Abstract. 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.

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Using biogeochemical analyses of sediments and porewaters, we investigate the legacy of a brief, intense period of eutrophication on sedimentary phosphorus (P) cycling in a boreal lake (Enonselkä basin, Lake Vesijärvi, Finland). Point-source sewage inputs in the twentieth century caused deoxygenation of the lake and accelerated the focusing of iron (Fe) and manganese (Mn) oxides into deeper areas. Early diagenesis under Fe–Mn-rich conditions now favors rapid burial of P in these areas, likely as a combination of both oxide-bound P phases and authigenic manganous vivianite. A new P budget for Enonselkä basin shows that P burial causes an annual drawdown of 1.2% (± 0.2%) of the surface sediment P inventory, supporting a long-term trend towards recovery since the construction of a wastewater treatment plant in the mid-1970s. However, remineralization of organic matter and associated dissolution of Fe–Mn oxides continues to regenerate P from a deep reactive layer (20–60 cm depth) deposited at the height of past eutrophication, leading to an upwards diffusive flux of dissolved phosphate towards the surface sediments. The magnitude of this flux is similar to that of external P loading to the lake. The combined incoming fluxes of P are likely to retard the complete recovery from eutrophication by decades, despite ongoing restoration actions.

Clastic-organic varved sediments from the boreal Lake Kalliojärvi, Central Finland, record changes in snow accumulation for more than 4000 years. The varve record was reconstructed using digital image analysis from 4132 varve yr BP to present with 2.2% counting error and is supported by paleomagnetic data. Two laminae are identified in a typical varve structure: (1) the minerogenic lamina, which accumulates during spring as a result of catchment erosion triggered by spring floods and (2) the organic lamina, which is composed of allochthonous and autochthonous organic matter that accumulates during summer, autumn, and winter. The minerogenic influx is related to variations in snow accumulation and follows the different phases of the North Atlantic Oscillation (NAO). Decreased snow accumulation is related to a weakened NAO phase. Thus, the minerogenic influx record provides additional information about NAO variation. The Fe/Mn ratio is related to changes in redox conditions at the lake floor. The oxygen availability in the lake floor depends on the duration of the ice cover during winter. Strengthened Siberian High (SH) causes colder autumn and winter temperatures and therefore leads to extended duration of ice cover. Fe/Mn can be considered as a proxy for SH. The sediment record suggests pronounced, generally positive but strongly fluctuating NAO phase, from ca. 4100 to 3200 varve yr BP. Periods of strengthened SH are observed at 3900–3600, 1900–1500, and 1200–750 varve yr BP. Our study suggests that NAO and SH operate individually, are not clearly linked, and do not entirely block each other.

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Clastic-biogenic varves are widely used for reconstructing past climate: in boreal environments, the accumulation of minerogenic clasts on the lake floor is generally considered a proxy for past variations in spring floods reflecting previous winter conditions. However, the physical mechanisms behind this winter climate sensitivity and the influence of catchment type on the varve formation are not fully investigated.Here, we present a winter climate record inferred from the clastic laminae of three lakes located on the region of fine-grained tills in eastern Finland spanning from AD 1890 to 1990. The minerogenic varve data is compared with instrumental meteorological and hydrological time series in order to investigate the physical link between winter and spring climate and minerogenic matter accumulation. Our analyses reveal that the climate-catchment mechanisms operating in the region of fine-grained tills in eastern Finland differ crucially from previously described climate catchment interactions on sand moraine-dominated catchments in central Finland. Usually the maximum river discharge in spring controls the clastic lamina formation. However, in contrast to earlier boreal varve records from central Finland, the clastic lamina formation in the studied region correlates negatively with spring temperatures and winter precipitation. This could be an artefact of varying catchment dynamics but also related to the regional climate. The lakes surrounded by catchments characterized by fine-grained tills are more sensitive to cold and dry winters. The differences in the sensitivity of varve characteristics to climate, highlights the importance of understanding the catchment dynamics in detail in order to better understand climatic forcing.

Sediment microfacies, geochemical l-XRFand X-ray density analyseswere conducted onvarved sediments from Lake Kortej€ arvi (eastern Finland) covering the last 2700 years. The varves comprise couplets of detrital and organic sublayers throughout the complete time-span. Based on their microfacies and stratigraphical position within a varve as well as comparisons with local discharge and meteorological data, thicker detrital layers are interpreted to reflect intensified snow-melt floods following more humid winters. Detailed comparisons with monthly to annually resolved North Atlantic Oscillation (NAO) indices back to AD 1049 (901 a BP) suggest that multidecadal increases in snowmelt layer thickness tend to be connectedwith a more positive phase of the NAO and, consequently, warmer winters. In contrast, distinct centennial intervals of thicker snow-melt layers from À40 to 170, 280 to 460 and 1900 to 2300 a BP as well as around 2600 a BP do not consistently correspond to a particular NAO phase, but coincide with extended sea-ice margins and a colder North Atlantic climate, causing intensified and southward shifted westerly cyclones. Our results point to a differential modification of North Atlantic winter hydroclimate working on varying time scales.