多種類指標重建全新世古環境變化: 以台南科學園區為例

Abstract

自全新世以來，台灣西南部的海岸線快速地變遷，廣闊的嘉南平原所保存的沉積物不僅紀錄了環境變遷的歷史，甚至可以在其東側的台南科學園區一帶發掘出距今4800年以來豐富的人類活動。而了解人類與環境相互作用之間複雜的關係對於幫助人類社會應對當前氣候變化的影響至關重要。本研究使用鑽探於台南科學園區西側的五根沉積物岩芯（50-100米，回收率>95%）與多種類沉積物-地球化學指標來建立早全新世以來台南科學園區一帶的環境變遷紀錄，透過加速器質譜儀碳十四定年建立年代框架後，進行1毫米解析度的X光螢光分析（X-ray fluorescence, XRF）為岩芯提供高解析度的元素組成資訊，並配合岩相分析來區分不同的沉積環境。此外，岩芯沉積物中所測得的總有機碳(total organic carbon, TOC)、總氮(total nitrogen, TN)、碳酸鈣含量與粒徑分析也同時一起進行後續的討論。 我們發現Log(Ca/Ti)可以作為指示海相沉積環境的代用指標，與總無機碳(碳酸鈣含量)呈現良好的相關性（r = 0.64, p < 0.0001）。而Log(Zr/Rb)則是與沉積物粒徑組成有著良好的相關性。其中，泥（r = -0.69, p < 0.0001）和粉砂（r = -0.72, p < 0.0001）有著顯著的負相關性，另外與砂（r = 0.76, p < 0.0001）也有著強烈的正相關。其中在五根岩芯中皆出現了Log(Ca/Ti)的平穩的訊號，指示出了一段從9300到7300年前的穩定海相沉積環境，推測為可能代表早全新世海進時期的溢浪三角洲相。除此之外，較高的TOC/TN也可以被發現在富含有機質的細顆粒泥質沉積物中，提供判斷沼澤相很好的依據。 早全新世以來的古河口-濱海沉積環境可以被此研究中的多種類指標健全地重建。XRF元素對數比可以以非破壞性的方式獲得建立環境指標，元素分析和雷射粒徑分析則可以得到特定層位的組成信息。因此，一個全面的沉積相判識方法可以透過與岩相相互驗證後被確立，並提供此地區後續研究可以被進行比對的參考。

Since the Holocene, the coastline of southwestern Taiwan has undergone rapid changes, and the sediment deposits preserved in the extensive Chianan Plain not only record the history of environmental changes but also reveal rich human activities from 4,800 cal BP in the Tainan Science Park region. Disentangling the complex relationship between humans and the environment is crucial for helping human society respond to the impacts of current climate change. Five long and continuous sedimentary cores (50-100 m penetration with >95% recovery rate) from the newly developed Tainan science park region have been retrieved in this study. We aim to apply multi-proxy methods to provide comprehensive environmental information in Tainan science park region since the early Holocene. The 1-millimeter resolution X-ray fluorescence (XRF) element profile offered enriched compositional information in the sediment cores that can support the lithofacies analysis to distinguish different sedimentary environments. Besides, the sedimentary composition of total organic carbon (TOC), total nitrogen (TN), and grain size analyses are applied with a well-established age model based on accelerator mass spectrometry radiocarbon dating. We find Ca/Ti ratio can serve as an indicator of the marine environment, which also has a positive correlation with the total inorganic carbon, which can represent the calcium carbonate content (r = 0.64, p < 0.0001). While the Zr/Rb proxy is linked with the variation of grain sizes, showing a significant negative correlation with clay (r = -0.69, p < 0.0001), silt (r = -0.72, p < 0.0001), and a positive correlation with sand (r = 0.76, p < 0.0001). Relatively constant Ca/Ti ratio in these five cores suggests a stable marine sediment supply during 9.3-7.3 kyr BP, which is inferred to represent washover fan facies during a transgressive phase. Furthermore, higher TOC/TN ratios are found in organic-rich fine-grained mud sediments, providing a reliable indicator for identifying marsh and lagoon facies. The early Holocene paleo-estuarine to nearshore depositional environment can be robustly reconstructed using the multi-proxy employed in this study. XRF logarithm ratios offer non-destructive method to establish environmental proxy, while elemental and grain size analyses provide compositional information for specific intervals. Therefore, a comprehensive sedimentary facies identification method can be established through cross-validation with lithofacies data, offering a comparable reference for other research in this region.