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Scientists identify flaws in using plankton shells for climate studies

Scientists at IISc have identified gaps in using certain plankton as a tool for determining the climactic history of a region. These marine plankton of the Foraminifera phylum have calcium carbonate shells. The shells are used for paleoclimactic studies. Studies on these shells found in oceans between 10°N and 40°S have seen a correspondance between the observed and expected values of Carbon and Oxygen isotopes in them. But, south of 40°S in the Southern Ocean, there is a disparity in these values. The research team from the Centre for Atmospheric and Oceanic Sciences led by Prof. Prosenjit Ghosh have studied this phenomenon and have put forth theories to address the inconsistency.

Foraminifera are a class of amoeboid organisms and are mostly marine. They excrete substances to form a shell around themselves. These shells have been used extensively for paleoclimatology. The research team at IISc selected Globigerina Bulloides, a foraminifera species that inhabits the oceans at a depth of 75m to 200m, for this study. The organism has a calcium carbonate shell and sinks to the ocean floor on its death, littering the ocean floor with its shell.

Significant information is available from calcium carbonate present in these shells. The oxygen isotope ratio, the relative abundance of oxygen atoms with different atomic masses, tells us about historic water temperatures. Similarly, the carbon isotope ratio provides information about the temperature and Dissolved Inorganic Carbon (DIC). These values can be calculated from the shells to determine the temperature and salinity of the water when the organism was alive. This provides us a window in to the region’s past and present climactic conditions.

The research team cruised on the ORV Sagar Nidhi to collect live specimen. They collected the shells at various locations between 10°N and 60°S using nets. They also collected water samples from the sea surface and sediment core samples. After baking the shells of the G. Bulloides, the team measured the amount of Oxygen 18 and Carbon 13 isotopes in them. These isotope values were then compared with the expected values.

The team found that in the regions between 10°N and 40°S, the two values matched within an error range of 0.25%. But, further south of 40°S, the values did not match quite well. “We have put forth three models or theories to address this inequality; the kinetic effect, genetic variability and increased productivity”, says Mr. Prasanna K, one of the members of the research team.

The researchers speculate genetic variations in the G. Bulloides species could be a reason. “We suspect that this deviation may be due to genetic effect on the calcification. More work is needed to confirm this”, adds Mr. Prasanna. The increased productivity hypothesis attributes the disparity to increased nutrient supply, upwelling and availability of light. “According to our estimates this process could take place in a short time. It might have taken around 5000 days of productivity to reach current values”, comments Mr. Prasanna.

 This study shows that the use of Foraminifera as a reliable proxy for paleoclimatic studies  needs corrections. We need to understand the reasons behind the inconsistencies before making these corrections. This research work tries to offer a possible explanation for these inconsistencies.

About the authors:

Prasanna K is a research student at Dr. Prosenjit Ghosh’s lab in the Centre for Atmospheric and Oceanographic Sciences at IISc.


Prosenjit Ghosh is an Associate Professor at the Centre for Atmospheric and Oceanographic Sciences at IISc.


About the paper:

The paper titled ‘Isotopic disequilibrium in Globigerina bulloides and carbon isotope response to productivity increase in Southern Ocean’ was published in the Nature journal. A link to the paper is provided below.