ALBERT

Beschreibung: Reconstructed sea surface temperatures (SSTs) derived from Mg/Ca measurements in nine encrusting coralline algal skeletons from the Aleutian archipelago in the northernmost Pacific Ocean reveal an overall increase in SST from 1665 to 2007. In the Aleutian SST reconstruction, decadal-scale variability is a transient feature present during the 1700s and early 1800s and then fully emerging post-1950. SSTs vary coherently with available instrument records of cyclone variance and vacillate in and out of coherence with multicentennial Pacific Northwest drought reconstructions as a response to SST-driven alterations of storm tracks reaching North America. These results indicate that an influence of decadal-scale variability on the North Pacific storm tracks only became apparent during the midtwentieth century. Furthermore, what has been assumed as natural variability in the North Pacific, based on twentieth century instrumental data, is not consistent with the long-term natural variability evident in reconstructed SSTs predating the anthropogenic influence.

Beschreibung: This study reports Mo isotopic compositions for fifty-two Palaeozoic granitic rocks with contrasting source affinities (A-, I- and S-type) from the Lachlan Fold Belt (LFB) and the New England Batholith (NEB), both in SE Australia, and three compositionally zoned plutons (Loch Doon, Criffell, and Fleet) located in the Southern Uplands of Scotland. The results show relatively large variations in δ98Mo for igneous rocks ranging from −1.73‰ to 0.59‰ with significant overlaps between different types. No relationships between δ98Mo and δ18O or ASI (Alumina Saturation Index) are observed, indicating that Mo isotopes do not clearly distinguish igneous vs. sedimentary source types. Instead, effects of igneous processes, source mixing, regional geology, as well as hydrothermal activity control the Mo isotope compositions in these granites. It is found that Mo is mainly accommodated in biotite and to a lesser extent in hornblende. Hornblende and Fe3+-rich minerals may preferentially incorporate light isotopes, as reflected by negative correlations between δ98Mo and K/Rb and [Fe2O3]. There is a positive correlation between initial 87Sr/86Sr and δ98Mo in I-type granitic rocks, reflecting the admixing of material from isotopically distinct sources. Granitic rocks from Scotland and Australia display strikingly similar curvilinear trends in δ98Mo vs. initial 87Sr/86Sr despite the differing regional geology. Localized hydrothermal effects can result in low δ98Mo in granite, as seen in three samples from Loch Doon and Criffell which have anomalously light δ98Mo of 〈−1‰. Based on this study, an estimate of δ98Mo = 0.14 ± 0.07‰ (95% s.e.) for the Phanerozoic upper crust is proposed. This is slightly heavier than basalts indicating an isotopically light lower crust and/or a systematic change to the crust resulting from subduction of isotopically light dehydrated slab and/or pelagic sediment over time.

Beschreibung: We investigated the controls of hydrography and of scavenging on the distribution of the particle reactive radionuclides 231Pa and 230Th in the water column and in surface sediments off Southwest Africa (Angola and Cape basins). Based on a vertical section of total 230Thex concentrations in the water column we show that small differences in the salinity between the North Atlantic Deep Water (NADW) in the Angola Basin and the NADW in the Cape Basin as well as the advection of NADW associated with the Namib Col Current are reflected in total 230Thex concentrations. These variable total concentrations are believed to reflect the flow path and mixing history of NADW with the NADW in the Angola Basin being relatively older and 230Th enriched compared to the NADW in the Cape Basin. In the area investigated we found high 231Paex/230Thex ratios (231Paex/230Thex 〉 0.093) in surface sediments at the continental margin and lower ones (231Paex/230Thex 〈 0.093) in the open ocean. Such a distribution is normally interpreted to result from high particle flux at ocean margins (boundary scavenging). However, the lack of any significant depletion of dissolved 230Th and 231Pa in the water column does not indicate extensive scavenging at the continental margin. High 231Paex/230Thex ratios are constrained to shallow waters depths (〈 2000 m) only and coincide with low fractionation between 231Pa and 230Th indicating that preferential scavenging of 231Pa on opal may have caused high 231Paex/230Thex ratios in the sediments. The observed close negative correlation (r2 = 0.82) between 231Paex/230Thex ratios in sediments and water depths is believed to reflect changes in the particle composition, i.e. a decrease in opal content with water depth. In the Angola and Cape basins the total 231Paex concentrations in NADW were the highest observed so far in the Atlantic Ocean, and they are attributed to the meridional export of 231Pa from the North Atlantic. This caused the average dissolved 231Pa/230Th in the Southeast Atlantic to be about a factor 2 higher when compared to the North Atlantic (Labrador Sea). These differences in the dissolved 231Pa/230Th were not reflected in 231Pa/230Th ratios of surface sediments because the fractionation is lower in the Labrador Sea compared to the Southeast Atlantic, i.e. fractionation counteracts changes in the dissolved 231Pa/230Th. This suggests that fractionation is more important for the determination of 231Paex/230Thex ratios in sediments than the meridional export of 231Pa from the North Atlantic.

