Search

The inference of past temperatures from a sedimentary pollen record depends upon the stationarity of the pollen-climate relationship. However, humans have altered vegetation independent of changes to climate, and consequently modern pollen deposition is a product of landscape disturbance and climate, which is different from the dominance of climate-derived processes in the past. This problem could cause serious signal distortion in pollen-based reconstructions. In the north-central United States, direct human impacts have strongly altered the modern vegetation and hence the pollen rain since Euro-American settlement in the mid-19th century. Using instrumental temperature data from the early 1800 s from Fort Snelling (Minnesota), we assessed the signal distortion and bias introduced by using the conventional method of inferring temperature from pollen assemblages in comparison to a calibration set from pre-settlement pollen assemblages and the earliest instrumental climate data. The early post-settlement calibration set provides more accurate reconstructions of the 19th century instrumental record, with less bias, than the modern set does. When both modern and pre-industrial calibration sets are used to reconstruct past temperatures since AD 1116 from pollen counts from a varve-dated record from Lake Mina, Minnesota, the conventional inference method produces significant low-frequency (centennial-scale) signal attenuation and positive bias of 0.8-1.7 °C, resulting in an overestimation of Little Ice Age temperature and likely an underestimation of the extent and rate of anthropogenic warming in this region. However, high-frequency (annual-scale) signal attenuation exists with both methods. Hence, we conclude that any past pollen spectra from before Euro-American settlement in this region should be interpreted using a pre-Euro-American settlement pollen set, paired to the earliest instrumental climate records. It remains to be explored how widespread this problem is when conventional pollen-based inference methods are used, and consequently how seriously regional manifestations of global warming have been underestimated with traditional pollen-based techniques.

Pollen and diatom assemblages from well-dated sediment cores from three lakes forming a west-to-east transect across the boreal forest in northwest Ontario (Canada) were used to evaluate the timing and nature of the movement of the prairie-forest ecotone (PFE) across the Holocene. Changes in vegetation, temperature, and effective moisture were inferred from pollen and pollen-based transfer functions. Analyses indicated site-specific vegetational and climate changes across short spatial distances, with prolonged prairie-like conditions during the middle Holocene at the westernmost site. Increased reconstructed temperatures at this westernmost site occurred from ∼9000 to 3000 cal yr BP, alongside increases in diatom-inferred lake levels beginning at ∼6000 cal yr BP. The abundance of Quercus peaked concurrently with rising lake levels before declining to trace levels by ∼3000 cal yr BP. Increases in the abundance of non-arboreal pollen between ∼8500 and ∼4500 cal yr BP at the more eastern lakes suggest relatively delayed and truncated PFE influence, before the reestablishment of primarily boreal taxa by ∼4500 cal yr BP, coincident with diatom-inferred increases in lake levels. This study shows that the PFE moved both farther east and north than previously determined, but generally agrees with established patterns in vegetation from other studied regions along the PFE. [ABSTRACT FROM AUTHOR]

Descriptions of regional climate expression require data from multiple lakes, yet little is known of how variation in records within morphometrically complex lakes may affect interpretations. In northeast Ontario (Canada), this issue was addressed using records of pollen, pigments, and diatoms in three sediment cores from two small boreal lakes spanning the last ~6000 years. Pollen analysis suggested warm conditions between ~6000 and ~4000 cal yr BP, coherent with previous assessments from boreal eastern Ontario and western Quebec. Analysis of phototrophic communities from fossil pigments and diatom valves suggested relatively eutrophic conditions with lower lake-levels during this interval. Generalized additive model trends identified significant regional changes in pollen assemblages and declines in pigment concentrations after ~4000 cal yr BP consistent with cooler and wetter climate conditions that resulted in regional lake oligotrophication and increased lake levels during the late-Holocene. Despite contemporaneous changes in pollen and pigment biomarkers across lakes, cores collected from adjacent basins of the same lake (Green Lake) did not show similar trends in fossil pigments likely reflecting preferential deposition of clay-rich allochthonous material in the deeper central basin and suggesting that regional signals in climate may be complicated by lake- or basin-specific catchment processes. [ABSTRACT FROM AUTHOR]

Historical changes in Holocene climate in northeastern Ontario were quantified using analyses of sedimentary pollen, diatoms, and pigments in a small boreal lake. Modern analog reconstructions of average temperature from Holocene pollen assemblages of Charland Lake showed temperature was ~2°C warmer than present conditions ~7800–4500 cal. yr BP, a time period consistent with the Holocene thermal maximum (HTM). Pollen data suggest a two-phase HTM: warm and dry conditions based on the presence of primarily Pinus spp., followed by warm and wet conditions based on increases in cedar. Overall, algal production was low during the HTM, as reflected by low concentrations of pigments and diatoms. In the late HTM, increases in cedar pollen and planktonic diatoms suggest sustained increases in water levels for the remainder of the Holocene. During the Post-HTM Period (~4500–2000 cal. yr BP), a period that was warmer than today but cooler than the HTM, overall pigment production was significantly higher than all other periods. However, changes in diatom species composition suggest this period was not uniform, with variation occurring between diatoms indicative of higher and lower nutrient levels. The last ~2000 cal. yr BP was less productive than the Post-HTM Period but more productive than the HTM with higher production from diatoms and cyanobacteria. This study suggests that the relationship between climate and lake water production can be quite complex, and that changes in temperature, precipitation, light, lake levels, and mixing patterns are among factors that are related to changes in subfossil phototroph assemblages. [ABSTRACT FROM AUTHOR]

