ORGANOGENIC DOLOMITE AS A PALEOCEANOGRAPHIC ARCHIVE - NEOPROTEROZOIC INSIGHTS?

Early diagenetic organogenic dolomite intervals occur episodically-to-cyclically throughout the Miocene Monterey Formation, but compositions have yet to be rigorously assessed stratigraphically or in relation to Miocene climate events. We present a detailed chemostratigraphic record (δ¹³C, δ¹⁸O, TOC, trace elements/REEs) of dolomite formation within a continuous Monterey core (offshore Santa Barbara Channel) exhibiting >100 distinct dolomite-cemented intervals. Despite dolomite horizons being physically separate from one stratum to the next, they display regular core-wide variations in δ¹³C, conceivably analogous to Neoproterozoic negative δ13C excursions associated with glacigenic intervals. The main Monterey depositional interval has entirely negative δ¹³C values (-2 to -16‰), whereas positive δ¹³C values (+2 to +9‰) occur in lithologic transitions to bounding formations. Dolomite and microfossil calcite similarly record late middle Miocene δ¹⁸O enrichment (~13.9-10 Ma) recognized globally in marine foraminiferal records. Dolomite δ¹³C mirrors sediment accumulation rate, with lightest values associated with both slowest sedimentation rates and highest TOC contents. Interestingly, lightest δ¹³C dolomite intervals correspond in time to distinct δ¹³C enrichment (~16-13.5 Ma) observed globally in open marine records; a potential smoking gun – linking organic matter burial to open ocean δ¹³C enrichment, prior to cooling and Antarctic ice expansion (Monterey Hypothesis). The persistent negative δ¹³C values of Monterey dolomites suggest formation within the zone of sulfate reduction, in turn tied to slow sediment accumulation rates. Fe, Mn, Al, and Ba all mimic δ¹³C variation, being less enriched where dolomites are more negative. Associated shale-normalized REE patterns also follow δ¹³C, being most like modern seawater (most negative Ce/Ce*, greatest HREE enrichment) where sediment accumulation rates were lowest. Monterey organogenic dolomites formed under steady state anoxic conditions thus appear to proxy several paleoceanographic processes. Neoproterozoic carbonatogenesis may have occurred under similar steady state anoxic conditions from similar microbes, begging the question - were negative δ¹³C excursions linked to sulfate availability?