Reconstruction of seasonal temperature variability in the tropical Pacific Ocean from the shell of the scallop, <i>Comptopallium radula</i>

International audienceWe investigated the oxygen isotope composition (d18O) of shell striae from juvenile Comptopallium radula (Mollusca; Pectinidae) specimens collected live in New Caledonia. Bottom-water temperature and salinity were monitored in-situ throughout the study period. External shell striae form with a 2-day periodicity in this scallop, making it possible to estimate the date of precipitation for each calcite sample collected along a growth transect. The oxygen isotope composition of shell calcite (d18Oshell calcite) measured at almost weekly resolution on calcite accreted between August 2002 and July 2003 accurately tracks bottom-water temperatures. A new empirical paleotemperature equation for this scallop species relates temperature and d18Oshell calcite: t(°C)=20.00(+/-0.61)-3.66(+/-0.39)x(d18Oshell calcite VPDB -d18Owater VSMOW) The mean absolute accuracy of temperature estimated using this equation is 1.0 °C at temperatures between 20 and 30 °C. Uncertainties regarding the precise timing of CaCO3 deposition and the actual variations in d18Owater at our study sites probably contribute to this error. Comparison with a previously published empirical paleotemperature equation indicates that C. radula calcite is enriched in 18O by ~0.7‰ relative to equilibrium. Given the direction of this offset and the lack of correlation between shell growth rate and d18Oshell calcite, this disequilibrium is unlikely to be related to kinetic isotope effects. We suggest that this enrichment reflects (1) a relatively low pH in the scallop's marginal extrapallial fluid (EPF), (2) an isotopic signature of the EPF different from that of seawater, or (3) Rayleigh fractionation during the biocalcification process. Relative changes in d18Oshell calcite reflect seawater temperature variability at this location and we suggest that the shell of C. radula may be useful as an archive of past seawater temperatures

Vertical structure, seasonal drawdown, and net community production in the Ross Sea, Antarctica

We calculate net community production (NCP) during summer 2005-2006 and spring 2006 in the Ross Sea using multiple approaches to determine the magnitude and consistency of rates. Water column carbon and nutrient inventories and surface ocean O-2/Ar data are compared to satellite-derived primary productivity (PP) estimates and C-14 uptake experiments. In spring, NCP was related to stratification proximal to upper ocean fronts. In summer, the most intense C drawdown was in shallow mixed layers affected by ice melt; depth-integrated C drawdown, however, increased with mixing depth. Delta O-2/Ar-based methods, relying on gas exchange reconstructions, underestimate NCP due to seasonal variations in surface Delta O-2/Ar and NCP rates. Mixed layer Delta O-2/Ar requires approximately 60 days to reach steady state, starting from early spring. Additionally, cold temperatures prolong the sensitivity of gas exchange reconstructions to past NCP variability. Complex vertical structure, in addition to the seasonal cycle, affects interpretations of surface-based observations, including those made from satellites. During both spring and summer, substantial fractions of NCP were below the mixed layer. Satellite-derived estimates tended to overestimate PP relative to C-14-based estimates, most severely in locations of stronger upper water column stratification. Biases notwithstanding, NCP-PP comparisons indicated that community respiration was of similar magnitude to NCP. We observed that a substantial portion of NCP remained as suspended particulate matter in the upper water column, demonstrating a lag between production and export. Resolving the dynamic physical processes that structure variance in NCP and its fate will enhance the understanding of the carbon cycling in highly productive Antarctic environments

Meteoric water and glacial melt in the southeastern Amundsen Sea: a time series from 1994 to 2020

