Bamboo coral Ba/Ca: Calibration of a new deep ocean refractory nutrient proxy

Abstract

It is poorly understood how intermediate water masses are affected by decadal scale climate via biogeochemical cycling, export production, and changes in circulation/ventilation. To this end, a geochemical nutrient proxy from deep-sea bamboo corals would provide decadal to centennial scale records of deep and intermediate-water nutrient dynamics. Seawater barium (Ba SW ) has a nutrient-like distribution in the water-column (similar to silicate), so Ba/Ca records in foraminifera, shallow water surface corals, and other deep-sea corals have been used to trace refractory nutrients. Here we present the first calibration of a nutrient proxy from skeletal barium preserved in the calcitic internodes of deep-sea bamboo corals, collected from intermediate water depths. A calibration was calculated from a broadly distributed suite of Isidella and Keratoisis corals spanning a silicate and Ba SW gradient (n = 33 corals; 300–2800 m): Ba / Ca b a m b o o c o r a l μmol/mol = 0.079 ± 0.008 ∗ Ba SW nmol/kg + 4.205 ± 0.870 r 2 = 0.77 ; n = 33 The strong linear correlation between Ba/Ca bamboo coral and Ba SW suggests that coral Ba/Ca is a reliable recorder of seawater barium (and, therefore, silicate). We find a distribution coefficient (D Ba ) for Ba/Ca bamboo coral of 1.3 ± 0.2, similar to that of other corals (surface and deep-sea dwelling) and inorganic calcium carbonate precipitation experiments (D Ba = 1.2–1.5). This suggests that Ba incorporation is primarily driven by cationic substitution in bamboo corals and holds promise as a globally applicable refractory nutrient proxy. We find interannual–decadal scale variability in a Ba/Ca bamboo coral timeseries from a California Margin coral (San Juan Seamount; 1295 m). These data suggest that additional high-resolution Ba/Ca bamboo coral records may reveal a connection between regional-scale intermediate water biogeochemistry and low-latitude surface ocean/atmospheric climate.