Pleistocene stratigraphy in the Lappajarvi meteorite crater in Ostrobothnia, Finland

Seventy-four meters of a 95-m-long drill core recovered from the Lappäjarvi crater, a meteoritic impact site in western Finland, consisted of Pleistocene sediments. These sediments refer to two events of glacial deposition (Saalian and Weichselian) interrupted by non-glacigenic freshwater sedimentation. The sediments contain abundant redeposited Holsteinian and Tertiary microfossils, and possibly represent a pre-Weichselian interstadial not described from elsewhere in Finland. The pollen flora indicates a mixed primary arctic to subarctic succession that followed deglaciation, i.e. the beginning of an interglacial or interstadial event. The secondary pollen component derives from an eroded interglacial deposit that can be interpreted as Holsteinian, or possibly Eemian, in age. The vegetation succession interpreted from the primary pollen flora reflects a transition from arctic conditions to subarctic birch forests. The diatom flora indicates a primary succession that can be observed clearly in the uppermost gyttja layer in which the rich alkaliphilous diatom flora refers to more or less eutrophic conditions. The diatom flora of sediments below the gyttja layer is composed of a primary component and a secondary, redeposited or relict component. The diatoms encountered are interglacial or Tertiary in origin. The results show that meteorite craters can provide long, representative stratigraphic sequences in glacially eroded Precambrian shield areas such as Finland.     

Doppler radar radial winds in HIRLAM. Part I: observation modelling and validation

An observation operator for Doppler radar radial wind measurements is developed further in this article, based on the earlier work and considerations of the measurement characteristic. The elementary observation operator treats radar observations as point measurements at pre-processed observation heights. Here, modelling of the radar pulse volume broadening in vertical and the radar pulse path bending due to refraction is included to improve the realism of the observation modelling.
The operator is implemented into the High Resolution Limited Area Model (HIRLAM) limited area numerical weather prediction (NWP) system. A data set of circa 133 000 radial wind measurements is passively monitored against the HIRLAM six-hourly background values in a 1-month experiment. No data assimilation experiments are performed at this stage. A new finding is that the improved modelling reduces the mean observation minus background (OmB) vector wind difference at ranges below 55 km, and the standard deviation of the radial wind OmB difference at ranges over 25 km.
In conclusion, a more accurate and still computationally feasible observation operator is developed. The companion paper (Part II) considers optimal super-observation processing of Doppler radar radial winds for HIRLAM, with general applicability in NWP.     

The Big Ice

Book reviewed in this article:
Fulton, Robert J. & Andrews, John T. (eds.) 1987: La calotte glaciaire laurentidienne-The Laurentide Ice Sheet. Géographie Physique et Quaternaire     

Middle Weichselian glacial event in the central part of the Scandinavian Ice Sheet recorded in the Hitura pit, Ostrobothnia, Finland

The Hitura open pit exposes a sedimentary sequence up to 50 m thick representing Late Saalian to Holocene glacial and non-glacial sediments. The sequence was investigated using sedimentological methods, OSL-dating and pollen and diatom analyses to reconstruct the Middle Weichselian (MWG) glacial event in the central part of the Scandinavian Ice Sheet (SIS). The results indicate that the sediment succession represents two entire glacial advance and retreat cycles. The lowermost deposits are Late Saalian esker and delta sediments overlain by sediments that correlate with the early Eemian lacustrine phase. Remnants of the Eemian soil post-dating the lacustrine phase were also observed. The area was ice-free during the entire Early Weichselian (EWG). The first glacial advance recorded in the sediments is related to the MWG. It started 79 kyr ago, deformed underlying sediments and deposited an immature till, including large detached sediment pods containing remains of organic material, soils and fluvial sediments representing allochthonous material from EWG ice-free stadials and interstadials. The glacial deposits are conformably overlain by glaciolacustrine and littoral accumulations, indicating MWG deglaciation between 62 and 55 kyr ago. Based on the fabric measurements from the till unit overlying the MWG sediments, ice advance during the Late Weichselian (LWG) was initially from the west and later from a north-northwesterly direction. The Hitura strata provide the first dating of the MWG deglaciation (55 to 62 kyr ago) from central parts of the SIS. It can be considered as a key site for studying the growth and decay of SIS during the poorly known early parts of the glaciation.     

