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。这一合作项目旨在解决有关意大利中三叠世莱特玛地台白云岩旋回地层学的持续的多学科辩论。拉特玛的600多米规模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。对地台内火山灰床的放射性同位素测年随后表明，各个循环的形成速度肯定要快得多，在2,000到4,000年内，甚至更短。这种校准间的差异构成了所谓的“拉特玛争议”。现在有了高精度的地质年代学，挑战是找到一个合理的解释来解释强烈旋回的拉特玛相的起源。以前的研究主要集中在相分析，以及海平面变化在旋回形成中的作用。该项目应用岩石磁学技术研究可能参与和/或影响沉积旋回性的一系列更全面的环境因素(例如，风沙作用)。对拉特玛系列40米断面的200个样品进行了磁化率、磁滞剩磁和饱和等温剩磁的初步研究，发现了一个相干信号，表明磁性矿物浓度随着旋回的变化而变化。该项目通过500米厚的拉特玛序列的主曝光，收集高分辨率的岩石磁学数据，以便对序列的短期和长期旋回性进行新的、客观的表征和彻底的采样。这项工作还将现有的磁极地层学数据库扩展到地台新位置的横断面，以帮助解决中三叠世白云岩相互冲突的生物磁性地层学。较短的放射性同位素时间刻度意味着地下钙质和块状帐篷带胶结的累积速率比第四纪类似物高出数量级。解决拉特马尔争议有助于国际地层学委员会为中生代开发天文时间刻度(ATS)的全球倡议。三叠纪在覆盖面上唯一的主要空白包括拉特玛地台占据的地层间隔；重要的是确认或拒绝天文强迫在拉特玛旋回形成中的作用。该项目汇集了三所大学，其中一所在意大利靠近田野的地区。

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.