This study evaluates the ability of diatoms to estimate past changes in temperature by directly comparing diatom-based inferences of mean July air temperature (July T) with measured values of century-long meteorological records. The comparison includes sediment cores from three lakes in northern Sweden (Alanen Laanijarvi, 365 in above sea level (a.s.l.), Lake 850, 850 in a.s.l. and Lake Njulla, 999 in a.s.l.) dated by radioisotopic methods (Pb-210, Cs-137) and records from meteorological stations in Kiruna and Abisko. In the sediment cores of all three study lakes, benthic species dominate the diatom assemblages with percent abundance ranging from 60 to 80%. Over the past century, the diatom-based quantitative July T inferences (weighted averaging partial least squares regression and calibration) based on a regional calibration set correspond in general closely with the meteorological records. In addition, the proportion of planktonic diatoms in the stratigraphy reflects the meteorological record as well. For example, in Alanen Laanijarvi, the proportion of planktonic diatoms (e.g. Cyclotella comensis) has increased markedly since 1985, coincident with a marked increase of measured July T (> 1degreesC) at the meteorological station in nearby Kiruna. In contrast, July T in Abisko has remained relatively constant since 1985 and relatively little changes are observed in diatom assemblages in the two nearby lakes, Lake 850 and Lake Njulla. Interestingly, periods of relatively weak correspondence between measured and inferred July T (at Lake 850 from 1910 to 1940, at Lake Njulla from 1955 to 1990) correspond with periods when diatom-inferred lake-water pH shows distinct trends of lake-water pH change. Because lake-water pH is a stronger factor influencing diatom community composition than July T, the accuracy of diatom-inferred July T appears to be reduced during periods when lake-water pH fluctuates. Overall, we conclude that diatoms can provide reliable estimates of July T over the past century from sediment samples in northern Sweden and the reliability of the inferences further increases when lake-water pH remains constant. One palaeoecological implication based on these findings is that the early Holocene is likely a problematic period for estimating July T from diatoms, because natural processes such as soil development, catchment stabilisation and weathering of bedrock resulted in a marked natural decline of lake-water pH. Similarly, short-term stochastic events, such as sediment in-wash due to erosional events, forest clearance and forest fires, also may lead to inaccurate diatom-temperature inferences through changes in lake-water pH. In order to identify periods that may affect the accuracy of diatom-based July T inferences, we suggest that diatom analyses should be combined with other palaeoecological proxy indicators (e.g. pollen, chironomids). (C) 2002 Elsevier Science B.V. All rights reserved.

Changes in intensity and composition of bioturbation and trace fossils in deep-sea settings are directly related to changes in environmental parameters such as food availability, bottom water oxygenation, or substrate consistency. Because trace fossils are practically always preserved in situ, and are often present in environments where other environmental indicators are scarce or may have been compromised or removed by diagenetic processes, the trace fossils provide an important source of paleoenvironmental information in regions such as the deep Arctic Ocean. Detailed analysis of X-ray radiographs from 12 piston and gravity cores from a transect spanning from the Makarov Basin to the Yermak Plateau via the Lomonosov Ridge, the Morris Jesup Rise, and the Gakkel Ridge reveal both spatial and temporal variations in an ichnofauna consisting of Chondrites, Nereites, Phycosiphon, Planolites, Scolicia, Trichichnus, Zoophycos, as well as deformational biogenic structures. The spatial variability in abundance and diversity is in close correspondence to observed patterns in the distribution of modern benthos, suggesting that food availability and food flux to the sea floor are the most important parameters controlling variations in bioturbation in the Arctic Ocean. The most diverse ichnofaunas were observed at sites on the central Lomonosov Ridge that today have partially ice free conditions and relatively high summer productivity. In contrast, the most sparse ichnofauna was observed in the ice-infested region on the Lomonosov Ridge north of Greenland. Since primary productivity, and therefore also the food flux at a certain location, is ultimately controlled by the geographical position in relation to ice margin and the continental shelves, temporal variations in abundance and diversity of trace fossils have the potential to reveal changes in food flux, and consequently sea ice conditions on glacial-interglacial time scales. Down core analysis reveal clearly increased abundance and diversity during interglacial/interstadial intervals that were identified through strongly enhanced Mn levels and the presence of micro- and nannofossils. Warm stages are characterized by larger trace fossils such as Scolicia, Planolites or Nereites, while cold stages typically display an ichnofauna dominated by small deep penetrating trace fossils such as Chondrites or Trichichnus. The presence of biogenic structures in glacial intervals clearly show that the Arctic deep waters must have remained fairly well ventilated also during glacials, thereby lending support to the hypothesis that the conspicuous brown layers rich in Mn which are found ubiquitously over the Arctic basins are related to input from rivers and coastal erosion during sea level high-stands rather than redox processes in the water column and on the sea floor. However, the X-ray radiograph study also revealed the presence of apparently post-sedimentary, diagenetically formed Mn-layers which are not directly related to Mn input from rivers and shelves. These observations thus bolster the hypothesis that the bioturbated, brownish Mn-rich layers can be used for stratigraphic correlation over large distances in the Arctic Ocean, but only if post sedimentary diagenetic layers can be identified and accounted for in the Mn-cycle stratigraphy. (C) 2012 Elsevier B.V. All rights reserved.

