Calcite Saturation State Research Articles

Distinct responses of planktonic foraminiferal B/Ca to dissolution on seafloor

AbstractWe have measured B/Ca in four core‐top planktonic foraminiferal species (Globigerinoides ruber (white), Globigerinoides sacculifer (without final sac‐like chamber), Neogloboquadrina dutertrei, and Pulleniatina obliquiloculata) from three depth transects (the Caribbean Sea, the southwestern Indian Ocean, and the Ontong Java Plateau) to evaluate the effect of dissolution on planktonic foraminiferal B/Ca. At each transect, G. ruber (w) and G. sacculifer (w/o sac) show decreasing B/Ca with increasing water depth. This decrease in B/Ca is accompanied with decreases in shell weights, Mg/Ca, and bottom water calcite saturation state. This indicates a postdepositional dissolution effect on B/Ca in these two species. The strong correlation observed between changes in B/Ca and bottom water calcite saturation state offers an approach to correcting for the dissolution bias. By contrast, B/Ca in N. dutertrei and P. obliquiloculata remains unchanged along depth transects, although shell weights and Mg/Ca display significant declines. Overall, our core‐top results suggest species‐specific dissolution effects on B/Ca in different planktonic foraminiferal species.

Temperature and altitudinal influence on karst dripwater chemistry: Implications for regional-scale palaeoclimate reconstructions from speleothems

The reconstruction of robust past climate records from speleothems requires a prior understanding of the environmental and hydrological conditions that lead to speleothem formation and the chemical signals encoded within them. On regional-scales, there has been little quantification of the dependency of cave dripwater geochemistry on meteorology (net infiltration, temperature), environmental and geographical factors (elevation, latitude, soil activity, vegetation cover, atmospheric aerosol composition) and geological properties of the aquifer (lithology, porosity and thickness). In the present study, we analysed over 200 karst waters collected in 11 caves of the Trentino region (NE Italy). The caves span sub-humid Mediterranean to cold-humid temperate climates and infiltration elevations (Zinf) ranging from 355 to 2400ma.s.l., corresponding to infiltration mean annual temperatures (MATinf) between 12 and 0°C. Since all the caves developed in pure carbonate rocks, soil pCO2 is found to be the main factor controlling the carbonate dissolution. For this reason, the parameters controlling the carbonate–carbonic acid system and calcite saturation state (SICC) are directly correlated with the MATinf, which influences the vegetation zones and eventually the production of CO2 in the soil. SICC linearly depends on MATinf (SICC=0.09 MATinf−0.4) and SICC=0 is reached at Zinf=1.66kma.s.l., corresponding to a MATinf=4.4°C. This point identifies the “speleothem limit” defined here as the elevation (or corresponding MATinf) above which no sparitic speleothem precipitation usually occurs. We demonstrate that due to temperature-forced changes in the soil and vegetation and subsequently SICC, the speleothem limit shifts to higher altitudes during maximum interglacial conditions. Speleothems from high altitude caves (1.5–2.5kma.s.l.) thus can identify optimum interglacial periods. By contrast, speleothems formed at lower altitudes are better suited as archives of hydrological proxies. At altitudes below 1.2kma.s.l., prior calcite precipitation (PCP) modifies percolating waters, particularly during periods of reduced infiltration. We introduce the use of the SiO2/Ca and SO4/Ca ratios in cave waters to complement Mg/Ca and Sr/Ca ratios as markers of PCP. SO4 and SiO2 are derived from atmospheric deposition and siliciclastic minerals in the soil zone, rather than carbonate host rocks (as in the case of Mg and Sr). By combing shifts to higher Mg/Ca, SiO2/Ca and SO4/Ca ratios along their characteristics PCP lines, we improve the robustness of the interpretation that this resulted from increasing PCP, rather than incongruent calcite dissolution (ICD). Our method permits the quantification of PCP between 0% and 40% for low elevation cave waters. This novel approach has important implications for speleothem-based paleoclimate studies where the distinction between PCP and ICD can be ambiguous and, in combination with Mg/Ca and Sr/Ca ratios, permits the quantification of net infiltration and/or rainfall amount from speleothem records.

