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

碳稳定同位素比值地层学：该方法在宾夕法尼亚期石灰岩中的测试该项目检验了石灰岩碳稳定同位素比值系统漂移可用作碳酸盐为主盆地的高分辨率地层对比工具的假设。众所周知，石灰岩中的碳酸盐成分可以保持海水中13C/12C的比值。这其中的原因很复杂，但其结果是记录了从5级冰川稳定海平面变化(10万年)的尺度上变化的同位素比率。在冰屋地球条件下，石灰岩记录了世界范围内的冰川稳定循环，即重复的浅向上岩性循环。我们的数据表明，同位素漂移反映了岩性的变化，因此可以用来对盆地内的石灰岩剖面进行更详细的对比。我们的目标是宾夕法尼亚伊利石灰岩和类似的鸟泉地层，它们出现在内华达州中部、东部和南部。我们将这种浅水碳酸盐盆地(S)称为伊利-鸟泉盆地。这些地层是在Morrowan和Atokan(宾夕法尼亚)时期，在冈德瓦南冰川期间，在低纬度的劳伦西亚西部开阔的陆架上沉积的。为了发展EBSB的高分辨率同位素地层学，我们测量和采样了许多紧密和广泛分布的剖面，根据生物地层学证据(分辨率~2-5My)，这些剖面已知的时间大致相等。充满浅水石灰岩的大型大陆盆地令人费解，它们在北美西部的石炭纪很常见。这些盆地包含非常厚的石灰岩部分，在某些例子中有1到3公里。根据我们的新数据，可以提出一些关于这些碳酸盐盆地的令人信服的问题。由已知水深(接近海平面)和高分辨率相对时间约束的详细沉降史揭示了沉降率。对比盆地不同地区的沉积速率，将得到盆地三维演化过程，并证明沉积堆积的构造作用力。我们或许还能对有关气候变化和冰屋地球的更基本问题做出贡献：碳酸盐循环告诉我们关于冰川进退期间海洋化学的什么信息？在盆地中，水深、石灰岩沉积和能量输送是如何在不同尺度上相互作用的？更长的、4级的海平面和海水化学循环也能相互关联吗？我们正在开发的开始回答这些问题的工具是旋回石灰岩剖面的碳同位素对比。

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.