The large and rapid release of CO2 into the atmosphere over the past two centuries and resultant uptake of a significant fraction of this by the oceans have resulted in measurable changes in ocean chemistry, most notably decreases in both pH and carbonate ion concentration. This change in atmospheric CO2 during the last 200 years and its concomitant effect on seawater chemistry are not unique but rather have occurred frequently throughout geologic history. Quantifying the response of the oceans to such naturally occurring changes in atmospheric CO2 provides valuable insights for modeling and predicting future changes in ocean chemistry associated with anthropogenic increases in CO2. This work examines three carbonate ion and pH proxies (weight, B/Ca and boron isotope composition of planktonic foraminiferal shells) using the 15-year time series of sediment trap samples and water column chemistry measurements collected biweekly as part of the Cariaco Basin ocean time series. These results will be compared to monthly measurements of upper water column carbonate ion and pH made at the time series site. High deposition rate, laminated sediments accumulating in the Cariaco Basin will be used to make a direct comparison between changes in these three foraminiferal pH proxies and the atmospheric CO2 record for the last two centuries, a period during which CO2 has increased by ~100 ppmv. The end product of the research will be an in-depth field-calibration of these widely used pH proxies. Funding supports a graduate student, and provides opportunities for undergraduate involvement in research.
Over the last two centuries atmospheric CO2 concentrations have increased from 280 ppm during the preindustrial period to 400 ppm at present, with the present concentration being significantly higher than any value determined from ice core records for the last 800,000 years. It is estimated that the global ocean has absorbed approximately a third of this anthropogenically produced CO2. This incorporation of atmospheric CO2 into seawater causes a decrease in pH, the phenomenon known as ocean acidification. This project utilized sediment trap samples and water column chemistry data collected as part of the Cariaco Basin ocean time series to evaluate and calibrate pH and carbonate ion proxies that can be used to reconstruct past changes in ocean acidification. Most notably, we have developed a new proxy, foraminiferal area density, for quantifying the relationship between calcification intensity and carbonate ion concentration. Our results clearly illustrate that the shell wall thickness of planktonic foraminifera varies as a function of carbonate ion content (Figure 1). The area density method is presented in a publication by Marshall et al. (2013). Brittney Marshall, a PhD student working with Thunell, is also a co-author on the paper by Henehan et al. (2013) that deals with the calibration of B isotopes in planktonic foraminifera as a proxy for past changes in ocean pH. In conjunction with Dr. David Black (Stony Brook University), we have documented how increasing CO2 concentrations in the oceans since the onset of the Industrial Revolution are preserved in the δ13C of planktonic foraminifera from both Cariaco Basin sediment trap samples and the seafloor sediment record. Broader Impacts: This award supported, in part, the research of two female students. Brittney Marshall is a 4th PhD year student and expects to complete her degree in December 2014. Jessica Holm, an undergraduate student, was supported by this grant for two years. Jessica graduated in May 2014 and will be pursuing graduate studies in the geosciences beginning Fall 2014. Publications from this award: Black, D., Thunell, R., Wejnert, K. and Astor, Y., 2011. Carbon isotope composition of Caribbean Sea surface waters: Response to the uptake of anthropogenic CO2, Geophysical Research Letters 38, L16609, doi: 10.1029/2011GL048538. Marshall, B., Thunell, R., Henehan, J., Astor, Y. and Wejnert, K., 2013. Planktonic foraminiferal area density as a proxy for carbonate ion concentration: A calibration study using the Cariaco Basin Ocean Time Series, Paleoceanography doi:10.1002/paleo.20034. Henehan, M., Rae, J., Foster, G., Erez, J., Prentice, K., Kucera, M., Bostock, H., Milton, J., Wilson, P., Marshall, B., and Elliott, T., 2013. Calibration of the boron isotope proxy in the planktonic foraminifera Globigerinoides ruber for use in paleo-CO2 reconstruction, Earth and Planetary Sciences Letters 364, 111-122.
