A speleothem record of Holocene climate variability from southwestern Mexico

Quaternary Research 75 (2011) 104–113

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Quaternary Research j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / y q r e s

A speleothem record of Holocene climate variability from southwestern Mexico Juan Pablo Bernal a,⁎, Matthew Lachniet b, Malcolm McCulloch c,1, Graham Mortimer c, Pedro Morales a, Edith Cienfuegos a a b c

Instituto de Geología, Universidad Nacional Autónoma de México, Ciudad Universitaria, Coyoacán, Mexico City, 04510, Mexico Department of Geoscience, University of Nevada, Las Vegas, Las Vegas, NV 89154-4010, USA Research School of Earth Sciences, The Australian National University, Canberra, ACT, 0200, Australia

a r t i c l e

i n f o

Article history: Received 4 November 2009 Available online 3 November 2010 Keywords: Stalagmite Mexico Holocene climate variability Oxygen isotopes ITCZ. ENSO

a b s t r a c t A paleoclimate reconstruction for the Holocene based upon variations of δ18O in a U–Th dated stalagmite from southwestern Mexico is presented. Our results indicate that the arrival of moisture to the area has been strongly linked to the input of glacial meltwaters into the North Atlantic throughout the Holocene. The record also suggests a complex interplay between Caribbean and Pacific moisture sources, modulated by the North Atlantic SST and the position of the ITCZ, where Pacific moisture becomes increasingly more influential through ENSO since ~ 4.3 ka. The interruption of stalagmite growth during the largest climatic anomalies of the Holocene (10.3 and 8.2 ka) is evidenced by the presence of hiatuses, which suggest a severe disruption in the arrival of moisture to the area. The δ18O record presented here has important implications for understanding the evolution of the North American Monsoon and climate in southwestern Mexico, as it represents one of the most detailed archives of climate variability for the area spanning most of the Holocene. © 2010 University of Washington. Published by Elsevier Inc. All rights reserved.

Introduction The tropics and subtropics play an important role in global climate modulation, as their hydrological cycle has a direct role in regulating rapid climate change (Schmidt et al., 2004) and they serve as conduits for atmospheric teleconnections between hemispheres. In particular, tropical and subtropical Central America provides a vital link between the Atlantic and Eastern Pacific basins, providing an avenue for the exchange of moisture (Leduc et al., 2007). This connection has been in place some time before the last glacial maximum (Lachniet et al., 2007). Extensive high-resolution paleoclimate information from the tropics and subtropics is thus essential to understand how the global climatic system has reached its current state, providing fundamental information on the geographical extent of climatic anomalies. Most paleoclimatic information from tropical and subtropical America has been derived from lake sediments (Metcalfe et al., 2000; Ortega et al., 2002; Hodell et al., 2008), soils (Sedov et al., 2001) and glacier dynamics (Lachniet and Vázquez-Selem, 2005; Lozano-García and Vázquez-Selem, 2005). High-resolution records from northern Mexico (Cleaveland et al., 2003; Barron et al., 2005; Cheshire et al., 2005; Pérez-Cruz, 2006) have shown the influence that El Niño– Southern Oscillation (ENSO) has upon the winter precipitation of

⁎ Corresponding author. Now at: Centro de Geociencias, Campus UNAM Juriquilla, Querétaro, Mexico 76230. Fax: + 52 55 56234317. E-mail address: [email protected] (J.P. Bernal). 1 Current address: The School of Earth and Environment, The University of Western Australia, Crawley 6009, Australia.

northern Mexico and the southern USA. However, similar records are scarce for central and southern Mexico. Consequently, it has not been possible to fully identify the extent of rapid climate fluctuations in tropical and subtropical North and Central America, in spite of being clearly identified in their corresponding marine environments (Haug et al., 2001; Peterson and Haug, 2006; Leduc et al., 2007), in tropical and subtropical South America (Wang et al., 2004; Cruz et al., 2005; Cruz et al., 2007), and other continental tropical settings (Wang et al., 2001). Moreover, most paleoclimatic records from the area have identified the influence from the Caribbean and Atlantic oceans onto the continent, but there are few records from Pacific Mexico that might provide information from possible feedback mechanisms from the Eastern Tropical Pacific. Characterization of the full extent of rapid climate fluctuations in the tropics and subtropics is of vital importance to clarify whether tropical climate is forced by climate from higher latitudes and/or controlled by an ENSO-type variability. Such information can also provide important clues on the mechanisms and strength of atmospheric teleconnections between hemispheres and/or oceanic basins. This is particularly true for the Holocene, where rapid climate fluctuations are used as analogues of disturbances to the thermohaline circulation (THC) stemming from outbursts of freshwater to the North Atlantic under interglacial conditions (Wiersma and Renssen, 2006). Here, we present the a high-resolution record of climate variability in southwestern Mexico from a U–Th dated stalagmite spanning most of the Holocene, giving the most detailed record of Holocene climate yet available for this area. Southwestern Mexico provides an

0033-5894/$ – see front matter © 2010 University of Washington. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.yqres.2010.09.002

