Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2000
Benthic foraminiferal δ13C, Cd/Ca, and Ba/Ca are important tools for reconstructing
nutrient distributions, and thus ocean circulation, on glacial-interglacial timescales.
However, each tracer has its own "artifacts" that can complicate paleoceanographic
interpretations. It is therefore advantageous to measure multiple nutrient proxies with the
aim of separating the various complicating effects. Zn/Ca is introduced as an important
aid toward this goal.
Benthic (Hoeglundina elegans) Cd/Ca ratios from the Bahama Banks indicate that the
North Atlantic subtropical gyre was greatly depleted in nutrients during the last glacial
maximum (LGM). A high-resolution Cd/Ca record from 965 m water depth suggests that
Glacial North Atlantic Intermediate Water formation was strong during the LGM,
weakened during the deglaciation, and strengthened again during the Younger Dryas cold
period. Comparison of Cd/Ca and δ13C data reveals apparent short-term changes in
carbon isotopic air-sea signatures.
Benthic foraminiferal Zn/Ca could be a sensitive paleoceanographic tracer because deep
water masses have characteristic Zn concentrations that increase about ten-fold from the
deep North Atlantic to the deep North Pacific. A "core top calibration" shows that Zn/Ca
is controlled by bottom water dissolved Zn concentration and, like Cd/Ca and BalCa, by
bottom water saturation state with respect to calcite Since Zn/Ca responds to a different
range of saturation states than Cd/Ca, the two may be used together to evaluate changes
in deep water carbonate ion (CO32-) concentration.
Zn/Ca and Cd/Ca ratios in the benthic foraminifer Cibicidoides wuellerstorfi exhibit large
fluctuations over the past 100,000 years in a deep (3851 m) eastern equatorial Pacific
sediment core. The data imply that bottom water CO32- concentrations were lowest during
glacial Marine Isotope Stage 4 and highest during the last deglaciation. LGM CO32- concentrations
appear to have been within a few μmol kg-1 of modern values.
Deep North Atlantic Cd/Ca ratios imply much higher nutrient concentrations during the
LGM. Although such data have usually been explained by a northward penetration of
Southern Ocean Water (SOW), it has been suggested that they could result from
increased preformed nutrient levels in the high-latitude North Atlantic or by increased
aging of lower North Atlantic Deep Water (NADW). Glacial Zn/Ca data, however,
require a substantially increased mixing with SOW and thus a reduction in NADW
formation. Large changes in carbon isotopic air-sea exchange are invoked to reconcile
benthic δ13C and trace metal data.
This work was supported by a JOIlUSSAC Ocean Drilling Fellowship (subgrant
JSG-CY 12-4), the R. H. Cole Ocean Ventures Fund, the Joint Program Education Office,
and the National Science Foundation (grants OCE-9402804 and OCE-9503135 to W.
Curry, and grant OCE-9633499 to D. Oppo).
Knorr (Ship : 1970-) Cruise KN159-5
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