ABSTRACT Conodont apatite δ18OV-SMOW values from Middle though Upper Pennsylvanian (Desmoinesian–Missourian) laminated, marine black shale units within cyclic deposits of intercalated terrestrial and marine strata (cyclothems) from the Illinois Basin (United States) were measured in order to evaluate their utility as a proxy for changes in the oxygen isotopic composition of the epicontinental Late Pennsylvanian Midcontinent Sea (LPMS). The average δ18OV-SMOW values of well-preserved monogeneric (Idiognathodus) separates of conodont apatite from 12 lithologic units representing nine cyclothems range from 17.0‰ to 20.1‰ and average 19.0‰ ± 0.4‰ (1σ). Within the limits of analytical uncertainty of stable isotope measurements, the stratigraphic distribution of conodont apatite δ18O values is nontrending; particularly, there is no significant shift in δ18O values across the Desmoinesian–Missourian boundary, a period that has been interpreted to preserve a shift toward a warmer climate, increased seasonality, a...
The Late Mississippian represents a time when Earth was thought to be an icehouse and was experiencing eustatic sea level changes similar to today. While there’s been a wealth of research done in the western equatorial Pangea for the Late Carboniferous and Permian, this study of the Pennington Formation, Tennessee, offers an opportunity to study less focused upon terrestrial early Carboniferous paleoenvironments from central equatorial Pangea, including the effects of diagenesis on paleoenvironmental proxies employed for paleosol research. New fieldwork of an outcrop outside Sparta, TN shows interbedded limestone and mudstone layers including four paleosol profiles that have been described and analyzed for their principle clay mineralogy. The paleosols preserve typical vertic features including slickensides, mukkara and wedge-shaped peds as well as low chroma color and are thus gleyed Vertisols. The gleyed nature of these paleosols is either the result of forming under waterlogged conditions seen today in soils forming proximal to shorelines or the result of diagenesis associated with sea level rising. The presence of Vertisols intercalated between limestones suggests a persistent influence of glacioeustacy in conjunction with highly seasonal climates during base-level lowstands and soil development which gave rise to pedoturbation and the characteristic suite of vertic morphologies seen in outcrop. Clay mineralogy dominated by illite and vermiculite suggests burial diagenesis. This contrasts sites from the upper Pennsylvanian which contain evidence for eustatic sea level change but are suggestive of more ever-wet conditions and recorded by the common occurrence of thick coal layers atop mineral-dominated paleosol profiles.
Royer et al. (2004), in their recent article in GSA Today, dispute the conclusion of Shaviv and Veizer (2003) that celestial forcing may have been the major driver of Phanerozoic climate, arguing instead for CO 2 as the dominant force.The δ 18 O of calcareous shells reflects the ambient temperature of seawater and the quantity of water locked in the polar ice caps, each contributing about one half to this signal, but lately it has been realized that seawater pH drives the δ 18 O in the opposite direction.Royer et al. utilized this for reconciling the GEOCARB III and the δ 18 O trend of Veizer et al. (2000) by assuming that any discrepancy of the two variables is due to pH.This is an interesting proposition that has some merit, but note that such a correction is entirely arbitrary, because we do not have any constraints for the pH of Phanerozoic seawater, except possibly some boron isotopes for the youngest portion of this record.Moreover, the pH correction itself basically reflects the GEOCARB III CO 2 model, such that the correlation obtained between the corrected δ 18 O and CO 2 cannot be claimed to be a CO 2 fingerprint.Furthermore, the model of Royer et al. does not consider the mitigating "ice volume" effect.Once included, the required pH correction (and GEOCARB III CO 2 levels) would have to be about doubled for CO 2 climate "driving" to be on par with the CRF.For all these, and the reasons listed in our detailed response (see www.gsajournals.org;go to "Online Journals," then "Online Forum"), we argue that the δ 18 O trend is still chiefly a reflection of the temperature history of the past oceans, controlled principally by the celestial driver.
