ALBERT

Description: We present whole rock 187Os/188Os data for the most mafic lavas along the Lesser Antilles arc (MgO = 5–17 wt.%) and for the subducting basalt and sediments. 187Os/188Os ratios vary from 0.127 to 0.202 in the arc lavas. Inverse correlations between 187Os/188Os and Os concentrations and between 187Os/188Os and indices of differentiation such as MgO suggests that assimilation, rather than source variation, is responsible for the range of Os isotopic variation observed. 87Sr/86Sr, La/Sm and Sr/Th are also modified by assimilation since they all correlate with 187Os/188Os. The assimilant is inferred to have a MORB-like 87Sr/86Sr with high Sr (〉700 ppm), low light on middle and heavy rare earth elements (L/M-HREE; La/Sm ∼2.5) and 187Os/188Os 〉 0.2. Such compositional features are likely to correspond to a plagioclase-rich early-arc cumulate. Given that assimilation affects lavas that were last stored at more than 5 kbar, assimilation must occur in the middle-lower crust. Only a high MgO picrite from Grenada escaped obvious assimilation (MgO = 17% wt.%) and could reflect mantle source composition. It has a very radiogenic 87Sr/86Sr (0.705) but a 187Os/188Os ratio that overlaps the mantle range (0.127). 187Os/188Os and 87Sr/88Sr ratios of the sediments and an altered basalt from the subducting slab vary from 0.18 to 3.52 and 0.708 to 0.714. We therefore suggest that, unlike Sr, no Os from the slab was transferred to the parental magmas. Os may be either retained in the mantle wedge or even returned to the deep mantle in the subducting slab.

Description: Particles sinking out of the euphotic zone are important vehicles of carbon export from the surface ocean. Most of the particles produce heavier aggregates by coagulating with each other before they sink. We implemented an aggregation model into the biogeochemical model of Regional Oceanic Modelling System (ROMS) to simulate the distribution of particles in the water column and their downward transport in the Northwest African upwelling region. Accompanying settling chamber, sediment trap and particle camera measurements provide data for model validation. In situ aggregate settling velocities measured by the settling chamber were around 55 m d(-1). Aggregate sizes recorded by the particle camera hardly exceeded I mm. The model is based on a continuous size spectrum of aggregates, characterised by the prognostic aggregate mass and aggregate number concentration. Phytoplankton and detritus make up the aggregation pool, which has an averaged, prognostic and size dependent sinking. Model experiments were performed with dense and porous approximations of aggregates with varying maximum aggregate size and stickiness as well as with the inclusion of a disaggregation term. Similar surface productivity in all experiments has been generated in order to find the best combination of parameters that produce measured deep water fluxes. Although the experiments failed to represent surface particle number spectra, in the deep water some of them gave very similar slope and spectrum range as the particle camera observations. Particle fluxes at the mesotrophic sediment trap site off Cape Blanc (CB) have been successfully reproduced by the porous experiment with disaggregation term when particle remineralisation rate was 0.2 d(-1). The aggregation-disaggregation model improves the prediction capability of the original biogeochemical model significantly by giving much better estimates of fluxes for both upper and lower trap. The results also point to the need for more studies to enhance our knowledge on particle decay and its variation and to the role that stickiness play in the distribution of vertical fluxes

Description: The Azores islands in the central North-Atlantic originate from a regional melting anomaly, probably created by melting hot, unusually hydrous and geochemically enriched mantle. Here, we present Hf, Pb and Os isotopic data in geochemically well-characterised primitive lavas from the islands Flores and Corvo that are located west of the Mid-Atlantic Ridge (MAR), as well as submarine samples from a subsided island west of Flores and from Deep Sea Drilling Project (DSDP) holes drilled in the western part of the Azores platform and beyond. These are compared to existing data from the Azores islands east of the MAR. The geodynamic origin of the two islands west of the ridge axis and furthest from the inferred plume centre in the central part of the plateau is enigmatic. The new data constrain the source compositions of the Flores and Corvo lavas and show that the western and eastern Azores mantle is isotopically similar, with the exception of an enriched component found exclusively on eastern São Miguel. Trace element ratios involving high field strength elements (HFSE) are distinctly different in the western islands (e.g. twofold higher Nb/Zr) compared to any of the islands east of the MAR. A similar signature is observed in MAR basalts to the south of the Azores platform and inferred to originate from (auto-) metasomatic enrichment of the sub-ridge mantle (Gale et al., 2011, 2013). In a similar fashion, low degree melts from an enriched source component may metasomatise the ambient plume mantle underneath the western Azores islands. Melting such a modified plume mantle can explain the chemical differences between lavas from the western and eastern Azores islands without the need for additional plume components. Recent re-enrichment and intra melting column modification of the upwelling mantle can cause local to regional scale geochemical differences in mantle-derived melts.

