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The overarching goal of SFB 574 is to understand the role and fate of volatiles and fluids in subduction zones. Both components have a major influence on, e.g., short- and long-term climate change, the geochemical evolution of the hydrosphere and atmosphere, as well as subduction-related natural hazards, such as earthquakes, volcanic eruptions and tsunamis, because they are cycled through the entire subduction system. The Chilean margin, which switched from erosion to accretion within the last several million years, has been chosen for the remainder of the SFB. Major goals of cruise SO-210 are: 1) to investigate and quantify the dewatering processes in the forearc of the central Chilean subduction zone, in particular the origin and output flux of vent fluids and volatiles, 2) to study biological processes fuelled by this discharge, 3) to use cold seep carbonates as a geochemical archive of cold seep activity, 4) to evaluate the role of forearc fluids in triggering mass wasting events that could generate tsunamis, 5) to characterize geochemically the subducting sediments to determine the input flux of climate-relevant volatiles and a variety of trace elements, and 6) to investigate the distribution of volcanic ashes to improve estimates on the volume of material emitted by volcanic eruptions and to date distinct events within the sedimentary sequence. Preliminary investigations by Chilean colleagues revealed definite indications of fluid venting which together with recent geophysical investigations are the basis for the present cruise. Here, we will present preliminary results and new discoveries made during the cruise.

Continuous seep carbonate cores were recovered with the BGS Rockdrill-I device offshore pacific Nicaragua and Costa Rica (Meteor 66/3). Three carbonate drill cores from the mud mounds Iguana, Perezoso, Baula V and one from the Jaco Scarp (JS) escarpment were used for mineralogical, petrological and isotope studies in order to decipher the late stage evolution of mound growth and related methane enriched fluid emanation. The combination of X-ray diffractometry and X-ray fluorescence (core scanner) analyses reveals that aragonite is the most dominant carbonate phase, followed by Mg-calcite and calcite. The proportions of Pyrrhotite and Pyrite within the cores indicate lower H2S concentrations within the mound systems relative to JS environment of formation. The presence of kutnohorite, associated struvite and monohydrocalcite can be most likely interpreted as product of microbial CO2 consuming activity (Idiomarina sp.; Gonzáles-Muñoz et al., 2008). Isotopic data show that JS is significantly lighter in !13C (-43 to - 56‰) than the mound samples (-22 to -36‰) indicating systematic variations in fluid composition and related methane source. The !18O values vary in the mound cores from 3.8 to 5.3‰ and in JS samples from 4.2 to 5.1‰ which correlate by their U/Th ages (Hammerich et al., 2007; Kutterolf et al., 2008) with major changes of seawater composition during the last 70,000 years. Fluorescence microscopy of aragonite cements, combined with electron microprobe data show multiple phases of crystal growth separated by residual organic matter which was attached to crystal surfaces during phases of stagnant or low fluid flow.

Cold seep ecosystems are often characterized by carbonate precipitation processes fueled by methane-rich fluids and microbial activity. Understanding and quantifying feedback mechanisms between methane sources, ocean chemistry, and climate requires detailed data about the dynamics of seafloor methane emanation throughout geological time. Carbonates from these ecosystems provide unique archives of marine methane emanation by their geobiological, geochemical, mineralogical, and structural inventory. Precise and high resolution geochronology of these archives provides new insights into the rate and duration of precipitation processes and the related microbial activity. In this compilatory study large carbonate samples from very different cold seep settings were investigated for the time scales of their formation and their specific precipitation environment. Beside new insights into growth structures, emplacement processes and initial approaches on high resolution geochemistry and biomarker analyses [1], special emphasis was given to the geochronological identification of paleo-seep-activity phases. The actual data set is spanning a wide range in space and time. It covers circum-Pacific settings (South China Sea, Costa Rica & Nicaragua, New Zealand), including more than 200 thousand years old archives ( Hydrate Ridge, off Oregon), and almost recent methane-related carbonates from Black Sea and Mediterranean Sea. On long time scales, the data indicates sea level decrease as an important enhancement factor for focused methane flux via destabilization of underlying gas hydrates upon hydraulic pressure release. Data from tectonically highly active settings imply structural changes as major control on initiation and position of cold seeps and their activation on short time scales [2, 3]. [1] Leefmann et al. (2008) BG. [2] Kutterolf et al. (2008) Geology, doi: 10.1130/G24806A [3] Liebetrau et al. (2010) MG, doi:10.1016/j.margeo.2010.01.003