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interesting and very sensitive area for climate variability studies: it is located just above the northern limit of the Intertropical Convergence Zone (ITCZ), and at the southernmost area directly affected by the North American Monsoon (NAM; Schmitz and Mullen, 1996; Higgins et al., 1999). Displacement of synoptic-scale climatic elements (e.g., ITCZ) is thus expected to have an almost immediate effect upon this area; consequently, expressions of climate change and variability are expected to be clearly imprinted on the geological record. Geological and climatic setting Stalagmite CBD-2 was collected from Cueva del Diablo (N18°11′, W99°55′), a cave located ~ 150 km north of the port of Acapulco in the state of Guerrero, Mexico (Figs. 1A–B), 1030 masl. The cave is hosted in a carbonate horizon of Aptian–Albian age (~ 112 Ma) and lies within the Guerrero Terrain, a series of sedimentary and volcaniclastic strata from the Cretaceous and Jurassic (Monod et al., 2000) that form the Sierra Madre del Sur (SMS). Cueva del Diablo cave is largely inactive, with very limited dripping and no evidence of speleogenesis even during the peak of the wet season. The stalagmite was found “dead” at the time of collection (January 2005), approximately 15 m from the entrance, in a site which is currently not isolated from the external environment. Records from a weather station 25 km west of the cave for the 1975–2004 period show a mean annual precipitation of 930 mm, averaging ~ 200 mm/month during the wet season and less than 10 mm/month during the dry season (Fig. 1C). Mean annual temperature for the area is 27.9°C, with maximum of 31–32°C during April and May, and minimum of 25.5°C between December and January. Current vegetation cover has some remnants from the native lowland deciduous forest (http:// www.atlasdemexico.gob.mx). A detailed vegetation survey (Piperno et al., 2007, supporting information) demonstrates oak species are only found at or above 1300 masl. The rainy season over southwestern Mexico is relatively short but intense, centered between May and October with a small decrease during July and August often referred as mid-summer drought (MSD) or “canícula” (Magaña et al., 1999). Rainfall is directly associated with the ITCZ reaching its northernmost position during the boreal summer (Amador et al., 2006). As in most tropical areas, summer rainfall is mostly convective in nature, and it is linked to the monsoonal circulation over southern Mexico (Higgins et al., 1999). The wet season begins when moist air from the ITCZ reaches southern Mexico facilitated by the trade winds and, in particular, by the Intra Americas Low-Level Jet; an intense easterly flow over southern Mexico and Central America, originated along the southern margin of the North Atlantic Subtropical High that develops during May–June (Amador et al., 2006). Upon crossing the isthmus, the jet winds curve to become southeasterly and transport moisture to southwestern Mexico from the eastern Tropical Pacific (Schmitz and Mullen, 1996). The convergence of the eastern and southeasterly flows results in intense convection over southwest Mexico, thus moisture reaching the area during the wet season can be sourced from either the Caribbean or the eastern tropical Pacific. As a result of the pronounced Mexican orography, a “wind shadow” is created over the adjacent Pacific Ocean west of Mexico and Central America, where winds are relatively weak, allowing the formation of the Eastern Pacific Warm Pool (EPWP) between March and October (Lavín et al., 2006), which, in conjunction with the strengthening of the ITCZ, triggers the onset of the North American Monsoon (Wang and Fiedler, 2006). The EPWP is an important area for cyclogenesis, providing an additional source of summer moisture for southwestern Mexico (Amador et al., 2006). During the boreal winter, when the ITCZ has migrated to lower latitudes, southward-moving polar air converges with warm, moist air from the Gulf of Mexico bringing some precipitation to the eastern side of Mexico, but rarely reaches southwestern Mexico. Instead, the winds are channeled through gaps in the Tehuantepec isthmus and

Fig. 1. A) Location of study area; dashed line denotes the average summer position of the Intertropical Convergence Zone, ITCZ, including some localities discussed in the text: LCN: Lake Chichancanab, TLN: Lake Tulane, and PP: Pink Panther cave. B) Detail of the study area, including other localities discussed in the text; CBD: Cueva del Diablo, PTZ: Pátzcuaro lake, ACP: Acapulco port, MXC: Mexico City, part of the Basin of Mexico, IZT: Iztaccihuátl volcano and VER: Veracruz city. C) Monthly precipitation in “El Caracol” dam, 25 km south from Cueva del Diablo for the 1975–2004 period. Error bars correspond to ± 1 standard deviation (data provided by Comisión Federal de Electricidad). MSD: mid-summer draught.

