δ18O composition of conodont apatite indicates climatic cooling during the Middle Pridoli

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Abstract

Phosphatic microfossils, such as conodonts and fish microremains (dermal scales) from the upper Silurian (Pridoli) of Lithuania have been studied for their oxygen isotope composition. The conodont colour alteration index of the biogenic apatite did not exceed 1.5 reflecting only a minor thermal alteration. Conodont δ18Oapatite values range from 17.7 to 19.2‰ V-SMOW, with the average values around 18.3‰. Fossil apatite of fish exoskeleton microremains from the same samples have lower δ18Oapatite values ranging from 15.2 to 17.4‰ V-SMOW, with the average values around 16.3‰ V-SMOW. Palaeoseawater temperatures calculated from conodont apatite δ18O range from 24.8–31.5 °C, those derived from fish δ18O give approximately 10 °C higher temperatures ranging from 32.6 to 38.1 °C. The lower δ18Oapatite values of fish apatite and thus the unrealistic high palaeotemperatures are interpreted as a result of diagenetic alteration. A significant positive shift in conodont apatite δ18O of + 1.1‰ at the boundary between the Vievis and Lapės formations is observed, coinciding with a major facies change. It is interpreted as a mid-Pridoli event within the Ozarkodina eosteinhornensis Bizone, or the Ozarkodina remscheidensis Biozone reflecting a major cooling event, which may have resulted in the formation of an ice sheet in high latitudes and a glacio-eustatic sea-level fall.

Introduction

The Silurian was characterized by several bioevents, the most prominent ones being the Ireviken, Mulde, and Lau Events, which have been documented in many regions (see compilation in Calner, 2008). Detailed studies by Lennart Jeppsson on Gotland have initiated investigations of these events on other palaeocontinents and resulted in recognition that the Silurian was not a climatically stable greenhouse period as considered earlier, but instead was characterized by major climatic changes (e.g. Calner, 2008). However, almost nothing is known yet about the palaeoclimatic history of the Pridoli.

Studies on Ordovician, Silurian, and Devonian conodonts (Wenzel et al., 2000, Joachimski and Buggisch, 2002, Joachimski et al., 2003, Joachimski et al., 2004, Bassett et al., 2007, Lehnert et al., 2007b, Trotter et al., 2008, Joachimski et al., 2009) revealed that conodont apatite has a high potential to retain the primary isotopic composition. Since conodonts are interpreted to have lived exclusively in the marine realm (Sansom et al., 1992, Donoghue et al., 2000), oceanic palaeotemperature and/or salinity variations can be reconstructed from their oxygen isotope composition. The oxygen isotopic composition of conodont apatite is supposed to be unaffected by increased burial temperature, with the exception of fluids, passing through the rocks during orogenic processes and regional metamorphism, which may lead to dissolution and partial exchange of phosphate, as well as overgrowth of conodont elements by phosphate from the solution. Transmission electron microscopy (TEM) investigations, and parallel geochemical studies suggest that cancellate albid crown of conodont element is the most resistant apatite tissue in respect to diagenetic alteration (Trotter and Eggins, 2006, Trotter et al., 2007). Fish exoskeletal microremains (scales) possess homologous teeth histology: a compact surface layer of diagenetically resistant enamel covers inner dentine and basal bone tissues (Märss et al., 2007). Oxygen isotopes of younger fossil fish apatite have already been successfully used to reconstruct variations in seawater palaeotemperature (Kolodny and Luz, 1991).

Conodont elements and fish microremains are very abundant in the upper Silurian sediments of Lithuania, the south-east part of the Baltic Palaeobasin, even if considering relatively small core samples. Biogenic apatite has been selected for this oxygen isotope study because the phosphatic fossils (both conodonts and fish) are interpreted not to have been affected by diagenesis due to tectonic stability and low thermal overprint of the Silurian strata (Verniers et al., 2008). This also can be certified by a conodont colour alteration index (CAI) ≤ 1.5, and a minimal fish scale colour alteration (Tway et al., 1986).

This paper presents the first oxygen isotope data based on studies of biogenic phosphate from the south-east part of the Baltic region, and reconstruction of climatic changes during the Late Silurian. In addition, the aim of this work was to test whether oxygen isotopes measured from the fish apatite can be used as a palaeoenvironmental proxy like these from the coeval conodont elements. The conodont apatite δ18O record suggests that prominent changes occurred at the boundary between the Vievis and Lapės formations.

Section snippets

Materials and analytical methods

Conodonts and fish remains have been picked from the upper Silurian (Pridoli) samples from the Gėluva-99 core section, Lithuania. Conodont elements and fish microremains (0.8 to 1 mg) have been dissolved in nitric acid and have been chemically converted into Ag3PO4, using the method described by Joachimski et al. (2009). Oxygen isotopes have been measured on CO generated by reducing trisilverphosphate using a high-temperature conversion-elemental analyzer (TC-EA). The analyzer has been connected

Geological setting

The south-east part of the Silurian Baltic Palaeobasin evolved as a part of a marginal sedimentary system during the Early Palaeozoic (Lazauskienė et al., 2003). A different sedimentary history and preservation of strata in the Lithuanian part of the Baltic Silurian Palaeobasin led to the subdivision of the basin into three main facies zones. From east to west, a very shallow marginal eastern, central, and deeper water western facies zones are differentiated (Paškevičius, 1997, Verniers et al.,

Biostratigraphic correlation

Two conodont biozones have been established in the Pridoli in the south-east Baltic region, the Ozarkodina eosteinhornensis eosteinhornensis interval zone, and the O. e. remscheidensis Biozone (Brazauskas, 1993). The O. e. eosteinhornensis Biozone corresponds to the lower part of the O. e. eosteinhornensis Biozone in the Carnic Alps (Brazauskas, 1993). Contrasting the first appearance (FAD) of O. e. eosteinhornensis in the lower O. crispa Biozone in the Carnic Alps, in the south-east Baltic

Results

The isotope values of the Late Silurian (Pridoli) conodonts and fish micro remains are shown in Fig. 3. The δ18O analyses have been performed on the specimen from the Gėluva-99 core section samples. The conodonts give δ18O values ranging from 17.7 to 19.2‰ V-SMOW with the average values around 18.3‰ V-SMOW. These δ18O values are quite similar to those reported by Joachimski et al. (2003) from Silurian conodonts (ranging from 17.5–19.5‰ V-SMOW). Fish fossils have lower oxygen isotope ratios than

Palaeoseawater temperature calculations

Palaeotemperatures have been calculated from the δ18O composition of the conodont and fish apatite using the temperature equation given by Kolodny et al. (1983)T°C=113.34.38(δ18Oapatiteδ18Oseawater)assuming an oxygen isotope composition of the Silurian seawater of −1‰ V-SMOW (ice-free world). The palaeotemperatures calculated from the δ18O values of the Pridoli conodont apatite range from 24.8 to 31.5 °C (Fig. 3). In comparison, palaeotemperatures derived from the δ18O of fish apatite range

Acknowledgements

We are particularly grateful to the Editors of this Volume, Dr. Thomas Servais (FRE 3298 CNRS Géosystèmes, Team of Palaeozoic Palaeontology and Palaeogeography, University of Lille – 1, France), and Dr. Alan Owen (Department of Geographical and Earth Sciences, University of Glasgow, UK) for their valuable advices. Dr. Alain Blieck (FRE 3298 CNRS Géosystèmes, Team of Palaeozoic Palaeontology and Palaeogeography, University of Lille – 1, France), Prof. Daniel Goujet (National Museum of Natural

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