Journal of Sedimentary Research, 2012, v. 82, 1006–1016 Current Ripples DOI: 10.2110/jsr.2012.85 DETERMINING THE DIAGENETIC CONDITIONS OF CONCRETION FORMATION: ASSESSING TEMPERATURES AND PORE WATERS USING CLUMPED ISOTOPES 1 2 3 1,3 SEAN J. LOYD, FRANK A. CORSETTI, JOHN M. EILER, AND ARADHNA K. TRIPATI 1Department of Earth and Space Sciences, University of California, Los Angeles, 595 Charles Young Drive East, Los Angeles, California 90095, U.S.A. 2Department of Earth Sciences, University of Southern California, 3651 Trousdale Avenue, Los Angeles, California 90089, U.S.A. 3Division of Geological and Planetary Sciences, California Institute of Technology, 1200 East California Boulevard, Pasadena, California 91125, U.S.A. e-mail: [email protected] 18 ABSTRACT: Carbonate-d O paleothermometry is used in many diagenetic studies to unravel the thermal history of basins. 18 18 However, this approach generally requires an assumed pore-water d O(d Opw) value, a parameter that is difficult to quantify in past regimes. In addition, many processes can change the original isotopic composition of pore water, which further 18 complicates the assignment of an initial d Opw and can lead to erroneous temperature estimates. Here, we use clumped-isotope thermometry, a proxy based on the 13C–18O bond abundance in carbonate minerals, to evaluate the temperatures of concretion formation in the Miocene Monterey Formation and the Cretaceous Holz Shale, California. These temperatures are combined 18 with established carbonate–water fractionation factors to calculate the associated d Opw. Results demonstrate that diagenetic processes can modify the d18O of ancient pore water, confounding attempts to estimate diagenetic temperatures using standard approaches. Clumped-isotope-based temperature estimates for Monterey Formation concretions range from , 17 to 35uC, up to , 12uC higher than traditional d18O carbonate–water paleothermometry when 18 18 d Opw values are assumed to equal Miocene seawater values. Calculated d Opw values range from +0.3 to +2.5% 18 (VSMOW)—higher than coeval Miocene seawater, likely due to d Opw modification accompanying diagenesis of sedimentary siliceous phases. Clumped-isotope temperatures for the Holz Shale concretions range from , 33 to 44uC, about 15 to 30uC 18 lower than temperatures derived using the traditional method. Calculated d Opw values range from 25.0 to 22.9% and likely reflect the influx of meteoric fluids. We conclude that the use of clumped isotopes both improves the accuracy of temperature 18 reconstructions and provides insight into the evolution of d Opw during diagenesis, addressing a longstanding conundrum in basin-evolution research. ÀÁ 3 {1 INTRODUCTION 1000lnacalcite-water~ 18:03|10 T {32:42 ð1Þ Carbonate is a common cementing material in sedimentary rocks, and the precipitation of carbonate cements is an important agent of ÀÁ 1000lna ~ 2:73|106 T{2z0:26 ð2Þ lithification. In siliciclastic rocks, carbonate cementation may be spatially dolomite-water discontinuous, producing conspicuous structures called concretions. Concretions commonly exhibit textural characteristics such as deflection where T represents the precipitation temperature in kelvins, and acalcite-water 13 13 of external laminae, and variable carbonate d C(d Ccarb) values and adolomite-water are the oxygen isotope fractionation factors between (Claypool and Kaplan 1974; Mozley and Burns 1993), suggesting that calcite and water and dolomite and water, respectively. This temperature- at least partial precipitation occurred relatively early and during dependent fractionation has been applied to concretions to estimate their progressive degradation of sedimentary organic matter. Carbonate formation temperatures, where the oxygen isotope composition of the fluid concretions are recognized in Phanerozoic sedimentary units of nearly from which the carbonate grew—an unknown parameter—is predicted. The 18 all ages and depositional environments, including those of the Miocene oxygen isotope composition of pore water (d Opw) in sediments can be Monterey Formation and Upper Cretaceous Holz Shale of California extremely variable and remains poorly constrained in most subsurface (Fig. 1). Given their diagenetic origin, concretions may plausibly form environments (Clayton et al. 1966; Hitchon and Friedman 1969; Perry et al. over a broad range of temperatures—from seafloor values that are near 1976; Hesse and Harrison 1981; Allan and Mathews 1982; Morton and bottom water temperatures (, 0–5uC) to 100uC or more. Land 1987; Wilkinson et al. 1992; Behl and Garrison 1994; Bohrmann et al. The differences between the oxygen isotope compositions of carbonate 1998). In the case of concretions in marine sediments, many studies have 18 18 minerals (d Ocarb) and the fluids from which they precipitate is related to generally assumed a d Opw equal to the mean value of contemporaneous the temperature of mineralization (Urey 1947). The most up-to-date seawater when calculating precipitation temperatures (i.e., Hudson 1978; estimates of the temperature dependence of oxygen isotope fractionation Kushnir and Kastner 1984; Hennessy and Knauth; 1985; Burns and Baker for calcite (Kim and O’Neil 1997) and dolomite (Vasconcelos et al. 2005) are 1987; Dix and Mullins 1987; Astin and Scotchman 1988; Thyne and Boles Published Online: December 2012 Copyright E 2012, SEPM (Society for Sedimentary Geology) 1527-1404/12/082-1006/$03.00 JSR CURRENT RIPPLES 1007 FIG. 1.