Mediterranean Outflow and surface water variability off southern Portugal during the early Pleistocene: A snapshot at Marine Isotope Stages 29 to 34 (1020–1135 ka)

Antje H.L Voelker,Emilia Salgueiro,Teresa Rodrigues,Francisco J Jimenez-Espejo,André Bahr,Ana Alberto,Isabel Loureiro,Maria Padilha,Andreia Rebotim,Ursula Röhl

doi:10.1016/j.gloplacha.2015.08.015

Abstract

Centennial-to-millennial scale records from IODP Site U1387, drilled during IODP Expedition 339 into the Faro Drift at 558m water depth, now allow evaluating the climatic history of the upper core of the Mediterranean Outflow (MOW) and of the surface waters in the northern Gulf of Cadiz during the early Pleistocene. This study focuses on the period from Marine Isotope Stages (MIS) 29 to 34, i.e. the interval surrounding extreme interglacial MIS 31. Conditions in the upper MOW reflect obliquity, precession and millennial-scale variations. The benthic δ18O signal follows obliquity with the exception of an additional, smaller δ18O peak that marks the MIS 32/31 transition. Insolation maxima (precession minima) led to poor ventilation and a sluggish upper MOW core, whereas insolation minima were associated with enhanced ventilation and often also increased bottom current velocity. Millennial-scale periods of colder sea-surface temperatures (SST) were associated with short-term maxima in flow velocity and better ventilation, reminiscent of conditions known from MIS 3. A prominent contourite layer, coinciding with insolation cycle 100, was formed during MIS 31 and represents one of the few contourites developing within an interglacial period. MIS 31 surface water conditions were characterized by an extended period (1065–1091ka) of warm SST, but SST were not much warmer than during MIS 33. Interglacial to glacial transitions experienced 2 to 3 stadial/interstadial cycles, just like their mid-to-late Pleistocene counterparts. Glacial MIS 30 and 32 recorded periods of extremely cold (<12°C) SST that in their climatic impact were comparable with the Heinrich events of the mid and late Pleistocene. Glacial MIS 34, on the other hand, was a relative warm glacial period off southern Portugal. Overall, surface water and MOW conditions at Site U1387 show a strong congruence with Mediterranean climate, whereas millennial-scale variations are closely linked to North Atlantic circulation changes.

Highlights

Reduced convection directly affects the intermediate-to-deep water masses contributing to the deep outflow from the Mediterranean Sea that is feeding the Mediterranean Outflow (MOW) in the Gulf of Cadiz and a “gap” in Holocene contourite layers in the Gulf of Cadiz has been linked to the formation of sapropel 1 (Rogerson et al, 2012)
As to be expected for a warmer water mass like the MOW, the benthic δ18O values of Site U1387 are lower (~1‰) than those recorded at North Atlantic Deep Water sites
Millennial-scale variability is seen in the Site U1387 δ18O records

Summary

Introduction

Within the longer cooling trend starting around 1800 ka (McClymont et al, 2013), MIS 31, which received the highest intensity summer insolation (567 W/m2) of the last 1800 ky (Laskar et al, 2004), stands out as a long lasting interglacial period that in many regions experienced surface water temperatures as warm as late Pleistocene interglacial MIS 5e, 9e or 11c (Medina-Elizalde and Lea, 2005; McClymont et al, 2008; Lawrence et al, 2010; Russon et al, 2011). Most earlyto-mid Pleistocene deep-sea records exhibit climate cycles at the precession (23 ky) frequency (e.g., Girone et al, 2013; McClymont et al, 2013). Associated with precession minima (insolation maxima) organicrich sapropel layers were formed in particular in the eastern Mediterranean Sea (e.g., Rossignol-Strick, 1985; Emeis et al, 2000). Reduced convection directly affects the intermediate-to-deep water masses contributing to the deep outflow from the Mediterranean Sea that is feeding the MOW in the Gulf of Cadiz and a “gap” in Holocene contourite layers in the Gulf of Cadiz has been linked to the formation of sapropel 1 (Rogerson et al, 2012). Borehole records of Pliocene sediments in the Gulf of Cadiz show a strong response to precession forcing, here related to rainfall patterns in the southern Spain (Sierro et al, 2000)

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Discussion

Conclusion