Changes in fish mercury concentrations over 20 years in an acidified lake subject to experimental liming

Lake Iso Valkjärvi (southern Finland, Europe) was divided in two with a plastic curtain in 1991. One half was neutralized with CaCO₃, and the other acted as a control. Mercury concentrations of perch (Perca fluviatilis) and northern pike (Esox lucius) in the limed and control side of the lake were studied both before and after the treatment. Average Hg concentrations of perch and pike were 0.40 and 1.2 μg g⁻¹ (ww) in the early 1980s and 0.25 and 0.72 μg g⁻¹ (ww) a decade later at the time of liming. Ten years after the liming the Hg concentrations of perch in the limed and control sides of the lake were 0.21 and 0.28 μg g⁻¹ (ww) and those of pike were 0.69 and 0.43 μg g⁻¹ (ww), respectively. Nitrogen isotope ratios (δ¹⁵N) for perch in the sampling period 2002–2004 showed wide variation suggesting variable trophic positions for individual fish. Pike formed two groups according to their δ¹⁵N-values, suggesting that zoobenthos dominated the diet of pike around 20 cm in length and fish that of the larger pikes. Because the δ¹⁵N-values of fish were at similar levels in the limed and control sides of L. Iso Valkjärvi, differences in food web structure cannot account for the different fish Hg concentrations. A more likely explanation is water quality induced differences in the dynamics and bioavailability of Hg, leading to decreased formation of methyl Hg.     

The deglaciation history of the Lake Superior region and its climatic implications

A zone of synchronous end moraines has been recognized in the Lake Superior region across northern Ontario and Michigan. The moraines were formed between 11,000 and 10,100 y.a. as cold climate resulted in successive halts in the general ice retreat. The cold climate is also indicated by the presence of tundra near Lake Superior until about 10,000 y.a. This episode is here referred to as the Algonquin Stadial. It was preceded and followed by rapid deglaciation. The Algonquin Stadial is comparable in age with the Younger Dryas Stadial of Europe, and indicates a reversal in the continuous trend toward a warmer climate during Late-Wisconsin (an) time. The apparent conflict between the present result (based on geologic evidence) and earlier pollen stratigraphical studies with no reversal is discussed.Glacial Lake Duluth formed in the western Lake Superior basin before 11,000 BP, followed by a series of Post-Duluth lakes between approximately 11,000 and over 10,100 BP. The Main Lake Algonquin stage in the Huron and Michigan basins terminated approximately 11,000 BP. The subsequent high-level post-Main Algonquin lakes, which were contemporaneous with the Post-Duluth lakes, existed in the southeastern Lake Superior basin. When the ice margin was along the north shore 9500 BP Lake Minong occupied the whole Lake Superior basin. By 9000 BP the ice had retreated north of Lake Superior-Hudson Bay divide.     

A review of analytical models of sea‐ice growth

Abstract

The modelling of sea‐ice growth is a classical problem in geophysics, which has been traditionally treated as one‐dimensional, considering only the vertical heat transfer. The modelling work commenced in the 1800s with analytic methods. These are very effective tools for examining the sea‐ice growth problem, providing a clear insight into the physical mechanisms and producing simple first‐order approximations for the ice thickness in various conditions. This paper describes the physical problem of sea‐ice growth, presents an analytical modelling framework for the problem and provides analytic solutions for different environmental conditions.     

Summanen structure: Further geological and geophysical evidence of a meteorite impact event in Central Finland

The Summanen structure is located in Central Finland and is one of Finland's 12 known meteorite impact structures. In 2017, the discovery of Summanen was based on numerous shatter cone boulders with planar deformation features (PDFs) and a circular electromagnetic anomaly, which is 2.6 km in diameter. The site was revisited in 2020 and 2022, and shatter cone-bearing outcrops were discovered. PDFs and feather features were identified in samples from these outcrops. A total of 38 PDF sets in 27 quartz grains resulted in rational crystallographic orientations concentrating on {10$\overline{1}$4}, {10$\overline{1}$3}, {10$\overline{1}$2}, and {11$\overline{2}$2}, implying shock pressures of 2–20 GPa. Gravity measurements were taken, and the electrical conductivity of the structure was studied. The gravimetric results revealed a circular negative anomaly of about 4 km in diameter, with an amplitude of −3.5 mGal. Excluding the gravitational effect of water and Quaternary sediments reduces the anomaly to −1.6 mGal. A bowl-shaped conductive layer, likely containing relict saline water in the impact-fractured bedrock, was identified to a depth of 240 m. Topographic and bathymetric data were combined to determine the impact's effect and interpret the level of erosion. Cobbles of sedimentary sand- and siltstones were found on the coastline of Lake Summanen. Based on their similarity to those found in the Söderfjärden impact crater with a Cambrian age, it is likely that these rocks and post-impact infill are also of a similar age.     

A preliminary archaeological and environmental study of pre‐Columbian burial towers at Huachacalla,Bolivian Altiplano

Chullpas are pre‐Columbian burial towers built by indigenous Aymaras on the Bolivian Altiplano. Bolivian chullpas date back to the Late Intermediate Period (A.D. 1000–1476) and the Late Horizon (A.D. 1476–1532). We recorded 228 chullpas among 84 sites in the Huachacalla region of west‐central Bolivia. In our study area, the chullpas are on debris flows and coarse alluvium in the proximal and medial segments of alluvial fans at the foot of two volcanoes. Grain‐size, element, and mineralogical analysis of chullpa wall material and local sediment revealed that the burial towers are composed of calcareous sand that is readily available in alluvial fan deposits near the sites. Our data suggest that the Aymaras considered environmental factors, such as drainage and stability of the soil, when they selected the locations of chullpas, whereas cultural factors played a significant role in chullpa architecture. © 2002 Wiley Periodicals, Inc.