Varved Lake Sediment Sequences Research Articles

Radiocarbon Wiggle Matching on Laminated Sediments Delivers High-Precision Chronologies

ABSTRACTHigh-resolution sediment chronologies with the best possible time control are essential for comparing palaeoecological studies with independent high-precision climatic, archaeological or historic data in order to disentangle causes and effects of past environmental, ecological and societal change. We present two varved lake sediment sequences from Moossee and Burgäschisee (Swiss Plateau) that have chronologies developed with Bayesian models and radiocarbon (14C) dating of terrestrial plant macrofossils extracted from sediment samples with constant age ranges. We illustrate the potential of high-resolution14C dating for the construction of robust, high-precision sediment chronologies. The mean 2σ age uncertainties were reduced to±19 cal yr for Moossee and to±54 cal yr for Burgäschisee over the entire period of 3000 cal yr, while 2σ uncertainties of only±13 cal yr and±18 cal yr respectively, were achieved for shorter time intervals. These precisions are better than or comparable to those of previous varve studies. Our results imply that a sophisticated subsampling strategy and a careful selection of short-lived and well-defined terrestrial plant remains are crucial to avoid outlying14C ages. A direct linkage between palaeoeological studies with dendrochronologically dated, local archaeological sites as well as a precise comparison with high-resolution climate proxy data have become feasible.

Holocene tephrostratigraphy of varved sediment records from Lakes Tiefer See (NE Germany) and Czechowskie (N Poland)

A detailed Holocene tephrostratigraphic framework has been developed for two predominately varved lake sediment sequences from NE Germany (Lake Tiefer See) and central N Poland (Lake Czechowskie). A total of thirteen tephras and cryptotephras of Icelandic provenance were detected and chemically fingerprinted in order to define correlatives and to integrate known tephra ages into the sediment chronologies. Out of these, three cryptotephras (Askja-AD1875, Askja-S and Hässeldalen) were identified in both records, thus allowing a detailed synchronization of developing high-resolution palaeoenvironmental proxy data. The early Holocene Saksunarvatn Ash layer and the middle Holocene Lairg-B and Hekla-4 cryptotephras in Lake Tiefer See are further important anchor points for the comparison with other high-resolution palaeoclimate records in Central and Northern Europe. Tentative correlations of cryptotephras have been made with a historical basaltic Grimsvötn eruption (∼AD890 – AD856) and three late Holocene rhyolitic eruptions, including the 2.1 ka Glen Garry and two unknown high-silicic cryptotephras of probably Icelandic provenance (∼1.9 cal ka BP).

Complexity of the 8 ka climate event in Sweden recorded by varved lake sediments

Mineral magnetic and carbon analyses of a continuous varved lake sediment sequence in west‐central Sweden (Lake Mötterudstjärnet) complement similar palaeoclimate proxies obtained from two varved lake sediment sequences in northern Sweden and one in central Finland. The varve chronology is supported by tephrochronology, palaeomagnetic secular variations and 14C AMS dating of terrestrial macrofossils. We apply a simple model in which the transport and deposition of catchment mineral matter reflect the amount of winter snow accumulation, spring snow‐melt and stream discharge. Our data show that winter snow accumulation was generally enhanced in Sweden between 8100 and 7750 cal. yr BP. If dating errors are taken into account, the 350‐year period of increased erosion is the geomorphic response to a multi‐centennial scale climatic cooling that occurred some time between 8500 and 7500 cal. yr BP. The most significant erosion event in central Sweden was centred at 8050 cal. yr BP. It lasted 150 years (between 8100 and 7950 cal. yr BP) and is equivalent to the most extreme Holocene climate anomaly in the northern hemisphere, known as the 8 ka or 8200 cal. yr BP climate event. Our high‐resolution paramagnetic susceptibility and ferrimagnetic grain‐size parameters suggest that snowpack accumulation increased most significantly in northern Sweden between 7900 and 7750 cal. yr BP. We suggest that this north–south difference was a response to the re‐establishment of moisture‐laden westerly air masses, as meridional Atlantic overturning circulation was re‐established at the beginning of the Holocene thermal maximum.

Occurrence of varved lake sediment sequences in Varmland, west central Sweden: lake characteristics, varve chronology and AMS radiocarbon dating

Varved lake sediments can be used to set multiple environmental proxies within a calendar year time scale. We undertook a systematic survey of lakes in the Province of Värmland, west central Sweden, with the aim of finding continuous varved lake sediment sequences covering the majority of the Holocene. In Fennoscandia, such sediments have previously only been recorded in northern Sweden and in southern and central Finland. By following a selective process and fieldwork we discovered three new varved sites (i.e. Furskogstjärnet, Mötterudstjärnet and Kälksjön). We found that lakes with varved sediments have several common lake morphometry properties and lake catchment characteristics such as maximum water depth, maximum water depth/lake surface area ratio, catchment soil types, altitude and number of inflows. Varve chronologies, supported by AMS‐14C dating and tephrochronology were established for two of the sediment profiles. These varve chronologies are the longest geological records with an annual resolution known to exist in Sweden. In Furskogstjärnet, the AMS‐ 14C dates based on terrestrial plant macrofossils at several levels deviate significantly from the varve based time‐depth curve. In Motterudstjarnet, a fully reasonable time‐depth model based on the 14C dates gives older ages in the lower part of the sequence compared to the varve chronology. These results highlight that seemingly acceptable AMS radiocarbon dates may be erroneous. They also point to the fact that varved lake sediments are reliable geological archives with respect to chronological control and accuracy. Thus, these archives should be of prime interest for studies of climate and environmental change undertaken with the aim of providing sub‐decadal resolution proxy data sets.

