Stable isotope compositions of organic carbon and contents of organic carbon and nitrogen of lacustrine sediments from sub-arid northern Tanzania

Abstract

The stable isotope compositions of organic carbon (OC), and contents of OC and nitrogen for four sediment cores recovered from lakes Makat (located in the Ngorongoro Crater), Ndutu and Masek (located in the Serengeti Plains) are used to document sources of organic matter (OM) and climatic changes in sub-arid northern Tanzania during the late Pleistocene-Holocene period. Accelerate mass spectrometer (AMS) 14C ages on total OM for sediments collected from the Ngorongoro Crater Lake indicate that the sedimentation rate is approximately 17 cm/ka. The 𗉝C values from the 20 cm long core (short core) show a downcore increase, whereas that of 500 cm long core (long core), show two peaks enriched in 13C and three peaks depleted in 13C. A general downcore increase in the 13C values for the short core suggests changes in the relative proportion of C3 and C4 with C4 fraction increasing downcore. Similarly, low and high peaks in the long core suggest changes in the relative proportion of C3 and C4 with low values having high proportion of C3 type of material, probably indicating changes in precipitation and lake levels in the area. Deposition of OM depleted in 13C took place during periods of high precipitation and high lake levels. Although high content of OC and nitrogen in some core sections are associated with elevated C/N ratio values, diagenetic alteration of isotope signature is unlikely to have caused OC isotope enrichment in sections having high contents of OC and nitrogen. The OC isotope record from Lake Ndutu shows a general downcore decrease in 𗉝C values and contents of OC and nitrogen. Furthermore, this general trend is punctuated by a period of above normal enrichment in 13C, with 𗉝C values being as high as -6‰. A downcore decrease in 𗉝C values and contents of OC and nitrogen could be attributed either to climatic variability or diagenetic effect. Moreover, a peak enriched in 13C probably indicates a period of reduced precipitation, where phytoplankton used dissolved bicarbonate rather than dissolved atmospheric CO2. Lake Mesak record shows two peaks depleted in 13C that are flanked by near constant values of about -20‰. These two peaks (depleted in 13C), which most likely have high proportion of C3 type of OM, are associated with low contents of OC and nitrogen. Generally, the contents of OC and nitrogen for this lake are lowest relative to other lakes.

Tanzania Journal of Science Vol. 32 (1) 2006: pp. 61-74