Matthew Lachniet, University of Nevada Las Vegas Juan Pablo Bernal, Universidad Autónoma de México

1. Multidecadal rainfall variability in Mesoamerica over the Little Ice Age and Medieval time from a Mexican speleothem Matthew Lachniet, University of Nevada Las Vegas Juan Pablo Bernal, Universidad Autónoma de México Yemane Asmerom, University of New Mexico Victor Polyak, University of New Mexico William P. Patterson, University of Saskatchewan

2. ENSO and the MCA • The El Niño/Southern Oscillation is the dominant source of interannual climate variability in the tropics and elsewhere • The Medieval Climatic Anomaly ca. 1200-700 yr BP (800-1300 A.D). was associated with temperature and hydroclimate anomalies in diverse locations • Paleoclimate data suggests a more La Niña-like climate state during the MCA • Modern studies link Western U.S. drought with La Niña • Mesoamerica also experienced severe droughts • Implicated in demise of Maya in late 9th century

3. ENSO State MCA climate(low-latitude) Cool/La Niña Warm/El Niño • ENSO-sensitive records from the tropics are ambiguous about El Niño or La Niña forcing during Medieval time Dry/El Niño Low upwelling /El Niño Cool/La Niña Less runoff /La Niña More runoff /El Niño Dry/La Niña

4. Objective and Hypothesis • Objective: • to constrain past rainfall variability within the eastern Pacific Intertropical Convergence Zone (ITCZ) and the southern portion of the North American Monsoon over the past two millennia • Hypothesis • We hypothesize that hydroclimate in southwestern Mexico will show sustained Medieval wet (e.g. La Niña-like) conditions.

5. Study Area • Southwestern Mexico is • On the northern fringe of the ITCZ • On the southern margin of the North American Monsoon

6. Western Hemisphere Warm Pool • Our study area is located next to the Western Hemisphere Warm Pool • SSTs > 28.5oC promote deep convection and high specific humidity • Is the moisture source for the southern part of the NAM

7. ENSO-sensitive climate • Correlation of Niño-3 SST with summer rainfall: • El Niño = dry conditions • La Niña = wet conditions • Wet and dry anomalies are -14.6% and +10.6 respectively (Higgins et al., 1999) Correlation of August through October precipitation anomalies with the Niño-3 SST index. Negative values (light grey shading enclosed with dashed contours) indicate dry conditions during warm El Niño events, throughout most of southern Mexico and Central America. From Wang et al. 2006.

8. Study area • Juxtlahuaca Cave, Gro. SW Mex • Water sample locations Veracruz Juxtlahuaca Cave

9. Modern Climate • Semi-arid seasonal climate: • Mean annual precipitation = 1163 mm/yr • PET = 2081 mm • Boreal summer wet season between June and October • MAT = 24.6oC

10. Seasonal amount effect • Surface water samples plot on the GMWL • Veracruz data show a clear ‘amount’ effect

11. Juxtlahuaca Cave and stalagmite JX-1

12. Methods • Stable Isotopes at the Las Vegas Isotope Science Lab (LVIS) at UNLV • 1 mm sampling interval • Corresponds to 2-3 yr resolution over MCA • U-series at the Radiogenic Isotope Lab at the University of New Mexico

13. U-series dating • Improved chronology relative to abstract with new 2010 dates • Very precise age dating: • High U concentrations • 4 to 8 ppm • High initial δ234U (>740) • Very low sensitivity to the initial thorium correction • Age errors <30 yr two σ. • Stalagmite JX-1 • 1115 mm tall • Upper 430 mm consists primarily of aragonite and is presented here • Hiatus between 620 and 364 yr BP, and no growth last 96 yr Tip

14. 1740-1690 (270 to 320) 1075 and 1150 (935 and 860) 1530-1245 (480 to 765) 364-96 (1646 to 1914) 2040-1840 (-50 to 150) 855-640 (1155-1370) 1740 -1870 (270-140) 1540 -1670 (470-340) 884 (1126) 1195 (815) 970 -1060 (1040-950) Stable Isotope Time Series Dry Intervals in yr BP (yr AD)

15. Wetter or drier Medieval Climate?(More La Niña- or El Niño–like?) • 1) Neither the LIA nor the MCA are particularly unusual over the past two millennia • 2) MCA relative to entire record: • Drier 44% of the time • Wetter 56% of the time • Generally unremarkable MCA • 3) MCA relative to most recent four centuries: • Drier 15% of the time • Wetter 85% of the time • More La Niña-like climate

16. Climate and Culture • How did climate vary over Maya cultural periods? • Pre-Classic Abandonment was wet • Maya hiatus was dry • Terminal Classic Drought was variable: 3 dry periods and two wet periods • Driest interval was ~1155 yr BP (855 AD), followed by a ‘double dip’ dry period at 1080 yr BP (930 AD) • We attribute to the Classic Maya Collapse in late 9th century

