No abstract

The rock magnetic properties of the Chinese loess and paleosols constitute a unique and sensitive record of East Asian paleoclimate through the Quaternary Period. Systematic variations in the concentration and grain size of the magnetic minerals in these sediments have produced systematic variations in the magnetic susceptibility signal, which can be easily and rapidly measured at many sites across the Loess Plateau. Variations in many other rock magnetic properties can be used to identify the key shifts in ferrimagnetic grain size, but magnetic susceptibility alone is sufficiently sensitive to record stadial and interstadial climate stages, as well as glaciations and interglaciations. Past changes in rainfall and monsoon activity for this region are reconstructed from the susceptibility variations. The susceptibility record is calibrated using the modern relationship between rainfall and pedogenic susceptibility on the Loess Plateau. Our rainfall reconstructions identify enhanced summer monsoonal activity in the Chinese Loess Plateau region in the early Holocene and the last interglaciation. In the presently semiarid western area of the plateau, annual precipitation in interglacial times was up to 80% higher than at present; in the more humid southern and eastern areas, values were up to 20% higher than today's levels. During the last glaciation, precipitation decreased across the entire plateau, typically by ∼25%. The relationship between pedogenic susceptibility, climate, and weathering age was examined over the Northern Hemisphere temperate zone and the observed positive correlation between rainfall and susceptibility indicates that climate, rather than soil age, is the predominant factor that controls pedogenic susceptibility enhancement in loess soils.

No abstract

No abstract

High‐resolution palaeolimnological data from a number of remote and nonpolluted lakes in Finnish Lapland reveal a distinct change in diatom assemblages. This parallels the post‐19th century Arctic warming detected by examination of long‐term instrumental series, historical records of ice cover and tree‐ring measurements. The change was predominantly from benthos to plankton and affected the overall diatom species richness. A particularly strong relationship was found between spring temperatures and compositional structure of diatoms. The change is irrespective of the lake type and catchment characteristics, and is reflected by several other biological indicators, such as chrysophytes and zooplankton, suggesting that entire lake ecosystems have been affected. No corresponding change in the diatom‐inferred lake‐water pH was observed; hence, atmospheric fallout of acid substances cannot have been the driving force for the observed biological change. The mechanism behind the diatom response is unclear, but it may be related to decreased ice‐cover duration, prolonged growing season and increased thermal stability. We postulate that 19th century Arctic warming, rather than acidic or other anthropogenic deposition, is responsible for the recent ecological changes in these high latitude lakes.

The origins of the magnetic susceptibility variations of the Chinese loess and paleosols are explored by scanning and transmission electron microscopy of magnetic extracts, and by magnetic modeling of magnetic hysteresis data, to provide quantified estimates of the major magnetic components. Microscopy identifies several distinct size and shape characteristics in the magnetic carriers. Lithogenic magnetites, intact and abraded, dominate the coarse-grained magnetic fraction. The smallest of the coarse grains is ∼ 2 μm. The remaining magnetic materal is ultrafine in size, with two types of magnetite particles present. Type A particles strongly resemble soil magnetites produced by inorganic precipitation. Type B particles, which occur rarely, are probably bacterial in origin. Quantitative modeling of these magnetic assemblages shows that over 90% of the susceptibility variations is accounted for by the superparamagnetic magnetite component. Compared to the loess units, the paleosols are richer in magnetite, particularly of superparamagnetic size, and have a threefold higher ratio of magnetite to hematite. We identify pedogenic formation of magnetite as the major contributor to the loess magnetic record. Matching this record against other paleoclimatic records, we find an extremely high correlation with the standard 18O record. The Chinese loess sequences record a very high resolution magnetic stratigraphy directly related to changing climate.

Global warming is one of the major issues with which mankind is being confronted, having vital ecological and economic consequences. Ice-cover, snow-cover and water temperatures in alpine catchments are controlled by air temperatures, and so are very susceptible to shifts in climate. Local factors such as wind exposure, shading, and snow patches that persist during cold summers can, however, modify the sensitivities of the relationships to air temperature. Thermistors exposed in 45 mountain lakes of the central Austrian Alps (Niedere Tauern) measured water temperatures during 1998 – 2003 at two or four hourly intervals. Degree-day and exponential smoothing models tuned with this data suggest we can anticipate extremely large temperature rises in some of the Niedere Tauern lakes in the coming century. Lakes at around 1500 to 2000 m altitude are found to be ultra-sensitive as they lie in the elevation range where changes in both ice-cover and snow-cover duration will be particularly pronounced. In the more extreme cases, our impact models predict a summer-epilimnion water-temperature rise of over 10 degrees. One example of a lake most at risk to future climate change is Moaralmsee. This lake is located at 1825 m a.s.l. on the northern slopes of the Niedere Tauern; its water temperature is likely to rise by 12 degrees. The projected water discharge, ice-cover duration and water temperature changes for the Tauern catchments in the coming century far exceed the variations experienced at any stage during the last ten thousand years

Oxygen isotope variations in Chinese stalagmites have been widely interpreted as a record of the amount of East Asian summer monsoonal rainfall. This interpretation infers decreasing monsoonal rainfall from the mid-Holocene and large, dipolar rainfall oscillations within glaciations. However, the speleothem d 18 O variations conflict with independent palaeoclimate proxies (cave d 13 C, loess/palaeosol magnetic properties, d 13 C alkanes), which indicate no systematic decline in rainfall from the mid-Holocene, and no glacial rainfall maxima. Using mass balance calculations (which incorporate seasonality effects in both d 18 O concentration and amount of precipitation), we demonstrate that the cave d 18 O variations cannot be accounted for by summer rainfall changes, or rainfall seasonality or winter cooling, but instead reflect changes in moisture source. A possible driver of the d 18 O variations in Chinese stalagmites is precessional forcing of inter-hemispheric temperature gradients, and resultant shifts in the position and intensity of the subtropical pressure cells. Through such forcing, Indian monsoon-sourced d 18 O may have dominated at times of high boreal summer insolation, and local Pacific-sourced moisture at low insolation. Suppression of summer monsoonal rainfall during glacial stages may reflect diminished sea and land surface temperatures and the radiative impacts of increased regional dust fluxes.