Calcite Saturation State Research Articles

Ocean carbon and nitrogen isotopes in CSIRO Mk3L-COAL version 1.0: a tool for palaeoceanographic research

Abstract. The isotopes of carbon (δ13C) and nitrogen (δ15N) are commonly used proxies for understanding the ocean. When used in tandem, they provide powerful insight into physical and biogeochemical processes. Here, we detail the implementation of δ13C and δ15N in the ocean component of an Earth system model. We evaluate our simulated δ13C and δ15N against contemporary measurements, place the model's performance alongside other isotope-enabled models and document the response of δ13C and δ15N to changes in ecosystem functioning. The model combines the Commonwealth Scientific and Industrial Research Organisation Mark 3L (CSIRO Mk3L) climate system model with the Carbon of the Ocean, Atmosphere and Land (COAL) biogeochemical model. The oceanic component of CSIRO Mk3L-COAL has a resolution of 1.6∘ latitude × 2.8∘ longitude and resolves multimillennial timescales, running at a rate of ∼400 years per day. We show that this coarse-resolution, computationally efficient model adequately reproduces water column and core-top δ13C and δ15N measurements, making it a useful tool for palaeoceanographic research. Changes to ecosystem function involve varying phytoplankton stoichiometry, varying CaCO3 production based on calcite saturation state and varying N2 fixation via iron limitation. We find that large changes in CaCO3 production have little effect on δ13C and δ15N, while changes in N2 fixation and phytoplankton stoichiometry have substantial and complex effects. Interpretations of palaeoceanographic records are therefore open to multiple lines of interpretation where multiple processes imprint on the isotopic signature, such as in the tropics, where denitrification, N2 fixation and nutrient utilisation influence δ15N. Hence, there is significant scope for isotope-enabled models to provide more robust interpretations of the proxy records.

Bottom water methane sources along the high latitude eastern Canadian continental shelf and their effects on the marine carbonate system

Measurements of dissolved methane (CH4) and carbonate system parameters were conducted in shelf and slope waters off Baffin Island and northern Labrador during September 2012 and August 2016 in order to investigate potential cold seeps and their influence on the marine carbonate system. Evidence of a strong near-bottom methane source was found at two sites along the Baffin Island Shelf (Scott Inlet and Cape Dyer) and above a pair of reported sea floor mounds in the Saglek Basin between Baffin Island and northern Labrador. Stable carbon isotope measurements of dissolved methane made at the Cape Dyer site give an estimated δ13C value of −71.2‰ for the source, consistent with microbial methanogenesis. The distributions of pCO2, pH and aragonite and calcite saturation states showed no evidence of enhanced ocean acidification in the vicinity of these sites, so it would seem that these seeps were not significant local sources of carbon dioxide, either as a component in the seep fluid or produced subsequent to release by water column microbial methane oxidation, nonetheless, aragonite saturation states of one or less were seen along the Baffin Island Shelf suggesting that ecosystems here may be vulnerable to further acidification. Spectrophotometric pH measurements of fresh samples in 2012 revealed a bottom water spike in pH at two seep sites that were not replicated by measurements of preserved seawater samples. We hypothesise that this was due to an unstable anionic compound present in seep fluid that can locally increase total alkalinity, pH and calcium carbonate saturation with HS− being a likely candidate, possibly to the benefit of calcifying organisms in an environment bordering on aragonite under-saturation.

Benthic fluxes from hypoxia-influenced Gulf of Mexico sediments: Impact on bottom water acidification

Benthic fluxes are reported from 8 sites on the shelf and slope of the Gulf of Mexico (GoM) in the region near the Mississippi River delta. Benthic landers, with the capability to incubate 3 chambers simultaneously, were deployed in August 2011 to establish the rate of phosphate, nitrate, ammonium, dissolved Si, Dissolved Inorganic Carbon (DIC) and alkalinity concentration change vs. incubation time (7–50 h) thereby providing flux estimates for these parameters. Stations located within 350 km of the Mississippi River mouth have nutrient and DIC fluxes that vary by a factor of 10 but show no longitudinal trends. Chambers deployed in low oxygen bottom water (<4 μM) do not have different flux values compared to chambers deployed in settings with higher bottom water oxygen (160 μM). At some stations, ammonium shows a change in flux with incubation time, yet no other species shows this change; hence neither a change in transport mechanism or reaction rate can explain the non-steady state behavior of ammonium. A positive correlation (r2 = 0.91) between the fluxes of dissolved Si and DIC imply a linkage between the diagenesis of biogenic Si and organic C throughout this region. The efflux of DIC and alkalinity in a ratio > 1 drives GoM bottom waters to a lower calcite and aragonite saturation state. Carbon isotopes of the DIC were measured on chamber waters and the calculated δ13C of the DIC source at 4 stations was similar, between −14 to −17‰ compared to typical δ13C values of bulk organic carbon in the GoM of −22‰. At a fifth station, the DIC added was calculated to have an isotope value of −31‰. Dissolved inorganic carbon isotope compositions that are significantly heavier or lighter than particulate organic carbon are unexpected and may be explained by a combination of inputs: the oxidation of average marine organic carbon and calcium carbonate dissolution, methanogenesis, and/or from oxidation of terrestrial lignin fractions.

