Late Cretaceous benthic foraminiferal assemblages from Demerara Rise, western tropical Atlantic

This paper is based on Santonian–Campanian sediments of Ocean Drilling Program Sites 1257 (2951 mbsl) and 1259 (2353 mbsl) from Demerara Rise (Leg 207, western tropical Atlantic, off Surinam). According to its position, Demerara Rise should have been influenced by the early opening of the Equatorial Atlantic Gateway and the establishment of a bottom-water connection between the central and South Atlantic Oceans during the Late Cretaceous. The investigated benthic foraminiferal faunas demonstrate strong fluctuations in bottom-water oxygenation and organic-matter flux to the sea-floor. The Santonian–earliest Campanian interval is characterised by laminated black shales without benthic foraminifera in the lowermost part, followed by an increasing number of benthic foraminifera. These are indicative of anoxic to dysoxic bottom waters, high organic-matter fluxes and a position within the oxygen minimum zone. At the shallower Site 1259, benthic foraminifera occurred earlier (Santonian) than at the deeper Site 1257 (Early Campanian). This suggests that the shallower site was characterised by fluctuations in the oxygen minimum zone and that a re-oxygenation of the sea-floor started considerably earlier at shallower water-depths. We speculate that this re-oxygenation was related to the ongoing opening of the Equatorial Atlantic Gateway. A condensed glauconitic chalk interval of Early Campanian age (Nannofossil Zone CC18 of Sissingh) overlies the laminated shales at both sites. This interval contains benthic foraminiferal faunas reflecting increasing bottom-water oxygenation and reduced organic-matter flux. This glauconitic chalk is strongly condensed and contains most of the Lower and mid-Campanian. Benthic foraminiferal species indicative of well-oxygenated and more oligotrophic environments characterise the overlying mid- to Upper Campanian nannofossil chalk. During deposition of the nannofossil chalk, a permanent deep-water connection between the central and South Atlantic Oceans is proposed, leading to ventilated and well-oxygenated bottom waters. If this speculation is true, the establishment of a permanent deep-water connection between the central and South Atlantic Oceans terminated Oceanic Anoxic Event 3 “black shale” formation in the central and South Atlantic marginal basins during the Early Campanian (Nannofossil Zone CC18) and led to well-oxygenated bottom waters in the entire Atlantic Ocean during the Late Campanian (at least from Nannofossil Zone CC22 onwards).本研究基于取自热带大西洋西部苏里南外海德马拉拉海隆（Demerara Rise）大洋钻探计划（Ocean Drilling Program, ODP）第207航次的1257站位（海平面以下2951米，meters below sea level, mbsl）与1259站位（海平面以下2353米，meters below sea level, mbsl）的桑托期-坎潘期（Santonian–Campanian）沉积物。根据其所处构造位置，德马拉拉海隆在白垩纪晚期应受赤道大西洋水道（Equatorial Atlantic Gateway）初始张开以及中大西洋与南大西洋之间底层水连通建立过程的影响。本次研究涉及的底栖有孔虫（benthic foraminifera）动物群记录显示，研究区底层水含氧量与海底有机质通量均存在剧烈波动。桑托期至早坎潘期地层段以底部缺失底栖有孔虫的纹层状黑色页岩为特征，随后底栖有孔虫丰度逐渐升高，这指示当时底层水处于缺氧至贫氧状态、有机质通量较高，且站位处于氧最小值带（oxygen minimum zone）内。在水深较浅的1259站位，底栖有孔虫的首现层位（桑托期）早于水深更深的1257站位（早坎潘期），这表明较浅站位的氧最小值带存在显著波动，且浅水区海底的复氧过程启动时间显著更早。我们推测该复氧过程与赤道大西洋水道的持续张开密切相关。早坎潘期（对应Sissingh划分的超微化石带CC18，Nannofossil Zone CC18）的海绿石白垩岩（glauconitic chalk）薄互层覆盖于两处站位的纹层状页岩之上，该地层段所含底栖有孔虫动物群反映出底层水含氧量升高、有机质通量降低。这套海绿石白垩岩沉积速率极慢，涵盖了大部分下坎潘期与中坎潘期地层。覆于其上的中-上坎潘期超微化石白垩岩以指示富氧且贫营养环境的底栖有孔虫物种为特征。在超微化石白垩岩沉积时期，学界普遍认为中大西洋与南大西洋之间形成了永久性深水连通通道，这使得底层水得以充分流通并保持富氧状态。若该推测成立，则中大西洋与南大西洋之间永久性深水连通通道的建立，终止了早坎潘期（对应超微化石带CC18）期间中大西洋与南大西洋边缘盆地内的大洋缺氧事件3（Oceanic Anoxic Event 3, OAE3）"黑色页岩"沉积，并在晚坎潘期（至少自超微化石带CC22起）使得整个大西洋海域的底层水均处于富氧状态。