Description: Benthic oxygen and nitrogen fluxes were quantified within the years 2012 to 2014 at different time series sites in the southern North Sea with the benthic lander NuSObs (Nutrient and Suspension Observatory). In situ incubations of sediments, in situ bromide tracer studies, sampling of macrofauna and pore water investigations revealed considerable seasonal and spatial variations of oxygen and nitrogen fluxes. Seasonal and spatial variations of oxygen fluxes were observed between two different time series sites, covering different sediment types and/or different benthic macrofaunal communities. On a sediment type with a high content of fine grained particles (o63 mm) oxygen fluxes of �15.5 to �25.1 mmol m�2 d�1 (June 2012), �2.0 to �8.2 mmol m�2 d�1 (March 2013), �16.8 to �21.5 mmol m�2 d�1 (November 2013) and �6.1 mmol m�2 d�1 (March 2014) were measured. At the same site a highly diverse community of small species of benthic macrofauna was observed. On a sediment type with a low content of fine grained particles (o63 mm) high oxygen fluxes (�33.2mmol m�2 d�1 August 2012; �47.2 to �55.1 mmolm�2 d�1 November 2013; �16.6 mmol m�2 d�1 March 2014) were observed. On this sediment type a less diverse benthic macrofaunal community, which was dominated by the large bodied suspension feeder Ensis directus, was observed. Average annual rain rates of organic carbon and organic nitrogen to the seafloor of 7.44 mol Cm�2 y�1 and 1.34 mol N m�2 y�1 were estimated. On average 79% of the organic bound carbon and 95% of the organic bound nitrogen reaching the seafloor are recycled at the sediment–water interface. & 2015 Elsevier Ltd. All rights reserved

Description: Estimating the amount of methane in the seafloor globally as well as the flux of methane from sediments toward the ocean–atmosphere system are important considerations in both geological and climate sciences. Nevertheless, global estimates of methane inventories and rates of methane production and consumption through anaerobic oxidation in marine sediments are very poorly constrained. Tools for regionally assessing methane formation and consumption rates would greatly increase our understanding of the spatial heterogeneity of the methane cycle as well as help constrain the global methane budget. In this article, an algorithm for calculating methane consumption rates in the inner shelf is applied to the gas-rich sediments of the Belt Seas and The Sound (North Sea–Baltic Sea transition). It is based on the depth of free gas determined by hydroacoustic techniques and the local methane solubility concentration. Due to the continuous nature of shipboard hydroacoustic measurements, this algorithm captures spatial heterogeneities in methane fluxes better than geochemical analyses of point sources such as observational/sampling stations. The sensibility of the algorithm with respect to the resolution of the free gas depth measurements (2 m vs. 50 cm) is proven of minor importance (a discrepancy of 〈10%) for a small part of the study area. The algorithm-derived anaerobic methane oxidation rates compare well with previous measured and modeling studies. Finally, regional results reveal that contemporary anaerobic methane oxidation in worldwide inner-shelf sediments may be an order of magnitude lower (ca. 0.24 Tmol year–1) than previous estimates (4.6 Tmol year–1). These algorithms ultimately help improve regional estimates of anaerobic oxidation of methane rates.

Description: In the eastern part of Lake Constance, the second largest pre-alpine Lake in Europe, about five hundred pockmarks (morphological depressions on the lake floor) were recently discovered of which ~ 40% release methane bubbles. The carbon isotopic composition of the escaping gas indicated that the methane is of biogenic origin. In our study, we investigated the fate of the released methane bubbles, i.e., the dissolution, oxidation or transport of the bubbles to the surface. At a littoral pockmark site (PM12, 12 m water depth) and a profundal pockmark (PM80, 80 m water depth), we analysed the dissolved methane concentrations and the methane isotopic carbon signature in the water column. At PM80, higher methane concentrations (up to 1523 nM), compared to the control site and the surface waters (225 ± 72 nM), were recorded only on some occasions and only in the bottom water, despite the fact that the released bubbles were dissolving within the hypolimnion based on bubble modeling. The isotope data suggest that most of the dissolved methane is oxidized below 40 m water depth. The isotopic signature of the methane in the surface water at PM80, however, differed from that of the methane in the hypolimnion; therefore, the surface methane at this profundal site is most likely an export product from the littoral zone. Assuming an initial bubble diameter of 5 mm, we calculated that these small bubbles would reach the surface, but approximately 96% of the methane would have dissolved from the bubble into the hypolimnion. At PM12, we observed higher concentrations of dissolved methane (312 ± 52 nM) with no significant differences between seasons or between control sites versus pockmark site. In the shallow water, divers estimated the bubble size to be 10 - 15 mm, which from a release depth of 12 m would barely dissolved in to the water column. The isotopic signature also indicated that there had been almost no methane oxidation in the shallow water column. Thus, the water depth of bubble release as well as the initial bubble size determine whether the methane enters the atmosphere largely unhindered (shallow site) or if the released methane is incorporated into the profundal water column.

