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Journal of the Geological Society; 2002; v. 159; issue.2; p. 215-224;
DOI: 10.1144/0016-764901-015
© 2002 Geological Society of London

Regular Article

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

STUART A. ROBINSON1, JULIAN E. ANDREWS2, STEPHEN P. HESSELBO1, JONATHAN D. RADLEY3,4, PAUL F. DENNIS2, IAN C. HARDING5 & PERCE ALLEN6

1 1Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK (e-mail: stuartr@earth.ox.ac.uk)
2 2School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK
3 3Palaeobiology Research Group, School of Earth, Environmental and Physical Sciences, University of Portsmouth PO1 3QL, UK
4 4The Warwickshire Museum, Market Place, Warwick CV34 4SA, UK
5 5School of Ocean and Earth Science, University of Southampton, Southampton Oceanography Centre, European Way, Southampton SO14 3ZH, UK
6 6PRIS, University of Reading, Reading RG6 6AB, UK

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 ({delta}13C= –15.0{per thousand}; {delta}18O= –6.3{per thousand}) compared to well-preserved micrite ({delta}13C= –10.2{per thousand}; {delta}18O= –4.0{per thousand}). Precise definition of {delta}13C 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.


Keywords: Early Cretaceous, calcrete, palaeoatmosphere, carbon dioxide, stable isotopes.




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