Skip to 0 minutes and 6 seconds JUSTIN DIX: Hi. My name is Dr. Justin Dix. And today, I’m going to give you an example of looking at a submerged landscape with marine geophysics. In many times, we would be wanting to look beneath the sea bed surface and use a certain set of techniques, but sometimes we will actually get landscapes exposed actually on the sea bed. Therefore, we can use conventional bathymetry, so just measuring the depths beneath the water’s surface, to actually image these ancient landscapes. And we get a lovely example of that within the Bristol Channel. Here the area is generally what we call sediment starved so there’s lots and lots of sediment that has been deposited since marine transgression. And therefore, we can see ancient remnants.
Skip to 0 minutes and 43 seconds In this case, the paleo Severn– a river system that runs all the way down the current Severn Estuary and through the Bristol Channel. It has many tributaries within it. And we can really reconstruct a very ancient landscape just from this surface data. There are elements of deposited sediments within it that we can use to date it. And we know that this land surface was last exposed about 10,000 years ago. So potentially, Mesolithic hunter- gatherers could occupy this terrain. This is a marine geophysical example of a submerged landscape just a little bit more typical. It’s from the Thames estuary in the southeast coast of England. And what we’re looking at is an ancient land surface here.
Skip to 1 minute and 21 seconds Again, a river system that’s running across the sea bed that we can actually see within the bathymetry in this particular package. This time though this wasn’t just active during the last lowstand, about 10,000 years ago. This has been reoccupied multiple times probably over the last 1 million years. But what’s interesting here is it disappears underneath a modern sandbank that’s built up over the last 6,000 or 7,000 years. Therefore, we need to use high-resolution sub-bottom profiling to be able to see beneath the sea bed and actually be able to trace the channel system. And most importantly, the very fine deposits that settle within it during both this period of time it was active and also during the transgression itself.
Skip to 2 minutes and 6 seconds So if we switch the swath bathymetry off, we can actually see the seismic lines that are penetrating beneath the sea bed surface underneath the actual sandbank. If we look at a window just focused on one of these seismic lines and we zoom in, we can actually see the ancient paleo channel buried beneath the actual sandbank itself and preserving very fine grain sediments, including potentially organics and peat, which can give us lots of information about the environment. It can give us a date. And in really exceptional circumstances it can actually preserve archaeological materials.
Marine geophysics for submerged landscape investigations
This video gives two contrasting examples of submerged landscapes imaged via a range of marine geophysical instruments.
The first is from the Bristol Channel, a system starved of sediment so ancient pre-transgression (sea-level flooding) land surfaces are still visible on the sea bed. Consequently, they can be imaged by swath bathymetry and other high resolution bathymetric techniques to allow us to produce an accurate terrain model of the area. The regional model shown in the video is actually produced by integrating a range of datasets (work done by the UKHO) to give a map at a 20 x 30 m bin resolution. In places higher resolution datasets (swath), as well as some sub-bottom data and cores, have enabled more detailed study of some of the buried sections of this land surface. Burial has resulted in preserved fine grained deposits, including peats, which has not only enabled us to date when this land surface was last exposed (just under 10,000 years ago), but also to provide a more detailed understanding of the palaeo-environment and even some potential indicators of human activity.
The second example is from the Thames estuary, where a large scale river system has been buried underneath a large sandbank. Here very high spatial resolution, swath bathymetry and sub-bottom profile (boomer) data has been acquired, as part of a large windfarm project, and from which a full 3D reconstruction of the channel system and the deposits it contains can be made. This system has again been cored, and sediments dated, but in this case the channels have evolved over multiple lowstands (periods of time when sea level was lower) over almost the last million years. These landscapes are therefore potentially contiguous with the deposits that contain the oldest evidence of hominin occupation in the British Isles (indeed North of the Alps) which are found on the adjacent Suffolk and Norfolk coast.
© University of Southampton, 2017