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.     

Radiocarbon wiggle matching of Swedish lake varves reveals asynchronous climate changes around the 8.2 kyr cold event

Snowball, I., Muscheler, R., Zillén, L., Sandgren, P., Stanton, T. & Ljung, K. 2010: Radiocarbon wiggle matching of Swedish lake varves reveals asynchronous climate changes around the 8.2 kyr cold event. Boreas, Vol. 39, pp. 720–733. 10.1111/j.1502‐3885.2010.00167.x. ISSN 0300‐9483. A series of nine radiocarbon measurements obtained at increments of 50 years was used to wiggle match varved sediments in Lake Kälksjön (west central Sweden) to the calendar year time scale provided by the tree‐ring‐derived radiocarbon calibration curve. The anchor point for a series of 400 varves known to span the ‘8.2 kyr cold event’ was derived from a combination of correlation analysis, Monte Carlo statistics and Bayesian age modelling. The GRIP and NorthGRIP ice‐cores were matched to the same absolute time scale by comparing ¹⁰Be data and tree‐ring Δ¹⁴C. The radiocarbon‐based wiggle match, organic carbon measurements, mineral magnetic parameters and XRF data reconstruct a distinct period of enhanced erosion in Kälksjön's catchment between 8066 ± 25 and 7920 ± 25 cal. yr BP. Prior to human impact, the increased erosion in the boreal environment is assigned to increased winter precipitation in the form of snow, which caused spring meltwater discharge to intensify. Our results suggest that the abrupt onset of increased winter precipitation in west central Sweden started at least 50 years after the onset of the ‘8.2 kyr cold event’ as defined by oxygen isotope data from Greenland. The study highlights the value of synchronized time scales in the reconstruction of abrupt climate changes based on proxy data.     

Garnet Compositions at Broken Hill, New South Wales, as Indicators of Metamorphic Processes

Electron microprobe analyses of garnets in the finely bedded‘banded iron formations’ (BIF) of the Willyama Complexat Broken Hill reveal marked compositional changes from onegarnet to the next on a scale of 1–2 mm. Further, systematicanalytical traverses across bedding and along bedding show thecompositions of the garnets to change markedly from one finebed to the next, but to remain extremely uniform within individualbeds. In view of the minuteness of the domains involved it appearsevident that compositional variation cannot be attributed tovariations in metamorphic pressures, temperatures or oxygenfugacities. Neither can they be attributed to variation in garnet-matrixpartition functions, as most of the garnets occur in one simplematrix—quartz. It is concluded that in spite of the high (sillimanite) gradeof the relevant metamorphism, any equilibration of garnet compositions,and hence any associated inter-grain metamorphic diffusion,has been restricted to a scale of less than 1 mm; that garnetcompositions here reflect original rock composition on an ultra-finescale, and have no connotations concerning metamorphic grade;that, hence, the garnets must derive from a single precursormaterial, earlier suggested to be a manganiferous chamositicseptechlorite; and that the between-bed variation: within-beduniformity of garnet composition reflects an original patternof chemical sedimentation—a pattern preserved with theutmost delicacy through a period of 1800 x 10⁶ years and a metamorphicepisode of sillimanite grade.