Cosmogenic 10 Be ages on the Pomeranian Moraine, Poland

Rinterknecht, V. R., Marks, L., Piotrowski, J. A., Raisbeck, G. M., Yiou, F., Brook, E. J. & Clark, P. U. 2005 (May): Cosmogenic ¹⁰Be ages on the Pomeranian Moraine, Poland. Boreas , Vol. 34, pp. 186–191. Oslo. ISSN 0300–9483.
We measured the ¹⁰Be concentrations in boulders collected from the Pomeranian Moraine in Poland, providing the first direct dating of the southern margin of the Scandinavian Ice Sheet (SIS) in the Polish Lowland. The mean age of 8 ¹⁰Be ages of the Pomeranian Moraine in northwestern Poland is 14.30.8 ¹⁰Be ka, while in northeastern Poland the mean age of 19 ¹⁰Be ages of the moraine is 15.00.5 ¹⁰Be ka. Given the excellent agreement between the two age groups, we calculate a mean age of 14.80.4 ¹⁰Be ka for final deposition of the Pomeranian Moraine of northern Poland. The age of the Pomeranian Moraine suggests that the southern margin of the SIS was near its maximum extent in Poland at a younger time than previously inferred, and that retreat from the moraine at 14.80.4 ¹⁰Be ka probably occurred in response to the onset of the Bølling interstade.     

Last Interglacial stratigraphy at Ristinge Klint, South Denmark

Past environmental changes in the Baltic area are discussed on the basis of foraminifera and ostracods as well as pollen and spores in marine sediments in cliff sections at Ristinge Klint, Langeland, southern Denmark. The sediment succession represents Jessen & Milthers' (1928) pollen zones d-g or Andersen's (1961, 1975) zones E2-E5, and a correlation with the annually laminated Bispingen sequence indicates that the sequence spans about 3400 years. Marine conditions seem to have occurred at c. 300-365 years after the beginning of the Eemian Interglacial, close to fully marine conditions developing by c. 2500 years. This early date of the marine ingression pre-dates that of most previous studies in the region by several hundred years, but it post-dates the initial marine ingression in the easternmost Baltic. A marked change in salinity at c. 650 years after the beginning of the Eemian was presumably caused by an opening of the Danish Belts. An indication of a major alteration in current activity is registered at c. 3000 years after the beginning of the interglacial. The recognition of the relative timing of these events may be significant for the understanding of the opening of connections between the North Sea, the Baltic and the White Sea.     

Composition and source of the clay-sized fraction of Saalian till in The Netherlands

The clay mineral composition of a number of Saalian till samples from The Netherlands shows a large variation in smectite and illite abundances. The samples were selected according to their source area, as inferred from coarse erratic components. Specimens containing primarily rock fragments indicative of East and East-Central Baltic areas are associated with no or low smectite percentages in their clay fractions. Those with mainly flint and rock fragments indicative of the West Baltic areas have high smectite contents in the clay. In general the smectite-rich material is found in the lower portions of the Saalian till bed, whereas the smectite-poor occurs in the upper portions. The linkage between very fine and coarse till components in the upper smectite-free parts of the till suggests that this entire till has been subjected to englacial transport over distances of at least 1000 km.     

Transport and accumulation of actinide elements in the near-shore environment: field and modelling studies

