Griffiths et al. (2017) analyzed several ponds and lakes from the Cape Herschel region of Ellesmere Island in order to "...explicitly examine the role of ice cover as the dominant driver of diatom ...assemblage change...". I reanalyze their data and suggest that their classification scheme, that they propose is due to differences in ice cover seasonality ("warm", "cool", "cold", and "oasis"), is confounded with other morphological and chemical variables that better explain the differences between the groups. The "cold" sites are the deepest (lakes) and differ from the small, shallow ponds that occasionally dry, which would therefore have different diatom assemblages and histories. The "oasis" sites are nutrient enriched and probably have more stable water supplies, thereby enabling an aquatic flora providing habitats for diatoms. A key part of their interpretation is that "warm" sites have responded more rapidly to recent climate change than "cold" or "cool" sites, but their chronologies do not allow for such a conclusion. There is no clear difference between "cool" and "warm" sites, and problems in dating the sequences means inferences about their histories are not supported by data. Their results, which are restricted to the past century, are contradicted by a Holocene sequence from the region.
This synthesis paper summarizes published proxy climate evidence showing the spatial and temporal pattern of climate change through the Holocene in Arctic Canada and Greenland. Our synthesis includes ...47 records from a recently published database of highly resolved Holocene paleoclimate time series from the Arctic (Sundqvist et al., 2014). We analyze the temperature histories represented by the database and compare them with paleoclimate and environmental information from 54 additional published records, mostly from datasets that did not fit the selection criteria for the Arctic Holocene database. Combined, we review evidence from a variety of proxy archives including glaciers (ice cores and glacial geomorphology), lake sediments, peat sequences, and coastal and deep-marine sediments. The temperature-sensitive records indicate more consistent and earlier Holocene warmth in the north and east, and a more diffuse and later Holocene thermal maximum in the south and west. Principal components analysis reveals two dominant Holocene trends, one with early Holocene warmth followed by cooling in the middle Holocene, the other with a broader period of warmth in the middle Holocene followed by cooling in the late Holocene. The temperature decrease from the warmest to the coolest portions of the Holocene is 3.0 ± 1.0 °C on average (n = 11 sites). The Greenland Ice Sheet retracted to its minimum extent between 5 and 3 ka, consistent with many sites from around Greenland depicting a switch from warm to cool conditions around that time. The spatial pattern of temperature change through the Holocene was likely driven by the decrease in northern latitude summer insolation through the Holocene, the varied influence of waning ice sheets in the early Holocene, and the variable influx of Atlantic Water into the study region.
•A synthesis of existing literature on Holocene climate change in Arctic Canada and Greenland.•Warmest-to-coldest millennium temperature change in the Holocene in Arctic Canada and Greenland is 3.0 ± 1.0 °C.•Thermal maximum conditions occurred earlier in high latitudes and at sites less influenced by North Atlantic circulation.
A pollen profile from a lake with varved sediments sampled at continuous 10-year intervals and spanning the past 1000 years was analyzed to understand the effects of climate change and anthropogenic ...activity on forests in southwestern Quebec. Pollen assemblages were dominated by arboreal taxa, primarily Pinus, Tsuga, Betula and Fagus. Between 990 and 1560 AD, pollen accumulation rates and percentages of hardwoods (Betula, Fagus, Acer, Ulmus, Tilia) and Tsuga were relatively high. At 1560 AD, PARs of many hardwood taxa (Fagus, Acer, Betula, Fraxinus, Ulmus) and Tsuga abruptly decreased, some remaining low for the remainder of the record (Tsuga, Fagus, Acer), but others increasing after 50 years (Betula, Fraxinus). An increase in non-arboreal pollen between 1810 and 2010 AD was caused by European settlement of the area. The transition in the pollen assemblages beginning at 1560 AD and a climate reconstruction based on these data shows an abrupt climate cooling had a significant impact on the pollen accumulation rates of the region within a couple of decades. A synthesis of this record with other high-resolution and well-dated pollen data from the conifer-hardwood forest of eastern North America shows consistent results across the whole area, indicating that very-high resolution pollen data can provide insight into multi-decadal climate variability and its impact on forest vegetation.
•Medieval Warm Period and Little Ice Age impacted vegetation of southwest Quebec.•Transition into the Little Ice Age was abrupt within a couple of decades.•High-resolution pollen data record multi-decadal to century climate changes.
