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).现代北冰洋被视为全球变化的晴雨表与全球变暖的放大器（格拉弗森等，2008，doi:10.1038/nature06502），因此过去北极气候变化记录对于古气候重建（palaeoclimate reconstruction）至关重要。目前对白垩纪晚期（6500万至9900万年前）温室期北冰洋的状态所知甚少，但该时期的沉积记录或可为理解未来近暖期的北冰洋演化行为提供关键线索。本文报道了采自北冰洋阿尔法脊的、具备季节分辨率的白垩纪沉积记录。该古沉积物捕集器（palaeo-sediment trap）为揭示白垩纪海洋生物碳泵（marine biological carbon pump）的运作机制提供了全新视角。其季节性初级生产以硅藻藻类为主，但并未如此前假说所述与上升流（upwelling）活动相关（基切尔与克拉克，1982，doi:10.1016/0031-0182(82)90087-6）。相反，初级生产发生于层化水体（stratified water column）之中，涉及特化适应的物种形成的藻华，其特征与现代北太平洋亚热带环流（North Pacific subtropical gyre）所记录的藻华相似（多尔等，2008，doi:10.1016/j.pocean.2007.10.002），亦与地中海腐泥层（Mediterranean sapropels）所指示的藻华特征一致（肯普等，1999，doi:10.1038/18001）。当前随着二氧化碳浓度升高与全球变暖加剧全球海洋层化（萨米恩托等，1998，doi:10.1038/30455），这种适应层化环境的生产模式可能会愈发普遍。我们关于季节性硅藻生产与通量的证据表明，当时夏季无海冰（sea ice），但硅藻软泥（diatom ooze）中产出的薄层陆源沉积物（terrigenous sediment）则与冬季间歇性存在海冰的情形相符，这一结果支持了白垩纪晚期北冰洋温度偏低的大量实证（福尔肯-朗等，2004，doi:10.1016/j.palaeo.2004.05.016；阿米奥等，2004，doi:10.1016/j.epsl.2004.07.015；奥托-布利斯纳等，2002，doi:10.1029/2001JD000821），而非近期提出的当时年均温达15℃的观点（詹金斯等，2004，doi:10.1038/nature03143）。