Central America, promoting upwelling, overturning and surface mixing off the Pacific Coast of southern Mexico and de-stabilizing the EPWP (Lavin et al., 2006). In addition to this, the upper-level northwesterly winds, whose low-level equivalent bring humidity to California and northwest Mexico, might reach some parts of central and southern Mexico, and is responsible for a small fraction of winter precipitation in southwest Mexico. ENSO plays an important control upon interannual rainfall variability over southern Mexico. In broad terms, warm ENSO events (El Niño) result in negative anomalies in summer precipitation in

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Table 1 U and Th activity ratios for the 9 different sub-samples from CBD-2 stalagmite. 230Th/238U were corrected for contributions from detrital material using a 2-point isochron (see text for details). [234U/238U]0 correspond to initial 234U/238U activity ratio, ρ = error correlation between [234U/238U]0 and 230Th-age. All uncertainties are ±2σ. Data in italics correspond to samples with elevated [234U/238U] suggesting the deposition of calcite under low-humidity conditions. Sample

Distance from base (mm)

[230Th/238U]

[234U/238U]

[232Th/238U]

238

CBD-2-15 CBD-2-4 CBD-2-12 CBD-2-6 CBD-2-3 CBD-2-5 CBD-2-7 CBD-2-8 CBD-2-1

142.55 119.38 102.59 84.72 68.32 65.14 16.19 13.99 0

0.020 ± 0.002 0.054 ± 0.002 0.069 ± 0.001 0.074 ± 0.001 0.078 ± 0.002 0.078 ± 0.001 0.094 ± 0.001 0.099 ± 0.002 0.103 ± 0.003

1.050 ± 0.004 1.052 ± 0.006 1.043 ± 0.005 1.048 ± 0.004 1.068 ± 0.003 1.044 ± 0.002 1.046 ± 0.003 1.068 ± 0.011 1.045 ± 0.002

0.0101 ±0.0001 0.0120 ± 0.0002 0.0069 ± 0.0002 0.0040 ± 0.00004 0.0030 ± 0.0001 0.0021 ± 0.00004 0.0023 ± 0.00005 0.0029 ± 0.0001 0.0026 ± 0.0002

215 ± 1.2 266 ± 4 315 ± 0.7 195 ± 1.3 317 ± 1 272 ± 1 252 ± 0.6 220 ± 4 318 ± 2.1

U(ng/g)

232

Th (ng/g)

6.65 ± 0.01 9.75 ± 0.02 6.63 ± 0.17 2.40 ± 0.01 2.88 ± 0.09 1.72 ± 0.03 1.79 ± 0.04 1.98 ± 0.01 2.5 ± 0.2

Age (ka) uncorrected

Age (ka) corrected

[234U/238U]0

ρ

2.12 ± 0.02 5.79 ± 0.17 7.53 ± 0.12 7.99 ± 0.14 8.24 ± 0.21 8.44 ± 0.13 10.25 ± 0.14 10.66 ± 0.25 11.27 ± 0.31

1.24 ± 0.09 4.71 ± 0.23 6.90 ± 0.12 7.62 ± 0.22 7.97 ± 0.33 8.25 ± 0.21 10.03 ± 0.27 10.36 ± 0.28 11.02 ± 0.52

1.050 ± 0.006 1.052 ± 0.008 1.044 ± 0.007 1.050 ± 0.006 1.069 ± 0.006 1.046 ± 0.006 1.047 ± 0.006 1.070 ± 0.013 1.047 ± 0.006

− 0.082 − 0.147 − 0.341 − 0.209 − 0.135 − 0.215 − 0.214 − 0.391 − 0.111

(Webster et al., 2007). The amount of isotopic change due to adiabatic cooling and precipitation associated with rising air masses for central Mexico has been estimated at −2.13‰/km (Cortés and Durazo, 2001), similar to the −2.4‰/km recently observed in Central America (Lachniet and Patterson, 2009). Ground and spring water samples from the Basin of Mexico collected at 3800 masl show an average of δ18O = −10.7‰ (Cortés et al., 1989), reflecting the 18O depletion due to the high altitude and transport across the continent. Consequently moisture transported from the Gulf of Mexico and the Caribbean over the Mexican Volcanic Belt towards southwestern Mexico should be, at least, as depleted in 18O as that from the Basin of Mexico due to the prior orographic distillation. In agreement with the observations in Veracruz (Webster et al., 2007), Guatemala and Belize (Lachniet and Patterson, 2009), it is expected that the summer rainwater δ18O

southern Mexico, while the cool ENSO phase (La Niña) brings additional rainfall (Magaña et al., 2003). The decrease of summer rainfall in southern Mexico during El Niño years is associated a shift in the walker circulation, stronger Inter-American Low-Level Jet (Amador et al., 2006), enhanced subsidence (Magaña et al., 2003), and a southward shift of the ITCZ. Additionally, higher SST anomalies decrease the land–sea thermal contrast, affecting the strength of the monsoonal circulation (Higgins et al., 1999). A link between ENSO and a reduced number of tropical storms making landfall during El Niño years has also been observed (Jauregui, 1995). There is limited information regarding the seasonal variations of rainfall δ18O in Mexico. For the 1962–1985 period, rainfall in Veracruz on the Gulf of Mexico (Fig. 1) had a mean weighted δ18O = −4.0‰ (IAEA, 2006), which is dependent on rainfall amount during summer

12

I

Hiatus

10

6

230

Th-Age (Ka)

II 8

4

2

0 0

20

40

60

80

100

120

140

160

Distance from base (mm)

I

II

Growth

1 mm Figure 2. Top: age model for stalagmite CBD-2. Hiatuses are denoted by shaded areas. All error bars correspond to ±2σ. All ages have been corrected from contributions for the presence of detrital 23Th using a two-point isochron (see text for details). Bottom: crossed-polars thin-section microphotographs of stalagmite CBD-2 demonstrating the presence of growth hiatuses during the 10.3 and 8.2 ka events. Scale bar is identical for both images.

