Paleoenvironmental Controls on the Abundance of ¹³C in Sedimentary Organic Carbon

J. M. Hayes, D. J. Hollander, S. G. Wakeham, and T. Pease

Variations in the ¹³C content of sedimentary organic molecules are often interpreted in terms of processes likely to have occurred in ancient water columns. The interpretations are based mainly on principles rather than on isotopic analyses of related compounds in modern water columns. Seeking to bridge the gap between interpretation and experience, we have analyzed lipids extracted from particulate organic carbon ("POC", two size fractions, >20µm and 20 - 0.2 µm) recovered from depths of 40, 100, 650 and 800 m in the water column of the Santa Monica Basin, offshore southern California and from underlying sediments. The sampling has been replicated six times in order to complete an annual cycle covering a range of upwelling conditions and levels of productivity.

Isotopic compositions of individual organic compounds and their relationships to hydrographic conditions indicate that the abundance of ¹³C in organic materials is related to availability of macronutrients as well as to concentrations of dissolved carbon dioxide. Coupled with observations by others, these observations indicate strong control of carbon isotopic compositions by rate of growth and by paleoceanographic conditions (e.g., upwelling) that regulate availability of nutrients. In general, values of ¹³^dgr for sedimentary materials match those for the most productive portions of the annual cycle and thus selectively record oceanographic conditions from that portion of each year.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California