On the sensitivity of Holocene talus-derived rock glaciers to climate change in the Ben Ohau Range,New Zealand

The morphology and surface ages of talus-derived rock glaciers are investigated to establish the timing of rock glacier formation in the central Southern Alps. Samples of rock weathering rinds show that all rock glaciers studied were formed during the Neoglacial period, but differences exist between sites in the number of new rock glacier lobes formed by Holocene climatic fluctuations. A qualitative conceptual model is proposed to explain rock glacier formation in terms of two thresholds. An external threshold relates to the presence of a cool climate capable of allowing internal ice to form within talus slopes. An internal threshold relates to the presence of sufficiently thick talus at a site to generate a shear stress capable of overcoming internal friction within the talus/ice mass. The model produces a non-steady-state response to explain why unmodified talus, single-lobed and double-lobed rock glaciers developed at adjacent sites under the same climatic regime. Individual landforms have different sensitivities to formation, which depend partly on the previous history of talus accumulation and rock glacier activity at a site. The model demonstrates how successive cool climate periods may be fully represented by rock glacier lobes at sensitive sites but under-represented at insensitive sites. Sensitivity (and therefore climatic representativeness) is favoured by high rates of debris supply. By implication, the timing of formation of rock glacier lobes in regions of prolonged cool climate and low debris production is less likely to correspond to the timing of climatic cooling and more likely to follow the ‘rules’ of deterministic chaos.     

Mid‐ and late Holocene tree population density changes in northern Fennoscandia derived by a new method using megafossil pines and their tree‐ring series

Changes in tree density are estimated for the northern forest‐limit region of Finnish Lapland over the past seven and a half millennia. This is done using dendrochronologically dated Scots pine megafossils and their tree‐ring series. Direct and indirect estimates of past tree density are derived from chronology sample size (CSS) and growth trend modelling (GTM) respectively. The latter is a new method, where the past levels of growth competition (the influence of nearby trees) are extracted from the behaviour of growth trends in cross‐dated tree‐ring series, trends that are expected to be driven by tree‐density control. Two records constructed from the same original data set but by different means (independently) correlate significantly. Both records show a tree‐density maximum around 3000–1750 BC , indicating relative warmth during that time. Another positive tree‐density anomaly occurred in accordance with the ‘Medieval Warm Period’, preceding the thinning at the forest‐limit due to the ‘Little Ice Age’. Copyright © 2005 John Wiley & Sons, Ltd.     

Post‐glacial vegetation reconstruction and a possible 8200 cal. yr BP event from the low arctic of continental Nunavut,Canada

Climate models suggest that the global warming during the early to mid‐Holocene may have partly resulted from the northward advance of the northern treeline and subsequent reduction of the planetary albedo. We investigated the Holocene vegetation history of low arctic continental Nunavut, Canada, from a radiocarbon‐dated sediment core from TK‐2 Lake, a small‐lake ca. 200 km north of the limit of the forest‐tundra. The pollen and loss‐on‐ignition data indicate the presence of dwarf shrub tundra in the region since the beginning of organic sedimentation at ca. 9000 cal. yr BP with dominance of Betula, especially since 8700 cal. yr BP. At 8100–7900 cal. yr BP the dominance of the shrub tundra was punctuated by a transient decline of Betula and coincident increases of Ericaceae undiff., Vaccinium‐type, and Gramineae. This suggests an abrupt disturbance of the Betula glandulosa population, approximately simultaneously with the sudden 8200 cal. yr BP event in the North Atlantic. However, in the absence of other sites studied in the area, linkage to the 8200 cal. yr BP event remains tentative. The lack of any evidence of forest‐tundra in the region constrains the northern limit of the mid‐Holocene advance of the forest‐tundra boundary in central northern Canada. Consequently, our results show that the climate models imposing a mid‐Holocene advance of the limit of the forest‐tundra to the arctic coast of Canada may have overestimated the positive climatic feedback effects that can result from the replacement of tundra by the boreal forest. Copyright © 2003 John Wiley & Sons, Ltd.     

Neoglacial chronology and floristics in the Middle Teton area,central Teton Range,Western Wyoming

The relative ages of Neogfacial deposits above 2900m near Cloudveil Dome, Middle Teton, and Teepe glaciers, central Teton Range, western Wyoming, were determined using topographic position, weathering features, lichenometry, vegetation characteristics, and soils. A three-fold deposit sequence is identified and correlated with the Gannett Peak, Audubon and Indian Basin Neoglacial advances described elsewhere in the Teton and nearby Wind River ranges. While soil profile morphology proved very useful in distinguishing deposits of different age, other age indicators such as clay mineralogy and soil chemistry, proved of only marginal value in age discrimination. This chronosequence, emplaced over the last ～5000yr, provides an excellent setting upon which to test changes in plant species composition and its use as a relative dating method. Vascular plant species composition cannot be used to discriminate deposits of the different advances, possibly because of random colonisation and establishment combined with extensive mass wasting during the Neoglacial. However, certain vegetation features, such as total cover and species densities, appear useful as relative dating methods.     

