Age and isotopes in bottom sediments from Core VER-99-1-St2, Baikal Lake

A simple, reliable, and efficient method has been elaborated for direct determination of isotopic composition of authigenic uranium in siliceous lacustrine sediments. The method is based on studying kinetics of selective extraction of authigenic uranium from sediments by weak solutions of ammonium hydrocarbonate followed by ICP-MS analysis of nuclides. To estimate contamination of authigenic uranium by terrigenous one contents of 232Th and some other clastogenic elements in the extracts were measured simultaneously. Selectivity of extraction of authigenic uranium from the sediments treated with 1% NH4HCO3 solution appeared to be no worse than 99%. The method was applied to analysis of isotopic composition of authigenic uranium at several key horizons of the earlier dated core from the Baikal Lake. Measurements directly show that 234U/238U values in Baikal water varied depending on climate, which contradicts existing hypotheses. Measured 234U/238U ratios in water of the paleo-Baikal match corresponding values reconstructed from isotopic data for total uranium in the sediments on supposition that U/Th ratio is constant in terrigenous fraction of the sediments. Direct experimental determination of total and authigenic nuclides in sediments enhances potentiality of the method for 234U-230Th dating of non-carbonate lacustrine sediments including those from the Baikal Lake within intervals corresponding to periods of glaciation, when sediments were rich in terrigenous components. Portions of terrigenous and authigenic uranium are well separated and we can study variability of sources of terrigenous matter and refine the earlier model for reconstructing climate humidity in the East Siberia.

Hydrochemical and microbiological properties of groundwater from pristine aquifer sampled at the Lower Rhine Embayment

The hydrochemistry and the microbial diversity of a pristine aquifer system near Garzweiler, Germany next to the open-pit lignite mine Garzweiler 1, were characterized. Hydrogeochemical and isotopic data indicate a recent activity of sulfate-reducing bacteria in the Tertiary marine sands. The community structure in the aquifer was studied by fluorescence in situ hybridization (FISH). Up to 7.3 x 10**5 cells/ml were detected by DAPIstaining. Bacteria (identified by the probe EUB338) were dominant, representing 51.9% of the total cell number (DAPI). Another 25.7% of total cell were affiliated with the domain Archaea as identified by the probe ARCH915. Within the domain Bacteria, the beta-Proteobacteria were most abundant (21.0% of total cell counts). Using genusspecific probes for sulfate-reducing bacteria (SRB), 2.5% of the total cells were identified as members of the genus Desulfotomaculum. This reflects the predominant role these microorganisms have been found to play in sulfatereducing zones of aquifers at other sites. Previously, all SRB cultured from this site were from the spore-forming genera Desulfotomaculum and Desulfosporosinus. Samples were taken after pumping for >= 40 min and after parameters such as temperature, pH, redox potential, oxygen and conductivity of the groundwater had remained stable for >= 15 min due to recharge of aquifer water. Hybridization and microscopy counts of hybridized and 4',6'-diamidino-2-phenylindole (DAPI)- stained cells were performed as described in Snaidr et al., (1997, http://aem.asm.org/content/63/7/2884.full.pdf). Means were calculated from 10 to 20 randomly chosen fields on each filter section, corresponding to 800-1000 DAPI stained cells. Counting results were always corrected by subtracting signals observed with the probe NON338. Formamide concentrations and oligonucleotide probes used please see further details.

(Table 2) Carbonate chemistry and isotopes from northern Cascadia margin samples

Most authigenic carbonates previously recovered from the Cascadia slope have 87Sr/86Sr signatures that reflect shallow precipitation in equilibrium with coeval seawater. There is also evidence for carbonate formation supported by fluids that have been modified by reactions with the incoming Juan de Fuca plate (87Sr/86Sr = 0.7071; Teichert et al., 2005, doi:10.1016/j.epsl.2005.08.002) or with terrigenous turbidites (87Sr/86Sr = 0.70975 to 0.71279; Sample et al., 1993, doi:10.1130/0091-7613(1993)021<0507:CCICFF>2.3.CO;2). We report on the strontium isotopic composition of carbonates and fluids from IODP Site U1329 and nearby Barkley Canyon (offshore Vancouver Island), which have strontium isotope ratios as low as 0.70539. Whereas the strontium and oxygen isotopic compositions of carbonates from paleoseeps in the uplifted Coast Range forearc indicate formation in ambient bottom seawater, several samples from the Pysht/Sooke Fm. show a 87Sr-depleted signal (87Sr/86Sr = 0.70494 and 0.70511) similar to that of the anomalous Site U1329 and Barkley Canyon carbonates. Our data, when analyzed in the context of published elemental and isotopic composition of these carbonates (Joseph et al., 2012, doi:10.1016/j.palaeo.2013.01.012 ), point to two formation mechanisms: 1) shallow precipitation driven by the anaerobic oxidation of methane (AOM) with d13C values as low as -50 per mil and contemporaneous 87Sr/86Sr seawater ratios, and 2) carbonate precipitation driven by fluids that have circulated through the oceanic crust, which are depleted in 87Sr. Carbonates formed from the second mechanism precipitate both at depth and at sites of deep-sourced fluid seepage on the seafloor. The 87Sr-depleted carbonates and pore fluids found at Barkley Canyon represent migration of a deep, exotic fluid similar to that found in high permeability conglomerate layers at 188 mbsf of Site U1329, and which may have fed paleoseeps in the Pysht/Sooke Fm. These exotic fluids likely reflect interaction with the 52-57 Ma igneous Crescent Terrane, which supplies fluids with high calcium, manganese and strontium enriched in the non-radiogenic nucleide. Tectonic compression and dehydration reactions then force these fluids updip, where they pick up the thermogenic hydrocarbons and 13C-enriched dissolved inorganic carbon that are manifested in fluids and carbonates sampled at Barkley Canyon and at Site U1329. The Crescent Terrane may have sourced cold seeps in this margin since at least the late Oligocene.

