The nitrogen isotopes of fossil Tahiti corals from the last deglaciation

Abstract:

Despite its vast geographic area, the South Pacific gyre has been minimally explored in terms of its changing conditions over glacial cycles, partly due to slow sedimentation and poor preservation of carbonate in its deep sea sediments. We have developed a sensitive and precise method for measuring the nitrogen isotopes (d15N) of organic matter protected within the skeletal aragonite of stony corals (coral skeletal d15N). Analyses of modern corals from 8 global sites suggest that the coral skeletal d15N first and foremost records the d15N of regional ocean nitrate, with a secondary effect of local reef productivity. In this study, we apply this new proxy to fossil corals collected during the Expedition 310 of International Ocean Discovery Program, investigating the nitrogen cycle of the South Pacific since the last deglaciation. We measured skeletal d15N in consecutive growth bands in 4 U-Th dated coral samples (with ages of 14.3 ka, 13.3 ka, 11.8 ka and 8.4 ka) from a single core. Within each coral, over the 3-8 consecutive bands sampled, inter-band d15N variation was 1-2.5‰. However, the band-averaged d15N from each dated sample showed <1‰ change between 14.3 and 8.4 ka (from oldest to youngest, 11.4±0.3‰, 10.8±0.7‰, 10.6±0.3‰, and 11.4±0.7‰), consistent with the high thermocline nitrate d15N in modern subtropical South Pacific. The implications of these results for the regional nitrogen cycle as well as the deglacial change in mean ocean nitrate d15N will be discussed.