Ian Snowball

Identifying landslide preconditions in Swedish quick clays—insights from integration of surface geophysical, core sample- and downhole property measurements

Quick-clay landslides are a serious geohazard in Canada, Norway and Sweden. Identification and mapping of quick clays are essential endeavours because the damage caused by an individual landslide can be large and costly, with potentially fatal consequences. We collected geophysical borehole and soil core data from an area prone to quick-clay landslides in southwestern Sweden. Methodologies included in situ and laboratory measurements, providing information about natural gamma radiation, sonic velocities, electrical conductivity, pH, physical grain size, elemental and mineral composition, magnetic properties, cation exchange capacity and fossil content. A stratigraphic thickness of almost 60xa0m enables us to study quick clays and their host environment in Sweden at unusually high resolution. Results identify the origin and location of reflections in nearby seismic lines and assign physico-chemical properties to the geological units present in the area. We show that coarse-grained layers are sandwiched between marine clays (some of which are quick clays). These layers function as a conduit for relatively fresh water that infiltrates the marine clays and chemically destabilizes them by leaching out their salts. The salinity distribution in the boreholes indicate that the groundwater movement is downwards, through the coarse-grained layer and towards the Göta river. The presence of these materials is important for the development of quick clays, although not a prerequisite. With the help of surface geophysical methods, the location of the coarse-grained layers can be known faster and more economically, which could be relevant for studying the potential for quick-clay landslide occurrence over large areas.

Geomagnetic and mineral magnetic characterization of the Anthropocene

Abstract Geomagnetic and mineral magnetic data provide geological indices that are both independent of human impact (i.e. geomagnetic) and respond to human-induced environmental impact (i.e. mineral magnetic). We provide the first discussion of such magnetic events for help in defining the Anthropocene. Within the Holocene, a potential geomagnetic marker for the Anthropocene is the low dipole latitude at c. 2700 cal a BP, which is associated with distinct palaeosecular variation features in northerly mid- to high-latitude sites. Mineral magnetic records from lake and marine sediments identify major deforestation and soil delivery events from catchment systems in many parts of the world during the last 4000 years. In Europe, clusters of these events occur around both 2600 cal a BP and AD 1100, the former coinciding with a low in geomagnetic field dipole latitude and peak intensity. Mineral magnetic records in peats and lake sediments can reflect particulate pollution from fossil fuel burning. The expansion of major coal burning began c. AD 1800 in western Europe and eastern North America, but around AD 1900 this expanded due to more widely distributed coal use, and this event is the most clear mineral magnetic marker for the base of the Anthropocene.

Echoes from the past: a healthy Baltic Sea requires more effort.

Integrated sediment multiproxy studies and modeling were used to reconstruct past changes in the Baltic Sea ecosystem. Results of natural changes over the past 6000xa0years in the Baltic Sea ecosystem suggest that forecasted climate warming might enhance environmental problems of the Baltic Sea. Integrated modeling and sediment proxy studies reveal increased sea surface temperatures and expanded seafloor anoxia (in deep basins) during earlier natural warm climate phases, such as the Medieval Climate Anomaly. Under future IPCC scenarios of global warming, there is likely no improvement of bottom water conditions in the Baltic Sea. Thus, the measures already designed to produce a healthier Baltic Sea are insufficient in the long term. The interactions between climate change and anthropogenic impacts on the Baltic Sea should be considered in management, implementation of policy strategies in the Baltic Sea environmental issues, and adaptation to future climate change.

Magnetostratigraphic importance of secondary chemical remanent magnetizations carried by greigite (Fe3S4) in Miocene sediments, New Jersey shelf (IODP Expedition 313)

Paleomagnetic and mineral magnetic analyses were carried out on Miocene clays from upper unit II at Sites M0027 and M0028 recovered during Integrated Ocean Drilling Program Expedition 313 on the New Jersey shallow shelf. A zone of mixed polarity in the lower section of Hole M0028A and dual overlapping magnetization components in upper Hole M0027A indicate that the sediments may have been chemically remagnetized during one or several events. Mineral magnetic investigations reveal that the magnetization is carried by the ferrimagnetic iron-sulfide greigite (Fe3S4), possibly with traces of titano-magnetite. We find that several changes in polarity coincide with variations in magnetic mineral grain size and/or concentration. We interpret these variations as different stages of greigite growth, which were triggered by changes in pore-water chemistry and/or upward migration of methane.

