Deep-sea Tephra Research Articles

Physical characteristics of tephra layers in the deep sea realm: the Campanian Ignimbrite eruption

Abstract Tephra deposits in the deep sea can survive undisturbed for long periods of time and, on regional scales, tend to be much better preserved than their subaerial counterparts. In this study, grain size distributions and thicknesses of tephra deposits from the Campanian Ignimbrite (CI) eruption (39 000 yr BP; magnitude c. 7.7) preserved in thirty-three deep sea cores are analysed to infer key eruption parameters. Distal deep sea tephra thickness data show an exponential decrease with distance from source. Such trends are difficult to identify in distal subaerial data owing to reworking and limited exposure. We find that tephra grain size distributions are much less affected by depositional environment than thickness, with trends that are consistent across distal subaerial, lacustrine and deep sea environments. The CI layer exhibits bimodal grain size distributions to distances of c. 1000 km, after which it becomes unimodal. Such trends can be related to different mechanisms of tephra transport in the atmosphere, whereby at proximal to medial distances the Plinian and co-ignimbrite phases produce distinct plumes. Within 150 and 900 km from source, Plinian tephra constitutes 40±5% of the deposit volume. Beyond this region where coarse particles are deposited, the plumes merge and fines derived largely from co-ignimbrite elutriation are spread in the atmosphere at velocities greater than the settling velocities of the particles.

Deep-sea ash layers reveal evidence for large, late Pleistocene and Holocene explosive activity from Sumatra, Indonesia

Deep-sea tephra layers sampled from sediment cores collected within, and adjacent to the Sunda trench of offshore Sumatra reveal evidence for five previously undocumented, and apparently large (minimum volume >0.6–>6.3km3; volcanic explosivity index values of 4–5) explosive eruptions over the last ~31,000years, with a presumptive source of mainland Sumatra. Chemical analysis of glass shards and 14C age constraints are used to distinguish the five tephra layers, as well as a sixth that likely correlates with the Youngest Toba tuff (YTT). The tephra layers are labeled V-1 through V-6 relative to their north-to-south positioning along the Sunda trench. The three tephra layers taken from cores west of central Sumatra (V-3, V-4, V-5) are well-constrained by 14C age determinations, whereas less reliable sedimentation-rate estimates are available for the northern (V-1, V-2) and southern (V-6) tephra layers. Deposition of the northernmost tephra, layer V-1, was likely accompanied by seismicity as two chemically indistinguishable tephras are separated by 12cm of course-grained turbidite. Layer V-2 shows a strong chemical resemblance to the YTT and age estimates do not rule out the correlation. With the exception of a likely correlation with the YTT, no other correlations were made between the tephras analyzed in this study with the marine or terrestrial record from the published literature. The most frequent, widespread, and youngest marine tephra layers were found in the central region of the study area. Layers V-3, V-4, and V-5 were all deposited within the last 17 thousand years with minimum eruptive volumes of >0.6 to >5.2km3. A complex depositional sequence of layer V-6 is estimated at ~27.5ka, and may be associated with Late Pleistocene ignimbrite volcanism of southern Sumatra. The ages and suggested minimum volumes represented by the deep-sea tephra layers are consistent with an active volcanic arc, and demonstrate the need for further terrestrial studies.

Correlation of Indian Ocean tephra to individual Oligocene silicic eruptions from Afro-Arabian flood volcanism

