Early Weichselian (MIS 5d and 5c) temperatures and environmental changes in northern Fennoscandia as recorded by chironomids and macroremains at Sokli,northeast Finland

Engels, S., Helmens, K. F., Väliranta, M., Brooks, S. J. & Birks, H. J. B. 2010: Early Weichselian (MIS 5d and 5c) temperatures and environmental changes in northern Fennoscandia as recorded by chironomids and macroremains at Sokli, northeast Finland. Boreas, Vol. 39, pp. 689–704. 10.1111/j.1502‐3885.2010.00163.x. ISSN 0300‐9483. A 25‐m‐long sediment record spanning the time from the Eemian to the Holocene was recovered from Sokli, northeast Finland. This study focuses on a 6‐m‐long sediment interval that is dated to the Early Weichselian period (MIS 5d and 5c) and consists of lacustrine and fluvial deposits. Using chironomid remains, botanical and zoological macroremains as well as sediment lithology, we were able to reconstruct past changes in the environment, including climate. The results indicate that the site was situated on a flood‐plain during the latter stages of MIS 5d (Herning Stadial) and that summer temperatures might have been ～6 °C lower than at present. Although this value should be treated with caution, as numerical analysis shows that it has a very poor fit‐to‐temperature, this low reconstructed value concurs with several other reconstructions that are available from western Europe. During MIS 5c (Brørup interstadial), the depositional environment changed into a lake system, initially with stratification of the water and subsequently with complete mixing and a strong influence of streams. Both chironomid‐based and macroremain‐based temperature inferences indicate past July air temperatures that were significantly higher than at present. This result is in contrast to other (low‐resolution) reconstructions from northern Fennoscandia that indicate past temperatures 6–7 °C lower than present using fossil coleopteran assemblages. However, several central European sites indicate that there was a phase during the Brørup interstadial that was characterized by high summer temperatures, and a comparison between the high‐resolution reconstructions from western Europe and the results presented in this study suggests that the north–south July air temperature gradient between the mid‐ and high‐latitudes was much weaker during the Brørup interstadial than it is at present. High solar insolation values (particularly the obliquity) during the Brørup interstadial might explain the low summer temperature gradient over the European continent. A return to fluvial conditions occurred in the upper parts of the sediment sequence, and, after a brief interval of gyttja deposition under cooling conditions, the site became glaciated during MIS 5b.     

Evolution of the Kap Edvard Holm Complex: a Mafic Intrusion at a Rifted Continental Margin

The Kap Edvard Holm Layered Series forms part of the East GreenlandTertiary Province, and was emplaced at shallow crustal level(at depths corresponding to a pressure of 1–2 kbar) duringcontinental break-up. It consists of two suites: a gabbro suitecomprising olivine and oxide gabbros, leucocratic olivine gabbrosand anorthosites, and a suite of wehrlites that formed fromthe intrusion of the gabbros during their solidification bya hydrous, high-MgO magma. Ion microprobe analyses of clinopyroxenereveal chemical contrasts between the parental melt of the wehrlitesuite and that of the gabbro suite. Thin sills (1–2 mthick) of the wehrlite suite, however, have clinopyroxene compositionssimilar to the gabbro suite, and were formed by interactionwith interstitial melts from the host layered gabbros. All evolvedmembers of the gabbro suite have elevated Nd, Zr and Sr concentrationsand Nd/Yb ratios, relative to the melt parental to the gabbrosuite. These characteristics are attributed to establishmentof a magma chamber at depths corresponding to a pressure of10 kbar, where melts evolved before injection into the low-pressuremagma chamber. Anorthosites of the gabbro suite are believedto have crystallized from such injections. The melts becamesupersaturated in plagioclase by the pressure release that followedtransportation to the low-pressure magma chamber after initialfractionation at 10 kbar. The most evolved gabbros formed bysubsequent fractionation within the low-pressure magma chamber.Our results indicate that high-pressure fractionation may beimportant in generating some of the lithological variationsin layered intrusions. KEY WORDS: fractionation; ion microprobe; layered intrusions; rift processes; trace elements ^*Corresponding author.     

