Description: In organic-rich sediments laid down in fresh water, much less diagenetic pyrite is formed than in analogous marine sediments because of the much lower concentrations of dissolved sulfate found in most fresh waters as compared to seawater. As a result, modern organic-rich freshwater sediments exhibit a much higher organic carbon-to-pyrite sulfur ratio (C/S) than marine sediments with similar organic contents. On this basis, C/S ratios can be used to distinguish ancient marine from freshwater (or slightly brackish) sedimentary rocks. This is demonstrated here for several Carboniferous shales and siltstones. The C/S technique cannot distinguish brackish-water sediments deposited under salinities greater than half that of seawater from marine sediments, as demonstrated by analyses of modern Chesapeake Bay sediments. Also, the method is not applicable to nearly pure limestones or to rocks low in organic matter (less than about 1% organic carbon). Saline (high sulfate) phases of ancient lakes can be distinguished from nonsaline phases using the C/S method.

Description: Glacial environments may provide an important but poorly constrained source of potentially bioavailable iron and manganese phases to the coastal ocean in high-latitude regions. Little is known about the fate and biogeochemical cycling of glacially derived iron and manganese in the coastal marine realm. Sediment and porewater samples were collected along transects from the fjord mouths to the tidewater glaciers at the fjord heads in Smeerenburgfjorden, Kongsfjorden, and Van Keulenfjorden along Western Svalbard. Solid-phase iron and manganese speciation, determined by sequential chemical extraction, could be linked to the compositions of the local bedrock and hydrological/weathering conditions below the local glaciers. The concentration and sulfur isotope composition of chromium reducible sulfur (CRS) in Kongs- and Van Keulenfjorden sediments largely reflect the delivery rate and isotope composition of detrital pyrite originating from adjacent glaciers. The varying input of reducible iron and manganese oxide phases and the input of organic matter of varying reactivity control the pathways of organic carbon mineralization in the sediments of the three fjords. High reducible iron and manganese oxide concentrations and elevated metal accumulation rates coupled to low input of “fresh” organic matter lead to a strong expression of dissimilatory metal oxide reduction evidenced in very high porewater iron (up to 800 lM) and manganese (up to 210 lM) concentrations in Kongsfjorden and Van Keulenfjorden. Sediment reworking by the benthic macrofauna and physical sediment resuspension via iceberg calving may be additional factors that promote extensive benthic iron and manganese cycling in these fjords. On-going benthic recycling of glacially derived dissolved iron into overlying seawater, where partial reoxidation and deposition occurs, facilitates the transport of iron across the fjords and potentially into adjacent continental shelf waters. Such iron-dominated fjord sediments are likely to provide significant fluxes of potentially bioavailable iron to coastal waters and beyond. By contrast, low delivery of reducible iron (oxyhydr)oxide phases and elevated organic carbon mineralization rates driven by elevated input of “fresh” marine organic matter allow organoclastic sulfate reduction to dominate carbon remineralization at the outer Smeerenburgfjorden sites, which may limit iron fluxes to the water column.

Description: The biogeochemical cycle of iron plays a key role in the ocean by delivering bioavailable iron that controls plankton productivity. Transport through the iron cycle occurs mainly as nanoparticulate (oxyhydr)oxides, which are physically and chemically intermediate between aqueous and particulate forms and can be directly or indirectly bioavailable. Iron nanoparticles transform with time to more stable forms by increased crystallinity, aggregation and growth, and they also alter to other nanominerals. These age transformations can be inhibited or reversed. The resulting aging-rejuvenation cycle first produces stability during long-distance transport and then reverses the process such that bioavailable and labile iron can be produced and delivered to the open ocean.

Description: Porewaters in site 680 Peru Margin sediments contain dissolved sulfide over a depth of approximately 70 m which, at a sedimentation rate of 0.005 cm/yr, gives a sediment exposure time to dissolved sulfide of about 1.4 Myr. Reactions with dissolved sulfide cause the site 680 sediments to show a progressive decrease in a poorly-reactive silicate iron fraction, defined as the difference between iron extracted by dithionite (FeD) at room temperature and that extracted by boiling concentrated HCl (FeH), normalised to the total iron content (FeT). Straight line plots are obtained for ln[(FeH - FeD)/FeT] against time of burial, from which a first order rate constant of 0.29 1/Myr (equivalent to a half-life of 2.4 Myr) can be derived for the sulfidation of this silicate iron. Comparable half-lives are also found for the same poorly-reactive iron fraction in the nearby site 681 and 684 sediments. This silicate Fe fraction comprises 0.8-1.0% Fe, only 30-60% of which reacts even with 1.5-3 million years exposure to dissolved sulfide. Diagenetic models based on porewater concentrations of sulfate and sulfide, and solid phase iron contents, at site 680 are consistent in indicating that this poorly-reactive iron fraction is only sulfidized on a million year time scale. Silicate iron not extracted by HCl can be regarded as unreactive towards dissolved sulfide on the time scales encountered in marine sediments.