HR: 08:45h
AN: PP21A-04    [Abstracts]
TI: Tracing redox processes during paleoclimatic changes in the Neoproterozoic: Stable chromium isotopic results from the Arroyo del Soldado Group (Ediacaran, Uruguay)
AU: * Frei, R
EM: robertf@geol.ku.dk
AF: Institute of Geopgraphy and Geology, University of Copenhagen, Oster Voldgade 10, Copenhagen, 1350, Denmark
AU: * Frei, R
EM: robertf@geol.ku.dk
AF: NordCEE - Nordic Center for Earth Evolution, Oster Voldgade 10, Copenhagen, 1350, Denmark
AU: Gaucher, C
EM: gaucher@chasque.net
AF: Departamento de Geologia, Facultad de Ciencias, Igua 4225, Montevideo, 11400, Uruguay
AB: Positive δ¹³C carbonate values, combined with the occurrence of Fe-rich cherts (oxide-facies BIF) and organic-rich black shales within the late Ediacaran (ca. 580-560 Ma) Yerbal Fm. of the Arroyo del Soldato Group (Uruguay) are compatible with paleoclimatic models which postulate that enhanced bioproductivity due to higher availability of nutrient (P, N, Fe) was essential for controlling Neoproterozoic glaciations. Tracing of associated redox processes (f.e. linked to oxygenation of bottom waters in restricted basins) that might have been responsible for the deposition of Fe-rich cherts (BIFs) is therefore an important tool to better understand the seawater changes during cold-warm periods. Besides the traditionally used Fe and Mo isotopic systems, the redox-sensitive element Cr (Cr(III); Cr(IV)) and its stable isotopes offer another complementary system to trace paleo-redox processes. We have applied Cr stable isotope systematics to a sequence of samples from a late Ediacaran sedimentary sequence in Uruguay, using a ⁵²Cr-⁵⁴Cr double spike (Schoenberg et al., Chem..Geol., subm.). The middle Yerbal Fm. is dominated by organic-rich, black shales and black dolostones (δ⁵³Cr = -0.05‰), followed by organic-rich cherts (δ⁵³Cr = +1.83 - +4.49 ‰) and BIF (δ⁵³Cr = -0.31 +0.90 ‰) gradually changing into Fe-bearing, organic-rich cherts and shales (δ⁵³Cr = -0.28 - -0.01 ‰), and another sequence with BIF and organic-rich cherts topped by carbonates of the lower Polanco Fm. (δ⁵³Cr = -0.17 to -0.27 ‰). The strongly positively fractionated Cr isotopic signatures in organic-rich and Fe-rich cherts in the Yerbal Fm. may point to significant oxidation processes either directly in the seawater column and/or during early diagenetic processes at the sediment-water interface. While these strongly positive δ⁵³Cr values are the first to be reported from Neoproterozoic sedimentary sequence, the exact nature of the chemical process that produced these anomalies is not yet understood. However, the occurrence of these anomalies in organic-rich and Fe-rich chemical sediments that were deposited in a period following a glacial (Gaskiers?) event is compatible with "Snowball Earth" scenarios whereby impulsive oxidation of the upper seawater was in response to ice cover retraction which allowed booming of the biosphere and concomitant oxidation of accumulated Fe²⁺ and subsequent precipitation of the Fe-oxyhydroxides to form the "BIF" during such epochs. Schoenberg et al. (subm.) The stable Cr isotope inventory of solid earth reservoirs determined by double-spike MC-ICP-MS. Chemical Geology
DE: 1505 Biogenic magnetic minerals
DE: 4870 Stable isotopes (0454, 1041)
DE: 4875 Trace elements (0489)
DE: 4924 Geochemical tracers
DE: 4928 Global climate models (1626, 3337)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting