Cenomanian benthic foraminifera of Demerara Rise

This study is based on Cenomanian sediments of Ocean Drilling Program (ODP) Sites 1258 and 1260 from Demerara Rise (Leg 207, western tropical Atlantic, off Suriname, ~1000 and ~500 m paleo-water depth, respectively). Studied sediments consist of laminated black shales with TOC values between 3 and 18% and include the Mid Cenomanian Event (MCE), a positive carbon isotope excursion predating the well-known Oceanic Anoxic Event 2 (OAE 2). Benthic foraminiferal assemblages of the continuously eutrophic environment at Demerara Rise are characterized by low diversities (<= 9 species per sample) and large fluctuations in abundances, indicating oxygen depletion and varying organic matter fluxes. Dominant species at both sites are Bolivina anambra, Gabonita levis, Gavelinella dakotensis, Neobulimina albertensis, Praebulimina prolixa, and Tappanina cf. laciniosa. Benthic foraminiferal assemblages across the MCE show a threefold pattern: (1) stable ecological conditions below the MCE interval indicated by relatively high oxygenation and fluctuating organic matter flux, (2) decreasing oxygenation and/or higher organic matter flux during the MCE with decreasing benthic foraminiferal numbers and diversities (Site 1258) and a dominance of opportunistic species (Site 1260), and (3) anoxic to slightly dysoxic bottom-water conditions above the MCE as indicated by very low diversities and abundances or even the absence of benthic foraminifera. Slightly dysoxic conditions prevailed until OAE 2 at Demerara Rise. A comparison with other Atlantic Ocean and Tethyan sections indicates that the MCE reflects a paleoceanographic turning point towards lower bottom-water oxygenation, at least in the proto-North Atlantic Ocean and in the Tethyan and Boreal Realms. This general trend towards lower oxygenation of bottom waters across the MCE is accompanied by ongoing climate warming in combination with rising sea-level and the development of vast shallow epicontinental seas during the Middle and Late Cenomanian. These changes are proposed to have favoured the formation of warm and saline waters that may have contributed to intermediate- and deep-water masses at least in the restricted proto-North Atlantic and Tethyan Ocean basins, poor oxygenation of the Late Cenomanian sediments, and the changes in benthic foraminiferal assemblages across the MCE.

本研究基于大洋钻探计划（Ocean Drilling Program, ODP）1258和1260站位的森诺曼期沉积物，这些站位位于德梅拉拉海隆（Demerara Rise，第207航次），地处热带大西洋西部、苏里南外海，古水深分别约为1000米和500米。研究沉积物为纹层状黑色页岩，总有机碳（Total Organic Carbon, TOC）含量介于3%至18%之间，包含中森诺曼事件（Mid Cenomanian Event, MCE）——这是一次早于著名大洋缺氧事件2（Oceanic Anoxic Event 2, OAE 2）的正碳同位素漂移。德梅拉拉海隆持续富营养化环境中的底栖有孔虫组合具有低多样性（每个样品≤9种）和丰度大幅波动的特征，指示缺氧状态与有机质通量的变化。两个站位的优势种包括Bolivina anambra、Gabonita levis、Gavelinella dakotensis、Neobulimina albertensis、Praebulimina prolixa及Tappanina cf. laciniosa。MCE期间的底栖有孔虫组合呈现三阶段模式：（1）MCE区间以下的生态条件稳定，指示底层水含氧较高且有机质通量波动；（2）MCE期间底层水含氧降低和/或有机质通量增加，导致底栖有孔虫数量与多样性减少（1258站位），且机会种占主导（1260站位）；（3）MCE区间以上的底层水处于缺氧至轻度低氧状态，表现为有孔虫多样性与丰度极低，甚至完全缺失。轻度低氧条件在德梅拉拉海隆持续至OAE 2发生。与大西洋其他区域及特提斯剖面的对比表明，MCE至少在原始北大西洋、特提斯域及北方域中，反映了古海洋学向底层水含氧降低转变的转折点。MCE期间底层水含氧降低的总体趋势，伴随着中晚森诺曼期持续的气候变暖、海平面上升及广阔浅海陆架海的形成。这些变化被认为有利于暖咸水的生成，此类水体可能至少在受限的原始北大西洋与特提斯海盆中，为中层及深层水团的形成做出贡献，同时导致晚森诺曼期沉积物缺氧，以及MCE期间底栖有孔虫组合的变化。