An alkalic volcanic suite of the Labrador Trough, Labrador

The Nimish Subgroup igneous suite is a linear belt of volcanic and plutonic rocks in the Dyke Lake area of the southern Labrador Trough. The volcanics are interbedded with the sediments of the Wishart and Sokoman Formations of the Aphebian aged, Knob Lake Group. The sokoman Formation forms a time stratigraphic horizon that separates the lower Petitsikapau Lake Formation from the upper Astray Lake formation of the Nimish Subgroup. The occurrence of these volcanics within the Knob Lake Group is unique relative to Labrador Trough stratigraphy, as elsewhere the Knob Lake Group is a dominantly sedimentary succession and volcanics are restricted to the younger Doublet Group. Stratigraphic relationships between the Nimish Subgroup and the Sokoman formation indicate contemporaneous volcanic, clastic and chemical sedimentary activity. The internal stratigraphy of the Sokoman Formation exhibits a three-fold subdivision that is broadly correlatable with similar subdivisions in the Schefferville "main ore zone", 30 miles to the northwest. A detailed facies and paleogeographic model relating the volcanic activity to iron formation deposition in the Dyke Lake is presented. The rocks of the Dyke Lake area have been affected by lower greenschist facies metamorphism during the Hudsonian orogenic event, circa 1735 my. Geochemical evidence indicates that the igneous rocks of the Nimish Subgroup have been metasomatized with large degrees of mobility in Na₂O, K₂O, CaO, MgO, SiO₂, FeO and Fe₂O₃ suspected. The "immobile trace elements", Ti, Zr, Nb, Y and Ga imply that the Nimish lavas are a mildly alkaline suite that has an alkali basalt-trachyandesite-comendite differentiation scheme. The rare earth element, REE, geochemistry of the Nimish Subgroup is supportive of the alkaline nature of the volcanics and has been used to model the fractional crystallization petrogenesis involved in the two volcanic cycles. The geological, geochemical and geophysical evidence indicates that the Nimish Subgroup lavas are possibly a rift facies, alkaline suite related to the tensional tectonic regime that preceeded the extrusion of voluminous tholeiitic lavas of the Doublet Group.

Environmental data from coastal sediments along natural CO2 gradients at a volcanic vent in Papua New Guinea

Natural CO2 venting systems can mimic conditions that resemble intermediate to high pCO2 levels as predicted for our future oceans. They represent ideal sites to investigate potential long-term effects of ocean acidification on marine life. To test whether microbes are affected by prolonged exposure to pCO2 levels, we examined the composition and diversity of microbial communities in oxic sandy sediments along a natural CO2 gradient. Increasing pCO2 was accompanied by higher bacterial richness and by a strong increase in rare members in both bacterial and archaeal communities. Microbial communities from sites with CO2 concentrations close to today's conditions had different structures than those of sites with elevated CO2 levels. We also observed increasing sequence abundance of several organic matter degrading types of Flavobacteriaceae and Rhodobacteraceae, which paralleled concurrent shifts in benthic cover and enhanced primary productivity. With increasing pCO2, sequences related to bacterial nitrifying organisms such as Nitrosococcus and Nitrospirales decreased, and sequences affiliated to the archaeal ammonia-oxidizing Thaumarchaeota Nitrosopumilus maritimus increased. Our study suggests that microbial community structure and diversity, and likely key ecosystem functions, may be altered in coastal sediments by long-term CO2 exposure to levels predicted for the end of the century.

(Table 2) Chemical composition of Fe-Mn nodules and volcanic glass in sediments of the Atlantic Ocean

Distribution pattern of titanium in Quaternary sediments of the Atlantic Ocean are examined on the base of 750 Ti and Fe determinations, and several dozen of complete chemical analyses. Analyses of surface sediment samples and sediment cores up to 6 m long were made. Stratigraphic levels from Middle Pleistocene to Holocene were identified from planktonic foraminifera. Distributions of Ti in recent and Pleistocene deposits were mapped. High titanium contents were found in sediments containing products of basalt vulcanism and in iron-manganese nodules. To determine origin of titanium concentrations in sediments, Ti/Fe ratios were calculated. Maximal values of this ratio were found in areas of basaltic volcanism and of intensive terrigenous sedimentation.

