The Bølling–Allerød Interstadial

Abstract

The Bølling–Allerød (B-A), the terrestrial counterpart of Dansgaard–Oeschger 1, is the first abrupt northern hemisphere climate warming episode of the last deglaciation. Several hypotheses have been proposed to explain this event: all involve the strengthening of the Atlantic Meridional Overturning Circulation (AMOC) prior or during the Heinrich Stadial 1/B-A transition and the consequent warming of the northern hemisphere. Terrestrial proxies (mainly pollen and speleothems) reflect a progressive increase in moisture availability from the Bølling to the Allerød in Europe contrasting with Greenland isotope records which show maxima in temperature and precipitation at the onset of the Bølling and a gradual decrease to the end of the Allerød. Changes in moisture availability in Europe are triggered by coupled interactions between the AMOC and changes in atmospheric dynamics led by the westerlies.

Full text

Chapter 6
The BøllingAllerød Interstadial
Filipa Naughton
1,2
, Marı´a F. Sa´nchez-Gon
˜i
3,4
, Amaelle Landais
5
, Teresa Rodrigues
1,2
,
Natalia Vazquez Riveiros
6
and Samuel Toucanne
6
1
Portuguese Institute for Sea and Atmosphere (IPMA), Lisboa, Portugal,
2
Center of Marine Sciences (CCMAR), Algarve University, Campus de
Gambelas, Faro, Portugal,
3
Environnements, Pale
´oenvironnements Oce
´aniques et Continentaux, Ecole Pratique des Hautes Etudes (EPHE, PSL),
Pessac, France,
4
University of Bordeaux, EPOC, Campus de Gambelas, Faro, France,
5
UMR8212, CEACNRSUVSQUPS, Laboratoire des
Sciences du Climat et de l’Environnement (IPSL), Gif-sur-Yvette, France,
6
Institut Franc¸ ais de Recherche pour l’Exploitation de la Mer (IFREMER),
Unite
´de Recherche Ge
´osciences Marines, Plouzane
´, France
Chapter outline
6.1 Definition, timing and causes of the BøllingAllerød
episode 45
6.2 The impact of B-A in the eastern North Atlantic and
Europe 47
References 48
6.1 Definition, timing and causes of the BøllingAllerød episode
The BøllingAllerød (B-A) interstadial, the first warm event of the last deglaciation in the northern hemisphere, was
originally described as consisting of three phases or chronozones (Mangerud et al., 1974): the Bølling and Allerød
warm interstadials and the Older Dryas cold stadial in-between. These phases were firstly identified in two Danish sites
based on plant remains: the Bølling and the Allerød sites (Hartz and Milthers, 1901; Iversen, 1942, 1954). The Older
Dryas was characterised by the presence of subarctic/arctic flora (e.g. Dryas octopetala) and the Allerød and Bølling
interstadials by the expansion of tree birch. Some high-resolution records from Europe further indicate that the B-A
was interrupted not only by the Older Dryas but also by the intra-Allerød cold event (e.g. Von Grafenstein et al., 1999;
Brauer et al., 2000;Hoek, 2009). We know now that vegetation changes associated with the B-A interstadial did not
occur synchronously outside this region and that the term B-A should not be generalised (e.g. Hoek, 2009). However,
as the B-A interstadial has been extensively used in records around the world and is now so firmly fixed in the litera-
ture, replacing it would be a big challenge (Hoek, 2009; Rasmussen et al., 2014). Therefore the INTIMATE event stra-
tigraphy recommends that the B-A can be used as a synonym for DansgaardOeschger 1 or Interstadial 1, but not as a
synonym for Greenland Interstadial 1 (GI-1) (Rasmussen et al., 2014). Also, because the B-A is subdivided into five
subevents in European records whilst the GI-1 is subdivided into seven subevents in Greenland ice, the INTIMATE
event stratigraphy group recommends to avoid giving names to these centennial-scale events because of the risk of mis-
interpretation (e.g. Rasmussen et al., 2014). Detecting these centennial-scale events is very challenging in some marine
and terrestrial sedimentary sequences due to their relative low temporal resolution and because some proxies are not
sensitive enough to respond to these abrupt cold spells.
We consider here that the B-A is the first northern hemisphere abrupt warming event of the last deglaciation that
occurred during an increase of boreal summer insolation, and precisely between Heinrich Stadial 1/Oldest Dryas and
the Younger Dryas (e.g. Severinghaus and Brook, 1999;Hoek, 2009;Denton et al., 2010;Naughton et al., 2016); that
is, 14.612.9 ka b2k (b2k: before CE 2000) or 14.712.85 years BP (BP: before present CE 1950) (Rasmussen et al.,
2014). In Antarctica, the B-A is synchronous with a cooling event; the Antarctic Cold Reversal (Stenni et al., 2001,
Pedro et al., 2016).
One of causes of the B-A abrupt warming episode in the northern hemisphere involves the ‘bipolar seesaw’ mecha-
nism, or the ‘heat piracy’ theory, associated with the meridional heat transport of the Atlantic Meridional Overturning
45
European Glacial Landscapes. DOI: https://doi.org/10.1016/B978-0-323-91899-2.00015-2
©2023 Elsevier Inc. All rights reserved.

