Skip to 0 minutes and 18 seconds BASTIEN LINOL: We’re here in southern South Africa, north of Port Elizabeth, on the site of Grassridge, where some fossilised oysters, their age is based on the fossil, Plio-Pleistocene, that’s about 1 to 5 million years old. The elevation is around 235 metre above present day 0. So the question we have is how did the sea came at this level? Did sea level rise, or did the continent uplifted, or what is the balance between the two?
Skip to 0 minutes and 55 seconds This oyster bed raised some oyster that are in place, so they have been fossilising in living position, such as this one. Where the two valves of the oysters are still together and showing up. So this indicate that the oyster has been fossilising in the living position– such as this one. So there is the two valves. The larger one is the bottom one, and then the final one go on top. So the animal was living here. And what’s characterised many of the fossils is the shells are very thick and large. And they are three, five– up to five centimetre thick. And that suggests that oyster was living under stress condition.
Skip to 1 minute and 43 seconds And it has to produce a lot of calcium carbonate, and more during at– more during the day, and this of less during the night. And then in between the shells there is this fine, muddy, sandy material that suggest some very low energy environment. This oyster has the two valves still in place. And on the valve and on many of the fossil, we can see there is no trace of epibiont of animal boring the shell. The shells are very thick. Here are the growing layers of the– in the fossils. And in between the matrix is this greenish, sandy mud that indicate an environment with low energy, such as an estuary.
Skip to 2 minutes and 36 seconds This is the Sundays’ River marsh. This is where we found the modern oyster, such as this one. They live in this muddy environment, in the estuary, in a place where the fresh water from the Sundays River meet with saline water from the ocean, and where we have a great volubility and salinity.
Skip to 3 minutes and 3 seconds [SOUND OF MACHINE RUNNING]
Skip to 3 minutes and 26 seconds We’re in the laboratory looking at the oyster fossil in the microscope to analyse its heterogeneities, the composition of the material with which we would be able to determine what was the condition in which the oyster were living at the time– the paleosalinity, for example, and the paleotemperature of the water.
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 models.
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 (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. 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 next phase of Bastien’s research involves a world-class and state of the art facility; the Centre for High Resolution Transmission Electron Microscopy at Nelson Mandela University. This work aims to identify which part of the fossil material is unaltered and thus most robust to date.
The new data indicate at least two episodes of marine transgression (where sea level rises relative to the land), during the Oligocene-Miocene (33.9 to 5 million years ago), and again during the Pliocene-Pleistocene (5 million years to 12,000 years ago).
All of this aside, 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 (5-1 Ma), 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 up to 400 m. Such large fluctuations in sea level change must have impacted on early human occupation.
© University of Southampton 2018