Identifying sunken slave ships with integrated maritime archaeology and geoscience
Associate Professor Richard Palin
Department of Earth Sciences, University of Oxford, Oxford, United Kingdom
Dr. David Conlin
National Park Service, Submerged Resources Center, Lakewood, Colorado, USA
An international collaboration
On January 8, 1455 Pope Nicolas V signed a Papal Bull granting Portugal the right to enslave any and all people they encountered south of Cape Bojar, on the northwest coast of Africa. For the next 450 years, a vast transnational industry encompassing most of the major European powers engaged in the shameful practice of human kidnapping and trafficking on an industrial scale—it was an industry that shaped the modern world. Between 1514 and 1866, more than 36,000 maritime voyages dedicated to the kidnapping of Africans occurred worldwide, hundreds of these voyages ended in shipwreck and catastrophe[1]. Little has been done to document these sites of painful history, and few have been excavated or documented to proper scientific standards.
Since 2015, an international network of researchers underwritten by the Slave Wrecks Project[2], have been searching for shipwrecks associated with the global slave trade in the waters of Africa. While the popular media often depicts shipwrecks that are definitively identified by one or two key artefacts, in reality, the true process of identification may rest upon a larger accumulation of circumstantial evidence. To make a convincing argument regarding the date, cultural affiliation and purpose of a shipwreck is an often arduous and frustrating task that draws from many different fields. At the University of Oxford, UK the Department of Earth Sciences has joined the quest to bring these wrecks, and their history back into memory with the application of cutting-edge geological science applied in novel ways.
Over the past nine months, Dr Richard Palin and colleagues at the Department of Earth Sciences, conducted research on six ballast stones recovered by archaeologists from the Slave Wrecks Project from a shipwreck in Africa in order to determine their geological provenance. Ballast stones are added and removed as ships complete different legs of their voyages and, with proper historical background, it is sometimes possible to correlate these physical specimens with locations noted in historical accounts of ship’s voyages. Deducing the geological history of the ballast stones recovered from the underwater wreck therefore offers a unique opportunity to determine where a ship had visited prior to sinking and is an important piece of circumstantial evidence regarding the potential identity of a shipwreck.
Analysing the ballast stones
The ballast stones collected by the team were made of basalt; a dark-coloured igneous rock that forms primarily on Earth today at mid-ocean ridges, where tectonic plates slowly spread apart, but also erupts from volcanoes that lie far away from plate boundaries, such as the Hawaiian Islands in the central Pacific Ocean. In the latter case, volcanism at the Earth’s surface is driven by rising columns of extremely hot mantle impinging on the base of the crust – commonly referred to as mantle plumes, or ‘hot spots’. Crucially, basalts that form in mid-ocean ridge and intraplate tectonic settings have distinctly different geological characteristics, especially in terms of their mineral contents, chemical compositions, and isotopic profiles.
“The critical step in working out whether the shipwreck discovered off the coast of Africa could be historically linked to a vessel involved in the trade of enslaved people was to determine whether the ballast stones had geological affinity to the island of Mauritius” says Richard. “Mauritius is part of the Mascarene Islands, which formed due to magmatism associated with the Réunion hotspot, which itself has been active for over 65 million years.” The mineralogical, geochemical, and isotopic characteristics of basalts erupted on the islands of Mauritius, Réunion, and Rodrigues are well documented in the scientific literature, and so Richard and his team collected equivalent data from the ballast stones to make comparisons. “We looked at the mineral contents, the major, minor, and trace element concentrations, and strontium and neodymium stable isotope ratios. This involved a wide variety of laboratory analyses, including making very thin slices of the ballast stones and viewing them under a polarising microscope, and crushing portions of the samples to a powder that was then analysed via mass spectrometry. The data were collected mostly here at Oxford, although a collaborator in Germany performed some of the geochemical work. I also consulted some of my colleagues around the UK to help with the data analysis and interpretation – it was truly a team effort!”
Archaeologists of the Slave Wrecks Project recovering ballast stones for analysis at University of Oxford’s Earth Sciences Laboratory
Did the team identify the ship?
Following data collection and comparison with published reference values, can the team now confirm that this wreck is one sought by the Slave Wrecks Project? “It’s not absolutely certain”, says Richard “but, that’s a common situation in geoscience – we often have to deal with incomplete datasets, and interpretations may be non-unique. Six ballast stones collected by the SWP were analysed, and three have strong petrological, geochemical, and isotopic similarities to basalt documented from Mauritius, so I would argue with confidence that they were collected there by a ship prior to its final voyage. However, two of the stones show characteristics that better match those found in basalts from Réunion, and one sample seems to be a complete wildcard, having isotopic characteristics that suggest formation above a subduction zone. This doesn’t fit the geological profile of the Mascarene Islands at all, unfortunately!” Even if all the ballast stones were from Mauritius, there still is the possibility that a ship currently under investigation that was not historically involved in the trade of enslaved people, but that followed a similar path, collected ballast from there as well. Barring an exceptional find from future archaeological excavations, the most likely result is the ship, and its history, will be identified from a preponderance of circumstantial evidence—in this, Oxford’s contribution to identifying the provenance of the basalts will be key.
The team is very happy with these results. Independent research performed on wood and lead recovered from the wreck will bring in additional lines of evidence, and hopefully they will all converge on a positive identification. As Dr. David Conlin, one of the SWP archaeologists observed, “Regardless of any final identification, we are very grateful and fortunate to have found highly talented and engaged partners like Dr. Palin and his team at the Earth Sciences Laboratory.”
A dark history and a brighter future
The story of the global trafficking in enslaved peoples is soberingly relevant to Black History Month. In the 241 years stretching from 1619 to 1860, an estimated 12.5 million people were abducted from their homes in Africa and taken to the New World; about 2 million of those souls perished during their voyage. The research performed at Oxford, effectively integrating geochemistry and geology with maritime archaeology will help to address a deeply important and meaningful piece of human history that has been largely ignored.
“Personally, I was honoured to have had the chance to contribute to this important work,” says Richard. “I’d love to take part in similar projects in the future.”
[1] https://www.slavevoyages.org/
[2] The Slave Wrecks Project is an ambitious international study of the impacts of the global slave trade via an examination of shipwrecks. The project is housed in, and supported by, the United States’ Smithsonian Museum of African American History and Culture. For details of the project and its scope go to https://nmaahc.si.edu/explore/initiatives/slave-wrecks-project