Soil organic matter as an important contributor to Late Quaternary sediments of the tropical West African continental margin

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Abstract

The contribution of soil organic matter (SOM) to continental margins is largely ignored in studies on the carbon budget of marine sediments. Detailed geochemical investigations of late Quaternary sediments (245–0 ka) from the Niger and Congo deep-sea fans, however, reveal that Corg/Ntot ratios and isotopic signatures of bulk organic matter (δ13Corg) in both fans are essentially determined by the supply of various types of SOM from the river catchments thus providing a fundamentally different interpretation of established proxies in marine sciences. On the Niger fan, increased Corg/Ntot and δ13Corg (up to −17‰) were driven by generally nitrogen-poor but 13C-enriched terrigenous plant debris and SOM from C4/C3 vegetation/Entisol domains (grass- and tree-savannah on young, sandy soils) supplied during arid climate conditions. Opposite, humid climates supported drainage of C3/C4 vegetation/Alfisol/Ultisol domains (forest and tree-savannah on older/developed, clay-bearing soils) that resulted in lower Corg/Ntot and δ13Corg (< −20‰) in the Niger fan record. Sediments from the Congo fan contain a thermally stable organic fraction that is absent on the Niger fan. This distinct organic fraction relates to strongly degraded SOM of old and highly developed, kaolinite-rich ferallitic soils (Oxisols) that cover large areas of the Congo River basin. Reduced supply of this nitrogen-rich and 12C-depleted SOM during arid climates is compensated by an elevated input of marine OM from the high-productive Congo up-welling area. This climate-driven interplay of marine productivity and fluvial SOM supply explains the significantly smaller variability and generally lower values of Corg/Ntot and δ13Corg for the Congo fan records. This study emphasizes that ignoring the presence of SOM results in a severe underestimation of the terrigenous organic fraction leading to erroneous paleoenvironmental interpretations at least for continental margin records. Furthermore, burial of SOM in marine sediments needs more systematic investigation combining marine and continental sciences to assess its global relevance for long-term sequestration of atmospheric CO2.

Introduction

Estimation of marine and terrigenous organic matter (OM) proportions in marine sediments forms the basis for calculations of global organic carbon budgets, the discussion of marine and terrigenous biomass as CO2 sinks, and paleoenvironmental reconstructions. Commonly used proxies in paleoceanography to assess the relative amounts of marine and terrigenous OM are e.g., the ratios of organic carbon to total nitrogen contents (Corg/Ntot) and the stable organic carbon isotopic composition (δ13Corg) of the sedimentary OM. The application of these proxies is based on the observation that terrestrial biomass (land plants) is depleted in nitrogen and 13C relative to marine OM (phytoplankton). Typical marine end-member values are −18 to −20‰ for δ13Corg and around 7 for Corg/Ntot ratios. The terrigenous end-member values are −27‰ and numbers >20, respectively. This conventional “two-component view” of OM in marine sediments however excludes two terrigenous OM fractions that may significantly bias both proxies: C4 plant matter and soil organic matter (SOM). Vascular land plants are classified according to their type of metabolism during carbon fixation. Most of the higher plants use the Calvin-Benson cycle of carbon fixation (C3 plants) that discriminates against the heavy carbon isotope (13C). The isotopic ratio of C3 biomass therefore ranges from −23 to −34‰, with an average value of about −27‰ (Meyers, 1997; Guillaume et al., 1999). Tropical grasses like Poaceae and Cyperaceae that grow in open vegetation habitats, i.e., in tree and grass savannah, in contrast, are mostly C4 plants using the Hatch-Slack cycle of carbon fixation that produce isotopic ratios between −9 and −17‰ (average: −12‰; Gearing, 1988; Sackett, 1989). SOM is typically enriched in nitrogen and depleted in 12C due to humification processes and microbial degradation occurring in the terrestrial environment (e.g., Hatcher et al., 1989; Zech et al., 1989, 1997; Christensen, 1996). Due to its reduced reactivity and bioavailability SOM reveals a high preservation potential in marine sediments. Organic geochemical investigations on the composition and reactivity of OM in Late Quaternary sediments from the Congo deep-sea fan have recently demonstrated that Corg/Ntot and δ13Corg are severely modified by stable (low-reactive) SOM exported through the Congo River during humid climates (Holtvoeth et al., 2003).

The central aim of this study is to detect changing quantities and qualities of terrigenous OM exported by the Niger River in response to insolation-driven climate fluctuations during the last 245 kyr. In particular, we address the presence of SOM and C4 plant matter and discuss their influence on bulk OM geochemical and isotopic properties (e.g., TOC, Corg/Ntot, Hydrogen Index, Oxygen Index, Tmax, and δ13Corg). We present new high-resolution bulk organic proxy records from sediments of the lower Niger deep-sea fan (core GeoB 4901) and compare them to the published data from the Congo fan (ODP Site 1075, Holtvoeth et al., 2003). This approach further incorporates inorganic geochemical data deduced from both African river fans (Schneider et al., 1997; Zabel et al., 2001). Inorganic continental proxy records from deep-sea fan sediments do not only document changes in chemical weathering intensity but also provide valuable although indirect records of soil erosion. Zabel et al. (2001) e.g., conclude that titanium/aluminium (Ti/Al) ratios of late Quaternary Niger fan sediments are rather determined by the delivery of kaolinite, a weathering product of silicate rocks, than by varying supply of Ti-bearing heavy minerals. They therefore propose that varying Ti/Al ratios mainly reflect the hydrological cycle, the intensity of chemical weathering, and the export of different types of soils (kaolinite-rich vs. kaolinite-poor) depending on the climatic conditions and the vegetation cover in the Niger catchment. Schneider et al. (1997) draw similar conclusions for the southern Congo deep-sea fan at site GeoB 1008. Based on the organic and inorganic geochemical deep-sea fan records as well as soil and vegetation patterns in central Africa we develop an alternative interpretation of Corg/Ntot and δ13Corg variability that can be tested on other tropical deep sea fan systems.

