• European Marine Science
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The calcareous nannofossil assemblages from sediment core DS97-2P from the Reykjanes Ridge have been investigated to document oceanographic changes in surface water during the Holocene. The recorded variations in coccolithophore species assemblages and accumulation rates indicate that the region was subjected to rapid changes of surface water masses throughout the entire Holocene. Coccolithophore assemblages generally are of low species diversity and consist mainly of Emiliania huxleyi and Coccolithus pelagicus ssp. pelagicus. Two major events occurred at 8.5-7. ka and at 4.5- 3.5. ka, showing higher coccolith accumulation rates, suggesting that the influence of relatively warm Atlantic waters via the Irminger Current was strong in the investigated area. The coccolithophore assemblages have been compared with diatom, foraminifer and sedimentological records within the same core. These data, supported by a comparison with previously published proxy records, add credit to the hypothesis that Holocene changes did not occur uniformly across the North Atlantic. The results have highlighted the Holocene pattern in the North Atlantic, as a period influenced by strong regionalism with discrepancies in the hydrographical trends and in the distribution of the planktonic proxies. © 2010 Elsevier B.V.

Subduction of a narrow slab of oceanic lithosphere beneath a tightly curved orogenic arc requires the presence of at least one lithospheric scale tear fault. While the Calabrian subduction beneath southern Italy is considered to be the type example of this geodynamic setting, the geometry, kinematics and surface expression of the associated lateral, slab tear fault offshore eastern Sicily remain controversial. Results from a new marine geophysical survey conducted in the Ionian Sea, using high-resolution bathymetry and seismic profiling reveal active faulting at the seafloor within a 140 km long, two-branched fault system near Alfeo Seamount. The previously unidentified 60 km long NW trending North Alfeo Fault system shows primarily strike-slip kinematics as indicated by the morphology and steep-dipping transpressional and transtensional faults. Available earthquake focal mechanisms indicate dextral strike-slip motion along this fault segment. The 80 km long SSE trending South Alfeo fault system is expressed by one or two steeply dipping normal faults, bounding the western side of a 500+ m thick, 5 km wide, elongate, syntectonic Plio-Quaternary sedimentary basin. Both branches of the fault system are mechanically capable of generating magnitude 6–7 earthquakes like those that struck eastern Sicily in 1169, 1542, and 1693. peer-reviewed

International audience; Subduction mega-thrust earthquakes in the SW Ryukyu trench pose a seismic and tsunami hazard. One of the objectives of this study is to estimate the downdip width of the seismogenic zone using numerical modeling to determine the temperature distribution along the plate interface. However, this approach depends strongly on the thermal parameters of the subducting slab. While the Philippine Sea plate (PSP) subducting beneath the central and eastern Ryukyu arc is of Eocene age (35-50 Ma), its age west of the Gagua Ridge is uncertain, with proposed ages ranging from Lower Cretaceous (140 Ma) to Upper Eocene (35 Ma). Since the sparse available heat flow data are insufficient to resolve this debate, both end-member hypotheses are tested as input parameters. We examined two transects at 122.5°E and 123.5°E on either side of the N-S trending, 4-km high, Gagua Ridge. The shallow forearc geometry is obtained from wide-angle seismic data. The deep slab geometry was obtained from hypocenter distribution and tomography. For an Eocene slab age, we obtain a 100 km and 110 km wide seismogenic zone (between the 150 °C and 350 °C isotherms) west and east of Gagua Ridge, respectively. This is in good agreement with the observed distribution of hypocenters. Using a Cretaceous slab west of Gagua Ridge predicts a deep seismogenic zone (25 km-60 km depth), inconsistent with observed thrust earthquakes. Tomographic images at 122.5°E and 123.5°E show a similar slab thickness of 70-80 km suggesting that the oceanic lithosphere has a young (Eocene) thermal age. The westernmost PSP (Huatung Basin) may have been thermally rejuvenated by mantle convection near the slab corner. The tectonic history since 6 Ma (transition from subduction to collision beneath Taiwan) may have also perturbed the thermal structure.

