FORT MCMURRAY, Alberta (AP) ¿ A Greenpeace protest shut down Royal Dutch Shell PLC's massive oil sands mine on Tuesday on the eve of Prime Minister Stephen Harper's visit with President Barack Obama. Two dozen Greenpeace members chained themselves to giant earth-moving equipment, shutting down Shell's massive Albian Sands oil sands mine in northwestern Alberta for several hours. Alberta's oil sands projects have been criticized as a growing source of greenhouse gas emissions. Shell temporarily suspended operations to ensure the safety of the activists and of Shell staff, while it negotiated with the protesters. Shell and the Royal Canadian Mounted Police were eager to see a peaceful end to the demonstration as it stretched on through most of the day. Shell spokesman Paul Hagel said the company invited the protesters to unchain themselves and come to a nearby office to discuss their concerns. Shell also offered them food, water and bug spray. RCMP Const. Karolina Malik said the protesters were very polite, so there was no rush to cut the chains or make arrests.