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A multi-proxy approach to drought reconstruction
Inga Labuhn, Valérie Daux, Dominique Genty
To cite this version:
Inga Labuhn, Valérie Daux, Dominique Genty. A multi-proxy approach to drought reconstruc-
tion. Quaternaire, AFEQ-CNF INQUA, 2016, 27 (3), pp.239-247. �10.4000/quaternaire.7632�. �hal-
02467720�
Quaternaire, 27, (3), 2016, p. 239-247
Manuscrit reçu le 16/10/2015, accepté le 26/12/2015
A MULTI-PROXY APPROACH
TO DROUGHT RECONSTRUCTION
n
Inga LABUHN
1,2
, Valérie DAUX
1
& Dominique GENTY
1
ABSTRACT
In palaeoclimate reconstructions, the combination of proxy records measured in different climate archives is challenging
because of the uncertainties associated with each proxy, but it can also help reduce some of these uncertainties. Here, we present a
novel approach to combine speleothem and tree ring proxies for a drought reconstruction of the last 640 years: a fluid inclusion δ
18
O
record from a stalagmite from Villars Cave (southwest France) and a tree ring cellulose δ
18
O record of Quercus spp. from the nearby
Angoulême area. The δ
18
O of the fluid inclusions is taken as an estimate of the δ
18
O of the trees’ source water. Then, the cellulose
and source water δ
18
O are used to calculate the leaf water isotopic enrichment, as well as relative humidity, which is the dominant
controlling factor of this enrichment. The reconstructed long-term trends in relative humidity differ from a previously published
reconstruction of moisture variability based on the tree ring record alone. Further measurements will be necessary to support either
reconstruction. Nevertheless, this investigation demonstrates the great potential for combining isotope proxies from speleothems and
tree rings to reconstruct both the low- and high-frequency variability of drought.
Keywords: tree rings, cellulose, speleothems, fluid inclusions, stable isotopes, oxygen (δ
18
O), palaeoclimate, drought, multi-proxy,
France
RÉSUMÉ
RECONSTITUTION DES PÉRIODES DE SÉCHERESSE PAR APPROCHE MULTI-INDICATEUR
En paléoclimatologie, la comparaison entre les différentes archives est complexe à cause des incertitudes associées à chaque
indicateur ; elle peut cependant aussi aider à réduire les incertitudes. Nous présentons ici une nouvelle approche pour combiner les
indicateurs paléoclimatiques issus des spéléothèmes et des cernes d’arbre pour reconstituer les sécheresses au cours des derniers
640 ans : le δ
18
O des inclusions fluides d’une stalagmite de la grotte de Villars (sud-ouest de la France) et le δ
18
O de la cellulose
des cernes de chênes (Quercus spp.) proches d’Angoulême. Le δ
18
O des inclusions fluides est considéré comme le δ
18
O de l’eau
souterraine qui alimente les arbres. Puis le δ
18
O de la cellulose et celui de l’eau souterraine sont utilisés pour calculer l’enrichis-
sement isotopique de l’eau de la feuille ainsi que l’humidité relative qui est le facteur dominant contrôlant cet enrichissement. La
variation de l’humidité relative reconstituée diffère des reconstitutions publiées basées uniquement sur les cernes d’arbre suggérant
la nécessité de faire d’autres reconstitutions pour mieux comprendre ces différences. Ce travail montre tout l’intérêt de comparer
deux archives pour tenter de reconstituer les périodes de sécheresse passées.
