Doctoral Dissertation Research: Paleo-Environmental History of San Francisco Bay Estuary Using Stable Carbon Isotopes and Pollen

人们对了解环境变化过程的兴趣日益浓厚，这突出表明需要更深入地了解关键地区和地方不同自然系统和人类系统之间的复杂相互作用。这个博士论文研究项目旨在通过稳定的碳同位素、化石花粉、沉积物分析和碳14定年，产生旧金山湾河口环境变化的历史，时间跨度为2000至3000年。本研究有两个不同但相互关联的主要目标：(1)阐明海湾河口环境变化的历史；(2)检验和完善基于现代分布的植被对环境变化的响应模型。旧金山河口沼泽植物种类的空间分布主要由盐度决定，其他物理和生物因素的重要性较低。沿着盐度梯度从San Pablo湾的西部边缘（最高盐度）到Suisun湾的东部边缘（接近淡水条件），从四个沼泽地点选择了一系列沉积物岩心。沼泽沉积物中有机碳的碳同位素组成（13C/12C）是源植物的直接反映，随着时间的推移，由于分解而发生微小变化。盐沼禾草（Distichlis spicata）和叶米草（Spartina foliosa）的光合作用均为c4型，具有不同的碳同位素特征。在沉积物记录中区分这些C4草种的能力是对花粉分析的重要补充，因为草花粉在科水平以下是难以区分的。本研究的记录将与其他已发表的河口古盐度记录进行比较，这些记录揭示了全新世晚期水盐度（相对于现在）的交替增减期。这种比较将确定特定地点植物物种的时间分布与现代空间分布的相似程度。然后，沼泽植被变化的记录可以用来扩大环境变化的记录。本研究对两个湿地的初步结果表明，植物组合对过去河口盐度的变化有一致的反应。这项研究的一个重要部分将是确定什么自然过程（包括构造活动、海平面上升或气候变化）是导致沉积物记录中检测到的变化的主要原因。将采用从几个地点建立高分辨率年表的策略。这些地点必须根据构造断层仔细选择，不仅要根据植被变化的时间进行比较，还要根据沉积速率进行比较。该方法的组成部分将是沉积物岩心的综合测年，采用放射性碳测年和铅-210测年。这个项目将产生重要和相关的记录，因为目前关于旧金山湾河口长期淡水流入率、盐度变化和植被反应的知识有限。早期发表的关于河口古盐度的研究在年代上是不完整的，并且没有包括植被的响应。这些方法在旧金山河口的成功应用可以推广到其他河口环境。作为博士论文研究改进奖，该奖项还将提供支持，使有前途的学生建立一个强大的独立研究生涯。

Growing interest in understanding the processes of environmental change have highlighted the need for deeper understanding of complex interactions among different natural and human systems in critical regions and localities. This doctoral dissertation research project aims to produce a history of environmental change in the San Francisco Bay Estuary using stable carbon isotopes, fossil pollen, sediment analysis and carbon-14 dating spanning the last 2,000 to 3,000 years. This research has two distinct but interrelated primary objectives: (1) to elucidate the history of environmental change in the Bay Estuary; and (2) to test and elaborate models of vegetation responses to environmental change which are based on modern distributions. The spatial distribution of marsh plant species in the San Francisco Estuary is primarily determined by salinity, with other physical and biological factors of lesser importance. A series of sediment cores have been recovered from four marsh sites selected along a salinity gradient extending from the western edge of San Pablo Bay (highest salinity) to the eastern edge of Suisun Bay (near fresh water conditions). The carbon isotopic composition (13C/12C) of organic carbon in marsh sediments is a direct reflection of the source plants, with only slight changes over time due to decomposition. The two salt marsh grasses, Distichlis spicata and Spartina foliosa, operate with the C4-type photosynthetic pathway, with distinct carbon isotope signatures. The ability to distinguish these C4 grass species in the sediment record is an important complement to pollen analysis, as grass pollen is indistinguishable below the family level. Records produced in this research will be compared with other published records of paleosalinity in the estuary, which have revealed alternating periods of increased and decreased water salinity (relative to the present) during the late Holocene. This comparison will ascertain the degree to which temporal distributions of plant species at specific sites are analogous to modern spatial distributions. The record of vegetation change from marshes can then be used to expand the record of environmental change. Preliminary results from two marsh sites in this research indicate that the plant assemblages have responded consistently to past changes in estuarine salinity. An important part of this research will be to determine what natural process (including tectonic activity, sea level rise or climate variation) was dominant in causing changes detected in the sediment records. A strategy that establishes high-resolution chronologies from several sites will be used. The sites must be carefully selected relative to tectonic faults and compared not only for timing of vegetation changes, but also for sedimentation rates. Integral to this approach will be comprehensive dating of the sediment cores, employing both radiocarbon dating and lead-210 dating.This project will result in records that are important and relevant because present knowledge about rates of long-term fresh water inflow, salinity variability, and vegetation responses in the San Francisco Bay Estuary is limited. Earlier published studies of Estuarine paleosalinity are chronologically incomplete and do not include vegetation responses. Successful application of combining these methods in the San Francisco Estuary can be transferred to other estuarine settings. As a Doctoral Dissertation Research Improvement award, this award also will provide support to enable a promising student to establish a strong independent research career.