Routine uncertainty propagation for the marine carbon dioxide system

Pairs of marine carbonate system variables are often used to calculate others, but those results are seldom reported with estimates of uncertainties. Although the procedure to propagate these uncertainties is well known, it has not been offered in public packages that compute marine carbonate chemistry, fundamental tools that are relied on by the community. To remedy this shortcoming, four of these packages were expanded to calculate sensitivities of computed variables with respect to each input variable and to use those sensitivities along with user-specified estimates of input uncertainties (standard uncertainties) to propagate uncertainties of calculated variables (combined standard uncertainties). Sensitivities from these packages agree with one another and with analytical solutions to within 0.01%; similar agreement among packages was found for the combined standard uncertainties. One package was used to quantify how propagated uncertainties vary among computed variables, seawater conditions, and the chosen pair of carbonate system variables that is used as input. The relative contributions to propagated uncertainties from the standard uncertainties of the input pair of measurements and various other input data (equilibrium constants etc) were explored with a new type of diagram. These error-space diagrams illustrate that further improvement beyond today's state-of-the-art measurement uncertainties for the input pair would generally be ineffective at reducing the combined standard uncertainties because the contribution from the constants is larger. Likewise, using much more uncertain measurements of the input pair does not always substantially worsen combined standard uncertainty. The constants that contribute most to combined standard uncertainties are generally K1 and K2, as expected. Yet more of the propagated uncertainty in the computed saturation states of aragonite and calcite comes from their solubility products. Thus percent relative combined standard uncertainties for the saturation states are larger than for the carbonate ion concentration. Routine propagation of these uncertainties should become standard practice.

Mapping of recent brachiopod microstructure: A tool for environmental studies.

Shells of brachiopods are excellent archives for environmental reconstructions in the recent and distant past as their microstructure and geochemistry respond to climate and environmental forcings. We studied the morphology and size of the basic structural unit, the secondary layer fibre, of the shells of several extant brachiopod taxa to derive a model correlating microstructural patterns to environmental conditions. Twenty-one adult specimens of six recent brachiopod species adapted to different environmental conditions, from Antarctica, to New Zealand, to the Mediterranean Sea, were chosen for microstructural analysis using SEM, TEM and EBSD. We conclude that: 1) there is no significant difference in the shape and size of the fibres between ventral and dorsal valves, 2) there is an ontogenetic trend in the shape and size of the fibres, as they become larger, wider, and flatter with increasing age. This indicates that the fibrous layer produced in the later stages of growth, which is recommended by the literature to be the best material for geochemical analyses, has a different morphostructure and probably a lower organic content than that produced earlier in life. In two species of the same genus living in seawater with different temperature and carbonate saturation state, a relationship emerged between the microstructure and environmental conditions. Fibres of the polar Liothyrella uva tend to be smaller, rounder and less convex than those of the temperate Liothyrella neozelanica, suggesting a relationship between microstructural size, shell organic matter content, ambient seawater temperature and calcite saturation state.

