Abstract

Supergene goethites from the Hamersley region of Western Australia were analyzed for δ18O and the results combined with both new and previously published (U-Th)/He ages to assess the paleoenvironmental signal recorded by surficial goethite in a long-term continental archive. δ18Ogoe values increase monotonically by ∼3‰ between 71 Ma and 5 Ma and appear little affected by changes in temperature or other characteristics of depositional environment. The temporal trend likely reflects the isotopic composition of meteoric water from which the goethite formed, which varied as Australia migrated ∼3000 km northward over the Cenozoic. Translating age to paleolatitude reveals that the δ18Ogoe data are consistent with a latitudinal gradient in the δ18Oprecip that, throughout the Cenozoic, remained largely unchanged from the modern gradient. There is a 5‰ offset between the latitudinal trends of δ18Ogoe and modern precipitation, approximately as expected from the water-goethite fractionation factor. Temporal variation in the δ18O of local meteoric water arising from changes in moisture source or pathway are not evident in the goethites, at least in part because topography is subdued and has evolved little in this tectonically quiescent landscape. Our results provide evidence that the efficiency of water transport from equator to pole that governs the latitudinal δ18O gradient of precipitation has been roughly constant over southern temperate latitudes for the entirety of the Cenozoic.

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