A substantial quantity of pollen grains produced, released and dispersed from terrestrial higher plants (as well as spores of lower plants) reaches the ocean mainly by fluvial and atmospheric transport processes, with oceanic currents playing a negligible role (Koreneva, 1966; Stanley, 1966). The relative contribution of one or other transport vector is regionally-dependent (Dupont, 2011). In the Arctic, for instance, pollen is also transported by sea ice scouring and sediment transport in addition to northeasterly winds (Mudie and McCarthy, 2006). Marine sediments located further than 300 km offshore are weakly influenced by rivers and the dominant pollen transport is aeolian (e.g., Hooghiemstra et al., 1988). In contrast, the pollen preserved in marine sediments located under the influence of river plumes (nepheloid layers) is mainly of fluvial origin (e.g., Heusser and Balsam, 1977; Cambon et al., 1997; Mudie and McCarthy, 2006). Once it arrives in ocean surface waters, pollen is ingested by planktonic organisms and later integrated into their fecal pellets or agglomerated with clays (Mudie and McCarthy, 2006; Beaudouin et al., 2007). Due to these processes, pollen buoyancy decreases and is little influenced by ocean currents. It thus becomes an integral part of marine snow and crosses the water column with a relatively high speed (estimated at ∼100 m/day in the Atlantic water column) before being deposited at the bottom of the oceans (Hooghiemstra et al., 1992). Furthermore, sediments under the influence of upwelling are particularly rich in pollen for two reasons: the intensification of the downward particle transport through the water column (Ratmeyer et al., 1999) and the better preservation in almost anoxic sediments (Dupont, 2011).

Pollen studies on modern deep ocean surface sediments from the Iberian margin (Turon, 1984a; Naughton et al., 2007), the Bay of Biscay (Turon, 1984b), the Mediterranean (Koreneva, 1971; Beaudouin et al., 2007), the African margin (Hooghmiestra et al., 1986; Dupont and Wyputta, 2003), and the western and eastern margins of North America and the Gulf of Mexico (Heusser and Balsam, 1977; Heusser, 1985; Heusser and Van de Geer, 1994) showed that pollen assemblages from the sediment surface of the ocean floor reflect an integrated image of the regional vegetation of the adjacent continent and, consequently, the climatic parameters under which this vegetation has developed. Palynological richness, i.e., the number of pollen types per sample standardized to a constant pollen sum, is similar in fossil samples from deep-sea cores, modern terrestrial and deep-sea surface samples, and fossil assemblages from lakes and estuaries, oscillating between 15 and 25 morphotypes with respect to a pollen sum of at least 100 terrestrial pollen grains (e.g., Sánchez Goñi and Hannon, 1999; Naughton et al., 2007). An ongoing study on the pollen representation of modern vegetation in moss-polsters along the Tajo basin shows that palynological richness ranges from 13 to 41 with respect to a pollen sum of 500 terrestrial pollen grains. In particular, Naughton et al. (2007) demonstrated by comparison of modern pollen spectra from the deep ocean, estuaries and continent that the pollen signal of the Iberian margin is similar to that found in estuarine sediments from western Iberia that reliably represents the broad regional vegetation of the related hydrographic basins (Figure 2). On the one hand, pollen assemblages from the northern part of the margin reflect the composition of temperate forest that develops in northwestern Iberia dominated by deciduous oaks. On the other hand, pollen samples from the southern part of the margin capture the composition of Mediterranean forest, i.e., warm-temperate forest dominated by sclerophyllous trees and shrubs that characterizes southern Iberia (Figure 2). In this region, pollen is mainly transported by the rivers because the dominant winds come from the north-west and the hydrographic Iberian basins are large and thus favor the transport of sediment load, including pollen (Dupont and Wyputta, 2003; Naughton et al., 2007). From a botanist’s point of view, this type of study has therefore a limitation for reconstruction of ancient local plant communities (Groot and Groot, 1966). However, the similarity of western European terrestrial pollen sequences and eastern North Atlantic deep-sea pollen records (e.g., Naughton et al., 2007 for the last 18,000 years in north-west Iberia; Fletcher and Sánchez Goñi, 2008 for the last 50,000 years in south-east Iberia; Sánchez Goñi et al., 2012 for the Last Interglacial in western France) demonstrate once more that marine pollen records provide a reliable image of the vegetation history of the adjacent landmasses (Groot and Groot, 1966). Moreover, the regional vegetation is directly linked to climate conditions as the present-day distribution of the major biomes is governed by climatic parameters (Bailey, 1998). For western Europe Gouveia et al. (2008) have recently shown the rapidity with which vegetation in this region responds to North Atlantic atmospheric processes, i.e., the westerlies.

