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

合作研究：验证晚奥陶世温室-冰屋转换的构造隆升-风化假说俄亥俄州立大学马修·萨尔茨曼斯蒂芬·莱斯利，詹姆斯·麦迪逊大学EAR-0745452，-0746181摘要人们普遍认为，奥陶纪末期(445-443 Ma)是极地地区大片大陆冰盖的时期。然而，围绕开始降温和从温室气候过渡的情况仍然存在争议。有人提出，降温是由大气中二氧化碳水平的降低带来的。奥陶系的二氧化碳可以通过埋藏有机碳或硅酸盐风化(埋藏碳酸盐碳)来降低。我们的研究将利用具有不同成因因素和响应时间(C、Sr、Nd)的多个同位素替代记录来研究奥陶纪气候与影响长期碳循环的过程之间的联系。PIS将主要在低纬度劳伦西亚两侧保存的厚厚的地层剖面中获得这些耦合的同位素记录，这些地层剖面由牙形刺生物地层学很好地确定了年代。以往的同位素研究发现，在奥陶纪气候过渡期间，碳酸盐岩的锶和碳同位素都发生了重大变化。PIS研究的目的是利用牙形石磷灰石中的钕同位素模式和有机质中的碳同位素模式来研究中晚奥陶世锶同位素大幅下降的原因和后果。由于受河流输入和海底热液蚀变平衡的控制，海水中的87Sr/86Sr常被用作古构造事件的代表。与锶一样，海水的Nd同位素组成也可以记录大陆风化的变化。然而，一个关键的不同之处在于，海底热液输入没有平衡海洋ND的库存。因此，~(143)Nd/~(144)Nd可以对海水~(87)Sr/~(86)Sr变化的来源提供独立的约束。PIS还将构建碳酸盐和有机质中碳同位素的配对记录，这些记录可能会提供由于硅酸盐风化变化或以前发现的有机碳埋藏事件而导致的大气二氧化碳相对变化的信息。此外，通过将C和ND结合起来，其停留时间比大洋间混合时间短得多，就有可能获得有关风化模式和营养循环的当地信息。

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.