ALBERT

Description: 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.

Description: We present strontium (Sr) isotope ratios that, unlike traditional 87Sr/86Sr data, are not normalized to a fixed 88Sr/86Sr ratio of 8.375209 (defined as δ88/86Sr = 0 relative to NIST SRM 987). Instead, we correct for isotope fractionation during mass spectrometry with a 87Sr–84Sr double spike. This technique yields two independent ratios for 87Sr/86Sr and 88Sr/86Sr that are reported as (87Sr/86Sr*) and (δ88/86Sr), respectively. The difference between the traditional radiogenic (87Sr/86Sr normalized to 88Sr/86Sr = 8.375209) and the new 87Sr/86Sr* values reflect natural mass-dependent isotope fractionation. In order to constrain glacial/interglacial changes in the marine Sr budget we compare the isotope composition of modern seawater ((87Sr/86Sr*, δ88/86Sr)Seawater) and modern marine biogenic carbonates ((87Sr/86Sr*, δ88/86Sr)Carbonates) with the corresponding values of river waters ((87Sr/86Sr*, δ88/86Sr)River) and hydrothermal solutions ((87Sr/86Sr*, δ88/86Sr)HydEnd) in a triple isotope plot. The measured (87Sr/86Sr*, δ88/86Sr)River values of selected rivers that together account for not, vert, similar18% of the global Sr discharge yield a Sr flux-weighted mean of (0.7114(8), 0.315(8)‰). The average (87Sr/86Sr*, δ88/86Sr)HydEnd values for hydrothermal solutions from the Atlantic Ocean are (0.7045(5), 0.27(3)‰). In contrast, the (87Sr/86Sr*, δ88/86Sr)Carbonates values representing the marine Sr output are (0.70926(2), 0.21(2)‰). We estimate the modern Sr isotope composition of the sources at (0.7106(8), 0.310(8)‰). The difference between the estimated (87Sr/86Sr*, δ88/86Sr)input and (87Sr/86Sr*, δ88/86Sr)output values reflects isotope disequilibrium with respect to Sr inputs and outputs. In contrast to the modern ocean, isotope equilibrium between inputs and outputs during the last glacial maximum (10–30 ka before present) can be explained by invoking three times higher Sr inputs from a uniquely “glacial” source: weathering of shelf carbonates exposed at low sea levels. Our data are also consistent with the “weathering peak” hypothesis that invokes enhanced Sr inputs resulting from weathering of post-glacial exposure of abundant fine-grained material.