Abstract
A downcore study of organic matter from three radiocarbon-dated Kasten cores collected between 11°15′S and 11°30′S at water depths of 186–580 m revealed that the main organic fraction consists of insoluble and non-hydrolyzable proto-kerogen. Twenty-six proto-kerogen samples were analyzed for stable carbon and nitrogen isotope ratios and element compositions. δ13C values range from −21.3 to −23.2‰, and H/C and O/C atomic ratios range from 1.2 to 1.6 and 0.22 to 0.49, respectively. These results indicate that the sedimentary organic matter on the Peru margin is mainly of marine origin. δ15N values were anomalously low (−0.5 to +6.2‰).
With increasing depth of burial, evidence for early stages of diagenetic alteration affecting proto-kerogens includes: decreasing hydrolyzable fraction and N/C ratios, and increasing S/C ratio. Progressive transformations in H/C and O/C ratios as described by a van Krevelen diagram, however, do not occur. It is proposed that the atomic composition of sedimentary organic matter influenced by upwelling is dependent on (1) the time detrital particles are exposed to oxygenated bottom waters, and (2) burial rate. Owing to episodic variations in late Quaternary oxygen conditions in Peru waters, the proto-kerogen H/C and O/C record reflects the degree of aerobic biodegradation prior to burial rather than post-depositional kerogen evolution. This agrees well with the bulk accumulation record and with climatic trends inferred from glacial, palynological and marine micropaleontological studies. Low δ15N values probably reflect the admixture of 14N-enriched mesopelagic or benthic biomass with detri-tal organic matter. One such mechanism of 14N enrichment may be N2-fixation by mat-forming sulfide biota.
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Reimers, C.E., Suess, E. (1983). Late Quaternary Fluctuations in the Cycling of Organic matter off Central Peru: A Proto-Kerogen Record. In: Suess, E., Thiede, J. (eds) Coastal Upwelling Its Sediment Record. NATO Conference Series, vol 10B. Springer, Boston, MA. https://doi.org/10.1007/978-1-4615-6651-9_25
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