A mortality kinetics approach to characterizing the fractionated exposure-mortality response relationship of radon progeny.

The utility of mortality kinetics analysis in evaluating mortality data from fractionated exposure studies was demonstrated using radon-progeny induced extra mortality as an example. Gompertz (log-hazard) functions were used to characterize the mortality of male SPF Wistar rats exposed to radon progeny at 100 WL and 1000 WL for total exposures ranging from 20 to 10,240 WLM. There was an upward parallel displacement of the Gompertz functions following the period of radon exposure. The shape of the Gompertz functions for the exposed animals was consistent with a Gompertz model of toxicity resulting from short-term exposure, resulting in non-repaired injury that summates with natural (aging) injury. The parallel upward displacements (epsilon ss) of the Gompertz functions showed an unexpected non-monotonic pattern for rats exposed at 1000 WL. The parallel upward displacements showed a sharp upward increase from 320 to 640 WLM, fell at 1280 WLM, and thereafter increased linearly to 10,240 WLM. These data suggest that the radon progeny exposure-mortality response is non-linear. In contrast, there was no significant parallel upward displacement of the Gompertz functions for rats exposed at 100 WL for total exposures of 20-1280 WLM, but a large displacement began at 2560 WLM total exposure.

A mortality kinetics approach to characterizing the relative biological effectiveness of short-term exposure to fission neutrons.

A Gompertz age-specific mortality rate model was developed for toxicity resulting from a short-term exposure to a toxicant resulting in nonrepaired injury that summates with natural (aging) injury. The model was applied to mortality data for male and female B6CF1 mice subjected to 60 weekly whole-body exposures to fission neutrons and gamma rays (W. F. Thomson and D. Grahn, Radiat. Res. 115, 347-360, 1988). There was no apparent increase in the age-specific mortality rate of mice during the 60-week period of fractionated exposures to fission neutrons. Cumulative toxicity was not exhibited in the Gompertz functions of mice exposed to fission neutrons until after termination of exposure. As a secondary issue, the Gompertz age-specific mortality rate model was modified to incorporate paradoxical "oversurvival" or longevity hormesis in mice exposed to fission neutrons. This modified model was compared to a model that has no allowance for oversurvival. The logarithmic-logistic function was used to relate the parallel upward displacement of the Gompertz function at steady state (i.e., the time after exposure ceased and the mortality effects of oversurvival dissipated) with dose. The data were shown to support the assumption of parallel upward displacement. The relative biological effectiveness (RBE) of fission neutrons compared to gamma rays was calculated using the steady-state displacement of the Gompertz functions. The RBE was defined as the ratio of doses D gamma/DN necessary to produce the same upward displacement by neutron radiation (DN) and gamma rays (D gamma). This definition is proposed as a useful measurement of RBE for studies of short-term exposures to fractionated radiation.

A Gompertz age-specific mortality rate model of toxicity from short-term whole-body exposure to fission neutrons in rats.

A Gompertz age-specific mortality rate model was developed for toxicity resulting from a single dose of a toxicant resulting in nonrepaired injury that summates with natural (aging) injury. The model was applied to mortality data for male Sprague-Dawley rats subjected to short-term whole-body exposure to fission neutrons (D. Chmelevsky et al., Radiat. Res. 98, 519-535, 1984; J. LaFuma et al., Radiat. Res. 118, 230-245, 1989). The logarithmic-logistic function was used to relate the displacement of the Gompertz function with dose. Analysis of the age-specific mortality rate provides a measure of total injury over time from combined neoplastic and non-neoplastic causes and can be used to calculate the relative biological effectiveness (RBE) of different radiation sources. It is a useful adjunct to conventional risk assessment paradigms, particularly for those toxicants such as fission neutrons which result in a large number of competing causes of death other than cancer.

Proximal impact deposits at the Cretaceous-Tertiary boundary in the Gulf of Mexico: a restudy of DSDP Leg 77 Sites 536 and 540.

Restudy of Deep Sea Drilling Project Sites 536 and 540 in the southeast Gulf of Mexico gives evidence for a giant wave at Cretaceous-Tertiary boundary time. Five units are recognized: (1) Cenomanian limestone underlies a hiatus in which the five highest Cretaceous stages are missing, possibly because of catastrophic K-T erosion. (2) Pebbly mudstone, 45 m thick, represents a submarine landslide possibly of K-T age. (3) Current-bedded sandstone, more than 2.5 m thick, contains anomalous iridium, tektite glass, and shocked quartz; it is interpreted as ejecta from a nearby impact crater, reworked on the deep-sea floor by the resulting tsunami. (4) A 50-cm interval of calcareous mudstone containing small Cretaceous planktic foraminifera and the Ir peak is interpreted as the silt-size fraction of the Cretaceous material suspended by the impact-generated wave. (5) Calcareous mudstone with basal Tertiary forams and the uppermost tail of the Ir anomaly overlies the disturbed interval, dating the impact and wave event as K-T boundary age. Like Beloc in Haiti and Mimbral in Mexico, Sites 536 and 540 are consistent with a large K-T age impact at the nearby Chicxulub crater.

Tektite-bearing, deep-water clastic unit at the Cretaceous-Tertiary boundary in northeastern Mexico.

The hypothesis of Cretaceous-Tertiary (K-T) boundary impact on Yucatán, Mexico, predicts that nearby sites should show evidence of proximal impact ejecta and disturbance by giant waves. An outcrop along the Arroyo el Mimbral in northeastern Mexico contains a layered clastic unit up to 3 m thick that interrupts a biostratigraphically complete pelagic-marl sequence deposited at more than 400 m water depth. The marls were found to be unsuitable for determining magnetostratigraphy, but foraminiferal biostratigraphy places the clastic unit precisely at the K-T boundary. We interpret this clastic unit as the deposit of a megawave or tsunami produced by an extraterrestrial impact. The clastic unit comprises three main subunits. (1) The basal "spherule bed" contains glass in the form of tektites and microtektites, glass spherules replaced by chlorite-smectite and calcite, and quartz grains showing probable shock features. This bed is interpreted as a channelized deposit of proximal ejecta. (2) A set of lenticular, massive, graded "laminated beds" contains intraclasts and abundant plant debris, and may be the result of megawave backwash that carried coarse debris from shallow parts of the continental margin into deeper water. (3) At the top, several thin "ripple beds" composed of fine sand are separated by clay drapes; they are interpreted as deposits of oscillating currents, perhaps a seiche. An iridium anomaly (921 +/- 23 pg/g) is observed at the top of the ripple beds. Our observations at the Mimbral locality support the hypothesis of a K-T impact on nearby Yucatán.