Estimating the frequency of volcanic ash clouds over northern Europe

Fine ash produced during explosive volcanic eruptions can be dispersed over a vast area, where it poses a threat to aviation, human health and infrastructure. Here, we focus on northern Europe, which lies in the principal transport direction for volcanic ash from Iceland, one of the most active volcanic regions in the world. We interrogate existing and newly produced geological and written records of past ash fallout over northern Europe in the last 1000 years and estimate the mean return (repose) interval of a volcanic ash cloud over the region to be 44 ± 7 years. We compare tephra records from mainland northern Europe, Great Britain, Ireland and the Faroe Islands, with records of proximal Icelandic volcanism and suggest that an Icelandic eruption with a Volcanic Explosivity Index rating (VEI) ≥ 4 and a silicic magma composition presents the greatest risk of producing volcanic ash that can reach northern Europe. None of the ash clouds in the European record which have a known source eruption are linked to a source eruption with VEI < 4. Our results suggest that ash clouds are more common over northern Europe than previously proposed and indicate the continued threat of ash deposition across northern Europe from eruptions of both Icelandic and North American volcanoes

Origin of depleted basalts during subduction initiation and early development of the Izu-Bonin-Mariana island arc: Evidence from IODP expedition 351 site U1438, Amami-Sankaku basin

The Izu-Bonin-Mariana (IBM) island arc formed following initiation of subduction of the Pacific plate beneath the Philippine Sea plate at about 52 Ma. Site U1438 of IODP Expedition 351 was drilled to sample the oceanic basement on which the IBM arc was constructed, to better understand magmatism prior to and during the subduction initiation event. Site U1438 igneous basement Unit 1 (150 m) was drilled beneath 1460 m of primarily volcaniclastic sediments and sedimentary rock. Basement basalts are microcrystalline to fine-grained flows and form several distinct subunits (1a-1f), all relatively mafic (MgO = 6.5–13.8%; Mg# = 52–83), with Cr = 71–506 ppm and Ni = 62–342 ppm. All subunits are depleted in non-fluid mobile incompatible trace elements. Ratios such as Sm/Nd (0.35–0.44), Lu/Hf (0.19–0.37), and Zr/Nb (55–106) reach the highest values found in MORB, while La/Yb (0.31–0.92), La/Sm (0.43–0.91) and Nb/La (0.39–0.59) reach the lowest values. Abundances of fluid-mobile incompatible elements, K, Rb, Cs and U, vary with rock physical properties, indicating control by post-eruptive seawater alteration, but lowest abundances are typical of fresh, highly depleted MORBs. Mantle sources for the different subunits define a trend of progressive incompatible element depletion. Inferred pressures of magma segregation are 0.6–2.1 GPa with temperatures of 1280–1470 °C. New 40Ar/39Ar dates for Site U1438 basalts averaging 48.7 Ma (Ishizuka et al., 2018) are younger that the inferred age of IBM subduction initiation based on the oldest ages (52 Ma) of IBM forearc basalts (FAB) from the eastern margin of the Philippine Sea plate. FAB are hypothesized to be the first magma type erupted as the Pacific plate subsided, followed by boninites, and ultimately typical arc magmas over a period of about 10 Ma. Site U1438 basalts and IBM FABs are similar, but Site U1438 basalts have lower V contents, higher Ti/V and little geochemical evidence for involvement of slab-derived fluids. We hypothesize that the asthenospheric upwelling and extension expected during subduction initiation occurred over a broad expanse of the upper plate, even as hydrous fluids were introduced near the plate edge to produce FABs and boninites. Site U1438 basalts formed by decompression melting during the first 3 Ma of subduction initiation, and were stranded behind the early IBM arc as mantle conditions shifted to flux melting beneath a well-defined volcanic front

Do peatlands or lakes provide the most comprehensive distal tephra records?

Despite the widespread application of tephra studies for dating and correlation of stratigraphic sequences (‘tephrochronology’), questions remain over the reliability and replicability of tephra records from lake sediments and peats, particularly in sites >1000 km from source volcanoes. To address this, we examine the tephrostratigraphy of four pairs of lake and peatland sites in close proximity to one another (<10 km), and evaluate the extent to which the microscopic (crypto-) tephra records in lakes and peatlands differ. The peatlands typically record more cryptotephra layers than nearby lakes, but cryptotephra records from high-latitude peatlands can be incomplete, possibly due to tephra fallout onto snow and subsequent redistribution across the peatland surface by wind and during snowmelt. We find no evidence for chemical alteration of glass shards in peatland or lake environments over the time scale of this study (mid- to late- Holocene). Instead, the low number of basaltic cryptotephra layers identified in distal peatlands reflects the capture of only primary tephra-fall, whereas lakes concentrate tephra falling across their catchments which subsequently washes into the lake, adding to the primary tephra fallout received in the lake. A combination of records from both lakes and peatlands must be used to establish the most comprehensive and complete regional tephrostratigraphies. We also describe two previously unreported late Holocene cryptotephras and demonstrate, for the first time, that Holocene Icelandic ash clouds frequently reached Arctic Sweden

