HR: 08:30h
AN: PP21A-03    [Abstracts]
TI: Mo Isotopes Record Destabilization of a Stratified Ocean at the Precambrian-Cambrian Boundary
AU: Wille, M
EM: wille@geo.unibe.ch
AF: Institute of Geological Sciences, University of Bern Baltzerstrasse 3, Bern, CH 3012, Switzerland
AU: * Nägler, T F
EM: naegler@geo.unibe.ch
AF: Institute of Geological Sciences, University of Bern Baltzerstrasse 3, Bern, CH 3012, Switzerland
AU: Schröder, S
EM: stefan.schroeder@total.com
AF: Department of Geology, University of Johannesburg Auckland Park, Johannesburg, 2006, South Africa
AU: Lehmann, B
EM: lehmann@min.tu-clausthal.de
AF: Insitute of Mineralogy, Technical University of Clausthal Adolph-Roemer-Str. 2A, Clausthal-Zellerfeld, 38678, Germany
AU: Kramers, J D
EM: kramers@geo.unibe.ch
AF: Institute of Geological Sciences, University of Bern Baltzerstrasse 3, Bern, CH 3012, Switzerland
AB: Here we present Mo isotope signatures in black shales from two sample sets (Ara group, Oman and Yangtze Platform, China) which were deposited at and shortly after the Precambrian-Cambrian boundary (PC-C). At the first view, the overall Mo isotopic signatures (delta98/95Mo) of the Early Cambrian black shales is 1.2 permil below recent ocean water, similar to the signature found in Mesoproterozoic shales (Arnold et al. 2004), indicating a larger proportion of Mo sedimentation under strongly euxinic conditions compared to recent oceans. A chemically stratified ocean with sulfidic deep waters and modestly oxygenated surface waters as proposed by Canfield (1998) for the Paleoproterozoic and Mesoproterozoic ocean, and Jiang et al. (2007) reported Carbon isotope data from the Ediacaran Yangtze platform (635-542 Ma) to be consistent with long-term deep ocean anoxia/euxinia. A stratified ocean therefore provides a plausible scenario to explain our new PC-C Mo isotope data. On closer inspection, a transient Mo isotopic signal following immediately after the PC-C boundary in both sample sets indicates a short but intense global non-steady state situation. In particular, a short term, drastic decrease of the Mo ocean inventory to almost zero is required to reconcile the observed Mo isotope data. Combined with the extreme Mo enrichment, found in the Chinese sulfide marker bed at the PC-C boundary, this signal has to be explained with a non-uniformitarian Mo scavenging mechanism. We put forward the hypothesis of mixing of oxidized, i.e. Mo rich surface waters with upwelling euxinic bottom water masses of the stratified ocean, as H2S is the most efficient Mo scavenging reagent. This scenario not only explains the transient isotopic signal, it can also be responsible for the sudden extinction of the Ediacaran fauna by H2S poisoning. In contrast, mass extinction scenarios like bolide impact, flood basalt eruptions or methane release, do not provide a direct explanation for the observed Mo isotope data.
DE: 1000 GEOCHEMISTRY
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 4800 OCEANOGRAPHY: BIOLOGICAL AND CHEMICAL (0460)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting