Tephra in Lake Sediments of Magadan Region

Abstract

Tephra of two generations has been identified in sediments of the numerous lakes lying from the coast of the Sea of Okhotsk in the south as far as the Indigirka River basin in the north. Tephra of different ages differs in area of occurrence, and in petromagnetic, geochemical, mineralogical, and thermomagnetic characteristics, thus enabling reliable identification in sections. The accumulation of lower tephra was due to an eruption that formed the caldera of Lake Kurilskoe in Kamchatka about 7600 B.P. It has been found in all lakes where sedimentation was occurring at that time. The character of tephra occurrence was controlled by the distance to the source, by morphologic and hydrodynamic parameters of the lakes, as well as by post-sedimentation processes. The tephra has a rhyolitic composition with silica concentrations of 75.6–75.9%; it is low-magnetic. The magnetic particles are mostly pseudo-single-domain ones. The tephra is dominated by titanomagnetites with mean concentrations of titanium within ∼3.6–6.5 wt %. The upper tephra only occurs in the northern Okhotsk region. It shows high magnetic properties and is dominated by low-titanium titanomagnetites (Ti = 2.6–3.2 wt %). The magnetic particles are pseudo-single-domain to multidomain (closer to multidomain). The magnetic susceptibility of the tephra layers, which is an order higher than that of the host sediments, is its leading distinctive feature. The tephra has a rhyodacitic composition with silica concentrations of 70.2–70.5%. Its age is about 3000 years. The tephra interbeds in lake sediments are important chronological and correlative markers of the Holocene.