Isotope ratios, trace elements and ultrastructure results of Mytilus galloprovincialis shells and experimental site data off Ischia (Italy)

Bivalve shells can provide excellent archives of past environmental change but have not been used to interpret ocean acidification events. We investigated carbon, oxygen and trace element records from different shell layers in the mussels Mytilus galloprovincialis combined with detailed investigations of the shell ultrastructure. Mussels from the harbour of Ischia (Mediterranean, Italy) were transplanted and grown in water with mean pHT 7.3 and mean pHT 8.1 near CO2 vents on the east coast of the island. Most prominently, the shells recorded the shock of transplantation, both in their shell ultrastructure, textural and geochemical record. Shell calcite, precipitated subsequently under acidified seawater responded to the pH gradient by an in part disturbed ultrastructure. Geochemical data from all test sites show a strong metabolic effect that exceeds the influence of the low-pH environment. These field experiments showed that care is needed when interpreting potential ocean acidification signals because various parameters affect shell chemistry and ultrastructure. Besides metabolic processes, seawater pH, factors such as salinity, water temperature, food availability and population density all affect the biogenic carbonate shell archive.

Concentrations of major-element oxides and minor and trace elements at DSDP Legs 87, 57, 56 and 31

Sediments from Sites 582 (11 samples), 583 (19 samples), 584 (31 samples), 294 (1 sample), 296 (9 samples), 297 (3 samples), 436 (11 samples), and 439 (3 samples) were analyzed by X-ray fluorescence and/or instrumental neutron activation analysis. Ten major elements and 24 minor and trace elements (including 7 rare earth elements) were determined with these methods. Geochemistry varies systematically with both the site location and sediment age. Such variations are explained in terms of changes in sedimentation processes caused by plate motion and changes in ocean currents.

Platinum group elements and geochemistry of altered oceanic crust of ODP Hole 140-504B

Primary chemical heterogeneity in the sheeted dike complex in Deep Sea Drilling Project Hole 504B makes these rocks unsuitable for conventional mass balance calculations in determining element mobility associated with hydrothermal alteration. Due to the original heterogeneity and variable degrees of fractionation in the dikes, an appropriate reference sample on which calculations can be based is difficult to find. Therefore, the use of incompatible element ratios is developed to evaluate geochemical changes during alteration(s). For example, on a Zr/Yb-La/Yb plot, scatter along a straight line suggests tapping of a variably depleted mantle source and deviation from the line suggests element mobility (gain or loss). Using this method, our data indicates that the hydrothermal evolution of the sheeted dike complex was accompanied by significant loss of Cu, Zn, and Ti and some loss of La. The sheeted dike complex has low platinum group element (PGE) concentrations and steep PGE patterns, typical of mid-ocean ridge basalts (MORBs) on the global scale. We propose that the unusual PGE patterns of MORBs cannot be entirely generated by a partial melting and sulfide segregation model; instead, these patterns in part must have been inherited from their mantle source. The Au data show no evidence for mobilization during hydrothermal alteration of the dikes.

Rare earth elements of modern seeps from the Gulf of Mexico, the Black Sea, and the Congo Fan

At marine seeps, methane is microbially oxidized resulting in the precipitation of carbonates close to the seafloor. Methane oxidation leads to sulfate depletion in sediment pore water, which induces a change in redox conditions. Rare earth element (REE) patterns of authigenic carbonate phases collected from modern seeps of the Gulf of Mexico, the Black Sea, and the Congo Fan were analyzed. Different carbonate minerals including aragonite and calcite with different crystal habits have been selected for analysis. Total REE content (SumREE) of seep carbonates varies widely, from 0.1 ppm to 42.5 ppm, but a common trend is that the SumREE in microcrystalline phases is higher than that of the associated later phases including micospar, sparite and blocky cement, suggesting that SumREE may be a function of diagenesis. The shale-normalized REE patterns of the seep carbonates often show different Ce anomalies even in samples from a specific site, suggesting that the formation conditions of seep carbonates are variable and complex. Overall, our results show that apart from anoxic, oxic conditions are at least temporarily common in seep environments.

Ordinary and duplicate analysis from major and trace elements of ODP Holes 146-888B and 168-1027B