Beschreibung: Hydrothermal vent deposits form on the seafloor as a result of cooling and mixing of hot hydrothermal fluids with cold seawater. Amongst the major sulfide and sulfate minerals that are preserved at vent sites, barite (BaSO4) is unique because it requires the direct mixing of Ba-rich hydrothermal fluid with sulfate-rich seawater in order for precipitation to occur. Because of its extremely low solubility, barite crystals preserve geochemical fingerprints associated with conditions of formation. Here, we present data from petrographic and geochemical analyses of hydrothermal barite from the Endeavour Segment of the Juan de Fuca Ridge, northeast Pacific Ocean, in order to determine the physical and chemical conditions under which barite precipitates within seafloor hydrothermal vent systems. Petrographic analyses of 22 barite-rich samples show a range of barite crystal morphologies: dendritic and acicular barite forms near the exterior vent walls, whereas larger bladed and tabular crystals occur within the interior of chimneys. A two component mixing model based on Sr concentrations and 87Sr/86Sr of both seawater and hydrothermal fluid, combined with 87Sr/86Sr data from whole rock and laser-ablation ICP-MS analyses of barite crystals indicate that barite precipitates from mixtures containing as low as 17% and as high as 88% hydrothermal fluid component, relative to seawater. Geochemical modelling of the relationship between aqueous species concentrations and degree of fluid mixing indicates that Ba2+ availability is the dominant control on mineral saturation. Observations combined with model results support that dendritic barite forms from fluids of less than 40% hydrothermal component and with a saturation index greater than ∼0.6, whereas more euhedral crystals form at lower levels of supersaturation associated with greater contributions of hydrothermal fluid. Fluid inclusions within barite indicate formation temperatures of between ∼120 °C and 240 °C during barite crystallization. The comparison of fluid inclusion formation temperatures to modelled mixing temperatures indicates that conductive cooling of the vent fluid accounts for 60–120 °C reduction in fluid temperature. Strontium zonation within individual barite crystals records fluctuations in the amount of conductive cooling within chimney walls that may result from cyclical oscillations in hydrothermal fluid flux. Barite chemistry and morphology can be used as a reliable indicator for past conditions of mineralization within both extinct seafloor hydrothermal deposits and ancient land-based volcanogenic massive sulfide deposits.