Eddy‐induced carbon transport across the Antarctic Circumpolar Current

AbstractThe implications of a mesoscale eddy for relevant properties of the Southern Ocean carbon cycle are examined with in situ observations. We explored carbon properties inside a large (~190 km diameter) cyclonic eddy that detached from the Subantarctic Front (SAF) south of Tasmania in March 2016. Based on remote sensing, the eddy was present for ~2 months in the Subantarctic Zone (SAZ), an important region of oceanic carbon dioxide (CO2) uptake throughout the annual cycle and carbon subduction (i.e., where mode and intermediate waters form), before it was reabsorbed into the SAF. The eddy was sampled during the middle of its life, 1 month after it spawned. Comparatively, the eddy was ~3°C colder, 0.5 practical salinity unit fresher, and less biologically productive than surrounding SAZ waters. The eddy was also richer in dissolved inorganic carbon (DIC) and had lower saturation states of aragonite and calcite than the surrounding SAZ waters. As a consequence, it was a strong source of CO2 to the atmosphere (with fluxes up to +25 mmol C m−2 d−1). Compared to the SAF waters, from which it originated, DIC concentration in the eddy was ~20 μmol kg−1 lower, indicating lateral mixing, small‐scale recirculation, or eddy stirring with lower‐DIC SAZ waters by the time the eddy was observed. As they are commonly spawned from the Antarctic Circumpolar Current, and as 50% of them decay in the SAZ (the rest being reabsorbed by the SAF‐N), these types of eddies may represent a significant south‐north transport pathway for carbon across the ACC and may alter the carbon properties of SAZ waters.

Coccolithophore haploid and diploid distribution patterns in the Mediterranean Sea: can a haplo-diploid life cycle be advantageous under climate change?

Coccolithophores are unicellular pelagic algae, capable of calcification. In the Mediterranean Sea, several species have a well-known haplo-diploid life cycle, alternating the production of different types of calcite plates, the holo- and hetero-coccoliths. We analyzed the distribution of both phases along a W-E Mediterranean transect during April 2011 and May 2013 (spring season), following strong environmental gradients in salinity, oxygen and nutrient concentration, temperature, carbonate chemistry and fluorescence. The proportion of holococcolithophores:heterococcolithophores of selected species varies not only vertically through the water column, but also longitudinally, following the main environmental gradients. Based on the environmental affinities of the coccolithophore life phases, we conclude that a dimorphic life cycle might provide the ability to adapt to the south-eastern (SE) Mediterranean environment, in conditions characterized by surface water with relatively high calcite saturation state, high temperature, stratification and nutrient limitation, and support the survival of species whose diploid phases are in contrast adapted to Atlantic or south-western (SW) Mediterranean conditions. Thus, a haplo-diploid life cycle could provide a way to adapt to environmental changes.

Ostracod calcite records the 18O/16O ratio of the bicarbonate and carbonate ions in water

The δ18O of ostracod valves is widely used to infer water δ18O and temperature. However, ostracod δ18O appears sensitive to other environmental variables. In addition, there is species-dependent ostracod calcite 18O enrichment, relative to slowly precipitated inorganic calcite under the same conditions. Together these uncertainties complicate ostracod paleoclimate reconstructions. This study presents a new understanding of the causes of ostracod δ18O variations based on a global database of published ostracod δ18O values in lake, marine and coastal environments, and from culture experiments. The database includes associated field/experiment host water parameters including temperature (−1 to 32°C), water δ18O (−13.2‰ to 4.3‰ VSMOW), pH (6.9–10.4), salinity (0–72g/kg), calcite saturation states (0.6–26), and dissolved inorganic carbon concentration [DIC] (0.9–54.3mmol/kg). The data show that: (1) the δ18O of marine and non-marine ostracods reflects the 18O/16O of the sum of host water CO32− and HCO3− ions. For example, at a given temperature, the δ18O of non-marine ostracods decreases by 4‰ to 6‰ as [CO32−]/[DIC] reaches 70%, depending on the ostracod species. In low [CO32−]/[DIC] settings (i.e. high HCO3−/CO32−), ostracod 18O/16O is close to the 18O/16O of HCO3− ions, which explains why on average ostracod δ18O is higher than the δ18O of inorganic calcite precipitated slowly under the same conditions. (2) Taxonomic offsets in ostracod δ18O vary with the host water [CO32−]/[DIC]. In environments where HCO3−≫CO32− (i.e. most freshwater lakes), the 18O/16O of Candonids is indistinguishable from the 18O/16O of HCO3− ions (difference of 0.10±0.16‰) while the 18O/16O of Cyprids is lower than the 18O/16O of HCO3− ions by −0.77‰ to −0.32‰, Cytherids by −0.88±0.29‰, and Limnocytherids by −1.12±0.05‰. (3) The sensitivity of ostracod δ18O to [CO32−]/[DIC] also varies with taxonomy. For each percent increase in [CO32−]/[DIC], Candonids δ18O decreases by −0.098±0.024‰, Cyprids by −0.075±0.004‰, Cytherids by −0.057±0.012‰, and Limnocytherids by −0.058±0.005‰. (4) The similarity in 18O/16O between ostracod calcite and the sum of host water ‘HCO3− and CO32−’ (despite species-specific offsets) suggests rapid precipitation of a finite DIC pool in the ostracod calcifying fluid. We propose an ostracod δ18O model whereby the more negative the 18O/16O of a given species relative to the sum of CO32− and HCO3−, the greater the isotopic equilibrium between the DIC pool and H2O in the ostracod calcifying fluid. (5) Higher host water salinities and pH induce higher [CO32−]/[DIC], resulting in lower ostracod δ18O, which explains the variable salinity and pH effects on ostracod δ18O. Our database and ostracod δ18O model shows that ostracod δ18O records from closed basin environments likely reflect high or variable [CO32−]/[DIC], rather than water temperature and δ18O alone. Our study reconciles contradictory observations of controls on ostracod δ18O and paves the way for improved paleo-environmental interpretations and reconstructions of past water [CO32−]/[DIC].

