STABLE ISOTOPE SIGNATURE OF MIDDLE DEVONIAN SEAWATER FROM HAMILTON GROUP BRACHIOPODS, CENTRAL NEW YORK STATE Bruce Selleck and Drew Koff Department of Geology, 13 Oak Drive, Colgate University, Hamilton, NY 13346;
[email protected] ABSTRACT: Stable isotope ratios of oxygen in articulate brachiopod shell low-magnesium calcity have been widely used as a proxy for seawater d18O through the Phanerozoic. It has been suggested that the generally negative d18O in Devonian brachiopods reliably records seawater isotope composition. In this study, stable isotope ratios of carbon and oxygen were obtained on well-preserved brachiopod shells from the Middle Devonian Hamilton Group of Central New York State. The sample set includes multiple specimens of two common genera, Spinocyrtia and Mucrospirifer. Spinocyrtia is a relatively thick-shelled genus that is found in sandy or firmmud bottom facies, whereas thinner-shelled Mucrospirifer had a somewhat broader environmental tolerance including organic-rich muddy substrates. Optical and scanning electron microscope images of shell material show remarkably well-preserved orginal biogenic microstructures, suggesting minimal diagenetic recrystallization of calcite. Consistent differences in stable isotope ratios between the two genera (Spinocyrtia mean d13C = 2.2; d18O = -6.7; Mucrospirifer mean d13C = 1.8; d18O = -8.1; all VPDB) suggest differing vital effects and/or cryptic diagenetic alteration. However, most samples show very limited or no alteration, and have stable isotope values in agreement with the Paleozoic data set of Veizer et al. (1999). Seawater in equilibrium with the Hamilton Group brachiopod calcite would have d18OSMOW of -4 to -10‰. Burial diagenetic calcite cement from the Hamilton Group has isotopic values that indicate precipitation from evolved basinal fluids with relatively more positive d18O than either modern or Devonian seawater.
INTRODUCTION The utility of biogenic calcite as a faithful recorder of ancient seawater temperature and stable isotopic character remains an important topic of geochemical research. The extensive database of Veizer et al. (1999) has been interpreted to demonstrate that low-magnesium calcite skeletons of brachiopods, and other low-Mg calcite fossils, record long-term secular changes in the isotopic composition of seawater. Change in d18O of seawater driven by isotopic exchange within mid-ocean ridge hydrothermal systems is proposed as the mechanism for generally increasing (more positive) values of seawater through geologic time. Long-term stability of d18O of seawater has been proposed by Muehlenbacks and Clayton (1976) based on studies of preserved seafloor hydrothermal systems in ophiolites, with the biogenic record of secular change questioned because of the presence of unrecognized or unappreciated diagenetic recrystallization. Brachiopods are favored as potential records of isotopic geochemistry because they and composed of lowmagnesium calcite and grow relatively slowly, and would therefore be expected to precipitate shell carbonate in isotopic equilibrium with ambient seawater. Studies of modern brachiopods (Parkinson et al. 2005) generally show that deepwater species do faithfully record seawater isotopic values and temperatures, while intertidal zone species have more variable isotopic signatures that may reflect disequilibrium crystallization of shell calcite during rapid episodes of growth that cause kinetic fractionation (Auclair et al. 2003). Kinetic factors may result in relatively more negative values of d18O in shell carbonate than would be expected from equilibrium precipitation (van Geldern et
al. 2006). Stable isotopic data from fossil Ca-phosphate skeletal material (e.g. conodonts) generally suggests that d18O of Paleozoic seawater was slightly more depleted than modern values, although the mechanisms of fractionation of O during precipitation of conodont phosphate are poorly understood. To better understand the cause of variation of d18O and d13C in ancient brachiopod calcite, the current study focuses on two common brachiopod genera, Mucrospirifer and Spinocyrtia, from the Middle Devonian Hamilton Group in central New York State (Fig. 1). GEOLOGY OF THE STUDY AREA The paleogeographic and tectonic framework for Middle Devonian sequence in eastern North America is welldocumented. Sevon and Woodrow (1985) and Ettensohn (1985) provide excellent summaries of the general evolution of the Catskill Delta Complex. Brett and Baird (1996) discuss regional sedimentary processes in the northern Appalachian Basin. Linsley (1994) provides a thorough stratigraphic and paleontological summary of the Hamilton Group and other Devonian sequences in New York. The Middle Devonian Hamilton Group of central New York was derived from the deposition of clastics sourced in the Acadian Highlands to the east (current geography) of the Appalachian foreland basin. In the study area the Hamilton Group is composed predominantly of fine-grained gray sandstone, mudstone, siltstone and organic-rich dark shales. The gray mudstone and siltstone represent deposition under normal marine, moderately oxygenated conditions, while relative deepening events have been proposed to explain
Northeastern Geology & Environmental Sciences, v. 30, no. 4, 2008, p. 330-343.
