Australia: The Land Where Time Began
Holocene Palaeoceanography, Bay of Biscay, Evidence of west-east links in the North Atlantic from dynocyst data
Paleoceanographical changes during the Holocene were reconstructed based on a study of core MD95-2002 recovered from the northern Bay of Biscay, which is marked by the direct influence of the northeastern return branch of the North Atlantic Drift. A strong influence of meltwater/freshwater from the proximal European sources, as well as the Laurentide Ice Sheet, which is more distal and experienced delayed deglaciation is revealed by palynological data, estimates of sea surface conditions, based on assemblages of dynocyst and stable isotope measurements in planktonic and benthic foraminifera. It is also indicated by the data that the setting of a climate optimum between 7 and 5.5 ka that was followed by a cooling trend, which is consistent with changes of insolation and other regional climate records. The reconstructions of sea surface conditions provide evidence of large amplitude changes at centennial to millennial time scales, when they are superimposed on the long term trends, with 7 cooling episodes and low salinity since 11 ka that are generally a match for episodes of dense sea ice cover in the Labrador Sea. Core MD95-2002 provides evidence pointing to strong linkages between the western and eastern North Atlantic, with the transfer of the sea ice and/or meltwater signal across the North Atlantic, probably in relation to the North Atlantic Oscillation (NAO) mode of variability.
High frequency climate variations during the last glacial period are highlighted by oxygen isotope records from Greenland ice cores (Dansgaard et al., 1993), which contrasts with the situation in the Holocene, which has long been considered to be relatively stable from a climatic point of view. In the context of the current global warming the scientific community has nevertheless intensified the study of climate changes during the present interglacial in order that the natural variability of the climate under a “warm” regime can be documented, as well as to better assess the actual impact of anthropogenic forcing. Special attention has been paid, in particular, to the North Atlantic and adjacent continental areas as they represent key areas in regard to polewards heat transport through the North Atlantic Drift (NAD) as it is a major component of the Atlantic Meridional Overturning Circulation (AMOC) (cf. Carton & Hakkinen, 2011).
The first to highlight climatic variability in the Holocene were (Bond et al., 1997) based on marine records of the northern North Atlantic (cf. Andersen et al., 2004a). Later, millennial scale variability of climate parameters has been detected in Holocene records that are based on various tracers that include coccoliths, foraminifers, and dinocysts (e.g. Giraudeau et al., 2010; Bond et al., 2001; Hall et al., 2004; Solignac et al., 2006, 2008). Some inconsistencies in the timing, periodicity and amplitude of the variations, as well as the signature of Early Holocene climate optimum and long-term trend have, however, been noticed by several authors (e.g. Eynaud et al., 2004; Solignac et al., 2006; de Vernal & Hillaire-Marcel, 2006). The amplitude and timing of the climatic optimum (Hypsithermal) differ between sites, though it seems along the main path of the NAD it has been well recorded and consistent (e.g. de Vernal & Hillaire-Marcel, 2006). Complex dynamics of ocean and climate in the North Atlantic during the Holocene are reflected by the heterogeneity of the Holocene records.
The goal of the present study, in this context, is to document further the variability of the climate of the present interglacial from reconstructed hydrographic conditions in the northern Bay of Biscay. In order to do this Zumaque et al. analysed core MD95-2002 that was retrieved from a site located under the direct influence of the northeastern return branch of the North Atlantic Drift (NAD) (Frew et al., 2000). High sedimentation rates, especially during Termination I, characterise the site, during which they reach up to 50 cm/ka (Zaragosi et al., 2001; Auffret et al. 2002; Zaragosi et al., 2006). In this paper Zumaque et al. reconstructed sea surface conditions based on the analysis of assemblages of dinocysts (e.g. Eynaud, 1999; Rochon et al., 1999; Penaud et al., 2009; de Vernal et al., 2013). Isotopic measurements of planktonic foraminifera (Globigerina bulloides and Globorotalia truncatulinoides) have been carried out in order to provide complementary information on the properties of subsurface water masses.
