Ca isotopes in carbonate sediment and pore fluid from ODP Site 807A; the Ca2+(aq)-calcite equilibrium fractionation factor and calcite recrystallization rates in Pleistocene sediments

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doi: 10.1016/j.gca.2007.03.006
Author(s): Fantle, Matthew S.; DePaolo, Donald J.
Author Affiliation(s): Primary:
University of California, Berkeley, Department of Earth and Planetary Sciences, Berkeley, CA, United States
Lawrence Berkeley National Laboratory, United States
Volume Title: Geochimica et Cosmochimica Acta
Source: Geochimica et Cosmochimica Acta, 71(10), p.2524-2546. Publisher: Elsevier, New York, NY, International. ISSN: 0016-7037 CODEN: GCACAK
Note: In English. 59 refs.; illus., incl. 2 tables, sketch map
Summary: The calcium isotopic compositions (δ44Ca) of 30 high-purity nannofossil ooze and chalk and 7 pore fluid samples from ODP Site 807A (Ontong Java Plateau) are used in conjunction with numerical models to determine the equilibrium calcium isotope fractionation factor (αs-f) between calcite and dissolved Ca2+ and the rates of post-depositional recrystallization in deep sea carbonate ooze. The value of αs-f at equilibrium in the marine sedimentary section is 1.0000±0.0001, which is significantly different from the value (0.9987±0.0002) found in laboratory experiments of calcite precipitation and in the formation of biogenic calcite in the surface ocean. We hypothesize that this fractionation factor is relevant to calcite precipitation in any system at equilibrium and that this equilibrium fractionation factor has implications for the mechanisms responsible for Ca isotope fractionation during calcite precipitation. We describe a steady state model that offers a unified framework for explaining Ca isotope fractionation across the observed precipitation rate range of ∼14 orders of magnitude. The model attributes Ca isotope fractionation to the relative balance between the attachment and detachment fluxes at the calcite crystal surface. This model represents our hypothesis for the mechanism responsible for isotope fractionation during calcite precipitation. The Ca isotope data provide evidence that the bulk rate of calcite recrystallization in freshly-deposited carbonate ooze is 30-40%/Myr, and decreases with age to about 2%/Myr in 2-3 million year old sediment. The recrystallization rates determined from Ca isotopes for Pleistocene sediments are higher than those previously inferred from pore fluid Sr concentration and are consistent with rates derived for Late Pleistocene siliciclastic sediments using uranium isotopes. Combining our results for the equilibrium fractionation factor and recrystallization rates, we evaluate the effect of diagenesis on the Ca isotopic composition of marine carbonates at Site 807A. Since calcite precipitation rates in the sedimentary column are many orders of magnitude slower than laboratory experiments and the pore fluids are only slightly oversaturated with respect to calcite, the isotopic composition of diagenetic calcite is likely to reflect equilibrium precipitation. Accordingly, diagenesis produces a maximum shift in δ44Ca of +0.1ppm for Site 807A sediments but will have a larger impact where sedimentation rates are low, seawater circulates through the sediment pile, or there are prolonged depositional hiatuses. Abstract Copyright (2007) Elsevier, B.V.
Year of Publication: 2007
Research Program: ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 06 Petrology, Sedimentary; Alkaline earth metals; Ca-44; Calcite; Calcium; Carbonate sediments; Carbonates; Cenozoic; Chemical composition; Chemical reactions; Clastic sediments; Diagenesis; Fluid phase; Geochemistry; Isotope fractionation; Isotopes; Leg 130; Mathematical methods; Metals; Numerical models; ODP Site 807; Ocean Drilling Program; Ontong Java Plateau; Ooze; Pacific Ocean; Phase equilibria; Pleistocene; Pore water; Quaternary; Recrystallization; Sediments; Siliciclastics; Stable isotopes; West Pacific
Coordinates: N033622 N033626 E1563730 E1563728
Record ID: 2008009451
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands