Collaborative Research: A new perspective on the driving forces of controversial Middle Triassic cyclostratigraphy: a rock magnetic approach

该奖项是根据2009年美国复苏和再投资法案（公法111-5）资助的。该合作项目解决了意大利中三叠统Latemar平台白云岩旋回地层学的多学科争论。Latemar超过600米尺度的浅海碳酸盐旋回最初被解释为天文驱动的2万年海平面振荡的产物。随后，对台地内的火山灰床进行的放射性同位素年代测定表明，各个周期的形成速度肯定要快得多，大约在2000年到4000年之间，甚至更短。这种校准间的差异构成了所谓的“Latemar争议”。现在有了高精度的地质年代学，面临的挑战是为强周期性拉末相的起源找到一个合理的解释。以前的研究主要集中在相分析和海平面变化在旋回形成中的作用。本项目应用岩石磁技术调查可能参与和/或影响沉积旋回的更全面的环境因素（例如，风成过程）。一项利用磁化率、非滞后剩磁和饱和等温剩磁对Latemar演替40 m样带的200个样品进行的初步研究揭示了一个相干信号，表明磁性矿物浓度随旋回变化。该项目通过500 m厚的Latemar层序的主要曝光，以高分辨率收集岩石磁性数据，以获得新的、客观的特征，并对层序的短期和长期旋回性进行全面采样。这项工作还将现有的磁极性地层学数据库扩展到平台新位置的样带，以帮助解决中三叠统白云岩的生物磁地层学冲突。解决Latemar争议很重要。较短的放射性同位素时间尺度表明，陆基钙质和块状山脚带胶结作用的累积速率比第四纪类似物高几个数量级。解决Latemar争议有助于国际地层学委员会开发中生代天文时间尺度（ATS）的全球倡议。三叠纪盖度上唯一的主要差距包括拉特玛地台所占据的地层间隙；确认或否定天文强迫在拉特玛旋回形成中的作用是很重要的。该项目汇集了三所大学，其中一所位于意大利，靠近该地区。

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).This collaborative project addresses an ongoing multidisciplinary debate concerning the cyclostratigraphy of the Middle Triassic Latemar platform Dolomites, Italy. The more than 600 meter-scale shallow-marine carbonate cycles of the Latemar were originally interpreted as the products of astronomically driven, 20,000 year sea-level oscillations. Radioisotope dating of ash-beds within the platform subsequently indicated that the individual cycles must have formed much faster, in 2,000 to 4,000 years, or even less. This inter-calibration discrepancy constitutes the so-called "Latemar controversy." With high-precision geochronology now available, the challenge is to find a reasonable explanation for the origin of the strongly cyclical Latemar facies.Prior research focused exclusively on facies analysis, and the role of sea level change in cycle formation. This project applies rock magnetic techniques to investigate a more comprehensive range of environmental factors (e.g., aeolian processes) that could have been involved in and/or influenced depositional cyclicity. A pilot study using magnetic measurements of magnetic susceptibility, anhysteretic remanent magnetism and saturation isothermal remanent magnetization of 200 samples from a 40-m transect of the Latemar succession reveals a coherent signal indicating magnetic mineral concentration variations in tune with the cycles. The project collects rock magnetic data at high resolution through the main exposure of the 500 m thick Latemar succession, for a new and objective characterization and thorough sampling of the succession's short and long term cyclicity. This work also expands an existing magnetic polarity stratigraphy database to a transect at a new location in the platform, to help resolve conflicting bio-magnetostratigraphies of the Middle Triassic Dolomites.Resolution of the Latemar controversy is important. The short radioisotope timescale implies accumulation rates of subaerial caliche and massive tepee-zone cementation that are orders of magnitude higher than known from Quaternary analogs.Addressing the Latemar controversy contributes to the International Commission on Stratigraphy global initiative to develop an Astronomical Time Scale (ATS) for the Mesozoic Era. The only major gap in coverage for the Triassic Period includes the stratigraphic interval occupied by the Latemar platform; it is important to confirm or reject the role of astronomical forcing in the formation of the Latemar cycles. This project brings together three universities, including one in Italy near the field area.