ALBERT

Description: International Ocean Discovery Program (IODP) Expedition 352 recovered a high-fidelity record of volcanism related to subduction initiation in the Bonin fore-arc. Two sites (U1440 and U1441) located in deep water nearer to the trench recovered basalts and related rocks; two sites (U1439 and U1442) located in shallower water further from the trench recovered boninites and related rocks. Drilling in both areas ended in dolerites inferred to be sheeted intrusive rocks. The basalts apparently erupted immediately after subduction initiation and have compositions similar to those of the most depleted basalts generated by rapid sea-floor spreading at mid-ocean ridges, with little or no slab input. Subsequent melting to generate boninites involved more depleted mantle and hotter and deeper subducted components as subduction progressed and volcanism migrated away from the trench. This volcanic sequence is akin to that recorded by many ophiolites, supporting a direct link between subduction initiation, fore-arc spreading, and ophiolite genesis.

Description: New biostratigraphical, geochemical, and magnetic evidence is synthesized with IODP Expedition 352 shipboard results to understand the sedimentary and tectono-magmatic development of the Izu–Bonin outer forearc region. The oceanic basement of the Izu–Bonin forearc was created by supra-subduction zone seafloor spreading during early Eocene (c. 50–51 Ma). Seafloor spreading created an irregular seafloor topography on which talus locally accumulated. Oxide-rich sediments accumulated above the igneous basement by mixing of hydrothermal and pelagic sediment. Basaltic volcanism was followed by a hiatus of up to 15 million years as a result of topographic isolation or sediment bypassing. Variably tuffaceous deep-sea sediments were deposited during Oligocene to early Miocene and from mid-Miocene to Pleistocene. The sediments ponded into extensional fault-controlled basins, whereas condensed sediments accumulated on a local basement high. Oligocene nannofossil ooze accumulated together with felsic tuff that was mainly derived from the nearby Izu–Bonin arc. Accumulation of radiolarian-bearing mud, silty clay, and hydrogenous metal oxides beneath the carbonate compensation depth (CCD) characterized the early Miocene, followed by middle Miocene–Pleistocene increased carbonate preservation, deepened CCD and tephra input from both the oceanic Izu–Bonin arc and the continental margin Honshu arc. The Izu–Bonin forearc basement formed in a near-equatorial setting, with late Mesozoic arc remnants to the west. Subduction-initiation magmatism is likely to have taken place near a pre-existing continent–oceanic crust boundary. The Izu–Bonin arc migrated northward and clockwise to collide with Honshu by early Miocene, strongly influencing regional sedimentation.

Description: We present an integrated age model for the incoming Cocos Plate sediments offshore Costa Rica. The data, collected over two IODP Expeditions (334 and 344), provides a medium­ to high­resolution record from the initial formation of the ocean crust in the Miocene to the present day. This study provides 〉50 age control points for the CRISP sediments from Sites U1381 and U1414. Although the two sites are just 10 km apart, there are distinct differences in the sediment and tephra record. Most notable is the presence of a hiatus at Site U1381. The hiatus, which is seen at other sites on the Cocos Plate, but not at Site U1414, may be related to erosion due to bottom water currents, mass wasting from Cocos Ridge subduction or may be related to the closure of the Central American Seaway (CAS). Sediment accumulation rates in the Miocene are comparable to modern abyssal plain rates. However, an increase is observed in the Pleistocene, when detritus from the forearc basin appears at Site U1414 ~2 Ma, shortly after the initiation of Cocos Ridge subduction. A tectonic model is presented that reconstructs the Cocos Plate, from its formation at 23 Ma to the present day. Eastern Equatorial Pacific (EEP) paleoceanographic events, such as the Miocene ‘carbonate crash’ and the Late Miocene­Early Pliocene ‘biogenic bloom’ observed at Site U1414, are also discussed.

