behaviour in the εNd tracer is reasonable and that further research into the fundamental chemistry underlying the marine neodymium cycle is necessary to increase confidence in assumptions of conservative εNd behaviour in the past.

The smallest known Palaeozoic mantis shrimp specimen, and possibilities for where to find more

Mantis shrimps are fierce predatory crustaceans in modern day oceans. Fossils of mantis shrimps are still a rarity. They occurred back in the Carboniferous, 300 million years ago, but already with quite a diversity of different forms. Interestingly, all these fossils are comparably large, usually several centimetres body length. From the Mesozoic, numerous mantis shrimp fossils have also been reported, the largest specimens being far in the centimetre range as well, but the smallest specimens being only about 4–5 mm. With this data set, we can reconstruct aspects of the life history for (at least some) Mesozoic mantis shrimps, but not for Palaeozoic forms. We report here a new fossil mantis shrimp specimen from the Carboniferous Bear Gulch limestones, USA. The specimen is quite small, in its rolled-up position about 10 mm across the laterally preserved body, but 25 mm in estimated total length. This is considerably shorter than any mantis shrimp so far known from the Palaeozoic. Hence, the specimen provides an important data point. Furthermore, it provides some indications where we could possibly find more specimens of comparable or even smaller size: The specimen resembles certain co-occurring fossils, representatives of Cyclida, in some aspects, suggesting that among the material of Cyclida we might find more small-sized mantis shrimps.

Influence of palaeo-redox and diagenetic conditions on the spatial distribution of Cambrian biotas: A case study from the upper Shuijingtuo Formation (Cambrian Series 2, Stage 3), Three Gorges area of South China

From the wake of the Marinoan glaciations to the early Cambrian, the global ocean experienced progressive but likely nonuniform and nonlinear oxygenation to levels that, although still low with respect to the modern ocean, and might be therefrom triggered the ecological expansion of animals. In this study, integrated palaeontological and geochemical analyses were conducted on the upper Shuijingtuo Formation (Cambrian Series 2, Stage 3) which outcrops western Yichang City, Hubei Province, South China, in order to assess the influence of oxygen on Cambrian ecosystem diversification in the local environment. To better understand the oxygen fluctuations in the water column over the Yangtze Block and their correlation with the spatial distributions of marine biota during the early Cambrian, three sections representing different environmental settings are investigated: from the carbonate epicontinental platform (Changkan section) to the intra-platform basin (Luojiacun and Qiaojiaping sections). The semi-quantitative analysis shows that the assemblages from the epicontinental platform are dominated by eodiscoid trilobites and brachiopods. Assemblages from the intra-platform basin differ in being dominated by articulated sponges and possible cnidarians, while brachiopods and trilobites are rare. Diagenetic effects on these observed patterns are assessed by SEM-coupled EDS analysis: they indicate that although such effects cannot be definitely discarded, the observed faunal distribution and relative abundance most probably reflect variations in environmental settings. The geochemical analysis (Mo-EF vs. U-EF) furthermore provides evidence of an oxygen gradient from the edge of the platform down to the intra-platform basin. Variations in palaeo-redox conditions between the different studied settings can be observed, which undoubtedly influenced the observed differentiation of benthic communities. Frequent anoxic events in the intra-platform basin were fatal for benthic communities, but dysoxic environments could have favored the persistence and even prosperity of metazoans such as sponges and possible cnidarians that can endure lower dissolved oxygen in the water column than other groups.

