Methane is a powerful greenhouse gas and its biological conversion in marine sediments, largely controlled by anaerobic oxidation of methane (AOM), is a crucial part of the global carbon cycle. However, little is known about the role of iron oxides as an oxidant for AOM. Here we provide the first field evidence for iron-dependent AOM in brackish coastal surface sediments and show that methane produced in Bothnian Sea sediments is oxidized in distinct zones of iron- and sulfate-dependent AOM. At our study site, anthropogenic eutrophication over recent decades has led to an upward migration of the sulfate/methane transition zone in the sediment. Abundant iron oxides and high dissolved ferrous iron indicate iron reduction in the methanogenic sediments below the newly established sulfate/methane transition. Laboratory incubation studies of these sediments strongly suggest that the in situ microbial community is capable of linking methane oxidation to iron oxide reduction. Eutrophication of coastal environments may therefore create geochemical conditions favorable for iron-mediated AOM and thus increase the relevance of iron-dependent methane oxidation in the future. Besides its role in mitigating methane emissions, iron-dependent AOM strongly impacts sedimentary iron cycling and related biogeochemical processes through the reduction of large quantities of iron oxides.

We present high-resolution records for oxygen isotopes of the planktic foraminifer Globigerinoides ruber (δ 18 O ruber ) and bulk sediment inorganic geochemistry for Holocene-age sediments from the southeast Mediterranean. Our δ 18 O ruber record appears to be dominated by Nile discharge rather than basin-scale salinity/temperature changes. Nile discharge was enhanced in the early to middle Holocene relative to today. The timing of the long-term maximum in Nile discharge during the early Holocene corresponds to the timing of maximum intensity of the Indian Ocean-influenced Southwest Indian summer monsoon (SIM). This coincidence suggests a major influence of an Indian Ocean moisture source on Nile discharge in the early to middle Holocene, while, presently, the Atlantic Ocean is the main moisture source. Nile discharge was highly variable on multicentennial time scale during the early to middle Holocene, being strongly influenced by variable solar activity. This solar-driven variability is also recorded in contemporaneous SIM records, however, not observed in an Atlantic Ocean-derived West African summer monsoon record from the Holocene. This supports the hypothesis that the Indian Ocean moisture source predominantly controlled Nile discharge at that time. Solar-driven variability in Nile discharge also influenced paleoenvironmental conditions in the eastern Mediterranean. Bulk sediment Ba/Al and V/Al, used as indicators for (export) productivity and redox conditions, respectively, varied both in response to solar forcing on multicentennial time scales. We suggest that changes in Nile discharge on these time scales have been concordant with nutrient inputs to, and shallow ventilation of, the eastern Mediterranean.

[1] XRF sediment core scanning technology is increasingly used to quantify sediment composition. The overall good correlation between biophilic halogen bromine (Br) and sedimentary total organic carbon (TOC) potentially allows the fast estimation of down core TOC profiles by XRF scanning. In order to test this approach we present data from the Arabian Sea and a Mediterranean brine basin, comparing XRF core scanning Br data with discrete sample TOC analyses. Overall, Br counts and TOC show a clear correlation, except when stable carbon isotope and C/N data indicate intervals characterized by enhanced input of terrestrial organic matter. Hence, solid phase Br is exclusively associated with marine organic matter (MOC) and can be used as a direct estimate of this parameter after a calibration is established. High pore water Br in the brine core steepens the Br/TOC correlation but after salt correction shows a nearly identical gradient to that of the Arabian Sea core.