Ice sheet mass loss from Antarctica is greatest in the Amundsen Sea sector, where “warm” modified Circumpolar Deep Water moves onto the continental shelf and melts and thins the bases of ice shelves hundreds of meters below the sea surface. We use nearly 1000 paired salinity and oxygen isotope analyses of seawater samples collected on seven expeditions from 1994 to 2020 to produce a time series of glacial meltwater inventory for the southeastern Amundsen Sea continental shelf. Deep water column salinity–δ¹⁸O relationships yield freshwater end-member δ¹⁸O values from -31.3+/-1.0% to -28.4+/-1.0%, consistent with the isotopic composition of local glacial ice. We use a two-component meteoric water end-member approach that accounts for precipitation in the upper water column, and a pure glacial meteoric water end-member is employed for the deep water column. Meteoric water inventories are comprised of nearly pure glacial meltwater in deep shelf waters and of >74 % glacial meltwater in the upper water column. Total meteoric water inventories range from 8.1±0.7 to 9.6±0.8 m and exhibit greater interannual variability than trend over the study period, based on the available data. The relatively long residence time in the southeastern Amundsen Sea allows changes in mean meteoric water inventories to diagnose large changes in local melt rates, and improved understanding of regional circulation could produce well-constrained glacial meltwater fluxes. The two-component meteoric end-member technique improves the accuracy of the sea ice melt and meteoric fractions estimated from seawater δ¹⁸O measurements throughout the entire water column and increases the utility for the broader application of these estimates

Carbon Isotopic Fractionation in Organic Matter Production Consistent With Benthic Community Composition Across a Coral Reef Flat

Carbon fluxes on coral reefs (net community production and net community calcification) aggregate the collective activity of all coral reef community members. This integrated approach provides powerful community-level insights, but is unable to resolve the finer-scale contributions of different reef functional groups to the community-scale rates. Tools are required to disaggregate the community-scale approaches and evaluate the performance of co-existing reef functional groups. Such assessments are necessary to improve forecasts of coral reef responses to global and local environmental change. We present results from a coral reef field study on One Tree Island reef in the Great Barrier Reef, off northeastern Australia, in September-October 2016 where we combined observations of total alkalinity, dissolved inorganic carbon (DIC), and the stable isotopic composition of dissolved inorganic carbon (δ13CDIC) to estimate carbon isotopic fractionation during organic matter formation. Portions of the reef with greater abundance of non-calcifying algae fractionated DIC ~5‰ more (stronger preference for 12C) during organic metabolism than did portions of the reef with a greater abundance of calcifiers. These results were consistent across a wide range of assumed isotopic fractionation factors for net calcification. We attribute the observed differences in carbon isotopic fractionation to the metabolic activities of the ecological community underlying each section of the reef, rather than to environmental factors such as light availability or water temperature. The patterns in carbon isotopic fractionation were generally consistent with inferred ratios of calcification to primary production in each reef zone, giving further confidence to our inference that differences in carbon isotopic fractionation may be related to differences in the ecological community on small spatial scales

Remote reef cryptobenthic diversity: Integrating autonomous reef monitoring structures and in situ environmental parameters

Coral reef sessile organisms inhabiting cryptic spaces and cavities of the reef matrix perform vital and varied functional roles but are often understudied in comparison to those on exposed surfaces. Here, we assess the composition of cryptobenthic taxa from three remote tropical reef sites (Central Indian Ocean) alongside a suite of in situ environmental parameters to determine if, or how, significant patterns of diversity are shaped by local abiotic factors. To achieve this, we carried out a point-count analysis of autonomous reef monitoring structure (ARMS) plate images and employed in situ instrumentation to recover long-term (12 months) profiles of flow velocity, wave heights, temperature, dissolved oxygen, and salinity, and short-term (3 weeks) profiles of light and pH. We recovered distinct environmental profiles between sampling sites and observed that ocean-facing reefs experienced frequent but short-lived cooling internal wave events and that these were key in shaping in situ temperature variability. By comparing temperature and wave height profiles recovered using in situ loggers with ex situ models, we discovered that global satellite products either failed to recover site-specific profiles or both over- and underestimated actual in situ conditions. We found that site choice and recruitment plate face (top or bottom) significantly impacted the percentage cover of bryozoans, gastropods, soft and calcified tube worms, as well as crustose coralline algae (CCA) and fleshy red, brown, and green encrusting macroalgae on ARMS. We observed significant correlations between the abundance of bryozoans, CCA, and colonial tunicates with lower mean temperature and higher mean dissolved oxygen profiles observed across sites. Red and brown encrusting macroalgae abundance correlated significantly with medium-to-high flow velocities and wave height profiles, as well as higher pH and dissolved oxygen. This study provides the first insight into cryptobenthic communities in the Chagos Archipelago marine-protected area and adds to our limited understanding of tropical reef sessile communities and their associations with environmental parameters in this region. With climate change accelerating the decline of reef ecosystems, integrating analyses of cryptobenthic organisms and in situ physicochemical factors are needed to understand how reef communities, if any, may withstand the impacts of climate change

