Integrated micropaleontological study of the Messinian diatomaceous deposits of the Monferrato Arc (Piedmont basin, NW Italy): New insights into the paleoceanographic evolution of the northernmost Mediterranean region
Introduction
The late Miocene was characterized by a global-scale increase of the biogenic, especially opaline, sedimentation in the oceans, the deep causes of which have been thoroughly debated but not yet fully clarified (e.g., Leclaire, 1978; Leinen, 1979; Sancetta, 1983; Farrell et al., 1995; Dickens and Barron, 1997; Dickens and Owen, 1999; Hermoyian and Owen, 2001; Diester-Haass et al., 2002, Diester-Haass et al., 2004, Diester-Haass et al., 2005; Diester-Haas et al., 2006; Cortese et al., 2004; Lyle and Baldauf, 2015; Herbert et al., 2016; Zhou et al., 2019). In the Mediterranean, this ‘biogenic bloom’ mostly occurred before the onset of the Messinian salinity crisis at ~5.97 Ma (e.g., Manzi et al., 2013), during an interval characterized by remarkable interactions between geodynamic, paleoclimatic and paleoceanographic processes (e.g., Pellegrino et al., 2018). Notably, the late Miocene intensification of the Africa-Eurasia tectonic convergence, in combination with glacio-eustatic forcing, led to the ongoing restriction of the Betic and Rifian corridors in the western Mediterranean. This restriction progressively limited the water mass exchanges with the Atlantic Ocean from ~8.5 Ma and increased the residence time of the Mediterranean deep waters after ~7.2 Ma (e.g., Krijgsman et al., 1999; Seidenkrantz et al., 2000; Kouwenhoven et al., 2003). The late Miocene paleoclimate of the Mediterranean was characterized by a long-term cooling trend punctuated by warm-humid and cold-dry phases mainly linked to the periodical migration of the inter-tropical convergence zone (ITCZ) and possibly of the North Atlantic storm tracks under the control of precession (e.g., Griffin, 2002; Gladstone et al., 2007; Böhme et al., 2011; Tzanova et al., 2015; Mayser et al., 2017; Kontakiotis et al., 2019; Modestou et al., 2019). Ultimately, these processes had remarkable effects on the physical (e.g., sea surface temperature, mixing and stratification) and chemical (e.g., nutrient availability, oxygen levels) properties of the water column and consequently on the planktonic and benthic Mediterranean marine biota (e.g., Moissette and Saint Martin, 1992; Kouwenhoven et al., 2003; Cornacchia et al., 2017; Hoffmann et al., 2020). Between 7 and 6 Ma, this scenario resulted in the diachronic, precessional-controlled, cyclic accumulation of sapropels, marls and diatomaceous deposits, nowadays exposed in Algeria, Cyprus, Greece, Italy, Morocco and Spain (see Pellegrino et al., 2018 and references herein). Traditional interpretations consider the upper Miocene laminated diatomites of the circum-Mediterranean region as being derived from the intensification of upwelling and/or from the establishment of deep anoxic conditions, in turn triggered by the progressive restriction of the Mediterranean marginal basins culminating with evaporite deposition at the onset of the Messinian salinity crisis (e.g., Sturani and Sampò, 1973; Moissette and Saint Martin, 1992; Krijgsman, 2002; Hüsing et al., 2009). Nevertheless, this assumption is challenged by investigations that demonstrate high diatom productivity rates in stratified waters and the excellent preservation of laminated diatom oozes even under well‑oxygenated waters, due to the tensile strength of rapidly-deposited diatom mats (e.g., Kemp and Baldauf, 1993; Pike and Kemp, 1999; Kemp et al., 2000; Kemp and Villareal, 2018).
