Bradley D. Cramer

The Late Wenlock Mulde positive carbon isotope (δ13Ccarb) excursion in North America

Abstract Marine carbonates from two well-studied areas of the Silurian of North America were analyzed for stable carbon isotope (δ13Ccarb) stratigraphy. A graptolite-bearing sequence from the eastern margin of the Panthallasic Ocean (Nevada) and a conodont-bearing sequence from the mid-continent epeiric seaway (Tennessee) were sampled for δ13Ccarb stratigraphy in order to improve the correlation between these areas and the Swedish island of Gotland, which has become the global standard for Wenlock conodont and carbon isotope stratigraphy. The Homerian (Late Wenlock) Mulde positive carbon isotope excursion serves as a useful chronostratigraphic marker for Homerian sequences, especially in regions such as the two included in this investigation, where zonal fossils are absent or poorly represented. In Nevada, using presently available biostratigraphic data, a detailed modern graptolite zonation cannot be applied due to a lack of several key species. Likewise, the zonally important conodont species are poorly represented in Tennessee. Our recognition of the dual-peaked Mulde δ13Ccarb excursion in North America allows improved correlation between these sequences and any other locality where the Mulde Excursion has been recorded in sufficient detail.

A revised 87Sr/86Sr curve for the Silurian: Implications for global ocean chemistry and the Silurian timescale

Recent recalibration of the Silurian timescale and improved global chronostratigraphic correlation of Silurian strata significantly altered the Silurian 87Sr/86Sr curve and the temporal extent of available data. Whereas previous Silurian 87Sr/86Sr composites showed a generally monotonic increase throughout the Silurian, revisions to the Silurian timescale now require a major increase in the rate of change in 87Sr/86Sr at or near the onset of the Gorstian Age of the Ludlow Epoch. Similarly, improved chronostratigraphic correlations between Silurian outcrops on Anticosti Island, Canada, and Gotland, Sweden, indicate that the middle part of the Telychian Age, which is roughly 10%–15% of the total duration of the Silurian period, is undersampled and underrepresented in Silurian 87Sr/86Sr composites. A revised Silurian 87Sr/86Sr curve based on 241 new and published analyses confirms the significant increase in the rate of change of 87Sr/86Sr toward more radiogenic values near the base of the Ludlow Series. On the basis of these data, we propose that the rapid trend toward more radiogenic 87Sr/86Sr values is indicative of increased weathering of old sialic crust exposed during the Silurian uplift of portions of Baltica, Laurentia, and Avalonia. Importantly, however, the actual rate of change of 87Sr/86Sr will remain equivocal until the durations of Silurian epochs and ages are better constrained.

High-precision U–Pb zircon age constraints on the duration of rapid biogeochemical events during the Ludlow Epoch (Silurian Period)

Precise determinations of the rates and durations of Palaeozoic biogeochemical events are largely unavailable. Here, we present two new high-precision U–Pb (zircon) dates from volcanic ash deposits from the Ludlow Series (Silurian System) of Podolia, Ukraine, that yielded weighted mean 206Pb/238U dates of 424.08 ± 0.20 (0.29) [0.53] Ma and 422.91 ± 0.07 (0.21) [0.49] Ma (analytical, tracer and total uncertainties). These new dates bracket the largest post-Cambrian global carbon cycle perturbation (Lau Excursion) and constrain the ‘Ludlow Rise’ in 87Sr/86Sr. These chronostratigraphically well-controlled dates improve the calibration of the Silurian time scale and provide the first determinations of the rates of biogeochemical change during the Ludlow Epoch. Supplementary material: U–Pb geochemical methods, data and CL imagery are available at http://www.geolsoc.org.uk/SUP18798.

Preliminary Estimation of Paleoproductivity via TOC and Habitat Types: Which Method Is More Reliable? —A Case Study on the Ordovician–Silurian Transitional Black Shales of the Upper Yangtze Platform, South China

