Palaeogeography, Palaeoclimatology, Palaeoecology 553 (2020) 109799 Contents lists available at ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo Integrated sedimentary, biotic, and paleoredox dynamics from multiple T localities in southern Laurentia during the late Silurian (Ludfordian) extinction event ⁎ Chelsie N. Bowmana, , Anders Lindskoga,b, Nevin P. Kozika, Claudia G. Richbourga, Jeremy D. Owensa, Seth A. Younga a Department of Earth, Ocean, & Atmospheric Sciences | National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32306, USA b Department of Geology, Lund University, Sölvegatan 12, SE-223 62 Lund, Sweden ARTICLE INFO ABSTRACT Keywords: The Silurian was a time of major climatic transition punctuated by multiple biotic crises and global carbon cycle Sulfur isotopes perturbations. The most severe of these biotic events was the late Silurian (Ludfordian) Lau/Kozlowskii ex- Iodine tinction event (LKE) and the associated Lau carbon isotope excursion (CIE). Although the extinction event and Microfacies Lau CIE are globally documented, the only records thus far of local and global marine paleoredox conditions Anoxia through this interval are from a single region in Scandinavia. Here we examine four sections along a bathymetric Euxinia transect of mixed carbonate-siliciclastic sediments from western Tennessee, USA. A novel approach using a Sea level multi-proxy dataset combining high-resolution geochemical data and microfacies analyses from multiple lo- calities explores the possibilities of local/regional-scale redox heterogeneities during a time of widespread en- vironmental upheaval on a global scale. Paired positive excursions recorded in carbonate carbon isotopes and carbonate-associated sulfate sulfur isotopes support recent work from carbonate and siliciclastic successions from Scandinavia, suggesting a global enhancement of organic carbon and pyrite burial driven by an expansion of euxinic (anoxic and sulfidic water column) conditions in the oceans during the mid-Ludfordian. Furthermore, positive excursions in organic carbon isotopes and pyrite sulfur isotopes reflect the global changes in redox. Stratigraphic trends in I/Ca ratios imply a local expansion of low-oxygen conditions, with low, but non-zero values during the rising limb and peak of the CIE. The fossil assemblages vary across the shelf and through the CIE interval. Stratigraphic changes in fossil assemblages and I/Ca are closely associated with local and global changes in oxygenation and sea level during the mid-Ludfordian. The collective data indicate significant biotic reorganization in response to changes in marine redox conditions and in conjunction with sea-level variation during the LKE interval, but detailed macroscopic biodiversity is currently unconstrained for this region. 1. Introduction is also considered the largest positive carbon isotope excursion in the Phanerozoic (Munnecke et al., 2003; see Calner, 2008 for review). The The Silurian Period was a climatically dynamic time in the middle Lau CIE is associated with the Lau/Kozlowskii extinction (LKE) event, Paleozoic marked by repeated extinctions, faunal reorganizations, and which is at least the tenth most severe extinction in Earth history with a carbon cycle perturbations. In less than 30 million years there were at loss of ~23% of marine genera (Bond and Grasby, 2017 and references least four globally documented positive carbon isotope excursions (CIE) therein). This extinction was first recognized in conodonts from car- greater than +4‰, indicating higher magnitude carbon-cycle volatility bonate platform successions as the ‘Lau event’ (Jeppsson, 1990) and in than any other period in the Phanerozoic (Bickert et al., 1997; Kaljo studies of graptolites in deeper-water shales as the ‘Kozlowskii event’ et al., 1997; Saltzman, 2001, 2002; Cramer and Saltzman, 2007; (Koren, 1993; Urbanek, 1993). As extinctions have been recognized in Saltzman and Thomas, 2012). The largest magnitude perturbation to an increasing variety of marine fauna, a curiously asynchronous pattern the Silurian global carbon cycle was the mid-Ludfordian Lau CIE, with of extinction has become apparent (e.g., Munnecke et al., 2003; peak excursion values averaging +5 to +8‰ and as high as +12‰; it Stricanne et al., 2006; compiled in Bowman et al., 2019). Extinctions in ⁎ Corresponding author. E-mail addresses: [email protected] (C.N. Bowman), [email protected] (A. Lindskog), [email protected] (N.P. Kozik), [email protected] (C.G. Richbourg), [email protected] (J.D. Owens), [email protected] (S.A. Young). https://doi.org/10.1016/j.palaeo.2020.109799 Received 17 January 2020; Received in revised form 10 May 2020; Accepted 11 May 2020 Available online 14 May 2020 0031-0182/ © 2020 Published by Elsevier B.V. C.N. Bowman, et al. Palaeogeography, Palaeoclimatology, Palaeoecology 553 (2020) 109799 benthic and nektonic taxa such as conodonts, fish (Eriksson et al., Silurian place the southern margin of Laurentia between 20 and 30°S 2009), and brachiopods (Talent et al., 1993) notably precede those of (Cocks and Scotese, 1991; Cocks and Torsvik, 2002; Torsvik and Cocks, planktic taxa such as graptolites and acritarchs (Stricanne et al., 2006). 