Abstract:

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.

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