mid-Holocene Warm Period Research Articles

Stratigraphy of the mid-Holocene black bands in Lakes Michigan and Huron: Evidence for possible basin-wide anoxia

The post-glacial history of the Great Lakes has involved several changes in lake levels throughout the latest Pleistocene and Holocene, resulting from the changing position of the retreating Laurentide ice sheet, outlet incision and isostatic rebound. The final lowering of lake levels occurred at approximately 7600 14C yr BP, after which lake levels began to rise again to the Nipissing highstand at approximately 4700 14C yr BP. During this time of rising lake levels, black bands of iron sulfide were being formed in the sediments of all five of the Great Lakes. These bands signify suboxic to anoxic conditions, at least within the sediments and possibly at the sediment-water interface, during the middle Holocene warm interval. During this interval, the climate was warmer and drier than present, possibly resulting in the occasional absence of seasonal turnover in the lakes. We examined a series of piston cores from northern Lakes Michigan and Huron and found that the black bands are correlatable among cores taken from within the same basin. The observation that the banding can be correlated suggests a basin-wide cause, near-bottom or sub-bottom anoxia in the northern Michigan and northern Huron sediments during the mid-Holocene warm period. The sedimentary and geochemical processes in the Great Lakes during the middle Holocene warm interval are good indicators of possible future scenarios for the lakes as a result of global warming, as 21st-century temperatures are predicted to reach similar levels due to increased concentrations of greenhouse gases.

The origin of remnant forest stands of Pinus tabulaeformis in southeastern Inner Mongolia

Pinus tabulaeformis is an endemic species in northern China. The northern edge of its distribution corresponds to the northern margin of the monsoon climate. Several stands of Pinus tabulaeformis beyond its major range of distribution were found on the southeastern edge of the Inner Mongolia Plateau. Phytosociological analysis shows that the community structure, species composition and regeneration ability of Pinus trees in these stands are quite different from those within its continuous range of distribution. This paper presents palynological evidence to explain how and when these remnant communities were formed. Pinus tabulaeformis entered the study area in the mid-Holocene as the summer monsoon intensified, and declined when the summer monsoon weakened. Climatic change was the driving factor for the migration of Pinus tabulaeformis. It might be supposed that the competition between pine and oak forest during the mid-Holocene warm period also affected the immigration of pine. The remnant stands of Pinus tabulaeformis were apparently favored by the microhabitat of the sandy soils in valleys when the climate became drier. The remnant stands of Pinus tabulaeformis help stabilize the stand dunes. The protection of these stands is an important task of natural conservation in the study area.

The Climate System History and Dynamics Program - CSHD: Introduction

The goal of the Climate System History and Dynamics (CSHD) Program is to improve understanding of the global climate system so as to increase our ability to predict future change in the global environment. Papers in this special issue represent examples of work accomplished during CSHD-Phase 1, which began in 1995 and ended in 1998. This special issue presents the work of ten groups of scientists, including groups interested in the reconstruction of past climate states using geologically derived proxy data and groups interested in the use of models to predict these states, combined in a focused effort to provide a Canadian contribution to the international Paleoclimate Model Intercomparison Project (PMIP). CSHD-Phase 1 primarily focussed on two "time slices:" the last glacial maximum (LGM) of the late Cenozoic (21 ka BP) and the mid-Holocene warm period (6 ka BP).

A diatom-inferred water depth reconstruction for an Upper Peninsula, Michigan, lake

