| 1. |
- Moros, M, et al.
(author)
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Were glacial iceberg surges in the North Atlantic triggered by climatic warming?
- 2002
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In: Marine Geology. - 0025-3227. ; 192:4, s. 393-417
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Journal article (peer-reviewed)abstract
- High-resolution physical, mineralogical, sedimentological and micropalaeontological studies were carried out on North Atlantic cores from the Reykjanes Ridge at 59degreesN and from the region southwest of the Faeroe Islands. All core sites are situated along the pathway of Iceland-Scotland Overflow Water (ISOW) and the various parameters measured display similar features. Previously identified carbonate oscillations [Keigwin and Jones (1994) J. Geophys. Res., 99, 12397-12410] in the time span back to the Marine Isotope Stage 5-4 transition and Late Glacial lithic events [Bond and Lotti (1995) Science, 267, 1005-1010], such as the Heinrich ice-rafting events, are all represented in the core records. Long-term trends and higher-frequency changes in ISOW intensity were reconstructed on the basis of various independent proxy records. The long-term trends in circulation match theoretical orbitally forced insolation changes. Our observed links between ice-rafted detritus (IRD) input, variations in sea surface temperature (SST) and circulation at greater depth point to the need to re-examine the origin of IRD events. We suggest that these events may have been triggered by enhanced, partly sub-surface, heat transport to the-north. Enhanced northward heat transport may have caused bottom melting of floating outlet glaciers and ice shelves, leading to increased iceberg discharge and ice sheet destabilization. This discharge. resulted in lower SST's and a lower temperature over Greenland. Thus, as shown by our records, this scenario implies a temporary de-coupling of surface processes and circulation at greater depth. A key feature is the occurrence of a-saw-tooth pattern in the marine data, which is similar to the Greenland ice core records. Moreover, the 'warming' theory of IRD events would explain the observed 'out-of-phase' relationship between the Greenland and Antarctic ice-core records and also the rapid establishment of higher temperatures over Greenland immediately after the cold phases (stadials) of the Dansgaard-Oeschger cycles.
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| 2. |
- Rasmussen, Tine, et al.
(author)
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The Faroe-Shetland Gateway: Late Quaternary water mass exchange between the Nordic seas and the northeastern Atlantic
- 2002
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In: Marine Geology. - 0025-3227. ; 188:1-2, s. 165-192
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Journal article (peer-reviewed)abstract
- Thirteen piston and gravity cores from the Faroe-Shetland area were investigated for their planktic and benthic foraminiferal and oxygen isotopic distributions. Eight time-slices between 18 ka BP and the present were reconstructed to study variations in surface and deep water exchange between the SE Norwegian Sea and the northeast Atlantic Ocean. Today, a relatively strong northward flow of warm North Atlantic surface water is counterbalanced by a southward outflow of newly convected cold bottom water, the Norwegian Sea Overflow Water. During the last glacial maximum at 18 ka BP both the surface and bottom flows were slow and the climate conditions were Arctic. The convection north of the Faroe area was weak and unstable. The first indication of the deglaciation is a decrease in the planktic oxygen isotope values discernible southwest of the Faroe Islands at 15.5 ka BP. The deglaciation proceeded northeast and eastward synchronous with a gradual intensification of northward flowing warmer Atlantic Intermediate Water along the sea bottom. Meltwater fluxes increased between 14 and 13 ka BP producing cold surface waters, and the climatic cooling was extreme. There was no southward overflow of cold bottom water during this time period and the exchange of water masses between the Nordic seas and the North Atlantic Ocean was essentially reversed, i.e. estuarine. During the Bolling Interstadial at 12.5 ka BP northward flowing warm surface water was present to the east of the Faroe-Shetland Channel, wedged below a tongue of polar water spreading from the northwest and reaching into the Faroe-Shetland Channel. Convection in the Nordic seas and overflow of cold deep water started during the Bolling Interstadial. The polar water spread more eastward and southward during the following cold spell, the Younger Dryas, around 10.3 ka BP. The polar water was overlying the warmer, but more saline Atlantic water, which flowed northward below the cold surface water. The overflow of cold bottom water was supposedly only slightly weaker than during the Bolling Interstadial. Strong inflow of warm surface water took place during the Early Holocene at 9.5 ka BP and relatively dense cold water flowed southward along the bottom. The rate of water mass exchange reached a maximum at 6.5 ka BP, when both the inflow of warm Atlantic surface water and the outflow of cold dense bottom water appear to have been stronger than today.
