Chemostratigraphy at DSDP Sites 386 (Bermuda Rise) and 144 (Demerara Rise), Implications for Euxinic Conditions During OAE-2

Details Chemostratigraphy at DSDP Sites 386 (Bermuda Rise) and 144 (Demerara Rise), Implications for Euxinic Conditions During OAE-2 Chemostratigraphy at DSDP Sites 386 (Bermuda Rise) and 144 (Demerara Rise), Implications for Euxinic Conditions During OAE-2

May 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agusmpp24a..06h&link_type=abstract

American Geophysical Union, Spring Meeting 2008, abstract #PP24A-06

Biology

0404 Anoxic And Hypoxic Environments (4802, 4834), 0448 Geomicrobiology, 0460 Marine Systems (4800), 0473 Paleoclimatology And Paleoceanography (3344, 4900)

Scientific paper

Chemostratigraphic studies of DSDP Site 386 on the Bermuda Rise and Site 144 on the Demerara Rise indicate that euxinic conditions developed at these deep-water sites during the time interval that corresponds to Oceanic Anoxic Event 2 (OAE 2). The data show a large increase in Fe/Al ratios, and dispersed pyrite aggregates (Site 386 Core 43, Section 3). Such findings at these deep oceanic sites are compatible with earlier studies showing that sediments in euxinic settings display increases in Fe/Al ratios due to the scavenging of dissolved Fe, and is also in agreement with previous Pr/Ph ratio of <1 (Simoneit, 1979). This study further shows that OM, previously believed to show bimodal distribution, which was used to argue in support of turbidity currents at Site 386 as transport mechanism for some of the OM, is predominantly derived from marine phytoplankton and cyanobacteria showing low thermal stress, supporting in situ derivation. Elemental analyses at Site 386 also show that relatively high Sr/CaO ratios are present before and after OAE 2, indicating an increased contribution of biogenic carbonates, but not during the C/T boundary event. When Cr is plotted against Al2O3 in conjunction with a solid line representing the Cr/Al2O3 ratio in average shale, half of the samples fall above and half fall below this line. The values that plot above this line are all from Cores 47, 44, 43, and 42, which contain higher TOC. Their strong Cr enrichment with respect to the average shale can be indicative of an algal source of the OM, as this biota preferentially concentrates Cr. Competitive exclusion due to dominance of opportunistic prokaryotic blooms in combination with oxygen depletion can be invoked to explain the conditions that developed and were unfavorable to most other organisms throughout the water column during OAE 2. Sediments from DSDP Site 144 also reveal increased molecular fossils indicative of green sulfur bacteria, which are further characteristic of euxinic conditions (Kuypers et al., 2002; Forster et al., 2004). These results are in agreement with earlier works that showed lipids at DSDP Site 144 are predominantly of an autochthonous origin with primary production as the dominant source (Simoneit and Stuermer, 1982). Biomarker data indicate that during OAE 2 similar paleoceanographic conditions prevailed at both DSDP Sites, 144 on the Demerara Rise and 386 on the Bermuda Rise. A. Forster, H. Sturt, P.A. Meyers, et al., in preliminary shipboard results from Sites 1257 and 1258, Leg 207, J. Erbacher, D.C. Mosher, M.J. Malone, et al., Proc. ODP, Init.Repts.207,1-22 [Online]: http://wwwodp.tamu.edu/publications/207_IR/VOLUME/ CHAPTERS/IR207_10.PDF. [Cited 2006-02-09] M.M.M. Kuypers, et al., Paleoceanography, 17, 4, 1051-1063 (2002) B.R.T. Simoneit, in Initial Reports of the Deep Sea Drilling Project, Vol. 43, Tucholke, B.E., Vogt, P.R., et al., 1979, Eds, Washington (U.S. Government Printing Office), pp. 643-650 (1979) B.R.T. Simoneit and D.H.Stuermer, in Cretaceous Oceans, S.O. Schlanger and M.B. Cita, Eds, Academic Press, New York, pp. 145-163 (1982)

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