Speleothem records decadal to multidecadal hydroclimate variations in southwestern Morocco during the last millennium

Earth and Planetary Science Letters 476 (2017) 1–10

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Earth and Planetary Science Letters www.elsevier.com/locate/epsl

Speleothem records decadal to multidecadal hydroclimate variations in southwestern Morocco during the last millennium Yassine Ait Brahim a,∗ , Hai Cheng b , Abdelfettah Sifeddine c,d,e , Jasper A. Wassenburg f,g , Francisco W. Cruz h , Myriam Khodri c,d , Lijuan Sha b , Núria Pérez-Zanón i , El Hassane Beraaouz a , James Apaéstegui j , Jean-Loup Guyot k , Klaus Peter Jochum g , Lhoussaine Bouchaou a a

Applied Geology and Geo-Environment Laboratory, Ibn Zohr University, Agadir, Morocco Institute of Global Environmental Change, Xi’an Jiaotong University, Xi’an 710049, China IRD-Sorbonne Universités (UPMC, CNRS, MNHN) UMR LOCEAN, Centre IRD, Bondy, France d LMI PALEOTRACES (IRD, UFF, UANTOF, UPCH, UPMC), Depto Geoq, Niteroi, RJ, Brazil e Departamento de Geoquimica, Universidade Federal Fluminense, Niteroi, RJ, Brazil f Institute of Geoscience, University of Mainz, Mainz, Germany g Climate Geochemistry Department, Max Planck Institute for Chemistry, Mainz, Germany h Instituto de de Geociências, Universidade de São Paulo, São Paulo, SP, Brazil i Climate Change Center (C3), Campus Terres de l’Ebre, Universitat Rovira i Virgili, Avda. Remolins, 13-15, Tortosa, Spain j Instituto Geofísico del Perú (IGP), Ate-Vitarte, Peru k GET, UMR5563, CNRS/IRD/UPS, 31400 Toulouse, France b c

a r t i c l e

i n f o

Article history: Received 7 June 2016 Received in revised form 24 July 2017 Accepted 27 July 2017 Available online xxxx Editor: H. Stoll Keywords: speleothem southwestern Morocco last millennium hydroclimate changes Atlantic Oscillation Sahara Low

a b s t r a c t This study presents the first well-dated high resolution stable isotope (δ 18 O and δ 13 C) and trace element (Mg and Sr) speleothem records from southwestern Morocco covering the last 1000 yrs. Our records reveal substantial decadal to multidecadal swings between dry and humid periods, consistent with regional paleorecords with prevailing dry conditions during the Medieval Climate Anomaly (MCA), wetter conditions during the second part of the Little Ice Age (LIA), and a trend towards dry conditions during the current warm period. These coherent regional climate signals suggest common climate controls. Statistical analyses indicate that the climate of southwestern Morocco remained under the combined influence of both the North Atlantic Oscillation (NAO) and the Atlantic Multidecadal Oscillation (AMO) over the last millennium. Interestingly, the generally warmer MCA and colder LIA at longer multidecadal timescales probably influenced the regional climate in North Africa through the influence on Sahara Low which weakened and strengthened the mean moisture inflow from the Atlantic Ocean during the MCA and LIA respectively. © 2017 Elsevier B.V. All rights reserved.

1. 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

*

Corresponding author at: Ibn Zohr University, Cité Dakhla, BP 8106, Agadir, Morocco. E-mail address: [email protected] (Y. Ait Brahim). http://dx.doi.org/10.1016/j.epsl.2017.07.045 0012-821X/© 2017 Elsevier B.V. All rights reserved.

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 south-

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Fig. 1. The mean rainfall amounts (mm/day) during the December–January–February (DJF) and June–July–August (JJA) seasons in Northwest Africa and Southwestern Europe, calculated from the “TRMM 3B42 daily V7” data for the period 1998–2015. Locations of the Moroccan paleorecords covering the last millennium are indicated in the maps: Ifoulki cave (white circle), Sediment core (red circle; McGregor et al., 2007), Piste cave (black circle; Wassenburg et al., 2013) and Tree-rings (green circle; Esper et al., 2007). (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

western 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. 2. 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 Tasroukht Plateau (e.g. dolines, lapiaz, holes) promotes a direct percolation of meteoric waters, which initiate underground karstic paths (including Ifoulki). The altitudes of High Atlas Mountains in Ifoulki cave area (around 1257 m.a.s.l.) allow a winter rainfall of ∼400 mm/yr (Bouchaou et al., 2002), partly due to the orographic effect. The distribution of rainfall events in Morocco is highly seasonal with recurrent severe droughts during the last decades (Bouchaou et al., 2011). The study area is climatologically influenced by the Atlantic Ocean, for which precipitation is highly sensitive to baroclinic activity over the North Atlantic during boreal winter (i.e. the NAO; Knippertz et al., 2003). The NAO shows a negative correlation with precipitation during the December–January–February– March season in the Mediterranean Mountains, including Morocco (López-Moreno et al., 2011; Wassenburg et al., 2016a). Between November and March, 80% of the annual rainfall is delivered. The annual mean temperature is ∼17 ◦ C with ∼8 ◦ C during winter and ∼19 ◦ C during summer). 3. 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

