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Abstract. Open questions remain around the Holocene variability of climate in Iceland, including the relative impacts of natural and anthropogenic factors on Late Holocene vegetation change and soil erosion. The lacustrine sediment record from Torfdalsvatn, north Iceland, is the longest known in Iceland (≤12000 cal a BP) and along with its high sedimentation rate, provides an opportunity to develop high-resolution quantitative records that address these challenges. In this study, we use two sediment cores from Torfdalsvatn to construct a high-resolution age model derived from marker tephra layers, paleomagnetic secular variation, and radiocarbon. We then apply this robust age constraint to support a complete tephrochronology (>2200 grains analyzed in 33 tephra horizons) and sub-centennial geochemical (MS, TOC, C/N, δ13C, and BSi) and algal pigment records. Along with previously published proxy records from the same lake, these records demonstrate generally stable terrestrial and aquatic conditions during the Early and Middle Holocene, except for punctuated disturbances linked to major tephra fall events. During the Late Holocene, there is strong evidence for naturally driven algal productivity decline beginning around 1800 cal a BP. These changes closely follow regional Late Holocene cooling driven by decreases in Northern Hemisphere summer insolation and the expansion of sea-ice laden Polar Water around Iceland. Then at 880 cal a BP, ~200 years after the presumed time of human settlement, a second shift in the record begins and is characterized by a strong uptick in landscape instability and possibly soil erosion. Collectively, the Torfdalsvatn record highlights the resilience of low-elevation, low-relief catchments to the pre-settlement soil erosion in Iceland, despite a steadily cooling background climate. The precisely dated, high-resolution tephra and paleoenvironmental record from this site can serve as a regional template for north Iceland. 

This dataset contains the raw data associated with the manuscript entitled: Aquatic Moss δ18O as a Proxy for Seasonally Resolved Lake Water δ18O, Northwest Greenland (Puleo et al., 2024). Reconstructing past climate seasonality is fundamental to understanding the nature of past climate changes. This is especially true in the Arctic, where climate is intensely seasonal and proxies that can distinguish climate conditions of multiple seasons in a single year are relatively rare. We propose that submerged aquatic mosses, which are abundant subfossils in some Arctic lake sediments and have distinctive seasonal growth morphologies, can be used to estimate past lake water oxygen isotope composition (δ18Olw) across multiple seasons. Aquatic mosses are abundant, well preserved, and grow continuously in Arctic lakes whenever light is available, with some species displaying unique seasonal morphologies influenced by water temperature. Although Greenland paleorecords support that aquatic moss oxygen isotope values (δ18Oom) reflect the δ18O values of lake water, no modern calibration between δ18Oom and δ18Olw exists in Greenland, as aquatic moss samples are composed largely, but not entirely, of cellulose. We present a modern δ18Oom vs. δ18Olw calibration using multiple moss species or morphotypes from eight lakes and ponds near Pituffik (Thule), northwest Greenland. We find strong linear relationships between the δ18Oom and δ18Olw values of multiple species or morphotypes across the range of relatively low δ18Olw values at Pituffik, and our results indicate isotopic fractionations are similar to those found previously at lower latitudes. To assess the potential of mosses as archives of seasonal δ18Olw values, we analyzed δ18Oom in season-specific segments of moss strands, with seasons identified based upon growth morphology. Moss inferred lake water δ18O values (δ18Olwom) are higher in autumn than spring or summer, likely due to increasing contributions of isotopically heavier precipitation and the cumulative effects of lake water evaporation throughout the ice-free season. For moss subsampled throughout summer, δ18Olwom values generally increased through the season in parallel with observed δ18Olw values. Potential temperature dependent fractionation effects during biosynthesis, however, remain unconstrained and should be further addressed with future research. Overall, these findings suggest that aquatic mosses from lake sediments could be used to directly resolve climate seasonality of the past. Puleo, P.J.K., Akers, P.D., Kopec, B.G., Welker, J.M., Bailey, H., Osburn, M.R., Riis, T., Axford, Y., 2024. Aquatic moss δ18O as a proxy for seasonally resolved lake water δ18O, northwest Greenland. Quaternary Science Reviews 334, 1-11. 

Abstract. As global warming progresses, changes in high-latitude precipitation are expected to impart long-lasting impacts on Earth systems, including glacier mass balance and ecosystem structures. Reconstructing past changes in high-latitude precipitation and hydroclimate from networks of continuous lake records offers one way to improve forecasts of precipitation and precipitation–evaporation balances, but these reconstructions are currently hindered by the incomplete understanding of controls on lake and soil water isotopes. Here, we study the distribution of modern water isotopes in Icelandic lakes, streams, and surface soils collected in 2002, 2003, 2004, 2014, 2019, and 2020 to understand the geographic, geomorphic, and environmental controls on their regional and interannual variability. We find that lake water isotopes in open-basin (through-flowing) lakes reflect local precipitation, with biases toward the cold season, particularly in lakes with sub-annual residence times. Closed-basin lakes have water isotope and deuterium excess values consistent with evaporative enrichment. Interannual and seasonal variabilities of lake water isotopes at repeatedly sampled sites are consistent with instrumental records of winter snowfall; summer relative humidity; and atmospheric circulation patterns, such as the North Atlantic Oscillation. Summer surface soil water isotopes span the entire range of seasonal precipitation values in Iceland and appear to be consistently overprinted by evaporative enrichment, which can occur throughout the year, although the sampling depths were shallower than rooting depths for many plant types. This dataset provides new insight into the functionality of water isotopes in Icelandic environments and offers renewed possibilities for optimized site selection and proxy interpretation in future paleohydrological studies on this North Atlantic outpost. 

