Carbonate Mineral Saturation States Research Articles

Carbonate Mineral Saturation State as the Recruitment Cue for Settling Bivalves in Marine Muds

After a pelagic larval phase, infaunal bivalves undergo metamorphosis and transition to the underlying sediments to begin the benthic stage of their life history, where they explore and then either accept or reject sediments. Although the settlement cues used by juvenile infaunal bivalves are poorly understood, here we provide evidence that carbonate saturation state is a significant chemical cue in both direct observation laboratory studies and field manipulations. In the laboratory, plantigrade-stage Mercenaria mercenaria (200 μm shell height) showed a significant positive relationship between percent burrowed and Ωaragonite, with an increasing probability of settlement with increasing saturation state. In the field, we increased bivalve recruitment by a factor of three in a 30-day field study by raising the pH (∼0.3) and saturation state of surface sediments by buffering sediments with crushed shell (CaCO3). The susceptibility of infaunal bivalves to dissolution mortality and the tight coupling of other sedimentary biogeochemical processes with carbonate dynamics suggest that mineral thermodynamics may be an overarching cue new settlers are responding to.

History of Seawater Carbonate Chemistry, Atmospheric CO2, and Ocean Acidification

Humans are continuing to add vast amounts of carbon dioxide (CO2) to the atmosphere through fossil fuel burning and other activities. A large fraction of the CO2is taken up by the oceans in a process that lowers ocean pH and carbonate mineral saturation state. This effect has potentially serious consequences for marine life, which are, however, difficult to predict. One approach to address the issue is to study the geologic record, which may provide clues about what the future holds for ocean chemistry and marine organisms. This article reviews basic controls on ocean carbonate chemistry on different timescales and examines past ocean chemistry changes and ocean acidification events during various geologic eras. The results allow evaluation of the current anthropogenic perturbation in the context of Earth's history. It appears that the ocean acidification event that humans are expected to cause is unprecedented in the geologic past, for which sufficiently well-preserved records are available.

Storm‐induced upwelling of high pCO2 waters onto the continental shelf of the western Arctic Ocean and implications for carbonate mineral saturation states

The carbon system of the western Arctic Ocean is undergoing a rapid transition as sea ice extent and thickness decline. These processes are dynamically forcing the region, with unknown consequences for CO2 fluxes and carbonate mineral saturation states, particularly in the coastal regions where sensitive ecosystems are already under threat from multiple stressors. In October 2011, persistent wind‐driven upwelling occurred in open water along the continental shelf of the Beaufort Sea in the western Arctic Ocean. During this time, cold (<−1.2°C), salty (>32.4) halocline water—supersaturated with respect to atmospheric CO2 (pCO2 > 550 μatm) and undersaturated in aragonite (Ωaragonite < 1.0) was transported onto the Beaufort shelf. A single 10‐day event led to the outgassing of 0.18–0.54 Tg‐C and caused aragonite undersaturations throughout the water column over the shelf. If we assume a conservative estimate of four such upwelling events each year, then the annual flux to the atmosphere would be 0.72–2.16 Tg‐C, which is approximately the total annual sink of CO2 in the Beaufort Sea from primary production. Although a natural process, these upwelling events have likely been exacerbated in recent years by declining sea ice cover and changing atmospheric conditions in the region, and could have significant impacts on regional carbon budgets. As sea ice retreat continues and storms increase in frequency and intensity, further outgassing events and the expansion of waters that are undersaturated in carbonate minerals over the shelf are probable.

Detection and projection of carbonate dissolution in the water column and deep‐sea sediments due to ocean acidification