Weichselian glacial stage in Murchisonfjorden, Nordaustlandet, Svalbard

Compared to the other islands in the Svalbard archipelago, Nordaustlandet offers only limited stratigraphical or sedimentological information on its Quaternary deposits. This article aims to fill the gap by presenting new results from glacial geological, sedimentological and chronological studies in the southern Murchisonfjorden area. Field data include reconnaissance mapping and detailed logging of vertical sections along cliff-face outcrops a few metres high adjacent to the present-day shoreline. Combined with OSL and AMS age determinations, these data provide evidence of three successive Weichselian sequences, each represented by the deposition of till followed by the accumulation of shallow marine deposits. Contrary to earlier conclusions, this study demonstrates that the area was occupied by a Late Weichselian glacier (LWG), although the LWG till is thin and discontinuous. Interstadial sublittoral sand related to the Mid-Weichselian interstadial was dated to 38–40 kyr, and an Early Weichselian interstadial to 76–80 kyr. The preservation of older sediments, multiple striae generations and abundant observations of weathered local bedrock material indicate weak glacial erosion within the study area. We suggest that the Late Weichselian glacier was relatively inactive and remained mainly cold-based until the deglaciation. The Isvika sections can be considered a new key site that offers further potential to improve our understanding of the Weichselian stage within the northwestern sector of the Barents–Kara Ice Sheet.     

Glacial dispersal of boulders in the James Bay Lowlands of Quebéc, Canada

Paleozoic fossiliferous limestones from the Hudson Bay area were dispersed southeastward a total distance of 110 km in the Québec part of theJames Bay Lowlands during the surging Cochrane flow event of the Laurentide Ice Sheet. The dispersal by the surging ice appears comparable to non-surging ice dispersal events elsewhere in terms of the dispersal index and the half distance of transportation; however, the total distance of transportation of the limestone clasts and their relatively high abundance far away from their source suggest that part of the transport of the debris during the surge was englacial, with the load later deposited as a surface mantle, or that the flow of the surging ice was limited to rapid basal sliding, with little or no internal shear within the ice mass. The glacial transport characteristics of earlier non-surging flow events in the same area were determined using the Total Transport Distance (TTD) method of measurements. Based on an indirect measurement of the half distance of transportation, the Selbaie till is characterized by longer transport distance than the Matheson till, and the Nouveau-Québec till has the shortest transport distance of all the tills of the area.     

Doppler radar radial winds in HIRLAM. Part II: optimizing the super-observation processing

Doppler radar radial wind observations are modelled in numerical weather prediction (NWP) within observation errors which consist of instrumental, modelling and representativeness errors. The systematic and random modelling errors can be reduced through a careful design of the observation operator (Part I). The impact of the random instrumental and representativeness errors can be decreased by optimizing the processing of the so-called super-observations (spatial averages of raw measurements; Part II).
The super-observation processing is experimentally optimized in this article by determining the optimal resolution for the super-observations for different NWP model resolutions. A 1-month experiment with the HIRLAM data assimilation and forecasting system is used for radial wind data monitoring and for generating observation minus background (OmB) differences. The OmB statistics indicate that the super-observation processing reduces the standard deviation of the radial wind speed OmB difference, while the mean vector wind OmB difference tends to increase. The optimal parameter settings correspond at a measurement range of 50 km (100 km) to an averaging area of 1.7 km² (7.3 km²).
In conclusion, an accurate and computationally feasible observation operator for the Doppler radar radial wind observations is developed (Part I) and a super-observation processing system is optimized (Part II).