HR: 0830h
AN: PP11A-0194    [PDF]
TI: Geochemistry of the Deep Water Bamboo Coral {\it Isidella}; Intermediate Depth and Surface Ocean Chemical Recorder
AU: * Spero, H J
EM: spero@geology.ucdavis.edu
AF: University of California Davis, Department of Geology, Davis, CA 95616 United States
AU: Jang, N A
EM: najang@ucdavis.edu
AF: University of California Davis, Department of Geology, Davis, CA 95616 United States
AU: Adkins, J F
EM: jess@gps.caltech.edu
AF: Caltech, Department of Geological and Planetary Sciences, Pasadena, CA 91125 United States
AB: Geochemical analyses of deep water corals have provided a wealth of data on past ocean circulation and chemical changes. Information obtained from these carbonate precipitating organisms generally reflects ambient conditions at the depth of growth. The bamboo coral, {\it Isidella} sp., belongs to a group of deep water Octocorals that live at intermediate ocean depths ($\sim200-1500$m) and produce a calcite skeleton that is divided by proteinaceous gorgonin internodes. Because, the calcite and organic regions of the skeleton are precipitated simultaneously, their chemistries are temporally coupled. Stable isotope, radiocarbon and $^{210}$Pb data were obtained from several specimens of {\it Isidella} sp. that were collected in fishing dredges from the outer continental shelf near Pt. Reyes, CA ($38\deg$N $123.4\deg$W $\sim$220 m). $^{210}$Pb analyses on one of the specimens suggests the coral was ~15-80 years old. $\delta^{13}$C and $\delta^{18}$O data from the calcite skeleton display the typical nonequilibrium covariation that has been described previously, thereby limiting the use of these data in reconstructing environmental temperatures. Although $\delta^{13}$C analyses of the organic internodes produced typical marine values of $-16.9\pm0.1\permil$ (n=17), $\delta^{15}$N values were unusually high, $13.8\pm0.4\permil$. Because the internode geochemistry records the organic chemistry of sinking particulate matter ingested by the coral, the enriched $\delta^{15}$N data reflect the chemistry of local upwelled NO$_{3}$ that was strongly influenced by subsurface denitrification. AMS analyses of the center and outer edge of the skeleton (branch diameter = 2.2 cm) and adjacent organic internodes (growth proceeds from center outwards) yield $^{14}$C ages of 2065 and $2000\pm$35 years for the calcite ($\Delta^{14}$C = -226.4 and -220.3$\permil$) and 785 and $765\pm$35 years for the organic node ($\Delta^{14}$C = -93.1 and -90.7$\permil$) respectively. The calcite AMS ages record the $^{14}$C reservoir age of upper N. Pacific thermocline waters whereas the organic data record the surface ocean reservoir age during the corals growth. The reservoir age difference is $\sim1250$ years. Comparison of these ages with data from a mussel collected along the Pt. Reyes coast in 1913 ($^{14}$C age = 710 years; M. Solomon unpub. data) suggest our coral specimen died is $\sim80$ years ago in agreement with the $^{210}$Pb data. These results demonstrate that the bamboo corals are a valuable archive of intermediate and surface ocean chemistry and could prove invaluable for reconstructing combined near surface circulation and intermediate ocean chemistry changes in the past.
DE: 1040 Isotopic composition/chemistry
DE: 4267 Paleoceanography
DE: 4271 Physical and chemical properties of seawater
DE: 4830 Higher marine organisms
DE: 4870 Stable isotopes
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2003 Fall Meeting