Rodney L. Stevens

Manganese authigenesis in the Landsort Deep, Baltic Sea

Abstract Manganese mineralogy, because of its redox sensitivity, is a valuable parameter for interpreting water-column anoxia and sediment diagenesis. In the Baltic, and especially the Landsort Deep, the relatively widespread occurrence of Mn-carbonates and the unique observations of authigenic Mn-sulphides in sedimentary environments need to be accounted for within the models proposed for Mn authigenesis in diverse settings. In this study stratigraphic changes in the geochemistry and mineralogy are documented in two gravity cores using petrographic microscopy, XRD, SEM-EDS, analyses of C and S content, 13 C isotopes, ICP-MS cation determinations, and electron microprobe element profiling. The lamination in the bottom sediments of the Landsort Deep suggests essentially continuous anoxic conditions at the sediment surface during the last ca. 4500 14 C years. The most striking variations in the sediments are related to the occurrence of two, Mn(II)-precipitate-rich, sapropel intervals (recent and 1390–1990 14 C years BP). Sapropel deposition is associated with relatively slow detrital sedimentation, allowing the concentration of organic matter. The sediment C org content is influenced by the variations in the primary production as well as the supply of terrestrial C org , but a major control appears to be detrital dilution. The precipitation of Mn-carbonate as individual aggregates of spherical particles, dispersed within clayey sediment matrix, and as laminae with semi-seasonal cyclicity is believed to be largely induced by the high alkalinity in connection with organic matter deposition and degradation. Where H 2 S has exceeded Fe availability, MnS has also precipitated, most commonly as well developed, hexagonal crystals, but also as the cubic variety, alabandite, which has not previously been reported from a sedimentary environment. An expansion of the anoxic bottom-water conditions during sapropel deposition is interpreted by correlation with laminated intervals on the slope of the deep basin. Budget calculations for the supply of Mn to the basin during Mn-rich sapropel formation can be better balanced by the addition of Mn from dissolution of oxides formed prior to the anoxic expansion. The importance of alkalinity for the precipitation of Mn-carbonates and sulphides does not exclude the need for considerable Mn concentrations, but the model argued in several recently published studies, stressing Mn availability as the major control, does not seem appropriate for the Landsort Deep, and may be too restrictive in many other settings as well.

Mineral magnetic and textural interpretations of sedimentation in the Skagerrak, eastern North Sea

Abstract The mineral magnetic properties of surface sediments in the northern Skagerrak show systematic trends which allow the identification of sediment provinces and the interpretation of transport pathways. This application is especially valuable considering the importance and difficulty of environmental analyses involving fine grained deposits. Magnetic susceptibility (χ) and saturation isothermal remanent magnetisation (SIRM) have different geographic patterns, but their individual trends are consistent in the Skagerrak muds (clay content 25–60%). Over the entire studied area the χ values correlate positively with general trends in particle size, whereas SIRM does not show particle size dependency. The present study confirms that the majority of sediments accumulating in the northern Skagerrak are supplied from the south. The mineral magnetic parameters facilitate detailed tracking of the counterclockwise transport of sediments around and into the Norwegian Trench.

The pre-Littorina diatom stratigraphy and sediment sulphidisation record from the west-central Baltic Sea: Implications of the water column salinity variations

Abstract The pre-Littorina section of a sediment core collected from the eastern slope of the Landsort Deep has been analysed with respect to diatoms and sedimentologic parameters. The occurrences of different Fe-sulphide phases are related to the depositional and diagenetic conditions that are reflected by Corg and S contents and the illite to quartz ratio. The later parameter is applied as a proxy for the sediment grain size. Diatom documentation allows the sub-division of the pre-Littorina stratigraphy into intervals corresponding to the Yoldia Sea and Ancylus Lake. The Yoldia Sea interval is additionally divided into three phases, with the middle phase corresponding to the period when the entire water column was brackish. During the first phase the bottom waters were brackish and surface waters fresh and during the last phase rapid freshening took place. The occurrence of distinct pockets and lenses with FeS in two portions of the pre-Littorina sediments is explained by the elevated Corg content in th...

Sediment color and weathered preglacial sources of Quaternary clays in southwestern Sweden

Abstract The factors influencing clay color have been investigated using chemical, X-ray and microscopic methods. The olive gray colors are believed to be caused by organic matter and greenish iron silicates. Iron oxides mask the colors of clastic minerals when present as a precipitate on fine-grained particles. The iron oxides produce brownish hues. These precipitates are interpreted to have mostly developed prior to glacial erosion and transport in a weathering environment. Iron-oxides absorbed on clay surfaces were either removed or preserved due to the relative reducing or oxidizing conditions in the depositional and diagenetic environments. Low concentrations of reactive organic matter in the early glacio-marine sediments allowed the oxides to persist in bands of brownish clay (winter) which alternate with gray bands (summer). Distally and stratigraphically upwards, reduction of iron precipitates results in dominantly gray colored marine deposits. The prevalent occurrence of vermiculite, smectite, ka...

