North Pacific late Holocene climate variability and atmospheric composition

A >8000 year-long ice core glaciochemical record from the summit plateau (5300 m a.s.l.) of Mt. Logan (60°35'N, 140°30'W), Yukon Territory, Canada, provides information about the natural variability and forcing mechanisms of late Holocene climate, and the natural and anthropogenic concentrations and sources of potentially toxic trace metals in the North Pacific atmosphere. To efficiently sample the 186 m-long ice core in ultra-clean conditions at high resolution, a novel continuous ice core melting system with discrete sampling was developed and extensively tested. The Mt. Logan core was subsequently melted at 1-4 cm/sample resolution and analyzed for major ions, trace elements and stable water isotope ratios (>40 analytes at 10 pg/l-1000 mug/l). A calibrated Mt. Logan sea-salt sodium proxy record for the intensity of the wintertime Aleutian Low (ALOW) pressure center reveals persistent El Nino-like conditions (strong ALOW) from ca. 650-900 A.D., 1300-1550 A.D., and ca. 1700-1998 A.D., and persistent La Nina-like conditions (weak ALOW) from ca. 900-1300 A.D. and ca. 1550-1700 A.D., consistent with El Nino-sensitive paleoclimate proxies from the Pacific basin. The ca. 1300 A.D. intensification of the ALOW is near-synchronous with a strengthening of the Siberian High and Icelandic Low, indicating a strong coupling between El Nino and Northern Hemisphere atmospheric circulation during the onset of the Little Ice Age. The intensified ALOW and Siberian High enhanced the emission and trans-Pacific transport of dust aerosols to Mt. Logan during the Little Ice Age. The Mt. Logan ALOW proxy and other Northern Hemisphere paleo-atmospheric circulation proxy records are statistically significantly correlated with atmospheric Delta14C records, supporting a significant influence of solar forcing on late Holocene Northern Hemisphere climate. Volcanic emissions accounted for ∼55-95% of the natural (pre-anthropogenic) concentrations of Pb, Bi and As, but rising (2- to 10-fold) anthropogenic emissions from Asia beginning ca. 1950 accounted for 63-92% of Pb, Bi, and As concentrations from 1980-1998. The distinctive trans-Pacific pollution record from Mt. Logan is indicative of rising Asian metal missions in contrast with falling metal emissions from North American and European sources.