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Fig. 2. (a) IFK1 speleothem indicating the possible hiatus (brown dashed line) and the location of U–Th dating samples and sample transects for carbon and oxygen isotopes (shaded blue) and trace elements (red line), and the approximate sample positions for the X-Ray Diffraction (orange). (b) Chronological model developed for the sample based on StalAge Algorithm. The gray lines represent the age model’s 95% confidence intervals. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

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. Furthermore, trace element data indicate that the Mg concentrations range from 6000 up to 14000 ppm. A secondary calcite (as a product from aragonite-tocalcite transformation) usually has similar Mg concentrations as the precursor aragonite (i.e. below 100 ppm). This result suggests that stalagmite IFK1 consists of primary calcite. The structure at the base of the stalagmite is the substrate on which the stalagmite precipitated (Fig. 2) and does not affect the record presented in this study. Stable isotope analyses (δ 18 O and δ 2 H) were performed on meteoric water and groundwater samples collected from springs near the Ifoulki cave over 3 campaigns (March 2014, June 2014 and April 2015). Rainwater samples were regularly collected for precipitation events between October 2014 and October 2015. Samples were shipped to the laboratory in double-capped polyethylene bottles. Water isotope analyses were performed in the Laboratory of Applied Geology and Geo-Environment at Ibn Zohr University (Morocco) using a Picarro L2120-i δ D/δ 18 O Isotopic Water Analyzer. Isotopic analyses are performed according to the International Atomic Energy Agency (IAEA) standards (IAEA-ILS). The values of the isotopic results are presented in the standard notation delta per mill (h VSMOW). Typical 1σ precision analysis is ±0.1h and ±1h for δ 18 O and δ 2 H respectively. The age-model of IFK1 is based on 23 U–Th dates (Fig. 2a). The U–Th analyses were carried out at the Isotope Laboratory in the University of Minnesota (U.S.A.) and at the Institute of Global Environmental Change of Xi’an Jiaotong University (China), using Neptune Plus multi-collector inductively coupled plasma mass spectrometry (ICP-MS) (Cheng et al., 2013) with 2σ errors of less than 1% for most ages (Table 1). The age model was constructed using the StalAge algorithm (Scholz and Hoffmann, 2011; Fig. 2b). 651 calcite micro-samples were drilled with a high resolution (0.1 mm) along the growth axis of IFK1. Conventional Isotope-Ratio Mass Spectrometry (IRMS) techniques were performed on these samples to measure stable oxygen and carbon isotope ratios (δ 18 O and δ 13 C).

IFK1 trace element concentrations were determined at the Max Planck Institute for Chemistry, Mainz, Germany, with a Thermo Finnigan Element 2 ICPMS coupled to a New Wave UP213 laser ablation system. The isotopes 25 Mg, 43 Ca, and 88 Sr were analyzed using linescans with a scanspeed of 10 μm/s and a 100 μm round spotsize. The NIST612 and MACS3 reference materials were used for calibration, and 43 Ca was used as an internal standard. For further methodological details the reader is referred to Jochum et al. (2007, 2012). Principal component analysis (PCA) was performed to extract the common variability between different paleorecords. In order to perform the PCA, time-series were interpolated to create a regular time step of one year and then normalized. Wavelet (Torrence and Compo, 1998) and spectral analysis (REDFIT) (Schulz and Mudelsse, 2002) were performed using the Past software (Hammer et al., 2001), to look for the most significant periodicities documented in IFK1-δ 18 O and δ 13 C time-series. Cross-wavelet analyses were used to test the consistency and coherence between our paleorecord and other paleoclimate reconstructions (Grinsted et al., 2004). 4. Results and interpretation 4.1. Stable isotope composition of precipitation and groundwater A Local Meteorological Water Line (LMWL) is defined based on δ 18 O and δ 2 H 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 δ 18 O signature of rainwater samples in Ifoulki cave area ranges from −0.67 to −8.69h, recorded between August and November respectively, suggestive of a seasonal effect on δ 18 O in precipitation. The LMWL presents a slope of 7.8, which is very similar to the Global Meteorological Water Line (GMWL, Craig, 1961). The mean value of deuterium excess in rainwater samples is 10h, confirming that the humid air masses in the region are mainly from the Atlantic as suggested in previous studies (Celle, 2000; Ouda et al., 2004; Ait Brahim et al., 2016). The seasonal effect on δ 18 O in precipitation is confirmed by the observational data from the Global Network of Isotopes in Precipitation (GNIP) of the IAEA. The GNIP station of Fès, with a 11 yrs long record of δ 18 O, precipitation and temperature, shows a signif-

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Table 1 U/Th dating results. *δ 234 U = ([234 U/238 U]activity − 1) × 1000. **δ 234 Uinitial was calculated based on 230 Th age (T ), i.e., δ 234 Uinitial = δ 234 Umeasured × e λ234× T . Corrected 230 Th ages assume the initial 230 Th/232 Th atomic ratio of 4.4 ± 2.2 × 10−6 . Those are the values for a material at secular equilibrium, with the bulk earth 232 Th/238 U value of 3.8. The errors are arbitrarily assumed to be 50%. ***B.P. stands for “Before Present” where the “Present” is defined as the year 1950 A.D. Sample number