LATE QUATERNARY MORAINES ALONG THE SEKHOKONG RANGE, EASTERN LESOTHO: CONTRASTING THE GEOMORPHIC HISTORY OF NORTH- AND SOUTH-FACING SLOPES

Considerable Quaternary environmental reconstruction for the high Drakensberg is based on geomorphological and sedimentological work undertaken along the northern aspects of the Sekhokong mountain range of eastern Lesotho. Given that no previous investigations have focused on the southern aspects, this paper documents the observed geomorphology and provides a more complete palaeo-environmental picture for this range. Data on the morphology, sedimentology and micromorphology for two linear debris ridges are presented. It is demonstrated that the two ridges are most likely moraines originating from a small niche glacier. The combined use of macro- and micro-scale sedimentology is proven to be an essential tool in ascribing a glacial process origin for the landforms, given the complex depositional history they have undergone. AMS ages obtained from the deposits (14 700 cal. yrs bp and 19 350 cal. yrs bp ) places these in the time-scale of the Last Glacial Maximum. The study demonstrates rather contrasting aspect-controlled palaegeomorphological environments along the Sekhokong range, which is also reflected in the dissimilar contemporary biophysical micro-environments. It is suggested that the south-facing slopes were dominated by glacial processes during the Last Glacial Maximum (LGM), as is evident from the moraines, while the proposition for previously described north-facing glacial cirques is rejected based on the absence of erosional/depositional evidence and greater insolation received on these warmerequator-facing slopes. Rather, we propose that the observed north-facing hollows are a product of a multitude of geomorphic processes spanning several tens to hundreds of thousands of years.     

中国南方新元古代可能后生动物成因的硅化和磷酸盐化球状微化石Tianzhushania

对中国南方贵州瓮安地区上新元古界陡山沱组两种类型沉积：燧石夹层(切片法研究)和磷灰岩／含磷碳酸盐岩(切片法和浸解法研究)中微化石的比较研究表明,磷酸盐化的Megasphaera ornata与燧石中保存的Tianzhushania tuberifera可能代表由不同矿化过程保存下来的同一生物分类。磷酸盐化标本的外层壁经常脱落,而暴露其具纹饰的中问壁。在一些孤立于围岩基质的磷酸盐化标本和在磷灰岩薄片所见到的标本上显现部分带刺的外壳保存,它们可以同燧石夹层薄片中发现的化石进行比较。磷酸盐化标本的中间壁瘤突表面上普遍见到的凹坑,相当于外层壁刺饰所遗留下的附着点。而具有刺饰的外层壁形态是Tianzhushania Yin and Li,1978属的典型特征,因此建议Tianzhushania arnate(xiao等,2000)Yin et al．,2004为该种的有效名称。而主要依据其中间壁的纹饰形态所得出的该种为后生动物休眠卵的认识与目前结果并不矛盾。但是,外层壁具刺饰的形态说明该种为浮游性的,而不是底栖性的生态类型。     

Seismic evidence of up to 200 m lake‐level change in Southern Patagonia since Marine Isotope Stage 4

Maar lake Laguna Potrok Aike is located north of the Strait of Magellan (south‐eastern Patagonia). Seismic reflection profiles revealed a highly dynamic palaeoclimate history. Dunes were identified in the eastern part of the lake at approximately 30 to 80 m below the lake floor, overlying older lacustrine strata, and suggest that the region experienced dry conditions probably combined with strong westerly winds. It is quite likely that this can be linked to a major dust event recorded in the Antarctic ice cores during Marine Isotope Stage 4. The dunes are overlain by a series of palaeo‐shorelines indicating a stepwise water‐level evolution of a new lake established after this dry period, and thus a change towards wetter conditions. After the initial, rapid and stepwise lake‐level rise, the basin became deeper and wider, and sediments deposited on the lake shoulder at approximately 33 m below present‐day lake level point towards a long period of lake‐level highstand between roughly 53·5 ka cal. bp and 30 ka cal. bp with a maximum lake level some 200 m higher than the desiccation horizon. This highstand was then followed by a regressional phase of uncertain age, although it must have happened some time between approximately 30 ka cal. bp and 6750 yrs cal. bp . Dryer conditions during the Mid‐Holocene are evidenced by a dropping lake level, resulting in a basin‐wide erosional unconformity on the lake shoulder. A second stepwise transgression between ca 5·8 to 5·4 ka cal. bp and ca 4·7 to 4 ka cal. bp with palaeo‐shorelines deposited on the lake shoulder unconformity again indicates a change towards wetter conditions.     