Assessing the reliability of foraminiferal Mg/Ca thermometry by comparing field-samples and culture experiments: a review

Trace elements incorporated into foraminiferal test carbonate are commonly used as palaeoproxies. For instance, benthic and planktonic Mg/Ca ratios are frequently used for reconstructing bottom and sea surface temperature (SST) changes, respectively. However, over the past few decades it has been shown that the incorporation of Mg 2+ into foraminiferal calcite is controlled by more than one environmental parameter, with significant variations of their sensitivities between culture and field-based studies. Opinions differ as to whether the laboratory conditions during culturing experiments or the natural conditions during field sampling (core-tops, sediment traps and plankton tows) better trace the wealth of information with improved accuracy. Laboratory culture experiments that isolate the effects of individual environmental parameters have been used to identify secondary controls on Mg uptake into planktonic foraminifer tests. However, field-based data (core-top sediments and plankton tows) from high salinity super-saturated settings have shown the additional presence of high-Mg inorganic precipitates leading to significant salinity (S) biases on the Mg/Ca palaeothermometer. Testing such synergistic effects between temperature, salinity and calcite saturation state would require an experimental design where all these parameters are varied systematically, but such experiments have yet to be conducted. Since the synergistic effects cannot presently be ruled out through culturing experiments, it is imperative to initially confirm the amplitude and geographic distribution of the detectable diagenetic precipitations (Scanning Electron Microscopy analysis) through field-based work, further quantify their importance (discrimination of distinct diagenetic stages and quantification of the diagenetic imprint) and finally estimate its potential effect on Mg/Ca-T calibration (e.g., overgrowth-corrected species-specific calibration equations). The example of the marginal high-salinity settings, among others, clearly highlights that the optimal use of Mg/Ca as a palaeotemperature proxy urgently requires the complementarity of both culture- and field-based data. To this end, we here present advantages and disadvantages to each approach. These insights reinforce the potential of the combined use of culture- and field-based foraminiferal studies, where possible, in order to minimize the observed inconsistencies, and to advance Mg/Ca thermometry by both providing a framework for better understanding the nature of Mg/Ca dependence on seawater temperature, and the effects of complicating factors.

Carbonate saturation state of surface waters in the Ross Sea and Southern Ocean: controls and implications for the onset of aragonite undersaturation

Abstract. Predicting when surface waters of the Ross Sea and Southern Ocean will become undersaturated with respect to biogenic carbonate minerals is challenging in part due to the lack of baseline high-resolution carbon system data. Here we present ~ 1700 surface total alkalinity measurements from the Ross Sea and along a transect between the Ross Sea and southern Chile from the austral autumn (February–March 2013). We calculate the saturation state of aragonite (ΩAr) and calcite (Ω Ca) using measured total alkalinity and pCO2. In the Ross Sea and south of the Polar Front, variability in carbonate saturation state (Ω) is mainly driven by algal photosynthesis. Freshwater dilution and calcification have minimal influence on Ω variability. We estimate an early spring surface water ΩAr value of ~ 1.2 for the Ross Sea using a total alkalinity–salinity relationship and historical pCO2 measurements. Our results suggest that the Ross Sea is not likely to become undersaturated with respect to aragonite until the year 2070.

Biogenic acidification reduces sea urchin gonad growth and increases susceptibility of aquaculture to ocean acidification

Decreasing oceanic pH (ocean acidification) has emphasised the influence of carbonate chemistry on growth of calcifying marine organisms. However, calcifiers can also change carbonate chemistry of surrounding seawater through respiration and calcification, a potential limitation for aquaculture. This study examined how seawater exchange rate and stocking density of the sea urchin Tripneustes gratilla that were reproductively mature affected carbonate system parameters of their culture water, which in turn influenced growth, gonad production and gonad condition. Growth, relative spine length, gonad production and consumption rates were reduced by up to 67% by increased density (9–43 individuals.m−2) and reduced exchange rates (3.0–0.3 exchanges.hr−1), but survival and food conversion efficiency were unaffected. Analysis of the influence of seawater parameters indicated that reduced pH and calcite saturation state (ΩCa) were the primary factors limiting gonad production and growth. Uptake of bicarbonate and release of respiratory CO2 by T. gratilla changed the carbonate chemistry of surrounding water. Importantly total alkalinity (AT) was reduced, likely due to calcification by the urchins. Low AT limits the capacity of culture water to buffer against acidification. Direct management to counter biogenic acidification will be required to maintain productivity and reproductive output of marine calcifiers, especially as the ocean carbonate system is altered by climate driven ocean acidification.

