PALEOTEMPERATURE AND SEASONALITY IN THE EARLY EOCENE SOUTHERN NORTH SEA BASIN INFERRED FROM FOSSIL FISH OTOLITHS

Shelf settings comprise invaluable information about the impact of paleoenvironmental change on the shallow marine ecosystem. We measured δ¹⁸O and δ¹³C ratios in fossil fish otoliths to assess temperature and salinity variability in the Ypresian (Ypr., 56.0-47.8 Ma) southern North Sea Basin (sNSB). Well-preserved otoliths were selected from U.K. and Belgian collections, representing a series of levels mainly within the Blackheath and Harwich (lower Ypr.), London Clay (middle Ypr.) and Tielt (upper Ypr.) Formations. Thirteen species were selected, of which 8 belong to demersal and probably non-migratory fish such as ophidiids, bythitids and congrids. Powders for bulk (mean annual temperature, MAT) and incremental (seasonality) analyses were generated by microdrilling the sagittal plane of embedded and polished otoliths.

Paleotemperatures are determined from the London Clay Fm. samples. This unit is composed of fine silts and clays deposited in an offshore environment (+/- 100 km from coast), unlikely to be influenced by freshwater influx. Mean bulk δ¹⁸O values reveal within-horizon consistency among different co-occurring taxa. The horizon means, including all taxa, range between -1.9 and -3.4 ‰. This translates to warm, albeit not extreme MAT’s between 21.3 and 28.5 °C during the 54-51.5 Ma interval. A paleowater composition of -1 ‰ is assumed, based on latitudinal correction and an open scenario for the Greenland-Norway Seaway. Based on the incremental sampling, low seasonality of 3-6 °C (about 1 to 1.5 ‰) is observed in the London Clay Fm.

By contrast, mean bulk δ¹⁸O values from the Harwich and Blackheath Fms. (55.5-55.0 Ma) are very negative, ranging from -5 to -7 ‰, indicating substantial mixing with low-salinity waters reduced by probably 3-4 ‰. The marine character of these marginal deposits is indicated by the presence of glauconite and marine mollusks. Restricted connection of the sNSB with the North Atlantic and an intense global water cycle shortly after the PETM both may explain these results. Samples from the shallow sandy Tielt Fm. (~51.0 Ma) bear values between -3 and -5 and are thought to be influenced by continental freshwater runoff. All marginal facies are characterized by a much higher intra-annual variability of 3-4 ‰, probably the result of seasonally paced freshwater influx.