Recent findings of natural strontium isotope fractionation have opened up a new field of research in non-traditional stable isotope geochemistry. While previous studies were based on data obtained by MC-ICP-MS we here present a novel approach combining thermal ionization mass spectrometry (TIMS) with the use of an 87Sr/84Sr double spike (DS). Our results for the IAPSO sea water and JCp-1 coral standards, respectively, are in accord with previously published data. The strontium isotope composition of the IAPSO sea water standard was determined as δ88/86Sr = 0.386(5)‰ (δ values relative to the SRM987), 87Sr/86Sr* = 0.709312(9) n = 10 and a corresponding conventionally normalized 87Sr/86Sr = 0.709168(7) (all uncertainties 2SEM). For the JCp-1 coral standard we obtained δ88/86Sr = 0.197(8)‰, 87Sr/86Sr* = 0.709237(2) and 87Sr/86Sr = 0.709164(5) n = 3. We show that by applying this DS-TIMS method the precision is improved by at least a factor of 2–3 when compared to MC-ICP-MS.

Widespread evidence of a +4–6-m sea-level highstand during the last interglacial period (Marine Isotope Stage 5e) has led to warnings that modern ice sheets will deteriorate owing to global warming and initiate a rise of similar magnitude by ad 2100 (ref. 1). The rate of this projected rise is based on ice-sheet melting simulations and downplays discoveries of more rapid ice loss2, 3. Knowing the rate at which sea level reached its highstand during the last interglacial period is fundamental in assessing if such rapid ice-loss processes could lead to future catastrophic sea-level rise. The best direct record of sea level during this highstand comes from well-dated fossil reefs in stable areas4, 5, 6. However, this record lacks both reef-crest development up to the full highstand elevation, as inferred7 from widespread intertidal indicators at +6 m, and a detailed chronology, owing to the difficulty of replicating U-series ages on submillennial timescales8. Here we present a complete reef-crest sequence for the last interglacial highstand and its U-series chronology from the stable northeast Yucatán peninsula, Mexico. We find that reef development during the highstand was punctuated by reef-crest demise at +3 m and back-stepping to +6 m. The abrupt demise of the lower-reef crest, but continuous accretion between the lower-lagoonal unit and the upper-reef crest, allows us to infer that this back-stepping occurred on an ecological timescale and was triggered by a 2–3-m jump in sea level. Using strictly reliable 230Th ages of corals from the upper-reef crest, and improved stratigraphic screening of coral ages from other stable sites, we constrain this jump to have occurred approx121 kyr ago and conclude that it supports an episode of ice-sheet instability during the terminal phase of the last interglacial period.

Methane-related carbonates from Hydrate Ridge typically show several macroscopically distinguishable mineral phases, namely whitish aragonite, lucent aragonite, and gray micrite. The relationship of these phases to particular microorganisms or biogeochemical processes is as yet unclear. We used a miniaturized biomarker technique on mg samples, combined with factor analysis and subsequent electron microprobe analysis, to study lipid biomarkers and chemical compositions of the individual phases. This allows us to identify particular mechanisms involved in the formation of the different carbonate precipitates. Our combined analysis of biomarkers and petrographic traits shows that most of the lipids related to the anaerobic oxidation of methane (>90% by weight) are concentrated within only a minor compartment (~20% by volume) of the Hydrate Ridge carbonates, the whitish aragonite. The patterns indicate that the whitish aragonite represents fossilized biofilms of methanotrophic consortia containing mainly archaea of the ANME-2 group and sulfate reducing bacteria, whereas the precipitation of the lucent aragonite may have lacked the immediate proximity of microorganisms during formation. By contrast, the gray micrite formed by incorporation of allochthonous organic and inorganic matter during carbonate precipitation induced by the anaerobic oxidation of methane involving ANME-1 archaea.

