A multi-proxy approach to drought reconstruction
Summary (3 min read)
1 - INTRODUCTION
- The ever-increasing number of palaeoclimate proxy records deepens their understanding of climate variability in the past on different temporal and spatial scales (Masson-Delmotte et al., 2013).
- Tree ring proxies can be well replicated and quantified by comparing them directly to meteorological variables from the year of ring formation.
- 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.
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).
- The bedrock in the region is a Jurassic limestone, and karstic features such as dolines, caves, and surface collapses characterize the landscape.
- Long-term monitoring series of precipitation and cave drip water d18O demonstrated that drip water corresponds to a weighted mean of pluri-annual precipitation at this site, without any evaporative enrichment or seasonal bias due to plant transpiration (Genty et al., 2014).
3.1 - COMPARISON OF THE FLUID INCLUSION AND CELLULOSE RECORDS
- For the comparison of fluid inclusions and cellulose, the d18O c time series was smoothed by a 25-year running mean.
- This interval was chosen because it likely corresponds 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 comparable.
3.2 - CALCULATION OF LEAF WATER ENRICHMENT AND RELATIVE HUMIDITY
- In order to calculate the leaf water isotopic composition and RH using the speleothem and tree ring isotope proxies, the authors suppose that the d18O fi represents the tree source water.
- The d18O fi values were linearly interpolated in order to obtain a time series of annual resolution.
- The resulting interpolated and smoothed fluid inclusion d18O values were supposed to be the source water d18O for the trees each year.
- The cellulose d18O time series, which had been normalized to a mean of zero (as published in Labuhn et al., 2016), was adjusted to the mean d18O value of recent cellulose (31 ‰).
- 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.
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
- Furthermore, even if the general increasing 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 δ18O values in both the fluid inclusions and the smoothed cellulose δ18O time series is about 2 ‰ (not shown).
- The underlying low-frequency component can be attributed to the variability of the source water (Labuhn et al., 2014).
- The dating of the stalagmite is less precise than the dating of the tree rings.
- The speleothem fluid inclusions represent the mean annual precipitation d18O, which is dominated by the d18O of winter precipitation because most precipitation falls during the winter months (Genty et al., 2014).
4.2 - CALCULATED LEAF WATER ENRICHMENT AND RELATIVE HUMIDITY
- The assumption that d18O fi represents the tree source water is supported by the fact that, at Villars Cave, there is no seasonal bias in the isotopic composition of the drip water due to the transpiration of the vegetation (Genty et al., 2014).
- The inter-annual variability of RH is well captured by the reconstruction, but average reconstructed RH values are too low (fig. 3).
- It assumes the kinetic isotope fractionation as water diffuses through stomata and through the boundary layer (e k in eq. 2) to be constant; but e k varies with the boundary layer conditions, which in turn depend on wind speed (Dongman et al., 1974; Burk & Stuiver, 1981).
- Furthermore, their calculation ignores the influence of a Péclet effect, which leads to a leaf water that is less enriched than predicted by the equations.
4.3 - DROUGHT RECONSTRUCTION WITH AND
- The observed summer drought index SPEI (JJA) and summer RH (JJA) are highly correlated during the period of observations (1961-2011) and are comparable to represent summer moisture conditions in the study area (fig. 4).
- The two reconstructions give a different picture of the long-term moisture changes in the past (fig. 5).
- The model for the tree-ring based drought reconstruction is well-verified with independent data, and correlations between the proxy and the reconstructed drought index are highly significant and stable throughout the 20th century, all of which gives strong indications for the validity of this reconstruction (Labuhn et al., 2016).
- The RH reconstruction supposedly captures better the low frequency.
5 - CONCLUDING REMARKS
- These investigations demonstrate the great potential in combining oxygen isotope ratios in speleothem fluid inclusions and tree ring cellulose to reconstruct moisture conditions, and the theoretical approach is established here.
- The combination of proxies might provide an estimate of past droughts that comprises both the low- and high-frequency variability, combining the strengths of the two climate archives while compensating their weaknesses when used alone.
- Further investigation will be necessary to better understand the differences between this multi-proxy reconstruction and the reconstruction based only on tree rings.
- These might provide additional oxygen isotope data from speleothem fluid inclusions and tree ring cellulose, which can be used to reconstruct drought patterns in the past and to test the validity of the approach demonstrated in this study.
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"A multi-proxy approach to drought r..." refers background in this paper
...…(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,…...
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125 citations
"A multi-proxy approach to drought r..." refers background in this paper
...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)....
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118 citations
"A multi-proxy approach to drought r..." refers background in this paper
...Furthermore, tree rings, in particular tree ring widths, are limited for reconstructing low-frequency climate variability 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)....
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117 citations
"A multi-proxy approach to drought r..." refers background in this paper
...…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…...
[...]