Quantitative Reconstruction of Sediment Provenance: A Geochemical Study of the Amazon Fan/Delta

保存在亚马逊水下三角洲和深海扇内的硅屑沉积物是在不同气候条件下来自亚马逊流域的。三角洲主要代表在热带条件下相对较近的沉积，而扇则代表在不同更新世冰期的沉积，当时陆架在低海平面时期被越过。地层约束样品最近才从ODP Leg 155获得。这一建议涉及定量评估在这些对比的气候条件下，这些沉积物的安第斯物源与克拉通物源的相对重要性。初步的岩石学、地球化学和同位素数据表明，安第斯山脉的物源占主导地位，但也表明，详细的物源史是复杂的。该研究将使用钕和铅同位素以及各种地球化学和岩石学技术来评估泥和砂的来源。对于古层序物源研究中最常用的岩性砂，我们将利用Pb和Nd测量的结合，进一步评估主要个体成分石英、长石和重矿物（以锆石为代表）的物源。这方面的研究有两个目的：(1)确定从整个岩石Pb和Nd同位素测量推断的物源是否可以应用于含Nd相对较少的主要矿物（石英和长石）或Pb（石英）；(2)确定碎屑锆石研究推断的物源是否与主要矿物（石英和长石）、大块砂和伴生泥浆推断的物源相似。根据这些数据，我们将能够以一定的精度重建砂和泥的来源，并评估克拉通对砂的作用是否大于对泥的作用，正如其他人所建议的那样，以及克拉通在对比气候条件下的作用是大还是小。这项研究将提供关于如何解释各种来源方法（例如岩石学、地球化学、放射性成因同位素）的局限性的基本信息。亚马逊流域系统是世界上最大的，了解该系统中沉积物的起源和历史本身就具有很大的意义。在扇形和三角洲上保存的沉积物之间的任何对比对于该排水系统的古气候学研究都具有重要意义。

9627908 McLennan Siliciclastic sediments preserved within the Amazon subaqueous delta and deep-sea fan were derived from the Amazon drainage basin under contrasting climatic conditions. The delta represents relatively recent deposition largely under tropical conditions whereas the fan represents deposition during various Pleistocene glacial stages when the shelf was by-passed during low sea level stands. Stratigraphically constrained samples have only recently been made available for the Amazon Fan from ODP Leg 155. This proposal is concerned with quantitatively evaluating the relative importance of Andean versus cratonic provenance for these sediments during these contrasting climatic conditions. Preliminary petrographic, geochemical and isotopic data suggest a dominance of Andean sources but also suggest that in detail the provenance history is complex. The study will evaluate the sources of both muds and sands using neodymium and lead isotopes and a variety of geochemical and petrographic techniques. For the sands, the lithology most often used in provenance studies of ancient sequences, we will further evaluate the provenance of the major individual components, quartz, feldspar and heavy minerals (using zircon as the proxy) using a combination of Pb and Nd measurements. The purpose of this aspect of the study is twofold: (1) to determine if the provenance inferred from whole rock Pb and Nd isotope measurements can be applied to the major minerals that contain relatively little Nd (quartz and feldspar) or Pb (quartz); (2) to determine if the provenance inferred from detrital zircon studies is similar to that inferred for major minerals (quartz and feldspar), bulk sands and associated muds. From these data, we will be able to reconstruct the provenance of both sands and muds with some precision and evaluate if the craton plays a greater role for sands that muds, as suggested by others, and whether the craton plays a greater or lesser role during contrasting climatic conditions. Th is study will provide fundamental information regarding limitations on how various approaches to provenance (e.g., petrography, geochemistry, radiogenic isotopes) can be interpreted. The Amazon drainage system is the largest in the world and understanding the origin and history of sediment derived from this system is of the great interest in its own right. Any contrasts between sediment preserved on the fan and delta would have significance for paleoclimatological studies of this drainage system.