The comparison between terrestrial and marine modern pollen samples to better interpret marine pollen records has been further improved and applied to south-western Africa (Urrego et al., 2015). In particular, modern pollen spectra were used to assess the distribution of Poaceae pollen abundance and other pollen taxa with potential indicator value for large biomes in southern Africa, and therefore climatic zones in this region. For instance, pollen percentages of Poaceae up to 70% reflect the Nama-karoo and its transition with the fined-leaved savanna. These results can substantially change the interpretation of the marine pollen fossil record collected off northwestern South Africa. Contrary to the studies that interpret the increase of Poaceae pollen percentages as the result of humidity-driven savanna expansions in southwestern Africa, the new pollen calibration study offers an alternative interpretation indicating that fine-leaved savanna developed in this region due to the aridity increase during the warm and humid Northern Hemisphere periods of the last interglacial (Urrego et al., 2015). This approach should be applied to other regions to improve the interpretation of marine pollen assemblages in terms of vegetation cover and composition. However, the quantification of the vegetation abundance from marine pollen assemblages to infer, among others, changes in albedo and their influence on climate change, remains a challenge. The REVEALS model was conceived to obtain quantitative reconstructions of regional vegetation cover around large lakes from pollen data, which resembles the spatial scale of marine pollen samples (Sugita, 2007). Nevertheless, this model only considers atmospheric pollen dispersal and deposition, thus disregarding the importance of pollen input from inlet streams and surface run-off, which are often the main vectors delivering pollen to the marine environment (Sugita, 2007).

The concentration (usually between 1,000 and 50,000 grains cm^-3) and taxonomical diversity of marine pollen assemblages are in general high enough to allow the quantitative estimation of annual and, more importantly, seasonal temperatures and precipitation (e.g., Sánchez Goñi et al., 2002, 2005, 2016a; Desprat et al., 2007; Combourieu Nebout et al., 2009) (Figure 3). Knowledge about past climate seasonality is crucial to understand natural climate variability (Carré and Cheddadi, 2017) and the sensitivity of vegetation to changes in this parameter makes pollen data particularly well-suited to track shifts in seasonality through time. Quantitative pollen-based climatic estimates are based on different techniques. The most frequently used is the Modern Analog Technique (MAT) based on a comparison of past assemblages to modern pollen assemblages through the calculation of the shortest weighted distance (Guiot, 1990). This method requires high-quality, taxonomically consistent modern pollen and climate datasets. The modern database includes different continental pollen spectra (moss polsters, soil surface samples, and lacustrine sediments) from Eurasia and northern Africa (Peyron et al., 1998; Bordon et al., 2009). This continental pollen database is, however, slightly modified to estimate climatic parameters from marine pollen samples. Since Pinus pollen is overrepresented in marine sediments (Heusser and Balsam, 1977; Hooghiemstra et al., 1988), this pollen type is excluded from the calculation of pollen percentages for marine pollen spectra as well as from the continental pollen database. As potential errors would be similar for adjacent samples of the same record, we consider that this approach is still valid for the quantification of climate change anomalies. The application of the MAT to estimate quantitatively climatic parameters for marine pollen records should be further improved by integrating marine modern pollen assemblages in the database, and by application of other reconstruction approaches that rely less strongly on the availability of suitable modern analogs, such as indicator approaches (Kühl et al., 2002) and biomisation (Prentice et al., 1996).

