Isotopic compositions of tropical East African flora and their potential as source indicators of organic matter in coastal marine sediments

The C and N stable isotope compositions of some flora of East Africa from c oastal Tanzania and Amboseli National Park (Kenya) are used to assess if th ey can be used as a terrestrial end member during the estimation of terrest rial fraction in coastal marine sediments. The results of C isotope composi tion of various tree leaves, which average -29.3 +/- 1.4 parts per thousand , indicate that these tropical higher land plant species follow a Calvin-Be nson or non-Kranz (C-3) type of metabolism. The results for grass species, which average -13.2 +/- 2.4 parts per thousand, indicate that most of them follow a Hatch-Slack or Kranz (C-4) type of metabolism. However, some of th e succulent plants from the Amboseli National Park have delta(13)C values t hat average -14.7 parts per thousand, an indication that they follow a CAM (Crassulacean Acid Metabolism) type of metabolism. The N isotope values are relatively higher than expected for the terrestrial organic material. The average delta(15)N values for both tree and grass samples are higher than 5 parts per thousand and fall within the range normally considered to be mar ine. The high enrichment in N-15 may be related to the environmental condit ions in which plants thrive. Plants growing in sandy, dry and overgrazed en vironments are expected to be enriched in N-15 owing to full utilisation of all available N species, regardless of their isotopic compositions. Other processes which may cause an enrichment in N-15 include adsorption by vario us types of clay minerals, supply of N-15-enriched nitrate through sea-spra y, and local denitrification, especially in swampy and lake margins where t he input of organic matter may be higher than the rate of decomposition. The stable isotopic compostion of organic C and N for surficial organic mat ter for the coastal marine sediments averages -17.0 +/- 0.9 parts per thous and and 5.4 +/- 1.1 parts per thousand, respectively. These values indicate a substantial contribution of C, plants and sea grasses. However, contribu tion of C, relative to that of sea grasses can not be evaluated owing to th e fact that there is no significant difference in the isotopic compositions between the two groups. In the savannah environment, where a contribution from the C, types of plan ts might be substantial, the delta(13)C value for a terrestrial end member needs to be established prior to evaluation of the terrestrially derived or ganic matter in the marine environment. Owing to a significant contribution of sea grasses to the total organic matter preserved in coastal marine sed iments, the stable isotopes of organic C seem to have a limited applicabili ty as source indicators in the East African coastal waters. Furthermore, th e results indicate that N stable isotopes seem to have a limited applicabil ity as source indicators in coastal waters of East Africa. However, more wo rk needs to be conducted to determine the terrestrial and sea grass end mem ber values for the coastal areas. (C) 1999 Elsevier Science Limited. All ri ghts reserved.