Paleoclimate and growth rates of speleothems in the northwestern Iberian Peninsula over the last two glacial cycles

Quaternary Research 80 (2013) 284–290

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Short Communication

Paleoclimate and growth rates of speleothems in the northwestern Iberian Peninsula over the last two glacial cycles Heather M. Stoll a,⁎, Ana Moreno b, Ana Mendez-Vicente a, Saul Gonzalez-Lemos a, Montserrat Jimenez-Sanchez a, Maria Jose Dominguez-Cuesta a, R. Lawrence Edwards c, Hai Cheng c,d, Xianfeng Wang c,1 a

Department of Geology, University of Oviedo, Oviedo 33005, Spain Instituto Pirenaico de Ecología (IPE)-CSIC, Avda. Montañana 1005, 50059 Zaragoza, Spain c Department of Earth Sciences, University of Minnesota, Minneapolis, MN 55455, USA d Institute of Global Environmental Change, Xian Jiaotong University, Xian 710049, China b

a r t i c l e

i n f o

Article history: Received 25 November 2012 Available online 20 June 2013 Keywords: Speleothem Stalagmite Holocene U/Th dates Paleoclimate

a b s t r a c t Speleothem growth requires humid climates sufficiently warm to stimulate soil CO2 production by plants. We compile 283 U/Th dates on 21 stalagmites from six cave systems in the NW coast of Spain to evaluate if there are patterns in stalagmite growth that are evidence of climatic forcing. In the oldest stalagmites, from marine oxygen isotope stage (MIS) 7–5, growth persists through the glacial period. Hiatuses and major reductions in growth rate occur during extreme minima in summer insolation. Stalagmites active during the last interglaciation cease growth at the MIS 5–4 boundary (74 ka), when regional sea-surface temperature cooled significantly. During MIS 3, only two stalagmites grew; rates were highest between 50 and 60 ka during the maximum in summer insolation. One stalagmite grew briefly at 41 ka, 36.5 and 28.6 ka, all during warm phases of the Dansgaard–Oeschger cycles. A pronounced Holocene optimum in stalagmite growth occurs from 9 to 6 ka. The cessation of most growth by 4.1 ka, coincident with broad increases in aridity over the Mediterranean and areas influenced by the North African Monsoon, suggest that regions such as NW Spain, with dominant Atlantic moisture sources, also experienced increased aridity at this time. © 2013 University of Washington. Published by Elsevier Inc. All rights reserved.

Introduction The growth of speleothems requires the delivery of water oversaturated in CaCO3 to a ventilated cave system. Both the infiltration rate of water and its oversaturation state with respect to CaCO3 in the cave atmosphere are related to climate, as described in modeling studies of stalagmite growth (Kaufmann and Dreybrodt, 2004). The hydrological balance (precipitation-evapotranspiration) determines the rates of dripwater flow to feed growing stalagmites. Dripwaters typically attain the greatest degree of oversaturation in CaCO3 by passing through soil and epikarst regions featuring pCO2 significantly higher than that of the atmosphere and of cave air. The degree of enrichment of CO2 in the soil and epikarst to a first order correlates with mean annual evapotranspiration (Brook et al., 1983). Recent work suggests that arboreal vegetation is especially effective at enriching infiltrating karst water in CO2(Breecker et al., 2012) and study of the Brown's Folly System has shown a strong increase in speleothem

⁎ Corresponding author. E-mail address: [email protected] (H.M. Stoll). 1 Present address: Division of Earth Sciences, Nanyang Technological University, 639798, Singapore.

growth following regrowth of natural forest and shrub coverage over the formerly unvegetated mine site (Baldini et al., 2005). We present here a compilation of 283 U/Th dates on 21 stalagmites from six cave systems in the NW coast of Spain to evaluate if there are significant patterns in stalagmite growth which are evidence of climatic forcing since marine oxygen isotope stage (MIS) 7. Ages of individual stalagmites may be subject to selection biases but ages of a large population of stalagmites from multiple caves within the same climatic setting are likely to be representative of major periods of speleothem growth during climatically favorable intervals. Changes in hydrological routing may also cause hiatus or surges in growth in individual stalagmites, but it is unlikely that such rerouting would similarly affect multiple stalagmites in different locations in the same cave or in different cave systems. In this region modern precipitation averages 1200 mm/yr (see Table 1) cave temperatures average 10–12°C, and caves are from low altitude ranges which were never glaciated, so variation in humidity and soil CO2 production in the past, rather than subfreezing temperatures, are expected to be dominant factors regulating speleothem growth. Methods Speleothems were collected from six different cave systems along the coastal region of NW Spain (Fig. 1, Table 1) in regions of similar

