Towards an orbitally-tuned Maastrichtian stage

最新的白垩纪见证了全球碳循环的扰动，人们认为这与气候变冷和潜在的短暂冰川有关。为了了解强迫机制、它们的全球意义以及区域对这些事件的反应，有必要改进我们目前可用的地层框架。马斯特里赫特天体年代学的最新进展使碳循环的变化能够调整到405 KYR长偏心周期的节拍器。然而，所有研究的时间序列都没有涵盖73 Ma到71 Ma之间的寒武纪-马斯特里赫期的转变，这是一个严重气候变冷的临界区间。法国西南部Tercis-les-Bains的旋回石灰岩序列记录了碳同位素的扰动，是被批准的马斯特里赫特基底的GSSP。然而，实际上碳同位素地层学的分辨率太低，无法评估地球外生碳循环的潜在轨道强迫机制。此外，生物地层事件的定义为钙质超微化石和浮游有孔虫的第一次和最后一次出现还没有得到准确的解决。在这里，我们建议在Tercis-les-Bain的GSSP剖面利用块状碳酸盐来发展天体年代学和高分辨率碳同位素地层学，以解决73-71 Ma之间轨道时间尺度上的短暂同位素事件。这种综合地层格架的建立对大陆架和海洋剖面以及低纬度和高纬度之间的全球对比具有重要意义。它还将通过提供与地磁极性时间尺度相关的精确边界定义，进一步允许重新评估全球海洋观测系统。该项目的成果将在轨道时间尺度上提供一个强有力的地层学框架，这是认识碳循环、海洋化学、环流和海平面变化以及海洋生态系统反应之间的时间关系所必需的。

The latest Cretaceous witnessed perturbations in the global carbon cycle assumed to have been associated to climatic cooling and potential ephemeral glaciations. In order to understand the forcing mechanisms, their global significance, and the regional response to these events it is necessary to improve our presently available stratigraphic framework. Recent advances in Maastrichtian astrochronology allowed for the tuning of carbon cycle variations to the metronome of the 405-kyr long eccentricity cycle. However, all studied time series do not cover the Campanian-Maastrichtian transition between 73 and 71 Ma, a critical interval of severe climatic cooling. The cyclic limestone succession at Tercis-les-Bains in SW France records carbon isotope perturbations and is the ratified GSSP for the base of the Maastrichtian. However, actually the resolution of carbon isotope stratigraphy is too low to assess potential orbital forcing mechanisms of the Earth’s exogenic carbon cycle. Further, the definition of biostratigraphic events as first and last occurrences of calcareous nannofossils and planktonic foraminifera is not precisely resolved yet. Here, we propose to develop an astrochronology together with a high-resolution carbon isotope stratigraphy at the GSSP section of Tercis-les-Bains using bulk-carbonates to resolve short-lived isotopic events on orbital timescales between 73 and 71 Ma. The development of such an integrated stratigraphic framework is important for global correlation between shelf and oceanic sections as well as between low and high-latitudes. It will further allow reassessing the GSSP by providing a precise boundary definition relative to the geomagnetic polarity time scale. Results of this project will provide a robust stratigraphic framework on orbital time scales necessary to recognize temporal relationships between the carbon cycle, changes in ocean chemistry, circulation and sea level, and the oceanic ecosystem response.