Integrated Paleontological, Geochemical, and Paleoceanographic Studies on the Taconic Foreland Basin

对纽约塔肯前陆盆地长约500万年的中奥陶世层序的研究旨在解决以下问题：从大陆碳酸盐台地向俯冲海沟外坡移动过程中，水温和含氧量如何随深度变化？海洋深层环流是由来自低纬度平台的温暖咸水下沉驱动的吗，就像这次和其他海平面最高时期所建议的那样？12年来关于该序列的地层学和古生物学的背景工作，包括正在进行的为期两年的可行性研究，为回答这些和相关问题提供了独特的机会，这些问题来自对水深范围广泛的底栖无脊椎动物贝壳的碳和氧同位素研究，以及来自沉积物的碳和硫分析。相对水深是根据沿着个别火山灰层的下坡横断面上的位置来测量的，并随着时间的推移，通过化石组合的梯度分析来测量。相对水温将根据一个有关节腕足动物(低镁方解石)、两个无关节腕足动物(磷酸钙)和其他被选择进行比较的类群的氧同位素组成来确定。显然，在结构和化学上保存完好的钙质腕足类在同位素上几乎没有变化：来自同一采样站的标本产生了紧密聚集的180个值，与保存最好的中、晚奥陶世材料的值相似。腕足类关节壳上斑点的密度(与现代形式中的温度成正比)与相对深度成正比，可以作为温度测量的温度标准。研究将有助于古海洋学重建和确定可能的资源勘探区域。

Studies on a roughly five million year long Middle Ordovician sequence in the Taconic Foreland Basin, New York are addressed to these questions: How did water temperature and oxygen content vary with depth in going from a continental carbonate platform down the outer slope of a subduction trench? Was oceanic deep circulation driven by sinking of warm, saline water from low-latitude platforms, as has been suggested for this and other times of highest sea stand? Twelve years' background work on the stratigraphy and paleontology of the sequence, including an ongoing two-year feasibility study, has made a unique opportunity for answering these and related questions from carbon and oxygen isotopic studies on shells of bathymetrically wide-ranging benthic invertebrates, and from carbon and sulfur analyses of sediments. Relative water depth is measured in terms of position on downslope transects along individual volcanic ash layers and, over time, through gradient analysis of fossil assemblages. Relative water temperature will be determined from oxygen isotopic composition in one articulate brachiopod (low-Mg calcite), two inarticulate brachiopods (calcium phosphate), and other taxa selected for comparison. Evidently, texturally and chemically well-preserved calcareous brachiopods are isotopically little-altered: specimens from the same sampling station yield tightly clustered 180 values similar to those of the best-preserved Middle and Late Ordovician material. The density of puncta on the articulate brachiopod's shell (proportional to temperature in modern forms) is proportional to relative depth, and may provide a check on 80O as a temperature measure. Research will aid in paleoceanographic reconstruction and in identification of possible areas for resource exploration.