origin and significance of isotope shifts in pennsylvanian carbonates

ORIGIN AND SIGNIFICANCE OF ISOTOPE SHIFTS IN PENNSYLVANIAN CARBONATES (ASTURIAS, NW SPAIN) ADRIAN IMMENHAUSER1, JEROEN A.M. KENTER1, GERALD GANSSEN1, JUAN R. BAHAMONDE 2, ARJAN VAN VLIET1, AND MARGOT H. SAHER1 1

Earth Sciences, Vrije Universiteit Amsterdam, De Boelelaan 1081, 1085 HV Amsterdam, The Netherlands e-mail: [email protected] 2 Departamento de Geologıá, Universidad de Oviedo, Jesu ´ s Arias de Velasco, s/n, 33005 Oviedo, Spain

ABSTRACT: The primary variability of the composition and properties of seawater is much greater in the shallow coastal zones than in the main body of ocean water. An inadequate understanding of this variability, as well as different diagenetic environments, severely limit the interpretation of the stable-isotope record of shoalwater carbonates. In order to investigate this primary and diagenetic variability along a Bashkirian–Moscovian platform-to-basin transect, d 13C and d 18O analyses have been performed on more than 1000 matrix micrite, carbonate cement, and brachiopod shell samples. In isotope analysis, these different carbonate materials tend to complement each other, inasmuch as they have different advantages and shortcomings. The resulting data reveal spatial trends in d 13C and d 18O signatures from platform top (lower values) to basin (higher values). In the case of d 13C from pristine brachiopods, this trend can be explained by the long residence time (aging) of platform-top water masses. In the case of brachiopod d 18O, this variance is interpreted to reflect temperature differences between warm surface and colder bottom water separated by a permanent thermocline at about 150 to 200 m beneath the shelf break. Micrite and marine cement isotopic values from the platform interior were reset (lowered) during pervasive early meteoric diagenesis. In contrast, micrite and marine cement isotopic values from the outer platform, slope, and basin show higher values close to the assumed Pennsylvanian seawater isotopic composition. This implies that isotopic data from shoalwater carbonates (including pristine brachiopod shells) might not necessarily reflect paleoceanographic trends of the open-ocean water masses because of changes in coastal water-mass isotope signature and interaction with early meteoric fluids.

INTRODUCTION

Shifts in the stable-isotope composition of marine carbonates in response to major climatic, paleoceanographic, or sea-level events are well known from the geologic record (e.g., Jenkyns 1980, Weissert et al. 1985, Beeunas and Knauth 1985, Ditchfield and Marshall 1989, Shackleton et al. 1993, MacManus et al. 1994, Mii et al. 1999, Kuypers et al. 1999). Amongst the factors that control the primary isotopic composition of carbonates are the geochemistry and temperature of the ambient seawater and vital effects. The primary variability of the composition and properties of seawater is greatest in the shallow coastal zones because of significant evaporation, meteoric dilution, and carbon transfer (Patterson and Walter 1994 and references therein). After deposition, the isotopic record of carbonates that precipitate from this seawater is affected by other factors. These include transformation of metastable carbonate polymorphs (aragonite, high-Mg calcite) into stable low-Mg calcite, early (meteoric) diagenesis, and finally late (burial) diagenesis. By definition, shallow platform-interior limestones tend to be more commonly affected by early meteoric diagenesis relative to platform margin, slope, and basinal environments (Kievman 1998). We took advantage of exceptional outcrops of an intact Pennsylvanian carbonate platform margin in the Sierra de Cuera in Asturias, NW Spain (Fig. 1; Bahamonde et al. 1997) and investigated a relatively short interval (late Bashkirian to early Moscovian) for an in-depth stable-isotope study along a platform-to-basin transect. Carbonate materials with an aragonitic JOURNAL OF SEDIMENTARY RESEARCH, VOL. 72, NO. 1, JANUARY, 2002, P. 82–94 Copyright q 2002, SEPM (Society for Sedimentary Geology) 1527-1404/02/072-87/$03.00

and high-Mg calcitic (matrix micrites), a high-Mg calcitic (marine cements), and a low-Mg calcitic (brachiopod shells) precursor mineralogy were used for isotopic analysis, because these different materials are affected differently by diagenesis and thus tend to complement each another. The main objective of this study was to examine the variability of the Pennsylvanian coastal seawater geochemistry and various diagenetic environments over time and paleodepth as recorded in different carbonate materials. REGIONAL SETTING

The Cantabrian Zone comprises the northwestern part of the Hercynian orogen in the Iberian massif. It consists of a thick succession of Paleozoic strata that were deformed into a set of thrust imbricates by thin-skinned tectonism during the middle and Late Carboniferous (Perez-Estaun et al. 1988). During the middle Carboniferous (Bashkirian–Moscovian), the Cantabrian Zone consisted of a marine foreland basin with a pronounced lateral instability and a strongly asymmetric profile (Agueda et al. 1991). The subsiding proximal zones were occupied by thick clastic wedges converging against the margins of an extensive carbonate shelf. This shelf developed in the more proximal and shallow areas (NE sector of Ponga Nappe and Picos de Europa domains; Colmenero et al. 1988). The carbonate platforms under investigation conformably overlie the Barcaliente Formation (Serpukhovian) and are built by the Valdeteja (Bashkirian) and Picos de Europa (Moscovian) formations (Fig. 2). The Ponga nappe and Picos de Europa domain were the last areas that were affected by the Variscan compression during Kasimovian times. The rock units in these areas were deformed into a set of imbricate thrust sheets with an east–west orientation (Fig. 1B, C; Marquıńez 1998). Although part of a thrust zone, and therefore bordered by faults, the internal structure of most platforms has been preserved. The dip of the bedding planes within the carbonate platforms varies between 70 and 908 but commonly approaches vertical (Fig. 1C). At the Sierra de Cuera (located in the NE sector of the Ponga Nappe), the Valdeteja Formation has a thickness that varies between 750 and 850 m and a predominantly progradational geometry, whereas the Picos de Europa Formation reaches a thickness close to 1 km and has a predominantly aggradational character. During the Moscovian, the Bashkirian platform sections were overlain by 0.7–1 km of marine carbonates. In the late Moscovian–early Kasimovian, i.e., about 8 Myr after deposition of the interval studied (time scale of Klein 1990), the platform under study was affected by the Variscian compression, became incorporated in a pile of imbricate thrust sheets, and was tilted to nearly vertical (Fig. 1C, D). The carbonate platform remained in this tilted position during its subsequent burial and uplift history; thus, all sections studied have identical post-tilting burial histories. After a Late Stephanian to Permian interval with semiarid, continental climatic conditions, the Jurassic transgression led to neo-autochthonous sediment deposition onto the rotated Paleozoic platforms, which continued into the Early Tertiary (Fig. 2; 300–400 m thick; Lepvier and Martinez-Garcia 1990). Uplift and erosion of most of the Mesozoic and parts of the Paleozoic units occurred during the Oligocene.

