2004 Denver Annual Meeting (November 7–10, 2004)

Paper No. 7
Presentation Time: 1:30 PM-5:30 PM


STILLWELL, Rebecca C., Geology, Scripps College, Scripps College Box 796, 1030 Columbia Ave, Claremont, CA 91711 and ABBOTT, Dallas H., Lamont Doherty Earth Observatory of Columbia Univ, 1000 RT 9W, Palisades, NY 10964, rstillwe@scrippscol.edu

As recorded in tree ring records, the year 535AD marks the worst climactic conditions of the last 1600 years. Proposed causes of this climactic downturn include a catastrophic eruption of Krakatau volcano or a bolide impact near the same location. Using magnetic susceptibility to locate ash layers in core VM19-149 (6.233°S 104.833°E) and VM28-358 (6.583°S 104.9°E) (67 and 79 km west of Krakatau, respectively) we have observed one layer, located within the top 10 cm of each core. This layer we believe to be from the explosive 1883 eruption, with a minimum thickness of 5 cm in VM28-358 and 7.5 cm in VM19-149. Below the layer, the cores have a low magnetic susceptibility, implying there is little to no ash. Pyrite nodules and pyritized marine microfossils are also evident throughout the ash layer in both cores sampled, suggesting they were distributed coincident with the ash and debris from an eruption of Krakatau. We infer that the pyrite was formed in the anoxic conditions of the submerged caldera of Krakatau, and distributed along with other ejecta during the 1883 eruption of the volcano. A deeper layer in VM28-358, at 112-140cm, also contains pyrite, suggesting it also originated at Krakatau. At typical sedimentation rates of 1-3 cm/kyr, this layer would be ~30,000-120,000 years old, occurring far too early to cause the 535AD climate change. Analyses by Sigurdsson and Beauregard of the composition of materials ejected by Krakatau during and prior to the 1883 eruption indicate a drop in iron content for the more recent eruption, providing the means to use chemical analysis to differentiate ash layers and indicating that magnetic susceptibility should also locate any older ash layers in the core. These have not been detected, suggesting an eruption of Krakatau circa 535AD was not sufficiently explosive to create a measurable ash deposit 67-78 km from the volcano, and thus not sufficient to affect world climate. The eruption resulting in the original caldera must then have occurred earlier, as de Neve (1985) has suggested. C14 dating and microprobe analysis are needed to confirm the date and composition of the ash layer. de Neve G A, 1985.In : Sastrapradja D et al (eds) Proc Symp., Jakarta: Lembaga Ilmu Pengetahuan Indonesia, 1: 35-46