Interglacial extension of the boreal forest limit in the Noatak Valley, Northwest Alaska: evidence from an exhumed river-cut bluff and debris apron

Numerous exposures of Pleistocene sediments occur in the Noatak basin, which extends for 130 km along the Noatak River in northwestern Alaska. Nk-37, an extensive bluff exposure near the west end of the basin, contains a record of at least three glacial advances separated by interglacial and interstadial deposits. An ancient river-cut bluff and associated debris apron is exposed in profile through the central part of Nk-37. The debris apron contains a rich biotic record and represents part of an interglaciation that is probably assignable to marine-isotope stage 5. Pollen spectra from the lower part of the debris apron closely resemble modern samples taken from the Noatak floodplain in spruce gallery forest, and macrofossils of spruce are also present at this level. Fossil bark beetles and carpenter ants occur higher in the debris apron. Mutual Climatic Range (MCR) estimates from the fossil beetles suggest temperatures similar to or warmer than today. Together, these fossils indicate the presence of an interglacial spruce forest in the western part of the Noatak Basin, which lies about 80 km upstream of the modern limit of spruce forest

Tundra and boreal forest of interior Alaska during terminal MIS 6 and MIS 5e

Two sites within the boreal forest of interiorAlaska shed light on the climate and vegetation of terminalmarine isotope stage (MIS) 6 (ca. 140–130 kyr ago) andMIS 5e (125–116 kyr ago). The Birch Creek and Koyukuklocalities are river-cut exposures with sediments datingfrom the penultimate glaciation (at least) to the present.Plant macrofossils, pollen, and beetles were analyzed atthese sites. Terminal MIS 6-aged samples indicate a coolerthan modern climate and the presence of shrub tundra.During MIS 5e, boreal forest grew at the sites and temperatureswere similar to modern times. However, theforest may also have been more mesic than today, asindicated by relatively abundant ferns. Winters may havebeen warmer than today, as suggested by beetle-basedclimatic reconstructions as well as the presence of twoextralimital taxa that today live in regions where wintertemperatures are up to 15 ?C warmer than at the sitelocalitie

Archéologie du cap Espenberg ou la question du Birnirk et de l’origine du Thule dans le nord-ouest de l’Alaska

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BrGDGT temperature reconstruction from interior Alaska: Assessing 14,000 years of deglacial to Holocene temperature variability and potential effects on early human settlement

Aeolian deposits in the middle Tanana Valley of central Alaska offer a well-preserved record of paleoenvironmental change since the deglacial period (c. 16,000–11,000 cal yr BP). These deposits also contain some of North America's oldest archaeological occupations (c. 14,000–13,000 cal yr BP), making this region critically important for understanding human migration into the high latitudes and the Americas. Major research questions involve assessing the magnitude of deglacial climatic change and the influence of climate on early human groups. This study uses branched glycerol dialkyl glycerol tetraethers (brGDGTs) from six loess-paleosol sequences to develop a quantitative paleotemperature record within terrestrial locations in the Tanana basin that are close to archaeological sites. BrGDGT-derived temperatures demonstrate a lack of cooler temperatures associated with deglacial conditions, making this region relatively “warm” compared to other parts of the globe. Additionally, our brGDGT record shows little coherent temperature change associated with deglacial climate variability (e.g., Bølling-Allerød, Younger Dryas), and Holocene temperatures are relatively stable as well, indicating that temperature fluctuation was not the main driver of environmental or archaeological change over time. We recommend averaging data across multiple terrestrial exposures to produce regional temperature reconstrutions. © 2023Open access articleThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Climate change and Arctic ecosystems: 2. Modeling, paleodata-model comparisons, and future projections

Large variations in the composition, structure, and function of Arctic ecosystems are determined by climatic gradients, especially of growing-season warmth, soil moisture, and snow cover. A unified circumpolar classification recognizing five types of tundra was developed. The geographic distributions of vegetation types north of 55°N, including the position of the forest limit and the distributions of the tundra types, could be predicted from climatology using a small set of plant functional types embedded in the biogeochemistry-biogeography model BIOME4. Several palaeoclimate simulations for the last glacial maximum (LGM) and mid-Holocene were used to explore the possibility of simulating past vegetation patterns, which are independently known based on pollen data. The broad outlines of observed changes in vegetation were captured. LGM simulations showed the major reduction of forest, the great extension of graminoid and forb tundra, and the restriction of low- and high-shrub tundra (although not all models produced sufficiently dry conditions to mimic the full observed change). Mid-Holocene simulations reproduced the contrast between northward forest extension in western and central Siberia and stability of the forest limit in Beringia. Projection of the effect of a continued exponential increase in atmospheric CO2 concentration, based on a transient ocean-atmosphere simulation including sulfate aerosol effects, suggests a potential for larger changes in Arctic ecosystems during the 21st century than have occurred between mid-Holocene and present. Simulated physiological effects of the CO2 increase (to &gt;700 ppm) at high latitudes were slight compared with the effects of the change in climate. <br/

Leaf area index for northern and eastern North America ad the Last Glacial Maximum: A data-model comparison

Aim To estimate the effects of full-glacial atmospheric CO2 concentrations and climate upon leaf area index (LAI), using both global vegetation models and palaeoecological data. Prior simulations indicate lowered LAIs at the Last Glacial Maximum (LGM), but this is the first attempt to corroborate predictions against observations. Location Eastern North America and eastern Beringia. Methods Using a dense surface pollen data set and remotely sensed LAIs from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument, we evaluate the ability of analogue-based techniques to reconstruct modern LAIs from pollen data. We then apply analogue techniques to LGM pollen records, calculate the ratio of LGM to modern LAIs (RLAI) and compare reconstructed RLAIs to RLAIs simulated by BIOME4. Sensitivity experiments with BIOME4 distinguish the effects of CO2 and climate on glacial LAIs. Results Modern LAIs are skilfully predicted (r2 = 0.83). Data and BIOME4 indicate that LAIs at the LGM were up to 12% lower than modern values in eastern North America and 60–94% lower in Beringia. In eastern North America, LGM climates partially counteracted CO2-driven decreases in LAI, while in Beringia both contributed to lowered LAIs. Main conclusions In both regions climate is the primary driver of LGM LAIs. The decline in eastern North America LAIs is smaller than previously reported, so regional vegetation feedbacks to LGM climate may have been less significant than previously supposed. CO2 exerts both physiological and community effects upon LAI, by regulating resource availability for leaf production and by influencing the competitive balance among species and hence the composition and structure of plant communities. Pollen-based reconstructions using analogue methods do not incorporate the physiological effect and so are upper estimates of full-glacial LAIs. BIOME4 sensitivity experiments indicate that the community and physiological effects together caused 10% to 20% decrease in LAIs at the LGM, so simulated RLAIs that are 80–100% of reconstructed RLAIs are regarded as consistent with data