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Title: Spatial and temporal reconstructions of surface deepwater flow in the subtropical NW Atlantic at sharp climatic transitions
Author: Evans, Helena Kay
ISNI:       0000 0004 2750 484X
Awarding Body: Cardiff University
Current Institution: Cardiff University
Date of Award: 2007
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A suite of deep-marine sediment cores recovered from the Blake Outer Ridge (BOR) in the subtropical North West Atlantic (28-34 N, 75-71 W) provide material for centennial to millennial scale investigations of abyssal circulation and surface ocean conditions during selected intervals in the last 130 kyr. Particular focus is placed on reconstructing the position and strength of the Deep Western Boundary Current (DWBC) at a high temporal resolution. Palaeocurrent dynamics are reconstructed using the 'sortable silt" (10-63 urn terrigeneous fraction) mean grain size sedimentological proxy. Paired Mg/Ca and 8 () records from the planktonic foraminifera G. ruber (white) are used to reconstruct deglacial sea surface temperature (SSI) and salinity variations, while benthic 513C data from Cibicidoides spp. document the interchange of northern- and southern source deepwater (NSW/SSW) in the subtropical Atlantic. Comparison of Holocene sediments with modern physical hydrographic measurements reveals a DWBC high velocity core between 3,000-4,000 m water depth. A deep position for the DWBC core below 3,500 m was also observed during the peak of the last interglaciation, marine isotope stage (MIS) 5e. The benthic 5I3C during these interglacial intervals reveals little chemical stratification and a water column dominated by NSW. A shallow Labrador Sea Water (LSW)-sourced secondary fast flowing DWBC core is also apparent during the Holocene, however during MIS 5e palaeocurrent data at intermediate depths suggest a weaker and possibly shallower position for the LSW-sourced secondary DWBC limb. The last glacial maximum and Younger Dryas reconstructions show similar hydrographic regimes with nutrient-depleted, vigorously flowing NSW above 2,500 m consistent with intermediate water formation. Northern sourced intermediate water was first apparent in the records presented at 111 kyr BP and is suggested to have persisted throughout the last glacial. Benthic 5I3C data suggest the presence of a highly stratified water column with an increasing influence of SSW with depth. Coupled suborbital oscillations in DWBC flow variability and palaeo-hydrography persist throughout the records. There is evidence for a broad-scale divergence in flow speed changes in the deep subtropical North Atlantic, with the presence of a vigorous, but poorly ventilated SSW mass below 4,200 m water depth during cold episodes of the last deglaciation and LGM, when shallower palaeocurrent and geochemical data suggest that NSW was suppressed. This is consistent with the operation of a bipolar see-saw effect. This study suggests a hitherto unrecognised degree of linkage between oscillations in subtropical North Atlantic SST and DWBC flow. During the last deglaciation the SST record is dominated by the position/strength of the Gulf Stream, while the effects of tropical heat retention are restricted prior to Heinrich event 1. A combination of meridional overturning strength, meltwater inputs and hydrological changes control salinity variability.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available