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for U–Th dating. Measurement of 238U–234U–232Th–230Th were done using a Finnigan Neptune MC-ICPMS at the Australian National University. Samples were dissolved in sub-boiled HNO3, and spiked with a 233U-229Th mixture. Uranium and Thorium were purified using the TRU-spec resin (Eichrom®, Darien, Il, USA) following the procedures described by McCulloch and Mortimer (2008). All age calculations were done using Isoplot 2.49t (Ludwig, 2001).

composition in southwestern Mexico is inversely proportional to the rainfall amount (Dansgaard, 1964). This is because southwestern Mexico is influenced by the same wind and climate patterns and moisture sources as Guatemala and Belize (Amador et al., 2006), where the amount effect has been observed to dominate the δ18O composition of rainfall and groundwater. Methods

Stable isotopes U-series dating The stalagmite was sampled at ~300-μm intervals over the growth axis using a microdrill to extract approximately 500 μg of calcite, resulting in δ18O time series with an average of ~ 50-yr resolution

Aliquots of approximately 200 mg of powdered calcite were extracted from different parts of the stalagmite over the growth axis

-7.0 -7.5

δ18OVPDB (‰)

-8.0

A -8.5 -9.0 -9.5 -10.0

B

-10.5

-11.5

δ13CVPDB (‰)

-11.0

-12.0

12

10

8

6

4

2

0

Age (ka) -10

δ13 CVPDB (‰)

BC δ13C= -8.89 + 0.311 x δ18O

-11

R2 = 0.201 p < 0.0001

-12

-13 -11

-10

-9

-8

-7

δ OVPDB (‰) 18

Figure 3. A) δ18OVPDB time series for stalagmite CBD-2. Light gray crosses are raw data and black continuous line is a 3-point moving average B). δ13CVPDB time series for CBD-2, light green crosses are raw data, dark green continuous line is a 3-point moving average. δ18O and δ13C values are referenced against the Vienna Pee Dee Belemnite. Shaded areas indicate hiatuses identified by petrographic inspection of the stalagmite. C) δ13CVPDB v.s. δ18OVPDB plot demonstrating lack of correlation between them.

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0.30

D

0.20

δ18OVPDB(‰) in gastropods

4.0

0.10

3.0

Drier

2.0

C -3.0

Drier

-4.0

(‰) in CBD-2

B

-5.0

-7.0

δ18OVPDB (‰) in PP-1

1.0

δ18OVPDB

% Ti in ODP 1002

0.40

Northward ITCZ

Drier

-8.0 -9.0

A

-10.0 12

11

10

9

8

7

6

5

4

3

2

1

Age (ka) Figure 4. Comparison of CBD-2 δ18O with other regional climatic records. Gray-shaded areas indicate periods when calcite deposition was interrupted. Triangles on top indicate age dated samples. Error bars correspond to ± 2σ. A) δ18OVPDB for CBD-2, gray: raw data, black: box-smoothed data using a 3-point moving average. B) δ18OVPDB from a stalagmite from Pink Panther cave, New Mexico (Asmerom et al., 2007); light blue: raw data; dark blue: smoothed data using a 10-point mobile average. C) Box-smothed record of gastropod δ18O from Lake Chichancanab in the Yucatan peninsula (Hodell et al., 1995). D) %Ti in marine core ODP 1002 from the Cariaco Basin (Haug et al., 2001).

between samples. δ13C and δ18O were measured in 160 samples following Kinga et al (2001), using a Finnigan MAT 253 mass spectrometer. Carbon and oxygen isotope compositions are reported relative to the Vienna Pee Dee Belemnite standard (VPDB), with typical uncertainties of ±0.2‰. Results Chronology Stalagmite CBD-2 has ~ 260 ng/g of 238U and less than 10 ng/g of Th; consequently, the average [232Th/238U] for CBD-2 is ~ 0.004 (Table 1). The measured activity ratios were corrected for contributions from detrital material by a two-point isochron and using 232Th as indicator of the allogenic material (Ludwig and Paces, 2002). Such approximation assumes that the detrital/allogenic fraction has the typical silicate composition, that is [232Th/238U] = 1.2 ± 0.6, similar to the Earth's crust (McDonough and Sun, 1995), and [230Th/238U] and [234U/238U] = 1.0 ± 0.10 (Ludwig and Paces, 2002). This correction has been successfully applied for dating stalagmites (e.g., Cruz et al., 2007) and “dirty” carbonates, such as those found in pedogenic environments (Ludwig and Paces, 2002; Sharp et al., 2003). A recent assessment of the performance of two- and three-point isochrons, and further comparisons with 10Be chronologies, has demonstrated the robustness of such correction (Fletcher et al., 2010). The uncertainties on the composition of the detrital material are quadratically propagated with those from the measurement and half-lives during the age calculations. The general validity of the twopoint isochron method used here is also somewhat justified by the 232

