Bomb Radiocarbon - Karst and Otoliths
Karst Complications - Floridan Aquifer and
Ancient Carbonates
Published 2020 in Radiocarbon
Influences of Upper Floridan aquifer waters on radiocarbon in the otoliths of gray snapper (Lutjanus griseus) in the
Gulf of Mexico
The otoliths (ear stones) of fishes are commonly used to describe the age and growth of marine and freshwater fishes. These non-skeletal structures are fortuitous in their utility by being composed of mostly inorganic carbonate that is inert through the life of the fish. This conserved record functions like an environmental chronometer and bomb-produced radiocarbon (14C) — a 14C signal created by atmospheric testing of thermonuclear devices — can be used as a time-specific marker in validating fish age. However, complications from the hydrogeology of nearshore marine environments can complicate 14C levels, as was the case with gray snapper (Lutjanus griseus) along the Gulf of Mexico coast of Florida. Radiocarbon of these nearshore waters is influenced by freshwater input from the karst topography of the Upper Floridan Aquifer — estuarine waters that are 14C-depleted from surface and groundwater inputs. Some gray snapper likely recruited to this kind of environment where 14C levels were depleted in the earliest otolith growth, although age was validated for individuals that were not exposed to 14C-depleted waters. Hence, use of otoliths to determine radiocarbon dates for historic human occupation from midden (refuse) sites must consider the life history of the fish species and regional hydrogeology.
Gray snapper (Lutjanus griseus) is a common sportfish in the Gulf of Mexico.
Gray snapper (Lutjanus griseus) otolith sections viewed with transmitted light for specimens GS-01 (panel A) and GS-08 (panel B). Growth zone structure is well defined for this species and can be age validated using bomb 14C dating (extraction of the core region with micromill in each section image), but the measured 14C values can be complicated by estuarine waters during the early life history of some gray snapper. GS-01 was consistently aged to 18 ± 1 years and was well supported from core 14C values. GS-08 was consistently aged to 31 ± 1 years but was not well supported from the initial core 14C value (14C-depleted). A second extraction at year-2 (a partial ring that is not visible in the section image) provided a 14C value near expected bomb 14C levels, indicating a habitat change during the first 2 years of life (estuary to marine). All extractions were performed with New Wave Research micromilling machine (Elemental Scientific Lasers, Bozeman, MT, USA; http://www.nwrlasers.com/milling/micromill/).
The Upper Floridan Aquifer feeds ancient freshwaters to the estuarine habitats of the Gulf of Mexico. These areas are settlement habitat of gray snapper. As a result, radiocarbon in otoliths can be affected by 14C-depleted waters from the karst topography.
Published: https://doi.org/10.1017/RDC.2020.29
