TY - JOUR

T1 - C, Sr, Nd isotope chemostratigraphy and zircon provenance of the Witvlei Group (Namibia)

T2 - Neoproterozoic glaciations and seawater evolution

AU - Hohl, Simon V.

AU - Rodler, Alexandra S.

AU - Viehmann, Sebastian

AU - Huang, Xiangtong

AU - Xu, Juan

AU - Gaucher, Claudio

AU - Germs, Gerard J.B.

AU - Hegenberger, Wulf

AU - Goderis, Steven

AU - Wei, Haizhen

AU - Frei, Robert

N1 - Publisher Copyright: © 2022 Elsevier B.V.

PY - 2022

Y1 - 2022

N2 - The Neoproterozoic witnessed several low latitude glaciations (i.e., the Sturtian, Marinoan and Gaskiers glaciations) and the geodynamic reorganization of cratonic blocks after the breakup of Rodinia. Trace element and stable isotope geochemistry from approximately coeval carbonate deposits on continental marginal sea and foreland basins (Yangtze Platform, China; Otavi and Nama Groups, Namibia) have shown fluctuating oxygen increase on the marine shelves. To gain an increasingly global picture of the bio-geochemical conditions in late Neoproterozoic seawater, we here present new redox-sensitive trace element, stable (C, O) and radiogenic (Nd, Sr) isotope records of carbonates from the Cryogenian Blaubeker and Court, and the Ediacaran Buschmannsklippe Formations (Witvlei Group, Namibia). Shale-normalised REE + Y patterns of post-Sturtian and post-Marinoan carbonates parallel modern seawater showing positive La, Gd and Y anomalies. Negative Ce anomalies argue for their preservation in increasingly more oxidising sea/porewater conditions in the Witvlei Basin from the Ediacaran on. While Cryogenian carbonates underwent radiogenic basin-fluid type overprinting, Ediacaran carbonates upsection record pristine Sr isotopic compositions that match the global Neoproterozoic seawater curve. Partly coupled negative correlations between Sr and Nd suggest long-term shifts in continental weathering and short-term changes in ocean circulation patterns. The δ13Ccarb values range from −7.2 to +3.5 ‰ and record a negative isotope excursion in the upper part of the Witvlei stratigraphy. This excursion might be equivalent to the ‘Shuram’ carbon isotope excursion (CIE). However, the causes, global extent and correlation of the Shuram CIE are still debated. Typical for carbonates of the putative Shuram excursion are less pronounced Ce anomalies, lower Y/Ho ratios, and lower bio-essential and redox-sensitive trace metal concentrations, arguing for periodic redoxcline oscillations in a redox-stratified late Neoproterozoic shelf environment. The overall long-term decrease in redox-sensitive element enrichments throughout the Witvlei Group argues for a progressively increasing, presumably biologically-driven metal cycling towards values typical for Phanerozoic carbonates. The combination of changes in local weathering flux and ambient redox conditions in the late Neoproterozoic ocean may have caused dynamic (bio)geochemical metal cycling, predating (and possibly promoting) the metazoan radiation documented in the overlying Nama Group.

AB - The Neoproterozoic witnessed several low latitude glaciations (i.e., the Sturtian, Marinoan and Gaskiers glaciations) and the geodynamic reorganization of cratonic blocks after the breakup of Rodinia. Trace element and stable isotope geochemistry from approximately coeval carbonate deposits on continental marginal sea and foreland basins (Yangtze Platform, China; Otavi and Nama Groups, Namibia) have shown fluctuating oxygen increase on the marine shelves. To gain an increasingly global picture of the bio-geochemical conditions in late Neoproterozoic seawater, we here present new redox-sensitive trace element, stable (C, O) and radiogenic (Nd, Sr) isotope records of carbonates from the Cryogenian Blaubeker and Court, and the Ediacaran Buschmannsklippe Formations (Witvlei Group, Namibia). Shale-normalised REE + Y patterns of post-Sturtian and post-Marinoan carbonates parallel modern seawater showing positive La, Gd and Y anomalies. Negative Ce anomalies argue for their preservation in increasingly more oxidising sea/porewater conditions in the Witvlei Basin from the Ediacaran on. While Cryogenian carbonates underwent radiogenic basin-fluid type overprinting, Ediacaran carbonates upsection record pristine Sr isotopic compositions that match the global Neoproterozoic seawater curve. Partly coupled negative correlations between Sr and Nd suggest long-term shifts in continental weathering and short-term changes in ocean circulation patterns. The δ13Ccarb values range from −7.2 to +3.5 ‰ and record a negative isotope excursion in the upper part of the Witvlei stratigraphy. This excursion might be equivalent to the ‘Shuram’ carbon isotope excursion (CIE). However, the causes, global extent and correlation of the Shuram CIE are still debated. Typical for carbonates of the putative Shuram excursion are less pronounced Ce anomalies, lower Y/Ho ratios, and lower bio-essential and redox-sensitive trace metal concentrations, arguing for periodic redoxcline oscillations in a redox-stratified late Neoproterozoic shelf environment. The overall long-term decrease in redox-sensitive element enrichments throughout the Witvlei Group argues for a progressively increasing, presumably biologically-driven metal cycling towards values typical for Phanerozoic carbonates. The combination of changes in local weathering flux and ambient redox conditions in the late Neoproterozoic ocean may have caused dynamic (bio)geochemical metal cycling, predating (and possibly promoting) the metazoan radiation documented in the overlying Nama Group.

U2 - 10.1016/j.precamres.2022.106600

DO - 10.1016/j.precamres.2022.106600

M3 - Journal article

AN - SCOPUS:85125147410

VL - 372

JO - Precambrian Research

JF - Precambrian Research

SN - 0301-9268

M1 - 106600

ER -