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Figure 1. Outline map showing outcrop belt of middle Devonian strata in central New York State with inset map illustrating sample localities. PS=Peterboro South; GR=Geer Road; QH=Quaker Hill; P=Pecksport; B=Bridgewater the presence of the organic-rich intervals. Coarse siltstone to fine sandstone units with storm event beds (coarse shell fragment horizons, hummocky cross-strata) cap 10-meter scale coarsening upwards sequences in the eastern portion of the study area. To the west, shallowing intervals are capped by bioclastic limestones and fossil-rich calcareous siltstones. These maximum shallowing facies are abruptly overlain by deeper water facies. Brett and Baird (1996) have interpreted the bioclastic limestones as representing relative clastic sediment starvation, and the contact of calcareous siltstones and limestones with overlying dark shale representing the fastest rates of sea level rise. Previous Studies of Hamilton Group Brachiopod Stable Isotopes Popp et al. (1986) examined textural, chemical, and stable isotopic characteristics of brachiopods in Middle Devonian limestones of North America including some samples from the Hamilton Group and concluded that Hamilton Group brachiopods are relatively resistant to diagenesis and therefore are likely to preserve primary isotope signatures. δ18OPBD values reported by Popp et al. (1986) range from -2.5 to -12.5‰; δ13CPBD values range from -4.8 to 4.8‰. Our results for Mucrospirifer and Spinocyrtia from central
New York State have a somewhat narrower range (δ18OPBD values from -5.0 to -10.6‰; δ13CPBD values from +2.8 to -0.8‰). Bates and Brand (1991) reported the results of isotopic analysis on brachiopods in the Hamilton Group of western New York State. Only minor isotopic variation between species led them to conclude that brachiopod calcite faithfully recorded environmental water conditions at the time of shell growth. Significant differences were observed in the isotopic signatures of shallow (mean values: δ13CPBD +2.97‰, δ18OPBD -3.83‰) and deep (mean values: δ13CPBD +5.01‰, δ18OPBD -2.85‰) water brachiopods. This δ18O distinction was attributed to temperature and salinity changes associated with water depth. The δ13C differences were attributed to varying amounts of organic matter associated with changes in water depth. The mean values of δ18O in our study (Mucrospirifer mean -8.1‰; Spinocyrtia mean 6.7‰) are significantly more negative than the results of Bates and Brand (1991). Goddéris et al. (2001) used oxygen isotopes of brachiopod shell calcite to constrain a model of δ18O, 87Sr/86Sr, δ13C, δ34S, and pCO2 through the Paleozoic, and reported δ18OPBD values in Middle Devonian brachiopods ranging from -4‰ 331
STABLE ISOTOPES HAMILTON GROUP BRACHIOPODS to -7.5‰ with a mean of -5.5‰. δ13CPBD for middle Devonian shell carbonate ranged from +2.2‰ to -0.5‰ with a mean of +0.8‰. Our results for δ18OPBD are generally somewhat more negative than the global sample set of Goddéris et al. (2001). van Geldern et al. (2006) documented changes in δ18OPBD and δ13CPBD through the Devonian using brachiopod samples from a number of globally distributed localities. Their data suggest that oxygen isotope values of shell carbonate became more negative (from -2.8 to -6.1‰) from Early through Late Devonian time, suggesting that changes in the isotopic composition of the Devonian oceans occurred during this interval. However, this Late Devonian oxygen isotope trend is not paralleled in conodont phosphate δ18OPBD data (Joachimski and Buggisch 2004). Our results are generally more negative in δ18OPBD than the van Geldern et al. (2006) data set. Selection of samples for the current study.-- Biogenic carbonate from brachiopods in the Hamilton Group in central New York was sampled for this study. Generalized facies characteristics of the sampled intervals are described in Table 1. The rock types from which fossils were extracted are relatively mud-rich facies, chosen to limit the potential degree of diagenetic recrystallization of fossil material overall. Care was taken to select the most well-preserved shell material during field sampling. The
