Antti E.K. Ojala

Cold event at 8200 yr B.P. recorded in annually laminated lake sediments in eastern Europe

A quantitative annual mean temperature reconstruction from an annually laminated lake-sediment sequence in Estonia, eastern Europe, shows a distinct cold period at 8400– 8080 yr B.P. (= before A.D. 2000); the timing is consistent with that seen in the Greenland ice-core data and various high-resolution records from western Europe. During maximal cooling at 8250–8150 yr B.P., the annual mean temperature in Estonia was ∼2.0 °C colder than prior to and ∼3.0 °C colder than after the cooling. The pollen-stratigraphic and sedimentological data suggest especially cold and snowy winter conditions. The duration and amplitude of the cold event agree with the modeled impact of a sudden freshening of the North Atlantic surface water and subsequent perturbation of the thermohaline circulation. Provided that the cold event was caused by a pulse of freshwater—from the melting Laurentide Ice Sheet—to the North Atlantic, the results indicate a strong teleconnection between the North Atlantic oceanic forcing and the east European climate at least up to long 26°E, mediated probably by the changing intensity of the zonal atmospheric circulation.

Testing the fidelity of sediment chronology: comparison of varve and paleomagnetic results from Holocene lake sediments from central Finland

Abstract A careful construction of independent varve and paleomagnetic chronologies from three varved lake sediment records from central Finland resulted in detailed and continuous age–depth profiles. Cumulative varve calculation errors were estimated to be ±3% for Lake Alimmainen Savijarvi (10,295±340 varve years), and ±1% for lakes Nautajarvi (9898±97 varve years) and Korttajarvi (9590±103 varve years). Paleomagnetic secular variation (PSV) curves demonstrate good consistency between these sites and within a margin of varve chronology error. They show similar declination and inclination features commonly described in the UK and Sweden. The timing of PSV patterns from Finnish varved sites agree well with UK curves between the present and 2000 BC (and in pre-6000 BC deposits) and with Swedish data between the present and 5000 BC. Combined inclination records from three independently dated Finnish varved sites possess an opportunity for accurate paleomagnetic comparisons within the margin of varve chronology errors and throughout the last 10,000 years.

Documentation of the physical properties of annually laminated (varved) sediments at a sub-annual to decadal resolution for environmental interpretation

This paper describes a method to digitize high-resolution physical properties of annually laminated lake sediments. We modified a water-acetone epoxy exchange method in order to preserve all the fine structural details that varved sequences provide. By applying sediment microstratigraphical investigations (X-ray radiography, SEM) and semi-automatic image analysis, we were able to establish detailed information about varve composition and structure. Mineral magnetic measurements were also applied for comparison. By comparing all available physical data at high resolution, it will be possible to identify environmental changes that has occurred in the lake drainage basin.

Integrated varve and pollen-based temperature reconstruction from Finland: evidence for Holocene seasonal temperature patterns at high latitudes

A detailed understanding of decadal to millennial-scale climate changes requires seasonal-scale (summer-winter) reconstructions of past precipitation and temperature fluctuations. Comparing seasonally resolved varve records with pollen-based sum of growing degree-days (GDD) reconstructions from Lake Nautajärvi, we examined the intra-annual nature of climate variability in central southern Finland during the Holocene. The organic varve record and the GDD reconstruction show roughly comparable trends supporting the interpretation that both proxies predominantly reflect summer temperatures in the study area. The records suggest low but rising early-Holocene (9500 to 8500 cal. yr BP) summer temperatures. The Holocene Thermal Maximum (HTM) in the GDD record dates to about 7500 to 4500 cal. yr BP, but the organic varve record along with reconstructed changes in vegetation composition, notably a peak of Tilia pollen percentages, indicate that during the HTM there was a trend towards a more continental climate with maximum mid-summer temperatures reached at 6500 to 4500 cal. yr BP. Both records reflect the start of the post-HTM cooling at about 4500 cal. yr BP, simultaneously with an increase of the amount of catchment erosion and mineral matter influx into the lake, suggesting gradually colder and/or longer winters with high net accumulation of snow. The organic varve record and the GDD record start to diverge at 2000 cal. yr BP, possibly owing to the human influence on catchment processes. The reconstructed mid-Holocene summer temperature peak deviates from the regional climate model outputs, which suggest highest summer temperatures during the early Holocene.

