Dating of shell fossils shows how shoreline changed during glacial-interglacial cycles

Specimens found 5-10 km inland were dated by electron spin resonance and served to refine the usual model, according to which the last time the sea level was higher than today was 120,000 years ago.

By José Tadeu Arantes | Agência FAPESP – On the coastal plain of Rio Grande do Sul, in Brazil’s South region, there is a privileged site from which to map the changes made to the shoreline by the rise and fall in sea levels during glacial-interglacial cycles. Its special nature is due to deposits of sediment produced by successive sea incursions, which have also created a vast system of lagoons in the area.

An investigation conducted by Renato Pereira Lopes, a researcher at the Federal University of Rio Grande do Sul (UFRGS), has refined the existing model by dating 35 marine shells of the bivalve Amiantis purpurata, found at depths of between 5 and 15 meters when boreholes were sunk in preparation for the installation of wind turbine towers, plus seven other shells collected from an irrigation channel. 

The shells were dated at the University of São Paulo’s Ribeirão Preto School of Philosophy, Sciences and Letters (FFCLRP-USP) by physicist Oswaldo Baffa and collaborators, with FAPESP’s support.

An article on the main findings of the study is published in the Journal of South American Earth Sciences.

“The datings to approximately 220,000 and 120,000 years ago matched the ages of the fossil-bearing deposits established by the usual model. The latter dating corresponds to the last time sea levels peaked, between 5 meters and 7 meters above the current level, owing to deglaciation. Given the low-gradient shore, each meter of elevation represents an inland intrusion of about 1 kilometer,” Lopes told Agência FAPESP.

“The presence of shells that are much older than the depositional systems shows that older marine deposits already existed on the coastal plain of Rio Grande do Sul and were completely eroded and churned over by subsequent marine transgressions. On the other hand, younger ages in the range of 100,000 years indicate that smaller sea level rises occurring later and not recognized hitherto also left geological and fossil records in the study area.”

Electron spin resonance (ESR) dating is not novel, but it is comparatively rarely used in geological research in Brazil. “Carbon-14 decay dating is more familiar, but works well only for specimens from up to 60,000 years ago. We used an alternative technique based on the effect of ionizing radiation on different materials,” Baffa said.

“Radiation produces ions, such as CO₂^-, SO₂^- and CO₃^3-, with one or more unpaired electrons. Each unpaired electron has a spin different from zero. By constructing a growth curve for the quantity of spins, we can determine the amount of radiation received and hence the amount of time the material was exposed to radiation.”

The article “Geological and taphonomic significance of electron spin resonance (ESR) ages of Middle-Late Pleistocene marine shells from barrier-lagoon systems of Southern Brazil” by Renato Pereira Lopes, Jamil Corrêa Pereira, Angela Kinoshita, Michelle Mollemberg, Fernando Barbosa Jr. and Oswaldo Baffa can be retrieved from: https://www.sciencedirect.com/science/article/abs/pii/S0895981120301188#!.