Ethan L. Grossman

OXYGEN AND CARBON ISOTOPE FRACTIONATION IN BIOGENIC ARAGONITE: TEMPERATURE EFFECTS

To better interpret the isotopic composition of ancient aragonitic fossils, stable-isotopic analyses have been performed on live and modern specimens of aragonitic foraminifera, gastropods and scaphopods. Samples were collected from the continental margins off southern California and Texas, U.S.A., and Mexico, and provide a range in ambient temperature of 2.6–22.0°C. We observed a strong covariance between the δ18O of the aragonitic foraminifera Hoeglundina elegans and that of coeval aragonitic mollusks. On the average, Hoeglundina was 0.2 ± 0.2‰ depleted in 18O relative to the mollusks, and 0.6 ± 0.3‰ enriched relative to the calcitic foraminifera Uvigerina. This enrichment in 18O of aragonite relative to calcite is similar to that observed in previous experimental and theoretical studies. The temperature dependences of mollusk and Hoeglundina δ18O-values were not notably different from that previously determined for inorganically precipitated calcite, and no significant temperature dependence in Hoeglundina-Uvigerina18O fractionation was observed. Of note is the temperature dependence of the δ13C of the biogenic aragonite. Relative to the dissolved inorganic carbon (DIC), the δ13C of Hoeglundina and the mollusks decreased by 0.11 and 0.13‰, respectively, per °C increase in temperature. The temperature dependence in Hoeglundina-DIC 13C enrichment, and the lack of it in Uvigerina-DIC enrichment, accounts for the temperature dependence in Hoeglundina-Uvigerina (calcitic) fractionation noted by us and previous workers. Isotopic differences between coeval specimens of these genera provide a rough measure of paleotemperature without requiring a knowledge of the isotopic composition of the paleo-ocean.

Carbon isotopic fractionation in synthetic aragonite and calcite: Effects of temperature and precipitation rate

An open-system chemostat technique was employed to study temperature and precipitation rate effects on carbon isotopic fractionation during the inorganic precipitation of aragonite and calcite from solutions held at constant chemical and isotopic composition. Calcite-bicarbonate enrichment factors (ϵcl-HCO−3) average 1.0 ± 0.2%. and are essentially constant throughout the range of temperatures (10, 25, and 40°C) and precipitation rates (1026 to 104.8 μmol m−2 h−1) studied. Calcite-CO2(g) enrichment factors (ϵcl-CO2) are temperature sensitive and are described by the equation ϵcl-CO2 = 11.98(±0.13) −0.12(±0.01) · T(°C). Aragonite-bicarbonate enrichment factors (ϵar-HCO−3) average 2.7 ± 0.6%. for temperatures of 10, 25, and 40°C and for rates from 102.3 to 104.1 μmol m−2 h−1 · ϵar-HCO−3 data are independent of temperature and display no precipitation rate effect at 10°C, a small negative relationship to rate at 25°C, and a small positive relationship to rate at 40°C. The relative weakness of the effect coupled with the lack of consistency in the trends suggests that ϵar-HCO−3 is independent of rate for most geologic processes. Aragonite-CO2(g) enrichment factors (ϵar-CO2) are temperature-sensitive and are described by the equation ϵar-CO2 = 13.88(±0.16) −0.13(±0.01) · T(°C). Aragonite-calcite fractionation (ϵar-cl) is 1.7 ± 0.4%. and is independent of temperature from 10 to 40°C. While the enrichment factors obtained here agree with those of some previous studies, temperature and rate effects do not. Our results, which benefit from better control on precipitate mineralogy and precipitation rate, are probably the most accurate to date.

Oxygen isotopes in meteoric calcite cements as indicators of continental paleoclimate

Because meteoric water δ 18 O values decrease with decreasing ambient temperature and increasing latitude, δ 18 O values of meteoric calcite cement should exhibit a similar relation with paleolatitude and be an indicator of continental paleotemperatures. To test this, we compiled isotopic and paleolatitude data for 20 meteoric cements and nine speleothems ranging in age from Devonian to modern and in paleolatitude from 3.5° to 83°. Mean δ 18 O values for meteoric cements and speleothems both show the same negative correlation with paleolatitude. The δ 18 O vs. latitude trend for these carbonates is almost identical to that predicted for modern inland environments, but differs from the trend for coastal environments. This suggests that the ground water controlling the ultimate composition of meteoric cement is derived predominantly from inland recharge. If it is assumed that the modern meteoric water δ 18 O vs. temperature relation is valid for the past and that insignificant evaporation occurred prior to carbonate precipitation, then coastal and inland paleotemperatures can be calculated from the δ 18 O values of meteoric calcite (δ 18 O mcl ) and seawater (δ 18 O sw ) by using the equations T coastal =13.3 ±32.6[-0.231- 0.0613(δ 18 O mcl + δ 18 O sw ) ½ and T inland =17.8 ±16.2[-0.572 - 0.1233(δ 18 O mcl + δ 18 O sw ) ½ , where T is temperature in °C. Calcite precipitated from coastal meteoric water at temperatures between 0 and 25 °C will exhibit a narrow range in δ 18 O(-6‰ to -4‰, where δ 18 O sw = 0‰). The δ 18 O of calcite precipitated from inland meteoric water will be sensitive to paleotemperature, ranging from -14‰ to -5‰ (where δ 18 O sw = 0‰) for temperatures of 0 to 25 °C.

