GEOTHERMAL SYSTEMS IN THE NORTHERN APENNINES, ITALY: MODERN ANALOGUES OF CARLIN-STYLE GOLD DEPOSITS

Carlin-type gold deposits in northern Nevada are inferred to overlie concealed late Eocene plutons, which are increasingly thought to have provided magmatic input to the meteoric water-dominated fluids from which the gold was precipitated. The Larderello, Monte Amiata, and Latera geothermal systems in the Northern Apennines of southern Tuscany and northern Latium, central Italy, may represent Pliocene to present-day analogues because of their demonstrated association with subsurface plutons and jasperoid-hosted antimony-gold mineralization. The plutons, which at depths of >5–7 km remain at least partially molten, continue to supply heat and magmatic fluids to the meteoric water-dominated geothermal systems. Formerly mined antimony deposits of Pliocene or younger age are exposed on the peripheries of the CO2 ± H2S-emitting geothermal systems, and antimony sulfides are still actively precipitating. Stibnite and submicroscopic gold in disseminated pyrite, along with Au/Ag of <0.5 and anomalous As, Hg, Tl, and Ba values, accompanied jasperoid formation in the Northern Apennines systems. Carlin-type mineralization in northern Nevada and the antimony-gold mineralization in the Northern Apennines are hosted by permeable carbonate rocks, particularly stratabound breccias, where they are intersected by steep normal or oblique-slip faults and confined beneath tectonically emplaced hydrologic seals. The Northern Apennines antimony-gold mineralization formed at shallow, epithermal depths, like that recently recognized in the southern Carlin trend of northern Nevada. Although underexplored, the Northern Apennines gold prospects are unlikely to ever attain the giant status of the Carlin-type deposits in northern Nevada, probably because of lower magmatic fertility (ilmenite-series rather than magnetite-series magmatism) and host-rock receptivity (less reactive iron). Nevertheless, shallow carbonate-rock aquifers within high-temperature, intrusion-related geothermal systems, be they extinct or still active, may be prospective for Carlin-style gold deposits.

Re-assessing copper and nickel enrichments as paleo-productivity proxies

Copper (Cu) and nickel (Ni) are elements frequently enriched in sedimentary deposits rich in organic matter (OM). In the marine environment, they are mainly supplied to the sediments in association with sedimentary OM. In current environments, a good correlation between the intensity of phytoplankton productivity and the quantities of Cu & Ni transferred to sediments made it possible to establish paleo-productivity calculations based on the contents of ancient sediments in these two metals. The present study is a re-evaluation of the value that can be attributed to these two metals as paleo-productivity proxies. The approach adopted here is based on the examination of a large database already available in the scientific literature. The choice was made to favor the examination of a large amount of data by simple means: comparisons of total organic carbon (TOC) content, enrichment in Cu & Ni (or even other trace metals), and value of the Fe:Al ratio that makes it possible to assess the availability of reactive iron. The basic idea is that the examination of a large number of geological formations thanks to the large database makes it possible to encompass all kinds of paleo-environmental settings, thus comprising an extreme variety of the factors conventionally involved in the mechanisms of accumulation of OM. The aim is to identify strong trends, valid in a large number of paleo-situations, which will have to be carefully taken into account in future detailed paleo-environmental reconstructions. It emerges from this study that, in many cases, Cu and Ni cannot be considered as faithfully reflecting the quantity of OM initially deposited. Several factors acting on the loss of Cu and Ni can be retained, and among them, (1) a rapid loss linked to the decomposition of the OM before the conditions conducive to sulfate-reduction set in; (2) a low abundance of reactive iron which limits the quantity of pyrite liable to form, which significantly hampers Cu & Ni fixation in sediments. If Cu & Ni are not reliably retained in the sediments, that is, proportional to the quantity of OM supplied to the sediment, the paleoenvironmental reconstitutions involving the concentrations of these metals may provide underestimated values of paleoproductivity. An interesting clue is the Fe:Al ratio that makes it possible to quickly know whether the values of the Cu & Ni enrichments are likely to be "abnormally" low.

Allele-specific mitochondrial stress induced by Multiple Mitochondrial Dysfunctions Syndrome 1 pathogenic mutations modeled in Caenorhabditis elegans

Multiple Mitochondrial Dysfunctions Syndrome 1 (MMDS1) is a rare, autosomal recessive disorder caused by mutations in the NFU1 gene. NFU1 is responsible for delivery of iron-sulfur clusters (ISCs) to recipient proteins which require these metallic cofactors for their function. Pathogenic variants of NFU1 lead to dysfunction of its target proteins within mitochondria. To date, 20 NFU1 variants have been reported and the unique contributions of each variant to MMDS1 pathogenesis is unknown. Given that over half of MMDS1 individuals are compound heterozygous for different NFU1 variants, it is valuable to investigate individual variants in an isogenic background. In order to understand the shared and unique phenotypes of NFU1 variants, we used CRISPR/Cas9 gene editing to recreate exact patient variants of NFU1 in the orthologous gene, nfu-1 (formerly lpd-8), in C. elegans. Five mutant C. elegans alleles focused on the presumptive iron-sulfur cluster interaction domain were generated and analyzed for mitochondrial phenotypes including respiratory dysfunction and oxidative stress. Phenotypes were variable between the mutant nfu-1 alleles and generally presented as an allelic series indicating that not all variants have lost complete function. Furthermore, reactive iron within mitochondria was evident in some, but not all, nfu-1 mutants indicating that iron dyshomeostasis may contribute to disease pathogenesis in some MMDS1 individuals.

