Elemental and isotopic compositions of organic carbon and nitrogen of recently deposited organic matter in Empakai crater and its implication for climatic changes in northern Tanzania
A 29 cm long core recovered from a water depth of 5 m in a small closed lake located in the Empakai crater northern Tanzania, is used to document the contents of organic carbon and nitrogen, stable isotopes composition of organic carbon and nitrogen, and C/N ratios and to infer climatic changes from these data. The δ 13C and δ15N values for this core, which averages -16.5±2.1l and 11.0±5.6l respectively, show a general downcore decrease below 5 cm. However, within this general trend, two peaks enriched in 13C are observable at about 8 cm and 13 cm. The peak at about 13 cm corresponds also to a zone depleted in 15N values. The general downcore trend can be attributed to systematic changes in relative proportion between C3 and C4 types of organic matter (OM), resulting from climatic changes or nutrient changes and shift between algae and higher plants. The lower most section containing the most depleted values can be attributed either to high input of autochthonous material owing to wetter/humid conditions or high contribution of C3 high land plant materials in response to increase in forest cover. The general downcore decrease in 13C values can also be interpreted to indicate that precipitation and vegetation cover in the area has been declining over time, and occasionally drought has been intense particularly during two zones that are characterized by enrichment in 13C. The upper 5 cm is characterized by near constant values of 13C and 15N with isotope values as high as -13.15‰ and 22.15‰ respectively. High enrichment in 15N in the upper 5 cm could be attributed to denitrification and ammonium volatilization processes, while enrichment in 13C in the upper 5 cm could be attributed to high proportion of C4 type of OM and/or utilization of HCO3- by phytoplankton during synthesis of OM. This site is also characterized by high concentrations of organic carbon (averaging 7.8±2.0%) and nitrogen (averaging 0.8±0.3%). A smell of hydrogen sulfide detected in water samples collected at approximately 20 m, suggest that there is enhanced preservation of OM in the lake under anoxic condition. The C/N ratios values, which average 12.2±2.3, increase downcore to the base of the core owing to preferential loss of nitrogen containing compounds during diagenesis.
Tanzania Journal of Science Vol. 30 (2) 2004: pp. 87-96