Carboniferous isotope stratigraphies of North America: Implications for Carboniferous paleoceanography and Mississippian glaciation

We present detailed isotope stratigraphies for Carboniferous time based on brachiopod shell calcite from the midcontinent region of North America. Evidence for shell calcite preservation includes (1) preservation of shell microstructure, (2) lack of cathodoluminescence, (3) low Si, Al, Fe, and Mn contents, (4) Na, Sr, and S contents comparable to those of modern brachiopod shells, and (5) δ13C and δ18O values higher than those of associated cements and matrix. The Carboniferous δ13C record for North America is characterized by three isotopic stages. The earliest stage, C1, follows a 2.0‰ increase in Kinderhookian time (early Tournaisian), from 1.5‰ to 3.5‰, and includes a brief and perhaps local late Kinderhookian excursion to 5.4‰. The δ13C values remain stable at 3.5‰ to 4‰ during stage C1, then decrease about 1‰ near the Meramecian-Chesterian boundary (Visean) to 2‰–3‰ (stage C2). Stage C2 ends with a 1‰–2‰ increase (C2-C3 transition) between middle Chesterian and early Morrowan time (Serpukhovian-Bashkirian). Stage C3 values remain mostly between 3‰ and 4.5‰ upsection to Virgilian strata (Gzhelian). Increases in δ13C probably reflect global increases in sedimentary organic carbon burial and suggest that rho CO2 declines in the earliest and middle Carboniferous strata. The middle Carboniferous δ13C shift of B. Popp, T. Anderson, and P. Sandberg, an ∼3‰ increase in European sections, occurs in North America (C2-C3 transition) but is limited to ∼1.5‰. This 1.5‰ increase was probably caused by increased organic carbon burial, whereas the additional ∼1.5‰ shift in European sections likely reflects changes in ocean circulation patterns associated with the closing of the Equatorial seaway. Based on the timing of the δ13C divergence between North America and Europe, the isolation of the Paleotethys began in late Chesterian time (Serpukhovian). The δ18O record can also be separated into the three stages. There is a 3‰ increase during Kinderhookian-Osagean time (Tournaisian), corresponding to the Devonian to Carboniferous transition to stage C1, a 3‰ decrease during Meramecian–early Chesterian time (Visean; C1-C2 transition), then a 2‰ increase in late Chesterian–early Morrowan time (Serpukhovian-Bashkirian; C2-C3 transition). The δ18O values then fluctuate between −1‰ and −3‰ (C3 stage) upsection to the Virgilian strata (Gzhelian). If global, the 2‰ to 3‰ δ18O shifts are compelling evidence for cooling and glaciation in Early Mississippian time, warming and deglaciation in Late Mississippian time, and a return to cool, glacial conditions in earliest Pennsylvanian time. The general correlation between δ13C and δ18O shifts suggests that cooling is associated with drawdown of atmospheric CO2.

Formation of the Isthmus of Panama

Independent evidence from rocks, fossils, and genes converge on a cohesive narrative of isthmus formation in the Pliocene. The formation of the Isthmus of Panama stands as one of the greatest natural events of the Cenozoic, driving profound biotic transformations on land and in the oceans. Some recent studies suggest that the Isthmus formed many millions of years earlier than the widely recognized age of approximately 3 million years ago (Ma), a result that if true would revolutionize our understanding of environmental, ecological, and evolutionary change across the Americas. To bring clarity to the question of when the Isthmus of Panama formed, we provide an exhaustive review and reanalysis of geological, paleontological, and molecular records. These independent lines of evidence converge upon a cohesive narrative of gradually emerging land and constricting seaways, with formation of the Isthmus of Panama sensu stricto around 2.8 Ma. The evidence used to support an older isthmus is inconclusive, and we caution against the uncritical acceptance of an isthmus before the Pliocene.

