TY - JOUR

T1 - Multiproxy isotope constraints on ocean compositional changes across the late Neoproterozoic Ghaub glaciation, Otavi Group, Namibia

AU - Rodler, Alexandra

AU - Frei, Robert

AU - Gaucher, C.

AU - Korte, Christoph

AU - Rosing, Salik Anders

AU - Germs, G. J. B.

PY - 2017

Y1 - 2017

N2 - Chromium isotopes are powerful in recording redox fluctuations in marine environments, potentially tracing past atmospheric oxygenation and continental weathering. We present δ53Cr signals with 87Sr/86Sr, δ13C and δ18O, and element abundances in carbonates of two shallow marine successions of the Otavi Group, Namibia, stretching over four stages from preglacial setting to retreat of the late Neoproterozoic Ghaub glaciation (∼635 Ma). The fluctuating but generally heavy δ53Cr signals indicate oxygenated shallow seawater prior to the glaciation (stage 1). Sustained high δ13Ccarb signatures at that time suggest organic matter formed a greater proportion of buried carbon, which implies a net increase in oxidant flux to the surface. The δ18O signals fluctuate, with a tendency towards lighter values approaching the Ghaub Fm. Correlating δ53Cr and δ13Ccarb values indicate disruptions affecting the carbon cycle and oxygen availability (stage 2). Prior to the Ghaub diamictite, a sequence of 87Sr/86Sr in the range typical of late Neoproterozoic seawater occurs with high Sr concentrations, low Mn/Sr and low Mg/Ca. Enhanced detrital input after the Ghaub glaciation (stage 3) indicates elevated weathering rates likely accompanied by enhanced nutrient influx. The δ53Cr–δ13Ccarb patterns are resolved in a two-step recovery towards the late glacial aftermath (stage 4), which is characterized by values similar to those of the early preglacial interval. There, a sequence of postglacial limestones also records typical late Neoproterozoic seawater 87Sr/86Sr ratios. The carbonate δ53Cr signatures at the base of the postglacial sequence are characterized by values even below the range of bulk silicate Earth (BSE). We hypothesize that this is due to (i) redox cycling of Cr in seawater, e.g. by (partial) reduction of Cr(VI) during microbial-mediated cap dolostone deposition and/or at the redox boundary of redox-stratified seawater and/or due to (ii) increased contribution of negatively fractionated Cr associated with an enhanced input of detrital-derived Cr from postglacial weathering and/or subaqueous hydrothermal influx. Previously it was demonstrated that carbonate δ53Cr signatures trace changes in continental weathering and provide valuable indication of fluctuating seawater redox conditions prior to and after one of the major syn-Marinoan glaciations. Furthermore, changes in δ53Cr values might scale with shifts in bioproductivity.

AB - Chromium isotopes are powerful in recording redox fluctuations in marine environments, potentially tracing past atmospheric oxygenation and continental weathering. We present δ53Cr signals with 87Sr/86Sr, δ13C and δ18O, and element abundances in carbonates of two shallow marine successions of the Otavi Group, Namibia, stretching over four stages from preglacial setting to retreat of the late Neoproterozoic Ghaub glaciation (∼635 Ma). The fluctuating but generally heavy δ53Cr signals indicate oxygenated shallow seawater prior to the glaciation (stage 1). Sustained high δ13Ccarb signatures at that time suggest organic matter formed a greater proportion of buried carbon, which implies a net increase in oxidant flux to the surface. The δ18O signals fluctuate, with a tendency towards lighter values approaching the Ghaub Fm. Correlating δ53Cr and δ13Ccarb values indicate disruptions affecting the carbon cycle and oxygen availability (stage 2). Prior to the Ghaub diamictite, a sequence of 87Sr/86Sr in the range typical of late Neoproterozoic seawater occurs with high Sr concentrations, low Mn/Sr and low Mg/Ca. Enhanced detrital input after the Ghaub glaciation (stage 3) indicates elevated weathering rates likely accompanied by enhanced nutrient influx. The δ53Cr–δ13Ccarb patterns are resolved in a two-step recovery towards the late glacial aftermath (stage 4), which is characterized by values similar to those of the early preglacial interval. There, a sequence of postglacial limestones also records typical late Neoproterozoic seawater 87Sr/86Sr ratios. The carbonate δ53Cr signatures at the base of the postglacial sequence are characterized by values even below the range of bulk silicate Earth (BSE). We hypothesize that this is due to (i) redox cycling of Cr in seawater, e.g. by (partial) reduction of Cr(VI) during microbial-mediated cap dolostone deposition and/or at the redox boundary of redox-stratified seawater and/or due to (ii) increased contribution of negatively fractionated Cr associated with an enhanced input of detrital-derived Cr from postglacial weathering and/or subaqueous hydrothermal influx. Previously it was demonstrated that carbonate δ53Cr signatures trace changes in continental weathering and provide valuable indication of fluctuating seawater redox conditions prior to and after one of the major syn-Marinoan glaciations. Furthermore, changes in δ53Cr values might scale with shifts in bioproductivity.

KW - (partial) Cr reduction

KW - Bioproductivity

KW - Carbonate Cr isotope composition

KW - Detrital contamination

KW - Neoproterozoic ocean oxygenation

U2 - 10.1016/j.precamres.2017.05.006

DO - 10.1016/j.precamres.2017.05.006

M3 - Journal article

AN - SCOPUS:85021204197

VL - 298

SP - 306

EP - 324

JO - Precambrian Research

JF - Precambrian Research

SN - 0301-9268

ER -