The southwestern Iberian margin is highly sensitive to changes in the
distribution of North Atlantic currents and to the position of oceanic
fronts. In this work, the evolution of oceanographic parameters from 812 to
530 ka (MIS20–MIS14) is studied based on the analysis of planktonic
foraminifer assemblages from site IODP-U1385 (37
Climate in the North Atlantic region is characterized by the continuous
poleward heat flow carried out by the oceanic circulation. The Gulf Stream
and the North Atlantic Current (NAC) transport warm and salty surface water,
originating in the tropical region, towards the polar ocean, the northeast
Atlantic, and along the western European margin, transferring heat and
moisture to the atmosphere during the process (e.g. McCartney and Talley,
1984; Ruddiman and McIntyre, 1984; Schmitz and McCartney, 1993; Rahmstorf,
1994; Chapman and Maslin, 1999). Surface circulation and associated heat
flow is pumped by the sinking of surface water in the subpolar region and
formation of the North Atlantic Deep Water (NADW). As a matter of fact, the
Atlantic meridional overturning circulation (AMOC) is responsible for
The NAC forms the transition zone between the cold and productive waters located north of the Arctic Front (AF) (e.g. Johannessen et al., 1994), and the warm and oligotrophic waters from the subtropical gyre in the south. Each water mass has distinct physical–chemical characteristics and specific planktonic foraminiferal assemblages (e.g. Bé, 1977; Ottens, 1991; Cayre et al., 1999). Various studies have shown that surface water characteristics in the midlatitude North Atlantic depend on the strength and position of the NAC and associated oceanic fronts (Calvo et al., 2001; Naafs et al., 2010; Voelker et al., 2010). During Pleistocene glacials, the AF migrated southward into the midlatitude North Atlantic (Stein et al., 2009; Villanueva et al., 2001), cold polar waters expanded to lower latitudes, and the NAC did not reach as far north as during interglacials (e.g. Pflaumann et al., 2003).
After MIS21, a northwestward shift in the position of the AF began (Hernandez-Almeida et al., 2013), that culminated at the end of MIS16, in a similar location to that of the present day (Wright and Flower, 2002). Coinciding with the final stage of this shift, a major reorganization of the meridional overturning circulation developed, related to increased NADW formation that resulted in deeper and southward penetration of this mass of water (Poirier and Billups, 2014). Both processes could have been related to the prolongation of glacials that occurred at the end of the mid-Pleistocene transition (MPT). This was the transitional period during which the Earth's climate system underwent a major change, non-linear 100 ky cycles appeared and superimposed over the more linear, orbital ones of 41 and 23 ky.
Although there is still no agreement over the initiation of the MPT (e.g.
Clark et al., 2006; Maslin and Brierley, 2015), strong 100 ky cycles have
been recorded since
Over the last glacial cycle, the Iberian margin recorded both peak
displacement events of the AF and periods of greater influence of
subtropical water from the Azores Current (AzC) (e.g. Martrat et al., 2007;
Eynaud et al., 2009; Salgueiro et al., 2010). There is also evidence that
polar to tropical planktonic foraminifers' assemblages co-occurred in a
latitudinal band around 35–40
The southwestern Iberian margin is a focal location for paleoclimate and
oceanographic research of the Quaternary (Hodell et al., 2013). Site
IODP-U1385 was drilled at the so-called Shackleton site (37
Today's surface water circulation in the North Atlantic (Fig. 1a) consists of two different branches. The NAC, after reaching the subpolar
ocean, drifts southwards along Europe, transporting the eastern North
Atlantic Central Water of subpolar origin (ENACWsp), formed north of
46
Sediments at site U1385 define a single, very uniform, lithological unit.
Calcareous muds and calcareous clays dominate the lithology. The relative
proportions of carbonate (23 %–39 %) and terrigenous materials are seen in
the sediment colour that varies from dark (i.e. more terrigenous) to light
(i.e. more calcareous). The average sedimentation rate for the section is
of
This study covers a section comprised between 67.2 and 94.6 crmcd (MIS14–MIS20). The age model (Hodell et al., 2015) is based on the correlation of the benthic oxygen isotope record to the global benthic LR04 isotope stack (Lisiecki and Raymo, 2005). For better comparison of our results with data from other North Atlantic sites, new age models were calculated for sites 980 and 607, based on correlations with the LR04 stack.
Sampling was performed every 20 cm, providing a 1.76 ky resolution on
average. A total of 147 samples, 1 cm thick, were freeze dried, weighed, and
washed over a 63
The microfaunal analysis focused on species and assemblages that are
associated with North Atlantic surface water masses (Appendices A and B).