Transects of surface sediment samples were taken in 4 lakes from the Sylvania Wilderness Area, Upper Peninsula of Michigan. These surface samples were compared with diatom samples from a core taken in the Northwest basin of Crooked Lake, also from the Sylvania Wilderness Area. Weighted Averaging calibration was used to reconstruct lake depths in Crooked Lake using the diatom microfossils from the core and the surface samples to infer past lake depth. During the early Holocene the lake was dominated by planktonic species and diatom-inferred water depth was large – approx. 13 m. At about 6700 BP inferred water depth was 2 m and samples were dominated by Fragilaria construens var. venter – a species characteristic of shallow parts of the surface sample transects. From 6700 to 5000 BP reconstructed water level was at its shallowest. From 5000 to 3000 BP it increased. This rise in water level was marked by increasing abundances of Aulacoseira ambigua and occurred at the same time increasing percentages of hemlock pollen indicate increasing available moisture. Modern water depth was reached about 3000 BP. The water level changes at Crooked Lake are consistent with regional climate changes in the Upper Midwest during the Holocene. The lake was shallowest during the mid-Holocene warm period documented by other investigators. It deepened as the Midwestern climate became cooler and wetter during the late Holocene.

Cyclic Development of Permafrost in the Peatlands of Northwestern Alberta, Canada

Vegetation in thermally subsided peatlands (collapse scars) grades from the wettest part near the collapsing peat bank with Sphagnum riparium to successively drier conditions with S. angustifolium and S. fuscum until the peat plateau surface is reached, covered with Picea mariana-lichen woodlands. The same sequence can be found in the peat stratigraphy, where the charred surface of the treed peat plateaus (sylvic peat) is overlain by a sequence of Sphagnum riparium, S. angustifolium, and S. fuscum, capped by sylvic peat. Often several such sequences occur in the peat stratigraphy, indicating periodic permafrost degradation, triggered by fires, and regeneration. Radiocarbon dates show that such cycles can be as short as 600 yr. The earliest incidence of permafrost in the study area was dated at 3700 yr BP, indicating the end of the mid-Holocene warm period and the onset of the current climatic regime.

Holocene Treeline Fluctuations in the Northern San Juan Mountains, Colorado, U.S.A., as Indicated by Radiocarbon-Dated Conifer Wood

An early to mid-Holocene warm period with greater than present-day monsoonal circulation is indicated by evidence in the northern San Juan Mountains of southwestern Colorado. Fifty radiocarbon ages of coniferous wood fragments, and several other radiocarbon ages of willow fragments, woody peat, and organic sediment associated with conifer remains, recovered from six sites at or above present-day treeline indicate that during the first half of the Holocene treeline was higher than present. Radiocarbon ages of coniferous wood fragments recovered from the remains of Lake Emma, a former glacial tarn above present-day treeline, indicate that between 9600 and 5400 BP treeline in the northern San Juan Mountains was at least 80 m higher than present. Furthermore, a radiocarbon age of a large spruce fragment with a complacent annual-ring record suggests that at about 8000 BP timberline may have been at least 140 m higher than present. These data suggest that during the early to mid-Holocene average July temperatures were at least 0.5 to 0.9?C higher than present. Radiocarbon ages and stratigraphy from bogs in the study area near present-day treeline suggest that between 5400 and 3500 BP treeline was near its present-day limits. After 3500 BP treeline was generally lower than present although short climatic ameliorations, with accompanying rises in treeline, may have occurred at about 3100 and 2400 BP. Deuterium concentrations previously determined for the cellulose of 18 conifer fragments from the Lake Emma site indicate a 70%/o decrease in deuterium from 9600 BP to present. These data indicate that the early Holocene increase in July solar radiation may have resulted in the intensification of the Arizona monsoon at that time.

The developmental history of peatlands at Muskiki and Marguerite lakes, Alberta

The peat stratigraphy of fens at Muskiki Lake (52°50′N, 116°51′W) and Marguerite Lake (54°38′N, 110°43′W) in central Alberta was examined to determine the developmental history of the fens on the basis of radiocarbon dating and bryophyte macrofossil analyses. Peat accumulation at Muskiki Lake peatland began about 9000 years BP via lake filling. Expansion of the peatland by paludification and vegetation changes, including string and flark formation, are secondary occurrences, the latter occurring subsequent to environmental changes, including mid-Holocene thermal events. Peat inception at Marguerite Lake began about 2400 years BP via paludification. This delay, relative to Muskiki Lake peatland, may be related to the mid-Holocene warm period. Fire activity and increased wetness, as well as autogenic successional processes, altered species composition and abundance.