Victor J. Polyak

Development and Disintegration of Maya Political Systems in Response to Climate Change

Maya and Climate Climate has affected the vitality of many different societies in the past, as shown by numerous records across the globe and throughout human history. One of the most obvious and spectacular examples of this is from the Classic Maya civilization, whose advanced culture left highly detailed records of all aspects of their existence between 300 and 1000 C.E. Kennett et al. (p. 788; see the cover) present a detailed climate record derived from a stalagmite collected from a cave in Belize, in the midst of the Classic Maya settlement. The fine resolution and precise dating of the record allows changes in precipitation to be related to the politics, war, and population fluctuations of the Mayans. A record of rainfall from a stalagmite in southern Belize provides a context for better understanding Maya civilization. The role of climate change in the development and demise of Classic Maya civilization (300 to 1000 C.E.) remains controversial because of the absence of well-dated climate and archaeological sequences. We present a precisely dated subannual climate record for the past 2000 years from Yok Balum Cave, Belize. From comparison of this record with historical events compiled from well-dated stone monuments, we propose that anomalously high rainfall favored unprecedented population expansion and the proliferation of political centers between 440 and 660 C.E. This was followed by a drying trend between 660 and 1000 C.E. that triggered the balkanization of polities, increased warfare, and the asynchronous disintegration of polities, followed by population collapse in the context of an extended drought between 1020 and 1100 C.E.

Solar forcing of Holocene climate: New insights from a speleothem record, southwestern United States

Holocene climate change has likely had a profound infl uence on ecosystems and culture. A link between solar forcing and Holocene climate, such as the Asian monsoon, has been shown for some regions, although no mechanism for this relationship has been suggested. Here we present the fi rst high-resolution complete Holocene climate record for the North American monsoon region of the southwestern United States (southwest) in order to address the nature and causes of Holocene climate change. We show that periods of increased solar radiation correlate with decreased rainfall, the opposite to that observed in the Asian monsoon, and suggest that a solar link to Holocene climate is through changes in the Walker circulation and the Pacifi c Decadal Oscillation and El Nino‐Southern Oscillation systems of the tropical Pacifi c Ocean. Given the link between increased warming and aridity in the southwest, additional warming due to greenhouse forcing could potentially lead to persistent hyperarid conditions, similar to those seen in our record during periods of high solar activity.

Tropical response to the 8200 yr B.P. cold event? Speleothem isotopes indicate a weakened early Holocene monsoon in Costa Rica

A δ 1 8 O monsoon rainfall proxy record from a U-Th-dated Costa Rican stalagmite (8840-4920 yr B.P.) documents an early Holocene dry period correlative with the high-latitude 8200 yr B.P. cold event. High δ 1 8 O values between ca. 8300 and 8000 yr B.P. demonstrate reduced rainfall and a weaker monsoon in Central America. A relatively wetter and more stable monsoon was established ca. 7600 yr B.P. The early Holocene dry event suggests a tropical-extratropical teleconnection to the 8200 yr B.P. cold event and a possible association of isthmian rainfall anomalies with high-latitude climate changes. The likely source of such a tropical anomaly is a decrease in Atlantic thermohaline circulation and atmospheric perturbations associated with drainage of proglacial lakes and freshwater discharge into the North Atlantic. A weaker monsoon at 8200 yr B.P. may be linked to wetland contraction and a decrease in methane observed in Greenland ice cores.

Prolonged wet period in the southwestern United States through the Younger Dryas

The Younger Dryas was one of the more dramatic climatic transitions ever recorded. How these types of climatic shifts are expressed in continental interiors is of primary scientific interest and of vital societal concern. Here we present a speleothem-based absolutely dated record (using uranium-series data) of climate change for the southwestern United States from growth chronology of multiple speleothems. The stalagmite growth represents the onset of wetter climate (12,500 yr B.P.) soon after the start of the Younger Dryas; the wetter climate persisted a millennium beyond the termination of the Younger Dryas. This wet cycle is likely related to a more southern positioning of the polar jet stream in response to cooler Northern Hemisphere climate. The end of the wet period coincides with the peak of the Holocene summer insolation maximum ca. 10,500 yr B.P. The Allerod (prior to the Younger Dryas), which corresponds to Clovis occupation in the southwestern United States, was drier in comparison and seems in line with a climatic contribution to megafauna extinction.

Age and Evolution of the Grand Canyon Revealed by U-Pb Dating of Water Table-Type Speleothems

The age and evolution of the Grand Canyon have been subjects of great interest and debate since its discovery. We found that cave mammillaries (water table indicator speleothems) from nine sites in the Grand Canyon showed uranium-lead dating evidence for an old western Grand Canyon on the assumption that groundwater table decline rates are equivalent to incision rates. Samples in the western Grand Canyon yielded apparent water table decline rates of 55 to 123 meters per million years over the past 17 million years, in contrast to eastern Grand Canyon samples that yielded much faster rates (166 to 411 meters per million years). Chronology and inferred incision data indicate that the Grand Canyon evolved via headward erosion from west to east, together with late-stage (∼3.7 million years ago) accelerated incision in the eastern block.