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Figure 1. Map of location of caves in northwest Spain from which stalagmites in Table 1 were sampled. Pando Cave is located b5 km from Cueva Rosa and is not shown with a separate symbol here.

modern mean annual temperature and precipitation. Stalagmites were selected to provide a representative population of growth in the caves, including older material with visible patinas as well as stalagmites with wet surfaces or active drips. Stalagmites were sectioned and 100 to 500 mg samples were drilled for dating as described in previous studies (Moreno et al., 2010). Dates from U-Th decomposition were obtained at the University of Minnesota using methodology described previously (Shen et al., 2002; Cheng et al., 2009) and measurements either with ICP-MS (Thermo-Finnigan ELEMENT) or MC-ICP-MS

(Thermo-Finnigan Neptune). Details of dates, U contents, and corrected initial δ234U ratios are given in Supplemental Table 1. We analyzed data from the total of 21 stalagmites for which we have a total of 283 dates. For 19 of these, dating constrains the basal and terminal ages and all visible growth hiatuses within the interval studied, and the age–depth relationships are illustrated in Supplemental Figure 1. Growth rates calculated for stalagmites feature uncertainty due to the paired uncertainty of each U/Th date. We employ a simple approach to minimize the effect of this uncertainty by

Table 1 Information on cave locations, bedrock and named stalagmites from each cave. Data on annual rainfall from http://idebos.bio.uniovi.es/GeoPortal/Atlas/Pan19702009.html, last accessed 28 April, 2013. Cave name or location (abbreviation used in Supplement Table 1)

Altitude (m) (cave entrance)

Coordinate

Annual precip. (mm/yr) (1970–2009)

Bedrock of cave

Thickness of limestone above stalagmite sample locations

Named stalagmites dated in this work

Pindal, Pimiango (PIN)

23

43°23′50″N 4°31′58″W

1250

Barcaliente Formation — Carboniferous dark sulfidic massive micritic limestone

10–35 m

Tito Bustillo, (TITB)

5

43°27′38″N 5°04′04″W

1185

15–60 m

La Vallina, Porrua (POR)

70

1230

Cueva Rosa, Calabrez (CAL)

121

43°24′36″N 4°48′24″W 43°26′37″N 5°08′25″W

Escalada Formation — Carboniferous light massive bioclastic boundstone (limestone) Barcaliente Formation

Candela, Gorda, Maria, Pinshort, Pin3, Ana, Paz Inma

1260

Escalada Formation

50 m

Cueva Fría, Piloña (PIL)

560

1350

Barcaliente Formation

>100 m

La Trapa, Oviedo

365

1080

Barcaliente Formation

>100 m

Tina

Pando (PAN)

123

43°17′20″N 5°16′52″W 43°18′26″N 5°56′45″W 43°27′14″N 5°06′57″W

Gael, Galia, Garth, Guillermina Angelines, Artemisa, Athenea, Neith, Romeo, Alicia Aida, Patricia, Sarla

1220

Escalada Formation

5m

Julieta

10–50 m

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calculating average growth rates over time intervals whose duration is at least 6 times the average uncertainty of the U/Th determination (see supplemental online file for details). Growth rates are never calculated over intervals containing a growth hiatus, so in proximity to growth hiatus growth rates may be calculated over a smaller span. The exact timing of transitions between different growth rates has a precision equivalent to the average temporal spacing between dated samples, for example, several ka during MIS 5–6. We compare stalagmite growth rates with insolation, regional estimates of sea surface temperature from alkenone undersaturation index (Uk37) in Iberian margin cores, and the abundance of arboreal pollen in sediment cores off the Iberian margin (Martrat et al., 2007; Sanchez-Goni et al., 2008). For the Holocene, where welldated terrestrial records are available, we also include climate proxies from lake records from the Pyrenees and the Cantabrian mountains. In the following discussion, stalagmites are described by their identifying name from Supplemental Table 1. In addition, we successively number these named stalagmites according to their order of appearance in the figures (from top to bottom, beginning in Fig. 2) so that each discussed stalagmite may be located quickly in Figures 2–4.

