Importance of hydrothermal vents in scavenging removal of ²³⁰Th in the Nansen Basin

In this study we present dissolved and particulate ²³⁰Th and ²³²Th results, as well as particulate ²³⁴Th data, obtained as part of the GEOTRACES central Arctic Ocean sections GN04 (2015) and IPY11 (2007). Samples were analyzed following GEOTRACES methods and compared to previous results from 1991. We observe significant decreases in ²³⁰Th concentrations in the deep waters of the Nansen Basin. We ascribe this nonsteady state removal process to a variable release and scavenging of trace metals near an ultraslow spreading ridge. This finding demonstrates that hydrothermal scavenging in the deep‐sea may vary on annual time scales and highlights the importance of repeated GEOTRACES sections

Transport van reactief silikaat uit Waddenzee sediment naar het bovenstaande water

In this work we have tried to find a value for the upward diffusive flux of dissolved silica from the sediment to the water of the Wadden Sea. Some aspects of the diffusion mechanism are discussed, especially the importance of the turbulent diffusion. Turbulent diffusion was simulated by flushing the sediment very slowly with seawater in the laboratory

Nutrients in the interstitial waters of the Southern Bight of the North Sea

Interstitial water samples from the Southern Bight of the North Sea were analysed for the nutrients NH4+, NO2-, NO3-, PO43- and Si. Ammonia, phosphate and silica concentrations in the upper 15 cm were on the average 2 to 3 times higher in a coastal zone of about 2 nautical miles wide than they were offshore. In this coastal zone the average organic carbon content was higher and the aerobic layer thinner. The major part of the high inter-core variance was also present between cores taken only 20 m apart. Nutrient concentrations were significantly lower in a station in a region with high sand waves. Average silica concentrations were in summer twice as high as they were in winter. Ambiguous results were obtained regarding a possible seasonal variation in NH4+ and PO43- concentrations. The general shape of depth profiles of nutrient concentrations was determined from 26 coastal and 21 offshore 35cm cores. Below 2,5cm, nitrate reduction was always found, indicating anoxic conditions. The increase with depth of ammonia, phosphate and silica was highest in the upper 2,5cm of the sediment. Fluxes of nutrients to the water column were estimated from concentration gradients in this upper layer and were compared with other nutrient inputs to the area. It was concluded that the relative contribution from regeneration in the sediments to the nutrient budgets was largest for silica. In summer, the sediment yields an estimated 30 to 50% of all silica inputs to the area

Resuspension and particle transport in the Benthic Nepheloid Layer in and near Fram Strait in relation to faunal abundances and 234Th depletion

The West Spitsbergen Current, flowing northward through Fram Strait, causes a benthic nepheloid layer (BNL) on the western slope of the Yermak Plateau. This BNL is weaker on the eastern side of the Plateau and absent on the Greenland side of the Fram Strait where the East Greenland Current flows south. In this BNL we find throughout a depletion of 234Th relative to its parent 238U, and we use this to study the particle dynamics in the BNL. The export flux from the ice-covered surface ocean and from a young bloom found in the ice-free waters off NE Greenland is shown to be negligible, allowing us to explain the 234Th depletion by interaction with the sediment alone. The depletion, balanced by a similar excess in the surface layer of the sediment, implies the existence of a settling-resuspension loop with an average particle residence time of 1-2 months.The asymmetry with a stronger resuspension loop on the western (80-120 mg m-2 d-1) than on the eastern side of the Yermak Plateau (1-15 mg m-2 d-1) is reflected in the numbers of species and individuals of suspension feeders in box core samples, and in epifauna abundance as has been estimated from video observations. The suspension feeders thus contribute to deposit the particles that are advected from more productive ice-free regions. This explanation is in agreement with the east-west asymmetry in the input of organic material to the sediments of the Yermak Plateau, which has been concluded by Soltwedel et al. (2000) from the distribution of pigments, bacterial activity and meiofauna abundances, observed in a concurrent study at the same stations.On the West Spitsbergen shelf, a very intensive BNL was monitored over one month with a moored filtration system. A part of the sustained high suspended load may be advected over long distances.This study gives an example how the tracer 234Th can help to determine to what extent suspended particles are in continuous exchange with the seafloor, and where biological mediation and chemical modification can be expected

Trapping efficiencies of sediment traps from the deep eastern North Atlantic: the 230Th calibration

Bottom-tethered sediment traps deployed in the deep eastern North Atlantic between 54°N 20°W and 33°N 20°W (L1, L2, L3), at the European continental margin at 49°N (OMEX) and off the Canary Islands (ESTOC) were investigated for the determination of 230Th trapping efficiencies. The ratios of 230Th flux measured in the traps (Fa) to the expected 230Th flux from the production rate of 230Th in the overlying water column (Fp) ranged between 0.09 and 1.26. For the traps with deployment periods >300 days the interannual variation of Fa/Fp ratios (different years but same location and water depth) were up to 10%, suggesting that the average 230Th flux to the sediment traps did not vary significantly. The influence of lateral advection on the 230Th flux was taken into account either by applying a mass balance of 230Th and 231Pa or by assuming a constant removal rate of 230Th from the water column, an assumption based on similar 230Th concentration-depth profiles observed at most locations investigated. 230Th trapping efficiencies were between 9 and 143%, showing a trend of increasing efficiencies with increasing water depth. No relation was found between current velocities and 230Th trapping efficiencies. Our investigations suggest that the observation of constant or even increasing particle flux rates with increasing water depths in several sediment trap arrays investigated may be a result of sediment trap biases. The correction for the trapping biases is important for the understanding of the regional differences in the particle flux in the eastern North Atlantic

Quasi-conservative behaviour of 10Be in deep waters of the Weddell Sea and the Atlantic sector of the Antarctic Circumpolar Current

We present the first transect of dissolved 10Be depth profiles across the Antarctic Circumpolar Current (ACC) in the Atlantic sector. North of the Polar Front the 10Be concentrations increase continuously from very low values at the surface to values of up to 1600 atoms/g at depth. Deep water 10Be concentrations of particular water masses are consistent with earlier results obtained further north. South of the Polar Front and in the Weddell Sea the distribution of 10Be is also characterised by low surface concentrations but below 1000 m depth the concentrations are relatively constant and significantly higher (up to 2000 atoms/g) than further north, probably as a result of mixing and advection of water masses of Pacific origin. Overall the deep water 10Be distribution is obviously not significantly affected by scavenging processes or ice melt and comparison with the density distribution suggests that 10Be can be viewed as a quasi-conservative tracer. This provides a tool for an improved understanding of the behaviour of other more particle reactive trace metals in the Southern Ocean such as 230Th: in deep waters north of the ACC/Weddell Gyre boundary (AWB) 10Be/230Th has a relatively constant value (1.7±0.3×109 atoms/dpm) over a wide density range whereas south of the AWB the ratio is significantly lower (1.1±0.2×109 atoms/dpm). This normalisation to 10Be corroborates that 230Th is enriched by 50% due to accumulation south of the AWB as a consequence of minimal particulate fluxes. The quasi-conservative behaviour deduced from our results also implies that 10Be can only be used as a tracer for Southern Ocean particle fluxes in the past if ocean circulation patterns and water mass residence times did not change significantly