It is clear that sea levels were higher around the world in the last warm period, some 100,000 years ago. However, fine details of the rise and fall of sea levels are rarely discernable in the geological record.
This winter researchers at Federal University of Rio Grande do Sul (Brazil) report an interesting case where detailed records are available . The site has a shallow incline that has been the site of coastal lagoons since the Pleistocene. As the sea level changed, the lagoons moved inland and out, leaving characteristic sediments filled with fossils of marine shellfish.
Different species flourish in water of specific depths, so the different layers indicate how deep the water was at that location at that time. The study focused on Amiantis purpurata, a bivalve that lives in shallow water. Their presence in sediments indicates the location was shallow water at the time the animal lived.
To fill in the picture, the researchers use electron spin resonance (ESR) dating. I’m not familiar with this technique (it emerged after I left my Anthro studies), but I gather that it the measurement detects the effects of radiation on electrons. In geological materials, these levels can be used to infer the age of the material. ESR has become widely used in paleontology because it can date tooth enamel or other fossils with a range of 10,000 to 300,000 years .
From the ESR data and stratigraphy, the researchers constructed a timeline of the coastline 100,000 to 300,000 years ago, i.e., how deep the water was at various geolocations over the period.
The most recent high sea level was about 10,000 to 20,000 years ago. This study finds a high water mark (“highstand”) of about 7 meters above today’s sea level at about 120,000 years ago, corresponding to a warming period. Much evidence of this 120 ka event was erased by the later ris . There were earlier high sea levels, but the evidence has been eroded even at this favorable location in Brazil.
The important point is that the ESR data was able to identify sea levels as far back as 120 ka, including possible smaller events not previously documented.
These findings are basically consistent with other evidence. The researchers argue that this study shows that fossil shells can survive 100,000 years, which is roughly the length of the high-low sea level changes in recent times. They also emphasize that this kind of evidence needs to be cross checked with other evidence. ESR dating itself is pretty finnicky, and, of course, fossil seashells might have been buried or uncovered in the intervening centuries.
- José Tadeu Arantes, Dating of shell fossils shows how shoreline changed during glacial-interglacial cycles, in Agência FAPESP – News, January 13, 2021. https://agencia.fapesp.br/dating-of-shell-fossils-shows-how-shoreline-changed-during-glacial-interglacial-cycles/34978/
- Rainer Grün, Electron spin resonance dating in paleoanthropology. Evolutionary Anthropology: Issues, News, and Reviews, 2 (5):172-181, 1993/01/01 1993. https://doi.org/10.1002/evan.1360020504
- Renato Pereira Lopes, Jamil Corrêa Pereira, Angela Kinoshita, Michelle Mollemberg, Fernando Barbosa, and Oswaldo Baffa, Geological and taphonomic significance of electron spin resonance (ESR) ages of Middle-Late Pleistocene marine shells from barrier-lagoon systems of Southern Brazil. Journal of South American Earth Sciences, 101:102605, 2020/08/01/ 2020. http://www.sciencedirect.com/science/article/pii/S0895981120301188