Strontium isotopes in various marine carbonates were determined using an “AXIOM” MC-ICP-MS in combination with a NewWave UP193 laser ablation unit. Using a modified measurement and data reduction strategy, an external reproducibility of 87Sr/86Sr ratios in carbonates of about 19 ppm (RSD) was achieved. For recent and sub-recent marine carbonates a mean radiogenic strontium isotope ratio 87Sr/86Sr of 0.709170 ± 0.000007 (2SE) was determined, which agrees well with the value of 0.7091741 ± 0.0000024 (2SE) reported for modern sea water (J. M. McArthur, D. Rio, F. Massari, D. Castrodi, T. R. Bailey, M. Thirlwall and S. Houghton, Palaeogeogr. Palaeoclimatol. Palaeoeco., 2006, 242(126), 2006). Compared to published laser-based methods, an improved accuracy and precision was achieved by applying a new data reduction protocol using the simultaneous responses of all isotopes measured. The latter is considered as a new principal approach for isotope ratio evaluation using LA-MC-ICP-MS. A major advantage of the presented method is the direct determination of the stable strontium isotope fractionation. Providing reproducible sample ablation, introduction into the plasma and stable plasma condition, this method excludes the efforts of a quantitative strontium recovery after ion chromatographic separation to avoid additional fractionation of the sample strontium due to chemical pre-treatment/separation (ion chromatography and solution preparation), and is therefore, together with the quicker sample preparation and spatially resolved analysis, advantageous when compared to published solution–nebulization bracketing-standard MC-ICP-MS methods for stable strontium isotope determination.

At convergent margins, fluids rise through the forearc in response to consolidation of the upper plate and dewatering of the subducting plate, and produce various cold-seep–related features on the seafloor (mud diapirs, mud mounds). At the Central American forearc, authigenic carbonates precipitated from rising fluids within such structures during active venting while typical mixed-mud sediments were ejected onto the surrounding seafloor where they became intercalated with normal pelagic background sediments, indicating that mud mounds evolved unsteadily through alternating active and inactive phases. Intercalated regional ash layers from Plinian eruptions at the Central American volcanic arc provide time marks that constrain the ages of mud ejection activity. U/Th dating of drill core samples of authigenic carbonate caps of mud mounds yields ages agreeing well with those constrained by ash layers and showing that carbonate caps grow inward rather than outward during active venting. Both dating approaches show that offshore Nicaragua and Costa Rica (1) active and inactive phases can occur simultaneously at neighboring mounds, (2) mounds along the forearc have individual histories of activity, but there are distinct time intervals when nearly all mounds have been active or inactive, (3) lifetimes of mounds reach several hundred thousand years, and (4) highly active periods last 10–50 k.y. with intervening periods of >10 k.y. of relative quiescence.

The stable 88Sr/86Sr-ratio has traditionally been considered to be a constant value in order to correct instrumental mass fractionation during measurement of the radiogenic strontium isotope ratio (87Sr/86Sr). However, recent high precision measurements (Fietzke and Eisenhauer, 2006) showed that the 88Sr/86Sr ratio of marine carbonates and certain silicates (deSouza et al., 2007) are not constant, but rather variable and temperature dependent. Variations of the 88Sr/86Sr-ratios are expressed in the usual δ-notation: δ88/86Sr=((88Sr/86Sr)sample/(88Sr/86Sr)NBS987-1)*1000, where δ88/86SrNBS987=0. The recent measurements also showed that the presently accepted (88Sr/86Sr)-value for seawater differs by about ~0.4 ‰ from the 88Sr/86Sr)NBS987 value. This in turn implies that the 87Sr/86Sr isotope ratio in seawater not corrected for natural isotope fractionation is also significantly different from its normalized value (87Sr/86Sr=0.70916). Correcting the 87Sr/86Sr ratio of seawater results in a 87Sr/86Sr value of about ~0.70930. First results from continental rocks and minerals indicate that they are significantly lighter than seawater by about 0.3 ‰ and presumably become fractionated due to dissolution, precipitation of secondary mineral phases and biological utilization ((deSouza et al., 2007), (Halicz et al., 2007)). The combination of δ88/86Sr and 87Sr/86Sr data provides a unique solution for three-component mixing processes and a way of distinguishing the different sources that supply Sr to seawater. Latter mixing has to account for the characteristic radiogenic ingrowth of the 87Sr/86Sr ratio as well for the natural fractionation related to chemical weathering, temperature and other yet not known effects. This may help to elucidate the link between continental weathering and atmospheric pCO2 in more detail on geological time scales. deSouza G., Reynolds B., and Bourdon B. (2007) Evidence for Stable Strontium Isotope Fractionation during Chemical Weathering. 17th Golds