Responses of the Deep Ocean Carbonate System to Carbon Reorganization During the Last Glacial-Interglacial Cycle
We present new deep water carbonate ion concentration ([CO3 2-]) records, reconstructed using Cibicidoides wuellerstorfi B/Ca, for one core from Caribbean Basin (water depth = 3623 m, sill depth = 1.8 km) and three cores located at 2.3-4.3 km water depth from the equatorial Pacific Ocean during the last glacial-interglacial cycle. The pattern of deep water [CO3 2-] in the Caribbean Basin roughly mirrors that of atmospheric CO2, reflecting a dominant influence from preformed [CO3 2-] in the North Atlantic Ocean. Compared to the amplitude of ~65 mol/kg in the deep Caribbean Basin, deep water [CO3 2-] in the equatorial Pacific Ocean has varied by no more than ~15 mol/kg due to effective buffering of CaCO3 on deep-sea pH in the Pacific Ocean. Our results suggest little change in the global mean deep ocean [CO3 2-] between the Last Glacial Maximum (LGM) and the Late Holocene. The three records from the Pacific Ocean show long-term increases in [CO3 2-] by ~7 mol/kg from Marine Isotope Stage (MIS) 5c to mid MIS 3, consistent with the response of the deep ocean carbonate system to a decline in neritic carbonate production associated with ~60 m drop in sea-level (the "coral-reef" hypothesis). Superimposed upon the long-term trend, deep water [CO3 2-] in the Pacific Ocean displays transient changes, which decouple with 13C in the same cores, at the start and end of MIS 4. These changes in [CO3 2-] and 13C are consistent with what would be expected from vertical nutrient fractionation and carbonate compensation. The observed ~4 mol/kg [CO3 2-] decline in the two Pacific cores at >3.4 km water depth from MIS 3 to the LGM indicate further strengthening of deep ocean stratification, which contributed to the final step of atmospheric CO2 drawdown during the last glaciation. The striking similarity between deep water [CO3 2-] and 230Th-normalized CaCO3 flux at two adjacent sites from the central equatorial Pacific Ocean provides convincing evidence that deep-sea carbonate dissolution dominantly controlled CaCO3 preservation at these sites in the past. Our results offer new a 70 nd quantitative constraints from deep ocean carbonate chemistry to understand roles of various mechanisms in atmospheric CO2 changes over the last glacial-interglacial cycle.
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- Quaternary Science Reviews