T. Felis et al., Vertical water mass mixing and plankton blooms recorded in skeletal stablecarbon isotopes of a Red Sea coral, J GEO RES-O, 103(C13), 1998, pp. 30731-30739
The environmental interpretation of the C-13/C-12 variations in the skeleto
ns of massive corals is still a matter of debate. A 19-year seasonal skelet
al C-13/C-12 record of a shallow-water Porites coral from the northern Red
Sea (Gulf of Aqaba) documents interannual events of extraordinarily large p
lankton blooms, indicated by anomalous C-13 depletions in the coral skeleto
n. These blooms are caused by deep vertical water mass mixing, convectively
driven in colder winters, which results in increased supplies of nutrients
to the surface waters. The deep vertical mixings can sometimes be driven b
y the cooling occurring throughout the Middle East after large tropical vol
canic eruptions. We therefore have evidence in our coral skeletal C-13/C-12
record for an indirect volcanic signal of the eruptions of El Chichon (198
2) and Mount Pinatubo (1991);. Deep mixing induced C-13/C-12 variations of
the dissolved inorganic carbon in the surface waters can be neglected at th
is location. We therefore suggest that the C-13 Skeletal depletions can be
best explained by changes in the coral's autotrophy-heterotrophy diet, thro
ugh increased heterotrophic feeding on zooplankton during the blooms. Incre
ased feeding on C-13-depleted zooplankton or increased heterotrophy at the
expense of autotrophy can both result in a C-13-depleted coral skeleton. Ho
wever, this suggestion requires more testing. If our conclusions are substa
ntiated, seasonal skeletal C-13/C-12 records of corals which change from au
totrophy under normal conditions to increased heterotrophy during bloom eve
nts may be used as indicators of ocean paleoproductivity at interannual res
olution, available from no other source.