DEEP WESTERN BOUNDARY CURRENT VARIABILITY IN THE SUBTROPICAL NORTHWEST ATLANTIC OCEAN DURING THE PAST 450 KYR

当今环境研究中最具争议的问题之一涉及当今气候制度的稳定性，特别是人类引起的变化是否足以导致气候的重大重组。一项重要的进展是确定了最近地质历史中突然和强烈的气候变化，说明源于自然强迫的气候事件也可能在与人类相关的时间尺度上发生。人们普遍认为，这些突变的气候变化与全球表层和地下洋流系统的行为变化有关，即海洋热盐环流（THC）。为了了解THC的自然变率，有必要从关键地点产生具有高千年尺度（1000年）时间分辨率的长（10万年）连续古气候记录。然而，由于海洋环境的普遍低沉积（一般为~2-3 cm/1000年），这种具有足够时间分辨率的长连续记录并不总是可用的。只有沉积物漂流的岩心，其沉积速率通常增加到~40厘米/1000年，才有可能创造这样的记录。这些沉积物漂移是由强大的底流产生的，它侵蚀了一个地方的物质，并将其沉积在另一个地方。漂移沉积物的缺点是它们可能含有很少的生物物质，例如通常用于重建地表水性质和深水块体横向分布的有孔虫。解决这一问题的一个潜在方法是研究与深海流动有关的物理参数，例如可选泥沙平均代理（SS-mean），即10-63微米陆源沉积物亚组分的平均粒度。有孔虫稳定同位素记录可能由于缺少有孔虫而呈斑块状，相比之下，ss平均记录通常是不间断的，因为陆源沉积物是连续沉积的。此外，沉积物的分选与沉积物颗粒在其中运输和沉积的深海洋流的物理流动速度直接相关，从而可以生成贯穿过去的古洋流速度的高分辨率记录。西北大西洋亚热带布雷克-巴哈马外脊（BBOR）沉积物漂移沉积提供了一个非常高分辨率的深西部边界流（DWBC）流动变化档案。作为THC的主要组成部分，DWBC将最近通气的水域（即北大西洋深水）带到赤道。以往的研究将粒度分析与有孔虫稳定同位素分析相结合，以重建DWBC在千年至米兰科维奇时间尺度上的强度和流动特征。然而，在过去的10万年冰期-间冰期旋回中，没有关于DWBC流量变化的更详细的时间和时间的连续记录。在这里，我们建议将重点放在近底部流速和DWBC的变化上，这些变化记录在ODP Leg 172， Site 1061（水深4047米）的深水BBOR上。我们将生成高分辨率（样本间隔约800年）的过去45万年的ss -平均和有孔虫稳定同位素记录，或从海洋同位素阶段12到全新世之间的间隔。为了通过这个区间确定DWBC的主要周期性变化，我们将使用光谱分析。这些统计数据中的非平稳性，即信号在长时间内振幅和频率的变化，将使用小波分析进行研究。这些发现将进一步加深我们对深海环流变率与气候系统边界条件之间联系的理解，从而减少当前围绕气候变率预测的不确定性。

One of the most controversial issues in environmental research today relates to the stability of the present-day climate regime and, in particular, whether human induced changes are sufficient to result in a significant restructuring of climate. An important development has been the identification of abrupt and intense climate changes in the recent geological past, illustrating that climate events originating from natural forcing may also occur on the timescales of relevance to humanity. These abrupt climate shifts are widely believed to involve changes in the behaviour of the global system of surface and subsurface ocean currents, the oceanic thermohaline circulation (THC). In order to understand the natural variability of the THC it is necessary to generate from key locations long (several 100,000 years) continuous palaeoclimate records with high millennial-scale (1000 years) temporal resolution. However, such long continuous records with sufficient temporal resolution are not always available due to the generally low sedimentation in marine environments (commonly ~2-3 cm/1000 years). Only cores from sediment drifts, where sedimentation rates are often increased up to ~40 cm/1000 years, present the possibility to create such records. These sediment drifts are produced by strong bottom currents, which erode material from one location and deposit it at another. The disadvantage of drift sediments is that they may contain little biogenic material, such as foraminifera that are commonly used to reconstruct surface water properties and the lateral distribution of deep water masses. A potential way around this problem is to investigate physical parameters related to deep ocean flow, such as the sortable silt mean proxy (SS-mean), which is the mean grain size of the 10-63 micron terrigenous sediment sub-fraction. In contrast to the foraminiferal stable isotope record, which might be patchy due to the lack of foraminifera, the SS-mean record is generally uninterrupted, since terrigenous sediments are deposited continuously. Furthermore, the sorting of the sediment is directly related to the physical flow speed of the deep ocean current in which the sediment grains are transported and deposited, which allows the generation of high-resolution record of palaeocurrent speed throughout the past. Sediment drift deposits at the Blake-Bahama Outer Ridge (BBOR) in the sub-tropical Northwest Atlantic Ocean present an archive for changes in the Deep Western Boundary Current (DWBC) flow at very high resolution. As a major component of the THC, the DWBC carries recently ventilated waters (i.e. North Atlantic Deep Water) equatorward. Previous studies have utilised grain size analyses in combination with foraminiferal stable isotope analysis in order to reconstruct strength and flow characteristics of the DWBC on millennial to Milankovitch timescales. However, no continuous record yet exists of the temporal variability and timing of DWBC flow variability in greater detail over the past few 100,000 year glacial-interglacial cycles. Here we propose to focus on changes in the near bottom flow speed and hydrography of the DWBC recorded at ODP Leg 172, Site 1061 (4047 m water depth) on the deep BBOR. We will generate high resolution (sample spacing of ~800 years) SS-mean and foraminiferal stable isotope records for the past 450,000 years, or the interval between Marine Isotope Stage 12 to the Holocene. In order to identify dominant periodic changes in the DWBC through this interval we will use spectral analysis. Non-stationarity in these statistics, i.e variation of the signals in both amplitude and frequency over long periods of time, will be investigated using wavelet analysis. The findings will further increase our understanding of the connections between variability in deep ocean circulation and the boundary conditions of the climate system, thus reducing current uncertainties surrounding the prediction of climate variability.