Millennial-scale Holocene hydrological changes in the northeast Atlantic: New insights from ‘La Grande Vasière’ mid-shelf mud belt

Abstract

A mid- to late-Holocene paleohydrological reconstruction from the northeast Atlantic is proposed through the study of a high-resolution sedimentary record from the northern continental shelf of the Bay of Biscay (BoB). Three foraminiferal species dominate the assemblages with Rosalina globularis showing an overall decrease in absolute and relative abundances from ~7 to 0.4 cal. ka BP, whereas the opposite trend is observed for Cibicides refulgens and Lobatula. These long-term patterns are interpreted as a response to the overall cooling trend and/or the progressive deepening of the water column because of the relative sea-level (RSL) rise. Foraminiferal δ18O and grain-size analyses show a significant shift around 3.5–2.5 cal. ka BP toward a heavier isotopic signature and finer sediments. We mainly link this change to enhanced contribution of continental freshwaters and fine sediments after the near-stabilization of the RSL rise. By reducing coastal accommodation spaces, this led to a better channelization of river outflows and probably to the formation of the modern winter thermohaline front. Superimposed on these long-term patterns, our data highlight strong millennial-scale variability (1250-year peak). Such cyclicity is consistent with several records tracing changes in rainfall and storminess regimes in northern Europe, and the dynamics of the subpolar gyre (SPG). We suggest a millennial time-scale control of a NAO-like (North Atlantic Oscillation) climatic process modulating continental humidity (and the associated river discharges) and SPG dynamics through wind stress. Spectral analyses reveal an additional 500-year frequency peak implying a possible solar forcing.

Oceanic versus continental influences over the last 7 kyrs from a mid-shelf record in the northern Bay of Biscay (NE Atlantic)

“…Present-day environmental conditions in northern Europe (i.e., continental climate, as well as sea-surface temperature and salinity changes) are thus strongly impacted by the NAO and associated changes in seasonal precipitation and fluvial discharge regimes (Castaing et al, 1999;Charria et al, 2013;Le Boyer et al, 2013;Tréguer et al, 2014). At the Holocene scale, previous studies have discussed the possible role of the "NAO-like" pattern in westerly storm activity within the north-eastern Atlantic area (van Vliet et al, 2014;Goslin et al, 2018Goslin et al, , 2019Pouzet et al, 2018) as well as in sea-surface and bottom hydrological conditions in the northern (Zumaque et al, 2017;Durand et al, 2018;Mojtahid et al, 2019) and southern BoBy (Garcia et al, 2013;Mojtahid et al, 2013;Mary et al, 2017).…”

“…in June 2010 (Dussud, 2010). The lithological log of the core, CaCO3-free grain-size analyses and XRF data (1 cm resolution) are fully described in Mojtahid et al (2019), allowing the precise characterization of the deposits through time. Benthic foraminiferal assemblages have also been published in Mojtahid et al (2019), with stable oxygen and carbon isotopes measured on the epibenthic species Cibicides refulgens, which was hand-picked from the 150-250 μm fraction (113 analyses, PSO, Plouzané, Table 2).…”

“…Previous studies suggested the existence of solar-induced cycles of 2,500 years throughout the Holocene (e.g., Debret et al, 2009). Superimposed on the latter, a 1,500-year (e.g., Bond et al, 1997Bond et al, , 2001Debret et al, 2007Debret et al, , 2009Wanner et al, 2008;Thornalley et al, 2009;Sorrel et al, 2012) or 1,250-year (Mojtahid et al, 2019) mode of periodicity was identified as having existed over the Middle to Late Holocene and attributed to internal forcings. Uncertainties nonetheless subsist concerning the main drivers of these latter modes and their different impacts according to the investigated area (e.g., Mayewski et al, 2004;Eynaud et al, 2018).…”

“…Along the western European margin, it is therefore critical to select archives mitigating the proximity of continental sources with non-biased sedimentary records. To that end, the northern shelf of the Bay of Biscay (around 100 m water depth), located in a transitional domain, appears ideal for investigating Holocene rapid and long-term paleoenvironmental changes with a combined land-sea approach (Naughton et al, 2007;Mojtahid et al, 2019).…”

“…In this study, we investigate the CBT-CS11 core, retrieved in the northern part of the Bay of Biscay (BoBy; northern part of the Grande Vasière mid-shelf sediment patch). Mojtahid et al (2019) recently published their results on benthic foraminiferal content, benthic oxygen and carbon stable isotopes on epibenthic species Cibicides refulgens, grain-size analyses and XRF data. Their stable oxygen isotope measurements on C. refulgens showed a significant shift around 3.5-2.5 ka BP, which they interpreted as resulting from the onset of the modern winter thermohaline front.…”