Dissolution of fossil fuel CO2 in seawater results in decreasing carbonate ion concentration and lowering of seawater pH with likely negative impacts for many marine organisms. We project detectable changes in carbonate dissolution and evaluate their potential to mitigate atmospheric CO2 and ocean acidification with a global biogeochemistry model HAMOCC forced by different CO2 emission scenarios. Our results suggest that as the anthropogenic CO2 signal penetrates into ocean interior, the saturation state of carbonate minerals will drop drastically – with undersaturation extending from the ocean floor up to 100–150 m depth in the next century. This will induce massive dissolution of CaCO3 in the water column as well as the sediment, increasing the Total Alkalinity (TA) by up to 180 μmol kg−1 at the surface and in the ocean interior over the next 2500 years. Model results indicate an inhomogeneous response among different ocean basins: Atlantic carbonate chemistry responds faster and starts recovering two millennia after CO2 emissions cease, which is not the case in the Pacific. CaCO3rain stops in the Pacific Ocean around 2230. Using an observation‐derived detection threshold for TA, we project detectable dissolution‐driven changes only by the year 2070 in the surface ocean and after 2230 and 2500 in the deep Atlantic and Pacific respectively. We show that different model assumptions regarding dissolution and calcification rates have little impact on future projections. Instead, anthropogenic CO2 emissions overwhelmingly control the degree of perturbation in ocean chemistry. In conclusion, ocean carbonate dissolution has insignificant potential in mitigating atmospheric CO2 and ocean acidification in the next millennia.

The role of ocean acidification in systemic carbonate mineral suppression in the Bering Sea

Ocean acidification driven by absorption of anthropogenic carbon dioxide (CO2) from the atmosphere is now recognized as a systemic, global process that could threaten diverse marine ecosystems and a number of commercially important species. The change in calcium carbonate (CaCO3) mineral saturation states (?) brought on by the reduction of seawater pH is most pronounced in high latitude regions where unique biogeochemical processes create an environment more susceptible to the suppression of ? values for aragonite and calcite, which are critical to shell building organisms. New observations from the eastern Bering Sea shelf show that remineralization of organic matter exported from surface waters rapidly increases bottom water CO2 concentrations over the shelf in summer and fall, suppressing ? values. The removal of CO2 from surface waters by high rates of phytoplankton primary production increases ? values between spring and summer, but these increases are partly counteracted by mixing with sea ice melt water and terrestrial river runoff that have low ? values. While these environmental processes play an important role in creating seasonally low saturation states, ocean uptake of anthropogenic CO2 has shifted ? values for aragonite to below the saturation horizon in broad regions across the shelf for at least several months each year. Furthermore, we also report that calcite became undersaturated in September of 2009 in the bottom waters over the shelf. The reduction in CaCO3 mineral saturation states could have profound implications for several keystone calcifying species in the Bering Sea, particularly the commercially important crab fisheries.

Real-time estimation of pH and aragonite saturation state from Argo profiling floats: Prospects for an autonomous carbon observing strategy

[1] We demonstrate the ability to obtain accurate estimates of pH and carbonate mineral saturation state (Ω) from an Argo profiling float in the NE subarctic Pacific. Using hydrographic surveys of the NE Pacific region, we develop empirical algorithms to predict pH and Ω using observations of temperature (T) and dissolved O2. We attain R2 values greater than 0.98 and RMS errors of 0.018 (pH), 0.052 (Ωarag), and 0.087 (Ωcalc) for data between 30–500 m, σθ < 27.1. After calibrating optode-based O2 data, we apply the algorithms to T and O2 data from an Argo profiling float to produce a 14 month time-series of estimated pH and Ωarag in the upper water column of the NE subarctic Pacific. Comparison to independent data collected nearby in 2010 indicates pH and Ωarag estimates are robust. Although the method will not allow detection of anthropogenic trends in pH or Ωarag, this approach will provide insight into natural variability and the key biogeochemical controls on these parameters. Most importantly, this work demonstrates that an assemblage of well-calibrated regional algorithms and Argo float data can be used as a low-cost, readily-deployable component of a global ocean carbon observing strategy.

