Uniquely low stable iron isotopic signatures in deep marine sediments caused by Rayleigh distillation.

Dissimilatory iron reduction (DIR) is suggested to be one of the earliest forms of microbial respiration. It plays an important role in the biogeochemical cycling of iron in modern and ancient sediments. Since microbial iron cycling is typically accompanied by iron isotope fractionation, stable iron isotopes are used as tracer for biological activity. Here we present iron isotope data for dissolved and sequentially extracted sedimentary iron pools from deep and hot subseafloor sediments retrieved in the Nankai Trough off Japan. Dissolved iron (Fe(II)aq) is isotopically light throughout the ferruginous sediment interval but some samples have exceptionally light isotope values. Such light values have never been reported in natural marine environments and cannot be solely attributed to DIR. We show that the light isotope values are best explained by a Rayleigh distillation model where Fe(II)aq is continuously removed from the pore water by adsorption onto iron (oxyhydr)oxide surfaces. While the microbially mediated Fe(II)aq release has ceased due to an increase in temperature beyond the threshold of mesophilic microorganisms, the abiotic adsorptive Fe(II)aq removal continued, leading to uniquely light isotope values. These findings have important implications for the interpretation of dissolved iron isotope data especially in deep subseafloor sediments.

Air handling in clean laboratory environments: the reason for anomalously high boron background levels.

The isotopic analysis of boron from material that carries only trace amounts of boron requires low boron blank levels. Large efforts are taken to keep blank levels low by purifying reagents. We performed exposure experiments and determined procedure blanks before and after the air-handling system was modified from silicate glass filters to polyethylene/polystyrole and active carbon filters. Our investigations demonstrate that the air-handling system may be the major source of boron blank in many laboratories, as the widely used silicate glass filters, which contain boron as a major component, in the form of borax and boric acid, release significant amounts of boron to the air.