consistency of the resulting δ18O time series with other, independently dated, paleoclimate records. All corrected ages are in stratigraphic order (Table 1) and are used to constrain an age model for CBD-2 by linearly interpolating between each dated layer (Fig. 2A). The sample grew between 11 ka and 1.24 ka; however, petrographic inspection of the stalagmite revealed the presence of two hiatuses (Fig. 2B), which were dated at 10.36± 0.28 to 10.03± 0.27 ka and 8.25 ± 0.21 ka to 7.97 ± 0.33 ka, respectively. After the second hiatus, the stalagmite grew continuously until 1.24 ka, when calcite deposition ended probably because of an alteration in drip routing. The possible climatic significance of such hiatuses is discussed further below. Between 11 and 6.9 ka, the stalagmite grew at an approximately constant rate of ~30 μm/yr (with the exception of the aforementioned hiatuses); after that period, calcite deposition rate decreased to less than 10 μm/yr. δ18O and δ13C composition The δ18O composition of CBD-2 varies between −10.3 and −6.9‰ (Fig. 3A), with clear cycles of ~ 1–2‰ amplitude, and ~ 1.5 ka frequencies throughout the growth period. Local minimums are observed at 11.0, 9.6, 8.7, 7.0, 5.0, 3.6, and 2.2 ka. An abrupt decrease in δ18O (~2‰) is observed at 7.3 ka, after which the record shows a trend towards higher δ18O values. Figure 3B shows the δ13C record for CBD-2, which broadly follows the cycles observed in the δ18O record. This, along with the proximity of the collection site to the entrance of the cave suggests that kinetic fractionation of O and C isotopes might be affecting the record. The limited correlation between δ18O and δ13C (R2 = 0.201, n = 161,

J.P. Bernal et al. / Quaternary Research 75 (2011) 104–113

180 y

0.8 0.6 False Alarm 90%

0.4 0.0

2

0.2

B

p b 0.0001 Fig. 3C) indicates that kinetic fractionation due to evaporation is not the most significant process affecting the isotope composition of the calcite (Hendy, 1971). Further tests to rule out kinetic fractionation were not carried out due to the difficulty of extracting samples from exactly the same layer. We note that δ18O records showing higher R2 correlation values between δ13C and δ18O, indication of kinetic fractionation (e.g., Treble et al., 2007; e.g., Cosford et al., 2008), are not necessarily devoid from valuable information for paleoclimate reconstruction (Lachniet, 2009). If kinetic fractionation affected the CBD-2 record more than what the poor correlation between δ18O and δ13C indicates, it is more likely to have happened during periods of low humidity, because evaporation within the cave would be more probable. In such a case, a significant correlation between δ13C and δ18O would be expected during such periods. To test this, we calculated the correlation for the 45 highest δ18O values (N−8.41‰) and their corresponding δ13C, and no significant correlation was obtained (R2 = 0.0209), strongly suggesting that no kinetic fractionation affected the isotopic composition of the stalagmite carbonate. Consequently, the large variations in δ18O in the CBD-2 record must reflect changes in the δ18O composition of local rainwater and the equilibrium temperature of the calcite:water fractionation at the time of crystallization. If temperature was the main factor forcing the δ18O variations during the growth of CBD-2, the large oscillations in δ18O observed in the record would represent unrealistic temperature variations, in some cases as large as 10°C (Kim and O'Neil, 1997). Therefore, the δ18O record from CBD-2 is interpreted to primarily reflect changes in the oxygen isotope composition of local precipitation, specifically, the amount of rainfall (Dansgaard, 1964). There is limited information on the geochemical criteria modulating variations in δ13C (Fairchild et al., 2006); typically such variations are interpreted to reflect changes in soil dynamics and vegetation cover (type and amount), although contributions from

Coherency, C XY

1545-1474 y 835 y 376 y

1000 542 (493)*

433

Power (dB)

(420)

100

235 (229)

2

χ = 90%

10

1

A 0

2

4

8x10 -3

6

Frequency (1/ka) Figure 5. A) Power spectra for CBD-2 δ18O record. The main periodicities above red noise level are highlighted with their corresponding wavelength. Between brackets and in italics are the periodicities reported for the 14C record of cosmogenic radiation influx (Stuiver and Braziunas, 1989; Sonett and Finney, 1990), those marked with a star are reported only by Sonett and Finney (1990). Dashed line indicates the red noise level (χ2 = 90%) above which periodicities are significant. B) Cross-spectral analysis of CBD-2 and PP1 δ18OVPDB records. Common periodicities above the 90% false alarm level have been highlighted. Dashed line is false alarm level with 90% significance.

% Sand El Junco

109

25 20 ENSO Frequency

15 10 5

C 7

4

5

20

Cooler NA 2

6

8

15

3

10

1

δ18OVPDB (‰) in CBD-2

5

B

-7.0

% Hematite-stained glass in MC52-V29-191

0

Drier

-8.0 -9.0

A

-10.0

-8.5‰ -9.1‰

12

11

10

9

8

7

6

5

4

3

2

1

Age (ka) Figure 6. Comparison of the CBD-2 record with records from the North Atlantic and Eastern Tropical Pacific. Yellow-shaded area highlights the period when ENSO modulation is more influential in southwest Mexico (see text for details). A) Same as in Figure 4. Horizontal dashed lines illustrate temporal δ18O average. B) Percent of Hematite-Stained Grains (%HSG) in three marine cores from the North Atlantic (Bond et al., 2001). Numbers 1–8 designate the millennial-scale cycles identified by Bond et al. (1997) and Bond et al. (2001). Vertical dashed lines show correspondence between the %HSG and δ18O records. C) El Junco % sand record, a proxy for ENSO frequency (Conroy et al., 2008).