Late Pleistocene Human Skeleton and mtDNA Link Paleoamericans and Modern Native Americans

American Beauty Modern Native American ancestry traces back to an East Asian migration across Beringia. However, some Native American skeletons from the late Pleistocene show phenotypic characteristics more similar to other, more geographically distant, human populations. Chatters et al. (p. 750) describe a skeleton with a Paleoamerican phenotype from the eastern Yucatan, dating to approximately 12 to 13 thousand years ago, with a relatively common extant Native American mitochondrial DNA haplotype. The Paleoamerican phenotype may thus have evolved independently among Native American populations. The differences between Paleoamericans and Native Americans likely resulted from local evolution. Because of differences in craniofacial morphology and dentition between the earliest American skeletons and modern Native Americans, separate origins have been postulated for them, despite genetic evidence to the contrary. We describe a near-complete human skeleton with an intact cranium and preserved DNA found with extinct fauna in a submerged cave on Mexico’s Yucatan Peninsula. This skeleton dates to between 13,000 and 12,000 calendar years ago and has Paleoamerican craniofacial characteristics and a Beringian-derived mitochondrial DNA (mtDNA) haplogroup (D1). Thus, the differences between Paleoamericans and Native Americans probably resulted from in situ evolution rather than separate ancestry.

Shrinking of the Colorado Plateau via lithospheric mantle erosion: Evidence from Nd and Sr isotopes and geochronology of Neogene basalts

Geochronologic data from the southern margins of the Colorado Plateau (western United States) show an inboard radial migration of Neogene basaltic magmatism. Nd and Sr isotopic data show that as basaltic volcanism migrates inboard it also becomes increasingly more asthenospheric. Strongly asthenospheric alkali basalt (e Nd > 4) appeared on the western plateau margin ca. 5 Ma, on the southeastern margin at 7 Ma, and is lacking from the plateau’s other margins. Tomographic data suggest that low-velocity mantle underlies almost all recent (younger than 1 Ma) basaltic volcanism in a ring around much of the Colorado Plateau at a depth of 80 km. The combined isotopic and tomographic data indicate that the low-velocity mantle is asthenosphere along the western and southeastern margins of the plateau, but modifi ed lithosphere around the remaining margins. Temporal and spatial patterns suggest a process by which upwelling asthenosphere is progressively infi ltrating and replacing lithospheric mantle, especially where Proterozoic boundaries exist. This model explains (1) the dramatic velocity contrast seen well inboard of the physiographic edge of the plateau, (2) the inboard sweep of Neogene magmatism, and (3) isotopic evidence that much (but not all) of the low-velocity mantle is asthenospheric. These data support models that ongoing uplift of the edges of the Colorado Plateau is driven by mantle processes.

A 2400 yr Mesoamerican rainfall reconstruction links climate and cultural change

Droughts are a recurring feature of Mexican climate, but few high-resolution data are available to test for climate-change forcing of Mesoamerican civilizations. We present a quantitative 2400 yr rainfall reconstruction for the Basin of Mexico, from a precisely dated and highly resolved speleothem, that documents highly variable rainfall over the past 2400 yr. Dry conditions peaked during a 150-yr-long late Classic (ca. 600–900 CE) (Common Era) mega drought that culminated at 770 CE which followed centuries of climatic drying that spanned the fall of the city of Teotihuacan ca. 550 CE. The wettest conditions in the 1450s CE were associated with fl ooding in the Basin of Mexico. Our data suggest that rainfall variability was likely forced by the El Nino–Southern Oscillation, and impacts on spring-fed irrigation agriculture may have been a stressor on Mesoamerican civilizations.

Climatic backdrop to the terminal Pleistocene extinction of North American mammals

North American terminal Pleistocene mammal extinctions are the subject of a long-running scientifi c debate. Although the role of climate has fi gured centrally, we lack clear knowledge of the timing and nature of terminal Pleistocene climate variability. Herein we document lengthy terminal Pleistocene drought in the southwestern United States (USA) using δ 13 C and δ 234 U effective moisture proxy data in speleothem calcite (stalagmite FS2) from Fort Stanton Cave, New Mexico, supplemented with age data from pool basin shelfstone speleothems from the Big Room in Carlsbad Cavern. This terminal Pleistocene drought, defi ned by a sharp rise in both δ 13 C and δ 234 U values, began just before 14.5 k.y. ago and lasted at least until 12.9 k.y. ago, when it was briefl y and only mildly interrupted by the Younger Dryas. The timing and length of this drought (~1500 yr) match the Northern Hemisphere Bolling-Allerod oscillation preserved in Greenland ice cores and exhibited in the δ 18 O record of stalagmite FS2. Rapid transition from cool moist Late Glacial to warm dry Holocenelike climatic conditions was likely unfavorable to many species of Pleistocene mammals in the southwestern USA. A climate-induced extinction implies that this last glacial cycle and its termination were more extreme than previous glacial cycles and/or glacial terminations.

Orbital control of western North America atmospheric circulation and climate over two glacial cycles

The now arid Great Basin of western North America hosted expansive late Quaternary pluvial lakes, yet the climate forcings that sustained large ice age hydrologic variations remain controversial. Here we present a 175,000 year oxygen isotope record from precisely-dated speleothems that documents a previously unrecognized and highly sensitive link between Great Basin climate and orbital forcing. Our data match the phasing and amplitudes of 65°N summer insolation, including the classic saw-tooth pattern of global ice volume and on-time terminations. Together with the observation of cold conditions during the marine isotope substage 5d glacial inception, our data document a strong precessional-scale Milankovitch forcing of southwestern paleoclimate. Because the expansion of pluvial lakes was associated with cold glacial conditions, the reappearance of large lakes in the Great Basin is unlikely until ca. 55,000 years into the future as climate remains in a mild non-glacial state over the next half eccentricity cycle.