Mots-clés
: cernes d’arbre, cellulose, spéléothèmes, inclusions fluides, isotopes stables, oxygène (δ
18
O), paléoclimat, sécheresses,
multi-indicateur, France
1 - INTRODUCTION
The ever-increasing number of palaeoclimate proxy
records deepens our understanding of climate variabi-
lity in the past on different temporal and spatial scales
(Masson-Delmotte et al., 2013). At the same time,
field and laboratory studies on the formation processes
of climate archive lead to a better comprehension of
the climatic and non-climatic information recorded
in the proxies, and contribute to a better evaluation of
the proxies’ limitations (e.g. Dreybrodt & Scholz 2011;
Offermann et al., 2011; Steen-Larsen et al., 2011;
Gessler et al., 2013; Genty et al., 2014; Labuhn et al.,
2014). However, it remains challenging to compare and
combine proxies from different climate archives, as all
are afflicted with their own uncertainties related to dating,
climate sensitivity, and linearity, and they are limited to
certain aspects of variations in climate. A multi-proxy
approach can give a broader perspective on past climate
change, and may help constrain the causes of variability
(Mann, 2002; Guiot et al., 2005; Li et al., 2010).
Tree rings have been the principal source of information
for hemispheric-scale temperature reconstructions of the
past millennium (Mann et al., 1999; Esper et al., 2002;
Briffa et al., 2004; D’Arrigo et al., 2006). Their advan-
tage lies in the annual resolution and the precise dating.
1
Laboratoire des Sciences du Climat et de l’Environnement (LSCE), laboratoire CEA/CNRS/UVSQ, Orme des Merisier,
FR-91191 GIF-SUR-YVETTE. Emails: valerie.daux@lsce.ipsl.fr, dominique.genty@lsce.ipsl.fr
2
Department of Geology, Lund University, Sölvegatan 12, SE-223 62 LUND. Email: inga.labuhn@geol.lu.se
1606-091-Mep3-2016.indd 239 23/09/16 14:48
240
Tree ring proxies can be well replicated and quantified
by comparing them directly to meteorological variables
from the year of ring formation. Their calibration is based
on the statistical relationships with climate (e.g. Fritts &
Guiot, 1990). Models for climate reconstruction based
on these relationships can be verified using independent
meteorological data, i.e. data that have not been used in
the calibration, and the variance explained by the model
can be quantified (e.g. Briffa et al., 1992). However, tree
ring proxies may be biased towards climate conditions
of the time of year that has the strongest influence on
tree growth. Tree-ring reconstructed changes in summer
climate, for example, might not be representative of
the evolution of the annual climate (Jones et al., 2003).
Furthermore, tree rings, in particular tree ring widths, are
limited for reconstructing low-frequency climate varia-
bility because of the standardization process and the
limited length of individual tree series (Cook et al., 1995;
Esper et al., 2004; Moberg et al., 2005).
Speleothems have the potential to provide a low-
frequency signal. However, their dating is less precise
compared to the trees. Despite the high analytical preci-
sion that is now achieved for U-Th dating (Hoffmann et
al., 2009; Cheng et al., 2013), and the possibility to count
annual growth layers in some speleothem samples (Genty
& Quinif, 1996; Tan et al., 2006; Baker et al., 2008; Shen
et al., 2013), slight changes in the growth rate or short
hiatuses increase the error in the chronology, making it
difficult to compare a stalagmite record to instrumental
data or to high-resolution proxy data from tree rings.
While adequate samples, which have a high growth rate
and a precisely dated lamination, enable a direct calibra-
tion by comparison with meteorological data (Genty &
Quinif, 1996; Proctor et al., 2000), in many cases the
calibration of speleothem proxies relies on an under-
standing of the cave processes, based on monitoring data,
laboratory experiments, and modeling exercises (Spötl et
al., 2005; Genty, 2008; Tremaine et al., 2011; Wacker-
barth et al., 2012; Treble et al., 2013). Lastly, an envi-
ronmental signal can be lagged and/or attenuated before
it is transmitted to the cave interior (Fairchild & Baker,
2012), whereas tree ring proxies often reflect the envi-
ronmental signal more directly, i.e. during the current
growing season.
Few previous studies have attempted to directly
compare tree rings and speleothems. Berkelhammer et
al. (2014) and Trouet et al. (2009) used tree ring and
speleothem proxy records from remote regions to study
teleconnections and atmospheric circulation indices.
