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Geophysical Research Letters
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Supporting Information for
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Black Sea temperature response to glacial millennial-scale climate variability
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Antje Wegwerth1, Andrey Ganopolski2, Guillemette Ménot3, Jérôme Kaiser1,
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Olaf Dellwig1, Edouard Bard3, Frank Lamy4, and Helge W. Arz1
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1Leibniz-Institute
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2Potsdam
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Institute for Climate Impact Research (PIK), Earth System Analysis, Potsdam, Germany.
3CEREGE,
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for Baltic Sea Research Warnemünde (IOW), Marine Geology, Rostock, Germany.
Aix-Marseille University, Collège de France, CNRS, IRD, Aix en Provence, France.
Alfred-Wegener-Institut Helmholtz-Zentrum für Polar und Meeresforschung, Bremerhaven, Germany.
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Contents of this file
Text S1 to S3
Figures S1 to S3
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Introduction
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The supplementary information provides additional information regarding the
contrasting temperature patterns during Dansgaard-Oeschger (DO) cycles and Heinrich
events (HE) in the southeastern (SE) and northwestern (NW) Black Sea. We also
present additional records confirming DO cycles in the Black Sea region and point out
temperature anomalies during individual cooling and warming events in Greenland, the
North Atlantic, the Mediterranean, and the SE Black Sea. Finally, the spatial patterns of
DO cycles and HE deduced from climate modeling are described in greater detail.
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Text S1. Contrasting TEX86 temperature patterns of DO cycles and HE in the NW
and SE Black Sea
As seen in Fig. S1, the TEX86-temperature record (from 40 ka BP to 9 ka BP) from
the NW Black Sea [Ménot and Bard, 2012] shows cooling during HE but no clear pattern
of DO cycles, in contrast to the record from the SE Black Sea (this study). These
differing temperature records initially suggest different impacts of the climate variability
of the Greenland DO cycles on the NW and SE Black Sea that were probably due to
differences in the regional atmospheric circulation. The DO cycle signatures from other
records in these regions [Allen et al., 1999; Tzedakis et al., 2004; Fleitmann et al., 2009;
Fletcher et al., 2010; Shumilovskikh et al., 2014], however, suggest that these
differences in the Black Sea temperatures were not climate-driven.
One reason might be that the NW coring site was subjected to higher terrestrial
inputs from the large and heterogeneous NW Black Sea drainage area. Thus, relatively
large amounts of isoprenoid GDGTs derived from Thaumarchaeota thriving in soils
[Schouten et al., 2013] could have biased the reconstructed temperatures. The
Branched and Isoprenoid Tetraether (BIT) index, defined as the normalized (0–1) ratio
between terrestrial-derived branched GDGTs and crenarchaeol [Hopmans et al., 2004],
is a useful tool for estimating terrestrial organic matter input. A BIT index close to 0 is
typical for open marine systems, while a BIT index close to 1 characterizes terrestrial
systems [Hopmans et al., 2004]. BIT values from the SE Black Sea (Fig. S1) are <0.3
during the entire investigated period, indicative of generally low inputs of soil-derived
isoprenoid GDGTs and providing evidence of the suitability of the TEX86
paleothermometer [Weijers et al., 2006]. BIT indices from the NW region [Ménot and
Bard, 2012] are in many cases, >0.3 (Fig. S1), indicating a higher terrigenous
contribution in that region. Even though the TEX86 temperatures were BIT-corrected, the
temperature record lacks a distinct DO cycle pattern but shows HE-related cooling. A
recent study [Liu et al., 2014] suggests that the branched GDGTs were derived from the
water column and/or the sediment of the brackish/marine Holocene Black Sea, thus
calling into question the application of the BIT as proxy for the input of soil-derived
organic matter. Whether this is also the case for the limnic/brackish phases of the Black
Sea is unclear.
Further TEX86 temperature reconstructions from other regions of the Black Sea may
clarify the different spatial patterns of temperature. Although certain changes during HE
are indicated in other regions around the Black Sea, for example, by some but not all
palynological data in Greece [Tzedakis et al., 2004] and in Anatolian Lake Van [Çağatay
et al., 2014], clear evidence of temperature changes (i.e. HE cooling) is not contained in
these records.
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Text S2. Temperature anomalies during individual cooling and warming events
The pattern of the temperature anomalies during cooling events (Fig. S2) shows
that cooling was generally stronger during HE than during most non-HE stadials at the
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Iberian Margin and in the Alboran Sea. Note that HE 5a was apparently milder than the
other HE, but this was most likely due to an undersampling during this interval. By
contrast, the temperature anomalies during the HE are generally within the range of
those determined for non-HE stadials in Greenland and the Black Sea. The individual
temperature departures from the long-term mean (average between 20 and 65 ka BP;
Fig. S2) supports the spatial pattern of the proxy-based HE extra cooling, as shown in
Fig. 3a, c in the main text. The GIS temperature anomalies (Fig. S3) show that warming
was slightly stronger during the GIS succeeding the HE. However, in contrast to the GS
temperature anomalies, this difference compared to non-HE associated GIS warming is
minor.
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Text S3. Spatial patterns of DO cycles and HE deduced from climate modeling
The difference between the spatial patterns of the temperature anomalies during
DO cycles and HE was previously explained by the existence of three distinct modes of
the glacial Atlantic Meridional Overturning Circulation (AMOC) [Ganopolski and
Rahmstorf, 2001]. According to Ganopolski and Rahmstorf [2001], the DO cycle pattern
corresponds to the difference between the interstadial (similar to the present) and nonHE stadial modes of the AMOC, which is weaker than the interstadial mode but is also
characterized by a more southward location of the deep-water formation area in the
North Atlantic and a more widely expanded sea-ice cover in Nordic Seas. The significant
reduction of the sea-ice cover during the transition from a non-HE stadial to an
interstadial mode of the AMOC strongly amplifies the direct temperature response to the
increased meridional ocean heat transport, especially during the winter. This concept is
additionally supported by the results of Li et al. [2005], which showed (Fig. 1B) that the
temperature effect of sea-ice removal from the Nordic Seas is similar to the DO pattern.
The HE temperature anomaly pattern represents additional cooling from the non-HE
stadial to the HE stadial. This cooling can be primarily attributed to a significant reduction
of the AMOC during HE, a conclusion supported by the larger magnitude of the bipolar
seesaw pattern during HE stadials [Margari et al., 2010]. The results reported by
Ganopolski and Rahmstorf [2001], who used a coarse-resolution model of intermediate
complexity (CLIMBER-2), were recently corroborated by those of modeling experiments
obtained with the high-resolution fully coupled general circulation model CCSM3 of
Zhang et al. [2014]. Figure 1b in that paper shows annual mean temperature differences
between experiments of “+0.02 Sv” and “−0.02 Sv”, which can be interpreted as the
difference between non-HE stadial and interstadial modes. The results are in
qualitatively good agreement with the warming pattern of the DO shown in Fig. 3a of
Ganopolski and Rahmstorf [2001]. In particular, simulations with the CCSM3 model
showed maximum temperature anomalies (>10°C) over the Nordic Seas as well as a
large change in the maximum sea-ice extent in the North Atlantic. The temperature
differences between experiments “+0.02 Sv” and “+0.2 Sv” (Xiao Zhang, personal
communication) may reflect the difference between HE stadials (“off” mode of the
AMOC) and non-HE stadials. Again, the pattern of additional HE cooling is in agreement
with the CLIMBER-2 model simulations shown in Fig. 3c [Ganopolski and Rahmstorf,
2001]. In spite of the strong additional reduction in AMOC strength determined in this
experiment, the magnitude of additional cooling is significantly smaller than the
differences between experiments “+0.02 Sv” and “−0.02 Sv”. Moreover, the maximum of
the temperature anomalies associated with the AMOC shutdown is located south of
Iceland. As described by Ganopolski and Rahmstorf [2001], a complete shutdown of the
AMOC in the CCSM3 model causes only modest additional cooling in Greenland and
has a very small effect on the maximum sea-ice extent. It is, however, important to note
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that, although the annual mean DO signal is stronger than the additional HE cooling, it is
strongest during the cold season whereas additional HE cooling is less seasonally
dependent. In addition, HE cooling occurred closer to the Equator and therefore should
have had a stronger impact on the position of the Intertropical Convergence Zone, which
is crucial for the strength of the Southeast Asian summer monsoon. This can explain
why in the monsoon proxies HE stadials differed from non-Heinrich stadials [Wang et al.,
2001].
Considering the slight differences between model-based and proxy-based
temperature changes during HE cooling (Fig. 3c), the temperature reconstructions (apart
from those in Greenland) may be biased towards summer, whilst the response to AMOC
changes is stronger during winter. This can explain why annual mean model
temperature changes are greater than those induced from data.
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Figure S1. Climate and environmental conditions in the Black Sea and northern Anatolia
during the last glacial. TEX86 mean annual lake surface temperature (LST) for the NW
Black Sea (MD042790; gray, BIT-corrected [Ménot and Bard, 2012]) and the SE Black
Sea, 25GC-1 (red, this study). The BIT index for the NW Black Sea (gray, [Ménot and
Bard, 2012]) and the SE Black Sea (red, this study) is shown. Crenarchaeol was
normalized to TOC and is an indicator of Thaumarchaeota productivity (this study). The
sum of all dinoflagellate cysts is a proxy for productivity [Shumilovskikh et al., 2014]. The
percentage of arboreal pollen reflects vegetation changes during DO cycles in Northern
Anatolia [Shumilovskikh et al., 2014]. δ18O serves as a record of Sofular stalagmites So1 and So-2 [Fleitmann et al., 2009]. The chronology for 25GC-1 was based primarily on
NGRIP (GICC05) and that of MD042790 on Hulu Cave speleothem record [Wang et al.,
2001; Ménot and Bard, 2012].
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Figure S2. Temperature anomalies for cooling during the individual HE (HE 2–6) and
non-HE stadials (Greenland Stadials GS 4–17) for Greenland ([Kindler et al., 2014],
NGRIP); the North Atlantic/Iberian Margin ([Martrat et al., 2007], core MD01-2444), the
western Mediterranean Sea/Alboran Sea ([Martrat et al., 2004], ODP Site 977A), and the
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SE Black Sea (this study). Each temperature anomaly describes the deviation during the
strongest cooling from the average temperature throughout the records between 20 and
65 ka BP. The average temperature during the investigated period is -44.9°C for
Greenland, 13.9°C for the Iberian Margin, 13.2°C for the Alboran Sea, and 7.0°C for the
Black Sea record.
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Figure S3. Temperature anomalies for warming during the individual Greenland
Interstadials (GIS) following HE (diamonds) and during GIS following non-HE stadials
(circles) for Greenland ([Kindler et al., 2014], NGRIP); the North Atlantic/Iberian Margin
([Martrat et al., 2007], core MD01-2444), the western Mediterranean Sea/Alboran Sea
([Martrat et al., 2004], ODP Site 977A), and the SE Black Sea (this study). Each
temperature anomaly describes the deviation during the strongest warming from the
average temperature throughout the records (between 20 and 65 ka BP). The average
temperature during the investigated period is -44.9°C for Greenland, 13.9°C for the
Iberian Margin, 13.2°C for the Alboran Sea, and 7.0°C for the Black Sea record.
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