Geochemistry of two shallow CO2 seeps in Shikine Island (Japan) and their potential for ocean acidification research

Shallow CO2 seeps, where CO2 gas is venting underwater, offer great potential for studies into the effects of ocean acidification at the ecosystem level. To our knowledge, only two tropical system and two temperate systems of such seeps have been described worldwide. Here we describe two new temperate systems: the Mikama Bay and Ashitsuke sites, located on Shikine Island, Japan. The Mikama Bay site is located in a shallow bay. Investigation of the gas and water chemistry showed that the gas contained 98% CO2 and up to 90 ppm H2S. Total alkalinity was constant in time and space with an average of 2265±10 μ mol kg−1. Mapping of Eh and pH showed that the low pH zones were the largest when currents were moderate. Under moderate currents, Eh values were globally higher and total sulfides concentration lower, supporting that a longer residence time of the bay water allow the oxidation of the sulfides to sulfates. Zones suitable for acidification studies: with a pH lower than 8.0, low saturation state of calcite and aragonite, and non-detectable sulfide concentration, can be defined a few meters from the main venting zone. The second site, Ashitsuke, is located in the inter-tidal zone on a shore composed of boulders. Several areas showed reduced pH sometimes restricted to a few meters and up to 20 m long along the shoreline. Temperature was higher in some of the reduced pH zones suggesting the presence of hot springs in addition to vents. This paper also highlights the need for discovering additional CO2 seeps, which by their nature often lack comparable replicates and can be confounded by factors other than CO2. In this regard, Japan offers great potential as it is home to numerous active volcanoes, representing potential venting sites in climates ranging from tropical to sub-polar.

Biogenic acidification drives density-dependent growth of a calcifying invertebrate in culture

A major factor often overlooked in the production of calcifying marine invertebrates in aquaculture is the effect of changes in the carbonate chemistry of culture water caused by organism metabolism and calcification. This study examined the effects of temperature, stocking density and seawater exchange rate on the survival and growth of the juvenile sea urchin, Tripneustes gratilla with particular reference to effects of these factors on the carbonate chemistry of culture water. Growth and survival of T. gratilla were highest at 26–28 °C. Growth rates, relative spine length, consumption rates and food conversion efficiency were reduced by up to 50 % by an increase in stocking density from 43 to 129 individuals m−2 and a reduction in seawater exchange rate from 3.0 to 0.3 exchanges h−1, but survival rates were unaffected. Analysis of the influence of seawater parameters indicated that total alkalinity (AT), calcite saturation state (ΩCa) and dissolved CO2 were the primary factors limiting growth at high densities and low seawater exchange rates. Uptake of bicarbonates for calcification and release of respiratory CO2 by T. gratilla appeared to be driving changes in carbonate chemistry. Our results show that reduced capacity of culture water to buffer acidification (low AT) can be a limiting factor in intensive production of calcifying marine invertebrates. This study demonstrates the importance of considering the influence of biogenic CO2 on seawater carbonate chemistry in aquaculture, particularly given the threat of reduced pH of source water due to ocean acidification, and also in experiments investigating the influence of anthropogenic CO2.

Processes determining the marine alkalinity and calcium carbonate saturation state distributions

Abstract. We introduce a composite tracer for the marine system, Alk*, that has a global distribution primarily determined by CaCO3 precipitation and dissolution. Alk* is also affected by riverine alkalinity from dissolved terrestrial carbonate minerals. We estimate that the Arctic receives approximately twice the riverine alkalinity per unit area as the Atlantic, and 8 times that of the other oceans. Riverine inputs broadly elevate Alk* in the Arctic surface and particularly near river mouths. Strong net carbonate precipitation results in low Alk* in subtropical gyres, especially in the Indian and Atlantic oceans. Upwelling of dissolved CaCO3-rich deep water elevates North Pacific and Southern Ocean Alk*. We use the Alk* distribution to estimate the variability of the calcite saturation state resulting from CaCO3 cycling and other processes. We show that regional differences in surface calcite saturation state are due primarily to the effect of temperature differences on CO2 solubility and, to a lesser extent, differences in freshwater content and air–sea disequilibria. The variations in net calcium carbonate cycling revealed by Alk* play a comparatively minor role in determining the calcium carbonate saturation state.

