Posts Tagged 'dissolution'

Control of CaCO3 dissolution at the deep seafloor and its consequences

Prediction of the neutralization of anthropogenic CO2 in the oceans and the interpretation of the calcite record preserved in deep-sea sediments requires the use of the correct kinetics for calcite dissolution. Dissolution rate information from suspended calcite-grain experiments consistently indicates a high-order nonlinear dependence on undersaturation, with a well-defined rate constant. Conversely, stirred-chamber and rotating-disc dissolution experiments consistently indicate linear kinetics of dissolution and a strong dependence on the fluid flow velocity. Here, we resolve these seeming incongruities and establish reliably the kinetic controls on deep-sea calcite dissolution. By equating the carbonate-ion flux from a dissolving calcite bed, governed by laboratory-based nonlinear kinetics, to the flux across typical diffusive boundary layers (DBL) at the seafloor, we show that the net flux is influenced both by boundary layer and bed processes, but that flux is strongly dominated by the rate of diffusion through the DBL. Furthermore, coupling that calculation to an equation for the calcite content of the seafloor, we show that a DBL-transport dominated model adeptly lysoclines adeptly, i.e., CaCO3 vs ocean depth profiles, observed across the oceans. Conversely, a model with only sediment-side processes fails to predict lysoclines in all tested regions. Consequently, the past practice of arbitrarily altering the calcite-dissolution rate constant to allow sediment-only models to fit calcite profiles constitutes confirmation bias. From these results, we hypothesize that the reason suspended-grain experiments and bed experiments yield different reaction orders is that dissolution rates of individual grains in a bed are fast enough to maintain porewaters at or close to saturation, so that the exact reaction order cannot be measured and dissolution appears to be linear. Finally, we provide a further test of DBL-transport dominated calcite dissolution by successfully predicting, not fitting, the in-situ pH profiles observed at four stations reported in the literature.

Continue reading ‘Control of CaCO3 dissolution at the deep seafloor and its consequences’

New methods for imaging and quantifying dissolution of pteropods to monitor the impacts of ocean acidification

Large-scale changes in climate and ocean ecosystems demand innovative and cost-effective ways to track changes in the marine environment and its living resources. During the past decade, ocean acidification has become recognized as a major threat to the biodiversity of marine ecosystems during the 21st century. However, an important constraint on modern ocean acidification research is the lack of accessibility to effective imaging techniques, as well as accurate analytical methods. Here, we compare several different microscopic techniques to evaluate the relative merits of each. Additionally, a new dissolution quantification method is developed that more completely assesses damage over an entire shell. These findings can help expand the toolbox for scientists engaged in studying the impacts of ocean acidification on marine invertebrates and enable more researchers to participate in this vital field.

Continue reading ‘New methods for imaging and quantifying dissolution of pteropods to monitor the impacts of ocean acidification’

In situ growth and bioerosion rates of Lophelia pertusa in a Norwegian fjord and open shelf cold-water coral habitat

Coral reef resilience depends on the balance between carbonate precipitation, leading to reef growth, and carbonate degradation, for example, through bioerosion. Changes in environmental conditions are likely to affect the two processes differently, thereby shifting the balance between reef growth and degradation. In cold-water corals estimates of accretion-erosion processes in their natural habitat are scarce and solely live coral growth rates were studied with regard to future environmental changes in the laboratory so far, limiting our ability to assess the potential of cold-water coral reef ecosystems to cope with environmental changes. In the present study, growth rates of the two predominant colour morphotypes of live Lophelia pertusa as well as bioerosion rates of dead coral framework were assessed in different environmental settings in Norwegian cold-water coral reefs in a 1-year in situ experiment. Net growth (in weight gain and linear extension) of live L. pertusa was in the lower range of previous estimates and did not significantly differ between inshore (fjord) and offshore (open shelf) habitats. However, slightly higher net growth rates were obtained inshore. Bioerosion rates were significantly higher on-reef in the fjord compared to off-reef deployments in- and offshore. Besides, on-reef coral fragments yielded a broader range of individual growth and bioerosion rates, indicating higher turnover in live reef structures than off-reef with regard to accretion-bioerosion processes. Moreover, if the higher variation in growth rates represents a greater variance in (genetic) adaptations to natural environmental variability in the fjord, inshore reefs could possibly benefit under future ocean change compared to offshore reefs. Although not significantly different due to high variances between replicates, growth rates of orange branches were consistently higher at all sites, while mortality was statistically significantly lower, potentially indicating higher stress-resistance than the less pigmented white phenotype. Comparing the here measured rates of net accretion of live corals (regardless of colour morphotype) with net erosion of dead coral framework gives a first estimate of the dimensions of both processes in natural cold-water coral habitats, indicating that calcium carbonate loss through bioerosion amounts to one fifth to one sixth of the production rates by coral calcification (disregarding accretion processes of other organisms and proportion of live and dead coral framework in a reef). With regard to likely accelerating bioerosion and diminishing growth rates of corals under ocean acidification, the balance of reef accretion and degradation may be shifted towards higher biogenic dissolution in the future.