Occurrence of varved lake sediment sequences in Värmland, west central Sweden: lake characteristics, varve chronology and AMS radiocarbon dating

Varved lake sediments can be used to set multiple environmental proxies within a calendar year time scale. We undertook a systematic survey of lakes in the Province of Varmland, west central Sweden, with the aim of finding continuous varved lake sediment sequences covering the majority of the Holocene. In Fennoscandia, such sediments have previously only been recorded in northern Sweden and in southern and central Finland. By following a selective process and fieldwork we discovered three new varved sites (i.e. Furskogstjarnet, Motterudstjarnet and Kalksjon). We found that lakes with varved sediments have several common lake morphometry properties and lake catchment characteristics such as maximum water depth, maximum water depth/lake surface area ratio, catchment soil types, altitude and number of inflows. Varve chronologies, supported by AMS- C-14 dating and tephrochronology were established for two of the sediment profiles. These varve chronologies are the longest geological records with an annual resolution known to exist in Sweden. In Furskogstjarnet, the AMS- C-14 dates based on terrestrial plant macrofossils at several levels deviate significantly from the varve based time-depth curve. In Motterudstjarnet, a fully reasonable time-depth model based on the C-14 dates gives older ages in the lower part of the sequence compared to the varve chronology. These results highlight that seemingly acceptable AMS radiocarbon dates may be erroneous. They also point to the fact that varved lake sediments are reliable geological archives with respect to chronological control and accuracy. Thus, these archives should be of prime interest for studies of climate and environmental change undertaken with the aim of providing sub-decadal resolution proxy data sets. (Less)

Geomagnetic field variations in northern Sweden during the Holocene quantified from varved lake sediments and their implications for cosmogenic nuclide production rates

Palaeomagnetic analyses were conducted on two varved lake-sediment sequences in northern Sweden. The magnetic properties of the sediment sequences are dominated by stable single-domain magnetite with characteristics typical of bacterial magnetosomes. Alternating field demagnetization measurements indicate that the single-domain magnetite is the dominant carrier of a stable natural remanent magnetization. Temporal variations in inclination and declination were obtained from a total of four cores and the data points were stacked according to their independent calendar-year (varve) ages. Statistically significant patterns in inclination and declination form a regional palaeomagnetic secular variation (PSV) curve, which possesses features that are identical in form to the UK Holocene PSV master curve. However, the calibrated radiocarbon ages of UK features identified prior to 1500 BC are approximately 500 years older than their Swedish varve-dated equiva lents, which points to dating errors and/or drifting of the geomagnetic field. The sediments meet the uniformity criteria proposed for palaeointensity reconstruction and estimates of relative geomagnetic field intensity are calibrated against global dipole-moment compilations. A calculated nuclide production curve is derived from the reconstructed geomagnetic field intensity, which empirically demonstrates the dominant modulation of cosmogenic nuclide production by dipole-moment between 5000 BC and ad 1500.

Rapid early-Holocene environmental changes in northern Sweden based on studies of two varved lake-sediment sequences

Mineral magnetic, organic carbon and pollen studies of two varved Holocene lake-sediment sequences in the boreal forest of northern Sweden indicate that signi” cant environmental changes took place between c. 6000 and 5700 bc. This interval is characterized by an increase in mineral-matter accumulation, which is a proxy for winter-snow accumulation, and a statistically signi” cant decrease in total pollen in‘ ux (predominantly Pinus, Betula and Alnus), which may re‘ ect lower spring and summer temperatures and increased frost frequency. Notable increases in the in‘ ux of deciduous tree species (including Quercus and Corylus) suggest a rapid change to warmer conditions between 5700 and 5600 bc. Given dating errors associated with the varve chronologies and the Greenland ice-core timescales, the cold interval can be considered to re‘ ect a regional (possibly global) climatic cooling, which is often referred to as the ‘8.2 kyr BP cooling event’. However, the younger age of the cold event in northern Sweden does not support the hypothesis of forcing by the sudden drainage of Laurentide glacial lakes into the North Atlantic, unless a minimal 300-year delay in ocean-atmospheric coupling is accepted. The data contribute to a complex picture of early-Holocene environmental change, in response to deglaciation of the Northern Hemisphere.

Bacterial magnetite in Swedish varved lake-sediments: a potential bio-marker of environmental change

Detailed mineral magnetic measurements were carried out on three varved lake-sediment sequences in Sweden, which extend to ca. 7000 BC. The comparison of the magnetic properties of the organic rich varved lake-sediments with their respective catchment materials indicates that the magnetic properties of the sediments are dominated by relatively high concentrations of single-domain magnetite magnetosomes produced by magnetotactic bacteria. The dimictic nature of the lakes, which help form and preserve the varves due to the weakly oxic or suboxic environment, also appear to be suitable for a high degree of magnetosome preservation. The concentration of the magnetosomes in two of the sequences, as determined by mass specific magnetic measurements, exhibits a positive linear relationship with the total organic carbon content and suggests that magnetic remanence measurements reflect lake productivity, via processes of organic matter accumulation and decomposition. Further research should focus on the potential to use the fossil magnetosomes as proxy-climate/environmental change indicators and as recorders of geomagnetic field variations and behaviour.