Abstract Understanding the response of sensitive Arctic and subarctic landscapes to climate change is essential to determine the risks of ongoing and projected climate warming. However, these responses will not be uniform in terms of timing and magnitude across the landscape because of site-specific differences in ecosystem susceptibility to climate forcing. Here we present a multi-proxy analysis of a sediment record from the 330-km2 lake TornetrÀsk to assess the sensitivity of the Fennoscandian subarctic landscape to climate change over the past 9500years. By comparing responses of this large-lake system to past climatic and environmental changes with those in small lakes in its catchment, we assessed when the magnitude of change was sufficient to affect an entire region rather than only specific sub-catchments that may be more sensitive to localized environmental changes such as, e.g., tree-line dynamics. Our results show three periods of regional landscape alteration with distinct change in sediment composition: i) landscape development following deglaciation and through the Holocene Thermal Maximum, 9500–3400calyr BP; ii) increased soil erosion during the Little Ice Age (LIA); and iii) rapid change during the past century coincident with ongoing climate change. The gradual landscape development led to successive changes in the lake sediment composition over several millennia, whereas climate cooling during the late Holocene caused a rather abrupt shift occurring within 100years. However, this shift at the onset of the LIA ( 750calyr BP) occurred >2000years later than the first indications for climate cooling recorded in small lakes in the TornetrÀsk catchment, suggesting that a critical ecosystem threshold was not crossed until the LIA. In contrast, the ongoing response to recent climate change was immediate, emphasizing the unprecedented scale of ongoing climate changes in subarctic Fennoscandia.

Abstract The melting of high-latitude permafrost peatlands is a major concern due to a potential positive feedback on global climate change. We examine the ecology of testate amoebae in permafrost peatlands, based on sites in Sweden (~200km north of the Arctic Circle). Multivariate statistical analysis confirms that water-table depth and moisture content are the dominant controls on the distribution of testate amoebae, corroborating the results from studies in mid-latitude peatlands. We present a new testate amoeba-based water table transfer function and thoroughly test it for the effects of spatial autocorrelation, clustered sampling design and uneven sampling gradients. We find that the transfer function has good predictive power; the best-performing model is based on tolerance-downweighted weighted averaging with inverse deshrinking (performance statistics with leave-one-out cross validation: R2=0.87, RMSEP=5.25cm). The new transfer function was applied to a short core from Stordalen mire, and reveals a major shift in peatland ecohydrology coincident with the onset of the Little Ice Age (c. AD 1400). We also applied the model to an independent contemporary dataset from Stordalen and find that it outperforms predictions based on other published transfer functions. The new transfer function will enable palaeohydrological reconstruction from permafrost peatlands in Northern Europe, thereby permitting greatly improved understanding of the long-term ecohydrological dynamics of these important carbon stores as well as their responses to recent climate change.