Beschreibung: Conflicting results have been reported for the stable Sr isotope fractionation, specifically with respect to the influence of temperature. In an experimental study we have investigated the stable Sr isotope systematics for inorganically precipitated and biogenic (coral) aragonite (natural and laboratory-cultured). Inorganic aragonite precipitation experiments were performed from natural seawater using the CO2 diffusion technique. The experiments were performed at different temperatures and different carbonate ion concentrations. 88Sr/86Sr of the inorganic aragonite precipitated in the experiments are 0.2‰ lighter than seawater, but showed no correlation to the water temperature or to CO32− concentration. Similar observations are made in different coral species (Cladocora caespitosa, Porites sp. and Acropora sp.), with identical fractionation from the bulk solution and no correlation to temperature or CO32− concentration. The lack of 88Sr/86Sr variability in corals at different environmental parameters and the similarity to the 88Sr/86Sr fractionation in inorganic aragonite may indicate a similar Sr incorporation mechanism in corals skeleton and inorganic aragonite, and therefore the previously proposed Rayleigh-based multi element model (Gaetani et al., 2011) cannot explain the process of Sr incorporation in the coral skeletal material. It is proposed that the relatively constant 88Sr/86Sr fractionation in aragonite can be used for paleo reconstruction of seawater 88Sr/86Sr composition. The seawater 88Sr/86Sr ratio reconstruction can be further used in calcite samples to reconstruct paleo precipitation rates.

Beschreibung: Deposits of coral-bearing, marine shell conglomerate exposed at elevations higher than 20 m above present-day mean sea level (MSL) in Bermuda and the Bahamas have previously been interpreted as relict intertidal deposits formed during marine isotope stage (MIS) 11, ca. 360–420 ka before present. On the strength of this evidence, a sea level highstand more than 20 m higher than present-day MSL was inferred for the MIS 11 interglacial, despite a lack of clear supporting evidence in the oxygen-isotope records of deep-sea sediment cores. We have critically re-examined the elevated marine deposits in Bermuda, and find their geological setting, sedimentary relations, and microfaunal assemblages to be inconsistent with intertidal deposition over an extended period. Rather, these deposits, which comprise a poorly sorted mixture of reef, lagoon and shoreline sediments, appear to have been carried tens of meters inside karst caves, presumably by large waves, at some time earlier than ca. 310–360 ka before present (MIS 9–11). We hypothesize that these deposits are the result of a large tsunami during the mid-Pleistocene, in which Bermuda was impacted by a wave set that carried sediments from the surrounding reef platform and nearshore waters over the eolianite atoll. Likely causes for such a megatsunami are the flank collapse of an Atlantic island volcano, such as the roughly synchronous Julan or Orotava submarine landslides in the Canary Islands, or a giant submarine landslide on the Atlantic continental margin.

Beschreibung: We investigated the influence of the composition of the vertical particle flux on the removal of particle reactive natural radionuclides (Th-230 and Pa-231) from the water column to the sediments. Radionuclide concentrations determined in sediment traps moored in the western, central and eastern Arabian Sea were related to the major components (carbonate, particulate organic matter (POC), opal, lithogenic material) of the particle flux. These data were combined with sediment trap data previously published from the Southern Ocean, Equatorial Pacific and North Atlantic [Z. Chase, R.F. Anderson, M.Q. Fleisher, P.W. Kubik, The influence of particle composition and particle flux on scavenging of Th, Pa and Be in the ocean, Earth Planet. Sci. Lett. 204 (2002) 215-229; J.C. Scholten, F. Fietzke, S. Vogler, M. Rutgers van der Loeff, A. Mangini, W Koeve, J. Waniek. P. Stoffers, A. Antia, J. Kass, Trapping efficiencies of sediment traps from the deep eastern North Atlantic: The Th-230 calibration, Deep Sea Research 1148 (2001) 2383-2408]. The correlations observed between the particle-dissolved distribution coefficients (K-d) of Th-230 and Pa-231 and the concentrations of the particle types depend on the sediment trap data set used. This result suggests that scavenging affinities of the nuclides differ between oceanic regions. Several factors (K-d values, reactive surface areas of particles, inter-correlations in closed data set) can, however, influence the observed relationships and thus hamper the interpretation of these correlation coefficients as a measure of relative scavenging affinities of the nuclides to the particle types investigated. The mean fractionation factor (F(Pa/Th)=K-d(Pa)/(K)d(Th)) from the Equatorial Pacific (F=0.11+/-0.03) is similar to that from the North Atlantic (F(Pa/Th)=0.077+/-0.026), and both are lower than the factors from the Arabian Sea (F(Pa/Th)=0.35+/-0.12) and from the Southern Ocean (F(Pa/Th) 0.87+/-0.4). For opal concentrations exceeding similar to60%, an increase in the fractionation factors is observed causing a higher mean fractionation factor for the Southern Ocean trap data set. For the other areas investigated, differences in the mean fractionation factors cannot be related to the particles types considered. In the Arabian Sea, seasonally variable Pa-231(ex)/Th-230(ex) ratios observed in the sediment traps as well as differences of the ratios between recently deposited phytodetritus (fluff) and normal surface sediments indicate seasonal changes in scavenging processes which the generally accepted reversible scavenging models do not envisage. We assume that variable sinking rates of particles, and/or particles not considered in this study (e.g. colloids, manganese oxides, transparent exopolymer particles) may play an important but as yet unexplored role in deep-water scavenging processes. (C) 2004 Elsevier B.V. All rights reserved.