During the last 60 years, several oceanographic cruises organized in the framework of the international programs IMAGES (International Marine Global changE Study) and ODP/IODP (International Ocean Drilling Program) have retrieved a large number of pollen-rich marine cores along different oceanic margins. Most of these cores come from oceanic rises and abyssal plains that are the most favorable areas for marine palynological work as terrigenous sediments are predominant (Groot and Groot, 1966). We have performed a literature survey to compile pollen records from deep-sea cores spanning the last 1 million years, finding 129 sites all over the world (Supplementary Table S1 and Figure 4). This compilation shows that there are 74 high-resolution pollen sequences (better than 1000 years between adjacent samples) and highlights the paucity of pollen records from the Indian (e.g., Rossignol-Strick, 1983) and the South Pacific (e.g., Montade et al., 2013; Seillès et al., 2016) Oceans. Likewise, almost half of the records (n = 57) do not cover periods older than the Last Glacial Maximum, and few sequences (n = 19) record several orbital climatic cycles. Therefore, the regional expression of long-term and rapid, millennial-scale, global climate changes is far from being well-documented, particularly in the Indian Ocean where only two out of the six available records cover the last climatic cycle (last 150,000 years) with a coarse time resolution (worse than 1000 years between consecutive samples). To fill this gap, IODP expedition 353 “Indian Monsoon Rainfall” collected a sedimentary sequence in 2015 on the eastern Indian margin, site U1446, which will enable to trace for the first time past vegetation and Indian summer monsoon variability over the last 1 million years.

Since 1995, the Marion Dufresne and Joides Resolution oceanographic ships have retrieved several high-resolution pollen-rich deep-sea sedimentary sequences from the eastern North Atlantic and the Mediterranean Sea within the framework of the IMAGES and IODP programs. These sequences are strategically located in the present-day Mediterranean and Atlantic regions, characterized by a markedly seasonal climate, with warm and dry summers and wet and mild winters, and a year-round humid climate with warm summers and cool winters, respectively. In winter, both regions are under the influence of the westerlies, but in summer the Azores high affects southern Iberia inducing a pronounced summer drought while the westerlies still play a prominent role from northern Iberia northward. Additionally, the region located below 40°N is affected by precession, and the subtropical ocean water currents (Ruddiman and Mcintyre, 1984).

At present, the seasonal distribution of precipitation, with a relatively abundant amount of rainfall from autumn to spring and a pronounced summer drought, determines the dominance of Mediterranean forest (Polunin and Walters, 1985; Gouveia et al., 2008). This forest comprises evergreen sclerophyllous woodland (evergreen Quercus, Olea, Pistacia, Phillyrea, Cistus) at low altitudes, and deciduous forest (mostly deciduous Quercus) at higher elevation. On the contrary, the occurrence of Atlantic forest, dominated by deciduous Quercus (oak), is today controlled by winter length and harshness, given that annual rainfall is high and summer drought absent or negligible (Polunin and Walters, 1985). Therefore, deciduous Quercus is included in both the ‘Atlantic forest’ and the ‘Mediterranean forest’ assemblages.

Pollen analysis of these European margin sequences has allowed the reconstruction of the responses of western European vegetation and climate to long- and short-term global climate changes of the Ice Ages.

Geological archives show that the Earth’s climate has experienced large changes during the last 2.58 million years (Shackleton and Opdyke, 1973; Hays et al., 1976), with alternating warm (interglacial) and cold (glacial) periods. During the glacial periods ice occupied large regions of the Northern Hemisphere resulting in low sea level stands while during interglacials the ice sheets only covered Greenland and sea level was similar or higher than at present. These changes in ice volume are identified in marine sedimentary sequences by measuring the oxygen isotope composition (δ¹⁸O) of benthic foraminifer carbonate shells, providing the basis of the marine isotopic stratigraphy. Depleted values define the interglacial periods sensu lato (s.l.) or odd numbered MIS, while heavy values mark the glacial periods corresponding to even-numbered MIS. Within interglacial periods, an alternation of ice retreats, called “e,” “c” and “a,” and ice advances, “d” and “b,” is observed, also resulting from orbital forcing. The interglacial period sensu stricto (s.s.) corresponds to the substage with minimal ice volume, often located just after deglaciation (Railsback et al., 2015).