Growth rates and relationship to climate Growth rates during MIS 7–5 (205–74 ka) and relationship to orbital forcing We first evaluate patterns of growth in the four stalagmites with longest total growth interval (Garth and Gael from La Vallina Cave, and Neith, Angelines from Cueva Rosa) and two stalagmites coeval with these (Athanea, Artemisa from Cueva Rosa) illustrated in Figure 2. The oldest stalagmite, Garth (1), begins growth at 205 ka and features a single 18 ka hiatus from 194 to 176 ka. The central age of this hiatus coincides with an extreme minimum (b 420 W/m2) in summertime insolation at 65°N and minima in SST in the Iberian margin. Around the time of termination II (ca. 130 ka), stalagmite growth commences in two other stalagmites, Gael (2) and Neith (3), and both feature a long hiatus commencing at 121 or 126 ka respectively, and lasting until 102 or 104 ka, respectively. The core age of this hiatus also coincides with another extreme minimum (b420 W/m2) in summertime insolation at 65°N and minima in Iberian margin SST; however, growth of the stalagmite Garth (1) persists during this interval. This interval also features growth in Athanea (5) and Artemisia

Figure 2. Long-term speleothem growth rates and climatic records over the past 200 ka. Upper panel shows Atlantic forest pollen in Iberian margin cores MD03-2697 and MD99-2331 (Sanchez-Goni et al., 2008). Middle panels show growth rates of stalagmites, with those from La Vallina Cave in Porrua (Garth (1) and Gael (2)) in the upper of the panels and below those from Cueva Rosa system in Calabrez (Neith (3), Angelines (4), Athanea (5), and Artemisa (6)). Lower panel indicates summer insolation for 65° N (bold line); sea surface temperatures for Portuguese margin MD01-2443 (red) and Alboran Sea ODP 977 (blue) (Martrat et al., 2007). Stippled bands highlight intervals with summer insolation >460 W/m2 and white rectangles indicate intervals with summer insolation b420 W/m2. Absolute dating of marine pollen and SST records is secured by 14C in the interval from 0 to 50 ka, but may be subject to greater chronological uncertainty in previous periods. Marine oxygen isotope stages (MIS) 1 to 7 are indicated.

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Figure 3. Detail of speleothem growth rates and climatic records from MIS 4 to MIS 1, over the last 70 ka. A. Atlantic forest pollen in Iberian margin cores MD03-2697 and MD99-2331 (Sanchez-Goni et al., 2008). B and C show growth rates of stalagmites Neith (3), Galia (7), Candela (8), and Julieta (9). In Galia (7), the brief growth phases at 36.5 and 28.6 ka are defined by a single date, so actual growth rate and duration are not defined; on the graph we show these intervals using the minimum growth rate measured in the stalagmite during the interval 49–54 ka. D. Summer insolation for 65° N (bold line) and sea surface temperatures for Portuguese margin (Martrat et al., 2007). Shaded bars highlight intervals with stalagmite growth. Marine oxygen isotope stages (MIS) 1 to 4 are indicated.

(6). During this interval, while cessation of stalagmite growth may be related to orbital-climatic forcing, different stalagmites, even within the same cave, appear to show different sensitivities to disruption of growth. During periods of stalagmite growth, temporal variations in growth rates within an individual stalagmite, at the scale of intervals calculated here, are of order 2- to 10-fold. Angelines (4) and Gael (2) grew very slowly (2.7 μm yr−1 and 6–11 μm yr−1, respectively) during the minimum in summer insolation centered at 90 ka, which is also minimum in Mediterranean SST. In contrast, these stalagmites grew much more rapidly (30–60 μm yr−1 and 23–55 μm yr−1, respectively) during the preceding and subsequent insolation maxima. In both Angelines (4) and Gael (2), maximum rates were attained slightly after the maximum insolation at 82 ka. Stalagmite Neith (3) grew most rapidly (52 μm yr−1) during the maximum in summer insolation at 126 ka, especially compared with minimum growth rates of the TII deglaciation (8 μm yr−1). Stalagmite Athanea (5) shows a similar brief peak growth rate at 126 ka. This peak coincides with the maximum Mediterranean SST estimates according to the marine age model (Martrat et al., 2007) and corresponds to the MIS 5e. The slowest average growth rates occur in Garth (1), and in this stalagmite there is no consistent relationship between growth rate and either insolation or temperature. Both the growth periods and growth rates suggest generally enhanced stalagmite growth conditions during periods of higher summer insolation. Summer insolation maxima may stimulate greater arboreal vegetation and greater vegetation productivity, increasing soil CO2