Schmidt‐hammer exposure‐age dating (SHD): application to early Holocene moraines and a reappraisal of the reliability of terrestrial cosmogenic‐nuclide dating (TCND) at Austanbotnbreen,Jotunheimen, Norway

Matthews, J. A. & Winkler, S. 2010: Schmidt‐hammer exposure‐age dating (SHD): application to early Holocene moraines and a reappraisal of the reliability of terrestrial cosmogenic‐nuclide dating (TCND) at Austanbotnbreen, Jotunheimen, Norway. Boreas, 10.1111/j.1502‐3885.2010.00178.x. ISSN 0300‐9483. Schmidt‐hammer exposure‐age dating (SHD) and terrestrial cosmogenic‐nuclide dating (TCND) are complementary techniques that can be used for mutual testing. SHD is low‐cost but requires local control points of known age and may be affected by local geological variation and other environmental factors that influence weathering rates. TCND is vulnerable to the occurrence of anomalous boulders, other geomorphological uncertainties and the effects of snow‐shielding at high altitudes. Both techniques are sensitive to post‐depositional disturbances if other than solid bedrock is sampled. SHD was applied to two moraine ridges beyond the Little Ice Age limit of Austanbotnbreen in the Hurrungane massif, southern Norway. Independent regional and experimental local age‐calibration curves were used to reappraise previous TCND results. Neither the two boulder surfaces nor their proximal bedrock surfaces could be differentiated statistically in terms of SHD exposure ages or their mean R‐values (±95% confidence intervals), which ranged from 40.73±1.72 to 43.34±0.69. The best of the independent regional‐calibration curves produced SHD exposure ages of 9413±723 and 9304±602 years, which are consistent with moraine formation early (c. 10.2 ka) and late (c. 9.7 ka) within the late‐Preboreal Erdalen Event. The current precision of SHD, as reflected in 95% confidence intervals of ±500–900 years, enables rejection of a Finse Event (c. 8.2 ka) age for either moraine. Results are consistent with a retracted Austanbotnbreen between the Erdalen Event and the Little Ice Age, and a modified model of Neoglaciation.     

Multi‐proxy evidence for early to mid‐Holocene environmental and climatic changes in northeastern Poland

Lauterbach, S., Brauer, A., Andersen, N., Danielopol, D. L., Dulski, P., Hüls, M., Milecka, K., Namiotko, T., Plessen, B., von Grafenstein, U. & DecLakes participants 2010: Multi‐proxy evidence for early to mid‐Holocene environmental and climatic changes in northeastern Poland. Boreas, 10.1111/j.1502‐3885.2010.00159.x. ISSN 0300‐9483. We investigated the sedimentary record of Lake Hańcza (northeastern Poland) using a multi‐proxy approach, focusing on early to mid‐Holocene climatic and environmental changes. AMS ¹⁴C dating of terrestrial macrofossils and sedimentation rate estimates from occasional varve thickness measurements were used to establish a chronology. The onset of the Holocene at c. 11 600 cal. a BP is marked by the decline of Lateglacial shrub vegetation and a shift from clastic‐detrital deposition to an autochthonous sedimentation dominated by biochemical calcite precipitation. Between 10 000 and 9000 cal. a BP, a further environmental and climatic improvement is indicated by the spread of deciduous forests, an increase in lake organic matter and a 1.7‰ rise in the oxygen isotope ratios of both endogenic calcite and ostracod valves. Rising δ¹⁸O values were probably caused by a combination of hydrological and climatic factors. The persistence of relatively cold and dry climate conditions in northeastern Poland during the first one and a half millennia of the Holocene could be related to a regional eastern European atmospheric circulation pattern. Prevailing anticyclonic circulation linked to a high‐pressure cell above the retreating Scandinavian Ice Sheet might have blocked the influence of warm and moist Westerlies and attenuated the early Holocene climatic amelioration in the Lake Hańcza region until the final decay of the ice sheet.     

