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, north‐west 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⁻¹) 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.

中文翻译：

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地中海最北端的中新世硅藻岩上部沉积物：对一个被忽视的古海洋学档案的层尺度调查

在中新世晚期，地中海地区的硅藻土积累急剧增加，这是由硅藻类沉积物的沉积所记录的。这些沉积物的精细层压可能记录在地中海盆地地球动力学演变的关键阶段发生的每年到次年的古海洋学过程，最终导致了墨西尼盐度危机。以前的研究人员已经将硅藻相解释为地中海盆地上升流上升和底部缺氧形成的产物。但是，直到现在，还没有做出任何努力来揭示这些沉积物在层尺度上的沉积学和微古生物学内容。本文介绍了基于系统扫描电镜的形态学研究的第一个案例研究，该形态学研究是在中新世晚期沉积在地中海盆地最北端（意大利西北皮埃蒙特盆地）的硅藻沉积物。本文使用一种非侵入性分析方法，描述并解释了六个生态学成分（薄片，叠层小包，非叠层间隔，洞穴，富含蛋白石的聚集体和混合颗粒结构）及其关系。在对这些沉积物进行层级研究之后，可以确定一个年度沉积周期，并确定堆积速率（本文描述并解释了六个生态学组成部分（薄片，叠层小包，非叠层间隔，洞穴，富含蛋白石的聚集体和混合颗粒结构）及其关系。在对这些沉积物进行层级研究之后，可以确定一个年度沉积周期，并确定堆积速率（本文描述并解释了六个生态学组成部分（薄片，叠层小包，非叠层间隔，洞穴，富含蛋白石的聚集体和混合颗粒结构）及其关系。在对这些沉积物进行层级研究之后，可以确定一个年度沉积周期，并确定堆积速率（大约50 cm kyr ^-1）推断对于受上升流积极影响的环境而言是非典型的。此处强调了硅藻瓣膜缠结在对生物扰动形成物理屏障方面所起的作用，它是负责保存硅藻土沉积物叠层织物的主要过程，挑战了深层缺氧的假定作用。这些结果表明，不能将皮埃蒙特盆地中新世晚期硅藻沉积视为上升强度和海底氧耗竭的副产品。