OCEAN ACIDIFICATION NOT LIKELY TO AFFECT THE SURVIVAL AND FITNESS OF TWO TEMPERATE BENTHIC FORAMINIFERAL SPECIES: RESULTS FROM CULTURE EXPERIMENTS

Research Article| October 01, 2014 OCEAN ACIDIFICATION NOT LIKELY TO AFFECT THE SURVIVAL AND FITNESS OF TWO TEMPERATE BENTHIC FORAMINIFERAL SPECIES: RESULTS FROM CULTURE EXPERIMENTS Anna McIntyre-Wressnig; Anna McIntyre-Wressnig Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA Search for other works by this author on: GSW Google Scholar Joan M. Bernhard; Joan M. Bernhard * Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA *Correspondence author. E-mail: jbernhard@whoi.edu Search for other works by this author on: GSW Google Scholar Johannes C. Wit; Johannes C. Wit Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA Search for other works by this author on: GSW Google Scholar Daniel C. Mccorkle Daniel C. Mccorkle Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA Search for other works by this author on: GSW Google Scholar Journal of Foraminiferal Research (2014) 44 (4): 341–351. https://doi.org/10.2113/gsjfr.44.4.341 Article history received: 18 May 2012 accepted: 08 May 2014 first online: 13 Jul 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Anna McIntyre-Wressnig, Joan M. Bernhard, Johannes C. Wit, Daniel C. Mccorkle; OCEAN ACIDIFICATION NOT LIKELY TO AFFECT THE SURVIVAL AND FITNESS OF TWO TEMPERATE BENTHIC FORAMINIFERAL SPECIES: RESULTS FROM CULTURE EXPERIMENTS. Journal of Foraminiferal Research 2014;; 44 (4): 341–351. doi: https://doi.org/10.2113/gsjfr.44.4.341 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyJournal of Foraminiferal Research Search Advanced Search Abstract Specimens of Bolivina argentea and Bulimina marginata, two widely distributed temperate benthic foraminiferal species, were cultured at constant temperature and controlled pCO2 (ambient, 1000 ppmv, and 2000 ppmv) for six weeks to assess the effect of elevated atmospheric CO2 concentrations on survival and fitness using Adenosine Triphosphate (ATP) analyses and on shell microfabric using high-resolution SEM and image analysis. To characterize the carbonate chemistry of the incubation seawater, total alkalinity and dissolved inorganic carbon were measured approximately every two weeks. Survival and fitness were not directly affected by elevated pCO2 and the concomitant decrease in seawater pH and calcite saturation states (Ωc), even when seawater was undersaturated with respect to calcite. These results differ from some previous observations that ocean acidification can cause a variety of effects on benthic foraminifera, including test dissolution, decreased growth, and mottling (loss of symbiont color in symbiont-bearing species), suggesting that the benthic foraminiferal response to ocean acidification may be species specific. If so, this implies that ocean acidification may lead to ecological winners and losers even within the same taxonomic group. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Morphology of <i>Emiliania huxleyi</i> coccoliths on the northwestern European shelf – is there an influence of carbonate chemistry?

Abstract. Within the context of the UK Ocean Acidification project, Emiliania huxleyi (type A) coccolith morphology was examined from samples collected during cruise D366. In particular, a morphometric study of coccolith size and degree of calcification was made on scanning electron microscope images of samples from shipboard CO2 perturbation experiments and from a set of environmental samples with significant variation in calcite saturation state (Ωcalcite). One bioassay in particular (E4 from the southern North Sea) yielded unambiguous results – in this bioassay exponential growth from a low initial cell density occurred with no nutrient enrichment and coccosphere numbers increased tenfold during the experiment. The samples with elevated CO2 saw significantly reduced coccolithophore growth. However, coccolithophore morphology was not significantly affected by the changing CO2 conditions even under the highest levels of perturbation (1000 μatm CO2). Environmental samples similarly showed no correlation of coccolithophore morphology with calcite saturation state. Some variation in coccolith size and degree of calcification does occur but this seems to be predominantly due to genotypic differentiation between populations on the shelf and in the open ocean.

Coccolithophores on the north-west European shelf: calcification rates and environmental controls

