Clay-cation composition of river sediments as evidence of wet versus dry paleoclimate

Abstract

Cation compositions (XNa, XCa, XMg) of clays in a fluvial, Late Pleistocene, clay-rich siliciclastic sedimentary unit are shown to reflect the composition of surface water at the time of deposition, with clays deposited from surface water in a semi-arid climate (evaporation > precipitation) having an XNa >8%, compared with XNa < 8% for clays deposited under wetter conditions (evaporation < precipitation). This is a product of evaporation in the dry climate that results in an increase in Na relative to Ca and Mg in the surface water during deposition. This trend is shown in fluvial sediments from the Texas Gulf Coast where both the present-day climate, and the climate during Late Pleistocene deposition, have a gradient from wet humid subtropical (mild temperate) in the north, to dry semi-arid subtropical in the south where evaporation exceeds precipitation for much of the year. The possibility that the clay-cation compositions could have been altered by meteoric water recharge after deposition was considered by comparing the ratios of the conservative ion Cl against Na, which undergoes cation exchange. It was found that the Na-rich clays currently present in the dry environment are releasing Na into the groundwater where they are enriched in Na relative to Cl, which strongly supports the original deposition of Na-rich clays rather than Na-poor clays in the south. This trend was not present in the northern sediments with the wetter climate, which supports the original deposition of Na-poor clays. These results support that the Na content relative to Ca and Mg in clays deposited with siliciclastic sediments from surface water in a fluvial environment could possibly be used as paleoclimate proxies to predict wet versus dry climates.