HR: 09:00h
AN: PP21A-05 INVITED    [Abstracts]
TI: Sulfur and Oxygen isotope variability across Cretaceous Ocean Anoxic Events
AU: * Turchyn, A V
EM: avturchyn@berkeley.edu
AF: UC Berkeley, McCone Hall Department of Earth and Planetary Science, Berkeley, CA 94720-4767, United States
AU: Schrag, D P
EM: schrag@eps.harvard.edu
AF: Harvard University, 20 Oxford St, Cambridge, MA 02138, United States
AU: Coccioni, R
EM: cron@info-net.it
AF: University of Urbino, Istituto di Geologia e Centro di Geobiologia, Universita degli Studi, Carlo Bo, Urbino, 61029, Italy
AU: Montanari, A
EM: sandro.ogc@fastnet.it
AF: Osservatorio Geologico di Coldigioco, Osservatorio Geologico di Coldigioco, Apiro, 61004, Italy
AB: The marine carbon and sulfur cycles are linked through bacterial sulfate reduction and organic matter oxidation in organic-rich sediments. Therefore, reconstructing temporal variability in the sulfur cycle remains an important goal for understanding changes in the carbon cycle, paleoalkalinity and paleoclimate. Traditionally the sulfur isotopic composition (δ³⁴S) of sulfur minerals has been used to explore changes in the biogeochemical sulfur cycle through Earth history. The δ³⁴S varies largely as a function of pyrite burial, its isotopic composition, and river input. Recent work has advanced the use of the oxygen isotopic composition of sulfate (δ¹⁸O_SO4) as a separate mechanism to probe changes in the sulfur cycle over time. The δ¹⁸O_SO4 varies with changes in the pathways of sulfate reduction and sulfide reoxidation in organic rich sediments. Thus the measurement of both the sulfur and oxygen isotope composition of sulfate minerals affords the possibility to explore variations in the biogeochemical sulfur cycle in precisely the location – organic rich sediments – where it is coupled to the carbon cycle. We will present data comparing the δ³⁴S and the δ¹⁸O_SO4) in marine barite from the middle Cretaceous. We will use this coupled isotope data to explore causes of temporal variability in the Cretaceous sulfur cycle. Unlike the δ³⁴S of marine barite across this time interval, the δ¹⁸O_SO4) shows rapid and large isotopic excursions associated with Ocean Anoxic Events (OAE). The δ¹⁸O_SO4) remains constant across OAE1b, increases after OAE 1c, decreases after OAE 1d, and then increases both during the Mid-Cenomanian event and OAE 2. These results suggest that there were distinct differences in the carbon cycle during different OAEs. We suggest that these differences may be due to euxinic (i.e. presence of H2S in the water column) versus anoxic oceans, which would impact the pathways of sulfide oxidation and therefore impact the δ¹⁸O_SO4). The δ³⁴S, on the other hand, is likely responding to a longer term changes in the amount or isotopic composition of pyrite being buried and does not respond to the rapid variability in the carbon cycle over this time.
DE: 0454 Isotopic composition and chemistry (1041, 4870)
DE: 0488 Sulfur cycling
DE: 4851 Oxidation/reduction reactions (0471)
DE: 4912 Biogeochemical cycles, processes, and modeling (0412, 0414, 0793, 1615, 4805)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting