Sulfur Isotope Composition of Organic Matter from the Monterey Formation: Implications for California Margin Redox Conditions in the Late Miocene

The isotopic composition of organic sulfur (δ³⁴S) is a potential recorder of past biogeochemical conditions that has, thus far, received relatively little attention. This study presents continuous organic δ³⁴S records from two basins of the organic-rich Miocene Monterey Formation, deposited under varying extents of restriction and spanning a combined time interval of c.14.5-6 Ma. In the relatively restricted San Joaquin basin, organic δ³⁴S values decreased progressively from +5‰ at c. 13 Ma down to near -20‰ at c. 6 Ma. These low organic δ³⁴S values are similar to those of the modern Santa Barbara Basin, and likely reflect an increasing contribution of low-δ³⁴S sulfide in the sediments and water column with progressive isolation of the basin and expanding anoxia. In marked contrast, the less restricted Santa Maria Basin exhibits organic δ³⁴S values that are >10‰ higher than in the coeval San Joaquin basin intervals. Upper siliceous shales of the Santa Maria Basin have an average δ³⁴S value of 24‰, the highest yet reported for marine organic sulfur and roughly 2-3‰ higher than Miocene seawater sulfate. They are separated from lower phosphatic shales of average δ³⁴S value 17‰ by a sharp lithostratigraphic contact at c. 11 Ma, over which the abrupt 7‰ increase in average organic δ³⁴S value occurs. This large and rapid shift in organic δ³⁴S implies a change in sulfurization regime that may in turn reflect a rapid transition in basin conditions. Possible environmental and biotic mechanisms underpinning these records are likely complex and will be discussed in the presentation.