CHANGES OF SEA ICE AND SURFACE WATER TEMPERATURE IN THE BERING SEA DURING DEGLACIAL TO HOLOCENE TIMES: EVIDENCE FROM IP25 AND ALKENONE DATA
Overall goal of our study of sediment material collected during RV Sonne Cruise 202 (INOPEX) in 2009 (Gersonde et al., Curise Report 2009), is the reconstruction of the short-term variability of sea-ice, sea-surface temperature (SST), primary productivity and terrigenous input in the subpolar North Pacific/Bering Sea and their relationship to global climate change, using organic-geochemical proxies (i.e. organic-geochemical bulk parameters and specific biomarkers such as: TOC, hydrogen indices; long-chain n-alkanes, sterols, alkenones; Uk37 and TEX86-Index; BIT-Index; HBIs, IP25, PIP25). In a first phase, these organic-geochemical proxies have been determined in surface sediments. The results show that the biomarker proxies reflect modern sea-ice and SST distributions as well as areas of increased primary productivity and increased input of terrigenous (organic) matter quite well. In a second phase of the project, the biomarkers have been determined in three selected sediment cores: Core SO202-18-6 (Umnak Plateau/Bering Sea; 60.127°N, 179.444°W; water depth 1105 m; core length 7.21 m; age interval 0 to 14 kyr.BP). Core SO202-07-6 (Detroit Seamount/western subpolar North Pacific; 51.272°N, 167.700°W; water depth 2340 m WD; core length 4.69 m; age interval MIS 1 to 3). Core SO202-27-6 (Patton Seamount/eastern subpolar North Pacific; 54.296°N, 149.600°W; water depth 2919 m; core length 2.91 m: age interval MIS 1 to 3). Here, we concentrate especially on the variability of sea-ice cover and sea-surface temperature, using the newly developed sea-ice proxy IP25 (Belt et al., 2007) and alkenone data, respectively, determined in the AMS14C-dated Core SO202-18-6. Based on these biomarker records, sea-ice cover and SST changed significantly in the northern Bering Sea during Deglacial-Holocene times. The Younger Dryas interval is characterized by extended sea-ice cover, coinciding with a drop in SST to 2-4°C. With the end of the Younger Dryas, between 460 and 420 cmbsf, sea-ice cover decreased with increasing SST. Between 420 and 120 cmbsf representing the early Holocene Thermal Maximum, IP25 is absent and maximum SST of about 6°C was reached. During the upper 120 cmbsf representing the late Holocene, IP25 occurred again and increased towards the top, paralleled by a decrease in SST of about 3°C. A very similar contemporaneous trend of increasing sea-ice cover during the late Holocene was recorded in the northern North Atlantic, paralleled by an advance of glaciers in Norway (Müller et al., 2009; 2012).