HR: 16:30h
AN: PP34A-03    [Abstracts]
TI: Phanerozoic Sedimentary C$_{org}$:P Ratios and Paleocean Ventilation
AU: * Algeo, T J
EM: Thomas.Algeo@uc.edu
AF: University of Cincinnati, Department of Geology 500 Geology/Physics Building, Cincinnati, OH 45221-0013 United States
AB: Phosphorus is probably the major limiting nutrient on marine primary productivity at geological timescales, although other elements (e.g., N, Fe) can be transiently biolimiting. It has been inferred that remineralization of organic P and upwelling of nutrient-rich deepwaters have the potential to stimulate primary productivity, and that measurements of sedimentary (C:P)$_{org}$ ratios in ancient sediments can yield insights on nutrient fluxes and productivity levels in paleoceans. However, recent work has shown that (1) sedimentary (C:P)$_{org}$ ratios exceed the Redfield ratio (106:1) in most environments as a result of preferential bacterial destruction of labile P-bearing compounds, and (2) the inorganic P fractions associated with Fe-bound and authigenic phosphate phases in anoxic sediments are largely of organic derivation, representing remineralized P that has been fixed through redox-related processes. These insights suggest that the most useful measure of nutrient regeneration is the nondetrital P fraction of sediments, a variable that is rarely determined but that can be adequately proxied by total P in anoxic facies, in which the detrital P fraction is typically small. In this study, C$_{org}$:P ratios were determined for 60 anoxic facies of Cambrian through Recent age. The Recent facies yield a mean C$_{org}$:P of 65ñ25, values similar to those for most anoxic facies of Mississippian and younger age. In contrast, many Cambrian-Devonian anoxic facies yield substantially higher C$_{org}$:P ratios, with the highest values (>400) in the Middle-Upper Cambrian and Middle-Upper Devonian. For the Phanerozoic as a whole, the logarithmic mean C$_{org}$:P ratio declines by a factor of four, from ~260:1 in the Cambrian to 65:1 in the Recent, a statistically robust result. Given the importance of redox controls on sedimentary P retention at a local scale, this pattern can be interpreted as a record of benthic redox conditions through time, i.e., paleocean ventilation. Although Recent anoxic facies are oxygen-depleted at present (i.e., an instantaneous condition), their relatively low C$_{org}$:P ratios suggest that, on a time-averaged basis at timescales associated with sedimentary P retention (i.e., 10$^{3}$-10$^{4}$ y), these environments are not nearly as anoxic as their Early-Middle Paleozoic counterparts. Anoxic facies of all ages experience episodic "freshening" events, in which small quantities of oxygen from surface waters are transferred below the chemocline by storm mixing, turbidite flows, or overspill into silled basins. Such freshening events have imparted an "oxic" C$_{org}$:P signature to Recent anoxic facies but not to Cambrian-Devonian anoxic facies. The key difference probably lies in atmospheric O$_{2}$ levels, which may have been sufficiently low during the Cambrian-Devonian that "freshening" events did little to mitigate oxygen-poor conditions in contemporaneous deepwaters. The abrupt mid-Paleozoic decline in sedimentary C$_{org}$:P ratios can be attributed to burial of large quantities of organic matter as marine black shales during the Devonian and as freshwater coals during the Carboniferous, effecting a permanent increase in atmospheric pO$_{2}$ and improved deep ocean ventilation.
DE: 4805 Biogeochemical cycles (1615)
DE: 4267 Paleoceanography
DE: 4802 Anoxic environments
DE: 1615 Biogeochemical processes (4805)
DE: 0325 Evolution of the atmosphere
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting