Award: OCE-1039503

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. Last Modified: 07/01/2014 Submitted by: Robert C Thunell