In light of these considerations, the micropaleontological investigation of the circum-Mediterranean diatomaceous sediments is fundamental in order to better understand the paleoceanographic processes responsible for their accumulation. In the central sector of the Mediterranean, the micropaleontological study of the upper Miocene diatomaceous deposits has been mainly focused on the well-exposed and continuous Sicilian sections of the Caltanissetta Basin. These studies aimed to establish cyclostratigraphic correlations with other circum-Mediterranean sites, particularly those located in Spain and Gavdos (e.g., Hilgen and Krijgsman, 1999; Pérez-Folgado et al., 2003). Conversely, the scattered sections outcropping along the Italian Peninsula (e.g., Pellegrino et al., 2018) have received less attention. Moreover, only a few works have attempted to compare both the siliceous and calcareous microfossils preserved in these sediments (e.g., Bonci et al., 1991; Gaudant et al., 2010). Paradoxically, this approach resulted in an underestimation of the role played by the silica-secreting biota in the paleoceanographic processes that occurred during an interval of significant increase in opaline deposition. To fill this gap and in order to better document the paleoceanographic processes within the water column and at the sediment-water interface involved in the accumulation of the diatomaceous deposits in a key-area of the Mediterranean basin, we analyzed the foraminifera, calcareous nannofossils, diatoms and associated siliceous microfossil assemblages of the upper Miocene diatom-bearing section of Pecetto di Valenza, located in the Piedmont Basin (NW Italy).
Section snippets
Regional setting
The Pecetto di Valenza section is located in the Monferrato Arc (Fig. 1A), representing the northernmost tectono-sedimentary domain of the Piedmont Basin. The latter is a large wedge-top basin filled with upper Eocene to Messinian sediments recording the geodynamic evolution of the Alps/Apennines junction (e.g., Piana, 2000). Since the early Miocene, the Piedmont Basin was affected by strong tectonic activity and the Monferrato Arc became a structural high along which diatomaceous sediments
Material and methods
The section exposed along the crossroad called ‘Strada della Guarnera’ (44°58′50.2”N, 8°40′30.8″E), next to the village of Pecetto di Valenza, was sampled for micropaleontological investigations. The upper homogeneous marl originally described by Sturani and Sampò (1973) and Pavia (1989) was not observed in the field, because of the thick vegetation and soil cover. The section was sampled every ~15 cm, obtaining four samples from the lower homogeneous marl, six samples from the laminated
Results
Well-preserved planktonic and benthic foraminifera (Figs. 2A, 3A–F) and calcareous nannofossils (Figs. 2B, 3G–O) have been identified within the section. Among the planktonic foraminifera, it is worth mentioning the occurrence of Turborotalita multiloba, a biozonal marker that supports the attribution of the Pecetto di Valenza section to the early Messinian (e.g., Lirer et al., 2019; Fig. 3A–B). Conversely, well-preserved siliceous microfossils have been identified only in the diatomaceous
Foraminifera
Based on the foraminiferal assemblages, four intervals can be recognized in the section, which are not coincident with the lithologic changes (Fig. 2A). These intervals are defined by sudden changes in benthic assemblages, while the planktonic assemblages show more gradual variation across the interval boundaries.
Benthic foraminifera primarily reflect changes in nutrient and oxygen content on the seafloor (e.g., Murray, 2006). In the lower part of the homogeneous marl (lowermost 2 samples, Fig.
Conclusions
The four main stages of biogenic sedimentation that occurred during the late Miocene in the northern part of the Mediterranean (Monferrato Arc) were characterized by a wide variability of both water column and seafloor conditions, under the control of precession-controlled climate oscillations. Our results suggest that the water column configuration evolved from well-mixed to periodically stratified, while the seafloor evolved from well‑oxygenated to slightly oxygen-depleted, but never anoxic.
Declaration of Competing Interest
None.
Acknowledgements
This study was supported by grants (ex-60% 2018 and 2019) to GC of the Università degli Studi di Torino. Associate Editor Xavier Crosta and two anonymous reviewers are warmly thanked for their useful comments and suggestions, that greatly improved the original version of the manuscript.
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