SIGNIFICANCE OF PENSYLVANIAN ISOTOPE SHIFTS

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FIG. 1.—A) Overview map and location of study area. B) Generalized geo-tectonic overview map (after Bahamonde et al. 1997). C) Schematic cross-section showing near-vertical tilting of thrust imbricates. D) Map view of the platform under study from aerial photographs. Note the position of the master correlation surface (mcs), the Bashkirian–Moscovian boundary (BMb), and sections on the platform-to-basin transect (1–8). LUBb 5 Lower to Upper Bashkirian boundary.

Platform Anatomy and Lithofacies The Asturian platform under study probably formed an isolated carbonate buildup, inasmuch as it is flanked by slopes at its western and eastern limit. Five general physiographic zones are observed: inner and outer platform, upper slope, lower slope, and toe of slope to basin (Fig. 1D). The inner platform (sections 1 and 2) is composed of stacked depositional cycles that reflect relative shoaling from subtidal to intertidal, locally overprinted by meteoric diagenetic features. Supratidal facies are conspicuously absent. Individual cycles are usually 5 to 12 m thick and have a basal packstone to grainstone interval dominated by algae, peloids, foraminifera, and crinoid hash. This basal interval has a thickness up to 7 m, is morphologically resistant, and is overlain by a recessive interval of packstone to wackestone with rare algae and calcispheres reflecting a restricted, la-

goonal environment. A horizon of grainstone with oncoids, coated grains (locally ooids), and Chaetetes caps the recessive interval. The outer platform (section 3) is dominated by crinoid–bivalve grainstone to rudstone that alternates with thin intervals of bioclastic grainstone with coated grains and, locally, meters-thick lenticular deposits of automicrite (clotted peloidal micrite) containing irregular voids filled by radiaxial fibrous calcite. The transition from platform to slope, the platform break, is relatively sharp. The flank is inclined at angles between 26 and 328, has a nearly planar upper slope and slightly concave-upwards lower slope, and a vertical relief between 650 to 850 m (Fig. 1D; figures apply to compacted successions). The upper slope, representing paleo-water depths of ca. 10 and 300 m below the shelf break (sections 4 and 5), is dominated by green algae (shallow part of upper slope) like Anthracoporella, Komia, and Donezella,

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A. IMMENHAUSER ET AL. established on the basis of fusulinid foraminifera, runs a few tens of meters above the correlation interval, and detailed biostratigraphic analysis of associated samples by E. Villa (University of Oviedo) indicates a constant distance between those two levels (Fig. 1D). The fusulinid zones and subzones established so far are Millerella, Profusulinella, and Fusulinella zones (Kenter et al. 2001). On the vertically rotated outcrops, the distances of each platform section from the shelf break and the depths of slope sections below the shelf break (paleobathymetry) were quantified directly from aerial photographs. Across this transect, we measured eight sections and sampled each of them for facies and stable isotope analysis in increments of 0.25 to 0.5 m (Fig. 1D). In most cases, the marker succession was located approximately in the middle of the section. Sampling for stable-isotope and trace-element data focused on matrix micrites (and grains), carbonate cements, and brachiopod shells. Carbonate Materials

FIG. 2.—Stratigraphic scheme of the Carboniferous strata in the Sierra de Cuera Region and the Ponga Nappe (after Bahamonde et al. 1997). Time scale after Klein (1990). Shaded area represents the interval investigated in this study.

and fenestellid bryozoans (along the entire upper slope), embedded in automicrite (clotted peloidal micrite) with irregular voids filled with marine cement. Sand-size, platform-derived grains are present in the automicrite facies or as thin lenses. Several thin-bedded, reddish intervals of alternating automicrite, crinoid rudstone, and wackestone with ostracods, ammonites, trilobites, brachiopods, and sponge spicules are present with a regular spacing of ca. 50 m. The presence of red detrital micrite with pelagic biota indicates sediment oxidation during periods of low rates of deposition (condensed facies). The lower slope, between ca. 300 and 800 m beneath the platform break (sections 6 and 7), is dominated by clast-supported and nearly mud-free, upper slope-derived breccia intervals of reworked, millimeter- to decimetersize, subangular clasts from the upper slope and thin grainstone lenses of platform-top-derived grains like ooids, algae, and crinoids. The toe of slope to basin (section 8 at ca. 800 m depth below the platform break; Fig. 1D) consists of an alternation of upper slope-derived rudstone tongues, rare platform-top-derived grainstone and hemipelagic argillaceous lime mudstone with sponge spicules. At this locality, sampling for isotope data focused on the lime mudstone between lithoclasts and on automicrites. The top of section 8 has probably been overprinted by Permian karsting, and the data from there are not considered any further. WORKING APPROACH, CARBONATE MATERIALS, AND METHODS

Working Approach Our working approach is based on a transect reaching from the platform top to the basin, a distance of nearly 7 km (Fig. 1D). In order to establish a correlation time interval that links limestones from the platform interior with coeval deposits at the toe of slope, a physically distinctive markerbed succession was used. This marker, being conspicuous because of its unusual thickness, light weathering color, and morphologically resistant nature, was traced in outcrop and on high-resolution aerial photographs using a hand-held GPS. The Bashkirian–Lower Moscovian boundary, well

