TY - JOUR
T1 - Oceanic redox conditions in the Late Mesoproterozoic recorded in the upper Vazante Group carbonates of São Francisco Basin, Brazil
T2 - Evidence from stable isotopes and REEs
AU - Azmy, Karem
AU - Sylvester, Paul
AU - de Oliveira, Tolentino F.
PY - 2009/2
Y1 - 2009/2
N2 - The Vazante Group consists of a late Mesoproterozoic (∼1.0-1.1 Ga) carbonate-dominated marine platform sequence in east-central Brazil. The upper part of the sequence consists of a glaciomarine diamictite unit overlain by a cap carbonate. The δ13C profile of the upper Vazante shows significant negative plunges, one preglacial (drop of ∼5.5‰ VPDB) and two postglacial (drops of ∼9 and 5‰VPDB, respectively). The C-isotope plunge in the preglacial carbonates is correlated with low Th/U ratios (0.1-1.4) and a negative Ce/Ce* shift (∼0.4), suggesting deposition under relative reducing conditions. In contrast, the C-isotope plunges in the postglacial carbonates are associated with high Th/U ratios (>2) and positive Ce/Ce* shifts (up to ∼1.5), thus reflecting oxidizing conditions. Variations in the redox conditions of the late Mesoproterozoic ocean, reflected by changes in the Th/U and Ce/Ce* ratios, are likely attributable to a combination of both global and local climatic and oceanographic changes, similar to what has been inferred for the Neoproterozoic oceans.
AB - The Vazante Group consists of a late Mesoproterozoic (∼1.0-1.1 Ga) carbonate-dominated marine platform sequence in east-central Brazil. The upper part of the sequence consists of a glaciomarine diamictite unit overlain by a cap carbonate. The δ13C profile of the upper Vazante shows significant negative plunges, one preglacial (drop of ∼5.5‰ VPDB) and two postglacial (drops of ∼9 and 5‰VPDB, respectively). The C-isotope plunge in the preglacial carbonates is correlated with low Th/U ratios (0.1-1.4) and a negative Ce/Ce* shift (∼0.4), suggesting deposition under relative reducing conditions. In contrast, the C-isotope plunges in the postglacial carbonates are associated with high Th/U ratios (>2) and positive Ce/Ce* shifts (up to ∼1.5), thus reflecting oxidizing conditions. Variations in the redox conditions of the late Mesoproterozoic ocean, reflected by changes in the Th/U and Ce/Ce* ratios, are likely attributable to a combination of both global and local climatic and oceanographic changes, similar to what has been inferred for the Neoproterozoic oceans.
KW - Brazil
KW - REE
KW - Redox conditions
KW - Th/U
KW - Vazante carbonates
UR - http://www.scopus.com/inward/record.url?scp=58549103903&partnerID=8YFLogxK
U2 - 10.1016/j.precamres.2008.10.003
DO - 10.1016/j.precamres.2008.10.003
M3 - Article
AN - SCOPUS:58549103903
SN - 0301-9268
VL - 168
SP - 259
EP - 270
JO - Precambrian Research
JF - Precambrian Research
IS - 3-4
ER -