Diatom abundances in sediment core CESAR_83-006

The modern Arctic Ocean is regarded as a barometer of global change and amplifier of global warming (Graversen et al., 2008, doi:10.1038/nature06502) and therefore records of past Arctic change are critical for palaeoclimate reconstruction. Little is known of the state of the Arctic Ocean in the greenhouse period of the Late Cretaceous epoch (65-99 million years ago), yet records from such times may yield important clues to Arctic Ocean behaviour in near-future warmer climates. Here we present a seasonally resolved Cretaceous sedimentary record from the Alpha ridge of the Arctic Ocean. This palaeo-sediment trap provides new insight into the workings of the Cretaceous marine biological carbon pump. Seasonal primary production was dominated by diatom algae but was not related to upwelling as was previously hypothesized (Kitchell and Clark, 1982, doi:10.1016/0031-0182(82)90087-6). Rather, production occurred within a stratified water column, involving specially adapted species in blooms resembling those of the modern North Pacific subtropical gyre (Dore et al., 2008, doi:10.1016/j.pocean.2007.10.002), or those indicated for the Mediterranean sapropels (Kemp et al., 1999, doi:10.1038/18001). With increased CO2 levels and warming currently driving increased stratification in the global ocean (Sarmiento et al., 1998, doi:10.1038/30455), this style of production that is adapted to stratification may become more widespread. Our evidence for seasonal diatom production and flux testify to an ice-free summer, but thin accumulations of terrigenous sediment within the diatom ooze are consistent with the presence of intermittent sea ice in the winter, supporting a wide body of evidence for low temperatures in the Late Cretaceous Arctic Ocean (Falcon-Lang et al., 2004, doi:10.1016/j.palaeo.2004.05.016; Amiot et al., 2004, doi:10.1016/j.epsl.2004.07.015; Otto-Bliesner et al., 2002, doi:10.1029/2001JD000821), rather than recent suggestions of a 15 °C mean annual temperature at this time (Jenkyns et al., 2004, doi:10.1038/nature03143).

现代北冰洋被视作全球变化的晴雨表与全球变暖的放大器（Graversen等，2008，doi:10.1038/nature06502），因此过往北极变化的相关记录对于古气候重建（palaeoclimate reconstruction）至关重要。目前人们对晚白垩世（6500万至9900万年前）温室期北冰洋的状态知之甚少，但该时期的沉积记录或可为未来近暖期的北冰洋演化模式提供关键线索。本研究首次公布了来自北冰洋阿尔法海岭（Alpha ridge）的白垩纪季节性分辨率沉积记录。该古沉积物捕集器（palaeo-sediment trap）为白垩纪海洋生物碳泵的运作机制提供了全新视角。研究发现，该区域季节性初级生产以硅藻（diatom algae）为主，但并非如先前假说（Kitchell与Clark，1982，doi:10.1016/0031-0182(82)90087-6）所提出的那样与上升流活动相关。相反，初级生产发生在分层水体中，涉及的特化物种水华与现代北太平洋副热带环流的水华特征相似，或与地中海腐泥层（sapropels）所指示的生态模式一致（Dore等，2008，doi:10.1016/j.pocean.2007.10.002；Kemp等，1999，doi:10.1038/18001）。当前，二氧化碳浓度升高与全球变暖正加剧全球海洋的水体分层（Sarmiento等，1998，doi:10.1038/30455），这种适应分层水体的生产模式或愈发普遍。我们通过季节性硅藻生产与通量的相关证据，证实了当时夏季无海冰的状态；而硅藻软泥中陆源沉积物的薄堆积层，则与冬季存在间歇性海冰的情况相符，这一结论支持了晚白垩世北冰洋温度偏低的大量相关证据（Falcon-Lang等，2004，doi:10.1016/j.palaeo.2004.05.016；Amiot等，2004，doi:10.1016/j.epsl.2004.07.015；Otto-Bliesner等，2002，doi:10.1029/2001JD000821），而非近期提出的当时年均温为15℃的观点（Jenkyns等，2004，doi:10.1038/nature03143）。