RIASSUNTO
Summary
Early in 2005 the US research vessel R/V Maurice Ewing, operated by Lamont-Doherty Earth Observatory at Columbia University, conducted an academic seismic survey of the Chicxulub impact crater in the shallow water offshore Yucatan, Mexico. This survey was the focus of a campaign by environmental activists, which enhanced local concerns in Mexico. A combination of US legal requirements and Mexican government restrictions resulted in highly restricted time windows in which the work could be conducted. Although the survey was ultimately successful, the costs escalated significantly, and the data return was less than originally planned. Here we analyse the nature of these costs, consider the extent to which these additional precautions helped to protect the environment, and examine strategies for minimizing unnecessary expenditure, preventing misinformation and ensuring environmental protection for future surveys.
Introduction Scientific target The Chicxulub crater is the best preserved of the three large (>150 km diameter) meteorite impact craters known on the surface of the Earth, and its timing at 65 My ago implicates it in the mass extinction event marking the end of the Mesozoic, including the extinction of the dinosaurs. Investigations of this crater are of interest to those studying the catastrophic environmental events that followed, as well as those with an interest in impact cratering in general, and in the response of the outer layers of the Earth to such a massive and high strain-rate deformation. The planned survey consisted of a grid of seismic reflection profiles, mainly focussed in a box 40 by 90 km in water depths of 15-30 m in the area offshore the port of Progreso; there were also some longer regional profiles. Shots from a tuned 20 airgun 6970 cu ins array were recorded on a 6 km streamer, and also on 25 ocean bottom seismometers and about 85 seismic stations on land, to give a 3D tomographic velocity model of the internal velocity structure of the upper part of the crater, the melt sheet and the layer of suevite, a welded impact breccia. Detailed knowledge of these features will allow calibration of impact models, constraints to be placed on the amount and nature of the ejecta, and a better understanding of whether the impact was oblique.
Environment
The Chicxulub crater is centred on the coastline of the Yucatan, and straddles the flat-lying onshore region and the gently sloping limestone continental shelf offshore. The water is exceptionally shallow to quite large distances offshore: for example the 10 m bathymetric contour is at about 15 km from shore, and the 30 m at about 60 km. The shoreline consists of a linear beach of carbonate sand, with significant westward longshore drift. The very shallow water is unattractive to the larger marine mammals, but there are small populations of Atlantic and spotted dolphins known to live in the area. There are several turtle breeding and feeding grounds on the Yucatan peninsular.
Campaign by environmentalists
The environmental campaign was led by Ben White of the Institute for Animal Welfare.