Having not read the book I am hesitant to comment, but did want to point out a few things to consider. If these are indeed what the book said, I would start being a bit suspicious.
First of all, if the mussel shells they founds were indeed Mytilus, those would have been
Mytilus galloprovincialis (one of the
M. edulis complex). These still are found in the Black Sea and are the dominant benthic invertebrate on the shelf regions there as well as in suitable habitats in the Mediterranean. They are in no way freshwater (although euryhaline).
Also, I am kind of stumped as to how they figured out the shells were 'sun bleached'. Any shell bleaches white very quickly in an aquatic environment without any sun exposure. The pigments in mollusk shells tend to be concentrated in the outer, very thin, shell layer (to test, take a file to a particularly colorful seashell) which is readily soluable in acidic seawater (like in sulfide rich and/or anoxic water). Have you ever seen shells with a yellow, brown or black outer layer that may flakes off when dried? This is called the periostracum. Fresh queen conchs have a very thin flaky one, freshwater mollusks sometimes have a heavy black periostracum. This is a proteinaceous (like hair or fingernails) shell layer that protects the shell from dissolution in acidic water. If you ever see the large FW clams or mussels they get in the Mississippi or Missouri (shells traditionally used for expensive buttons, cameos, beads, etc) you sometimes see a shallow hole on the black outside showing white or nacreous pit. This results from a chip in the periostracum and acidic action of water eating into the shell.
I also wonder about the 'sapropel'. From your wiki link I gather it is the same thing we call "organic rich mud", "organic ooze", etc. In my career in marine biology and teaching both physical and chemical oceanography & limnology I haven't heard this term. It is not in any of the textbooks I have available. Anyway, it is no surprise to find preserved organics in this material. From the Burgess Shale fossils to fjords here in Alaska it is common to find well preserved organic material. The sulfides and lack of oxygen make decomposition very slow. This is especially seen in cellulose and lignin. A paper I have by Popov from 1931 describes the anoxic layer in the Black Sea deep water regions.
Popov, AM (1931) Distribution of Fishes in the Black Sea With Reference to Bottom Conditions: Based on Observations Made Chiefly Off the Southern Coasts of Crimea. Ecology 12(3):468-475.
Doctor Bashir: "Of all the stories you told me, which were true and which weren't?"
Elim Garak: "My dear Doctor, they're all true"
Doctor Bashir: "Even the lies?"
Elim Garak: "Especially the lies"
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