Supplemental Material: Late Cenozoic tephrochronology of the Mount Diablo area within the evolving plate-tectonic boundary zone of northern California

Terminology relating to tephra and tephra layer nomenclature, methods of sampling tephra in the field, laboratory treatment of tephra samples for analysis, methods of chemical analysis of tephra and radiometric dating (40Ar/39Ar), and methods of data evaluation<br>

Supplemental Material: Late Cenozoic tephrochronology of the Mount Diablo area within the evolving plate-tectonic boundary zone of northern California

Terminology relating to tephra and tephra layer nomenclature, methods of sampling tephra in the field, laboratory treatment of tephra samples for analysis, methods of chemical analysis of tephra and radiometric dating (40Ar/39Ar), and methods of data evaluation<br>

Supplemental Material: Late Cenozoic tephrochronology of the Mount Diablo area within the evolving plate-tectonic boundary zone of northern California

Terminology relating to tephra and tephra layer nomenclature, methods of sampling tephra in the field, laboratory treatment of tephra samples for analysis, methods of chemical analysis of tephra and radiometric dating (40Ar/39Ar), and methods of data evaluation<br>

How do the grain size characteristics of a tephra deposit change over time?

Volcanologists frequently use grain size distributions (GSDs) in tephra layers to infer eruption parameters. However, for long-past eruptions, the accuracy of the reconstruction depends upon the correspondence between the initial tephra deposit and preserved tephra layer on which inferences are based. We ask: how closely does the GSD of a decades-old tephra layer resemble the deposit from which it originated? We addressed this question with a study of the tephra layer produced by the eruption of Mount St Helens, USA, in May 1980. We compared grain size distributions from the fresh, undisturbed tephra with grain size measurements from the surviving tephra layer. We found that the overall grain size characteristics of the tephra layer were similar to the original deposit, and that distinctive features identified by earlier authors had been preserved. However, detailed analysis of our samples showed qualitative differences, specifically a loss of fine material (which we attributed to ‘winnowing’). Understanding how tephra deposits are transformed over time is critical to efforts to reconstruct past eruptions, but inherently difficult to study. We propose long-term, tephra application experiments as a potential way forward.

A Holocene tephra layer within coastal aeolian deposits north of Caleta Olivia (Santa Cruz Province, Argentina)

In this paper we illustrate the stratigraphy, geochronology, and geochemistry (major, minor, trace elements and Sr-isotopes) of a Holocene tephra layer found within coastal sedimentary deposits north of Caleta Olivia (Santa Cruz Province, Argentina). The stratigraphic succession comprises beach deposits with basal erosive surface resting on the local substrate (“Formación Patagonia”) followed by a poorly developed paleosoil. The paleosoil is covered by a lenticular fine-grained (Mdφ: 5.2, 0.027 mm), well sorted (σφ: 1.2) volcanic ash layer and aeolian sands. The geochemical composition of shard fragments points to an origin from the Hudson volcano, located in the southern Andes, ca. 400 km to the west. The geochemistry, Sr-isotopes and the radiometric constraints (younger than the age of the underlying marine layer dated at ca. 4,100 a cal BP) further allow correlating this tephra with the so-called H2 eruption (ca. 3,900 a cal BP). This finding is of interest owing to the poor preservation potential of tephra within the Late Holocene sedimentary deposits of the Atlantic coast of Patagonia and represents the first finding of H2 eruption in this area, improving our knowledge of the dispersion of the fine-grained distal deposit of the Hudson volcanic explosive activity, thus allowing a better estimate of the eruptive dynamics and the risks associated with the Hudson volcano.

Age of the oldest Homo sapiens from eastern Africa

Efforts to date the oldest modern human fossils in East Africa, from Omo-Kibish and Herto in Ethiopia, have drawn on a variety of chronometric evidence, including 40Ar/39Ar ages of stratigraphically-associated tuffs. The generally-accepted ages for these fossils are ca. 196 thousand years (ka) for the Kibish Omo I and ca. 160-155 ka for the Herto hominins. However, stratigraphic relationships and tephra correlations that underpin these estimates have been challenged. Here, we report new geochemical analyses that link the Kamoya Hominin Site (KHS) Tuff, which conclusively overlies the Kibish Formation member containing Omo I, with a major explosive eruption of Shala volcano in the Main Ethiopian Rift. By dating the proximal deposits of this eruption, we obtain a new minimum age for the Omo fossils of 212±16 ka. Contrary to previous arguments, we also show that the KHS Tuff does not correlate with another widespread tephra layer, the Wadaido Vitric Tuff (WAVT), and therefore cannot anchor a minimum age for the Herto fossils. Shifting the age of the oldest known Homo sapiens fossils in eastern Africa to before ~200 ka is consistent with several independent lines of evidence for greater antiquity to the modern human lineage.

