Relationships between organic carbon preservation and mineral surface area in Amazon Fan (Holes 932A and 942A)

Abstract

Mineral surface area (SA), neutral sugar (aldose) concentrations, and organic carbon content (OC) were measured in two holes from Ocean Drilling Program (ODP) Leg 155 (Holes 932A and 942A) and compared to analogues from the modem Amazon River and Delta. Nearly all (>90%) of the OC in the ODP holes was tightly associated with the mineral phase; the two phases could not be quantitatively separated after sonication and density fractionation. Organic matter in Holes 932A and 942A can thus be functionally defined as adsorbed. There is a relationship between OC and a sediment transport pathway prior to deposition. When OC is normalized to a mineral surface area (OC:SA), sediments that were rapidly deposited and strongly influenced by the river had average OC:SA ratios of 0.46 and 0.39 mgCm-2 for Holes 932A and 942A. Sediment deposited indirectly through nepheloid transport had lower OC:SA ratios (average 0.32 and 0.24 mgCm-2). For comparison, OC:SA ratios of suspended sediments from the Amazon River average 0.67 mgCm-2, and modern Amazon Delta sediments have a loading of -0.30 mgCm-2. Although aldose concentrations in Hole 932A decreased downcore, corresponding molecular compositional patterns do not suggest extensive in situ diagenesis. Rather, the aldose composition throughout Hole 932 lithologic Subunits IIA and IIB is similar to that of fine suspended particulate materials carried by the modern Amazon River, which are largely composed of degraded material derived from plants and riverine microorganisms. The aldose data support isotopic and lignin compositional evidence for a terrestrial-riverine source of organic matter in Unit II of Hole 932A, as suggested by Goni (this volume).