Radiocarbon constraints on the Holocene flood deposits of the Ning‐Zhen Mountains,lower Yangtze River area of China

Lying athwart both the temperate and subtropical zones, the Ning‐Zhen Mountains are particularly prone to extreme floods in the summer months when cold fronts collide with the subtropics‐derived warm airmasses. The Holocene flood deposits in the region may provide a long‐term perspective on hydrographical change and its palaeoclimatic implications. Radiocarbon dates on carbonised wood preserved in flooding sediments reveal that the region has experienced a number of catastrophic floods throughout the middle Holocene. These extreme flooding events cluster into three periods: (i) 9200–8200 cal. yr BP, (ii) 7600–5800 cal. yr BP and (iii) 5200–4000 cal. yr BP, corresponding to the times when the East Asian monsoon has been intensified under warm conditions. The significant falls in flooding frequency around 8200 cal. yr BP, 5800 cal. yr BP and 4000 cal. yr BP may have resulted from substantial reduction in regional precipitation, probably associated with meridional displacement of the planetary frontal system. Copyright © 2003 John Wiley & Sons, Ltd.     

Holocene landscape change at Carn Dubh,near Pitlochry,Perthshire, Scotland

Detailed palynological analyses, including pollen preservation and charcoal counts, and sedimentological analyses (loss-on-ignition, percentage dry weight, dry bulk density and humification), supported by 15 ¹⁴C dates, were applied to a small upland (350 m OD) peat basin near Pitlochry, east-central Grampians, Scotland. Stratigraphical analyses and radiocarbon dating were also undertaken on the valley peat surrounding the basin. Organic sediment began to accumulate in the basin and on the valley floor at ca. 9800 yr BP. ‘Recurrence surfaces’ within the valley floor peat correlate with the end of a trend to increased aridity in the deep basin, from 8900 to 8500 yr BP. Pine may have colonised the hills, with birch, before the migration of hazel at 9150 yr BP. From around 8000 yr BP elm grew on the base-rich soils. Alder may have been present by 7000 yr BP but it did not expand until immediately after the first major anthropogenic disturbance, at 4800 yr BP. Above 8000 yr BP silt was commonly deposited in the basin by streams, but several phases of exceptionally intense inwashing are identified, and at least two of these correlate with periods of increased precipitation. Grazing pressure during the elm decline marks the beginning of a long history of generally low intensity pastoralism, interspersed with periods of heightened anthropogenic activity.     

Decadal to millennial cyclicity in varves and turbidites from the Arabian Sea: hypothesis of tidal origin

We have analyzed a Late Holocene record, almost 5000 years long, consisting of varved sediments deposited in the oxygen-minimum zone (OMZ) off Pakistan. We searched for cyclicity in the series of varve thickness (“varve” cycles), of unusually large excursions in varve thickness (“agitation” cycles), and of abundance of turbidites (“turbidite” cycles). We found the following high-frequency cycles (periods between 10 and 100 years) in one or several of the three types of series as follows: near 12.4, 14–15, 16.8, 18.6 (strong, agitation), 25–26 (strong, turbidite), 29–31 (strong, agitation), 39 (varve), 44 (strong, turbidite), 51–54 (strong, agitation), 56 (strong, varve), 64 (strong, turbidite), 69, 77 (strong, turbidite), 82 (very strong, agitation), and 95 years (strong, varve). Low-frequency cycles center around 99–115, 125 (very strong, varve), 164, 177, 202, 242–255 (strong, agitation and turbidite), 280 (strong, varve; doubled, turbidite), 340–370 and 460–490 years.Some cycles of varve thickness match the cyclicity of turbidite frequency (12.3, 14–15, 25–26, 245–255 years) but similarities between spectra are not striking. Taken as a whole, however, the sequence of cycles detected (by autocorrelation and standard Fourier analysis) seems to contain a large proportion of multiples of the basic tidal cycles 4.425 (lunar perigee cycle) and 9.3 years (lunar half-nodal cycle). This impression is supported by testing the three binned spectra for whole-number multiples and fractions as well as whole-number beat structure. We therefore propose that a large proportion of the cyclicity detected can be ascribed to tidal action. Our record also contains evidence for the presence of the 1470-year cycle previously reported from the glacial-age Greenland ice record. The main harmonics of this Greenland cycle can be tied to the pattern of periods seen in the varved sediments. We hypothesize that tidal action produces the cycle, and that the reason for its great length is the requirement that maximum tidal activity has to fall into a narrow seasonal window to be geologically effective.     

The effect of geomorphological setting on Holocene lake sediment variability,northern Swedish Lapland

Comparison of catchment geomorphology and lithostratigraphical analysis of sediments in two small neighbouring alpine lakes show that the minerogenic influx into the lakes has varied significantly during the Holocene, despite similarities in environmental setting. One lake contains a homogeneous organic‐rich sediment sequence whereas the sediment of the other lake is laminated and has a higher minerogenic content. X‐ray radiographs are used to visualise lithostratigraphical structures and provide high‐resolution density data. We find that moderate differences in geomorphology and process activity in the lakeshore region around alpine lakes can significantly affect the lake sediment composition. Minerogenic sediment accumulation rates vary strongly over time, owing to different depositional processes, which complicate temporal reconstructions. We also find that non‐glacial processes deposit minerogenic sediment layers with similar characteristics (high density, low organic content) as layers interpreted as having a glaciofluvial origin. This has implications for palaeoclimate studies based on proglacial lacustrine sediment. Our results indicate that erosion of surface sediments in the catchments characterised the early Holocene. A low and constant minerogenic inflow indicates that stable environmental conditions (with little fluvial erosion) were established in the catchments during the middle Holocene. The variability in sediment composition increased again in the late Holocene, possibly as a result of short‐term climate fluctuations superimposed on a general climate deterioration trend. Copyright © 2003 John Wiley & Sons, Ltd.