Multidecadal rainfall variability in Mesoamerica over the Little Ice Age and Medieval time from a Mexican speleothem

Matthew Lachniet, University of Nevada Las VegasMatthew Lachniet, University of Nevada Las VegasJuan Pablo Bernal, Universidad Autónoma de MéxicoJuan Pablo Bernal, Universidad Autónoma de MéxicoYemane Asmerom, University of New MexicoYemane Asmerom, University of New MexicoVictor Polyak, University of New MexicoVictor Polyak, University of New MexicoWilliam P. Patterson, University of SaskatchewanWilliam P. Patterson, University of Saskatchewan

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

MCA climate(low-latitude)

ENSO-sensitive records from the tropics are ambiguous about El Niño or La Niña forcing during Medieval time

Cool/La Niña

Warm/El Niño

Dry/El Niño

Low upwelling /El Niño

Cool/La Niña

Less runoff /La NiñaMore runoff /El Niño

Dry/La Niña

ENSO State

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.

Study Area

Southwestern Mexico is On the northern

fringe of the ITCZ

On the southern margin of the North American Monsoon

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

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.

Study area

Juxtlahuaca Cave, Gro. SW Mex Water sample locations

Juxtlahuaca Cave

Veracruz

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

Colotlipa climate data

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Seasonal amount effect

Veracruz data show a clear ‘amount’ effect

Mexico surface waters

dD = 8.4 * d 18O + 13.4R2 = 0.95

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Surface water samples plot on the GMWL

Juxtlahuaca Cave and stalagmite JX-1

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

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

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930

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0 500 1000 1500 2000 2500Age (yr BP)

Heig

ht (m

m)

Likely hiatus @ 1080 mm

Black zone @ 639-672 mm

Tip

Stable Isotope Time SeriesDry Intervals in yr BP (yr AD)

970 -1060(1040-950)

884(1126)

1540 -1670(470-340)1195

(815)

1740 -1870(270-140)

1740-1690 (270 to 320)

2040-1840(-50 to 150)

1530-1245 (480 to 765)

1075 and 1150(935 and 860)

855-640 (1155-1370)

364-96 (1646 to 1914)

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

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

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

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

New stalagmite

2010 field work Stalagmite JX-6 All aragonite Super-precise

dating No hiatuses

over past 2000 years

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th (m

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Possible hiatus @ 985 mm

LIA

May

a Pe

riod

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

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

Agradecimentos a

NSF P2C2 ProgramNSF EAR/IF Program to fund LVIS Lab

Docents of Juxtlahuaca Cave UNLV

UNAM UNM

Optional Slides

Comparison to Temperature records

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

Mexico

Sta. Barbara Basin

Gulf of California

Panama

Palmyra

Comparison to hydroclimate records

Most similar to the Galapagos Lake record (El Junco) and Peru margin lithics

Mexico

Galapagos

Peru Margin

Peru Margin

Pallcacocha, Ecuador

Is solar variability driving Mexico rainfall?

Perhaps

Similar to Total Solar Irradiance proxy of Bard

Dissimilar to residual 14C

But JX-1 d18O time series lacks solar spectra…

Ambiguous evidence for a solar control on Mexico rainfall

High-Res Late Holocene records

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

Carbon/Oxygen Scatterplots

JX-1 d18O/d13C by mineralogy

Covariance in both aragonite and calcite layers

Common environmental forcing, fast degassing, or another process?

JX-1 Cross plot by mineralogy

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Calcite Layer 1

Calcite Layer 2

Calcite Layer 3

Calcite Layer 4

Aragonite Layer 1

Aragonite Layer 2

Aragonite Layer 3

Aragonite Layer 4

Aragonite Layer 5

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.