Deep-water dissolved iron cycling and reservoir size across the Ediacaran-Cambrian transition

The majority of the deep ocean was likely under ferruginous conditions during the first four billion years of Earth's history. As the atmosphere was gradually oxygenated, the sources, sinks, redox cycling, and reservoir size of dissolved iron in the deep ocean are likely to have changed dramatically. Whether deep water was thoroughly oxygenated by the time of the Ediacaran-Cambrian transition, and the relationship of this oxygenation to the Cambrian explosion, remains debated. To explore the degree of oceanic oxygenation and its effect on Cambrian explosion, we measured the iron isotopic composition (δ56Fe) of bulk rock (i.e., cherts and mudstones/shales) through the Piyuancun and Hetang formations, using samples collected from the Chunye-1 core, on the Lower Yangtze Block in western Zhejiang. The limited variation in δ56Fe values (<0.7‰) and low FeT/Al ratios (<0.77) in euxinic samples show that the deep-water Fe2+ reservoir was quite limited, and likely similar to that of the modern ocean, during the latest Ediacaran and Cambrian Stages 1–3. Iron isotope results, combined with published data from sections on the Middle and Upper Yangtze Block, record a general decline in seawater δ56Fe values from >0.55‰ during the end-Ediacaran and Cambrian Stages 1–3 to <0‰ during Cambrian Stage 4. Seawater δ56Fe values in the lower and middle Hetang Formation range between 0 and 0.2‰, suggesting that the riverine dissolved and suspended flux and/or aeolian dust was the predominant source of highly reactive iron to the deep basin. Positive deep-water δ56Fe values, above 0.55‰ during the terminal Ediacaran and Cambrian Stages 1–3, likely reflect a basin where pyritization, rather than oxidation, was the predominant sink for deep-water ferrous Fe. Thus, we infer that only the shallow water was sufficiently oxygenated to support complex metazoans and the evolutions of skeletons, and that atmospheric oxygen levels were not high enough to directly oxygenate deep water environments during the Cambrian explosion.

Interactions of thallium with marine phytoplankton

Thallium (Tl) is the most toxic metal for mammals, likely due to its chemical similarity to the bioessential potassium (K). The biogeochemical cycle of Tl is susceptible to perturbation from a number of anthropogenic activities. Tl accumulation in the food chain may cause chronic Tl poisoning from dietary intake for human beings. However, Tl accumulation in the marine biosphere has largely been overlooked because high concentrations of K are thought to inhibit any uptake of Tl by organisms. Here, for the first time, high accumulation of Tl in the cytosol by the ubiquitous and abundant marine phytoplankton Emiliania huxleyi was found, making it readily transferable to higher trophic levels. Dose-response and physiological data were presented from a range of 9 species of phytoplankton under a wide range of Tl concentrations (1 ng L−1 to 1 mg L−1), spanning modern open ocean concentrations of 10–20 ng L−1, and higher concentrations in the coastal and fresh waters. Of all phytoplankton studied here, the cyanobacteria Synechococcus and the haptophytes Isochrysis galbana and Pavlova granifera have the largest tolerance to Tl toxicity, while the chlorophyte Micromonas pusilla has the least tolerance to elevated Tl. Haptophytes, especially isochrysidales, accumulated significantly more Tl intracellularly (up to 300-fold higher) than chlorophytes, making them good candidates for Tl bioremediation studies. Potassium transporters from E. huxleyi are found to be different from all other phytoplankton in this study based on analyses of the proteome. Since Tl is well known to be taken into cells by mistake through potassium channels, the difference of K channels in E. huxleyi may be responsible for its highest accumulation of Tl. As one of the most widely distributed phytoplankton in the ocean, the bioaccumulation of Tl in E. huxleyi may transfer and concentrate through the marine food chain and have significant impacts on the Tl biogeochemical cycle in the modern ocean.

A rare earth element and Nd isotopic investigation into the provenance and deposition of the Dahongliutan banded iron formation and associated carbonates, NW China: Implications on Neoproterozoic seawater compositions