Betancourt et al. (2002) compared annual band widths
in tree rings and a stalagmite from the same site and
found no correspondence, but their approach has been
criticized because even if the banding in both archives is
annual, the band width in each proxy might not depend
on the same influences (Baker & Genty 2003). However,
when tree ring and speleothem proxies from the same
region respond to the same dominant factors, e.g.
droughts, they can show similar patterns (Sinha et al.,
2011; Wassenburg et al., 2013). Managave (2014) used
a model to investigate to what extent the oxygen isotopic
composition of tree ring cellulose and speleothem calcite
can be correlated if they have the same source water. The
author determined that a correspondence between these
proxies is likely when the variation in the d
18
O of preci-
pitation is high compared to the variation induced by the
influences on each single proxy, i.e. the cave tempera-
ture and equilibrium conditions during precipitation for
calcite d
18
O, and relative humidity, leaf temperature and
the isotopic composition of atmospheric water vapor for
cellulose d
18
O.
This article explores the potentials and limits of a
multi-proxy climate reconstruction based on speleothem
and tree ring proxy records from the southwest of
France, a region characterized by recurrent drought
periods that might prove to be particularly vulnerable to
the consequences of global warming (Moisselin et al
.,
2002; Itier 2008; Lemaire et al., 2010; Levrault et al.,
2010). Identifying the patterns of moisture variability in
the past may help evaluate the possible future extent of
droughts in a changing climate. Both archives have in
common that we can measure oxygen isotope ratios in
their components: in cellulose from the wood of tree rings
(d
18
O
c
), and in water that is incorporated in speleothem
calcite, the so-called fluid inclusions (d
18
O
fi
). Here, we
combine a d
18
O
fi
record from a speleothem from Villars
Cave (Labuhn et al., 2015) and a d
18
O
c
chronology from
the nearby Angoulême (Labuhn et al., 2016) in order to
reconstruct both low and high frequency drought variabi-
lity during the last 640 years. The oxygen in both proxies
originates from the precipitation feeding the soil water
reservoir, which subsequently infiltrates into the cave, or
is tapped by the trees. We therefore hypothesize that it
is possible to use the d
18
O
fi
as an independent estimate
of the source water d
18
O for the trees. In a first step, we
investigate the co-variation of the fluid inclusion and the
cellulose d
18
O time series. In a second step, we assume
that the d
18
O
fi
represents the d
18
O of the tree source water,
and, using d
18
O
c
and d
18
O
fi
, we calculate the isotopic
enrichment of the leaf water above the source water, as
well as relative humidity (RH), which is the dominant
controlling factor of this enrichment. Lastly, we compare
this reconstruction of relative humidity with a previous
drought reconstruction based only on tree rings (Labuhn
et al., 2016).
2 - STUDY SITES AND DATA
The study sites Villars Cave and Angoulême are
situated in the southwest of France at 50 km distance,
in similar geological and climatological settings (fig. 1).
As both are located at approximately the same altitude
(100-175 m a.s.l.) and distance from the coast (~150 km),
and no important relief separates them, they experience
similar variations in temperature, moisture conditions
(Météo-France, 2009), and in the isotopic composition
of precipitation (Millot et al., 2010). The bedrock in the
region is a Jurassic limestone, and karstic features such
as dolines, caves, and surface collapses characterize the
landscape.
1606-091-Mep3-2016.indd 240 22/09/16 14:54
241
The crossdated, annually resolved chronology of tree
ring cellulose d
18
O from Angoulême (45°44’N, 0°18’E)
has been constructed using living trees and building
timbers of oak (Quercus spp.), and covers the period
from 1360 to 2004 (Labuhn et al., 2014; 2016). Inter-
annual variations in d
18
O
c
at this site are dominated by
the atmospheric conditions that influence leaf water
enrichment (Labuhn et al., 2014), but the underlying
low-frequency variability are likely linked to variations
in the source water d
18
O.
