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Scientists visit meteorite impact crater in the wilds of Bolivia

By Timothy J. Killeen¹
1. Missouri Botanical Garden

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In perhaps the remotest and wildest part of the Bolivian lowlands, in an area hundreds of kilometers from the closest town, NASA scientists have identified what they believe to be the youngest complex meteorite impact crater on earth. Based on what is known about the geology of the region, they believe the meteorite slammed into the Earth sometime between 5,000 and 20,000 years ago, making it the youngest "large" impact crater on Earth. The crater is approximately 8 km across and was produced by the collision of a meteorite about 200 m in diameter. The impact released energy of approximately 500 to 1000 megatons of TNT; in comparison, a hydrogen bomb is equivalent to only one megaton, while the atomic bomb that exploded over Hiroshima, Japan released only 20 kilotons. The crater was originally identified in the mid-1980s with satellite imagery, but a previous attempt to visit the site in 1987 was unsuccessful due to the remoteness of the locality.

The goal of the second expedition was to verify the existence of the crater (scientists are 95% confident that it is) and to gather data from the site on the nature of the meteorite that caused it. The expedition was organized by Tim Killeen, formerly with the Missouri Botanical Garden, and Compton Tucker, a specialist in remote sensing technology with NASA's Goddard Space Flight Center. Both Tucker and Killeen have been working in Bolivia for the past several years mapping vegetation types, as part of an effort to understand the processes that lead to the development of different habitats. Accompanying them was Peter Wazelewski, a specialist in meteorite impact craters, who was interested in visiting the only (potential) crater known to exist in alluvial sediments. All other craters are found in hard rock formations, since craters formed by meteroite impacts in unconsolidated sediments are erased quickly by erosion and sedimentation.

The expedition reached the crater impact site after traveling by jet airliner, small airplane, motor boat, dugout canoe, and finally by cutting a 15 km long trail through the forest. Field data gathered during the expedition supports the hypothesis that the circular feature is a meteorite impact crater. The rings visible on the satellite image correspond to slight ridges not more than 2 m in elevation, but sufficiently higher to support upland forest vegetation, while the interior of the crater is either inundated savanna or flooded forest. However, only sophisticated methods using seismology and magnetometry can definitively prove the existence of a meteorite or meteorite fragments buried beneath hundreds of meters of alluvial deposits.

NASA scientists were accompanied by biologists from the Missouri Botanical Garden, The Field Museum of Natural History, and the Noel Kempff Mercado Natural History Museum in Santa Cruz, Bolivia. The area is especially interesting to them because it is in a region unspoiled by modern man and has never been visited by biologists. The only community within a 200 km radius of the crater is a small village of about 40 people of the Araona indigenous group, which is situated just 30 km upstream on the Río Manupari (see Figure 1 and Figure 2). The biological specimens collected have been incorporated into the growing collections from northern Bolivia (see Table 1 for a list of bird species collected or observed).

The expedition also allowed Killeen, Tucker and a Bolivian student, Teddy Siles, to study the past ecological history of the region. The crater is situated on the interface of the Amazon forest and the vast open savannas of eastern Bolivia. A preliminary evaluation of the images identified patches of low forest north of the current savanna – forest edge, leading them to hypothesize that the forest –forest ecotone has migrated South in recent history. The expedition provided them with a unique opportunity to test this hypothesis using soil samples collected along a transect across the savanna forest ecotone.

Soil organic matter (SOM) provides a record of the vegetation that occupied a site in the past. The grass species in the savanna at Noel Kempff Park use the C4 metabolic pathway and therefore contains an elevated amount of the stable carbon isotope 13C when compared to the forest plants. The relative amount of 13C is quantified as the ratio of 13C to 12C and expressed as a per mil deviation from a geological standard (dC13). Savanna grass tissue has a dC13 value of approximately -12, while forest plants range from -25 to -30. Over time, the SOM at a site will mirror the stable carbon isotope value of the vegetation growing on it. For example, SOM in an area, which has been covered with tropical savanna grassland will have a dC13 value approaching that of the live savanna grass tissue. In the event of a change in vegetation, the SOM will serve as an indicator of past vegetation at the site. If SOM with a dC13 value of between -20 and -25 were found under savanna vegetation, it would indicate that the area was previously occupied by forest vegetation.

Preliminary evidence from the soils samples taken indicates that the Amazon forest has expanded southward in recent geological time, an indication of past climate change (Figure 3). Even the most northern soil pits (M-1, M-2 and M-3) taken some 20 km North of the current boundary showed some evidence of C4 grasses in the lowest (and oldest) part of the soil profile, while the proxy sample taken in open grassland south of the airstrip near the Araona Village (M-7) showed no evidence of recent forest cover. The transitional sites all show evidence of past movement in the forest – savanna ecotone. The two sites that are currently forested (M-4 and M-5) had dC13 values in the upper horizon typical of forest vegetation, while deeper soil profiles were more like those found in the savanna soils (M-7). Similarly, the savanna patch found in the center of the crater had top soil characteristics intermediate to open grassland and closed forest, reflecting the its grass sward and the presence of a numerous shrubs and herbaceous forbs (Table 2).

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