New insights into the climate of northern Iberia during the Younger Dryas and Holocene: The Mendukilo multi-speleothem record

New insights into the climate of northern Iberia during the Younger Dryas and Holocene: The Mendukilo multi-speleothem record J.L. Bernal-Wormull a , b , * , A. Moreno a , M. Bartolom e a , M. Arriolabengoa c , C. P erez-Mejías d , E. Iriarte e ,C.Os  acar b ,C.Sp € otl f , H. Stoll g , I. Cacho h , R.L. Edwards i , H. Cheng d , j a Pyrenean Institute of Ecology - CSIC, 50059, Zaragoza, Spain b Department of Earth Sciences, University of Zaragoza, C/ Pedro Cerbuna 12, 50009, Zaragoza, Spain c Department of Geology, University of the Basque Country, Leioa, Spain d Institute of Global Environmental Change, Xi'an Jiaotong University, 710054, Xi'an, China e Laboratory of Human Evolution-IsoTOPIK Stable Isotope Laboratory, Department of History, Geography &Communication, Edificio de IþDþi, Universidad de Burgos, Pl. Misael Ba~ nuelos s/n, 09001, Burgos, Spain f Institute of Geology, University of Innsbruck, Innrain 52, 6020, Innsbruck, Austria g Department of Earth Sciences, ETH Zürich, 8092, Zürich, Switzerland h GRC Geoci encies Marines, Dept. Din amica de la Terra i de l'Oce a, Facultat de Ci encies de la Terra, Universitat de Barcelona, 28080, Barcelona, Spain i University of Minnesota, Minneapolis, MN, 55455, USA j Institute of Earth Environment, Chinese Academy of Sciences, Xi'an, China article info Article history: Received 7 November 2022 Received in revised form 25 January 2023 Accepted 10 February 2023 Available online xxx Handling Editor: Mira Matthews Keywords: Holocene Younger dryas Iberian Peninsula Speleothem North Atlantic Stable isotopes Abrupt changes abstract Recent hydroclimate studies on the Iberian Peninsula have shown a complex regional pattern in timing and intensity of climate change spanning the Younger Dryas and the Holocene. These changes are due to multifaceted interactions between climate variability that characterizes the Atlantic Ocean region and hydroclimatic processes associated with the Mediterranean climate, thus making it difficult to reconstruct centennialand millennial-scale variability in rainfall. In this study we present a composite and continuous isotopic record ( d 13 C and d 18 O) consisting of four stalagmites from Mendukilo cave (MEN composite) in the western Pyrenees covering the Younger Dryas and the entire Holocene. This record reveals millennial-scale shifts in carbon isotopes in response to changes in the hydroclimate in the northern part of Iberia. The MEN oxygen isotopes show little variation on millenial time scales but reveal centennial changes that correlate with North Atlantic events (e.g., the 8.2 kyr BP cooling event). We observe a delay in the onset of humid conditions in the early Holocene and a subsequent trend towards drier and colder conditions between 6.0 and 2.5 kyr BP. This new, high-resolution and replicated speleothem record denotes the complex connection that exists between the North Atlantic and Western Europe during last millennia and the strong regional heterogeneity of the hydroclimate of Iberia during this time. ©2023 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). 1. Introduction The transition from the cold and dry Younger Dryas to the early Holocene was one of the most rapid warming events on a global scale. This warming was associated with a resumption of the Atlantic Meridional Overturning Circulation (AMOC) (McManus et al., 2004). In the North Atlantic, the onset of the Holocene was punctuated by short cold periods due to variable meltwater input. At latitudes below 45  N, including the Mediterranean region, deglacial warming was influenced by orbitally driven changes (Renssen et al., 2009) that modulated Holocene hydroclimate variability. The general evolution from wetter to drier climatic conditions between the early and late Holocene throughout southern Europe (Roberts et al., 2019) is consistent with the longterm change in insolation, and hence it is influenced by seasonality (Wanner et al., 2008). In fact, the Holocene Mediterranean climate has been traditionally divided into three intervals *Corresponding author. Pyrenean Institute of Ecology - CSIC, 50059, Zaragoza, Spain. E-mail address: [email protected] (J.L. Bernal-Wormull). Contents lists available at ScienceDirect Quaternary Science Reviews journal homepage: www.elsevier.com/locate/quascirev https://doi.org/10.1016/j.quascirev.2023.108006 0277-3791/©2023 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Quaternary Science Reviews 305 (2023) 108006 according to water availability: (1) a first part characterized by humid conditions (11.7e7.0 kyr BP), (2) the mid-Holocene with increased hydroclimatic variability but in general higher temperatures (7.0e5.5 kyr BP), and (3) a decrease in humidity since ~5.5 kyr BP associated with the onset of