Tjeerd C.E. van Weering

The Avaatech XRF Core Scanner: technical description and applications to NE Atlantic sediments

Abstract X-ray fluorescence (XRF) core scanning provides rapid high-resolution (down to 1 mm) records of chemical composition on split sediment cores. The measurements are non-destructive and require very limited sample preparation. The new Avaatech XRF Core Scanner, operational since 2002, covers the atomic mass range from Al to U. Instrument parameters, especially tube voltage, can be adjusted to provide optimum settings for selected elements or sets thereof. Owing to the nature of the surface of split sediment cores, particularly effects resulting from sample inhomogeneity and surface roughness, results are semiquantitative, yet provide reliable records of the relative variability in elemental composition downcore. Selected case studies from diverse sedimentary settings in the NE Atlantic Ocean illustrate a range of applications of XRF logging data. These include preliminary stratigraphic interpretations (glacial-interglacial cycles), provenance studies of the terrigenous sediment fraction, lithological characterization, early diagenetic processes and distinction between carbonate phases (aragonite v. calcite).

Organic carbon in shelf seas: sinks or sources, processes and products

Abstract The preservation of C org in various shelf seas from different geological, hydrological and climatological settings is compared. Most of these shelves have been studied extensively over the last two to three decades, often within the framework of large (inter)national research projects. Various factors (physical oceanography, chemical conditions, sedimentation rates) that could influence the preservation of C org are discussed. Not only recent conditions, but also longer time scale fluctuations (glacials versus inter-glacials, sea level variations) are dealt with. Most (>95%) of the C org introduced onto shelves by primary production and imported from the oceans and continents is mineralized in the water column and sediments. It appears that the role of shelves as sinks for C org is sometimes overestimated. Large areas of the continental shelves do not show any accumulation of organic matter under present day conditions. Only locally, where hydrological and sedimentological conditions are favourable for organic matter accumulation considerable amounts of C org are buried. On longer time scales the role of shelves as sinks for C org is limited, even more than under recent conditions. Continental slopes, canyons and deep-sea fans are thought to be the main sinks for C org .

Giant carbonate mud mounds in the southern Rockall Trough

Seismic profiles, sidescan sonar data, bottom photography and sampling proved for the first time that there are giant carbonate mounds (bioherms) in the Rockall Trough. The Pelagia Mounds, on the northern Porcupine Bank, are relatively isolated whereas most of the Logachev Mounds, on the southeast Rockall Bank, form a field of closely spaced, contiguous mounds. They are all found on the upper slope at depths between about 500 and 1200 m and as a result of this study about 500 have been mapped. They have a variety of shapes and the larger ones are very steep-sided, up to 350 m high and 2 km wide at the base. Sediment samples show that the mounds consist of pale coloured muds, mainly aragonite, usually with live and/or dead cold-water corals at the seafloor and with buried dead corals. Coral thickets appear to be detected as a brush-like signature on high resolution profiles and as a speckled pattern on high resolution sidescan sonar records. Shelly sands are found on the seabed between the mounds. Currents in the mound areas are strong enough to transport sands along slope as medium-sized sand waves and to prevent pelagic deposition, thus providing surfaces for initial coral settlement by the winnowing of fines to leave glacial dropstones and exposed rocks. These observations show that the poleward-directed upper slope current of the eastern North Atlantic extends further south than hitherto known and that there is a southwesterly directed current on Rockall Bank. There are waves on the Logachev Mounds, with a wavelength of about 20–30 m, that are thought to be moulded in carbonate muds by across-slope directed internal tidal currents and/or cascading currents. Mounds develop above erosional surfaces seen on seismic data. The huge amount of carbonate mud seems to be produced by the rapid growth and breakdown of cold-water corals living in a very favourable environment. Intermediate nepheloid layers formed upstream by the strong currents may provide the food source for the deep water suspension feeders. Conditions for giant mound growth require an ideal balance between current speed and sedimentation rate, among other factors. No evidence for the suggested link to methane seepage has been found so far.

Role of cold-water Lophelia pertusa coral reefs as fish habitat in the NE Atlantic

The rate of discovery of reefs of the cold-water coral Lophelia pertusa (Linnaeus, 1758) has been remarkable, and attributable to the increased use of underwater video. These reefs form a major three-dimensional habitat in deeper waters where little other ‘cover’ for fish is available. They are common in the eastern North Atlantic, and occur at least in the western North Atlantic and off central Africa. There are also other non-reef records of Lophelia in the Atlantic, and in Indian and Pacific oceans. Thus, not only are these reefs a significant habitat on a local scale, but they may also provide an important habitat over a very wide geographic scale.

Carbonate removal by acidification causes loss of nitrogenous compounds in continental margin sediments

Abstract This study reports on measurements of organic carbon (Corg) and total nitrogen (Ntot) in surface sediments originating from 6 transects along the northwest European continental margin. After elimination of carbonates by an acidification technique using sulphurous acid, both elements were analysed in the same sediment sample using an elemental analyser. Corg and Ntot in the sample were comparatively low, ranging between 1 and 10 mg C and 0.2 to 1 mg N g−1 dry sediment. In a second analysis, the samples were analysed without acid addition, resulting in Ntot concentrations of 0–50% higher compared to their acidified counterparts. As a consequence, molar C/N ratios derived from the analysis of Corg and Ntot in the acidified sample ranged between 6 to 11, while the Ntot separate analysis reduced C/N ratios to 6 to 8. It is suggested that the addition of sulphurous acid to eliminate inorganic carbon volatilises nitrogenous organic matter.