238

IFK1-3 IFK1-7 IFK1-9 IFK1-11 IFK1-13 IFK1-15 IFK1-20 IFK1-23 IFK1-24 IFK1-27 IFK1-31 IFK1-36 IFK1-38 IFK1-39 IFK1-44 IFK1-47 IFK1-49 IFK1-54 IFK1-59 IFK1-62 IFK1-63 IFK1-66 IFK1-68

245.7 247.0 308.9 328.5 298.2 313 201.9 201.7 195.3 241.5 322.8 282.0 331.8 257.6 379.1 320.0 329 310 271 345 284.9 371.2 719.0

232

U (ppb)

230

Th/232 Th (atomic ×10−6 )

Th (ppt)

±0.5 ±0.9 ±0.4 ±1.1 ±0.4 ±1 ±0.7 ±0.8 ±0.2 ±0.9 ±0.4 ±0.3 ±1.2 ±0.3 ±1.0 ±0.4 ±1 ±1 ±1 ±0 ±0.6 ±0.4 ±3.0

315 61 105 80 273 564 183 170 464 189 118 79 401 355 178 52 123 401 911 73 167 37 1266

±7 ±3 ±5 ±4 ±6 ±11 ±4 ±5 ±10 ±5 ±4 ±3 ±9 ±8 ±4 ±3 ±4 ±9 ±19 ±3 ±5 ±3 ±26

25 147 124 255 79 62.7 150 176 69 215 486 687 172 157 488 1451 686.0 208.4 87.0 1378.7 501 3111 194

±5 ±45 ±38 ±44 ±12 ±2.5 ±14 ±20 ±9 ±16 ±27 ±35 ±9 ±7 ±12 ±94 ±33.3 ±9.0 ±3.8 ±70.5 ±25 ±287 ±5

δ 234 U*

230

(measured) 1047 1024.6 1046.2 1054.4 1035.5 1024.4 1035.3 1000.5 993.5 972.8 975.8 961.4 949.0 947.7 909.0 931.7 918.8 942.2 906.8 904.5 897.8 891.2 876.6

±4.5 ±5.4 ±2.6 ±5.4 ±2.5 ±3.1 ±5.4 ±5.8 ±2.3 ±6.1 ±2.5 ±2.1 ±5.3 ±2.2 ±4.0 ±2.3 ±5.1 ±5.9 ±5.6 ±2.8 ±2.9 ±2.2 ±6.0

Th/238 U (activity) 0.0020 0.0022 0.0026 0.0038 0.0044 0.0069 0.0083 0.0090 0.0099 0.0102 0.0108 0.0116 0.0126 0.0131 0.0139 0.0144 0.0155 0.0164 0.0177 0.0176 0.0178 0.0189 0.0207

±0.0004 ±0.0007 ±0.0008 ±0.0006 ±0.0007 ±0.0002 ±0.0007 ±0.0010 ±0.0012 ±0.0007 ±0.0005 ±0.0005 ±0.0006 ±0.0005 ±0.0002 ±0.0005 ±0.0006 ±0.0006 ±0.0007 ±0.0005 ±0.0007 ±0.0005 ±0.0003

230

Th Age (yr) (uncorrected) 105 119 136 200 234 370 443 492 543 567 596 647 709 738 798 814 884 922 1019 1014 1028 1097 1209

±5.0 ±36 ±42 ±34 ±36 ±13 ±40 ±56 ±67 ±41 ±29 ±25 ±32 ±31 ±11 ±29 ±33 ±35 ±39 ±29 ±42 ±32 ±19

230

Th Age (yr) (corrected) 87 115 131 196 221 344 430 479 508 555 590 643 691 718 791 812 879 903 968 1011 1019 1095 1181

±24 ±36 ±42 ±34 ±37 ±22 ±41 ±56 ±71 ±42 ±29 ±25 ±34 ±34 ±12 ±29 ±33 ±37 ±53 ±29 ±42 ±32 ±27

δ 234 UInitial ** (corrected) 1048 1025 1047 1055 1036 1025 1037 1002 995 974 977 963 951 950 911 934 921 945 909 907 900 894 879

±4 ±5 ±3 ±5 ±2 ±3 ±5 ±6 ±2 ±6 ±3 ±2 ±5 ±2 ±4 ±2 ±5 ±6 ±6 ±3 ±3 ±2 ±6

230

Th Age (yr B.P.)*** (corrected) 25 53 69 134 159 282 368 417 446 493 528 581 629 656 729 750 817 841 906 949 957 1033 1119

±24 ±36 ±42 ±34 ±37 ±22 ±41 ±56 ±71 ±42 ±29 ±25 ±34 ±34 ±12 ±29 ±33 ±37 ±53 ±29 ±42 ±32 ±27

Fig. 3. Relationship between deuterium and oxygen isotopes. The samples are sorted according to their type (groundwater and rain samples collected at Agadir and Wintimdouine). The dashed yellow line indicates the GMWL (Craig, 1961).

