Carbon Stable Isotope Ratio Stratigraphy: A Test of the Method in Pennsylvanian Limestones

碳稳定同位素比地层学：宾夕法尼亚石灰岩方法的测试

• 批准号: 1038702
• 负责人: James Trexler
• 金额: $ 5.97万
• 依托单位: Board of Regents, NSHE, obo University of Nevada, Reno
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2010
• 资助国家: 美国
• 起止时间: 2010-09-01 至 2012-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1038702&HistoricalAwards=false
• 关键词: Carbon; Stable; Isotope; Ratio; Stratigraphy

Carbon Stable Isotope Ratio Stratigraphy: A Test of the Method in Pennsylvanian LimestonesThis project tests the hypothesis that systematic excursions in limestone carbon stable isotope ratios can be used as a high-resolution stratigraphic correlation tool in carbonate-dominated basins. It is well known that carbonate components of limestone preserve the 13C/12C ratio in seawater. The reasons for this are complex, but the result is a record of isotopic ratios that vary at scales as short as 5th-order glacioeustatic sea-level changes (100,000 years). During ice-house Earth conditions, limestones record glacioeustatic cycles world-wide as repeated shallowing-upward lithologic cycles. Our data show that isotopic excursions mirror lithologic changes, and therefore can be used to correlate limestone sections within basins in great detail. Our target is the Pennsylvanian Ely Limestone and equivalent Bird Spring Formation that crop out throughout central, eastern, and southern Nevada. We refer to this collection of shallow-water carbonate basin(s) as the Ely-Bird Spring basin (EBSB). These strata were deposited during Morrowan and Atokan (Pennsylvanian) time at low latitudes on the open-marine shelf of western Laurentia, during Gondwanan glaciation. In order to develop the high-resolution isotopic stratigraphy of the EBSB we are measuring and sampling a number of closely and widely spaced sections which are known to be roughly time-equivalent based on biostratigraphic evidence (resolution ~2-5My). Large continental basins filled with shallow-water limestone are enigmatic, and they are common in the Carboniferous of western North America. These basins contain very thick sections of limestone, 1 to 3 kilometers in some examples. Based on our new data, a number of compelling questions can be asked about these carbonate basins. Detailed subsidence histories constrained by known water depth (near sea-level) and high-resolution relative timing reveal rates of subsidence. Comparing rates from different parts of the basin will result in three-dimensional basin evolution, and evidence of the tectonic forcing of sediment accumulation. We may also be able to contribute to more fundamental questions about climate change and ice-house Earth: What do the carbonate cycles tell us about ocean chemistry during glacial advance and retreat? How do water depth, limestone sedimentation, and transport energy interact at different scales in the basin? Can longer, 4th order, cycles of sea-level and marine water chemistry also be correlated? The tool we are developing to begin to answer these questions is carbon isotope correlation of cyclothemic limestone sections.

碳稳定同位素比值地层学：宾夕法尼亚系灰岩碳稳定同位素比值方法的验证本项目验证了灰岩碳稳定同位素比值系统偏移可以作为碳酸盐岩为主盆地高分辨率地层对比工具的假设。众所周知，石灰岩的碳酸盐组分在海水中保存了13C/12C比值。其原因很复杂，但结果是同位素比率的记录，其变化范围短至5阶冰川上升海平面变化（10万年）。在冰库地球条件下，石灰石记录了世界范围内反复的浅向上的岩性旋回。我们的数据表明，同位素漂移反映了岩性变化，因此可以用来非常详细地关联盆地内的石灰岩剖面。我们的目标是宾夕法尼亚伊利石灰岩和等效的鸟泉地层，它们遍布内华达州中部，东部和南部。我们将这组浅水碳酸盐岩盆地称为伊利鸟泉盆地（EBSB）。这些地层沉积于Morrowan和Atokan（宾夕法尼亚）时期，在冈瓦纳冰川时期，在低纬度的劳伦西亚西部的开阔陆架上。为了发展高分辨率的同位素地层学，我们测量和采样了许多紧密和广泛间隔的剖面，这些剖面在生物地层学证据的基础上大致是时间等效的（分辨率~2-5My）。充满浅水石灰岩的大型大陆盆地是神秘的，它们在北美西部石炭纪很常见。这些盆地含有非常厚的石灰岩，在一些例子中有1到3公里。根据我们的新数据，这些碳酸盐岩盆地可以提出许多令人信服的问题。受已知水深（接近海平面）和高分辨率相对时间限制的详细沉降历史揭示了沉降速率。比较盆地不同部位的速率将得出三维盆地演化，并提供构造强迫沉积的证据。我们或许还能对气候变化和冰库地球的更基本问题做出贡献：在冰川前进和后退期间，碳酸盐循环告诉了我们什么关于海洋化学的信息？在不同尺度下，盆地的水深、灰岩沉积和输运能量是如何相互作用的？更长的四阶海平面循环和海水化学是否也有关联？为了回答这些问题，我们正在开发的工具是循环石灰岩剖面的碳同位素对比。