Seawater carbonate chemistry in the future ocean, 2008

Future anthropogenic emissions of CO2 and the resulting ocean acidification may have severe consequences for marine calcifying organisms and ecosystems. Marine calcifiers depositing calcitic hard parts that contain significant concentrations of magnesium, i.e. Mg-calcite, and calcifying organisms living in high latitude and/or cold-water environments are at immediate risk to ocean acidification and decreasing seawater carbonate saturation because they are currently immersed in seawater that is just slightly supersaturated with respect to the carbonate phases they secrete. Under the present rate of CO2 emissions, model calculations show that high latitude ocean waters could reach undersaturation with respect to aragonite in just a few decades. Thus, before this happens these waters will be undersaturated with respect to Mg-calcite minerals of higher solubility than that of aragonite. Similarly, tropical surface seawater could become undersaturated with respect to Mg-calcite minerals containing ?12 mole percent (mol%) MgCO3 during this century. As a result of these changes in surface seawater chemistry and further penetration of anthropogenic CO2 into the ocean interior, we suggest that (1) the magnesium content of calcitic hard parts will decrease in many ocean environments, (2) the relative proportion of calcifiers depositing stable carbonate minerals, such as calcite and low Mg-calcite, will increase and (3) the average magnesium content of carbonate sediments will decrease. Furthermore, the highest latitude and deepest depth at which cold-water corals and other calcifiers currently exist will move towards lower latitudes and shallower depth, respectively. These changes suggest that anthropogenic emissions of CO2 may be currently pushing the oceans towards an episode characteristic of a 'calcite sea.'

Seawater carbonate chemistry, clearance rates, respiration rates, condition index and cellular turnover (RNA: DNA) of Juvenile King Scallop, Pecten maximus in a laboratory experiment

The decline in ocean water pH and changes in carbonate saturation states through anthropogenically mediated increases in atmospheric CO2 levels may pose a hazard to marine organisms. This may be particularly acute for those species reliant on calcareous structures like shells and exoskeletons. This is of particular concern in the case of valuable commercially exploited species such as the king scallop, Pecten maximus. In this study we investigated the effects on oxygen consumption, clearance rates and cellular turnover in juvenile P. maximus following 3 months laboratory exposure to four pCO2 treatments (290, 380, 750 and 1140 µatm). None of the exposure levels were found to have significant effect on the clearance rates, respiration rates, condition index or cellular turnover (RNA: DNA) of individuals. While it is clear that some life stages of marine bivalves appear susceptible to future levels of ocean acidification, particularly under food limiting conditions, the results from this study suggest that where food is in abundance, bivalves like juvenile P. maximus may display a tolerance to limited changes in seawater chemistry.

Seawater conditions during the experiment in May 2011 at the sampling sites off Vulcano Island

The impacts of ocean acidification on coastal biofilms are poorly understood. Carbon dioxide vent areas provide an opportunity to make predictions about the impacts of ocean acidification. We compared biofilms that colonised glass slides in areas exposed to ambient and elevated levels of pCO2 along a coastal pH gradient, with biofilms grown at ambient and reduced light levels. Biofilm production was highest under ambient light levels, but under both light regimes biofilm production was enhanced in seawater with high pCO2. Uronic acids are a component of biofilms and increased significantly with high pCO2. Bacteria and Eukarya denaturing gradient gel electrophoresis profile analysis showed clear differences in the structures of ambient and reduced light biofilm communities, and biofilms grown at high pCO2 compared with ambient conditions. This study characterises biofilm response to natural seabed CO2 seeps and provides a baseline understanding of how coastal ecosystems may respond to increased pCO2 levels.

Trace-element contents in ash samples from DSDP Leg 19 sediments

The Sr, Rb, Ti, and Zr concentrations of 16 volcanic ash samples from Leg 19 of the Deep Sea Drilling Project were determined by X-ray fluorescence. The age of each ash sample had been established previously by faunal criteria and had been confirmed by fission-track dating. Variations in the trace-element concentrations through the past 8 m.y. are clearly seen. Seven of the ashes are older than 4 m.y., have low TiO2 contents, and have Sr concentrations of less than 200 ppm; they are thus similar to tholeiitic basalts of island arcs. Nine ashes are younger than 4 m.y. and are similar in trace-element content to andesite. Magmatic evolution of the Aleutian arc over the past 8 m.y. is clearly shown.

The influence of temperature and seawater carbonate saturation state on 13C-18O bond ordering in bivalve mollusks

The shells of marine mollusks are widely used archives of past climate and ocean chemistry. Whilst the measurement of mollusk delta 18O to develop records of past climate change is a commonly used approach, it has proven challenging to develop reliable independent paleothermometers that can be used to deconvolve the contributions of temperature and fluid composition on molluscan oxygen isotope compositions. Here we investigate the temperature dependence of 13C-18O bond abundance, denoted by the measured parameter Delta 47, in shell carbonates of bivalve mollusks and assess its potential to be a useful paleothermometer. We report measurements on cultured specimens spanning a range in water temperatures of 5 to 25 °C, and field collected specimens spanning a range of -1 to 29 °C. In addition we investigate the potential influence of carbonate saturation state on bivalve stable isotope compositions by making measurements on both calcitic and aragonitic specimens that have been cultured in seawater that is either supersaturated or undersaturated with respect to aragonite. We find a robust relationship between Delta 47 and growth temperature. We also find that the slope of a linear regression through all the Delta 47 data for bivalves plotted against seawater temperature is significantly shallower than previously published inorganic and biogenic carbonate calibration studies produced in our laboratory and go on to discuss the possible sources of this difference. We find that changing seawater saturation state does not have significant effect on the Delta 47 of bivalve shell carbonate in two taxa that we examined, and we do not observe significant differences between Delta 47-temperature relationships between calcitic and aragonitic taxa.

Seawater carbonate chemistry, survival rate, shell size, calcification rate of the planktonic foraminifer Neogloboquadrina pachyderma (sinistral) in a laboratory experiment

The present study investigated the effects of ocean acidification and temperature increase on Neogloboquadrina pachyderma (sinistral), the dominant planktonic foraminifer in the Arctic Ocean. Due to the naturally low concentration of [CO3] 2- in the Arctic, this foraminifer could be particularly sensitive to the forecast changes in seawater carbonate chemistry. To assess potential responses to ocean acidification and climate change, perturbation experiments were performed on juvenile and adult specimens by manipulating seawater to mimic the present-day carbon dioxide level and a future ocean acidification scenario (end of the century) under controlled (in situ) and elevated temperatures (1 and 4 °C, respectively). Foraminifera mortality was unaffected under all the different experiment treatments. Under low pH, N. pachyderma (s) shell net calcification rates decreased. This decrease was higher (30 %) in the juvenile specimens than decrease observed in the adults (21 %) ones. However, decrease in net calcification was moderated when both, pH decreased and temperature increased simultaneously. When only temperature increased, a net calcification rate for both life stages was not affected. These results show that forecast changes in seawater chemistry would impact calcite production in N. pachyderma (s), possibly leading to a reduction of calcite flux contribution and consequently a decrease in biologic pump efficiency.