Abstract Glaciated watersheds are regions of intense physical and chemical weathering. In order to gain new insight on subglacial weathering processes, we measured uranium and radium isotopes from a proglacial river draining the Greenland Ice Sheet (GrIS). Time series samples were collected from the spring to mid-summer, a time period during which subglacial drainage pathways are thought to transition from slow-inefficient (distributed) to fast-efficient (channelized) systems. The 228Ra/226Ra activity ratio of the dissolved load varied from 5.2 ± 0.9–16.9 ± 3.6, which was significantly higher than the 228Ra/226Ra ratio of a suspended sediment load sample of 2.1 ± 0.07 and crustal values of ~1. The high 228Ra/226Ra in the dissolved load relative to the source rock material is indicative of mineral surface weathering induced by rapid and continuous flushing of the subglacial drainage network during the course of the melt season and those prior. The 234U/238U ratio (δ234U) varied between 33 and 106‰ with a discharge-weighted mean of 67‰; the seasonal evolution of δ234U did not correlate to geochemical indicators of subglacial meltwater storage time. An experiment designed to measure changes in δ234U with increasing meltwater storage times found that δ234U in the dissolved phase decreased rapidly with increasing storage time. Similarly, samples collected along a transect moving downstream from the ice sheet terminus had decreasing δ234U values from 63 to 15‰ further indicating that with increased weathering, the δ234U of meltwater decreases. Coupled with the relatively low δ234U and high 228Ra/226Ra, U appears to be impacted by rapid chemical weathering of subglacial and suspended sediments. The Leverett River discharge weighted U concentration was 0.13 nM; if this system is considered representative of the broader GrIS, then the total dissolved U flux from the GrIS would be on the order of 6.4 × 104 mol/y. Using a similar set of assumptions, the dissolved 228Ra and 226Ra flux from the GrIS was ~1.1 × 1014 dpm/y and ~ 5.5 × 1013 dpm/y, respectively. These estimates suggest that the 226Ra flux to the ocean from the GrIS is globally significant and that the 228Ra flux in particular is larger than most river inputs.

A detailed record of late Pleistocene deglaciation followed by mid-Holocene ice shelf breakup and late Holocene re-growth is contained in continental shelf sediments in the northern Larsen area, northeast Antarctic Peninsula. The zero age of core tops is confirmed by new and published 210Pb profiles, and 70 accelerator mass spectrometer (AMS) 14C dates on bulk organic carbon define sedimentation rates of 7.6–92 cm/ka. The varied geology in the local ice drainage basins facilitates the use of ice-rafted debris (IRD) provenance in determining the presence or absence of ice shelves. All inshore cores contain an interval of non-local IRD in the post-glacial section, demonstrating widespread ice shelf breakup in the mid-Holocene. Both breakup and re-growth may have taken centuries and there are no widespread debris layers associated with breakup. Cores beyond and up to 30 km inside the historical ice shelf limit exhibit a varied IRD provenance throughout the Holocene, suggesting the maximum ice shelf limit may date only from the Little Ice Age. Benthic foraminiferal assemblages are related to water masses and position on the continental shelf and have been modified by taphonomic processes. Nevertheless we discern a deglaciation signal in Prince Gustav Channel of a calcareous spike in predominantly agglutinated assemblages, and this is repeated at the time of mid-Holocene ice shelf breakup. The inferred mid-Holocene warm period occurred later in the northern Larsen area than on the west coast of the Antarctic Peninsula.