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atmospheric CO2, and changes in the rock dissolution conditions and calcite precipitation in the epikarst cannot be ruled out as additional sources of δ13C variations (Richards and Dorale, 2003; Fairchild et al., 2006). The δ13C record in CBD-2 (Fig. 3B) varies between −12.5 and −10.5‰, which is consistent with speleothem development underneath a C3 vegetation cover (Richards and Dorale, 2003). Such scenario implies that no significant change in vegetation type has occurred above the cave during the Holocene. The 13C record might also reflect changes on vegetation density, soil respiration, and prior calcite precipitation during the period of speleothem growth throughout the Holocene and thus, it is difficult to pinpoint to a single dominant process behind the observed variability without a detailed study on the process currently affecting the local epikarst. Nevertheless, we note that since kinetic fractionation is not the most significant process modulating the isotopic composition of the carbonate, it is very likely that any process behind the observed variability, had a strong climatic modulation, as suggested by the similar variability between δ13C and 18O. Discussion and climatic implications The δ18O record from CBD-2 suggests that the climatic evolution of southwestern Mexico has been complex and reflects the combination of many influences due to its geographic location underneath the convergence of synoptic-scale climatic patterns. This is demonstrated in Figure 4, where a comparison between the CBD-2 δ18O record with other contemporaneous regional and global paleoclimate records is presented. Comparison with regional records Figure 4A shows that the CBD-2 record has a ~2‰ drop between 7.3 and 7.1 ka, which suggests a rather abrupt change to moister conditions in southwest Mexico. Hodell et al. (1995) found that the δ18O record of terrestrial gastropods in Lake Chichancanab, in the Yucatan Peninsula shows a shift which is similar in magnitude and timing to our record (Fig. 4C). Additionally, a speleothem record from Venado cave in Panama also shows a ~ 0.7‰ shift in the calcite δ18O (Lachniet et al., 2004). Both have been interpreted to mark the initiation of humid climate for Mexico and Central America. The close agreement of the CBD-2 δ18O record with the Yucatan and Panama records strongly suggests that such a monsoon increase also affected southwestern Mexico, and it is also coincident with the ITCZ reaching its northernmost position during the Holocene “thermal maximum” (Fig. 4D), as recorded by the %Ti in marine core ODP 1002 from the Cariaco Basin (Haug et al., 2001). Figure 4 also compares the CBD-2 δ18O record with the δ18O record from a speleothem collected in the Pink Panther Cave in New Mexico (hereafter PP1 record, Fig. 4B), an area where modern summer precipitation is derived from the NAM, and winter rainfall is transported from the Pacific Ocean with the incursion of northwesterly winds. Periods of decreased rainfall in the PP1 record were observed to coincide with periods of increased solar radiation throughout the Holocene (Asmerom et al., 2007). Figure 4 shows that during the early and mid-Holocene (11 to 4.3 ka), the CBD-2 and PP-1 δ18O records are synchronous; i.e. periods of low and high humidity in both areas are coincident, which is confirmed by the correlation coefficient between both records (r = 0.3666). The match among records can be further improved by adjusting the chronology within the chronological error, this results in a correlation coefficient between both records r = 0.535 for the 11 to 4.3 ka period. During the late Holocene, however, the correlation between PP1 and CBD-2 is lost (r = 0.089), a result whose implications are discussed below. Asmerom et al. (2007), demonstrated that the PP1 record is coherent with 14C records of cosmogenic radiation influx during the Holocene, supporting the interpretation that the arrival of moisture to southwest

