The Holocene

The Neoglacial was a period of cold that lasted more than 2000 years during the mid-Holocene, from approximately 4700 to 2500 years ago. Although proxy data from a number of sources document the regional onset and duration of cold conditions in the Northern Hemisphere during this period, none have suggested an expansion of sea ice in the Bering Sea. Here we provide new evidence that Neoglacial sea ice expansion in the Bering Sea was substantial enough to have altered the distribution of North Pacific pinnipeds and cetaceans, using prehistoric skeletal remains recovered from an archaeological site on the island of Unalaska in the eastern Aleutians (Amaknak Bridge, occupied from c. 3500—2500 yr BP (radiocarbon years before present, uncalibrated)). Comprehensive archaeozoological analysis of the Amaknak Bridge fauna indicates that sea ice in the Bering Sea must have reached a more southerly position at the height of the Neoglacial and persisted longer than it does today. We infer from this evidence that for most of the Neoglacial period, sea ice must have surrounded the Pribilof Islands until early summer and blocked the Bering Strait until late summer. Such an expansion and seasonal persistence of sea ice would have prevented fur seals from using the Pribilofs as a summer breeding rookery and whales from making summer migrations into arctic waters to feed, as they do today. We suggest this expansion of sea ice in the Bering Sea during the Neoglacial may explain several unresolved phenomena of mammalian distributions, genetic partitioning and extinctions in the North Pacific.

The analyses of high-resolution seismic data, swath bathymetry data and sediment cores from the basin Svensksunddjupet in Isfjorden, Spitsbergen, reveal the deposits of repeated slope failures. Most of the slope failures occurred during three periods at c. 9650 cal. BP, 8350—8200 cal. BP and 3000 cal. BP. Suggested factors causing instability of the slopes include earthquake activity resulting from rapid isostatic uplift, excess pore pressure caused by relative sea-level fall and/or the presence of gas as well as climatic changes. Mass-transport deposits at the mouth of an incision at the southern slope of Svensksunddjupet are suggested to mainly relate to runoff from the land-based glacier Vardebreen. The increase of the mass-transport activity may indicate that Vardebreen either formed or increased in size at c. 7000 cal. BP.

The relationship between isotope ratios in precipitation and lake waters was monitored by bi-weekly measurements taken over a five and a half year period between May 2000 and September 2005 (a period of 1964 days) at Lochnagar, a remote mountain lake in eastern Scotland. Short-term changes in the oxygen isotope composition of lake water (δ¹⁸O_L) at Lochnagar follow a seasonal pattern similar to isotopes in local precipitation (δ¹⁸O_P), however changes in catchment residence time, snow accumulation, lake ice cover and lake stratification modify the temporal structure of δ¹⁸O_L over the seasonal cycle. Of particular importance is precipitation amount, which controls catchment and lake residence times, and determines the degree of phase lag and amplitude change between δ¹⁸O_L and δ¹⁸O_P. A simple mass balance model replicates these effects and demonstrates that the degree of phase lag and amplitude reduction is predictable given known input/volume ratio. The implications of these observations for the use of δ¹⁸O records in palaeoclimatology are important, since it is rare that authigenic and biogenic minerals or organic compounds (from which δ¹⁸O and/or δ²H can be measured) are produced in a lake evenly throughout the year.

This study attempts to reconstruct the Holocene glacier history of Breheimen, southern Norway from four stream-bank mire sites and a downstream lake sediment core near Sota Sæter, based on lithostratigraphy, sediment properties (loss-on-ignition (LOI), magnetic susceptibility (MS), grain size characteristics) and 69 radiocarbon dates. Late-Preboreal deglaciation is indicated by lake-floor accumulation of organic matter (gyttja) largely replacing minerogenic sediment before 9000 cal. BP. A distinct minerogenic layer is thought to represent glacier expansion during the 8.2 ka Finse Event. Following this event until c . 2200 cal. BP, glaciers were mostly smaller than today, indicated by slow accumulation of mainly gyttja with predominantly thin minerogenic layers. A trough in LOI and corresponding small peaks in different silt fractions indicate possible glacial expansion from c. 3500 to 3000 cal. BP. The shorter c. 7500 year stream-bank mire record suggests brief, distinct episodes of Neoglacial glacier activity at c. 5600 and 3650 cal. BP. After c. 2200 cal. BP, up to three possible glacier expansion events are indicated in the lake core compared with up to eight represented in the mire sequences, suggesting greater sensitivity of the latter to small-scale glacier expansion. Marked `suppressed' Neoglacial expansion until after c. 2200 cal. BP contrasts with the more typically reported start at 6000—5300 cal. BP elsewhere in southern Norway. Methodological implications are discussed including: factors responsible for erroneous radiocarbon dates in mire and lake core stratigraphies; high spatial variability in mire stratigraphy; determining the amplitude of glacier contraction episodes; isolating the glacial signal from background `noise'; and further improvement of dating.

Past vegetation dynamics and human impact from the Lateglacial to the present are reconstructed by pollen analysis of a core 12 m long from the raised bog Egelsee, central Switzerland (770 m a.s.l.). The depth—age model of the core is based on 29 AMS ¹⁴C dates. The oldest dated macrofossil has an age of 13 080 ± 105 radiocarbon yr BP (15 370 cal. yr BP); extrapolation leads to a basal age of the core of 16 200 cal. yr BP. The biostratigraphy shows a typical vegetation development for Central Europe from open pioneer vegetation to Betula-Pinus forests in the Lateglacial, thermophilous mixed deciduous forests in the early Holocene, then mesophilous Abies-Fagus forests, and finally to the present cultural landscape. The initial population expansion of Abies alba may have coincided with climatic change at c. 8500 cal. yr BP. The mass expansion of Abies alba at 8100 cal. yr BP may have been triggered by the 8.2-ka event and coincided with the empirical limit of Fagus silvatica, indicating initial population expansions. The succession of ecosystems with different plant composition is confirmed by detrended correspondence analysis (DCA): the main changes in the DCA axis 1 and 2 correspond to the transition phases between the different ecosystems. DCA axis 1 has an eigenvalue of λ₁ = 0.46 and is significantly correlated (r = 0.82) to the non-arboreal pollen percentage curve. Rarefaction analysis shows a high pollen diversity in the Lateglacial, lower pollen diversity in the Holocene before human impact, and increasing pollen diversity correlated with increasing human impact. Very little is known about the archaeology of the montane belt in central Switzerland. This study provides new data concerning human impact and settlement history in this area. Human activities are indicated by cereal pollen (earliest Hordeum/Triticum type at 4700 cal. yr BP). Distinct pulses of anthropogenic forest clearances can be observed from the Neolithic period onwards.

This study examines the variability of the Holocene landscape and the extent of human impact on the vegetation of Bela krajina region (southeastern Slovenia). Pollen and microcharcoal records of two small palaeoecological sites Mlaka and Griblje (G3), located just c. 10 km apart, indicate that human impact, manifested as forest clearance and burning, was significant throughout the Holocene and led to change of forest composition, impoverishment of soils, increased biodiversity and formation of a mosaic landscape with significant differences between the two sites. On both sites the strongest human impact associated with Fagus decline and the appearance of taxa characteristic for meadows, fields and pastures was detected from approximately 6000 cal. yr. BP onwards. However, differences between study sites were significant. At Mlaka, located on predominantly limestone bedrock, the human impact was very intensive and the present-day open landscape was formed by the Mediaeval period at c. 1000 cal. BP. In comparison the landscape around Griblje (on sandy bedrock) remained predominantly forested to the present.

To unravel the evolution of a dry valley in the Belgian loess area soils and sediments along a slope catena were studied in detail. A 67 m long trench was opened from the upper slope to the centre of the valley bottom. The exposed soils and sediments showed evidence of severe soil erosion and other human disturbances that significantly changed the valley topography. The early-Holocene slope gradient was up to 25%, whereas now it is less than 10%. In the thalweg, a remnant of the early-Holocene soil was found underneath colluvial deposits, which were more than 3 m thick. A chronology for the valley evolution was established based on AMS ¹⁴C dating of charcoal fragments and optical dating of colluvial sediments. The first sediment deposition occurred in the early Iron Age, with an average sedimentation rate of approximately 3.4 ± 1.3 t/ha per yr. This increased to c. 5.4 ± 2.2 t/ha per yr during the Roman Period and further to 18.0 ± 2.2 t/ha per yr in the Middle Ages. Although sediment accumulation in the valley was substantial, soil-erosion processes were mainly low-magnitude and signs of gullying are absent in the thalweg until the last few centuries.

