Publication Date:
2022-05-25
Description:
Author Posting. © American Geophysical Union, 2006. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 7 (2006): Q11004, doi:10.1029/2005GC001216.
Description:
The Sr/Ca ratio of biogenic carbonate is widely used as a proxy for paleotemperature. This application is supported by empirical calibrations of Sr/Ca as a function of temperature, but it is also known that Sr uptake in calcite gauged by KdSr=(Sr/Ca)calcite/(Sr/Ca)solution is affected by other variables, including bulk precipitation rate (Kd Sr increases with increasing precipitation rate). There are no data from controlled experiments specifically addressing the effect of radial growth rate of individual crystals on Kd Sr. For this reason, we conducted two series of experiments to explore Sr partitioning at varying growth rates: (1) growth from a CaCl2–NH4Cl–SrCl2 solution by diffusion of CO2 from an ammonium carbonate source (“drift” experiments) and (2) “drip” precipitation of calcite on a substrate, using a steady flow of CaCl2–SrCl2 and Na2CO3 solutions, mixed just before passage through a tube and dripped onto a glass slide precoated with calcite (“cave-type” experiments). The growth rates of individual crystals were determined by periodic monitoring of crystal size through time or, roughly, by comparison of the final size with the duration of the experiment. Electron microprobe analyses across sectioned crystals grown in the drift experiments show that the concentration of Sr is high in the center (where radial growth rates are highest) and decreases systematically toward the edge. The center-to-edge drop in Sr concentration is a consequence of the slowing radial growth rate as individual crystals become larger. In general, high crystal growth rate (V) enhances Sr uptake in calcite due to a type of kinetic disequilibrium we refer to as “growth entrapment.” The apparent Kd Sr ranges from 0.12 to 0.35 as V increases from 0.01 nm/s to 1 μm/s at 25°C.
Description:
This work was supported by the National Science Foundation through grants EAR-9804794 and EAR-0337481 to E. B. Watson.
Keywords:
Calcite
;
Partitioning
;
Fluid
;
Growth rate
;
Strontium
;
Temperature
Repository Name:
Woods Hole Open Access Server
Type:
Article
Format:
1815612 bytes
Format:
application/pdf
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