Loess and dust. Jonathan A. Holmes Environmental Change Research Centre

Transcription

1 Loess and dust Jonathan A. Holmes Environmental Change Research Centre Why is dust important? Mineral dust is an important constituent of the solid load in Earth's atmosphere, the total atmospheric aerosol loading being both a function of, and a factor affecting climatic change. Human actions have progressively enhanced atmospheric dust loading, especially in the past century, so that both natural and human-induced effects of atmospheric dust on climatic change require more detailed research. (Derbyshire, 2003) 1

2 Loess Definition and distribution Silt formation Loess-palaeosol sequences and palaeoclimatic information Chronology Palaeoclimatic information from the C. European loess Palaeoclimatic information from the Chinese loess Correlations with marine and ice-core records Conclusions What is loess? Windblown silt deposit Consists chiefly of quartz, feldspars, mica, clay minerals and carbonate Blankets pre-existing relief Commonly found in continental interiors Thickest deposits well over 100 m thick Unstratified unless reworked by fluvial processes, but may show pedogenic and diagenetic alteration 2

3 Loess distribution (From Pye, 1984) Origin of silt Glacial model Silt formed by subglacial grinding: silt is eluviated from till by meltwater, deposited in the proglacial environment and deflated by the wind Desert model Silt formed by desert processes such as salt weathering and low-frequency high energy fluvial activity,and then deflated by the wind High mountain model Silt formed by high-energy geomorphological processes in high mountain regions, including mechanical weathering, mass movement, high-energy fluvial processes and glacial activity. This is followed by transport over short to medium distances and then deflation by the wind. 3

4 Western Loess Plateau, China Eastern Loess Plateau, China 4

5 St. Romain, Normandy Kashmir, Northern India 5

6 Stratigraphic sequence through loess and palaeosols Soil Warm, humid Loess Cold, arid Late Pleistocene -Holocene loess, Burzahom, Kashmir Palaeoclimatic indicators in loess sequences Sediment type (reflects depositional environment and, in some cases, extent of reworking by water) Soil type (reflects past vegetation and hence climate) Grain size (reflects wind energy and direction) Land snails (species composition reflect environment, especially vegetation) Pollen (reflects past vegetation, although pollen preservation is often poor) Magnetic susceptibility (reflects the types and concentrations of magnetic minerals, which are related to source and postdepositional alteration) 6

7 Loess Chronology Radiocarbon dating of palaeosol Luminescence dating of loess Magnetic polarity stratigraphy Correlation, especially with the marine isotope record Palaeoenvironmental Information from the Central European Loess Sedimentology Loess - windblown silt Pellet sands - reworked loess (hillwash deposits) Markers - fine-grained calcareous silt (dust storms?) Soils Steppe soils (chernozems) Brown earths Snail faunas From information in Kukla (1975) 7

8 Central European Loess Important loess sites in C. Europe. Stippling denotes extent of Quaternary glaciers and ice sheets (Kukla, 1975) Central European Loess Soil and loess stratigraphy from a site near Brno, Czech Republic (Kukla, 1975) 8

9 Central European Loess Summary stratigraphy of loess sequences around Prague and Brno, Czech Republic (Kukla, 1975) The loess Plateau, China 9

10 Xifeng, Central Loess Plateau Stratigraphy of the Xifeng loess site showing lithostratigraphy, magnetic polarity stratigraphy and magnetic susceptibility (from Liu et al., 1985) Magnetic susceptibility of the Chinese loess Magnetic susceptibility of a sediment or soil reflects the types and concentrations of magnetic minerals In the Chinese loess sequence, magnetic susceptibility is: Higher in soils Lower in the loess Models to explain higher magnetic susceptibility in the soils Pedogenic formation of magnetic minerals within the soils Translocation of magnetic minerals and accumulation within the soils Accumulation from subaerial dustfall Consensus that magnetic variability is a summer monsoon proxy 10

11 Late Quaternary grain-size variations in the Chinese loess Louchuan, Chinese Loess Plateau (Xiao et al., 1995, in Porter 2001) 11

12 Correlation with the marine isotope record Composite marine oxygen-isotope record and proposed correlation with the loess-palaeosol sequence at Baoji, NC China (Rutter et al., 1990) Correlations with marine, lacustrine and ice-core records Late Quaternary dust flux over the Chinese Loess Plateau, Japan, NW Pacific and Greenland (Porter, 2001) 12

13 Chinese loess records of East Asian monsoon intensity Conceptual model of loess and palaeosol development in NC China associated with a typical Quaternary climatic cycle (from Porter et al., 1992) Chinese loess records of Asian monsoon intensity East Asia, showing the landward limit of Indian and East Asian monsoon-derived rainfall at present (a) and during the Younger Dryas stadial (b) (From An et al., 1993) 13

14 But, past dust flux records are not derived solely from loess deposits Dust in deep-sea sediment and lake sediments J. Adkins et al., Paleoceanography 21, 4203 (2006) F. A. Street-Perrott et al., Holocene 10, 293 (2000) Figure from Holmes, 2008, Science, 320,

15 Dust in polar ice cores Temperature variations, CO 2 level and dust from the Vostok Ice core from Antarctica (Petit et al., 1999) Conclusions Loess-palaeosol sequences provide long, semi-continuous records of continental climate Major loess deposition started about 2.6 MaBP, coincident with the onset of northern hemisphere glaciation, although dust deposition in China can be traced back to the Miocene (>5Ma BP) Cyclical alternations of loess and palaeosols represent periods of dry/cold and warm/humid climate Good correspondence between the marine isotope record and loess palaeosol sequences, especially in the loess plateau of China Central Asian loess provides a detailed record of variations in both summer (magnetic susceptibility) and winter (grain size) monsoons over the past few million years Dust is also preserved in some lake sediments, marine sediments and ice cores, although calculated mass accumulation rates are typically lower than those from loess deposits 15