—Study site locations. The insets in the map of California indicate the general areas of Parts A and B. A) Monterey Formation and B) Holz Shale localities. MDO 5 Montan˜a de Oro. 1989; Morad and Eshete 1990). However, sedimentary processes that provides a way to assess the applicability of the clumped-isotope proxy in 18 affect d Opw values can invalidate this assumption. In fact, previous concretions. The results demonstrate that temperatures derived from 18 researchers have recognized anomalously low d Ocarb values in clumped-isotope thermometry can deviate markedly from those calculat- carbonate concretions (Hudson and Friedman 1974; Hudson 1978; ed using conventional d18O thermometry with assumed pore-water Irwin et al. 1980; Coleman and Raiswell 1981; Burns and Baker 1987; compositions of 0%, the approximate value of contemporaneous Mozley and Burns 1993). These authors have stressed the unlikelihood seawater d18O. The discrepancy between the two approaches can be of precipitation at high temperatures as the cause, largely due to the attributed to the errors that arise when the pore-water composition is presence of shallow indicators (e.g., laminae deflection, high minus- estimated using oversimplified and therefore inaccurate assumptions. cement porosity, etc.) and inferred the influence of processes that lead to Data show that independently determined temperatures provide the 18 18 depleted d Opw. Raiswell and Fisher (2000) summarized the apparent potential to track pore-water processes that perturb d Ofluid and better disagreement among these shallow indicators and actual concretion characterize diagenetic carbonate-forming environments. formation depths (and temperatures). Some authors have suggested specific mechanisms responsible for the isotopic depletions; however, GEOLOGIC CONTEXT without a method by which to isolate the effects of temperature from 18 The Monterey Formation d Opw, the interpretations are somewhat speculative. Carbonate clumped-isotope thermometry can be used to constrain the The Miocene Monterey Formation is a hemipelagic, organic-rich and temperatures of carbonate growth independently of fluid d18O, and thus largely siliceous deposit that crops out along much of the California coast offers a way to 1) determine the temperature of carbonate formation and (see Fig. 1 for study site locations) (Bramlette 1946; Isaacs 1981; 2) unambiguously calculate the ancient fluid d18O (Ghosh et al. 2006; Schwalbach and Bohacs 1991; Behl and Garrison 1994). The large Schauble et al. 2006; Eiler 2007; Eagle et al. 2010; Tripati et al. 2010; Eiler depositional area of the Monterey Formation during the Miocene led to 18 2011), including d Opw (Huntington et al. 2011). The abundance of the development of highly variable depocenters which have been 13 18 16 mass 47 CO2 (mostly the C– O– O isotopologue) that is produced interpreted to range from gradual slopes and sediment-starved plains upon phosphoric-acid digestion of carbonate minerals is reported using (Isaacs 2001) to border-land basin type (e.g., Blake 1981) systems. Among the notation D47, and represents the abundance of this isotopologue in a these differing depositional environments, the Monterey Formation sample relative to a stochastic distribution (in units of permil, %). exhibits characteristics consistent with anoxic bottom waters, including Theoretical (Schauble et al. 2006; Guo et al. 2009; Eagle et al. 2010), laminated sediments, a general lack of bioturbation, the scarcity of experimental (Ghosh et al. 2006), and field-based (Ghosh et al. 2006; benthic macrofossils, and relatively high organic contents (e.g., Isaacs Eagle et al. 2010; Tripati et al. 2010; Thiagarajan et al. 2011; Zaarur et al. 2001). Of course the differences in depositional environment and its 2011) studies have shown that values of D47 in calcite, aragonite, and temporal variation have produced both spatial and stratigraphic dolomite are inversely correlated to the temperature of carbonate heterogeneity in the Monterey Formation, and as a result potentially 18 precipitation.