Coupling primary production and terrestrial runoff to ocean acidification and carbonate mineral suppression in the eastern Bering Sea

Water column pH and carbonate mineral saturation states were calculated from dissolved inorganic carbon (DIC) and total alkalinity data collected over the eastern Bering Sea shelf in the spring and summer of 2008. The saturation states (?) of the two most important carbonate minerals, calcite (?calcite) and aragonite (?aragonite) were strongly coupled to terrestrial runoff from the Yukon and Kuskokwim rivers, primary production in the surface waters, and remineralization of organic matter at depth over the shelf. In spring, before ice melt occurred, pH over the shelf was largely confined to a range of 7.9–8.1 and ?calcite and ?aragonite ranged from 1.5 to 3.0 and 0.8 to 2.0, respectively. At the stations closest to river outflows, aragonite was undersaturated in the water column from the surface to the bottom. During the summer sea ice retreat, high rates of primary production consumed DIC in the mixed layer, which increased pH and ?calcite and ?aragonite. However, ?calcite and ?aragonite decreased by ?0.3 in the bottom waters over the middle and outer shelf. Over the northern shelf, where export production is highest, ?aragonite decreased by ?0.35 and became highly undersaturated. The observed suppression and undersaturation of ?calcite and ?aragonite in the eastern Bering Sea are correlated with anthropogenic carbon dioxide uptake into the ocean and will likely be exacerbated under business-as-usual emission scenarios. Therefore, ocean acidification could threaten some benthic and pelagic calcifying organisms across the Bering Sea shelf in the coming decades.

Carbonate mineral saturation states along the U.S. East Coast

To assess the impact of ocean acidification on the carbonate chemistry of the shelf waters off the southeastern United States (South Atlantic Bight [SAB]), we measured carbonate mineral saturation states from January 2005 to May 2006. The findings reveal that aragonite (Ωarag: 2.6–4.0) and calcite (Ωcal: 4.1–6.0) saturation states were considerably higher than those recently reported along the West Coast of North America. Different water mass age between the Atlantic and Pacific Oceans during global ocean circulation is the primary reason for the higher carbonate mineral saturation states in the SAB than along the West Coast. The contrasting water temperatures in the two coasts contribute to such differences. Both upwelling and freshwater discharge also play important roles in controlling saturation state. Carbonate mineral saturation in the surface water of the West Coast is strongly controlled by the upwelling of high‐salinity, low‐temperature, low‐oxygen, and low‐pH deep water. In comparison, saturation states in the surface water of the SAB coast are rarely affected by upwelling. Instead, they are strongly influenced by the input of low‐saturation‐state water from rivers. Continued increases of atmospheric CO2 under the Intergovernmental Panel on Climate Change B1 emission scenario will decrease the carbonate mineral saturation states by up to 40% by the end of this century, and aragonite will approach undersaturation near the coast.

The Arctic Ocean marine carbon cycle: evaluation of air-sea CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; exchanges, ocean acidification impacts and potential feedbacks

Abstract. At present, although seasonal sea-ice cover mitigates atmosphere-ocean gas exchange, the Arctic Ocean takes up carbon dioxide (CO2) on the order of −66 to −199 Tg C year−1 (1012 g C), contributing 5–14% to the global balance of CO2 sinks and sources. Because of this, the Arctic Ocean has an important influence on the global carbon cycle, with the marine carbon cycle and atmosphere-ocean CO2 exchanges sensitive to Arctic Ocean and global climate change feedbacks. In the near-term, further sea-ice loss and increases in phytoplankton growth rates are expected to increase the uptake of CO2 by Arctic Ocean surface waters, although mitigated somewhat by surface warming in the Arctic. Thus, the capacity of the Arctic Ocean to uptake CO2 is expected to alter in response to environmental changes driven largely by climate. These changes are likely to continue to modify the physics, biogeochemistry, and ecology of the Arctic Ocean in ways that are not yet fully understood. In surface waters, sea-ice melt, river runoff, cooling and uptake of CO2 through air-sea gas exchange combine to decrease the calcium carbonate (CaCO3) mineral saturation states (Ω) of seawater while seasonal phytoplankton primary production (PP) mitigates this effect. Biological amplification of ocean acidification effects in subsurface waters, due to the remineralization of organic matter, is likely to reduce the ability of many species to produce CaCO3 shells or tests with profound implications for Arctic marine ecosystems

Ocean acidification and biologically induced seasonality of carbonate mineral saturation states in the western Arctic Ocean

