A core-top study of dissolution effect on B/Ca in Globigerinoides sacculifer from the tropical Atlantic: potential bias for paleo-reconstruction of seawater carbonate chemistry.

It has been recently shown that B/Ca in planktonic foraminiferal calcite can be used as a proxy for seawater pH. Based on the study of surface sediments (multi-cores) retrieved along a depth transect on the Sierra Leone Rise (Eastern Equatorial Atlantic), we document the decrease of B/Ca and Mg/Ca of Globigerinoides sacculifer shells with increasing water depth and dissolution. This effect of dissolution on B/Ca may potentially represent a severe bias for paleo-pH reconstructions using this species. Samples of G. sacculifer were analyzed independently at two laboratories for B/Ca and Mg/Ca. Both sets of results show a systematic decrease of B/Ca and Mg/Ca along the depth transect, with an overall loss of ∼14 µmol/mol (∼15%) for B/Ca and of ∼0.7 mmol/mol (∼21%) for Mg/Ca between the shallowest (2640 m) and the deepest (4950 m) sites. Because of this dissolution effect, surface water pH reconstructed from B/Ca of G. sacculifer decreases by ∼0.11 units between the shallowest site and the deepest site, a magnitude similar to the expected glacial/interglacial surface water pH changes.

The marine carbon system and ocean acidification during Phanerozoic time

The global CO₂-carbonic acid-carbonate system of seawater, although certainly a well-researched topic of interest in the past, has risen to the fore in recent years because of the environmental issue of ocean acidification (often simply termed OA). Despite much previous research, there remain pressing questions about how this most important chemical system of seawater operated at the various time scales of the deep time of the Phanerozoic Eon (the past 545 Ma of Earth’s history), interglacial-glacial time, and the Anthropocene (the time of strong human influence on the behaviour of the system) into the future of the planet. One difficulty in any analysis is that the behaviour of the marine carbon system is not only controlled by internal processes in the ocean, but it is intimately linked to the domains of the atmosphere, continental landscape, and marine carbonate sediments.

For the deep-time behaviour of the system, there exists a strong coupling between the states of various material reservoirs resulting in an homeostatic and self-regulating system. As a working hypothesis, the coupling produces two dominant chemostatic modes: (Mode I), a state of elevated atmospheric CO₂, warm climate, and depressed seawater Mg∕Ca and SO₄∕Ca mol ratios, pH (extended geologic periods of ocean acidification), and carbonate saturation states, and elevated Sr concentrations, with calcite and dolomite as dominant minerals found in marine carbonate sediments (Hothouses, the calcite-dolomite seas), and (Mode II), a state of depressed atmospheric CO₂, cool climate, and elevated seawater Mg∕Ca and SO₄/Ca ratios, pH, and carbonate saturation states, and low Sr concentrations, with aragonite and high magnesian calcites as dominant minerals found in marine carbonate sediments (Icehouses, the aragonite seas).

Investigation of the impacts of deglaciation and anthropogenic inputs on the CO₂–H₂O–CaCO₃ system in global coastal ocean waters from the Last Glacial Maximum (LGM: the last great continental glaciation of the Pleistocene Epoch, 18,000 year BP) to the year 2100 shows that with rising sea level, atmospheric CO₂, and temperature, the carbonate system of coastal ocean water changed and will continue to change significantly. We find that 6,000 Gt of C were emitted as CO₂ to the atmosphere from the growing coastal ocean from the Last Glacial Maximum to late preindustrial time because of net heterotrophy (state of gross respiration exceeding gross photosynthesis) and net calcification processes. Shallow-water carbonate accumulation alone from the Last Glacial Maximum to late preindustrial time could account for ~24 ppmv of the ~100 ppmv rise in atmospheric CO₂, lending some support to the ‘‘coral reef hypothesis’’. In addition, the global coastal ocean is now, or soon will be, a sink of atmospheric CO₂, rather than a source. The pHT (pH values on the total proton scale) of global coastal seawater has decreased from ~8.35 to ~8.18 and the CO₃^2- ion concentration declined by ~19% from the Last Glacial Maximum to late preindustrial time. In comparison, the decrease in coastal water pHT from the year 1900 to 2000 was ~8.18 to ~8.08 and is projected to decrease further from about ~8.08 to ~7.85 between 2000 and 2100. During these 200 years, the CO₃^2- ion concentration will fall by ~ 45%. This decadal rate of decline of the CO₃^2- ion concentration in the Anthropocene is 214 times the average rate of decline for the entire Holocene!