Continue reading ‘In situ growth and bioerosion rates of Lophelia pertusa in a Norwegian fjord and open shelf cold-water coral habitat’

Assessing annual variability in the shell thickness of the pteropod Heliconoides inflatus in the Cariaco Basin using micro-CT scanning

Pteropods have been nicknamed the canary in the coal mine for ocean acidification because they are predicted to be among the first organisms to be affected by future changes in ocean chemistry. This is due to their fragile, aragonitic shells and high abundances in polar and sub-polar regions where the impacts of ocean acidification will manifest first. For pteropods to be used most effectively as indicators of ocean acidification, their natural variability in the modern ocean needs to be quantified and understood. Here, we measured the shell condition (i.e., the degree to which a shell has dissolved) and shell characteristics, including size, number of whorls, shell thickness, and shell volume (i.e., amount of shell material) of nearly fifty specimens of the pteropod species Heliconoides inflatus from a sediment trap in the Cariaco Basin, Venezuela sampled over an 11-month period. The water in the Cariaco Basin is supersaturated with respect to aragonite year-round, and hydrographic and chemical properties vary seasonally due to the movement of the Inter Tropical Convergence Zone (ITCZ). Shell condition was assessed using with two methods: the Limacina Dissolution Index (LDX) and the opacity method. The opacity method captured changes in shell condition only in the early stages of dissolution, whereas the LDX recorded dissolution changes over a much larger range. Shell condition did not deteriorate with the length of time in the sediment trap. Instead, the most altered shells occurred in samples collected in September and October when water temperatures were warmest, and the amount of organic matter degradation in the water column was likely to have been the greatest. Shells of H. inflatus varied in size, number of whorls, and thickness throughout the year. The number of whorls did not correlate with shell diameter, suggesting that shell growth is plastic. H. inflatus formed shells that were 40 % thicker and 20 % larger in diameter when nutrient concentrations were high during times of upwelling, compared to specimens sampled from the oligotrophic rainy season. This study produces a baseline dataset of the variability in shell characteristics of H. inflatus in the Cariaco Basin and establishes a methodology for generating similar baseline records for pteropod populations globally.

Continue reading ‘Assessing annual variability in the shell thickness of the pteropod Heliconoides inflatus in the Cariaco Basin using micro-CT scanning’

Reef dissolution : rates and mechanisms of coral dissolution by bioeroding sponges and reef communities