The fluid inclusion d
18
O measurements were acquired
from a stalagmite from Villars Cave (45°26’N, 0°47’E;
Labuhn et al., 2015). The record covers the last
2,300 years, and the fluid inclusion samples have a reso-
lution of approximately 25 years. The stalagmite was
dated by the U-Th and
14
C methods, as well as by laminae
counting. Long-term monitoring series of precipitation
and cave drip water d
18
O demonstrated that drip water
corresponds to a weighted mean of pluri-annual preci-
pitation at this site, without any evaporative enrichment
or seasonal bias due to plant transpiration (Genty et al.,
2014). An investigation of corresponding modern fluid
inclusion samples indicated that the isotopic composition
of the infiltrating water was preserved in the fluid inclu-
sions (Labuhn et al., 2015).
Monthly relative humidity (RH) data was obtained
from Météo-France for Cognac (1961-2012, station
no. 16089001), which is the longest available RH
record from the study area. The standardized precipi-
tation evapotranspiration index (SPEI; Beguería et al.,
2010; Vicente-Serrano et al., 2010) was obtained from
http://sac.csic.es/spei/database.html, and the time
series for the Angoulême grid cell (1901-2011) was
extracted.
3 - METHODOLOGY
3.1 - COMPARISON OF THE FLUID INCLUSION
AND CELLULOSE RECORDS
For the comparison of fluid inclusions and cellulose,
the d
18
O
c
time series was smoothed by a 25-year running
mean. This interval was chosen because it likely corres-
ponds to the time period integrated in a fluid inclusion
sample, as deduced from sample size, stalagmite growth
rate and the infiltration time of the water from the surface
to the cave (Labuhn et al., 2015). Both time series were
transformed to z-scores (i.e. the differences of each value
and the mean value of the time series, divided by the
standard deviation) to make their variations more compa-
rable.
3.2 - CALCULATION OF LEAF WATER ENRICH-
MENT AND RELATIVE HUMIDITY
In order to calculate the leaf water isotopic composi-
tion and RH using the speleothem and tree ring isotope
proxies, we suppose that the d
18
O
fi
represents the tree
source water. The d
18
O
fi
values were linearly interpo-
lated in order to obtain a time series of annual resolu-
tion. Then, this time series was smoothed using a 25-year
running mean. The resulting interpolated and smoothed
fluid inclusion d
18
O values were supposed to be the
source water d
18
O for the trees each year. The cellulose
d
18
O time series, which had been normalized to a mean of
zero (as published in Labuhn et al., 2016), was adjusted
to the mean d
18
O value of recent cellulose (31 ‰).
The oxygen isotopic composition of cellulose can
be related to the isotopic composition of source water
Fig. 1: Study site locations.
The square indicates the location of Villars Cave, where the stalagmite was sampled, the circle indicates the location of the tree ring chronology from
Angoulême, and the triangle indicates the location of the meteorological station in Cognac.
Fig. 1 : Localisation des sites d’étude. Le carré indique la localisation de la grotte de Villars, le cercle indique la localisation de la chronologie des
cernes d’arbre à Angoulême, et le triangle indique la localisation de la station météorologique de Cognac.
1606-091-Mep3-2016.indd 241 22/09/16 14:54
242
(δ
18
O
sw
) and leaf water (δ
18
O
lw
) (Sternberg et al., 1986;
Yakir & DeNiro, 1990):
δ
18
O
c
= 0.42 x (δ
18
O
sw
+ e
wc
) + 0.58 x (δ
18
O
lw
+ e
wc
)
(Eq. 1)
where e
wc
, the fractionation factor between water and
carbonyl oxygen, is approximately equal to 27 ‰. Thus,
using our δ
18
O
c
record and the interpolated fluid inclusion
time series as an estimate of δ
18
O
sw
, we can calculate δ
18
O
lw
.