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| 3. |
- Schwarzer, K, et al.
(author)
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Coastline evolution at different time scales - examples from the Pomeranian Bight, southern Baltic Sea
- 2003
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In: Marine Geology. - 0025-3227. ; 194:1-2, s. 79-101
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Journal article (peer-reviewed)abstract
- Sedimentological and morphological changes on the upper and lower shoreface during relatively stable sea-level highstand conditions have been investigated in the Pomeranian Bight, southern Baltic Sea, at time scales ranging from storm events to millennia. In order to cover that variety of time scales, different methods have been applied. Seasonal variations in the morphology of the upper shoreface were measured accurately using the tracer stick method. The ratio of breaking waves and energy dissipation due to wave breaking are the main forces controlling redeposition on the upper shoreface with the depth of disturbance up to three times the net change. The impact of single storm events can be observed from sidescan sonar mosaics to remain on the decadal scale. Aerial photographs covering the upper shoreface show that the location of gates, channel-like systems where water masses move offshore created during storm events, also remain stable over decades. Sedimentological and geomorphological variations and changes on the lower shoreface are only measurable on the century to millennium scale because the main driving forces are longlasting processes like sea-level fluctuations or neotectonics. Data on these scales have much more uncertainty in their relationship to forcing functions than data at shorter time scales. Because the effects of coastal processes active on different time scales can interact, comprehensive understanding of large-scale coastal behavior requires investigations from short events to long-term processes. (C) 2002 Elsevier Science B.V. All rights reserved.
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| 4. |
- Ask, Maria
(author)
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Mechanical tests on claystone from Ocean Drilling Program Hole 1070A (Leg 173) : implications for elevatedpore-fluid pressure in sediments within the ocean-continent transition zone, West Iberia
- 2001
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In: Marine Geology. - 0025-3227 .- 1872-6151. ; 177:3, s. 395-410
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Journal article (peer-reviewed)abstract
- Laboratory geotechnical deformation tests on a claystone test specimen from 619 m depth in Ocean Drilling Program Hole 1070A provide evidence of low effective yield stresses and capillary pressures (1.2–1.5 MPa) in the sediment section near the acoustic basement. Effects from negative pore-fluid pressure, and the presence of expansive clay minerals probably account for the drastic change in the mechanical behavior of the sample below and above the yield stress. The results from the geotechnical tests, together with high porosity values, suggest that the pore-fluid pressure is high in these basal sediments. The intense fracture/vein development in the basement suggests that the pore-fluids originate from deeper sections in the basement. Porosity anomalies are found near the basement in three boreholes drilled in the western part of the Iberia Margin. The anomalies are interpreted to signal elevated pore-fluid pressure. Consequently, a zone of overpressure possibly exists in the western part of the ocean-transition zone of the West Iberia margin, which thickens towards the west.
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| 5. |
- Boström, Kurt, et al.