17. Time Spectra • 7-8 yr ENSO band • Decadal (9-14) • Multidecadal (16-50) spectral density • Solar variability? • has 22 yr at 99% CI • 11 yr at 80% CI • lacks 88 and 210 yr • Possible forcings include • A) Pacific Decadal Oscillation • B) Atlantic Multidecadal Oscillation • C) Low-frequency ENSO

18. Wet conditions = calcite • 3 thick layers of calcite are present in the stalagmite • Begin in wet periods, and transition into dry periods • Suggestive of secondary recrystallization without loss of d18O signal during wet events • Complicates the climate story, thus need for replication

19. New stalagmite • 2010 field work • Stalagmite JX-6 • All aragonite • Super-precise dating • No hiatuses over past 2000 years

20. Was Medieval Climate locked into a La Niña-like climate state? • Our data suggest • 1) large hydroclimate variability • 2) generally wetter climate relative to LIA • 3) but with persistent droughts • Most severe drought at 855 A.D. contemporaneous with Classic Maya Collapse and another during Maya hiatus • The MCA if interpreted in terms of ENSO: • More La Niña-like climate than last 4 centuries • BUT not unusual relative to past 2 millennia

21. Conclusions • MCA hydroclimate in Mexico: • 1) Highly variable rainfall • 2) A more La Niña-like climate • 3) Similar in mean and variability to preceding millennium • Evidence suggests that MCA climate in the eastern Pacific warm pool region was not locked into a La Niña-like climate state, but rather associated with high variability and a cooler temperature state

22. Agradecimentos a NSF P2C2 Program NSF EAR/IF Program to fund LVIS Lab Docents of Juxtlahuaca Cave UNLV UNAM UNM

23. Optional Slides

24. Comparison to Temperature records Mexico • Wet then dry Mexico while SBB and Palmyra are cool • Supports La Niña-like climate • Conflicts with mean state of climate from the Panama and GOC records • CMC = Classic Maya Collapse • PCA = pre-Classic abandonment • Hiatus = Maya hiatus Sta. Barbara Basin Gulf of California Panama Palmyra

25. Comparison to hydroclimate records Mexico • Most similar to the Galapagos Lake record (El Junco) and Peru margin lithics Galapagos Peru Margin Peru Margin Pallcacocha, Ecuador

26. Is solar variability driving Mexico rainfall? • Perhaps • Similar to Total Solar Irradiance proxy of Bard • Dissimilar to residual D14C • But JX-1 d18O time series lacks solar spectra… • Ambiguous evidence for a solar control on Mexico rainfall

27. High-Res Late Holocene records

28. Hendy tests show no d18O enrichment along growth layers Seven ‘Hendy’ tests for oxygen isotopes from 10 to 34 mm away from axis in individual growth bands support equilibrium calcite

29. Carbon/Oxygen Scatterplots • JX-1 d18O/d13C by mineralogy • Covariance in both aragonite and calcite layers • Common environmental forcing, fast degassing, or another process?

30. Abstract • We present high-resolution data for a precisely-dated aragonite/calcite stalagmite collected from southwestern Mexico that documents pronounced variability in rainfall on multi-decadal to centennial time scales. The oxygen isotope and mineralogical variations in the stalagmite document alternating wet and dry periods over the interval between 1800 yr BP and present, with a hiatus between 840 and 550 yr BP. The climate of the region is currently semi-arid and rainfall variability is strongly linked to El Niño (dry) and La Niña (wet). The stalagmite record indicates relatively dry conditions over the modern and Little Ice Age (LIA) periods when aragonite with relatively high δ18O values was precipitated. During and prior to the Medieval Climatic Anomaly (MCA), δ18O values alternated rapidly between conditions much wetter and drier than today, indicating greater rainfall variability relative to the last several centuries. Three calcite layers likely formed during wet intervals when cave drip waters were initially under-saturated in aragonite, at approximately 1590-1546, 1164-1204, and 846 to 1050 yr BP, the youngest being coincident with a wet spell during the MCA. The interval between 1200 and 1590 yr BP was consistently dry, similar to the LIA and modern periods. The δ18O data display spectral power in multi-decadal bands, which suggest that Mexican rainfall on the northern fringe of the eastern Pacific ITCZ responds to multi-decadal scale forcing, possibly via the Pacific Decadal Oscillation and/or the Atlantic Multidecadal Oscillation. As a whole, wet conditions (La Niña-like) were more common during the MCA than the Little Ice Age, but that the driest interval during the entire record occurred 890-910 A.D. within the MCA. This dry period we correlate with the Terminal Classic Drought in Mesoamerica. Our data suggests that Medieval climate was not locked into a sustained La Niña state but rather fluctuated rapidly and drastically about a stable mean state.