A puzzle from the past

How can you tell if the sea level has fallen or if the continent has risen? How precise are measurements of prehistoric sea level?

In this video Dr Bastien Linol, a geologist from the Africa Earth Observation Network at Nelson Mandela University, explores a fossil oyster bed which is 250 metres above present-day sea level and explains part of his research which aims to answer these two questions. You can explore an interactive 360 image of Grassridge for yourself if you wish to.

Global sea level is known to have fluctuated by several hundreds of meters over the past 100 million years as indicated by marine fossils preserved at various elevations on the continents. However, the exact amplitude of these sea-level changes is uncertain and, in reality, a discrepancy of about 200 m exists between different estimates by different scientists.

Here, strontium isotope stratigraphy is used to date fossils from marine environments preserved at relatively high elevations (150 to 350 m above present-day sea level) along the southern coast of South Africa. The strontium isotope composition of the ocean is known to have changed through time, as measured from marine fossils (foraminifera, shark teeth, sea urchins, corals etc…) of different ages that record the strontium isotope composition of the seawater in which they were living. Using the global curve of strontium in the ocean through time, we can therefore tell what is the age of an unknown marine fossil. We can tell whether a fossil is 1 Million years old, 5 Million years old or even 50 million years old.

The Grassridge locality in the video is special because oysters are largely predominant and very few other fossil types have been found. Moreover in this outcrop some of the oysters are preserved in living position, which indicate we find them under in-situ conditions.

Pristine fossils are collected and taken back to the lab. Bastien uses high-resolution Transmission Electron Microscopy to identify which part of the fossil material is unaltered and thus most robust to date. Mass spectrometry analyses were undertaken at the MIT Radiogenic Isotope Laboratory. The strontium isotope results date the oysters bed at Grassridge being 1 to 5 million years old, depending on the amount of possible subsequent contamination by fresh water.

The new data indicate at least two episodes of marine transgression (where sea level rises relative to the land) in the record.

If we assume that the subcontinent of southern Africa has not moved up or down during the period between which the oysters were deposited at Grassridge (1 to 5 million years ago), and when some of the first prehistoric humans colonized the now submerged continental shelf south of South Africa, (150 m below present-day sea level for the last 100 thousand years) this study suggests maximum amplitudes of sea-level change of several 100 m. Such large fluctuations in sea level change must have significantly impacted on early human occupation.