Oxygen isotope fractionation in the CaCO3-DIC-H2O system

The oxygen isotope ratio (δ18O) of inorganic and biogenic carbonates is widely used to reconstruct past environments. However, the oxygen isotope exchange between CaCO3 and H2O rarely reaches equilibrium and kinetic isotope effects (KIE) commonly complicate paleoclimate reconstructions. We present a comprehensive model of kinetic and equilibrium oxygen isotope fractionation between CaCO3 and water (αc/w) that accounts for fractionation between both (a) CaCO3 and the CO32− pool (αc/CO32-), and (b) CO32− and water (αCO32-/w), as a function of temperature, pH, salinity, calcite saturation state (Ω), the residence time of the dissolved inorganic carbon (DIC) in solution, and the activity of the enzyme carbonic anhydrase. The model results suggest that: (1) The equilibrium αc/w is only approached in solutions with low Ω (i.e. close to 1) and low ionic strength such as in the cave system of Devils Hole, Nevada. (2) The sensitivity of αc/w to the solution pH and/or the mineral growth rate depends on the level of isotopic equilibration between the CO32− pool and water. When the CO32− pool approaches isotopic equilibrium with water, small negative pH and/or growth rate effects on αc/w of about 1–2‰ occur where these parameters covary with Ω. In contrast, isotopic disequilibrium between CO32− and water leads to strong (>2‰) positive or negative pH and growth rate effects on αCO32-/w (and αc/w) due to the isotopic imprint of oxygen atoms derived from HCO3−, CO2, H2O and/or OH−. (3) The temperature sensitivity of αc/w originates from the negative effect of temperature on αCO32-/w and is expected to deviate from the commonly accepted value (−0.22±0.02‰/°C between 0 and 30°C; Kim and O’Neil, 1997) when the CO32− pool is not in isotopic equilibrium with water. (4) The model suggests that the δ18O of planktic and benthic foraminifers reflects a quantitative precipitation of DIC in isotopic equilibrium with a high-pH calcifying fluid, leading to a relatively constant foraminifer calcite δ18O-temperature relationship (−0.21±0.01‰/°C). The lower average coral δ18O data relative to foraminifers and other calcifiers is best explained by the precipitation of internal DIC derived from hydrated CO2 in a high-pH calcifying fluid and minimal subsequent DIC-H2O isotopic equilibration. This leads to a reduced and variable coral aragonite δ18O-temperature relationship (−0.11 to −0.22‰/°C). Together, the model presented here reconciles observations of oxygen isotope fractionation over a range of CaCO3-DIC-H2O systems.

Remote effects of mixed layer development on ocean acidification in the subsurface layers of the North Pacific

