Description: We present a comprehensive study showing new results from a shallow gas seep area in approximate to 40 m water depth located in the North Sea, Netherlands sector B13 that we call Dutch Dogger Bank seep area. It has been postulated that methane presumably originating from a gas reservoir in approximate to 600 m depth below the seafloor is naturally leaking to the seafloor. Our ship-based subbottom echosounder data indicate that the migrating gas is trapped in numerous gas pockets in the shallow sediments. The gas pockets are located at the boundary between the top of the Late Pliocene section and overlying fine-grained sediments, which were deposited during the early Holocene marine transgression after the last glaciation. We mapped gas emissions during three R/V Heincke cruises in 2014, 2015, and 2016 and repeatedly observed up to 850 flares in the study area. Most of them (approximate to 80%) were concentrated at five flare clusters. Our repeated analysis revealed spatial similarities of seep clusters, but also heterogeneities in emission intensities. A first calculation of the methane released from these clusters into the water column revealed a flow rate of 277 L/min (SD=140), with two clusters emitting 132 and 142 L/min representing the most significant seepage sites. Above these two flare clusters, elevated methane concentrations were recorded in atmospheric measurements. Our results illustrate the effective transport of methane via gas bubbles through a approximate to 40 m water column, and furthermore provide an estimate of the emission rate needed to allow for a contribution to the atmospheric methane concentration.

Description: Extensive investigations of sedimentary barium were performed in the southern South Atlantic in order to assess the reliability of the barium signal in Antarctic sediments as a proxy for paleoproductivity. Maximum accumulation rates of excess barium were calculated for the Antarctic zone south of the polar front where silica accumulates at high rates. The correspondence between barium and opal supports the applicability of barium as a proxy for productivity. Within the Antarctic zone north of today's average sea ice maximum, interglacial vertical rain rates of excess barium are high, with a maximum occurring during the last deglaciation and early Holocene and during oxygen isotope chronozone 5.5. During these periods, the maximum silica accumulation was supposedly located south of the polar front. Glacial paleoproductivity, instead, was low within the Antarctic zone. North of the polar front, significantly higher barium accumulation occurs during glacial times. The vertical rain rates, however, are as high as in the glacial Antarctic zone. Therefore there was no evidence for an increased productivity in the glacial Southern Ocean.

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: 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