Holocene, Paleogeography, NGRIP ice core, Mawmluh Cave stalagmite, stratigraphic subdivision, 8.2‐ and 4.2‐ka events, Holocene, Paleogeography, NGRIP ice core, Mawmluh Cave stalagmite, stratigraphic subdivision, 8.2‐ and 4.2‐ka events