During the IMAGEs 101 expedition of RV Marion Dufresne in May-July 1995 (Bassinot & Labeyrie, 1996; Auffret et al., 2002) core MD95-2002 (47o27’N; 08o32’W), was retrieved. In the Bay of Biscay the deep relief constitutes a seawards prolongation of the Berthois Spur which forms a morphological boundary along the Celtic and Armorican margins (Bourillet et al., 2006) that splits from the shelf towards the Celtic fan and the Armorican Fan eastwards. On the French Atlantic Margin these fans are 2 of the major deep sea fans (Zaragosi et al., 2000). They were fed mainly by discharges from the “Fleuve Manche palaeoriver” which drained a large part of northwestern Europe at times of low stands of eustatic sea level. This important palaeosystem, comprised on the English Channel, a portion of the continental slope that is “canyon-dominated” and the 2 deep sea turbidite systems (i.e. Celtic and Armorican Fans), this important fluvial palaeosystem system extended from the southern North Sea to the Bay of Biscay (cf. Toucanne et al., 2011).
At the present, the surface waters are directly influenced by the warm NAD, which contributes to the North Atlantic Gyre (e.g. Sutton & Allen, 1997). Sea surface temperature (SST) and sea surface salinity (SSS) are 11.7 ± 0.6oC and 35.54 ± 0.05 and 17.5 ± 1.0oC and 35.58 ± 0.10 in winter and summer, respectively (World Ocean Atlas, 2001; Conkright et al., 2002). The warm and salty waters of the Eastern North Atlantic Water (ENAW), that is present in the water column to a depth of 800 m, is also carried by the Slope Current (SC) (e.g. Lazure et al., 2008).
From about 800 m to 1,300 m below the Slope Current, a branch of the warm though very salty (35.7) Mediterranean Overflow Water (MOW) overlies the Labrador Sea Waters (LSW), which is characterised by salinity that ranges from 35 – 35.5 (Cossa et al., 2004). In the eastern part of the Bay of Biscay diluted Labrador Sea Water signal is episodically induced by diapycnal mixing that is favoured by the proximity of the continental slope (van Aken, 2000).
The complexity of the ocean-atmosphere-cryosphere interactions in the boreal Atlantic is illustrated by the study of the sea surface conditions recorded from core MD95-2002 during the Holocene, as was highlighted previously at other key locations (e.g. Bond et al., 1997, 2001; Hall et al., 2004; Anderson et al., 2004b; Solignac et al., 2004, 2006; Giraudeau et al., 2010; Morley & Rosenthal, 2014; Thornalley et al., 2013). This paper shed light on a few regional characteristics of hydrographical changes beyond the findings of other studies.
1) Evidence of strong stratification of the upper levels of the water column in the Bay of Biscay, especially prior to 7 ka, as the site was submitted to freshwater advection from the proximal European continent as well as distal Laurentide Ice Sheet (LIS).
2) Variations during the Holocene that correspond to the classical palaeoclimatic subdivisions of the Holocene with a climatic optimum between 7 and 5.5 ka, is highlighted by the reconstruction of sea surface conditions, though the identification of it is equivocal as a result of its signal being hidden by the meltwater inputs of the remnant Laurentide Ice Sheet.
3) The record shows that the long term trends are superimposed to larger amplitude changes on time scales of centennial to millennial; 7 episodes of cooling and low salinity have been identified at site MD95-2002 and correspond to episodes of enhanced sea level cover in the Labrador Sea. At these periods enhanced westerlies are likely to have lowered SSTs at both sites and favoured sea ice advection and related meltwater from south Greenland to the northern Bay of Biscay. Such a phenomenon, probably linked to the variability of the North Atlantic Oscillation, is suggested by Zumaque et al. to possibly have had a huge impact on the AMOC as weakenings of the production of the Iceland-Scotland Overflow Water are also recorded during these very same periods.
Zumaque, J., et al. (2017). "Holocene paleoceanography of the Bay of Biscay: Evidence for west-east linkages in the North Atlantic based on dinocyst data." Palaeogeography, Palaeoclimatology, Palaeoecology 468: 403-413.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|