Element and Sr isotope composition of interstitial waters in sediments of ODP Leg 129

Interstitial water samples from Leg 129, Sites 800, 801, and 802 in the Pigafetta and Mariana basins (central western Pacific), have been analyzed for major elements, B, Li, Mn, Sr, and 87Sr/86Sr. At all sites waters show enrichment in Ca and Sr and are depleted in Mg, K, Na, SO4, B, alkalinity, and 87Sr compared to seawater. These changes are related to alteration of basaltic material into secondary smectite and zeolite and recrystallization of biogenic carbonate. Water concentration depth profiles are characterized by breaks due to the presence of barriers to diffusion such as chert layers at Sites 800 and 801 and highly cemented volcanic ash at Site 802. In Site 800, below a chert layer, concentration depth profiles are vertical and reflect slight alteration of volcanic matter, either in situ or in the upper basaltic crust. Release of interlayer water from clay minerals is likely to induce observed Cl depletions. At Site 801, two units act as diffusion barrier and isolate the volcaniclastic sediments from ocean and basement. Diagenetic alteration of volcanic matter generates a chemical signature similar to that at Site 800. Just above the basaltic crust, interstitial waters are less evolved and reflect low alteration of the crust, probably because of the presence in the sediments of layers with low diffusivities. At Site 802, in Miocene tuffs, the chemical evolution generated by diagenetic alteration is extreme (Ca = 130 mmol, 87Sr/86Sr = 0.7042 at 83 meters below seafloor) and is accompanied by an increase of the Cl content (630 mmol) due to water uptake in secondary hydrous phases. Factors that enhance this evolution are a high sediment accumulation rate, high cementation preventing diffusive exchange and the reactive composition of the sediment (basaltic glass). The chemical variation is estimated to result in the alteration of more than 20% of the volcanic matter in a nearly closed system.

Biomass and stable isotope composition of plankton in five size fractions along a zonal transect (24°N) from the Canary Islands to Florida in January-March 2011

The spatial variability of biomass and stable isotopes in plankton size fractions in the upper 200 m was studied in a high spatial resolution transect along 24°N from Canary Islands to Florida (January - March 2011) during Leg 8 of the Malaspina-2010 expedition (http://www.expedicionmalaspina.es) to determine nitrogen and carbon sources. Plankton samples were collected by vertical tows of a microplankton net (40 mm mesh size) and a mesoplankton net (200 mm mesh size) through the upper 200 m of the water column. Sampling was between 10:00 and 16:00 h GMT. Plankton was separated into five size fractions (40 - 200, 200 - 500, 500 - 1000, 1000 - 2000 and > 2000 mm) by gentle filtration of the samples by a graded series of nylon sieves (2000, 1000, 500, 200 and 40 mm). Large gelatinous organisms were removed before filtration. Aliquots for each size fraction were collected on pre-weighed glass-fibre filters, dried (60°C, 48 h) and stored in a desiccator before determination of biomass (dry weight), carbon and nitrogen content and natural abundance of stable carbon and nitrogen isotopes ashore. Vertical advection of waters predominated in lateral zones while the central Atlantic (30-70°W) was characterized by a strong stratification and oligotrophic surface waters. Plankton biomass was low in the central zone and high in both eastern and western sides, with most of the variability due to either large (>2000 µm) and small plankton (<500 µm). Carbon isotopes reflected mainly the advection the deep water in lateral zones. Stable nitrogen isotopes showed a nearly symmetrical spatial distribution in all fractions, with the lowest values (delta15N <1per mill) in the central zone, and were inversely correlated to carbon stable isotopes (delta13C) and to the abundance of the nitrogen-fixer Trichodesmium. Diazotrophy was estimated to account for >50% of organic nitrogen in the central zone, and even >30% in eastern and western zones. The impact of diazotrophy increased with the size of the organisms, supporting the wide participation of all trophic levels in the processing of recently fixed nitrogen. These results indicate that atmospheric sources of carbon and nitrogen prevail over deep water sources in the subtropical North Atlantic and that the zone influenced by diazotrophy is much larger than reported in previous studies.

Mineralogy, chemistry and stable isotope composition of hydrothermal altered sheeted dikes of ODP Hole 111-504B

During ODP Leg 111 Hole 504B was extended 212 m deeper into the sheeted dikes of oceanic Layer 2, for a total penetration of 1288 m within basement. Study of the mineralogy, chemistry, and stable isotopic compositions of the rocks recovered on Leg 111 has confirmed and extended the previous model for hydrothermal alteration at the site: axial greenschist hydrothermal metamorphism was followed by seawater recharge and subsequent off-axis alteration. The dikes are depleted in 18O (mean delta18O = +5.1 ? +/- 0.6 ?) relative to fresh mid-ocean ridge basalt. Oxygen isotopic data on whole rocks and isolated secondary minerals indicate temperatures during axial metamorphism of 250°-350°C and water/rock ratios about one. Increasing amounts of actinolite with depth in the dike section, however, suggest that temperatures increased downward in the dikes. Pyrite + pyrrhotite + chalcopyrite + magnetite was the stable sulfide + oxide mineral assemblage during axial alteration, but these minerals partly re-equilibrated later at temperatures less than 200°C. The dikes sampled on Leg 111 contain an average of 500 ppm sulfur, slightly lower than igneous values. The delta34S values of sulfide average 0?, which indicates the presence of basaltic sulfide and incorporation of little or no seawater-derived sulfide into the rocks. These data are consistent with models for the presence of rock-dominated sulfur in deep hydrothermal fluids. The presence of anhydrite at 1176 m within basement indicates that unaltered seawater can penetrate to significant depths in the crust during recharge.

