Thomas E. Yancey

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.

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.

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.

Stable-isotope stratigraphy of brachiopods from Pennsylvanian shales in Texas

Stable-isotope stratigraphies have been generated for brachiopods from five Late Pennsylvanian sections (upper Winchell cycle, Finis shale, Colony Creek shale, Necessity cycle, and Wayland shale) in north-central Texas to examine the relationship between isotopic composition, paleoenvironment, and sea-level change. Nonluminscent calcite from more than 300 specimens of 4 species of brachiopod ( Composita subtilita, Crurithyris planoconvexa, Eridmatus texana, Neospirifer cameratus ) was analyzed. Sedimentological and paleontological data are used as evidence for paleoenvironmental change. Higher δ 18 O values of brachiopod shells occur in intervals of greatest paleodepth as indicated by fossil content and lithology. These higher δ 18 O values reflect a decrease in bottom temperature with increased water depth. Normalized for paleodepth, the δ 18 O values of the brachiopods are similar in all of the units except the Wayland shale, for which they are ∼0.5‰ higher. This suggests either cooler temperatures or greater continental ice volume during the time of Wayland deposition. Brachiopod δ 13 C values do not correlate with paleodepth. The δ 13 C values of Composita are consistently higher than those of Crurithyris and Eridmatus . Composita δ 13 C values are similar in the upper Winchell cycle, Necessity cycle, and Wayland shale, averaging 4.8‰ to 4.9‰. In contrast, Crurithyris δ 13 C values range from 2.5‰ in the Necessity cycle to 4.4‰ in the Wayland shale; Eridmatus values are also lowest in the Necessity cycle (3.4‰) and greatest in the Wayland shale (3.9‰). These differences between species do not correlate with susceptibility to diagenesis and appear to reflect a pore-water influence on semi-infaunal ( Crurithyris ) forms. The δ 13 C values of Composita shells suggest that the Late Pennsylvanian ocean off eastern Laurussia was enriched in 13 C by about 1‰ relative to the ocean off western Laurussia.

Stable carbon and oxygen isotope shifts in Permian seas of West Spitsbergen - Global change or diagenetic artifact?

We performed petrographic, cathodoluminescence, electron-microprobe, and isotopic analyses of brachiopod shells from the Permian Kapp Starostin Formation in West Spitsbergen to reevaluate the >9‰ negative shift in δ13C and δ18O values reported in 1989 by M. Gruszczynski, S. Halas, A. Hoffman, and K. Malkowski. The δ13C and δ18O values within shells typically decrease with increasing luminescence, indicating diagenesis. Nonluminescent (NL) shell δ13C and δ18O values are 4.3‰ and 6.2‰ higher, respectively, than those of associated cements and matrix. For the same stratigraphic interval, δ13C and δ18O values of the NL shells are equal to, or substantially greater than, those reported by Gruszczynski et al. For the interval where those authors saw a 10‰ δ13C shift, our mostly NL Spiriferella polaris shells only yield a 1.5‰ shift. Gruszczynski et al. reported a 9‰ δ18O shift, whereas we observe almost none. Our results strongly suggest that the >9‰ isotopic shifts reported in Gruszczynski et al. are diagenetic artifacts. On the other hand, their Kazanian-Tatarian δ13C maximum of 7.5‰ is substantiated by our data. This Late Permian 13C maximum represents the highest spiriferid brachiopod δ13C values in the Phanerozoic and, within stratigraphic uncertainty, correlates with the whole-rock δ13C maximum in East Greenland and northwestern Europe. The δ13C shift may reflect changes in global storage of organic carbon indicated by coal-volume changes in the Late Permian.

Reevaluation of conflicting Eocene tropical temperature estimates: Molluskan oxygen isotope evidence for warm low latitudes

Oxygen isotope data from planktonic foraminifera for the warm Eocene epoch suggest that tropical sea-surface temperatures (SSTs) may have been cooler than at present. Such data have stimulated various explanations involving, e.g., major changes in ocean heat transport. However, the planktonic data disagree with terrestrial climate proxies, which suggest significantly warmer low-latitude temperatures. We examined this discrepancy by analyzing seasonal oxygen isotope variations in shallow-marine mollusks from the Mississippi Embayment. Results indicate that mean annual SSTs decreased from 26–27 °C in the early Eocene to 22–23 °C in the Oligocene, agreeing well with temperatures inferred from terrestrial climate proxies. These cooling trends, with more significant winter cooling (5 °C) than summer cooling (3 °C), are consistent with the predicted consequences of decreasing atmospheric CO 2 concentration through the Paleogene, suggesting that atmospheric CO 2 change was a major controlling factor for Paleogene climate change. That winter SST estimates from the mollusks agree well with the foraminiferal SST estimates suggests that planktonic foraminiferal growth in low latitudes occurred mainly during the cooler winter months throughout the Eocene. We hypothesize that the unusual hydrography of Eocene oceans shifted foraminiferal productivity primarily to winter, biasing foraminiferal SST estimates of mean annual SSTs.