Geochemistry of serpentinized peridotites from the Mariana Forearc Conical Seamount, ODP Leg 125: Implications for the element recycling at subduction zones

Recent examinations of the chemical fluxes through convergent plate margins suggest the existence of significant mass imbalances for many key species: only 20–30% of the to-the-trench inventory of large-ion lithophile elements (LILE) can be accounted for by the magmatic outputs of volcanic arcs. Active serpentinite mud volcanism in the shallow forearc region of the Mariana convergent margin presents a unique opportunity to study a new outflux: the products of shallow-level exchanges between the upper mantle and slab-derived fluids. ODP Leg 125 recovered serpentinized harzburgites and dunites from three sites on the crests and flanks of the active Conical Seamount. These serpentinites have U-shaped rare earth element (REE) patterns, resembling those of boninites. U, Th, and the high field strength elements (HFSE) are highly depleted and vary in concentration by up to 2 orders of magnitude. The low U contents and positive Eu anomalies indicate that fluids from the subducting Pacific slab were probably reducing in nature. On the basis of substantial enrichments of fluid-mobile elements in serpentinized peridotites, we calculated very large slab inventory depletions of B (79%), Cs (32%), Li (18%), As (17%), and Sb (12%). Such highly enriched serpentinized peridotites dragged down to depths of arc magma generation may represent an unexplored reservoir that could help balance the input-output deficit of these elements as observed by Plank and Langmuir (1993, 1998) and others. Surprisingly, many species thought to be mobile in fluids, such as U, Ba, Rb, and to a lesser extent Sr and Pb, are not enriched in the rocks relative to the depleted mantle peridotites, and we estimate that only 1–2% of these elements leave the subducting slabs at depths of 10 to 40 km. Enrichments of these elements in volcanic front and behind-the-front arc lavas point to changes in slab fluid composition at greater depths

Redox transfer at subduction zones: insights from Fe isotopes in the Mariana forearc

info:eu-repo/semantics/publishe

Duration of ammonite zones and characteristic Jurassic fossil taxa from the Pliensbachian–Aalenian successions of Bulgaria

The relative durations of ammonite biozones and selected ammonite taxa, as well as some bioevents associated with coeval Lower–Middle Jurassic benthic taxa are represented herein. Based on linked Sr-isotope data, we found a notable difference in the time spans of the zones and in the relative longevity of the guiding ammonite genera around the Pliensbachian/Toarcian (P/T) boundary. The last occurrences of the well-known late Pliensbachian bivalve taxa Gryphaea gigantea and Pseudopecten aequivalvis were dated at 184 Ma and 183.8 Ma, respectively. Recently defined Toarcian–Aalenian autochtonous brachiopod taxa (Bulgariarhynchia and Capillirhynchia brezenensis) were found to have significant taxonomic durations (2 Ma and 5 Ma). The Sr-isotope data also revealed ~2 Ma duration of seawater warming and 0.2 Ma fossil hiatus around the P/T boundary

Paleoenvironmental conditions recorded by 87Sr/86Sr, δ13C and δ18O in late Pliensbachian–Toarcian (Jurassic) belemnites from Bulgaria

The late Pliensbachian–Toarcian (Jurassic) sedimentological, paleontological and geochemical (belemnite 87Sr/86Sr, δ13C and δ18O) record is examined in two Eastern Tethyan (Bulgarian) locations. This interval contains the well-known early Toarcian ocean anoxic event (T-OAE) and its manifestation and temporal context is examined in Bulgaria. Many of the features seen in south-western Europe are identified: collapse of carbonate platform productivity at the Pliensbachian/Toarcian boundary, the T-OAE (a short pulse of euxinic deposition in the Falciferum Zone), an early Toarcian rapid warming event seen in the belemnite δ18O record that peaked around the Falciferum/Bifrons Zone boundary. The long-recognized positive δ13C excursion in the late Falciferum Zone is also seen but a precursor, sharp δ13C negative excursion seen around the Tenuicostatum/Falciferum Zone boundary in most organic carbon records is not seen in the belemnite data, a curious absence noted from other belemnite records. Subsequent perturbations in 87Sr/86Sr, δ13C and δ18O suggest that there may be more global isotopic excursions in the Early Jurassic. On the other hand, belemnite Sr isotope values from Bulgaria are in accord with those recorded in Western Europe and hence, demonstrating its value as a chronostratigraphic tool