(2006), Pp. 85–90. Article

GFF volume 128 (2006), pp. 85–90. Article 13 The Late Wenlock Mulde positive carbon isotope (δ Ccarb) excursion in North America BRADLEY D. CRAMER1, MARK A. KLEFFNER2 and MATTHEW R. SALTZMAN1 Cramer, B.D., Kleffner, M.A. & Saltzman, M.R., 2006: The Late Wenlock Mulde positive carbon isotope excursion in North America. GFF, Vol. 128 (Pt. 2, June), pp. 85–90. Stockholm. ISSN 1103-5897. Abstract: Marine carbonates from two well-studied areas of the Silurian of North America were analyzed 13 for stable carbon isotope (δ Ccarb) stratigraphy. A graptolite-bearing sequence from the eastern margin of the Panthallasic Ocean (Nevada) and a conodont-bearing sequence from the mid-continent epeiric 13 seaway (Tennessee) were sampled for δ Ccarb 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 13 represented in Tennessee. Our recognition of the dual-peaked Mulde δ Ccarb excursion in North America allows improved correlation between these sequences and any other locality where the Mulde Excursion has been recorded in sufficient detail. Keywords: Wenlock, Ludlow, Silurian, carbon isotope, stratigraphic correlation, conodont, graptolite, extinction. 1 Department of Geological Sciences, The Ohio State University, 125 S. Oval Mall, Columbus, Ohio 43210, USA; [email protected]; [email protected] 2 Department of Geological Sciences, The Ohio State University at Lima, 4240 Campus Drive, Lima, Ohio 45804, USA; [email protected] Manuscript received 15 August 2005. Revised manuscript accepted 3 May 2006. Introduction 13 The well-studied Swedish island of Gotland has become the The two protracted positive carbon isotope (δ Ccarb) excursions global standard for conodont biostratigraphy and carbon isotope (Ireviken and Mulde Excursions) can serve as reliable chronos- stratigraphy of the Wenlock (Jeppsson 1997; Bickert et al. 1997; tratigraphic markers for Wenlock successions. The Sheinwoodian Calner & Jeppsson 2003; Jeppsson & Calner 2003; Munnecke (Early Wenlock) Ireviken Excursion reaches values as high as et al. 2003; Calner et al. 2004; Calner et al. 2005; Calner et al. +5.00‰ (VPDB) and spans the lower half of the Sheinwoodian this volume). The numerous carbon isotope investigations of (Bickert et al. 1997; Munnecke et al. 2003; Cramer & Saltzman Wenlock sequences have promoted global climate research sur- 2005; Calner et al. 2006; Kaljo & Martma this volume). The rounding the two major positive carbon isotope excursions that Homerian (Late Wenlock) Mulde Excursion is a dual-peaked took place during the Wenlock (see Calner et al. 2004) however excursion with values approaching +4.00‰ (VPDB) in some the inability to precisely correlate sections outside of Baltica localities (Corfield et al. 1992; Por´bska et al. 2004; Calner et with the Gotland sequence has hindered accurate global climate al. 2006) and is associated with the ʻBig Crisisʼ graptolite ex- reconstructions of this unstable phase of the Silurian. In many tinction (“der Grossen Krise” Jaeger 1959; 1991; Quinby-Hunt sections world-wide, available conodont data is often insuffi- & Berry 1991). Here, we sampled marine carbonates from a cient to use the detailed zones recognized on Gotland (Calner & graptolite-bearing sequence in central Nevada (Simpson Park Jeppsson 2003). Likewise, the classical data available for many I), and a conodont-bearing sequence in north-central Tennessee 13 graptolitic sections do not contain all of the now recognized (McCrory Lane) at regular stratigraphic intervals for δ Ccarb 13 zonal species (e.g. Por´bska et al. 2004). In instances such as stratigraphy. By locating the Mulde δ Ccarb excursion in the this, where biostratigraphic correlation between geographically study areas, we can constrain more precisely the timing of global distant sections that often lack key species leaves a measure of climate changes recorded in strata deposited during this interval uncertainty, another means of correlation is desirable. of the Silurian. 13 86 Cramer et al.: The Late Wenlock Mulde positive carbon isotope (δ Ccarb) excursion in North America GFF 128 (2006) by assigning a δ13C value of +1.95‰ and a δ18O value of –2.20‰ to NBS19. Reproducibility was checked by replicate analysis of laboratory standards and is better than ±0.05‰. Samples were analyzed at the Erlangen University stable isotope laboratory in Germany. Results and Discussion Simpson Park I, Nevada The well-known graptolite-bearing succession of the Roberts Mountain Formation in the Simpson Park Range, Nevada, was studied in detail for graptolite biostratigraphy by Berry & Murphy (1975). The Mulde Excursion is well developed in the Fig. 1. Wenlock paleogeography (after Woodcock 2000; Cocks 2001; Roberts Mountain Formation with baseline values of +0.50‰ Johnson