Thermal gradients in the North Atlantic are reconstructed by calculating the
difference between the sea surface temperature (SST) from two sites. The
site 607 was used as the starting point and compared with sites 980 for the
latitudinal gradient (SST
This estimation of thermal gradients is possible because all the SST records used for this work are based on planktonic foraminifers' census counts. Nevertheless, prior to the comparison, interpolation was applied to obtain records with the same age points.
Relative abundance of planktonic foraminiferal species
and assemblages in IODP-U1385 through MIS14–MIS20, and comparison with benthic
isotope data from the same site.
Except in the eighth climate cycle (MIS19–MIS18),
The NAC assemblage (Ottens, 1991) is the most abundant one at this site
(Fig. 2), indicating that the ENACWsp dominates the surface oceanography in
the area through the time series. This assemblage does not keep a similar
interglacial–glacial pattern through the whole study interval but changes
its behaviour at
The warm surface (WS) assemblage (Vautravers et al., 2004) is typical of the subtropical water transported eastwards by the AzC. In U1385, this assemblage has shown a clear interglacial–glacial pattern only since Termination TVIII, its percentage decreasing gradually during MIS17–MIS16 until the glacial maximum (Fig. 2e). Comparing glacial stages, MIS20 records the highest average relative abundance (16.8 %) and MIS14 the lowest (8.7 %). Termination TIX records the most abrupt decrease of this assemblage (15 % drop), while at TVI it even increases (5 % rise). At the beginning of each interglacial, the percentage of this assemblage rises rapidly, suggesting that the AzC strengthens rapidly in the area after terminations.
Comparison of records from the midlatitude (IODP-U1385;
ODP-607) and the subpolar (ODP-980) North Atlantic. Benthic
The location of sites 607 and 980 along the main core of the NAC towards the high latitudes of the North Atlantic (Fig. 1a) allowed us to monitor past changes in the northward heat transport, using planktonic foraminifer assemblages and SST reconstructions from both sites. By contrast, planktonic foraminifer assemblages at site U1385 are more influenced by the advection of heat to the northeastern Atlantic through the easternmost branches of the NAC, and especially by the AzC, that originates in the tropics and flows towards the Iberian Peninsula following the northern margin of the subtropical gyre (Fig. 1a). In consequence, with these three strategic sites, we can monitor changes in the main circulation systems of the northeast Atlantic during the mid-Pleistocene and estimate the heat advection to the north (SST gradient between sites 607 and 980) and to the northeast Atlantic (SST gradient between sites 607 and U1385) (Fig. 3f–g).
During both glacials, progressive cooling is recorded at sites 607 and 980 (Fig. 3f). Though the cooling is more pronounced at the higher latitude, the SST gradient between both sites is not very high and decreases largely towards the end of glacial stages (Fig. 3g). In contrast, the Iberian margin remained relatively warm during most of MIS20 and a large part of MIS18 (Fig. 3f), which undoubtedly reflects a continuous flow of the AzC to this region, as also indicated by the WS assemblage record (Fig. 2e).
At the subpolar latitude of site 980, the presence of polar water has increased
rapidly since glacial inceptions, as informed by very high percentages of
While the northward flow of heat decreased progressively along both
glacials, the heat flow towards the Iberian margin continued in the early
part of glacial MIS18 and, especially, during MIS20, indicating a very
active AzC during both glacials. This current advected warm water eastward
and deflected northward along the Iberian margin, similarly to
today's IPC (Fig. 1a), probably overflowing the polar water
mass, as the co-occurrence of polar and subtropical fauna suggests (Fig. 2b, e). The advection of the warm AzC to site U1385 was only interrupted at
Terminations TIX and TVIII, and at deglaciation MIS18e/d, when massive surges
of very cold and low-salinity surface waters reached the area, which was
registered by peaks of the polar species
The very low SST at the midlatitude site 607 and the low latitudinal thermal gradient during glacial maxima MIS20a, MIS18e, and MIS18a (Fig. 3f–g) suggest either a complete shutdown of the NAC core flux or a southward or southeastward diversion of this current as glacial conditions progressed. Nevertheless, the low thermal gradient between sites 607 and U1385 (Fig. 3g) implies that the southwest Iberian margin was always under the influence of the warmer AzC.
