A late Quaternary chironomid-inferred temperature record from the Sierra Nevada, California, with connections to northeast Pacific sea surface temperatures

Chironomid remains from a mid-elevation lake in the Sierra Nevada, California, were used to estimate quantitative summer surface water temperatures during the past ∼15,000 yr. Reconstructed temperatures increased by ∼3°C between lake initiation and the onset of the Holocene at ∼10,600 cal yr BP (calibrated years before present). Temperatures peaked at 6500 cal yr BP, displayed high variability from 6500 to 3500 cal yr BP, and stabilized after 3500 cal yr BP. This record generally tracks reconstructed Santa Barbara Basin sea surface temperatures (SSTs) through much of the Holocene, highlighting the correspondence between SST variability and California land temperatures during this interval.     

Late Pleistocene Glaciations in the Northwestern Sierra Nevada, California

Pleistocene fluvial landforms and riparian ecosystems in central California responded to climate changes in the Sierra Nevada, yet the glacial history of the western Sierra remains largely unknown. Three glacial stages in the northwestern Sierra Nevada are documented by field mapping and cosmogenic radionuclide surface-exposure (CRSE) ages. Two CRSE ages of erratic boulders on an isolated till above Bear Valley provide a limiting minimum age of 76,400±3800 ¹⁰Be yr. Another boulder age provides a limiting minimum age of 48,800±3200 ¹⁰Be yr for a broad-crested moraine ridge within Bear Valley. Three CRSE ages producing an average age of 18,600±1180 yr were drawn from two boulders near a sharp-crested bouldery lateral moraine that represents an extensive Tioga glaciation in Bear Valley. Nine CRSE ages from striated bedrock along a steep valley transect average 14,100±1500 yr and suggest rapid late-glacial ice retreat from lower Fordyce Canyon with no subsequent extensive glaciations. These ages are generally consistent with glacial and pluvial records in east-central California and Nevada.     

Late Holocene vegetation and climate oscillations in the Qaidam Basin of the northeastern Tibetan Plateau

Pollen evidence from sediment cores at Hurleg and Toson lakes in the Qaidam Basin was obtained to examine vegetation and climatic change in the northeastern Qinghai-Tibetan Plateau. The chronologies were controlled by ²¹⁰Pb and ¹³⁷Cs analysis and AMS ¹⁴C dating. Pollen assemblages from both lakes are dominated by Chenopodiaceae (∼ 40%), Artemisia (∼ 30-35%) and Poaceae (∼ 20-25%), with continued occurrence but low abundance of Nitraria, Ephedra, and Cyperaceae. Artemisia/Chenopodiaceae (A/C) pollen ratios from two lakes show coherent large oscillations at centennial timescale during the last 1000 yr. A/C ratios were high around AD 1170, 1270, 1450, 1700 and 1920, suggesting that the vegetation was more “steppe-like” under a relatively moist climate than that during the intervening periods. Wet-dry climate shifts at the two lakes (2800 m asl) are in opposite phases to precipitation changes derived from tree-ring records in the surrounding mountains (> 3700 m asl) and to pollen and snow accumulation records from Dunde ice core (5300 m asl), showing that a dry climate in the basin corresponds with a wet interval in the mountains, especially around AD 1600. This contrasting pattern implies that topography might have played an important role in mediating moisture changes at regional scale in this topographically complex region.     

Holocene environmental change and vegetation contraction in the Sonoran Desert

Two types of microtopographic features (plant scar mounds and plant scar depressions) on surfaces of barren desert pavements provide a unique record of the former presence of large perennial plants. Evidence of bioturbation by burrowing animals extends more than 1 m beneath each type of plant scar, indicating that both features originated as large bioturbation mounds. Formation of bioturbation mounds in desertscrub environments is generally restricted to areas beneath widely separated, large perennial plants. The contrasting forms of plant scars (mounds vs. depressions) represent time-dependent changes following disappearance of the large plants and eventual cessation of bioturbation. Plant scar mounds represent a geologically recent episode of plant mortality, whereas plant scar depressions represent the disappearance of plants at a considerably earlier time, possibly at the Pleistocene-Holocene transition. Contrasting spatial distributions of the two kinds of plant scars indicate that vegetation on alluvial fans has progressively contracted from a more diffuse, former vegetation cover, yielding the wide, barren pavement surfaces present today. In less arid portions of the Sonoran Desert, spatial distribution of recent plant mortality due to persistent, severe drought provides an analog of the progressive loss of plants from different parts of the landscape in the past.     

