Framboids from the Cariaco Basin and Demerara Rise were prepared into lamellar foils for STEM ima... more Framboids from the Cariaco Basin and Demerara Rise were prepared into lamellar foils for STEM imaging analysis using a dual-beam scanning electron microscope (SEM; Thermo Fisher Scientific Helios Nanolab 600i) with focused-ion bean capabilities (FIB), at the Environmental Molecular Sciences Laboratory (EMSL) at Pacific National Northwest Laboratory (PNNL). Framboids of interest were identified with SEM and conventional FIB techniques were used for the extraction and preparation of the framboids. The approach is summarized for the Cariaco Basin specimen here, with the same approach applied to the Demerera Rise specimens. First, a protective Pt-C capping layer was deposited as a rectangular section of the identified framboid surface (e.g., ∼4 × ∼15 μm) by electron beam induced deposition (~150 nm) followed by ion-beam induced deposition (~350 nm) using the dual-beam FIB microscope, further protecting the framboids from Ga + ion damage and Ga contamination during ion milling. The framboid substructure was viewed in cross-section by trenching both sides of the rectangular region of interest to a depth ~10 µm by ion beam milling, using an 30kV accelerating voltage and ~3nA current. After substructures of interest were identified, the section was extracted with an OmniProbe micromanipulator and transferred to a Cu TEM ½ grid and thinned to <100 nm. The same preparation approach was applied to the Demerara Rise specimen.
The early diagenetic interplay between reactive iron, sulfur, and organic matter in the bathymetr... more The early diagenetic interplay between reactive iron, sulfur, and organic matter in the bathymetrically isolated Santa Monica Basin (SMB) sediments are investigated in this study. We explore solid-phase and porewater profiles from the basin supplemented with a transect from 71 to 907 meters water depth that includes oxygenated (>60 μM O2) bottom waters near the coast and oxygen-deficient waters (~4 µM O2) in the basin. The geochemical data of the basin sediments are further scrutinized by means of reactive transport modeling. The results show that the basin sediments do not follow the traditional geochemical signatures of oxygen-deficient settings. A lack of dissolved sulfide accumulation and sulfurized iron persists despite the sediments being deposited under reducing conditions (without bioturbation/bioirrigation), strong organic carbon input (TOC up to 5.0 wt%), and active dissimilatory sulfate reduction. Not only did we find an exceptional enrichment in highly reactive Fe in the surface sediments (~45% of total Fe), but the enrichment of reactive Fe, including ferrihydrite, persists downcore and coexists with high levels of dissolved Fe. The enhanced preservation of Fe oxides and lack of iron-sulfide precipitation is in part explained by detection via Mössbauer spectra of iron oxides bounded to organic matter (Fe[III]-OM coprecipitates). The modeled Fe budget shows that most of the Fe oxides in the surface sediments are internally recycled by upward diffusion and subsequent oxidation of Fe 2+. Sulfide oxidation coupled to Fe reduction effectively precludes sulfide accumulation while enhancing build-up of dissolved Fe, fueling the Fe cycle within the first 5 cm depth. Continuous reoxidation of Fe 2+ enhances the formation of Fe(III)-OM coprecipitates, limiting the amount of reactive organic matter. In the unavailability of labile organic matter, other than within the uppermost layers, the organic-rich sediment profiles are dominated by iron cycling that limits the production and preservation of sulfides and enhances the preservation of Fe oxides and organic carbon. This study highlights key local controls on Fe availability in marginal basins and describes an intricate biogeochemical C-Fe-S cycling in modern and possibly ancient marine systems with important implications for Fe availability in the marine realm.
Geochemical proxies used widely to reconstruct global paleodepositional systems require further c... more Geochemical proxies used widely to reconstruct global paleodepositional systems require further calibration and validation in a wider range of oxygen-poor settings. The redox threshold values associated with various proxies (e.g., Fespeciation, trace-metal enrichments) can vary considerably among depositional systems and, for this reason, geochemical proxies should be scrutinized in multiple modern depositional systems of diverse redox characteristics-both stable and dynamic. Here, we provide a detailed study of Saanich Inlet, a semi-restricted fjord-like basin noted for high-frequency redox variation. Bottom water and sediment samples were collected in July 2019 when complete anoxia developed below 130 m water depth. We present data from the seasonally anoxic basin (200 m) and the oxygenated margin (100 m) to compare how spatiotemporal variations in redox condition impact the cycling of iron, sulfur, and trace metals in the bottom waters as recorded in bulk sediments, porewaters, and pyrite. We examined key biogeochemical drivers of early diagenetic reactions mainly via stable sulfur isotopes (δ 34 S), trace metal content, and iron-speciation. Additionally, we performed 57 Fe Mössbauer Spectroscopy analysis of iron mineralogical phases to cross-validate with widely used wet-chemical sequential Fe extraction methods. Given that Mössbauer Spectroscopy measures minerals directly rather than the response of a mineral to a chemical reagent, it offers an independent analytical method that can characterize and quantify different iron (oxyhydr)oxides, sulfides, sulfates, carbonates, silicates, amorphous colloids, and nanoparticles. The result is direct qualitative and quantitative estimates of the precipitation pathways and transformations of redox-sensitive iron phases. Additionally, this technique offers important novel insights into the dominant pyrite precursor pathway (i.e., FeS x species), a question long debated in the field. Chiefly, this study enables a direct comparison between biogeochemically dynamic oxic and anoxic environments along a transect recording differing depositional redox but similar detrital inputs. Most specifically, this novel calibration combining water column, solid phase, porewater, and pyrite data will provide new insights into the early diagenetic reactions that define the pyrite trace element compositions and Fe speciation data that are often used to interpret ancient environments.
Comparison of whole rock and discrete pyrite geochemistry as complementary tracers of ancient ocean chemistry: An example from the Neoproterozoic Doushantou Formation, China
Framboids from the Cariaco Basin and Demerara Rise were prepared into lamellar foils for STEM ima... more Framboids from the Cariaco Basin and Demerara Rise were prepared into lamellar foils for STEM imaging analysis using a dual-beam scanning electron microscope (SEM; Thermo Fisher Scientific Helios Nanolab 600i) with focused-ion bean capabilities (FIB), at the Environmental Molecular Sciences Laboratory (EMSL) at Pacific National Northwest Laboratory (PNNL). Framboids of interest were identified with SEM and conventional FIB techniques were used for the extraction and preparation of the framboids. The approach is summarized for the Cariaco Basin specimen here, with the same approach applied to the Demerera Rise specimens. First, a protective Pt-C capping layer was deposited as a rectangular section of the identified framboid surface (e.g., ∼4 × ∼15 μm) by electron beam induced deposition (~150 nm) followed by ion-beam induced deposition (~350 nm) using the dual-beam FIB microscope, further protecting the framboids from Ga + ion damage and Ga contamination during ion milling. The framboid substructure was viewed in cross-section by trenching both sides of the rectangular region of interest to a depth ~10 µm by ion beam milling, using an 30kV accelerating voltage and ~3nA current. After substructures of interest were identified, the section was extracted with an OmniProbe micromanipulator and transferred to a Cu TEM ½ grid and thinned to <100 nm. The same preparation approach was applied to the Demerara Rise specimen.
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Papers by Daniel Gregory