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Bioapatite in mammalian teeth is readily preserved in continental sedimentsand represents a very important archive for reconstructions of environmentand climate evolution. This project provides a comprehensive data base ofmajor, minor and trace element and isotope tracers for tooth apatite using avariety of microanalytical techniques. The aim is to identify specificsedimentary environments and to improve our understanding on the interactionbetween internal metabolic processes during tooth formation and externalnutritional control and secondary alteration effects. Here, we use theelectron microprobe to determine the major and minor element contents offossil and modern molar enamel, cement and dentin from Hippopotamids. Mostof the studied specimens are from different ecosystems in Eastern Africa,representing modern and fossil lacustrine (Lake Kikorongo, Lake Albert, andLake Malawi) and modern fluvial environments of the Nile River system.Secondary alteration effects - in particular FeO, MnO, SO3 and Fconcentrations – are 2 to 10 times higher in fossil than in modern enamel;the secondary enrichment of these components in fossil dentin and cement iseven higher. In modern and fossil enamel, along sections perpendicular tothe enamel-dentin junction (EDJ) or along cervix-apex profiles,P2O5 and CaO contents and the CaO/P2O5 ratios are veryconstant (StdDev ∼1%). Linear regression analysis reveals tightcontrol of the MgO (R2∼0.6), Na2O and Cl variation (for bothR2>0.84) along EDJ-outer enamel rim profiles, despite largeconcentration variations (40% to 300%) across the enamel. These minorelements show well defined distribution patterns in enamel, similar in allspecimens regardless of their age and origin, as the concentration of MgOand Na2O decrease from the enamel-dentin junction (EDJ) towards theouter rim, whereas Cl displays the opposite trend.Fossil enamel from Hippopotamids which lived in the saline Lake Kikorongohave a much higher MgO/Na2O ratio (∼1.11) than those from theNeogene fossils of Lake Albert (MgO/Na2O∼0.4), which was a largefresh water lake like those in the western Branch of the East African RiftSystem today. Similarly, the MgO/Na2O ratio in modern enamel from theWhite Nile River (∼0.36), which has a Precambrian catchment ofdominantly granites and gneisses and passes through several saline zones, ishigher than that from the Blue Nile River, whose catchment is the Neogenevolcanic Ethiopian Highland (MgO/Na2O∼0.22). Thus, particularlyMgO/Na2O might be a sensitive fingerprint for environments where riverand lake water have suffered strong evaporation.Enamel formation in mammals takes place at successive mineralization frontswithin a confined chamber where ion and molecule transport is controlled bythe surrounding enamel organ. During the secretion and maturation phases theepithelium generates different fluid composition, which in principle, shoulddetermine the final composition of enamel apatite. This is supported byco-linear relationships between MgO, Cl and Na2O which can beinterpreted as binary mixing lines. However, if maturation starts aftersecretion is completed, the observed element distribution can only beexplained by equilibration of existing and addition of new apatite duringmaturation. It appears the initial enamel crystallites precipitating duringsecretion and the newly formed bioapatite crystals during maturationequilibrate with a continuously evolving fluid. During crystallization ofbioapatite the enamel fluid becomes continuously depleted in MgO andNa2O, but enriched in Cl which results in the formation of MgO, andNa2O-rich, but Cl-poor bioapatite near the EDJ and MgO- andNa2O-poor, but Cl-rich bioapatite at the outer enamel rim.The linkage between lake and river water compositions, bioavailability ofelements for plants, animal nutrition and tooth formation is complex andmultifaceted. The quality and limits of the MgO/Na2O and other proxieshave to be established with systematic investigations relating chemicaldistribution patterns to sedimentary environment and to growth structuresdeveloping as secretion and maturation proceed during tooth formation

biogeosciences

Chemical composition of modern and fossil Hippopotamid teeth and implications for paleoenvironmental reconstructions and enamel formation – Part 1: Major and minor element variation