the current Mediterranean-type climate (Jalut et al., 2009;Magny et al., 2011). In the case of the Iberian Peninsula which is influenced by two contrasting climate regimes (Atlantic and Mediterranean), the hydroclimate evolution during the deglacial warming and the Holocene was characterized by a strong regional heterogeneity (e.g., Carri on et al., 2010). Thus, the observed changes in continental records (lake sediments and speleothems) (e.g., Gonz alez-Samp eriz et al., 2017;Morell on et al., 2018;Moreno et al., 2017) in Iberia are not consistent in space or time with the simplified picture outlined for the Mediterranean region leading to apparent contradictions in temperature and precipitation reconstructions that need to be understood on a regional scale. Different records in the region have highlighted that the sometimes contradictory pattern of climate proxy results may be due to seasonality changes in rainfall (Baldini et al., 2019; Morell on et al., 2009;Moreno et al., 2017;Walczak et al., 2015), potentially due shifts in the position and strength of the Azores High. Northern Iberia shows large environmental changes along a WE transect controlled by the altitude and proximity to the ocean, resulting in a large temperature and humidity variability during the Holocene (Finn eetal.,2019). For example, the Holocene vegetation and climate succession reconstructed from lakes and peatbogs in NW Iberia and the Eurosiberian region (excluding eastern Iberia) reveals the traditional Holocene regional tripartite pattern (Allen et al.,1996;L opez-Merino et al., 2012;Moreno et al., 2011): Lake records from the Pyrenees show a progression from a dominantly Atlantic pattern in the western sector (Gonz alezSamp eriz et al., 2006) to a strong Mediterranean imprint in the central region (P erez-Sanz et al., 2013). Consistently, lake records present in the central-southern region of the pre-Pyrenees show a delay in the onset of wet conditions (9.5 kyr BP) and a decrease in humidity after 5e4.5 kyr BP (Morell on et al., 2009), a pattern that is supported by terrestrial proxies obtained from western Mediterranean marine records (Fletcher et al., 2013;Frigola et al., 2007). Several Holocene speleothem records have also been obtained from caves in northern Spain, but some are discontinuous (e.g., Serra do Courel; Railsback et al., 2011) or do not cover the entire interglacial (Martín-Chivelet et al., 2011;Rossi et al., 2018). In general, orbital forcing clearly exerted a first-order control on speleothem growth, as shown by an increase in the relative abundance of speleothems (Stoll et al., 2013). Holocene speleothem records also revealed the important role of the Atlantic Ocean in shaping centennial-scale oscillations (Domínguez-Villar et al., 2017;Smith et al., 2016), while a compilation of speleothem records from northern Spain highlighted the role of rainfall seasonality (Baldini et al., 2019). Here, we present four new stalagmites collected from Mendukilo cave in northern Iberia, which provide a well-replicated, highresolution and continuous record of northern Iberian climate from the beginning of the Younger Dryas to present-day. Stable isotopes, combined with trace elements and supported by a detailed monitoring of the cave, allow reconstructing temperature and amount of rainfall during last 12,700 years, thus addressing the role of different mechanisms in causing centennial-scale oscillations, such as the 8.2 kyr event. The new data is discussed in both site-specific and regional contexts, together with other published records from the Atlantic margin of Europe, thus offering new insights into the Iberian climate evolution on millennial to centennial time scales since the Younger Dryas. 2. Site description Mendukilo cave is located at 750 m a.s.l. and 40 km from the Cantabrian coast (Fig. 1A and B) in a site of community importance within the lands of the Astitz council (Larraun valley, Navarra), on the slopes of the eastern part of the Sierra de Aralar (42  58 0 25 00 N, 1  53 0 45 00 W; Fig. 1B). The cave developed within reef limestones of Lower Cretaceous (Urgonian, Albian-Aptian) age at the eastern boundary of the Basque-Cantabrian basin. The climate of this region is temperate with cool summers (West coast maritime climate following the K€ oppen-Geiger classification). Rain, fog and drizzle are abundant, making this area one of the rainiest locations in Spain (1365 mm of annual precipitation). Rainfall is concentrated in fall, winter and spring, and temperatures are mild, without reaching extremes values, reflecting the proximity of the Atlantic Ocean (Supp. Material Fig. S1). The surroundings of the cave are made up of small karstic mountain plateaus and dolines, with meadows in the highest parts and forests of beech, oak and abundant moss covering the slopes. Soils are well developed, rich in clay, and usually less than 50 cm in thickness. The cave shows a descending morphology and consists of seven main rooms (Entrance hall, Intermediate gallery, Laminosin gallery, Los Lagos gallery, Dragon hall, Guerrero and Caballo gallery's) (Fig. 1C and D). The total passage length is 869 m and the vertical extension is 59 m (see also Supp. Material). 