and dripwater saturation state. In addition, more humid climates in this precessional configuration may also contribute to the cyclic growth. Similarly, most phases of fluvio-glacial terrace aggradation in the Pyrenean valleys (Lewis et al., 2009) (Benito et al., 2010) and in nonglaciated regions (Fuller et al., 1998) correspond to periods of enhanced humidity at the insolation maxima. Of the four stalagmites whose growth continues into MIS 5c–a, growth ceases either temporarily or permanently around 74–73 ka at the MIS 5–4 boundary, or slightly earlier (79 ± 1.4 ka) in Neith (3). The climate forcing strong enough to terminate growth in all sampled stalagmites during this interval includes: (1) the onset of another extreme minimum in summertime insolation, (2) a pronounced shift to much lower abundance of arboreal pollen in marine cores (Sanchez-Goni et al., 2008), as well as (3) a more permanent decrease in temperatures at the onset of MIS 4 (Martrat et al., 2007). Caves south of the Cantabrian Mountains, in a more continental climate regime with different modern precipitation origin, also show evidence of initiation of growth at the onset of MIS 5 and cessation of growth slightly before the MIS 4–5 boundary (Muñoz-García et al., 2007). Growth phases during MIS 4–2 None of the stalagmites which began growing during MIS 5 feature continuous growth through glacial stages MIS 4 or MIS 3 (Fig. 3). This contrasts with the situation for stalagmite Garth (1), which began growing during the interglacial of MIS 7, and continued to grow during most of the glacial MIS 6, including the penultimate

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Figure 4. Speleothem growth rates and climatic records over the past 14 ka. A. arboreal pollen in Lago Enol lake core (dashed line), located in the Cantabrian Mountains, b80 km south of the cave locations (Moreno et al., 2011) and magnetic susceptibility record from Base de la Mora lake core (solid line) from the Pyrenees (Perez-Sanz et al., in press), together with lake levels in Zoñar lake (Martin-Puertas et al., 2011). Middle panels B–E show growth rates of stalagmites. B. Pindal Cave. Points with arrows indicate where only basal dates of onset of growth are available. C. Cueva Rosa cave system, and also stalagmite Tina from La Trapa Cave. D. La Vallina system (Galia, Guillermina) and from Tito Bustillo (Inma). E. Cueva Fria system, solid line indicates dated portion and dashed line indicates age to base extrapolating constant growth rates from the dated span. F. Summer insolation for 65° N (smooth curve); sea surface temperatures for Portuguese margin MD01-2443 (Martrat et al., 2007). Shaded band highlights interval of optimum stalagmite growth. For clarity new sequence numbers are assigned to Neith and Galia in this plot compared to Figure 3, so that here they may be grouped with others from the same cave.

glacial maximum. SST records from the Alboran sea reach similar cold levels during the MIS 6 and MIS 4–2 glacial periods. Likewise, the Portuguese margin is affected by IRD during both MIS 6 and MIS 4–2 (Martrat et al., 2007). However, climate records from terrestrial environments suggest differences in moisture availability between the MIS 6 and MIS 2 glacial periods. Recent geomorphological and chronological data indicate an important phase of karst subsidence and fluvial aggradation during MIS 6 in clear relation to moister conditions (Benito et al., 2010), suggesting the potential for greater humidity during MIS 6 than MIS 4–2. In addition, exokarstic (tufa) deposits in NE Spain also support a more moderate continental climate during MIS 6 compared to MIS 2(Valero-Garcés et al., 2008; Sancho et al., 2010; Lozano et al., 2012).

