Reconnaissance 14C Dating and the Evaluation of Mg/Li as a Temperature Proxy in Bamboo Corals from the California Margin

Abstract:

In the face of anthropogenically induced climate changes, it is becoming increasingly important to develop high-resolution paleoceanographic records that may elucidate how ocean conditions may shift in coming decades. Recently, bamboo corals (gorgonian octocorals) have been proposed as archives of intermediate ocean conditions. This study used 'reconnaissance' radiocarbon analysis to identify the nuclear bomb ¹⁴C spike in the proteinaceous nodes of bamboo corals and to quantify radial growth rates and ages of corals spanning the eastern Pacific oxygen minimum zone (OMZ) (790-2055 m). Preliminary data suggest that these corals exhibit a wide range of growth rates (9.4-350 μm/yr) that are non-linear over time and decrease with coral age and depth. Records of Mg/Li were investigated in these corals, given that previous studies have demonstrated positive correlations between Mg/Li and temperature in benthic foraminifera and surface and deep-sea aragonitic corals, with a reduced influence of vital effects over Mg/Ca. Intracoral reproducibility observed for replicate Mg/Li timeseries within each sample (p=0.6±0.2, n=6) and strong correlations between Mg/Ca and Li/Ca (0.9±0.1, n=6) indicate similar environmental or biological drivers of Mg and Li incorporation in bamboo corals. Given the strong positive correlations between Mg/Li and water temperature across a depth transect (r²=0.87, n=6), increasing Mg/Li observed over the growth history of each of the corals more likely reflects declining growth rates resulting in decreased Li incorporation over time rather than cooling of California Margin intermediate waters. Reductions in growth rate over the lifespan of each coral (~100+ years) may be a function of natural coral growth patterns or changes in carbonate chemistry, oxygen, or food supply in a sensitive OMZ coral ecosystem.