87Sr/86Sr evidence from the epeiric Martin Ridge Basin for enhanced carbonate weathering during the Hirnantian

Dongping Hu,Stanley C Finney,Yanan Shen,Xiaolin Zhang,Wei Huang,Lian Zhou,Megan Shen,Yongsheng Liu,Danielle Shen

doi:10.1038/s41598-017-11619-w

Abstract

A pronounced positive δ13C excursion in the Hirnantian Age has been documented globally, reflecting large perturbations of carbon cycling in the Late Ordovician oceans. Increased organic-carbon burial or enhanced carbonate weathering during glacioeustatic sea-level regression has been proposed to account for this anomalous C-isotope excursion. To test the two competing hypotheses, we measured 87Sr/86Sr and δ13C of carbonates from the Copenhagen Canyon section in Nevada, USA. Our data reveal two rapid negative 87Sr/86Sr shifts that coincide with two prominent positive δ13C excursions and glacial advances. Numerical model simulations suggest that enhanced weathering of carbonates driven by glacio-eustatically controlled sea-level fall is required to produce the observed drops of 87Sr/86Sr and the coeval large positive δ13C excursions, possibly with or without increased organic carbon burial.

Highlights

The interpretation of the HICE is important in our understanding of the causes of the Late Ordovician glaciation and paleo-environment in which the mass extinction occurred
We report δ13C and 87Sr/86Sr data of carbonates from the Copenhagen Canyon section
The δ13C values of the upper part of the Katian Stage vary between 0‰ and 3.2‰ and are followed by a typical HICE with a large positive δ13C excursion of ~7‰ (Fig. 2, Table S1)

Summary

Introduction

The interpretation of the HICE is important in our understanding of the causes of the Late Ordovician glaciation and paleo-environment in which the mass extinction occurred. We carry out high-resolution analyses of 87Sr/86Sr and δ13C for carbonates from the Copenhagen Canyon section in central Nevada, USA (Fig. 1). The Ordovician 87Sr/86Sr of carbonates have provided insights into paleo-climatic changes[29,30,31,32,33]. Few high-resolution 87Sr/86Sr measurements on the Hirnantian carbonates have been carried out to reconstruct continental weathering history during the glaciation. We report δ13C and 87Sr/86Sr data of carbonates from the Copenhagen Canyon section. Integrated with numerical modeling, our results can be used to test the competing hypotheses about the HICE and provide new insights into weathering history during the Hirnantian glaciation. On the basis of the stratigraphic patterns of lithofacies and fossil records, a sea-level curve has been reconstructed for the Hirnantian Stage at Copenhagen Canyon[11]. Overlying the exposure surface, wackestone, packstone, and dark gray lime mudstone with chert suggest post-glacial flooding

Methods

Results

Conclusion