icant seasonal variation, with values ranging from −5.7 to −3.2h recorded between November and April respectively (Fig. S2). Due to the scarce summer precipitation in Morocco, no δ 18 O measurements are available for precipitation during the months of June, July and August. However, on a seasonal timescale, δ 18 O co-varies with both temperature and precipitation, whereas the lowest δ 18 O values are observed during the winter season and are associated with relatively cold and humid conditions and vice-versa. However, the temperature effect on δ 18 O values is less expressed than the amount effect on an inter-annual timescale (Fig. 4). In fact, from the anomalies of the mean climatology, the correlation between δ 18 O and temperature is void (r = 0.01; p < 0.01; n = 60), while it is higher for precipitation (r = −0.4; p < 0.01; n = 60). The rainfall amount control of δ 18 O variations in Morocco is well documented in Taza and Bab Bou Idir (Wassenburg et al., 2016a) but has also been observed for Moroccan GNIP stations in Rabat, and Béni Mellal. Although the scarce measurements in these stations, which cover a period of only two years (2001–2002), could limit the proper quantifications of the temperature influence

Fig. 4. Relationship between anomalies from the mean seasonal cycle of δ 18 O with temperature and precipitation from Fès GNIP station (N33◦ 58 ; W4◦ 59 ). The observational monthly records for δ 18 O in precipitation, temperatures and rainfall covers the 1994–2004 years period. The anomalies are relative to the 11-yrs climatology from 1994–2004.

at inter-annual timescale. Indeed, the δ 18 O in precipitation data from these stations confirm the opposite relationship between the weighted values of monthly rainfall and monthly δ 18 O with an-

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nual precipitation (Fig. S3a). Significant correlations are observed between rainfall amount and δ 18 O in precipitation in the GNIP data from Bab Bou Idir (r = −0.85; p < 0.01; n = 10) and Fès (r = −0.44; p < 0.01; n = 60). The rainfall amount effect is less expressed in Rabat and Béni Mellal stations, which may be related to the limited amount of datapoints. Although the amount effect does not explain the total variance of δ 18 O at the GNIP stations, the regression line between rainfall amount and δ 18 O from data from the GNIP station in Fès is significant (Fig. 4a). In contrast, the correlation between temperature and δ 18 O in precipitation is very low in all GNIP stations (Fig. 4b and Fig. S3b). Such results do not rule out the temperature effect on δ 18 O in precipitation in Morocco, but do suggest that the amount effect is more prominent (Ait Brahim et al., 2016). Groundwater samples, collected from springs of the same Jurassic limestone formation where Ifoulki cave was formed, also present isotopic ratios close to the GMWL with no significant evaporation effect (Fig. 3). The mean isotopic values of groundwater (−5.8h) is very close to the mean value of δ 18 O observed in precipitation in Ifoulki cave area (−5.58h), suggestive of a rapid infiltration and insignificant evaporation taking place in the karstic system that supplies the drip waters forming modern speleothems in Ifoulki cave. 4.2. Ifoulki speleothem record The twenty-three U–Th dates of IFK1 speleothem (Table 1) reveal that the sample covers the period 790–1953 AD with dates ranging from AD 831 ± 27 to 1925 ± 25 and a short hiatus observed around 1290 AD. However, this hiatus is probably less than 10 yrs and cannot be resolved by dating. The mean growth rate for the entire record is 0.08 mm/yr. Sampling resolution of 0.1 mm for IFK1 speleothem yields a mean data resolution of 1.3 yrs, varying between 0.8 and 2.8 yrs, except for the period between 1668 ± 22 and 1816 ± 34 which presents the lowest growth rate (0.03 mm/yr) and data resolution of 3 to 9 yrs. The round 100 μm spotsize for the trace elements averages approximately 1.3 yrs of stalagmite growth. However, the linescan method provides a measurement every 1.2 s, which corresponds to a sampling resolution of 12 μm with a scanspeed of 10 μm/s. The IFK1 record covers the period from A.D 790 ± 27 to 1953 ± 24 and does not show a significant long-term trend (Fig. 5). The high amplitude of δ 18 O values observed in IFK1 cannot be accounted for by the temperature dependence of isotope fractionation between calcite and drip water in the cave. For instance, a decrease of more than 3h observed during the LIA would require an increase of the cave air temperature of about 12 ◦ C, by considering the temperature-dependent fractionation of −0.24h/◦ C between calcite and water (Friedman and O’Neil, 1977). This temperature range is entirely inconsistent with changes in mean annual air temperature outside the cave, which at most changed over a few degrees for this period. In addition to local temperature effects, changing moisture source may also affect precipitation δ 18 O (Lachniet, 2009). According to the global map of δ 18 O by LeGrande and Schmidt (2006), the North Atlantic, being the main source of moisture at Ifoulki Cave, shows an isotopic gradient from north to south, with values of approximately 0h in the north to max. 1.4h in the south. However, southwestern winds are uncommon in Morocco (Knippertz et al., 2003) due to the position of the Azores High, we therefore do not expect the southern part of the North Atlantic to be a potential moisture source for precipitation at Ifoulki Cave. However, within the likely source of precipitation at the region of Ifoulki cave, there still is a possible variability of 0.4–0.5h. In addition to changes in the source region, rain-out effects, and temperature and humidity at the time of evaporation or

Fig. 5. Trace element (a: Sr; b: Mg) and stable isotope (c: δ 13 C; d: δ 18 O) time series of IFK1 speleothem. (e): the first component of the PCA between δ 13 C and Mg.

