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Changing climate conditions affect dust emissions and the global dust cycle, which in turn affects climate and biogeochemistry. In this study we use observationally constrained model reconstructions of the global dust cycle since the Last Glacial Maximum, combined with different simplified assumptions of atmospheric and sea ice processing of dust-borne iron, to provide estimates of soluble iron deposition to the oceans. For different climate conditions, we discuss uncertainties in model-based estimates of atmospheric processing and dust deposition to key oceanic regions, highlighting the large degree of uncertainty of this important variable for ocean biogeochemistry and the global carbon cycle. We also show the role of sea ice acting as a time buffer and processing agent, which results in a delayed and pulse-like soluble iron release into the ocean during the melting season, with monthly peaks up to ~17 Gg/month released into the Southern Oceans during the Last Glacial Maximum (LGM).

Southern Ocean (SO) marine primary productivity (PP) is strongly influenced by the availability of iron in surface waters, which is thought to exert a significant control upon atmospheric CO2 concentrations on glacial/interglacial timescales. The zone bordering the Antarctic Ice Sheet exhibits high PP and seasonal plankton blooms in response to light and variations in iron availability. The sources of iron stimulating elevated SO PP are in debate. Established contributors include dust, coastal sediments/upwelling, icebergs and sea ice. Subglacial meltwater exported at the ice margin is a more recent suggestion, arising from intense iron cycling beneath the ice sheet. Icebergs and subglacial meltwater may supply a large amount of bioavailable iron to the SO, estimated in this study at 0.07-0.2 Tg yr-1. Here we apply the MIT global ocean model (Follows et al., 2007) to determine the potential impact of this level of iron export from the ice sheet upon SO PP. The export of iron from the ice sheet raises modelled SO PP by up to 40%, and provides one plausible explanation for seasonally very high in situ measurements of PP in the near-coastal zone. The impact on SO PP is greatest in coastal regions, which are also areas of high measured marine PP. These results suggest that the export of Antarctic runoff and icebergs may have an important impact on SO PP and should be included in future biogeochemical modelling. copyright © Author(s) 2014.

Glacial and iceberg sediments contain nanoparticulates of schwertmannite, ferrihydrite, and goethite formed where pyrite was oxidized by dissolved oxygen in aqueous subglacial environments. Schwertmannite, typically found in acid mine drainage, only forms at low pH by the oxidative weathering of pyrite. Stoichiometric models show that these conditions can be created in closed-system microenvironments containing at least [10.sup.-4] M dissolved oxygen, where volume ratios of water/pyrite are ~[10.sup.-5] to [10.sup.-6]. Ferrihydrite is the only product at higher and lower volume ratios. In these microenvironments the oxidation of pyrite to form schwertmannite is possible in several decades. Schwertmannite and ferrihydrite are metastable in contact with water and are transformed to goethite in

The authors trace the evolution of the sulfur cycle. Topics include bacterial metabolisms, anoxygenic photosynthesis, and ecosystems.

Pyrite formation is associated with fossils where the organic material decays by sulphate reduction, or where the carbonate skeleton acts either as a nucleation substrate or induces iron sulphide precipitation by dissolution. These requirements define specific combinations of porewater chemistry and saturation state with respect to carbonates and iron sulphides. A review of modern marine sediments suggests that near-surface porewaters are always over-saturated, or close to saturation, with iron sulphides but are alternately rich in either dissolved iron or dissolved sulphide. However, significant variations in carbonate saturation state may occur, with porewaters being undersaturated in the early stages of sulphate reduction, but subsequently becoming oversaturated until the later stages of methanogenesis. Here undersaturation may return if there is a substantial input of carbon dioxide, and if little alkalinity has arisen from iron reduction. Organic matter pyritization requires that soft-tissue material decays by sulphate reduction to release dissolved sulphide, with dissolved iron supplied by the surrounding porewaters. The modern sediment studies therefore suggest that organic matter pyritization can occur only during the earliest stages of sulphate reduction, where there are iron-rich porewaters which are saturated with iron sulphides. Pyrite replacement of carbonate shell material requires porewater undersaturation with respect to carbonates and analogy with modern sediments suggests that these conditions occur either during the earliest stages of sulphate reduction, or when methanogenesis produces undersaturation. Between these periods porewaters are saturated with respect to carbonates and pyrite of different morphologies can precipitate on carbonate shells. Sulphur isotope data provide support for these associations of porewater chemistry and style of fossil pyritization. The best-described examples are for soft tissue pyritization in Beecher's Trilobite Bed (Ordovician) and the Hunsruck Slate (Devonian), where the isotopic data are consistent with rapid, early pyritization in iron-rich porewaters. More data are needed on shell pyritization with the range of isotopic compositions indicating formation through all stages of diagenesis. Keywords: pyritization, fossils, sulphur, isotopes.

