北極海中部古海洋學變化： 來自生痕化石和生物標記代理指標之見解

Abstract

北極海冰在調節地球的能量收支、海氣氣體交換、海洋生產力以及全球溫鹽環流中扮演著重要角色。研究其演變及其與環境參數在地質時間尺度上的交互作用是關鍵的研究領域。要理解北極海冰的變化，需要發展新型代理指標並改進傳統指標，因為現有方法在北極地區仍面臨挑戰且理解有限。本論文通過兩項重點研究，探討了北極地區晚更新世和全新世期間海冰條件的變化及其對有機質（OM）分布和產量的影響。 第一項研究利用新穎的行為遺跡指數（Ethological Ichno Quotient, EIQ），檢測晚更新世海冰範圍變化下，底棲生物挖掘行為（行為組）的變化，通過分析岩芯中生痕化石及生物擾動的變化來驗證假設。研究發現，在海冰覆蓋減少的時期，沉積物攝食型生痕增多，而在厚實的多年生海冰下，化能共生型遺跡生痕占主導地位。這些發現證實了耶爾馬克高原（Yermak Plateau）在冰期間存在更多開放水域條件的觀點，並突顯了生物擾動在不同海冰條件下連接底棲與浮游生態系統中的作用。 北極海冰分布還影響從邊緣海域到北極中心有機質和沉積物的運輸。生物標記為研究有機質運輸和分布有前景的代理指標，但在最後一個冰期循環中，其時空變化及來源在北極中心仍不明確。為解決這一問題，第二項研究集中於建立生物標記代理指標並探討其在北極中心的分布及來源。在此研究中，對北極中心的九個岩芯進行了多種代理指標分析，包括總有機碳（TOC）、正烷烴（n-alkane）及甘油二烷基甘油四醚脂（GDGTs）。研究結果顯示，用於確定北極中心支鏈GDGT (brGDGTs)來源的兩個指數得出了相反的結果：MIS 3–1期間升高的IIIa/IIa值表明海洋來源，而#ringstetra比率則指向陸源。然而，由於IIIa/IIa比率對pH變化的敏感性更高，在北極中心地區可能是更可靠的指標，可以準確反映海洋環境中brGDGT的產生。TOC和生物標記濃度在靠近西伯利亞大陸棚的中央羅蒙諾索夫脊（CLR）岩芯中最高，隨著距離大陸棚越遠而逐漸減少。這些空間模式結合北極中心低生產力，表明海洋和陸源生物標記通過海冰漂移從西伯利亞大陸棚運輸至北極中心，該過程在MIS 3–1期間持續，但在冰期中強度有所減弱。此外，冰期中北極中心的高BIT指數值可能反映了大陸棚上crenarchaeol和brGDGT生產的變化和/或crenarchaeol在運輸過程中的選擇性降解。 總體而言，本論文通過改進傳統代理指標和開發新指標，增進了對晚更新世-全新世北極中心古海洋變化的研究，提供了關於過去海冰變化及其時間演化的寶貴見解，並克服了北極代理指標發展中的挑戰。

Arctic sea ice is a critical component in regulating the earth's energy balance, ocean-air gas exchange, marine productivity and the global thermohaline circulation. The evolution and interactions of the Arctic sea ice with environmental parameters over geological timescales are key areas of research. Understanding this Artic sea ice variability requires developing novel proxies and improving traditional ones, as existing methods in the Arctic remain challenging and poorly understood. This thesis explores the variability of sea ice conditions and their influence on the distribution and production of organic matter (OM) in the Arctic during the late Pleistocene and Holocene, explored through two focused studies. The first study examined downcore trace fossils and bioturbation variation using the novel Ethological Ichno Quotient (EIQ) to evaluate the hypothesis that different activities of burrowing benthic organisms (ethological groups) vary in response to the changes in the sea ice extent during the late Pleistocene. The findings revealed an increase in deposit-feeding traces during periods of decreased sea ice cover and a dominance of chemosymbiotic traces under thick perennial sea ice. These findings support the idea of enhanced open-water conditions on the Yermak Plateau during glacial periods and highlight the role of bioturbation in linking benthic and pelagic ecosystems under varying sea ice regimes. Arctic sea ice distribution also impacts the transport of sediment and OM from marginal seas to the central Arctic. Biomarkers offer a promising proxy to study OM transport and distribution, but their spatio-temporal variation and sources in the central Arctic during the last glacial cycle remain unclear. To address this, the second study focused on establishing biomarker proxies and understanding their distribution and sources in the central Arctic. In this study, a multiproxy analysis, including total organic carbon (TOC), glycerol dialkyl glycerol tetraethers (GDGTs) and n-alkanes was conducted on nine sediment cores collected from the central Arctic region. Two indices used for determining the source of branched GDGT (brGDGTs) in the central Arctic yielded contrasting results: elevated IIIa/IIa values during MIS 3–1 suggest a marine origin, while #ringstetra ratios indicate a terrigenous source. However, the IIIa/IIa ratio may be more reliable in the central Arctic due to its better sensitivity to pH changes, making it a stronger indicator of brGDGT production in the marine environment. TOC and biomarker concentrations were highest in the Central Lomonosov Ridge (CLR) cores, closer to the Siberian shelves, and decreased moving further from the shelves. These spatial patterns, coupled with the reduced productivity in the central Arctic, suggest that both marine- and terrestrial-derived biomarkers were transported via sea ice drift from the Siberian shelves, a process that persisted through MIS 3–1, albeit with reduced intensity during glacial periods. Additionally, elevated Branched Isoprenoid Tetraether (BIT) index values in the central Arctic during glacial periods likely reflect changes in brGDGT and crenarchaeol production along the shelves and/or the preferential breakdown of crenarchaeol during the transport. Overall, this thesis advances the study of paleoceanographic changes in the central Arctic by improving our understanding of sea ice variability during the late Pleistocene–Holocene and contributing to the refinement of traditional proxies as well as the development of new ones. It addresses the challenges of proxy development in the Arctic, providing valuable insights into past sea ice variability and its temporal evolution.