Matrix Micrite.—Matrix micrite is a polygenic material. At deposition, the porosity of modern lime muds occupies about half the sediment volume. Depending on the degree of compaction and on the volume and type of cement occluding this pore space, the sample appears uniform, but a considerable part of its volume may be an early marine or a late diagenetic product, respectively (Dickson and Coleman 1980). Depending on the ratio of depositional versus diagenetic carbonate material in the sample, the stable-isotope data will deviate to variable degrees from the primary isotopic composition at deposition. Nevertheless, previous studies have demonstrated that, under favorable conditions, matrix-micrite chemostratigraphy is a proxy for global climatic variability (e.g., Stoll and Schrag 2000), seawater isotopic composition (e.g., Jenkyns 1980), or the global carbon cycle (e.g., Weissert et al. 1985). Matrix-micrite chemostratigraphy, however, focuses on characteristic isotope signatures rather than on absolute values. About 1000 analyses of d13C and d18O were performed on Bashkirian carbonates (see JSR data repository). Sample selection focused mainly on detrital matrix (allo)micrite but also on automicrite. Detrital or allomicrite is a petrologic and geochemical average of the original sediment. In basinal sections, the source of detrital micrite might be the platform, the slope, or plankton. The data from such samples might thus not be representative for the isotopic composition of the basinal setting. This problem can be avoided by incorporating isotope data from automicrites. Automicrite is microcrystalline Ca-carbonate formed in situ in close association with nonliving organic substrates (Neuweiler et al. 1999). Automicrite is distinctively different from detrital micrite by its clotted or accretionary, laminar fabric, which often defies gravity by coating the roofs of cavities etc. While microdrilling matrix subsamples from cut rock faces, we attempted to avoid sparry cement and vein material that could be recognized with a hand lens. Investigation of thin sections, however, revealed that most of the matrix micrites contain fine skeletal components and small inclusions of carbonate cements. We thus collected a second data set of microscopically uniform matrix micrites, skeletal components, micrites with intercalated cements (‘‘recrystallized micrites’’), and marine and later cement phases using a computer-controlled microsampling system as described in Dettman and Lohmann (1995). This second data set focused on intrasample isotopic variations. Carbonate Cements.—Tobin et al. (1996) suggested that translucent fibrous calcite cement might reflect ambient seawater geochemistry. In the Sierra de Cuera, marine fibrous cements occur as several superimposed isopachous rims of radial-fibrous (including radiaxial) calcite lining interparticle pores created by the sheltering effects of framework organisms (e.g., fenestellid bryozoans). Single rims range from 0.6 to 3 mm, and their cumulative thickness may attain 4 cm. The fibrous cements are generally nonluminescent but occasionally are patchily luminescent and often overlain by nonluminescent to banded-luminescent scalenohedral cements (Fig.


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FIG. 3.—Carbonate cements. A) Crossed-nicols view (5 mm wide) of fibrous and scalenohedral calcites and of saddle dolomites occluding framework cavity at the outer platform. B) Cathode luminescence (CL) view of part A, showing the transition of nonluminescent fibrous (f) and scalenohedral (sc) calcites to bright-luminescent and banded-luminescent scalenohedral cements. Center of pore is occluded by bright-red-luminescent saddle dolomite (sa). C) CL view (2.5 mm wide) of well developed nonluminescent scalenohedral calcites (sc) with bright- and banded luminescent outer margins. D) Transmitted light view (5mm wide) of microstalactitic meteoric cement (m) beneath crinoid fragment. Note dark outer rim stained by terrestrial humic acids (arrow).

3). In the centers of pores, luminescent scalenohedral cements pass into bright-luminescent saddle dolomites and dull-luminescent blocky sparite (Fig. 3B, C). This diagenetic succession is characteristic of many Mesozoic and Paleozoic carbonate platforms, with the non-bright-dull luminescence sequence interpreted to be either meteoric (Meyers 1978) or shallow burial in origin (Fig. 3C; Zeeh et al. 1995; Kaufmann 1997). In the case under study, a shallow burial origin is assumed, because we find this luminescence pattern also in toe-of-slope carbonates, i.e. well beyond the reach of early meteoric fluids. The Pennsylvanian fibrous marine cements under study consist of cloudy, low-Mg calcite and contain abundant microdolomite inclusions. The lack of aragonite remnants, presence of microdolomite inclusions, and the shape of crystals are indicative of a high-Mg precursor mineralogy that stabilized to low-Mg calcite and microdolomite (Lohman and Meyers 1977). Sampling for stable-isotope data focused on the first two generations of fibrous cements. In addition to samples from fibrous cements, we collected stable-isotope data from early diagenetic meteoric microstalactitic cements related to subaerial exposure surfaces (Fig. 3D), shallow to late burial scalenohedral calcites, and late diagenetic saddle dolomites. These data were used to gain an understanding how early meteoric or burial diagenesis possibly influenced the preservation of marine isotope values. Brachiopod Shells.—Stable-isotope data from carefully screened brachiopod shells are considered to be one of the most reliable proxies for the ambient seawater isotopic composition (e.g., Grossman et al. 1996;

Bruckschen et al. 1999). In order to select pristine material, we used petrographic and geochemical screening methods similar to those described by Mii et al. (1999). Well-preserved brachiopod shells of the same species, however, are rare in Pennsylvanian shoalwater sections and very rare in deeper settings. The shells of 75 productid brachiopods (Echinoconchus cf. punctatus, Kozlowskia sp., Reticulata sp.) collected in sections 1, 3, 5, 6, and 7 were investigated under a cold-stage cathode luminescence microscope. Twenty-three nonluminescent shells from the mid to lower slope and twenty-seven non-luminescent shells from the inner platform and the platform break were checked under a scanning electron microscope (SEM). Twelve specimens from the platform top to upper slope and eleven specimens from the middle to lower slope showed no discernible diagenetic effects, and their inner shell layer was analyzed for their isotopic and traceelement composition (archived in JSR data repository, see Acknowledgments section). Methods Carbonate subspecimens (10 to 20 micrograms on average) were analyzed on a VG Prism II ratio mass spectrometer for carbon and oxygen isotope ratios. Repeated analyses of carbonate standards show a reproducibility of better than 0.1‰ for d18O and better than 0.05‰ for d13C. Duplicate samples scatter on the order of 6 0.1‰ or less for d18O and d13C. All isotope results are reported in ‰ relative to the VPDB standard and are available in JSR’s data repository (see Acknowledgments). The organic

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FIG. 4.—A) Comparison of Asturian brachiopod isotope values with published data. B) Strontium and manganese abundances from the inner layers of Asturian brachiopod shells used in this study. Shaded area is region of modern brachiopod shell material (from Veizer et al. 1999).

carbon content in most samples is too low for reliable d13Corg analysis. Minor-element analysis (Ca, Sr, Mg, Mn, Fe) was obtained from brachiopod and cement samples by electron microprobe analysis operating at 20 kV, beam current 0.015 mA, and 13 mm beam diameter (detection limits for Sr 5 200 ppm, Mg 5 100 ppm, Mn 5 200 ppm, Fe 5 200 ppm). Manganese abundances were also measured by inductively coupled plasma atomic emission spectrometry (ICP-AES) after dissolution of ; 1.5 mg of powdered sample in 1 N HNO3 with subsequent dilution to an ; 0.1 N HNO3 sample solution (detection limit for Mn 5 40 ppm). Cathodoluminescence (CL) on carbonate cements and brachiopod shells was conducted on a cold-stage cathode luminescence operating under 10 to 14 kV accelerating voltage, 200 to 300 mA beam current, and a beam diameter of 4 mm. The textural preservation of brachiopod shells was investigated under a SEM. Quantification of the different cement phases and paleo-porosity was undertaken using digitized images of polished slabs (15 cm 3 15 cm) and graphic software (Canvas 6). RESULTS OF STABLE-ISOTOPE AND MINOR-ELEMENT ANALYSIS

d13C Variation from Different Carbonate Materials The d13C values of the 23 pristine brachiopod shells plot within two clearly distinct populations (Fig. 4A). One data set, sampled from specimens in the inner and outer platform (n 5 11) shows relatively low values of 4.0 to 4.7‰ (mean 4.3‰; standard deviation (s) 5 0.2‰). Brachiopod shells from the mid to lower slope (n 5 12) display higher values of 5.2 to 5.6‰ (mean 5.3‰; s 5 0.1‰). We use these brachiopod data as a benchmark for (near) primary marine carbonate values. The comparison of these brachiopod data from Asturias with published data sets is also shown in Figure 4A. The d13C values from fibrous calcites (n 5 27) and nonluminescent