condensation also affect the δ 18 O of precipitation, which complicates the signal recorded in the stalagmite (Lachniet, 2009). Stable isotope ratios of oxygen (δ 18 O) and carbon (δ 13 C) (Fig. 5c and 5d) are well correlated along the growth axis (R 2 = 0.49, n = 651). A positive correlation between speleothem δ 18 O and δ 13 C may be explained by drip rate related fractionation processes that force both δ 18 O and δ 13 C towards more positive values under lower drip rates (Deininger et al., 2012; Mühlinghaus et al., 2009; Romanov et al., 2008). Lower drip rates may be the result of higher evaporation occurring under drier climate conditions. Furthermore, evaporation itself may force drip water δ 18 O towards more positive values. Similarly, lower rainfall amounts are associated with more positive δ 18 O values in precipitation, as shown in Fig. 4. In addition, δ 18 O and δ 13 C often co-vary to some extent, because precipitation and temperature have a major influence on the biological soil activity, which in turn controls the δ 13 C signal (Genty et al., 2006; Hellstrom et al., 1998). We emphasize, however, that in a semi-arid region like southwestern Morocco, soil and vegetation productivity are strongly limited by precipitation, whereas temperature only plays a minor role. Thus, the drip related fractionation processes, the biological activity in soil, evaporation and the rainfall amount effect all influence the isotopic signal in the same direction under drier (more humid) climate conditions, amplifying the signal variations without causing any damage to its quality (Couchoud, 2008). Prior calcite precipitation (PCP) is the process of calcite precipitation before the drip water reaches the stalagmite surface. PCP is enhanced during periods of drier climate conditions due to increased degassing of CO2 from the drip water at the cave ceiling or if the drip water encounters empty voids in the epikarst above the cave. Enhanced PCP results in higher δ 13 C and higher Mg/Ca and potentially higher Sr/Ca ratios in the drip water and stalagmite (Stoll et al., 2012; Fairchild et al., 2000). Comparison with IFK1 trace element data (Fig. 5a–b), shows important similarities between especially IFK1 Mg and δ 13 C, which suggests that PCP exerts a strong control on δ 13 C (Wassenburg et al., 2012; Johnson et al., 2006). However, the Sr record does not reveal a

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PCP signal as clear as Mg and δ 13 C. This may be related to variable sources with different Sr concentrations that contribute to the dripwater Sr/Ca ratios, whereas the source of Mg is dominated by the dolomitic host rock of the cave. This result suggests that the δ 18 O variability that has no counterpart in δ 13 C reflects a different process as rainfall amount. A Principal Component Analysis (PCA) was performed between Mg and δ 13 C time-series, in order to extract the PCP-related common variability of both variables. We emphasize that the PCA, results in the loss of (multi) decadal variability due to potential problems with aligning trace element and isotope transects and required interpolation steps. Nevertheless, the first principal component (PC1) explains 73% of the total variance. Considering the large coherence between Mg and δ 13 C, we interpret this component and the (multi) decadal variability in δ 13 C as a precipitation variability record, whereas more negative values reflect more humid conditions and vice-versa. A comparison between PC1, δ 18 O, δ 13 C, Mg and Sr is given in Fig. 5. The PC1 time series shows high coherence with the δ 18 O record. However, positive peaks in the δ 18 O record are observed around 1040 AD, 1190 AD and 1770 AD and do not replicate in the PC1 record. These peaks can therefore not be explained by the amount effect but are related to other processes affecting precipitation δ 18 O, like changes in the moisture source region, and changes in temperature and humidity conditions at the time of isotope fractionation taking place between the moisture source and the cave site. The alternations of increasing and decreasing stable isotope and trace element values are observed on decadal to multidecadal timescales. The MCA and the LIA were defined in Morocco as ∼900–1350 AD and LIA ∼1500–1850 AD respectively (Esper et al., 2007; Wassenburg et al., 2013). The period before the MCA, presents δ 18 O and δ 13 C values as high as during the MCA (∼800 AD) followed by relatively low values around 850–970 AD. The MCA is characterized by a long period of overall dry conditions (∼1010–1200 AD), interrupted by a shorter and relatively humid period (∼1200–1280) and followed by a dry period (∼1280–1420 AD) showing the driest peak of the entire record around 1350 AD as shown in the 1st PC. Hence, the first period of the MCA–LIA transition (∼1350–1500 AD) is characterized by dry conditions (∼1350–1420 AD), while the second period is humid (∼1420–1500 AD). As per the LIA, the first period (∼1500–1640 AD) is generally a dry phase. Afterwards, values drop towards the lowest values, with peaks around ∼1680 AD and ∼1810 AD, indicating the longest humid period of the entire record which extends until ∼1900 AD. During the LIA, the most humid conditions of the entire record are observed in the 1st PC and δ 18 O at ∼1680 AD and ∼1810 AD. Finally, the 20th century presents a trend towards drier climate conditions. 5. Discussion 5.1. 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 be related to detrital components or the occupation of crystal defect sites, PCP or prior aragonite precipitation (PAP; Wassenburg et al., 2016b; Jamieson et al., 2016) strongly impact drip water Mg/Ca. Therefore, a PCA analysis is performed between Piste cave Mg and Sr records to facilitate the comparison between GP5 and IFK1 records

