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Orbital Pacing and Ocean Circulation-Induced Collapses of the Mesoamerican Monsoon Over the Past 22,000 Y

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Orbital Pacing and Ocean Circulation-Induced Collapses of the Mesoamerican Monsoon Over the Past 22,000 Y Orbital pacing and ocean circulation-induced collapses of the Mesoamerican monsoon over the past 22,000 y Matthew S. Lachnieta,1, Yemane Asmeromb, Juan Pablo Bernalc, Victor J. Polyakb, and Lorenzo Vazquez-Selemd aDepartment of Geoscience, University of Nevada, Las Vegas, NV 89154; bDepartment of Earth and Planetary Science, University of New Mexico, Albuquerque, NM 87131; cCentro de Geociencias, Universidad Nacional Autónoma de México, Querétaro 76230, Mexico; and dInstituto de Geografía, Universidad Nacional Autónoma de México, Coyoacán, Mexico City 04510, Mexico Edited by James P. Kennett, University of California, Santa Barbara, CA, and approved April 19, 2013 (received for review January 2, 2013) The dominant controls on global paleomonsoon strength include circulation was disrupted by freshwater forcing into the North summer insolation driven by precession cycles, ocean circulation Atlantic, whereas the SASM intensity strengthened during sta- through its influence on atmospheric circulation, and sea-surface dials, suggesting a hemispheric antiphasing in tropical rainfall temperatures. However, few records from the summer North linked to migrations in the latitude of the ITCZ. However, the American Monsoon system are available to test for a synchronous relative orbital and ocean circulation influence on the Meso- response with other global monsoons to shared forcings. In parti- american monsoon is unconstrained, limiting our ability to test cular, the monsoon response to widespread atmospheric reorgani- hypotheses of monsoon forcing (3), the peopling of the Americas zations associated with disruptions of the Atlantic Meridional (9), the Holocene domestication of maize and squash in Mexico Overturning Circulation (AMOC) during the deglacial period (10), climate’sinfluence on past civilizations (11), and to forecast remains unconstrained. Here, we present a high-resolution and future monsoon variations in an anthropogenically altered radiometrically dated monsoon rainfall reconstruction over the climate (12). past 22,000 y from speleothems of tropical southwestern Mexico. The data document an active Last Glacial Maximum (18–24 cal ka B.P.) Results 18 monsoon with similar δ O values to the modern, and that the Our 22-ka monsoon reconstruction (Fig. 1) is a well-dated and monsoon collapsed during periods of weakened AMOC during high-resolution paleorainfall record of the Mesoamerican mon- Heinrich stadial 1 (ca. 17 ka) and the Younger Dryas (12.9–11.5 ka). soon, and is based on 2,230 stable oxygen isotope (δ18O) values The Holocene was marked by a trend to a weaker monsoon that and 50 U-series dates (see SI Text for methods; Figs. S1 and S2; was paced by orbital insolation. We conclude that the Mesoamer- Table S1). The Juxtlahuaca Cave stalagmites presented here ican monsoon responded in concert with other global monsoon were recovered ∼750 m from the entrance (927 m), in the Sierra regions, and that monsoon strength was driven by variations in Madre del Sur of southwestern Mexico. Juxtlahuaca Cave pres- the strength and latitudinal position of the Intertropical Conver- ents an ideal cave climate with constant temperature (24.2 °C) and gence Zone, which was forced by AMOC variations in the North constant 100% relative humidity (11, 13, 14). To facilitate com- Atlantic Ocean. The surprising observation of an active Last Glacial parison of monsoon strength for different time intervals, the spe- Maximum monsoon is attributed to an active but shallow AMOC leothem δ18O records were corrected for changes in the δ18Oof and proximity to the Intertropical Convergence Zone. The emergence the ocean, which is the moisture source for rainfall in southwestern of agriculture in southwestern Mexico was likely only possible after Mexico and other global monsoon regions (SI Text). The effect of monsoon strengthening in the Early Holocene at ca. 11 ka. this δ18O sea water correction is to make δ18Ovalues∼1% more negative during the Last Glacial Maximum (LGM), when ice stalagmite | paleoclimatology | plant domestication | cave | volume was maximal, and with a negligible correction for today. Sierra Madre del Sur Further, to test for links between Mesoamerican rainfall and hemispheric forcings, we compared our reconstruction (Fig. 1) he North American Monsoon (NAM) dominates the summer with the 231Pa/230Th ratio in subtropical North Atlantic sediments Tmoisture budget for most of Mexico and the southwestern (15), a proxy for AMOC intensity (SI Text), Greenland Ice Sheet United States, but despite its proximity to the Atlantic Meridi- δ18O, a proxy for temperature and atmospheric circulation (6), ice onal Overturning Circulation (AMOC) center of action in the volume (16), and sea-surface temperature in the tropical North North Atlantic Ocean, it remains one of Earth’s least understood Atlantic Ocean (17) to show the sequence of deglacial climate monsoons because of a lack of