Skip to 0 minutes and 18 secondsBASTIEN 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 secondsThis 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 secondsAnd 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 secondsThis 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 secondsWe'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 land 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 current sea level and explains part of his research study 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. However, the exact amplitude of these sea-level changes is uncertain and in reality a discrepancy of about 200 m exists between different models.
In the research study, strontium isotopes were used to date fossils from marine environments preserved at 150 to 350 m above present-day sea level along the southeast coast of southern Africa. Strontium isotopes in the ocean change through time and the fossils form recording the isotope composition of the time of burial. We can therefore tell whether a fossil is 1 Million years old, 5 Million years old or even 50 million years old.
Although various fossils such as oysters, shark teeth, sea urchins and corals have been found at many sites, the Grassridge locality in the video is special as oysters are dominant and very little else has been found. The oysters are often found in life position, with thick shells that indicate a life lived in stressful environmental conditions.
Pristine fossils are collected and taken back to the laboratories at Nelson Mandela University for processing and further study. Some mass spectrometry analysis was undertaken at the MIT Radiogenic Isotope Laboratory which date the oysterbed at Grassridge as being 5 to 1 Million years old.
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 fossil material is unaltered since burial and which parts have been laid down since burial and have a secondary signature that allows us to identify sea water flooding associated with rapid sea level rise.
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).
We can safely assume that the landmass of southern Africa has not moved upwards in this time, therefore this study suggests maximum amplitudes of sea-level change of up to 450 m between 5 Million years and the last 100 thousand years. This is a period when humans emerged in the fossil record and humans first colonised the region and such large fluctuations in sea level change must have impacted on early human occupation.
© University of Southampton 2017