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27Microanalysis of epoxy resin-embedded sediments is used to demonstrate the presence 28 of authigenic iron (Fe) (II) phosphates and manganese (Mn)-calcium (Ca)-carbonate-29 phosphates in the deep euxinic basins of the Baltic Sea. These minerals constitute major burial 30 phases of phosphorus (P) in this area, elevating the total P burial rate above that expected for 31 a euxinic depositional environment. Particle shuttles of Fe and Mn oxides into the deep 32 euxinic basins act as drivers for P-bearing mineral authigenesis. While Fe (II) phosphates are 33 formed continuously in the upper sediments following the sulfidization of Fe-oxyhydroxides 34 and release of associated P, Mn-Ca-carbonate-phosphates are formed intermittently following 35 inflow events of oxygenated North Sea water into the deep basins. The mechanism of Fe (II) 36 phosphate formation differs from previously reported occurrences of vivianite formation in 37 marine sediments, by occurring within, rather than below, the sulfate-methane transition zone. 38The spatial distribution of both authigenic phases in Baltic sediments varies in accordance 39 with the periodic expansion of anoxia on centennial to millennial timescales. The results 40 highlight the potential importance of authigenic P-bearing minerals other than carbonate 41 fluorapatite for total P burial in euxinic basins. 42 43 44 45 46 47 48 70 conditions also lowers the potential of sediments to 'trap' P close to the sediment-water 71 interface (Mortimer, 1941), further increasing the efficiency of P regeneration.72 73 Sediment P burial represents the long-term output of the marine P cycle, and thus has 74 the potential to break the feedback loop between P regeneration, high biological production 75 4 and low-oxygen conditions. Phosphorus may be buried in sediments within various distinct 76 fractions. These include organic matter, detrital phosphate minerals and authigenic mineral 77 phases (Ruttenberg, 1992). Authigenic P-bearing phases buried in sediments include P 78 associated with Fe-oxyhydroxides which resist reduction in the upper sediments (Reed et al., 79 2011), calcium (Ca)-phosphates such as carbonate fluorapatite (e.g. Ruttenberg and Berner, 80 1993) and P associated with manganese (Mn)-Ca-carbonates (Mort et al., 2010; Suess, 1979), 81 and finally Fe (II) phosphates such as vivianite (e.g. Burns, 1997).82 83 Carbonate fluorapatite (CFA), Ca 10 (PO 4 ) 6-x (CO 3 ) x (OH,F) 2+x , forms either by direct 84 nucleation from saturated porewaters (Van Cappellen and Berner, 1989), via precursor phases 85 (Gunnars et al., 2004; Jahnke et al., 1983) or by conversion of calcite microfossils within the 86 sediments (Manheim et al., 1975). CFA formation has been shown to occur in a range of 87 coastal and deep-sea environments worldwide (Filippelli and Delaney 1996; Ruttenberg and 88 Berner 1993; Slomp et al., 1996) and is considered the most important authigenic sink for P in 89 the marine realm, accounting for an estimated 50% of global P burial (Ruttenberg, 1993). 90 91 In contrast t...

Hypoxia (oxygen concentrations of <2 ml/L) and so-called "dead zones" are a growing concern in coastal marine environments. The Baltic Sea is a shelf sea that is highly sensitive to hypoxia, and may serve as a laboratory for studying the interplay between natural and anthropogenic forcing of redox conditions in the global coastal zone. Past occurrences of hypoxia in the Baltic Sea have been shown by previous studies, but high-resolution, quantitative reconstructions of past hypoxia intensity are lacking. Here we present bulk sediment geochemical records from the deep basins of the Baltic Sea that show multicentennial oscillations during intervals of past hypoxia, suggesting rapid alternations between hypoxic and relatively oxic conditions. While the onset of past hypoxic events was likely forced by climatic variability, these events intensifi ed and terminated rapidly due to feedbacks in the phosphorus (P) cycle. The modern intensity of hypoxia is similar to several past events, suggesting that hypoxia in the Baltic Sea has a maximum potential intensity. However, using ultrahigh-resolution laser ablation-inductively coupled plasma-mass spectrometry scanning of sediment blocks, we show that modern hypoxia intensifi ed more rapidly than any past event. This confi rms the role of anthropogenic nutrient loading in driving this system into its current hypoxic state.

Hypoxia has occurred intermittently over the Holocene in the Baltic Sea, but the recent expansion from less than 10 000 km2 before 1950 to >60 000 km2 since 2000 is mainly caused by enhanced nutrient inputs from land and atmosphere. With worsening hypoxia, the role of sediments changes from nitrogen removal to nitrogen release as ammonium. At present, denitrification in the water column and sediments is equally important. Phosphorus is currently buried in sediments mainly in organic form, with an additional contribution of reduced Fe-phosphate minerals in the deep anoxic basins. Upon the transition to oxic conditions, a significant proportion of the organic phosphorus will be remineralized, with the phosphorus then being bound to iron oxides. This iron-oxide bound phosphorus is readily released to the water column upon the onset of hypoxia again. Important ecosystems services carried out by the benthic fauna, including biogeochemical feedback-loops and biomass production, are also lost with hypoxia. The results provide quantitative knowledge of nutrient release and recycling processes under various environmental conditions in support of decision support tools underlying the Baltic Sea Action Plan.

Nutrient over-enrichment is one of the classic triggering mechanisms for the occurrence of cyanobacteria blooms in aquatic ecosystems. In the Baltic Sea, cyanobacteria regularly occur in the late summer months and form nuisance accumulations in surface waters and their abundance has intensified significantly in the past 50 years attributed to human-induced eutrophication. However, the natural occurrence of cyanobacteria during the Holocene is debated. In this study, we present records of cyanobacteria pigments, water column redox proxies, and nitrogen isotopic signatures for the past ca. 8000 years from Baltic Sea sediment cores. Our results demonstrate that cyanobacteria abundance and nitrogen fixation are correlated with hypoxia occurring during three main intervals: (1) ca. 7000–4000 B.P. during the Littorina transgression, (2) ca. 1400–700 B.P. during the Medieval Climate Anomaly, and (3) from ca. 1950 A.D. to the present. Issues of preservation were investigated, and we show that organic matter and pigment profiles are not simply an artifact of preservation. These results suggest that cyanobacteria abundance is sustained during periods of hypoxia, most likely because of enhanced recycling of phosphorus in low oxygen conditions.