The δ
18
O
lw
is related to relative humidity (RH) as
follows (Dongman et al., 1974):
δ
18
O
lw
= δ
18
O
sw
x (1 – RH) + δ
18
O
v
x RH + e*+ e
k
x (1 - RH)
(Eq. 2)
where δ
18
O
v
is the oxygen isotopic composition of
atmospheric water vapor, and e* and e
k
are the equili-
brium and kinetic fractionation factors. At 20°C, e* is
equal to 9.7 ‰ (Horita & Wesolowski, 1994); e
k
is equal
to 16 ‰ / 21 ‰ / 32 ‰ for turbulent/laminar/static boun-
dary conditions respectively (Burk & Stuiver, 1981).
Under normal European summer conditions, the water
vapor is, on average, approximately in isotopic equili-
brium with soil water (Förstel & Hutzen, 1982). Thus:
δ
18
O
v
= δ
18
O
sw
– e*
(Eq. 3)
Combining Equations (2) and (3) gives the equation to
calculate RH:
δ
18
O
lw
– δ
18
O
sw
RH = 1 – –––––––––––––
e*+ e
k
(Eq. 4)
4 - RESULTS AND DISCUSSION
4.1 - COMPARISON OF THE FLUID INCLUSION
AND CELLULOSE RECORDS
The δ
18
O records in tree ring cellulose from Angoulême
and in speleothem fluid inclusions from Villars Cave
display some common trends: a decrease from 1500 to
1700, an increase from 1750 to present, and a marked
peak around 1720 (fig. 2). However, there is also a period
where the two series show opposing trends, between
1360 and 1500. Furthermore, even if the general increa-
sing trend in the most recent period is apparent in both
records, the large increase in the cellulose time series
from 1750 to 1850 and the subsequent rapid decrease are
not seen in the fluid inclusions. The range of δ
18
O values
in both the fluid inclusions and the smoothed cellulose
δ
18
O time series is about 2 ‰ (not shown). However, the
common peak at 1720 is about twice as large in the fluid
inclusions.
The co-variation in both records can be ascribed to a
common source water δ
18
O, which is controlled by the
average δ
18
O of precipitation in the region. Although the
δ
18
O
c
is strongly influenced by leaf water enrichment on an
inter-annual scale, the underlying low-frequency compo-
nent can be attributed to the variability of the source water
(Labuhn et al., 2014). Furthermore, enhanced evapora-
tion and transpiration during dry periods might cause an
increase of δ
18
O in both proxies (e.g. Bar-Matthews et al.,
1996; Denniston et al., 1999; Sternberg, 2009; Gessler et
al., 2013). Several factors may contribute to the disagree-
ment between cellulose and fluid inclusions:
(1) - The isotopic composition of the source water can
be modified before it becomes preserved in the proxies.
The modern calibration indicates that speleothem fluid
inclusions from Villars cave correspond to a weighted
mean of pluri-annual precipitation, without any signi-
ficant modification (Genty et al., 2014), but this might
not hold true for the past. The δ
18
O
c
, on the other hand,
reflects partly the isotopic composition of the source
water, and partly the isotopic composition of enriched
leaf water (eq. 1). Their relative contribution to the δ
18
O
c
has been estimated (Roden et al., 2000; Cernusak et al.,
2005), but is likely to vary over the growing season, as
well as over longer time periods (Gessler et al., 2009;
Offermann et al., 2011);
(2) - The δ
18
O
c
time series might not capture well the
low frequency variability in precipitation/source water
δ
18
O. A correction had to be applied for offsets in average
Fig. 2: Comparison of the speleothem fluid inclusion and tree ring cellulose δ
18
O time series (z-scores).
The cellulose δ
18
O chronology has been smoothed by a 25-year running mean.
Fig. 2 : Comparaison des séries temporelles du δ
18
O des inclusions fluides des spéléothèmes et de la cellulose des cernes d’arbre (z-scores). La chrono-
logie de δ
18
O de la cellulose a été lissée par une moyenne glissante de 25 ans.
1606-091-Mep3-2016.indd 242 27/09/16 09:46