Using the outputs of projections under the highest emission scenario of the representative concentration pathways performed by Earth system models (ESMs), we evaluate the ocean acidification rates of subsurface layers of the western North Pacific, where the strongest sink of atmospheric CO2 is found in the mid-latitudes. The low potential vorticity water mass called the North Pacific Subtropical Mode Water (STMW) shows large dissolved inorganic carbon (DIC) concentration increase, and is advected southwestward, so that, in the sea to the south of Japan, DIC concentration increases and ocean acidification occurs faster than in adjacent regions. In the STMW of the Izu-Ogasawara region, the ocean acidification occurs with a pH decrease of ~0.004 year−1 , a much higher rate than the previously estimated global average (0.0023 year−1), so that the pH decreases by 0.3–0.4 during the twenty-first century and the saturation state of calcite (ΩCa) decreases from ~4.8 down to ~2.4. We find that the ESMs with a deeper mixed layer in the Kuroshio Extension region show a larger increase in DIC concentration within the Izu-Ogasawara region and within the Ryukyu Islands region. Comparing model results with the mixed layer depth obtained from the Argo dataset, we estimate that DIC concentration at a depth of ~200 m increases by 1.4–1.6 μmol kg−1 year−1 in the Izu-Ogasawara region and by 1.1–1.4 μmol kg−1 year−1 in the Ryukyu Islands region toward the end of this century.

Spatial patterns of phytoplankton composition and upper-ocean biogeochemistry do not follow carbonate chemistry gradients in north-west European Shelf seas

A key difficulty in ocean acidification research is to predict its impact after physiological, phenotypic, and genotypic adaptation has had time to take place. Observational datasets can be a useful tool in addressing this issue. During a cruise in June–July 2011, measurements of upper-ocean biogeochemical variables, climatically active gases and plankton community composition were collected from northwestern European seas. We used various multivariate statistical techniques to assess the relative influences of carbonate chemistry and other environmental factors on these response variables. We found that the spatial patterns in plankton communities were driven more by nutrient availability and physical variables than by carbonate chemistry. The best subset of variables able to account for phytoplankton community structure was the euphotic zone depth, silicic acid availability, mixed layer average irradiance, and nitrate concentration (59% of variance explained). The spatial variations in phytoplankton and coccolithophores species composition were both found to be more strongly associated with nutrients and physical variables than carbonate chemistry, with the latter only explaining 14 and 9% of the variance, respectively. The plankton community composition and contribution of calcifying organisms was not observed to change under lower calcite saturation state (Ω) conditions, although no regions of undersaturation (Ω &amp;lt; 1) were encountered during the cruise. Carbonate chemistry played a more prominent, but still secondary, role in determining dinoflagellate and diatom assemblage composition (20 and 13% of total variance explained, respectively). Nutrient and physical variables also explained more of the spatial variations of most climatically active gases and selected biogeochemical response variables, although some also appeared to be influenced by carbonate chemistry. This observational study has demonstrated that ocean acidification research needs to be set in context with other environmental forcing variables to fully appreciate the primary, or indeed secondary, role that increasing fugacity of carbon dioxide has on biological communities and associated biogeochemical rates.

The experimental determination of REE partition coefficients in the water-calcite system

To better quantify the behaviour of rare earth elements (REE) in low temperature natural systems, calcite was precipitated from NaCl-bearing aqueous solutions in the presence of La and Yb using a constant addition technique at pH ~6 and 25°C under steady-state conditions. The presence of REE in the fluid significantly slows calcite growth rates. Partition coefficients (DREE), determined from analysis of the aqueous solution compositions, range from 102.8 at the lowest aqueous La concentration to 101.4 at the highest aqueous concentration of Yb. DREE was found to correlate with aqueous REE concentrations, as well as with the calcite saturation state. Experimental results, together with previous spectroscopic evidence, suggest the incorporation of REE3+ into the Ca2+ site of calcite while maintaining charge balance by the concurrent incorporation of Na+. The value of the equilibrium constants for the corresponding exchange reaction was determined for La, Eu and Yb by combining our results with previously published experimental data. Subsequently, these constants were used together with a lattice strain model to estimate corresponding values for the other REE. Application of this exchange model suggests that DREE vary significantly with the composition of natural waters including seawater. Such variations are likely the origin for the large differences of DREE values reported in the literature based on experimental and field evidence.

Fluoride and iodine enrichment in groundwater of North China Plain: Evidences from speciation analysis and geochemical modeling