Isotopic records of brachiopod shells from the Russian Platform — evidence for the onset of mid-Carboniferous glaciation

We performed isotopic analyses of Carboniferous brachiopod shells from the Russian Platform to examine global and regional environmental change along the western and eastern margins of Laurussia during the formation of Pangea, and specifically to examine the isotopic evidence for the onset of mid-Carboniferous glaciation. Shell preservation was evaluated from shell microstructure, cathodoluminescence, trace element content and isotopic comparison with matrix material. Most interior nonluminescent (NL) prismatic shell appears to be chemically well preserved as indicated by low to undetectable Si, Al, Fe and Mn contents. With minor exception, NL shell δ13C and δ18O values are higher than those of corresponding cements and matrix. The δ13C record for the Russian Platform clearly shows a 3‰ increase (from 2.4±0.7‰ to 5.5±0.6‰) in the Serpukhovian or Early Bashkirian. This shift was first reported by Popp et al. [Popp, B.N., Anderson, T.F., Sandberg, P.A., 1986. Brachiopods as indicators of original isotopic compositions in some Paleozoic limestones. Geol. Soc. Am. Bull. 97, 1262–1269.], but had not been documented for a single region. Widespread occurrence of high δ13C values in the Late Carboniferous support the interpretation of Popp et al. with regard to this shift as a record of increased burial of organic carbon. North American sections show a mid-Carboniferous δ13C shift of only ∼1.5‰. We hypothesize that the reduced δ13C shift reflects enhanced upwelling on the epicontinental seas of North America after the closing of the seaway between Laurussia and Gondwana. The δ18O record for the Russian Platform shows a 1.8‰ increase in the mid-Carboniferous correlative with increased occurrence of glacial sediments and a drop in sea level. As a first approximation, ice volume calculations suggest that ∼0.7‰ of the mid-Carboniferous δ18O shift is due to changes in seawater δ18O, and ∼1.1‰ is due to 5°C cooling. Concurrent positive δ13C and δ18O shifts provide evidence for a relationship between mid-Carboniferous glaciation and burial of organic carbon, presumably through changes in atmospheric CO2 levels.

Stable isotopes in Late Pennsylvanian brachiopods from the United States: Implications for Carboniferous paleoceanography

Stable isotopic analyses have been performed on nearly 500 nonluminescent brachiopod shells from Kansas, New Mexico, and Texas to evaluate temporal and geographic variability in the carbon and oxygen isotopic record for late Pennsylvanian time. Brachiopod specimens were collected from Missourian and Virgilian shales and examined in thin section for preservation of microstructure and absence of cathodoluminescence as a primary test for shell preservation. Regional variations are observed in the δ18O and δ13C values of nonluminescent brachiopod shells of the same genera. Average δ18O values for the three genera analyzed ( Crurithyris , Composita , and Neospirifer ) are highest in Kansas (≈-1.9‰), intermediate in Texas (≈-2.3‰), and lowest in New Mexico (≈-3.6‰). Vital effect on the δ18O of these genera appears minimal. δ18O data and other evidence suggest warmer temperatures for the shallow New Mexico localities and slightly higher salinity for the Kansas sea relative to the Texas sea. The δ13C values of Composita average about 1‰ higher than those of co-occurring Crurithyris and Neospirifer , suggesting microhabitat differences or vital effects. Preservation of this species effect argues for preservation of original δ13C values. Average δ13C values are highest in Texas, intermediate in Kansas, and lowest in New Mexico. Although these values range from 2.6‰ to 4.9‰, for individual genera the regional variation in δ13C averages less than 1‰. Data for nonluminescent brachiopods and marine cements reveal a mid-Carboniferous δ13C increase of 2‰ in Paleotethyan sea water. This increase is not seen in samples from the North American epicontinental seas, which opened to the Panthalassa ocean. This regional difference in δ13C appears to be due to changes in ocean circulation associated with the closing of the equatorial seaway and formation of Pangea.

Stable isotope fractionation in live benthic foraminifera from the southern California Borderland