Rare earth element and Nd isotopic data for the recently discovered Neoproterozoic Dahongliutan banded iron formation (BIF) and associated carbonates are employed to infer aspects of the provenance and depositional environment, including ambient seawater compositions. The geochemical evidence is suggestive of deposition of the Dahongliutan marble in an epicontinental shallow–marine environment with riverine influx and low–T hydrothermal input, with restricted exchange to the open ocean. In contrast, the Dahongliutan BIF formed in a more open setting, with high–T hydrothermal input and old continental materials addition. The clearly distinct but negative εNd (0.593 Ga) values indicate that Neoproterozoic seawater at Dahongliutan could be spatially inhomogeneous with respect to 143Nd/144Nd ratios, owing to inefficient vertical and horizontal mixing of seawater closely related to the riverine Nd input derived from the weathering of different types and ages of continental crusts of the Western Kunlun orogenic belt and the Tarim Block. The Nd isotopic mass balance calculations of the Dahongliutan BIF indicate that most (>68%) of the Nd were derived from continental source materials transported by river water. In conjunction with previously published geochemical data of Neoproterozoic BIFs worldwide, we evaluate the chemical compositions of seawater where they formed, represented by a seawater reservoir strongly influenced by continentally derived freshwaters (>10%) mixed with open ocean and minor (<5%) high–T hydrothermal fluids as indicated by Sm/Yb and Eu/Sm ratios. By comparing the Nd isotopic features of Neoproterozoic BIFs, it is concluded that similar to modern oceans, the Neoproterozoic ocean was not well–mixed with respect to its Nd isotopic composition and continental source materials transported by river water dominated the great majority of Nd in Neoproterozoic seawater.

Limited Archaean continental emergence reflected in an early Archaean 18O-enriched ocean

The origin and evolution of Earth’s biosphere were shaped by the physical and chemical histories of the oceans. Marine chemical sediments and altered oceanic crust preserve a geochemical record of these histories. Marine chemical sediments, for example, exhibit an increase in their 18O/16O ratio through time. The implications of this signal are ambiguous but are typically cast in terms of two endmember (but not mutually exclusive) scenarios. The oceans may have been much warmer in the deep past if they had an oxygen isotope composition similar to that of today. Alternatively, the nature of fluid–rock interactions (including the weathering processes associated with continental emergence) may have been different in the past, leading to an evolving oceanic oxygen isotope composition. Here we examine approximately 3.24-billion-year-old hydrothermally altered oceanic crust from the Panorama district in the Pilbara Craton of Western Australia as an alternative oxygen isotope archive to marine chemical sediments. We find that, at that time, seawater at Panorama had an oxygen isotope composition enriched in 18O relative to the modern ocean with a δ18O of 3.3 ± 0.1‰ VSMOW. We suggest that seawater δ18O may have decreased through time, in contrast to the large increases seen in marine chemical sediments. To explain this possibility, we construct an oxygen isotope exchange model of the geologic water cycle, which suggests that the initiation of continental weathering in the late Archaean, between 3 and 2.5 billion years ago, would have drawn down an 18O-enriched early Archaean ocean to δ18O values similar to those of modern seawater. We conclude that Earth’s water cycle may have gone through two separate phases of steady-state behaviour, before and after the emergence of the continents. The water cycle was in two different steady states, before and after continental emergence, as recorded in the decreasing oxygen isotope values of seawater since the Archaean, according to an inverse geochemical model of the oceanic crustal record.

Mesoproterozoic oxygenated deep seawater recorded by early diagenetic carbonate concretions from the Member IV of the Xiamaling Formation, North China

Oxygen availability is crucial for the evolution of eukaryotes in geological history. However, the evolution of mid-Proterozoic oceanic–atmospheric redox conditions remains heavily debated, e.g., [O2] <0.1–1% PAL vs. >4–8% PAL (present atmospheric level). In order to further constrain the surface oxygen levels during Mesoproterozoic, an investigation on the I/(Ca + Mg) ratios of deep-water (>100 m) early diagenetic carbonate concretions from the shale-predominated Member IV of the Xiamaling Formation (~1.4 Ga) in three sections of the North China Platform was conducted. The results show that more than half (36/47) of the I/(Ca + Mg) values obtained from the concretions are higher than 0.5 μmol/mol (avg. 0.68), indicating non-negligible iodate retained in the porewaters where the concretions were formed. Compared with the iodine redox cycle and oxygen transportation pathway from bottom seawater to surface sediments in modern oceans, the oxygen concentration of the bottom seawater was estimated to be higher than 16–22 μM, and the minimal atmospheric oxygen level should be higher than 6–9% PAL. This result differs from the previous estimation that the atmospheric oxygen level is lower than 0.1–1% PAL, but is consistent with the estimation of >4–8% PAL at ~1.4 Ga.

