Speleothem records decadal to multidecadal hydroclimate variations in southwestern Morocco during the last millennium
Introduction
Southwestern Morocco is located in a semi-arid region and is thus highly sensitive to climate change on interannual to multidecadal timescales. Most rainfall in this area occurs during the winter season, whereas summers are generally dry. The regional climate is characterized by pronounced year to year variations in precipitation, which are highly correlated with the winter NAO index (Knippertz et al., 2003, López-Moreno et al., 2011). At decadal scale, it was found that large oscillations of North Atlantic sea surface temperature (SST) (i.e. the AMO) have impacts on the regional climate in the North Atlantic region (Knight et al., 2006, Gastineau and Frankignoul, 2014). Although Knight et al. (2006) indicate that the effect of AMO on SW Moroccan climate is very small, only little is known about the long-term influence of AMO during the last millennium. Superimposed on those variations at interannual to multi-decadal scales, climate projections predict a substantial increase of extreme heats and droughts during the coming decades in this region, which is already under severe water stress (Bouchaou et al., 2011). It is however not clear what can be attributed to natural climate variability and to anthropogenic forcing as hydroclimate variations observed in areas such as southwestern Morocco are highly influenced by the North Atlantic climate modes, like NAO and possibly AMO.
Since observational data sets are too short to resolve properly natural modes of variability acting on decadal to multidecadal timescales, high resolution paleoclimate reconstructions are the only alternative to reconstruct climate variability in the remote past (Novello et al., 2012). Hydroclimate reconstructions during the last millennium from various natural archives in the North of Morocco have shown regionally consistent climate features during the Medieval Climate Anomaly (MCA) and the Little Ice Age (LIA) (Esper et al., 2007, Détriché et al., 2009, Wassenburg et al., 2013). These paleorecords show that the MCA was generally drier in Northern Morocco as compared to the LIA, which was characterized by a recovery to wetter conditions. These very low frequency hydroclimate variations have been interpreted as being the result of a persistent positive (negative) state of the NAO during the MCA (LIA) (Trouet et al., 2009, Olsen et al., 2012, Baker et al., 2015). However, Wassenburg et al. (2013) indicated that the MCA was characterized by more frequent positive phases of the NAO and a recent proxy NAO reconstruction shows no persistent positive NAO conditions during the MCA (Ortega et al., 2015).
Thus, the climate processes explaining the MCA and LIA in northern Africa are still poorly understood because of the lack of well-dated and high-resolution paleoclimate records, which are able to resolve the climate variation at a multi-year timescale. Moreover, apart from the sediment core SST reconstruction off the Moroccan coast by McGregor et al. (2007), all the previous Moroccan paleoclimate studies are located in Northern Morocco. Therefore, the investigation of hydroclimate variations during the last millennium in Southwestern Morocco is certainly required. Hence, we present here the first continental paleoclimate record in Southwestern Morocco based on a well-dated speleothem from Ifoulki cave in the Western High Atlas Mountains (Fig. 1). The Ifoulki cave record spans the last 1200 yrs with a high resolution. This allows exploring the hydroclimate variability at multi-annual to centennial timescales. In addition, the Ifoulki paleorecord is discussed in the light of existing continental and marine paleoclimate proxies from Morocco in order to find potential relationships between atmospheric and oceanic large-scale climate modes and reconstructed dry/wet periods.
Section snippets
Study area and modern climatology
The Ifoulki cave (N30°42′29″; W09°19′39″; 1256 m.a.s.l.) is located in the Western High Atlas Mountains of southwestern Morocco within the karstified Tasroukht plateau. The Tasroukht plateau is a mature karst without dense vegetation cover and the litter is virtually absent (Junger and Faille, 2011). The karstic network of the cave, with a length of 433 meters (Fig. S1), is developed within white dolomitic limestones from the Upper Jurassic. The abundance of exo-karstic forms at the surface of
Material and methods
Stalagmite “IFK1” (∼10 cm tall) was collected from the bottom of Ifoulki cave, in order to make sure that the stalagmite has grown under conditions of minimal temperature fluctuations and highest relative humidity. The analysis of two powdered samples with X-Ray Diffraction indicates that the sample is calcitic. The stalagmite is macroscopically translucent. A mosaic fabric (typical for secondary calcite) or acicular fabric/needle like crystals (associated with aragonite) are not observed.
Stable isotope composition of precipitation and groundwater
A Local Meteorological Water Line (LMWL) is defined based on and analyses of 28 event-based rainwater samples (Fig. 3) sampled at two stations (Agadir and Wintimdouine) nearby Ifoulki cave between October 2014 and October 2015. The isotopic signature of rainwater samples in Ifoulki cave area ranges from −0.67 to −8.69‰, recorded between August and November respectively, suggestive of a seasonal effect on in precipitation. The LMWL presents a slope of 7.8, which is very
Comparison with regional paleoclimate records
A comparison between the IFK1 records and another trace element (Mg and Sr) speleothem record, by Wassenburg et al. (2013), (hereafter named “GP5”) from Piste cave (known as “Grotte de Piste”; 34°N, 04°W, 1260 m.a.s.l.) in Northern Morocco is shown in Fig. 6, where the increase (decrease) of Sr (Mg) is interpreted as a result of increasing precipitation and vice-versa (Wassenburg et al., 2013), although Mg is not incorporated into the crystal lattice of aragonite speleothems (like GP5) and may
Conclusions
A new high resolution U–Th dated stalagmite stable isotope and trace element record is investigated in this study. The speleothem was collected from the Ifoulki cave in southwestern Morocco where rainfall occurs mainly during the winter in relation to NAO in the modern period. The new stalagmite and 1st principal component based on IFK1 Mg and , covering the last 1200 yrs, suggests that southwestern Morocco is currently undergoing drought conditions which began in the early 20th
Acknowledgments
This work was supported by the CLIMACTE Tripartite Cooperative Project (IRD-France/CNPq-Brazil/APGMV-Africa grant 457400/2012-9), the PRIMO: IRD-CNPq project, and the German Research Foundation (DFG; WA3532/1-1) and the strong collaboration between the Laboratory of Speleological studies at the Institute of Geoscience in the University of Sao Paulo (Brazil), the Isotope Laboratory in the University of Minnesota (U.S.A.), the Institute of Global Environmental Change in Xi'an Jiaotong University
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