Two long and continuous pollen-rich sedimentary sequences are available from the southwestern and northwestern Iberian margin covering the last 1.5 million years and 425,000 years, respectively. These Iberian margin sequences are each composed of three cores: the southern sequence is a composite of cores MD95-2042, MD01-2443, and IODP U1385 located at around 37°N (Figure 5; Sánchez Goñi et al., 1999, 2016b; Roucoux et al., 2006; Chabaud et al., 2014; Oliveira et al., 2016), and the northern one is a composite of cores MD99-2331, MD01-2447, and MD03-2697 collected at 42°N (Desprat et al., 2005, 2006, 2007, 2009; Sánchez Goñi et al., 2005, 2008; Naughton et al., 2007, 2009, 2016). The pollen analysis of the 425,000-year northern sequence is completed (Desprat et al., 2017) while that of the southern site is in progress (Figure 5). Further north, the sequence MD04-2845 collected on the European margin, off western France, spans the last 140,000 years (Sánchez Goñi et al., 2008, 2012).

Both Iberian margin pollen sequences show an alternation of glacial periods dominated by Mediterranean steppe in southern Iberia (that we call semi-desert although it rarely included true semi-desert taxa such as Lygeum) and steppe/heathlands to the north, and interglacial periods when Mediterranean and Atlantic forests expanded over southern and northern Iberia, respectively. Tree pollen percentages are a good estimate for the magnitude of the interglacial in both Iberian regions. The magnitude of past Mediterranean forest expansions is interpreted as an indicator of the amount of winter precipitation in southern Iberia, given that relatively warm temperatures and summer drought occurred. The magnitude of the Atlantic forest expansion during different interglacials will mainly reflect the amplitude of the warming above 40°N, as forest development in the Atlantic domain requires a climate with a growth period of 4–6 months, and a mild winter period of 3–4 months (Polunin and Walters, 1985). Two or even three major phases of forest expansion occurred during all these interglacials, related with ice volume minima during sub-stages “a,” “c,” and “e” within each MIS (Figure 5). However, forest development in general reached its maximum during the earliest phase of each stage, concomitant with the highest sea level, indicating that warmest and wettest conditions occurred then, and subsequently identifying this phase with the terrestrial interglacial sensu stricto.

Pollen records from the Iberian margin show that the magnitude of forest development substantially differed from one interglacial to another in the south (MIS 19 to MIS 1, Mediterranean forest pollen = 40 to 80%, Figure 5), while in the north the magnitude of the forest expansion was similar (Atlantic forest pollen = 80%; Desprat et al., 2017). These data suggest that temperatures were similarly warm during the last five interglacials in northwestern Iberia, whereas the amount of winter precipitation in the south was quite variable. The Holocene (MIS 1, ∼10 ka), the last interglacial (MIS 5e, ∼128 ka) and MIS 17c (∼690 ka) would have been the wettest followed by MIS 9e (∼335 ka), MIS 11c (∼415 ka) and MIS 19c (∼784 ka), and MIS 7e (∼240 ka) being the driest. Excluding the Lake Orhid pollen record (Sadori et al., 2016), these results coincide with what is observed in the European sequences of Tenaghi Philippon and Praclaux located above 40°N (Reille et al., 2000; de Beaulieu et al., 2001; Tzedakis et al., 2006), which show similar expansions of the temperate forest during the interglacials of the last 400,000 years. So far no long terrestrial pollen sequence exists below 40°N for comparison with the southwestern Iberian margin pollen record.

The comparison of the Iberian observations with those at higher latitudes illustrates the regional variability of the magnitude and impacts of climate change. Due to the combined forcing of insolation and GHG, climatic models indicate that MIS 5e, MIS 9e, and MIS 11c were the warmest at the highest latitudes, associated with the strongest melting of ice sheets (Yin and Berger, 2012, 2015). This scenario is in marked contrast with data from southwestern Europe showing that these interglacials were not particularly warm. Our pollen data suggest that substantial differences in precipitation occurred across the different interglacials. Nevertheless, it is difficult to propose a relationship between warming at high latitudes due to GHG increase and dryness in the Mediterranean region, contrary to what climate models project for the end of this century (Hoerling et al., 2012). Identifying forcing factors responsible for the interglacial diversity in terms of precipitation in the Mediterranean region is a subject of debate. A good candidate is the orbital parameter of precession that influences regions below 40°N (Ruddiman and Mcintyre, 1984). Precession controls the amplitude of the seasonality and, therefore, the amount of winter precipitation in subtropical regions as recently suggested by models (Yin and Berger, 2012) and observations (e.g., Sánchez Goñi et al., 2008; Oliveira et al., 2016, 2017).

