Other data sources - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 195

generally be distinguished visually, but magnetic measurements are more
quantitative: sediment ages can be determined by means of the magnetic polarity
time scale, and the magnitude of climate changes can be related to the magnitude
of variation in magnetic properties. Soils deposited during interglacials are about
200 times more magnetic than loess deposited during glacials. Increased magnetic
susceptibility values may result (in principle) from either a higher concentration
of magnetic iron-bearing minerals in interglacial wind-borne dust, or formation of
such minerals from preexisting non-magnetic or magnetic materials, as a result of
chemical changes during soil formation.
A number of papers have been published reporting studies of the magnetic
properties of loess (e.g., Florindo et al., 1999 who reported on a core representing
150,000 years). However,

the data from these various studies tend to be

qualitative.

6.3.2 Rock magnetism in lake sediments

It has been proposed that the magnetic susceptibility of rocks buried in lake
sediments can provide a proxy record of past temperatures. However, the descrip-
tion of how it works is not very clear. There exist so-called maar-diatreme
phreatomagmatic explosion craters which form when magma rises close to the
surface and interacts explosively with groundwater. This leaves behind a crater
that may fill with water, forming a lake. Such lakes store sediments that have
measurable age vs. depth characteristics.
Thouveny et al. (1994) investigated lake sediments at two such sites in France.
Cores

50m in length were taken. It was claimed that past cold climates tended
to preserve residual magnetism in the rocks in sediments, whereas warmer climates
would have reduced magnetic susceptibility—although it is not clear to this writer
how this occurs. Nevertheless, the authors made measurements to determine the
concentrations of magnetite (Fe 3 O 4 ) and titano-magnetite (Fe 3 x Ti x O 4 ) vs. depth
and converted this to age by independent radioisotope measurements. The results
are shown in Figure 6.7 . The susceptibility measurements seem to match up
moderately well with Greenland ice core measurements, suggesting that

>

the

cooling during the Ice Age was widespread across the NH.

6.4 POLLEN RECORDS

Wilson et al. (2000) provide a good overview of the use of pollens as climate
proxies. Pollen from flowering plants and conifers as well as spores from ferns,
horsetails, and mosses provide microscopic grains that are very resistant to decay
and often occur well preserved in sediments in bogs and lakes. The abundance and
distribution of such pollens provides insights into regional climates at various
times in the past. Figures 1.1 and 1.2 illustrate how changing climates affect the
distribution of plant life across the globe. The proportion of different types of
pollen and spores in sediments depends on the amounts produced by various