Northeastern Atlantic cold-water coral reefs and climate

U-series age patterns obtained on reef framework-forming cold-water corals collected over a nearly 6000-km-long continental margin sector, extending from off Mauritania (17 degrees N; northwest Africa) to the southwestern Barents Sea (70 degrees N; northeastern Europe), reveal strong climate influences on the geographical distribution and sustained development of these ecosystems. Over the past three glacial-interglacial cycles, framework-forming cold-water corals (Lophelia pertusa and Madrepora oculata) seem to have predominantly populated reefs, canyons, and patches in the temperate East Atlantic and the Mediterranean Sea. Above 50 degrees N corals colonize reefs in the northern East Atlantic primarily during warm climate periods with the biogeographic limit advancing from similar to 50 degrees N to similar to 70 degrees N. We propose that north-south oscillations of the biogeographic limit of reef developments are paced by ice ages and may occur synchronously with north-south displacement of cold nutrient-rich intermediate waters and surface productivity related to changes of the polar front.

Relationship of surface sediments and benthic foraminiferal distribution patterns in the Norwegian Channel (northern North Sea)

Abstract Our study of the distribution of benthic foraminifers in surface sediment samples from the Norwegian Channel has shown that eight species and five different assemblages are important. The assemblages are dominated byUvigerina peregrina, Bolivina skagerrakensis, Cassidulina laevigata, Elphidium excavatum andBulimina marginata, respectively.Hyalinea balthica, Melonis barleeanum andTrifarina angulosa are also abundant. The first four assemblages and their dominant species can be related to certain water masses with known characteristics. Two species are related to certain types of substrate;M. barleeanum seems to prefer a fine-grained substrate, whileT. angulosa occurs most abundantly where the content of silt and clay is low.B. skagerrakensis andH. balthica occur most abundantly in sediments with a high content of organic carbon, whileT. angulosa shows inverse correlation to the organic carbon content.

Living (stained) benthic foraminifera within surficial sediments of the Skagerrak

Abstract An analysis of living (stained) benthic foraminifera within four box cores raised from water depths of 74 to 621 m in the Skagerrak shows that faunal patterns at different water depths are strikingly different within this relatively shallow, organicrich marginal sea environment. Although some genera show vertical stratification within the sediments, a number of taxa are found over wide intervals or have different patterns between cores. The vertical distributional data are compared with deep-sea data from the eastern margin of North America. Benthic foraminifera from a deep core at 530 m in the Skagerrak have similar faunal patterns within the surficial sediments as found in the deep sea, but shallow cores at 74 and 212 m have more complex patterns. This difference between shallow and deep cores within the Skagerrak and the North Atlantic is attributed to higher biological activity in the shallow environments, which creates a greater amount of heterogeneity within the shallow-water sediments than is found in deeper environments. The faunal data suggest that the use of benthic foraminifera for paleoenvironmental reconstructions in shallow, organicrich regions will be complicated in future studies. The ecology of individual species is difficult to assess because of the complex faunal patterns observed within the surficial sediments. Geochemical studies for estimating bottom water oxygen or organic carbon flux will also be difficult, since these studies assume that taxa occupy particular microhabitats within the surficial sediments.

Recent sedimentation and organic carbon burial in a shelf sea: the North Sea

To obtain a carbon budget of the North Sea, recent sedimentation rates and organic carbon and nitrogen concentrations were determined on 27 box cores from the shallow North Sea (with a water depth < 200 m and a surface area of 500 × 103 km2). The sedimentation rates in these box cores range from 5 to 35 cm · 100 year−1, Corg concentrations are < 0.05 − 1%. Based on these numbers and published data the total amount of Corg buried in the shallow North Sea is calculated as ~ 1 × 105 tons · year−1. In large parts of the shallow North Sea recent sedimentation is (nearly) absent. Reworking of relict deposits and partial mixing of these deposits with small amounts of recent fine grained sediments by biological and physical processes and human activities (beam trawl fishery) result in apparent sedimentation rates which are likely to exceed the real net sedimentation rates considerably. The results of this study suggest that the majority of the organic matter is exported from the shelf over the shelf edge into the Norwegian Sea and into inner shelf deeps (Skagerrak and Norwegian Channel). Coarse grained shelf sediments are only of minor significance for the preservation of organic matter.

Recent sediment accumulation, organic carbon burial and transport in the northeastern North Sea

Abstract Organic carbon and nitrogen and sedimentation rate determinations were made of boxcores from the Norwegian Channel, North Sea. The geographical distribution of recent sedimentation areas were defined by analysis of published data and 3.5 kHz penetrating echo sounder data. The annual dry bulk sediment accumulation in the northeastern North Sea is established at 74 × 10 6 tons. As the average organic carbon content for the Norwegian Channel and Skagerrak/Kattegat sediments is 0.6 and 1.8%, respectively, the organic carbon accumulation rate in the northeastern North Sea could be calculated. The total organic carbon accumulation in the Norwegian Channel and Skagerrak/northern Kattegat is 0.17 × 10 6 tons · yr −1 and 0.83 × 10 6 tons · yr −1 , respectively. Less than 10% of this is accounted for by local primary production. This means that more than 90% of the organic carbon buried in the sediments of the northeastern North Sea must have been supplied from different sources. Terrigenous sources supply 20% of the organic carbon. The remainder is marine organic matter produced elsewhere in the North Sea, or imported from the Atlantic Ocean, Norwegian Sea and the Baltic Sea. Storm wave and -current induced (near) bottom nepheloid layers are the main likely mechanism to transport fine grained sediments and associated organic matter from the North Sea plateau into the Kattegat, Skagerrak and Norwegian Channel.