More recently, geological archives have also shown that millennial-scale changes punctuated this long-term climatic variability at the global scale as these changes are registered in North Atlantic SSTs (Voelker and Workshop Participant, 2002; Martrat et al., 2007) as well as in air temperatures over Greenland (North Greenland Ice-Core Project Members [NGRIP], 2004; Barker et al., 2011) and Antarctica (Augustin et al., 2004). These changes occur independently of the boundary climate state, i.e., in glacial and interglacial periods, and are generally associated with changes in ice volume (Siddall et al., 2003), GHG concentration (Knutti et al., 2004; Spahni et al., 2005; Loulergue et al., 2008), and AMOC (Oppo et al., 2006; Lynch-Stieglitz, 2017). The magnitude and frequency of these short-term climatic oscillations were larger when ice caps reached a critical mass (McManus et al., 1999) even if they were not the trigger (Barker et al., 2015).

So far, the best studied millennial-scale climate variability is that observed during the last glacial period (MIS 4, MIS 3 and MIS 2, i.e., ∼73–14.7 ka) characterized by a series of warming and cooling events called Dansgaard-Oeschger (D-O) cycles that were first identified in the atmosphere of Greenland and usually lasted 500–2,000 years (Dansgaard et al., 1984). These cycles recorded in the δ¹⁸O of the ice cores were characterized by a large (7 to 16°C) and rapid (within a few decades) warming event followed by a progressive decrease in temperature and a final abrupt cooling (Wolff et al., 2010). The warming and progressive cooling phase is termed Greenland Interstadial (GI), and the final cooling leading to the cold phase termed Greenland Stadial (GS). The GI phases lasted between 100 and 2,600 years (Wolff et al., 2010). Also during the last glacial period, repeated iceberg discharges cooled the surface of the North Atlantic (Heinrich, 1988; Bond et al., 1993). On the one hand, massive iceberg discharges from the Laurentide ice sheet, the so-called Heinrich events (HE), occurred with a cyclicity of 7,000–10,000 years while, on the other hand, weaker discharges coincided with iceberg fragmentation from the British-Icelandic-Scandinavian (BIS) ice cap (Elliot et al., 2001). We define a HE as the time period synchronous with the deposition of the coarse sediment Heinrich layer in a given region following the iceberg discharge while the Heinrich Stadial (HS) is the cold interval associated with the HE (Sánchez Goñi and Harrison, 2010). Some GSs encompass the HSs while the others are associated with the BIS minor iceberg discharges. These cold intervals are related to decreases in the AMOC (McManus et al., 2004; Lynch-Stieglitz, 2017) but the cause of the ice sheet collapse remains a subject of debate (e.g., Alvarez-Solas and Ramstein, 2011; Barker et al., 2015). The duration of the iceberg discharge has been simulated to be between 50 and 200 years (Roche et al., 2004) and data show that the HSs lasted longer, up to 3000 years (Sánchez Goñi and Harrison, 2010). HE and HS time intervals coincide in the Ruddiman belt, the preferential zone of IRD deposition that extends roughly between 45 and 50°N, while this is not the case outside (Naughton et al., 2009). Since the discovery of these millennial-scale climate changes, D-O cycles and HEs, the paleoclimatic community has dedicated major efforts to investigate the regional expression of this variability, the oceanic and atmospheric mechanisms involved in its transmission, and the interaction with long-term climate forcing such as the orbital parameters and ice volume. The following sub-section summarizes the great contribution that palynological research conducted on marine sediments has done so far to increase current understanding of these processes in western Europe.

In this section, we firstly focus on the specific mechanisms linking changes in Earth’s orbit to Ice Ages taking as a test bed the onset of the last glaciation (i.e., MIS 5a/4 transition: ∼80–70 ka), when one of the largest ice accumulations of the last 250,000 years occurred.

Ruddiman and McIntyre (1979) showed unequivocally that the ice growth in the northern hemisphere was contemporaneous with persistent warmth and high salinity in the subpolar North Atlantic (44–54° N) when boreal summer insolation was decreasing. This apparent paradox was interpreted as the development of a strong thermal gradient between a warm subpolar North Atlantic and a cold nearby land leading to an increase of moisture. This moisture was transported toward the north by storm tracks and fell as snow following the decrease in insolation. However, this theoretical model involving a warm ocean-cold land thermal contrast was not confirmed at that time due to the lack of data. Also, the interglacial/glacial transitions were then thought to be gradual but we know now that they were punctuated by millennial-scale events. Thus, one may wonder whether sub-orbital climatic variability, i.e., D-O cycles and HEs, affected this orbitally-controlled ice growth.