genus Mucrosprifer is a relatively thin-shelled form that inhabited a range of muddy to fine sandy environments in the Hamilton Group. The thin shell was predicted to be more responsive to diagenetic recrystallization if such processes occurred after deposition. The genus Spinocyrtia is a more thick-shelled form that is generally found in sandier, firm-bottom facies in the Hamilton Group, although its occurrence overlaps with that of Mucrospirifer. The thicker shell of Spinocyrtia should make this form more resistant to diagenetic alteration. In general, diagenetic recrystallization at elevated temperatures during burial in contact with connate seawater will result in precipitation of calcite with relatively more negative d18O compared to primary values. Likewise, recrystallization of shell carbonate in contact with meteoric water will cause more negative d18O values. Results of analysis indicate that the calcite of Mucrospirifer generally exhibits greater negative δ13C and δ18O values than that of Spinocyrtia. This relationship is interpreted within the context of the different physiological controls and varying environmental parameters existing between the two brachiopod species. METHODS Five field sites were selected for study and sample collection (Table 1; Figs. 1, 2). Sections were measured
Table 1. Generalized facies characteristics of the sampled intervals.
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Figure 2. Schematic stratigraphic cross-section of Hamilton Group strata in the study area showing approximate location of intervals from which brachiopod samples were obtained. Locality keys as in Figure 1. and described to assess sedimentological features and determine distribution of fossils. Samples were extracted at the outcrop sites using hammer and chisel; the freshest, least weathered materials were collected and individual shells removed and cleaned using a low-power binocular microscope and dental pick. Where necessary, shell material was gently broken and freed from matrix. Millimetersize fragments of shell material were then selected for further preparation. Samples from the Colgate University collections from the Peterboro South locality were also included in this study because this locality is well-known for excellent preservation of shell microstructure and unusual preservation of soft-bodied annelid worms (Cameron 1967, 1968; Carter 1980, 1990). Individual shells from this locality were subsampled along the growth axis from hinge area (pedicle opening) to shell margin. Samples of coarsely
crystalline calcite spar cement were collected from shelter voids within articulated brachiopods at the Bridgewater and Peterboro South localities. Approximately 100 mg of shell and spar cement for each sample was ground using mortar and pestle, and heated in open crucibles at 300°C for 1 hour to oxidize any organic carbon present. Samples were then re-ground and stored for analysis. Prepared samples were analyzed for carbon and oxygen and carbon SIRA at the Stable Isotope laboratory at New York State University at Albany. The samples were placed in septum vials and loaded into a heated rack with temperature controlled to ± 0.1°C. 100% phosphoric acid was sequentially needle injected producing CO2 which was then passed to a Micromass Optimagas-source triplecollector mass spectrometer for determination of stable 333
STABLE ISOTOPES HAMILTON GROUP BRACHIOPODS isotope ratios. Regular standardization with NBS-7 and replicate analysis of samples indicate precision of better than 0.1% for both d13C and d18O. All isotopic data are reported relative to PDB. RESULTS
exhibited minor twinning within prismatic shell layer calcite crystals (Fig. 3B), suggesting minor structural deformation and perhaps recrystallization during burial. Optical microscopic evidence for recrystallization of original shell material was not seen in any other of the 22 samples examined in thin-section.