(marine) blocky spar (n 5 4) similarly plot in two separate fields (Fig. 5). Fibrous calcites from section 1 (n 5 11) show low values between 20.2 and 1.9‰ (mean 0.9‰; s 5 0.7‰); blocky nonluminescent spar from section 1 (n 5 4) shows values between 2.2 and 2.7 (mean 2.4‰; s 5 0.2‰). In contrast, marine fibrous cements from the slope sections 4 through 8 (n 5 16) plot in a narrow range of 3.3 to 5.3‰ (mean 5.0‰; s 5 0.2‰). Meteoric pendant cements from section I (n 5 4) are depleted in 13C (20.9‰; s 5 0.03‰). Late burial cement phases (luminescent scalenohedral and blocky calcites and saddle dolomites; n 5 24) from all localities uniformly plot in a narrow d13C range of 0.6 to 2.1‰ (mean 1.1‰; s 5 0.4‰; Fig. 5). The isotopic composition of Bashkirian matrix micrites shows a strong correlation between d13C values and their positions in the platform-to-basin transect (Fig. 6). The inner-platform sections 1 and 2 have mean d13C values of 1.2 and 1.3‰ (s 5 0.7‰). The outer-platform section 3 averages 4.2‰. Sections on the slope (4 to 7) show consistently higher mean values between 5.1 and 5.3‰, and basin section 8 has a mean d13C composition of 3.8‰ (although parts of Section 8 underwent Permian karst diagenesis). The isotopic composition of the flank and parts of the toe-of-slope section 8 (which were not overprinted by late karst; Fig. 6) oscillates around values that are slightly more positive than 5‰. Isotope shifts towards lower values are recognized in all sections. On the platform, the excursions are gradual depletions in 13C sharply overlain by carbonate rocks that have higher d13C values. The amplitudes of these platform excursions vary between 1.3‰ and 4‰. These platform shifts are related to exposure horizons, as discussed in a subsequent section. d18O Variation from Different Carbonate Materials As with carbon, oxygen isotope values of brachiopod shells (n 5 23) plot within two separate populations (Fig. 4A). Specimens from the inner


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narrow d13C and a wide d18O range. The brachiopod data from the platform top plot within the upper part of the micrite field delineated by section 3 and, those from the slope fit in the micrite data field of sections 4 to 7 (Fig. 8). Data from radiaxial fibrous cements collected on the platform show a trend comparable to matrix micrites, whereas those from the slope and basin define a narrow field within the scatter of slope-derived matrix micrites. Intrasample Isotopic Variability

FIG. 5.—Cross-plot of d13C and d18O values from Asturian cement phases. Shaded fields are fibrous marine calcites. Numbers refer to sections (see Fig. 1). M 5 early (meteoric) cements from section 1; B 5 late (burial) cements from all localities.

In order to reveal the variability of matrix micrite stable-isotope composition within a rock sample, subsamples were drilled from individual thick sections (; 200 micrometers thick) for comparison. We selected samples whose ‘‘bulk’’ micrite matrix represent the low and high end members in d13C and d18O values. Data collection focused on matrix micrites that were microscopically uniform and on recrystallized micrites that contained intercalated luminescent microsparite. Results from section 1 (Fig. 9) are shown as an example and indicate the following. (1) Subsamples of microscopically uniform micrite closely match the data from ‘‘bulk micrites’’ from the same specimen. The variability for d13C and d18O is always smaller than 0.75‰ and 0.5‰, respectively. The average scatter related to the matrix data set, relative to subsamples, is thus below 0.5‰. (2) The difference in isotopic composition between recrystallized micrite (i.e., patchy micrite replacement by luminescent microspar) and micrite is in the order of 0.25 to 0.5‰. (3) Stratigraphic shifts in the d13C and d18O composition of bulk matrix micrites are mirrored in the isotopic composition of the intrasample matrix subsamples. For instance, the increase in d18O for ‘‘bulk’’ matrix samples 98 and 102 in section 1 are also reflected in an increase in the high d18O values of micrite subsamples. Trace Elements

and outer platform (n 5 11) exhibit a range of 23.1 to 24.3‰ (mean 23.8‰; s 5 0.4‰). Specimens from the mid to lower slope (n 5 12) display a range of 21.0 to 21.8‰ (mean 21.4‰; s 5 0.3‰). d18O values of fibrous cements (n 5 27) mirror the trend shown by brachiopod data (Fig. 5). Fibrous cements from section 1 have values between 25.2 and 27.8‰ (mean 26.5‰; s 5 0.8‰). Specimens of fibrous cement collected in section 4 show a comparable d18O range (Fig. 5). In contrast, d18O values of fibrous cements from sections 5 through 8 range between 20.3 and 23.8‰ (mean 22.5‰; s 5 1.4‰). Meteoric pendant cements in section 1 (n 5 4) are low in d18O (range 27.8 to 28.2; mean 28‰; s 5 0.1‰). Late burial (scalenohedral) cements from sections 5 through 8 are depleted in 18O (range 210.4 to 29.8; mean 210.2‰; s 5 0.28‰; Fig. 5). Oxygen isotope values from matrix micrites show a strong correlation between mean d18O values and the position on the platform-to-basin transect (Fig. 7). The general picture is that of two end members, with low d18O values for platform and uppermost slope samples (mean 25.6‰; s 5 2.2‰), and high d18O values for the mid and lower slope (mean 22.8‰; s 5 2.1‰) and basin (mean 20‰; s 5 0.9‰) samples. The d18O values of sections 4 through 7 oscillate between the isotopic composition of those end members with intervals of upper-slope-derived and platform-derived material retaining the low shoalwater values and intervals of in situ slope deposits reflecting the high basinal values. Patterns in d C versus d O 13