ABSTRACT New total organic carbon (TOC) data from the two Ordovician–Silurian transitional graptolite-bearing black shale intervals, the Wufeng Formation and the Longmaxi Formation in Central Guizhou and West Hubei, respectively, as well as previously reported TOC data from the same intervals in other places on the Yangtze platform of South China, have been used to produce an initial estimate of the primary paleoproductivity via a conventional inverse method (i.e., Rpp-inverse). The values of the Rpp-inverse are estimated to be 32 (43–21) gC/(m2·a) (Wufeng Formation) and 21 (27–16) gC/(m2·a) (Longmaxi Formation). Also, simultaneously, the habitat types (i.e., HT; cf., BA: benthic assemblage) and their temporal and spatial changes have been documented from the same succession, and an initial estimate of the primary paleoproductivity has been produced using a forward method (i.e., Rpp-forward). Being bounded mainly by the peritidal to inner-shelf environment shelly-facies or mixed-facies successions with BA1 to BA3 faunas both at the top and the base, which indicates the habitat types from HT II1 to HT III2, the biohabitat type of the two graptolite-bearing black shale intervals can be limited to HT III to HT IV, corresponding to the inner shelf to the outer shelf, with depths from roughly 60 m to 200—300 m. Based on the current data from the South China Sea and the southern part of the East China Sea, values of Rpp-forward should be about 100 to 400 gC/(m2·a). The difference in the results via the two methods suggests that paleoproductivity estimates from the geological strata need to be made cautiously, with particular attention paid to the paleogeographic setting, oxic-anoxic conditions, as also the preservation factor of organic carbon.

Phanerozoic pO2 and the early evolution of terrestrial animals

Concurrent gaps in the Late Devonian/Mississippian fossil records of insects and tetrapods (i.e. Romers Gap) have been attributed to physiological suppression by low atmospheric pO2. Here, updated stable isotope inputs inform a reconstruction of Phanerozoic oxygen levels that contradicts the low oxygen hypothesis (and contradicts the purported role of oxygen in the evolution of gigantic insects during the late Palaeozoic), but reconciles isotope-based calculations with other proxies, like charcoal. Furthermore, statistical analysis demonstrates that the gap between the first Devonian insect and earliest diverse insect assemblages of the Pennsylvanian (Bashkirian Stage) requires no special explanation if insects were neither diverse nor abundant prior to the evolution of wings. Rather than tracking physiological constraint, the fossil record may accurately record the transformative evolutionary impact of insect flight.

Correlation of the Waco Member of the Alger Shale Formation (Silurian; Llandovery; Telychian) in east-central Kentucky and south-central Ohio

New and published stratigraphic data are integrated herein to resolve the age and correlation of the Waco Member (Alger Shale Formation), a problematic Telychian lithostratigraphic unit exposed in east-central Kentucky (United States). This interval is correlated with strata presently assigned to the Dayton Member of the Drowning Creek Formation in south-central Ohio and north-central Kentucky, where recent conodont studies refer these beds to the Pterospathodus eopennatus Superzone, a conclusion that is reinforced by an interval of elevated stable carbon isotope (δ13C) values, consistent with the lower Telychian “Valgu” Excursion. This correlation is strengthened by observations of facies and new carbonate carbon isotope (δ13Ccarb) data generated from localities in south-central Ohio and, for the first time in publication, from the type area of the Waco in east-central Kentucky. It is now clear that both lithostratigraphic units are substantially older than the recently redefined Dayton Formation of western Ohio, and a formal revision of nomenclature is needed. A series of revised correlations are proposed: the basal carbonate bed of the Waco in its type area is correlated to the lower “white” division of the “Dayton Member” in southern Ohio; an interval of fossiliferous shales, found in typical exposures of the Waco, is correlated to the upper “orange” division of the “Dayton” in outcrops further north. Continued use of the term “Dayton Member of the Drowning Creek Formation” at sections south and east of Greene County, Ohio, should be abandoned in favour of “Waco Member of the Alger Shale”.

Sequence boundaries and chronostratigraphic gaps in the Llandovery of Ohio and Kentucky: The record of early Silurian paleoceanographic events in east-central North America

New and published data are integrated herein to resolve the age and stratigraphic relationships for problematic strata of the Aeronian and Telychian (Llandovery; Silurian) in Ohio and Kentucky (USA). At least two major depositional sequences were traced along the eastern flank of the Cincinnati Arch; these are separated by a regionally angular unconformity with complex topography. Underlying units are progressively truncated to the northwest while overlying strata change facies, condense, and onlap in the same direction. The basal unit of the upper sequence is the Waco Member of the Alger Shale Formation in Kentucky and southern Ohio and the Dayton Formation in western Ohio. A persistent, positive carbonate carbon isotope (δ13Ccarb) excursion associated with the mid-Telychian Valgu Event is recognized in the upper subunit of the Waco Member; the absence of a comparable signal in the Dayton Formation corroborates interpretations that it is significantly younger. The correlations proposed here can be used to understand the nuanced depositional history and chronostratigraphic completeness of the lower Silurian in eastern North America. This framework can be used to characterize sea-level history and local conditions that prevailed during global paleoenvironmental events.