Holocene stratigraphy of the Ångermanälven River estuary, Bothnian Sea

This study explores the Holocene depositional succession at the IODP Expedition 347 sites M0061 and M0062 in the vicinity of the Ångermanälven River estuary in the Bothnian Sea sector of the Baltic Sea in northern Scandinavia. Site M0061 is located in a coastal offshore setting (87.9 m water depth), whereas site M0062 is fully estuarine (69.3 m water depth). The dataset comprises acoustic profiles and sediment cores collected in 2007 and late 2013 respectively. Three acoustic units (AUs) were recognized. Lowermost AU1 is interpreted as a poorly to discontinuous stratified glaciofluvial deposit, AU2 as a stratified conformable drape of glaciolacustrine origin, and AU3 as a poorly stratified to stratified mud drift. A strong truncating reflector separates AU2 and AU3. Three lithological units (LUs) were defined in the sediment cores. LU1 consists of glaciofluvial sand and silt gradating into LU2, which consists of glaciolacustrine varves. A sharp contact interpreted as a major unconformity separates LU2 from the overlying LU3 (brackish-water mud). In the basal part of LU3, one debrite (site M0061) or two debrites (site M0062) were recognized. Information yielded from sediment physical properties (magnetic susceptibility, natural gamma ray, dry bulk density), geochemistry (total carbon, total organic carbon, total inorganic carbon and nitrogen), and grain size support the LU division. The depositional succession was formally subdivided into two alloformations: the Utansjö Alloformation and overlying Hemsön Alloformation; the Utansjö Alloformation was further subdivided into two lithostratigraphic formations: the Storfjärden and Åbordsön formations. The Storfjärden (sandy outwash) and Åbordsön (glaciolacustrine rhythmite) formations represent a glacial retreat systems tract, which started at ca. 10.6 kyr BP. Their deposition was mainly controlled by meltwater from the retreating ice margin, glacio-isostatic land uplift and the regressive (glacial) lake level. The Hemsön Alloformation (organic-rich brackish-water mud) represents a period of forced regression, starting possibly at ca. 9.5 kyr BP. At about 7 kyr BP, brackish water reached the study area as a result of the mid-Holocene marine flooding of the Baltic Sea Basin, but the rapid land uplift soon surpassed the associated (Littorina) transgression. Changed near-bottom current patterns, caused by the establishment of a permanent halocline, and the reduced sediment consistency caused by increased organic deposition resulted in a sharp and erosional base of the brackish-water mud. Estuarine processes and salinity stratification at site M0062 started to play a more important role. This study applies a combined allostratigraphic and lithostratigraphic approach over the conventional Baltic Sea stages. This approach makes it more straightforward to study this Baltic Sea deglaciation–postglacial sequence and compare it to other formerly glaciated shallow sea estuaries.

Bulk sediment 14C dating in an estuarine environment – How accurate can it be?

Abstract Due to a lack of marine macrofossils in many sediment cores from the estuarine Baltic Sea, researchers are often forced to carry out 14C determinations on bulk sediment samples. However, ambiguity surrounding the carbon source pathways that contribute to bulk sediment formation introduces a large uncertainty into 14C geochronologies based on such samples, and such uncertainty may not have been fully considered in previous Baltic Sea studies. We quantify this uncertainty by analyzing bulk sediment 14C determinations carried out on densely spaced intervals in independently dated late-Holocene sediment sequences from two central Baltic Sea cores. Our results show a difference of ~600?14C?yr in median bulk sediment reservoir age, or R(t)bulk, between the two core locations (~1200?14C?yr for one core, ~620?14C?yr for the other), indicating large spatial variation. Furthermore, we also find large downcore (i.e., temporal) R(t)bulk variation of at least ~200?14C?yr for both cores. We also find a difference of 585?14C?yr between two samples taken from the same core depth. We propose that studies using bulk sediment 14C dating in large brackish water bodies should take such spatiotemporal variation in R(t)bulk into account when assessing uncertainties, thus leading to a larger, but more accurate, calibrated age range. (Less)

Combined Land and River High-resolution Reflection Seismic Imaging of an Area Prone to Quick-clay Landslides in Sweden

Quick-clay landslides are common in northern countries and the delineation of these materials is important when planning urban areas. High-resolution reflection seismic data were acquired on land along four profiles in an area prone to quick-clay landslides in southwest Sweden in 2013. These data complement previous investigations that show the influence of the underlying coarse-grained layers in the formation and thickness of the quick clays. The intercalation of the different glacial and postglacial sediments, and the structural information of the subsurface provide clues to the possible causes of a landslide. Apart from the land data, river seismic data, using one and six channels, were obtained. All the seismic sections show a clear undulating bedrock reflection, and signs of bedrock faults at shallower depth that maybe important in the generation of quick clays and landslides. The coarse-grained layer reflection is delineated on the land data, and several filled channels can be distinguished along the river. The presence of the coarse-grained layer indicates the possible large extension of the quick clays in the study area, which needs to be confirmed using geotechnical investigations.