ALBERT

Beschreibung: Submarine groundwater discharge (SGD) to the ocean supplies Sr with less radiogenic 87Sr/86Sr than seawater, and thus constitutes an important term in the Sr isotope budget of the modern ocean. However, few data exist for Sr in coastal groundwater or in the geochemically dynamic subterranean estuary (STE). We examined Sr concentrations and isotope ratios from nine globally-distributed coastal sites and characterized the behavior of Sr in the STE. Dissolved Sr generally mixed conservatively in the STE, although large differences were observed in the meteoric groundwater end-member Sr concentrations among sites (0.1–24 μM Sr). Strontium isotope exchange was observed in the STE at five of the sites studied, and invariably favored the meteoric groundwater end-member signature. Most of the observed isotope exchange occurred in the salinity range 5–15, and reached up to 40% exchange at salinity 10. Differences in fresh groundwater Sr concentrations and isotope ratios (87Sr/86Sr = 0.707–0.710) reflected aquifer lithology. The SGD end-member 87Sr/86Sr must be lower than modern seawater (i.e., less than 0.70916) in part because groundwater Sr concentrations are orders of magnitude higher in less-radiogenic carbonate and volcanic island aquifers. A simple lithological model and groundwater Sr data compiled from the literature were used to estimate a global average groundwater end-member of 2.9 μM Sr with 87Sr/86Sr = 0.7089. This represents a meteoric-SGD-driven Sr input to the ocean of 0.7–2.8 × 1010 mol Sr y−1. Meteoric SGD therefore accounts for 2–8% of the oceanic Sr isotope budget, comparable to other known source terms, but is insufficient to balance the remainder of the budget. Using reported estimates for brackish SGD, the estimated volume discharge at salinity 10 (7–11 × 1015 L y−1) was used to evaluate the impact of isotope exchange in the STE on the brackish SGD Sr flux. A moderate estimate of 25% isotope exchange in the STE gives an SGD Sr end-member 87Sr/86Sr of 0.7091. The brackish SGD Sr flux thus accounts for 11–23% of the marine Sr isotope budget, but does not appear sufficient to balance the ∼40% remaining after other known sources are included. Substantial uncertainties remain for estimating the SGD source of Sr to the global ocean, especially in the determination of the volume flux of meteoric SGD, and in the paucity of measurements of groundwater Sr isotope composition in major SGD regions such as Papua New Guinea, the South America west coast, and West Africa. Consequently, our global estimate should be viewed with some caution. Nevertheless, we show that the combined sources of meteoric SGD and brackish SGD coupled with isotope exchange in the STE may constitute a substantial component (∼13–30%) of the modern oceanic 87Sr/86Sr budget, likely exceeding less radiogenic Sr inputs by sedimentary diagenesis and hydrothermal circulation through the mid-ocean ridge system. Temporal variation in SGD Sr fluxes and isotope composition may have contributed to fluctuations in the oceanic 87Sr/86Sr ratio over geologic time.

Beschreibung: 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.

Beschreibung: Major- and trace-element as well as Pb-isotope data are presented for greenschist- to amphibolite-facies greenstones from two locations: (1) metabasalt-breccias from the Galfipagos Rift at 100°W; and (2) metabasalt-breccias and unbrecciated greenstones from the Chile Ridge at 38°S. Greenstone-breccias from both locations display stockwork-like sulfide mineralizations related to the upflow portion of a hydrothermal convection system. Whereas Galfipagos Rift stockwork sulfides belong to the "normal" type containing Cu-Fe-sulfides, Chile Ridge stockwork sulfides are galena-rich Pb-Zn _ Cu-sulfides and represent a previously unknown type of sulfide mineralisation in a MORB environment. Geochemically, the greenstones can be divided into two types: (1) The unbrecciated galena-free greenstones from both Chile Ridge and Galfipagos Rift show at least a 10-20-fold Pb enrichment compared to fresh MORB. With respect to similar Pb enrichment measured in greenstones from two other locations, i.e. DSDP Hole 504B and Galfipagos Rift near 86°W, we suggest that this may be a general feature of all stockwork-mineralised oceanic greenstones. (2) The galena- and quartz-rich metabasalt-breccias from the Chile Ridge are up to 3000-fold enriched in Pb (up to 1000 Ixg/g Pb in the whole-rock analyses) compared to MORB and indicate Pb mineralisation two orders of magnitude higher than that of the "normal" greenstone-type. A mass-balance calculation carried out using crustal column thickness of 3000 m with a 200-m-thick greenstone layer and 0.15 m of galena-beating breccias shows that at the Chile Ridge ~ 42% of the entire Pb is concentrated in the greenstones. This suggests that the rest of the crustal column is depleted in 58% of its primary Pb content. This degree of depletion matches well with previous calculations that a 56% depletion of Pb in oceanic crust subjected to mantle-recycling via subduction would be necessary to yield a HIMU mantle source within 2 Ga. Despite the need for future investigations into the extent and volume of the Pb enrichment in Chile Ridge greenstones, we believe that this process of major Pb redistribution is capable of creating huge volumes of oceanic crust that on average are extremely Pb-depleted and which when recycled would produce the HIMU source. Keywords: Chile Ridge; Galfipagos Ridge; Hydrothermal alteration; Greenstones; Geochemistry; Mineralogy