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 modern 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 Goñi (this volume).

Perspectives on the Terrestrial Organic Matter Transport and Burial along the Land-Deep Sea Continuum: Caveats in Our Understanding of Biogeochemical Processes and Future Needs

“…Consistent to this suggestion, enhanced preservation of terrestrial OC has been noted in sediments accumulated in glacial periods in the Amazon Fan (Goñi et al, 1997;Keil et al, 1997), and in the Mississippi Cone in the Gulf of Mexico (Newman et al, 1973). Indeed, terrestrial organic carbon deposited at the mouth of the Amazon River under present-day conditions was transferred almost entirely to the deep-sea fan during the LGM (Schlünz et al, 1999).…”

“…Ninety percent of the OC burial in the ocean occurs in deltaic and margin sediments and is associated with mineral particles largely of clay-silt sizes (Hedges and Keil, 1995). Moreover, this terrestrial OC associated with fine minerals of large surface area can be exchanged with marine OM as it enters the coastal ocean Keil et al, 1997). This export to burial offset of terrestrial OC implies that most terrigenous OM delivered to the oceans must be efficiently remineralized, and that the ocean is operating as a net heterotrophic system, accumulating less sedimentary OC than it receives via riverine input (Smith and Mackenzie, 1987;Blair and Aller, 2012).…”

“…During low sea level glacial times, the absence of significant continental shelves can facilitate the transfer of both marine and terrestrial OM from the coastal zone to the deep sea through downslope transport and nepheloid layers and therefore bypassing temporary storage and partial remineralization on shelves, and increasing burial efficiencies (e.g., Keil et al, 1997). Consistent to this suggestion, enhanced preservation of terrestrial OC has been noted in sediments accumulated in glacial periods in the Amazon Fan (Goñi et al, 1997;Keil et al, 1997), and in the Mississippi Cone in the Gulf of Mexico (Newman et al, 1973).…”

Loss of organic matter from riverine particles in deltas

“…The types and amounts of organic matter buried in marine sediments are therefore highly sensitive to the fraction of deposition which occurs within deltas, versus other continental margin sites. In particular, efficient transfer of riverine POM to nondeltaic locations during low sea level stands could help explain the elevated concentrations of terrigenous organic matter often found in glacial-age river cones and abyssal plains (Newman et al, 1973;Goni, 1997;Keil et al, 1997), and could thereby shift the balance of the global carbon cycle on millennial timescales.…”

Changes in the composition of organic matter from prodeltaic sediments after a large flood event (Po River, Italy)

“…Because a large proportion of the humic substances in soils form clay-organic complexes, humified OC is generally adsorbed to fine sediments (Mayer, 1994;Keil et al, 1997;Mayer, 1999;Laird et al, 2001). The highest clay percentages were measured in December 2000 (p < 0.001 and p < 0.01, in the northern and central region, respectively), and are probably responsible for the elevated contents of soil-derived OM evidenced by the biochemical parameters.…”