Abstract Ocean circulation changes along the continental shelf of the Nordic and Barents Seas have been investigated in order to reconstruct regional changes in the inflow of Atlantic Water (AW) through the last 16,000 calibrated (cal) years (yr) B.P. We have selected five time-slices representing the late glacial (16,000–15,000 cal yr B.P.), the Bolling-Allerod warm interstadials (14,500–13,500 cal yr B.P.), the Younger Dryas cold stadial (12,500–11,500 cal yr B.P.), the early Holocene (9500–7500 cal yr B.P.) and the late Holocene (4000–2000 cal yr B.P.). Twelve previously published records of the distribution of benthic foraminifera faunas and ice-rafted debris have been compiled. The earliest sign of Atlantic Water inflow was recorded at the northern Iceland shelf at 16,000–15,000 cal yr B.P. The inflow of warm AW to the Nordic Seas shelf has been persistent since, but with variable strength and geographic pattern. An apparent zonal seesaw pattern in the strength of the Norwegian Atlantic Current (NwAC) and the Irminger Current (IC) during the late glacial, Bolling-Allerod and Younger Dryas periods was found. During the Holocene, no zonal differences in the inflows of NwAC and IC were found. A strong meridional gradient with warmer conditions at lower latitudes and relatively cold conditions at high northern latitudes existed.

Fast spherical harmonic synthesis (SHS) at multiple points based on the lumped coefficients approach is a very well-established technique. However, this method cannot be applied to SHS at irregular surfaces, as the points must be regularly spaced and refer to a regular surface such as the sphere or the ellipsoid of revolution. In this paper we present a MATLAB ? -based graphical user interface software for ultra-high degree (e.g. tens of thousands or even higher) SHS on grids at irregular surfaces, like the Earth surface. This software employs the highly efficient lumped coefficient approach for SHS at regular surfaces and the Taylor series expansions to continue the functionals to the irregular surfaces, e.g. the Earth surface. The generalized idea of continuing functionals using the Taylor series was presented by Hirt (2012) (J. Geod. 86, 729-744). We took advantage of the software GrafLab (Bucha and Janak, 2013. Comput. Geosci. 56, 186-196), which employs the lumped coefficients approach, and developed a new software isGrafLab (Irregular Surface GRAvity Field LABoratory). Compared to the commonly used "two loops" approach, the factor of increased computational speed can reach a value of several hundreds. isGrafLab allows accurate evaluation of 38 functionals of the geopotential on grids at irregular surfaces. High orders of the Taylor series can be used for the continuation. The new software offers all the other options available in GrafLab, such as the employment of three different approaches to compute the fully normalized associated Legendre functions, the graphical user interface or the possibility to depict data on a map. HighlightsA new MATLAB-based software for spherical harmonic synthesis is presented.Spherical harmonic synthesis is efficiently performed on grids at irregular surfaces.The program employs the lumped coefficients approach and Taylor series expansions.The program has been developed with a graphical user interface.

Semi-continuous, linear accumulations of poorly-sorted debris are present on the surface of the remnant Larsen-A Ice Shelf, Antarctic Peninsula. These accumulations form a complex of debris bands extending parallel to the front of the ice shelf for several kilometres. Landsat imagery shows that the debris bands originated as lateral moraines along the Nordenskjöld Coast. Almost 80% of clasts sampled from these debris accumulations have shape/roundness characteristics consistent with glacier transport in the zone of basal traction. Angular and very angular clasts account for 15% and 22% of clasts in the pebble- and cobble/boulder-sized fractions, respectively, and originated by rockfall from valley/nunatak sides, with subsequent passive glacier transportation. Lithological analysis indicates that the debris is derived locally from the Nordenskjöld Coast, Cape Fairweather region and interior of the Antarctic Peninsula. Episodic melt-out and resedimentation of this debris from the front of the ice shelf would deliver pulses of coarse-grained sediment to the sea floor. Therefore, coarse-grained debris can also be released along the calving margin of small polar ice shelves fringing mountainous terrain, and could potentially be confused with sediment deposited at the grounding line of Antarctic ice-shelves. Sedimentological criteria to differentiate between these environments are proposed in this paper.