Abstract. Coccolithophores are a key functional group in terms of the pelagic production of calcium carbonate (calcite), although their contribution to shelf sea biogeochemistry, and how this relates to environmental conditions, is poorly constrained. Measurements of calcite production (CP) and coccolithophore abundance were made on the north-west European shelf to examine trends in coccolithophore calcification along natural gradients of carbonate chemistry, macronutrient availability and plankton composition. Similar measurements were also made in three bioassay experiments where nutrient (nitrate, phosphate) and pCO2 levels were manipulated. Nanoflagellates (< 10 μm) dominated chlorophyll biomass and primary production (PP) at all but one sampling site, with CP ranging from 0.6 to 9.6 mmol C m−2 d−1. High CP and coccolithophore abundance occurred in a diatom bloom in fully mixed waters off Heligoland, but not in two distinct coccolithophore blooms in the central North Sea and Western English Channel. Coccolithophore abundance and CP showed no correlation with nutrient concentrations or ratios, while significant (p < 0.01) correlations between CP, cell-specific calcification (cell-CF) and irradiance in the water column highlighted how light availability exerts a strong control on pelagic CP. In the experimental bioassays, Emiliania-huxleyi-dominated coccolithophore communities in shelf waters (northern North Sea, Norwegian Trench) showed a strong response in terms of CP to combined nitrate and phosphate addition, mediated by changes in cell-CF and growth rates. In contrast, an offshore diverse coccolithophore community (Bay of Biscay) showed no response to nutrient addition, while light availability or mortality may have been more important in controlling this community. Sharp decreases in pH and a rough halving of calcite saturation states in the bioassay experiments led to decreased CP in the Bay of Biscay and northern North Sea, but not the Norwegian Trench. These decreases in CP were related to slowed growth rates in the bioassays at elevated pCO2 (750 μatm) relative to those in the ambient treatments. The combined results from our study highlight the variable coccolithophore responses to irradiance, nutrients and carbonate chemistry in north-west European shelf waters, which are mediated by changes in growth rates, cell-CF and species composition.

Global dissolution effects on planktonic foraminiferal Mg/Ca ratios controlled by the calcite-saturation state of bottom waters

Mg/Ca ratios of planktonic foraminiferal tests are important tools for reconstructing past ocean temperatures at different levels of the upper water column. Yet numerous studies suggest a significant influence of calcite dissolution on Mg/Ca ratios lowering their initial signal recorded within a planktonic foraminiferal habitat. To determine the effect of dissolution, this study presents Mg/Ca ratios of eight planktonic foraminiferal species from the South China Sea sediment surface. Continuously decreasing with increasing water depth, the Mg/Ca ratios also decrease with calcite-saturation states close to and below saturation (bottom water Delta[CO32-] 40 mu mol kg(-1)). This preservation pattern compares well with examples of Mg/Ca dissolution from the tropical Atlantic Ocean and is independent of the foraminiferal species. Merging a global data set by separate normalization of 79 Mg/Ca data sets from the Pacific, Atlantic, and Indian Oceans, which removes thermal differences between the ocean regions and foraminiferal species, enabled us to quantify a global decrease in planktonic foraminiferal Mg/Ca ratios of 0.054 +/- 0.019 mu mol mol(-1) per mol kg(-1) below a critical threshold for dissolution of 21.3 +/- 6.6 mu mol kg(-1). The absolute decline in Mg/Ca ratios, which is similar for all species, affects temperature estimates from (sub-)thermocline species more strongly than those from shallow dwellers. The water depth of this critical threshold in the global oceans shoals from >3.5 km in the North Atlantic to <0.5 km in the North Pacific based on calculations of the global calcite-saturation state from 6321 hydrographic stations. Above this critical threshold Mg/Ca ratios are well preserved, and paleotemperature estimates are broadly unaffected by dissolution.

Regional variability of acidification in the Arctic: a sea of contrasts

Abstract. The Arctic Ocean is a region that is particularly vulnerable to the impact of ocean acidification driven by rising atmospheric CO2, with potentially negative consequences for calcifying organisms such as coccolithophorids and foraminiferans. In this study, we use an ocean-only general circulation model, with embedded biogeochemistry and a comprehensive description of the ocean carbon cycle, to study the response of pH and saturation states of calcite and aragonite to rising atmospheric pCO2 and changing climate in the Arctic Ocean. Particular attention is paid to the strong regional variability within the Arctic, and, for comparison, simulation results are contrasted with those for the global ocean. Simulations were run to year 2099 using the RCP8.5 (an Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5) scenario with the highest concentrations of atmospheric CO2). The separate impacts of the direct increase in atmospheric CO2 and indirect effects via impact of climate change (changing temperature, stratification, primary production and freshwater fluxes) were examined by undertaking two simulations, one with the full system and the other in which atmospheric CO2 was prevented from increasing beyond its preindustrial level (year 1860). Results indicate that the impact of climate change, and spatial heterogeneity thereof, plays a strong role in the declines in pH and carbonate saturation (Ω) seen in the Arctic. The central Arctic, Canadian Arctic Archipelago and Baffin Bay show greatest rates of acidification and Ω decline as a result of melting sea ice. In contrast, areas affected by Atlantic inflow including the Greenland Sea and outer shelves of the Barents, Kara and Laptev seas, had minimal decreases in pH and Ω because diminishing ice cover led to greater vertical mixing and primary production. As a consequence, the projected onset of undersaturation in respect to aragonite is highly variable regionally within the Arctic, occurring during the decade of 2000–2010 in the Siberian shelves and Canadian Arctic Archipelago, but as late as the 2080s in the Barents and Norwegian seas. We conclude that, for future projections of acidification and carbonate saturation state in the Arctic, regional variability is significant and needs to be adequately resolved, with particular emphasis on reliable projections of the rates of retreat of the sea ice, which are a major source of uncertainty.

