The coral δ15N record of terrestrial nitrate loading varies with river catchment land use

Abstract

We analysed the nitrogen isotopes in two coral cores (δ15Ncoral) from the mouth of the Todoroki River, Ishigaki Island, Japan, to examine whether the δ15Ncoral reflects the run-off of nitrate related to the land use in the river catchment. The two coral cores were used to examine the seasonal variation in δ15Ncoral for 14 years (CORE1; 1993–2007) and the annual variation of δ15Ncoral for 52 years (CORE2; 1958–2010). In CORE1, the 5-month running mean of δ15Ncoral was positively correlated with that of monthly precipitation, excluding all strong precipitation events (>150 mm d−1). In CORE2, the δ15Ncoral mean in the earlier period (1958–1980) was 1.0 ‰ greater than that in the later period (1981–2010). The annual averages of δ15Ncoral are positively correlated with the total precipitation in the rainy season (May–June) for both time periods. The difference in the δ15Ncoral between the earlier and later periods is probably caused by the land use changed from paddy fields with 15N-rich manure to sugar cane fields in the early 1980s. Although some uncertainties still remain regarding the precision of δ15N coral proxy records, this study emphasises the clear potential for their use in reconstructing terrestrial nitrate discharge records from corals.