Live specimens of benthic foraminifera have been analyzed from surface sediment samples collected from 108 to 1134 m depth in the Southern California Borderland. The temperature and isotopic composition of the ambient water and dissolved inorganic carbon have also been measured to permit comparison of foraminiferal compositions to expected equilibrium values. The species analyzed break down into four groups which reflect the influence of mineralogy (aragonite vs. calcite), biological fractionation (“vital effect”), and microhabitat effects on isotopic composition. The aragonitic Hoeglundina elegans show an enrichment in 18O relative to equilibrium calcite of 0.41%0 in shallower water increasing to at least 1.06%0 in deeper water. The 13C enrichment of H. elegans relative to the dissolved inorganic carbon (DIC) is also depth dependent, increasing from 1.27%0 at 108 m to 2.13%0 at 940 m. This depth dependence results from the change in composition of the DIC; the 13C composition of H. elegans remains constant with depth for some unknown reason. One possible explanation is that the change in composition of the DIC is balanced by an increase in Hoeglundina—HCO−3 fractionation with decreased temperature. The δ13C values of the Cassidulina species are almost identical to those of the ambient dissolved inorganic carbon (DIC) and reflect the decrease in the δ13C of the DIC with depth. The δ13C values of species not in 18O equilibrium vary independently of the changing composition of the DIC. There is a correlation between δ13C and δ18O among coeval species, consistent with the model which explains vital effect as the result of incorporation of isotopically light metabolic CO2 into the shell. The magnitude of disequilibrium is relatively constant for 18O but not for 13C, suggesting that in isotopic stratigraphy studies, it may be difficult to “calibrate” the δ13C of different species as has been done with δ18O. There appears to be a relationship between taxonomic group and vital effect. The foraminifera in approximate 18O equilibrium with the water, including species of Cassidulina, Bolivina, Uvigerina, and Globobulimina, are members of the family Cassidulinidae or superfamily Buliminacea. Those not in 18O equilibrium, including species of Pyrgo, Quinqueloculina, Triloculina, and Lenticulina, belong to the families Miliolidae and Vaginulinidae. No disequilibrium species have been found as yet in the low oxygen environments of the borderland. This observation and published findings on the ultrastructure of benthic foraminifera suggest that isotopic “behavior” may be related to a foraminifers ability to exchange gases rapidly with its ambient water.

Chemical Variation in Pennsylvanian Brachiopod Shells--Diagenetic, Taxonomic, Microstructural, and Seasonal Effects

ABSTRACT To improve our ability to use minor and trace element (MTE) variation in biotic carbonates as diagenetic and paleoenvironmental indicators, we performed electron probe microanalysis on more than 100 Late Pennsylvanian brachiopod shells from Texas, Kansas, Missouri, and New Mexico. Texturally preserved specimens of the genera Crurithyris, Composita, and Neospirifer from all three regions were analyzed, as were Eridmatus specimens from Texas. Twenty measurements were made in two transects across each shell. Shell microstructure and cathodoluminescence were described for each spot analyzed. Three modern shells were analyzed for comparison. Diagenesis, as indicated by cathodoluminescence and/or absence of microstructure, tends to enrich shells in Fe and Mn (X/Ca >= 0.7 mmol/mol) and deplete shells in Na and S. Mg content shows no consistent trend with diagenesis. In fabric-retentive, nonluminescent shell areas, Mg, Na, and S contents vary twofold to sevenfold depending on taxonomy, microstructure, and season. Overall, taxonomy is the dominant factor controlling MTE composition. Na and S concentrations are consistently highest in Crurithyris and Eridmatus, intermediate in Neospirifer, and lowest in Composita. In taxa with mixed microstructure (Composita, Neospirifer), secondary fibrous layer calcite contains 1.5 to 2 times more Na than does interlayer prismatic calcite. Thus whole-shell Na contents of these taxa depend on the proportion of fibrous and prismatic shell. Seasonal cycles are revealed in MTE transects across growth lines. Mg, Na, and S contents commonly vary by more than a factor of two between maxima (presumably summer) and minima (winter) within the same shell. Retention of taxonomic, microstructural, and seasonal effects in shell chemistry argues for preservation of original chemistry in fabric-retentive, nonluminescent Paleozoic brachiopod shells.

Bacterial production of methane and its influence on ground-water chemistry in east-central Texas aquifers

Geochemical and isotopic data for methane and ground water indicate that gaseous hydrocarbons in Eocene aquifers in east-central Texas form by bacterial processes. The δ13C values of methane from five wells in the clay-rich Yegua and Cook Mountain Formations range from -71‰ to -62‰. Methane from ten wells in the cleaner sands of the Sparta and Queen City Formations have δ13C values between -57‰ and -53‰. The carbon isotopic difference between methanes from the Yegua and Sparta aquifers is comparable to the isotopic difference in sedimentary organic matter from outcrops of the units, suggesting substrate control on the δ13C of bacterial methane. Hydrogen isotopic compositions of methane from the aquifers are similar, averaging -181‰. This high value suggests methane production predominantly by CO2 reduction. The δ13C of dissolved inorganic carbon (DIC) in high bicarbonate waters increases from about -20‰ to 0‰ with increasing DIC. Mass-balance calculations indicate that the DIC added to the ground water has δ;13C values as high as 10‰. This 13C-enriched carbon is predominantly derived from CO2 production by fermentation and anaerobic oxidation reactions combined with CO2 consumption by CO2 reduction. This process is responsible for high bicarbonate contents in these and probably other Gulf Coast ground waters.