So, in order to understand how this spike in carbon isotope happens you need to understand
something about the carbon isotope in the ocean, the cycle of carbon in the ocean.
In the top ocean we have a lot of productivity, a lot of photosynthesis.
Now this photosynthesis consumes all the lighter carbon because it's easier to incorporate the
lighter carbon into organic tissues if you consume the organic carbon into the organic tissues by
implication the waters around you the carbonate pool or the inorganic carbon pool
becomes heavier. So, the shells of planktic forums that live in the surface of the ocean will tend to
have a more positive carbon isotope value than the organic matter. Now that organic matter of course
will be eaten by other organism and eventually it will sink usually it's sinks as fecal pellets
that's known as packaging. That's a the technical term for it and that fecal pallet sinks through
the water column when it gets to the bottom of the water column it consumes the oxygen
by oxidizing the organic matter and releasing some of that carbon back into the water. So, typically
the deep water are actually lighter in carbon isotopes because of that export of the light
carbon isotope that is produced in the surface ocean, so the deep water are characterized by this
much lighter carbon isotope value and if you look at this figure here you can see that in the
Cretaceous at the very base of the plot we have the surface ocean that's basically a record of
microfossil. So, that's a carbonate record and as I’ve explained the surface dwelling forams
essentially have a very heavy delta c13 because all the c12 has been consumed by photosynthesis.
However, the species that live in the intermediate and deep portion tend to have a lighter delta c13
signature because of this respiration of the organic carbon that was sinking through the water
column and gets oxidized and released the c12 in the water and that's typical of the ocean, that's
what you would observe in the ocean today. Now, once we go to the Cretaceous Tertiary Boundary,
we have the extinction event right here, but then we go into what is known as the Strangelove
ocean. What is the Strangelove ocean? Well, look at this the surface dwelling forams all of a
sudden, the carbon isotope shifts to zero, or to let's say 1.5, to much lighter values
by extension, or by contrast if you want the deep dwelling species their value shifts now
to more heavy values. So, it's just as if during the Strangelove ocean, the production of light
carbon in the surface ocean was drastically reduced and so the export of carbon to the deep
ocean no longer existed. Now we have an ocean that is completely not stratified,
completely homogeneous in terms of carbon isotope values from the surface to the bottom
and that's exactly the interpretation here. The interpretation is that the impact of the bolide
the meteorite essentially ceased all production in the ocean, or most production in the ocean and
you no longer have this packaging and export production of shallow water photosynthesis to
the deep ocean and that's in part because we went into what is known as a nuclear winter with a lot
of dust in the atmosphere blocking sunlight and of course then in the Danian we have a recovery,
and we see that the surface water becomes heavier. So, that's a resumption of photosynthesis,
meanwhile the deeper water again become more negative because we have export of light carbon
but notice that the recovery is not complete the surface ocean is never as heavy in terms
of delta c13 as it was in the Cretaceous and the deep ocean never becomes as light and that's
the Strangelove ocean. So, now you think that in modern stratigraphy there no longer are any
room for controversies the way we would have controversies in the 19th century like Cedric
and Merchantson that's not completely true because sometimes correlation can be tricky. So, here
we're looking at a correlation of the KT boundary in the Gulf of Mexico from Bass River, which is
in New Jersey on the left, to Blake nose, which is in the northwest Atlantic and the Demerara rise,
which is in the middle of the Atlantic and you can see here that one of the issues is that we
have fairly thick interval of the KT transition boundary where we find spherule those spherule layers
and you remember the map I showed you that's expected because we're close from the impact,
so we have a lot of spherules and we can actually correlate this,
but the problem becomes more acute when you look at the detail of those sediments and Gerter
Keller and co-author in 2002 did exactly that they looked at the detail of the section in the Gulf of
Mexico and in particular in Mexico and they found actually several layers of spherules. So, you have
one layer here at the base they're represented in this dark grey material,
but you have four layers in total of those spherules. So, their interpretation was could
that actually indicate multiple impacts, can we be looking at not one impact of a meteorite,
but several impacts that came in close proximity because maybe it was a meteorite shower and that
paper really threw the whole scientific world in turmoil because this was a big debate at the time
where did we have only one impact, did we have multiple impact, did we have maybe multiple impact
and volcanism, what exactly caused the end of the dinosaurs and the rise of the mammals. Well
the answer to the question actually lies in an industry well that was drilled by Pimex the
Yax-1 well, which is right at the edge if you look at this beautiful concentric circle right at
the edge of the Chicxulub impact crater and to make a long story short in this particular well
you see lots of evidence for cross stratification linked to storm deposits and this is right
after the impact you have evidence for storm deposit. In fact, you have evidence for
mega tsunami. So, from this well and from this sedimentology we understood that after the
impact what happened is you had a ton of tsunami deposits being formed close to the impact crater,
which makes total sense if you imagine the power of this impact would have a tremendous impact on
the ocean. So, now the conventional stratigraphy is to say that if you look at El Kef in Tunisia
the base of the ejecta layer there is only represented by the red iridium-rich layer, whereas
in the Gulf of Mexico the layer itself is several meters thick and because it represents tsunami
deposits it's really mixed you have several layers of ejecta deposits mixed with marine sediments and
linked again with ejecta deposits and that's why the confusion existed for a few years
where some author said well maybe we have multiple impact because we have ejecta deposits mixed with
marine sediments whereas other authors said no it's only one impact, we have one impact crater
it cannot be multiple impacts and as you can see good stratigraphy and good sedimentology are key
to resolve these issues in earth's history and this is the end of this particular lecture and
it is also the end of this online course. I hope to see you in another one of my online classes and
of course maybe one day on the Chikyū or the Jordes Resolution.