The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/Virgo GW170817. IV. Detection of Near-infrared Signatures of r-process Nucleosynthesis with Gemini-South

NSF [AST-1411763, AST-1714498, PHY-1707954, AST-1518052]; NASA [NNX15AE50G, NNX16AC22G, NAS 5-26555]; Department of Energy; Office of Energy Research, Office of High Energy and Nuclear Physics, Divisions of Nuclear Physics, of the U.S. Department of Energy [DE-AC02-05CH11231]; Gordon and Betty Moore Foundation [GBMF5076]; Office of Science of the U.S. Department of Energy [DE-AC02-05CH11231]; Kavli Foundation; Danish National Research Foundation; Niels Bohr International Academy; DARK Cosmology Centre; Heising-Simons Foundation; Alfred P. Sloan Foundation; David and Lucile Packard Foundation; NASA by Space Telescope Science Institute; [GS-2017B-Q-8]; [GS-2017B-DD-4]; [DE-SC0017616]