Age of Paleo-otoliths - Bomb Radiocarbon

Mid-Holocene fringing coral reefs - excavations for assemblage assays

Tuesday, December 31, 2019

Reconstructing reef fish communities using 

Abstract: Little is known about long-term changes in coral reef fish communities. Here we present a new technique that leverages fish otoliths in reef sediments to reconstruct coral reef fish communities. We found over 5,400 otoliths in 169 modern and mid-Holocene bulk samples from Caribbean Panama and Dominican Republic mid-Holocene and modern reefs, demonstrating otoliths are abundant in reef sediments. With a specially-built reference collection, we were able to assign over 4,400 otoliths to one of 56 taxa (35 families) though mostly at genus and family level. Many otoliths were from juvenile fishes for which identification is challenging. Richness (by rarefaction) of otolith assemblages was slightly higher in modern than mid-Holocene reefs, but further analyses are required to elucidate the underlying causes. We compared the living fish communities, sampled using icthyocide, with the sediment otolith assemblages on four reefs finding the otolith assemblages faithfully capture the general composition of the living fish communities. Radiocarbon dating performed directly on the otoliths suggests that relatively little mixing of sediment layers particularly on actively accreting branching coral reefs. All otolith assemblages were strongly dominated by small, fast-turnover fish taxa and juvenile individuals, and our exploration on taxonomy, functional ecology and taphonomy lead us to the conclusion that intense predation is likely the most important process for otolith accumulation in reef sediments. We conclude that otolith assemblages in modern and fossil reef sediments can provide a powerful tool to explore ecological changes in reef fish communities over time and space. 

Plot of the radiocarbon (14C) reference records available from hermatypic corals in the Caribbean Sea (Loess curve fit) with otolith 14C data from sediment cores plotted on the correlated dates of formation. For most measurements the values crossed the regional 14C reference curve with potential dates from the bomb 14C rise or decline periods, with the exception of two samples (S1 Table). These fish were a short-lived species (gobies, Gobiidae) and posited to cover a negligible age (<2 years of growth). Dates of formation for rise and decline periods were from a simple linear regression of coral 14C values that were coincident with the Loess curve for each time span. The outliers were one 14C value that is centered on the peak at 1976 and the other is a pre-bomb otolith that was aged to median year of formation of 1550 AD (2 sigma = 1490–1640; CALIB Rev7.1.0 and see S2 Text).