SEM and Optical Petrography.-- Examination of thinsections of brachiopod shells from most localities showed uniformly consistent preservation of original shell microstructure. However, one sample containing thick Spinocyrtia shells from the Bridgewater locality revealed minor fractures with 20-60 mm thick veinlets of calcite crosscutting otherwise well-preserved secondary layer prismatic calcite (Fig. 3A). These veinlets connect with wispy, stylolitic clay seams in the surrounding carbonatecemented sandy siltstone matrix. This same shell also
Scanning Electron Microscopy.-- Subsamples of shell material from all of the brachiopod materials sent for stable isotope analyses were also examined using secondary electron imaging. Typical images are shown in Figure 3C and 3D, and reveal the remarkable level of preservation of shell microstructure in Hamilton Group brachiopods. Figure 3D illustrates the preservation of both primary and secondary shell layer calcite. In all samples examined, microscopic evidence for diagenetic recrystallization, such as dissolution pitting or coarsening of primary calcite
Figure 3. A. Optical photomicrograph (cross-polarized light) of Spinocyrtia in calcareous siltstone/fine sandstone matrix, Bridgewater locality. Arrow indicates veinlet of secondary calcite cross-cutting original prismatic layer microstructure. B. Optical photomicrograph (cross-polarized light) of Spinocyrtia shell showing strain-induced twinning of prismatic layer calcite crystals, Bridgewater locality. C. Secondary electron scanning electron microscope image of broken Mucrospirifer shell, Peterboro South locality. Note excellent preservation of original shell microstructure. D. Secondary electron scanning electron microscope image of broken Mucrospirifer shell, Pecksport locality. Note preservation of original shell fabric in contact with silty shale matrix. 334
SELLECK AND KOFF crystals, was very rare. SEM-cathodoluminescent imaging of polished thinsections of brachiopods revealed no luminescent areas, but clearly post-depositional calcite cement in shelter voids is also non-luminescent, so that this criterion could not be used to distinguish areas of burial diagenetic replacement of original shell calcite. Based upon both optical and SEM study, there is very limited evidence for recrystallization of original shell calcite; however, the presence of cryptic dissolution-reprecipitation processes cannot be ruled out. Stable Isotope Results.-- Results of stable isotope analyses of brachiopods from the Hamilton Group, and of calcite spar, are presented in Table 2. While these results are broadly
similar to the ranges for Devonian brachiopods reported by Popp et al. (1986) and Bates and Brand (1991) for central and western New York Hamilton Group brachiopods, there are differences. Our results for Mucrospirifer and Spinocyrtia from central New York State have a somewhat narrower range (δ18OPBD values from -5.0 to -10.6‰; δ13CPBD values from +2.8 to -0.8‰) than the values reported by Popp et al. (1986), whose data include samples from both central and western New York State, and a greater number of genera. The mean values of δ18O in our study (Mucrospirifer mean -8.1‰; Spinocyrtia mean -6.7‰) are more negative than the results of Bates and Brand (1991) from Athyris, Mucrospifer and Mediospirifer from Genesee and Erie County localities. Our results for δ18OPBD are generally somewhat more negative than the global sample
Table 2. Results of stable isotope analyses of brachiopods from the Hamilton Group, and of calcite spar.
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STABLE ISOTOPES HAMILTON GROUP BRACHIOPODS Table 2. Results of stable isotope analyses of brachiopods from the Hamilton Group, and of calcite spar (continued).
set of Goddéris et al. (2001). The results of a world wide sampling of Devonian brachiopods reported by van Geldern et al. (2006) include Lower and Middle Devonian values that are somewhat more positive (d18O in the range -2.8 to -3.5‰) and relatively more negative Middle to Upper Devonian values (d18O in the range -4.3 to -6.1 per mil). For the Hamilton Group brachiopods analyzed in this study, comparison of d13C and d18O as shown in Figure
4, indicates that there is considerable overlap among the samples, but that Spinocrytia samples overall are slightly more positive in oxygen and carbon stable isotope ratio. Table 3 presents t-statistic data that supports the hypothesis that the two genera are different with regard to d18O and d13C. A significant cluster of values for Spinocrytia are the most positive d13C and d18O values determined in this study, and form a discrete field in which no values of Mucrospirifer occur.
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Figure 4. Bivariate plot of δ13C vs. δ18O (VPDB standard) for all brachiopod samples analyzed in this study. Note the cluster of values for Spinocyrtia that appear to define a discrete cluster in which no Mucrospirifer values overlap.
