Oral Presentation Australian Society for Fish Biology Conference 2017

Fish ear bones record the bomb radiocarbon decline in an upwelling area off southern Australia (#62)

Gretchen L. Grammer 1 2 , Stewart J. Fallon 3 , Peter J. Hawthorne 1 , Chris Izzo 2 4 , Rachel Wood 3 , Jonathan Carroll 1 , Bronwyn M. Gillanders 2
  1. SARDI Aquatic Sciences, West Beach, SA, Australia
  2. School of Biological Sciences, University of Adelaide, Adelaide, SA, Australia
  3. Radiocarbon Facility, Research School of Earth Sciences,, The Australian National University, Canberra, ACT, Australia
  4. Fisheries Research and Development Corporation (FRDC), Adelaide, SA, Australia

Biogenic carbonates are successfully used to track radiocarbon (Δ14C) evolution through time, and we use otoliths (ear bones) of fish to establish a new record of the bomb Δ14C decline (since ca. 1980 to the present) in an upwelling area of the southeastern Indian Ocean. Marine surface waters were enriched with 14C during the 1950-60s when thermonuclear weapons testing was at its zenith (bomb Δ14C). Upwelling events ventilate ocean surface waters with subsurface water lower in radiocarbon Δ14C through a wind-driven, oceanographic process. We assayed otolith carbonate microsampled from ocean perch (Helicolenus percoides) for 14C with accelerator mass spectrometry. The ocean perch otolith Δ14C record extended from 1994 to 2012 and displayed a general decline of Δ14C levels through time (mean Δ14C 1994: 75.04‰; mean 2012: 7.57‰). Water depths represented by the otolith records ranged from 42.9 to 118.9 m, and age-based depth segregation between juvenile (<6 yrs old) and adult fish was not evident. Ocean perch Δ14C values were lower (62.69 to 82.53‰) than reported seawater values (90.1 to 95.5‰) in depths <200 m in adjacent non-upwelling areas. Comparisons of ocean perch D14C with published Indo-Pacific coral records showed the ocean perch Δ14C record was lower than Δ14C values in coral of tropical surface water, but had higher levels than coral in areas of equatorial upwelling. The ocean perch Δ14C record characterises temperate marine waters well mixed throughout the year by both upwelling and downwelling and with sources of upwelled waters being formed at higher latitudes. Documenting levels of Δ14C in the marine environment along the coast of southern Australia has improved existing 14C data relating to ocean circulation in the southeastern Indian Ocean.