Alternatively, since the stalagmite Garth (1) is unique among our population of dated stalagmites in its maintenance of very low (nearly always b20 μm yr−1) and stable growth rates for long periods, it may result from a diffuse drip system whose growth was less sensitive to small climate variations in either temperature or humidity. Episodic growth during MIS 4–2 occurs in several stalagmites, but always during a warm phase of the Dansgaard–Oeschger (D–O) cycles (Fig. 3). Neith (3), which began growing during MIS 5, grew only during one brief episode during MIS 3, from 54 to 50 ka. Stalagmite Galia (7) began growing at the MIS 4–3 boundary (60 ka) and continued until 49 ka, but subsequently grew only in very brief pulses from 41 to 40.6 ka, and at 36.5 and 28.6 ka. The stalagmite Candela (8) began

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growth at 26 ka and grew until 18 ka, then resumed growth from 15.5 to 11.6 ka. The stalagmite Julieta (9) grew rapidly from 22 to 21 ka during the LGM. These growth periods all coincide with intervals in which Portuguese margin SST was at least 13.7 ° C, except for the brief 36.5 ka pulse which is within dating uncertainty of the SST in this range during D–O events 7 or 8 (Fig. 3). However, not all intervals of SST >13.7° during MIS 3 corresponded with growth, which may in part reflect the limited resolution of growth phases given only two dated stalagmites from MIS 3. The periodic cessation of growth in stalagmites in this region, such as that in Candela during the cold “Mystery Interval” (Moreno et al., 2010), may be caused by low temperatures which reduced soil CO2 production, and by reduction in precipitation which is correlated with the low temperatures (Stoll et al., 2012).

289

(Frigola et al., 2007; Naughton et al., 2007; Fletcher and Sánchez Goñi, 2008). The drying recorded in stalagmites could reflect a modest reduction in precipitation, which nonetheless may have caused significantly lower infiltration into the karst system if high rates of evapotranspiration were favored by the warm temperatures and more forested ecosystems. The observation of a significant decrease in effective moisture in NW Spain is significant because this region is dominated by precipitation from Atlantic extratropical systems (Gimeno et al., 2010). While models for strong orbital modulation of summer monsoon systems are widely established, mechanisms for orbital modulation of extratropical systems remain to be explored. One possible factor is that the maximum in summertime insolation would coincide with a minimum in wintertime insolation. This may have favored a more equatorward position of the polar front and a shift of more extratropical cyclone activity toward the Iberian Peninsula.

Growth in the Holocene (MIS 1) Early Holocene growth optimum The Holocene is marked by a strong optimum in stalagmite growth conditions between 9 and 6 ka (Fig. 4). In the three stalagmites growing during the earliest Holocene, Candela (8), Maria (10), Alicia (15), growth rates are more than triple around 10–8 ka compared to immediately preceding rates. Growth commences in another seven stalagmites by early mid-Holocene (Neith (18) 9.9 ka, Tina (19) 9.3 ka, PIN03 (12) 9.3 ka, Galia (21) 8.5 ka, Romeo (17) 8.1 ka, Rosa (16) 8.1 ka, Gorda (11) 7.4 ka) indicating a clear climate amelioration at that time. Optimal conditions for stalagmite growth began to diminish around 6 ka. In those stalagmites for which dating resolution permits definition of multiple growth phases, growth rates decreased around 6 ka (Maria (10), Romeo (17), Alicia (15), Gorda (11); resolved here between 6.2 and 5.8 ka) whereas growth ceased in two others at this time (Tina (19) at 6.3 ka and Rosa (16) at 5.8 ka). In Candela (8), growth had already ceased at 7 ka. Conditions become progressively less favorable for stalagmite growth, crossing thresholds at which growth stops in other stalagmites between 5.3 and 4.1 ka (Neith (18) 5.3 ka, Romeo (17) 5.3 ka, Alicia (15) 4.9 ka, Gorda (11) 4.4 ka, Maria (10) 4.3 ka, Pin03 (12) 4.1 ka). Of the ten stalagmites growing in the mid-Holocene, only one, Galia (21), continued to grow during the interval from 4.1 to 3.2 ka, persisting until 0.9 ka. This stalagmite featured the lowest and most stable growth rates during the mid-Holocene, potentially indicating growth fueled uniquely by diffuse flow at very low drip rates and more stable dripwater saturation state, and therefore lower sensitivity to small climate variations. Stalagmite growth during the Holocene was clearly favored by the warm climate of present interglacial. However, we infer that the peak in stalagmite growth from 9 to 6 ka, and the cessation of most growth by 4.1 ka, was not driven by regional temperatures, which appear to have remained stable since the onset of the Holocene with only slight decrease in the last 6 ka (Martrat et al., 2007). This temperature decrease of b 1°C could have affected only slight influence on soil CO2 or stalagmite growth kinetics, unless unusually warm summers significantly enhanced vegetation productivity and the saturation state of infiltrating dripwater. The cessation of stalagmite growth is unlikely to reflect anthropogenic deforestation which at that time was of limited extent and very localized (Carrión et al., 2010) and unlikely to exert similar effects in multiple cave locations simultaneously. We favor a regional drying as the dominant explanation for the pattern of stalagmite growth in the early and mid-Holocene. The optima in stalagmite growth coincides with the second half of the early Holocene maximum in summer insolation, which is the most humid period in most Eastern Mediterranean lake sequences (Roberts et al., 2011). In the Pyrenees, lake records suggest a similar increase in aridity around 5 ka from the magnetic susceptibility record (Perez-Sanz et al., in press) (Fig. 4). Lake records and marine cores around the Iberian margin have suggested a shift to drier conditions in the mid-Holocene (Santos et al., 2000; Reed et al., 2001; Leira, 2005; Gonzalez-Samperiz et al., 2008)

