Carbon dioxide concentrations in Earth’s atmosphere have fluctuated over time. Measurements in ice cores show that, over the past 800,000 years, the concentration of carbon dioxide in the atmosphere tracks the increasing and decreasing extent of ice sheets.
It is thought that storage and release of CO2 into and from the deep ocean is primarily responsible for these changes across the glacial cycles. At a smaller scale, the surface ocean has areas that release CO2 and others that absorb it from the atmosphere, in large part depending on the amount of biological productivity, which is tied to nutrient availability. The Arabian Sea has a semiannual seasonal surface water circulation, which reverses during the monsoon cycle when large amounts of fresh water drain into the eastern Arabian Sea from streams and rivers. This reversal affects nutrient uptake and productivity, and thus carbon dioxide uptake.
Naik et al. use boron isotopes and other chemical signals from planktonic foraminifera shells in seabed core samples from the Arabian Sea to reconstruct the pH of the surface water through time. Increased uptake of CO2 lowers the pH of the surface water, and thus the calculated surface water pH can be used in conjunction with salinity and other parameters to determine past carbon dioxide levels. The results show that the monsoon was weak, and CO2 uptake was reduced, during the Last Glacial Maximum but increased in strength after about 14,000 years ago. (Geophysical Research Letters, doi:10.1002/2015GL063089, 2015)
—Catherine Minnehan, Freelance Writer
Citation: Minnehan, C. (2015), Correlating monsoon strength with boron isotopes, Eos, 96, doi:10.1029/2015EO037427. Published on 19 October 2015.