11 Paleolimnological Reconstructions: New Insights from the Tried and True to the Unique and New (Posters)

Thursday, 31 March 2016
8:00 AM - 5:30 PM
Exhibit Hall

A COMPARATIVE ANALYSIS OF AUTHIGENIC CLAYS AND BULK GEOCHEMISTRY: OLDUVAI GORGE, TANZANIA

Paper number: 11-7
Program: Theme Sessions
Time: 8:00 AM – 5:30 PM

Authors

Abstract

Paleoenvironmental reconstructions are critical to fully understanding paleoclimate change and hominin evolution. Authigenic clays have been identified as a potential proxy for paleoclimate change because Mg-rich clays form in saline and alkaline conditions. With these minerals as a proxy, we can identify fluctuations between saline and freshwater conditions in East African paleolake basins. Authigenic clays formed in the Olduvai Basin during the late Pleistocene and early Pliocene. Between ~1.9 -1.7 ma, Olduvai was a closed-hydrologic basin into which Al- and Fe- rich detrital clays were transported from surrounding soils. Mg-rich smectite formed within the basin at times due to chemical alteration in alkaline and saline conditions. In this study, 40 samples were chosen from Locality 80 in the center of the paleolake and analysed to characterize the geochemical facies of the fine-grained authigenic material.

Submicron clay fraction XRD patterns were collected by progressive centrifugation. In addition to oriented (air-dried and glycolated) clay slides, randomly oriented clay fractions were used to investigate their octahedral composition based on the position of the 060 peaks. Such peaks indicate a predominant trioctahedral composition of the clay (i.e. Mg-rich clay).

These samples were pressed into KBr pellets and FTIR analyses were conducted. Observations suggest that the IR patterns (e.g. peaks at wavenumbers 816 and 654 cm-1) are strongly related to the amounts of dioctahedral and trioctahedral smectite. Electron microprobe data were collected for further geochemical characterization.

Bulk geochemistry (e.g. MgO/(Al2O3+Fe2O3)) correlates well with the position of the 060 XRD peaks, suggesting that the bulk composition is controlled by authigenic material. Variations in both XRD and FTIR also show fluctuating Mg- and Al- enrichment within the authigenic clays, suggesting the geochemical facies of the clays may reflect changes in the paleoenvironment. When plotted on a time-series chart for the locality, electron microprobe analyses of the authigenic clays correspond very well to the variations in previously collected bulk XRF geochemistry. Therefore, authigenic clays have the potential to be used as proxies for paleoenvironmental identification.

Geological Society of America Abstracts with Programs. Vol. 48, No. 3

doi: 10.1130/abs/2016SE-273661

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