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Tom Lipka had a good discussion provoker with this statement:
> 'Multiple Iridium spikes" need also be taken in the context of the
depositional history of the stratum in question. I believe at least one (Hell
Creek?? I forget where ) had more than one Ir spike but can be explained by
the fact that the formation was created by a fluvial stream complex. In such a
highly variable depositional setting, not only fossils can be reworked but so
can the geochemical signature of an impact.
So Tom has brought up the important point of how physical processes may cause
the appearance of multiple iridium spikes in a stratigraphic sequence. I'd
like to bring in another point from the perspective of an ichnologist by
asking about biogenic processes. How many of the K/T boundary sections have
trace fossils crossing the boundary or otherwise show evidence of
bioturbation? Certainly there were some major bioturbators (no offense meant
by that word - it just means that these organisms mixed up sediment a lot)
during the Cretaceous and following in the Tertiary. This means that workers
looking at a K/T boundary section have to make sure that iridium wasn't
"piped" down into a lower part of the section through a burrow system (i.e.,
the trace fossils _Thalassinoides_ or _Ophiomorpha_) or simply mixed into a
broad interval by burrowing activity. Because this burrowing happened in both
marine and nonmarine environments, this might partially explain Norm MacLeod's
observation:
> As for Ir, it's distribution is very curious. There are multiple Ir
anomalies in many marine sections (e.g., Braggs, Brazos River) where the
fluvial mechanism Tom alludes to is not operable.
Because I have not seen references to these processes in what little
literature I've read on the subject (and the discussion here on PaleoNet), I'm
curious whether (or how) biogenic processes are accounted for in analyses of
the precision of iridium zones at these boundary sections. Any takers?
Anthony J. Martin
Geosciences Program, Emory University
Atlanta, Georgia USA
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