HR: 09:45h
AN: B21A-08    [PDF]
TI: Toward Understanding Sr/Ca and Mg/Ca Signatures in Biogenic Calcite: Nanoscale Observation of Two Distinct Types of Interaction Between Impurity Ions and Growing Calcite
AU: Dove, P M
EM: dove@vt.edu
AF: Dept. of Geosciences, Virginia Tech, Blacksburg, VA 24061 United States
AU: * Wasylenki, L E
EM: lew@vt.edu
AF: Dept. of Geosciences, Virginia Tech, Blacksburg, VA 24061 United States
AU: Wilson, D S
EM: dawilso5@vt.edu
AF: Dept. of Geosciences, Virginia Tech, Blacksburg, VA 24061 United States
AU: De Yoreo, J J
EM: deyoreo1@llnl.gov
AF: Dept. of Chem. and Mat. Sci., LLNL, Livermore, CA 94551 United States
AB: Sr/Ca and Mg/Ca are widely used as paleotemperature indicators in biogenic carbonates. Although Sr and Mg have similar chemical behavior in some systems, we show here that they have very different mechanisms of interaction with growing calcite crystals. Minor element signatures in biominerals are complex products of inorganic and biological constraints; understanding how each impurity is influenced by the conditions of growth is key to robust interpretation of paleoenvironments. Using fluid cell atomic force microscopy, we measured step velocities for layer growth of abiotic calcite in supersaturated solutions containing NaCl, CaCl$_{2}$, NaHCO$_{3}$, and varying amounts of Mg or Sr. For Mg, step velocities monotonically decrease with increasing [Mg] in solution. Sr displays strikingly different behavior by {\it increasing} growth rates at low concentrations. To our knowledge, this is the first documented case of an impurity enhancing growth rates compared to the pure system. At higher concentrations, Sr pins kink-sites, blocking growth completely. With increasing concentrations of either impurity, acute (negative) step edges become rounded first, followed by rounding of obtuse (positive) step edges at higher concentrations. We conducted long-term growth experiments with identical conditions in order to measure impurity contents with the electron microprobe. Our results show that both Mg and Sr have a distinct preference for incorporation on obtuse (positive) flanks of growth hillocks. This contrasts with the work of Paquette and Reeder (1995; GCA 59:735), who found Mg partitioning strongly into acute (negative) flanks, while Sr partitioned into positive domains of calcite grown from solutions with high ionic strength and containing multiple impurities at once. This discrepancy suggests that the partitioning of Mg may not be independent of the presence or absence of other impurities. This study demonstrates the critical importance of understanding molecular-scale interactions between trace impurities and vicinal calcite faces. Our ability to unambiguously interpret trace element signatures depends on thorough quantification of the major controls on their incorporation in biominerals.
DE: 1045 Low-temperature geochemistry
DE: 1615 Biogeochemical processes (4805)
DE: 3620 Crystal chemistry
DE: 4267 Paleoceanography
DE: 4875 Trace elements
SC: Biogeosciences [B]
MN: 2003 Fall Meeting