Late Holocene growth Dating results suggest that stalagmite growth during the last several millennia has been less widespread than during the Holocene optimum. This result is in spite of our sampling strategy which has deliberately sought stalagmite growth in the last several millennia by seeking wet and apparently fresh stalagmites in several different caves. In four of the five caves in which multiple stalagmites were sampled (and one in which a single stalagmite was sampled), the mid-Holocene optimum growth period is represented, but active growth over the last several millennia is found in only two of these caves (Fig. 4; Guillermina (20), in La Vallina Cave and Pinshort (13) in Pindal Cave). One explanation for this trend would be reduced humidity in recent millennia compared to the mid-Holocene optimum. The cave with the highest frequency of active stalagmite growth, Cueva Fria, Piloña, is one in which drier conditions may favor stalagmite growth. A stream passes through one end of the cave and periodically floods the gallery in which stalagmites grow, in the process reworking parts of the 0.5–2 m thick sand deposits on the cave floor and in terraces. In more humid periods, more frequent or longer duration inundation of this gallery, evidenced by a level of continuous Mn staining on cave walls which is higher than most stalagmites, may have inhibited stalagmite growth. The onset of growth in the two long stalagmites from this cave, Patricia (24) and Aida (23) (Fig. 4), could not be directly dated by U/Th methods due to abundance of detrital material near the base. However, the oldest date in each is 2.6 ka and extrapolation of the average growth rate in this period over the older portion of the stalagmites suggests that their deposition began around 4 ka. The age model for Patrizia is constrained additionally by four 14C ages, which yield a linear growth rate (Stoll and Asturias cave team, 2011). Growth in Pinshort (13) and Inma (22) began at 3.1 ka. Of those stalagmites growing during the last several millennia, Pinshort (13) exhibits slow and very stable growth rates, whereas in Galia (21) and Inma (22) growth rates are highly variable growth and/or episodic. With the existing set of samples, we do not observe any clear, reproducible trends in growth rates or growth hiatuses. On this time scale, in addition to regional climatic trends, local anthropogenic changes in land use may also influence soil and epikarst CO2 levels and hence the saturation state of dripwater, contributing to variations in growth conditions among different caves. Cueva Fria, with the most widespread active modern growth, lies beneath undisturbed, native forest. In contrast, Pindal, Tito Bustillo, and La Vallina caves lie beneath land heavily altered by human settlement over the past centuries and currently covered by pasture, planted allochthonous Eucalyptus groves, and encina (Quercus ilex) and chestnut groves. The end of growth in Galia (21) at about 0.8 ka does coincide with the culmination of a long drying trend in Iberian lacustrine record of Zoñar (MartinPuertas et al., 2011) (Fig. 4A). South of the Cantabrian Mountain chain, intermittent Late Holocene growth phases have also been recognized (Martín-Chivelet et al., 2011).