Abstract: Changes in vegetation were tracked from a well-dated sediment core from a boreal lake, Lake 239, at ∼200-year resolution over the Holocene. This presently oligotrophic lake is located ∼100-km east from the present-day parkland-forest ecotone in northwestern Ontario. Near-shore sediment core transects from Lake 239 have previously shown this lake was at least 8-m lower than present in the mid-Holocene, or ∼58% less lake volume in comparison to today. Large shifts were expected in the terrestrial vegetation if the low lake levels were related to climate. The core from Lake 239 shows increases in the relative abundance and concentration of pollen such as Cupressaceae and Ambrosia, indicating a more open boreal forest between ∼4500–8000 cal yr BP. Pollen-based inferences of average, summer and winter temperatures suggest that temperatures were on average up to 1–2 °C warmer than today, with winter temperatures up to 4 °C warmer. The pollen inferences also suggest enhanced precipitation, likely in the summer, but with an overall increase in evaporation and evapotranspiration resulting in reduced effective moisture. To assess regional climate changes, pollen-based reconstructions of temperature and precipitation were developed and synthesized from sediment cores from eight previously published lakes, from which pollen sites were available to both the west and east of Lake 239, spanning present-day prairie lakes to forested lakes up to 300 km east of the prairie-boreal ecotone. All sites show shifts in pollen assemblages that indicate a warm mid-Holocene period; prairie sites west of the Experimental Lakes Area (ELA) show mid-Holocene decreases in precipitation relative to today, whereas sites near or east of ELA show consistent increases in precipitation, but with increased temperatures and enhanced evaporation during the mid-Holocene. [Copyright &y& Elsevier]

Abstract: Drought is endemic to the North American Great Plains, causing severe economic consequences. However, instrumental climate data only exist from ca AD 1890, and limited tree-ring, paleolimnological, archeological and eolian records document the last two millennia. To address this lack of monitoring and paleoclimatic data, the pollen preserved in the varved sediments of Lake Mina, Minnesota, on the northeastern border of the Great Plains, were analyzed. May and February mean monthly temperatures and “annual precipitation minus potential evapotranspiration” were reconstructed at a 4-year resolution using a pre-settlement pollen–climate calibration set. The period of the so-called Little Ice Age (LIA) (AD 1500–1870) was colder than the Medieval Climate Anomaly (MCA) (AD 1100–1500) in west-central Minnesota. Winter temperatures in the LIA declined more than summer ones. The pollen record suggests that the LIA occurred in three phases: an initial cold phase from AD 1505 to AD 1575, a warmer phase, and then a very cold phase from AD 1625 to AD 1775. There were severe droughts detected in the Lake Mina record from AD 1660 to AD 1710 and AD 1300 to AD 1400, suggesting that high-resolution pollen records can detect events previously defined from the tree-ring records. This latter century-scale drought is concurrent with the widely reported “AD 1250–1400 mega-drought”, which exceeds the severity of 20th century droughts. [Copyright &y& Elsevier]

The Holocene Thermal Maximum (HTM) is relatively well-understood both in terms of its external drivers and timing of its expression in different regions across the globe, and has previously been used to investigate lake ecosystem response to a warm and drier climate. However, connections between mid-to-late Holocene climate, associated drivers, and lake response are less understood, particularly in western North America (WNA) where complex topography and differences in proximity to the Pacific coast result in spatially variable responses to ocean-atmosphere processes. Here, we present a multi-proxy paleolimnological record spanning 11,000 years from a small lake in the semi-arid southern Interior Plateau (SIP) of British Columbia. Sub-fossil diatom, trace element (μXRF via ITRAX), and pollen data indicate that our study lake was shallow, nutrient-poor, and poorly oxygenated during the HTM. High effective moisture (EM) is inferred throughout the mid-Holocene (∼6500–2000 calibrated years before present (cal yr BP)), followed by a return to low EM after 2000 cal yr BP. Synthesis of additional SIP diatom records show similarities in EM throughout the Holocene, but most notably, the decline in EM at ∼2000 cal yr BP. Comparison of patterns in the SIP with additional climate proxy data across WNA, including lake sediment δ18O records that are interpreted to represent changes in Pacific ocean-atmosphere processes, suggest that shifts in the mean state of the Pacific Decadal Oscillation (PDO) and the El Niño Southern Oscillation (ENSO), and associated strength and position of the Aleutian Low, are linked with late Holocene declines in EM throughout the SIP. • A multi-proxy paleolimnological record of Holocene environmental change from interior B.C. is presented. • Diatom and micro-X-ray fluorescence data indicate the lake was shallow, nutrient-poor, and anoxic during the HTM. • Effective moisture was higher during the mid-Holocene (∼6500–2000 cal yr BP) compared to the late Holocene. • Diatom records from proximal interior B.C. lakes show coherent declines in effective moisture at ∼2000 cal yr BP. • WNA proxy data demonstrates the importance of PDO/ENSO dynamics on mid-to-late Holocene effective moisture. [ABSTRACT FROM AUTHOR]