Description: Abstract In the Arctic Seas, the West Spitsbergen continental margin represents a prominent methane seep area. In this area, free gas formation and gas ebullition as a consequence of hydrate dissociation due to global warming are currently under debate. Recent studies revealed shallow gas accumulation and ebullition of methane into the water column at more than 250 sites in an area of 665 km2. We conducted a detailed study of a subregion of this area, which covers an active gas ebullition area of 175 km2 characterized by 10 gas flares reaching from the seafloor at~245 m up to 50 m water depth to identify the fate of the released gas due to dissolution of methane from gas bubbles and subsequent mixing, transport and microbial oxidation. The oceanographic data indicated a salinity-controlled pycnocline situated ~20 m above the seafloor. A high resolution sampling program at the pycnocline at the active gas ebullition flare area revealed that the methane concentration gradient is strongly controlled by the pycnocline. While high methane concentrations of up to 524 nmol L−1 were measured below the pycnocline, low methane concentrations of less than 20 nmol L−1 were observed in the water column above. Variations in the δ 13 C CH 4 values point to a 13C depleted methane source (~−60‰ VPDB) being mainly mixed with a background values of the ambient water (~−37.5‰ VPDB). A gas bubble dissolution model indicates that ~80% of the methane released from gas bubbles into the ambient water takes place below the pycnocline. This dissolved methane will be laterally transported with the current northwards and most likely microbially oxidized in between 50 and 100 days, since microbial CH4 oxidation rates of 0.78 nmol d−1 were measured. Above the pycnocline, methane concentrations decrease to local background concentration of ~10 nmol L−1. Our results suggest that the methane dissolved from gas bubbles is efficiently trapped below the pycnocline and thus limits the methane concentration in surface water and the air–sea exchange during summer stratification. During winter the lateral stratification breaks down and fractions of the bottom water enriched in methane may be vertically mixed and thus be potentially an additional source for atmospheric methane.

Description: In coastal waters and the ocean silicic acid (Si(OH) 4 ) is a key nutrient for primary producers ( e.g . diatoms) and other siliceous organisms, because it is required for the formation of frustules and other hard parts made of biogenic silica (bSi). Especially in shallow waters like the southern North Sea, dissolution of bSi in surface sediments and the re fl ux of silicic acid from sediments into the water column is an important feedback mechanism for sustaining primary production. We investigated the temporal variability of benthic silicic acid fl uxes and the recycling ef fi ciency of bSi in surface sediments of the Helgoland Mud Area (southern North Sea). For this purpose we used different methods including a benthic chamber lander system for in situ fl ux studies of Si(OH) 4 , ex situ sediment incubations, pore water studies and sediment analysis. Our in situ measurements revealed considerable temporal variations with low silicic acid fl uxes in winter (0.3 – 1.0 mmol m � 2 d � 1 in March 2013 and 2014), increased fl uxes of 2.0 – 4.0 mmol m � 2 d � 1 in November 2013, and high fl uxes in June and August 2012 (3.6 – 8.3 mmol m � 2 d � 1 ). The relevance of biological mediated transport for the recycling of Si(OH) 4 was underlined by comparing in situ and ex situ sediment incubations, pore water studies, as well as depth pro fi les of benthic macrofauna. Mass budget calculations indicate that about 1.7 – 2.2 mol bSi m � 2 settle annually at the sea fl oor, off which about 60 – 81% are recycled within surface sediments and transported back into the water column

Description: Mapping radon (222Rn) distribution pat- terns in the coastal sea is a widely applied method for localizing and quantifying submarine groundwater discharge (SGD). While the literature reports a wide range of successful case studies, methodical problems that might occur in shallow wind-exposed coastal settings are generally neglected. This paper evalu- ates causes and effects that resulted in a failure of the radon approach at a distinct shallow wind-exposed location in the Baltic Sea. Based on a simple radon mass balance model, we discuss the effect of both wind speed and wind direction as causal for this fail- ure. We show that at coastal settings, which are domi- nated by gentle submarine slopes and shallow waters, both parameters have severe impact on coastal radon distribution patterns, thus impeding their use for SGD investigation. In such cases, the radon approach needs necessarily to allow for the impact of wind speed and wind direction not only during but also prior to the field campaign.