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A puzzle from the past

How do we study prehistoric changes in sea level? Watch Dr Bastien Linol explain a puzzle from South Africa's geological history.
BASTIEN LINOL: We’re here in southern South Africa, north of Port Elizabeth, on the side of Grassridge. There are 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 zero. So the question we have is, did the sea come at this level? Did sea level rise, or did the continent uplifted, or what is the balance between the two?
This oyster bed, there is some oyster that are in place, so they have been fossilised 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 fossilised in living position. Such as this one. So there is the two valves. The larger one is a bottom one, and then a thinner 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 up to 5 centimetres thick.
And that suggests that oyster was living under stress condition, and it has to produce a lot of calcium carbonate– more during the day, and dissolve less during the night. This oyster has the two valves still in place. And on the valve– and on many of the fossils– we see there is no trace of epibiont, of animal boring the shell. Here are the growing layers of the fossils, and in between the matrix is this greenish sandy mud that indicate an environment with low energy, such as an estuary.
This is a Sundays River [INAUDIBLE].. This is where we found the modern oysters 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 the saline water from the ocean, and where we have a great variability in salinity.
We’re in the laboratory looking at the oyster fossil in the microscope to analyse its heterogeneity, the composition of the material with which we will be able to determine what was the condition in which the oyster were living at the time. The paleo salinity, for example, and the paleo temperature of the water.

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.

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Exploring Our Ocean

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