Impact of sea‐ice processes on the carbonate system and ocean acidification at the ice‐water interface of the Amundsen Gulf, Arctic Ocean

From sea‐ice formation in November 2007 to onset of ice melt in May 2008, we studied the carbonate system in first‐year Arctic sea ice, focusing on the impact of calcium‐carbonate (CaCO3) saturation states of aragonite (ΩAr) and calcite (ΩCa) at the ice‐water interface (UIW). Based on total inorganic carbon (CT) and total alkalinity (AT), and derived pH, CO2, carbonate ion ([CO32−]) concentrations and Ω, we investigated the major drivers such as brine rejection, CaCO3 precipitation, bacterial respiration, primary production and CO2‐gas flux in sea ice, brine, frost flowers and UIW. We estimated large variability in sea‐ice CT at the top, mid, and bottom ice. Changes due to CaCO3 and CO2‐gas flux had large impact on CT in the whole ice core from March to May, bacterial respiration was important at the bottom ice during all months, and primary production in May. It was evident that the sea‐ice processes had large impact on UIW, resulting in a five times larger seasonal amplitude of the carbonate system, relative to the upper 20 m. During ice formation, [CO2] increased by 30 µmol kg−1, [CO32−] decreased by 50 µmol kg−1, and the ΩAr decreased by 0.8 in the UIW due to CO2‐enriched brine from solid CaCO3. Conversely, during ice melt, [CO32−] increased by 90 µmol kg−1 in the UIW, and Ω increased by 1.4 between March and May, likely due to CaCO3 dissolution and primary production. We estimated that increased ice melt would lead to enhanced oceanic uptake of inorganic carbon to the surface layer.

Geologic history of seawater: A MAGic approach to carbon chemistry and ocean ventilation

We explore the relationship between atmospheric O2 and CO2 evolution and seawater chemistry, with particular focus on the CO2–carbonic acid system and ocean ventilation, over the Phanerozoic Eon using a coupled biogeochemical Earth system model (MAGic). This model describes the biogeochemical cycles involving the major components of seawater (Ca, Mg, Na, K, Cl, SO4, CO2HCO3CO3), as well as components (O2, Fe, P, organic C, reduced S) central to long-term ecosystem productivity. The MAGic calculations show that the first-order input fluxes from weathering of continental rocks of Ca, Mg, and dissolved inorganic carbon (DIC) to the ocean varied in a cyclical manner over the Phanerozoic. The cyclicity is mainly the result of the impact of changing atmospheric CO2 levels, and hence temperature and runoff, on these fluxes, reflecting the nature of hothouse (greenhouse, high CO2 and warm) versus icehouse (low CO2, cool, and continental glaciation) conditions during the Phanerozoic. Uptake of DIC by seafloor basalt–seawater reactions also varied in a corresponding fashion to the weathering fluxes. The fluxes of Ca, Mg, DIC and other seawater constituents removed in oceanic sinks were also calculated and hence with calculated inputs and outputs of seawater constituents, the changes in seawater chemistry through Phanerozoic time could be obtained. Seawater pH increased irregularly during the Phanerozoic from just above 7 in the Cambrian Period, approaching modern average values in the most recent several millions of years. Calcite saturation state also increased with decreasing age. Both pH and calcite saturation state trends exhibited a cyclic overprint of hothouse and icehouse environmental conditions. Dissolved sulfate changed in a cyclical manner reflecting mainly variations in weathering and accretion rates and redox conditions, whereas dissolved potassium exhibited little variation in concentration.Using our “standard” model results for the chemistry of seawater and changes in atmospheric CO2 and O2 as the basis for a series of sensitivity experiments, we vary the ventilation rate of the global ocean, and quantify the resulting changes in terms of processes such as net primary production, organic carbon burial and oxidation, pyrite weathering, and sulfate reduction. We use these preliminary results to discuss how changes in ocean ventilation affect atmospheric CO2 and O2, and in turn exert changes in the sulfur, organic carbon, and inorganic carbon systems. We postulate that periods of slow plate accretion rates, associated with lower atmospheric CO2, vigorous deep water formation, cooler, drier climatic conditions and greater poleward temperature gradients are more likely to be associated with a strong thermohaline circulation, and thus “enhanced” global ocean mixing. Conversely, periods of higher accretion rates, higher CO2, higher average global temperatures with more equable poleward gradients, and higher sea levels resulting in extensive continental inundation, would be more likely to be coincident with times of reduced mixing of the global ocean. It is important to recognize that the scale of these changes depends on how major tectonic cycles (controlling chemical weathering, CO2 and temperature) in turn affect nutrient supply, global ocean productivity, and global ocean thermohaline circulation. The key to elucidating these changes lies in an understanding of the relationship between long-term tectonic evolution, which leads to changes in climate, sea level, and the global distribution of continental landmasses and the sedimentary environments they host, and the circulation of the global ocean.