To better understand the enrichment of fluoride and iodine in groundwater at North China Plain (NCP), speciation analysis and geochemical modeling were conducted to identify the key hydrochemical processes controlling their mobilization in groundwater system. Groundwater fluoride and iodine concentrations ranged from 0.18 to 5.59mg/L and from 1.51 to 1106μg/L, respectively, and approximately 63% and 32.3% of groundwater fluoride and iodine were higher than the guidelines for drinking water (1.5mg/L and 150μg/L). High fluoride concentration (>1.5mg/L) can be detected in groundwater from the flow-through and discharge areas of NCP, and high iodine groundwater (>150μg/L) is mainly scattered in the coastal area. Na-HCO3/Cl type water resulted from water-rock interaction and seawater intrusion favors fluoride and iodine enrichment in groundwater. Speciation analysis results indicate that (1) fluoride complexes in groundwater are dominated by free fluoride, the negative charge of which favors fluoride enrichment in groundwater under basic conditions, and (2) iodide, iodate and organic iodine co-occur in groundwater at NCP with iodide as the dominant species. The geochemical modeling results indicate that groundwater fluoride is mainly associated with the saturation states of fluorite and calcite, as well as the adsorption equilibrium onto goethite and gibbsite, including the competitive adsorption between fluoride and carbonate. Groundwater iodine is mainly controlled by redox potential and pH condition of groundwater system. Reducing condition favors the mobilization and enrichment of groundwater iodide, which has the highest mobility among iodine species. Under reducing condition, reductive dissolution of iron (oxy)hydroxides is a potential geochemical process responsible for iodine release from sediment into groundwater. Under (sub)oxidizing condition, as groundwater pH over the ‘point of zero charge’ of iron (oxy)hydroxides, the lowering adsorption capacity of groundwater iodide/iodate on minerals leads to the release of sediment iodine into groundwater.

Effect of calcite precipitation on stable strontium isotopic compositions: Insights from riverine and pool waters in a karst cave

Radiogenic and stable Sr isotope ratios (87Sr/86Sr, δ88/86Sr) serve as potential tracers for evaluation of weathering sources and associated physicochemical processes in terrestrial environments. Despite carbonate weathering sourcing two-thirds of the total continental Sr flux to the ocean, however, few attempts have been aimed at studying stable Sr isotope fractionation in carbonate-dominated aquifers. Here we present a monthly-resolved monitoring dataset of major ion concentrations and Sr isotope ratios for karst riverine and pool waters at the Luofang Cave in the Central China to understand the potential controls responsible for water δ88/86Sr compositions under various climatic conditions. With the exception of some pool waters in the dry period, significantly heavier δ88/86Sr were detected in the riverine and pool waters (0.20‰ to 0.42‰) compared to bedrock δ88/86Sr (0.24‰ to 0.28‰), which can't be interpreted solely by lithology. A sequential leaching experiment of the local soils suggests limited stable Sr isotope fractionation during incongruent silicate mineral dissolution. Local plant utilization and biological activity in the soil overlying the cave are possible processes for causing the observed heavier δ88/86Sr in cave water. However, air partial pressure of CO2 in cave, monitoring water pH, and Sr isotopic mass balance calculations suggest that plant utilization is unlikely to be the primary control on the heavier water δ88/86Sr. On the other hand, significant stable Sr isotope fractionation was found between the cave dripping waters and calcite precipitates, and the Δ88/86Srcar-aq was estimated to be −0.15‰±0.07‰ (2SD). Precipitation of secondary calcites might be a potential process responsible for the heavier δ88/86Sr in the waters. The negative correlation between δ88/86Sr and Sr/Na ratios in the cave waters is consistent with a scenario of preferential incorporation of the light Sr isotope by solid phase. Furthermore, by combining the calcite saturation states and water pH, our results suggest that calcite precipitation is the most likely process controlling δ88/86Sr compositions in cave water. We demonstrate that secondary calcite precipitation in carbonate-dominated catchments could be important in controlling the riverine water δ88/86Sr compositions.

Variability and Transport of Inorganic Carbon Dioxide in a Tropical Estuary

Marine ecosystems are a known net source of greenhouse gases emissions. On the other hand, the geochemical fluxes, particularly from the developing countries estuarine areas, are poorly evaluated. In the present study, temporal and longitudinal water samplings were taken hourly and seasonally during full semidiurnal tidal-cycles, along the Caravelas estuary, located in northeast region of Brazil. It was analyzed the pH, total alkalinity, aragonite saturation state, calcite saturation state, dissolved inorganic carbon, aqueous CO2 and partial pressure of CO2 found in estuarine waters. Results showed that the total alkalinity values decreased from the estuary mouth up-river during the dry season, which reflects the low buffering capacity of river water. The estuarine total alkalinity and dissolved inorganic carbon fluxes were significantly higher in the dry season during the spring tide. The spring and neap tides were typically oversaturated with CO2 in both the dry and rainy seasons. The highest average values were found in the dry season for both the neap and spring tides, while the CO2 air-water flux values throughout the dry and rainy seasons, retained a positive variation range. Finally, the annual surface water flux of dissolved inorganic carbon revealed to be positive suggesting dissolved inorganic carbon exportation to coastal water.