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Bashkirian matrix micrites document a characteristic pattern based on the range in d13C and d18O and the locality studied (Fig. 8). Platform sections 1 and 2 plot in a comparably narrow d13C and d18O field. Section 3 at the platform break displays higher d13C values and more variable d18O. Slope sections 4 through 7 and basin section 8 are also characterized by a

Strontium and manganese abundances in brachiopod shells are commonly used as an estimate of diagenetic alteration (Brand and Veizer 1980). As seen in Figure 4B, the 23 brachiopod shells, that passed the petrographic screening tests show Sr abundances ranging from 820 to 2108 ppm (mean 5 1298; s 5 396 ppm) and manganese abundances ranging from below detection limit to 193 ppm (mean 127; s 5 55ppm). These values plot in the field of modern low-Mg calcite shells (Veizer et al. 1999). The Sr, Mn, and Fe abundances in fibrous marine cements and of scalenohedral cements are commonly below detection limit. DISCUSSION

Seven major factors must be considered in order to interpret the nature of lateral and stratigraphic shifts in the isotope data from the Pennsylvanian carbonates under study. These are, in chronological order: (1) the ambient sea-water isotopic composition; (2) seawater paleo-temperature, (3) the original (precursor) mineralogy, (4) possible isotopic fractionation due to metabolic effects; (5) early meteoric diagenesis; (6) rock-fluid interaction during burial diagenesis; (7) ‘‘late’’ meteoric diagenesis (late Stephanian to Permian exposure and karsting). Carbon-Isotope Shifts Isotopic Composition of Paleo-Tethyan Seawater.—Owing to diagenetic rock–fluid interaction, isotope data from matrix micrites may not reflect primary values of the precipitated aragonitic or high-Mg-calcitic mud. The same holds true for fibrous cements, which originally were high-Mg calcite and now are low-Mg calcite, although they are probably more conservative in terms of their primary isotope values than micrites (Tobin et al. 1996). We follow previous authors (e.g., Bruckschen et al. 1999 and references therein) and use estimates of the Bashkirian primary marine

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FIG. 6.—Stratigraphic plots of d13C for matrix micrites (points) and the range of brachiopod shells (B, shaded gray). Means and standard deviations (s) of micrites are given below each section. Refer to the schematic sketch of the platform and slope for location of sections. E 5 exposure surface; cs 5 master correlation surface (showing estimated error bar of 6 10 m).

carbonate d13C and d18O from pristine brachiopods as benchmarks of primary isotope values. The brachiopod values allow a tentative assessment as to what degree the isotope shifts in fibrous cements and matrix micrites from Asturias might reflect a geochemical signal from the Bashkirian ocean, or alternatively, are early to late diagenetic artifacts. In the case of brachiopod data, the variance observed between platform interior, shelf break, and slope appears to be a primary signal (about 1.3‰; Fig. 4), because it is consistently recorded in all species used for this study (Echinoconchus sp, Kozlowskia sp., Reticulata sp.). The interspecies consistency means that it is not a vital effect related to a specific species. Aging of Platform-Top Water and Precursor Mineralogy.—Patterson and Walter (1994) found that seawater on the Bahamas platform (salinity range 34–38‰) is depleted in 13C by up to 4‰ when compared to openocean surface water. They linked this to the input of 12C from remineralized organic carbon during the long residence time of seawater on the platform. Assuming that the mechanism proposed by Patterson and Walter (1994) is applicable to the Asturian case study, ‘‘aging’’ of the restricted-platformtop water may explain the relatively low d13C values of carbonate specimens collected in sections 1 and 2. Patterson and Walter (1994) dealt, however, with seawater chemistry and did not include isotope data on actual Holocene sediments (mainly aragonite) from the Great Bahama Bank. Those aragonitic sediments exhibit d13C values on the order of 4‰, meaning that they are actually higher (not lower) than the typical 2‰ value of ocean water (R.K. Matthews, personal communication 1999). These relations are important, because aragonite mud is enriched in 13C by roughly 2.7‰ relative to bicarbonate (Romanek et al. 1992), thus it appears that Bahamas aragonite sediment does not preserve the aging effect. This does not necessarily hold true, however, for all calcium carbonates. Assuming

no metabolic effects, brachiopod shells are precipitated in equilibrium with the seawater geochemistry (James et al. 1997) and thus are likely to record the low platform-top isotopic composition of aged seawater. The carbon isotope data from micrites might also be reflecting differences in original sediment mineralogies for the platform interior versus the margin and slope. For example, Wiggins (1986) used spatial differences in Sr–Mg ratios to conclude that the interior of the Pennsylvanian Marble Falls platform of Texas hosted largely aragonitic mud (higher d13C values) whereas the margin hosted predominantly low-Mg and high-Mg calcite mud (lower d13C values). The Asturian d13C micrite values, however, exhibit the opposite pattern: they are low in the platform interior and high at its margin (Figs. 6, 8). This implies that the spatial distribution of calcite versus aragonite lime muds cannot explain the isotope shifts in the Asturian platform under study. This is further supported by the low d13C values of fibrous cements and brachiopods in the platform interior, which had highMg calcite and low-Mg calcite precursors, respectively. Judging from brachiopod d13C values, however, the less stable fibrous cements and matrix micrites have further been lowered by about 3‰, i.e., they underwent a subsequent diagenetic overprint (Figs. 6, 8). Early Meteoric Diagenesis.—A likely mechanism that could cause early diagenetic 13C depletion of the bulk of platform-top micrites and fibrous cements is meteoric diagenesis associated with high-frequency, low-amplitude oscillations in sea level that episodically exposed the shallow-platform-interior sections 1 and 2 (Fig. 6). Isotopically light meteoric water and light soil zone CO2 commonly cause a marked depletion in 13C and 18O in carbonates beneath exposure horizons (Allan and Matthews 1982). Section 3, located at the platform break (Fig. 6), possibly stayed submerged in seawater for a greater length of time, because it was located in a some-


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FIG. 7.—Stratigraphic plots of d18O for matrix micrites (points) and the range of brachiopod shells (B, shaded gray). Means and standard deviations (s) of micrites given below each section. Refer to the schematic sketch of the platform and slope for location of sections. The position of the inferred paleo-thermocline is also indicated. See Figure 6 for key to abbreviations.