(author)
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Geochemistry and origin of ferromanganese concretions in the Gulf of Bothnia
- 1982
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In: Marine Geology. - : Elsevier BV. - 0025-3227 .- 1872-6151. ; 50:1-2, s. 1-24
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Journal article (peer-reviewed)abstract
- Ferromanganese concretions cover large areas of the Gulf of Bothnia. They are flat to well-rounded, the rounded ones being richer in oxyhydroxides of iron and manganese. Rounded and ellipsoidal nodules, particularly those in the northern Gulf of Bothnia, are richest in Mn, Ni, Ba and Cu, which probably coexist in a Mn oxyhydroxide phase. Flat nodules are enriched in Fe, P, rare earths and As, probably associated with an Fe oxy-hydroxide component. Aluminum, V, Cr and Ti occur in still another phase. The sediments of the gulf generally consist of a 10-50 mm-thick layer of oxidized surface sediment, enriched in Mn, Ba, P and Ni lying on top of reduced sediments which are diagenetically depleted in these elements. The remobilized elements have redeposited in the nodules, but this process cannot explain the origin of all the nodular material. Some released Mn, Ba and Ni furthermore enter into suspended phases, which eventually leave the Baltic Sea. The economic value of the nodules in the Gulf of Bothnia is probably limited at present.
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| 6. |
- Glaccum, R., et al.
(author)
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(Na, K)-phillipsite: Its stability conditions and geochemical role in the deep sea
- 1976
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In: Marine Geology. - : Elsevier BV. - 0025-3227 .- 1872-6151. ; 21:1, s. 47-58
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Journal article (peer-reviewed)abstract
- Phillipsite occurs in the deep sea in areas of very slow sedimentation and as the final alteration product of basalt, suggesting that it is thermodynamically stable. The thermodynamic data for gibbsite, kaolinite, K-feldspar, Na-feldspar, analcime and H4 SiO4 (aq), as well as the activities of seawater constituents are reasonably well known. These values and estimated entropies for Na- and K-phillipsites permit an estimate of the free energy of formation (at 25°C) for Na-phillipsite (5337.6 ± 6.7 kcal/mole) and for K-phillipsite (5382.4 ± 1.9 kcal/mole). The decomposition of phillipsite to analcime at 250°C yields the same result, suggesting that phillipsite is indeed a stable mineral. Phillipsite does not regulate the concentration of Na+ and K+ in seawater, but probably affects the pH of bottom waters and the composition of interstitial waters. No indications exist of "frozen in" equilibria from hydrothermal reactions at 100-200°C as could be expected by submarine volcanism.
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| 7. |
- Ingri, Johan
(author)
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Geochemistry of ferromanganese concretions in the Barents Sea
- 1985
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In: Marine Geology. - : Elsevier BV. - 0025-3227 .- 1872-6151. ; 67:1-2, s. 101-119
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Journal article (peer-reviewed)abstract
- Ferromanganese concretions from ten stations in the Barents Sea have been analysed for 24 elements. The deposits occur as discoidal and flat concretions and as coatings, in the latter case on lithified or detrital material or as extensive pavements on the Svalbard shelf. The concretions are compositionally similar to Baltic concretions but differ considerably from deep-ocean nodules, particularly in Cu, Ni and Co contents. Statistical analyses reveal distinct correlations between Mn, Na, Ba, Ni and Cu; the Mn-rich coatings showed enrichment of Mo, Zn and possibly Co in a Mn-phase. The iron phase holds high concretions of P and As. Two iron-rich concretions with high contents of P, Ca, Sr, Y, Yb and La were found east and northeast of Spitsbergen Banken, probably indicating upwelling of nutrient-rich, cold polar water along the Svalbard shelf.
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| 8. |
- Sohlenius, Gustav, et al.