Calcium carbonate (CaCO3) mineral saturation states for aragonite (Ωaragonite) and calcite (Ωcalcite) are calculated for waters of the Chukchi Sea shelf and Canada Basin of the western Arctic Ocean during the Shelf‐Basin Interactions project from 2002 to 2004. On the Chukchi Sea shelf, a strong seasonality and vertical differentiation of aragonite and calcite saturation states was observed. During the summertime sea ice retreat period, high rates of phytoplankton primary production and net community production act to increase the Ωaragonite and Ωcalcite of surface waters, while subsurface waters become undersaturated with respect to aragonite due primarily to remineralization of organic matter to CO2. This seasonal “phytoplankton‐carbonate saturation state” interaction induces strong undersaturation of aragonite (Ωaragonite = &lt;0.7–1) at ∼40–150 m depth in the northern Chukchi Sea and in the Canada Basin within upper halocline waters at ∼100–200 m depth. Patches of aragonite undersaturated surface water were also found in the Canada Basin resulting from significant sea ice melt contributions (&gt;10%). The seasonal aragonite undersaturation of waters observed on the Chukchi Sea shelf is likely a recent phenomenon that results from the uptake of anthropogenic CO2 and subsequent ocean acidification, with seasonality of saturation states superimposed by biological processes. These undersaturated waters are potentially highly corrosive to calcifying benthic fauna (e.g., bivalves and echinoderms) found on the shelf, with implications for the food sources of large benthic feeding mammals (e.g., walrus, gray whales, and bearded seals). The benthic ecosystem of the Chukchi Sea (and other Arctic Ocean shelves) is thus potentially vulnerable to future ocean acidification and suppression of CaCO3 saturation states.

Ocean acidification of the Greater Caribbean Region 1996–2006

The global oceans serve as the largest sustained natural sink for increasing atmospheric carbon dioxide (CO2) concentrations. As this CO2 is absorbed by seawater, it not only reacts causing a reduction in seawater pH (or acidification) but also decreases the carbonate mineral saturation state (Ω), which plays an important role in calcification for many marine organisms. Ocean acidification could affect some of the most fundamental biological and geochemical processes of the sea in coming decades. Observations obtained in situ from Volunteer Observing Ships and multiple geochemical surveys have been extended using satellite remote sensing and modeled environmental parameters to derive estimates of sea‐surface alkalinity (AT) and carbon dioxide partial pressure (pCO2,sw). Pairing estimates of AT and pCO2,sw have permitted characterization of the changes in sea‐surface Ω, which have transpired over the past decade throughout the Greater Caribbean Region as a consequence of ocean acidification. The results reveal considerable spatial and temporal variability throughout the region. Despite this variability, we observed a strong secular decrease in aragonite saturation state (Ωarg) at a rate of approximately −0.012 ± 0.001 Ωarg yr−1 (r2 = 0.97, P &lt; 0.001).

Application of multiple geochemical indicators, including the stable isotopes of water, to differentiate water quality evolution in a region influenced by various agricultural practices and domestic wastewater treatment and disposal

Spatial and temporal variations in groundwater chemistry indicate that the use of low TDS lake water for irrigation, on land located just south of the City of Dixon, Solano County, California, is primarily responsible for improving groundwater quality with regards to salts. The stable isotopes of water further support this finding and suggest that TDS concentrations decrease as groundwater evolves to a more highly evaporated state. This seemingly contradictory finding was primarily attributed to infiltration of low TDS Lake Berryessa surface water, which has an isotopic signature indicative of an evaporated source and is used extensively for irrigation in the area, mixing with poorer quality locally recharged shallow groundwater. Geochemical modeling using the program PHREEQC further supports the anthropogenic aquifer freshening hypotheses through computed reductions in the saturation state of carbonate minerals in the vicinity of land irrigated by lake derived water, which is undersaturated with regards to modeled carbonates. Additionally, δ 18O and δ 2H were found to be useful in estimating climatic variables such as temperature and humidity, illustrating the potential for applying these models in hydrologic investigations within the area. It was however found that USDA NRCS soils data and measured water chemistry were not well correlated and thus the use of soils classifications to assess potential groundwater quality impacts was of limited utility.

Carbonate mineral saturation state and cratonic limestone accumulation

Carbonate mineral saturation state and cratonic limestone accumulation