Weathering variability and aluminium interlayering: clay mineralogy of podzol profiles in till and glaciofluvial deposits, SW Sweden

Abstract Clay mineralogy is used as a reflection of trends in soil profile development in till and glaciofluvial deposits in SW Sweden. A major weathering products is vermiculite. Smectite occures in E horizons at sites with low pH and where soil mineralogy provided less acid neutralization through weathering. Low pH conditions have caused Al leaching and subsequent precipitation as hydroxy-Al interlayers in vermiculite. Changes in Al-interlayering between soil horizons have been evaluated by calculating a gravity centre of the clay mineral peaks on XRD spectra, which change in response to K-saturation and heating. The presence of interlayers has an important effect upon the cation exchange capacity of vermiculite.

A generalized lithofacies model for glaciomarine and marine sequences in the Göteborg area, Sweden

Abstract Strong stratigraphic trends can be recognized at many, if not most, of the valley localities below the marine limit in the Goteborg area. The deposits reflect the deglaciation and the isostatically induced regression following the last Weichselian glaciation. The generalized sequence of paleoenvironments and their associated deposits are used as an initial basis for comparing three stratigraphic examples from SW Sweden. These interpreted sections, in turn, are used to improve the paleogeographic model by specifying typical variations in important environmental parameters not originally included in the model. Ice-front proximity and meltwater drainage are decisively involved in the supply of sediments to both proximal and distal glaciomarine settings. Climatic variations further determine the extent of this supply and the balance between the contribution from the glacial source and the increasingly important supply from local erosion. Sedimentologic facies are described and discussed with respect ...

A new sand-silt-clay triangle for textural nomenclature

Abstract A classification scheme is presented which takes consideration for the sedimentation and frequency of different grain-size mixtures and the dominating influence of clay content upon sediment character. The classification triangle is more detailed and is less uniformly divided than most other schemes. This is required to convey the important changes within different sediment mixtures. This classification is particularly useful when working with unlithified, finer-grained sediments, and attempts to make field and laboratory descriptions more continuous with each other.

Historical trends in Hg, Pb and Zn sedimentation in the central shelf area of Portugal

Temporal records of excess 210Pb, and the determination of major (Al and Ca) and trace elements (Zn, Cr, Ni, Pb, Cu and Hg) in two sediment box-cores, collected in the central area of the Portuguese shelf of North of the Nazare canyon (offshore from the Lis River), allow evaluation of the deposition of various chemical elements normally associated with anthropogenic activities. In order to compensate for the natural sediment variability, heavy metal contents were normalised to Al. Temporal variations of Hg, Pb and Zn (Al-normalised) show an increasing trend since the beginning of the 1920’s, recording the development of industrial activities. Enrichment factors (EF) were calculated to estimate the level of contamination in these sediments. Mercury is the element with the highest average EF values (EF = 3), followed by Pb (EF = 1.5) and Zn (EF = 1.2). The results indicate that since 1991 64% of total Hg, 44% of total Pb and 24% of total Zn are derived from anthropogenic sources. The average anthropogenic fluxes of Hg, Pb and Zn (0.008, 3, 6 μg cm-2yr-1, respectively) for the last 40 years in a ca. 400 km2 deposition area represent a total accumulation of approximately 30, 12000 and 24400 kg per year of Hg, Pb and Zn, respectively. These results indicate that despite the high-energy conditions and the generally sandy nature of the Portuguese shelf sediments, it is possible to identify significant anthropogenic enrichments in some areas of sediment accumulation. These contaminants are not necessarily related to immediate sources but may instead indicate atmospheric and or marine transport from more distant sources.

Source, transport, and weathering influences upon grain-size and heavy mineral trends in glacial deposits of southwestern Sweden

Abstract Heavy mineral variability and regional trends are interpreted in terms of sediment source, transport sorting and weathering processes at 22 sites in tills, glaciofluvial deposits and reworked sediments in southwestern Sweden. Heavy minerals make up 4-11% of the coarsesilt fraction (45-63 μm) of the tills and reworked sediments, while significant enrichment (up to 100%) has occurred in glaciofluvial deposits. The most abundant minerals are hornblende and epidote. Minor amounts of apatite, garnet, titanite, and zircon are usually present. The geographic distribution of the heavy minerals relates to three factors. The local Precambrian bedrock has considerable amounts of metamorphic minerals, such as epidote, due to repeated periods of low-grade metamorphism. Secondly, mineral sorting during sediment transport and deposition has enriched heavy minerals within individual deposits and specific fractions. Finally, chemical and physical weathering of the surficial sediments has selectively depleted cert...

Clay mineralogy of agricultural soils (Ap horizon) in Västergötland, SW Sweden

Abstract The mineralogy of the clay‐size fraction of 15 samples from cultivated soils was determined using X‐ray diffraction methods. All of the samples contain mica, vermiculite, kaolinite and mixed‐layer minerals, but the proportions of these vary. The regional mineralogic trends are discussed in relation to the possible influences of grain size, relief, sediment‐source variations, time of exposure and climate. The geologic factors are largely related to the geographic distribution of the former glaciomarine, marine and terrestrial environments that successively existed in SW Sweden following the Weichselian glaciation. Although all of these factors have been involved to some extent, there is a tendency for weathering products (kaolinite, vermiculite and mixed‐layer minerals) to be most abundant in areas with greater relief, precipitation and exposure time. However, the grain‐size variations that we documented in this study would directly influence permeability and the intensity of weathering processes ...