The crater is situated on the interface of the Amazon forest and the vast open savannas of eastern Bolivia. A preliminary evaluation of the Landsat images identified patches of low forest north of the current savanna – forest edge, leading Killeen and Tucker to hypothesize that the forest –forest ecotone has migrated South in recent history. The 1998 expedition provided 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 13 C when compared to the forest plants. The relative amount of 13 C is quantified as the ratio of 13 C to 12 C and expressed as a per mil deviation from a geological standard (dC 13 ). Savanna grass tissue has a dC 13 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 dC 13 value approaching that of the live savanna grass tissue.

Sampling locations are indicated by the white arrows

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 dC 13 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 . 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 numerous shrubs and herbaceous forbs.

Preliminary information based on the 1998 expedition - Tim and Compton are experts in understanding species diversity and consequently the source of the Landsat signatures.

The websites below will give a good picture of the basis for the power of the stable C isotope research
http://www.esd.ornl.gov/people/amthor/carbon.html
http://hjs.geol.uib.no/marinegeology/chapter7-4-2.shtml