CAPE TOWN, South Africa (AFP) — The discovery of a wrecked slave ship off the South African coast, in which more than 200 captives drowned, marks a milestone in the study of the slave trade, the Smithsonian Institution said Monday.

The wreck of the Sao Jose-Paquete de Africa, a Portuguese vessel that sank off Cape Town on its way to Brazil in 1794, will be the focus of two ceremonies in the city on Tuesday, the US institution said in a news release.

“This discovery is significant because there has never been archaeological documentation of a vessel that foundered and was lost while carrying a cargo of enslaved persons,” said Lonnie G. Bunch, founding director of the Smithsonian’s National Museum of African American History and Culture (NMAAHC).

About 400 slaves captured in the southeast African nation of Mozambique were on board when the ship foundered on submerged rocks about 100 metres (yards) from shore.

The crew and some of the captives were rescued. The surviving slaves were sold in the Cape.

Artefacts recovered from the ship will be handed over on long-term loan by South Africa’s Iziko Museums to the NMAAHC.

At the same time, soil from the Sao Jose’s point of embarkation — Mozambique Island — will be deposited at the wreck site by divers from Mozambique, South Africa and the United States.

“Perhaps the single greatest symbol of the trans-Atlantic slave trade is the ships that carried millions of captive Africans across the Atlantic never to return,” said Bunch.

The wreck was first discovered by amateur South African treasure hunters in the 1980s, but was wrongly identified as an earlier Dutch vessel, the Smithsonian said.

Once researchers found the captain’s account of the shipwreck in the Cape archives in 2010, new interest developed in the site and it was confirmed to be the Sao Jose.

“The Sao Jose is all the more significant because it represents one of the earliest attempts to bring East Africans into the trans-Atlantic slave trade — a shift that played a major role in prolonging that tragic trade for decades,” Bunch added.

More than 400,000 East Africans are estimated to have made the journey from Mozambique to Brazil between 1800 and 1865.

Iron ballast to weigh down the ship and its human cargo, and a wooden pulley block, were retrieved this year under the Slave Wrecks Project, a global partnership among museums and research institutions.

The wreck site is between two reefs battered by strong swells, creating challenging conditions for archaeologists, and only a small percentage of the site has been excavated so far, the Smithsonian said.