HR: 11:35h
AN: B21G-06 INVITED     [PDF]
TI: Observations and Experiments on Carbonate Secretion in "Calcite Seas": Why Massive Chalk Deposits Formed in Late Cretaceous Time
AU: * Stanley, S M
EM: stanley@jhu.edu
AF: Morton K. Blaustein Department of Earth and Planetary Sciences, Johns Hopkins University, Charles & 34th Streets, Baltimore, MD 21218 United States
AU: Ries, J B
EM: jries@jhu.edu
AF: Morton K. Blaustein Department of Earth and Planetary Sciences, Johns Hopkins University, Charles & 34th Streets, Baltimore, MD 21218 United States
AU: Hardie, L A
EM: hardie@ekman.eps.jhu.edu
AF: Morton K. Blaustein Department of Earth and Planetary Sciences, Johns Hopkins University, Charles & 34th Streets, Baltimore, MD 21218 United States
AB: It is now well-established that oscillations in the Mg/Ca ratio of seawater, driven by changes in spreading rates along mid-ocean ridges, have determined the mineralogy of nonskeletal marine carbonate precipitation throughout earth history. Low-Mg calcite has formed at Mg/Ca ratios $<$ 2 ("calcite seas"), and aragonite has formed at ratios $>$ 2 ("aragonite seas"). High-Mg calcite (mole % Mg $>$ 4) has formed by itself at Mg/Ca ratios of 1-2, and along with aragonite at ratios above 2. We report here on experiments rooted in the paleontological observation that the carbonate mineralogy of major reef-building and sediment-producing organisms has tended to correspond to that of nonskeletal precipitates throughout Phanerozoic time. Earlier experiments showed that, with changes in the Mg/Ca ratio of ambient seawater, the Mg content of calcite in coralline algae varies like that of nonskeletal calcite precipitates. New experiments reveal that many kinds of calcite-secreting marine animals exhibit similar mineralogical trends, but with varying partition coefficients. Other experiments address the effects of seawater chemistry on productivity. One can predict that secretion of calcium carbonate will facilitate growth of algae because it releases carbon dioxide that can be used in photosynthesis. Indeed, we found that Halimeda, a major producer of aragonite sediment in present-day aragonite seas, grows less rapidly when living at ambient Mg/Ca ratios below the modern marine level of 5.2. Conversely, calcareous nannoplankton, which secrete calcite, become much more productive when ambient Mg/Ca ratios are below unity and associated concentrations of Ca are high. Such conditons characterized the extreme calcite seas of Late Cretaceous time, when nannoplankton formed massive chalk deposits throughout the world. Thus, we attribute the formation of these deposits, which gave the Cretaceous Period its name, to seawater chemistry. Calcareous nannoplankton in modern seas are adapted to low nutrient levels and are not limited by iron. We conclude that the productivity of present-day calcareous nannoplankton is limited by the high Mg/Ca ratio and/or low absolute concentration of Ca in the modern ocean. Experiments now in progress will examine the relative importance of these two factors.
DE: 0330 Geochemical cycles
DE: 1030 Geochemical cycles (0330)
DE: 1050 Marine geochemistry (4835, 4850)
DE: 1615 Biogeochemical processes (4805)
DE: 3035 Midocean ridge processes
SC: Biogeosciences [B]
MN: 2003 Fall Meeting