Nuclear facilities in coastal locations often discharge low‐level liquid wastes into the sea and the radioisotopes in these discharges are of interest both in assessing possible environmental impacts and as tracers for coastal processes. The distributions of a range of artificial radionuclides, derived from the authorized discharges from British Nuclear Fuels (BNFL) Sellafield, have been determined in the sediments of an intertidal salt marsh in the Esk Estuary, Cumbria, UK. Where published discharge histories exist (for ¹³⁷Cs, ²³⁸Pu, ^239,240Pu and ²⁴¹Am), the sediment core‐profile distributions of these radionuclides have been compared with the releases from Sellafield, and consistent values of the accumulation rate (0·226 ± 0·007 g cm^?2 yr^?1) are obtained. A quantitative model has been developed, describing association of radionuclides with suspended particulate material, which is then accumulated and mixed in an offshore mud patch before resuspension and deposition in the salt marsh. The model has been used to describe radionuclide distributions observed in both the mud patch and the salt marsh, and to identify isotopes for which post‐depositional remobilization or solution transport from the discharge point are important. The behaviour of the commonly studied isotopes (¹³⁷Cs, ²³⁸Pu, ^239,240Pu and ²⁴¹Am) is similar to that observed at this and other nearby locations. The activation product isotope ²³⁶U is enhanced in these sediments over the natural baseline by four to eight orders of magnitude, and the results suggest that Sellafield‐derived uranium is comparably mobile to ¹³⁷Cs in these sediments although the processes governing the behaviour of these two elements may be different. In situ production of ²⁴¹Am by decay of its ²⁴¹Pu parent has generated only 17% of the current sediment inventory of this isotope, insufficient to account for the increase over the last 20–25 years, and suggesting that the input material for these sediments is preferentially enriched in Am relative to Pu during transport from the offshore mud patch. The discharge history of ²⁴⁴Cm, which is unknown, has also been reconstructed from the sediment profile and the model.     

Volatile Components, Magmas, and Critical Fluids in Upwelling Mantle

The phase diagram for lherzolite–CO₂–H₂O providesa framework for interpreting the distribution of phase assemblagesin the upper mantle with various thermal structures, in differenttectonic settings. Experiments show that at depths >80 km,the near-solidus partial melts from lherzolite–CO₂–H₂Oare dolomitic, changing through carbonate–silicate liquidswith rising temperatures to mafic liquids; vapor, if it coexists,is aqueous. Experimental data from simple systems suggest thata critical end-point (K) occurs on the mantle solidus at anundetermined depth. Isobaric (T–X) phase diagrams forvolatile-bearing systems with K elucidate the contrasting phaserelationships for lherzolite–CO₂–H₂O at depths belowand above a critical end-point, arbitrarily placed at 250 km.At levels deeper than K, lherzolite can exist with dolomiticmelt, aqueous vapor, or with critical fluids varying continuouslybetween these end-members. Analyses of fluids in microinclusionsof fibrous diamonds reveal this same range of compositions,supporting the occurrence of a critical end-point. Other evidencefrom diamonds indicates that the minimum depth for this end-pointis 125 km; maximum depth is not constrained. Constructed cross-sectionsshowing diagrammatically the phase fields intersected by upwellingmantle indicate how rising trace melts may influence trace elementconcentrations within a mantle plume. KEY WORDS: mantle solidus; critical end-point; dolomitic magma; diamond inclusions; critical fluids     

Pleistocene Underplating and Metasomatism of the Lower Continental Crust: a Xenolith Study

Xenoliths from Engeln–Kempenich in the East Eifel volcanicfield (Germany) comprise gabbroic to ultramafic cumulates, andmeta-igneous and meta-sedimentary granulite- to amphibolite-facieslithologies. They provide evidence for Pleistocene heating andmetasomatism of the lower continental crust by mafic magmas.The metamorphic xenoliths were divided into three types: (1)primitive type P, which are little affected by metasomatic replacementstructures; (2) enriched type E1 defined by metasomatic replacementof primary pyroxene and garnet by pargasitic amphibole and biotite;(3) enriched type E2 defined by breakdown of hydrous phases.Type E rocks are geochemically related to type P and cumulatexenoliths by compositional trends. During modal metasomatism,type E rocks were oxidized. Type E1 rocks were typically enrichedin Rb, Th, U, Nb, K, light rare earth elements (LREE) and Zr,and E2 enriched in Rb, Th, U, Nb, K, REE, Zr, Ti and Y, relativeto type P rocks. Formation of the hydrous, chlorine-bearingphases amphibole and scapolite containing glass and fluid inclusionsin the E1 rocks provides evidence for a water and Cl-bearingfluid phase coexisting with silicate melt. Accordingly, we calculated10 mol % H₂O back into the CO₂-dominated fluid inclusions, inagreement with experimental data on the composition of a fluidphase coexisting with mafic alkaline melts at elevated pressure.Primary CO₂-dominated fluid inclusions coexisting with glassinclusions in metamorphic corona phases and neoblasts, and incumulate xenoliths, have overlapping densities. Fluid inclusionbarometry using the corrected densities indicates that bothcumulates and metamorphic xenoliths originated from the samedepth at 22–25 km (650 ± 50 MPa). This is interpretedas being a main magma reservoir level within the upper partof the lower crust close to the Conrad discontinuity, wherethe xenoliths represent wall-rocks. The Conrad discontinuityseparates an upper-crustal layer, consisting of preferentiallyductile granodioritic and tonalitic gneisses, and more brittlelower-crustal mafic granulites. The brittle–ductile transitionappears to be a preferred level of magma stagnation. KEY WORDS: continental lower crust; fluids; magma chamber; metasomatism; xenoliths     

