Comparing sediment accumulation below the unproductive South Pacific Gyre (IODP expedition 329) with the highly productive central equatorial Pacific

IODP探险队329在南太平洋环流（SPG）中心沿两条横断面的七个地点对深海沉积物进行了取样。在这次探险之前，SPG包含了从未用科学海洋钻探探索过的最大部分海底。因此，这次考察和陆上巡航后研究将在广阔的前沿推进科学认识。远征329号的主要目的是记录低生产力海洋中心沉积物中微生物生命的范围和性质。SPG有时被描述为地球上最大的海洋沙漠。它的中心比任何其他环流中心离大陆更远。该环流的表面叶绿素浓度和初级生产力低于世界海洋的其他区域。在这个纬度，风沙最少。平均沉积速率是地球上最低的，比赤道太平洋的速率低1-2个数量级。因此，SPG是一个理想的区域，用于探索在一个开放海洋环流的极低活动中心的沉积性质。这项研究将比较和对比非生产性南太平洋环流下的沉积物积累与高产的中赤道太平洋的积累。不同的物理环境以及与SPG以下粘土为主的沉积物相关的非常不同的颗粒组成和大小应该导致有趣的差异。大多数深海沉积研究都集中在相对靠近海岸和在主要上升流区之下的地点，那里的生物生产力和流向海底的有机通量一般都很高。与对大陆边缘和斜坡的影响相比，对弱底流和相关深水过程对海洋盆地沉积的影响所知甚少。SPG下方沉积过程的记录需要进行一系列的分析。我将使用地震研究和现场调查数据（克鲁斯KNOX-02RR，在R/V Revelle上），我将执行SediGraph粒度分析。颗粒大小的变化可以揭示深海沉积物的许多方面，包括沉积条件，并可以提供相对流速的指标。来自这个偏远地区的粒度数据也可以提供关于风成风输入和过去大气环流的重要信息。此外，颗粒尺寸的测量将为329远征队的基本目标提供有价值的信息。特别是，它将有助于确定最缺乏有机物的沉积物中的海底生命是否在很大程度上由水的原位辐射分解（水的分子解离，H2O，由放射性衰变产生的高能α粒子引起）所产生的H2所滋养。深海沉积物中的大多数海底生物都是由有机物降解的产物维持的，因此这将是一个重要的发现。放射性溶解速率将由船上的科学人员根据放射性元素浓度来确定，他们需要粒度数据，因为粒度影响水的放射性溶解速率。由于富含粘土的沉积物比其他深海沉积物含有更高浓度的放射性元素，并且在细粒沉积物中，大多数α和β的产生发生在孔隙水的显著范围内，因此预计这些深海粘土比其他沉积环境产生更高的水放射性溶解速率。由于这些沉积物的独特性质，特别是它们的有机物含量非常低，我们还将进行一项试点研究，以确定使用特定生物标志物（即化石分子）重建未来古海洋学和古气候的潜力。这项研究将加深我们对深海沉积物如何积聚的理解。迄今为止，对远洋沉积物进行精确粒度分析的报道相对较少。

IODP Expedition 329 cored deep-sea sediments at seven sites along two transects in the center of the South Pacific Gyre (SPG). Before this expedition, the SPG contained the largest portion of the seafloor that was never explored with scientific ocean drilling. Therefore, this expedition and the onshore post-cruise studies will advance scientific understanding across a broad front. The primary purpose of Expedition 329 is to document the extent and nature of microbial life in the sediments beneath the low-productivity heart of the ocean.The SPG is sometimes described as Earth's largest oceanic desert. Its center is farther from continents than the center of any other gyre. Surface chlorophyll concentrations and primary productivity are lower in this gyre than in other regions of the world ocean. Wind-blown dust is at a minimum at this latitude. Mean sedimentation rates are among the lowest that occur on Earth, 1-2 orders of magnitude lower than rates in the equatorial Pacific. The SPG is thus an ideal region for exploring the nature of sedimentation in the extreme low-activity centre of an open-ocean gyre. This research will compare and contrast sediment accumulation below the unproductive South Pacific Gyre with accumulation in the highly productive central equatorial Pacific. The different physical environment as well as the very different particle compositions and sizes associated with clay-dominated deposits below the SPG should lead to interesting differences. The majority of deep-sea sedimentation studies have focused on sites relatively close to shore and beneath major upwelling zones, where biological productivity and organic flux to the seafloor are generally high. Little is known about the effects of weak bottom currents and related deep-water processes on sedimentation in ocean basins compared with their effects on continental margins and slopes.Documentation of sedimentation processes below the SPG will require a range of analyses. I will use the seismic studies and site survey data (Cruise KNOX-02RR, aboard the R/V Revelle) and I will perform SediGraph grain size analyses. Variations in particle size can shed light on many facets of deep-sea sediments including depositional conditions and can provide indicators of relative flow speed. Grain size data from this remote location can also provide crucial information on eolian inputs and past atmospheric circulation.In addition, the grain size measurements will contribute valuable information to the fundamental objectives of Expedition 329. In particular, it will help determine whether subseafloor life in the most organic-poor sediment is nourished to a significant extent by H2 from in-situ radiolysis of water (the molecular dissociation of water, H2O, caused by energetic alpha-particles produced by radioactive decay). Most subseafloor life in deep-sea sediments is maintained by products of organic matter degradation so this would be an important finding. Radiolysis rates will be determined from radioactive element concentrations by the scientific shipboard party, who require grain size data because particle size influences the rate of water radiolysis. As clay-rich sediment contains much higher concentrations of radioactive elements than other deep-sea sediment and, in fine-grained sediment, most alpha and beta production occurs within striking range of pore water, these deep-sea clays are anticipated to yield much higher rates of water radiolysis than other sedimentary environments.Due to the unique nature of these sediments, in particular their very low organic matter content, we will also conduct a pilot study to determine the potential for future paleoceanographic and paleoclimatic reconstructions using specific biomarkers (i.e. fossil molecules).This research will deepen our understanding of how pelagic sediments accumulate in the deep ocean. Relatively few precise grain size analyses of pelagic sediments have been reported so far.