Chemical composition of modern and fossil hippopotamid teeth and implications for paleoenvironmental reconstructions and enamel formation – Part 2: Alkaline earth elements as tracers of watershed hydrochemistry and provenance

[摘要] This study demonstrates that alkaline earth elements in enamel ofhippopotamids, in particular Ba and Sr, are tracers for water provenance andhydrochemistry in terrestrial settings. The studied specimens are permanentpremolar and molar teeth found in modern and fossil lacustrine sediments ofthe Western Branch of the East African Rift system (Lake Kikorongo, LakeAlbert, and Lake Malawi) and from modern fluvial environments of the NileRiver.

Concentrations in enamel vary by two orders of magnitude for Ba (120–9336 μg g⁻¹)as well as for Sr (9–2150 μg g⁻¹). The variations arepartially induced during post-mortem alteration and during amelogenesis, butthe major contribution originates ultimately from the variable waterchemistry in the habitats of the hippopotamids which is controlled by thelithologies and weathering processes in the watershed areas. Amelogenesiscauses a distinct distribution of MgO, Ba and Sr in modern and fossilenamel, in that element concentrations increase along profiles from theouter rim towards the enamel–dentin junction by a factor of 1.3–1.9. Theseelements are well correlated in single specimens, thus suggesting that theirdistribution is determined by a common, single process, which can bedescribed by closed system Rayleigh crystallization of bioapatite in vivo.

Enamel from most hippopotamid specimens has Sr/Ca and Ba/Ca which aretypical for herbivores. However, Ba/Sr ranges from 0.1 to 3 and varies onspatial and temporal scales. Thus, Sr concentrations and Ba/Sr in enameldifferentiate between habitats having basaltic mantle rocks or Archeancrustal rocks as the ultimate sources of Sr and Ba. This provenance signalis modulated by climate change. In Miocene to Pleistocene enamel from theLake Albert region, Ba/Sr decreases systematically with time from 2 to 0.5.This trend can be correlated with changes in climate from humid to arid, invegetation from C₃ to C₄ biomass as well as with increasingevaporation of the lake water. The most plausible explanation is that Bamobility decreased with increasing aridification due to preferentialdeposition with clay and Fe-oxide-hydroxide or barite on the watershed ofLake Albert.

[学科分类] 地球化学与岩石