3. Methods 3.1. Rainfall monitoring In order to explore the relationship between the isotopic composition of rainfall ( d 18 O r and d D r ) and the climate parameters outside the cave, rainwater was sampled in the University of the Basque Country (Bilbao), an area with a mainly Atlantic source of rainfall and 95 km away from Mendukilo cave (Fig.1). The sampling was carried out between July 2018 and February 2021 and 144 rain events were sampled. These samples were analyzed for their stable isotopic composition using cavity ring-down spectroscopy (PICARRO L2130-i) at the Pyrenean Institute of Ecology (Zaragoza). The results are reported in per mil with respect to Vienna Standard Mean Ocean Water (V-SMOW) and the reproducibility of the measurements is typically 0.1 ‰ for d 18 O and 0.5 ‰ e1 ‰ for d D. The isotopic results were compared to the air temperature measured at the Bilbao airport meteorological station on the day of the respective rain event (Supp. Material Fig. S2). 3.2. Cave monitoring: sampling and analyses Mendukilo cave was monitored on a seasonal basis for temperature, humidity, pCO 2 , dripwater composition and farmed carbonates from 2018 to 2021. Cave-air temperature and relative humidity were recorded using HOBO®U23 pro v2 data loggers in three different galleries (Laminosin, Dragon hall and Guerrero; Fig. 1D). Additionally, pCO 2 was measured using a pSense Portable CO 2 meter (model AZ-0001) at the same sampling points. Precipitation and temperature data obtained from the Aldatz meteorological station, located 5 km northeast of Mendukilo cave (Fig. 1B), were used for comparison. Dripwater samples were obtained from four different drip sites (MEN-A, MEN-B, MEN-D and MEN-E; Fig.1D) and, additionally, four pluviometers (RAIN-O-MATIC-HOBO coupled with a HOBO data logger UA-003-64) were used to monitor drip rate (Fig. 1D). All dripwater samples were analyzed for oxygen ( d 18 O drip ) and hydrogen ( d D drip ) isotope composition via cavity ring-down J.L. Bernal-Wormull, A. Moreno, M. Bartolom e et al. Quaternary Science Reviews 305 (2023) 108006 2 spectroscopy (PICARRO L2130-i) at the Pyrenean Institute of Ecology (Zaragoza). Results are reported in per mil with respect to Vienna Standard Mean Ocean Water (V-SMOW) and the reproducibility of the measurements is typically 0.1 ‰ for d 18 O and 0.5 ‰ e1 ‰ for d D(Fig. 2). Farmed carbonate precipitated on glass plates (at drip sites MEN-A, MEN-D and MEN-E and additionally at MEN-F and MEN-G sites without drip monitoring) was analyzed for oxygen and carbon isotopes ( d 18 O farmed and d 13 C farmed , reported as ‰ with respect to the Vienna Pee Dee Belemnite (VPDB) standard). The samples were recovered seasonally. The MEN-E, MEN-F and MEN-G monitoring sites coincide with the locations of the stalagmites examined in this study (MEN-3, MEN-4 and MEN-5 respectively; MEN-2 was not beneath an active drip site). The first batch of isotopic analyses (7 samples) was analyzed at the University of Innsbruck (Austria) using a ThermoFisher Delta V Plus linked to a GasBench II, following the methodology described in Sp€ otl (2011). The long-term reproducibility (1 sigma) of d 18 O is 0.08 ‰ and 0.06 ‰ for d 13 C(Sp€ otl, Fig. 1. - A) Location of Mendukilo cave (red square) and other records of the Iberian Peninsula cited in this work (yellow squares ¼cave sites; green stars ¼marine records; purple hexagon ¼lake record; blue circle ¼ice cave): [1] La Garma Cave; [2] Cueva de Asiul; [3] Kaite Cave; [4] El Pindal Cave; [6] El Soplao Cave; [7] Cova de Arcoia; [8] El Refugio Cave; [9] Ejulve Cave; [10] Molinos cave; [11] ODP976; [12] ALB-2; [13] Basa de la Mora lake; [14] A294 Ice Cave. B) Regional setting with the location of the cave (red star), meteorological station (yellow square; where the temperature and rainfall databases for this work were obtained) and nearby major cities (black hexagons). C) Digital elevation model and plan view of the cave (pink). D) Geomorphological map of Mendukilo cave showing locations of dripwater and speleothem samples. J.L. Bernal-Wormull, A. Moreno, M. Bartolom e et al. Quaternary Science Reviews 305 (2023) 108006 3 Fig. 2. Monitoring results from Mendukilo cave from November 2018 to May 2021. A) Daily and monthly precipitation outside the cave (Aldatz meteorological station). B) Drip rate of the different sampling points inside the cave. C) Daily surface average temperature outside the cave. D) Cave temperature and E) CO 2 concentration at the Laminosin (light blue line and dots), Dragon (grey line and dots) and Guerrero (blue line and dots) galleries. F) Drip water stable isotope composition of the different drip sites. G) d 18 O and H) d 13 C values of farmed calcite for the different monitoring sites (see Fig. S3 in the Suppl. for more information). J.L. Bernal-Wormull, A. Moreno, M. Bartolom e et al. Quaternary Science Reviews 305 (2023) 108006 4 2011). The rest of the samples were analyzed at the Iso TOPIK Laboratory (University of Burgos; 29 samples) and at the University of Barcelona (10 samples) following similar procedures and identical equipment (Fig. 2). 