Deciphering the fallout of tephra on glaciers in past eruptions, the case history of the Katla 1918 eruption

&lt;p&gt;Explosive eruptions in ice-covered volcanoes may deposit large volumes of tephra on the glaciated slopes.&amp;#160; The tephra can influence surface ablation and alter mass balance.&amp;#160; Ice melting by an eruption can change glacier geometry and temporarily alter the flow of outlet glaciers.&amp;#160; Conversely, when assessing the size of past tephra-producing eruptions in an ice-covered volcano the glacier complicates such estimates.&amp;#160; The effects of ice flow, dilation and shear need to be considered.&amp;#160; A tephra layer may get buried in the accumulation area, be transported by glacier flow and progressively removed over years-to-centuries by ice flow, eolian transport of exposed tephras and sediment transport in glacial rivers.&amp;#160; Here we report on a case study from the M&amp;#253;rdalsj&amp;#246;kull ice cap that covers the upper parts of the large Katla central volcano in south Iceland.&amp;#160; Most eruptions start beneath the 300-700 m thick ice cover within the Katla caldera, melt large volumes of ice and cause major j&amp;#246;kulhlaups.&amp;#160; They also produce tephra layers that are preserved in soils around the volcano.&amp;#160; The most recent eruption in Katla occurred in October-November 1918, when a large tephra layer was deposited in a 3-weeks long eruption. By using a combination of (1) data obtained at or near the vent area within the SE-part of the Katla caldera in the year following the eruption, (2) mapping of the tephra as exposed at the present time in the ablation areas in the lower parts of the outlet glaciers, and (3) simple models of ice flow based on balance velocities and knowledge of mass balance, we estimate the location of fallout and the original thickness of the presently exposed tephra.&amp;#160; Photos taken in the vent area in 1919 indicate a tephra thickness of 20-30 m on the ice surface proximal to the vents. &amp;#160;The greatest thicknesses presently observed, 30-35 cm, occur where the layer outcrops in the lowermost parts of the ablation areas of the K&amp;#246;tluj&amp;#246;kull and S&amp;#243;lheimaj&amp;#246;kull outlet glaciers.&amp;#160; A fallout location within the Katla caldera is inferred for the presently exposed tephra, as estimates of balance velocities imply lateral transport since 1918 of ~15 km for K&amp;#246;tluj&amp;#246;kull, ~11 km for S&amp;#243;lheimaj&amp;#246;kull and about 2 km for the broad northern lobe of Sl&amp;#233;ttj&amp;#246;kull. &amp;#160;The calculations indicate that ice transport with associated dilation of the glacier through the accumulation areas has resulted in significant thinning. &amp;#160;&amp;#160;Thus, the layer that is now 0.3-0.35 m thick in the fastest flowing outlets is estimated to have been four to seven times thicker when it fell on the accumulation area within the ice-filled caldera.&amp;#160; In contrast, changes have been minor in the slowly moving Sl&amp;#233;ttj&amp;#246;kull. &amp;#160;These findings allow for the construction of an isopach map for the glacier.&amp;#160; The results indicate that just under half of the total airborne tephra produced in the eruption fell within the M&amp;#253;rdalsj&amp;#246;kull glacier, with the remaining half spread out over a large part of Iceland.&amp;#160; These methods potentially allow for reconstruction of several tephra layers from ice-covered volcanoes in Iceland and elsewhere.&amp;#160;&lt;/p&gt;

Sliding layer estimation of shallow landslides on Aso volcanic mountains in Japan based on tephra layer-physical properties of soil

Shallow landslides occur frequently on the Aso volcanic mountains. The soil materials on the Aso volcanic mountains consist of tephra layers formed by volcanic activities. This study is aimed to specify the physical properties of soil that correlate with the sliding layer of a shallow landslide on the volcanic mountain area. Tephra layers consist of kuroboku and scoria layers and the differences between these layers were specified using the physical properties of soil methods. Results showed that the plasticity index and the fine fraction content can be used for estimating the sliding layer in the Aso volcanic area.