Anthropogenic stressors to freshwater environments have perpetuated water quality and quantity challenges for communities across Arctic Canada, making drinking water resources a primary concern for ...northern peoples. To understand the ecological trajectory of lakes used as freshwater supply, we conducted a paleolimnological assessment on two supplemental sources in Igloolik, Nunavut, Arctic Canada. A stratigraphic examination of biological indicators (Insecta: Diptera: Chironomidae) allowed for paleotemperature reconstructions with decadal and centennial resolution over the past 2000 years. Between 200 and 1900 CE, the sub-fossil chironomid community was comprised of cold-water taxa, such as Abiskomyia, Micropsectra radialis-type, and Paracladius. Reconstructed temperatures were consistent with known climate anomalies during this period. A rapid shift in the composition of the chironomid assemblages to those with higher temperature optima (Chironomus anthracinus-type, Dicrotendipes, and Tanytarsus lugens-type) in the late 20th century was observed in both systems. Our results demonstrate that these ecosystems are undergoing marked transformations to warmer, more nutrient-rich environments, and suggest that water sustainability pressures will likely continue in tandem with ongoing climate change. To contextualize the influence of recent warming and elucidate the status of freshwater resources over the longer term, paleolimnological methods can be usefully applied as components of vulnerability assessments.
Reconstructing climates of the past relies on a variety of evidence from a large number of sites to capture the varied features of climate and the spatial heterogeneity of climate change. This review ...summarizes available information from diverse Holocene paleoenvironmental records across eastern Beringia (Alaska, westernmost Canada and adjacent seas), and it quantifies the primary trends of temperature- and moisture-sensitive records based in part on midges, pollen, and biogeochemical indicators (compiled in the recently published Arctic Holocene database, and updated here to v2.1). The composite time series from these proxy records are compared with new summaries of mountain-glacier and lake-level fluctuations, terrestrial water-isotope records, sea-ice and sea-surface-temperature analyses, and peatland and thaw-lake initiation frequencies to clarify multi-centennial- to millennial-scale trends in Holocene climate change. To focus the synthesis, the paleo data are used to frame specific questions that can be addressed with simulations by Earth system models to investigate the causes and dynamics of past and future climate change. This systematic review shows that, during the early Holocene (11.7–8.2 ka; 1 ka = 1000 cal yr BP), rather than a prominent thermal maximum as suggested previously, temperatures were highly variable, at times both higher and lower than present (approximate mid-20th-century average), with no clear spatial pattern. Composited pollen, midge and other proxy records average out the variability and show the overall lowest summer and mean-annual temperatures across the study region during the earliest Holocene, followed by warming over the early Holocene. The sparse data available on early Holocene glaciation show that glaciers in southern Alaska were as extensive then as they were during the late Holocene. Early Holocene lake levels were low in interior Alaska, but moisture indicators show pronounced differences across the region. The highest frequency of both peatland and thaw-lake initiation ages also occurred during the early Holocene. During the middle Holocene (8.2–4.2 ka), glaciers retreated as the regional average temperature increased to a maximum between 7 and 5 ka, as reflected in most proxy types. Following the middle Holocene thermal maximum, temperatures decreased starting between 4 and 3 ka, signaling the onset of Neoglacial cooling. Glaciers in the Brooks and Alaska Ranges advanced to their maximum Holocene extent as lakes generally rose to modern levels. Temperature differences for averaged 500-year time steps typically ranged by 1–2 °C for individual records in the Arctic Holocene database, with a transition to a cooler late Holocene that was neither abrupt nor spatially coherent. The longest and highest-resolution terrestrial water isotope records previously interpreted to represent changes in the Aleutian low-pressure system around this time are here shown to be largely contradictory. Furthermore, there are too few records with sufficient resolution to identify sub-centennial-scale climate anomalies, such as the 8.2 ka event. The review concludes by suggesting some priorities for future paleoclimate research in the region.
•We summarize diverse Holocene paleoclimate records from Alaska and NW Canada.•Proxy types include midges, pollen, biogeochemistry, glaciers, isotopes, lake levels.•Early Holocene (11.5–8.0 ka) temperatures were highly variable, but cold on average.•Regional average temperature increased to a maximum between 7 and 5 ka.•Priorities for future paleoclimate research in the region are suggested.
Climatic changes during the late Quaternary have resulted in substantial, often abrupt, rearrangements of terrestrial ecosystems, but the relationship between these environmental changes and ...prehistoric human culture and population size remains unclear. Using a database of archaeological radiocarbon dates alongside a network of paleoecological records (sedimentary pollen and charcoal) and paleoclimatic reconstructions, we show that periods of cultural and demographic change in the northeastern United States occurred at the same times as the major environmental-climatic transitions of that region. At 11.6, 8.2, 5.4, and 3.0 kyr BP (10³ calendar years before present), changes in forest composition altered the distribution, availability, and predictability of food resources which triggered technological adjustments manifested in the archaeological record. Human population level has varied in response to these external changes in ecosystems, but the adoption of maize agriculture during the late Holocene also resulted in a substantial population increase. This study demonstrates the long-term interconnectedness of prehistoric human cultures and the ecosystems they inhabited, and provides a consolidated environmental-cultural framework from which more interdisciplinary research and discussion can develop. Moreover, it emphasizes the complex nature of human responses to environmental change in a temperate region.