Estimating carbonate parameters from hydrographic data for the intermediate and deep waters of the Southern Hemisphere oceans

Abstract. Using ocean carbon data from global datasets, we have developed several multiple linear regression (MLR) algorithms to estimate alkalinity and dissolved inorganic carbon (DIC) in the intermediate and deep waters of the Southern Hemisphere (south of 25° S) from only hydrographic data (temperature, salinity and dissolved oxygen). A Monte Carlo experiment was used to identify a potential density (σθ) of 27.5 as an optimal break point between the two regimes with different MLR algorithms. The algorithms provide a good estimate of DIC (R2=0.98) and alkalinity (R2=0.91), and excellent agreement for aragonite and calcite saturation states (R2=0.99). Combining the algorithms with the CSIRO Atlas of Regional Seas (CARS), we have mapped the calcite saturation horizon (CSH) and aragonite saturation horizon (ASH) for the Southern Ocean at a spatial resolution of 0.5°. These maps are more detailed and more consistent with the oceanography than the previously gridded GLODAP data. The high-resolution ASH map reveals a dramatic circumpolar shoaling at the polar front. North of 40° S the CSH is deepest in the Atlantic (~ 4000 m) and shallower in the Pacific Ocean (~ 2750 m), while the CSH sits between 3200 and 3400 m in the Indian Ocean. The uptake of anthropogenic carbon by the ocean will alter the relationships between DIC and hydrographic data in the intermediate and deep waters over time. Thus continued sampling will be required, and the MLR algorithms will need to be adjusted in the future to account for these changes.

Permeability evolution due to dissolution and precipitation of carbonates using reactive transport modeling in pore networks

[1] A reactive transport model was developed to simulate reaction of carbonates within a pore network for the high-pressure CO2-acidified conditions relevant to geological carbon sequestration. The pore network was based on a synthetic oolithic dolostone. Simulation results produced insights that can inform continuum-scale models regarding reaction-induced changes in permeability and porosity. As expected, permeability increased extensively with dissolution caused by high concentrations of carbonic acid, but neither pH nor calcite saturation state alone was a good predictor of the effects, as may sometimes be the case. Complex temporal evolutions of interstitial brine chemistry and network structure led to the counterintuitive finding that a far-from-equilibrium solution produced less permeability change than a nearer-to-equilibrium solution at the same pH. This was explained by the pH buffering that increased carbonate ion concentration and inhibited further reaction. Simulations of different flow conditions produced a nonunique set of permeability-porosity relationships. Diffusive-dominated systems caused dissolution to be localized near the inlet, leading to substantial porosity change but relatively small permeability change. For the same extent of porosity change caused from advective transport, the domain changed uniformly, leading to a large permeability change. Regarding precipitation, permeability changes happen much slower compared to dissolution-induced changes and small amounts of precipitation, even if located only near the inlet, can lead to large changes in permeability. Exponent values for a power law that relates changes in permeability and porosity ranged from 2 to 10, but a value of 6 held constant when conditions led to uniform changes throughout the domain.