Chemical and isotope composition of carbonate nodules and host bottom sediments from Core DM9-666, Gulf of California

Results of mineralogical and isotopic analyzes of sulfur and carbon in carbonate nodules and host bottom sediments and results of 14C measurement in carbonate nodules are reported. It is proved that the carbonate nodules formed 11-22 thousand years ago in anaerobic diagenesis of bottom sediments rich in organic matter. Isotopic light metabolic carbon dioxide was a source of carbonate for nodules. It formed during microbial degradation of organic matter of bottom sediments.

Oxygen and carbon isotope data for benthic foraminifera from DSDP Site 94-608 and ODP Site 208-1264

Small biserial foraminifera were abundant in the early Miocene (ca. 18.9-17.2 Ma) in the eastern Atlantic and western Indian Oceans, but absent in the western equatorial Atlantic Ocean, Weddell Sea, eastern Indian Ocean, and equatorial Pacific Ocean. They have been assigned to the benthic genus Bolivina, but their high abundances in sediments without evidence for dysoxia could not be explained. Apertural morphology, accumulation rates, and isotopic composition show that they were planktic (genus Streptochilus). Living Streptochilus are common in productive waters with intermittent upwelling. The widespread early Miocene high Streptochilus abundances may reflect vigorous but intermittent upwelling, inducing high phytoplankton growth rates. However, export production (estimated from benthic foraminiferal accumulation rates) was low, possibly due to high regeneration rates in a deep thermocline. The upwelled waters may have been an analog to Subantarctic Mode Waters, carrying nutrients into the eastern Atlantic and western Indian Oceans as the result of the initiation of a deep-reaching Antarctic Circumpolar Current, active Agulhas Leakage, and vigorous vertical mixing in the Southern Oceans.

(Table 1) Oxygen and carbon isotope compositions of altered carbonates from DSDP Hole 6-53 from the western Pacific Ocean

In the lower part of DSDP core 53.0, partly recrystallized carbonate sediments and well recrystallized limestone breccias of Oligo-Miocene age are associated with altered volcanic flows, lithified tuffs, and tuff breccias, suggesting that carbonate alteration was the result of thermal metamorphism. However, the oxygen isotope compositions of these carbonates (-3.4 to +0.6 per mil rel. PDB) are not compatible with recrystallization and isotope exchange with sea water at high temperatures. Evaluating the effects of the composition of the water which exchanged with the carbonates and of carbonate-water isotope exchange in closed systems yields the following approximate maximum temperature of recrystallization: limestone breccias, 100°C; calcite veins rimming breccia clasts, 30°C; and unconsolidated sediments overlying the breccias, 20°C. Therefore, the volcanics at site 53.0 must have been emplaced into the primary carbonate sediments at relatively low temperatures. Subsequent carbonate alteration was probably a consequence of chemical changes in ambient pore waters resulting from the submarine weathering of volcanic material.

The magnesium isotope composition of diagenetic dolomites from ODP Hole 112-685A and 201-1230A of the Peru Margin

The magnesium isotope composition of diagenetic dolomites and their adjacent pore fluids were studied in a 250 m thick sedimentary section drilled into the Peru Margin during Ocean Drilling Program (ODP) Leg 201 (Site 1230) and Leg 112 (Site 685). Previous studies revealed the presence of two types of dolomite: type I dolomite forms at ~ 6 m below seafloor (mbsf) due to an increase in alkalinity associated with anaerobic methane oxidation, and type II dolomite forms at focused sites below ~ 230 mbsf due to episodic inflow of deep-sourced fluids into an intense methanogenesis zone. The pore fluid delta 26Mg composition becomes progressively enriched in 26Mg with depth from values similar to seawater (i.e. -0.8 per mil, relative to DSM3 Mg reference material) in the top few meters below seafloor (mbsf) to 0.8 ± 0.2 per mil within the sediments located below 100 mbsf. Type I dolomites have a delta 26Mg of -3.5 per mil, and exhibit apparent dolomite-pore fluid fractionation factors of about -2.6 per mil consistent with previous studies of dolomite precipitation from seawater. In contrast, type II dolomites have delta 26Mg values ranging from -2.5 to -3.0 per mil and are up to -3.6 per mil lighter than the modern pore fluid Mg isotope composition. The enrichment of pore fluids in 26Mg and depletion in total Mg concentration below ~ 200 mbsf is likely the result of Mg isotope fractionation during dolomite formation, The 26Mg enrichment of pore fluids in the upper ~ 200 mbsf of the sediment sequence can be attributed to desorption of Mg from clay mineral surfaces. The obtained results indicate that Mg isotopes recorded in the diagenetic carbonate record can distinguish near surface versus deep formed dolomite demonstrating their usefulness as a paleo-diagenetic proxy.

