Premium
Variations in Mid‐ to Late Holocene Nitrogen Supply to Northern Great Barrier Reef Halimeda Macroalgal Bioherms
Author(s) -
McNeil Mardi,
Nothdurft Luke,
Erler Dirk,
Hua Quan,
Webster Jody M.
Publication year - 2021
Publication title -
paleoceanography and paleoclimatology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.927
H-Index - 127
eISSN - 2572-4525
pISSN - 2572-4517
DOI - 10.1029/2020pa003871
Subject(s) - halimeda , oceanography , holocene , geology , paleoceanography , upwelling , reef
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 km 2 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 NO 3 ‐ ‐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 15 N/ 14 N ratio of skeletal‐bound organic N (δ 15 N‐skeletal organic material [SOM]) in modern and fossil Halimeda sediment cores. Average Halimeda skeletal δ 15 N‐SOM was 6.28 ± 0.26‰, consistent with δ 15 N‐NO 3 ‐ 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 δ 15 N‐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 δ 15 N‐SOM variation reflects mid‐ to late Holocene changes in regional climate and intensified El Niño activity that likely facilitated elevated N 2 fixation in the WTSP, thereby lowering thermocline δ 15 N‐NO 3 ‐ . 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.