High‐resolution carbon budgets on a Palau back‐reef modulated by interactions between hydrodynamics and reef metabolism

Diel trends in carbon chemistry and hydrodynamic regimes were measured on a coral‐dominated back‐reef community in the Republic of Palau, western Pacific. Observed diel ranges over 5 d were pH 7.92–8.09, partial pressure of carbon dioxide 33.4–52.7 Pa, measured total alkalinity 2080–2272 µmol kg−1, total dissolved inorganic carbon 1763–1939 µmol kg−1, aragonite saturation state 2.84–3.98, and calcite saturation state 4.44–5.67. The combination of reef metabolism and hydrodynamic regimes drives this variability. We report net community calcification (NCC) and net community production (NCP) rates of −20.4 (± 7.8) to 41.2 (± 37.7) mmol CaCO3 m−2 h−1 and 77.3 (± 95) to 64 (± 22) mmol C m−2 h−1, respectively. Using a control volume approach, we show that NCC and NCP rate estimates are quite sensitive to the effective control volume dimensions. At this site, tracking vertical mixing of water masses within the control volume shows that active water mixing is often confined to a region 2–3 m above the corals. Vertical carbon mass fluxes into and out of the benthic environment were unexpectedly large for the slow current flow rates observed, suggesting that coral morphology or reef rugosity may affect mass transport of carbon and nutrients. The ranges of NCC and NCP values highlight the need for additional field work on various reefs to tune and optimize the control volume approach to achieve its full potential for reef metabolism monitoring.

Glacial deep ocean sequestration of CO2 driven by the eastern equatorial Pacific biologic pump

The potential influence of low latitude ocean primary productivity on glacial atmospheric carbon dioxide levels has proven challenging to deduce using mass accumulation rates (MARs) of biogenic particulates in deep sea sediment cores. Benthic foraminiferal B/Ca serves as a proxy for past seawater calcite saturation state, and thereby provides a fresh perspective on this outstanding paleoceanographic problem. Here we employ Cibicidoides wuellerstorfi B/Ca in the Panama Basin region of the eastern equatorial Pacific (EEP) to investigate the nature of deep tropical Pacific carbon storage over the past 50 ka BP. We present evidence for persistently lower deep Panama Basin calcite saturation state, reflecting an increase in total carbon dioxide storage, during the last ice age relative to the Holocene. These results reflect the modification of inflowing deep waters by overlying export productivity, and support the concept of an invigorated glacial EEP soft-tissue pump possibly driven by oceanic nutrient (iron and silica) redistribution. Benthic Cibicidoides spp. carbon-13 is consistent with this conclusion by exhibiting substantially lighter values during glacial time, reflecting the accumulation of metabolic carbon dioxide in the deep tropical Pacific. Counterintuitively, downcore application of the Globorotalia menardii calcite fragmentation index (MFI) reveals enhanced glacial sedimentary calcite preservation in the Panama Basin. Together these results point towards a systematic decoupling of bottom water chemistry from biogenic burial fluxes: the crux of the aforementioned traditional paleoproductivity problem.

Mg/Ca and Mn/Ca ratios in benthic foraminifera: the potential to reconstruct past variations in temperature and hypoxia in shelf regions

Abstract. Shelf and coastal regions are exceptionally important for many countries as they provide the main habitat for many economically important fish and shellfish species. With ongoing climate change and human-induced eutrophication the shelf regions are especially affected, resulting in increased temperatures and stratification as well as oxygen depletion of the bottom waters. In order to be able to predict the magnitude of these changes in the future, it is necessary to study how they varied in the past. Commonly used foraminiferal climate and environmental proxies, e.g., stable isotopes and trace metal/Ca ratios, that are applied in open-ocean settings are not necessarily applicable in shelf regions, either as faunas are significantly different or as conditions can change much faster compared to the open ocean. In this study we explore the use of Mg/Ca as paleothermometer and Mn/Ca as a potential proxy for changing dissolved oxygen conditions in bottom water on the benthic foraminifera Bulimina marginata and Globobulimina turgida. Living specimens were collected from the Skagerrak and the Gullmar Fjord (SW Sweden); the latter is hypoxic for several months a year. As the specimens were alive when collected, we assume it unlikely that any diagenetic coatings have already significantly affected the trace metal/Ca ratios. The Mg/Ca ratios are similar to previously published values but display much larger variation than would be expected from the annual temperature change of less than 2 °C. An additional impact of the difference in the calcite saturation state between the Skagerrak and the Gullmar Fjord could explain the results. Mn/Ca ratios from G. turgida can potentially be related to variations in dissolved oxygen of the habitat where the foraminifera calcify. Samples from the Skagerrak display increased Mn/Ca in specimens that lived deeper in the sediment than those that lived near the surface. G. turgida samples from the low-oxygen Gullmar Fjord showed significantly increased Mn/Ca, being highest when bottom water dissolved oxygen was at a minimum. Our study suggests that trace metal/Ca ratios in benthic foraminifera from shelf regions have the potential to record past variations in bottom water temperature and dissolved oxygen concentrations, but an additional impact of the inorganic carbonate chemistry cannot be excluded.