Description: The 1Myr tephra records of IODP (International Ocean Discovery Program) Holes U1436A and U1437B in the Izu-Bonin fore- and reararc were investigated in order to assess provenance and eruptive volumes, respectively. In total, 304 tephra samples were examined and 260 primary tephra layers were identified. Tephra provenance was determined by means of major and trace element compositions of glass shards and distinguished between Japan and Izu-Bonin arc origin of the tephra layers. A total of 33 marine tephra compositions were correlated to the Japan arc and 227 to the Izu arc. Twenty marine tephra layers were correlated between the two drilling sites. Additionally, we defined eleven correlations of marine tephra deposits to major widespread Japanese eruptions; from the 1.05Ma Shishimuta-Pink Tephra to the 30ka Aira-Tn Tephra, both from Kyushu Island. These eruptions provide independent time markers within the sediment record and six correlations were used to date tephra layers from Japan in Hole U1436A to establish an alternative age model for this hole. Furthermore, the minimum distal tephra volumes of all detected events were calculated, which enabled the comparison of the tephra volumes that derived from the Japan and the Izu-Bonin arcs. For some of the major Japanese eruptions these are the first volume estimations that also include distal deposits. All of the Japanese tephras derived from events with eruption magnitude Mv≥5.6 and three of the investigated eruptions reach magnitudes Mv≥7. Volcanic events of the Izu-Bonin arc have mostly eruption magnitudes Mv≤5.

Description: Icelandic explosive volcanic eruptions such as the 2010 Eyjafjallajökull eruption have far-reaching impacts. Tephra has been found as far as Northern Continental Europe and Greenland. On Iceland, however, erosion and ice cover limit the preservation, particularly of pre-Holocene volcanic deposits. We use the marine sedimentary archive offshore southeast Iceland, which preserves information about the depositional fans at medial distances from the volcanic sources, to infer past eruption frequencies and geochemical characteristics of the volcanic systems, contributing to Icelandic volcanic hazard assessment and to the stratigraphic framework used for palaeoceanographic reconstructions. Here we report the analysis of four sediment gravity cores of ~5 to 10 m lengths, obtained during RV Poseidon Cruise 457, at distances of 60 to 180 km southeast of Iceland between 755 m and 1610 m water depths. In addition to prominent tephra layers, the inter-core correlation is supported by color-scans and tied in with the δ18O Greenland Ice-core record, providing the age model. We analyzed major element compositions of volcanic glass by electron microprobe. Using geochemical fingerprinting and sedimentary observations, we identified ~50 basaltic primary ash layers. Background sediment includes 〈15% rhyolitic to basaltic-andesitic shards. Although reworking hampers tephrochronological interpretation in the Holocene part of the record, the high abundance of the bimodal Vedde-type tephra implies strong activity of the Katla volcanic system during this time period. Tephras of unknown composition and the “Faroe Marine Ash zones” II and III, including the ~27 ka Fugloyarbanki tephra, provide insight into the Late Pleistocene volcanic activity at the central part of the Icelandic rift zone. The deposits can be traced back 68 ka to the volcanic systems of Grímsvötn-Lakagígar,Kverkfjöll, Bárðarbunga-Veiðivötn, Katla and Hekla. Our results extend their eruption record further back in time than currently inferred from terrestrial Iceland and in more detail than the far-distant records.

