Atmospheric pCO 2 and depositional environment from stable-isotope geochemistry of calcrete nodules (Barremian, Lower Cretaceous, Wealden Beds, England)

Nodular soil carbonates (calcretes) are present in overbank facies of Lower Cretaceous, non-marine Wealden Beds (Wessex Formation) of southern England. Field evidence suggests that these calcretes formed mostly under semi-arid Mediterranean-type climatic conditions. Typical calcrete fabrics, identified petrographically, include floating detrital grains, corroded grain margins and circumgranular cracks defining peds. Localized alteration of primary micrites is mainly associated with large cracks where early non-ferroan diagenetic cementation and neomorphism was focused. Diagenetic ferroan calcites occur as void fills and yield relatively light carbon-isotope and oxygen-isotope compositions ( δ 13 C= −15.0‰; δ 18 O= -6.3‰) compared to well-preserved micrite ( δ 13 C= -10.2‰; δ 18 O= -4.0‰). Precise definition of δ 13 C values for well-preserved micrites allow estimation of partial pressure of atmospheric CO 2 ( p CO 2 ) for the early Barremian of 560 ppmV using a published diffusion-reaction model. The data suggest that atmospheric CO 2 was low during the mid-Early Cretaceous before rising to a previously defined mid-Cretaceous high. Data from calcretes in the Weald Clay highlight the need for selection of appropriate material and careful evaluation before p CO 2 calculations are attempted. The Weald Clay samples come from marshy palaeoenvironments where ingress of atmospheric CO 2 into the soil-zone was either reduced or prevented.