United States is solar-forced. Spectral analysis of the CBD-2 δ18O record using REDFIT (Schulz and Mudelsee, 2002) reveals the presence of three periodic signals above the noise level (χ2 = 90%) with frequencies of 0.00191, 0.0023 and 0.00425 a−1, equivalent to 522, 433 and 235 yr, respectively (Fig. 5A), which are correlated to the 492, 429 and 229 yr periodic signals present in records of solar radiation flux (Stuiver and Braziunas, 1989; Sonett and Finney, 1990). We note that some of these signals are not significant in the PP1 record (229 yr) and, due to the age resolution of our sampling we are unable to test for higher frequency periodicities. Cross-spectral analysis between CBD-2 and PP1 using SPECTRUM (Schulz and Stattegger, 1997), reveals that both records share periodic signals of 1545-1474, 835 and 376 yr (Fig. 5B), none of which was above the red noise level in their corresponding power spectra of either record. We note, however, that the signal corresponding to 1545-1474 yr is coincident with the observed periodicity in SST in the North Atlantic during the Holocene (Bond et al., 1997; Bond et al., 2001), and strongly suggests that the arrival of moisture to both areas is partially driven by the events in the North Atlantic. Moreover, the correlation between the CBD-2 and PP1 records during the early and mid-Holocene suggests a common climatic driver for both areas. One of such drivers might be the NAM, whose moisture sources from the Gulf of California and the Gulf of Mexico (Adams and Comrie, 1997), are substantially modulated by SST in the North Atlantic (Hu and Feng, 2008; Feng et al., 2010). The North Atlantic influence Probably one of the best records of North Atlantic SST variability throughout the Holocene is the percentage of Hematite-stained grains (%HSG, Fig. 6B) found in three marine cores from the North Atlantic (Bond et al., 1997; Bond et al., 2001). The %HSG is a proxy for changes in the amount and trajectories of glacial ice and/or sea ice circulating in the surface waters of the North Atlantic (Bond et al., 2001). In broad terms, %HSG is inversely proportional to SST in the North Atlantic; i.e. an increase in %HSG is indicative of cooler North Atlantic, whilst low % HSG indicates a warm North Atlantic surface water. Figure 6 shows that the variability in the CBD-2 δ18O record during the early and midHolocene is coeval with the North Atlantic marine cores, supporting the hypothesis of a link between North Atlantic SST and summer rainfall in southwestern Mexico. The correlation between both records for the period 11–4.3 ka yields an acceptable correlation coefficient (r = 0.3756), which is remarkable considering the different origins of the proxies and records. Similarly to the PP1 record, after 4.3 ka the correlation between the CBD-2 and %HSG is lost (r = 0.09). The synchroneity between %HSG from the North Atlantic with the δ18O record in CBD-2 indicates that low SST in the North Atlantic results in precipitation over southwestern Mexico with relatively high δ18O, while a warm North Atlantic leads to 18O-depleted rainfall. Because 90% of the precipitation over southwestern Mexico corresponds to summer rainfall, which is intimately associated with the position of the ITCZ (Amador et al., 2006), changes in the CDB-2 δ18O record are interpreted to reflect changes in the strength and latitudinal position of the ITCZ throughout the Holocene and, thus, the amount and source of moisture reaching southwestern Mexico. Additional features in the CBD-2 record providing evidence for a link between the North Atlantic and southwestern Mexico include the presence of two hiatuses centered at the height of major climatic events taking place in the Northern Atlantic. The first hiatus, centered at ~10.3 ka, is followed by a significant drop in δ18O, suggesting an abrupt return from dry to humid conditions. The hiatus is coeval with lower lake levels in Pátzcuaro, western Mexico (Bradbury, 2000) and central Mexico (Caballero et al., 2002), although such paleohydrological conditions were not recognized by the authors as being influenced by the North Atlantic. The 10.3 ka event is the second most intense climatic anomaly in the Holocene, with manifestations in Greenland (Stuiver et al., 1995) as well as north and central Europe

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(Matthews et al., 2000; Bjorck et al., 2001; Heiri et al., 2004). Moreover, high %HSG values in the North Atlantic marine cores (Bond et al., 2001) during the period encompassed by the hiatus, indicate lower SST for the North Atlantic (Fig. 6). The second hiatus, at 8.2 ka, coincides with the largest climatic anomaly in the Holocene, produced from the catastrophic input of freshwater from proglacial lakes into the North Atlantic (Wiersma and Renssen, 2006). This resulted in an abrupt slowing of the ocean thermohaline circulation (THC) and had climatic expressions at a global scale, including central Mexico, where glacial advances in the Iztaccihuátl volcano, in the Mexican Volcanic Belt are apparently coincident with the 8.2 ka event (Lozano-García and Vázquez-Selem, 2005). The presence of hiatuses in stalagmites can be the result of many hydrological and physical processes within and around the cave and associated epikarst (Hill and Forti, 1997). However, additional evidence supporting the hypothesis that hiatuses in the CBD-2 stalagmite were the result from abrupt changes in the hydrological conditions above the cave, stems from the U-series data: dated samples near the hiatuses (CBD-2-8 and CBD-2-3 in Table 1) have higher [234U/238U]0 than the rest of the samples, indicating that drip water was more enriched in 234U during the time around the cessation of calcite growth than in the rest of the Holocene. Such departures from secular equilibrium in drip water have been associated with low growth rates in stalagmites (Zhou et al., 2005), because dissolution of the easily accessible 234U located in α-recoil tracks is preferred to “old” U located within the crystal structure of the minerals during times of low water availability in the epikarst (Osmond and Ivanovich, 1992). In contrast [234U/238U] approaching secular equilibrium is associated with speleothems with high growth rate, because higher water availability in the epikarst promotes full mineral dissolution, hence, higher proportion of “old” U near or at secular equilibrium. While it is difficult to draw climatic conclusions on a regional scale from one stalagmite, the above results indicate that the hydrological conditions above the speleothem were significantly disrupted during the time of the two largest climatic anomalies of the Holocene. This suggests that the cooling of the North Atlantic SST and slowing of the THC during the 10.3 and 8.2 ka events might have inhibited the arrival of moisture to southwestern Mexico, most likely stemming from the southward migration of the ITCZ. This interpretation is in agreement with other high-resolution records from Central America indicating that the southwards migration of the ITCZ during cold North Atlantic stadials (such as Heinrich events) reduce precipitation in Central and South America (Hodell et al., 2008), but in contrast to records from Florida that suggest warm wet Heinrich events (Grimm et al., 2006). Such discrepancy might stem from the localized origin of precipitation in Florida or heterogeneities in SST on the Western Atlantic Ocean in the Florida coastline as suggested by Grimm et al (2006). The suggestion that Holocene climate variability in southwestern Mexico has been strongly influenced by the events in the North Atlantic is consistent with marine records from the Eastern Pacific (Schmidt et al., 2004; Benway et al., 2006; Leduc et al., 2007). These indicate an increase in sea-surface salinity during periods when the ITCZ has retreated southward, for example during the last glacial period, during Heinrich and Dansgaard–Oescher events, indicating that the input of fresh cold water to the North Atlantic results in poor contributions from Caribbean moisture to the Pacific Ocean, and hence to southwestern Mexico. The role of ENSO As mentioned above, from ~ 4.3 onwards the correlation between the CBD-2 vs. PP1 and %HSG records is lost. Moreover, the cyclicity in the CBD-2 record is somewhat dampened, and δ18O values are generally higher than those during the preceding two and a half millennia by about 0.6‰ (Fig. 6A). Such shift implies a decrease in

