Chemical composition and isotopic ratios of basic lavas from Iceland and the surrounding ocean floor

Major and trace dement data are used to establish the nature and extent of spatial and temporal chemical variations in basalts erupted in the Iceland region of the North Atlantic Ocean. The ocean floor samples are those recovered by legs 38 and 49 of the Deep Sea Drilling Project. Within each of the active zones on Iceland there are small scale variations in the light rare earth elements and ratios such as K/Y: several central complexes and their associated fissure swarms erupt basalts with values of K/Y distinct from those erupted at adjacent centres; also basalts showing a wide range of immobile trace element ratios occur together within single vertical sections and ocean floor drill holes. Although such variations can be explained in terms of the magmatic processes operating on Iceland they make extrapolations from single basalt samples to mantle sources underlying the outcrop of the sample highly tenuous. 87Sr/86Sr ratios measured for 25 of the samples indicate a total range from 0.7028 in a tholeiite from the Reykjanes Ridge to 0.7034 in an alkali basalt from Iceland and are consistent with other published ratios from the region. A positive correlation between 87Sr/86Sr and Ce/Yb ratios indicates the existence of systematic isotopic and elemental variations in the mantle source region. An approximately fivefold variation in Ce/Yb ratio observed in basalts with the same 87Sr/86Sr ratio implies that different degrees and types of partial melting have been involved in magma genesis from a single mantle composition. 87Sr/86Sr ratios above 0.7028, Th/U ratios close to 4 and La/Ta ratios close to 10 distinguish most basalts erupted in this part of the North Atlantic Ocean from normal mid-ocean ridge basalt (N-type MORB) - although N-type MORB has been erupted at extinct spreading axes just to the north and northeast of Iceland as well as the presently active Iceland-Jan Mayen Ridge. Comparisons with the hygromagmatophile element and radiogenic isotope ratios of MORB and the estimated primordial mantle indicate that the mantle sources producing Iceland basalts have undergone previous depletion followed by more recent enrichment events. A veined mantle source region is proposed in preference to the mantle plume model to explain the chemical variations.

Age determination and grwoth rates of stalagmites and stalactites from the Okshola cave, Fauske, northern Norway

The sensitivity of terrestrial environments to past changes in heat transport is expected to be manifested in Holocene climate proxy records on millennial to seasonal timescales. Stalagmite formation in the Okshola cave near Fauske (northern Norway) began at about 10.4 ka, soon after the valley was deglaciated. Past monitoring of the cave and surface has revealed stable modern conditions with uniform drip rates, relative humidity and temperature. Stable isotope records from two stalagmites provide time-series spanning from c. 10380 yr to AD 1997; a banded, multi-coloured stalagmite (Oks82) was formed between 10380 yr and 5050 yr, whereas a pristine, white stalagmite (FM3) covers the period from ~7500 yr to the present. The stable oxygen isotope (delta18Oc), stable carbon isotope (delta13Cc), and growth rate records are interpreted as showing i) a negative correlation between cave/surface temperature and delta18Oc, ii) a positive correlation between wetness and delta13Cc, and iii) a positive correlation between temperature and growth rate. Following this, the data from Okshola show that the Holocene was characterised by high-variability climate in the early part, low-variability climate in the middle part, and high-variability climate and shifts between two distinct modes in the late part. A total of nine Scandinavian stalagmite delta18Oc records of comparable dating precision are now available for parts or most of the Holocene. None of them show a clear Holocene thermal optimum, suggesting that they are influenced by annual mean temperature (cave temperature) rather than seasonal temperature. For the last 1000 years, delta18Oc values display a depletion-enrichment-depletion pattern commonly interpreted as reflecting the conventional view on climate development for the last millennium. Although the delta18Oc records show similar patterns and amplitudes of change, the main challenges for utilising high-latitude stalagmites as palaeoclimate archives are i) the accuracy of the age models, ii) the ambiguity of the proxy signals, and iii) calibration with monitoring data.