Table 3. T-statistic data that supports the hypothesis that the two genera are different with regard to d18O and d13C.
Figure 5 illustrates the variation in stable isotope ratios within the two brachiopod genera by locality. Values of Spinocyrtia from the Bridgewater locality, where minor recrystallization of shell material was observed in thinsection, overlap with the values of Geer Road samples. Note that samples from Peterboro South, a locality known for excellent preservation of primary shell carbonate appear to be somewhat lighter in terms of both d13C and d18O than Spinocyrtia from the other two localities. Values for Mucrospirifer show no consistent variation by locality. Stable isotope values for subsamples of individual shells from the Peterboro South locality are illustrated in Figure 6. No trend along the growth axis was observed, and the range of values within individual shells is similar to the range for the genus as a whole. The mean values of all samples of the two genera, plus calcite spar cement are plotted in Figure 7. As expected, the calcite spar values are much more negative in d18O, typical of burial diagenetic calcite precipitated at elevated temperature, or calcite precipitating from meteoric fluids. The relatively more negative d13C values are attributable to derivation of carbon in carbonate from organic carbon reservoirs during burial. The trend toward more negative values is shown via the arrow labeled “expected diagenetic trend”. Calculation of equilibrium temperatures.-- Using the averaged data from Table 3 and applying the constants of O’Neill et al. (1969), the isotopic composition for equilibrium precipitation of shell calcite can be calculated, assuming a range of values of temperature of the water from which precipitation took place, and assuming that the 337
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Figure 5. A. Bivariate plot of δ13C vs. δ18O for Spinocyrtia samples, grouped by locality. B. Bivariate plot of δ13C vs. δ18O for Mucrospirifer samples, grouped by locality. measured values of shell carbonate isotope ratios reflect original shell precipitation from seawater. As shown in Figure 8, the calculated equilibrium temperatures for shell carbonate are unrealistically high (45-65°C) if the original seawater d18O is assumed to be 0‰SMOW. Also shown in Figure 8 is a range of seawater temperature and oxygen isotope values calculated using data from the cluster of the most positive values of Spinocyrtia that might be assumed to represent the least diagenetically altered shell material (see Table 3 for values). Based upon these calculations, for temperatures that are in the 25-35°C range (reasonable assuming the subtropical location of the region in Middle
Devonian time), a seawater d18Osmow in the range of 6 to -3‰ is required for the most positive range of shell carbonate isotope values. This result is similar to the values of Givetian to Late Devonian seawater determined by van Geldern et al (2006). The calculated d18Osmow of water in equilibrium with spar cement at 140°C is +2.7 to 3.9‰, more positive than modern seawater. A temperature of 140°C represents the minimum burial temperature reached by the Devonian rocks in central New York, based upon regional paleotemperature studies (Sarwar and Friedman 1995). Higher assumed temperatures
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Figure 6. A. Bivariate plot of δ13C vs. δ18O for subsamples of individual Spinocyrtia shells from the Peterboro South locality. Note that the range of values for individual shell subsmaples (e.g. PS-2) covers nearly the entire range of all subsamples. B. Plot of δ13C vs. δ18O for subsamples of individual Mucrospirifer shells from Peterboro South locality.
of cement precipitation require values of water that are increasingly more positive (e.g. if the assumed temperature of burial cement precipitation is 160°C, the calculated equilibrium water is +4.3 to 5.5‰). DISCUSSION Comparison with other results for Devonian Brachiopods.-As noted above, our results are broadly comparable to other published stable isotope data for Devonian brachiopod shell carbonate, but differ in some important aspects. The data of Popp et al (1986) for the Middle Devonian of eastern
North America include a broader range of genera than the current study, and not surprisingly their results show a much greater range of variation in both carbon and oxygen isotopic values. The data from Bates and Brand (1991) is from a relatively narrower geographic range and included three genera, and the range of variation in the stable isotope data is less than the Popp et al. (1986) data set, and the data reported here. The relatively more negative oxygen isotope values in our data and that of Popp et al (1986) may reflect differences in burial signature or possibly primary seawater or temperature controls. However, the use of 339
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Figure 7. Bivariate plot of mean values of δ13C vs δ18O for all Mucrospirifer, Spinocyrtia, Spinocyrtia subset (see text) and calcite spar samples. Error bars are 1σ.