Holocene beaver damming, fluvial geomorphology, and climate in Yellowstone National Park, Wyoming

We use beaver-pond deposits and geomorphic characteristics of small streams to assess long-term effects of beavers and climate change on Holocene fluvial activity in northern Yellowstone National Park. Although beaver damming has been considered a viable mechanism for major aggradation of mountain stream valleys, this has not been previously tested with stratigraphic and geochronologic data. Thirty-nine radiocarbon ages on beaver-pond deposits fall primarily within the last 4000 yr, but gaps in dated beaver occupation from ~ 2200–1800 and 950–750 cal yr BP correspond with severe droughts that likely caused low to ephemeral discharges in smaller streams, as in modern severe drought. Maximum channel gradient for reaches with Holocene beaver-pond deposits decreases with increasing basin area, implying that stream power limits beaver damming and pond sediment preservation. In northern Yellowstone, the patchy distribution and cumulative thickness of mostly < 2 m of beaver-pond deposits indicate that net aggradation forced by beaver damming is small, but beaver-enhanced aggradation in some glacial scour depressions is greater. Although 20th-century beaver loss and dam abandonment caused significant local channel incision, most downcutting along alluvial reaches of the study streams is unrelated to beaver dam abandonment or predates historic beaver extirpation.     

Reconstruction of the long-term fire history of an old-growth deciduous forest in Southern Québec, Canada, from charred wood in mineral soils

Charcoal particles are widespread in terrestrial and lake environments of the northern temperate and boreal biomes where they are used to reconstruct past fire events and regimes. In this study, we used botanically identified and radiocarbon-dated charcoal macrofossils in mineral soils as a paleoecological tool to reconstruct past fire activity at the stand scale. Charcoal macrofossils buried in podzolic soils by tree uprooting were analyzed to reconstruct the long-term fire history of an old-growth deciduous forest in southern Québec. Charcoal fragments were sampled from the uppermost mineral soil horizons and identified based on anatomical characters. Spruce (Picea spp.) fragments dominated the charcoal assemblage, along with relatively abundant wood fragments of sugar maple (Acer saccharum) and birch (Betula spp.), and rare fragments of pine (Pinus cf. strobus) and white cedar (Thuja canadensis). AMS radiocarbon dates from 16 charcoal fragments indicated that forest fires were widespread during the early Holocene, whereas no fires were recorded from the mid-Holocene to present. The paucity of charcoal data during this period, however, does not preclude that a fire event of lower severity may have occurred. At least eight forest fires occurred at the study site between 10,400 and 6300 cal yr B.P., with a dominance of burned conifer trees between 10,400 and 9000 cal yr B.P. and burned conifer and deciduous trees between 9000 and 6300 cal yr B.P. Based on the charcoal record, the climate at the study site was relatively dry during the early Holocene, and more humid from 6300 cal yr B.P. to present. However, it is also possible that the predominance of conifer trees in the charcoal record between 10,400 and 6300 cal yr B.P. created propitious conditions for fire spreading. The charcoal record supports inferences based on pollen influx data (Labelle, C., Richard, P.J.H. 1981. Végétation tardiglaciaire et postglaciaire au sud-est du Parc des Laurentides, Québec. Géographie Physique et Quaternaire 35, 345-359) of the early arrival of spruce and sugar maple in the study area shortly after deglaciation. We conclude that macroscopic charcoal analysis of mineral soils subjected to disturbance by tree uprooting may be a useful paleoecological tool to reconstruct long-term forest fire history at the stand scale.     

Molluscs as evidence for a late Pleistocene and early Holocene humid period in the southern coastal desert of Peru (14.5°S)

The southern Peruvian coastal desert around Palpa, southern Peru (14.5°S) is currently characterized by hyper-arid conditions. However, the presence of two species of molluscs (Scutalus, Pupoides) and desert-loess deposits indicates the past development of semi-desert and grassland ecosystems caused by a displacement of the eastern desert margin due to hydrological changes. Radiocarbon dating shows that the transition to a semi-arid climate in the southern Peruvian coastal desert took place during the Greenland interstadial 1, ∼ 13.5 cal ka BP. At the beginning of the Holocene, the mollusc fauna vanished due to increasing humidity and the development of grasslands. Dust particles were fixed by the grasses, as indicated by abundant Poaceae phytoliths, and desert loess was formed. The humid period we observe here is out of phase with the palaeoenvironmental records from the Titicaca region, which indicates dry conditions at that time. This paper offers a new idea for this contradiction: an orbitally driven meridional shift of the Bolivian high might have altered the moisture supply across the Andes.