Description: Biogenic structures in Late Quaternary sediments from the southwestern Iberian continental slope were studied by using X-ray images from two cores from 580 and 1750 m water depth. Eight different ichnocoenoses were observed: indistinct bioturbation, Planolites-dominated, Thalassinoides-dominated, Chondrites-dominated, Planolites and Thalassinoides-dominated, pyritized microburrows such as Trichichnus and ‘Mycellia’-dominated, Chondrites, Trichichnus, and ‘Mycellia’-dominated and Zoophycos. Variations of the ichnocoenoses within the cores show a striking correlation with climatically induced changes in the hydrographical regime, i.e. current strength and bathymetric position of the Mediterranean Outflow Water (MOW). Comparison of the response of the ichnocoenoses to changes in bottom-water conditions and substrate between the two cores studied indicate that bottom-water oxygenation and enrichment of particulate organic matter at the base of the MOW layer are the primary factors controlling the ichnocoenoses. The traces even recorded short-term climatic changes such as the Younger Dryas cold event. The spatial and temporal distribution patterns are in good agreement with earlier models of the MOW history, which gives reason to see a refinement of trace fossils as a complementary tool for paleoceanographical studies.

Description: A one-dimensional model "ADAM" is presented that allows the prognostic computation of the interactions between mineral particles (dust) and biologically formed aggregates. The model couples a 7-compartment biogeochemical component (NO(3), NH(4), phytoplankton aggregates, zooplankton, detritus, carbon, and chlorophyll) and a 4-compartment component for the tracing of mineral particles: single free particles in the water, particles aggregated with phytoplankton, incorporated in zooplankton, and attached to detritus. It resolves both annual and daily cycles of plankton and the fate of dust from eolian import into the ocean via biological activity, aggregation and disaggregation to sedimentation at the sea floor. The model results suggest that particle scavenging is essentially occurring in the mixed layer, where biological activity and shear aggregation regulate the formation of the aggregates. The aggregates interact intensively with the suspended pool of dust particles, and sink through the upper main thermocline with increasing speed. Particle break up and organic matter degradation are important mechanisms for particle cycling in the intermediate and deeper layers. The model predicts an 80% decrease of the annual carbon flux between 100 m and 3000 m depth. The vertical profile of Al-contents in suspended particulates and the annual average vertical flux of particulate organic matter are fairly well reproduced by the model, as well as the seasonal cycles of carbon and dust fluxes in the ocean interior. (C) 2010 Elsevier B.V. All rights reserved.

Description: The Miocene to Pleistocene Limon Group of Costa Rica is a mixed carbonate–siliciclastic succession that formed in association with the emergence of the Central American Isthmus. Our study focuses on a lower Late Pliocene reef unit, the newly excavated Contact Cut, which is located at the contact between the siliciclastic sediments of the Rio Banano Formation and the mixed reefal and coral bearing deposits and siliciclastic sediments of the Quebrada Chocolate Formation. The siliciclastic sediments were deposited in a thick, deltaic setting sourced by erosion of the Cordillera de Talamanca. Deposits of the Limon Group preserve a sequence of progressively shallowing, near-shore sediments that were exposed by uplift during the early to middle Pleistocene. The Contact Cut outcrop shows the first reef sequence in the stratigraphic sequence and thus illustrates the reestablishment of Caribbean coral reef predominance in the Neogene. It shows extensive reef growth during a rise in sea level and a slight progradation during the succeeding sea-level highstand. Three stages of reef evolution are recognized based on faunal diversity. The Contact Cut reef complex is comparable to the time equivalent reef of the Las Islas roadcut, situated west of Limon, which shows a rapid burial of the corals by siliciclastics. Both reefs document a distinct facies diversification during the final stages of the closing of the Central American Seaway. The reefs developed in an environment stressed by siliciclastic input, which ultimately caused a decrease in coral diversity and abundance followed by a temporary demise of the reefs. The biotic composition of the patch reefs that occurred during the sea-level rise, Las Islas and Contact Cut, did not differ from the reefs that developed during the final highstand in sea level, the reefs of the overlying Moin formation (Limon Group). Differences in the position on the shelf relative to the source of the siliciclastics might have been the cause for the different response to the rise in sea level of the transgressive reefs, with a very fast give up scenario for Las Islas reef and a catch up followed by a give up phase for Contact Cut reef.

