SEMINAR: Reminder: MGG Student Seminar at 12 today!

[aoehlert@rsmas.miami.edu]

Please join us for the geochemistry edition of MGG STUDENT SEMINAR at 12
today in 343 CIMAS Conference room!

How are carbon and oxygen isotope records in meteoric altered carbonates
of Plio-Pleistocene rocks?

Yulaika Hernawati and Peter K. Swart

Studying the exposed Plio-Pleistocene carbonate terraces in southern
Dominican Republic presents a good opportunity to learn how diagenetic
overprints of meteoric water influence the isotopic values of carbon and
oxygen. The short cores were drilled through the 15 and 30 meter terraces.
Generally, the data from short cores mostly show large variations in
carbon isotope values, but a narrow range of oxygen isotopes. The data
from the higher 30 m terrace shows a narrower variation of oxygen isotopes
than the lower terrace. Since oxygen values depend on meteoric water that
has penetrated through carbonates, this might indicate that as carbonate
rocks are exposed longer, more meteoric waters have altered the initial
oxygen values and ultimately made the values more uniform. However, a
distinguishable variation of oxygen isotopes in the 15 m terrace reveals
that change in local climate could also generate different oxygen values.
The meteoric diagenesis zone is believed to be the zone with the best
prospect for high porosity and permeability, so the results of this study
will be useful in developing the predictive model for the distribution of
meteoric and mixing zone-induced porosity in carbonate reservoir rocks
(Allan & Matthews, 1982).



Using Stable Isotope and Sr/Ca Ratios to Assess the Paleoecology and
Extinction Selectivity of Mio-Pliocene Free-Living Corals

Sean T. Murray, James S. Klaus, Peter K. Swart, Donald F. McNeill

A major extinction event affected free living, solitary and
flabello-meandroid corals in Caribbean around the Pliocene/Pleistocene
boundary. The two surviving members of this group in the region are
colonial zooxanthellates. In order to determine if this symbiotic
relationship preferentially aided in the survival of these two species, 8
specimens from 4 extinct species of free living corals from the Cibao
Basin in the Dominican Republic were tested isotopically for indicators of
a symbiotic relationship with zooxanthellae. These were then compared to
the isotopic signature of known zooxanthellate and azooxanthellate free
living corals from the same region and time period. The technique utilized
was developed from Stanley and Swart (1995) and looks at correlations
between  ä18O and ä13C values. It was found that Trachyphilia bilobata,
Manicina jungi, and Manicina grandis all displayed isotopic signatures
similar to zooxanthellate corals, while Placocyathus costatus displayed an
azooxanthellate isotopic covariance. These results suggest that a
symbiotic relationship alone was not enough to stem the wave of extinction
that hit free living corals at this time period. Other possibilities that
need to be looked into are the effects on survival probability between
colonial and solitary corals and habitat selection.




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