Lack of evidence for enhanced preservation of sedimentary organic matter in the oxygen minimum of the Gulf of California: Comment and Reply

Abstract

The role of bottom-water oxygenation in the degradation and preservation of marine sedimentary organic matter is the subject of controversy. Two opposing theories have been proposed to explain the typically higher organic carbon concentrations found in laminated, fully anoxic sediments and sedimentary rocks, when compared to their bioturbated, more oxygenated counterparts. One theory holds that preservation of organic matter is enhanced in laminated sediments because of slower net rates of organic carbon decomposition under fully anoxic conditions (e.g., Cranfield, 1989, 1992). Alternatively, it has been suggested that organic-rich laminated sediments reflect high levels of primary productivity in the surface waters and, consequently, increased proportions of organic matter in the particulate flux deposited at the water-sediment interface, rather than enhanced preservation in the sediments (e.g., Pedersen and Calvert, 1990). Calvert et al (1992) compared data on modern surface sediments of the Guaymas Basin accumulating insides as well as outside the zone where the oxygen minimum impinges on the sea floor. The choice of sediments deposited in close proximity minimizes the effects related to variations in the nature and mass flux or organic matter reaching the water-sediment interface. The data were collected on the first 2-5 cm of gravity cores. With themore » high sediment-accumulation rates in the Guaymas Basin (see Calvert et al., 1992), organic matter in the sediments analyzed has been subjected to early diagenetic processing for only short periods of time, at most 10-20 years. Calvert et al. reported the absence of systematic variations of the total-organic-carbon concentrations and Rock-Eval hydrogen and oxygen indices with bottom-water oxygen concentrations. This led them to conclude that bottom-water oxygen levels exert no major influence on the concentration and composition (in terms of the H/C and O/C ratios) of organic matter in the sediments. 10 refs.« less