3.3. Mendukilo stalagmites: petrography, trace elements and stable isotope analyses Four stalagmites were collected from the Guerrero gallery (Fig. 1D), a deep gallery around 200 m from the cave entrance. Except for MEN-2 these stalagmites were located underneath active drips fed by different fracture networks and dripping conditions (section 4.1). The stalagmites were cut parallel to their growth axis, and the central segment of the slab of each speleothem was sampled for UeTh dating, stable isotopes, and major and trace elements. In MEN-3 and MEN-4, the opposite slab was used to obtain thin sections for a petrographic study (Fig. 3A). MEN-4 is macroscopically very similar to MEN-2 and MEN-5; thus these two last stalagmites were not petrographically analyzed. A total of 1451 samples for d 18 O and d 13 C analysis were obtained along the central axis at 1 mm increments from stalagmites MEN-2, MEN-3, MEN-4 and MEN-5. Isotopic analyses were performed at the University of Innsbruck, using a ThermoFisher Delta V Plus linked to a GasBench II, following the methodology described in Sp€ otl (2011). The long-term reproducibility (1 sigma) of the d 18 O analyses is 0.08 ‰ and 0.06 ‰ for d 13 C(Sp€ otl, 2011). All values are reported as per mil with respect to the Vienna Pee Dee Belemnite standard (VPDB). In addition to the isotope samples, 267 carbonate samples were analyzed for trace elements. Samples were taken at 5 mm intervals and measured for Mg/Ca and Sr/Ca ratios at ETH Zurich (Agilent QQQ 8800) using a standardization approach similar to that reported in Stoll et al. (2022). 3.4. UeTh dating and age model development: integration into a composite record A total of 41 powder samples were drilled for uranium-series dating from distinct growth layers along the central growth axis of each speleothem using a handheld drill and a tungsten carbide drill bit. After chemical separation, U and Th isotope measurements were performed using a MC-ICP-MS (Thermo-Finnigan Neptune Plus) at the University of Xi'an and the University of Minnesota (USA), following a previously described methodology (Cheng et al., 2013). To calculate corrected ages an initial 230 Th/ 232 Th atomic ratio of 4.4 ±2.2 10 6 was used. Fig. 3. A) Thin sections showing the main fabrics of stalagmites MEN 4 (columnar fabric [C]) and MEN 3 (transition between columnar, columnar microcrystalline [Cm] and dendritic [d] fabrics). B) Dating results (black dots with error bars) and age models of the four stalagmites (green line) with their corresponding error limits (red lines) as obtained using StalAge. Changes in growth rate (blue line) are also indicated. J.L. Bernal-Wormull, A. Moreno, M. Bartolom e et al. Quaternary Science Reviews 305 (2023) 108006 5 The individual age model for each stalagmite has been obtained using the software StalAge (Scholz and Hoffmann, 2011)(Fig. 3B). Later, the individual isotope profiles of the four stalagmites were integrated into a single one (MEN composite record) using the software Iscam (Fohlmeister, 2012). This program looks for the highest correlation between two or more dated proxy signals within age uncertainties using a linear interpolation between adjacent UeTh dates. Two composite records have been constructed by applying the d 13 C and then the d 18 O data from the different stalagmites of the Mendukilo cave. The isotopic values between the different stalagmites in the overlapping intervals correlate well with each other in each of the composites ( d 13 C and d 18 O), but finally it has been decided to use the d 13 C composite (and the resulting age model) throughout this work, since it presents a greater variability in its isotopic values over the last 13 kyr BP. Finally, the age model that produced the highest coherence (applying a smoothing of 100 years) was selected and subsequently the data from other proxies of this work ( d 18 O and trace elements) have been assigned a chronology based on this new d 13 C composite age model. 