Foraminifera from the borehole Altdorf (SE Germany): Proxies for Ottnangian (early Miocene) palaeoenvironments of the Central Paratethys

Abstract

During the Ottnangian (early Miocene), the shallow epicontinental sea of the Upper Marine Molasse in the Western and Central Paratethys realm experienced major sea-level fluctuation leading to significant environmental changes. However, the effects of these changes remain largely unexplored because detailed palaeoenvironmental analyses of the respective time interval are scarce. We here present a thorough analysis of the benthic foraminiferal composition (36 samples, 5616 individuals, and 90 taxa) from a 105.25 m thick marine Ottnangian succession of the borehole Altdorf and from two coeval outcrops (Neuhofen, Oberschwärzenbach), all located in the western Central Paratethys. Studies on the planktonic foraminifera and calcareous nannoplankton were conducted as well. We show that benthic foraminiferal assemblages clearly differ with regard to species composition, diversity indexes, relative abundances of infauna/epifauna, and oxygen indicators. We introduce five foraminiferal biofacies, which together are suggestive of a transgressive–regressive development and several distinct environmental changes from the early to middle Ottnangian. The planktonic foraminiferal assemblages from Altdorf and the Central Paratethys suggest cold–temperate surface water. However, high nutrient availability and taphonomic factors such as selective dissolution, could also have affected the planktonic assemblages. In most samples, the relative abundance of infaunal/epifaunal benthic foraminifera, planktonic foraminifera and calcareous nannoplankton clearly indicates nutrient-rich waters. We did not find indicators for upwelling (as a possible source for the nutrient flux) and therefore assume that the high nutrient levels indicate large-scale river influx in a humid climate. Based on our foraminiferal biofacies, we postulate the presence of a particularly high fluviatile runoff and high organic matter supply resulting in a moderate oxygen depletion of marine bottom waters for the period of relative sea-level highstand in the early Ottnangian. Strong oxygen depletion of bottom waters occurs in the middle Ottnangian, and is mainly caused by salinity stratification and a reduced water exchange to the open ocean during that time. Moreover, our data demonstrate that P/B ratios for early and middle Ottnangian samples only partially support the palaeobathymetric reconstructions based on the benthic foraminifera. We suggest that fluviatile input and estuarine circulation strongly affected the relative abundances of planktonic foraminifera, besides tidal and other marine currents.