Calibrated pollen accumulation rates as a basis for quantitative tree biomass reconstructions

Recent investigations show that the pollen accumulation rate (PAR) of the common tree taxa is directly related to the biomass and, by inference, to the population size of the taxa around the study site. Fossil PAR records preserved in lakes provide therefore a potential proxy for quantitative biomass and population reconstructions. We use the high-resolution PAR records obtained from two accurately dated lake sediment cores in Finland to generate quantitative Holocene biomass records for Pinus, Picea and Betula, the most common tree taxa of the European Boreal forest. PAR values were calibrated to biomass values by comparing the modern PAR values with the modern biomass values and assuming a linear relationship between the past PAR and biomass values. The obtained PAR and biomass values and trends are remarkably coherent between the two records. Pinus has a stable Holocene biomass size and its modern biomass, about 20 t/ha, corresponds with the natural Pinus biomass in the study regions. In contrast, Picea immigrated from the East during the mid Holocene, had a maximum biomass, 50—60 t/ha, at 3500—1000 cal. yr BP, and declined strongly during the last 1500—1000 years as a result of increased human activity and related rise of fire frequency. Thus, the modern Picea biomass in the study regions, about 22 t/ha, is only 35—40% of the natural Picea biomass. The results of this pilot study demonstrate the potential of the calibrated PAR data in quantitative biomass and population reconstructions. Such reconstructions can provide fresh insights into the structure of past plant communities and, when combined with records reflecting palaeoclimates, natural disturbances, and human activity, can help to disentangle the long-term importance of different enviromental drivers to changes in plants populations and ecosystems.

Climate change and bird impact as drivers of High Arctic pond deterioration

The environmental history since the end of the Little Ice Age of the bird-influenced pond Fugledammen (Hornsund, Svalbard, 77°N) was inferred from a 1-m sediment core using paleolimnological methods. The aim was to track long-term environmental changes and to evaluate the limnological consequences of catchment development in this extremely sensitive landscape. A special focus is given to the impacts of climate change and the observed increase in bird populations in the catchment. The late nineteenth century was characterized by littoral scraping/filtering Cladocera together with vegetation-associated chironomids. The invertebrate community became less diverse towards the twentieth century. Planktonic filter-feeder cladocerans replaced the littoral taxa and collector–gatherers became the most abundant chironomid feeding group. In the more recent sediment layers, invertebrate diversity decreased further but the number of individuals (biomass) increased. Daphnia showed a progressive increase that is typical for similar Arctic ponds in Svalbard, where nutrient loading has increased due to growing bird populations in the catchment. The decreases in vegetation-associated invertebrates, biodiversity, and functional diversity suggest that turbidity has increased and oxygen availability and light penetration decreased in the lake. The paleoecological record is in agreement with the sediment physical and geochemical evidence, indicating that in-lake productivity has strongly increased towards the present. These changes are concurrent with the recent climate warming in Svalbard suggesting that, in addition to longer ice-free season and increased water temperature, the increased air temperatures have various indirect catchment-mediated influences on the aquatic community through changes in bird-population size.

Urban point sources of nutrients were the leading cause for the historical spread of hypoxia across European lakes

Significance Using a compilation of data arising from over 1,500 European watersheds, we have identified the relative role of different drivers in initiating hypolimnetic hypoxia, a critical indicator of lake health. In particular, our regional synthesis of laminated lake sediments indicated a significant acceleration in the spread of lacustrine hypoxia in the 1900s, which occurred well before the general use of commercial fertilizers in the mid-20th century and the onset of supraregional climate warming in the 1970s. The spread of hypoxia was best explained by urban expansion and the associated intensification of anthropogenic point sources of phosphorus, whereby changes in lifestyle increased the discharge of nutrients from treated and raw sewage, and ultimately led to enhanced lacustrine biological productivity. Enhanced phosphorus (P) export from land into streams and lakes is a primary factor driving the expansion of deep-water hypoxia in lakes during the Anthropocene. However, the interplay of regional scale environmental stressors and the lack of long-term instrumental data often impede analyses attempting to associate changes in land cover with downstream aquatic responses. Herein, we performed a synthesis of data that link paleolimnological reconstructions of lake bottom-water oxygenation to changes in land cover/use and climate over the past 300 years to evaluate whether the spread of hypoxia in European lakes was primarily associated with enhanced P exports from growing urbanization, intensified agriculture, or climatic change. We showed that hypoxia started spreading in European lakes around CE 1850 and was greatly accelerated after CE 1900. Socioeconomic changes in Europe beginning in CE 1850 resulted in widespread urbanization, as well as a larger and more intensively cultivated surface area. However, our analysis of temporal trends demonstrated that the onset and intensification of lacustrine hypoxia were more strongly related to the growth of urban areas than to changes in agricultural areas and the application of fertilizers. These results suggest that anthropogenically triggered hypoxia in European lakes was primarily caused by enhanced P discharges from urban point sources. To date, there have been no signs of sustained recovery of bottom-water oxygenation in lakes following the enactment of European water legislation in the 1970s to 1980s, and the subsequent decrease in domestic P consumption.

