Iridium

Iridium enrichment (or concentrations of iridium above background levels) may provide the ideal criterion for correlating marine and nonmarine K-T boundary sections worldwide (Berry, 1984). High concentrations of iridium were first reported by Alvarez et al. (1980) at the marine K-T boundary (as recognized then) in Italy and Denmark, and later elsewhere worldwide (Alvarez et al., 1982, 1984). Alvarez et al. (1980) proposed that the iridium was too concentrated to have been terrestrially derived and therefore must be the record of a bolide impact. The effects of this impact on latest Cretaceous organisms were interpreted to be the causal factor in terminal Cretaceous extinctions, in both marine and terrestrial realms.

According to an impact scenario, extraterrestrially derived iridium would have been dispersed into the air after impact, and eventually incorporated into sedimentary basins within 100 years or less. Iridium-enriched sediment at K-T boundary sections worldwide would be the record of this virtually instantaneous event in earth history, and would thereby provide a basis for worldwide correlation (Berry, 1984). However, others argued that K-T iridium enrichment was deposited during a period of intense volcanism lasting 10,000 to 100,000 years (Officer and Drake, 1985; Officer et al., 1987; Crocket et al., 1988). Reports of high concentrations of iridium during recent volcanic emissions from Kilauea Volcano, Hawaii (Zoller et al., 1983), support the possibility that K-T iridium enrichment may be terrestrially derived. Also, the concentrating effects of micro-organismal activity may have had a role in the formation of iridium anomalies (Dyer et al., 1989).

Shocked quartz (Bohor et al., 1984; 1987a) and microspherules (Smit and Klaver, 1981; Montanari et al., 1983; Montanari, 1986) are often associated with iridium anomalies and may represent impact-derived products. However, shocked quartz may originate from intense volcanism (Carter et al., 1986; but see Alexopoulos et al., 1988), and microspherules may have a volcanic origin as well (Naslund et al., 1986). Also, microspherules from three marine K-T sites have been interpreted to represent diagenetic infillings of organic spheres, not impact or volcanism products (Hansen et al., 1986).

The source(s) of the iridium, shocked quartz, and microspherules near the K-T boundary no doubt will be debated for years. In this study, iridium enrichment is used as a correlation tool, whatever its source. Iridium enrichment is present at the proposed K-T boundary stratotype at El Kef, Tunisia (Kuslys and Krahenbulh, 1983). It also has been reported from many of the terrestrial stratigraphic sections known to span the K-T transition in the western interior of North America (Orth et al., 1981, 1982; Smit and Van der Kaars, 1984; Nichols et al., 1986; Lerbekmo and St. Louis, 1986; Lerbekmo et al., 1987; Bohor et al., 1987b). Other sections spanning the K-T transition, most notably those in eastern Garfield and western McCone counties in eastern Montana (where Bug Creek local faunas are located), have been sampled for iridium, but have not yielded a high level of iridium enrichment. In any case, the critical premise is that the marine and nonmarine iridium enrichments are records of the same discrete event, which allows precise correlation.

Several challenges to worldwide iridium anomaly correlations have been proposed. For example, one hypothesis suggests that K-T extinctions were caused by multiple impacts, perhaps comet showers (Hut et al., 1987). Also, the marine (worldwide distribution) and terrestrial (North American distribution) K-T iridium anomalies may have been deposited by different mechanisms (Schmitz, 1988). If the marine and nonmarine anomalies were formed by different events, or if they were caused by multiple impacts, temporal correlation becomes suspect. However, iridium enrichment could still be an effective correlation tool for terrestrial K-T sections within North America.

Unfortunately, sections spanning the K-T transition in McCone County have been sampled for iridium enrichment without success (Smit et al., 1987). The nearest K-T iridium enrichment is 50 km to the west, at the Lerbekmo site in Garfield County. With the absence of records of iridium enrichment in McCone County, direct correlation of K-T sections to the marine K-T boundary cannot be accomplished. Pollen extinction then becomes the most precise correlation tool available for determining the local K-T boundary.

It has been implied that an iridium enrichment is present in the lower "Z" coal at Russell Basin, McCone County (Sloan et al., 1986; Rigby, 1989), a few kilometers north of the McGuire Creek study area. However, this iridium concentration is 40-80ppt, or only 2-3 times background (Fastovsky and Dott, 1986), and is similar to normal background levels such as those reported from Saskatchewan (5-60ppt, Nichols et al., 1986), North Dakota (25ppt, Johnson et al., 1989), and the Raton Basin in New Mexico (10-30ppt, Orth et al., 1981). A concentration of 40-80ppt is not anomalous, and reference to an iridium anomaly in McCone County should be discontinued.