For coral reefs to persist, the rate of CaCO3 production must be greater than the rate of erosion to enable positive growth. Negative impacts of global change (ocean acidification and warming) and local stressors (eutrophication, overfishing) on accretion co-occur with positive effects of these changes on bioerosion capacity and chemical dissolution by excavating euendolithic organisms. This is especially relevant for reefs characterised with low calcifying rates as they will tip faster into net loss. The Caribbean reefs suffered from a decrease by up to 80% in scleractinian coral cover in the past 50 years, their configuration bears very little resemblance with reefs pre1980s, in terms of benthic composition, coral cover and structural complexity. Specifically, excavating sponges can contribute up to 90% of the total macroborer activity on coral reefs and their rates of bioerosion are positively affected by pCO2. The overarching aim of this thesis was to quantify and understand the accretion and loss terms of coral reef communities with a focus on the interactions of anthropogenic ocean acidification and eutrophication with bioerosion by coral-excavating sponges.The use of incubations was central in this piece of work. Changes in the chemical composition of the water overlying sponges and reef communities indicate the relative contribution of metabolic processes such as net calcification/dissolution and net respiration/production. However, we first used fluorescence microscopy to investigate the underlying mechanisms of CaCO3 dissolution by excavating sponges. It revealed that they promote CaCO3 dissolution by decreasing pH at the sponge/coral interface. The high [H+] at this site is achieved through delivery of low-pH vesicles by the etching cells. The enzyme carbonic anhydrase, which is responsible for significantly increasing the speed of the reversible reaction H2O+CO2↔H++HCO3−, has been shown to be associated to the sponge’s etching processes and is therefore thought to play a role in the dissolution of CaCO3. By blocking its activity whilst incubating sponges and analysing the rate of dissolution, CA was found to play an important role in speeding up protonation of HCO3− ions at the dissolution site, enabling CO2 to diffuse out of the etching area. When exposed to different ranges of ocean acidification and eutrophication, bioerosion rates increased with both variables but no synergistic relation was revealed. Incubations performed at the community level around Saba and Curacao yielded net community calcification (NCC) rates which were lower than those reported for reef flats worldwide. Still, Saba coral reefs are considered relatively pristine sites compared to the average within the wider Caribbean. Around Curaçao, incubations on reef assemblages dominated by coral yielded even lower NCC rates. Incubations of other benthic assemblages that currently characterized shallow Caribbean reef substrate (such as bioeroding sponges, benthic cyanobacterial mats and sand) all resulted in net dissolution. For both Saba and Curaçao, results suggest that reef calcification on these sites is barely able to compensate the CaCO3 losses due to dissolution from other opportunistic benthic residents. With the ongoing global and local pressures, the delicate balance between CaCO3 accretion and loss is likely to tip.

Continue reading ‘Reef dissolution : rates and mechanisms of coral dissolution by bioeroding sponges and reef communities’

Reduced continental weathering and marine calcification linked to late Neogene decline in atmospheric CO2

The globally averaged calcite compensation depth has deepened by several hundred metres in the past 15 Myr. This deepening has previously been interpreted to reflect increased alkalinity supply to the ocean driven by enhanced continental weathering due to the Himalayan orogeny during the late Neogene period. Here we examine mass accumulation rates of the main marine calcifying groups and show that global accumulation of pelagic carbonates has decreased from the late Miocene epoch to the late Pleistocene epoch even though CaCO3 preservation has improved, suggesting a decrease in weathering alkalinity input to the ocean, thus opposing expectations from the Himalayan uplift hypothesis. Instead, changes in relative contributions of coccoliths and planktonic foraminifera to the pelagic carbonates in relative shallow sites, where dissolution has not taken its toll, suggest that coccolith production in the euphotic zone decreased concomitantly with the reduction in weathering alkalinity inputs as registered by the decline in pelagic carbonate accumulation. Our work highlights a mechanism whereby, in addition to deep-sea dissolution, changes in marine calcification acted to modulate carbonate compensation in response to reduced weathering linked to the late Neogene cooling and decline in atmospheric partial pressure of carbon dioxide.

Continue reading ‘Reduced continental weathering and marine calcification linked to late Neogene decline in atmospheric CO2’

Living coral tissue slows skeletal dissolution related to ocean acidification

Climate change is causing major changes to marine ecosystems globally, with ocean acidification of particular concern for coral reefs. Using a 200 d in situ carbon dioxide enrichment study on Heron Island, Australia, we simulated future ocean acidification conditions, and found reduced pH led to a drastic decline in net calcification of living corals to no net growth, and accelerated disintegration of dead corals. Net calcification declined more severely than in previous studies due to exposure to the natural community of bioeroding organisms in this in situ study and to a longer experimental duration. Our data suggest that reef flat corals reach net dissolution at an aragonite saturation state (ΩAR) of 2.3 (95% confidence interval: 1.8–2.8) with 100% living coral cover and at ΩAR > 3.5 with 30% living coral cover. This model suggests that areas of the reef with relatively low coral mortality, where living coral cover is high, are likely to be resistant to carbon dioxide-induced reef dissolution.

Continue reading ‘Living coral tissue slows skeletal dissolution related to ocean acidification’


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