Widespread silicic pyroclastic eruptions of the Oligocene Afro-Arabian flood volcanic province (ignimbrites and airfall tuffs) produced up to 20% of the total flood volcanic stratigraphy (>6×10 4 km 3). Volumes of individual ignimbrites and tuffs exposed on land range from ∼150 to >2000 km 3 and eight major units (15–100 m thick) were erupted in <2 Myr, placing these amongst the largest-magnitude silicic pyroclastic eruptions on Earth. They are compositionally distinctive time-stratigraphic markers which were deposited as co-ignimbrite ashfall deposits on a near-global scale around the time of the Oi2 cooling anomaly at ∼30 Ma. Two ignimbrites from the lower part of the flood volcanic succession in Yemen have been correlated to: (a) the conjugate rifted margin of Ethiopia (>500 km distant); and (b) to two deep sea ash layers sampled by ODP Leg 115 in the Indian Ocean ∼2700 km to the southeast. This correlation is based on whole rock analyses of silicic units for isotope ratios (Pb, Nd) and rare earth element compositions, in conjunction with novel in situ Pb isotope laser ablation multicollector inductively coupled plasma mass spectroscopy analysis of groundmass and glass shards. Compositional diversity preserved on the scale of individual ash shards in these deep sea tephra layers record chemical heterogeneity present in the silicic magma chambers that is not evident in the welded on-land deposits. Ages of the ash layers can be established by correlation to precisely dated on-land ignimbrites, and current evidence suggests that while these eruptions may have exacerbated already changing climatic conditions, they both marginally post-date the Oi2 global cooling anomaly.

Deep sea tephra from Nisyros Island, eastern Aegean Sea, Greece

Abstract Isopach maps have been constructed onland for the proximal deposits of the caldera-forming pyroclastic eruptions from the Quaternary volcano Nisyros and are augmented by a newly recognized Nisyros ash layer in the SE Aegean. The Lower Pumice eruption had a NE-SW dispersal direction with onland isopachs from 1–7m thickness and a vent position on the eastern side of the ancestral cone. A 3mm thick layer of Lower Pumice ash is found 75km away to the SW of the island at a depth of 133cm. This layer yields a Lower Pumice age estimate of 35ka. The Upper Pumice had a NNE dispersal direction with onland isopachs from 1–7m thickness and a vent position on the northern side of the island. No Upper Pumice ash has been found in the core record. Preliminary volume estimates derived from the isopach maps and the lack of regionally correlatable Nisyros ash deposits in the Aegean core record suggest that the caldera formation on the island was incremental and tephrostratigraphic markers suggest it occurred between c. 35 and 31 ka ago.

Single crystal 40Ar‐39Ar dating of a Late Quaternary paroxysm on Kos, Greece: Concordance of terrestrial and marine ages

The late Quaternary plinian eruption forming the Kos Plateau Tuff on the island of Kos, Greece, was one of the largest in the Mediterranean. A precise date for this eruption would significantly improve the chronology of the deep‐sea tephra layer W‐3, because reliable correlation of W‐3 with the KOS Plateau Tuff has been previously established. We have dated sanidine from the eruption at 161.3±1.1 ka using the laser‐fusion single crystal 40Ar‐39Ar technique. This age represents a significant refinement over both conventional and highly precise (Cassignol method) K‐Ar ages, and the 40Ar‐39Ar age is shown to be in remarkably good agreement with the age of W‐3 inferred from recent astronomically calibrated oxygen isotope chronologies.

A Review of Late Quaternary Deep-sea Tephras Around Japan

Studies on late Quaternary deep-sea tephras are briefly reviewed in oceans and seas of the world and around the Japanese islands. A great number of modern research has been carried out on deep-sea tephra layers especially in the eastern Mediterranean Sea, Atlantic Ocean, Caribbean Sea and Pacific Ocean. Researches focus on the magnitude and type of individual explosive volcanism, the mechanism of deposition of tephra, correlation of macroscopic tephra layers and biostratigraphic dating of tephra, and resulted in disclosing several explosive volcanic eruptions of extraordinarily great magnitude such as the eruption of Campanian Y 5 ash in the Mediterranean, Los Chocoyos ash around Central America, Wairakei ash around New Zealand and Toba ash in the Bengal Bay and Indian Ocean.In deep-sea sediments around the Japanese islands there are numbers of tephra layers, which are characterized and identified by petrographic and chemical methods. In the Sea of Japan six tephra layers have been identified, three of which are alkalic and are the products of major plinian eruptions of two volcanoes situated on and near the Korean Peninsula: Baegdusan and Ulreung-do. The other three are subalkalic and are corelated with large-scale eruptions from caldera volcanoes in Kyushu: Kikai, Aira and Aso. They are recognized not only in the Sea of Japan but also in the East China Sea and northwest Pacific Ocean, together with widespread tephras derived from Ata and Kikai calderas. All of them are co-ignimbrite ash fall deposit, and well dated by radiometric and stratigraphic methods. Palaeooceanography of late Quaternary around Japan are successfully studied with the aid of such widespread and time-marker tephras. There remain several tephras unidentified around Japan. In addition detailed volcanological and interdisciplinary studies of deep-sea tephras will be one of the problems to be studied in near future.

