ELLEN WULFHORST TRIBUTES to victims of the transatlantic slave trade can be found in museums and through statues, but a new proposal is calling for a memorial that can neither be visited nor even seen. A virtual memorial of ribbons on maps of the Atlantic deep seabed could honor the estimated 1.8 million Africans who died at sea during the trans-oceanic slave trade, said a proposal published this month in the Journal of Marine Policy. "It would be on a map ... they can't visit it," said Phillip Turner, a science policy consultant who worked on the paper as a…