Trace Metal Record of a 200-Year-Old Deep-Sea Bamboo Coral (Isidella sp.)

High resolution records of past oceanic conditions can be constructed from the annually secreted calcite laminations of deep-sea gorgonian corals. Previous research has shown that deep-sea gorgonians incorporate both surface organic matter and nutrients from the surrounding water into their coral skeleton, making them ideal recorders of long-term ocean variability of surface and intermediate water. In this study we examined a 200-year-old bamboo coral (Isidella sp.) that was live collected by bottom trawl in the summer of 2000 on the Oregon continental margin at a water depth of 1148m. We explored how annual changes in upwelling strength, circulation and surface productivity are reflected in the trace metal concentrations recorded in the carbonate skeleton of the bamboo coral. To determine trace metal concentrations, laser ablation inductively coupled plasma mass spectrometry was employed at a resolution of 10microns on multiple radial transects of the coral cross section. Minor element abundances were determined on the same transects by electron microprobe (EMP) analysis. We constructed an age model by counting peaks in the ratio of magnesium to calcium abundances obtained from the EMP. Uranium series dating methods were then used to verify the age model. The concentrations of phosphorus (P), barium (Ba) and cadmium (Cd) showed considerable variation through time. Initial time series data of phosphorus to calcium (P/Ca) ratios indicates strong variability at the decadal scale, potentially reflecting varying nutrient availability. Cadmium to calcium (Cd/Ca) ratios also showed strong variability at the decadal scale. However, periods of increased P/Ca did not always correspond to elevated Cd/Ca, suggesting that P and Cd concentrations were not controlled by the same processes. The record of barium to calcium (Ba/Ca) ratios was poorly correlated to both P/Ca and Cd/Ca and showed irregular episodes of increased Ba/Ca. These irregular episodes may indicate disturbance events, such as earthquakes, during the life of the coral. Continuing work will attempt to elucidate the controls of P, Ba and Cd concentrations preserved in the carbonate skeleton of the bamboo coral.