Ontogenetic disparity in early planktic foraminifers

Abstract. Changes in morphology during ontogeny can have profound impacts on the physiology and biology of a species. Studies of ontogenetic disparity through time are rare because of the lack of preservation of developmental stages in the fossil record. As they grow by incremental chamber accretion and retain evidence of growth in their shell, planktic foraminifera are an ideal group for the study ontogenetic disparity through the evolution of a higher taxon. Here, we quantify different developmental stages in Jurassic foraminifers and infer the evolutionary implications of the shape of these earliest representatives of the group. Using a Zeiss Xradia micro-CT scanner, the development of Globuligerina bathoniana and Globuligerina oxfordiana from the Bathonian sediments of Gnaszyn, Poland, and Globuligerina balakhmatovae and Globuligerina tojeiraensis from the Kimmeridgian Tojeira Formation of Portugal was reconstructed. Disparity is low through the early evolution of planktic foraminifers. The number of chambers and range in surface area per unit volume are lower than in modern specimens. We interpret this morphology as an indication of opportunistic behaviour. The low morphological plasticity during the juvenile stage suggests that strong constraints on the juveniles, described in the modern ocean, were already acting on Jurassic specimens. The high surface area per unit volume in these developmental stages points towards the need to satisfy a higher metabolic demand than in the adult specimens. We are interpreting the lower chamber numbers as indicative of short life cycles and potentially rapid reproduction, both of which may have allowed these species to exploit the nutrient-rich waters of the Jurassic Tethys Ocean.

Marine clade sensitivities to climate change conform across timescales

Rapid climate change is postulated to cause marine extinctions, especially among climate-sensitive clades, traits and regions1–6. This premise is based on two hypotheses: (1) known individual physiological sensitivities scale up to macroecological selectivity patterns4,7,8 and (2) ancient hyperthermal events are appropriate models to anticipate ecological winners and losers of anthropogenic climate change9. Yet these hypotheses have largely escaped quantitative appraisal. Here we show that experimental responses of modern marine ectotherms to single and combined climate-related stressors (such as seawater warming, hypoxia and acidification) align with Phanerozoic fossil extinction regimes across clades and functional traits. Of climate-related stressors, the synergistic interaction between warming and hypoxia10, encumbering aerobic metabolism, has the greatest potency as a proximate driver of extinction. All else being equal8, this synergy particularly imperils modern warm-water organisms. Modern–fossil agreement is strongest at intermediate–high extinction intensities and hyperthermal events but may fail at extreme extinction events, perhaps due to rising prominences of, and interactions among, additional biotic and abiotic stressors. According to results from marine ectotherms, clade-based sensitivity of individuals to climate-related stressors scales up from subannual experiments and decadal range-shift response magnitudes11, to extinction selectivity patterns at ancient climate-related stressor events and the Phanerozoic durations of genera. Climate change induced warming, hypoxia and acidification threaten marine species. Experimental work shows that the susceptibility of clades to climate-related stressors in the modern ocean is related to their extinction risk in the fossil record, which could allow prediction of future responses.

