2003 Seattle Annual Meeting (November 2–5, 2003)

Paper No. 4
Presentation Time: 8:00 AM-12:00 PM


BORGES, Joniell B. and SAGEMAN, Bradley B., Geological Sciences, Northwestern Univ, Locy Hall, 1850 Campus Drive, Evanston, IL 60208, joniell@earth.northwestern.edu

The recognition that mineral surface processes play important roles in the transport and burial of organic carbon (OC) in marine environments has added another factor to the list of controls on OC burial. After decades of debate between production and preservation models, one recent study suggested that mineral surface area (MSA) may be the dominant control on OC enrichment in ancient shales. Clearly MSA needs to be critically and comparatively evaluated. In this study fine-grained strata from organic carbon rich units representing different ages and depositional systems were analyzed for MSA. The studied samples reflect a broad range of lithologies including non-calcareous shales, calcareous shales, and marly shales or marlstones. Total organic carbon (TOC) content varied from <1 to >15 weight percent in the samples. Those with TOC > 3% were designated "black shales". Mineral surface area was assessed using the ethylene glycol monoethyl ether method and the results were compared to a pre-existing suite of geochemical data (e.g., redox sensitive trace metals, such as Mo). This comparative analysis was directed at understanding the role MSA plays relative to other possible controls on organic matter enrichment, such as production, preservation, and bulk accumulation rate. Results indicate a weak correlation between TOC and MSA for the complete data set. The correlation improves when samples are separated by lithology, and the best correlation (r2=0.50) is found in the calcareous shales. As a group, however, the TOC-enriched "black shales" showed the poorest correlation. In contrast, the complete sample set showed a relatively strong correlation of TOC with Mo content, which is interpreted as a proxy for pore water sulfide and thus enhanced organic matter preservation. Therefore, although MSA may play some role in OC accumulation in the sampled units, other factors such as the flux of organic matter into the sulfate reduction zone and concomitant sulfide generation, which severely limits the exposure time of OC to enhanced degradation, appear to be more important determinants for OC burial. Further work will focus on the mineralogy of the samples in comparison to the relatively smectite-rich lithologies that were previously documented with high correlation of MSA to TOC.