Stable isotope records (d 18 O and d 13 C) of Lower-Middle Jurassic belemnites from the Western Balkan mountains (Bulgaria): Palaeoenvironmental application Lubomir Metodiev * , Elena Koleva-Rekalova Geological Institute, Bulgarian Academy of Sciences, Acad. G. Bonchev Street Bl. 24, 1113 Sofia, Bulgaria article info Article history: Available online 18 April 2008 abstract Stable O and C isotope data of 110 Upper Pliensbachian-Lower Bajocian belemnites have been obtained and used to attempt a reconstruction of palaeotemperature and its variation in two epicontinental depositional environments from the Western Balkan mountains (Bulgaria). The samples were collected from 3 sections with high-resolution ammonite subdivision. Initially taphonomic, cathodoluminescence and geochemical analyses were used for evidence of diagenetic alteration. Non-luminescent parts of the belemnite rostra have been sampled for isotope analyses and 76 samples, having d 18 O< 4‰ (PDB), d 13 C> 0.5‰ (PDB), Fe < 250 ppm, Mn < 50 ppm, Sr > 950 ppm and Sr/Mn ratio > 80 were used for palaeotemperature interpretations. The O and C isotope data generally exhibit lit- tle stratigraphical variability with minor fluctuations, however, there are prominent posi- tive C isotope excursions and coeval negative O isotope shifts detected in the Lower Toarcian Tenuicostatum, Falciferum and Bifrons Zones. The O isotope data, interpreted in terms of palaeotemperature, revealed relatively high seawater temperatures during the Toarcian, Aalenian and Early Bajocian, with detectable temperature rises during the Early Toarcian (maximum value of 29.6 °C). Both C isotope maxima and O isotope minima are used as evidence of the Early Toarcian anoxic event reported from many localities of the same age and in similar facies in Western Europe. In the study the latter is recognized as 3 episodes, which are closely related with the seawater temperature maxima. This isotope record pattern is considered as a consequence of a global Tethyan transgression during the Early Toarcian. Ó 2008 Published by Elsevier Ltd. 1. Introduction Oxygen and C isotope research (d 18 O and d 13 C) em- ployed for palaeoenvironmental interpretations has a long history that can be traced back to the middle of the last century. There is agreement that O isotope val- ues of marine carbonates are a function of the O isotope composition and temperature of the ambient seawater (Broecker and Peng, 1982). The d 13 C values in carbonates reflect the 13 C/ 12 C ratio of CO 2 dissolved in seawater as well as indicating the source of the C in the CO 2 . If the d 18 O and d 13 C values of marine carbonates do indeed form in equilibrium with the seawater, and if there has been no later diagenetic alteration, they can be used for palaeotemperature reconstructions and discrimina- tion of palaeoceanic events. Empirical and experimen- tally derived equations are drawn for carbonate palaeotemperature estimations (e.g., O’Neil et al., 1969; Hays and Grossman, 1991; Slen et al., 1996). The theo- retical basis and analytical techniques of carbonate iso- tope geochemistry are undergoing progressive refinement enabling the recognition of anomalies (excur- sions) of the O and C isotope record, which are consid- ered to reflect the existence of palaeoclimatic changes of global significance. 0883-2927/$ - see front matter Ó 2008 Published by Elsevier Ltd. doi:10.1016/j.apgeochem.2008.04.010 * Corresponding author. Fax: +359 2 872 46 38. E-mail address: lubo@geology.bas.bg (L. Metodiev). Applied Geochemistry 23 (2008) 2845–2856 Contents lists available at ScienceDirect Applied Geochemistry journal homepage: www.elsevier.com/locate/apgeochem