In terms of the modern problem of ocean acidification and its effects, the “other CO₂ problem”, we emphasise that most experimental work on a variety of calcifying organisms has shown that under increased atmospheric CO₂ levels (which attempt to mimic those of the future), and hence decreased seawater CO₃^2- ion concentration and carbonate saturation state, most calcifying organisms will not calcify as rapidly as they do under present-day CO₂ levels. In addition, we conclude that dissolution of the highly reactive carbonate phases, particularly the biogenic and cementing magnesian calcite phases, on reefs will not be sufficient to alter significantly future changes in seawater pH and lead to a buffering of the CO₂-carbonic acid system in waters bathing reefs and other carbonate ecosystems on timescales of decades to centuries. Because of decreased calcification rates and increased dissolution rates in a future higher CO₂, warmer world with seas of lower pH and carbonate saturation state, the rate of accretion of carbonate structures is likely to slow and dissolution may even exceed calcification. The potential of increasing nutrient and organic carbon inputs from land, occurrences of mass bleaching events, and increasing intensity (and perhaps frequency of hurricanes and cyclones as a result of sea surface warming) will only complicate matters more. This composite of stresses will have severe consequences for the ecosystem services that reefs perform, including acting as a fishery, a barrier to storm surges, a source of carbonate sediment to maintain beaches, and an environment of aesthetic appeal to tourist and local populations. It seems obvious that increasing rates of dissolution and bioerosion owing to ocean acidification will result in a progressively increasing calcium carbonate (CaCO₃) deficit in the CaCO₃ budget for many coral reef environments. The major questions that require answers are: will this deficit occur and when and to what extent will the destructive processes exceed the constructive processes?

Effects of seawater temperature and pH on the boring rates of the sponge Cliona celata in scallop shells

Warmer, more acidic water resulting from greenhouse gas emissions could influence ecosystem processes like bioerosion of calcifying organisms. Based on summer-maxima values (temperature = 26 °C; pH = 8.1) at a collection site in New York (40°56″ N, 72°30″ W), explants of the boring sponge Cliona celata Grant, 1826 were grown for 133 days on scallop shells in seawater ranging from current values to one scenario predicted for the year 2100 (T = 31 °C; pH = 7.8). High water temperature had little effect on sponge growth, survival, or boring rates. Lower pH slightly reduced sponge survival, while greatly influencing shell boring. At pH = 7.8, sponges bored twice the number of papillar holes and removed two times more shell weight than at pH = 8.1. Greater erosion resulted in weaker scallop shells. This study suggests that lower seawater pH may increase boring rates of C. celata in shellfish, with potentially severe implications for wild and farmed shellfish populations.

Benthic foraminifera show some resilience to ocean acidification in the northern Gulf of California, Mexico

Extensive CO₂ vents have been discovered in the Wagner Basin, northern Gulf of California, where they create large areas with lowered seawater pH. Such areas are suitable for investigations of long-term biological effects of ocean acidification and effects of CO₂ leakage from subsea carbon capture storage. Here, we show responses of benthic foraminifera to seawater pH gradients at 74–207 m water depth. Living (rose Bengal stained) benthic foraminifera included Nonionella basispinata, Epistominella bradyana and Bulimina marginata. Studies on foraminifera at CO₂ vents in the Mediterranean and off Papua New Guinea have shown dramatic long-term effects of acidified seawater. We found living calcareous benthic foraminifera in low pH conditions in the northern Gulf of California, although there was an impoverished species assemblage and evidence of post-mortem test dissolution.

