Orbital pacing and ocean circulation-induced collapses of the Mesoamerican monsoon over the past 22,000 y

Abstract

The dominant controls on global paleomonsoon strength include summer insolation driven by precession cycles, ocean circulation through its influence on atmospheric circulation, and sea-surface temperatures. However, few records from the summer North American Monsoon system are available to test for a synchronous response with other global monsoons to shared forcings. In particular, the monsoon response to widespread atmospheric reorganizations associated with disruptions of the Atlantic Meridional Overturning Circulation (AMOC) during the deglacial period remains unconstrained. Here, we present a high-resolution and radiometrically dated monsoon rainfall reconstruction over the past 22,000 y from speleothems of tropical southwestern Mexico. The data document an active Last Glacial Maximum (18-24 cal ka B.P.) monsoon with similar δ 18O values to the modern, and that the monsoon collapsed during periods of weakened AMOC during Heinrich stadial 1 (ca. 17 ka) and the Younger Dryas (12.9-11.5 ka). The Holocene was marked by a trend to a weaker monsoon that was paced by orbital insolation. We conclude that the Mesoamerican monsoon responded in concert with other global monsoon regions, and that monsoon strength was driven by variations in the strength and latitudinal position of the Intertropical Convergence Zone, which was forced by AMOC variations in the North Atlantic Ocean. The surprising observation of an active Last Glacial Maximum monsoon is attributed to an active but shallow AMOC and proximity to the Intertropical Convergence Zone. The emergence of agriculture in southwestern Mexico was likely only possible after monsoon strengthening in the Early Holocene at ca. 11 ka.stalagmite | paleoclimatology | plant domestication | cave |

Glacial reduction of the North American Monsoon via surface cooling and atmospheric ventilation

“…Single‐forcing simulations clearly show the reduction of NAM strength in response to LGM boundary conditions, in contrast to some records that suggest a strong LGM NAM [ Lachniet et al , ]. However, the monsoon is not fully suppressed in our simulations, suggesting that the same fundamental climatological regimes existed in the Southwest during the last glacial period.…”

“…Some researchers posit that SSTs in the GoC were too cold to support a monsoonal circulation during the LGM [ Barron et al , ; Metcalfe et al , ]. Monsoon changes have also been linked to the position of the Intertropical Convergence Zone (ITCZ) [ Lachniet et al , ; Jiang et al , ]. Alternatively, the LGM NAM may have been entirely suppressed by the westerlies [ Thompson and Anderson , ].…”

The global monsoon across timescales: coherent variability of regional monsoons

“…The idea of a mega-monsoon first appeared in 1973, when Pamela Robinson drew the hypothetical position of the ITCZ for the late Permian and early Triassic with a 40-degree range of seasonal migration and a single extremely intense monsoon system (Lamb, 1977). Although the cross-equatorial mega-continent provided tectonic settings for the development of the mega-monsoon, the circulation itself could penetrate into the continental interior only with the presence of a pronounced highland (Parrish, 1993).…”

The deglaciation of the mountains of Mexico and Central America

“…Overall, glacial advances of central Mexico seem to be controlled by regionally cold conditions somewhat modulated by moisture (see Lachniet et al, 2013). The LLGM coincides with cold and wet conditions, the latter produced by a strong monsoon.…”

“…The LLGM coincides with cold and wet conditions, the latter produced by a strong monsoon. Considering the regional evidence of drying around 17 ka, it is possible that glacier recession and ELA ascent of ~100 m between ~18 ka (Hueyatlaco-1 moraines) and ~17 ka (Hueyatlaco-2) on Iztaccíhuatl (but not on lower mountains located near the oceans), resulted from lower precipitation, inasmuch as temperatures remained low in connection with Heinrich stadial 1 (Lachniet et al, 2013). From this perspective, it seems plausible that the low ELA represented by Hueyatlaco-2 moraines and the late deglaciation of the mountains of central Mexico resulted from low temperatures associated with Heinrich stadial 1 (17-15 ka), low enough to compensate for dry conditions in the region.…”