BASIN-SCALE VARIABILITY OF SULFUR ISOTOPE RECORDS FROM A MIXED CARBONATE-SILICICLASTIC SUCCESSION (ORDOVICIAN-SILURIAN STRATA OF ANTICOSTI ISLAND, QUEBEC)

We present sulfur isotope records of carbonate-associated sulfate (δ³⁴S_CAS) from multiple outcrops of upper Ordovician – lower Silurian strata preserved across Anticosti Island, Québec. The strata span the Hirnantian Stage of the Ordovician Period, which is associated with mass extinction, glacioeustatic sea level change, and a major carbon cycle perturbation (the globally documented Hirnantian positive carbon isotope excursion). Published sequence stratigraphy, chitinozoan biostratigraphy, and inorganic carbon isotope chemostratigraphy (δ¹³C_carb) provide independent means to correlate sections throughout the Anticosti Island outcrop belt during this interval.

Time series δ³⁴S_CAS records from different locations within the basin are divergent. The limestone- and shale-dominated western sector is characterized by relative stability in δ³⁴S_CAS (ca. +23‰) through the strata containing the positive δ¹³C_carb excursion. In the east, where siliciclastic sands are mixed with limestone, the δ³⁴S_CAS curve across the correlative interval fluctuates over ca. 30‰ in less than ten meters of stratigraphy. The δ³⁴S_CAS record of east-central sector of the island contains fluctuations of similar magnitude, but the curve includes a ca. 25‰ negative excursion not observed in the east.

We suggest that these discordant δ³⁴S_CAS records are due to differential diagenetic histories across the various sectors of the basin. Specifically, we attribute the rapid stratigraphic variability and lateral heterogeneity seen in the central and eastern parts of the basin to several possible post-depositional processes, and not, e.g., to low sulfate concentrations or pervasive euxinia in Ordovician-Silurian marine basins. Independent geochemical proxies indicate that the eastern end of the island suffered more diagenesis at higher temperatures. Consequently, we hypothesize that the data collected from the western end of the basin offer a more accurate history of the sulfur isotopic composition of seawater during this time. These observations of intra-basinal sulfur isotope variability highlight the significant impact of post-depositional processes on the δ³⁴S_CAS values of what otherwise are exceptionally preserved carbonate rocks.