HR: 09:45h
AN: H41J-08 INVITED    [Abstracts]
TI: Calcium Isotopes in Marine Sediments and Soils: Paleoceanography, Diagenesis, and Soil Processes
AU: * DePaolo, D J
EM: depaolo@eps.berkeley.edu
AF: Dept. of Earth & Planetary Science, University of California, MS4767, Berkeley, CA 94720- 4767, United States
AU: * DePaolo, D J
EM: depaolo@eps.berkeley.edu
AF: Earth Sciences Division, E.O. Lawrence Berkeley National Laboratory, Berkeley, CA 94720, United States
AU: Fantle, M S
EM: 10073692
AF: Deaprtment of Geosciences, Pennsylvania State University, University Park, PA 16802, United States
AU: Turchyn, A
EM: avturchyn@berkeley.edu
AF: Dept. of Earth & Planetary Science, University of California, MS4767, Berkeley, CA 94720- 4767, United States
AU: Ewing, S
EM: saewing@nature.berkeley.edu
AF: Dept. of Earth & Planetary Science, University of California, MS4767, Berkeley, CA 94720- 4767, United States
AU: Yang, W
EM: wenbo@berkeley.edu
AF: Dept. of Earth & Planetary Science, University of California, MS4767, Berkeley, CA 94720- 4767, United States
AB: Calcium has 6 stable isotopes covering a wide mass range from 40 to 48. Because it is a critical component of the carbon cycle and a major constituent of common minerals like calcite, apatite, and gypsum, there is growing interest in understanding its stable isotope fractionation patterns in nature. The first work on Ca isotopes emphasized fractionation in food chains and in the formation of biogenic mineral matter in vertebrates. More recent studies have emphasized the fractionations observed in marine carbonate deposits, plants, and hydrological systems. Laboratory experiments show that Ca isotopes are fractionated during precipitation of calcite and aragonite from aqueous solution. Comparison with natural carbonate indicates that the fractionations are probably kinetic in origin although the exact mechanisms are not understood. There is little difference between the fractionation observed in inorganic and organic systems; precipitation rate (or solution oversaturation) seems to be the controlling factor, rather than temperature. One promising aspect of Ca isotopes is in their application to marine carbonate diagenesis. Especially when combined with studies of Sr, C, S, U, and O isotopes in deep sea sediments, Ca isotopes can give unique information on calcite dissolution and recrystallization rates. In a pure carbonate section from DSDP site 807A, Ca isotopes in pore fluids are found to differ greatly from seawater values and require that the equilibrium fractionation factor for 44Ca/40Ca between calcite and dissolved carbonate is almost exactly 1.0000. This fractionation corresponds to very slow calcite deposition rates at near-equilibrium conditions, implying that this fractionation factor may be generally applicable to diagenetic calcite precipitation, distinguishing it from biogenic precipitation, for which the factor is about 0.9985. This property can also be used to establish the dissolution rates of young carbonate sediments in the uppermost few meters of the sediment column. A deep-sea section where calcite is present in subequal proportions with clay, and in which there is also organic matter and rapid sulfate reduction, has a different pattern. Pore water 44Ca/40Ca values are closer to those of seawater, suggesting that there is virtually no calcite dissolution below a few meters depth. Analysis of the available data on fractionation during calcite precipitation suggests that the fractionation is controlled by attachment kinetics at the mineral surface. The observations also require that the presence of clay and/or organic material change the calcite dissolution rates by a large factor. Studies of Ca isotopes in hyper-arid soils and in alkaline lakes also show evidence of inorganic Ca isotope fractionation, probably controlled by relatively rapid, non-equilibrium precipitation of calcite, gypsum, or anhydrite during dessication events.
DE: 0454 Isotopic composition and chemistry (1041, 4870)
DE: 0473 Paleoclimatology and paleoceanography (3344, 4900)
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 3022 Marine sediments: processes and transport
DE: 4825 Geochemistry
SC: Hydrology [H]
MN: 2007 Fall Meeting