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

9615775迪尔彻这个项目旨在开发一个从古新世中期(约60 Ma)到早始新世末期(约49 Ma)的非海洋气候的初步框架。始新世早期(约49-53/54 Ma)被普遍认为是第三纪最温暖的时期，可能是白垩纪-第三纪界线之后的一段短暂时期。需要研究始新世早期热最佳期之前的气候发展，以便了解最佳期是代表气候的突然变化，还是先前确定的趋势的顶峰。此外，与气候变化的海洋记录不同，非海洋记录提供了季节性温度变化和降水制度变化的证据。这些数据对于全面评估基于大气环流模型的预测是必要的，特别是如果如所建议的那样，始新世早期的热最佳是由大量大气二氧化碳造成的。以前在墨西哥湾(密西西比)海湾得克萨斯州部分的Wilcox群(约49-60 Ma)中曾报道过一些树叶化石组合，但在野外观察到了更多的化石树叶组合，仍有待收集。这些叶子组合在战略上可以与相互交错的海洋浮游层序有关，也可以与普遍接受的海平面曲线有关。在某些情况下，需要在不能直接追踪到海洋部分的非海洋部分之间进行孢粉对比。德克萨斯威尔科克斯植被位于约30oN的古纬度，将首次在如此低的纬度提供关于古新世和始新世早期非海洋性气候和气候变化的长期记录。所代表的植被是泛滥平原的植被。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，雨影在西部内陆的树叶组合中是有问题的，不会是一个因素。用于古环境解释的PAL植物学/孢粉学分析的传统方法包括根据化石最接近的现存近亲来确定化石，以及植物对气候的耐受性随着时间的推移而不变的可疑假设。这一程序将是一项耗时多年的非常耗时的努力，即使如此，这些推论也可能在理论上受到质疑。相比之下，收集、整理和分配Wilcox树叶给单个物种可以在相对较短的时间内完成。不再使用最接近的古气候推断方法，而是用气候-叶分析多变量程序(CLIP)来分析叶组合。这一方法于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。该方法通过典型对应分析(CCA)对31种严格定义的叶片外貌特征状态进行分析，CCA是一个日益广泛应用于生态学研究的多变量程序。对已记录气象数据的站点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得出的估计值，包括年平均温度、平均冷月和暖月温度、生长季长度、生长季降水、相对湿度和季节性干旱。初步分析表明，德克萨斯州威尔科克斯的年平均温度在古新世中晚期出现了上升。分析还表明，得克萨斯州中、晚古新世的特点是降水非常丰富，但在始新世早期热最佳结束时，气候状况已明显出现季节性干旱。?？

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. ??