Australia: The Land Where Time Began

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Deep-sea CaCo3 sedimentation - Response to Shutdown of the Atlantic Meridional Overturning Circulation (AMOC)

The authors1 used an Earth system model to study preservation and burial distribution changes of calcium carbonate (CaCO3) in sediments of the deep ocean that are associated with the atmospheric pCO2 response to the shutdown of the Atlantic Meridional Overturning Circulation (AMOC). The results of the study indicate that the inflow of freshwater forced the shutdown of the AMOC with the result that there was a significant decrease of CaCO3 content in the sediments of the North Atlantic, a consequence of reduced carbonate ion concentrations in bottom water and reduced production of CaCO3 in the surface waters. There is a separation in time of the main impacts of both these effects, in the first 1,000 years following the application of the forcing the reduced production of CaCO3 dominates the reduced CaCO3 burial. Atmospheric pCO2 increases by 11 ppm when significant overturning circulation in the Atlantic, largely as a result of decreased POC Export and a biological pump that is weakening. According to the authors1 increased CaCO3 burial in the Pacific Ocean largely efficiently buffers the reduced preservation in the Atlantic. This results in the  pCO2, that is caused by the reorganisation of the burial of CaCO3 in the sediments of the deep sea, is small, 1 ppm, leaving the overall global burial and the alkalinity of the ocean changed only minimally.

Authors'1 Summary

In the Genie-1 earth system model the AMOC shutdown, that was forced by freshwater inflow to the North Atlantic promotes a decreased burial flux by 86 % in the sediments of the North Atlantic, which is mainly controlled by 2 factors. One involves the reduction of carbonate ion concentration in the bottom water, enhancing the dissolution of  CaCO3 in the sediments, reducing the burial flux. There is also weakening of the production of CaCO3 at the surface that then decreases deposition of CaCO3 and its burial in the sediments. 10,000 years following the application of the freshwater forcing the effects on the deposition and dissolution change on the burial flux change are 26 % and 74 %. The findings of the authors1 indicate that the first 1,000 years are the most significant, a period during which the burial flux depends nearly completely on the reduced production of CaCO3, The implication of this is that 2 events, the Younger Dryas and Heinrich Events, that were triggered by the discharge of fresh water, may be responsible for decreasing the sedimentation of CaCO3 through lowered production of CaCO3, instead of a lowered concentration of carbonate in bottom water, which acts on a time-scale that is actually longer than the fresh water events.

Atmospheric pCO2 is depleted by 11 ppm in an open system by the depleted AMOC, a response similar to that in a closed system, both systems exhibiting a weakened biological pump. Carbonate compensation by 1 ppm reduces atmospheric pCO2, highlighting the feedback strength of carbonate preservation in the deep seasediments, in regulating atmospheric CO2 and climate.

Sources & Further reading

  1. Chikamoto, Megumi O., Katsumi Matsumoto, and Andy Ridgwell. "Response of Deep-Sea Caco3 Sedimentation to Atlantic Meridional Overturning Circulation Shutdown." Journal of Geophysical Research: Biogeosciences 113, no. G3 (2008): n/a-n/a.
Author: M. H. Monroe
Last Updated 10/02/2013

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