News Article | February 15, 2017
Using the largest set of radiocarbon dates ever obtained from a single Maya site, archaeologists have developed a high-precision chronology that sheds new light on patterns leading up to the two major collapses of the ancient civilization. Archaeologists have long puzzled over what caused what is known as the Classic Maya collapse in the ninth century A.D., when many of the ancient civilization's cities were abandoned. More recent investigations have revealed that the Maya also experienced an earlier collapse in the second century A.D. -- now called the Preclassic collapse -- that is even more poorly understood. University of Arizona archaeologist Takeshi Inomata and his colleagues suggest in a new paper, to be published in the Proceedings of the National Academy of Sciences, that both collapses followed similar trajectories, with multiple waves of social instability, warfare and political crises leading to the rapid fall of many city centers. The findings are based on a highly refined chronology developed by Inomata and his colleagues using an unprecedented 154 radiocarbon dates from the archaeological site of Ceibal in Guatemala, where the team has worked for over a decade. While more general chronologies might suggest that the Maya collapses occurred gradually, this new, more precise chronology indicates more complex patterns of political crises and recoveries leading up to each collapse. "What we found out is that those two cases of collapse (Classic and Preclassic) follow similar patterns," said Inomata, the paper's lead author and a professor in the School of Anthropology in the UA College of Social and Behavioral Sciences. "It's not just a simple collapse, but there are waves of collapse. First, there are smaller waves, tied to warfare and some political instability, then comes the major collapse, in which many centers got abandoned. Then there was some recovery in some places, then another collapse." Using radiocarbon dating and data from ceramics and highly controlled archaeological excavations, the researchers were able to establish the refined chronology of when population sizes and building construction increased and decreased at Ceibal. While the findings may not solve the mystery of why exactly the Maya collapses occurred, they are an important step toward better understanding how they unfolded. "It's really, really interesting that these collapses both look very similar, at very different time periods," said Melissa Burham, one of three UA anthropology graduate students who co-authored the paper. "We now have a good understanding of what the process looked like, that potentially can serve as a template for other people to try to see if they have a similar pattern at their (archaeological) sites in the same area." Inomata and his UA colleagues -- anthropology professor Daniela Triadan and students Burham, Jessica MacLellan and Juan Manuel Palomo -- worked with collaborators at Ibaraki University, Naruto University of Education and the Graduate University for Advanced Studies in Japan, and with Guatemalan archaeologists and students. Radiocarbon dating was done at Paleo Laboratory Company in Japan and at the Accelerator Mass Spectrometry Laboratory in the UA Department of Physics. "Radiocarbon dating has been used for a long time, but now we're getting to an interesting period because it's getting more and more precise," said Inomata, who also is an Agnese Nelms Haury Chair in Environment and Social Justice at the UA. "We're getting to the point where we can get to the interesting social patterns because the chronology is refined enough, and the dating is precise enough." Inomata's research was funded in part by the National Science Foundation, National Endowment for the Humanities, National Geographic Foundation, the Alphawood Foundation and the UA's Agnes Nelms Haury Program in Environment and Social Justice.
Duxbury J.,University of Vermont |
Bierman P.R.,University of Vermont |
Portenga E.W.,University of Vermont |
Portenga E.W.,University of Glasgow |
And 4 more authors.
American Journal of Science | Year: 2015
We use cosmogenic 10Be analysis of fluvial sediments and bedrock to estimate erosion rates (104-105 year timescale) and to infer the distribution of post-orogenic geomorphic processes in the Blue Ridge Province in and around Shenandoah National Park, Virginia. Our sampling plan was designed to investigate relationships between erosion rate and lithology, mean basin slope, basin area, and sediment grain size. Fifty-nine samples were collected from a variety of basin sizes (<1-3305 km2) and average basin slopes (6-24°) in each of four different lithologies that crop out in the park: granite, metabasalt, quartzite, and siiciclastic rocks. The samples include bedrock (n = 5), fluvial sediment from single-lithology basins (n = 43), and fluvial sediment from multilithology basins (n = 11): two multilithology samples are from rivers with tributary streams draining the eastern and western slopes of the park, respectively (Rappahannock and Shenandoah Rivers), and two samples are temporal replicates. In one sample of each lithology, we measured 10Be in four different grain sizes from fine sand to gravel. Inferred erosion rates for the medium sand fraction of all fluvial samples from all lithologies range from 3.0 to 21 m/My. The area-weighted mean erosion rate for single-lithology basins in the Park is 12.2 m/My. Single-lithology erosion rate ranges for fluvial samples are: granite, 7.0 to 20 m/My; metabasalt, 3.8 to 21 m/My; quartzite, 3.8 to 15 m/My; and silicicla.stic rocks, 5.2 to 15 m/My. Multilitholo2gy basins erode at rates between 3.0-16 m/My. The Shenandoah River basin (3305 km2) is eroding at 6.6 m/My. Bedrock erosion rates range from 1.8 to 11 m/My across all lithologies, with a rnean of 6.5 ± 4.3 m/My. Grain-size specific 10Be analysis of four samples showed no consistent trend of concentration with grain size. Cosmogenic analysis of bedrock and sediment from the Shenandoah National Park area allows us to speculate about why some parts of the Appalachian Mountains erode more slowly and sorne more rapidly. Overall, it appears that steep drainage basins erode more rapidly than gently sloped basins. Climate and lithology may also influence basin-scale rates of erosion as suggested by the difference in average erosion rates east and west of the divide and the difference between the erosion rates of quartzite- and granite-dominated basins. Data are conflicting in regards to the evolution of relief over time. Analyses made of exposed bedrock along ridgelines suggest that such rock is eroding either more slowly than adjacent drainage basins (Susquehanna River, Shenandoah National Park region) or at similar rates (Great Smoky Mountains) providing a mechanism for growing relief at the scale of individual ridgelines. However, considering relief on a landscape or physiographic province scale, by comparing erosion rates of the highlands versus the lowlands, suggests that relief of the range as a whole is either steady or very slowly decreasing over multi- millennial timescales. The presence of significant erosion rate/slope relationships negates a broad Hackian view of the landscape because there is not uniform erosion across this landscape. The aspect-erosion rate and slope-erosion rate relationships present in the Shenandoah area suggest that the landscape is not fully adjusted to rock strength.