Non-lethal effects of ocean acidification on the symbiont-bearing benthic foraminifer Amphistegina gibbosa

The responses of marine taxa to ocean acidification are varied, with, for example, some exhibiting decreased and some increased calcification rates. Experiments were conducted to assess the effect of elevated atmospheric carbon dioxide concentrations on the survival, fitness, shell microfabric and growth of Amphistegina gibbosa, a symbiont-bearing, coral-reef dwelling, benthic foraminiferal species that precipitates low-Mg calcite tests, using CO₂ partial pressure ( pCO₂) levels similar to those likely to occur in shallow marine pore waters in the decades ahead. Specimens were cultured at constant temperature and controlled pCO₂ (ambient, 1000 parts per million by volume [ppmv], and 2000 ppmv) for 6 wk, and total alkalinity and dissolved inorganic carbon were measured every 2 wk to characterize the carbonate chemistry of the incubations. Foraminiferal survival and cellular energy levels were assessed using adenosine triphosphate analyses, and test microstructure and growth were evaluated using high resolution scanning electron microscopy and image analysis. Fitness and survival were not directly affected by elevated pCO₂ and the concomitant decrease in pH and calcite saturation states (Ω_c). Test growth was not affected by elevated pCO₂. However, areas of dissolution were observed after 6 wk, even though Ω_c was >1 in all treatments; the fraction of test area dissolved increased with decreasing Ω_c. Test dissolution occurred only in small, well defined patches that appeared to be distributed randomly over the whole test surface. Similar dissolution was observed in offspring produced in the 2000 ppmv pCO₂ treatments. The long-term ecological consequences of the effects observed are not yet known.

Calcareous plankton and geochemistry from the ODP site 1209B in the NW Pacific Ocean (Shatsky Rise): new data to interpret calcite dissolution and paleoproductivity changes of the last 450 ka

The high-resolution, multi-proxy investigation of microfossil and isotopic data from Shatsky Rise (ODP Site 1209B, NW Pacific Ocean) is presented to evaluate the potential of calcareous nannofossil assemblages as dissolution and paleoproductivity proxies over the last 450 ka.

To identify the best nannofossil index to evaluate dissolution (in particular, under polarized light microscope), we calculate and compare the different nannofossil and planktonic foraminiferal dissolution-indices from our original dataset. The results demonstrate that the most reliable and reproducible nannofossil dissolution index is the Nannofossil Dissolution Index (NDI) proposed by Marino et al. (2009), particularly for records prior to 250 ka.

The NDI data from the studied Site 1209B represent evidence of preservation maxima mainly during deglaciations, whereas dissolution peaks are recorded at the onset of glacial phases or during severe interglacials. These fluctuations are demonstrated to be basin-wide features in the Pacific. The synchronous timing in the fluctuations of the preservation indices, which consistently lagged behind the oxygen isotope cycles, clearly demonstrates the basin-wide changes in the ocean chemistry during the glacial-interglacial transitions. The Mid-Brunhes Dissolution Event, which was recorded in the Western Pacific at depths below the lysocline at the Marine Isotope Stage (MIS) 11, is not detectable at our relatively shallower site.

At the studied site, the intervals of high productivity generally coincide with the time of good preservation and light carbon-isotope values and vice versa. Therefore, carbonate undersaturation and changes in ocean chemistry (carbonate ion concentration) rather than the variations in the organic carbon flux appear to have controlled the pattern of CaCO₃ preservation.

In addition to the characterization of the dissolution proxies, changes in the calcareous nannofossil assemblages were used to evaluate the primary productivity fluctuations at the mid-latitudes of the NW Pacific over the last 450 ka. The results highlight a general decrease of paleoproductivity during the entire time period as well as shorter glacial/interglacial fluctuations from the base-core upwards. We interpret these features to be variations in the thermocline and nutricline dynamics related to the northward migration of the Kuroshio Extension, which was triggered by the Mid-Brunhes Event and may have caused deepening of the thermocline/nutricline at the site. The spectral and wavelet analyses performed on the microfossil database prove that the variations in paleoproductivity and carbonate dissolution over the last 450 ka were primarily driven by the glacial-interglacial variability (100 ka periodicity) and by the obliquity-controlled changes.