Pilgrimstad revisited – a multi‐proxy reconstruction of Early/Middle Weichselian climate and environment at a key site in central Sweden

Wohlfarth, B., Alexanderson, H., Ampel, L., Bennike, O., Engels, S., Johnsen, T., Lundqvist, J. & Reimer, P. 2010: Pilgrimstad revisited – a multi‐proxy reconstruction of Early/Middle Weichselian climate and environment at a key site in central Sweden. Boreas, 10.1111/j.1502‐3885.2010.00192.x. ISSN 0300‐9483. The site Pilgrimstad in central Sweden has often been cited as a key locality for discussions of ice‐free/ice‐covered intervals during the Early and Middle Weichselian. Multi‐proxy investigations of a recently excavated section at Pilgrimstad now provide a revised picture of the climatic and environmental development between ～80 and 36 ka ago. The combination of sedimentology, geochemistry, OSL and ¹⁴C dating, and macrofossil, siliceous microfossil and chironomid analyses shows: (i) a lower succession of glaciofluvial/fluvial, lacustrine and glaciolacustrine sediments; (ii) an upper lacustrine sediment sequence; and (iii) Last Glacial Maximum till cover. Microfossils in the upper lacustrine sediments are initially characteristic for oligo‐ to mesotrophic lakes, and macrofossils indicate arctic/sub‐arctic environments and mean July temperatures >8 °C. These conditions were, however, followed by a return to a low‐nutrient lake and a cold and dry climate. The sequence contains several hiatuses, as shown by the often sharp contacts between individual units, which suggests that ice‐free intervals alternated with possible ice advances during certain parts of the Early and Middle Weichselian.     

SCHMIDT‐HAMMER EXPOSURE AGES FROM PERIGLACIAL PATTERNED GROUND (SORTED CIRCLES) IN JOTUNHEIMEN,NORWAY, AND THEIR INTERPRETATIVE PROBLEMS

Periglacial patterned ground (sorted circles and polygons) along an altitudinal profile at Juvflya in central Jotunheimen, southern Norway, is investigated using Schmidt‐hammer exposure‐age dating (SHD). The patterned ground surfaces exhibit R‐value distributions with platycurtic modes, broad plateaus, narrow tails, and a negative skew. Sample sites located between 1500 and 1925 m a.s.l. indicate a distinct altitudinal gradient of increasing mean R‐values towards higher altitudes interpreted as a chronological function. An established regional SHD calibration curve for Jotunheimen yielded mean boulder exposure ages in the range 6910 ± 510 to 8240 ± 495 years ago. These SHD ages are indicative of the timing of patterned ground formation, representing minimum ages for active boulder upfreezing and maximum ages for the stabilization of boulders in the encircling gutters. Despite uncertainties associated with the calibration curve and the age distribution of the boulders, the early‐Holocene age of the patterned ground surfaces, the apparent cessation of major activity during the Holocene Thermal Maximum (HTM) and continuing lack of late‐Holocene activity clarify existing understanding of the process dynamics and palaeoclimatic significance of large‐scale sorted patterned ground as an indicator of a permafrost environment. The interpretation of SHD ages from patterned ground surfaces remains challenging, however, owing to their diachronous nature, the potential for a complex history of formation, and the influence of local, non‐climatic factors.     

Ice‐free conditions in eastern Fennoscandia during early Marine Isotope Stage 3: lacustrine records

Helmens, K. F. & Engels, S. 2010: Ice‐free conditions in eastern Fennoscandia during early Marine Isotope Stage 3: lacustrine records. Boreas, 10.1111/j.1502‐3885.2010.00142.x. ISSN 0300‐9483. The traditional notion that Fennoscandia was glaciated throughout Marine Isotope Stages (MIS) 4–2, from c. 70 kyr BP to the deglaciation 15–10 kyr BP ago, has been challenged during the last decade. Recent studies have shown that climate and environmental settings during MIS 3 were more dynamic than previously assumed, and lacustrine sediment bodies indicate open‐water conditions for several sites in eastern Fennoscandia. In this study, three sediment sequences from western, eastern and northeast Finland are compared in detail with respect to their chronology, vegetation reconstruction and climatic inferences. OSL‐dating places the sediments in early MIS 3. Pollen evidence suggests the presence of isolated birch trees and open birch forest close to the retreating ice margin, in contrast to vegetation reconstructions from central Europe, which indicate tree‐less vegetation. Furthermore, reconstructions of climate using transfer functions have yielded surprising results, indicating present‐day summer temperatures in northeast Finland. The combined results suggest ice‐free and warm conditions in major parts of eastern Fennoscandia in early MIS 3, possibly during Greenland Interstadial (GIS) 14 around 53 kyr BP ago.