Fig. 6. Comparison between Ifoulki record and other Moroccan paleorecords covering the last millennium. (a): Ifoulki records, δ 18 O (black) and the PC1 between δ 13 C and Mg (blue). Corresponding dating positions and uncertainties are shown at the bottom. (b, c and d): Piste cave speleothem records, dating positions and uncertainties by Wassenburg et al. (2013), Sr (purple), Mg (orange) and the PC1 between Mg and Sr (bold). (e): Tree-ring ScPDSI (Esper et al., 2007; Wassenburg et al., 2013) with the bold curve corresponding to the 10 yrs regular interpolation. Orange and blue shadings indicate the MCA and LIA periods respectively. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

(Fig. 6b). Interestingly, there is a substantial similarity between IFK1 and GP5 1st PC at decadal to centennial timescales throughout most of the record. The MCA is characterized by dry conditions compared to the LIA in both speleothems. The 1st PC of GP5 shows three dry phases, of which the ones around 1050 AD and 1375 AD occur within error of the two long dry phases observed in the 1st PC of IFK1. Before the LIA, both GP5 and IFK1 indicate a wet phase around 1460 AD. The onset towards the humid conditions of the LIA is synchronous in the 1st PC’s of IFK1 and GP5. The dry phase recorded in GP5 around 1280 AD is observed in the IFK1 Mg record and coincides with a macroscopically visible hiatus in IFK1 (Fig. 2a). This suggests that changes in precipitation in those areas is governed by a common regional climate forcing. However, during the period before the MCA, humid conditions are only recorded in IFK1, while the GP5 record shows a hiatus related to dry conditions (Wassenburg et al., 2013). In addition, the dry phase around 1200 AD during the MCA in GP5 is not replicated in IFK1. Overall, the IFK1 record replicates many climate patterns observed in the Piste cave over the last millennium, regardless of the different types of tracers and study regions (Northwestern Middle Atlas and Occidental High Atlas; ∼634 km away). The IFK1 record also shows coherent variations with other regional paleoclimate records. A tree-ring Self-calibrating Palmer Drought Severity Index (ScPDSI) record from the North of Morocco originally published by Esper et al. (2007) and updated by

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Wassenburg et al. (2013) shows a pronounced change from drier conditions during the medieval times to a pluvial period after about 1400 AD (Fig. 6e). This onset of the transition towards the pluvial period is in phase with the onset towards more humid conditions recorded in IFK1 and GP5. However the return to drier conditions is earlier in the scPDSI record and lasts longer compared to the dry phase recorded in IFK1 and GP5 around 1560 AD. It is also observed that the LIA is shorter with higher amplitude in the variance in IFK1 than in the North Moroccan records. It is possible that these differences are related to the slightly different seasons recorded in IFK1 (winter) and the tree ring scPDSI (February–June). After the LIA, both records end by a shift towards relatively drier conditions in the 20th century. The long-term trend toward drier conditions around A.D. 1800 is not clearly captured in the tree-ring record. The hydrologic conclusions of this study from southern Morocco and that of previous studies from northern Morocco are in agreement with the evidence inferred from a sediment core from Lake Afourgagh, located approximately 540 Km NE of Ifoulki cave, where a low stand is observed between 890 and 1201 AD within the MCA period (Détriché et al., 2009). The low stand was followed by more humid conditions during the LIA in Morocco. In the Iberian Peninsula, a magnetic susceptibility record from the Douro mud patch in NW Portugal (Abrantes et al., 2011) and from marine and lake proxy records (Moreno et al., 2012) suggest that the MCA was drier compared to the LIA. Even though some continental paleorecords in the NW Iberian peninsula suggest more humid conditions during the MCA, the proxy records from Morocco and the Iberian Peninsula generally agree that the MCA was relatively dry compared to the LIA (Wassenburg et al., 2013).

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Fig. 7. Comparison of the Ifoulki records (δ 18 O and PC1 of δ 13 C and Mg) (c) with the reconstructed NAO index (a) (Ortega et al., 2015), AMO index (b) and SST anomalies (inversed axis) (d) (McGregor et al., 2007) including dating uncertainties. Bold curves correspond to the 10 yrs regular interpolation of each time series. Orange and blue shadings show the timing of the MCA and LIA periods as in Esper et al. (2007), and Wassenburg et al. (2013) IFK1 record respectively. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

