快速造山作用對氣候訊號保存之影響

Abstract

本研究使用多種類代用指標，如黏土礦物學、有機碳穩定碳同位素、碳氮比以及磁感率分析台灣西部前陸盆地中新世晚期至上新世早期之桂竹林層樣本，分析結果指出隨著台灣島抬升初期，在中-上新世交界時已成為西部前陸盆地之重要沉積物來源，比過去研究認定之結果提早了約二百萬年。至上新世早期，台灣島成為西部前陸盆地最主要的沉積物供應來源，快速的沉積速率加上不斷增加的盆地容納空間為其淺海相地層中氣候訊號的保存提供了理想的條件。透過桂竹林層伽馬射線記錄的時序分析證實了岩層中的軌道力氣候循環紀錄的存在，並指出快速隆起的造山帶演化造成不同軌道週期的紀錄保存的差異。台灣島的持續成長導致沉積物通量增加、盆地容納量增加以及半封閉海峽的形成，以上狀況皆有助於增強歲差驅動之水文氣候變化紀錄的保存。台灣島的演變亦影響了南中國海的東亞夏季季風代用指標紀錄，對比西部前陸盆地及東亞夏季季風的各種代用指標，包含伽瑪射線、有機碳穩定碳同位素、赤鐵礦與針鐵礦比例及磁感率記錄，可發現台灣島的抬升與逐漸向西南遷移與呂宋島向西北遷移的狀況導致了西南向海流將沉積物從台灣島輸送至南中國海，並在一千公里之外的紀錄中顯示赤鐵礦/針鐵礦比值下降。總體而言，這項研究證實了淺海沉積紀錄可用於解決軌道力氣候振盪之謎團，並展示了不同軌道週期訊號如何在造山運動的不同階段被紀錄在淺海沉積紀錄中。此外，本研究並指出與氣候無關的控制變因有可能產生「錯誤」的氣候特徵，例如構造及地球動力驅動的沉積物輸送機制和沉積物來源的變化必須予以考慮，以便正確解讀古氣候記錄。

Multi-proxy analyses of clay mineralogy, δ13Corg and C/N of organic matter, and mass-specific magnetic susceptibility of late Miocene–early Pliocene Kueichulin Fm of the Taiwan Western Foreland Basin (WFB) indicate that Taiwan became a major sediment source to the WFB near the Miocene-Pliocene transition, with the initial emergence of Taiwan, about two million years earlier than previously recognized. By the early Pliocene, Taiwan became the overwhelmingly dominant source of sediment to the WFB. This rapid sedimentation rate coupled with increasing basin accommodation space provided ideal conditions for the preservation of climate signals in its shallow-marine strata. Time-series analysis of gamma-ray records through the Kueichulin Fm confirmed the presence of orbital climate cycles imprinted in the rock record and show that the evolution of the rapidly uplifting orogen influenced the preservation of different orbital frequencies. The growth of Taiwan resulted in elevated sediment flux, increased basin accommodation, and the formation of a semi-sheltered strait, all of which served to enhance preservation of precession-driven hydroclimate variability. The evolution of Taiwan also influenced East Asian Summer Monsoon (EASM) proxy records in the South China Sea. Comparison of time-equivalent gamma-ray, δ13Corg, hematite/goethite, and magnetic susceptibility records from the WFB and EASM proxy records shows that the uplift and southwest migration of Taiwan and northwest migration of Luzon resulted in the formation of southwest-flowing currents that transported sediment from Taiwan towards the South China Sea, which is recorded as a decline in hematite/goethite values in proxy records >1000 km away. Overall, the findings of this study confirm that shallow-marine sedimentary archives can be used for resolving orbital climate oscillations, and demonstrates how different orbital signals are imprinted in the shallow-marine record at different stages of orogenesis. Additionally, this research also demonstrates that climate-independent controls, such as tectonic- and geodynamic-driven shifts in sediment transport mechanisms and sediment source, have the potential to produce “false” climate signatures, and must be considered in order to properly interpret paleoclimate records.