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Isotope ratio analyses of trace elements are applied to tooth enamel, ostrich eggshell, and other archaeological hard tissues to infer mobility and other aspects of hominin and animal paleoecology. It has been assumed that these highly mineralized tissues are resistant to diagenetic alteration, but this is seldom tested and some studies document diagenetic alteration over brief time spans. Here, we build on existing research on Maximum Threshold Concentrations (MTCs) to develop screening tools for diagenesis that can inform heavy isotopic analyses. The premise of the MTC approach is that archaeological tissues are likely contaminated and unsuitable for isotope ratio analysis when they exceed characteristic modern concentration ranges of trace elements. Furthermore, we propose a new metric called the Maximum Threshold Ratio (MTR) of 85Rb/88Sr or whole element Rb/Sr, which can be measured simultaneously with 87Sr/86Sr during laser ablation (LA) MC-ICP-MS or applied during post hoc screening of specimens. We analyzed 56 enamel samples from modern Kenyan mammals and 34 modern ostrich eggshells from South Africa, Namibia, and the United States by solution ICP-MS, as well as a subset of shells using LA-MC-ICP-MS. Our results indicate that thresholds are consistent across taxa at a single location, but likely vary across locations. Therefore, MTCs and MTRs need to be tissue and locality specific, but not necessarily taxon-specific. Other important differences are observed between the inner and outer surfaces of the eggshells and between LA and solution ICP-MS. This exploratory study provides guidelines for building reference thresholds to screen enamel and eggshell for diagenesis potentially impacting biogenic isotope ratios. 

Despite very low concentrations of cobalt in marine waters, cyanobacteria in the genus Prochlorococcus retain the genetic machinery for the synthesis and use of cobalt-bearing cofactors (cobalamins) in their genomes. We explore cobalt metabolism in a Prochlorococcus isolate from the equatorial Pacific Ocean (strain MIT9215) through a series of growth experiments under iron- and cobalt-limiting conditions. Metal uptake rates, quantitative proteomic measurements of cobalamin-dependent enzymes, and theoretical calculations all indicate that Prochlorococcus MIT9215 can sustain growth with less than 50 cobalt atoms per cell, ∼100-fold lower than minimum iron requirements for these cells (∼5,100 atoms per cell). Quantitative descriptions of Prochlorococcus cobalt limitation are used to interpret the cobalt distribution in the equatorial Pacific Ocean, where surface concentrations are among the lowest measured globally but Prochlorococcus biomass is high. A low minimum cobalt quota ensures that other nutrients, notably iron, will be exhausted before cobalt can be fully depleted, helping to explain the persistence of cobalt-dependent metabolism in marine cyanobacteria. 

Abstract. Bioactive trace metals are critical micronutrients for marinemicroorganisms due to their role in mediating biological redox reactions,and complex biogeochemical processes control their distributions.Hydrothermal vents may represent an important source of metals tomicroorganisms, especially those inhabiting low-iron waters, such as in thesouthwest Pacific Ocean. Previous measurements of primordial 3Heindicate a significant hydrothermal source originating in the northeastern (NE)Lau Basin, with the plume advecting into the southwest Pacific Ocean at1500–2000 m depth (Lupton etal., 2004). Studies investigating the long-range transport of trace metalsassociated with such dispersing plumes are rare, and the biogeochemicalimpacts on local microbial physiology have not yet been described. Here wequantified dissolved metals and assessed microbial metaproteomes across atransect spanning the tropical and equatorial Pacific with a focus on thehydrothermally active NE Lau Basin and report elevated iron and manganeseconcentrations across 441 km of the southwest Pacific. The most intensesignal was detected near the Mangatolo Triple Junction (MTJ) and NortheastLau Spreading Center (NELSC), in close proximity to the previously reported3He signature. Protein content in distal-plume-influenced seawater,which was high in metals, was overall similar to background locations,though key prokaryotic proteins involved in metal and organic uptake,protein degradation, and chemoautotrophy were abundant compared to deepwaters outside of the distal plume. Our results demonstrate that tracemetals derived from the NE Lau Basin are transported over appreciabledistances into the southwest Pacific Ocean and that bioactive chemicalresources released from submarine vent systems are utilized by surroundingdeep-sea microbes, influencing both their physiology and their contributionsto ocean biogeochemical cycling.