Paleoclimate reconstruction for the East African Rift from geochemical studies of mammalian teeth
Final Report Abstract
The Neogene was a period of long-term global cooling and increasing climatic variability on astronomical time scales. Lake systems strongly depend on rainfall patterns and size or geographical distribution of river networks. To unravel environmental change and watershed dynamics in the western branch of the East African Rift System (EARS) during the Late Neogene, we use proxy data (δ18 O, δ 13 C, trace elements, radiogenic Sr and Pb isotope ratios) of Hippopotamidae teeth sampled from the Lake Albert region, Uganda. In cases where precise age constraints are required, we started to numerically calibrate fossils or sediments via U+Th/He dating on goethitic ooids from stratigraphically relevant iron stone layers. Additionally, there are ongoing 4 He/3 He thermochronometrical experiments in order to gain information on the timetemperature evolution of samples and their basin environments. The trace element distribution in fossil teeth indicates that diagenetic overprint has to be considered in dentine and enamel. Dentin has high but homogeneous concentrations of REE, Y, Sr, U, Ba, Pb (>10 times PAAS) and appears to be equilibrated with surrounding pore waters. The outer enamel is enriched to different degrees in many trace elements, but towards the dentine they reach a plateau where most concentrations vary between less than 1 to 10 ppb. The trace element distribution along this plateau probably represents a primary biological fingerprint which has the potential to monitor migration pathways and palaeoenvironmental changes. The Sr concentration in enamel varies between 100 and 4000 ppm among different molar teeth. Typically, the SrO/CaO ratio systematically increases from the outer rim towards the dentin. This is considered to reflect the maturation process which follows the apposition of bioapatite forming enamel. Therefore, Sr concentration and particularly Sr isotope ratios in enamel from Hippopotamidae teeth represent a suitable proxy for local and regional water chemistry and vegetation. Stable carbon isotope (δ 13 C) compositions of enamel from single teeth are compositionally uniform, which may indicate that no dietary changes took place during the course of a year. In contrast, stable oxygen isotopes exhibit pronounced intra-annual variability (3 ‰) as a response to changes in ingested water compositions. On geological time scales δ 13 C compositions reflect a transition from pure C3 browsers (-11 ‰ PDB) at 5 to 6 Ma towards C4 dominated grazers (0 ‰ PDB) at 2.0 to 2.5 Ma. Average oxygen stable isotope (δ18 O) compositions rise from 26 ‰ at 5 to 6 Ma to a maximum of 32 ‰ SMOW at 2.0 to 2.5 Ma. Increasing δ18O values suggest enhanced evaporation of the lake due to rising aridity. This is in agreement with a synchronous spread of C4 vegetation in the reac h of Hippopotamid populations. Synchronous increase in C4 diet has been observed in hippopotamuses from Turkana Basin (eastern EARS). These results underline suggestions made previously that Hippopotamid feeding strategy is indeed more opportunistic than inferred from ethological observation. The Sr isotopic composition of enamel displays a large variation over geological time. The 87 Sr/86 Sr ratio is 0.714 about 5 Ma ago, reaches a maximum of 0.717 at about 2.3 Ma and decreases from there on to about 0.708. Sr and O isotopic compositions correlate with each other on the geological time scale plausibly explainable by the following circumstances: The Sr isotopic composition dominantly reflects the changes of the water chemistry of the lake, and is therefore a powerful tool for tracing ancient hydrological networks. The large variation of the Sr isotope composition can be explained if the lake is fed by different sources: water draining Cenozoic volcanic terrains have low 87 Sr/86 Sr (~ 0.704), whereas Proterozoic-Archaean terrains of the rift flanks have high 87 Sr/86 Sr (> 0.718). Thus, the increasing 87 Sr/86 Sr from about 5.2 to 2.3 Ma, suggests that water supply from volcanic terrains ceases (Virunga area or the Easter catchment of the rift) and the local, Archaean run-off dominated the lake water chemistry. Consistent with the concurrent increase of δ18O, this suggests that increasing aridity and evaporation of lake water on a regional scale, interrupts the axial river network and local river discharge becomes dominant. The decrease of 87 Sr/86 Sr starting at about 2 Ma indicates new water supply from volcanic rock dominated terrains, which could reflect a tectonic restructuring of the rift valley or the initiation of the young Toro-Ankole igneous province. Palaeoclimate records from rift systems are governed by global climate forcing mechanisms and interacting geodynamics. Our study of the chemical and isotope record of tooth enamel from mammals permits the identification of these local and global environmental changes, in the western EARS, covering annual to geological time scales.
Publications
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(2006) Paleoclimate reconstruction for the East African Rift from geochemical studies of mammalian teeth.- Internal RIFTLINK Meeting, Darmstadt
Brachert, T.C.
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(2007) First insights into climate impact on faunal composition and distribution of molluscan and mammalian communities in the East African Rift.- EAAPP conference, Nairobi
Melcher, M., Schultheiß, M., Brachert, T.C., Albrecht, C. Kullmer, O., Schrenk, F.
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(2007) Neogene high-resolution climate records from mammalian teeth: potential and limitations.- Internal RIFTLINK meeting, Frankfurt
Brachert, T.C., Melcher, M., Kullmer, O., Schrenk, F.
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(2007) Paleoclimate reconstruction for the East African Rift from geochemical studies of mammalian teeth.- EGU Conference, Vienna
Brachert, T.C., Jacob, D.E., Kullmer, O., Mertz, D.F., Schrenk, F., Ssemmanda, I.
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(2007) Stable isotope variability in Hippopotamid molar teeth from the Ugandan Late Neogene,- Geologische Vereinigung, Jahrestagung
Brachert, T.C., Jacob, D.E., Kullmer, O., Mertz, D.F., Schrenk, F., Ssemmanda, I.
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(2007): Umweltdynamik im späten Miozän: Möglichketen und Limitierungen der Rekonstruktion.- Institut für Geophysik und Geologie, Universität Leipzig, 10.07.2007
Brachert, T.C.
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(2008) Chemical and isotope compositions of Neogene Hippopotamidae teeth from Lake Albert (Uganda): Implications for environmental change.- RIFTLINK Conference, Neustadt/W.
Brügmann, G.
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(2008) Chemical Composition of Hippopotamid Molar Teeth from the Ugandan Late Neogene.- Internal RIFTLINK meeting, Darmstadt
Brügmann, G.
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(2008) Neogene high-resolution climate records from mammalian teeth: potential and limitations.- Institut für Geowissenschaften (Prof. Dr. M. Strecker), 21.2.2008, Universität Potsdam
Brachert, T.C.
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(2008) Sub-annual stable isotope variability of mammalian tooth enamel: Low-crowned hippopotamid molars.- RIFTLINK Conference, Neustadt/W.
Brachert, T.C., Brügmann, G., Jacob, D.E., Kullmer, O., Mertz, D.F., Schrenk, F., Ssemmanda, I.
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(2008): Stable and radiogenic isotope ratios as tracer for paleoenvironmental reconstructions: Results from the Late Neogene of the Albertine Rift, Uganda.- Berkeley Geochronology Center & UC Berkeley, 11.06.2008
Mertz, D.F.
