Calcareous nannofossil biostratigraphy of the M. del Casino section (northern Apennines, Italy) and paleoceanographic conditions at times of Late Miocene sapropel formation

Abstract

A detailed quantitative calcareous nannofossil analysis has been performed on 138 samples from the astronomically dated Monte del Casino section with the aim to identify and precisely date the most important calcareous nannofossil events across the Tortonian=Messinian boundary in the Mediterranean, and to unravel paleoceanographic conditions at times of sapropel formation during the Late Miocene. From the biostratigraphic perspective, the genus Amaurolithus provides three successive first occurrences (FOs): A. primus, A. cf. amplificus and A. delicatus, dated at 7.446, 7.434 and 7.226 Ma, respectively. Other bioevents include the base and top of the 'small reticulofenestrids' Acme, dated at 7.644 and 6.697 Ma, and the FO, FCO and LO of R. rotaria, dated at 7.405, 7.226 and 6.771 Ma. These events appear to be useful in improving biostratigraphic resolution in the Tortonian-Messinian boundary interval, at least for the Mediterranean. Quantitative analysis revealed changes in the calcareous nannofossil assemblage associated with the sapropels. The observed fluctuations suggest a single mechanism for sapropel formation in the Mediterranean during the late Neogene. Sapropels are characterized by a decrease in the total number of coccoliths, interpreted mainly as a reduction in calcareous nannofossil production due to increased siliceous plankton production during spring blooms; and an increase in reworked specimens, interpreted to reflect enhanced continental input via river run-off. An increase in abundance of the genus Rhabdosphaera can be explained by opportunistic behavior at the end of the spring bloom when nutrient levels start to become impoverished. As far as sea surface water temperature indicators are concerned, warm water D. pentaradiatus shows positive fluctuations in sapropels while cooler water D. intercalaris and C. pelagicus show negative fluctuations.

Phyto_ and zooplankton paleofluxes during the deposition of sapropel S1 (eastern Mediterranean): Biogenic carbonate preservation and paleoecological implications

“…Being primary producers and sediment forming, calcareous nannofossils are of great significance in paleoecological-paleoenvironmental reconstructions. The composition of assemblages in the Holocene has been quantified in terms of relative abundance (Violanti et al, 1991;Castradori, 1992Castradori, , 1993Crudeli and Young, 2003;Crudeli et al, 2004) and as the number of calcareous nannofossils mm À 2 (Negri et al, 1999;Negri and Giunta, 2001;Principato et al, 2003). These studies provided significant information on environmental variations during the deposition of sapropel S1.…”

Late Miocene calcareous nannofossil biotratigraphy and astrobiochronology for the Mediterranean region

“…In addition, we obtained a detailed quantitative calcareous nannofossil biostratigraphy for the La Sardella section, located in eastern central Italy and straddling the Tortonian/Messinian boundary interval. Finally, we compare and discuss the resulting calcareous nannofossil astrobiochronology with data from other Mediterranean sections covering partly the same time interval (Sprovieri et al 1996, Negri et al 1999, Negri and Villa 2000, Bonomo 2001) and with the calcareous nannofossil magneto-biochronology from low-latitude oceanic reference sections (Raffi et al 1995, Backman andRaffi 1997).…”

“…Very small-sized nannofossils (smaller than 3µm) were not included in the countings, due to difficulties encountered in identifying them as specific taxa and, hence, evaluating their abundance. Therefore, the abundance pattern of the so-called "small reticulofenestrids" (placoliths with the long axis < 3µm) was not quantitatively evaluated following the methodology of Negri et al (1999). These authors estimated the abundance of small reticulofenestrids by counting the specimens present in a single field of view, a method that we consider statistically to be of little reliability.…”

“…While some authors reported highly questionable findings of marker species and continued to recognize the standard zones in nannofossil studies of the Mediterranean (Mazzei 1985, Cosentino et al 1997) other recent studies have attempted to establish a regional biostratigraphic framework for the Mediterranean, with the purpose of solving problems of biostratigraphic classification and correlation with the open ocean in late Miocene time (Flores et al 1992, Negri et al 1999, Negri and Villa 2000. The validity of such proposals will be discussed in the following chapters.…”

“…We compared our biochronology with data available from other Mediterranean sections (Sprovieri et al 1996, Negri et al 1999, Negri and Villa 2000, Bonomo 2001, and with the nannofossil biochronology from low-latitude oceanic reference sections (Raffi et al 1995, Backman and Raffi 1997). Our results evidence differences and similarities: some of the recently proposed events in the Tortonian-Messinian (e.g., FO and FCO of Reticulofenestra rotaria Theodoridis 1984, FO of Amaurolitus delicatus Gartner and Bukry 1975) proved not to be reliable and do not improve the biostratigraphic resolution by calcareous nannofossils in the time interval under consideration.…”