Carbon Cycling in the World's Deepest Blue Hole

AbstractBlue holes are unique geomorphological features with steep biogeochemical gradients and distinctive microbial communities. Carbon cycling in blue holes, however, remains poorly understood. Here we describe potential mechanisms of dissolved carbon cycling in the world's deepest blue hole, the Yongle Blue Hole (YBH), which was recently discovered in the South China Sea. In the YBH, we found some of the lowest concentrations (e.g., 22 μM) and oldest ages (e.g., 6,810 years before present) of dissolved organic carbon, as well as the highest concentrations (e.g., 3,090 μM) and the oldest ages (e.g., 8,270 years before present) of dissolved inorganic carbon observed in oceanic waters. Sharp gradients of dissolved oxygen, H2S, and CH4 and changes in bacterially mediated sulfur cycling with depth indicated that sulfur‐ and/or methane‐based metabolisms are closely linked to carbon cycling in the YBH. Our results showed that the YBH is a unique and easily accessible natural laboratory for examining carbon cycling in anoxic systems, which has potential for understanding carbon dynamics in both paleo and modern oceans—particularly in the context of global change.

Response of benthic foraminifera to pH changes: Community structure and morphological transformation studies from a microcosm experiment

Abstract Marine calcifying organisms, such as foraminifera, are threatened by the declining pH in the modern ocean. Benthic foraminifera are abundant, widespread and occur in diverse populations in the intertidal environment. However, to date, no studies have been conducted on the response of the intertidal foraminiferal community to pH under laboratory culture experiment. In this study, we cultured the entire foraminiferal community with the natural sediments from the intertidal area of the Yellow Sea at five pH values (8.5, 8.0, 7.5, 7.0 and 6.5, NBS scale). After four months' incubation, all living specimens (stained by rose-Bengal) were picked and identified. A total of 2246 living benthic foraminiferal specimens belonging to 15 species were analyzed, among which 1962 individuals were cultured and 284 ones were sampled before culturing. We calculated the community parameters under different pH, which showed both foraminiferal abundance and species richness decreased with the decline in pH. We analyzed the response of three foraminiferal taxa with different test types (hyaline, porcelaneous and agglutinated). The hyaline (e.g., Ammonia aomoriensis) and porcelaneous (e.g., Quinqueloculina seminula) foraminifera showed significant positive correlation with pH. In contrast, the agglutinated taxa (e.g., Ammoglobigerina globigeriniformis) showed significant negative response. For detecting the response of individual species to pH, body size and abnormal morphology of dominant species were measured and analyzed. Morphometric analysis of 1919 specimens showed the maximum length of hyaline and porcelaneous species decreased under low pH treatments (

Enhanced Organic Carbon Burial in Sediments of Oxygen Minimum Zones Upon Ocean Deoxygenation

Oxygen minimum zones (OMZs) in the ocean are expanding. This expansion is attributed to global warming and may continue over the next 10 to 100 kyrs due to multiple climate CO$_\text{2}$-driven factors. The expansion of oxygen-deficient waters has the potential to enhance organic carbon burial in marine sediments, thereby providing a negative feedback on global warming. Here, we study the response of dissolved oxygen in the ocean to increased phosphorus and iron inputs due to CO$_\text{2}$-driven enhanced weathering and increased dust emissions, respectively. We use an ocean biogeochemical model coupled to a general ocean circulation model (the Hamburg Oceanic Carbon Cycle model, HAMOCC 2.0) to assess the impact of such regional deoxygenation on organic carbon burial in the modern ocean on time scales of up to 200~kyrs. We find that an increase in input of phosphorus and iron leads to an expansion of the area of the OMZ impinging on continental margin sediments and a significant decline in bottom water oxygen in the open ocean relative to pre-industrial conditions. The associated increase in organic carbon burial could contribute to the drawdown of $\sim$1600~Gt of carbon, which is equivalent to the total amount of CO$_\text{2}$ in the atmosphere predicted for the year 2100 in a business as usual scenario, on time scales of up to 50 kyrs. The corresponding areal extent of sediments overlain by bottom waters with little or no oxygen as estimated by the model is not very different from the minimum area estimated for two major oceanic anoxic events in Earth's past. Such events were associated with major perturbations of the oceanic carbon cycle, including high rates of organic carbon burial. We conclude that organic carbon burial in low oxygen areas in the ocean could contribute to removal of anthropogenic CO$_\text{2}$ from the atmosphere on long time scales.