5.2. Climatic interpretation The long-term consistency of Ifoulki record with previous studies from Northern Morocco, and the Iberian Peninsula, highlights the potential of these records to provide a robust regional reconstruction of precipitation changes. The replication of these multiproxy signals confirms that the information from paleorecords is accounted for by climate. Herein, the IFK1 δ 18 O and PC1 (Mg-δ 13 C) records are compared with a multi-proxy based NAO reconstruction (Fig. 7a) from Ortega et al. (2015) (NAOORT ). This NAO index outperforms the bi-proxy index of Trouet et al. (2009) (NAOMS ). However, the Western Mediterranean (NW Africa and the Iberian Peninsula), which is considered as a key NAO region, is highly underrepresented in NAOORT , with only one tree-ring record from Northern Morocco. NAOORT suggests that the MCA was generally represented by more positive (less negative) phases of the NAO when compared to the LIA. Accordingly, the climatic response revealed in all the Moroccan paleoclimate proxies is a transition from generally dry conditions during the MCA to wetter conditions in the LIA. However, there is only little coherence between IFK1 and NAOORT on centennial to (multi-)decadal timescales during the last millennium (Fig. 7a). These are the positive phase of the NAO around 1565 AD and the trend towards positive NAO conditions after 1800 AD, which both coincide with relatively dry conditions in Morocco. During the MCA, a wet phase centered on ∼1120 AD is indicated by both speleothems (IFK1 and GP5), which overlaps with a peak in the ScPDSI reconstruction. However, the wet phase recorded in speleothem GP5 seems to cover a longer time period. This period coincides with wet periods recorded in the Douro mud patch and lake records from the Iberian Peninsula (lake Arreos, Estanya, Basa de la Mora; Moreno et al., 2012), indicating a regional response to large scale atmospheric circulation patterns of NAO negative conditions, even though this is not observed in NAOORT (Ortega et al., 2015).

The inconsistencies with NAOORT might be attributed to the non-stationary behavior of the NAO (Jung et al., 2003; Lehner et al., 2012; Wang et al., 2012; Wassenburg et al., 2016a; Baker et al., 2015) or to the fact that the Western Mediterranean is underrepresented in NAOORT . Another possibility is the presence of other leading modes of rainfall variability in southwestern Morocco during the last millennium. Spectral analyses of IFK1 speleothem time-series were performed to show significant periodicities. Due to potential problems with aligning the isotope and trace element transects and interpolation steps, the (multi) decadal variability is lost in the PC1 record. However, the large coherence between δ 13 C and Mg points to a strong PCP control on δ 13 C, such that only the spectral analysis of δ 13 C is used. The δ 13 C is indeed a proxy for effective rainfall, and the difference between the δ 18 O and δ 13 C would be interpreted as isotope fractionation processes taking place somewhere between the moisture source and the cave site. In addition, vegetation in Ifoulki cave region is limited by rainfall, which reinforces the δ 13 C sensitivity as a proxy for effective rainfall. The spectral analyses of IFK1 δ 18 O and δ 13 C indicate significant decadal to multi-decadal periodicities between 15 and 62 yrs, and a high power around 200 yrs (Fig. 8a and 8b). However, because of age uncertainties, we conservatively refrain from interpreting periodicities of less than 50 yrs. Wavelet analyses further show that statistically significant multi-decadal periodicities between 40 and 90 yrs are persistent throughout the δ 18 O record (Fig. 8c.). These are also visible in the δ 13 C record, although slightly less pronounced (Fig. 8d). The multi-decadal periodicities observed in the IFK1 record point to frequencies related to the AMO (Delworth and Mann, 2000; Enfield and Cid-Serrano, 2010). In fact, the decadal patterns of SST change imply atmospheric dynamical responses to winddriven ocean circulation by the North Atlantic Oscillation–Arctic Oscillation (Mann et al., 2009; Gastineau and Frankignoul, 2014; McCarthy et al., 2015). Moreover, the MCA was found to be

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Fig. 8. Spectral analysis of the time series of δ 18 O (a) and δ 13 C (b) Ifoulki records; Green, yellow and red dashed lines represent the confidence levels of 90%, 95% and 99% respectively. Peaks that exceed the 99% confidence level are labeled with their periods in years. Wavelet analysis of the Ifoulki record δ 18 O (c) and δ 13 C (d); black contours indicate the 95% significance level. Analyses were performed with the Past software (Hammer et al., 2001) using REDFIT of Schulz and Mudelsse (2002), which uses the Lomb-Scargle periodogram for unevenly spaced data. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

dominated by warm SST anomalies in the North Atlantic region (Keigwin, 1996), which correspond to a positive state of the AMO (Feng et al., 2008; Mann et al., 2009). Indeed, the Atlantic forcing of multidecadal droughts in West Africa (Shanahan et al., 2009) and the impact of AMO on rainfall distribution and intensity over the African Sahel (Nicholson et al., 2007) have already been documented, whereas the summer rainfall increases over the Sahel during warm AMO years as part of a dipole structure with the Guinea coast (Martin and Thorncroft, 2014). However, the specific influence of the AMO on hydroclimate variability in North Africa is poorly known. According to Knight et al. (2006), the large multidecadal oscillations in the Atlantic SST have impacts on regional north Atlantic climate including the Sahel and Northeastern Brazil, especially during the months March–May through the latitudinal displacements of Inter Tropical Convergence Zone (ITCZ). However, precipitation occurs mainly in winter in Morocco and is modulated at multi-decadal timescales by the North–South swings of the sub-tropical jet stream and latitudinal position of the storm tracks much like at inter-annual timescale. McCarthy et al. (2015) showed that the NAO affects the AMO on decadal timescales, through ocean circulation changes in the intergyre region of the North Atlantic. Gastineau and Frankignoul (2014) have shown that positive (negative) AMO phases are associated with negative (positive) NAO-like atmospheric patterns for the observational period. This would result in more moisture inflow and wetter conditions in Morocco during the winter season and vice-versa. However, the AMO index reconstruction (Mann et al., 2009) reveals negative AMO anomalies during the LIA, so this is not in line with Gastineau and Frankignoul (2014), since this is a wet period in Morocco. This suggests that the relationship between the NAO and AMO was more complex during the last millennium. The AMO potentially exerts an influence on water vapor δ 18 O because changes in SST impact isotope fractionation taking place in the moisture source region. Warmer SST may lead to higher δ 18 O precipitation (Lachniet, 2009). The AMO index should thus show coherence with IFK1 δ 18 O. Indeed, this is the case, in particular during the LIA (Fig. 7b), for the positive δ 18 O peaks around 1050 AD and 1750 AD, and the positive trend observed for