Abstract. Greenland Ice Sheet (GrIS) outlet glaciers are currently losing mass, leading to sea level rise. Reconstructions of past outlet glacier behavior through the Holocene help us better understand how they respond to climate change. Kiattuut Sermiat, a southern Greenland outlet glacier near Narsarsuaq, is known to have experienced an unusually large Late Holoceneadvance that culminated at ∼1600 cal yr BP and exceeded theglacier's Little Ice Age extent. We report sedimentary records from twolakes at slightly different elevations in an upland valley adjacent toKiattuut Sermiat. These reveal when the outlet glacier's surface elevationwas higher than during the Little Ice Age and constrain the associatedoutlet glacier surface elevation. We use bulk sediment geochemistry,magnetic susceptibility, color, texture, and the presence of aquatic plantmacrofossils to distinguish between till, glaciolacustrine sediments, andorganic lake sediments. Our 14C results above basal till recordingregional deglaciation skew slightly old due to a reservoir effect but aregenerally consistent with regional deglaciation occurring ∼ 11 000 cal yr BP. Neoglacial advance of Kiattuut Sermiat is recorded by deposition of glaciolacustrine sediments in the lower-elevation lake, which we infer was subsumed by an ice-dammed lake that formed along the glacier's margin just after ∼ 3900 cal yr BP. This timing is consistent with several other glacial records in Greenland showing neoglacial cooling driving advance between ∼ 4500–3000 cal yr BP. Given that glaciolacustrine sediments were deposited only in the lower-elevation lake, combined with glacial geomorphological evidence in the valley containing these lakes, we estimate the former ice margin's elevation to have been ∼ 670 m a.s.l., compared with ∼ 420 m a.s.l. today. The ice-dammed lake persisted until the glacier surface fell below this elevation at ∼ 1600 cal yr BP. The retreat timing contrasts with overall evidence for cooling and glacier advance in the region at that time, so we infer that Kiattuut Sermiat's retreat may have resulted from reduced snowfall amounts and/or local glaciological complexity. High sensitivity to precipitation changes could also explain the relatively limited Little Ice Age advance of Kiattuut Sermiat compared with the earlier neoglacial advance. 

This dataset contains subfossil chironomid (Diptera: Chironomidae) species counts and the corresponding chironomid-inferred summer temperatures from a sediment core recovered from Lake N14 in southern Greenland. The record covers the period from approximately 13,800 to 9,900 years ago (cal BP). These data were generated for the study named below, which should be consulted for details and cited when using these data. Medeiros, A.S., Chipman, M., Francis, D.R., Hamerlik, L., Langdon, P., Puleo, P.J.K., Schellinger, G., Steigleder, R., Walker, I.R., Woodroffe, S., and Axford, Y. 2022. A continent-scale chironomid training set for reconstructing arctic temperatures. Quaternary Science Reviews 294, 107728. DOI 10.1016/j.quascirev.2022.107728. 

This dataset includes chironomid species assemblage data and air temperature estimates from 400+ lakes across northern North America, Greenland, Iceland, and Svalbard to inform interpretations of Holocene subfossil chironomid assemblages used in paleolimnological reconstruction. This calibration-set was developed by re-identifying and taxonomically harmonizing chironomids in previously described surface sediment samples, with identifications made at finer taxonomic resolution than in original publications (which are cited in the publication describing this dataset, Medeiros et al. 2022 Quaternary Science Reviews, and should be cited by dataset users). Site summer air temperatures are newly estimated with a consistent method using the WorldClim 2.1 gridded bioclimatic dataset. The large geographic coverage of this dataset is intended to provide climatic analogs for a wide range of Holocene climates in the northwest North Atlantic region and North American Arctic, including Greenland. For many of these regions, modern calibration data for paleoclimate proxies are sparse despite keen interest in paleoclimate reconstructions from high latitudes. Dataset users should consult and cite the following source publication: Medeiros, A.S., Chipman, M., Francis, D.R., Hamerlik, L., Langdon, P., Puleo, P.J.K., Schellinger, G., Steigleder, R., Walker, I.R., Woodroffe, S., and Axford, Y. 2022. A continent-scale chironomid training set for reconstructing arctic temperatures. Quaternary Science Reviews 294, 107728. DOI 10.1016/j.quascirev.2022.107728.