Multi-Proxy Investigations into Mediterranean Outflow Water Variability and Sedimentary Processes on the West Iberian Margin Through the Late Pleistocene

Abstract

Mediterranean Outflow Water (MOW) adds salt and density to open ocean intermediate waters and is therefore an important motor of Atlantic meridional overturning circulation (AMOC) and climate variability. However, the variability in strength and depth of MOW on geological timescales is poorly documented. West Iberian Margin sediments provide important reference records in palaeoceanographic studies, climate events in the marine realm recorded here can be correlated to both the Antarctic and Greenland ice cores as well as to European terrestrial records. These sediments, therefore, provide an opportunity to document MOW variability in high-resolution, underpinned by strong age control.
Here, I present new detailed records, with excellent age control, of MOW variability from ~416 ka to present from rapidly accumulated marine sediments recovered from the West Iberian Margin during Integrated Ocean Drilling Program (IODP) Expedition 339. The records of X‐ray fluorescence (XRF), physical grain size, and paleocurrent information from the anisotropy of magnetic susceptibility (AMS) indicate (i) a close relationship between the orientation of principle AMS axes and glacial‐interglacial cycles and (ii) two distinct regimes of MOW behaviour over the last ~416 kyr in grain‐size and AMS variability at orbital (mainly precession) and suborbital timescales. It is inferred that late Pleistocene variability in MOW strength and depth were strongly climate influenced and that changes in circum‐Mediterranean rainfall climate were likely a primary control.
A suite of environmental and rock magnetic records from IODP Site U1391 sediments are presented. Different forms of magnetite with varying grain size are inferred as being the dominant magnetic phase in the sediments. From ~130 ka to present, the grain size and concentration of magnetite were strongly controlled by northern high latitude variability. During some stadials in this interval an additional high coercivity component is present, possibly representing haematite derived from North African dust. In sediments older than ~130 ka, evidence of reductive diagenesis is seen across environmental and rock magnetic proxies, and is particularly strong in sediments deposited during marine isotope stages (MIS) 9 and 11. These conditions likely result from a combination of increased productivity and changes in sedimentation rates, associated with vertical migration of the Mediterranean outflow water.
XRF (i.e., ln(Zr/Rb)) and magnetic grain size (i.e., ARM/) proxy records from IODP Site U1391 sediments are identified as best representing different physical size fractions of sediment, 33 – 43 m and 8 – 19 m (and likely sizes below 5 m), respectively. Utilising the high-resolution grain size records as proxy of MOW strength in combination with strong chronology; frequency and phasing analysis uncover the relative importance of different climate forcings on MOW. Evidence suggests that spring insolation forcing of MOW strength variability at Site U1391 between ~370 and 60 ka, likely linked with intensification of late-winter North Atlantic storm track activity at precession minima. There is also evidence that this mechanism is impacted upon by the interference of meltwater pulses with North Atlantic circulation.

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Identifiers

ORCID for Matthew Nichols: orcid.org/0000-0001-7219-4594

ORCID for Chuang Xuan: orcid.org/0000-0003-4043-3073

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