4. Results 4.1. Monitoring The average annual rainfall in the area (1921e2021) is 1365 mm (Supp. Material Fig. S1). Rainfall amount during the monitored years shows an important interannual variability, with a decreasing annual amount from 2019 (1931 mm) to 2020 (1371 mm) (Fig. 2A). Drip data (November 2018 to May 2021) reveal that the MEN-D and MEN-E drip sites were continuously dripping. They show a lower drip-rate variability between different seasons compared to the MEN-A and MEN-B drip sites, which tend to almost cease in summer (Fig. 2B). Episodes of highest drip rates per day recorded by the MEN-A and MEN-B loggers correlate with periods of intense daily (>50 L/m 2 ) and monthly rainfall (winter and fall in 2019, Fig. 2A). Drip sites MEN-A, MEN-B and MEN-D are fed by low discharge seasonal drips with a medium to high variability, with a maximum discharge rate of 2.49 ml min 1 , 0.15 ml min 1 and 0.17 ml min 1 and a coefficient of variation of 134, 90 and 59, respectively (following Smart and Friederich (1987), Supp. Material Fig. S3). On the other hand, MEN-E is fed by seepage flow (low variability, coefficient of variation of 21) showing the lowest maximum discharge rate of 0.06 ml min 1 . In the case of drips without drip rate monitoring (MEN-F and MEN-G) they have been categorized as continuous drips (not seasonal) in the different monitoring campaigns inside the cave and with an apparent similar behavior between MEN-E and MEN-D drips. The classification of drips based on discharge rate and coefficient of variation has been shown to be useful to discern its influence on the isotopic composition of the dripwater and of the farmed calcite (P erez-Mejías et al., 2018). The air temperature inside Mendukilo cave varies between 8.4 and 9.2  C during the year, i.e. 3.4  C lower than the mean annual surface air temperature in the monitored years (11.8e12.6  C, Aldatz meteorological station) showing a delay of about 10e14 days in relation to variations in surface temperature (Fig. 2C and D). Only the temperature sensor installed in the Guerrero room, far from the entrance, recorded a constant temperature (9.1e9.2  C). pCO 2 values follow the seasonal temperature pattern inside the cave, being higher in summer and lower in winter. For the studied period, d 18 O drips and d D drips values for the four drip sites range from 8.7 to 7.3 ‰ and 53.8 to 42.0 ‰ , respectively. The mean d 18 O drips values for these drip sites are between 7.6 ‰ (MEN-D and MEN-E) and 8.0 ‰ (MEN-A and MEN-B) and, in general, winter values are lower than summer values (Fig. 2F). Considering the average values for all drip sites during summer (7.6 ‰ ) and winter (8.1 ‰ ), the average seasonal variability is 0.5 ‰ . Adding to this variability, a slight trend towards less negative values is observed between 2018 and 2021 associated with a decreasing amount of rainfall as described above. The exception is the MEN-E drip site which fed the MEN-3 stalagmite and records a stable drip rate that does not respond to seasons or rain events and shows a low isotopic variability (Fig. 2). The seasonality recorded in the dripwaters (low variability) values was also observed in rainfall (high variability; Fig. S2), although it was much more reduced in the first one due to homogenization once water enters into the epikarst. Carbonate precipitated on glass slides all year round and the d 18 O farmed and d 13 C farmed values range from 6.8 ‰ to 4.6 ‰ and from 12.0 ‰ to 8.3 ‰ , respectively (five different sampling points, Fig. 2G and H). Except for drips MEN-E, MEN-F and MEN-G, farmed calcite results show a tendency towards less negative values (statistically significant for d 18 O farmed ) during the second half of 2020 and the first part of 2021 (mean d 18 O farmed 5.3 ‰ ) compared to the period 2018e2020 (mean d 18 O farmed 6.3 ‰ ;Fig. 2G). Comparing d 18 O drip with d 18 O farmed of each drip site shows that calcite precipitated close to isotopic equilibrium. The amount of rainfall is thus likely influencing d 18 O farmed on an interannual scale but the seasonal impact is less pronounced. This seems to be true for d 13 C farmed as well, although the MEN-D drip site shows seasonal variability in the 2020e2021 time interval where the highest d 13 C farmed values coincide with low pCO 2 , thus suggesting enhanced degassing and possible prior calcite precipitation (PCP). An interesting feature is that d 13 C farmed shows less negative values for the seepage drip and nearby drips of the same cave gallery (MEN-F and MEN-G) in comparison to faster drips (Fig. 2H). 