what deeper setting. Nevertheless, section 3 also records at least two exposure events. A similar diagenetic model was proposed for the late Pleistocene of the Great Bahama Bank, where numerous small-scale sea-level events are recorded on the platform top but not at the margin (Kievman 1998). If Pennsylvanian soil-zone CO2 is responsible for the low d13C of sections 1 and 2, then the related exposure events left few discernible petrographic traces. The petrographically altered horizons may be very thin and difficult to recognize, or they may have been removed during transgression, whereas the geochemical signal of exposure was preserved in the underlying rocks. Intervals in sections 1 through 3 that bear petrographic evidence for meteoric diagenesis are related to the major shifts in micrite d13C and are indicated with an ‘‘E’’ in Figure 6. These negative shifts often appear as double peaks (‘‘vadose pairs’’). This can occur where early cementation of former exposure surfaces has locally created aquicludes (Matthews and Frohlich 1998). Relation between Lithofacies, Petrophysical Properties, and Isotopic Composition.—Lithofacies pattern and the distribution of volumetrically important skeletal components that might be the source material of detrital carbonate mud are factors that can influence the isotopic composition of marine carbonates (Veizer 1983). In section 2, we tested the relation between biota abundance and the isotopic composition of matrix micrites (Fig. 10). There, it appears that intervals in the section that show abruptly fluctuating distributions of volumetrically important fossil groups (mainly crinoids) display high-frequency isotope fluctuations, whereas a more uniform distribution of biota coincides with low-frequency isotope shifts (Fig. 10). Statistical analysis, however, reveals a poor correlation coefficient (r) between isotopic values and crinoid abundance (d13C, r 5 0.072; d18O, r 5 0.165). Marine cement phases show uniformly low volumes in the platform in-

terior (3 to 7%) and uniformly high volumes at the platform break and the slope (10 to 50%; Table 1). Fluctuations in the abundance of bioclasts or marine cements thus are not likely causes for stratigraphic shifts in the stable-isotope composition of matrix micrite. In terms of lithofacies distribution, the measured sections are composed of a number of distinct facies types that characterize the platform interior, the outer platform, the slope, and the basin settings (Bahamonde et al. 1997; Kenter et al. 2001). As seen in Figure 10, however, isotope shifts are observed in rocks of uniform texture, and uniform isotope values are observed across some textural lithofacies boundaries. This suggests that lithofacies are not a factor in the observed isotopic patterns. Stratigraphic variations in carbonate content and bulk porosity may influence the stratigraphic shifts in micrite d13C via dilution and concentration effects. Overall, porosity of more than 130 plug samples from all sections (Table 1) shows mean values of 2.5% (s 5 0.58%); the mean carbonate content is 93%, and the mean bulk density is 2.7 g/cm3. There is little stratigraphic variability. A correlation between porosity and mean isotopic composition of the carbonates is not obvious. It is thus concluded that the petrophysical properties do not produce the observed shifts in micrite d13C. Effects of Burial Diagenesis on Carbon Isotope Composition.—The large majority of the micrite samples taken for isotope analysis are from massive carbonate rocks. Significant burial alteration of the d13C of the micrites seems unlikely, because of the large mass of carbon in the limestones relative to the small mass of carbon in any potential diagenetic fluid. Magaritz (1983) found that d13C was not appreciably decreased until the water–rock ratio was raised to 1000 or more. From petrographic observations, none of the micrites with lower d13C values are any more recrystallized than those with higher d13C values. The volume and distribution of burial cement phases ranges between 1 to 7%, both spatially and strati-

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A. IMMENHAUSER ET AL. relation of shoalwater carbon-isotope stratigraphy with slope and basinal sections is a problematic task. Oxygen-Isotope Shifts

FIG. 8.—Cross-plot of d13C and d18O values. Gray shaded regions are matrix micrites for sections 1 through 8. brs 5 brachiopods from slope localities; brp 5 brachiopods from platform localities. Heavy dotted lines delineate data for fibrous calcite; thin dotted lines indicate micrite values directly beneath Bashkirian exposure horizons in section 3.

graphically, and could not be correlated to any pattern in isotopic composition (Table 1). Summary and Implications.—Using proposed Baskhirian marine carbonate values (e.g., Bruckschen et al. 1999) as a benchmark for this study, d13C values of brachiopod shells, marine carbonates, and matrix micrites from slope sections 4 to 7 are invariant and close to the inferred seawater values (Figs. 4, 6). This implies that these slope samples stabilized under the influence of marine fluids, retained values close to their primary isotopic composition, and underwent only minor late diagenetic overprint. In contrast, the isotopically lighter (altered) d13C values of micrite and fibrous cement samples in platform sections 1 and 2 (and to a lesser degree of section 3) are interpreted to have been affected by meteoric diagenesis. The d13C values of prisitine brachiopod shells from platform section 1 are (on average) one permil lower than d13C values from pristine slope brachiopods. This might reflect primary changes in coastal water-mass isotope signature (aging of seawater). The main implication of this is that the cor-

In the marine and the early diagenetic environment, the diagenetic stabilization of metastable aragonite and high-Mg calcite to stable low-Mg calcite generally results in 18O depletion (Banner and Hanson 1990). Subsequent alteration may also occur during shallow burial because of compaction and pressure solution until the rock is completely lithified. Once lithified, 18O depletion is generally much less pronounced (Veizer et al. 1999), unless the rocks are entering the field of low-grade metamorphism. Oxygen Isotope Composition and Temperature of Paleo-Tethyan Seawater.—Previous authors (e.g., Adlis et al. 1988; Mii et al. 1999; Veizer et al. 1999 and references therein) argued that the inner fibrous layer of pristine brachiopod shells reflects the temperature and the oxygen isotope composition of ambient seawater when the shell formed. Several lines of argument suggest that this might hold true for the d18O values obtained from Asturian brachiopod shells in this study. Evidence for this is the excellent textural preservation of the brachiopod shells from the platform top and the slope sections as recognized under SEM. This is further supported by the nonluminescent appearance of the shell material. Another line of argument is based on trace-element data. Trace-element data of diagenetically altered shell material are commonly characterized by decreased Sr and increased Mn contents (Brand and Veizer 1980). The Sr and Mn concentrations of the Asturian brachiopod shells, however, are within the range of modern low-Mg calcite shells (Fig. 4B). Furthermore, Sr and Mn values of shell material from all localities across the transect plot in the same field. In contrast, d18O data from shells collected on the platform top differ consistently by about 2.3‰ from shells found on the mid to lower slope (Fig. 4A). It is difficult (but not impossible; cf. Banner and Hanson 1990) for diagenesis to have lowered the oxygenisotope values of brachiopod shells on the platform top by several permil without affecting their trace-element composition. Finally, d18O values of Asturian brachiopod shells plot within the ‘‘heavy range’’ of published Late Carboniferous primary brachiopod data (Fig. 2), which is difficult to explain by diagenetic reequilibration of Asturian brachiopods. If the d18O values from Asturian brachiopod shells are in fact a preserved primary signal, then two interpretations are tenable. The first is that the d18O of surficial water masses recorded a considerable influx of isotopically light meteoric water from the platform interior down to about 150 m depth below the shelf break (i.e., section 4 in Fig. 7). The alternative interpretation is that the basinal water masses were cooler, and thus enriched in d18O, relative to the surficial water. For three reasons, we reject the first interpretation (meteoric runoff). First, plate-tectonic reconstructions indicate that the platform under study was located near the equator during the Bashkirian and Moscovian. This implies that the oxygen isotope fractionation of meteoric water was less pronounced than at higher latitudes. Second, the Sierra de Cuera platform was not attached to a mainland but formed an isolated carbonate buildup. Meteoric influx via river systems is thus not expected. Third, brachiopod d18O values show little spatial variability between the platform-interior section 1 (where meteoric runoff is expected to be most dominant) and the platform margin section 3 (where meteoric runoff is expected to be least dominant). On the basis of these considerations, we favor the interpretation that differences in brachiopod d18O reflect cooler basinal water masses. Using the paleo-temperature equation of Kim and O’Neil (1997), the difference in brachiopod d18O translates in a temperature gradient of about 98C between surficial and basinal water. This might imply that the higher d18O measured in sections 5 to 8 are related to cold basinal water as contrasted to the warmer surficial water in the platform-top sections 1 to 4. If this holds true, then the permanent paleo-thermocline between the warm surficial and cold basinal water would have been between sections 4 and 5,


