PHOSPHORUS GEOCHEMISTRY AND C:P RELATIONSHIPS IN PALEO LAKE MABABE, BOTSWANA

Phosphorus is an important nutrient element that likely limits ocean primary productivity on geologic time scales. For this reason, it is important to understand factors that influence the delivery of P to the oceans, such as weathering of P-rich minerals, such as apatite, and P retention in soil and terrestrial aquatic environments including landscape development and watershed changes. Sediment samples were collected from the Okavango Rift Zone at two different locations (MabX and Mab6) in the now dry paleo Lake Mababe, Botswana. Organic carbon data (Gamrod et al., 2008) suggests discrete intervals of enhanced organic matter burial possibly related to dry intervals within the watershed, which may have enhanced internal lake processes that became dominant over fluxes from the landscape to Lake Mababe. What is unclear is whether this enhanced organic matter burial is related to increased production or increased preservation within the lake.

Detailed phosphorus geochemistry, using a sequential extraction technique (SEDEX), helps to evaluate terrestrial P cycling and data may be able to help differentiate between internal lake processes and external sediment supply related to watershed processes. SEDEX isolates different sedimentary components commonly found in sediments: occluded P (oxide –associated and adsorbed), mineral P, and organic P. In addition, Lake Mababe sediments contain high concentrations of diatoms. For this reason, an additional step has been added to the SEDEX reaction scheme to evaluate diatom-associated phosphorus, following Latimer et al. (2006). The determination of diatom-associated phosphorus is still in progress. Comparisons between phosphorus and carbon geochemistry as well as determinations of biogenic silica will help to evaluate the biogeochemical cycling of nutrients and differentiate between watershed and internal lake processes that led to the burial of organic-rich sediments within paleo Lake Mababe.