Over the past 200 years, the ocean has taken up roughly 50% of the carbon dioxide (CO2) released into the atmosphere by mankind. As CO2 invades the ocean, the seawater becomes less alkaline and the pH drops, a process termed ?ocean acidification?. Concurrently, the saturation state of seawater with respect to carbonate minerals such as calcite and aragonite falls, which is likely to impact calcification rates in many calcareous organisms on a short time scale. On longer time scales, anthropogenic CO2 will also impact the benthic environment, as a significant fraction of the CO2 will react with CaCO3 in deep-sea sediments and be neutralized to bicarbonate ions (fossil fuel neutralization). These consequences of ocean acidification are critical for the future of marine ecosystems as well as for the fate of natural carbon sinks, the latter being vital to predicting future atmospheric CO2 levels. While effects on calcifying organisms have been observed in laboratory and mesocosm experiments, the ecosystem response on a global scale is hitherto unknown. Similar uncertainties exist regarding large-scale rates of deep-sea carbonate dissolution. Fortunately, both of these processes can be detected and quantified via their effect on ocean chemistry: production and dissolution of CaCO3 change the total alkalinity of seawater.
In this project, researchers at the University of Hawaii will employ a synthesis of modeling and ocean chemistry data in order to address the following questions: (1) Does ocean acidification lead to a decline in marine calcification on a global scale, and if so, what is the magnitude and time scale of the decline? (2) What is the rate of fossil fuel neutralization by large-scale carbonate dissolution in deep-sea sediments? They will use the 3-D global biogeochemical ocean model HAMOCC (which includes a detailed sediment module) to forecast changes in alkalinity due different scenarios of changes in surface calcification and rates of deep-sea carbonate dissolution. The outcome will be compared to changes in ocean chemistry derived from data of repeat hydrographic surveys. For a given scenario, they will calculate the location and time at which transient changes in total alkalinity will exceed the natural variability and identify target regions for future ocean chemistry programs that are critical for computing the exact magnitude of acidification effects from field data. They will then be able to design a tool for early detection of large-scale effects of ocean acidification observable in the field and to project the future role of deep-sea carbonate dissolution during the coming millennia.
In terms of broader impacts, this project will provide insights into the short- and long-term effects of invasion of anthropogenic CO2 into the ocean. The aim is to help guide ocean carbon cycle observations in the near future in order to detect the large-scale response of marine calcification and deep-sea carbonate dissolution to ocean acidification at an early stage. The proposed work will include a graduate student project and will provide educational opportunities for undergraduate students from underrepresented groups at the University of Hawaii.