Planktonic foraminifera abundances and isotopes of Late Quaternary sediments off New Zealand

A series of cores from east of New Zealand have been examined to determine the paleoceanographic history of the late Quaternary in the SW Pacific using planktonic foraminiferal data. Distinct shifts of species can be seen between glacial and interglacial times especially south of Chatham Rise east of South Island. Foraminiferal fragmentation ratios and benthic/planktonic foraminiferal ratios both show increased dissolution during glacials, especially isotope stage 2 to the south of Chatham Rise. The present-day Subtropical Convergence appears to be tied to the Chatham Rise at 44°S, but during glacial times this rise separated cold water to the south from much warmer water to the north, with an associated strong thermal gradient across the rise. We estimate that this gradient could have presented as much as an 8°C temperature change across 4° of latitude during the maximum of the last ice age. There is only weak evidence of the Younger Dryas cool event, but there is a clear climatic optimum between 8 and 6.4 ka with temperatures 1°-2°C higher than the present day. The marine changes compare well with vegetational changes on both South and North Island.

Stable chlorine isotopes in pore waters from DSDP Hole 79-546 and ODP Hole 131-808C

Stable Cl isotope ratios, measured in marine pore waters associated with the Barbados and Nankai subduction zones, extend significantly (to ~-8 per mil) the range of d37Cl values reported for natural waters. These relatively large negative values, together with geologic and chemical evidence from Barbados and Nankai and recent laboratory data showing that hydrous silicate minerals (i.e., those with structural OH sites) are enriched up to 7.5 per mil in 37Cl relative to seawater, strongly suggest that the isotopic composition of Cl in pore waters from subduction zones reflects diagenetic and metamorphic dehydration and transformation reactions. These reactions involve clays and/or other hydrous silicate phases at depth in the fluid source regions. Chlorine therefore cannot be considered geochemically conservative in these systems. The uptake of Cl by hydrous phases provides a mechanism by which Cl can be cycled into the mantle through subduction zones. Thus, stable Cl isotopes should help in determining the extent to which Cl and companion excess volatiles like H2O and CO2 cycle between the crust and mantle.

Geochemistry and Mg-isotopes of carbonate sediments and porefluids of ODP Site 130-807

Deep-sea pore fluids are potential archives of ancient seawater chemistry. However, the primary signal recorded in pore fluids is often overprinted by diagenetic processes. Recent studies have suggested that depth profiles of Mg concentration in deep-sea carbonate pore fluids are best explained by a rapid rise in seawater Mg over the last 10-20 Myr. To explore this possibility we measured the Mg isotopic composition of pore fluids and carbonate sediments from Ocean Drilling Program (ODP) site 807. Whereas the concentration of Mg in the pore fluid declines with depth, the isotopic composition of Mg in the pore fluid increases from -0.78 per mil near the sediment-water interface to -0.15 per mil at 778 mbsf. The Mg isotopic composition of the sediment, with few important exceptions, does not change with depth and has an average d26Mg value of -4.72 per mil. We reproduce the observed changes in sediment and pore-fluid Mg isotope values using a numerical model that incorporates Mg, Ca and Sr cycling and satisfies existing pore-fluid Ca isotope and Sr data. Our model shows that the observed trends in magnesium concentrations and isotopes are best explained as a combination of two processes: a secular rise in the seawater Mg over the Neogene and the recrystallization of low-Mg biogenic carbonate to a higher-Mg diagenetic calcite. These results indicate that burial recrystallization will add Mg to pelagic carbonate sediments, leading to an overestimation of paleo-temperatures from measured Mg/Ca ratios. The Mg isotopic composition of foraminiferal calcite appears to be only slightly altered by recrystallization making it possible to reconstruct the Mg isotopic composition of seawater through time.

Mass of particulate carbon and nitrogen in suspended matter as a function of volume filtered in several oceanic areas

We address two issues in the determination of particulate carbon and nitrogen in suspended matter of aquatic environments. One is the adsorption of dissolved organic matter on filters, leading to overestimate particulate matter. The second is the material loss during filtration due to fragile algal cells breaking up. Examples from both laboratory cultures and natural samples are presented. We recommend using stacked filters in order to estimate thefirst and filtering different volumes of water in order to evaluate the second.