(Continued)
46 PART | II Climate changes during the Last Deglaciation in the Eastern North Atlantic region

Circulation (AMOC) (e.g. Crowley, 1992;Broecker, 1998;Rahmstorf, 2002). During the preceding Heinrich Stadial 1
(HS 1), the massive discharges and melting of northern hemisphere ice sheets triggered the weakening of the AMOC
and the reduced northward heat transport, explaining the opposing behaviour between the two hemispheres (e.g.
Crowley, 1992;Broecker, 1998). The contrasting signal in both hemispheres, during the B-A, with warm in the north
and cooling in the south would be explained by a more vigorous AMOC, facilitating the northward ocean heat transport
(e.g. Crowley, 1992;Broecker, 1998;Rahmstorf, 2002). Some authors suggest that the interruption of meltwater dis-
charges into the North Atlantic have favoured the resumption of the AMOC and the consequent warming of the north-
ern Hemisphere causing the HS 1/B-A transition (e.g. Liu et al., 2009;Lohmann et al., 2016;Ng et al., 2018). Some
authors (Gregoire et al., 2016, Ng et al., 2018) suggest that this warming could have triggered Laurentide ice sheet
melting and the collapse of the Greenland and Iceland ice sheets at around 14.5 ka and a subsequent 1422 m sea-level
rise (Lambeck et al., 2014; Gregoire et al., 2016; Telesinski et al., 2014; Norðdahl and Ingo
´lfsson, 2015). This meltwa-
ter pulse would reduce rather than invigorate the AMOC after 14.5 ka; but this was not detected in marine records (Ng
et al., 2018). Other authors, based on several transient simulations of the last deglaciation, show that the increase of
CO
2
had a pivotal role and contributed to an abrupt increase in the AMOC despite meltwater input into the North
Atlantic (Liu et al., 2009; Shakun et al., 2012; Zhang et al., 2017; Obase and Abe-Ouchi, 2019). Some authors further
suggest that the AMOC resumption during the B-A was triggered by a warming at intermediate depths of the North
Atlantic during the preceding HS 1 as a result of the seesaw effect (Thiagarajan et al., 2014; Su et al., 2016).
Regardless of the role of each driver, data and climate simulations agree with a more vigorous state of the AMOC
and warming in the northern hemisphere during the B-A (e.g. McManus et al., 2004;Liu et al., 2009;Ng et al., 2018).
Climate simulations further suggest an increase of moisture in Europe as a response to the AMOC strengthening (e.g.
Rahmstorf, 2006).
6.2 The impact of B-A in the eastern North Atlantic and Europe
Sea surface temperature (SST) estimates from alkenones and planktic foraminifera assemblages reveal that the B-A was
marked by an abrupt SST increase of 410 %
oC in the eastern North Atlantic mid-latitudes (western Iberian Margin and
W Mediterranean Sea) (Fig. 6.1) (e.g. Bard et al., 2000;Cacho et al., 2001;Pailler and Bard, 2002;Martrat et al., 2007;
2014;Rodrigues et al., 2010;Salgueiro et al., 2014;Naughton et al., 2016). An abrupt warming was also detected in
several records from the Bay of Biscay by the decrease of the polar planktic foraminifera Neogloboquadrina pachyder-
ma abundances (Fig. 6.1) (e.g. Zaragosi et al., 2001). An increase of 4C in SST based on Mg/Ca ratios of planktic fora-
minifera was also recorded in the central and western North Atlantic mid-latitudes (Carlson et al., 2008; Repschla
¨ger
et al., 2015). The reduction of meltwater fluxes from Eurasian Ice sheets is testified by a decrease in the Channel River
runoff (Fig. 6.1)(Zaragosi et al., 2001; Me
´not et al., 2006). At this time, the AMOC became strongly vigorous, as sup-
ported by the
231
Pa/
230
Th records from the western North Atlantic mid-latitudes and the compiled North Atlantic dataset
(McManus et al., 2004; Ng et al., 2018). In Greenland, air temperatures increased by about 810C(Fig. 6.1)(Buizert
et al., 2014). The increase of atmospheric summer temperature of B35C is also detected in northern and southern
Europe (Renssen and Isarin, 2001; Dormoy et al., 2009), and supported by the oxygen-isotope ratios of deep-dwelling
ostracods from Lake Ammersee (southern Germany) (Von Grafenstein et al., 1999). The B-A warming led the expan-
sion of forest in the Iberian Peninsula (Fig. 6.1), southern France and central Europe, as shown by pollen records (e.g.
Litt and Stebich, 1999;Hoek, 2009;Fletcher et al., 2010;Naughton et al., 2016 and references therein) and by the δ
13
C
decrease in speleothem records (Genty et al., 2006; Moreno et al., 2010). However, pollen and speleothems records
indicate that the southern and central European temperate forests expanded progressively and the δ
13
C decreased gradu-
ally from the Bølling to the Allerød, contrasting with the Greenland temperature pattern that shows the warmest peak at
L
FIGURE 6.1 (A) Iberian margin alkenone-derived Sea surface temperature (SST) records (U
k
37 SST) (Bard et al., 2000; Pailler and Bard, 2002;
Martrat et al., 2007); (B) Iberian margin planktic foraminifera-derived SST records (Salgueiro et al., 2014; Naughton et al., 2016); (C) abundance of
planktic polar foraminifera Neogloboquadrina pachyderma from western Iberia and French margins (Salgueiro et al., 2014; Zaragosi et al., 2001); (D)
meltwater discharges (C37:4) from western Iberian and French margins (Bard et al., 2000; Me
´not et al., 2006; Martrat et al., 2007); (E) isoprenoid tet-
raether (BIT) index from western French margin (Me
´not et al., 2006); (F) ice-rafted debris (IRD) in the western Iberian margin (Bard et al., 2000;
Naughton et al., 2016) and (G) in the western French margin (Me
´not et al., 2006); (H) temperate forest (TF), (I) heathland and (J) semidesert plants
(SD) abundances in NW Iberian margin record MD032697 (Naughton et al., 2016); (K) channel river flood (number of flood events per 250 years)
(Toucanne et al., 2015); (L) ex231Pa0/ex230Th0 from composite North Atlantic records (blue line) (black bold line: smoothed record) (Ng et al.,
2018) and from the western North Atlantic (light blue) (OCE326-GGC5; McManus et al., 2004); (M) Greenland temperature (Buizert et al., 2014);
(N) Atmospheric CO
2
reconstructed from West Antarctic Ice Sheet Divide ice core (Marcott et al., 2014). Heinrich Stadial 1 (HS 1), pre-HS 1 and
Younger Dryas: light blue bands.
The BøllingAllerød Interstadial Chapter | 6 47

