Paleoecologic and paleoceanographic interpretation of δ18O variability in Lower Ordovician conodont species

Conodont δ18O is increasingly used to reconstruct Paleozoic–Triassic seawater temperature changes. Less attention has been paid to δ18O variation in time slices across paleoenvironments, within sample assemblages, or for reconstructing the thermal structure of Paleozoic oceans. Furthermore, there have been few independent tests of conodont ecologic models based on biofacies and lithofacies distributions. Here we present the first test of ecologic models for conodonts based on δ18O values of a Laurentian Lower Ordovician (Floian) shelf edge–upper slope assemblage in debrites of the proximal lower slope Shallow Bay Formation, Cow Head Group, western Newfoundland. Nine species yield a 1.6–1.8‰ intra-sample δ18O variability based on mixed tissue and white matter-only analyses, equivalent to an ~7–8 °C range. Linear mixed models demonstrate statistically significant differences between the δ18O of some species, supporting the interpretation that an isotopic and temperature gradient is preserved. By considering conodont δ18O in a geologic context, we propose an integrated paleoecologic and paleoceanographic model with species tiered pelagically through the water column, and confirm the utility of conodonts for water-mass characterization within Paleozoic oceans. INTRODUCTION The δ18O records of carbonate fluorapatite conodont elements are increasingly utilized to estimate Paleozoic–Triassic seawater temperatures, and have led to global-scale hypotheses concerning biodiversification and mass extinction, reef evolution, climate change, and glacioeustasy (e.g., Trotter et al., 2008, 2016; Joachimski et al., 2009; Elrick et al., 2013; Rosenau et al., 2012). Despite these advances, little research has addressed variation of conodont δ18O in relation to conodont ecology and oceanic thermal configurations. Conodont morphological diversity and their peritidal to deep-water facies distribution in the marine rock record implies occupancy of multiple ecologic niches, which likely also represent a variety of seawater temperatures (Sweet, 1988). A number of models for conodont ecology have been proposed that suggest they either lived tiered pelagically in the water column (Seddon and Sweet, 1971), or that the majority were segregated by depth as nektobenthos (Barnes and Fåhræus, 1975). For the Ordovician, these models have been combined to help explain conodont distributions (Zhen and Percival, 2003) but, to date, conodont δ18O has not been used to test the models directly. Conodont paleothermometry assumes precipitation of fluorapatite in oxygen-isotopic equilibrium with seawater, no species-vital effects, and retention of primary oxygen-isotopic signatures in the fossil record (see the review in MacLeod, 2012). Thus, conodont δ18O variability is interpreted as representing an environmental signal, most commonly seawater temperature. Although some researchers report no significant isotopic distinction between Ordovician taxa from temperature-differentiated oceanic realms (Buggisch et al., 2010), others have considered the importance of isotopic variability between taxa to reflect varying occupation of temperature-stratified water masses (e.g., Herrmann et al., 2010). The extent of inter-species δ18O variability is known from two approaches. Analyses by thermal conversion–elemental analyzer–isotope ratio mass spectrometry (TC-EA-IRMS) of silver phosphate from conodont fluorapatite have demonstrated significant species δ18O offsets within a single ‘conodont’ sample (~1–1.5‰; e.g., Herrmann et al., 2010; Rosenau et al., 2012). Analyses by secondary ionization mass spectrometry (SIMS) identified δ18O offsets of up to 2‰ across shelf taxa from an Ordovician sample (Wheeley et al., 2012), and 0.9‰ variability between genera in a Devonian basin (Narkiewicz et al., 2017). These ~1–2‰ taxonomic offsets equate to significant temperature ranges (~4–8 °C) when converted using phosphate-temperature equations (e.g., Lécuyer et al., 2013). Therefore, it is important to consider how isotopic variability in samples relates to autecology or other controls (Wheeley et al., 2012), especially because conodont δ18O is being used more widely in studies of seawater temperature evolution (e.g., Trotter et al., 2016). A potential complicating factor to an ecologic explanation of δ18O species variability is that of differential δ18O from the constituent hard tissues of conodont elements: the white matter (albid crown), hyaline crown, and basal body (e.g., Zhang et al., 2017). TC-EA-IRMS analyses have established that δ18O variability increases when the basal body is included (Wenzel et al., 2000), but this tissue is rarely preserved and therefore unlikely to be a source of δ18O variability. Some authors report no significant δ18O differences between conodont crown tissues via SIMS (Trotter et al., 2016), whereas others have (Wheeley et al., 2012; Zhang et al., 2017). The differing δ18O values between conodont tissues may relate to carbonate content, although this effect is probably minimal as the majority of oxygen is contained within diagenetically stable phosphate (Wheeley et al., 2012). Basal body tissue has the most carbonate, with lesser amounts in hyaline crown, and it is not detected in albid crown (Trotter and Eggins, 2006). The high spatial resolution of SIMS analyses enables targeting of tissues and evaluation of where analyses have occurred and, along with multiple analyses of individual conodont elements, enables potential histological bias to be overcome. This paper has two aims: (1) to appraise the SIMS conodont δ18O variability of a speciesrich sample from a setting with potential for seawater-column temperature differences, and (2) to contextualize these data in an integrated paleoecologic and paleoceanographic model, thus testing the potential for determining the paleothermometry of ancient oceans, and conodont ecologic models.