Nitrogen isotope, total organic carbon and element concentration data for piston core M77/2-024-5 from the Peruvian continental margin

Piston core M77/2-024-5 was retrieved during the M77/2 cruise of Research Vessel Meteor in December 2008. Total organic carbon concentrations were determined using a Carlo Erba Element Analyzer (NA1500). Prior to analysis carbon bound to carbonate minerals was removed by leaching the sediment with 1 M HCl. Bulk nitrogen isotope ratios were determined using a Carlo Erba Element Analyzer (NA1500) coupled to a DeltaPlusXL isotope ratio mass spectrometer. Major and trace metals were analyzed after microwave-assisted (CEM MARS-5) acid digestion (HCl, HNO3 and HF) by inductively coupled plasma optical emission spectrometry (aluminum, titanium and iron) (Teledyne Leeman Prodigy) and inductively coupled plasma mass spectrometry (molybdenum and uranium) (THERMO X-Series 2).

Sediment and pore water geochemistry of sediment cores, Potter Cove, King George Island

Redox-sensitive trace metals (Mn, Fe, U, Mo, Re), nutrients and terminal metabolic products (NO3-, NH4+, PO43-, total alkalinity) were for the first time investigated in pore waters of Antarctic coastal sediments. The results of this study reveal a high spatial variability in redox conditions in surface sediments from Potter Cove, King George Island, western Antarctic Peninsula. Particularly in the shallower areas of the bay the significant correlation between sulphate depletion and total alkalinity, the inorganic product of terminal metabolism, indicates sulphate reduction to be the major pathway of organic matter mineralisation. In contrast, dissimilatory metal oxide reduction seems to be prevailing in the newly ice-free areas and the deeper troughs, where concentrations of dissolved iron of up to 700 µM were found. We suggest that the increased accumulation of fine-grained material with high amounts of reducible metal oxides in combination with the reduced availability of metabolisable organic matter and enhanced physical and biological disturbance by bottom water currents, ice scouring and burrowing organisms favours metal oxide reduction over sulphate reduction in these areas. Based on modelled iron fluxes we calculate the contribution of the Antarctic shelf to the pool of potentially bioavailable iron (Feb) to be 6.9x10**3 to 790x10**3 t/yr. Consequently, these shelf sediments would provide an Feb flux of 0.35-39.5/mg/m**2/yr (median: 3.8 mg/m**2/yr) to the Southern Ocean. This contribution is in the same order of magnitude as the flux provided by icebergs and significantly higher than the input by aeolian dust. For this reason suboxic shelf sediments form a key source of iron for the high nutrient-low chlorophyll (HNLC) areas of the Southern Ocean. This source may become even more important in the future due to rising temperatures at the WAP accompanied by enhanced glacier retreat and the accumulation of melt water derived iron-rich material on the shelf.

Chemistry of Jurassic/Cretaceous black shales

Black shale samples of Jurassic to Cretaceous age recovered during the 'Norwegian Shelf Drilling Program' between 1987 and 1991 from Sites 7430/10-U-01 (Barents Sea), 6814/04-U-02 (Norwegian Shelf near the Lofoten) and 6307/07-U-02 (Norwegian Shelf near Trondheim) were analyzed for major and trace elements. These laminated black shales are characterized by high total organic carbon (TOC) and total sulfur (TS) contents as well as by significant enrichments in several redox-sensitive and/or sulfide-forming trace metals (Ag, Bi, Cd, Co, Cr, Cu, Mo, Ni, Re, Sb, Tl, U, V, and Zn). Enrichment factors relative to 'average shale' are comparable to those found in Cenomanian-Turonian boundary event (CTBE) black shales and Mediterranean sapropels. The Re content is high in the studied black shales, with maximum values up to 1221 ng/g. Re/Mo ratios averaging 2.3*10**-3 are close to the seawater value. High trace metal enrichments and Re/Mo ratios close to the seawater value point to a dominantly anoxic and sulfidic water column during black shale formation. Interbeds with higher Re/Mo ratios, especially in high-resolution sampled core sections, point to brief periods of suboxic conditions. Additionally, enhanced Zn concentrations in the black shales from the Barents Sea support the assumption that hydrothermal activity was also high during black shale deposition. Trace metal signatures of black shales at different drill sites on a transect along the Norwegian Shelf are not only influenced by water depth but also by their location in the boreal realm. Metal enrichments are higher in the northern compared to the southern sites. Volgian (=Tithonian 151-144 Ma BP) black shales exhibit elevated trace metal contents in comparison to their Berriasian (144-137 Ma BP) counterparts. This probably reflects a change in the circulation pattern during periods of black shale formation. Therefore different paleoceanographic conditions, probably controlled by climatic change linked to the transgression of the paleo-sealevel and the North Atlantic opening, may have developed from the Volgian to the Berriasian.