Shallow erosion beneath turbidity currents and its impact on the architectural development of turbidite sheet systems

Erosion by turbidity currents changes the morphology of the sea floor. The relief of the scoured surface may affect the dynamics of the flow and thereby the pattern of deposition; this could, in turn, affect flow and deposition patterns in subsequent events. This study investigates shallow, centimetre to decimetre scale erosion beneath turbidite sheet sandstones of the Oligocene Macigno Formation of North‐west Italy, where erosion and deposition are variably coupled at the bed scale in a net‐aggradational setting. The research focus was on: (i) the recognition of scour edges and erosive surfaces; (ii) quantification of spatial differences in the amount of erosion; and (iii) an investigation of how this differential erosion can be compensated by the deposits directly overlying the erosional surfaces. Where they can be observed, scour edges commonly have sills of the overlying sandstone intruding beneath blocks and wings of the substrate that is being eroded. A consequence of this de‐laminating scouring style is that erosional surfaces are bedding parallel when followed away from the scour edges, giving the appearance of normal conformable bed bases. Despite their cryptic nature, such bedding‐parallel scour surfaces can be recognized by comparing serial detailed sedimentary logs (here, 16 bed‐parallel scour surfaces were identified in a succession comprising 95 beds). Different styles of compensation by the overlying turbidite beds are defined based on differential sedimentation inside and outside of the scour relief. It is found that differential erosion is on average under‐compensated by differential sedimentation. In some cases, the overlying deposits anti‐compensate, being thinner at the location where more erosion has occurred. Unequal spatial distribution of differential erosion in the study area combines with sedimentary under‐compensation to result in a trend of accumulating section thickness differences over multiple beds. In one ca 25 m thick package, the maximum cumulative change in lateral gradient during some 20 events reached 0·17°, before being reset by a single event. This process can be interpreted either as a lobe compensation effect, or as a scour enhancement effect, depending on the orientation of the palaeohorizontal datum. If allowed to proceed, the latter process could force the system past a channellization threshold, prompting a change from sheet to channelled architecture. This type of shallow substrate scouring and differential deposition is likely to be an important process in the build‐up of sheet turbidite sandstone units and could play a major role in autocyclic adjustment of local sea‐floor gradients.     

Importance of volcanic glass alteration to sediment stabilization: offshore Japan

A minor amount (ca 1 wt%) of amorphous silica cement sourced from volcanic glass inhibits consolidation of hemipelagic sediment approaching the Nankai Trough subduction zone throughout the Shikoku Basin. The distribution and nature of the cement were examined via secondary and backscattered electron imaging. The amorphous silica occurs as altered material in contact with volcanic glass, coating grains (including grain contacts) and filling pores. Based on chemical and petrographic evidence, the cement is probably sourced from volcanic glass; this is in contrast to a previous suggestion that this silica cement is sourced dominantly from biogenic silica. Amorphous silica sourced from disseminated volcanic glass shards has the ability to form a thin coating on clay‐dominated sediment throughout the Shikoku Basin. Measured amorphous silica content in hemipelagic sediments suggests that the cementing process is active throughout the Shikoku Basin (at sites separated by >500 km). The cementation process may occur in other locations where sediment containing hydrated disseminated volcanic glass is buried sufficiently for heat to facilitate alteration (i.e. Central America, Cascadia and the Gulf of Alaska).