• journal_title：Journal of the Geological Society
• Contributor：STUART A. ROBINSON ; JULIAN E. ANDREWS ; STEPHEN P. HESSELBO ; JONATHAN D. RADLEY ; PAUL F. DENNIS ; IAN C. HARDING ; PERCE ALLEN
• Publisher：Geological Society of London
• Date：2002-
• Format：text/html
• Language：en
• Identifier：10.1144/0016-764901-015
• journal_abbrev：Journal of the Geological Society
• issn：0016-7649
• volume：159
• issue：2
• firstpage：215
• section：Regular Article

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 (δ¹³C= −15.0‰; δ¹⁸O= –6.3‰) compared to well-preserved micrite (δ¹³C= –10.2‰; δ¹⁸O= –4.0‰). Precise definition of δ¹³C 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.