Telychian (Llandovery, Silurian) conodonts from the LaPorte City Formation of eastern Iowa, USA (East-Central Iowa Basin) and their implications for global Telychian conodont biostratigraphic correlation

Conodonts from the LaPorte City Formation of eastern Iowa (East-Central Iowa Basin) indicate an early to middle Telychian age for the formation. Conodonts diagnostic of the Pterospathodus eopennatus Superzone, Pterospathodus eopennatus ssp. n. 2 Zone, and Pterospathodus amorphognathoides angulatus Zone were recovered, allowing for the first direct comparison of the stratigraphic ranges of conodont species in the Illinois and Baltic Basins. A heretofore undescribed species of Pseudolonchodina similar to Pseudolonchodina fluegeli occurs in the LaPorte City Formation. It is distinguished from Pseudolonchodina fluegeli by the absence of fused denticles on Pa elements and discrete to nearly-discrete denticles on the other elements. Due to the fragmented nature of the specimens, the species is left in open nomenclature. Wurmiella? polinclinata polinclinata ranges much lower in the East-Central Iowa Basin (Pt. eopennatus ssp. n. 2 Zone) than the Baltic Basin, and therefore cannot be used as an index fossil diagnostic of the Pt. am. amorphognathoides Zone in global correlations. This study documents the utility of the small limestone formations on the northwest flank of the East-Central Iowa Basin for refining global Silurian conodont biostratigraphic zonation. Christopher B.T. Waid. Department of Earth and Environmental Sciences, University of Iowa, 115 Trowbridge Hall, Iowa City, IA 52242, USA; Present address: Ohio Geological Survey, 2045 Morse Rd. Building C, Columbus, OH 43229, USA. christopher.waid@dnr.ohio.gov Bradley D. Cramer. Department of Earth and Environmental Sciences, University of Iowa, 115 Trowbridge Hall, Iowa City, IA 52242, USA. bradley-cramer@uiowa.edu

Who Will Build the 21st Century? Addressing Critical Demographic Gaps in the Geosciences

GSA Today, v. 25, no. 12, doi: 10.1130/GSATG243GW.1. Bradley D. Cramer, Dept. of Earth and Environmental Sciences, University of Iowa, Iowa City, Iowa 52242, USA, bradley-cramer@ uiowa.edu; Katherine J. Lewandowski, Dept. of Geology/ Geography, Eastern Illinois University, Charleston, Illinois 61920, USA, kjlewandowski@eiu.edu; Arthur Goldstein, Bartlett College of Science & Mathematics, Bridgewater State University, Bridgewater, Massachusetts 02325, USA, arthur.goldstein@ bridgew.edu; Pranoti Asher, American Geophysical Union, Washington, D.C. 20009, USA, pasher@agu.org; Jeffrey Ryan, School of Geosciences, University of South Florida, Tampa, Florida 33620, USA, ryan@usf.edu; David I. Schofield, British Geological Survey, Greenmeadow Springs, Tongwynlais, Cardiff CF15 7NE, UK, dis@bgs.ac.uk; Rex Buchanan, Kansas Geological Survey, University of Kansas, Lawrence, Kansas 66047, USA, rex@ kgs.ku.edu; Richard Denne, Marathon Oil, Houston, Texas 77056, USA, radenne@marathonoil.com; William I. Ausich, School of Earth Sciences, The Ohio State University, Columbus, Ohio 43210, USA, ausich.1@osu.edu; Thijs R.A. Vandenbroucke, Evo-EcoPaleo, UMR 8198 du CNRS, Université Lille 1, Villeneuve d’Ascq 59655, France, and Dept. of Geology and Soil Sciences, Ghent University, Ghent 9000, Belgium, Thijs.Vandenbroucke@UGent.be; Sherman Lundy, BMC Aggregates L.C., Elk Run Heights, Iowa, 50707, USA, sherml@bmcaggregates.com; Tyler Priest, Dept. of History and Dept. of Geographical and Sustainability Sciences, University of Iowa, Iowa City, Iowa 52242, USA, tyler-priest@ uiowa.edu; and Ryan J. Clark, Iowa Geological Survey, University of Iowa, Iowa City, Iowa 52242, USA, ryan-j-clark@uiowa.edu