Coral carbonate-bound isotopes reveal monsoonal influence on nitrogen sources in Southeastern China's Greater Bay Area from the mid-Holocene until the Anthropocene.

Most anthropogenic nitrogen (N) reaches coastal waters via rivers carrying increasing loads of sewage, fertilizer, and sediments. To understand anthropogenic N impacts, we need to understand historical N-dynamics before human influence. Stable isotope ratios of N preserved in carbonates are one way to create temporal N records. However, records that span periods of human occupation are scarce, limiting our ability to contextualize modern N dynamics. Here, we produce a fossil-bound N-record using coral subfossils, spanning 6700 years in China's Greater Bay Area (GBA). We found that during the mid-to-late Holocene, the GBA's coastal N was dominated by fluvial sources. The weakening of the Asia monsoon throughout the late-Holocene decreased river outflow, leading to a relative increase of marine nitrate. This source shift from riverine-to-ocean dominance was overprinted by anthropogenic N. During the late 1980s to early 1990s, human development and associated effluent inundated the coastal system, contributing to the decline of coral communities. [Display omitted] • Carbonate-bound nitrogen from coral faithfully tracked marine nitrogen-source changes over the last 6,700 years. • Coral carbonate-bound δ ¹⁵N values decreased from the mid-Holocene to the late-Holocene by ~2 ‰. • Decline in δ ¹⁵N values is driven by a decrease in the East Asian monsoon intensity. • Elevated δ ¹⁵N values caused by anthropogenic N are unique over 6,700 years, and coincide with regional coral declines. [ABSTRACT FROM AUTHOR]