Variations in mid‐ to late Holocene nitrogen supply to northern Great Barrier Reef Halimeda macroalgal bioherms

The northern Great Barrier Reef (GBR) Halimeda bioherms have accumulated on the outer continental shelf from calcium carbonate algal sediments over the past ∼10,000 years and cover >6,000 km2 of shelf area. As such, Halimeda bioherms play a key role in the shallow marine carbon cycle over millennial timescales. The main source of nitrogen (N) to these bioherms is thought to be westward transport of upwelled NO3--rich water from the Coral Sea. However, the primary N source has not been traced geochemically, and we have no understanding of any temporal variation. Here, we reconstruct patterns of N supply to Halimeda bioherms in the GBR since the mid-Holocene using the 15N/14N ratio of skeletal-bound organic N (δ15N-skeletal organic material [SOM]) in modern and fossil Halimeda sediment cores. Average Halimeda skeletal δ15N-SOM was 6.28 ± 0.26‰, consistent with δ15N-NO3- from western tropical South Pacific (WTSP) thermocline waters. Thus, geochemically validating shelf-break upwelling of an oceanic N source that regulates bioherm spatial distribution. Halimeda δ15N-SOM decreased by 1‰–2‰ from 5,000 to 2,000 cal. yr BP, reaching a minima of 5.5‰ that persisted for almost 1,000 years. The Halimeda δ15N-SOM variation reflects mid- to late Holocene changes in regional climate and intensified El Nino activity that likely facilitated elevated N2 fixation in the WTSP, thereby lowering thermocline δ15N-NO3-. Thus, Halimeda skeletal material provides a valuable high-resolution geochemical archive of past oceanographic and climatic processes over centennial to millennial timescales, complementing existing paleoclimate proxy records.