台西南深海底栖有孔虫及其5万年来冷泉微生境变迁记录

为探讨冷泉区底栖有孔虫组合特征、受控因素及其冷泉微生境随时间的变迁,本文对台西南海盆取自九龙甲烷礁和海洋四号区冷泉区的973-4和973-5两根岩心展开了底栖有孔虫及其壳体氧碳同位素研究。测年结果揭示两根岩心为晚更新世约5万年来海洋氧同位素期(MIS)MIS 3至MIS 1早期的沉积序列。两个站位共识别了底栖有孔虫79属233种,优势类别在973-4组合中为Uvigerina(23.3%)、Bulimina(10.71%)和Cibicidoides(9.87%),在973-5组合中是Bulimina(20.6%),两站位的组合优势和常见属种均以内生类别为主。有孔虫分异度显然同时受到正常深海环境因子TOC和沉积物粒度的影响。总体上,优势和常见类群与TOC相关性较弱,但与δ¹⁸O_{Uvigerina spp.}有不同程度的相关性,说明有孔虫还受冷泉特殊营养物质和流体因子影响。5万年来,973-4和973-5站位底栖有孔虫组合生活的冷泉微生境,经历了由双壳-自生碳酸盐岩(MIS 3至MIS 2早期)向双壳-菌席(MIS 2晚期至MIS 1早期)的变迁。底栖有孔虫的优势类群也随微生境的变迁而演替,如973-4站位MIS 3-MIS 1的优势类群依次为U.peregrina、Cibicidoides-Bulimina、U.vadescens和Cibicides,973-5站位为Chilostomella+ Globobulimina、Cibicidoides、Bulimina。有孔虫壳体氧碳同位素特征也随时间改变,从MIS 3到MIS 2早期,在自生碳酸盐岩水岩交换背景下,具有富δ¹⁸O和亏损δ¹³C特点(3.5‰~4.49‰,-2‰~-0.2‰); MIS 2晚期—MIS 1早期因双壳和菌席的生物地球化学作用影响,具有略富集δ¹⁸O和略微亏损δ¹³C的特征(2.5‰~3.5‰,-1‰~-0.1‰)。自5万年来两个区甲烷渗漏逐渐减弱,其间发生了几次增强事件。973-4站位记录了1次持续时间约10 ka的增强事件(35~25 ka);973-5站位记录3次(45 ka,35 ka,14~12 ka)。其中,45 ka时海底上涌的甲烷通量可能最大,在海底表面形成水合物。35 ka时的甲烷喷溢增强事件可能为区域性事件。

Deep sea benthic foraminifera from the Taixinan Basin and changes of their cold seep microhabitats during the past 50000 years

Benthic foraminifera and their shell oxygen and carbon isotopes from Core 973-4, located on the slope of the Jiulong Methane Reefs cold seep area, and Core 973-5 on the edge of the Haiyang No.4 Site in the Taixinan Basin, were studied to understand the characteristics of fauna assemblages, their ecological factors, and the change in microhabitats with time. The ages of sediments from the two cores were determined to be 50 ka, which were assigned to the late Pleistocene Marine Oxygen Isotope Stage MIS 3 to early MIS 1. In total, 233 species of 79 genera were identified for the benthic faunas from the two cores. The dominant taxa from Core 973-4 were Uvigerina (23.3%),Bulimina (10.71%) and Cibicidoides (9.87%), while from Core 973-5 was Bulimina (20.6%). Generally, these two faunas were dominated by infauna taxa. Foraminiferal diversity was influenced by TOC and sediment grain sizes. A weak correlation between dominant (or common) taxa with TOC and varying degree of correlation with δ¹⁸O_{Uvigerina spp.} indicated that foraminifera taxa were also affected by other special nutrition and fluids in the cold seep areas. Over the last 50 ka, the microhabitats of benthic foraminiferal assemblages from Cores 973-4 and 973-5 changed from bivalve shell debris-authigenic carbonates (MIS 3 to early MIS 2) to bivalve-bacterial mats (late MIS 2 to early MIS 1). Correspondingly, from MIS 3 to MIS 1, the dominant taxa of foraminiferal assemblages from Core 973-4 changed from U.peregrina to Cibicidoides-Bulimina then to U.vadescens-Cibicides and from Core 973-5 changed from Chilostomella+Globobulimina to Cibicidoides then to Bulimina. Oxygen and carbon isotopic profiles of foraminiferal shells also changed with time, showing enriched δ¹⁸O (3.5‰-4.49‰) and depleted δ¹³C(-2.0‰ to -0.2‰), as results of exchanging ambient pore fluids with authigenic carbonates during the period of MIS 3 to early MIS 2 and biogeochemical processes in the microhabitats of bivalve and bacterial mats during the time of late MIS 2 to early MIS 1. Generally, the activity of methane seeping decreased gradually during the past 50 ka in these two cold seep regions. Several enhanced seeping events during this period were indicated by abnormal δ¹⁸O and δ ¹³C values. One persistently enhanced seeping event, lasted about 10 kyr (35-25 ka), was seen in Core 973-4. In Core 973-5, three short events (at 45 ka, 35 ka, and 14-12 ka) were found, among them the 45 ka event may be the strongest one as methane flux reached near the surface of the sea floor to form gas hydrates near the seafloor, where as the 35 ka event was inferred as a regional one.