The impact of postdepositional alteration on iron- and molybdenum-based redox proxies

Ratios of (un)reactive iron species, authigenic molybdenum contents (Moauth), and molybdenum isotope compositions (δ98Moauth) in sedimentary rocks are geochemical proxies that are widely used to reconstruct past marine redox states, which have been calibrated in modern marine settings covering oxic to euxinic conditions. However, syn- and postdepositional processes can result in alterations and ambiguities of proxy-derived redox signals that can challenge the validity of paleoreconstructions. We present new data from modern organic-rich sediments of two oxygen minimum zone settings in the Gulf of California and the Peruvian margin. The results show that Mo is fully immobilized shortly after deposition by reaction with hydrogen sulfide (H2S) produced during organoclastic sulfate reduction. Thus, any H2S produced deeper in the sediment (e.g., by sulfate reduction coupled to anaerobic methane oxidation) leaves the initially deposited Mo concentrations and δ98Mo signatures unaltered, which supports the robustness of Mo-based redox proxies. In contrast, the Fe speciation data reveal continued pyritization due to constant exposure of Fe minerals to H2S. Importantly, both Fe bound to oxides and carbonates (highly reactive Fe) and also poorly reactive Fe (e.g., sheet silicates) undergo pyritization during early diagenesis. This process generates Fe-based proxy signatures that falsely imply ferruginous or euxinic conditions.

Cross-Shore and Depth Zonations in Bacterial Diversity Are Linked to Age and Source of Dissolved Organic Matter across the Intertidal Area of a Sandy Beach

Microbial communities and dissolved organic matter (DOM) are intrinsically linked within the global carbon cycle. Demonstrating this link on a molecular level is hampered by the complexity of both counterparts. We have now investigated this connection within intertidal beach sediments, characterized by a runnel-ridge system and subterranean groundwater discharge. Using datasets generated by Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) and Ilumina-sequencing of 16S rRNA genes, we predicted metabolic functions and determined links between bacterial communities and DOM composition. Four bacterial clusters were defined, reflecting differences within the community compositions. Those were attributed to distinct areas, depths, or metabolic niches. Cluster I was found throughout all surface sediments, probably involved in algal-polymer degradation. In ridge and low water line samples, cluster III became prominent. Associated porewaters indicated an influence of terrestrial DOM and the release of aromatic compounds from reactive iron oxides. Cluster IV showed the highest seasonality and was associated with species previously reported from a subsurface bloom. Interestingly, Cluster II harbored several members of the candidate phyla radiation (CPR) and was related to highly degraded DOM. This may be one of the first geochemical proofs for the role of candidate phyla in the degradation of highly refractory DOM.

Cryoturbation leads to iron-organic carbon associations along a permafrost soil chronosequence in northern Alaska

In permafrost soils, substantial amounts of organic carbon (OC) are potentially protected from microbial degradation and transformation into greenhouse gases by association with reactive iron (Fe) minerals. As permafrost environments respond to climate change, increased drainage of thaw lakes in permafrost regions is predicted. Soils will subsequently develop on these drained thaw lakes, but the role of Fe-OC associations in future OC stabilization during this predicted soil development is unknown. To fill this knowledge gap, we have examined Fe-OC associations in organic, cryoturbated and mineral horizons along a 5500-year chronosequence of drained thaw lake basins in Utqiaġvik, Alaska. By applying chemical extractions, we found that ~17 % of the total OC content in cryoturbated horizons is associated with reactive Fe minerals, compared to ~10 % in organic or mineral horizons. As soil development advances, the total stocks of Fe-associated OC more than double within the first 50 years after thaw lake drainage, because of increased storage of Fe-associated OC in cryoturbated horizons (from 8 to 75 % of the total Fe-associated OC stock). Spatially-resolved nanoscale secondary ion mass spectrometry showed that OC is primarily associated with Fe(III) (oxyhydr)oxides which were identified by 57Fe Mössbauer spectroscopy as ferrihydrite. High OC:Fe mass ratios (>0.22) indicate that Fe-OC associations are formed via co-precipitation, chelation and aggregation. These results demonstrate that, given the proposed enhanced drainage of thaw lakes under climate change, OC is increasingly incorporated and stabilized by the association with reactive Fe minerals as a result of soil formation and increased cryoturbation.