The climate of the early Paleogene is characterized by short-scale temperature variations which are superimposed on a general trend of rising temperatures culminating during the late early Eocene (early Eocene climatic optimum, EECO). These include several transient periods of abrupt climate warming or ‘hyperthermals’, such as the PETM (~55 Ma). Profound proxy development is needed to successfully extract shorter-scale variability from suitable records and unravel its underlying mechanisms. This study assesses and extends the use of fossil fish otolith O and C stable isotopes as a paleotemperature and seasonality proxy for early Paleogene marginal marine sedimentary environments. Well-known limitations include the lack of accurate estimates for the oxygen isotope composition of ambient water, and potential bias when applying paleotemperature equations. Moreover, taxon inconsistencies for both O and C were observed, complicating data interpretation (Vanhove et al., 2011). A single locality test case in the southern North Sea Basin has been performed to address this observation (Egem, Belgium, coastal sands). In each of four fossiliferous levels sampled, the same three demersal otolith species were analyzed (Platycephalus janeti, Paraconger papointi and “genus Neobythitinorum” subregularis). Cross-plots of δ18O and δ13C isotopes show three statistically different data clouds, corresponding to the three taxa. Several processes can cause such discrepancies. The most likely option is the influence of freshwater influx. According to this interpretation, Paraconger sp. and Platycephalus janeti lived in coastal areas prone to freshwater influx, while “genus Neobythitinorum” subregularis inhabited more distal realms. This is confirmed by similar analyses on Callista sp. and Venericardia sp. bivalves of the same locality, because these were deposited relatively in situ compared with otoliths, which predominantly arrive in the sediment after post-mortem predation-related transport. Taxon-sensitive differential diagenesis is disproved by SEM, cold cathodoluminescence and X-ray diffraction investigations, revealing the presence of pristine aragonite in all cases. Bias resulting from variability in the amount of summer or winter carbonate deposition is contradicted by visual inspection of growth ring thicknesses, and cyclical incremental stable isotope patterns of individual growth bands. Taxon inconsistencies were not described previously by authors working on the same taxa and in the same area, hence the paleoecological interpretation of this data could indicate enhanced runoff and freshwater influx during the EECO relative to later time intervals, or the presence of a large river mound close the investigated location. Temperature calculations based on “genus Neobythitinorum” subregularis reveal mean annual temperatures around 27.5 °C and a seasonality of 9 °C for the EECO interval. Given the mentioned assumptions, future directions should include other quantitative, preferably salinity-independent paleotemperature proxies to test these data interpretations. VANHOVE D., STASSEN P., SPEIJER R. P. and STEURBAUT E., 2011. Assessing paleotemperature and seasonality during the early Eocene climatic optimum (EECO) in the Belgian Basin by means of fish otolith stable O and C isotopes. Geologica Belgica, 14(3-4): 143-158.
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Since Alvarez et al. (1980) found new evidence for the impact of catastrophic events on earth’s biota, hypothesis and theories explaining the fossil record (re)gained a lot of attention. The extraterrestrial origin of the anomalous iridium concentrations seemed highly controversial at first, but nowadays the Chicxulub ‘accident’ has become the marker for the start/base of the Paleogene. Its pivotal role in the Mesozoic-Cenozoic faunal turnover cannot be refuted (Schulte et al 2010). However, alternative theories remain being published. Of these, the Deccan volcanism with its widespread flood basalts stepped prominently forward as one of the main triggers, especially when trying to explain the gradual diversity decline within the fossil record. The inconsistencies between the proposed theories generally root in too narrowly geographically and geologically spread datasets. This applies to most fossil groups, and especially to the ammonoids (Class Cephalopoda, °Early Devonian – †Late Cretaceous). A compilation of ammonoid occurrences of Late Maastrichtian age published by Kiessling & Claeys (2002) evidenced the lack of a globally well distributed dataset. In this compilation, North Africa was left as a blind spot, while Tunisia had been the centre of the K/Pg mass extinction debate for almost three decades, e.g. with the definition of the GSSP for the base of the Paleogene at El Kef. Both at the GSSP and several other sections in the Tunisian Trough Basin, ammonoids were found within the topmost meters of the Maastrichtian, until very close to the K/Pg boundary level. About 900 uppermost Maastrichtian ammonoids were collected, all from within the last 420.000 years of the Cretaceous. With 22 species on record, belonging to 18 genera and 10 families, and with representatives of each of the four large ammonoid suborders (Phylloceratina, Lytoceratina, Ammonitina and Ancyloceratina), the Tunisian fauna demonstrates that ammonoids were both taxonomically and morphologically diverse until their very end. An updated version of the compilation of latest Maastrichtian ammonoid occurrences documents at least 53 species, 29 genera and 13 families in the ultimate half million year of the Cretaceous, in many more localities and occurring in a wide variety of settings. When the Tunisian ammonoid species richness data are plotted next to all time constraints of the possible causes, the possibility of Deccan flood basalt volcanism negatively influencing ammonoid diversity must be refuted. A major extinction caused by the Chicxulub impact seems the most plausible theory at present. Through inducing a mass kill of the marine plankton, the juvenile ammonoids lost their primary food source leading to their final extinction. Alvarez, L.W., Alvarez, W., Asaro, F., Michel, H.V., 1980. Extraterrestrial cause for the Cretaceous-Tertiary extinction. Science, 208, 1095-1108. Kiessling, W., Claeys, P., 2002. A geographic database approach to the KT Boundary. In Buffetaut, E., Koeberl, C. (Eds), Geological and Biological Effects of Impact Events, Springer-Verlag Berlin, 83-140. Schulte, P. & 40 authors, 2010. The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene Boundary. Science 327, 1214-1218.
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