Figure 8. Calculated δ18Osmow of seawater in equilibrium with brachiopod shell calcite based on mean values of Spinocyrtia and Mucrospirifer (dashed line area), and on the mean values of the Spinocyrtia data subset with the most positive isotope values (solid line area). Range of calculated values for each band is based on 1σ range from mean shell calcite values. different brachiopod genera in these studies makes detailed comparisons of the data problematic, because vital effect differences between genera may be more important than other environmental factors or burial history (Parkinson et al. 2005). Our results for δ18OPBD on Hamilton Group brachiopods are generally somewhat more negative than the global sample set of Goddéris et al. (2001) and the Devonian
data of van Geldern et al. (2006). This difference may reflect subregional ocean water composition or temperature differences, or burial diagenesis. Differences between genera.-- The differences in stable isotope values for the two genera examined in this study could indicate a primary difference based upon vital effects related to ecology and/or shell secretion mechanisms. Isotopic analyses of modern brachiopods (Parkinson et
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SELLECK AND KOFF al 2005) demonstrate that most species generate shell carbonate in near equilibrium with sea water, although some species show variation between primary and secondary shell layers. Auclair et al (2003) proposed that non-equilibrium variation in d18O and d13C within a single shell of a modern terebratulid was related to kinetic and metabolic effects. Based upon the range of variation among individual shell samples of modern brachiopods and Devonian brachiopods from China, Lee and Wan (2000) concluded that biogenic fractionation (vital effects) were not a factor for the Devonian shell materials. For the Middle Devonian Hamilton Group brachiopods in the current study, the mean d18O value for Mucrospirifer is approximately 1.4‰ more negative than the mean value for Spinocyrtia, and this difference is statistically significant. Ignoring the possibility of diagenetic alteration, this difference might be attributable to vital effects or differences in life habits. The thinner-shelled, less massive Mucrospirifer may have grown at a different rate than the more robust Spinocyrtia. Ecological differences could also account for the isotopic distinctions. The mean d13C of Mucrospirifer samples is ~1.4‰ more negative than Spinocyrtia, and could be related to the inferred preference of Mucrospirifer for softer mud substrates where relatively isotopically negative organic carbon may have contributed more significantly to the carbonate budget during shell growth. An alternative interpretation of the differences between the two genera, and generally consistent with the isotopic data, is that the Mucrospirifer isotopic values reflect a greater degree of diagenetic alteration. As shown in Figure 7, the mean values for Mucrospirifer lie along the expected trend that is defined by the relatively more negative values of calcite spar cement. This pattern has been widely observed during burial diagenesis and recrystallization of biogenic carbonate (Lohmann 1988). Diagenetic changes in stable isotope values of primary biogenic carbonate require dissolution of shell material and growth of new carbonate from solution, or filling of primary void space within shells. Solid state diffusion exchange of oxygen and carbon between shell and surrounding water occurs at rates which are orders of magnitudes too slow, even with elevated temperature during burial diagenesis, to cause resetting of primary shell values (O’Neil 1986). Recrystallization (meaning dissolution and reprecipitation) at elevated temperatures and/or in contact with meteoric water causes the values of d18O relatively more negative than original values of calcite precipitated from seawater. Given the relatively thinner shell of Mucrospirifer, relatively greater alteration during burial diagenesis would be expected, and indeed this is the pattern observed in the mean values as shown in Figure 7. However, petrographic and SEM evidence for recrystallization of the Hamilton Group brachiopods selected for analysis is very limited, unless the recrystallization process was accomplished under
