Atmospheric pCO2 and depositional environment from stable-isotope geochemistry of calcrete nodules (Barremain, Lower Cretaceous, Wealden Beds, England)

Robinson, SA, Andrews, JE, Hesselbo, SP, Radley, JD, Dennis, PF, Harding, I and Allen, P (2002) Atmospheric pCO2 and depositional environment from stable-isotope geochemistry of calcrete nodules (Barremain, Lower Cretaceous, Wealden Beds, England). Journal of the Geological Society, 159. pp. 215-224. ISSN 0016-7649

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Abstract

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 (d13C= –15.0‰; d18O= –6.3‰) compared to well-preserved micrite (d13C= –10.2‰; d18O= –4.0‰). Precise definition of d13C values for well-preserved micrites allow estimation of partial pressure of atmospheric CO2 (pCO2) for the early Barremian of 560 ppmV using a published diffusion-reaction model. The data suggest that atmospheric CO2 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 pCO2 calculations are attempted. The Weald Clay samples come from marshy palaeoenvironments where ingress of atmospheric CO2 into the soil-zone was either reduced or prevented.

Item Type: Article
Faculty \ School: Faculty of Science > School of Environmental Sciences
University of East Anglia > Faculty of Science > Research Groups > Environmental Earth Sciences
University of East Anglia > Faculty of Science > Research Groups > Marine and Atmospheric Sciences
University of East Anglia > Faculty of Science > Research Groups > Geosciences and Natural Hazards
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Depositing User: Rachel Snow
Date Deposited: 03 Mar 2011 09:03
Last Modified: 25 Jul 2018 07:32
URI: https://ueaeprints.uea.ac.uk/id/eprint/25565
DOI: 10.1144/0016-764901-015

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