Constraints on palaeoenvironments in the Lower Cretaceous Wealden of southern England, from the geochemistry of sphaerosiderites

Sphaerosiderites are millimetre-scale concretions of FeCO₃ that form predominantly in waterlogged environments and are of use in palaeoenvironmental and palaeohydrological studies. We present petrographic, elemental and stable-isotopic (δ¹³C, δ¹⁸O) data from sphaerosiderites collected from the Lower Cretaceous Wealden sediments of southern England. The sphaerosiderites are composed of very pure FeCO₃, with only small amounts of Ca, Mg and Mn present, suggesting that they are well preserved and were precipitated from fresh groundwater. δ¹³C values display a relatively large range from about -5 to -30‰ (VPDB). In contrast, δ¹⁸O values are relatively invariant (1σ = 0.83‰), with an average of -3.01‰ (VPDB). The relationship between carbon and oxygen suggests that the sphaerosiderites faithfully record palaeo-groundwater δ¹⁸O. Likely δ¹⁸O values for groundwater were calculated using a published, experimentally derived siderite-water fractionation equation and plausible assumed values for palaeotemperature. The average estimate was -4.9‰ (SMOW). The Wealden sphaerosiderite oxygen-isotope values are comparable with other early Cretaceous data from similar palaeolatitudes but lower than estimated Holocene values for equivalent latitudes. This observation may indicate greater rainfall at 30°N during the Cretaceous relative to the Holocene and also suggests relative stability of palaeoclimatic conditions during the early Cretaceous at this palaeolatitude.