HOLOCENE BENTHIC FORAMINIFERAL RECORD FROM A HIGH-ARCTIC FJORD, NORDAUSTLANDET, SVALBARD

Kubischta, F., Knudsen, K.L., Ojala, A.E.K. and Salonen, V.‐P., 2011. Holocene benthic foraminiferal record from a high‐arctic fjord, Nordaustlandet, Svalbard. Geografiska Annaler: Series A, Physical Geography, 93, 227–242. DOI: 10.1111/j.1468‐0459.2011.00434.x Abstract This study presents the results of benthic foraminiferal analyses of a marine core record from about 100 m water depth in the fjord environment of Isvika bay, Nordaustlandet, Svalbard. During the Last Glacial Maximum, the bay was glaciated by the north‐western sector of the Svalbard‐Barents Sea Ice Sheet. The foraminiferal assemblages reflect glaciomarine conditions during deglaciation and early Holocene (about 11-250–9640 cal. yr bp) followed by glacier‐distal environment in an open fjord with relatively high influence of Atlantic waters from the open ocean (9640–7430-cal. yr bp). A subsequent foraminiferal indication of a glacial advance was followed by deposition of a diamicton unit within a short time period at 5800–5700 cal. yr bp. The mid‐ and late Holocene were characterized by assemblages indicating restricted open‐ocean connection and gradually more severe conditions, which are considered to be caused partly by a gradual cooling and freshening of the Atlantic water inflow to the shelf area and partly by a continuous isostatic uplift of the area. The foraminiferal record shows that the environmental changes in Murchisonfjorden are not just an effect of changes in solar insolation but are also influenced by local processes, as well as changes in the Atlantic water inflow to the northern Svalbard shelf and to the fjord via Hinlopenstretet.

LiDAR DEM detection and classification of postglacial faults and seismically-induced landforms in Finland: a paleoseismic database

Abstract During the last decades, postglacial faults (PGFs) have been found in northern Fennoscandia, the first fault scarps being discovered in western Finnish Lapland in the 1960s. With LiDAR-based digital elevation models (DEMs), a new and accurate remote sensing mapping methodology has been acquired. It allows the relatively rapid and low-cost detection and mapping of late- or PGFs and, for instance, mapping of landslides from areas where they have not previously been recognized. We describe the approach of the Geological Survey of Finland to the systematic search for (screening) and mapping of PGFs, paleolandslides, and other morphological features of Quaternary deposits related to post- and late-glacial seismic activity in Finland. The observations have been collected and classified into a file geodatabase with ArcGis (© ESRI) using a procedure that includes several steps. We also provide examples from western and northern Finland of how sites of late- and postglacial fault scarps and landslides have been detected and described from LiDAR DEM data.

Seasonal formation of clastic-biogenic varves: the potential for palaeoenvironmental interpretations

Clastic-biogenic varved lake sediments from Nautajärvi, Finland, covering the last ca. 10 000 years have previously been investigated with respect to varve chronology, laminae composition and thickness variations, palaeomagnetic secular variation, the pollen record and the diatom composition of selected sections. In this study, we provide detailed information on the seasonal sedimentation of different varve components and palaeolimnological indicators (diatoms, chrysophycaean cysts) using near-bottom sediment-trap monitoring. The monitoring results confirm previous interpretations of the formation of clastic-biogenic varves in Fennoscandian lakes. The results also indicate that seasonal sediment fluxes correspond with environmental changes. Higher snow storage in winter and the discharge intensity during the following spring snowmelt clearly increase the suspended sediment load, transportation and net accumulation of detrital mineral matter during spring and early summer. A prolonged autumn and subsequent freezing and thawing cycles in winter, conversely, result in an incremental but slow accumulation of assorted mineral matter and organic particles during winter, whereas the subsequent spring flooding and detrital sediment yield are diminished and thinner clastic laminae are formed. The accumulation rates of diatoms and chrysophyte cysts suggest that these algal groups are more dependent on seasonal processes (e.g. spring and autumnal overturn) than on rapid, short-lived environmental episodes such as the spring discharge peak.