(author)
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Development of anoxia during the Holocene fresh-brackish water transition in the Baltic Sea
- 2001
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In: Marine Geology. - : Elsevier. - 0025-3227 .- 1872-6151. ; 177, s. 221-242
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Journal article (peer-reviewed)abstract
- One of the most pronounced environmental changes during the Holocene Baltic Sea history was the transition from the freshwater Ancylus Lake to the brackish water Litorina Sea. The establishment of brackish conditions during this transition (the A/L) was caused by an interplay of sea level rise and subsidence of sills in the Danish Straits. The northward progression of salt water influence caused the gradual and transgressive development of a halocline which obstructed vertical water circulation in the deep depositional basins. It caused changes in surface water properties (mirrored by diatom flora and productivity levels), in redox conditions of bottom waters, in organic carbon preservation of sediments, and in nutrient cycling. In seven cores from the Arkona, Bornholm and Gotland Basins, the A/L transition was WC dated and studied in high-resolution samples for minor and major element composition. The earliest marine inflows were small and can only be detected by sedimentary properties in the southern Baltic. Further north the salinity increase was gradual and retarded; only when the connection through the Danish Straits was well established the entire Baltic proper became brackish with a stratified water column. This development took altogether 2000 C-14 yr (c. 9000-7000 BP). Diatom analyses indicate a fast increase in salinity c. 7000 C-14 yr BP, which coincides with a transgressive phase in the straits, In the Gotland Basin, deposition of laminated sediments and periodically euxinic conditions were established first at the deepest bottoms, and rose to shallower water depth as the basin was gradually filled with dense brackish water. The laminated sequences have high organic carbon contents, which is attributed to better preservation under anoxic conditions. Litorina sediments from the Arkona and Bornholm Basins are bioturbated even though conditions became more reducing also in these sediments during the A/L transition. The development of reducing conditions during the A/L transition probably caused Pre-mobilization from the sediments and a decrease in the rates of denitrification. Both factors increased primary productivity. A comparison between isochronous sediments from different basins shows that certain elements (Mo, Cu, V and Cd) are enriched sediments deposited during predominantly anoxic conditions.
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| 9. |
- Sohlenius, Gustav, et al.
(author)
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Holocene history of the Baltic Sea as recorded in a sediment core from the Gotland Deep
- 1996
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In: Marine Geology. - : Elsevier. - 0025-3227 .- 1872-6151. ; 134, s. 183-201
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Journal article (peer-reviewed)abstract
- A 4 m long sediment core from the Gotland Deep, Baltic Sea, was investigated by means of biostratigraphical and chemical parameters and C-14 datings. This multidisciplinary approach allows us to evaluate the regional changes in salinity and redox conditions during the Holocene. According to the diatom assemblages and C-14 datings, the sediments were deposited in the brackish and freshwater phases of the Yoldia Sea (10,300-9600 C-14 yr B.P.), the freshwater Ancylus Lake (9600-8000 C-14 yr B.P.) and in the brackish water Litorina Sea (8000-3000 C-14 yr B.P.). Of several palaeosalinity indicators evaluated, the diatoms are the least ambiguous. Nevertheless the B content and, under some premises, the C/S ratio may be used for palaeosalinity information. Iron sulphides in the lowermost freshwater sediments formed below the prevailing, oxic sediment-water interface. Pyrite in the uppermost freshwater sediments was to a large extent formed in Litorina time, by diffusion of Sigma H2S from overlying brackish water sediments. The Ancylus Lake/Litorina Sea transition can be traced by the marked changes in both the siliceous microfossil assemblages and in the chemical properties of the sediments. An early Litorina phase is characterised by increased primary production, progressively increasing salinity and the development of euxinic conditions. In the Litorina Sea laminated clay gyttjas (def. as a clay containing 6-30% organic matter) containing pyrite and laminae of Ca-rhodochrosite, (Mn,Ca)CO3, accumulated. Both these minerals formed in close connection with sediment deposition. The rhodochrosite laminae formed during occasional inflow of oxygen rich, marine water through the Danish Straits. Rhodochrosite is by far the most common mineral in the laminae and formed on average with a periodicity of two to three years.
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| 10. |
- Aldahan, AA, et al.
(author)
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Be-10 records from sediments of the Arctic Ocean covering the past 350 ka
- 1997
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In: MARINE GEOLOGY. - : ELSEVIER SCIENCE BV. - 0025-3227. ; 144:1-3, s. 147-162
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Journal article (peer-reviewed)abstract
- Records of Be-10, Be-9, mineralogy and grain size were obtained from two cores collected by the Polarstern Expedition 1991 in the southern Nansen Basin (Core 2213-6) and the Yermak Plateau (Core 2208-2). The accumulation of sediments examined started from
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