During the MIS 5a/4 transition, the direct land-sea correlation of the core MD04-2845, located at 45°N in the northern subtropical Atlantic, shows that the increase in ice volume at orbital scale was contemporaneous with a progressive cooling in western Europe, as indicated by the replacement of temperate forest by boreal forest (mainly Abies and Picea). The simultaneous stabilization of warm SST in the Bay of Biscay demonstrates for the first time a long-term increase of the thermal warm sea-cold land gradient toward the MIS 4 glacial (Sánchez Goñi et al., 2013) (Figure 11). At sub-orbital scale, a decoupling with cooling on land synchronous with warming in SST is furthermore observed (Figure 12). Three successive episodes of strong land-sea thermal gradient and another three with weak gradients occurred during the MIS 5a/4 transition, superimposed on the long-term increasing trend in the thermal gradient (Sánchez Goñi et al., 2013).

The strong land-sea thermal gradients coincided with the North Atlantic cold events C20, C19, and C18′ marked by moderate iceberg discharges in the western side of the North Atlantic (McManus et al., 1994) and related to Greenland cold episodes GS 21, GS 20, and GS 19 (Rasmussen et al., 2014) while the weak gradients coincided with the warm episodes in the North Atlantic and Greenland, GI 20, GI 19, and GI 18. Whereas the events C20, C19, and C18′ were cold in the subpolar gyre and cold conditions installed in western Europe, the northern subtropical gyre remained warm. Increased snowfall in northern Europe and subsequent ice growth resulted from the high rates of moisture production resulting from the abovementioned strong land-sea thermal gradient and its transport by northward tracking storms. Southward displacement of tundra by 10° in latitude during cold phases C20, C19 and C18′, as suggested by boreal forest colonization of western Europe, probably amplified ice growth owing to the increase in surface albedo (Crucifix and Loutre, 2002). Weak gradients slowed down the process but still allowed ice accumulation. The marine and pollen palaeoclimatic records from core MD99-2331, off north-western Iberia, show the same orbital and sub-orbital increases in the land-sea thermal gradient, excluding during C19 (Figure 12). In contrast, further south, in the south-western Iberian margin, the thermal gradient remained weak throughout the MIS 5a/4 transition. In summary, the direct comparison between marine and terrestrial records from the European margin demonstrates for the first time a long-term increase in the thermal gradient between the cold air and warmer sea, and three short intervals of even more pronounced thermal gradients during the last entering in glaciation. This synergy between orbital and millennial-scale variability provided a substantial source of moisture that was transported, through northward-tracking storms, to feed ice sheets in colder Greenland, northern Europe, and the Arctic.

The origin of the short-lived climate changes is still a subject of debate. On the one hand, some authors propose that high-latitude processes such as iceberg discharges and changes in the AMOC would have forced the rapid climate changes observed in the mid and low latitudes of the North Atlantic Ocean throughout the Pleistocene (Ganopolski and Rahmstorf, 2001). On the other hand, other authors put forward a low-latitude forcing mechanism via the harmonics of precession. Actually, the tropical latitudes receive over the course of the year a daily irradiance characterized by a double maximum that originates in the equatorial insolation cyclicities at 5,500 and 11,000 years (Berger et al., 2006). A direct consequence of this process would be a larger latitudinal thermal gradient and the enhanced transport of warmth and moisture by either atmospheric (westerlies) or oceanic circulation (subtropical gyre) from equatorial to high latitudes in the North Atlantic (Berger et al., 2006).

