COLLABORATIVE RESEARCH: TESTING THE TECTONIC UPLIFT-WEATHERING HYPOTHESIS FOR THE LATE ORDOVICIAN GREENHOUSE-ICEHOUSE TRANSITION

摘要奥陶纪末期（445-443 Ma）是极地大陆大冰原形成的时期，这一观点被广泛接受。然而，关于开始变冷和从温室气候转变的情况仍然存在争议。有人提出，冷却是由大气中二氧化碳水平的降低带来的。奥陶系二氧化碳可以通过埋藏有机碳或硅酸盐风化作用（埋藏碳酸盐碳）来降低。本研究将利用不同成因和响应时间（C、Sr、Nd）的多个同位素代用记录，探讨奥陶纪气候与影响长期碳循环过程之间的联系。pi将获得这些耦合同位素记录，主要是在保存在低纬度劳伦提亚相对两侧的厚地层剖面中，这些剖面通过牙形石生物地层学确定年代。以前的同位素研究已经发现碳酸盐的锶和碳同位素在奥陶纪气候转变过程中发生了重大变化。PIs的研究旨在利用牙形石磷灰石中的钕同位素模式和有机质中的C同位素模式来解决中晚奥陶世Sr同位素大幅下降的原因和后果。海水87Sr/86Sr通常被用作古构造事件的代表，因为它受河流输入和海底热液蚀变之间的平衡控制。和Sr一样，海水的Nd同位素组成可以记录大陆风化的变化。然而，一个关键的区别是，海洋的Nd库存不受海底热液输入的平衡。因此，143Nd/144Nd可以为海水87Sr/86Sr变化提供独立的来源约束。pi还将构建碳酸盐和有机质中C同位素的配对记录，这可能提供有关硅酸盐风化变化或先前确定的有机碳埋藏事件导致的大气二氧化碳相对变化的信息。此外，通过将C与Nd耦合，可以获得有关风化模式和养分循环的当地信息，而Nd的停留时间比洋间混合时间短得多。

COLLABORATIVE RESEARCH: TESTING THE TECTONIC UPLIFT-WEATHERING HYPOTHESIS FOR THE LATE ORDOVICIAN GREENHOUSE-ICEHOUSE TRANSITIONMatthew Saltzman, Ohio State UniversityStephen Leslie, James Madison UniversityEAR-0745452, -0746181AbstractIt is widely accepted that the end of the Ordovician period (445-443 Ma) was a time of large continental ice sheets in polar regions. However, the circumstances surrounding the initiation of cooling and the transition from a greenhouse climate remains controversial. It has been proposed that cooling was brought about by a lowering of atmospheric carbon dioxide levels. Ordovician carbon dioxide could be lowered either by burial of organic carbon or by silicate weathering (burial of carbonate carbon). Our research will use multiple isotopic proxy records with different causal factors and response times (C, Sr, Nd) to investigate links between Ordovician climate and processes affecting the long term carbon cycle. PIs will obtain these coupled isotopic records primarily in thick stratigraphic sections preserved on opposite sides of low-latitude Laurentia that are well-dated by conodont biostratigraphy. Previous isotopic studies have identified significant changes in both strontium and carbon isotopes of carbonates through the Ordovician climate transition. PIs research aims to address the cause and consequences of the large drop in Sr isotopes in the Middle-Late Ordovician using neodymium isotope patterns in conodont apatite and C isotopes in organic matter. The 87Sr/86Sr of seawater is commonly used as a proxy for paleotectonic events because it is controlled by a balance between riverine inputs and seafloor hydrothermal alteration. Like Sr, the Nd isotopic composition of seawater can record changes in continental weathering. However, a key difference is that the oceanic inventory of Nd is not balanced by seafloor hydrothermal inputs. Thus, 143Nd/144Nd can provide independent constraints on the sources of seawater 87Sr/86Sr variations. PIs will also construct paired records of C isotopes in carbonate and organic matter that may potentially provide information on relative changes in atmospheric carbon dioxide that resulted from changes in silicate weathering or previously identified organic carbon burial events. In addition, by coupling C with Nd which has a residence time considerably shorter than interocean mixing times, it is possible to obtain local information on weathering patterns and nutrient cycling.