One of the world's most famous asteroid craters, the Chicxulub crater, has been the subject of research for about twenty years. The asteroid impact that formed it probably put an end to the dinosaurs and helped mammals to flourish. Together with an Anglo-American team, an ETH Zurich researcher has studied the most recent deposits that filled the crater. The results provide accurate dating of the limestones and a valuable basis for archaeologists to research the Maya.
The discovery of the Chicxulub asteroid crater was detective work: in 1980, based on iridium anomalies in clay sediments – which could only be formed extraterrestrially – the American physicist Walter Alvarez postulated a devastating asteroid impact (see box) at the transition from the Cretaceous to the Paleogene around 65 million years ago. Another ten years passed before the associated crater was discovered on the Yucatan peninsula. Research work since then has focused mainly on the structure of the crater, which has been buried in a layer of sediment up to two kilometres thick since its formation and which can only be studied using boreholes or geophysical methods. Little is known about the sediments close to the surface. Most of the geological maps also originate from the time before the Chicxulub crater was discovered, and do not completely reflect the geology.
Crater ring as a geological boundary
Together with American and English researchers, Adrian Gilli, Senior Lecturer at the Geological Institute of ETH Zurich, has now filled in a few of the gaps in the knowledge about the near-surface rock deposits. Gilli says, “The crater ring of the Chicxulub crater is scarcely recognisable in the terrain.” This is despite the fact that it is distinctly different, geo-morphologically speaking, from the crater interior: the ring, which is about five kilometres wide and has a radius of approximately 90 kilometres around the port of Chicxulub, is criss-crossed by fractures that also occur frequently outside the crater ring. The limestone along these faults has been riddled with holes and eroded by rain and groundwater. A process known as karstification has to date created about 3000 circular collapses forming small basins filled with groundwater. The Maya called these basins “d’zonot”, or “cenotes”, regarding them as a direct connection to the underworld and using such places as sacrificial sites.
Crater sediments distinctly younger
Previously, the rocks outside the crater ring were suspected to be older than those inside, but now researchers have for the first time been able to determine their precise age using a method based on the isotope ratios of strontium 87 to strontium 86 in the limestone, thus confirming their suspicions. The rock samples inside the ring gave an age of between 2.3 and 6 million years. On the other hand, the rocks outside the ring showed more variable strontium isotope ratios and are from 10 to 33 million years old. Gilli suspects that the crater basin had been covered by seawater for a prolonged time, which allowed the more recent sediments to be deposited there.
The results help to understand the geology better and to re-draw and refine the outdated maps. Archaeology research can also benefit from the geologists’ work: for example, the life of the Maya, whose important settlements Mayapán und Chichén Itzá are located in these two geologically different regions, can be better researched on the basis of the strontium data.
Gilli explains, “We really wanted to carry out a purely geo-archaeological study in which, initially, the crater played no part at all.” The aim was to refine an earlier strontium isotopes study of the rocks around the former Maya sites by adding 72 new samples to enable the identification of small-scale variations in the isotope ratio. This is because the transitions between regions with a different ratio of strontium 87 to strontium 86 can be particularly important: Gilli says, enthusiastically, “We can obtain a very large amount of information about the lifestyle of the people from settlements at such boundaries, such as the Maya city of Mayapán.”
Geology and archaeology complement each other
For instance, the fine-mesh geological measurement network in conjunction with the biological strontium isotope values makes migration movements visible. It could yield knowledge about where the Maya obtained their building materials or where they cultivated their maize. This is because the weathering of rocks carries strontium into the soil and from there into plants. When eaten by humans or animals in food, strontium instead of calcium is incorporated into teeth when they are formed in childhood. If detailed strontium isotopes maps are available, this enables the region in which a person grew up to be determined. Gilli says, “It gets particularly exciting if the origin of the rulers of the various Maya towns can be determined. This in turn allows conclusions to be drawn about the social structures of the Maya.” The scientist is convinced that the geological analyses of the strontium isotope ratios form an important basis for the work of the archaeologists, who are working ever more frequently with the biological strontium isotopes from bones or teeth.
Chicxulub asteroid crater
The asteroid with a diameter of about ten kilometres that created the Chicxulub crater most probably caused the worldwide extinction of species at the boundary between the Cretaceous and the Palaeogene. The event caused the extinction of about 75 percent of all species. Its effect was due mainly to the fact that it impacted into rocks rich in carbonate and anhydrite. This hurled massive amounts of carbon dioxide (CO2) and sulphur dioxide (SO2) into the atmosphere. It is estimated that between 200 and 3000 cubic kilometres of sediments vaporised, releasing 35 to 700 million tons of sulphur and 10 billion tons of CO2 into the atmosphere. The ozone layer would probably have been destroyed within a very short time, and there must have been complete darkness over the Earth for months due to the dust in the atmosphere. This had serious short- and long-term consequences for the climate and biosphere. The crater that remains has a diameter of 180 kilometres and extends from the Yucatán peninsula to the Gulf of Mexico.
The first evidence of the Maya in Central America can be traced back to 3000 B.C. Over the course of history, their cultural centres moved from the uplands to the lowlands and finally to the north of the Yucatán. Chichén Itzá in the north-west of the Yucatán peninsula flourished during the classical period, whereas Mayapán was the last Maya capital in the north of the peninsula. When the Spanish arrived in the late 15th century, the centres of post-classical Maya culture were in the extreme north of the Yucatán, while the central lowlands remained only thinly populated. Today, around 6.1 million Maya still live on the Yucatán peninsula and in Belize, Guatemala and Honduras.
Gilli A. et al.: Geological and archaeological implications of strontium isotope analysis of exposed bedrock in the Chicxulub crater basin, northwestern Yucatán, Mexico. Geology (2009), 37, 723-726, doi:10.1130/G30098A.1
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