Seasonal variations in the nitrogen isotope composition of Okinotori coral in the tropical western Pacific: A new proxy for marine nitrate dynamics

To demonstrate the utility of coral skeletons as a recorder of nitrate dynamics in the surface ocean, we collected coral skeletons of Porites lobata and determined their nitrogen isotope composition (δ¹⁵N _coral) from 2002 to 2006. Skeletons were collected at Okinotori Island in southwestern Japan, far from any sources of terrestrial nitrogen. Nitrogen isotope compositions along the growth direction were determined at 800 m intervals (∼1 month resolution) and compared against the skeletal carbon isotope composition (δ¹³C_coral-carb), barium/calcium ratio (Ba/Ca), and Chlorophyll-a concentration (Chl-a). From 2002 to 2004, ratios of the δ¹⁵N_coral varied between +0.8 and +8.3‰ with inverse variation to SST (r =-0.53). Ba/Ca ratios and Chl-a concentrations were also observed to be high during seasons with low SST. These results suggested that the vertical mixing that occurs during periods of low SST carries nutrients from deeper water (δ¹⁵N_DIN; +5∼+6‰) to the sea surface. In 2005 onward, δ ¹⁵N_coral and Ba/Ca ratios also had positive peaks even in high SST during periods of transient upwelling caused by frequent large typhoons (maximum wind speed 30 m/s). In addition, low δ¹⁵N _coral (+0.8∼+2.0‰) four months after the last typhoon implied nitrogen fixation because of the lack of typhoon upwelling through the four years record of δ¹⁵N_coral. Variations in the δ¹³C_coral-carb and δ¹⁵N _coral were synchronized, suggesting that nitrate concentration could control zooxanthellae photosynthesis. Our results suggested that δ¹⁵N_coral holds promise as a proxy for reconstructing the transport dynamics of marine nitrate and thus also a tool for estimating nitrate origins in the tropical and subtropical oceans.