4.2. Petrography, chronology and stable isotopes of the Mendukilo stalagmites Stalagmites MEN-2 (31.0 cm), MEN-4 (25.6 cm) and MEN-5 (61.0 cm) consist of coarsely crystalline calcite and are macroscopically homogeneous without any signal of recrystallization. They are made of columnar fabric, lack growth hiati and do not show macroscopically visible laminae (Fig. 3A). Stalagmite MEN-3 (28.0 cm) is an exception showing a more porous, columnar microcrystalline fabric passing into a dendritic type at the base (the first 4.0 cm) as well as close to the top (between 18.5 and 28.0 cm from the base - Fig. 3A). According to Frisia (2015), this transition suggests a change from a relatively slow and constant discharge to more variable drip rates. UeTh dating revealed that these four stalagmites grew continuously over different intervals of the Holocene and Younger Dryas (YD). Taking together, they cover continuously the entire Holocene and the YD (i.e., since 12.8 kyr BP) with a good overlap (Fig. 3): MEN-2 grew between 12.8 and 6.3 kyr BP, MEN-3 between 6.0 and 0 kyr BP, MEN-4 covers the last 3.0 kyr, and MEN-5 spans the last 8.8 kyr. The dating of the base and top parts of stalagmite MEN-3 was challenging given the change in petrography (see above; Supp. Material Table S1). Measured U concentrations in Mendukilo stalagmites range between 73 and 350 ppb, and the measured 230 Th/ 232 Th activity ratio varies between 20 and 9500. Each of the MEN stalagmite age models have fairly low uncertainty of the corrected ages ranging between 0.005 and 0.250 kyr BP (the average error of the MEN-2, MEN-4 and MEN-5 age models are 0.055, 0.041 and 0.040 kyr BP respectively). In the case of MEN-3 the errors are larger fluctuating between 0.060 and 0.800 kyr BP (average error of 0.212 kyr BP). Average growth rates of MEN-3 (145 m m/yr) and MEN-4 (224 m m/yr) are higher than for MEN-2 (68 m m/yr) and MEN-5 (77 m m/yr), indicating higher speleothem J.L. Bernal-Wormull, A. Moreno, M. Bartolom e et al. Quaternary Science Reviews 305 (2023) 108006 6 growth rates during the late Holocene in this cave compared to the YD and early Holocene (Fig. 3). The d 18 O and d 13 C values of the four stalagmites range from 6.3 to 4.3 ‰ and from 10.4 to 4.4 ‰ , respectively, with similar amplitudes in the overlapping intervals (Suppl. Material Fig. S4 and Fig. S5). This replication supports the use of these isotopic profiles as reliable proxy records. The only exception is the upper part of stalagmite MEN-3 which differs markedly from MEN-4 and MEN-5 in its stable isotope values (Suppl. Material Fig. S4 and Fig. S5). Given the fact that this part of MEN-3 also shows a different type of calcite fabric we excluded this part. 4.3. Composite record from Mendukilo cave The MEN composite generated with Iscam for both d 13 C and d 18 O isotopic records covers the last 12.7 kyr BP with an average growth rate of 130 m m/yr. The d 13 C composite record shows values between 9.5 and 4.5 ‰ (MEN d 13C mean ¼6.6 ‰ ; MEN d 13C STD ¼0.8 ‰ ;Fig. 4). In the case of d 18 O the values are characterized by a lower amplitude and variability, ranging from 6.3 to 4.3 ‰ (MEN d 18 O mean ¼5.4 ‰ ; MEN d 18 O STD ¼0.3 ‰ ;Fig. 4). For the composite we included the stalagmites from the youngest to the oldest one to maximize the correlation coefficient. The d 13 C correlation between MEN-3 and MEN-4 is high (r ¼0.97) but they only intersect at a gap of about 0.5 kyr; when adding MEN-5 the correlation decreases (r ¼0.37) reflecting bad interrelation in the overlapping intervals of stalagmites MEN-5, MEN-4 and MEN-3 during the last 3 kyr (Suppl. Material Fig. S4 and Fig. S5). Thus, paleoclimatic interpretations for the last 3 kyr should be viewed with caution. Finally, when combining this composite with the MEN-2 record, the final correlation is very high (r ¼0.91). The MEN composite data contains results from mathematical interpolation, therefore, abrupt variations in the MEN composite should be treated and interpreted with caution and always taking into account the isotopic (Fig. S4 and Fig. S5) and trace element (Fig. S6 and Fig. S7) records of each stalagmite separately. The composite record produced by Iscam shows that the YD starts with opposite trends when comparing both isotopic records (Fig. 4). The d 13 C values increase (from 6.7 to 5.6 ‰ ) and those of d 18 O decrease (from 4.6 to 5.2 ‰ ) at the onset of the YD. The values remain stable during the YD, and at its end the d 13 C values gradually decrease (from 5.6 to 7.8 ‰ ) while the d 18 O values increase abruptly (from 5.2 ‰ in the YD to 4.3 ‰ at 11.6 kyr BP) reaching the highest values of the entire record at the YD-Holocene transition (Fig. 4). MEN composite d 13 C data show a significant trend towards more negative values during the first part of the Greenlandian, from 6.0 ‰ at 11.7 kyr BP to 8.1 ‰ at 10 kyr BP (Fig. 4). Afterwards, the values remain stable, only interrupted by a short interval of higher values between 9.3 and 9.1 kyr BP. The highest d 18 O values of the composite are present in the early Greenlandian and show a trend towards lower values (as the d 13 C record) but lasting until the end of the Greenlandian (from 5.0 ‰ at 11.68 kyr BP to 5.7 ‰ at 8.23 kyr BP). During the Northgrippian (8.2e4.2 kyr BP) the d 13 C record shows a progressive trend towards higher values: from 8.0 ‰ at 7.9 kyr BP to 4.8 ‰ at 4.1 kyr BP (Fig. 4). During the first part of the Northgrippian (8.2e6.5 kyr BP) the d 18 O values are very negative and increase in the