https://doi.org/10.1130/G46571.1 Manuscript received 28 May 2019 Revised manuscript received 19 July 2019 Manuscript accepted 6 August 2019 © 2019 Geological Society of America. For permission to copy, contact [email protected]. Published online 30 August 2019 Linking the progressive expansion of reducing conditions to a stepwise mass extinction event in the late Silurian oceans Chelsie N. Bowman1, Seth A. Young1, Dimitri Kaljo2, Mats E. Eriksson3, Theodore R. Them II4, Olle Hints2, Tõnu Martma2 and Jeremy D. Owens1 1Department of Earth, Ocean and Atmospheric Science–National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32306, USA 2Department of Geology, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia 3Department of Geology, Lund University, Sölvegatan 12, SE-223 62 Lund, Sweden 4Department of Geology and Environmental Geosciences, College of Charleston, Charleston, South Carolina 29424, USA ABSTRACT fauna and a resur gence of microbially mediated The late Ludlow Lau Event was a severe biotic crisis in the Silurian, characterized by sedimentary facies (e.g., Talent et al., 1993; resurgent microbial facies and faunal turnover rates otherwise only documented during the Jeppsson, 2005; Calner, 2005, 2008; Eriksson “big fve” mass extinctions. This asynchronous late Silurian marine extinction event preceded et al., 2009). an associated positive carbon isotope excursion (CIE), the Lau CIE, although a mechanism Despite the magnitude and complexity of for this temporal offset remains poorly constrained. Here, we report thallium isotope data the LKE, its mechanistic underpinnings are from locally reducing late Ludlow strata within the Baltic Basin to document the earliest not well constrained. An expansion of reduc- onset of global marine deoxygenation. The initial expansion of anoxia coincided with the on- ing conditions has been implicated as a po- set of the extinction and therefore preceded the Lau CIE. Additionally, sulfur isotope data tential driver of the observed stepwise extinc- record a large positive excursion parallel to the Lau CIE, interpreted to indicate an increase tion (e.g., Munnecke et al., 2003; Stricanne in pyrite burial associated with the widely documented CIE. This suggests a possible global et al., 2006). This hypothesis is also used to expansion of euxinia (anoxic and sulfdic water column) following deoxygenation. These data explain the possibility of extensive burial of are the most direct proxy evidence of paleoredox conditions linking the known extinction to organic carbon, resulting in the Lau positive the Lau CIE through the progressive expansion of anoxia, and most likely euxinia, across carbon isotope excursion (CIE; e.g., Saltzman, portions of the late Silurian oceans. 2005), but this cannot explain the temporal offset between the LKE and the Lau CIE. Fur- INTRODUCTION graptolite studies of deeper-water shale succes- ther, organic carbon burial can be affected by High rates of evolutionary turnover and sions (termed the Kozlowskii event; Koren’, other factors (e.g., Canfeld, 1994). Variations severe, punctuated extinctions of marine taxa 1993; Urbanek, 1993), herein referred to as the in eustatic sea level and carbonate weather- were hallmarks of the Silurian (e.g., Jeppsson, Lau/Kozlowskii extinction (LKE). The LKE ing rates have also been invoked as potential 1998; Crampton et al., 2016). These events is at least the tenth largest e xtinction event mechanisms for driving positive CIEs (e.g., occurred during the transition from the Late in Earth history, with ∼23% loss of genera Hirnantian CIE; Kump et al., 1999). A global Ordovician icehouse to Devonian greenhouse (e.g., Bond and Grasby, 2017, and references expansion of reducing conditions, however, worlds, when a dynamic ocean-atmosphere therein). In addition to conodonts and grapto- provides a kill mechanism and can be tested system oscillated between cool and warm lites, it affected a wide range of marine taxa, using combined traditional and novel paleo- conditions (e.g., Jeppsson, 1998). Recurrent including brachiopods (Talent et al., 1993), redox proxies. extinctions in graptolites and conodonts were fshes (Eriksson et al., 2009), and acritarchs This study investigated the relationship be- associated with the transitions between the (Stricanne et al., 2006). Extinctions of indi- tween the LKE and Lau CIE in the context of alternating climate states, the most notable vidual taxonomic groups were asynchronous, changing marine redox conditions in Upper Si- being the globally documented late Ludlow with documented extinctions in benthic and lurian (Ludfordian Stage) strata from the Baltic Lau extinction (Jeppsson, 1998; Calner, 2005; nektonic groups preceding the planktic organ- Basin. In