Deciphering Earth History: Mapping the Micron-Scale Spatial Distribution and Speciation of Sulfur in Ordovician Carbonates

Abstract:

Isotopic measurements of sulfate and sulfide phases preserved in carbonates document secular changes in the sulfur cycle and help shape our understanding of the redox evolution of Earth’s surface over geologic time. However, as the isotopic record of ancient oceanic conditions becomes better resolved, reports of coeval but discordant geochemical/isotopic proxies are becoming increasingly common. Such varied data could arise from (i) primary differences in the chemistry of the water column from which these sediments were deposited; (ii) geochemical alteration during physical reworking as sediments are being deposited; or (iii) as the result of secondary alteration of geochemical signals after deposition and lithification. As a bulk-rock proxy, d³⁴S signals can consist of multiple origins of sulfate and sulfide within carbonate minerals. Deciphering these phases is critical in order to extract meaningful information about the depositional and diagenetic environment in which the samples formed.

Here, we use X-ray spectromicroscopy to map the distribution of primary and secondary S-bearing sedimentary phases at the micron-scale in a well-characterized suite of Ordovician-aged (∼444 million years ago) carbonate strata from Anticosti Island, Quebec. The high-resolution maps of sulfate variability show differences between major phases (e.g., clasts vs. cement), as well as subtle differences in sulfate concentrations between fossil clades (e.g., crinoids vs. gastropods). Further, we can distinguish the sulfate content of different stages of calcite cement, helping to constrain the diagenetic history and relate specific cements with the chemistry of the waters from which they formed. In conjunction with secondary ionization mass spectrometry (SIMS) δ³⁴S measurements, this work can distinguish isotopic signatures between primary and diagenetic phases, enhancing our ability to reconstruct biogeochemical sulfur cycling over Earth history using δ³⁴S isotopic signatures.