A 5,000 year record of Holocene nitrate source to the Great Barrier Reef lagoon, using the isotopic composition of calcareous macroalgae Halimeda

The northern Great Barrier Reef (GBR) Halimeda bioherms have accumulated on the outer continental shelf from calcium carbonate algal sediments since ~10,000 years BP and cover 6095 km² of shelf area. As such, Halimeda bioherms play a key role in the shallow marine carbon cycle over millennial timescales, yet the biogeochemical processes regulating bioherm formation and distribution are poorly understood. The main source of NO₃- to these bioherms is thought to be westward transport of upwelled deep ocean water from the Coral Sea intruding onto the continental shelf through narrow inter-reef passages. Upwelling and intrusion has been demonstrated in oceanographic field and modelling experiments, but a direct geochemical connection between the NO₃- signature in the water column, and assimilation by Halimeda biomass has not been established. Here we present the first geochemical evidence of the source of nitrogen supply to Halimeda bioherms, allowing us to better understand the drivers of bioherm formation and distribution in the northern Great Barrier Reef (GBR).

From Halimeda bioherm sediment cores collected from the outer continental shelf, we measured the isotopic composition (δ¹⁵N) of skeletal bound organic NO₃- in the Halimeda CaCO₃ crystal lattice, using the persulfate/denitrifier method developed for foraminifera and coral investigations. Four cores were sampled at ~50 cm intervals (n=44) and coupled with ¹⁴C AMS radiocarbon dating and ICP-OES trace element chemistry. The average skeletal δ¹⁵N isotopic composition was 6.28 ‰, consistent with the δ¹⁵N of deep ocean nitrate in the Coral Sea. This finding provides the first direct geochemical evidence in support of cool-water, nutrient-rich upwelling as the likely mechanism for NO₃- delivery to Halimeda bioherms. We also observed an ~2 ‰ decrease in δ¹⁵N from 5000 to 1000 years ago, suggesting an increase in availability of upwelled nutrients. We conclude that 1) upwelling and intrusion onto the shelf controls bioherm distribution in the GBR, 2) the frequency and magnitude of upwelling has varied over the past 5000 years, and 3) Halimeda bioherms provide a valuable high-resolution proxy archive of past oceanographic processes.