Conference proceeding
A 5,000 year record of Holocene nitrate source to the Great Barrier Reef lagoon, using the isotopic composition of calcareous macroalgae Halimeda
American Geophysical Union Fall Meeting, Vol.2019
American Geophysical Union 2019 fall meeting (San Francisco, California, 09/12/2019 - 13/12/2019)
12/2019
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Abstract
The northern Great Barrier Reef (GBR) Halimeda bioherms have accumulated on the outer continental shelf from calcium carbonate algal sediments since approximately 10,000 years BP and cover 6095 km (super 2) 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 NO (sub 3) - 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 NO (sub 3) - 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 (delta (super 15) N) of skeletal bound organic NO (sub 3) - in the Halimeda CaCO (sub 3) crystal lattice, using the persulfate/denitrifier method developed for foraminifera and coral investigations. Four cores were sampled at approximately 50 cm intervals (n=44) and coupled with (super 14) C AMS radiocarbon dating and ICP-OES trace element chemistry. The average skeletal delta (super 15) N isotopic composition was 6.28 ppm, consistent with the delta (super 15) N 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 NO (sub 3) - delivery to Halimeda bioherms. We also observed an approximately 2 ppm decrease in delta (super 15) N 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.
Details
- Title
- A 5,000 year record of Holocene nitrate source to the Great Barrier Reef lagoon, using the isotopic composition of calcareous macroalgae Halimeda
- Creators
- Jody Webster - University of Sydney, Geocoastal Research Group, School of Geosciences Sydney, N.S.W. AUS AustraliaMardi McNeilLuke D NothdurftDirk ErlerQuan HuaAnonymous
- Publication Details
- American Geophysical Union Fall Meeting, Vol.2019
- Conference
- American Geophysical Union 2019 fall meeting (San Francisco, California, 09/12/2019 - 13/12/2019)
- Publisher
- American Geophysical Union
- Identifiers
- 991012978868102368
- Academic Unit
- Centre for Coastal Biogeochemistry; Faculty of Science and Engineering; Science
- Language
- English
- Resource Type
- Conference proceeding