doi:10.1016/j.gca.2005.07.017 Coexisting goethite and gibbsite from a high-paleolatitude (55°N) Late Paleocene laterite: Concentration and 13 C/ 12 C ratios of occluded CO 2 and associated organic matter NEIL J. TABOR* and CRAYTON J. YAPP Department of Geological Sciences, Southern Methodist University, Dallas, Texas 75275-0395, USA (Received January 25, 2005; accepted in revised form July 18, 2005) Abstract—A Late Paleocene (60 Ma BP) lateritic soil from Northern Ireland (the Antrim paleosol, herein referred to as Nire) contains coexisting goethite, gibbsite, phyllosilicate, and hematite. The Fe(III) oxides exhibit pisolitic and Liesegang-type morphologies that are mutually exclusive in hand specimens. X-ray diffraction (XRD) measurements of Al substituted for Fe in goethite indicate two populations: (1) low-Al, Liesegang-type goethites (0 mol% Al) and (2) high-Al, pisolitic goethites (9 to 24 mol% Al). Selective dissolution and incremental vacuum dehydration-decarbonation were used to determine the concentration and  13 C values of CO 2 occluded in the respective structures of the goethites and gibbsites in this complex mixture of Nire lateritic minerals. The Fe(CO 3 )OH component in the high-Al goethites appears to retain a proxy carbon isotopic record of vadose zone CO 2 in the ancient soil. The  13 C values of CO 2 occluded in coexisting goethites and gibbsites indicate that these minerals did not form in equilibrium with the same environmental CO 2 . The measured mole fractions (X) of Fe(CO 3 )OH in the high-Al goethites range from 0.0059 (0.0005) to 0.0077 (0.0006) and correspond to soil CO 2 concentrations of 28,000 to 37,000 ppmV. The average values of X and  13 C for the four high-Al goethites are 0.0067  0.0007 and -20.1  0.5‰, respectively. The  13 C value of the organic matter undergoing oxidation in this midlatitude (55°N) Late Paleocene soil appears to have been -28.2‰. Taken together, these data indicate an atmospheric CO 2 concentration of 2400 ppmV (1200 ppmV) at 60 Ma BP. The inferred high concentration of atmospheric CO 2 would have been coincident with the warm global climate of the Late Paleocene and is consistent with the idea that CO 2 plays an important role in climate variation. Copyright © 2005 Elsevier Ltd 1. INTRODUCTION Lateritic soils typically form in the warm and humid condi- tions found in the modern low-latitude tropics (Krishnaswamy, 1986; however, see Taylor, et al., 1992). Yet, lateritic profiles of Late Paleocene age found in southern Australia and Tasmania (Taylor et al., 1992), Ireland (Hill et al., 2000), and Iceland (Nilsen, 1978) formed at southern and northern paleolatitudes of 50° to 60° (Scotese and Golonka, 1992). Today these high- latitude regions lie within the cool and humid climates of the temperate zone, a region in which laterites are not currently forming (e.g., UNFAO, 1991). This apparent dichotomy of soil type and latitude suggests that these ancient laterites formed under conditions of high-latitude warmth in the Late Paleocene. Such high-latitude warmth could be related to high concentra- tions of CO 2 in the Late Paleocene atmosphere (Wing, 1995; Zachos et al., 2001). However, biologic and geologic proxies as well as global carbon flux balance models provide contradic- tory estimates of the concentration of CO 2 in the Late Paleo- cene atmosphere, ranging from 300 ppmV to 3500 ppmV (e.g., Berner, 1994; Ekart et al., 1999; Pearson and Palmer, 2000; Retallack, 2001; Royer et al., 2001; Demicco et al., 2003; Tyrrell and Zeebe, 2004). The Fe(CO 3 )OH component in solid solution in goethite provides information on the concentration and  13 C value of CO 2 in the immediate vicinity of goethite crystallization, and in certain paleosols, may be used to infer atmospheric CO 2 con- centrations (Yapp and Poths, 1991; Yapp and Poths, 1992; Yapp and Poths, 1993; Hsieh and Yapp, 1999; Yapp, 2001; Yapp, 2002; Yapp, 2004). Similar types of information are recorded in CO 2 occluded in pedogenic gibbsite (Schroeder and Melear, 1999). In this paper, we present analyses of the con- centrations and  13 C values of CO 2 evolved from coexisting goethites, gibbsites, and organic matter from a Late Paleocene laterite from Northern Ireland. The results are discussed in terms of soil CO 2 partial pressures, CO 2 in the Late Paleocene atmosphere, and the implications of the carbon isotope ratios for equilibrium between goethite and gibbsite in this system. 2. BACKGROUND AND METHODS 2.1. Field Area and Sample Collection The geology of the sample area is summarized by Hill et al. (2000). Briefly, in Northern Ireland, tertiary basaltic lavas rest on a Cretaceous chalk (the Ulster White Limestone Formation). There are three stratigraphic units recognized in the lavas. From oldest to youngest they are (1) the lower basalt formation (LBF), (2) the interbasaltic formation (IBF), and (3) the upper basalt formation (UBF). In the sample area, the IBF consists of two laterite horizons separated by a basalt (the Causeway tholeiite member, CTM). The upper laterite rests on the CTM and is termed the Ballylagan member (Hill, et al., 2000). Figure 1a is an outcrop photograph showing the upper 6 m of the Ballylagan member. The age of the laterite is relatively well-constrained by 40 Ar- 39 Ar dates of 61.0 (0.6) Ma and 58.3 (1.1) Ma for tertiary lavas that underlie and overlie the interbasaltic formation (Thompson, 1985). The laterite of the Ballylagan member developed at a paleolatitude of 55°N (Scotese and Golonka, 1992) as a result of extensive surficial chemical weathering of the parent basalt in a warm, humid climate (e.g., Montford, 1970). * Author to whom correspondence should be addressed (ntabor@ mail.smu.edu). Geochimica et Cosmochimica Acta, Vol. 69, No. 23, pp. 5495–5510, 2005 Copyright © 2005 Elsevier Ltd Printed in the USA. All rights reserved 0016-7037/05 $30.00 + .00 5495