second part (mean 5.2 ‰ between 6.5 and 4.2 kyr BP). The lowest d 18 O values of the MEN composite record are present by extreme values of 6.3 ‰ at 7.0 and 8.1 kyr BP. The d 18 O variability during the Meghalayan (4.2 kyr BP to present) is small and lacks a trend. d 13 C values show the highest values from Fig. 4. Mendukilo isotope composites for d 13 C (upper panel; note inverted y-axis) and d 18 O (lower panel) for the last 13 kyr obtained using Iscam combining four stalagmites. The Holocene division follows Walker et al. (2019). J.L. Bernal-Wormull, A. Moreno, M. Bartolom e et al. Quaternary Science Reviews 305 (2023) 108006 7 4.2 to 2.5 kyr BP, but by comparing the d 13 C values of the composite with those of stalagmites MEN-3 and MEN-5 it is possible to detect that there is a distortion in the former during the time interval of 4.2 and 3.5 kyr BP. This over-exaggeration of the d 13 C data in the composite is clearly an artifact of the mathematical algorithm, so that interpretations at millennial and centennial scales during that time interval must take into consideration the d 13 C values shown by stalagmites MEN-3 and MEN-5 separately from the composite. The MEN d 13 C composite shows a rapid shift towards lower isotope values at 2.5 kyr BP (Fig. 4). Mg/Ca ranges between 0.40 and 1.60 mmol/mol (Suppl. Material Fig. S6) and shows greater variations in trends among the four stalagmites. Sr/Ca measured in the four stalagmites show similar temporal patterns and are spanning the same range of values (0.12e0.06 mmol/mol) (Suppl. Material Fig. S7). Sr/Ca values are low during the YD and increase during the Greenlandian, reaching high values around 10 kyr BP. This is followed by a decreasing trend until 3.5 kyr BP and an increasing trend until modern conditions. Sr/Ca is anticorrelated with d 13 C at the millennial time scale (correlation coefficient 0.37, n ¼1288, p ≪0.01) (Suppl. Material Fig. S8). The Ba/Ca ratio of the MEN composite shows strong similarities with Sr/Ca, but correlation among the four stalagmites is less good, so we only use Sr/Ca to compare with other proxies. In particular, the trends during the last 3000 years are not well replicated among the four speleothems and thus not considered for discerning the climate signal. We also assess the potential effect of in-cave processes such as PCP on the d 13 C record looking for the most robust proxy trends associated to more constant Mg/Ca ratios of the MEN-5 stalagmite (Suppl. Material Fig. S9). 5. Discussion 5.1. Controls on MEN geochemical proxy data Trace elements in speleothems are useful paleoclimate indicators in caves where seasonal and long-term changes in water balance result in large and systematic change in Mg/Ca, Sr/Ca and Ba/Ca in dripwater associated with PCP during the dry season (Fairchild and Treble, 2009). In Mendukilo stalagmites, Mg/Ca and Sr/Ca show a contrasting pattern (Suppl. Material Fig. S6 and Fig. S7), indicating that PCP cannot be the dominant control on both ratios. One explanation could be a significant non-bedrock source of Mg or Sr which varied temporally. In caves very close to the coast, such as Pindal Cave with galleries 200 m from the sea cliff, marine aerosols can be a significant source of Mg in the Holocene (Moreno et al., 2010). However, in the cave of Mendukilo, 50 km from the Atlantic coast and at 750 m elevation, surrounded by mountain ranges of higher altitude, a dominant marine aerosol contribution of Mg is not expected, and temporal changes in Mg/Ca are unlikely to reflect changes in marine aerosol Mg delivery. An alternative explanation for the differences in Mg and Sr could be decoupled temporal changes in the partitioning coefficient of either Sr or Mg. Many studies have documented variations in the Sr partitioning coefficient driven by calcite growth rate (Lorens, 1981;Nielsen et al., 2013;Tan et al., 2014;Tang et al., 2008;Tesoriero and Pankow, 1996), and this is a common explanation for the decoupling of Mg/Ca and Sr/Ca in stalagmites (Stoll et al., 2012;Warken et al., 2018). We observe that Sr/Ca in Mendukilo stalagmites positively correlates with growth rate (correlation coefficient ¼0.55, n ¼1288, p ≪0.01) deduced from the UeTh age models (Suppl. Material Fig. S10), suggesting that growth rate contributed to the Sr/Ca variations. Consequently, we infer that Sr/ Ca may be driven by the growth rate effect superimposed on PCP, whereas Mg/Ca may reflect temporal variations in PCP. In fact, Sr/ Mg of stalagmites allows isolating the correlation between Sr/Ca and growth rate (Suppl. Material Fig. S8). Since stalagmites fed by different drip routes may have different sensitivities to PCP, Mg/Ca of coeval stalagmites may not covary as is the case in this work. On the contrary, the existence of common drivers of dripwater saturation by soil pCO 2 may lead to more reproducible variations in growth rate and