Both latitudinal and longitudinal thermal gradients (Fig. 3g) inform of
drastic rearrangement of North Atlantic circulation starting at MIS17. SST
at site 607 was much warmer than during MIS19, although both interglacials
were similar, according to
The unusually high longitudinal thermal gradient registered during MIS17 was
due to the prolonged deglaciation of MIS18 that continuously advected polar
water along the Iberian margin (Martin-Garcia et al., 2015), resulting in
very cold SST and high percentages of
MIS16 was a very prolonged glacial with extensive ice sheets; nevertheless,
polar waters did not extend to the midlatitude ocean, as suggested by the
low percentages of
The latitudinal thermal gradient for most of MIS16, and the whole MIS14, was
notably higher than during MIS20–MIS18 (Fig. 3g). This great SST decrease,
between sites 607 and 980, must be the result of a significant heat loss to
the atmosphere and associated release of water vapour along the path of the
NAC during both MIS16 and MIS14. This water vapour release provided the
necessary moisture to continue ice-sheet growth, opposite to what had happened
during previous glacials. Also contrary to glacials MIS20 and MIS18, when
the surface water at the subpolar site 980 progressively cooled towards
glacial maxima without important millennial-scale oscillations (Fig. 3f); in
glacials MIS16 and MIS14, the surface ocean circulation was very variable
and the AF migrated northward–southward to site 980 very frequently (Fig. 3c–d).
During short time periods, the NAC reached this subpolar site,
conveying heat to the northern-latitude Atlantic (Fig. 3e). However, this
oscillation of the AF never affected middle latitudes, according to the
fairly mild SST, and low percentage of
At the midlatitude ocean site 607, SSTs during MIS16 and MIS14 were very different from those recorded in MIS20 and MIS18 (Fig. 3f). While in the older glacials SST decreased towards glacial maxima, this trend is not observed during MIS16 and MIS14, and warm SST was recorded also during glacial maxima.
Although warmer SST were recorded through the midlatitude North Atlantic, a
negative thermal gradient still prevailed during MIS16–MIS14 between sites 607
and U1385 (Fig. 3g), indicating a continuous heat flow toward the southwest
Iberian Peninsula. This suggests that this region remained under the influence of the
subtropical AzC during most parts of glacials MIS16 and MIS14, as it also did
during MIS20, based on the mild SST registered at that time (Fig. 3f).
Contrary to previous glacials, the NAC remained vigorous at site U1385 during
MIS16, except at
Assuming a close correlation between the rate of AMOC and benthic
The increased production of NADW, during glacials after MIS16 with respect to
previous ones, triggered the advection of relatively warm NAC towards
subpolar latitude, providing additional humidity to the area and thus
enhancing the growth of ice sheets, which led to the prolonged and extreme
glaciation of MIS16, one of the first and most prominent glacials of the
“100 ky world”. In addition, the intermittent advection of this warm water
made ice sheets more vulnerable to internal instabilities, with the
subsequent release of icebergs registered in the North Atlantic during MIS16
(e.g. Wright and Flower, 2002; Hodell et al., 2008). The interaction
between a more intense AMOC and ice-sheet instabilities, registered by rapid
migrations of the AF north and south of site 980 (Fig. 3c–d), resulted in
punctual events of sharp reduction of the NADW formation, like that at
This vigorous AMOC mode recorded in MIS14 was the culmination of a sequence
of increasing deepening of the overturning circulation cell that was initiated
in MIS22, and was registered by a tendency towards higher benthic
By studying planktonic foraminiferal assemblages from the Iberian margin
(IODP-U1385) for the interval 812–530 ka and comparing them with records
from other sites between 41 and 55
Variations of microfaunal assemblages associated with surface currents indicate a major change in the general North Atlantic circulation during this interval, coinciding with the definite establishment of the 100 ky climate phasing. In surface, this change consisted in the redistribution of water masses and associated SST that happened linked to the northwestward migration of the AF during MIS16, and was related to the increasing NADW formation trend that was initiated in MIS22.
Prior to MIS16, the AMOC rate was very low, especially during glacials, the AF was at a southerly position, and the NAC diverted southeastwards, developing steep south–north and east–west thermal gradients, and blockading the arrival of warm water, with associated moisture, to the high-latitude North Atlantic.
During MIS16, the NADW formation increased with respect to previous glacials, especially during glacial maxima, which resulted in the northwestward AF shift and enhanced surface circulation, allowing the arrival of the relatively warm NAC to subpolar latitudes and increasing the moisture availability to continuing the ice-sheet growth, which would have worked as a positive feedback to prolong the duration of glacials to 100 ky cycles.
The available datasets used for this article can be accessed at the NOAA/World Data Service for Paleoclimatology archive
(
Planktonic foraminifer species used in this study.
Faunal composition of both the NAC and the warm
surface assemblages at site U1385 through the study interval.
The authors declare that they have no conflict of interest. Edited by: Marit-Solveig Seidenkrantz Reviewed by: two anonymous referees