I think that manganese can contribute to the black as well.  Decay of
the animal can contribute to the lack of oxygen, but sometimes live
shells have a fair amount of black.  It depends on the habitat.
Lucines pump anoxic water from below for their bacterial symbionts and
oxic water from above for themselves, so where they live the
oxic-anoxic boundary is fairly shallow in the sediment.

On Sun, Jun 17, 2012 at 6:57 AM, Marlo Krisberg <[log in to unmask]> wrote:
> Seeking comments from Conch-Lers regarding the process that blackens dead
> shells.  Susan has pretty well described it, but I’m wondering if some of
> you can provide more details.
>
>
>
> “…shells sometimes get buried in sediments in a layer that has no oxygen,
> and then bacteria living in the sediment produce iron sulfides which can
> turn a shell partly or completely black.”
>
>
>
> http://z14.invisionfree.com/Conchologist_Forum/index.php?showtopic=2424
>
>
>
> Marlo



--
Dr. David Campbell
Collections Assistant
The Paleontological Research Institution
1259 Trumansburg Road
Ithaca NY 14850

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