CAMP CENTURY SUBGLACIAL SEDIMENT PRESERVES EVIDENCE FOR DEGLACIATION OF NORTHWESTERN GREENLAND DURING MIS 11: IMPLICATIONS FOR PALEO-SEA LEVEL

Accelerating Greenland Ice Sheet (GrIS) melt is contributing to sea-level rise. Past times when the GrIS was smaller and global mean sea level (GMSL) was higher than today are analogues for the future. Ice core basal material age constraints suggest a reduced GrIS at least once since ~1 Ma, but the timing and extent of such ice sheet retreat(s) is unknown. One possibility was during Marine Isotope Stage (MIS) 11c (424-395 ka), a long interglacial period marked by GMSL 6-13 m higher than present that required a smaller GrIS. Marine archives suggest an ice-free and forested southern Greenland during MIS 11c; however, direct terrestrial evidence of retreated ice-sheet extent during this time has been elusive. As part of an international team examining the Camp Century ice core subglacial sediment, we show evidence of an ice-free period in the last 400 kyr in northwestern Greenland. Replicate fading-corrected infrared stimulated luminescence (IRSL) ages from different grain-size fractions of the same sample indicate that the upper-most subglacial sediment was last exposed to sunlight ~390 ka, consistent with an ice-free event during MIS 11c. The IRSL ages considered with cosmogenic ²⁶Al/¹⁰Be measurements mandate no more than ~13 kyr of surface exposure under ice-free conditions. Ice-sheet models that maintain ice-free conditions at Camp Century require at least 1.6 m of sea level equivalent contribution from Greenland. Mineralogical and multiple geochronometric analyses (apatite U-Th/He, hornblende ⁴⁰Ar/³⁹Ar, and zircon, apatite, rutile U-Pb) indicate different weathering histories but indistinguishable provenance between the upper-most sediment and underlying, older sediment in the Camp Century ice core; this implies that the IRSL ages record an ice sheet advance that reworked and buried local surface material as climate cooled. Preservation of fossil vegetation in the same subglacial sediment indicates that tundra emerged in northwestern Greenland during MIS 11c, while boreal forest inhabited southern Greenland coevally. Resolving the extent of GrIS retreat during MIS 11c will further constrain the GMSL budget, and ultimately improve our understanding of past GrIS stability. Nevertheless, the demise of northwestern and southern GrIS during MIS 11c highlights ice-sheet sensitivity to prolonged warmth.