Analysis of the geochemical variability in lepisosteid scales from the Fontllonga-3 site (early Danian, Tremp Formation, South Central Pyrenees, Spain): Implications for palaeoenvironmental studies

Abstract

Rare Earth Element (REE), Ba/Ca, Sr/Ca and δ 18O PO4 analyses have been performed on the ganoine and isopedine of different types of isolated scales (anterior, middle, posterior flank scales and dorsal body margin scales) from juvenile lepisosteids from the Fontllonga-3 site (early Danian, Tremp Formation, South-Central Pyrenees, Spain) with the aim of detecting the most suitable kind of scale to be used in palaeoclimate studies. The REE flat-shape patterns obtained in lepisosteid scales from Fontllonga-3 are indicative of the absence of late stage diagenesis and recrystallization and have confirmed the identification of the palaeoenvironment where the Fontllonga-3 lepisosteids inhabited to be that of an estuary. The Ba/Ca palaeothermometer shows that posterior flank scales record on average 4.52 °C (ganoine) and 3.49 °C (isopedine) higher temperatures when compared to middle flank scales, which may be related to the fact that posterior flank scales grow preferentially during the warm season, whilst middle flank scales, which have the greatest number of layers of ganoine per unit of time and grow all year round, record a greater amplitude with respect to palaeoenvironmental and palaeoclimatic variations. Temperature values obtained from anterior flank scales follow a pattern similar to that showed by middle flank scales, whilst temperature values provided by dorsal scales resemble the pattern furnished by posterior flank scales. The same relationship between scale position and temperature but to a lesser degree (1.48 °C) is also evident in the calculated δ 18O PO4 temperatures. The Ba/Ca palaeothermometer was applied by considering Ba/Ca ratios of the water for different types of environments and it has been found out that a ratio similar to that of marine waters yielded the best approximation to δ 18O PO4 temperatures (29.82 ± 3.76 °C). The mean Ba/Ca temperature value obtained from the analysis of all the ganoine samples (26.96 ± 5.72 °C) is also consistent with the temperature of the water in which extant relatives of osteoglossiform fish, whose remains have been found in Fontllonga-3 along with lepisosteid remains, inhabit nowadays (24–40 °C, with mean values of 27 to 32 °C). The mean Ba/Ca temperature values furnished by the isopedine are lower than that obtained on the ganoine which may be due to minor diagenetic alteration. On account of these results, it is recommended that a distinction should be made between different types of scales as this could provide information concerning the summer growth temperature (posterior flank scales) as well as the annual temperature range (middle flank scales). Furthermore, it is recommended that ganoine be used in all studies as it appears to be the most resistant tissue in lepisosteid scales to diagenetic alteration.