The upper Miocene diatomaceous sediments of the northernmost Mediterranean region: A lamina‐scale investigation of an overlooked palaeoceanographic archive

Abstract

During the late Miocene the Mediterranean experienced a dramatic intensification of opaline accumulation, recorded by the deposition of diatomaceous sediments. The fine lamination of these deposits potentially records annual to sub-annual palaeoceanographic processes that occurred during a critical phase of the geodynamic evolution of the Mediterranean basin, which eventually led to the Messinian salinity crisis. The diatomaceous facies has been interpreted by previous researchers as the product of intensified upwelling currents and of bottom anoxia formation in the Mediterranean basin. However, until now, no efforts have been made to unravel the sedimentological and micropalaeontological content of these deposits at the lamina-scale. This paper presents the first case study of a systematic scanning electron microscope-based morphological investigation of the diatomaceous sediments deposited during the late Miocene at the northernmost offshoot of the Mediterranean basin (Piedmont Basin, northwest Italy). Using a non-invasive analytical approach, six faciological components (laminae, laminated packets, non-laminated intervals, burrows, opal-rich aggregates and mixed pelletal structures) and their relationships are described and interpreted herein. Following the lamina-scale study of these sediments, an annual sedimentary cycle could be identified and an accumulation rate (ca 50 cm kyr À1) inferred that is atypical for a setting actively influenced by upwelling. The role played by the entanglement of diatom valves in creating a physical barrier to the bioturbation is here emphasized as the main process responsible for the preservation of the laminated fabric of diatomaceous sediments, challenging the supposed role of deep anoxia. These results suggest that the late Miocene diatomaceous deposition in the Piedmont Basin cannot be univocally considered as a by-product of upwelling intensification and seafloor oxygen depletion.

From biogenic silica and organic matter to authigenic clays and dolomite: Insights from Messinian (upper Miocene) sediments of the Northern Mediterranean

“…It has been shown that tensile‐strength deriving from the close entanglement of diatom valves (Fig. 4) is able to inhibit benthic bioturbation (Kemp & Baldauf, 1993; Bodén & Backman, 1996; Pike & Kemp, 1999; Shimada et al ., 2008; Pellegrino et al ., 2020b).…”

“…Similarly, low TOC contents of the diatomaceous shales in spite of high biosiliceous productivity agree with the notion that organic matter was mostly degraded aerobically, as hypothesized for other Neogene Mediterranean (e.g. Pérez‐Folgado et al ., 2003; Pellegrino et al ., 2020a,b) and global (e.g. White et al ., 1992; Koizumi & Yamamoto, 2018) diatomaceous deposits.…”

Tiny, glassy, and rapidly trapped: The nano-sized planktic diatoms in Messinian (late Miocene) gypsum

“…The diatom assemblage of the studied gypsum samples differs from that of the pre-MSC (Pellegrino et al, 2018) diatomaceous deposits of the Mediterranean area. The latter are dominated by Thalassionema nitzschioides, which is found in biogenic laminae reflecting a relatively well-mixed water column where nutrients previously trapped at depth were reinjected into surface waters (Pellegrino et al, 2020). This diatom species is underrepresented in gypsum.…”