Organic matter in river‐influenced continental margin sediments: The land‐ocean and climate linkage at the Late Quaternary Congo fan (ODP Site 1075)

Abstract

[1] Late Quaternary sections (1.2 Ma) of ODP-Site 1075 from the Congo deep-sea fan are investigated to reconstruct variations of terrigenous organic matter supply to the eastern equatorial Atlantic. To characterize the organic matter (OM) with regard to marine and terrigenous amounts we used elemental analysis (C, N, S), stable carbon isotopes (bulk d 13 C org ), Rock-Eval pyrolysis, and terrigenous biomarkers (lignin phenols from CuO oxidation). The records of total organic carbon (TOC) contents, C org /N tot ratios, bulk OM degradation rates (C org /C org *), and the ratios of hydrocarbons (HC) from low-mature versus HC from high-mature OM (lm/hm) reveal pronounced cyclic changes in OM abundance, preservation, and reactivity that are closely related to the precessional controlled record of insolation, and thus, to variations in upwelling intensity and fluvial run-off. Primary productivity off the Congo is stimulated by both, enhanced nutrient supply in response to trade-induced upwelling during arid African climates (insolation minima) and fluvial nutrient delivery during humid stages (following insolation maxima), especially due to the contribution of dissolved silica that is taken up preferably by diatoms. However, results stemming from a multiparameter approach reveal that the fluvial supply of degraded OM and black carbon (BC) associated with fine-grained sediments from soil erosion is a decisive factor for the preservation of marine OM and, in addition, significantly influences the geochemical signature of bulk and terrigenous OM. Riverine and eolian supply of C 4 plant matter, as well as enhanced concentrations of BC, during arid and arid-to-humid transitional climate stages, may lead to a severe underestimation of terrigenous organic carbon, if its amount is calculated from bulk isotopic ratios using binary end-member models. During the humid stages, it is the massive supply of 13 C-enriched soil OM with low C org /N tot ratios that may suggest a mainly marine composition of bulk OM. In fact, terrigenous OM governs bulk OM geochemical signatures in the sediments of the Congo deep-sea fan, a result that is contradictory to earlier studies, especially to the conventional interpretation of the bulk d 13 C org signal.

Bacteriohopanepolyols in tropical soils and sediments from the Congo River catchment area

“…This is in accord with other studies from the Congo deep sea fan. Holtvoeth et al (2003) used a binary mixing model approach to determine that between 18% and 61% of bulk OM in ODP 1075 is of continental origin. Similarly, Weijers et al (2009) used a 3 end member mixing model to determine that between 38% and 52% of OC within GeoB 6518-1 is of terrigenous (soil) origin.…”

Guidelines for Rock–Eval analysis of recent marine sediments

“…It has been increasingly applied to the characterization of organic matter (OM) in soils (Di Giovanni et al, 2000;Disnar et al, 2003;Hetényi et al, 2005;Sebag et al, 2006;Graz et al, 2012;Saenger et al, 2013;Hétényi and Nyilas, 2014), the detection of black carbon (Copard et al, 2006;Poot et al, 2009) and even the analysis of pure organic products (Carrie et al, 2012). It has also been applied for several decades to the study of recent lacustrine sediments (Campy et al, 1994;Di Giovanni et al, 1998;Meyers and Lallier-Vergès, 1999;Ariztegui et al, 2001;Steinmann et al, 2003;Sanei et al, 2005;Jacob et al, 2004;Boussafir et al, 2012;Zocatelli et al, 2012;Lavrieux et al, 2013;Sebag et al, 2013, amongst others) or recent marine sediments (Peters and Simoneit, 1982;Hussain and Warren, 1991;Calvert et al, 1992;Combourieu et al, 1999;Ganeshram et al, 1999;Ozcelik and Altunsoy, 2000;Holtvoeth et al, 2001Holtvoeth et al, , 2003Holtvoeth et al, , 2005Tamburini et al, 2003;Baudin et al, 2007Baudin et al, , 2010Kim et al, 2007;Marchand et al, 2008;Tribovillard et al, 2008Tribovillard et al, , 2009Biscara et al, 2011;…”

Precession driven changes in terrestrial organic matter input to the Eastern Mediterranean leading up to the Messinian Salinity Crisis

“…forests of North Africa (Holtvoeth et al, 2003) supplied the Pissouri depositional system via 481 enhanced Nile River input (Fig.9). This drove the n-alkane δ 13 C values lower (Fig.…”