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Title:
Chemical and Isotope Compositions of Neogene Hippopotamidae Teeth From Lake Albert (Uganda): Implications for Environmental Change
Authors:
Brugmann, G. E.; Brachert, T. C.; Ssemmanda, I.; Mertz, D. F.
Affiliation:
AA(Institute of Geosciences Johannes Gutenberg University, Johann-J. Becher-Weg, Mainz, D55099, Germany ; ), AB(Institute for Geophysics and Geology University of Leipzig, Talstr. 36, Leipzig, D04103, Germany ; ), AC(Geology Department Makerere University, P.O. Box, Kampala, 7062, Uganda ; ), AD(Institute of Geosciences Johannes Gutenberg University, Johann-J. Becher-Weg, Mainz, D55099, Germany ; )
Publication:
American Geophysical Union, Fall Meeting 2008, abstract #PP11D-01
Publication Date:
12/2008
Origin:
AGU
AGU Keywords:
1040 Radiogenic isotope geochemistry, 1041 Stable isotope geochemistry (0454, 4870), 1637 Regional climate change, 4924 Geochemical tracers
Abstract Copyright:
(c) 2008: American Geophysical Union
Bibliographic Code:
2008AGUFMPP11D..01B

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 (Lake Albert, Uganda) during the Late Neogene, we use proxy data (trace elements, O, C and Sr isotopes) from Hippopotamidae teeth. Laser ablation ICPMS profiles in enamel measured from the outside rim towards the dentin show an asymmetric trace element distribution in that the concentrations continuously decrease by up to 5 orders of magnitude within a distance of about 1 mm until a minimum is reached (<10 ppb for REE, Y, U). From thereon concentrations stay rather constant or even increase until about 100 μm in front of the dentin where concentrations rise sharply. This concentration minimum represents the least altered part of the enamel and it probably represents a primary biological fingerprint which has the potential to monitor migration pathways and palaeoenvironmental changes. On geological time scales δ13C compositions reflect a transition from pure C3 browsers (-11 per mil PDB) at 5 to 6 Ma towards C4 dominated grazers (0 per mil PDB) at 2.0 to 2.5 Ma. The oxygen stable isotope (δ18O) composition of enamel rises from 26 per mil at 5 to 6 Ma to a maximum of 32 per mil SMOW at 2.3 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 reach of Hippopotamid populations. The Sr isotopic composition of enamel displays a large variation and 87Sr/86Sr 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. Thus, Sr and O isotopic compositions correlate with each other on the geological time scale. This is plausible if the Sr isotopic composition of Hippopotamid enamel 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 87Sr/86Sr (~ 0.704), whereas Proterozoic-Achaean terrains of the rift flanks have high 87Sr/86Sr (>0.718). Thus, the increasing 87Sr/86Sr from 5.2 to 2.3 Ma, suggests that water supply from volcanic terrains ceases and the local, Achaean run-off dominated the lake water chemistry. Consistent with the concurrent increase of ?18O, 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 87Sr/86Sr 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 on geological time scales. investigation.
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