Neogene Tephrochronology from Site 758 of the Northern Ninetyeast Ridge: Indonesian Arc Volcanism of the Past 5 Ma

Abstract

A tephrochronology of the past 5 Ma is constructed with ash layers recovered from Neogene sediments during drilling at ODP Leg 121 Site 758 on northern Ninetyeast Ridge. The several hundred tephra layers observed in the first 80 m of cores range in thickness from a few millimeters to 34 cm. Seventeen tephra layers, at least 1 cm thick, were sampled and analyzed for major elements. Relative ages for the ash layers are estimated from the paleomagnetic and δ 18 θ chronostratigraphy. The ash layers comprise about 1.7% by volume of the sediments recovered in the first 72 m. The median grain size of the ashes is about 75 µm, with a maximum of 150 µm. The ash consists of rhyolitic bubble junction and pumice glass shards. Blocky and platy shards are in even proportion (10%-30%) and are dominated by bubble wall shards (70%-90%). The crystal content of the layers is always less than 2%, with Plagioclase and alkali feldspar present in nearly every layer. Biotite was observed only in the thickest layers.The major element compositions of glass and feldspar reflect fractionation trends. Three groupings of ash layers suggest different provenances with distinct magmatic systems. Dating by δ 18 θ and paleomagnetic reversals suggests major marine ash-layer-producing eruptions (marine tephra layers > 1 cm in thickness) occur roughly every approximately 414,000 yr. This value correlates well with landbased studies and dates of Pleistocene Sumatran tuffs (average 375,000-yr eruptive interval). Residence times of the magmatic systems defined by geochemical trends are 1.583, 2.524, and 1.399 Ma. The longest time interval starts with the least differentiated magma.The Sunda Arc, specifically Sumatra, is inferred to be the source region for the ashes. Four of the youngest five ash layers recovered correlate in time and in major element chemistry to ashes observed on land at the Toba caldera.

Retracted: Early hominin biogeography in Island Southeast Asia

“…149 The Australasian Strewnfield and OTT are closely related in time. At ODP site 758, northwest of Toba, Australasian microtektites and OTT are associated in ash layers D and E. 254 YTT is found in undersea deposits across the Indian Ocean, as well as the Arabian and South China Sea. 255 YTT dates to about 73,500 years 6 3,000 years 148 or 73,000 years 6 4,000 years ago.…”

Nature, source and composition of volcanic ash in surficial sediments around the Zhongsha Islands

“…All of the brown volcanic glass shards which fall into the field of mafic rocks are different from all of the colorless volcanic shards which fall into the field of felsic rocks, and volcanic scoria is plotted in the transitional field between these two fields (Fig.2). The total amount in weight analyzed by EPMA suggests that there is a little water content which may be ascribed to hydration of volcanic ashes in seawater (Dehn et al, 1991;Shane et al, 1995;Westgate et al, 1998;Pattan et al, 1999;Gasparotto et al, 2000;Liang et al, 2000). For ease of comparison, we recalculated these analyzed total amounts into 100 wt%.…”

Relative geomagnetic intensity as a tool for high resolution stratigraphy

“…The tephra layers (A, C, and D) within the core were probably correlated with the youngest Toba tephra, and the C and D tephras of Dehn et al (1991), respectively. Modified from Suganuma et al (Tauxe et al, 2006).…”