Quantifying glacial carbonate compensation in the deep Pacific using trace metals in benthic foraminifera
T. Marchitto, J. Lynch-Stieglitz, S. Hemming
Talk presented at 2003 EGS-AGU-EUG Joint Assembly
Abstract. One mechanism called upon to explain a significant portion of the atmospheric CO2
lowering during the last glaciation is "carbonate compensation," whereby the oceanic
ratio of alkalinity to DIC is increased by the dissolution of seafloor carbonates. This
mechanism predicts transient deep ocean CO32- drops and dissolution events during
glacial inceptions, and CO32- spikes and preservation events during glacial terminations.
Such events have been recorded by various dissolution proxies, but quantification
of the associated CO32- excursions (and therefore their impact on the carbon
cycle) is not straightforward. Here we apply an independent approach that relies on
the observation that Zn and Cd incorporation into benthic foraminiferal calcite is a
function of bottom water saturation state. This behavior is hypothesized to result from
a relationship between saturation state and the degree to which the foraminifer’s internal
calcification reservoir is isolated from seawater. In the deep (3400 m) eastern
equatorial Pacific, the highest C. wuellerstorfi Zn/Ca and Cd/Ca values (and thus the
highest seawater CO32- concentrations) during the past 150 kyr occurred during Terminations
I and II. More subtle negative excursions occurred during MIS 5 and 4. We
also find Termination I Zn/Ca and Cd/Ca peaks in the western equatorial Pacific (4000
m). At each site, the deglacial CO32- spike is further reflected in the average weights
of planktonic foraminifera. If we assume that seawater Zn and Cd concentrations remained
relatively constant across the deglaciation, then the inferred CO32- excursion
can explain roughly one third of the glacial-interglacial atmospheric CO2 difference.
We independently constrain deep water circulation changes (which could influence
seawater Zn and Cd) using Nd isotopic measurements on Fe-Mn oxides.
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