Hot off the press! Variations in Mid‐ to Late Holocene Nitrogen Supply to Northern Great Barrier Reef Halimeda Macroalgal Bioherms

McNeil et al. 2021 – Paleoceangraphy and Paleoclimatology.

Please join me in congratulating GRG PhD student Mardi McNeil (based at Queensland University of Technology in Brisbane) and the team on the publication of our new paper in the journal Paleoceangraphy and Paleoclimatology. The paper represents another wonderful contribution to the Project “HALO” (HALimeda bioherm Origins, function and fate in the northern Great Barrier Reef) – our wider research program focused on the Halimeda Bioherms and inter-reef areas of the Great Barrier Reef.

McNeil, M., Nothdurft, L., Erler, D., Hua, Q., & Webster, J. M. (2021). Variations in mid- to Late Holocene nitrogen supply to northern Great Barrier Reef Halimeda macroalgal bioherms. Paleoceanography and Paleoclimatology, 36(2), e2020PA003871. doi:

Nitrogen isotope records from Halimeda algal bioherms in the northern Great Barrier Reef have recorded a 5000-year history of nutrient supply and climatic change since the mid-Holocene to present.

The study analysed the ratio of 15N/14N of organic nitrogen bound within Halimeda skeletal segment grains. Halimeda is a calcareous green macroalgae that is a prolific carbonate sediment producer in tropical reef environments. The Great Barrier Reef Halimeda bioherms accumulate on the low-energy inter-reef seafloor, just westward (landward) of the outer barrier coral reefs. These carbonate build-ups initiated off a Pleistocene antecedent substrate, so Halimeda bioherm vibrocores preserve a sedimentary record of Holocene deposition and environmental change.

The nitrogen isotope values for inter-reef Halimeda bioherms were enriched compared to typical shallow coral reef Halimeda. This record provides a geochemical fingerprint that the main nutrient supply to the inter-reef bioherms comes from an oceanic thermocline source in the western tropical South Pacific Ocean.

Over the 5000-year Halimeda nitrogen isotope record, we found a consistent pattern of variation across the four cores analysed in the study. The Halimeda record corresponds with an almost 1000-year period of intensified El Niño activity and reduced monsoonal precipitation that is evident in other climate proxy records from the northern Australian region in the mid- to late Holocene.

Key Points from the paper:

  • Well-preserved fossil Halimeda is a valuable geochemical proxy archive of millennial-scale oceanographic and climatic processes
  • Nitrogen (N) supply to Halimeda bioherms originates from western tropical South Pacific thermocline waters since at least the past 5,000 years
  • Halimeda 15N signature records a 1‰–2‰ change in δ15N-skeletal organic material (SOM) concurrent with regional climate variation and the El Niño Southern Oscillation

This paper represents a really fundamental contribution as it demonstrates for the first time how these amazing Halimeda bioherms can be used as paleoceanographic proxies. Bravo Mardi!!




Image source: Emma Kennedy (UQ)

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