Plant Wax Biomarkers in Fluvial-Lacustrine Sediments from the Omo-Turkana and Awash Basins in Eastern Africa
Abstract
Stable isotope analysis of terrestrial plant biomarkers has emerged over the past decade as a powerful tool for reconstructing vegetation and hydroclimate from lacustrine and marine sediments in eastern Africa. Presently, no comprehensive biomarker records exist from late Miocene to late Pleistocene terrestrial sediments in eastern Africa. Fluvial-lacustrine deposits in the Omo-Turkana and Awash Basins located in Kenya and Ethiopia host a diverse faunal record that is rich in hominin taxa. Biomarker records from these deposits could provide continuous, high-resolution terrestrial paleoclimate records that would enable direct comparison of climate and biotic change through time, fill gaps in the discontinuous isotopic record from paleosol carbonates, and compliment existing biomarkers records from marine sediments. The heterogeneity of fluvial-lacustrine deposits compared to marine and continuous lacustrine deposits requires rigorous assessment of biomarker preservation and abundance prior to isotopic analysis. We analyzed fluvio-lacustrine sediments from the Sagantole, Hadar, and Busidima Fms. in the Awash Basin and the Shungura and Nachukui Fms. in the Omo-Turkana Basin to assess the feasibility of generating continuous isotopic records of vegetation and hydroclimate from these basins. We measured the distribution, abundance, and where possible, preliminary δD and δ13C values of n-alkanes and n-alkanoic acids extracted from sediments. The carbon preference index (CPI) serves as an indicator for biomarker preservation. The CPI for long-chain n-alkane and n-alkanoic acids is generally >2, suggesting suitable biomarker preservation in most cases. Abundances vary significantly in both n-alkanes and n-alkanoic acids, with higher concentrations in lacustrine versus fluvial samples. About two-thirds of the samples have concentrations sufficient for δD analysis given sample size (<200 g) and current analytical requirements, whereas δ13C analysis is feasible for a majority of samples because of the smaller sample requirements. Preliminary results show a range of ca. 95‰ in δD values. In many samples, a large unresolved complex mixture (UCM) is present in the aliphatic fraction. The UCM co-elutes with short and mid-chain n-alkanes and can interfere with isotopic measurement of the long chain n-alkanes unless removed via urea adduction or molecular sieve. A promising application of paleosol biomarkers is combining biomarker δD values with pedogenic carbonate clumped isotope paleothermometry and δ18O values from the same sample or within the same soil unit. We are exploring how δD, δ18O, and clumped isotope data from these isotope systems can be used to evaluate aridity, changes in moisture source, or changes in precipitation in past environments. Overall, we find fluvial-lacustrine deposits are promising biomarker archives, but caution that careful screening and assessment is required.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFMPP34B..05U
- Keywords:
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- 0473 BIOGEOSCIENCES Paleoclimatology and paleoceanography;
- 0486 BIOGEOSCIENCES Soils/pedology;
- 1041 GEOCHEMISTRY Stable isotope geochemistry;
- 1055 GEOCHEMISTRY Organic and biogenic geochemistry