Horng Sheng Mii

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 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.

Late Pennsylvanian seasonality reflected in the 18O and elemental composition of a brachiopod shell

To evaluate the potential for the study of Late Pennsylvanian seasonality, detailed isotopic and elemental analyses were performed on a single specimen of the brachiopod Neospirifer dunbari . Shell preservation was evaluated by petrographic and cathodoluminescence microscopy. Carbonate powders from 112 spots were collected from the sectioned shell for isotopic analyses, and 369 spots on the complementary thin section were analyzed by electron microprobe for chemical composition. In the nonluminescent part of the shell, Mg/Ca ratios were between 0.001 and 0.012, and Na/Ca and S/Ca ratios ranged from 0.003 to 0.012 and 0.003 to 0.021, respectively. Values of δ 18 O vary between -2.3‰ and -1.1‰. Contours of 18 O, Mg, Na, and S concentrations parallel growth bands and reveal a record of 1½ to 2 cycles. Mg, S, and Na contents varied nversely with δ 18 O. This trend is opposite to the expected diagenetic trend and is consistent with the temperature dependences of Mg content and δ 18 O, thus implying preservation of shell chemistry. The ∼1.2‰ range in δ 18 O values suggests a seasonal temperature variation of 5 to 6 °C (assuming no change in the δ 18 O of the water). This high seasonality for the tropical epicontinental sea of Kansas supports climate-model predictions of enhanced continentality in Pangean climate. Detailed stable isotope and element concentration profiles across growth bands of brachiopod shells can provide quantitative records of Paleozoic seasonality.

21st-century rise in anthropogenic nitrogen deposition on a remote coral reef

From air to shining sea Nitrogen is an essential nutrient for phytoplankton growth. Nitrogen is primarily supplied to the surface ocean by mixing from below. However, as fertilizer use and combustion of fossil fuels rise, the atmosphere is expected to become an increasingly important source. Ren et al. measured nitrogen isotopes in organic matter from a South China Sea coral (see the Perspective by Boyle). Their findings suggest that atmospheric deposition of anthropogenic nitrogen began right at the end of the 20th century. This pathway now supplies nearly one quarter of the annual nitrogen input to the surface ocean in this region. Science, this issue p. 749; see also p. 700 Atmospheric deposition of anthropogenic nitrogen has become a major nitrogen source in the South China Sea. With the rapid rise in pollution-associated nitrogen inputs to the western Pacific, it has been suggested that even the open ocean has been affected. In a coral core from Dongsha Atoll, a remote coral reef ecosystem, we observe a decline in the 15N/14N of coral skeleton–bound organic matter, which signals increased deposition of anthropogenic atmospheric N on the open ocean and its incorporation into plankton and, in turn, the atoll corals. The first clear change occurred just before 2000 CE, decades later than predicted by other work. The amplitude of change suggests that, by 2010, anthropogenic atmospheric N deposition represented 20 ± 5% of the annual N input to the surface ocean in this region, which appears to be at the lower end of other estimates.

Obliquity pacing of the western Pacific Intertropical Convergence Zone over the past 282,000 years.

The Intertropical Convergence Zone (ITCZ) encompasses the heaviest rain belt on the Earth. Few direct long-term records, especially in the Pacific, limit our understanding of long-term natural variability for predicting future ITCZ migration. Here we present a tropical precipitation record from the Southern Hemisphere covering the past 282,000 years, inferred from a marine sedimentary sequence collected off the eastern coast of Papua New Guinea. Unlike the precession paradigm expressed in its East Asian counterpart, our record shows that the western Pacific ITCZ migration was influenced by combined precession and obliquity changes. The obliquity forcing could be primarily delivered by a cross-hemispherical thermal/pressure contrast, resulting from the asymmetric continental configuration between Asia and Australia in a coupled East Asian–Australian circulation system. Our finding suggests that the obliquity forcing may play a more important role in global hydroclimate cycles than previously thought.

Uppermost cretaceous to middle oligocene carbon and oxygen isotope stratigraphy of southwest pacific: Holes 1121B and 1124C, ODP leg 181

Abstract Oxygen and carbon isotopic ratios of bulk sediments from ODP Leg 181, Holes 1121B and 1124C, in the Southwest Pacific were measured. The isotopic signals are mainly contributed by calcareous nannofossils with minimal diagenetic alteration. A complete section of the late Paleogene age between 60.7 and 57.5 Ma was recovered from Hole 1121B. However, the Paleogene sedimentary sequence of Hole 1124C was truncated by three major hiatuses: late Paleocene to middle Eocene (59–42 Ma), middle Eocene to early Oligocene (40–33.5 Ma), and early Oligocene to middle Oligocene (31.3–27.5 Ma). The middle Eocene shows the most negative δ18O values (c. ‐0.8‰) compared to the early Paleocene (c. ‐0.2 to ‐0.3‰) and Oligocene (c. 0.6–0.9‰). The δ18O pattern is consistent with previous understanding of the Paleogene paleoclimate: a warmth optimum in the early‐middle Eocene followed by a major glaciation in the early Oligocene at c. 34 Ma. The hiatus of 33.5–40 Ma indicates that the Tasmanian Gateway had deepened enough by 33.5 Ma, allowing the breakthrough of cold, bottom water and consequently the formation of the Deep Western Boundary Current (DWBC). With the aid of independent biochronological and magnetochronological markers, the Paleocene carbon isotopic profiles were correlated with that of DSDP 577 in the North Pacific. Both sites record the early part of the Paleocene carbon isotopic maximum event, while only Hole 1124C extends back to the early Paleocene and latest Cretaceous. A short hiatus of 60.5–62.5 Ma age may exist. Although the Cretaceous/Tertiary boundary is not directly recorded, a significant cooling trend across the boundary is evident. The surface water became warmer after 64.5 Ma, and reached a stable warmth level during 64–59 Ma. A major cooling took place during c. 59–57 Ma in the late Paleocene. The temperature gradients between the two sites (ODP 1121 and 1124, paleolatitudes 64°S versus 53°S) are estimated to be c. 2°C. Together with the oxygen isotopic profiles of North Pacific (DSDP 577, paleolatitude 7°N) and eastern Indian Ocean (ODP 761B, paleolatitude 32°S), the overall pattern suggests that the temperature gradients between the high latitudes and the subtropics increased substantially during this cooling period.

Nonlinear climatic sensitivity to greenhouse gases over past 4 glacial/interglacial cycles.

The paleoclimatic sensitivity to atmospheric greenhouse gases (GHGs) has recently been suggested to be nonlinear, however a GHG threshold value associated with deglaciation remains uncertain. Here, we combine a new sea surface temperature record spanning the last 360,000 years from the southern Western Pacific Warm Pool with records from five previous studies in the equatorial Pacific to document the nonlinear relationship between climatic sensitivity and GHG levels over the past four glacial/interglacial cycles. The sensitivity of the responses to GHG concentrations rises dramatically by a factor of 2–4 at atmospheric CO2 levels of >220 ppm. Our results suggest that the equatorial Pacific acts as a nonlinear amplifier that allows global climate to transition from deglacial to full interglacial conditions once atmospheric CO2 levels reach threshold levels.