Assessing modern deep-water ages in the New Zealand region using deep-water corals

The radiocarbon levels of 19 deep-sea corals from seamounts and plateaus in the New Zealand region were determined to assess their ability to record contemporaneous radiocarbon levels in surrounding (in situ) waters. Four species of subthermocline-dwelling corals from 366 to 1680 m water depth were collected between 1964 and 2000 using dredge hauls. Carbonate directly underlying living polyps was analyzed for radiocarbon content. Corals underlying subantarctic waters to the south of the South Island and subtropical waters to the north of New Zealand were collected from 426 to 1680 m water depth and had Δ 14C levels ranging from −11‰ to −146‰ (45–1200 14C years). These radiocarbon levels are in good agreement with the radiocarbon levels of the regional subthermocline waters, as determined for the ∑CO 2 in the waters by GEOSECS in 1974 and WOCE in 1992. The differences in Δ 14C levels in corals sampled in the 1960s and 1990s trace a pattern of changing bomb radiocarbon influence on the carbon in subthermocline waters. The subtropical frontal zone, an area known for significant eddies and mesoscale mixing, sits astride the Chatham Rise to the east of New Zealand, and radiocarbon profiles there are correspondingly more complex. Corals collected on the Chatham Rise were retrieved from 366 to1100 m and document Δ 14C levels ranging from −116‰ to +12‰ (945 14C years before present to significantly bomb-influenced). Corals sampled from depths shallower than ∼500 m tended to have higher and more consistent levels of bomb-carbon input, whereas corals sampled from depths deeper than ∼800 m show little or no bomb-carbon influence. Published measurements of the radiocarbon levels of the dissolved inorganic carbon from a transect taken in 1988 across the Chatham Rise indicate localized injection of surface waters containing bomb radiocarbon to depths as great as 600 m, which appears to be faithfully recorded in the corals. These results indicate that deep corals record even decadal-scale changes in the radiocarbon signal of the water in which they grew, making them excellent recorders of deep-water radiocarbon past and present.