Aeolian sand in two small lakes within a Lake Michigan coastal-dune complex southwest of Holland, Michigan, provides a more detailed higher-resolution history of dune activity, during the later half of the Holocene, than do combined palaeosol and OSL ages from the dunes themselves. The sand signal from four cores within these lakes consists of visible sand laminae and invisible sand peaks within mud and sapropel units. Early in the history of dune growth the sand may have come from backdunes immediately adjacent to the lakes, but in the upper portions of the core aeolian sand is interpreted as having been transported from 30—40 m high parabolic dunes ~200 m from the lakes. All but one episode of dune mobility suggested by radiocarbon-dated dune palaeosols or OSL-dated dune sand are represented by a peak in the radiocarbon-dated lacustrine sand signal. However, sand peaks occur in the lacustrine record that do not correspond to palaeosol or OSL ages, suggesting that the sand record from small lakes gives a more complete record of aeolian activity. Heterogeneity in sand deposition related to bathymetry suggests that the study of multiple cores from the same lake will yield the best results . No strong correlation between rising lake levels on the Lake Michigan relative lake level curve and aeolian activity was found, although such a correlation has been reported elsewhere.

Palaeolimnological analyses from Lake PW03, a small, dilute lake on crystalline bedrock on Prescott Island, Nunavut, Canada (73°07′ N, 96°41′ W, 243 m a.s.l.), provide a sensitive record of the impacts of Holocene climatic change on diatom communities. A high-resolution sequence from 3500 cal. BP to present indicates that a significant change in diatom assemblages takes place at 2300 cal. BP, when alkaliphilous fragilarioids and planktonic taxa are replaced by benthic, epiphytic and acidophilous diatoms. These changes coincide with the onset of neoglacial cooling and likely reflect more persistent ice-cover and associated acidification of lake water. Maximum diatom species richness occurs during this period of transition. Dense bryophyte communities in the littoral zone provide habitats to sustain diatom production even under the cooler conditions following 2300 cal. BP. The concentration of diatom valves declines at 500 cal. BP, coincident with cooling at the time of the `Little Ice Age', and minor changes in species composition take place at that time. The sediments deposited over the past 150 years show modest increases in fragilarioids and planktonic diatoms, in response to a longer ice-free season under a warmer climate. Diatom valve production increases significantly in these recent sediments but increases in diversity are relatively small. Diatom diversity, production and the species composition of diatom communities do not change simultaneously in response to climatic changes.

Pollen, loss on ignition and magnetic susceptibility analyses provide a high-resolution palaeoenvironmental record from Lago Puerto Arturo, Peten, Guatemala. The presence of Zea pollen ~2650 BC provides a latest date for the arrival of maize agriculture to the region. The following 3600 years are marked by significant opening of the forest and episodic pulses of erosion. During the early Preclassic, around 1450 BC, all proxies indicate an abrupt increase in human activity, coincident with archaeological evidence of early settlement. Three discrete periods of decreased human activity are indicated by cessations of landscape disturbance. Such decreased human activity likely reflects periodic local population decline. These events coincide with times of cultural transition in the Maya lowlands and correspond to the terminal phases of the middle Preclassic, late Preclassic and late Classic periods. There is no evidence for human activity in the area following the late Classic abandonment.

Minimum blue intensity measurements of resin-extracted Pinus sylvestris (L.) samples, conducted using a flat-bed scanner and commercially available software, are shown to provide a robust and reliable surrogate for maximum latewood density. Blue intensity data from 15 trees, from three stands, are reported relative to a standard blue-scale in a manner similar to grey-scale calibration in x-ray densitometry. The resulting time series are highly correlated with x-ray densitometry data generated from the same samples and preserve the same high level of signal strength. Sensitivity to summer climate variables is similar to that identified in the relative density record, demonstrating that minimum blue intensity can also be used for the study of climate change. While not a replacement for the powerful range of x-ray densitometry techniques, blue intensity provides an inexpensive and accessible alternative for accessing palaeoclimatic information.