As the Cordilleran and Laurentide Ice Sheets retreated, North America was colonized by human populations; however, the spatial patterns of subsequent population growth are unclear. Temporal frequency ...distributions of aggregated radiocarbon (14C) dates are used as a proxy of population size and can be used to track this expansion. The Canadian Archaeological Radiocarbon Database contains more than 35,00014C dates and is used in this study tomap the spatiotemporal demographic changes of Holocene populations in North America at a continental scale for the past 13,000 y. We use the kernel method, which converts the spatial distribution of14C dates into estimates of population density at 500-y intervals. The resulting maps reveal temporally distinct, dynamic patterns associated with paleodemographic trends that correspond well to genetic, archaeological, and ethnohistoric evidence of human occupation. These results have implications for hypothesizing and testing migration routes into and across North America as well as the relative influence of North American populations on the evolution of the North American ecosystem.
A high‐resolution diatom analysis of a varved sediment sequence from Lac Brûlé, southwestern Québec, was used to study temporal patterns of environmental change in the late Holocene. Key periods of ...interest in the record included the Medieval Warm Period (~800–1300 CE), the Little Ice Age (~1450–1850 CE), and post‐European settlement (~1850–present). Subfossil diatom assemblages were compared to previously published pollen, cladocera, and sediment records from Lac Brûlé, revealing complex dynamics between terrestrial vegetation succession, nutrient fluxes, and trophic interactions. Generalized additive models showed a response to long‐term climate variability in the diatom record, although it was not the most influential driver of community changes at Lac Brûlé. Catchment‐mediated processes instead played the largest role in governing the structure of diatom assemblages in the lake. For example, nutrient loading following a local fire in the watershed at 1345 CE led to an abrupt and significant increase in Fragilaria spp. Human activity associated with deforestation and the Wallingford‐Back Mine (1924–1972 CE) also had strong impacts on the landscape, which triggered further responses in the aquatic communities of Lac Brûlé.
A compilation of published and new limnological data from 1489 shallow lakes and ponds in northern Canada, sampled between 1979 and 2009, revealed significant patterns that correlated with landscape ...features and climate. Lakes and ponds underlain by Archean or Proterozoic bedrock had lower specific conductivity and pH. Vegetation cover had a lesser influence on these parameters. Forested landscapes tended to have higher phosphorus and nitrogen, as did younger rock types. Dissolved organic carbon was higher, but dissolved inorganic carbon was lower in forested regions. Phytoplankton biomass of the surface waters, as estimated by chlorophyll a concentrations, was positively correlated with July air temperature and nutrients, and was higher in forested relative to polar desert regions. There were no significant differences in the measured limnological variables between shallow (<2 m depth) and deep lakes (>2 m); however, all water chemistry parameters were negatively correlated with depth. Despite large variability within and among regions, spatial trends in water chemistry were associated with geology, vegetation, and climate at a continental scale.
High-elevation ecosystems, such as those on Mount Kenya are undergoing significant changes, with accelerated glacial ice losses over the twentieth century creating new space for alpine plants to ...establish. These ecosystems respond rapidly to climatic variability and within decades of glacial retreat, Afroalpine pioneering taxa stabilize barren land and facilitate soil development, promoting complex patches of alpine vegetation. Periglacial lake sediment records can be used to examine centennial and millennial scale variations in alpine and montane vegetation compositions. Here we present a 5300-year composite pollen record from an alpine tarn (4370 m asl) in the Hausberg Valley of Mount Kenya. Overall, the record shows little apparent variation in the pollen assemblage through time with abundant montane forest taxa derived and transported from mid elevations, notably high abundances of aerophilous Podocarpus pollen. Afroalpine taxa included Alchemilla, Helichrysum and Dendrosenecio-type, reflecting local vegetation cover. Pollen from the ericaceous zone was present throughout the record and Poaceae percentages were high, similar to other high elevation pollen records from eastern Africa. The Oblong Tarn record pollen assemblage composition and abundances of Podocarpus and Poaceae since the late Holocene (~4000 cal yr BP-present) are similar to pollen records from mid-to-high elevation sites of nearby high mountains such as Mount Elgon and Kilimanjaro. These results suggest a significant amount of uphill pollen transport with only minor apparent variation in local taxa. Slight decreasing trends in alpine and ericaceous taxonomic groups show a long-term response to global late Holocene cooling and a step decrease in rate of change estimated from the pollen assemblages at 3100 cal yr BP in response to regional hydroclimatic variability. Changes in the principal component axis scores of the pollen assemblage were coherent with an independent mid-elevation temperature reconstruction, which supported the strong influence of uphill pollen transport from montane forest vegetation and association between temperatures and montane vegetation dynamics. Pollen accumulation rates showed some variability related to minerogenic sediment input to the lake. The Oblong Tarn pollen record provides an indication of long term vegetation change atop Mount Kenya showing some decreases in local alpine and ericaceous taxa from 5300-3100 cal yr BP and minor centennial-scale variability of montane taxa from mid elevation forests. The record highlights potentials, challenges and opportunities for the use of proglacial lacustrine sediment to examine vegetation change on prominent mountain massifs.
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