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

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.