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Conclusions During MIS 7–5, stalagmite growth was nearly continuous. Stalagmites exhibit different thresholds for growth and do not all exhibit synchronous hiatuses, but all hiatuses and pronounced (>3-fold) reductions in growth rate occur during extreme minima in summer insolation. All stalagmites with growth during MIS 5c–a ceased growth at or slightly before the MIS 5–4 boundary (74 ka), coinciding with a major reduction in SST from 18 to below 14 ° C. Growth resumed periodically in MIS 3 in the interval between 50 and 60 ka, which coincides with a maxima in summer insolation and slightly warmer SSTs. Very brief growth pulses occurred in one at 41 ka, 36.5 and 28.6 ka, all intervals coinciding with SST >13.8, but not all periods with temperature over this threshold featured stalagmite growth. In any case, speleothem growth during MI S3 is especially scarce. A pronounced optimum in stalagmite growth from 9 to 6 ka, and the cessation of most growth by 4.1 ka, was not driven by regional temperatures but by moisture availability. Acknowledgments This work was funded by grant MEC CGL2006 13327-Co4-02, MEC CGL2010 16376, and FICYT IB08-072C1, and a DuPont Young Professor award (to H.M.S.). The work was conducted in collaboration with the GRACCIE-Consolider CSD2007-00067. U/Th dating was conducted by H.M.S., A.M., A.M.-V., and S.G-L. under the direction of R.L.E., H.C., and X.W. Field collection was conducted by H.M.S., A.M.-V., S.G-L., M.J-S., and M.J.D.-C. H.M.S. and A.M. wrote the paper. Appendix A. Supplementary data Supplementary data to this article can be found online at http:// dx.doi.org/10.1016/j.yqres.2013.05.002. References Baldini, J.U.L., McDermott, F., Baker, A., Baldini, L.M., Mattey, D.P., Railsback, L.B., 2005. Biomass effects on stalagmite growth and isotope ratios: a 20th century analogue from Wiltshire, England. Earth and Planetary Science Letters 240, 486–494. Benito, G., Sancho, C., Pena, J.L., Machado, M.J., Rhodes, E.J., 2010. Large-scale karst subsidence and accelerated fluvial aggradation during MIS6 in NE Spain: climatic and paleohydrological implications. Quaterary Science Reviews 29, 2694–2704. Breecker, D., Quinn, A., Quade, J., Banner, J.L., Ball, C., Meyer, K., 2012. The source of carbon in cave air CO2 under mixed woodland and grassland vegetation. Goldschmidt, Montreal. Brook, G.A., Folkoff, M.E., Box, E.O., 1983. A world model of soil carbon-dioxide. Earth Surface Processes and Landforms 8, 79–88. Carrión, J.S., Fernández, S., González-Sampériz, P., Gil-Romera, G., Badal, E., Carrión-Marco, Y., López-Merino, L., López-Sáez, J.A., Fierro, E., Burjachs, F., 2010. Expected trends and surprises in the Lateglacial and Holocene vegetation history of the Iberian Peninsula and Balearic Islands. Review of Palaeobotany and Palynology 162, 458–475. Cheng, H., Fleitmann, D., Edwards, R.L., Wang, X.F., Cruz, F.W., Auler, A.S., Mangini, A., Wang, Y.J., Kong, X.G., Burns, S.J., Matter, A., 2009. Timing and structure of the 8.2 kyr BP event inferred from delta O-18 records of stalagmites from China, Oman, and Brazil. Geology 37, 1007–1010. Fletcher, W.J., Sánchez Goñi, M.F., 2008. Orbital- and sub-orbital-scale climate impacts on vegetation of the western Mediterranean basin over the last 48,000 yr. Quaternary Research 70, 451–464. Frigola, J., Moreno, A., Cacho, I., Canals, M., Sierro, F.J., Flores, J.A., Grimalt, J.O., Hodell, D.A., Curtis, J.H., 2007. Holocene climate variability in the western Mediterranean region from a deepwater sediment record. Paleoceanography 22. Fuller, I.C., Macklin, M.G., Lewin, J., Passmore, D.G., Wintle, A.G., 1998. River response to high-frequency climate oscillations in southern Europe over the past 200 k.y. Geology 26, 275–278. Gimeno, L., Nieto, R., Trigo, R., Vicente-Serrano, S., Lopez-Moreno, J.I., 2010. Where does the Iberian Peninsula moisture come from? An answer based on a lagrangian approach. Journal of Hydrometeorology 11, 421–436.

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