Earthquake-induced barium anomalies in the Lisan Formation, Dead Sea Rift valley, Israel

Abstract

Prominent barium concentration anomalies that appear within earthquake brecciated layers (seismites) in the late Pleistocene lacustrine Lisan Formation in the Dead Sea Basin (DSB) are described and discussed here for the first time. Chemical analyses of samples from vertical profiles through the seismites display asymmetric Ba concentration peaks. The peaks start a few centimeters above the seismite's base and gradually rise to maxima reaching some 1000 ppm Ba, before falling off to background values (around 100 ppm), or abutting against the upper boundary of the breccia layer. High resolution SEM and electron microprobe analyses disclose that the Ba in the anomalies resides within prismatic crystallites (mostly < 2 µm in length) composed of a Ba(Sr)SO 4 mineral (designated “BM” henceforth). These are lacking altogether in the normal (non-seismic) underlying and overlying sediments. The crystallites are much smaller than the adjacent, supporting matrix grains of the gradually-bedded seismite, and show no size-sorting relationship with the latter. The peaks of the anomalies reflect higher population density, rather than larger crystal sizes, of the BM crystallites therein. Mass balance calculations show that the mass of Ba 2+ dissolved in the lake above a unit area of the seismites was mostly several times larger than that found in the seismite. The concentration of Ba 2+ in DSB Ca-chloride brines is mostly lesser than that in the DSB Lake, ruling out the former as a source of Ba to the anomalies. We propose that, during earthquakes, the uppermost bottom sediment layers in the DSB Lake were shaken and re-suspended into the overlying brine. The larger, faster-settling fragments and grains remained almost intact or were rapidly removed, unaffected, from the slurry. However, the finer grains remained in suspension for longer periods, allowing nucleation and growth of BM crystallites on their surfaces from the surrounding brine before reaching the bottom. The lag of Ba trapping behind the breccia accumulation and the asymmetrical peak shapes of the anomalies are accounted for by decreasing dilution of the Ba-rich finer particles by Ba-poor coarse grains during seismite accumulation, as reflected by the graded bedding of the seismite layers. The supply rate of Ba 2+ to the lake by freshwater and brines was more than sufficient to account for the buildup of Ba in recurring seismites separated by seismically quiescent intervals as short as 100 yr.