Dispersed rhyolitic tephra from New Zealand in deep-sea sediments of the Southern Ocean

Dispersed rhyolitic tephra in deep-sea piston cores collected from the South Pacific sector of the Southern Ocean have previously been considered to originate from the Balleny Islands, Antarctica. Chemical composition of the materials suggests that a source in New Zealand is more likely. A strong correlation exists between paleomagnetically determined ages for these deep-sea tephra layers and fission-track ages determined on volcanic glasses and zircons from tephra sequences in Wanganui Basin and Cape Kidnappers, New Zealand. The correlation suggests that the tephra extend more than 5000 km east and south of New Zealand.

Sedimentology of the Minoan deep-sea tephra layer in the Aegean and Eastern Mediterranean

The Minoan eruption of Santorini resulted in deposition of pyroclastic material over a large area of the Aegean Sea and Eastern Mediterranean. The eruptive activity commenced with a plinian phase, which was followed by a phreatomagmatic phase, and ended with numerous pyroclastic flows. We present a sedimentological study of the Minoan ash deposits found in cores taken throughout the region. The cores are divided into three groups: (1) those from the sub-marine slopes immediately surrounding Santorini; (2) those from the Aegean Sea where the topography consists of many steep-sided basins; and (3) those from the Eastern Mediterranean where the topography is relatively flat and smooth. This last group is designated the Distal Ash Zone. The deposits from the Aegean Sea have been remobilised as turbidity currents. Grain-size distribution analysis reveals that these deposits are bimodal, the Md ø of the coarse population increasing away from source, whilst that of the fine population remains uniform irrespective of distance from source. The bimodality is caused by a combination of aggregation of fine particles in ash clouds produced during the last two phases of the eruptive activity and mixing of the coarse plinian and fine phreatomagmatic and co-ignimbrite deposits within the turbidity current. An unusual feature found in two of the cores is a coarse, density sorted layer beneath the Bouma “a” division of several turbidite units. This is interpreted as an example of a flow head deposit as described by Sparks and Wilson (1983). The deposits from the Distal Ash Fall Zone are fine grained and unimodal, containing phreatomagmatic and co-ignimbrite material. They show only very minor remobilisation of the original air-fall ash, due to the flat topography of the area. The Md ø of the deposit remains uniform irrespective of distance from source and is very similar to that of the fine population found in the bimodal Aegean deposits. This is interpreted as the result of aggregation of fines in the phreatomagmatic and co-ignimbrite ash clouds. The deposits in both areas have been extensively bioturbated. A study of dispersed ash in the cores to reconstruct the original thickness of the ash layers suggests that ash layers less than 0.62 cm thick will be completely dispersed in the surrounding sediment, whilst those thicker than 2.5 cm will be preserved as discrete layers, the base being unaffected by bioturbation. The deposits on the submarine slopes of Santorini were not originally deposited as air-fall ash, but are derived from the erosion of coastal exposures of non-welded ignimbrite. They form storm-sand layers and are typical of the type of deposit that may be formed by rapid erosion of fresh pyroclastic material after the eruption of an island-arc volcano.

