Collaborative Research: (ESH) Late Paleocene and Early Eocene Nonmarine Climate in the Gulf of Mexico Basin, Texas

该项目旨在为中古新世（约60 Ma）至早始新世末期（约49 Ma）的非海洋气候建立一个初步框架。早始新世（约49-53/54 Ma）是第三纪最温暖的时期，除了白垩纪-第三纪界线之后的一段短暂时期。需要对始新世早期热最佳期之前的气候发展进行研究，以了解最佳期是否代表了气候的突然变化，还是先前确定的趋势的高潮。此外，与海洋记录的气候变化不同，非海洋记录提供了季节温度变化和降水制度变化的证据。这些数据对于充分评估基于一般环流模式的预测是必要的，特别是如果，正如已经提出的那样，始新世早期的热最佳是由大量的大气CO2造成的。以前在墨西哥湾（密西西比）海湾的德克萨斯州部分的威尔科克斯群（约49-60 Ma）中报道过一些化石叶子的组合，但在野外观察到的更多，仍有待收集。这些叶片组合可以在策略上与舌间海洋浮游生物序列有关，也与普遍接受的海平面曲线有关。在某些情况下，需要在不能直接追溯到海洋剖面的非海洋剖面之间进行孢粉对比。德克萨斯Wilcox植被位于古纬度~30oN，将提供古新世和始新世早期在如此低纬度地区的非海洋性气候和气候变化的第一个长期记录。所代表的植被是洪泛区的植被。这些组合都代表低海拔植被，因此古海拔变量保持不变。此外，在西部内陆的树叶组合中存在问题的雨影也不会是一个因素。用于古环境解释的植物/孢粉学分析的传统方法包括根据其最近的近亲来确定化石，以及对植物的气候耐受性随时间不变的可疑假设。这一程序将是一项耗时多年的工作，即使到那时，推论也可能在理论基础上受到质疑。相比之下，收集、管理和分配威尔科克斯树叶到单个物种可以在相对较短的时间内完成。本文将采用CLAMP （Climate-Leaf Analysis Multivariate Program）对叶片组合进行分析，而不是使用古气候推断的最近生活亲属法。该方法于1990年首次提出，在某些方面仍处于发展阶段，拟议的研究将对此作出贡献。该方法采用典型对应分析（Canonical Correspondence Analysis， CCA）这一越来越广泛应用于生态学研究的多变量程序，对31种严格定义的叶片面相性状状态进行分析。对在有气象数据记录的气象站附近收集的现代样本进行分析，可以用各种气象参数校准从CLAMP得出的估计，包括年平均温度、冷月和暖月平均温度、生长季节长度、生长季节降水、相对湿度和季节性干旱。初步分析表明，在古新世中晚期，德克萨斯州威尔科克斯地区的年平均气温有所上升。分析还表明，尽管德克萨斯州古新世中晚期降水非常丰富，但在始新世早期热最佳期结束时，气候状况出现了明显的季节性干旱。?？

9615775 Dilcher This project aims to develop an initial framework for nonmarine climates from the mid-Paleocene (ca. 60 Ma) through the end of the early Eocene (ca. 49 Ma). The early Eocene (ca. 49-53/54 Ma) was by consensus the warmest period during the Tertiary, except possibly for a brief period after the Cretaceous-Tertiary boundary. The climatic developments preceding the early Eocene thermal optimum need to be studied in order to understand whether the optimum represented an abrupt change in climate or was a culmination of previously established trends. Further, unlike the marine records of climate change, the nonmarine record provides evidence of changes in seasonal temperatures and changes in precipitation regimes. Such data are necessary to fully evaluate predictions based on General Circulation Models, especially if, as has been suggested, the early Eocene thermal optimum resulted from high amounts of atmospheric CO2. Some assemblages of fossil leaves have been previously reported from the Wilcox Group (ca. 49-60 Ma) of the Texas part of the Gulf of Mexico (Mississippi) Embayment, but many more have been observed in the field and remain to be collected. These leaf assemblages can be related strategically to the intertonguing marine planktic sequence and, as well, related to the generally accepted sea-level curves. In some instances, palynological correlations will need to be made between nonmarine sections that cannot be directly traced into marine sections. The Texas Wilcox vegetation was a paleolatitude of ~30oN and will provide the first long record of nonmarine climate and climatic change during the Paleocene and early Eocene at such a low latitude. The vegetation represented is that of a floodplain. The assemblages all represent low altitude vegetation, and thus the variable of paleoaltitude is held constant. Further, rain shadows, which are problematic in leaf assemblages from the Western Interior, would not have been a factor. Traditional methods of pal eobotanical/palynological analyses for paleoenvironmental interpretations involve determination of fossils in terms of their nearest living relatives and the questionable assumption of unchanging climatic tolerances for plants through time. This procedure would represent a very time-consuming effort of many years' duration, and, even then, the inferences could be questioned on a theoretical basis. In contrast, collection, curation, and assignment of the Wilcox leaves to individual species can be done in a comparatively short time. Instead of using the nearest living relative method of paleoclimatic inferences, the leaf assemblages will be analyzed by means of CLAMP (Climate-Leaf Analysis Multivariate Program). This method was first proposed in 1990 and is, in some respects, still in a developmental stage, to which the proposed research will contribute. The method analyzes 31 rigorously defined physiognomic character states of leaves by means of Canonical Correspondence Analysis (CCA), a multivariate program increasingly and widely employed in ecological studies. Analysis of modern samples collected proximal to stations where meteorological data have been recorded allows estimates derived from CLAMP to be calibrated with various meteorological parameters, including mean annual temperature, mean cold-month and warm-month temperatures, growing season length, growing season precipitation, relative humidity, and seasonal drought. Preliminary analyses suggest that mean annual temperatures in the Texas Wilcox showed an increase during the mid- to late Paleocene. The analyses also indicated that, whereas the mid- to late Paleocene in Texas was characterized by very abundant precipitation, by the end of the early Eocene thermal optimum the climatic regime had pronounced seasonal drought. ??