HR: 16:00h
AN: B14B-01 INVITED     [Abstracts]
TI: Thermodynamic and Kinetic Controls on Element Incorporation into Aragonite
AU: * Gaetani, G A
EM: ggaetani@whoi.edu
AF: Dept Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 United States
AU: Cohen, A L
EM: acohen@whoi.edu
AF: Dept Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 United States
AB: Much of what we know about the behavior of Earth's climate system is based on proxy records preserved in biogenic carbonates. This approach is inherently limited by uncertainty concerning the extent to which biological processes influence skeletal composition. A crucial first step to resolving this ambiguity is the establishment of a quantitative understanding of the thermodynamic and kinetic controls on the compositions of abiogenic carbonates. New experimentally determined aragonite-seawater partition coefficients, combined with the lattice strain equation of Blundy and Wood [1], indicate that the composition of biogenic aragonite are controlled primarily by the compatibility of cations in the near-surface region of a growing crystal rather than by crystal-fluid equilibrium. Experiments to determine the partitioning of the alkaline earth cations Mg$^{2+}$, Ca$^{2+}$, Sr$^{2+}$, and Ba$^{2+}$ between abiogenic aragonite and seawater were carried out over a temperature range of 15 $\deg$ to 45 $\deg$C following the protocol of Kinsman and Holland [2]. After each experiment, the seawater was analyzed for $^{25}$Mg, $^{48}$Ca, $^{86}$Sr and $^{138}$Ba using ICP-MS. Individual aragonite crystals were analyzed for $^{24}$Mg, $^{42}$Ca, $^{88}$Sr, and $^{138}$Ba by SIMS ion microprobe. Elemental ratios were converted to concentration using analyses of Ca carried out by electron microprobe. Partitioning varies systematically with cation radius and temperature. However, several lines of evidence suggest that surface equilibrium rather than crystal-fluid equilibrium is controlling element incorporation. First, the compatibilities of Sr$^{2+}$ and Ba$^{2+}$ in aragonite are equal to, or greater than, that of Ca$^{2+}$, which is the essential structural constituent and should be the most compatible cation. Second, the effective Young's moduli derived from our data (13-17 GPa) are much lower than values for aragonite (76-144 GPa). This indicates that the aragonite grains precipitated in our experiments are enriched in trace components (impurities) relative to the concentrations expected from crystal-fluid equilibrium. Our data are inconsistent with diffusive fractionations in a boundary layer adjacent to the growing crystal because compatible elements (i.e. Sr$^{2+}$; Ba$^{2+}$) are enriched rather than depleted in our experimental run products. A boundary layer effect would drive all partition coefficients toward unity, whereas the partition coefficients for Mg$^{2+}$, Sr$^{2+}$, and Ba$^{2+}$ determined in our experiments diverge from 1. Our data appear consistent with the surface enrichment model of Watson [3,4], in which the composition of the crystal reflects equilibrium element concentrations in the near-surface region of the crystal. References: [1] Blundy and Wood (1994) Nature 372:452-454. [2] Kinsman and Holland (1969) Geochim Cosmochim Acta 33:1-17. [3] Watson (1996) Geochim Cosmochim Acta 60:5013-5020. [4] Watson (2004) Geochim Cosmochim Acta 68:1473-1488.
DE: 4835 Inorganic marine chemistry
DE: 1045 Low-temperature geochemistry
DE: 0400 Biogeosciences
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting