Reanalysis data show an increasing trend in Arctic precipitation over the 20th century, but changes are not homogenous across seasons or space. The observed hydroclimate changes are expected to continue and possibly accelerate in the coming century, not only affecting pan-Arctic natural ecosystems and human activities, but also lower latitudes through the atmospheric and ocean circulations. However, a lack of spatiotemporal observational data makes reliable quantification of Arctic hydroclimate change difficult, especially in a long-term context. To understand Arctic hydroclimate and its variability prior to the instrumental record, climate proxy records are needed. The purpose of this review is to summarise the current understanding of Arctic hydroclimate during the past 2000 years. First, the paper reviews the main natural archives and proxies used to infer past hydroclimate variations in this remote region and outlines the difficulty of disentangling the moisture from the temperature signal in these records. Second, a comparison of two sets of hydroclimate records covering the Common Era from two data-rich regions, North America and Fennoscandia, reveals inter- and intra-regional differences. Third, building on earlier work, this paper shows the potential for providing a high-resolution hydroclimate reconstruction for the Arctic and a comparison with last-millennium simulations from fully coupled climate models. In general, hydroclimate proxies and simulations indicate that the Medieval Climate Anomaly tends to have been wetter than the Little Ice Age (LIA), but there are large regional differences. However, the regional coverage of the proxy data is inadequate, with distinct data gaps in most of Eurasia and parts of North America, making robust assessments for the whole Arctic impossible at present. To fully assess pan-Arctic hydroclimate variability for the last 2 millennia, additional proxy records are required.

Here, we present results from the most comprehensive compilation of Holocene peat soil properties with associated carbon and nitrogen accumulation rates for northern peatlands. Our database consists of 268 peat cores from 215 sites located north of 45°N. It encompasses regions within which peat carbon data have only recently become available, such as the West Siberia Lowlands, the Hudson Bay Lowlands, Kamchatka in Far East Russia, and the Tibetan Plateau. For all northern peatlands, carbon content in organic matter was estimated at 42 ± 3% (standard deviation) for Sphagnum peat, 51 ± 2% for non- Sphagnum peat, and at 49 ± 2% overall. Dry bulk density averaged 0.12 ± 0.07 g/cm 3 , organic matter bulk density averaged 0.11 ± 0.05 g/cm 3 , and total carbon content in peat averaged 47 ± 6%. In general, large differences were found between Sphagnum and non- Sphagnum peat types in terms of peat properties. Time-weighted peat carbon accumulation rates averaged 23 ± 2 (standard error of mean) g C/m 2 /yr during the Holocene on the basis of 151 peat cores from 127 sites, with the highest rates of carbon accumulation (25–28 g C/m 2 /yr) recorded during the early Holocene when the climate was warmer than the present. Furthermore, we estimate the northern peatland carbon and nitrogen pools at 436 and 10 gigatons, respectively. The database is publicly available at https://peatlands.lehigh.edu .

Andersson, S. & Schoning, K. 2010: Surface wetness and mire development during the late Holocene in central Sweden. Boreas, Vol. 39, pp. 749–760. 10.1111/j.1502‐3885.2010.00157.x. ISSN 0300‐9483.Late Holocene mire development and surface wetness changes have been studied in a small mixed mire located in central Sweden. Today the mire is characterized by a mainly ombrotrophic centre dominated by Sphagnum mosses, with Carex content increasing towards the more minerotrophic mire margins. Two peat sequences extracted from the central ombrotrophic part were investigated for stratigraphy, humification, testate amoebae analysis, C/N ratio and δ13C and δ15N stable isotopes. Three main stages of mire development are identified, with the first stage, between c. 4200 and 2600 cal. yr BP, characterized by water‐logged conditions, suggesting a minerotrophic fen stage. The second stage, between c. 2600 and 1000 cal. yr BP, is characterized by more ombrotrophic conditions and Sphagnum‐dominated vegetation. The onset of the prominent change at c. 2600 cal. yr BP could have been initiated by climate change coincident with a change in solar activity. The last stage, between c. 1000 and 50 cal. yr BP, is dominated by more ombrotrophic conditions, suggesting increased precipitation. This study shows that the response of hydrological proxies in a mixed mire during its development towards more ombrotrophic conditions might result in conflicting results, a finding that needs to be considered in palaeoenvironmental reconstructions from mires that change between ombrotrophic and minerotrophic settings.

Biological and geochemical proxies from two lakes and two peat sites in central Sweden are used to test if the inferred climate change can be connected to previously reported temperature anomalies, ...

Stable isotopes (δ18O and δ13C) of lacustrine carbonates (Chara spp. algae and Pisidium spp. molluscs) from a lake sedimentary sequence in central Sweden were analysed to infer changes in lake hydrology and climate during the late Holocene. Results from analysis of lake water isotopes (δ18O and δ2H) show that Lake Blektjärnen water isotope composition is responsive to the balance between evaporation and input water (E/I ratio). A high E/I ratio results from a dry and probably warmer climate, decreasing the relative importance of precipitation input. Under such conditions evaporation and atmospheric equilibration probably enrich lake water in 18O and 13C, respectively, which is reflected in the isotopic composition of the carbonates in the lake. From the relatively positive Chara δ18O values we infer that conditions were dry and warm between 4400 and 4000 cal. a BP, whereas more negative values indicate that conditions were wetter and probably cooler between 4000 and 3000 cal. a BP. A drier climate is inferred from more positive values between 2500 and 1000 cal. a BP. However, a successive depletion after ca. 1750 cal. a BP, also detected in several other δ18O records (carbonate and diatom), suggest increasingly wetter conditions in Scandinavia after that time, which is probably related to increased strength of the zonal flow. Copyright © 2010 John Wiley & Sons, Ltd.

A mid-Holocene tephra, LBA-2 has been found in a peat bog in central Sweden. Geochemical analyses suggest an origin in an evolved volcanic centre, most likely the Snaefellsnes volcanic centre in western Iceland. The geochemistry of the LBA-2 tephra is similar to the youngest of three silicic tephra layers from Snaefellsjokull, Sn-1 dated to 1780 cal yr BP. However, wiggle-match dating indicates an age of 3550-3650 cal yr BP, close in age to the Hekla-S/Kebister tephra (3720 cal yr BP), previously found in several sites in Scandinavia. Detailed geochemical analyses and dating is lacking for the mid-Holocene Sn-2 tephra in Iceland but it is likely that the geochemical composition is similar as the Sn-1 tephra. The LBA-2 tephra is tentatively correlated with the Sn-2 tephra and we also suggest that the 'x' tephra layer (ca 3500 BP) found in the Dyngjufjoll area, central Iceland (Sigvaldason et al. 1992) can be correlated with the Sn-2 tephra.

The middle Holocene Hekla-S/Kebister tephra originates in the Hekla volcanic system on SW Iceland. The distal distribution of the tephra includes the Faroe Islands, Shetland and Central Sweden, indicating a main dispersal towards the east. The chemical composition of the tephra follows the pattern of other major eruptions of Hekla, and ratios between selected oxides may in some cases allow separation from other major Holocene tephras from Hekla. Tephra from the Plinian phase dominates in eastern sites, while tephra also from later phases is found in the Faroe sites. Wiggle-matching of radiocarbon dates around the tephra in a Swedish peat-bog suggests an age around 3720 cal. yr BP (3750—3700 cal. yr BP), which is in accordance with previous attempts to date this tephra. This is within a period with significant climate changes in NW Europe and opens possibilities for exact comparisons of peat and lake sediment records from different geographical areas.