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40264-AC2
Carbon and Phosphorous Cycling at the Sediment-Water Boundary in a Deep Lacustrine Basin
Phosphorus has the potential to be a limiting or co-limiting nutrient for primary production in lacustrine systems. Because of its status as a potentially important nutrient for plant growth, changes in the causes that control phosphorus burial could influence other chemical cycles. The initial focus of our work was to examine the factors that might influence phosphorus burial within a deep lacustrine basin, Lake Malawi. Our work is being extended to include Lake Tanganyika, which lies to the north of Malawi. These lakes lie within the East African Rift. Lakes Tanganyika and Malawi, for example, contain sediment with ages up to 5 - 15 million years (Johnson and Odada, 1996). With a volume of 8,400 km3, area of 28,800 km2, and maximum depth of 706 m, Lake Malawi is the fourth largest freshwater lake in the world by volume (Herdendorf, 1982; Hecky et al., 1996). Interplay among the lake's physical, chemical, and biological dynamics result in a chemical stratification at approximately 200 - 250 m water depth, above which sediments are bathed in oxygenated waters and below which sediments are exposed to anoxic water (Eccles, 1974; Gonfiantini et al., 1979; Brown et al., 2000). This stratification may lead to redox-sensitive trace metal signatures within the sediment (Brown et al., 2000). Much like Lake Malawi, Lake Tanganyika is chemically stratified at roughly 200 m water depth (Edmond et al., 1993).
The samples used for this study cover a range of reducing character as implied by the relative abundance of metals that are sensitive to the presence or absence of oxygen (e.g., uranium). The Lake Malawi sites, which are intended to constitute a transect through the Lake Malawi oxygen gradient were chosen for phosphorus and trace metal analyses. Phosphorus concentrations at all sites generally show elevated concentrations in the upper 11 cm and decrease with depth. The two sites having the highest phosphorus concentrations appear to be under the influence of less reducing conditions at the surface as compared to two other sites. This interpretation is based on the generally lower sedimentary uranium enrichments at the sites having higher phosphorus concentrations. At the site having the highest average phosphorus enrichment in the upper 11 cm of sediment, this upper layer appears to have lower average carbon to phosphorus ratios than the other sites. This observation of comparatively high phosphorus concentrations, and relatively low carbon to phosphorus ratios may suggest an enhancement of phosphorus burial, relative to carbon under less reducing conditions. However, despite there being some indication that there are lower phosphorus concentrations and higher carbon to phosphorus ratios with increasing abundance of metals associated with reducing conditions, this relationship is not uniform or straightforward within the data set, implying that other factors aside from sedimentary reducing conditions may be influencing sedimentary phosphorus, carbon, or trace element burial. There are multiple possible interpretations of surface enrichments and their decreases with depth, including a change in the lake's phosphorus budget (e.g., Heckey et al., 2003), a recent authigenic P enrichment mechanism, or another diagenetic process. We are currently extending our project to include Lake Tanganyika, and this work will include analyses of trace metal stable isotopes. Like Malawi, Tanganyika is chemically stratified and sampling this chemical gradient allows for an improved understanding of the relationship between aquatic system reducing character and element burial.
As part of this project, a postdoctoral associate joined the PI's research team. In addition to our work on phosphorus, this postdoctoral associate also introduced into our laboratory a technique for analyzing molybdenum isotopes in sediments. Our initial work suggests that molybdenum isotopes may be at least partially sensitive to the reducing character of sedimentary systems, and we intend to employ this technique on our samples from Lake Tanganyika. This postdoctoral associate also developed a technique for examining the Ge isotope composition of terrestrial high-temperature fluids. Both the molybdenum and germanium work resulted in published manuscripts acknowledging partial support from this project.
References:
Table of Contents Map showing location of Lake Malawi is modified from Wikipedia, which references the CIA's World Factbook.
Brown, E.T., et al., 2000. Geochim. Cosmochim. Acta 64: 3515-3523.
Eccles, D. 1974 Limnol. Oceanogr. 19: 730-742
Edmond, J.M., et al., 1993. Limnol. Oceanogr. 38: 725-738.
Gonfiantini, R., et a., 1979. In: C.H. Mortimer (ed.). Isotopes in Lake Studies. International Atomic Energy Commission, pp. 195-207.
Hecky, R.E., et al., 1996. In: T.C. Johnson and E. Odada (eds.). The Limnology, Climatology, and Paleoclimatology of the East African Lakes. Gordon and Breach Publishers, Amsterdam, pp. 205-224.
Hecky, R.E., et al., 2003. J. Great Lakes Res. 29(sup2):139-158.
Herdendorf, C.E. 1982. J. Great Lakes Res. 8: 379-412.
Johnson, T.C. and E.O. Odada, eds, 1996. The limnology, climatology and paleoclimatology of the East African lakes. Gordon and Breach Pubs. Australia.
