Soil organic matter as an important contributor to Late Quaternary sediments of the tropical West African continental margin
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)
- et al.
Role of the soil matrix and minerals in protecting natural organic materials against biological attack
Org. Geochem.
(2000) The supply and accumulation of silica in the marine environment
Geochim. Cosmochim. Acta
(1981)- et al.
Soil organic matter (SOM) characterization by Rock-Eval pyrolysisScope and limitations
Org. Geochem.
(2003) - et al.
An assessment of the soil resources of Africa in relation to productivity
Geoderma
(1997) - et al.
Stabilisation of soil organic matter by interactions with minerals as revealed by mineral dissolution and oxidative degradation
Org. Geochem.
(2003) - et al.
Black carbon in density fractions of anthropogenic soils of the Brazilian Amazon region
Org. Geochem.
(2000) - et al.
Adsorption and desorption of different organic matter fractions on iron oxide
Geochim. Cosmochim. Acta
(1995) - et al.
Lignin and carbohydrate alteration in particle-size separates of an Oxisol under tropical pastures following native savanna
Soil Biol. Biochem.
(1995) - et al.
Soil organic matter dynamics in tiger bush (Niamey, Niger)—Preliminary results
Acta Oecol.
(1999) - et al.
The chemical structure of highly aromatic humic acids in three volcanic ash soils as determined by dipolar dephasing NMR studies
Geochim. Cosmochim. Acta
(1989)