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FIG. 9.—Comparison of d13C and d18O data from ‘‘bulk’’ matrix micrite with nonluminescent matrix, recrystallized (luminescent) matrix, and marine, meteoric, and burial cements in section 1. Symbols define the thick section sampled; numbers define the type of sample. Inset shows plot of d13C and d18O data from section 1. Samples used for intrasample variability study are indicated.

i.e., in a depth range between 80 and 250 m beneath the shelf break, where a pronounced shift in oxygen isotope composition is observed (Fig. 7). Typical present-day mean temperature profiles for low-latitude settings show a permanent thermocline that commences at 200–300 m and extends down to 1000 m depth. There is an average present-day low-latitude temperature gradient of 98C over a depth range from 0 to about 300 m. These

observations suggest that the inferred temperature gradient with depth obtained from Asturian brachiopod d18O data is plausible. Meteoric Diagenesis.—In terms of d18O values, meteoric diagenesis causes depletion in 18O due to the influence of isotopically light rainwater (Allan and Matthews 1982). In fact, d18O values of platform-interior sections must have been reset during early meteoric diagenesis unless the

FIG. 10.—Relationship between lithofacies, micrite isotopic composition (black dots), and point-counting data of volumetrically important bioclasts. Note rapidly oscillating distribution of skeletal components and isotope values in lower half of section 2 (shaded gray). For. 5 foraminifera, Alg. 5 algae, Bra. 5 Brachiopods. In the facies column, B 5 boundstone, M 5 mudstone, W 5 wackestone, P 5 packstone, G 5 grainstone.

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A. IMMENHAUSER ET AL. TABLE 1.—Marine, late (burial), and present-day cement volumes

Platform Domain Inner platform

Marine Cement Main phases

Upper slope

Isopachous rims, syntaxial overgrowth cement Isopachous rims, fibrous calcite, syntaxial overgrowth cement Isopachous rims, fibrous calcite, syntaxial overgrowth cement Fibrous calcite, botryoidal calcite

Lower slope

Fibrous calcite, botryoidal calcite

Toe of slope and basin

Fibrous calcite (between breccia clasts)

Outer platform Platform break

Marine Cement Volume (%)

Burial Cement Main Phases

562

Blocky, syntaxial overgrowth cement

8–9 6 5 10–50 10–50 10–50 5–10

meteoric fluids had the same oxygen isotope composition as seawater, which is unlikely. Resetting occurs because of the higher water–rock ratio of meteoric water with respect to oxygen relative to carbon (Banner and Hanson 1990). This is supported by the low d18O values of meteoric cements in section 1 (28‰; Fig. 5). As noted previously, the platforminterior sections 1 and 2, located in the shallowest setting, were exposed during low-amplitude sea-level events. Sections at the platform margin or the uppermost slope, located in deeper settings, emerged only during sealevel drops of considerably larger amplitude. Soreghan and Giles (1999) documented amplitudes of Late Carboniferous glacio-eustasy in excess of 100 meters. Using this value as a benchmark, we might expect to see some degree of meteoric alteration down to about 100 m below the shelf break (i.e., in sections 1 through 4). It is noteworthy that means of matrix micrite values in sections 1 through 4, located in a bathymetric range of about 0 to 80 m, are in fact depleted in 18O relative to values obtained in ‘‘deeper’’ sections 5 through 8 (Fig. 7). In sections 1 and 2, micrite d18O values do not shift beneath exposure surfaces (Fig. 7). This pattern of invariant d18O coupled with variable d13C possibly means that all of the micrite has been isotopically reequilibrated with meteoric water (the ‘‘meteoric water lines’’ of Lohman 1988). On the basis of these considerations, it is possible that the high-Mg precursor mineralogy of fibrous calcites in sections 1 and 2 stabilized predominantly to low-Mg calcite under the influence of ‘‘light’’ meteoric fluids. In contrast,

FIG. 11.—Relationship between d18O and upper-slope-derived and platformderived (white), and in situ slope deposits (light and dark gray) in section 5.

Scalenohedral calcite, blocky calcite, saddle dolomite, syntaxial overgrowth cement Scalenohedral calcite, blocky calcite, saddle dolomite, syntaxial overgrowth cement Scalenohedral calcite, blocky calcite, saddle dolomite Scalenohedral calcite, blocky calcite, saddle dolomite Scalenohedral calcite, blocky calcite, saddle dolomite

