Journal of the Geological Society, London, Vol. 159, 2002, pp. 215–224. Printed in Great Britain. Atmospheric pCO 2 and depositional environment from stable-isotope geochemistry of calcrete nodules (Barremian, Lower Cretaceous, Wealden Beds, England) STUART A. ROBINSON 1 , JULIAN E. ANDREWS 2 , STEPHEN P. HESSELBO 1 , JONATHAN D. RADLEY 3,4 , PAUL F. DENNIS 2 , IAN C. HARDING 5 & PERCE ALLEN 6 1 Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK (e-mail: stuartr@earth.ox.ac.uk) 2 School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK 3 Palaeobiology Research Group, School of Earth, Environmental and Physical Sciences, University of Portsmouth PO1 3QL, UK 4 The Warwickshire Museum, Market Place, Warwick CV34 4SA, UK 5 School of Ocean and Earth Science, University of Southampton, Southampton Oceanography Centre, European Way, Southampton SO14 3ZH, UK 6 PRIS, 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 ( 13 C= 15.0‰;  18 O= –6.3‰) compared to well-preserved micrite ( 13 C= –10.2‰;  18 O= –4.0‰). Precise definition of  13 C values for well-preserved micrites allow estimation of partial pressure of atmospheric CO 2 (pCO 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 pCO 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. Keywords: Early Cretaceous, calcrete, palaeoatmosphere, carbon dioxide, stable isotopes. The Early Cretaceous was a period of large-scale fluctuation in the isotopic composition of both the marine and terrestrial carbon reservoirs (e.g. Weissert & Bréhéret 1991; Lini et al. 1992; Hasegawa 1997; Menegatti et al. 1998; Gröcke et al. 1999). Over short periods of time, perturbations in the global carbon cycle reflected by the isotopic data probably had a significant affect on the amount of carbon dioxide present in the Early Cretaceous atmosphere (Lini et al. 1992; Gröcke et al. 1999; Jenkyns 1999). In addition, the Early Cretaceous is thought to have been a time of elevated partial pressure of atmospheric CO 2 (pCO 2 ), which was possibly related to increased rates of mid-ocean ridge spreading and associated volcanism (e.g. Jones & Jenkyns 2001). In order to study palaeoenvironmental conditions within the so-called ‘Cretaceous Greenhouse’ (e.g. Spicer & Corfield 1992; Larson & Erba 1999) it is desirable to have a detailed record of atmospheric pCO 2 change through time. During the last decade the  13 C of soil carbonate has been used as a proxy for estimating palaeoatmospheric pCO 2 in pre-Miocene times (e.g. Cerling 1991, 1992, 1999; Andrews et al. 1995; Mora et al. 1996; Ekart et al. 1999). These estimates have all been made using versions of Cerlingws (1984, 1991, 1992, 1999) diffusion model where the  13 C of pristine soil carbonate is key. Use of the Cerling model provides a method to verify the values predicted from long-term carbon-cycle models (e.g. Berner 1991, 1994; Tajika 1999; Wallmann 2001). The present study focuses on Lower Cretaceous (Wealden) palaeosols from southern England, providing new data to help assess the variability of atmospheric pCO 2 through the Cretaceous as a whole. Geological setting During the Early Cretaceous, non-marine Wealden sediments were deposited across much of what is now southern England. These sediments comprise variegated sandstones and mud- stones of fluvial and lagoonal origin (Allen 1975, 1981). Two stratigraphic schemes exist for the English onshore Wealden strata: one for the Weald area and another for the Isle of Wight and Dorset (Figs. 1 and 2; Allen & Wimbledon 1991). In the Weald, the cyclic alluvial and lagoonal sediments of the Hastings Beds (formally a group within the Wealden Supergroup in the Weald area) give way to the dominantly lacustrine-lagoonal sediments of the Weald Clay (formally a group) at approximately the Valanginian-Hauterivian 215