An Iridium Abundance Anomaly at the Palynological Cretaceous-Tertiary Boundary in Northern New Mexico

Abstract

An iridium abundance anomaly, with concentrations up to 5000 parts per trillion over a background level of 4 to 20 parts per trillion, has been located in sedimentary rocks laid down under freshwater swamp conditions in the Raton Basin of northeastern New Mexico. The anomaly occurs at the base of a coal bed, at the same stratigraphic position at which several well-known species of Cretaceous-age pollen became extinct.

A sequence of events across the Cretaceous-Tertiary boundary

“…Criteria (1) would place the boundary at the iridium level, (2) about 3 cm higher up section, and (3) at 34 cm above the Ir anomaly. It is the authors opinion that it is most useful to define the K/T boundary at the mass-extinction level, because it is worldwide, coincides exactly with the iridium anomaly respectively ETC-rich layer both in marine rocks and on land (with pollen extinctions [13,21], and it is probably synchronous. Doubts about the synchronism of the K/T boundary, expressed by some [14,15,22] are based 159 on the alleged occurrence of the K/T boundary in different polarity intervals.…”

The reciprocal interaction of angiosperm evolution and tetrapod herbivory

“…Palynofloras and megafloras from immediately above the K/T boundary are now known from several parts of western North America (Orth et al, 1981;Tschudy et al, 1984;Nichols et al, 1986;Wolfe and Upchurch, in press). …”

“…Earliest Paleocene angiosperm assemblages are only now in the process of being described (Wolfe and Upchurch, 1986, in press), but immediately post K/T boundary vegetation has an early successional appearance (Orth et al, 1981;Nichols et al, 1986). The angiosperms involved may have been dominantly deciduous (Wolfe and Upchurch, in press).…”

“…Whatever its cause, the terminal Cretaceous event had a major effect on the vegetation of a large part of North America. Floras from the few cm just above the boundary are dominated by ferns (Orth et al, 1981;Tschudy et al, 1984;Nichols et al, 1986; although see Lerbekmo and St. Louis, 1986); in Colorado, U.S.A., the subsequent few meters of sediment preserve dispersed cuticle and megafossils that indicate early successional angiosperms (Wolfe and Upchurch, 1986, in press). Wolfe and Upchurch (1986, in press) suggest this records vegetational succession following regional devastation of Maestrichtian plant communities.…”

Palynostratigraphy of the Cretaceous–Paleogene in the Austral Basin, SW Santa Cruz Province, Argentina

“…The evidence of a comparable extinction to that of the fauna in the flora is ambiguous at least in the Southern Hemisphere (Diéguez, 2003). In the Northern Hemisphere in various sections of North America, and also in Japan and Europe there was a violent and rapid decline in the abundance and diversity of various plant groups across the area (Orth et al, 1981). This process was followed by a significant increase in the concentration of spores of ferns, an event known as the "fern spike".…”