Total organic carbon content, methane isotopic composition and hydrocarbon characteristics of ODP Leg 180 sites

The organic geochemistry of Sites 1108 and 1109 of the Woodlark Basin, offshore Papua New Guinea, was studied to determine whether thermally mature hydrocarbons were present in the penetrated section and, if present, whether they are genetically related to the penetrated "coaly" interval. Both the organic carbon and pyrolysis data indicate that there is no significant hydrocarbon source-rock potential at Site 1108. The hydrocarbons encountered during drilling appear to be indigenous and not migrated products or contaminants. In contrast, the coaly interval at Site 1109 contains zones with significant hydrocarbon-generation potential. Several independent lines of evidence indicate that the coaly sequence encountered at Site 1109 is thermally immature. The Site 1108 methane stable-carbon isotope composition does not display a clear trend with depth as would be expected if it was solely reflecting a maturation profile. The measured isotopic composition of methane has most probably been altered by fractionation during sample handling and storage. This fractionation would result in isotopically heavier values than would be obtained on free gas. The organic geochemical data gathered indicate that Site 1108 can be safely revisited and that the organic-rich sediments encountered at Site 1109 were not the source of the gas encountered at Site 1108.

Stable oxygen and carbon isotope ratios of foraminifera from Oligocene to Miocene sediments of DSDP Hole 39-354 from Ceara Rise, West Altlantic (Table 1)

The oxygen- and carbon-isotope compositions of planktic and benthic foraminifera and calcareous nannofossils from Middle Oligocene-Early Miocene Equatorial Atlantic sediments (DSDP Site 354) indicate two important paleoceanographic changes, in the Late Oligocene (foraminiferal Zone P.21) and in the Early Miocene (foraminiferal Zone N.5). The first change, reflected by a delta18O increase of 1.45? in Globigerina venezuelana, affected only intermediate pelagic and not surface, deep or bottom waters. The second change affected surface and intermediate waters, whereas deep and bottom waters showed only minor fluctuations. In the case of the former the isotope effect of the moderate ice accumulation on the Antarctic continent is amplified in the Equatorial Atlantic by changes in the circulation pattern. The latter paleoceanographic change, reflected by a significant increase in 18O in both planktic and benthic forms (about 1.0? and 0.5?, respectively), may have been caused by ice volume increase and temperature decrease. Both oxygen- and carbon-isotope compositions indicate a marked depth-habitat stratification for planktic foraminifera and calcareous nannofossils. Three different dwelling groups are recognized: shallow Globigerinoides, Globoquadrina dehiscens, Globorotalia mayeri and nannofossils; intermediate Globigerina venezuelana; and deep Catapsydrax dissimilis. The comparison of foraminifera and calcareous nannofossils suggests that the isotopic compositions of nannofossils are generally controlled by the same parameters which control the isotopic composition of shallow-dwelling foraminifera, but the former are more enriched in 18O.

Calcium isotope composition of Oceanic Anoxic Events 1a and 2 sediments

Calcium-isotope ratios (d44/42Ca) were measured in carbonate-rich sedimentary sections deposited during Oceanic Anoxic Events 1a (Early Aptian) and 2 (Cenomanian-Turonian). In sections from Resolution Guyot, Mid-Pacific Mountains; Coppitella, Italy; and the English Chalk at Eastbourne and South Ferriby, UK, a negative excursion in d44/42Ca of ~0.20 per mil and ~0.10 per mil is observed for the two events. These d44/42Ca excursions occur at the same stratigraphic level as the carbon-isotope excursions that define the events, but do not correlate with evidence for carbonate dissolution or lithological changes. Diagenetic and temperature effects on the calcium-isotope ratios can be discounted, leaving changes in global seawater composition as the most probable explanation for d44/42Ca changes in four different carbonate sections. An oceanic box model with coupled strontium- and calcium-isotope systems indicates that a global weathering increase is likely to be the dominant driver of transient excursions in calcium-isotope ratios. The model suggests that contributions from hydrothermal activity and carbonate dissolution are too small and short-lived to affect the oceanic calcium reservoir measurably. A modelled increase in weathering flux, on the order of three times the modern flux, combined with increased hydrothermal activity due to formation of the Ontong-Java Plateau (OAE1a) and Caribbean Plateau (OAE2), can produce trends in both calcium and strontium isotopes that match the signals recorded in the carbonate sections. This study presents the first major-element record of a weathering response to Oceanic Anoxic Events.