Oxygen Isotopes in Authigenic Clay Minerals: Toward Building a Reliable Salinity Proxy

D Gebregiorgis,J S Delaney,A Simpson,G M Ashley,D M Deocampo,J Cuadros,E J Beverly,F J Longstaffe

doi:10.1029/2019gl085576

Abstract

AbstractMost clay minerals in sedimentary environments have traditionally been considered to be of detrital origin, but under certain conditions, authigenic clay minerals can form at low temperature through the transformation of precursor clays or as direct precipitates from lake water. Such clay minerals can hold important information about the prevailing climatic conditions during the time of deposition. We present the first quantitative reconstruction of salinity in paleolake Olduvai based on the oxygen‐isotope composition of authigenic clay minerals. We provide a framework illustrating that the isotopic signature of authigenic lacustrine clay minerals is related to the isotopic composition of paleo‐waters, and hence to paleosalinity. This new paleosalinity proxy shows that the early Pleistocene East African monsoon was driven by combinations of precession and obliquity forcing and subsequent changes in tropical sea surface temperatures. Such quantitative lacustrine paleosalinity estimates provide a new direction of research for modeling ecosystem change based on an ecologically relevant parameter.

Highlights

Understanding past climate variability is prerequisite to better predictive climate models for simulating past changes in Earth's climate
We show that the weakening or strengthening of the early Pleistocene East African monsoon was related to changes in the Earth's orbit
Significant disparities are found among closely proximal East African Rift valley lakes in the timing and magnitude of response to well‐ known past global climate events such as cooling at the Last Glacial Maximum and warming in the Mid‐ Holocene (Chevalier et al, 2017)

Summary

Introduction

Understanding past climate variability is prerequisite to better predictive climate models for simulating past changes in Earth's climate. Significant disparities are found among closely proximal East African Rift valley lakes in the timing and magnitude of response to well‐ known past global climate events such as cooling at the Last Glacial Maximum and warming in the Mid‐ Holocene (Chevalier et al, 2017). This challenge is exacerbated by the lack of robust paleohydrological proxies from continental basins that quantitatively record past climate change. New proxies that overcome such challenges are needed to improve our ability to accurately simulate past climates and thereby improve future climate prediction

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