Description: A novel technique was applied to estimate differences in core shortening in three gravity cores taken at the same core location on the Portuguese continental slope using different coring devices. No obvious deformational features are visible in the fresh core sediments; the isotope stratigraphy and abundance maxima of ice-rafted debris, representing North Atlantic Heinrich-events, indicate identical stratigraphic range in all three cores. However, one of the cores is significantly shorter than the others. X-ray radiographs of the cores reveal that the shorter core shows typical deformation structures, whereas the two other cores are lacking signs of deformation. This serious disturbance had likely gone unnoticed had it not been for the X-ray radiographs. As an approach to semi-quantitatively estimate the core shortening, we used the fragmentation and displacement of the pyritized trace fossil Trichichnus that is easily recognizable in X-ray radiographs through its high contrast. The Trichichnus data indicate that a shortening of 50–60% occurred in the lower part of the shorter core. This estimate is in good agreement with variations in apparent sedimentation rates for the interval considered. Accurate flux rates are essential for our understanding marine biogeochemical cycles in general and the marine budgets of nutrients such as carbon and phosphorus in particular. X-ray radiographs are very useful in assessing the intactness of the sedimentary records and the presented method has potential to become a valuable tool in correcting sedimentation rates in disturbed gravity cores.

Description: The July–August 2008 phreatomagmatic eruption of Okmok Volcano produced ~ 0.26 km3 (DRE) of phenocryst-poor (1 to 2 vol.%) basaltic andesite ejecta, compositionally distinct from the basalt erupted during 1997 (51.90 wt.% SiO2). Analyzed juvenile products are tan to dark gray vesicular lapilli (scoria), and dense, purple-black bombs. Whole-rock compositions cluster tightly (54.97 ± 0.25 wt.% SiO2). The eruption also produced mafic ash containing basaltic groundmass glasses (52 wt.% SiO2) and olivine-hosted melt inclusions (down to 47 wt.% SiO2). The scoria and early-erupted ash contain compositionally similar plagioclase, clinopyroxene, and olivine phenocrysts. Olivine phenocrysts in the scoria and ash are not in equilibrium with the basaltic andesite whole-rock composition. Olivine-hosted melt inclusions yield 0.11 (± 0.04) to 3.61 (± 1.24) wt.% total H2O by μ-FTIR, with an average of 1.23 ± 0.68 (1σ) wt.%. Three inclusions contain CO2 = 37 to 49 ppm with the rest below detection. Solubility model-derived inclusion entrapment/re-equilibration depths extend from near surface to 4.6 (± 2.5) km, in agreement with recent geophysical studies. The 2008 eruption was triggered by an influx of melt-rich basalt originating from the 3 to 6 km storage region beneath the center of the caldera, which intersected a shallower, more evolved magma body beneath Cone D. Our study concludes that the Okmok magma system is “mush-column” like, containing multiple magma bodies with a common and frequent replenishment source, but segregated with unique geochemical signatures.

Description: Cosmopolitan nature of the marine bryozoan Electra pilosa was studied to clarify geographic structure and to outline evolution and phylogeography of the species. Several local populations from the Northeast Atlantic (North Sea and Baltic Sea), Arctic (Barents Sea and White Sea) and Indo-West Pacific (New Zealand) were compared. In addition, we examined the closely related species E. posidoniae from the Mediterranean Sea. Phylogenetic analysis based on both 16S and 18S rDNA indicate that the Indo-West Pacific E. pilosa is a sister species to the Atlantic–Mediterranean clade, with the latter including the species E. posidoniae and the Atlantic population of E. pilosa. The topology of the phylogenetic tree leads us to conclude that E. pilosa is a paraphyletic species group relative to E. posidoniae, and a molecular dating of its divergence is consistent to geologic events associated with the closure of the Tethys Sea.

Description: We present integrated relative production rates for cosmogenic nuclides in rock surfaces, which take into account reported variations of the geomagnetic field intensity during the past 800,000 yr. The calculations are based on the model simulating cosmic ray particle interactions with the Earth’s atmosphere given by Masarik and Beer [“Simulation of particle fluxes and cosmogenic nuclide production in the Earth’s atmosphere,” J. Geophys. Res. 104(D10), 12099–12111, 1999]. Corrections are nearly independent on altitude between sea level and at least 5000 m. The correction factors are essentially identical for all stable and radioactive cosmogenic nuclides with half-lives longer than a few hundred thousand years. At the equator, integrated production rates for exposure ages between ∼40,000 to 800,000 yr are 10 to 12% higher than the present-day values, whereas at latitudes 〉40°, geomagnetic field intensity variations have hardly influenced in situ cosmogenic nuclide production. Correction factors for in situ 14C production rates differ from those of longer-lived nuclides. They are always smaller than ∼2% because the magnetic field intensity remained rather constant during the past ∼10 kyr, when the major fraction of the 14C extant today was produced.