Dolomite-rich coralline algae in reefs resist dissolution in acidified conditions

Coral reef ecosystems develop best in high-flow environments but their fragile frameworks are also vulnerable to high wave energy. Wave-resistant algal rims, predominantly made up of the crustose coralline algae (CCA) Porolithon onkodes and P. pachydermum^{1, 2}, are therefore critical structural elements for the survival of many shallow coral reefs. Concerns are growing about the susceptibility of CCA to ocean acidification because CCA Mg-calcite skeletons are more susceptible to dissolution under low pH conditions than coral aragonite skeletons³. However, the recent discovery⁴ of dolomite (Mg_0.5Ca_0.5(CO₃)), a stable carbonate⁵, in P. onkodes cells necessitates a reappraisal of the impacts of ocean acidification on these CCA. Here we show, using a dissolution experiment, that dried dolomite-rich CCA have 6–10 times lower rates of dissolution than predominantly Mg-calcite CCA in both high-CO₂ (~ 700 ppm) and control (~ 380 ppm) environments, respectively. We reveal this stabilizing mechanism to be a combination of reduced porosity due to dolomite infilling and selective dissolution of other carbonate minerals. Physical break-up proceeds by dissolution of Mg-calcite walls until the dolomitized cell eventually drops out intact. Dolomite-rich CCA frameworks are common in shallow coral reefs globally and our results suggest that it is likely that they will continue to provide protection and stability for coral reef frameworks as CO₂ rises.

Oceanography: a sea butterfly flaps its wings

Ocean acidification is predicted to harm the ocean’s shell-building organisms over the coming centuries. Sea butterflies, an ecologically important group of molluscs in the Arctic and Southern oceans, are already suffering the effects.

Extensive dissolution of live pteropods in the Southern Ocean

The carbonate chemistry of the surface ocean is rapidly changing with ocean acidification, a result of human activities¹. In the upper layers of the Southern Ocean, aragonite—a metastable form of calcium carbonate with rapid dissolution kinetics—may become undersaturated by 2050 (ref. 2). Aragonite undersaturation is likely to affect aragonite-shelled organisms, which can dominate surface water communities in polar regions³. Here we present analyses of specimens of the pteropod Limacina helicina antarctica that were extracted live from the Southern Ocean early in 2008. We sampled from the top 200 m of the water column, where aragonite saturation levels were around 1, as upwelled deep water is mixed with surface water containing anthropogenic CO₂. Comparing the shell structure with samples from aragonite-supersaturated regions elsewhere under a scanning electron microscope, we found severe levels of shell dissolution in the undersaturated region alone. According to laboratory incubations of intact samples with a range of aragonite saturation levels, eight days of incubation in aragonite saturation levels of 0.94–1.12 produces equivalent levels of dissolution. As deep-water upwelling and CO₂ absorption by surface waters is likely to increase as a result of human activities^{2, 4}, we conclude that upper ocean regions where aragonite-shelled organisms are affected by dissolution are likely to expand.

Structural and functional vulnerability to elevated pCO2 in marine benthic communities

The effect of elevated pCO₂/low pH on marine invertebrate benthic biodiversity, community structure and selected functional responses which underpin ecosystem services (such as community production and calcification) was tested in a medium-term (30 days) mesocosm experiment in June 2010. Standardised intertidal macrobenthic communities, collected (50.3567°N, 4.1277°W) using artificial substrate units (ASUs), were exposed to one of seven pH treatments (8.05, 7.8. 7.6, 7.4, 7.2, 6.8 and 6.0). Community net calcification/dissolution rates, as well as changes in biomass, community structure and diversity, were measured at the end of the experimental period. Communities showed significant changes in structure and reduced diversity in response to reduced pH: shifting from a community dominated by calcareous organisms to one dominated by non-calcareous organisms around either pH 7.2 (number of individuals and species) or pH 7.8 (biomass). These results were supported by a reduced total weight of CaCO₃ structures in all major taxa at lowered pH and a switch from net calcification to net dissolution around pH 7.4 (Ω_calc = 0.78, Ω_ara = 0.5). Overall community soft tissue biomass did not change with pH and high mortality was observed only at pH 6.0, although molluscs and arthropods showed significant decreases in soft tissue. This study supports and refines previous findings on how elevated pCO₂ can induce changes in marine biodiversity, underlined by differential vulnerability of different phyla. In addition, it shows significant elevated pCO₂-/low pH-dependent changes in fundamental community functional responses underpinning changes in ecosystem services.