PRESERVATION OF ORIGINAL SEDIMENTARY SULFUR ISOTOPE RECORDS UNDER POST-DEPOSITIONAL THERMAL ALTERATION

Abstract

The sedimentary sulfur record is critical for interpreting how marine environments have evolved throughout Earth’s history because the burial of reduced sulfur species (i.e. in pyrite and kerogen) impacts global climate through drawdown of atmospheric CO2 and buildup of O2 levels. Furthermore, depositional conditions, such as ocean chemistry, microbial activity, and redox conditions in deep geologic time are recorded in the pyrite sulfur isotope composition (i.e. δ34S). Microbial sulfate reduction, a key step in pyrite formation, imparts an isotopic fractionation that is preserved in pyrite δ³⁴S. This process occurs in low-oxygen environments when other metabolic processes, such as aerobic respiration, are unavailable to organisms. Thus, the deposition of sulfur as pyrite in the sedimentary record can indicate periods of anoxia in marine environments through time. However, it remains unclear whether original δ34S signals are retained after deep burial and exposure to the geothermal gradient, and later stage metamorphism. This is important to distinguish as use of the sedimentary sulfur record is limited by the uncertainty in how these records are impacted by post-depositional alteration. In this work I seek to assess how well sedimentary δ34S records are preserved under high temperature diagenesis through metamorphic conditions, using in-situ sulfur isotope analyses of δ34S of individual pyrite grains. Samples were collected within a single stratigraphic horizon of the Upper Cretaceous Pierre Shale (Raton Basin, NM) along a transect perpendicular to a later igneous intrusion, in which heating decreased with distance from the dike. I collected Secondary Ionization Mass Spectrometry (SIMS) data on individual pyrite grains and additional mineralogical and lithological constraints (e.g. x-ray diffraction, total organic carbon content, and total sulfur content). I find that sedimentary sulfur isotope signatures in the Pierre Shale remain preserved

across metamorphic grades up to ~300°C. Heterogeneous fluid flow produced an irregular spatial distribution of sulfide minerals across the outcrop as pyrite reacted to form pyrrhotite, and increasing bulk sulfur contents toward the dike suggests magmatic sulfur addition. Although heating recrystallized pyrite from framboids to larger crystals, sulfur-rich magmatic fluids may have limited sulfur exchange, potentially contributing to the preservation original δ34S signature across the gradient. This work supports interpretations of measured δ34S values of sediments in contact metamorphic setting, increasing confidence in δ34S measurements on rocks previously deemed unsuitable for analysis and interpretation.