Iceberg discharge to the Chukchi shelf during the Younger Dryas

a r t i c l e i n f o The extent of glaciation in northwestern Alaska, the source of sediment supply to the Chukchi shelf and slope, and the movement of sea ice and icebergs across the shelf during the last glacial maximum (LGM) remain poorly constrained. Here we present geophysical and geological data from the outer Chukchi margin that reveal a regionally extensive, heavily ice-scoured surface ∼5–8 m below the modern seafloor. Radiocarbon dating of this discrete event yields age estimates between 10,600 and 11,900 14 C yr BP, indicating the discharge event occurred during the Younger Dryas. Based on mineralogy of the ice-rafted debris, the icebergs appear to be sourced from the northwestern Alaskan margin, which places important constraints on the ice extent in northern Alaska during the LGM as well as existing circulation models for the region. Introduction Although recent studies of ice grounding have been conducted along the Chukchi borderland and greater Arctic Ocean regions (Polyak, et al., 2001; Jakobsson, et al., 2005), the shallow Chukchi shelf remains relatively unexplored. Numerous questions persist concerning the extent of glaciation in northwestern Alaska during the last glacial maximum (LGM), the source of sediment supply to the Chukchi shelf, and the movement of sea ice and/or icebergs across the shelf. Evidence of iceberg scouring and ice-rafted debris on the outer Chukchi shelf presented here provide an important proxy for deglacial circulation patterns, as well as insight into ice extent across the western Arctic during recent glaciations. Regional ice cover in the Arctic strongly affects albedo, which has climatic feedbacks that can play an important role in oceanic and atmospheric circulation (Aagaard and Carmack, 1989). The question of ice extent during the LGM has particular significance in the coherence of global circulation models for glacial periods, the results of which are strongly influenced by ice-extent input parameters (Smith, et al., 2003; Zweck and Huybrechts, 2005). Methods CHIRP subbottom data were acquired aboard the USCGC Healy on the outer Chukchi shelf (Fig. 1) in 2002, using the Scripps Institution of Oceanography EdgeTech X-Star CHIRP subbottom sonar. The subbottom profiles were acquired using a 1-to 6-kHz CHIRP signal with a 50-ms sweep. The core locations are correlated with the subbottom profiles by projecting the core location orthogonally onto the plane of the subbottom profile and this projection distance is noted in each figure. Two-way travel time was converted to depth …