Diatoms Are Selective Segregators in Global Ocean Planktonic Communities.

Diatoms are a major component of phytoplankton, believed to be responsible for around 20% of the annual primary production on Earth. As abundant and ubiquitous organisms, they are known to establish biotic interactions with many other members of plankton. Through analyses of cooccurrence networks derived from the Tara Oceans expedition that take into account both biotic and abiotic factors in shaping the spatial distributions of species, we show that only 13% of diatom pairwise associations are driven by environmental conditions; the vast majority are independent of abiotic factors. In contrast to most other plankton groups, on a global scale, diatoms display a much higher proportion of negative correlations with other organisms, particularly toward potential predators and parasites, suggesting that their biogeography is constrained by top-down pressure. Genus-level analyses indicate that abundant diatoms are not necessarily the most connected and that species-specific abundance distribution patterns lead to negative associations with other organisms. In order to move forward in the biological interpretation of cooccurrence networks, an open-access extensive literature survey of diatom biotic interactions was compiled, of which 18.5% were recovered in the computed network. This result reveals the extent of what likely remains to be discovered in the field of planktonic biotic interactions, even for one of the best-known organismal groups.IMPORTANCE Diatoms are key phytoplankton in the modern ocean that are involved in numerous biotic interactions, ranging from symbiosis to predation and viral infection, which have considerable effects on global biogeochemical cycles. However, despite recent large-scale studies of plankton, we are still lacking a comprehensive picture of the diversity of diatom biotic interactions in the marine microbial community. Through the ecological interpretation of both inferred microbial association networks and available knowledge on diatom interactions compiled in an open-access database, we propose an ecosystems approach for exploring diatom interactions in the ocean.

Trace Metal and Cd Isotope Systematics of the Basal Datangpo Formation, Yangtze Platform (South China) Indicate Restrained (Bio)Geochemical Metal Cycling in Cryogenian Seawater

The behaviour of bioavailable trace metals and their stable isotopes in the modern oceans is controlled by uptake into phototrophic organisms and adsorption on and incorporation into marine authigenic minerals. Among other bioessential metals, Cd and its stable isotopes have recently been used in carbonate lithologies as novel tracer for changes in the paleo primary productivity and (bio)geochemical cycling. However, many marine sediments that were deposited during geologically highly relevant episodes and which, thus, urgently require study for a better understanding of the paleo environment are rather composed of a mixture of organic matter (OM), and detrital and authigenic minerals. In this study, we present Cd concentrations and their isotopic compositions as well as trace metal concentrations from sequential leachates of OM-rich shales of the Cryogenian basal Datangpo Formation, Yangtze Platform (South China). Our study shows variable distribution of conservative and bioavailable trace metals as well as Cd isotope compositions between sequential leachates of carbonate, OM, sulphide, and silicate phases. We show that the Cd isotope compositions obtained from OM leachates can be used to calculate the ambient Cryogenian surface seawater of the restricted Nanhua Basin by applying mass balance calculations. By contrast, early diagenetic Mn carbonates and sulphides incorporated the residual Cd from dissolved organic matter that was in isotopic equilibrium with deep/pore waters of the Nanhua Basin. Our model suggests that the Cd isotopic composition of surface seawater at that time reached values of modern oxygenated surface oceans. However, the deep water Cd isotope composition was substantially heavier than that of modern fully oxygenated oceans and rather resembles deep waters with abundant sulphide precipitation typical for modern oxygen minimum zones. This argues for incomplete recycling of Cd and other bioavailable metals shortly after the Sturtian glaciation in the redox stratified Cryogenian Nanhua Basin. Our study highlights the importance of sequential leaching procedures when dealing with impure authigenic sediments such as OM-rich carbonates, mudstones, or shales to achieve reliable trace metal concentrations and Cd isotope compositions as proxies for (bio)geochemical metal cycling in past aquatic systems.