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 (delta C-13, delta O-18) 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. delta C-13 values display a relatively large range from about -5 to -0% (VPDB). In contrast, delta O-18 values are relatively invariant (1 sigma = 0.83%), with an average of -3.01 parts per thousand ( VPDB). The relationship between carbon and oxygen suggests that the sphaerosiderites faithfully record palaeo-groundwater delta O-18. Likely delta O-18 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 degrees N during the Cretaceous relative to the Holocene and also suggests relative stability of palaeoclimatic conditions during the early Cretaceous at this palaeolatitude.