古菌脂膜脂質代用指標對沈積物中甲烷濃度的敏感性

Abstract

溫室氣體排放會造成氣候變遷，甲烷為溫室氣體的一種。在地質時間尺度上天然氣水合物分解釋放出的甲烷對全球氣候有影響。欲了解氣候變化， Zhang et al. (2011) 建立用於追蹤過去是否含有天然氣水合物解離和現存甲烷的代用指標–Methane Index (MI) 。 MI 為基於沈積物Glycerol dialkyl glycerol tetraethers (GDGTs) 相對分佈所建立，其 GDGTs 來源為古菌脂膜脂質化合物。海洋沈積物 GDGTs 主要由浮游性 Thaumarchaeota 提供，其相對分佈可做為海水溫度的代用指標，稱 TEX86 (TetraEther indeX of 86 carbon atoms)。而與甲烷相關的 Euryarchaeota 膜脂內含的 GDGTs 分佈不同於 Thaumarchaeota ，因此當 Euryarchaeota 大量存在於沈積物時會影響沈積物的 GDGTs 相對分佈從而影響 MI 。正常海洋沈積物 MI 值通常小於 0.3 ，當沈積物MI值大於 0.3 時，被認為是天然氣水合物影響環境的典型特徵。 MI 不僅用於偵測是否含有甲烷還作為控制 TEX86 的數據質量，但與甲烷濃度之間的關係尚未經過測試和量化。因此本研究選用來自熱液、冷泉、泥火山含不同甲烷濃度 (0 ~ 1650μM) 的沈積物對 MI 的可用性進行測試。結果表明海洋沈積物顯示正常海洋環境中典型 GDGTs 分佈其 MI 值小於 0.3 ，且不隨甲烷濃度 (0 ~ 400 μM) 變化；相反地，陸地泥火山甲烷範圍介於 150 ~ 1650 μM ，GDGTs 分佈特徵為 GDGT-0 ~ 2 相對較高， MI 值高於 0.7 。 進一步對比海洋與陸地沈積物的古菌群落，發現冷泉沈積物古菌 Thaumarchaeota 和 Euryarchaeota 相對含量約各半，但 GDGT-0 ~ 3 相對含量卻不比受甲烷影響區域高。另擇表層海洋沈積物進行 TEX86 計算，將重建溫度對比近數十年的海水溫度，發現重建溫度皆落在表水至水深 100 公尺的範圍內。由此表示此區域海洋沈積物中大部分的 GDGTs 皆來自浮游性 Thaumarchaeota ，而不受陸源或沈積物中嗜甲烷古菌 Euryarchaeota 製造的 GDGTs 影響。此發現與樣本的低Branched and Isoprenoid Tetraethers (BIT) 和 MI 值一致。由結果得出 MI 可以用於評估 TEX86 於海水溫度重建的可用性，但對於甲烷濃度的敏感度取決於沈積環境。

The emission of greenhouse gases such as methane can cause climate change. On geological time scale, methane released by the dissociation of gas hydrate is known to have consequences on global climate. To understand the link between methane and climate change, Zhang et al. (2011) established Methane Index (MI), a proxy used to track hydrate and methane in the past. MI is based on the GDGT distribution in marine sediments. In marine sediments, GDGTs are mainly originated from planktonic archaea Thaumarchaeota, and its relative distribution can be used as a proxy for seawater temperature, called TEX86. The GDGT profile in methane-associated Euryarchaeotal membrane lipids are different from that in Thaumarchaeotal membrane, thus when Euryarchaeota exists in large quantities in marine sediments it will affect the sedimentary GDGT distributions, hence also the MI value. MI value in normal marine sediments is typically < 0.3. Sedimentary MI value > 0.3 is considered typical of gas hydrate impacted setting. MI is used not only to detect the presence of methane but also to monitor the applicability of sedimentary TEX86 for seawater temperature reconstruction. However, the relationship between methane concentration has not been tested nor quantified. In this study, we selected sediments from diverse settings (hydrothermal, cold seep, mud volcano) on and off Taiwan, containing a range of methane concentrations (0 ~ 1650 μM) to assess the applicability of MI. The results show that the distribution of GDGTs in marine sediments off Taiwan belong to a normal marine environment. At these sites, MI values are less than 0.3 and do not change with methane concentration (0 ~ 400 μM). In contrast, GDGT-0 ~ 2 are relatively high and the MI values are higher than 0.7 for mud volcano samples with 150 to 1650 μM methane concentration. By further comparing with the archaeal community in marine and terrestrial sediments, we found that the relative abundance of Thaumarchaeota and Euryarchaeota are approximately 1:1 in cold seep sediment, but the relative content of GDGT-0~3 is not high as is typical for hydrate-impacted sediments. Further, temperatures reconstructed from TEX86 in surface marine sediments fall within the range of decadal average of seawater temperature for surface water to a depth of 100 meters. This shows that the majority of the GDGTs in the marine sediments originate from planktonic Thaumarchaeota, and are not affected by the GDGTs produced by the methanotrophic Euryarchaeota in the sediment. This finding is consistent with the low BIT and MI values in these samples. Our findings suggest that MI can be used to assess the applicability of TEX86 for seawater temperature reconstruction, but its sensitivity to methane concentration depends on the sedimentary environment.