2017). The Western Tennessee Shelf area was bounded by the Nashville Widespread development of ‘anachronistic’ (both microbially-mediated Dome to the east, the Reelfoot Rift Embayment to the west/northwest, and abiotic) carbonate facies has also been linked to the LKE as evi- and the continental margin to the south (present-day directions; dence of environmental stress and ecosystem collapse (e.g., Calner, Fig. 1B). The upper Silurian rocks exposed in the study area form a 2005). Much of the work on the record of biotic change during the LKE transect from shallower- to deeper-ramp settings approximately from has been done on the paleocontinents of Baltica (Latvia, Sweden; e.g., the southeast to northwest, towards the Reelfoot Rift Embayment Jeppsson, 1990; Stricanne et al., 2006; Eriksson et al., 2009; Urbanek (Barrick et al., 2010). et al., 2012) and Gondwana (Australia, Czech Republic; e.g., Jeppsson The upper Silurian (Ludfordian) Brownsport Formation is exposed et al., 2007; Lehnert et al., 2007; Manda et al., 2012). Comparatively in multiple roadcuts in the area northwest of Waynesboro, TN, along little is known about the Laurentian expression of this extinction event. highways US-64, US-641, and TN-228 near the Tennessee River. The High-magnitude positive CIEs can result from increased weathering Brownsport Formation conformably overlies the Dixon Formation, of carbonate platforms during sea-level lowstand (e.g., Berner, 1994; which is comprised largely of red and green mottled calcareous mud- Kump et al., 1999) and/or from enhanced organic carbon burial via stones (Amsden, 1949; Broadhead and Gibson, 1996). It is conformably productivity- or preservation-driven expansion of anoxia (Arthur et al., overlain by the Decatur Limestone, a typically light grey, massively 1988; Kump and Arthur, 1999; Cramer and Saltzman, 2005, 2007; Gill bedded crystalline limestone (Amsden, 1949). The Brownsport Forma- et al., 2011a; Owens et al., 2018). A gradual expansion of reducing tion itself is subdivided into three members, in ascending stratigraphic conditions has been proposed as a kill mechanism for the LKE, which order: Beech River, Bob, and Lobelville (Pate and Bassler, 1908; can account for the asynchronous nature of the extinction event and Broadhead and Gibson, 1996; Barrick et al., 2010). The Beech River and also the global CIE (e.g., Munnecke et al., 2003; Stricanne et al., 2006). Lobelville members are lithologically similar, being composed pri- A recent study using thallium isotopes from the Baltic Basin has shown marily of argillaceous wackestones and packstones that are commonly that global marine deoxygenation predates the Lau CIE and is coeval interbedded with calcareous shales; coarser-grained, locally sandy, with the onset of the LKE (Bowman et al., 2019). Additionally, parallel packstones and grainstones make up the Bob Member (Pate and Bassler, positive carbon and sulfur isotope excursions are documented following 1908; Amsden, 1949; Broadhead and Gibson, 1996; Barrick et al., the initial global deoxygenation, indicating that enhanced organic 2010). There has been detailed discussion on the decision to break up carbon burial was closely associated with the expansion of euxinic the Brownsport into finer-scale, formal lithologic units (e.g., Pate and conditions (anoxic with water-column sulfide) in the late Silurian Bassler, 1908; Amsden, 1949; Broadhead and Gibson, 1996; Barrick oceans (Bowman et al., 2019). et al., 2010), largely because the Bob Member is not readily identifiable During global scale biogeochemical events, such as the Lau CIE, in all exposures of the Brownsport Formation and the Beech River and only an increase in the fraction of ocean floor experiencing reducing Lobelville members are lithologically similar (Amsden, 1949). Some conditions is proposed (i.e., not anoxia or euxinia throughout the entire studies have tried to break up the Brownsport Formation based on ocean). Many previous studies have used geochemical box modeling to macrofaunal assemblages (e.g., Pate and Bassler, 1908), but these estimate the extent of marine anoxia and/or euxinia during similar subdivisions appear to be inconsistent between sections (see also results events, determining that