SULFUR CYCLING IN THE LATE MIDDLE ORDOVICIAN: IMPLICATIONS FOR OCEAN CIRCULATION AND THE ONSET OF LATE ORDOVICIAN GLACIATION

Mechanisms that triggered widespread glaciation after a stable greenhouse climate in the Early-Middle Ordovician are controversial, making it important to establishing a reliable time-series record of marine chemistry across this critical transition. Here, we present paired S-isotope (trace sulfate and pyrite) and Sr-isotope data for Middle-Late Ordovician marine carbonates of Western Newfoundland (Table Head Group) and the Argentine Precordillera (Las Chacritas and Aguaditas formations). Biostratigraphic constraints indicate that basal strata of the Aguaditas Fm. overlaps in age with the youngest strata of the Table Head Group, and that the Darriwilian-aged Las Chacritas Fm. is coeval with the transition from Table Head to Aguaditas strata.

Pre-Darriwilian aged strata of Newfoundland and Argentina preserve C-isotope trends that record a stable greenhouse climate and S-isotopes that suggest a strong, yet fluctuating oxycline that likely controlled H₂S removal via pyrite formation. Here, S-isotope data record a robust transition in Δ³⁴S(SO4-PY) from ca. 26‰ in pre-Darriwilian Table Head strata to near zero in the Darriwilian-aged upper Table Head Group, Las Chacritas Fm. and lower Aguaditas Fm., finally reaching values of ca. -4‰ — i.e., δ³⁴S(PY) heavier than δ³⁴S(SO4). Values then rise to ca. 5‰ in the Sandbian-aged upper Aguaditas Fm. This dramatic fall in Δ³⁴S(SO4-PY) values coincides with a globally recognized drop in ⁸⁷Sr/⁸⁶Sr from 0.7087 to 0.7078 (Qing et al. 1998; Shields et al. 2003), suggesting a major shift in weathering regime during the Middle-Late Ordovician.

Models for sulfur cycling assume most marine sulfide is derived from bacterial sulfate reduction of an oxidized marine sulfur reservoir. This model requires that Δ³⁴S(SO4-PY) not surpass zero even under extremely low sulfate conditions. Data presented here thus requires either an addition of isotopically enriched sulfur to the pyrite reservoir or an addition of depleted sulfur to the marine sulfate reservoir. We suggest that rapid sea floor spreading, along with the onset of more vigorous oceanic circulation associated with global cooling, may have resulted in disruption of the greenhouse oxycline, flooding of platforms with isotopically light bacterially-mediated hydrogen sulfide, and massive oxidation of this reservoir.