Late Ordovician Seawater Sulfate δ34S in Well-preserved Brachiopods

Abstract:

In the end-Ordovician Hirnantian stage, pyrite-sulfur and carbon isotope excursions coincide with a mass extinction and major glaciation [1]. The sulfur isotopic composition of Carbonate Associated Sulfate (δ³⁴S_CAS) is routinely measured as a proxy for the composition of ancient seawater sulfate, which offers a measure of Earth’s surface redox balance during this time. However, the variance among bulk-rock CAS samples commonly greatly exceeds analytical precision. CAS analytical techniques typically require large samples (normally >30g CaCO₃), so integration of different synsedimentary and diagenetic components may explain this scatter.

Using a new sulfur isotope MC-ICP-MS analytical technique [2], we analyzed 5-10mg of well-preserved secondary layer fibrous calcite of brachiopod fossils collected from Late Ordovician-early Silurian sections on Anticosti Island, Canada, and compared these values to those obtained from texture-specific sampling of other carbonate phases found in these rocks. Brachiopod secondary layer calcites show a range of δ³⁴S_CAS values with only 2‰ variability—much less than the 21‰ range observed among less well-preserved and/or diagenetic components. Notably, micrite— a phase often valued for CAS analysis— is as much as 15‰ lower than brachiopods in the same hand sample. Recrystallized fossils and late diagenetic cements are enriched up to 6‰ from brachiopods. Thus, diagenetic processes can strongly impact and overprint CAS signals, even in carbonate rocks of exceptionally low thermal maturity.

Our analysis of brachiopod CAS from these sections indicates that there was no marine sulfate excursion coeval with the Hirnantian carbon isotope excursion and glacial maximum. These observations support Jones and Fike’s interpretation of the noisier bulk-rock δ³⁴S_CAS record from Anticosti Island [1]. In addition, we measured the sulfur isotopic composition of CAS in a modern brachiopod to confirm that it provides a robust archive of seawater sulfate. A modern Terebratalia transversa from San Pedro, CA (20.52±0.11‰, V-CDT) is very slightly depleted relative to modern seawater (20.97±0.10‰, V-CDT) [2]. We are further exploring the range of modern brachiopod CAS.

[1] Jones & Fike (2013) EPSL 363, 144-155. [2] Paris et al. (2013) Chem. Geol. 345, 50-61.