Records of continental weathering for the past 15 Ma from paired analysis of lithium and neodymium isotopes in planktonic foraminifera

Anand, Pallavi; James, Rachael and Mokadem, Fatima (2010). Records of continental weathering for the past 15 Ma from paired analysis of lithium and neodymium isotopes in planktonic foraminifera. In: 10th International Conference on Paleoceanography, 29 Aug - 3 Sep 2010, San Diego, USA.


Lithium (Li) shows great potential as a tracer of continental weathering because its two isotopes have a large relative mass difference, it is unaffected by biological activity and it is only slightly incompatible during magmatic processes so tends to be relatively uniformly distributed in the Earth’s crust. The continental weathering signature is transferred via rivers to the oceans, where it mixes with Li derived from high-temperature hydrothermal vents. Changes in the Li and Li isotope signature of seawater are recorded in the tests of ancient foraminifera preserved in marine sediments.

Li shows conservative behaviour in seawater, with a residence time of ~1 million years (Ma). On the other hand, neodymium (Nd) in the ocean is predominantly derived from continental weathering [1]; there is no hydrothermal input. Neodymium has a shorter residence time (600-2000 years) and different water masses have a distinct Nd isotopic composition due to local input from continental sources. Recent studies of Nd isotopes in planktonic foraminifera have shown that they record the Nd isotope signature of bottom water [2]; however, our analyses of reductively cleaned Holocene planktonic foraminifera from ODP Site 925 (Equatorial Atlantic) suggests that they record the Nd isotope composition of surface water which is consistent with earlier work [e.g., 3].

Here, we present records of Li and Nd isotopes derived from planktonic foraminifer from ODP site 925 for the past 15 Ma, at a resolution of ~0.5 Ma. Both Li and Nd isotopic records show similar patterns over this time period. The pattern of the 7Li record agrees well with published data. However, the overall shift in 7Li that we record here is larger (~10-12‰) than previously observed (~5-6‰). The record shows a sharp increase in 7Li (~10‰) between 13.7 and 8.7 Ma, and a second step increase of ~1-2‰ between 4.3 and 3.9 Ma. The paired εNd record from the same samples records the surface water signature of the Atlantic Ocean, and generally increases from 15 Ma to the present-day. The overall increase in 7Li (~10-12‰) and εNd (5ε units; from -17 to -11.5) suggests changes in weathering intensity and oceanic boundary conditions, respectively. We will also present records of past variations in foraminiferal trace element/Ca ratios, measured on the same samples.

[1] S. J. Goldstein & S. B. Jacobsen, Chem. Geol. 66 245-272 (1987), [2] N. L. Roberts, A. M. Piotrowski, J. F. McManus & L. D. Keigwin, 327 DOI: 10.1126/science.1178068 (2009), [3] K. W. Burton & D. Vance, EPSL 176 425-441 (2000).

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