The micropaleontological record of marine early Eocene oil shales from Jordan

Abstract

Core OS-28 located in central Jordan offers an expanded record of calcareous nannofossils and benthic foraminifera for the early Eocene of the southern Tethys. The oil shale succession of the studied interval covers calcareous nannofossil biozones NP10, NP11, NP12 and NP13. Microfossils document the biotic response of calcareous nannofossils and benthic foraminifera to short and long term warming trends of the early Eocene following the Paleocene-Eocene Thermal Maximum (PETM). On a global scale, two early Eocene short term shifts to higher atmospheric CO2 concentration are recorded by negative δ13C excursions. These are known as Eocene Thermal Maximum 2 and 3 (ETM2, ETM3). Stable isotope records (δ13Corg, δ13Ccarb, δ18Ocarb) from the study area do not show these negative excursions, which elsewhere can be correlated with the two hyperthermal events known from early Eocene successions. The absence of negative δ13C signals otherwise typical for ETM2 and ETM3 is explained by a diagenetic alteration of the isotope signal after deposition.High abundances of the nannofossil taxon Sphenolithus spp. in biozone NP10 indicate less fertile conditions for this interval than for the overlying biozone NP11. Throughout biozones NP11 to NP13 Coccolithus pelagicus and Toweius spp. are the dominant taxa of the assemblages, suggesting a mesotrophic environment. Surface waters experienced a temperature change from warm to moderate-warm. The benthic foraminifera in calcareous nannofossils biozone NP10 and in the lower part of NP11 reflect oligo-mesotrophic and moderate to well oxygenated bottom water conditions. Throughout the upper part of biozones NP11, NP12 and NP13, the benthic foraminifera are characterized by an increasing abundance of Anomalinoides zitteli, Lenticulina spp. and Valvulineria scrobiculata. This coincides with enriched total organic matter content (TOC), indicating a shift to mesotrophic conditions and a moderate oxygen supply of the bottom waters. Based on the micropaleontological observations made in core OS-28 the early Eocene is first characterized by an increase and subsequently by a decrease of nutrients and oxygen. The composition of both, benthic and planktic communities was rather controlled by variations of regional environmental parameters than by global ones. These include strong runoff, climatic variability and the architecture of the basin.