Burial Cement Volume (%) 1–2 1–2

Present Day Porosity Volume (%) 2.5 6 1 2.5 6 1

2–3 6 1

2.5 6 1

3–5 6 2

2.5 6 1

3–5 6 2

2.5 6 1

2–3 6 1

2.5 6 1

fibrous calcites in sections 3 through 8, showing higher d18O values, were thus most likely stabilized in ‘‘heavier’’ seawater (Fig. 8). Variations in early meteoric and marine diagenesis might also explain the highly variable oxygen isotope composition of matrix micrites on the mid-slope to lower-slope sections. There, intervals of the sections that are built by upper-slope-derived and platform-derived carbonate material (Fig. 11) show characteristically low values around 26‰, which is suggestive of an imported meteoric signal. Conversely, intervals that formed in situ on the slope (bafflestone and automicrite), beyond the reach of early meteoric diagenesis, show high open-water values near 0‰ (Figs. 7, 11). Burial Diagenetic Overprinting.—After deposition, all sections under study underwent comparably shallow Moscovian burial on the order of 1 km and less (Fig. 2; Lepvier and Martinez-Garcia 1990). It is possible that most of the carbonate sediments became lithified owing to compaction and pressure solution during this Moscovian burial, and thus burial diagenesis may have affected micrite d18O values. As discussed by Veizer et al. (1999), the 18O depletion of carbonate rocks once lithified is generally less pronounced than before and during lithification. Judging from the presence of saddle dolomites, a burial temperature field of 60–1508C (Radke and Mathis 1980) is deduced. Assuming an average geothermal gradient and a burial depth of 1 km, a burial temperature of about 358C results. This implies that saddle dolomite in Asturian carbonates formed at its lower depth–temperature limit. Vertical changes in the fluid–rock ratio during burial diagenesis, which can result from differences in preburial pore volume related to fabric or compaction, can influence the magnitude of diagenetic alteration (Algeo et al. 1992). Parts of the sections with a larger open pore volume might have permitted circulation of larger volumes of isotopically light burial fluids and therefore underwent greater 18O depletion. Such differences in preburial porosity and thus fluid–rock ratio could explain the d18O shifts in the Bashkirian slope sections 4 through 7 (Figs. 7, 11). There is, however, an inverse relationship between burial cement volume (i.e., pre-burial pore volume) and micrite d18O composition. The limestones that contain the largest volume of burial cement (Table 1), i.e., the outer platform and the slope sections (3–5%), show high d18O values. Limestones from the innerplatform sections that have a small burial cement volume (1–2%) show low d18O values (Table 1). This suggests that the distribution of preburial pore volume was not a major controlling factor affecting the d18O of matrix micrites. Another potential mechanism of burial overprinting of d18O values is recrystallization of matrix micrites with depth and/or addition of burial cements within the open pore space of the matrix micrites with depth. However, the difference in oxygen isotope composition between micrite and patchily recrystallized luminescent micrite in section 1 is on the order of only 0.25 to 0.5‰ (Fig. 9). This implies that burial recrystallization of matrix micrites alone is insufficient to explain the oxygen isotope variability between platform top and basin.

SIGNIFICANCE OF PENSYLVANIAN ISOTOPE SHIFTS In the late Moscovian–early Kasimovian, i.e., about 8 Myr after deposition, the Bashkirian carbonate rocks were rotated in vertical thrust imbricates. The spatial and stratigraphic isotope shifts under study were most likely established prior to this rotation. Summary and Implications.—Our data suggest that the d18O variability of pristine brachiopod shells is a preserved primary signal. The favored interpretation is that brachiopod shells from slope settings recorded higher d18O values due to cooler basinal water masses. Micrite from platform sections reflects an invariant d18O coupled with variable d13C. This possibly means that all of the micrite has been isotopically reequilibrated with the oxygen isotope signature of meteoric water. Sediments that were shed downslope from the platform margin exported these low meteoric d18O values. In slope sections, the lower (meteoric) d18O of these transported sediments contrasts with the higher (marine) d18O of in situ sediments and marine cements. It appears that shallow burial diagenesis (# 1 km) has not erased the dominant early diagenetic d18O values of matrix micrites and marine cements on the platform top. Two main implications follow: First, pristine brachiopod shells collected in different bathymetric levels may reflect variations in d18O due to temperature differences of the ambient seawater. This must be kept in mind when using brachiopod data from different settings for geochemical studies. Second, metastable aragonite and high-Mg calcite mineralogies that stabilized in an early meteoric or marine diagenetic environment largely preserve their isotopic signatures under shallow burial conditions. CONCLUSIONS

Across a platform-to-basin transect, d13C isotope data from matrix micrites, marine fibrous cements, and pristine brachiopod shells are one to several permil lower relative to samples from the platform margin and the slope. In the case of d13C from brachiopod shells, this trend can be explained by changes in coastal water-mass isotope signature. This is of relevance for the application of brachiopod d13C from shoalwater settings. These data might not necessarily reflect global and paleoceanographic changes in carbon chemostratigraphy. Variability in micrite and marinecement d13C between platform-interior and basin can be explained by early diagenetic stabilization of aragonite and high-Mg calcite in the presence of soil-zone CO2 during exposure events. Oxygen isotope signatures of pristine brachiopod shells, matrix micrites, and marine fibrous cements from shallow platform settings are statistically depleted in 18O compared to samples taken in a bathymetric range of about 150 m and deeper. In the case of pristine brachiopod shells, this trend can be explained by a steep temperature gradient between a warm mixed surface-water layer and cooler bottom-water masses. This result suggests that care must be taken if brachiopod-shell data from different bathymetric levels are combined in order to reconstruct d18O trends of the ambient seawater. Matrix micrite and marine-cement d18O values were lowered, mainly during pervasive early meteoric diagenesis and, to a minor degree, during subsequent shallow burial diagenesis. Platform-top-derived sediments exported their low (meteoric) d18O signature downslope where they contrast the higher (marine) d18O values of in situ slope sediments (automicrites). In the case of this Asturian study, it appears that shallow burial diagenesis has not affected or only mildly affected, those carbonate sediments that stabilized in meteoric or marine fluids. This implies that, under favorable conditions, the isotopic values of in situ micrites and marine cements from slope to basin settings can serve as proxies for ambient seawater geochemistry. In contrast, Asturian matrix micrite and marine cement data from shallow marine settings predominantly reflect early meteoric signals. ACKNOWLEDGMENTS

We would like to thank H. Vonhof for support in the stable-isotope laboratory, E. Villa (University of Oviedo) for analysis and refinement of fusulinid biostratigraphy, and L. Matıńez-Chacon (University of Oviedo) for determination of brachio-

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pod species. W.J. Lustenhower analyzed the minor-element composition of brachiopod and cement samples. These analyses were provided by the Vrije Universiteit Amsterdam and by NOW, the Netherlands Organization for Scientific Research. Some of the platform-top samples were measured at the GEOMAR Research Center for Marine Geosciences in Kiel, Germany. We acknowledge the numerous controversial comments by S.J. Burns, J.A.D. Dickson, E. Grossman, C. Lećuyer, R.K. Matthews, G. Della Porta, W. Schlager, L. Soreghan, H. Weissert, and an anonymous JSR reviewer. JSR editor D. Budd and technical editor J.B. Southard rigorously criticized a previous version of this manuscript. The data described in this paper have been archived, and are available in digital form, at the World Data Center-A for Marine Geology and Geophysics, NOAA/NGDC, 325 Broadway, Boulder, CO 80303; (phone 303-497-6339; fax 303-497-6513; E-mail: [email protected]; URL http://ngdc.noaa.gov/mgg/sepm/jsr/). REFERENCES

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