Description: This cumulative work summarizes seven manuscripts published between 2007 and 2012. These studies use marine and on-shore tephrostratigraphy as a tool to quantify and identify the timing, extent, and causes of geological processes taken place at subductions zones. In many subduction-related regions on Earth, highly explosive plinian volcanic eruptions generate buoyant, tephra bearing eruption columns capable of penetrating up to 40 km into the stratosphere, where they reach a neutral level of buoyancy and spread laterally. Such eruption clouds drift with the prevailing wind over nearby oceans, gradually dropping their ash load over areas that sometimes can be larger than 106 km2. The resulting ash layers are best preserved in non-erosive marine environments and thus provide the most complete record of volcanic activity. Wide aerial distribution across sedimentary facies boundaries, near-instantaneous emplacement, correlative chemical signatures, and the presence of minerals suitable for radiometric dating make ash layers an excellent stratigraphic marker in marine sediments and provide constraints on the temporal evolution of both, the volcanic source region and the ash-containing sediment facies. On-shore stratigraphic successions of tephra layers are generally based on the distinct composition of tephras. In west-central Nicaragua for example (section 2.1), late Pleistocene to Holocene tephras were emplaced by highly explosive eruptions, with a combined erupted mass of 184 Gt (DRE), that are distributed into 9 dacitic to rhyolitic eruptions (84%) and 4 basaltic to basaltic-andesitic eruptions (16%). Widespread eruptive masses from explosive volcanism are usually underestimated, even when the most distal parts of the on-shore distribution fans, normally not preserved in terrestrial environments, are included. If on-shore tephras can be correlated to offshore deposits like those in Central America (sections 2.2 and 2.3), the revised erupted magma mass show that the tephras account for 65% of the total arc magma output. This enables the minimum estimation of long-term average magma production rate at each volcano and over whole arcs. Using their unique compositional signatures, tephras facilitate the determination of provenance as well as the reconstruction of emplacement processes of volcanoclastic marine sediments, in accordance with regional geotectonic settings (section 2.4). Ash layers in marine sediments offshore Central America can provide time constraints for submarine landslides at the continental slope, as they probably act as weak layers where sliding initiates (section 2.5). Variations in the sedimentation rates on the slope, constrained by bracketing tephras of known age, can be attributed to periods of intense erosion on land likely triggered by tectonic processes. In the case of the incoming plate these changes can be due to changes in bend-faulting activity across the outer rise, which elicit erosion and re-sedimentation. Additionally, ash layers in Central America can help determine the duration of active and inactive periods in the multi-stage growth history of fluid venting sites (section 2.6). Cyclicity in the marine tephra record along the Pacific Ring of Fire yields a statistically significant detection of a spectral peak at the obliquity period, which is related to crustal stress changes associated with ice age mass redistribution and therefore supports the presence of a causal link between variations in ice age climate, continental stress field, and volcanism (section 2.7). To summarize, the seven manuscripts presented here highlight the benefit of tephrostratigraphy as a major tool in geology, and show that the tephra record on-shore and, especially in the marine environment, have a spectrum of possible applications to decipher the causes and temporal variability of geological processes.

Description: Mildly explosive volcanic eruptions on the deep seafloor have been demonstrated in several cases. In contrast, we present here evidence for highly explosive activity producing diatreme-like ejecta on the surrounding seafloor at the Charles Darwin Volcanic Field (CDVF) located at about 3600 m water depth on the lower southwestern slope of the Cape Verdean Island of Santo Antão. We examined the 1 km diameter Kolá volcanic crater using photogrammetric reconstructions derived from ROV-based imaging followed by 3D quantification using a novel remote sensing workflow. The measured and calculated parameters of physical volcanology derived from the 3D model allow us to get a handle on explosive volcanic processes on the deep seafloor. Kolá crater comprises a complicated lithologic succession of highly fragmented deposits, including spheroidal juvenile lapilli that may contain quenched carbonatite melts, and were likely formed by spray granulation. The deposits comprise numerous well-rounded clasts of MORB-type gabbroic country rocks with diameters up to 20 cm, probably entrained and abraded by fluidization within the vent, that were laterally transported for hundreds of meters through water. In spite of the great depth, the Kolá crater features dense but highly fragmented volcanic deposits with an unexpected combination of large clast sizes and wide clast dispersal. This suggests an energetic eruptive environment which may have similarities with that seen in maar-type eruptions on land. The occurrence of carbonatite in the juvenile clasts provide evidence for a CO2-rich magmatic fluid involved in the explosive events.

Description: Sediment gravity cores recovered during the RV METEOR cruise M80/3 in 2010 around the northwestern end of the Cape Verde Archipelago contain three widespread hyaloclastic tephra layers. One of these layers occurs in two sediment cores 40 km apart. The blocky shapes of the vesicle-poor/-free glass-shards clearly indicate their origin from a subaqueous eruption. There are three potential sources in the northwestern Cape Verdean Seamount Province: (1) the Nola Seamount, (2) the Sodade Seamount and (3) the Charles Darwin Volcanic Field. Using geochemical fingerprinting the hyaloclastic glass-shards could be unambiguously correlated to the Charles Darwin Volcanic Field. This is a deep-sea volcanic field consisting of at least 14 eruption centers all at 〉2,850 m below sea level, located about 100 km east of the core locations. Previous studies have documented widespread tephra distributions from relatively shallow (〈 500 mbsl) submarine explosive eruptions, but here we record such a widespread tephra from a deep-sea (probably 〉3000 mbsl) eruption. We discuss the mechanisms of formation and far transport of the hyaloclastic particles.