extraordinary conditions that permitted perfect replacement at a crystal-fiber by crystal-fiber level, preserving primary shell microstructure. In addition, the locality at which evidence of recrystallization was observed, Bridgewater, yields samples of Spinocyrtia with stable isotope values that are not anomalous among the entire data set, as shown in Figure 5. (The sample of Spinocrytia that contained petrographic evidence of recrystallization was not included in the stable isotopic analyses.) The locality with the best preservation of delicate fossil materials is Peterboro South. Some samples of both genera from this locality have the most negative isotopic signatures of all brachiopods analyzed. Again, diagenetic recrystallization cannot be ruled out as a significant factor in the variation in the stable isotope values, but if present, the effects on optical and SEM properties are cryptic, and the effects on the stable isotope system must vary without consistent pattern among localities and between individual samples. Inferences from paleotemperature-paleoseawater calculations: Given the lack of convincing evidence of wholesale diagenetic alteration of primary shell carbonate by burial processes, the stable isotopic values for the brachiopods may be reliably used as modestly robust records of primary seawater during the Middle Devonian in central New York. As shown in Figure 8, the d18O and temperature ranges for primary seawater calculated from the data require that d18O of the seawater must have been in the range of -5 to -10‰ SMOW, or significantly more negative than modern seawater. As noted above, a cluster of values from Spinocyrtia samples that does not overlap with Mucrospirifer values might be chosen to represent the least diagenetically altered biogenic shell carbonate among the data set. This data subset was used in calculation of the most positive seawater d18O values (Figure 8 – area outlined with solid line) that could reasonably generated. The calculations using this data subset still yield isotopic values for Middle Devonian seawater (-3 to -7‰ SMOW) that are considerably more negative than modern seawater. These results are consistent with a number of studies based upon stable isotopic analyses of Paleozoic biogenic carbonate, and strongly support the notion that ancient seawater was isotopically more negative in terms of oxygen than modern seawater. While local effects of meteoric water input might be called upon for sediments that were deposited in marginal marine environments, there is no direct evidence to suggest that dilution by freshwater was a significant factor in the shelf and basinal sediments of the Hamilton Group in central New York. Burial Diagenetic Cement: The d13C and d18O values for the two samples of coarse spar cement are reasonable values for calcite of burial diagenetic origin. As noted above, the d18O values for calculated equilibrium waters at realistic burial temperatures for the region are significantly more positive than the values calculated from the brachiopod data. This 341
STABLE ISOTOPES HAMILTON GROUP BRACHIOPODS suggests that the burial calcite precipitated from waters that were isotopically more positive in d18O than the middle Devonian seawater from which the brachiopods grew. This pattern is common in sedimentary basins and is related to the isotopic exchange that occurs as a result of dissolutionprecipitation reactions during burial. Sediments represent a reservoir of relatively heavier oxygen isotopes and during aqueous-solid reactions, the water becomes more positive in d18O. The isotopic evolution of basinal fluids during burial, generally causing the water to become isotopically more positive in d18O, and the range of temperatures at which minerals precipitate during burial diagenesis, are factors which cannot be easily taken into account when interpreting isotopic data from sedimentary systems. Both the temperature of mineral precipitation and the composition of the water influence isotopic signatures. Data external to the isotope system are required to adequately constrain the interpretation of isotopic values of carbonate cements. CONCLUSIONS The brachiopods Mucrospirifer and Spinocyrtia from the Hamilton Group of central New York show excellent preservation of original shell microstructure. Evidence for secondary burial alteration of the shell structure of these brachiopods is rare or cryptic. Stable isotope values of oxygen and carbon for the two genera are similar to the values for middle Devonian brachiopods determined by other workers. The differences between the two genera are statistically significant, and may be attributed to differences in vital effects, or possibly, susceptibility to diagenetic alteration. Oxygen isotope values of middle Devonian seawater calculated from brachiopod values at reasonable life temperatures are in the range -4 to -10‰, significantly more negative than modern seawater. This result is similar to the findings of other studies of Paleozoic biogenic marine carbonate. Coarsely crystalline calcite cement from two localities in the Hamilton Group has stable isotope values reflecting precipitation under burial conditions, at temperatures in the 140-160°C range, from waters with relatively positive d18O values, compared to either Devonian or modern seawater. ACKNOWLEDGMENTS We deeply appreciate the assistance of our colleagues Sarah Miller, Kelsi Olson, Kristi Woodworth, Kevin Kelly, and Emily Constantine, who provided sample materials, measured sections and made initial data interpretations in the Marine Environments Seminar, at Colgate University in the fall term, 2003. Stephen Howe of the Stable Isotope Laboratory of the State University at Albany provided helpful advice regarding sample preparation and performed
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