Environmental drivers of coccolithophore abundance and calcification across Drake Passage (Southern Ocean)

Abstract. Although coccolithophores are not as numerically common or as diverse in the Southern Ocean as they are in subpolar waters of the North Atlantic, a few species, such as Emiliania huxleyi, are found during the summer months. Little is actually known about the calcite production (CP) of these communities or how their distribution and physiology relate to environmental variables in this region. In February 2009, we made observations across Drake Passage (between South America and the Antarctic Peninsula) of coccolithophore distribution, CP, primary production, chlorophyll a and macronutrient concentrations, irradiance and carbonate chemistry. Although CP represented less than 1 % of total carbon fixation, coccolithophores were widespread across Drake Passage. The B/C morphotype of E. huxleyi was the dominant coccolithophore, with low estimates of coccolith calcite (∼ 0.01 pmol C coccolith−1) from biometric measurements. Both cell-normalised calcification (0.01–0.16 pmol C cell−1 d−1) and total CP (&lt; 20 µmol C m−3 d−1) were much lower than those observed in the subpolar North Atlantic where E. huxleyi morphotype A is dominant. However, estimates of coccolith production rates were similar (0.1–1.2 coccoliths cell−1 h−1) to previous measurements made in the subpolar North Atlantic. A multivariate statistical approach found that temperature and irradiance together were best able to explain the observed variation in species distribution and abundance (Spearman's rank correlation ρ = 0.4, p &lt; 0.01). Rates of calcification per cell and coccolith production, as well as community CP and E. huxleyi abundance, were all positively correlated (p &lt; 0.05) to the strong latitudinal gradient in temperature, irradiance and calcite saturation states across Drake Passage. Broadly, our results lend support to recent suggestions that coccolithophores, especially E. huxleyi, are advancing polewards. However, our in situ observations indicate that this may owe more to sea-surface warming and increasing irradiance rather than increasing CO2 concentrations.

Seasonality in planktic foraminifera of the central California coastal upwelling region

Abstract. The close association between planktic foraminiferal assemblages and local hydrography make foraminifera invaluable proxies for environmental conditions. Modern foraminiferal seasonality is important for interpreting fossil distributions and shell geochemistry as paleoclimate proxies. Understanding this seasonality in an active upwelling area is also critical for anticipating which species may be vulnerable to future changes in upwelling intensity and ocean acidification. Two years (2012–2014) of plankton tows, along with conductivity–temperature–depth profiles and carbonate chemistry measurements taken along the north-central California shelf, offer new insights into the seasonal dynamics of planktic foraminifera in a seasonal coastal upwelling regime. This study finds an upwelling affinity for Neogloboquadrina pachyderma as well as a seasonal and upwelling associated alternation between dominance of N. pachyderma and Neogloboquadrina incompta, consistent with previous observations. Globigerina bulloides, however, shows a strong affinity for non-upwelled waters, in contrast to findings in Southern California where the species is often associated with upwelling. We also find an apparent lunar periodicity in the abundances of all species and document the presence of foraminifera even at very low saturation states of calcite.

Coccolithophore responses to environmental variability in the South China Sea: species composition and calcite content

Abstract. Coccolithophore contributions to the global marine carbon cycle are regulated by the calcite content of their scales (coccoliths) and the relative cellular levels of photosynthesis and calcification rates. All three of these factors vary between coccolithophore species and with response to the growth environment. Here, water samples were collected in the northern basin of the South China Sea (SCS) during summer 2014 in order to examine how environmental variability influenced species composition and cellular levels of calcite content. Average coccolithophore abundance and their calcite concentration in the water column were 11.82 cells mL−1 and 1508.3 pg C mL−1, respectively, during the cruise. Water samples can be divided into three floral groups according to their distinct coccolithophore communities. The vertical structure of the coccolithophore community in the water column was controlled by the trophic conditions, which were regulated by mesoscale eddies across the SCS basin. The evaluation of coccolithophore-based calcite in the surface ocean also showed that three key species in the SCS (Emiliania huxleyi, Gephyrocapsa oceanica, Florisphaera profunda) and other larger, numerically rare species made almost equal contributions to total coccolith-based calcite in the water column. For Emiliania huxleyi biometry measurements, coccolith size positively correlated with nutrients (nitrate, phosphate), and it is suggested that coccolith length is influenced by light and nutrients through the regulation of growth rates. Larger-sized coccoliths were also linked statistically to low pH and calcite saturation states; however, it is not a simple cause and effect relationship, as carbonate chemistry was strongly co-correlated with the other key environmental factors (nutrients, light).