Gas hydrates of nonpressurized (GC) and pressurized (DAPC) sediment cores from the Hakon Mosby Mud Volcano, SW Barents Sea

The occurrence of gas hydrates at submarine mud volcanoes (MVs) located within the gas hydrate stability zone (GHSZ) is controlled by upward fluid and heat flux associated with MV activity. Determining the spatial distribution of gas hydrates at MVs is crucial to evaluate their sensitivity to known episodic changes in volcanic activity. We determined the hydrocarbon inventory and spatial distribution of hydrates at an individual MV structure. The Håkon Mosby Mud Volcano (HMMV), located at 1,250 m water depth on the Barents Sea slope, was investigated by combined pressure core sampling, heat flow measurements, and pore water chemical analysis. Quantitative pressure core degassing revealed gas-sediment ratios between 3.1 and 25.7, corresponding to hydrate concentrations of up to 21.3% of the pore volume. Hydrocarbon compositions and physicochemical conditions imply that gas hydrates incipiently crystallize as structure I hydrate, with a dissociation temperature of around 13.8°C at this water depth. Based on numerous in situ measurements of the geothermal gradient in the seabed, pore water sulfate profiles and microbathymetric data, we show that the thickness of the GHSZ increases from less than 1 m at the warm center to around 47 m in the outer parts of the HMMV. We estimate the total mass of hydrate-bound methane stored at the HMMV to be about 102.5 kt, of which 2.8 kt are located within the morphological Unit I around the center and thus are likely to be dissociated in the course of a large eruption.

Chemical composition of volcanic glasses from bottom sediments of the Transpacific profile

Lithofacial types of sediments formed in certain geographic and physical-chemical conditions of the Pacific Ocean are distinguished and characterized. It is shown that the regular change of bottom sediment types forming a genetic series from the coast to the pelagic zone clearly demonstrates a leading role of biogenic-terrigenous sedimentation in their formation. In the pelagic zone of the ocean erosion of islands and seamounts, basalt volcanism of anticlinal uplifts, as well as exhalative contribution play some role in addition to the main source of terrigenous and pyroclastic material from continents. These sources do not change, but only complicate terrigenous sedimentation in the studied area of the ocean.

3D Attenuation Tomography of the Volcanic Island of Tenerife (Canary Islands)

Peer reviewed

Frontier exploration and the North Atlantic Igneous Province : new insights from a 2.6 km offshore volcanic sequence in the NE Faroe–Shetland Basin

Peer reviewed

3D Attenuation Tomography of the Volcanic Island of Tenerife (Canary Islands)

Peer reviewed

Frontier exploration and the North Atlantic Igneous Province : new insights from a 2.6 km offshore volcanic sequence in the NE Faroe–Shetland Basin

Peer reviewed

3D Attenuation Tomography of the Volcanic Island of Tenerife (Canary Islands)

Peer reviewed

Geochemistry and radiogenic isotope ratios of the volcanic sequences from Izu-Bonin Arc, Japan

New radiogenic isotope and trace element data are presented for the volcanic sequences along 600 km of the active Izu-Bonin arc, the Oligocene Izu arc, and their associated rift basins. As with many intra-oceanic island arcs, the Pliocene-Recent Izu-Bonin frontal-arc lavas are highly depleted in Zr, Nb and the rare-earth elements relative to typical mid-ocean ridge basalt (MORB), indicating that the mantle wedge source has undergone a previous episode of melting. Ratios between these elements (such as Nb/Zr and La/Sm), as well as 143Nd/144Nd, do not vary significantly along the length of the frontal-arc. These parameters suggest that each of the arc volcanoes is derived from similar melt fractions of the mantle wedge. However, Ba/Zr, Ba/Rb and 87Sr/86Sr increase along the frontal-arc to the north. This leads us to propose that a variable enrichment in Ba and radiogenic Sr is superimposed on the mantle wedge. Sr-Nd and Pb-Nd isotope variation indicate that both Sr and Pb become more radiogenic after fluid addition. However, Pb isotope ratios do not correlate with increases in Pb concentration or ratios such as Ba/Zr and Nb/Pb. In other words, the Pb isotopic composition of the arc lavas appears to be independent of the amount of Pb introduced by subduction fluids into the mantle source. This buffering of Pb isotopes along the frontal-arc means that the isotopic composition of the lavas is indistinguishable from that of the fluid. Isotopic mixing models presented for the arc are only illustrative of the many plausible combinations of components and quantities. Despite this, we are able to determine that the mantle wedge has isotopic characteristics similar to Indian Ocean MORB, and that the subduction-fluid solute is primarily derived from subducted oceanic basalt with a <2% contribution from subducted sediment. Lavas in the Oligocene Izu arc and fore-arc basin were derived from a mantle wedge of similar composition to the active arc. Despite levels of Pb enrichment comparable to those of the modern arc, the Pb isotopes of the Oligocene volcanics indicate a lower sediment input into the melting region.

