Kilwa Marine Geophysics Survey, 2014

I have just returned from Tanzania working with Dr Ted Pollard from the British Institute in Eastern Africa and Elgidius Ichumbaki from the University of Dar es Salaam. Ted (also an honorary researcher at St Andrews) has been researching coastal archaeological sites along the southern Tanzanian islands near World Heritage site of Kilwa for over 10years.  His studies have documented settlements and trade back to Middle Stone Age with extensive developments from the 14C and 15C based on gold trade  that originated in modern Zimbabwe. To date, most of his work has been focused around recording intertidal sites in relation to the changing geomorphology that will have dictated the land use.  He has shown how river channels and harbours have silted up, settlements abandoned both as a result of nature and from human pressure.  There is a fascinating story in this coast line and one that despite the obvious immediate differences (its hot and the water is warm) has many similarities to the work that many of us are conducting around the North Sea.  

Kilwa Fort, Kilwa World Heritage Site, Tanzania

So this year Ted wanted to investigate the sub-tidal part of the study area.  In order to do this I have brought out our SwathPlus 468kHz bathymetric sidescan.  This is the sonar that we have been using up in Orkney, Scotland (see earlier posts) to successfully map the submerged topography (the drowned landscape) but also using to map different types of biological habitat on the seafloor.  It was deployed with a TSS DMS205 motion reference unit, sound velocity probe, Vector Hemisphere GPS and Topcon RTK dGPS.  Unfortunately here I do not have the advantage of bringing a custom survey vessel so we are relying on the collaboration with the Tanzanian Antiquities Department.  Check the video clip to see how the survey went and get a feel for the place. The boat required a fair amount of impromptu rigging and a couple of visits to a local woodworking shop - here my demands on yokes to go around the keel of a boat were a welcome change from making bed frames I think.

Charles Okeny from BIEA surveying using the SwathPlus bathymetric sonar

Over the course of the two weeks we managed to get a fair amount of the inner harbours covered including some great detail around the old fort and palace. Water depths ranged down to 65m and from the mapping it appears that there are a number of submerged coral terraces plus some great sediment features. 

As with any geophysics, it is only as good as the ground truth and so there was nothing for it here but to jump into the water to get some.  Whereas I am usually a fan of putting down an ROV (remotely operated camera) there was just not the facilities to do this and so we hired kit from a local dive centre and had to do the honourable thing – diving around coral reefs can be a hardship, honest!

The final map of bathymetry showing a deep (>60m) channel with steep coral shelf breaks

Dr Ted Pollard 

The result of the day of targeted dives proved out many of the sonar targets and gave some valuable information on seafloor type.  It also gave us the first confirmation of archaeological significance with a large pottery scatter at one site.  Ted will now be following this up with careful analysis/identification of the pottery.

Hominin footprints at Happisburgh (written by Dr Martin Bates, University of Lampeter)

View from clifftop looking towards the area of footprints revealed by the coastal erosion.

Well it is out there now after nearly a year of sitting on the news. The announcement last week at the British Museum that the world’s oldest footprints (outside Africa) had been discovered in Norfolk has made the news across the world. Again the small settlement of Happisburgh is in the news for the exciting discoveries about our Prehistoric past. From the BBC news to the tabloid newspapers, web media sites to blogs and tweets (even an appearance on Radio 4 Thought for the Day) the evidence is now available to all to examine. Our scientific findings were published last week on PLOS ONE and this shows that a small group of children and adults walked across the mudflats some 900,000 years ago. But how were these traces discovered? It happened last May when we were undertaking work for the British Museum as part of their English Heritage funded work to map the channels associated with the archaeological remains that exist beneath the cliffs of sands and silts laid down by ice sheets some half a million years ago.

Dr Richard Bates surveying area of footprints. Note the cliffs in the background are deposits (sands and silts) left by ice following retreat of the ice sheet that buried the surface with the footprints. Today the erosion by the sea is removing the sands and silts to reveal the deposits containing the archaeology and footprints.

We have been working on this project for the last 2 years or so and it was in the downtime while we were waiting for the geophysical equipment to take its readings that we saw what appeared to be odd patterns in the silts deposited in a former tidal channel. This system was probably part of the mouth of the Thames at that time. These traces were strange, elongated shapes totally unlike any of the natural ripple marks and mud cracks that occurred on other surfaces at the site. In fact they appeared very similar to features known from Holocene (last 10,000 years) sites in the Severn Estuary that had been identified as human footprints. We pointed this out to the archaeologists on the project and quickly a team was mobilised over the next couple of weeks to visit the beach to record these features and sample them for analysis. It is unlikely that these prints are the only ones preserved in the area and as the coast retreats further it is likely that more such surfaces will be exposed so the hunt is now on for more. Surprisingly there are no animal prints mixed in with the humans despite the fact the bones recovered from the site show a range of different animals were present with the humans. It has been hard to keep quiet about these discoveries for nearly 12 months knowing their importance but scientific rigor has to take precedent and that before announcing to the world their presence we had to be sure that these really were ancient footprints. Now we have the data and hopefully more will come to light when conditions are right to reveal them.

 

Close up detail of individual footprints at Happisburgh.

Chew Bahir, ERT 2013

I am just about to leave for Tanzania to do some offshore archaeological surveying with Ted Pollard from BIEA (I will be blogging on this on my return) but I thought before I went I would post a short video that was compiled during a field campaign late last year to the Rift Valley in southern Ethiopia. 

I made this trip together with Dei Huws (Bangor Uni) and Tigistu Haile (Addis Ababa Uni) as part of a much larger, international project that is drilling a number of locations throughout East Africa that are linked to key Hominid sites.  The overall aim of this is to be able to provide dated information about environment and climate at key stages of hominin development and migration.

The Chew Bahir site is located in SW Ethiopia close to the boarder with Kenya.  The large valley generally drains to the south into Lake Stephanie but the northern part where we hope to drill our core is only flooded on an infrequent basis.  As with many of arid areas, when it rains it does so often causing drastic and violent floods from the surrounding uplands that wash coarse river gravels far out into the generally flat valley floor.  Since our drilling objective is to obtain as simple a geologic sequence as possible the first task is to locate an appropriate site that has not been affected by the alluvial fans on the valley side or any of the ancestral rivers that migrate across the valley bottom. 

So, in November we made a preliminary field visit to the site armed with magnetic and electrical resistivity tomography/imaging (ERT) equipment in order to find appropriate sites.  Our choice of locations for the geophysics was based on some regional background work on geophysical signatures and also on some local knowledge kindly shared by Tullow Oil Plc.

The site is quite special and like nowhere I have worked before as you can see from the video.  We managed a couple of sites during our survey, one in the centre of the basin, the preferred core location, and one over an alluvial fan sequence at the edge of the basin.  The ERT gave some great results despite the arid conditions mainly I think because of the relatively high conductivity of the lake sediments – likely a result of the periodic wetting and drying. The alluvial fan was readily recognised in the ERT sections at the basin edge but no signatures of this were recorded at the centre of the basin where a fairly uniform sequence was modelled.

The next stage from us it to return to do a seismic reflection survey in order to image the top 400m (the core depth) in much greater detail.