Effects of Co-Varying Diel-Cycling Hypoxia and pH on Growth in the Juvenile Eastern Oyster, Crassostrea virginica

Shallow water provides important habitat for many species, but also exposes these organisms to daily fluctuations in dissolved oxygen (DO) and pH caused by cycles in the balance between photosynthesis and respiration that can contribute to repeated, brief periods of hypoxia and low pH (caused by elevated pCO2). The amplitude of these cycles, and the severity and duration of hypoxia and hypercapnia that result, can be increased by eutrophication, and are predicted to worsen with climate change. We conducted laboratory experiments to test the effects of both diel-cycling and constant low DO and pH (elevated pCO2) on growth of the juvenile eastern oyster (Crassostrea virginica), an economically and ecologically important estuarine species. Severe diel-cycling hypoxia (to 0.5 mg O2 L-1) reduced shell growth in juvenile oysters, as did constant hypoxia (1.2 and 2.0 mg O2 L-1), although effects varied among experiments, oyster ages, and exposure durations. Diel-cycling pH reduced growth only in experiments in which calcite saturation state cycled to ≤0.10 and only during the initial weeks of these experiments. In other cases, cycling pH sometimes led to increased growth rates. Comparisons of treatment effects across multiple weeks of exposure, and during a longer post-experiment field deployment, indicated that juvenile oysters can acclimate to, and in some cases compensate for initial reductions in growth. As a result, some ecosystem services dependent on juvenile oyster growth rates may be preserved even under severe cycling hypoxia and pH.

TEST APPEARANCE OF THE PLANKTONIC FORAMINIFERPULLENIATINA OBLIQUILOCULATAAS AN INDICATOR OF CALCITE DISSOLUTION IN DEEP-SEA SEDIMENTS

Deep-sea calcareous sediments are subject to dissolution that can alter the appearance and geochemistry of planktonic foraminiferal tests and faunal assemblages. Assessment of the impact of dissolution is often based on assumptions [e.g., position of a sediment sample with respect to the seawater calcite-saturation horizon (CSH)] and elaborate techniques (e.g., scanning-electron microscopy). To assess dissolution effects on calcareous sediments, we weighed and photographed tests of Pulleniatina obliquiloculata from South China Sea (SCS) sediment-surface samples spanning deposition above the CSH down to the calcite-compensation depth. Controlled by the calcite-saturation state of bottom waters (Δ[CO 3 2− ] bw ), the observed decrease in test weights by ~0.3 μg per μmol kg −1 was accompanied by decreasing Mg/Ca ratios (~0.12 mmol mol −1 per μg) and increasing stable oxygen isotope values δ 18 O; ~−0.13‰ per μg). Concomitant changes in test appearance demonstrated the potential of P. obliquiloculata to indicate distinct preservation stages. We present an economical, fast and non-destructive guideline to evaluate sediment preservation based on P. obliquiloculata appearance under a light microscope. In addition to the initial P. obilquiloculata test preservation, characterized by a smooth and shiny surface of slightly brownish-pinkish color in samples deposited in bottom waters with a Δ[CO 3 2− ] bw >21 μmol kg −1 (no dissolution), three dissolution stages can be distinguished: 1) Gentle dissolution (Δ[CO 3 2− ] bw ~10–21 μmol kg −1 , little alteration of planktonic foraminiferal Mg/Ca and δ 18 O) indicated by slightly brownish-pinkish tests with smooth and shiny surfaces only slightly damaged by fissures on top of the pores along the test’s periphery; 2) Moderate dissolution (Δ[CO 3 2− ] bw ~0–10 μmol kg −1 , clear alteration of Mg/Ca and δ 18 O) with extensive disintegration and coexistence of both slightly brownish-pinkish tests and white tests with initially smooth surfaces already decayed to small prismatic units; 3) Severe dissolution (Δ[CO 3 2− ] bw −1 , significant alteration of planktonic foraminiferal Mg/Ca and δ 18 O) indicated by only white tests showing several calcite layers, holes and broken final chambers.