Testing the Silicic Acid Leakage Hypothesis as a cause of CO2 drawdown during Marine Isotope Stage 4

The ‘Silicic Acid Leakage Hypothesis’ (SALH) is a mechanism by which the increased supply of silicic acid to the low latitude ocean allows diatoms (silica producers) to outcompete coccolithophorids (carbonate producers). This would result in a decrease in the export of carbonate, and drawdown of atmospheric CO2 through changes in surface- and whole-ocean alkalinity. Here I test the SALH as a potential cause of CO2 drawdown during glacial Marine Isotope Stage (MIS) 4 (~70-59 ka). Firstly, I measure opal (biogenic silica) accumulation rates in a suite of cores from the equatorial Atlantic, to determine whether the export productivity of diatoms increased during MIS 4. I found that opal accumulation rates increased ~100% in MIS 4 relative to interglacial MIS 5a (~84-77 ka), in agreement with the SALH; however the timing of the changes make the SALH unlikely to be the direct cause of the CO2 drawdown. I then measured the calcium carbonate accumulation rates in the same suite of cores and found that carbonate accumulation decreased in MIS 4 relative to MIS 5a, also in agreement with the SALH. However, I found that this decrease may have been the result of enhanced carbonate dissolution. I also tested the SALH directly by reconstructing changes in the silicic acid concentration of AAIW, by using neodymium and silicon isotopic ratio measurements. I found that AAIW conveyed an increased amount of silicic acid into the western tropical Atlantic during MIS 4, and that the timing of this increase was coeval with increases in both low latitude opal accumulation, and also opal accumulation in the northwest Atlantic. Lastly, I tested the amount of atmospheric CO2 drawdown which could have been attributed to the SALH mechanism during MIS 4 by using a box model, and I estimated that ~35-50 ppmv of CO2 drawdown is achievable.

How to cite
Griffiths James D (2012). Testing the Silicic Acid Leakage Hypothesis as a cause of CO2 drawdown during Marine Isotope Stage 4. PhD Thesis, University of Cardiff. https://archimer.ifremer.fr/doc/00495/60642/

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