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

过去 450 KYR 期间副热带西北大西洋深西边界海流变化

• 批准号: NE/F007558/1
• 负责人: Ian Hall
• 金额: $ 4.1万
• 依托单位: CARDIFF UNIVERSITY
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2008
• 资助国家: 英国
• 起止时间: 2008 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FF007558%2F1
• 关键词: DEEP; WESTERN; BOUNDARY; CURRENT; VARIABILITY

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.

当今环境研究中最具争议的问题之一是涉及当今气候制度的稳定性，尤其是人类诱发的变化是否足以导致气候重大重组。一个重要的发展是在最近的地质过去确定了突然的气候变化和强烈的气候变化，这表明源自自然强迫的气候事件也可能发生在与人类相关的时间表上。这些突然的气候转移被广泛认为涉及全球表面和地下洋流的行为的变化，即海洋热 - 热盐循环（THC）。为了了解THC的自然变异性，有必要从关键位置（数十万年）连续的古气候记录产生，并具有高千禧一代（1000年）的时间分辨率。但是，由于海洋环境中的沉积通常较低（通常约为2-3 cm/1000年），因此并非总是可以使用足够的时间分辨率记录。仅来自沉积物漂移的岩心，这些岩心通常会增加〜40 cm/1000年的沉积率，这是创建此类记录的可能性。这些沉积物漂移是由强底电流产生的，该电流从一个位置侵蚀材料并将其存放在另一个位置。漂移沉积物的缺点是它们可能包含很少的生物材料，例如通常用于重建地表水性质和深水质量的横向分布的有孔虫。围绕此问题的一种潜在方法是研究与深海流有关的物理参数，例如可排序的淤泥均值代理（SS均值），这是10-63微米地陆泥土亚汇率的平均晶粒尺寸。与有孔虫稳定的同位素记录相反，由于缺乏有孔虫，因此可能是斑点的，因为陆源沉积物是连续沉积的，因此SS均值记录通常不间断。此外，沉积物的排序与深海电流的物理流速直接相关，其中沉积物晶粒被运输和沉积，这使得过去允许在过去的高分辨率记录中产生高分辨率的记录。亚热带西北大西洋亚热带的布雷克 - 巴哈马外脊（BBOR）的沉积物漂移沉积物为深度分辨率的深度西部边界电流（DWBC）流动的变化提供了档案。作为THC的主要组成部分，DWBC最近载有通风的水（即北大西洋深水）。先前的研究已将晶粒尺寸分析与有孔虫稳定的同位素分析结合使用，以便重建DWBC在千禧一代到Milankovitch Timescales上的强度和流动特性。但是，在过去的100,000年冰川间冰期周期中，尚无连续记录DWBC流动变异性的时间变异性和时间变化的时间。在这里，我们建议专注于在ODP腿172，站点1061（4047 m的水深）上记录的DWBC的近底流速和水文的变化。在过去的450，000年中，我们将产生高分辨率（样本间距约为800年）和有孔虫稳定的同位素记录，或者在全新世的海洋同位素阶段12之间的间隔。为了通过此间隔确定DWBC中的显着周期性变化，我们将使用光谱分析。这些统计数据中的非平稳性，即长时间幅度和频率中信号的变化，将使用小波分析研究。这些发现将进一步提高我们对深海循环变异性与气候系统边界条件之间的联系的理解，从而减少了围绕气候变异性预测的当前不确定性。