The roseau ash: Deep-sea tephra deposits from a major eruption on Dominica, lesser antilles arc

Abstract Two extensive marine tephra layers recovered by piston coring in the western equatorial Atlantic and eastern Caribbean have been correlated by electron microprobe analyses of glass shards and mineral phases to the Pleistocene Roseau tuff on Dominica in the Lesser Antilles arc. Tephra deposition and transport to the deep sea was primarily controlled by two processes related to two different styles of eruptive activity: a plinian airfall phase and a pyroclastic flow phase. A plinian phase produced a relatively thin (1–8 cm) airfall ash layer in the western Atlantic, covering an area of 3.0 × 105 km2 with a volume of 13 km3 (tephra). The majority of the airfall tephra was transported by antitrade winds at altitudes of 6–17 km. Aeolian fractionation of crystals and glass occurred during transport resulting in an airfall deposit enriched in crystals relative to the source. Mass balance calculation based on crystal/glass fractionation indicates an additional 12 km3 of airfall tephra was deposited outside the observed fall-out envelope as dispersed ash. Discharge of pyroclastic flows into the sea along the west coast of Dominica initiated subaqueous pyroclastic debris flows which descended the steep western submarine flanks of the island. 30 km3 of tephra were deposited by this process on the floor of the Grenada Basin up to 250 km from source. The Roseau event represents the largest explosive eruption in the Lesser Antilles in the last 200,000 years and illustrates the complexity of primary volcanogenic sedimentation associated with a major explosive eruption within an island arc environment.

Deep-Sea Tephra from the Azores during the past 300,000 Years: Eruptive Cloud Height and Ash Volume Estimates

Background and Purposes Deep-sea tephra has been studied intensively because of its diverse geological uses. Tephra studies provide information on petrochemistry and petrogenesis, volcanicity and regional tectonism, possible climatic variation, magnetic properties, ambient wind directions and velocities, regional tephrochronology, depositional processes and origin of volcanogenic sediments, eruptive cloud heights, chemical correlation of tephra layers between land and abyssal regions, and micro-morphological indications on glass shards of eruptive cloud heights. Hitherto, deep-sea tephrochronology was based solely on visible ash layers in cores, Limiting Identification to a downwind distance of, 1,000 km. By using our technique for the separation and counting of deep-sea dispersed tephra (Huang and others, 1975a), we have obtained evidence that a single ash layer can be correlated between cores 3,000 km apart at high latitudes in the South Pacific (Huang and others, 1973, 1975b). The downwind particle-size variations can be used to estimate eruptive cloud heights and paleoexplosivities (Shaw and others, 1974; Huang and others, 1975b).

Distribution, age and chemical composition of tephra layers in deep-sea sediments off western Indonesia

Tephra layers occur in deep-sea sediments of the northeastern Indian Ocean, adjacent to western Indonesian are. The layers range in age from Recent to Late Miocene. Relative abundance of light and heavy mineral species in all tephra layers have been determined, and pure glass shards from representative samples have been analyzed chemically for major oxides. On the basis of the chemical data, three distinct provinces can be recognized: (1) an extensive province of rhyolitic tephra layers, ranging in age back to Late Miocene, is found adjacent to Sumatra; (2) a more restricted province of dacitic layers, adjacent to Sunda Strait and western Java; and (3) a province of andesitic layers, found adjacent to eastern Java and the Lesser Sunda Islands. Chemical composition of tephra layers in each province remains constant with time. As an example, tephra layers from the rhyolitic province are characterized by a high and restricted range of SiO 2 (75–77%) when expressed on an H 2O-free basis. Tephra layers recovered from the study area were examined for chemical evidence of secondary alteration. The analyses revealed that H 2O is the only major oxide in the glass shards which increases progressively with the age of the tephra layers regardless of the bulk composition. H 2O, however, reaches a “saturation point” of 4–5% in the layers 250–400 thousands of years old and remains constant to the oldest recovered tephra layer (7.5 m.y. old). The decrease in silica content in deep-sea tephra layers eastward along the Indonesian volcanic arc coincides with a similar eastward decrease in average silica content in Indonesian lavas. A relatively high silica content in lavas from Sumatra, with associated ignimbrites and their deep-sea ash-fall equivalents is closely linked to thick pre-Cenozoic crust. In the portion of the arc to the east of Sumatra, the crust is Cenozoic and thin. Difference in silica content of both the lavas and deep-sea tephras along the Indonesian arc is considered in regard to the hypothesis of “magma filtering” which is based on the contrasting density gradients of ascending magma and the upper crust.