Beschreibung: Zooxanthellate scleractinian corals are known as archives for temporal variations of climate variables, such as sea surface temperature, salinity or productivity. The use of azooxanthellate cold-water corals as potential archives for intermediate water mass properties and climate variability was tested recently. However, the correlation of established proxies such as delta O-18 and delta C-13 with temperature is difficult since there is no direct temperature equation applicable as in shallow-water corals. Other temperature proxies such as Sr/Ca, Mg/Ca and U/Ca are influenced by the complex microstructure of the aragonite skeleton, the rate of calcification, and other vital effects observed for coral species. For the first time we show that the stable strontium isotope ratio delta Sr-88/86 incorporated in the skeletons of the cold-water coral species Lophelia pertusa portrays the ambient seawater temperature. The temperature sensitivity from live samples collected along the European continental margin covering a temperature range from 6 degrees to 10 degrees C is 0.026 +/- 0.003%omicron/degrees C (2 sigma standard error) which is a sensitivity similar to the tropical shallow-water coral record of Pavona clavata. This indicates a similar fractionation process of strontium for both, zooxanthellate and azooxanthellate corals. For coral aragonite the delta Sr-88/86 ratio may serve as a new paleo-temperature proxy and introduces new perspectives in paleoceanography with respect to intermediate water dynamics.

Beschreibung: Our recent paper [McMurtry, G.M., Tappin, D.R., Sedwick, P.N., Wilkinson, I., Fietzke, J. and Sellwood, B., 2007a. Elevated marine deposits in Bermuda record a late Quaternary megatsunami. Sedimentary Geol. 200, 155–165.] critically re-examined elevated marine deposits in Bermuda, and concluded that their geological setting, sedimentary relations, micropetrography and microfaunal assemblages were inconsistent with sustained intertidal deposition. Instead, we hypothesized that these deposits were the result of a large tsunami that impacted the Bermuda island platform during the mid-Pleistocene. Hearty and Olson [Hearty, P.J., and Olson, S.L., in press. Mega-highstand or megatsunami? Discussion of McMurtry et al. “Elevated marine deposits in Bermuda record a late Quaternary megatsunami”: Sedimentary Geology, 200, 155–165, 2007 (Aug. 07). Sedimentary Geol. 200, 155–165.] in their response, attempt to refute our conclusions and claim the deposits to be the result of a +21 m eustatic sea level highstand during marine isotope stage (MIS) 11. In our reply we answer the issues raised by Hearty and Olson [Hearty, P.J., and Olson, S.L., in press. Mega-highstand or megatsunami? Discussion of McMurtry et al. “Elevated marine deposits in Bermuda record a late Quaternary megatsunami”: Sedimentary Geology, 200, 155–165, 2007 (Aug. 07). Sedimentary Geol. 200, 155–165.] and conclude that the Bermuda deposits do not provide unequivocal evidence of a prolonged + 21 m eustatic sea level highstand. Rather, the sediments are more likely the result of a past megatsunami in the North Atlantic basin.