Natural variation of pelagic carbonate production during Cenozoic warm periods

Abstract

The biological carbon uptake, called biological compensation, have been shown to have a huge potential to affect the capacity of the ocean to absorb (anthropogenic) carbon dioxide, and so equilibrate the global carbon budget and hence climate. Since the pelagic calcite flux is made of two fundamentally different components, coccolithophore algae and planktonic foraminifera, understanding of the process of biological compensation requires knowledge of variability of their relative contribution to the total pelagic calcite flux. The aspects of the pelagic carbonate production that have changed through time and the mechanisms explaining the observed carbonate flux variability remain, despite their importance, largely unconstrained. In order to evaluate the orbital and long geological time scale variability of the pelagic carbonate production, I generated new high-resolution records of carbonate accumulation rate, using marine sediments deposited in the equatorial Atlantic Ocean (Ceará Rise) at ODP Site 927, across four warm climates intervals ranging from the Neogene to the Quaternary. I find that the relative contribution of the two groups to the total pelagic carbonate production remains relatively constant on long geological time scales, shows a high orbital time scale variability (factor of two), and is not driving the changes in total pelagic carbonate production. I conclude that at the studied location, the main driver of the pelagic carbonate changes, for both the planktonic foraminifera and the coccoliths were changes in population growth, with a shift in the composition of the communities. The observed dominant periodicities in carbonate accumulation rate indicate that the two groups responded to local changes in factors affecting their productivity, rather than to global climate modulations. On both time scales, the observed changes were large enough to affect the marine inorganic carbon cycle and thus the ocean’s capacity to absorb inorganic carbon.