Transformation of ancient organic carbon in exposed organic-rich black shale of the Monterey Formation, Naples Beach, CA
Abstract
The outcrop of the Miocene Monterey Formation at Naples Beach, California, offers a unique opportunity to study the interaction of shale-hosted microbial communities with ancient organic matter. This biogenic sedimentary oil-producing formation is rich in organic carbon, sulfur and phosphorus. Strata here are sub-vertical and exposed along a shoreline cliff face where visually apparent weathering increases from the base to the top of the cliff. Weathering by physical, chemical and microbiological degradation processes are potential controls on transformation of ancient recalcitrant organic carbon in black shales. To investigate the interplay among oil-rich shale organic chemistry, mineralogy, and microbial ecology, we characterized patterns in weathering, biodegradation, and microbial diversity associated with samples collected from vertical transects representing approximately two million years of geologic time within the Carbonaceous Marl Member of the Monterey Formation.
We sampled rock material excavated to a depth of 10 cm into the cliff along bedding planes spanning lower bedrock to upper soil horizon. We characterized rock chemistry (XRF, XRD, XANES, SEM), microbial communities (16S rRNA gene sequencing), and hydrocarbon composition (GC/MS). We determined the extent of hydrocarbon biodegradation using phytane/n-C18 ratios where higher values indicate more degradation. We observed a significant loss of sulfur and pyrite in more weathered upper cliff and surficial rock samples. Although rock chemistry is heterogenous across outcropped strata, chemistry suggests a strong influence by sea spray, and marine halophilic bacteria and archaea are abundant in microbial assemblages. Hydrocarbons within the weathered outcrop are heavily biodegraded compared to an unweathered drill core from the Monterey Formation, suggesting the potential for past or active transformation of hydrocarbons. We did not detect obvious hydrocarbon-degrading microorganisms in our sequence analysis which may indicate the exhaustion of bioavailable ancient organic matter. However, we detected novel Proteobacteria and Flavobacteria phylotypes which may play a role in hydrocarbon degradation, and we are currently investigating this possibility using incubation experiments with Naples Beach shale.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.B23H2630C
- Keywords:
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- 0414 Biogeochemical cycles;
- processes;
- and modeling;
- BIOGEOSCIENCESDE: 0439 Ecosystems;
- structure and dynamics;
- BIOGEOSCIENCESDE: 0463 Microbe/mineral interactions;
- BIOGEOSCIENCESDE: 0470 Nutrients and nutrient cycling;
- BIOGEOSCIENCES