This low latitude forcing has also been suggested during periods of low eccentricity, when precession changes are muted, such as the last 45,000 years and MIS 19, centered at 800,000 years ago, based only on paleoceanographic evidence (e.g., McIntyre and Molfino, 1996; Ferretti et al., 2015). Our recent land-sea direct correlation studies at the IODP Site U1385 (south-western Iberian margin) have provided firm evidence on the transport of energy from equatorial regions to the high latitudes by means of the westerlies during the MIS 19 and MIS 11, the astronomically closest analogs to the present interglacial. During MIS 19 and MIS 11, Mediterranean forest pollen percentages indicate two and three long-term major Mediterranean forest expansions, respectively, following maxima in insolation (Figure 13 for MIS 19 as an example). These major forest episodes point to the occurrence of a well-established Mediterranean climate, with higher winter precipitations and warmer and drier summers. Forest expansion in south-western Iberia therefore indicates that the prevailing zonal configuration of the westerlies during these interglacials brought precipitation to this region (Oliveira et al., 2016; Sánchez Goñi et al., 2016b). During MIS 19c and MIS 11c, a long-term decrease in the Mediterranean forest cover indicating a progressive cooling and drying trend contrasts with the SST record showing quite stable and warm conditions (around 18°C) in the subtropical gyre. During these intervals, freshwater input is not detected on this margin and the progressive increase in the local benthic foraminifer δ¹³C indicates good deep water ventilation (Oliveira et al., 2016; Sánchez Goñi et al., 2016b). These long-term trends were punctuated by major contractions of the Mediterranean forest coeval with increases in Mediterranean semi-desert plants, which may indicate intervals when the westerlies were slightly deflected toward the north with a subsequently reduced influence in south-western Iberia. Repeated meridional shifts in the westerlies during MIS 19c and MIS 11c would have implied successive large masses of warm and moist air reaching the high latitudes of the North Atlantic via the western boundary current of the North Atlantic subtropical gyre. At sub-orbital time scales a decoupling also occurred during MIS 19c and MIS 11c with several drying and cooling events on land concomitant with stable warm SST.

The direct comparison of marine and pollen proxies shows therefore a clear air-sea decoupling during MIS 19c and MIS 11c at orbital and millennial time-scales. In contrast, the other cool and dry events during MIS 19b-19a and MIS 11b-11a were contemporaneous with SST cooling in the subtropical gyre and IRD deposition at in the subpolar gyre (IODP Site U1314 and ODP Site 980), which evidences the input of large amounts of icebergs in the subpolar North Atlantic (Oppo et al., 1998; Alonso-Garcia et al., 2011). Interestingly, the atmospheric cool and dry events during MIS 19c and MIS 11c were not associated with freshwater pulses in either the subtropical or subpolar gyres indicating that the cause of these events was not primarily related to high latitude ice-sheet dynamics. Fourier spectral analysis applied to the pollen and marine records, excluding the benthic foraminifer δ¹⁸O record, show dominant 5,000- and 10,000-year cyclicities for MIS 19 and MIS 11, likely related with the fourth and second harmonics of precession, respectively (Oliveira et al., 2016; Sánchez Goñi et al., 2016b; Oliveira, 2017). For MIS 19c, the comparison of the Mediterranean forest pollen record and its 5-kyr bandpass filter output curve with the variations in the largest amplitude of the seasonal cycle reconstructed at the equator shows good correspondence between the two records although the magnitude of changes was larger during MIS 19b-19a than during MIS 19c (Sánchez Goñi et al., 2016b). This good correspondence gives support to the low latitude origin of the observed repeated shift of the westerlies in southwestern Iberia at 37°N likely related to the harmonics of precession.

The stronger forest contractions during MIS 19b-19a and MIS 11b-11a would be explained by the regional SST cooling linked to the freshwater fluxes arriving at the subpolar gyre and reducing the AMOC. These data show once more that cold and dry episodes occurred independently of the amount of ice volume (Oppo et al., 1998; Desprat et al., 2009). In contrast, their intensity and duration are modulated by positive feedback mechanisms on the AMOC associated with ice dynamics and, particularly, with freshwater pulses from iceberg discharges and melting (Barker et al., 2015; Oliveira et al., 2016).

During the MIS 19c/19b transition the three cold land-warm sea decoupling events coincide with three increases in the benthic δ¹⁸O values concomitant with decreases in sea level. This observation suggests that for a period of low amplitude changes in insolation a tropically-driven strong warm sea-cold land contrast at millennial-timescale in the western European margin likely contributed to the increase of moisture. Similarly to that described previously for the MIS 5a/4 transition, these repeated millennial-scale episodes of moisture increase fed the ice caps through northward storm tracks and triggered the onset of the successive glacial period, at around 774,000 years ago.