Section snippets

Study site, material, methods, and stratigraphy

Gravity core GeoB 4901 was taken in 1997 during RV Meteor cruise 41/1 from the Niger deep-sea fan (2°40.7′ N, 6°43.2′ E) at 2184 m water depth (Schulz et al., 1998, Fig. 1). The sediments consist mainly of dark greenish-gray clay-bearing ooze containing remains of diatoms and foraminifera as well as terrestrial siliceous components. From 13.6 to 19.3 m core depth the alternation of greenish black and dark olive colors reveal variable amounts of OM. No evidence was found for turbidites or

Results

Late Quaternary records from elemental analysis (TOC, CaCO3, Corg/Corg*, Corg/Ntot) and δ13Corg are shown in Figure 2. In general, most records show two complete short eccentricity cycles of 100 kyr each that are superimposed by higher-frequency fluctuations (precession) and a distinct shift centered around Termination II at ∼127 ka. The new geochemical and isotopic data are compared to the records of insolation at 15°N and Ti/Al previously reported by Zabel et al. (2001). Low Ti/Al ratios

Late Quaternary sedimentation on the Niger fan

The Ti/Al record from the Niger core shows a clear relation to northern hemisphere summer insolation (15°N) with maxima in kaolinite supply lagging insolation maxima by ∼4.9 kyr (Zabel et al., 2001). According to Zabel et al. (2001) elevated supply of kaolinite-rich terrigenous material is directly linked to humid African climate. A shift to lower Ti/Al ratios in sediments younger than 127 ka (Termination II) from average values of 0.055 to 0.051 supports the conclusion of generally more humid

Conclusions

Organic geochemical records of late Quaternary sediments from the Niger deep-sea fan at site GeoB 4901 provide evidence that a dominant part of the sedimentary OM is of terrestrial origin. The positive correlation of TOC with Ti/Al ratios documenting fluvial kaolinite supply supports the conclusion that a significant part of the terrigenous organic fraction is of soil origin (SOM) probably protected by clay minerals. Based on that we further propose that the positive correlation of Corg/Ntot

Acknowledgments

We are grateful to Helga Heilmann, Renate Henning, and Marco Klann for technical assistance in the laboratories. We thank Fabienne Marret, Gerard Versteegh, one anonymous reviewer, Marcel Kuypers, and Matthias Zabel for their constructive remarks and helpful discussions on a former version of this manuscript. This study was funded by the Deutsche Forschungsgemeinschaft, grant Wa 1036/5.

Associate editor: J. J. Middelburg

References (47)

  • L. Haumaier et al.

    Black carbon - possible source of highly aromatic components of soil humic acids

    Org. Geochem.

    (1995)
  • K. Kaiser et al.

    The role of DOM sorption to mineral surfaces in the preservation of organic matter in soils

    Org. Geochem.

    (2000)
  • P.A. Meyers

    Organic geochemical proxies of paleoceanographic, paleo-limnologic and paleoclimatic processes

    Org. Geochem.

    (1997)
  • P.J. Müller et al.

    An automated leaching method for the determination of opal in sediments and particulate matter

    Deep-Sea Res. I

    (1993)
  • J. Runge

    Holocene landscape history and palaeohydrology evidenced by stable carbon isotope (δ13C) analysis of alluvial sediments in the Mbari valley (5°N/23°E), Central African Republic

    Catena

    (2002)
  • K.L. Sahrawat

    Fixed ammonium and carbon-nitrogen ratios of some semi-arid tropical Indian soils

    Geoderma

    (1995)
  • D. Solomon et al.

    Land use effects on soil organic matter properties of chromic luvisols in semi-arid northern TanzaniaCarbon, nitrogen, lignin and carbohydrates

    Agric. Ecosyst. Environ.

    (2000)
  • M. Zabel et al.

    Late Quaternary climate changes in central Africa as inferred from terrigenous input to the Niger fan

    Quat. Res.

    (2001)
  • W. Zech et al.

    Changes in aromaticity and carbon distribution of soil organic matter due to pedogenesis

    Sci. Total Environ.

    (1989)
  • W. Zech et al.

    Factors controlling humification and mineralization of soil organic matter in the tropics

    Geoderma

    (1997)
  • A. Adegbie

    Reconstruction of paleoenvironmental conditions in equatorial Atlantic and the Gulf of Guinea Basins for the last 245 ka

    (2001)
  • B.T. Christensen

    Physical fractionation of soil and organic matter in primary particle size and density separates

    Adv. Soil Sci.

    (1992)
  • B.T. Christensen

    Carbon in primary and secondary organomineral complexes

  • Cited by (0)

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