Granulometry, mineralogy and geochemistry of sediments from ODP Hole 169-1037B and Site 169-1038

Central Hill is in the northern part of the Escanaba Trough, which is a sediment-filled rift of southern Gorda Ridge. Central Hill is oriented north-south and is associated with extensive sulfide deposits. Hydrothermal alteration of sediment from Site 1038 was studied through analyses of mineralogy and the chemistry and oxygen isotopic compositions of one nearly pure clay sample. In addition, Site 1037 was drilled to establish the character of the unaltered sedimentary sequence away from the hydrothermal centers of the Northern Escanaba Trough Study Area (NESCA). Mineralogy of the clay-size fraction of turbiditic and hemipelagic sediments of Hole 1037B are predominantly quartz, feldspar, pyroxene, illite, chlorite, and smectite, representing continental-derived material. Cores from Hole 1038I, located within the area of Central Hill but away from known active vent areas, recovered minor amounts of chlorite/smectite mixed-layer clay in the fine fraction, indicating a low-temperature hydrothermal alteration. The 137.4-m-thick sediment section of Hole 1038G is located in an area of low-temperature venting. The uppermost sample is classified as chlorite/smectite mixed layer, which is underlain by chlorite as the dominant mineral. The lowermost deposits of Hole 1038G are also characterized by chlorite/smectite mixed-layer clay. In comparison to Hole 1038I, the mineralogic sequence of Hole 1038G reflects increased chloritization. Intensely altered sediment is almost completely replaced by hydrothermal chlorite in subsurface sediments of Hole 1038H. Alteration to chlorite is characterized by depletion in Na, K, Ti, Ca, Sr, Cs, and Tl and enrichment in Ba. Further, Eu depletion reflects a high-temperature plagioclase alteration. A chlorite 18O value of 2.6 indicates formation at a temperature of ~190°C. It is concluded that the authigenic chlorite in Hole 1038H formed by an active high-temperature fluid flow in the shallow subsurface.

(Table DR1) Trace element contents in basalts from ODP Hole 33-317A

Numerous large igneous provinces formed in the Pacific Ocean during Early Cretaceous time, but their origins and relations are poorly understood. We present new geochronological and geochemical data on rocks from the Manihiki Plateau and compare these results to those for other Cretaceous Pacific plateaus. A dredged Manihiki basalt gives an 40Ar-39Ar age of 117.9+/-3.5 Ma (2 sigma), essentially contemporaneous with the Ontong Java Plateau ~2500 km to the west, and the possibly related Hikurangi Plateau ~3000 km to the south. Drilled Manihiki lavas are tholeiitic with incompatible trace element abundances similar to those of Ontong Java basalts. These lavas may result from high degrees of partial melting during the main eruptive phase of plateau formation. There are two categories of dredged lavas from the Danger Islands Troughs, which bisect the plateau. The first is alkalic lavas having strong enrichments in light rare earth and large-ion lithophile elements; these lavas may represent late-stage activity, as one sample yields an 40Ar-39Ar age of 99.5+/-0.7 Ma. The second category consists of tholeiitic basalts with U-shaped incompatible element patterns and unusually low abundances of several elements; these basalts record a mantle component not previously observed in Manihiki, Ontong Java, or Hikurangi lavas. Their trace element characteristics may result from extensive melting of depleted mantle wedge material mixed with small amounts of volcaniclastic sediment. We are unaware of comparable basalts elsewhere.