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rainfall amount which is not consistent with the slight warming of the North Atlantic suggested by the decrease in % HSG (Fig. 6B). The rainfall reduction, however, is contemporaneous with the southward migration of the ITCZ since the mid-Holocene observed at the Cariaco basin (Figure 4D; Haug et al., 2001). The loss of the correlation with PP1 and the North Atlantic records suggests the presence of additional climate features, such as ENSO, impacting upon the arrival of moisture to southwestern Mexico. In fact, numerous paleoclimate records indicate that El Niño activity increased ca. 5 ka (Moy et al., 2002; Conroy et al., 2008), and the higher δ18O values at Diablo may reflect the increasing El Niño frequency. Figure 6 compares the CBD-2 δ18O record with the % of sand (Fig. 6C) in a sediment core from El Junco Lake, Galapagos Islands, which has been interpreted to reflect Holocene ENSO activity since sand abundance reflects the number and intensity of rainfall events associated with El Niño (Conroy et al., 2008). From 4.3 ka onwards, an increase on the % sand is the result from the increasing ENSO activity in the eastern tropical Pacific, which is coincident with the δ18O shift observed in our record (Figs. 6A–C). In southern Mexico, the warm phase of ENSO is associated with negative rainfall anomalies (Magaña et al., 2003). Consequently, the observed shift in δ18O suggesting a reduction of rainfall in southwestern Mexico, is interpreted to be the result from greater ENSO activity, and lesser rainfall reaching southwestern Mexico during the late Holocene. Consequently, the onset of ENSO ~ 4.3 ka ago lead to the partial decoupling of southwest Mexico from the North Atlantic, and established the climatic conditions prevailing until today. Conclusions The CBD-2 δ18O record represents the most detailed record yet of climatic conditions in southwest Mexico throughout the Holocene, and reveals a complex climatic history. During the early and midHolocene, the amount of rainfall reaching Cueva del Diablo was driven by the events in the North Atlantic. The record indicates that incursion of glacial waters into and cooling of the North Atlantic, lead to a more southerly ITCZ, decreasing the delivery of Caribbean moisture to the area (Vellinga and Wood, 2002; Dahl et al., 2005; Broccoli et al., 2006), hampering the advection of humidity towards southwestern Mexico, and the development of the NAM. In contrast, a warm North Atlantic resulted in more Caribbean precipitation reaching Cueva del Diablo, as the ITCZ reaches a more northerly position, enhancing rainfall in southwest Mexico. Significant disruptions on the hydrology above the Cueva del Diablo are coincident with the main climatic anomalies of the Holocene, i.e. the 10.3 and 8.2 ka events. Such disruptions were probably the result from a reduction on the arrival of moisture from the Caribbean, documented in other localities in Mexico (LozanoGarcía and Vázquez-Selem, 2005) and as a consequence of the cooling of the North Atlantic and slowing of the THC. The record indicates that southwest Mexico has gone through two major climatic transitions during the Holocene (7.2 and 4.3 ka), both implying significant reorganization in the ocean/atmosphere interactions, which are contemporaneous with the north–south migration of the ITCZ. The transition at 4.3 ka is more subtle but perhaps more relevant, as it marks the point from which that the arrival of moisture to southwest Mexico was decoupled from the events in the North Atlantic and increasingly modulated by the Eastern Tropical Pacific via ENSO, leading to the establishment of the modern climatic conditions. Our results are important in the context of climate change, as they indicate that an increase of glacial/fresh water flowing into the North Atlantic have had a significant effect upon the amount of rainfall reaching southwestern Mexico, providing important evidence for climate models that indicate a southward migration of the ITCZ after such events (Vellinga and Wood, 2002; Dahl et al., 2005; Zhang and Delworth, 2005; Broccoli et al., 2006).

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