Paleoproductivity of the northwestern Tethyan margin (Anthering section, Austria) across the Paleocene-Eocene transition

Sediments in the expanded Anthering Section at the northwestern Tethyan margin were deposited in an abyssal environment, at the continental rise to the south of the European plate. The section contains deposits from calcareous nannoplankton Zones NP9 and NP10 and displays the global negative carbon isotope excursion (CIE) in the upper part of Zone NP9. Associated with the CIE is a strong three-fold increase in the rate of hemipelagic sedimentation. This suggests an increased input of siliciclastic suspended material into the basin, which is indicative of enhanced continental runoff at that stratigraphic level. Concurrent acmes of siliceous plankton and dinoflagellate cysts indicate that a high input of dissolved nutrients affected even open marine settings and caused eutrophication of surface waters. The associated high flux of organic carbon to the seafloor resulted in oxygen-depleted conditions and caused the total extincton of benthic faunas. However, impoverished foraminifera faunas indicate that a change of ecological conditions started shortly before the CIE. 133 Egger, H., Fenner, J., Heilmann-Clausen, C, Rogl, F., Sachsenhofer, R.F., Schmitz. B., 2003, Paleoproductivity of the northwestern Tethyan margin (Anthering Section, Austria) across the Paleocene-Eocene transition, in Wing, S.L., Gingerich, P.D., Schmitz, B., and Thomas, E., eds., Causes and Consequences of Globally Warm Climates in the Early Paleogene: Boulder, Colorado, Geological Society of America Special Paper 369, p. 133–146. © 2003 Geological Society of America. *E-mails: Egger—eggjoh@cc.geolba.ac.at; Fenner—J.Fenner@bgr.de; Heilmann-Clausen—claus.heilmann@geo.au.dk; Rogl—fred.roegl@nhm-wien.ac.at; Sachsenhofer—sachsenh@unileoben.ac.at; Schmitz—birger@gvc.gu.se. Thomas, 1998, for a review), a rapid diversification of planktonic foraminifera (Lu and Keller, 1993), a global bloom of the dinoflagellate genus Apectodinium (Crouch et al., 2001), a turnover in calcareous nannoplankton (Bybell and Self-Trail, 1994), and a major turnover in land mammals (Wing et al., 1991). In the South Atlantic and Antarctica and in some Tethyan sections a high influx of kaolinite has been interpreted as an indicator for a change to more humid climatic conditions or, alternatively, as an effect of enhanced erosion under more arid conditions (see Thiry, 2000, for a review). Over extensive areas in the southern and northern Tethys and Peri-Tethys, the CIE is coeval with sapropelic sedimentation (Speijer et al., 1997; Gavrilov et al., 2000; Schmitz et al., 1997a). Biogenic barium, which can be used as a proxy for paleoproductivity, indicates a dramatic increase in surface water fertility in the Eastern Tethys (Schmitz et al., 1997b) and in the North and South Atlantic (Bains et al., 2000). However, studies of microfossil assemblages, including these at sites where elevated barite accumulation has been documented, suggest that surface productivity actually decreased during the δ13C excursion (Dickens et al., 2001). From the expanded and continuous sedimentary record of the Anthering Section (Fig. 1) an acme of siliceous plankton (Egger et al., 2000b) can be interpreted either as an indicator of high surface-water productivity or as a result of better preservation conditions for siliceous plankton. The main objective of this paper is to explore the significance of the faunal and floral changes at the level of the CIE at Anthering. Since the influx of suspended terrestrial material into the basin is essential for the preservation of organic matter as well as for the level of dissolved nutrients in the surface water, we calculated hemipelagic sedimentation rates that can be used as a proxy for the amount of fluvial discharge. We studied temporal relationships between high sedimentation rates, acmes of planktic assemblages and the benthic foraminifera extinction event (BFEE). In order to discriminate between effects of preservation and signals of changed primary productivity, we focus on the ecological significance of Apectodinium and large diatom species (Trinacria spp. and Craspedodiscus spp.). ENVIRONMENTAL SETTING The 250 m thick upper Paleocene to lower Eocene deposits of the Austrian Anthering Section, spanning calcareous nannoplankton Zones NP9 and NP10, represent the most expanded marine sedimentary record of this interval known to date. These sediments comprise the youngest part of the Rhenodanubian Flysch that was deposited on the continental rise to the south of the European plate (Fig. 2), which was the main source for the siliciclastic detritus entering the basin. The section is composed of carbonaceous mud-turbidites with intervening hemipelagic shales, which indicate deposition below the calcite compensation depth. Paleo-water depth estimations by Butt (1981), using foraminifera assemblages, range between 3000 and 5000 m. 134 H. Egger et al. Figure 1. Position of the Anthering section, location of outcrops (A–N) and biostratigraphy of outcrops in terms of the calcareous nannoplankton standard zonation (NP Zones) of Martini (1971) and their further subdivision by Aubry (1996). For the Paleogene, a warm seasonal climate with alternating wet and dry conditions is indicated by the strong dominance of smectite in the clay mineral assemblages of hemipelagic shale. Close to the CIE, slightly increased percentages of kaolinite suggest a change to a more humid climate with high precipitation rates and associated enhanced continental runoff (Egger et al., 2002). This interpretation is consistent with terrestrial palynomorph assemblages in samples taken immediately above the onset of the CIE. These contain elements of the subtropical/tropical vegetation similar to the recent flora of Southeast Asia and southeast North America (Draxler, 2000).