the 20th century. Moreover, the AMO frequencies are more pronounced in the δ 18 O record compared to the δ 13 C record, which might be explained by the SST effect on oxygen isotope fractionation. Although the interpretation of speleothem δ 18 O usually represents a combination of processes, our conclusions are supported by Smith et al. (2016) and Deininger et al. (2016) who also observed speleothem δ 18 O variability in phase with changes in the North Atlantic Ocean circulation. Furthermore, McGregor et al. (2007) reconstructed SST for the last 2500 yrs, based on alkenones from a Moroccan sediment core in Cape Ghir (30◦ 50.7 N, 10◦ 5.9 W; ∼70 km away from Ifoulki cave). According to McGregor et al. (2007), the SST in southwestern Morocco is not only dominated by the NAO, but also by the land-sea pressure contrast that affects upwelling along the coast of Northwest Africa. Such changes in coastal SST’s certainly affected regional southwestern Moroccan climate. Furthermore, the NAO-forced SST pattern suggests negative SST anomalies west of Morocco during positive NAO modes (Marshall et al., 2001). Although we acknowledge that the chronology of the SST record from McGregor et al. (2007) is associated with large age uncertainties, warm SST coincide with relatively wet conditions in IFK1 for several phases (∼980 AD, ∼1270 AD, ∼1460 AD, ∼1690 AD, and ∼1880 AD), while cold SST peaks (∼1100 AD ∼1330 AD and during the 20th century) coincide with dry conditions in IFK1. This is consistent with the NAO-forced SST pattern from Marshall et al. (2001). The potential link between IFK1 and local SST could be explained by opposite signs between air temperature and the coastal SST in Cape Ghir. The relative increase (decrease) in surface air temperature over land relative to the ocean during the MCA (LIA) would accentuate (weaken) the low-pressure cell over land, while a high-pressure center develops offshore. In fact, according to Speth et al. (1978), this land-sea pressure gradient co-varies with SSTs in the northwest African coast and warm (cold) surface air temperature over the Eurasian continent and the Sahara landmass strengthens (weakens) the Sahara Low (i.e. Haarsma et al., 2005). This would result in sustained/weakened moisture inflow toward Ifoulki cave from the Atlantic Ocean, and potentially

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explains the opposite climate signals between SW and North Morocco around 1200 AD. In order to test the multi-decadal influence of different atmospheric and oceanographic variables on the hydroclimate variability in southwestern Morocco, cross-wavelet analyses were performed for the IFK1 record of δ 18 O and δ 13 C (Fig. S4). Results indicate significant multi-decadal co-variabilities between different paleorecords and the IFK1 record, regardless of the age uncertainties of different records. Based on these results, we hypothesize that the climate signals during the last millennium in southwestern Morocco remained under the combined influence of both the AMO and the NAO at decadal timescale. At longer multi-decadal timescales, the contrasted warm MCA and cold LIA in the North Hemisphere extra tropics probably impacted the mean climate in southwestern Morocco through the influence on Sahara Low which either weakened or strengthened the mean moisture inflow from the Atlantic Ocean (Liu et al., 2014). 6. 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 δ 18 O and 1st principal component based on IFK1 Mg and δ 13 C, covering the last 1200 yrs, suggests that southwestern Morocco is currently undergoing drought conditions which began in the early 20th century. We show that these dry conditions are the driest of the last four centuries, the stalagmite stopped growing after the year 1953 AD as a likely response to dry conditions. The δ 18 O and the PC1 confirm that the MCA in Morocco was generally marked by dry conditions in comparison to the LIA. This has been demonstrated by previous paleoclimate studies in the north of Morocco and the Iberian Peninsula and is now also confirmed for southwest Morocco by the Ifoulki record. The results allow testing the robustness of AMO and NAO influence on hydroclimate variability during the last millennium in Morocco. We highlight intense responses to regional and global controls of the NAO, AMO and Sahara Low over the hydroclimate variability in southwestern Morocco. These results offer new insights on decadal scale hydro-climate variations during the pre-industrial periods in the drought-prone region of southwestern Morocco. This will help characterize the base line of natural climate variations and allow improved detection of the global warming influence on recent and future climate change. Acknowledgements 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 (China) and the Laboratory of Applied Geology and GeoEnvironment in Ibn Zohr University (Morocco). We would like to express our gratitude to the CNRST-Morocco for the scholarship awarded to Yassine Ait Brahim. We are also grateful to Augusto Auler and the Associations of Speleologists in Agadir (ASS and ASA) for their help with the field work. The authors also thank the editor and reviewers for their constructive comments which helped improving the paper.

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