Deep-sea tephra from the Azores during the past 300,000 years: Eruptive cloud height and ash volume estimates: Summary

Deep-sea tephra from the Azores during the past 300,000 years: Eruptive cloud height and ash volume estimates: Summary

Duration of large-magnitude explosive eruptions deduced from graded bedding in deep-sea ash layers

Thick silicic volcanic ash layers commonly observed hundreds of kilometres from potential source areas have resulted from large-magnitude explosive eruptions that have no historical equivalents. We have developed a model that predicts the duration of these eruptions from the vertical size grading of feldspar phenocrysts near the base of deep-sea tephra layers. The size grading is a function of the release time of the particles, their settling velocity, the water depth at the site of the ash layer, and the duration of the eruption. The model has been tested on two layers of the Worzel D ash in the eastern equatorial Pacific. This ash layer is the distal counterpart of the rhyolitic Los Chocoyos ash-flow tuff and H-tephra layer associated with the formation of the Lake Atitlan caldera in Guatemala. The estimated duration of the eruption is 20 to 27 d. Calculations using published estimates of the volume of erupted material yield an average magma-discharge rate of about 240,000 m 3 /s. This rate is approximately equivalent to that recorded in the 1956 eruption of Bezymianny and the 1912 eruption of Katmai.

The exceptional magnitude and intensity of the Toba eruption, sumatra: An example of the use of deep-sea tephra layers as a geological tool

The eruption of Toba (75,000 years BP), Sumatra, is the largest magnitude eruption documented from the Quaternary. The eruption produced the largest-known caldera the dimensions of which are 100 × 30 km and which is surrounded by rhyolitic ignimbrite covering an area of over 20,000 km2. The associated deep-sea tephra layer is found in piston cores in the north-eastern Indian Ocean covering a minimum area of 5 × 106 km2. We have investigated the thickness, grain size and texture of the Toba deep-sea tephra layer in order to demonstrate the use of deep-sea tephra layers as a volcanological tool. The exceptional magnitude and intensity of the Toba eruption is demonstrated by comparison of these data with the deep-sea tephra layers associated with the eruptions of the Campanian ignimbrite, Italy and of Santorini, Greece in Minoan time. The volume of ignimbrite and distal tephra fall deposit produced in the Toba eruption are comparable, a total of at least 1000 km3 of dense rhyolitic magma. In contrast the volume of dense magma produced by the Campanian and Santorini eruptions are approximately 70 and 13 km3 respectively. Thickness versus distance data on the three deep-sea tephra layers show that eruptions of smaller magnitude than Santorini are unlikely to be preserved as distinct tephra layers in most deep-sea cores. In proximal cores all three tephra layers show two distinct units: a lower coarse-grained unit and an upper fine-grained unit. We interpret the lower unit as a plinian deposit and the upper unit as a co-ignimbrite ash-fall deposit, indicating two major eruptive phases. The Toba tephra layer is coarser both in maximum and median grain size than the Campanian and Santorini layers at a given distance from source. These data are interpreted to indicate a very high cruption column, estimated to be at least 45 km. We have applied a method for estimating the duration of the Toba eruption from the style of graded-bedding in deep-sea tephra layers. Studies of two cores yield estimates of 9 and 14 days. The eruption column height and duration estimates both indicate an average volume discharge rate of approximately 106 m3/sec. The Toba eruption therefore was not only of exceptional magnitude, but also of exceptional intensity.