ALBERT

Description: Early diagenetic dolomite beds were sampled during the Ocean Drilling Programme (ODP) Leg 201 at four reoccupied ODP Leg 112 sites on the Peru continental margin (Sites 1227/684, 1228/680, 1229/681 and 1230/685) and analysed for petrography, mineralogy, δ13C, δ18O and 87Sr/86Sr values. The results are compared with the chemistry, and δ13C and 87Sr/86Sr values of the associated porewater. Petrographic relationships indicate that dolomite forms as a primary precipitate in porous diatom ooze and siliciclastic sediment and is not replacing the small amounts of precursor carbonate. Dolomite precipitation often pre-dates the formation of framboidal pyrite. Most dolomite layers show 87Sr/86Sr-ratios similar to the composition of Quaternary seawater and do not indicate a contribution from the hypersaline brine, which is present at a greater burial depth. Also, the δ13C values of the dolomite are not in equilibrium with the δ13C values of the dissolved inorganic carbon in the associated modern porewater. Both petrography and 87Sr/86Sr ratios suggest a shallow depth of dolomite formation in the uppermost sediment (〈30 m below the seafloor). A significant depletion in the dissolved Mg and Ca in the porewater constrains the present site of dolomite precipitation, which co-occurs with a sharp increase in alkalinity and microbial cell concentration at the sulphate–methane interface. It has been hypothesized that microbial ‘hot-spots’, such as the sulphate–methane interface, may act as focused sites of dolomite precipitation. Varying δ13C values from −15‰ to +15‰ for the dolomite are consistent with precipitation at a dynamic sulphate–methane interface, where δ13C of the dissolved inorganic carbon would likewise be variable. A dynamic deep biosphere with upward and downward migration of the sulphate–methane interface can be simulated using a simple numerical diffusion model for sulphate concentration in a sedimentary sequence with variable input of organic matter. Thus, the study of dolomite layers in ancient organic carbon-rich sedimentary sequences can provide a useful window into the palaeo-dynamics of the deep biosphere.

Description: Dissolved lead (Pb) is mainly supplied to the oceans by physical and chemical weathering on the continents. The short residence time of Pb in seawater on the order of only a few decades makes its isotopic compositions an excellent tracer for local continental inputs. Lead was found to be incongruently released during early chemical weathering on the continents (Erel et al., 1994), often generating a more radiogenic runoff signal compared to the bulk rock compositions (Gutjahr et al., 2009; Kurzweil et al., 2010; Crocket et al., 2012; Crocket et al., 2013). In addition, the presence of abundant ice-rafted detrital material (IRD) may also release a highly radiogenic signature in high latitude settings (Kurzweil et al., 2010; Crocket et al., 2012). In the (sub-)Antarctic marine environment, authigenic Pb isotope records from core top sediments offer the possibility of assessing spatial seawater Pb isotopic variability of subglacial Antarctic runoff. Furthermore, palaeo-seawater Pb isotope records extracted from authigenic Fe-Mn oxyhydroxides will likely record periods of enhanced iceberg calving, freshwater input, and/or associated circulation changes. Since the leaching method for extracting authigenic Pb from Antarctic proximal bulk sediments has not been studied to date, we firstly evaluated and refined existing reductive leaching methods (Gutjahr et al., 2007; Blaser et al., 2016;) for efficient and reliable chemical extraction of bottom seawater Pb isotope signals from Weddell Sea and Southern Ocean core top sediment samples. We investigated the effects of (i) the MgCl2 pre-treatment, (ii) the effectiveness of chelates as well as (iii) exposure time of sediments to reducing reagents on the Pb isotopic signals. Chelate EDTA shows stronger complexation ability to Pb than DTPA and can significantly prevent Pb from readsorption back onto sediment surfaces during leaching as described in previous studies (Gutjahr et al., 2007). We also found that leaching without extended (〉20 min) shaking, hence only agitating sediments for less than a minute on a vortex mixer to help sediment disperse into leaching solution, can extract quantities of Pb as extracted with via leaching for 20 minutes in a shaker. Using this short-term “vortexing” method, reproducible and in most cases accurate isotopic ratios identical or close to seawater signals can be obtained. Therefore we suggest using the vortexing method with EDTA and without MgCl2 pre-treatment to recover authigenic Pb from Antarctic ice shelf-proximal bulk sediments. Employing this new method, we present Pb isotope records from 90 core top sediment samples from the Weddell Sea and the Atlantic sector of Southern Ocean covering ~4000 km of the Weddell Sea Antarctic continental margin. Furthermore, first results are presented from IODP Site 1094 delineating the authigenic Pb isotopic evolution over the past 140 ka tracing Antarctic ice sheet dynamics and Southern Ocean circulation. References: Blaser, P., Lippold, J., Gutjahr, M., Frank, N., Link, J.M., Frank, M., 2016. Extracting foraminiferal seawater Nd isotope signatures from bulk deep sea sediment by chemical leaching. Chemical Geology 439, 189-204. Crocket, K.C., Foster, G.L., Vance, D., Richards, D.A., Tranter, M., 2013. A Pb isotope tracer of ocean-ice sheet interaction: the record from the NE Atlantic during the Last Glacial/Interglacial cycle. Quaternary Science Reviews 82, 133-144. Crocket, K.C., Vance, D., Foster, G.L., Richards, D.A., Tranter, M., 2012. Continental weathering fluxes during the last glacial/interglacial cycle: insights from the marine sedimentary Pb isotope record at Orphan Knoll, NW Atlantic. Quaternary Science Reviews 38, 89-99. Erel, Y., Harlavan, Y., Blum, J.D., 1994. Lead isotope systematics of granitoid weathering. Geochimica et Cosmochimica Acta 58, 5299-5306. Gutjahr, M., Frank, M., Halliday, A.N., Keigwin, L.D., 2009. Retreat of the Laurentide ice sheet tracked by the isotopic composition of Pb in western North Atlantic seawater during termination 1. Earth and Planetary Science Letters 286, 546-555. Gutjahr, M., Frank, M., Stirling, C.H., Klemm, V., Flierdt, T., Halliday, A.N., 2007. Reliable extraction of a deepwater trace metal isotope signal from Fe-Mn oxyhydroxide coatings of marine sediments. Chemical Geology 242, 351-370. Kurzweil, F., Gutjahr, M., Vance, D., Keigwin, L., 2010. Authigenic Pb isotopes from the Laurentian Fan: Changes in chemical weathering and patterns of North American freshwater runoff during the last deglaciation. Earth and Planetary Science Letters 299, 458-465.