Oceanic sediments deposited at high rate close to continents are dominated by terrigenous material. Aside from dilution by biogenic components, their chemical compositions reflect those of nearby continental masses. This study focuses on oceanic sediments coming from the juvenile Canadian Cordillera and highlights systematic differences between detritus deriving from juvenile crust and detritus from old and mature crust. We report major and trace element concentrations for 68 sediments from the northernmost part of the Cascade forearc, drilled at ODP Sites 888 and 1027. The calculated weighted averages for each site can then be used in the future to quantify the contribution of subducted sediments to Cascades volcanism. The two sites have similar compositions but Site 888, located closer to the continent, has higher sandy turbidite contents and displays higher bulk SiO2/Al2O3 with lower bulk Nb/Zr, attributed to the presence of zircons in the coarse sands. Comparison with published data for other oceanic sedimentary piles demonstrates the existence of systematic differences between modern sediments deriving from juvenile terranes (juvenile sediments) and modern sediments derived from mature continental areas (cratonic sediments). The most striking systematic difference is for Th/Nb, Th/U, Nb/U and Th/Rb ratios: juvenile sediments have much lower ratios than cratonic sediments. The small enrichment of Th over Nb in cratonic sediments may be explained by intracrustal magmatic and metamorphic differentiation processes. In contrast, their elevated Th/U and Nb/U ratios (average values of 6.87 and 7.95, respectively) in comparison to juvenile sediments (Th/U ~ 3.09, Nb/U ~ 5.15) suggest extensive U and Rb losses on old cratons. Uranium and Rb losses are attributed to long-term leaching by rain and river water during exposure of the continental crust at the surface. Over geological times, the weathering effects create a slow but systematic increase of Th/U with exposure time.

Lignin, phenols, and some chemical elements in aerosols and bottom sediments from the Tropical North Atlantic

A study of composition of biomarkers (lignin and phenols) in aerosols and bottom sediments from the Tropical North Atlantic was carried out. It was shown that organic matter of aerosols was mostly composed of products of terrestrial plants (arboreal fibers, pollen, and spores). Biomarker composition in the aerosols and in the bottom sediments was practically similar, which proved delivery of terrigenous organic matter to the ocean via the atmosphere.

Chemical composition and accumulation rates of chemical elements in metalliferous sediments from axial zones of the East Pacific Rise and the Mid-Atlantic Ridge

In the monograph metalliferous sediments of the East Pacific Rise near 21°S are under consideration. Distribution trends of chemical, mineral and grain size compositions of metalliferous sediments accumulated near the axis of this ultrafast spreading segment of the EPR are shown. On the basis of lithological and geochemical investigations spatial and temporal variations of hydrothermal activity are estimated. Migration rates of hydrothermal fields along the spreading axis are calculated. The model of cyclic hydrothermal process is suggested as a result of tectono-magmatic development of the spreding centre.

Geochemistry, isotopes, trace elements and fluid inclusion geothermometry at DSDP Hole 83-504B

Stockwork-like metal sulfide mineralizations were found at 910-928 m below seafloor (BSF) in the pillow/dike transition zone of Hole 504B. This is the same interval where most physical properties of the 5.9-m.y.-old crust of the Costa Rica Rift change from those characteristic of Layer 2B to those of Layer 2C. The pillow lavas, breccias, and veins of the stockwork-like zone were studied by transmitted and reflected light microscopy, X-ray diffraction, and electron microprobe analysis. Bulk rock oxygen isotopic analyses as well as isolated mineral oxygen and sulfur isotopic analyses and fluid inclusion measurements were carried out. A complex alteration history was reconstructed that includes three generations of fissures, each followed by precipitation of characteristic hydrothermal mineral parageneses: (1) Minor and local deposition of quartz occurred on fissure walls; adjacent wall rocks were silicified, followed by formation of chlorite and minor pyrite I in the veins, whereas albite, sphene, chlorite and chlorite-expandable clay mixtures, actinolite, and pyrite replaced igneous phases in the host rocks. The hydrothermal fluids responsible for this first stage were probably partially reacted seawater, and their temperatures were at least 200-250° C. (2) Fissures filled during the first stage were reopened and new cracks formed. They were filled with quartz, minor chlorite and chlorite-expandable clay mixtures, traces of epidote, common pyrite, sphalerite, chalcopyrite, and minor galena. During the second stage, hydrothermal fluids were relatively evolved metal- and Si-rich solutions whose temperatures ranged from 230 to 340° C. The fluctuating chemical composition and temperature of the solutions produced a complex depositional sequence of sulfides in the veins: chalcopyrite I, ± Fe-rich sphalerite, chalcopyrite II ("disease"), Fe-poor sphalerite, chalcopyrite III, galena, and pyrite II. (3) During the last stage, zeolites and Mg-poor calcite filled up the remaining spaces and newly formed cracks and replaced the host rock plagioclase. Analcite and stilbite were first to form in veins, possibly at temperatures below 200°C; analcite and earlier quartz were replaced by laumontite at 250°C, whereas calcite formation temperature ranged from 135 to 220°C. The last stage hydrothermal fluids were depleted in Mg and enriched in Ca and 18O compared to seawater and contained a mantle carbon component. This complex alteration history paralleling a complex mineral paragenesis can be interpreted as the result of a relatively long-term evolution of a hydrothermal system with superimposed shorter term fluctuations in solution temperature and composition. Hydrothermal activity probably began close to the axis of the Costa Rica Rift with the overall cooling of the system and multiple fracturing stages due to movement of the crust away from the axis and/or cooling of a magmatic heat source.