Oxygen, strontium, and neodymium isotope composition of fossil shark teeth as a proxy for the palaeoceanography and palaeoclimatology of the Miocene northern Alpine Paratethys

Abstract

The Upper Marine Molasse sediments of southern Germany were deposited during the Early to Middle Miocene, a period of significant change for the global Miocene palaeoceanography, palaeoclimate, and the regional palaeogeography because of the ongoing Alpine–Himalayan orogeny. To address the influence of the Alpine uplift on climate and oceanography of the northern Alpine molasse basin, a combined O-, Sr-, and Nd-isotope study was undertaken on fossil teeth from sharks ( Synodontaspis sp., Galeocerdo cf. aduncus, Chaenogaleus sp.), and button-like teeth from the bony fish of the Upper Marine Molasse ( Chrysophrys molassica). Average δ 18O values for hydroxyapatite of teeth from two sites are 20.3‰ (1 σ=0.6) and 21.2‰ (1 σ=0.5), while their average 87Sr/ 86Sr and 143Nd/ 144Nd ratios are 0.70850 (±0.00001, 2 σ) and 0.70882 (±0.00003, 2 σ), and 0.51220, and 0.51223 (both at ±0.00001, 2 σ), respectively. By comparison to the Sr-evolution curve for the global ocean, the Sr-isotope compositions suggest ages of about 22 and 17 Ma for the teeth. These ages are in good agreement with the depositional ages, which suggests open marine conditions. Open marine conditions are also compatible with (1) lower average δ 18O values and 87Sr/ 86Sr ratios for the one site compared to higher δ 18O values and higher 87Sr/ 86Sr ratios for the other site, (2) estimates for seawater temperatures which are similar to other estimates from deposits of similar latitude and age, (3) a comparison of seawater temperatures with average ambient air temperature estimates deduced from continental palaeoclimate indicators. Nd-isotope compositions are compatible with an influence of the North Atlantic on the Paratethys. Assuming constant salinity and seawater oxygen isotope composition, a 4°C cooling can be inferred from average δ 18O values of the teeth between 22 and 17 Ma. This is similar to trends observed for foraminiferal records from the Mediterranean Tethys, but different from foraminiferal and molluscan records of the North Atlantic and the North Sea during this period, and allows for the possibility that the Alpine orogeny had an influence on the regional, circum-Alpine climate.