Average rates of Holocene chemical weathering and `Little Ice Age' glacial erosion of bedrock are estimated across the glacier—foreland boundary of an outlet glacier of the Jostedalsbreen ice-cap, southern Norway. Estimates are derived from two types of evidence: (1) the heights of quartz veins above adjacent surfaces of granitic or granodioritic gneiss; and (2) Schmidt-hammer R-values of rock hardness. Average and maximum rates of surface lowering of gneiss surfaces by chemical weathering are 1.63 and 3.61 mm/ka, respectively, whereas many quartz veins exhibit negligible weathering after c. 9700 years. `Little Ice Age' glacial erosion rates are at least two orders of magnitude greater than the chemical weathering rates, except in a narrow marginal zone inside, but close to, the `Little Ice Age' limit, where patches of weathered surface survive. Bedrock surfaces outside and well inside the `Little Ice Age' glacier—foreland boundary are shown to have potential for improved calibrated-age dating using R-values, compared with moraine surfaces.

Radiocarbon dating has been previously suggested to be applicable to pedogenic carbonate. However, until now the validity of radiocarbon dates from pedogenic carbonate remained poorly understood because of the relatively low resolution of chronological controls. In this study we compared the radiocarbon dates (a total of 21) of thin (0.2—0.3 mm in thickness) laminae of pedogenic carbonate coatings on clasts from Holocene archaeological sites in the eastern Mediterranean and south Siberia with independently estimated age-ranges for these sites. We obtained a high correlation between the oldest laminae of coatings with the site ages. The ¹⁴C ages of pedogenic carbonate were systematically younger by c. 0.3—1.6 ka (mean c. 0.75 ka) compared with archaeological sites. This finding suggests that pedogenic carbonate — despite potential complicating factors — can be dated more accurately with ¹⁴C than has been generally appreciated.

Knowledge on processes of charcoal transportation is crucial for fire reconstruction based on sedimentary charcoal. Charcoal is susceptible to long-distance transport by water. A lake basin with a large and long catchment area is likely to accumulate charcoal from many fires, not only those produced by fires nearby the lakeshore. Here we test the potential of charcoal transportation by analysing sedimentary charcoal accumulated in an underground lake within a karstic massif. Fires cannot spread around the lake, nor within the karstic massif. Organic materials, including charred particles, are generated several kilometres from the lake on the karstic plateau above. The pattern of sedimentary charcoal shows that the underground lake records continuously produced charcoal by wild fires or human-made biomass burning (slash-and-burn, charcoal kilns) over centuries and millennia, but also stored charcoal from eroded soils. Although the charcoal series shows a certain high variability signal, fire frequency reconstruction cannot be performed owing to chronological uncertainties. The charcoal accumulation corresponds to a more or less regular background input. Such background input is empirically well described in palaeo-fire reconstruction, but was never experimentally displayed. This study provides evidence that the pattern (surface, length, slope, etc.) of catchment areas is crucial for interpreting sedimentary charcoal series. Large catchment areas draining long rivers are not suitable for high-resolution and spatially precise fire reconstructions.

Reconstructions of solar activity prior to AD 1600 rely on estimates of the past production rates of cosmogenic radionuclides in the Earth's atmosphere. These rates are modulated by the strength of the solar magnetic field and the intensity of the Earth's geomagnetic dipole. One of the largest uncertainties associated with these reconstructions originates from palaeointensity data and to show how errors in palaeomagnetic data influence reconstructions of solar activity we present different radiocarbon based reconstructions of the solar magnetic modulation parameter (Φ) during the Holocene. We considered geomagnetic field changes according to (i) a global archaeomagnetic compilation, (ii) a geomagnetic field model covering the last 7000 years and (iii) a regional sediment relative palaeointensity record. Comparison between alternative solar modulation reconstructions shows that the uncertainties in palaeomagnetic data prevent us from making robust conclusions about solar variability at timescales longer than a few hundred years. Our analyses indicate that the geomagnetic dipole field dominates radiocarbon production at timescales longer than 500 years although a combination of solar and geomagnetic modulation could apply to all timescales. Long-term trends depend on which palaeomagnetic data set is used for correction and there are additional unexplained differences between ¹⁰Be and ¹⁴C records that question results about long-term changes in solar activity. The higher-resolution geomagnetic field records, which are usually very much smoothed in stacked records, also indicate that part of the shorter-term changes (~500 yr) in the radiocarbon production rate could be attributed to the geomagnetic dipole field. To test this hypothesis and improve our knowledge of past solar activity a higher resolution palaeomagnetic reconstruction of geomagnetic dipole moment is required.