GEOCHEMICAL CHARACTERIZATION OF SOME LAKE MALAWI TEPHRA LAYERS

The Rungwe Volcanic Province (RVP) is located in the East African Rift (EAR) with initial volcanic activity beginning between 19 and 17 Ma, and the main phase of volcanism occurring over the past 9 Ma. Lake Malawi is located at the southern end of the EAR and its northern tip is about 40 km to the southeast of the southernmost volcano of the RVP (Keijo Volcano). Lake Malawi is estimated to have formed more than 7 Ma and has recorded some of the regional volcanic activity through its many volcanic ash layers preserved in sediments, at least over the past ~100,000 years. In 2005, the Lake Malawi Drilling Project recovered sediment cores from two sites: one in the northern basin (site MAL05-2 at 9°58'56.60"S, 34°11'9.17"E) and one in the central basin (site MAL05-1 at 11°17'39.60"S, 34°26'9.00"E). Ten tephra layers from site MAL05-2A were analyzed using single grain glass analysis for the abundance of major and trace elements. The geochemical data are used to determine whether these individual layers can be distinguished based on chemical composition. In addition, three tephra layers from nearby site MAL05-2B and one tephra layer from site MAL05-1C in the central basin were analyzed and correlated with tephra layers found in site MAL05-2A. These layers range in age from about 40 to 75 ka.

Samples were analyzed for chemical composition using both wavelength-dispersive spectrometry on an electron microprobe (WDS-EMP) and energy-dispersive spectrometry on a scanning electron microscope (EDS-SEM). The geochemical data obtained from these methods show that significant chemical differences exist among at least some of the tephra layers and thus these are able to be uniquely characterized. These unique characteristics allow these layers to be correlated between cores creating isochronous stratigraphic markers, improving the age precision of Lake Malawi cores.