Argon analyses of volcanic rocks from the Japan Sea floor

Volcanic rocks recovered from the Japan Sea during ODP Legs 127 and 128 were analyzed by 40Ar-39Ar whole-rock stepwise-heating experiments. All three experiments on samples from Site 795 in the Japan Basin revealed disturbed age spectra, but they are consistent with crystallization ages of 15 to 25 Ma for the samples. At Site 797 in the Yamato Basin, three of the five samples showed plateau ages of 18-19 Ma. At Site 794 in the northern Yamato Basin, three of the five samples revealed concordant age spectra of 20-21 Ma. The radiometric age results are consistent with the estimated ages for the oldest sediments at Site 797 based on the biostratigraphy, but are significantly older than those of the oldest sediments at Site 794. However, the radiometric ages are concordant with previously inferred ages for the formation of the Japan Sea floor based on radiometric age data from dredged igneous rocks from the Japan Sea. The present results indicate that formation of the Japan Sea floor started at least 19-20 Ma ago and give more precise age constraints.

Microbialites and lipid biomarker from coral reefs off the volcanic islands Tahiti and Vanuatu and from the nonvolcanic sites Belize and the Maldives

The occurrence of microbialites in post-glacial coral reefs has been interpreted to reflect an ecosystem response to environmental change. The greater thickness of microbialites in reefs with a volcanic hinterland compared to thinner microbial crusts in reefs with a non-volcanic hinterland led to the suggestion that fertilization of the reefal environment by chemical weathering of volcanic rocks stimulated primary productivity and microbialite formation. Using a molecular and isotopic approach on reef-microbialites from Tahiti (Pacific Ocean), it was recently shown that sulfate-reducing bacteria favored the formation of microbial carbonates. To test if similar mechanisms induced microbialite formation in other reefs as well, the Tahitian microbialites are compared with similar microbialites from coral reefs off Vanuatu (Pacific Ocean), Belize (Caribbean Sea, Atlantic Ocean), and the Maldives (Indian Ocean) in this study. The selected study sites cover a wide range of geological settings, reflecting variable input and composition of detritus. The new lipid biomarker data and stable sulfur isotope results confirm that sulfate-reducing bacteria played an intrinsic role in the precipitation of microbial carbonate at all study sites, irrespective of the geological setting. Abundant biomarkers indicative of sulfate reducers include a variety of terminally-branched and mid chain-branched fatty acids as well as mono-O-alkyl glycerol ethers. Isotope evidence for bacterial sulfate reduction is represented by low d34S values of pyrite (-43 to -42 per mill) enclosed in the microbialites and, compared to seawater sulfate, slightly elevated d34S and d18O values of carbonate-associated sulfate (21.9 to 22.2 per mill and 11.3 to 12.4 per mill, respectively). Microbialite formation took place in anoxic micro-environments, which presumably developed through the fertilization of the reef environment and the resultant accumulation of organic matter including bacterial extracellular polymeric substances (EPS), coral mucus, and marine snow in cavities within the coral framework. ToF-SIMS analysis reveals that the dark layers of laminated microbialites are enriched in carbohydrates, which are common constituents of EPS and coral mucus. These results support the hypothesis that bacterial degradation of EPS and coral mucus within microbial mats favored carbonate precipitation. Because reefal microbialites formed by similar processes in very different geological settings, this comparative study suggests that a volcanic hinterland is not required for microbialite growth. Yet, detrital input derived from the weathering of volcanic rocks appears to be a natural fertilizer, being conductive for the growth of microbial mats, which fosters the development of particularly abundant and thick microbial crusts.

Chemical composition of tephra from the Greenland-Iceland-Norwegian-Sea

Explosive ocean island volcanism in the Greenland-Iceland-Norwegian Sea (GIN Sea) is indicated by marine tephra layers at 10-300 ka. Peaks of explosive volcanism occurred in oxygen isotope stages 8, 7, 5 and 1. The depositional age of the tephra was estimated using the oxygen isotope stratigraphy and dating of marine records. Geochemical analyses of the tephra layers show that all originate from Iceland. Here we report the characteristics of tephra from these major Icelandic events in 30 deep-sea cores from the GIN Sea. Our findings provide constraints on the distribution of tephra from the eruption source. For the Vedde Ash (oxygen isotope stage 1) we estimate a minimum fallout area of 2*10**5 km**2, stretching from central Greenland in the west and southern Sweden in the east, to 71°N in the GIN Sea. The magnitude of the eruption and the regional wind conditions controlled the extent and concentrations of these ash fallout events. Oceanic circulation and differential settling may have affected the distribution and final deposition of ash particles such as bubble wall shards.