Sr/Ca. Speleothem d 13 C is regulated by both soil/vegetation processes as well as in-cave processes. The d 13 C of DIC in dripwater is initially set by the d 13 C of soil CO 2 , which is more negative when the soil pCO 2 is high under warm and moist conditions which favor high heterotrophic and autotrophic respiration. The d 13 C of DIC in dripwater may be modified slightly during carbonate dissolution, but the low dead carbon percentage in most recent stalagmites, including those from Northern Iberia, suggests that dissolution commonly occurs in open systems and the impact on the d 13 Cof DIC is limited (Lechleitner et al., 2021). In the cave or karst cavities, the coupled processes of degassing and PCP can generate a significant positive shift in d 13 C of DIC before the water reaches the stalagmite. The modern d 13 C farmed for the high drip location (11 to 12 ‰ ) is in the range of d 13 C expected for CO 2 in soils with a dominant C3 vegetation in a warm and humid climate (Lechleitner et al., 2021), potentially affected by only limited PCP. The modern d 13 C farmed for the seepage location (MEN-E) and other nearby drips of the same gallery (MEN-F and G) are less negative (9to10 ‰ ) and these drips may have experienced some degassing and PCP and/or a greater extent of precipitation from the arriving drip (in situ PCP), and/or a more closed dissolution regime. Seasonal changes in d 13 C farmed at higher and intermediate drip rate sites may reflect seasonal variations in soil pCO 2 or in the significance of PCP. Drip rate monitoring and farmed calcite data suggest that the MEN composite record is not seasonally biased but rather reflects continuous, year-round deposition. Continuous calcite deposition is also supported by the columnar fabric and the absence of visible laminae as well as the lack of dissolution features (i.e., the drip water was always supersaturated with respect to calcite). Long-term changes in d 13 C values are rather unrelated to the Mg/Ca pattern in the MEN stalagmites. For example, in the d 13 C MEN5 record, the main long-term trends exhibit similar Mg/Ca ratios (low Mg/Ca variability between the different d 13 C larger anomalies; Suppl. Material Fig. S9). Therefore, the temporal trends in d 13 C are most likely driven unaffected by PCP, although the seepage drips may be characterized by some PCP. We suggest that the long-term trends in d 13 C are driven by climatic processes controlling soil pCO 2 , i.e. soil temperature and soil moisture. Some studies suggest that the temperature component dominates over soil pCO 2 , such during MIS 3 and 4 in Villars cave in SW France (Genty et al., 2006) and during the last deglaciation in El Pindal cave in NW Spain (Moreno et al., 2010). Comparison of the Mendukilo composite d 13 C record with paleotemperature reconstructions from Pyrenean lakes (e.g., Tarrats et al., 2018) and SST records (Catal a et al., 2019;Martrat et al., 2014) supports a temperature influence at millennial scale (Fig. 5). Therefore, higher (lower) d 13 C values are arguably linked to colder (warmer) and/or dryer (wetter) intervals with reduced (enhanced) soil respiration and vegetation productivity. We note that the inverse correlation between d 13 C and Sr/Ca in the MEN stalagmites (Suppl. Material Fig. S10), while opposite to the trend expected from PCP control, is consistent with the expected relationship of low d 13 C in warm and humid periods of high soil CO 2 which also lead to higher dripwater oversaturation and higher growth rates promoting higher Sr/Ca. Variation in the dripwater and stalagmite d 18 O record may reflect changes in d 18 O of the surface ocean in the moisture source area, as well as changes in the atmospheric processes which J.L. Bernal-Wormull, A. Moreno, M. Bartolom e et al. Quaternary Science Reviews 305 (2023) 108006 8 Fig. 5. Pal eoenvironmental records from the Iberian Peninsula and Morocco covering the YD and the Holocene. A) Stacked record of normalized d 13 C stalagmite data from Kaite cave; Cueva del Cobre; Cueva Mayor from northern Iberia (Martín-Chivelet et al., 2011). B) d 13 C record of the SIR-1 speleothem from El Soplao cave in northern Iberia (Rossi et al., 2018). C) Summer insolation at 42.D) d 13 C record of the BG speleothem from westernmost Iberia (Buraca Gloriosa Cave, Portugal) (Thatcher et al., 2020). Note that amplitude of d 13 C axis in this record (high variability) is not the same compared to the rest of Iberian stalagmite records. E) MEN d 13 C composite record (this work). F) G. bulloides Mg/Ca SST (ALBJ.L. Bernal-Wormull, A. Moreno, M. Bartolom e et al. Quaternary Science Reviews 305 (2023) 108006 9 Fohlmeister, J., 2012. A statistical approach to construct composite climate records of dated archives. Quat. Geochronol. 14, 48e56. https://doi.org/10.1016/ j.quageo.2012.06.007. 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