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Strontium isotopic variations of Neoproterozoic seawater: implications for crustal evolution
Y Asmerom1, S B Jacobsen, A H Knoll
1Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138.
Geochimica Et Cosmochimica Acta
|January 1, 1991
Summary
This study presents new strontium (Sr) isotope data from Neoproterozoic carbonates, refining our understanding of seawater composition between 790-850 million years ago. The findings reveal significant changes in hydrothermal flux and continental erosion rates during this period.
Area of Science:
- Geochemistry
- Isotope Geology
- Paleoceanography
Background:
- The Neoproterozoic era (1000–541 million years ago) is marked by significant environmental and tectonic changes.
- Strontium (Sr) isotopes in marine carbonates serve as a proxy for seawater composition and global geochemical cycles.
- Previous studies have provided limited high-resolution Sr isotope data for the Neoproterozoic, hindering detailed reconstructions.
Purpose of the Study:
- To generate high-precision strontium (Sr) isotopic data from Neoproterozoic Shaler Group carbonates.
- To reconstruct the Sr isotopic composition of seawater between approximately 790 and 850 million years ago.
- To model changes in seafloor spreading rates (hydrothermal flux) and continental erosion during the Proterozoic.
Main Methods:
- Sr isotopic analysis (87Sr/86Sr) on low 87Rb/86Sr ratio carbonates from the Shaler Group, Victoria Island.
- Geochemical analysis (Delta 18O, Mn, Ca, Mg, Sr) to identify and exclude altered samples.
- Integration of lithostratigraphy, radiometric dating, and microfossil data for age constraints.
- Coupled Nd and Sr isotope modeling to infer hydrothermal flux and continental erosion rates.
Main Results:
- Established a seawater 87Sr/86Sr ratio curve for 790–850 Ma, varying between 0.70676 and 0.70561.
- Identified a minimum 87Sr/86Sr value of 0.70561 around 830 Ma.
- Modeled a peak in hydrothermal flux around 830 Ma and a peak in continental erosion around 570 Ma.
- Correlated these geochemical peaks with major tectonic events, including supercontinental break-up and orogenesis.
Conclusions:
- The Sr isotope data provide a valuable stratigraphic tool for correlating Neoproterozoic successions globally.
- The reconstructed Sr isotope variations reflect significant shifts in the balance of mantle and continental inputs to the oceans.
- The modeled peaks in hydrothermal flux and erosion are linked to major crustal evolution events during the Proterozoic and early Phanerozoic.
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