the onset of GI 1 (Fig. 6.1). The temperature increase is clearly evident in most of the North Atlantic, European and
Greenland records, and agrees with a more vigorous AMOC at the onset of the Bølling (Fig. 6.1). Therefore, the con-
trasting pattern observed in central and southern European pollen and speleothem records with that of Greenland could
be the result of changes in moisture availability rather than in temperature (e.g. Naughton et al., 2016). The continuous
increase of moisture availability from the Bølling to the Allerød was also detected in other regions, such as in the west-
ern and central Mediterranean region (e.g. Dormoy et al., 2009;Desprat et al., 2013). Since the AMOC was vigorous at
the onset of the Bølling, it should be expected that moisture delivery was highest at the beginning of this interval in
Europe. However, changes in moisture delivery result from complex interactions between ocean moisture sources and
modifications in atmospheric dynamics (e.g. Naughton et al., 2009; 2019). The relative deficit in precipitation at the
onset of the Bølling, under a more vigorous AMOC mode, can be explained by changes in the position and shape of the
jet stream, which is responsible for the delivery of moisture over Europe via the westerly winds. The jet stream was
probably displaced further north in response to the North Atlantic high latitude warming with a more meandric form at
the onset of the Bølling. This would favour the transfer of high amounts of moisture to the North Atlantic high latitudes
lowering the precipitation in central and southern Europe (Naughton et al., 2016). This hypothesis has been proposed
for previous D-O warmings and is supported by δD and deuterium excess data from Greenland ice cores (Masson-
Delmotte et al., 2005; Gon
˜i et al., 2009), even if the climatic boundary conditions are not the same during the Marine
Isotope Stage 3 (B6026 ka) and the last deglaciation.
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50 PART | II Climate changes during the Last Deglaciation in the Eastern North Atlantic region