et al. 2001). Grey areas denote land. Capital letters refer to and highs approaching +2.50‰ during both peaks of the excur- major emergent landmasses: L – Laurentia; B – Baltica; S – Siberia; G sion (Fig. 2; Table 1). The graptolite data of Berry & Murphy – Gondwana. Arrows point to locations of the two sampled sections in Nevada and Tennessee. (1975) allow the recognition of the Monograptus testis and M. flemingii-P. dubius Zones of Por´bska et al. (2004), but the rest of the zonally useful species for higher in the sequence were not reported from Nevada with the exception of G. nassa and Pristiograptus ludensis (Berry & Murphy 1975). The carbon isotope curve follows the graptolite correlation of Por´bska et al. (2004) in the lower portions of the sequence Geological background where zonally useful species are abundant. As predicted by the The two sampled sections were located on the paleocontinent of correlation of Por´bska et al. (2004), the most rapid positive rate 13 Laurentia, with Nevada (Simpson Park I) located on the western of change during the first peak in δ Ccarb took place shortly after margin facing the Panthallasic Ocean, and Tennessee (McCrory the disappearance of the M. flemingii, and the first peak occurred Lane) located towards the southern end of the mid-continent near the FAD of G. nassa. Graptolites present higher in the se- epeiric sea (Fig. 1). Both sections were located between 15° and quence are problematic however, when compared with the carbon 35° S latitude (Witzke & Scotese 1990; Woodcock 2000). isotope data. From the first peak and the first positive identifica- The Mulde Excursion has been well-correlated to both grap- tion of G. nassa, the excursion extends more than 10 meters up tolite and conodont biostratigraphy (Jeppsson & Calner 2003; section before the end of the second peak. The problem arises Calner & Jeppsson 2003; Por´bska et al. 2004; Calner et al. with the occurrence of Neodiversograptus nilssoni, which marks 2006). In graptolite-bearing sequences, the fastest rate of posi- the base of the Ludlow Series, only 2.4 meters above G. nassa 13 tive change in δ Ccarb occurs shortly after the disappearance of and well within the center of the Mulde Excursion as recorded in Monograptus flemingii with the first peak occurring near the first this section. Previously, the Mulde Excursion was considered to appearance datum (FAD) of Gothograptus nassa. The low-point have occurred entirely within the Wenlock (Corfield et al. 1992; between the peaks occurs near the boundary between the Pristi- Por´bska et al. 2004; Calner et al. 2004). Our data therefore in- ograptus dubius-G. nassa Zone and the overlying Colonograp- dicate one of several possibilities; (1) that the Mulde Excursion tus praedeubeli Zone. The second peak appears to be restricted actually crossed the Wenlock-Ludlow boundary and should be primarily to the C. praedeubeli Zone (Por´bska et al. 2004). In re-correlated with respect to Silurian time and that all other sec- conodont-bearing sequences, the first peak occurs in the Ozarko- tions where the Mulde Excursion has been recorded need to be dina bohemica longa Zone with the low-point coming near the likewise adjusted; (2) that N. nilssoni was misidentified by Berry boundary with the overlying Kockelella ortus absidata Zone. & Murphy (1975); (3) that N. nilssoni first occurred in Nevada The second peak is restricted to the Ctenognathodus murchisoni substantially earlier than elsewhere in the world; or (4) that a Zone (Calner et al. 2005). significant break in deposition occurred just below the first occurrence of N. nilssoni cutting out the second peak of the Mulde Excursion and serendipitously replacing it with another isotopic Methods excursion of similar magnitude in the Early Ludlow. Marine carbonates have been shown to faithfully preserve the None of the four possible explanations suggested for the prob- 13 13 primary marine carbon isotopic δ Ccarb signature (compare lematic second peak of the δ Ccarb curve in the Simpson Park I Munnecke et al. 1997 with Bickert et al. 1997), and the use of section clearly best explains the distribution of N. nilssoni rela- fine-grained carbonates for carbon isotope stratigraphy has been tive to it in that section: (1) Based on the work in the classical demonstrated in every system of the Paleozoic (Saltzman 2005). British sections by Corfield et al. (1992), and the fact that nei- Analyzed powders were micro-drilled from micritic matrix ther N. nilssoni, nor any other graptolite indicative of the Early whenever possible, but some samples were too coarse grained Ludlow, was found during the Mulde Excursion in Poland by to guarantee that no skeletal material was sampled.

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