40Ar/39Ar Evidence for a 17 ka Geomagnetic Field Excursion at Changbaishan Volcano, Northeastern China

Eleven new 40Ar/39Ar incremental heating experiments on sanidine phenocrysts in three alkali basalt lava flows atop the northern rim of the Tianchi crater on Changbaishan Volcano, Northeastern China (42.027 deg. N; 128.069 W; 2600 masl), indicate that they were erupted less than 18 ka. The upper two of these dated flows correlate with lavas from which measurements on oriented core samples reveal transitional VGPs that fall over Japan, as well as remarkably low paleointensities (Zhu et al., 2000, JGR). On the basis of of a single 40Ar/39Ar experiment on one of the flows from which the paleomagnetic results were obtained, it has long been thought that these lavas record the Blake excursion at ca. 123 ka (Zhu et al., 2000, JGR). However, the raw 40Ar/39Ar data, details of the analytical procedures, and information on the neutron fluence standard were never published for this sample, making its 40Ar/39Ar age impossible to evaluate. Our new 40Ar/39Ar results include 8 plateau ages from the two transitionally magnetized lavas that give a weighted mean age of 17.0 +/- 1.1 ka (2 sigma). Thus, these lava flows record a geomagnetic field excursion that is not only far younger than the Blake excursion, but also significantly younger than lavas that record the Laschamp and Mono Lake excursions which have been 40Ar/39Ar dated at UW-Madison at 40.7 +/- 1.0 and 31.8 +/0 1.8 ka. There are three important implications: (1) Changbaishan Volcano was far more active immediately following the abrupt termination of the global Last Glaciation maximum at about 17-18 ka than previously thought. These eruptions might be linked to deglaciation of the volcano. Coupled with recent 40Ar/39Ar dating at the Chinese Academy of Sciences of a major ash fall associated with formation of the Tianchi crater at 1.13 ka (Wang, 2011, INQUA congress abstract), our findings suggest that the volcano may pose hazards that have been underappreciated, (2) A 17 ka age for the geomagnetic field behavior recorded by these lavas may provide the first direct radioisotopic evidence that the excursional behavior recorded in Hawaii by lavas in the SOH1 drill core, and in a handful of surficial outcrops, is a global feature of geodynamo behavior, and (3) If this interpretation is correct, our findings bolster the evidence that during the Bruhnes chron 13 well-dated excursions are temporally clustered during two periods with 7 occurring between 17-220 ka, and 6 others between 520-720 ka. Most of these excursions occur during highly resolved minima in the PISO-1500 global stack of relative paleointensity.