high-resolution summer-sensitive events that typify North Atlantic paleoclimatic change (Fig. 1). proxy records. In particular, the paleoclimatic history of the Over the past 22 ky Mexico speleothem δ18O values are tropical sector (south of 20°N) of the NAM, which we informally marked by intermediate values of −8to−9% Vienna PeeDee EARTH, ATMOSPHERIC, term the Mesoamerican monsoon because of its distinction from Belemnite (VPDB) during both the last 2 ky and the LGM. In AND PLANETARY SCIENCES the more northerly “core” sector (1) and its links to the broader contrast, high δ18O values (−6.3%) characterize Heinrich stadial cultural area (SI Text) remains poorly known due to conflicting 1 (HS1) and the Younger Dryas (YD) intervals (Fig. 1). Low interpretations of lacustrine proxy data (2). However, common forcings on the global paleomonsoon (3) suggest that the Meso- american monsoon should be sensitive to both precessional-scale Author contributions: M.S.L. and J.P.B. designed research; M.S.L., Y.A., J.P.B., and V.J.P. orbital forcing and ocean circulation variations via changes in performed research; Y.A. and V.J.P. contributed new reagents/analytic tools; M.S.L., Y.A., the latitudinal position of the Intertropical Convergence Zone J.P.B., V.J.P., and L.V.-S. analyzed data; and M.S.L., Y.A., J.P.B., V.J.P., and L.V.-S. wrote the paper. (ITCZ) (3). Reconstructions of the East Asian Monsoon (EAM) The authors declare no conflict of interest. and South American Summer Monsoon (SASM) from radio- This article is a PNAS Direct Submission. metrically dated speleothems (cave calcites) (3–5) showed that Data deposition: The speleothem δ18O data reported in this paper have been deposited monsoon strength was paced by changes in orbital insolation at the National Climatic Data Center, National Oceanic Atmospheric Administration at ∼21,000-y orbital precession time scales, and exhibited mil- Paleoclimatology Web site, www.ncdc.noaa.gov/paleo/paleo.html. lennial-scale climate variability linked by atmospheric tele- 1To whom correspondence should be addressed. E-mail: [email protected]. – connections to the North Atlantic region (6 8). Weakest EAM This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. intervals coincided with cold Heinrich events when ocean 1073/pnas.1222804110/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1222804110 PNAS | June 4, 2013 | vol. 110 | no. 23 | 9255–9260 Downloaded by guest on October 3, 2021 weakest during the deglacial cold episodes HS1 and the YD (highest δ18O values). We refer to these three monsoon states as “active,”“strong,” and “weak,” respectively. Discussion Forcing of the LGM Mesoamerican Monsoon. The observation of similar modern and LGM Mesoamerican monsoon strength is a surprising result because of the lower specific humidity asso- ciated with cooler temperatures, such as is indicated by 3–4°C sea-surface temperature (SST) depression relative to modern in the Cariaco Basin (17) and lower land-surface temperatures (LST) on the Yucatan Peninsula (20). Thus, the low LGM δ18O values may be explained by a similar degree of air-mass rainout as today. The stalagmite δ18O values also contain a temperature imprint: a decrease in LST of ∼3–4 °C is responsible for making the stalagmite δ18O values higher by ∼0.6–0.8% due to the water–carbonate precipitation temperature (13), leading to ap- parently slightly weaker monsoons during cold periods. How- ever, this influence on the stalagmite δ18O profile (+0.6 to + 0.8%) is small relative to the observed magnitude of δ18O changes related to monsoon strength (ca.3–4%), and it would have been offset by the decrease in δ18O values of precipitation during cold periods due to the atmospheric temperature effect. Because our observations contradict a simple thermodynamic forcing of monsoon strength via atmospheric water vapor content, an alternative mechanism for maintaining an active LGM monsoon must be considered. A key similarity between today and the LGM is that local summer insolation at 20°N reached precessional-scale minima, and we suggest that the similar monsoon strength was forced by nearly equal summer insolation despite the high ice volume, lower temperature, and lower CO2 (21) boundary conditions at the LGM (Fig. 1). Today, monsoon strength is positively corre- lated both with the magnitude of the ocean-to-land temperature gradient (ΔT ) and proximity to the ITCZ, and we hy- Fig. 1. Comparison of Mesoamerican monsoon strength to proxies for SST-LST paleoclimatic change in the North Atlantic Ocean. (A) North Greenland Ice pothesize that similar processes affected the Mesoamerican Sheet Project (NGRIP) δ18O values on the Greenland Ice Core Chronology monsoon during the LGM in response to similar orbital forcing. 2005 (GICC05) (6). (B) SST from Mg/Ca ratios in the Cariaco Basin, Venezuela However, insolation forcing is unlikely to be the only driver on (17). (C) 231Pa/230Th ratio, a proxy for AMOC (15). (D) LR04 marine benthic monsoon strength during deglaciation, because weak monsoon 18 foraminifera δ O stack (16), a proxy for global ice volume. (E)CO2 record intervals during the YD and HS1 also occurred at times of higher from Antarctica (21).
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