Bubble Stripping as a Tool To Reduce High Dissolved CO₂ in Coastal Marine Ecosystems

High dissolved CO₂ concentrations in coastal ecosystems are a common occurrence due to a combination of large ecosystem metabolism, shallow water, and long residence times. Many important coastal species may have adapted to this natural variability over time, but eutrophication and ocean acidification may be perturbing the water chemistry beyond the bounds of tolerance for these organisms. We are currently limited in our ability to deal with the geochemical changes unfolding in our coastal ocean. This study helps to address this deficit of solutions by introducing bubble stripping as a novel geochemical engineering approach to reducing high CO₂ in coastal marine ecosystems. We use a process-based model to find that air/sea gas exchange rates within a bubbled system are 1–2 orders of magnitude higher than within a nonbubbled system. By coupling bubbling-enhanced ventilation to a coastal ecosystem metabolism model, we demonstrate that strategically timed bubble plumes can mitigate exposure to high CO₂ under present-day conditions and that exposure mitigation is enhanced in the more acidic conditions predicted by the end of the century. We argue that shallow water CO₂ bubble stripping should be considered among the growing list of engineering approaches intended to increase coastal resilience in a changing ocean

Carbonate composition of Pliocene-Pleistocene sediments of the Maledives

Ocean Drilling Program (ODP) Leg 115 post-cruise research was focused on two Maldives sites, more precisely on the top 108 m of Hole 716B (water depth, 540 m), equivalent to the past 3.5 m.y., and the top 19.5 m of Hole 714A (water depth, 2195 m), equivalent to the past 0.55 m.y. These sediments consist of mostly unaltered and undisturbed, turbidite-free, periplatform ooze. Results of our research are compared with existing data on Hole 633A (water depth, 1681 m), drilled in the Bahamas during ODP Leg 101, using age/depth models built on the basis of oxygen isotope, nannofossil, and magnetic stratigraphies. Climate-induced, long-term (roughly 0.5 m.y.) aragonite cycles, superposed on short-term (roughly 0.04 and 0.1 m.y.) aragonite cycles, have been established at least during the past 2.0 m.y., in the Maldives and the Bahamas. Our most interesting result is the clear correlation among the aragonite long-term cycles in the Maldives and the Bahamas and the carbonate-preservation, long-term cycles from the open Pacific, Indian, and North Atlantic oceans. The mid-Brunhes dissolution interval, corresponding to the youngest preservation minima of the carbonate-preservation, longterm cycles, is clearly defined by fine aragonite minimum values in the deep periplatform sites, and by maximum fragmentation of pteropod tests in the shallow sites. Aragonite and planktonic d18O records, usually in phase during the late Pleistocene, display, further back in time, discreet intervals where the two records do not match with one another. Major mismatches between both records occur synchronously in the Maldives and Bahamas periplatform sites and seem to correspond to extreme events of either carbonate-preservation or dissolution in the deep pelagic carbonate sites of the equatorial Pacific Ocean. Based on our findings, short- and long-term aragonite cycles can no longer be explained only by variations of aragonite input from the nearby shallow carbonate banks, in response to their alternate flooding and exposure through cyclic sea-level fluctuations. The aragonite long-term cycles in the periplatform environments are interpreted as carbonatepreservation cycles at intermediate-water depths. Their occurrence shows, therefore, that the carbonate chemistry of the entire water column has been influenced by long-term (0.5 m.y.) cyclic variations during the past 2.0 m.y. These major changes of the water-column carbonate chemistry are linked to the climate-induced carbon cycling among the different atmospheric, oceanic, and sedimentary carbon reservoirs