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Impact iridium (how much was there?)



>>> Earl Wood <candles@jps.net> wrote:
> I would like to know if anyone has calculated the amount of Iridium
> needed to cover the entire earths surface , at the depth
> associated with the K C
> boundary ?

That should be K/T boundary, or even C/T (see Alvarez
et al.'s 1980 paper...they write it "C/T").


> and if so does anyone have these figures , or a
> reference that would be available to Joe citizen ?


The total amount (weight) of atmospheric-pumped iridium can be
calculated by finding the *average* weight of iridium in
a cubic centimeter of the layer containing the iridium "spike"
(averaged from various localities around the world). Then,
multiplying that value by the total cubic cm of the entire iridium-bearing
air-fall bed over the entire world.

I don't have time to find all the data from the 30 or so
boundary sites, but I will provide you with two of them.

First, a couple complicating factors.  The so-called "boundary
bed" that comprises the effects of the K/T impact is actually
composed of more than one layer.  The lower layer should
be ignored in your calculations.

The boundary bed consists of an ejecta layer, often referred
to as "the boundary clay", and an overlying layer
often called "the magic layer" ("impact bed" of Retallack (1996)).
The boundary clay is composed of material blown out of the crater
that was of a size somewhat greater than dust (alvarez et al., 1995).
A lot of the boundary clay is composed of glassy spherules
(micro-tektites) and impact-shocked quartz grains.
Because of the original size of this material (sand size and greater),
this material was all deposited within a period of one hour or less
after the impact.  It was later altered to clay minerals.
This layer is highly variable in thickness, ranging from
almost a foot-thick hundreds of miles from the crater (in Haiti),
down to 1-2 centimeters thick in Montana and Wyoming, which
are thousands of miles from the crater.  This layer doesn't
contain much iridium.

The overlying clay layer (the "magic" layer; see Bohor et al.
1987) contains the iridium spike.  This clay layer represents
slow air-fall deposition, probably over a period of months, of
the iridium-containing dust-cloud that was pumped into the
stratosphere by the convective mushroom cloud from the blast.

The way that the Ir concentrations were written-out varies
from author to author, so I will leave the necessary
calculations for you to do.

1) At Snow Creek Montana, the concentration of iridium in the
air-fall (non-heavy ejecta) portion of the boudary bed is
11.7 nanograms per gram of sediment (+/- 10 %).  The
thickness of the claystone is 1.0 to 1.5 cm in that region.
Kaolinite clay has an average compacted density of_____ gm/cm^3
(sorry, I don't have my copy of Dana near me; any good
mineralogy book will have the specific gravity of compacted
kaolinite).

2) In the Fish Clay in Denmark, there is an Ir concentration of
72 X 10 ^-9 grams, per cubic centimeter of clay (Alvarez et al.
1980:1099).  The layer here is about 2.5 cm thick.

3) The surface area of the earth is calculated as on an oblate
spheroid, and is:   510.0501 X 10^6 km^2


Caveats:

Your result will be very rough, because the air-fall layer
thickness is/was undoubtably non-uniform over the entire earth
(local weather conditions, jet-stream patterns, etc.). For instance,
it is possible that the polar regions did not recieve nearly as
much fall-out as the equatorial and temperate regions of the earth.

But it should be fun to work out, anyway.  Please put your answer
in kilograms.  You have one day to finish. There will be no make-up
exam.  And I don't grade on the curve (I don't even grade!).
Cheers,
                     <pb>

--
                 Phil Bigelow
                 bh162@scn.org