OCEANOGRAPHY: Mechanical Problems for Isotope Tracers

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Science  10 Aug 2001:
Vol. 293, Issue 5532, pp. 1017a
DOI: 10.1126/science.293.5532.1017a

Elements with intermediate oceanic residence times—shorter than the time it takes for the oceans to mix but long enough for regional isotopic compositional differences to be affected by the movement of large water masses—are useful tracers of oceanic circulation in the past. The paleoceanographic distributions of lead and neodymium can be reconstructed from the crusts of deep-sea ferromanganese nodules.

von Blanckenburg and Nägler have measured the Pb and Nd isotopic compositions of sea water for the past 7 million years in a suite of such crusts and analyzed their data with a box model. They conclude that the changes in the isotopic composition of North Atlantic Deep Water during the Neogene were caused primarily by changes in erosion related to continental glaciation, rather than by changes in the source of the oceanic inputs or in thermohaline circulation. They support this conclusion with results from leaching experiments designed to simulate the effects of mechanical erosion on continental sources of Pb and Nd, and they suggest that the transition from chemical weathering to mechanical weathering accompanying the growth of large ice sheets could cause important isotopic changes in oceanic tracer compositions. — HJS

Paleoceanography 16, 424 (2001).

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