The pyritization of fossils in the Lower Devonian Hunsruck Slate, Germany, occurred in the later stages of authigenic mineralization due to the absence of organic matter and the presence of sulfur (S) and iron. An analysis of S isotope and C-S-Fe elemental data reveals that the slates containing fossils possess low amounts of pyrite S and organic substances but high levels of total and HCl-soluble Fe. The presence of highly enriched 34S in the adjacent slates indicates continued fossil pyritization due to exceptional diagenetic conditions.

Iron monosulfides and pyrite (total Cr-reducible S-TRS) in the bottom sediments of the exceptionally sulfide-rich euxinic fjord, Framvaren, south Norway, are unusually enriched in 34S. Most of these TRSs probably form in bottom waters, where reactive detrital iron phases encounter isotopically heavy dissolved sulfide. Enrichment in 34S in Framvaren is a result of intense bacterial reduction of a sulfate reservoir whose exchange with the open sea is limited by shallow sills and a low-salinity surface-water layer. 34S-enriched TRS in the deposits of ancient euxinic environments is commonly interpreted as evidence for early diagenetic sulfide formation where residual, less reactive iron phases were in contact with isotopically heavy dissolved sulfide (sulfate-depleted pore waters) for prolonged periods during burial. However, the Framvaren data indicate that isotopically heavy pyrite may form in relatively shallow (80-180 m), super-anoxic bottom-water pools. In contrast, the presence of isotopically light pyrite does not indicate an absence of restricted conditions, but only that most iron sulfide formation was completed before 34S-enriched sulfide could be encountered in the water column.

Sulphur was incorporated into Jurassic marine shales mainly through contact with dissolved sulfides or by microbial processes in oxygenated waters. Pyrite formation occurred at the same time, but reactive iron was not a significant influence on sulphur incorporation. This conclusion follows from analysis of sulfur isotopes.

A detailed approach to the low temperature geochemical cycle of iron is presented by quantifying the major continental fluxes and marine sediment sinks in terms of solid phase speciation. A comparable iron speciation data for suspended sediment from a selection of thirty-four world rivers was used for the study.

Pore-water sulphate concentrations show marked increases at depths [is greater than] 50m at ODP sites 888 and 890/889 from the Cascadia Margin accretionary wedge. In the uppermost 10 m sulphate concentrations decrease with depth and sulphate [[Delta].sup.34]S and [[Delta].sup.18]O increase as sulphate is removed by bacterial sulphate reduction. Isotopic data show that sulphate formed below 50 m results from oxidation of early diagenetic pyrite and that oxygen in the sulphate molecules is derived from pore water. [Fe.sup.3+] in the sediment is the probable oxidizing agent. The increased sulphate concentrations stimulate bacterial sulphate reduction at depths of 70-250 m and are thus important in sustaining deep bacterial activity. Keywords: anoxic environment, pore water, pyrite, oxidation, stable isotopes.

Existing interpretations of cement textures and isotopic compositions may significantly under-estimate the depth and duration of concretionary growth. Minus-cement porosities can commonly under-estimate depths of concretionary growth for some, or all, of the following reasons; (i) cements might not passively replace host sediment porosity, (ii) non-cement carbonate phases (such as replaced bioclastic carbonate) can be significant, (iii) sediment compaction models over-estimate rates of porosity loss at shallow ([is less than] 500 m) depths and (iv) cementation can create a framework that prevents compaction and preserves porosity. Cement textures can be used to distinguish two modes of growth; concentric growth, where successive layers of cement are added to the outer surface (radius increases with time), and pervasive growth, where cement crystals grow simultaneously throughout the concretion volume (little or no radius increase with time). Cement textures of siderite concretions are mostly consistent with pervasive growth, but many calcite microsparite concretions show no diagnostic textural features and could grow either concentrically or pervasively. Concretionary cementation, whether concentric or pervasive, occurred such that there was accessible porosity which could be filled by later cements. Pervasive growth in particular is associated with the retention of substantial amounts of porosity which may be filled by chemically and isotopically distinct phases. The resulting chemical gradients across concretions may then reflect variations in the relative proportions of early and later cements more than variations in porewater composition. Carbon isotope data from modern sediments show that dissolved carbonate in the methanogenic zone has a continuum of values from - 30% to + 15%, and thus overlaps [sup.13]C-depleted values normally considered characteristic of sulphate reduction. Many concretions previously thought to have grown entirely during sulphate reduction may therefore have continued cementation during methanogenesis, indicating a deeper and more prolonged cementation history. The necessary carbonate supersaturation for concretionary growth could either occur throughout the porewaters (the equilibrium model), or be generated in situ by organic matter decay (the local-equilibrium model), or created where external fluids are introduced (the fluid-mixing model). Keywords: concretions, carbonates, precipitation, [sup.18]O, [sup.13]C.