order to reconstruct the evolution of Crampton et al., 2016). This extinction was frst isms (e.g., Munnecke et al., 2003; Stricanne global marine redox conditions, we measured recognized using conodonts from carbonate et al., 2006; Calner, 2008). The LKE shares thallium (Tl) isotopes, manganese (Mn) con- 34 platform successions (termed the Lau Event; similar characteristics to the “big fve” mass centrations, and pyrite sulfur isotopes (δ Spyr) e.g., Jeppsson and Aldridge, 2000) and then in extinctions, such as the survival of disaster from a distal shelf/slope setting (Latvia), and CITATION: Bowman, C.N., et al., 2019, Linking the progressive expansion of reducing conditions to a stepwise mass extinction event in the late Silurian oceans: Geology, v. 47, p. 968–972, https://doi.org/10.1130/G46571.1 968 www.gsapubs.org | Volume 47 | Number 10 | GEOLOGY | Geological Society of America Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/47/10/968/4830621/968.pdf?casa_token=bVkh5kLY-Z0AAAAA:359HkSvelJv13qxZCterkUhCPVlSWGIJq3TGaDdjftR2LMRqK1rxLmTc8uiLAhi7IzCyGQ by Florida State University user on 05 April 2020 GEOLOGIC SETTING isotopes (ε205Tl = [{R /R } – 1] × 104) Estonia sample reference Land In the late Silurian, the Baltic Basin was lo- have been used to investigate the earliest onset of Shallow Shelf cated in a tropical, epicratonic seaway on the global marine deoxygenation during Mesozoic Deep Shelf Latvia southern margin of the paleocontinent Bal- CIEs (Ostrander et al., 2017; Them et al., 2018). Ocean tica (e.g., Eriksson and Calner, 2008; Fig. 1; During Mn-oxide precipitation, Tl is adsorbed Fig. DR1 in the GSA Data Repository1). with a large positive isotope fractionation, thus Lithuania The northern and eastern edges of the basin leaving seawater isotopically lighter (as re- P-20 were delineated by rimmed carbonate shelves viewed in Nielsen et al., 2017). Precipitation and U-1 with parallel facies belts ranging from lagoonal burial of Mn-oxides require oxic bottom-water Sweden deposits in the north to deep shelf shales and conditions, the expanse of which represents the marls in the south, deepening toward the Rheic dominant seawater control of Tl isotope com- Poland Ocean (Eriksson and Calner, 2008). The Ud- position on time scales <∼5 m.y. (Nielsen et al., dvide-1 drill core and nearby outcrops on the 2017; Owens et al., 2017). The global seawater island of Gotland, Sweden, are predominantly Tl isotope signal is recorded in euxinic marine composed of carbonates from the shallow shelf settings and in anoxic waters with sulfde near area of the basin (for more details, see Eriks- the sediment-water interface from basins that Rheic Ocean son and Calner, 2008). The Priekule-20 drill are well connected to the open ocean (Owens core from southwestern Latvia predominantly et al., 2017). Consequently, reconstruction of the 0km 100 200 o consists of shales and marls from a correlative Tl isotope composition of late Silurian seawater N 30 S deep shelf setting (details in Kaljo et al., 1997). provides evidence for initial changes in global marine oxygenation by tracking the burial fux Figure 1. Paleogeographic reconstruction of METHODS AND RESULTS of Mn-oxides relative to the extinction, CIE, the late Silurian Baltic Basin region (modi- Two study localities were analyzed for δ13C and additional redox proxies (e.g., Ostrander fed from the Blakey Europe Series, Silurian records, organic or inorganic (micrite), to in- et al., 2017; Them et al., 2018). Importantly, a ca. 425 Ma; https://www2.nau.edu/rcb7/). vestigate carbon cycle dynamics. Pyrite sulfur large Tl isotope fractionation is not associated Locations of Gotland (Sweden) localities and the Latvian Priekule-20 drill core are was extracted from shale samples using a wide- with the burial of other Mn-bearing minerals marked by yellow stars. Detailed discussion ly accepted chromium reduction method, and (e.g., sulfdes, carbonates). Constraints on lo- of correlation and biostratigraphy of our two CAS was extracted from carbonates following cally reducing conditions using an independent localities can be found in the Data Repository standard methods. Full details of all analytical geochemical proxy are necessary to interpret Tl (see footnote 1). methods are given in the Data Repository. Sul- isotopes as a temporal seawater signature and to fur isotopes were analyzed to investigate global avoid contamination via local Mn-oxides (Ow- 34 carbonate-associated sulfate (CAS) sulfur iso- pyrite burial (δ SCAS) and the potential imprints ens et al., 2017). Low total Mn concentrations 34 34 topes (δ SCAS) from a shallow shelf setting (Got- on local signatures (δ Spyr). Sedimentary Tl [Mn] are indicative of locally reducing condi- land,