Geochemical Markers of the Cretaceous-Tertiary Boundary Event at Brazos River, Texas, USA

The Cretaceous-Tertiary boundary sites around the Gulf of Mexico are close to the Chixculub impact site and are relatively well studied, yet much remains to be learned about them. Therefore, the first integrated study of carbon, soot, and fullerenes in a Cretaceous-Tertiary boundary section was undertaken at the Brazos-1 site on the Brazos River in Texas at the most complete section of end Cretaceous and basal Paleocene deposits on the Texas segment of the Gulf Coast area. Up to 409 ppm of native sulfur (So) were serendipitously discovered in a spherule-bearing unit of the BR-1 section, and lesser amounts were found in spherule-bearing units of nearby Brazos riverbed sections in a section on Darting Minnow Creek. The isotopic composition, δ33S = −12.97‰, δ34S = −24.89‰, and δ36S = −46.4‰, implies that this So cannot have come to Earth by the impactor that formed the Chicxulub crater, but, most likely, was produced by sulfate-reducing bacteria during a local, transient bacterial bloom for which the sulfate was provided by CaSO4-bearing spherules. Carbon and soot were determined in twelve samples representing all units of BR-1 from the Cretaceous Corsicana/Kemp Formation to the Tertiary Kincaid Formation. A significant increase of C and soot contents, up to 2.2×104 ppm and 1.4×104 ppm, respectively, occurs in a sandy bed at the top of the KT complex. Fullerenes were determined in fifty-four samples from all units of the same BR-1 section. Less than 1 ppb was reliably detected at the same sandy bed where the strongest Ir anomaly of the section is known to occur. It is suggested here that the Chicxulub impact 65 Ma ago ignited local wildfires that produced C, soot, and fullerene, which settled onshore, or near-shore, whence they were transported to the Brazos site by coastal flooding and associated sediment-laden water plumes moving offshore.

Giant alatoform bivalves in the Upper Triassic of western North America

ABSTRACT. Large, alatoform bivalves, Wallowaconcha raylenea gen. et sp. nov., from the Upper Triassic of north-eastern Oregon, are described and placed in a new family, Wallowaconchidae, within the Megalodontoidea, which also contains the families Megalodontidae and Dicerocardiidae (herein transferred). Major character innovations of wallowaconchids are the internal partitioning of wings and development of non-articulating thin vanes on the hingeplate. The wallowaconchid hinge, which changed during ontogeny, differs greatly from the hinge of other bivalves. Wallowaconchids probably evolved from a species of the megalodontid Triadomegalodon. Although wallowaconchids are homeomorphs of the alate Permian alatoconchid bivalves, hinge structure shows that these two groups are unrelated. Large wings on wallowaconchids were used for snowshoe support. They may have utilized elaborate vanes and wing chambers to culture microbial symbionts, either microalgae or bacteria. These bivalves are endemic to displaced island arc terranes in western North America, occurring in Yukon, Canada (Stikine terrane), Oregon, USA (Wallowa terrane), and Sonora, Mexico (Antimonio terrane). They occupied environmental niches similar to those of large megalodontid bivalves of Triassic tropical provinces.

Larval shells of Late Palaeozoic naticopsid gastropods (Neritopsoidea: Neritimorpha) with a discussion of the early neritimorph evolution

Extant neritimorphs with planktotrophic larval development have a convolute smooth larval shell which is internally resorbed. The oldest known larval shells of this type are of Triassic age. Well-preserved Late Palaeozoic neritimorph specimens have larval shells of two or more rapidly increasing well separated whorls. These larval shells resemble planktotrophic caenogastropod larval shells. This type of larval shell is possibly plesiomorphic in neritimorphs and caenogastropods. Permian/Pennsylvanian neritimorphs (Naticopsis, Trachyspird) have smooth larval shells (Naticopsidae) or larval shells with strong axial ribs (Trachyspiridae new family). The convolute low-spired round shell shape of modern neritimorphs is causally linked with the resorption of the inner teleoconch and protoconch whorls. Modern neritimorph shells with a uniform, undifferentiated inner lumen have probably evolved from naticopsid ancestors which lack resorption. It is possible that an elevated spire, deep sutures and protruding spiral larval shells would have made such internally undifferentiated shells more vulnerable for mechanical destruction and prédation. Suggestions that coiling evolved independently in neritimorphs and other Gastropoda are unlikely and contrast with the fossil record. The modern neritid larval shell has probably evolved from relatively low-spired smooth naticopsid larval shells like those reported here.KurzfassungHeutige Neritimorpha mit planktotropher Larvalentwicklung besitzen eine konvolute, glatte Larvalschale, deren Windungen innen resorbiert sind. Die ältesten bekannten Larvalschalen dieses Typs sind triassischen Alters. Gut erhaltene spätpaläozoische neritimorphe Gastropoden besitzen Larvalschalen mit zwei oder mehr wohl separierten, schnell zunehmenden Windungen. Diese Larvalschalen gleichen planktotrophen Larvalschalen von Caenogastropoden. Dieser Larvalschalentyp ist möglicherweise bei Neritimorpha und Caenogastropoda plesiomorph. Permische und oberkarbonische Neritimorphe (Naticopsis, Trachyspird) haben glatte Larvalschalen (Naticopsidae) oder Larvalschalen mit starken Axialrippen (Trachyspiridae, neue Familie). Die stark kon volute niedrig gewundene, runde Schalengestalt der modernen Neritimorphen entwickelte sich zusammen mit der Resorbierung der Innenwindung des Teleoconchs und Protoconchs bzw. Beides bedingte einander. Die moderne Neritimorphenschale mit einem einheitlichen, undifferenzierten Innenraum lässt sich von einem naticopsiden Vorfahren herleiten, der noch nicht resorbiert war. Eine herausgehobene Spira, tiefe Suturen sowie eine gewundene herausragende Larvalschale hätten eine intern resorbierte Schale vermutlich anfällig für mechanische Verletzung und Angriffe von Räubern gemacht. Die Annahme, die Aufwindung habe sich bei Neritimorphen unabhängig von anderen Schnecken entwickelt ist unwahrscheinlich und steht im Widerspruch zum Fossilbericht. Die Larvalschale der modernen Neritiden entwickelte sich vermutlich aus einer glatten, niedrig gewundenen Larvalschale, wie sie hier bei karbonischen Naticopsiden dokumentiert wird.