Ignorant carbon isotope/carbon cycle question

[Paracelsus]

From same book (Extinction), Erwin talks about assessing the earth's carbon cycle via carbon isotope ratios in marine sediments, on the principle that living organisms preferentially take up the [12]C isotope, leaving ocean waters relatively enriched in [13]C.

Well, when marine organisms die, they fall to the bottom of the ocean and eventually become incorporated into sedimentary rock. Given that these organisms are part of the sedimentary rock that is being analyzed for its [12]C/[13]C ratio and the assumption that all of these fossils are probably not separated out of the rock before analysis, wouldn't that relative enrichment in [13]C come out in the wash, so to speak, as the carbon from the organisms enriched in [12]C would be analyzed along with the carbon resulting from inorganic processes that resulted in deposits enriched in [13]C?

Or do I have all of this bass-ackwards?

[Neverfly]

From same book (Extinction), Erwin talks about assessing the earth's carbon cycle via carbon isotope ratios in marine sediments, on the principle that living organisms preferentially take up the [12]C isotope, leaving ocean waters relatively enriched in [13]C.

Well, when marine organisms die, they fall to the bottom of the ocean and eventually become incorporated into sedimentary rock. Given that these organisms are part of the sedimentary rock that is being analyzed for its [12]C/[13]C ratio and the assumption that all of these fossils are probably not separated out of the rock before analysis, wouldn't that relative enrichment in [13]C come out in the wash, so to speak, as the carbon from the organisms enriched in [12]C would be analyzed along with the carbon resulting from inorganic processes that resulted in deposits enriched in [13]C?

Or do I have all of this bass-ackwards?

A fossil of a creature is not made of organics but of the minerals (mostly and usually) that fill the gap as the organics decay away. What we are left with is an impression like a plaster cast, not a mummy or preserved specimen.

[Romanus]

So far as I know, the C12/C13 ratio in fossils is less important than that in the sediments themselves; hence, only one baseline is being used.

The process is a little different for O16/O18 ratios, which in fact rely directly on carbonate remains, but that's another post...