7: Soil and Loess Flashcards
What is the main problem with using soils as global paleoclimate archives?
Not very well preserved usually
What are the two main special situations of high accumulation rates which can preserve soil in the paleoclimate record?
Floodplains, windblown dust (loess)
What is the primary location of paleoclimatic archives of soil, and why is this location timeless?
South of the furthest reach of glacial ice sheets, thus band across asia – glaciers do not reach to erode archive
Where is loess most often formed?
Periglacial regions – marginal to ice cover
When is loess most readily formed in periglacial conditions?
Cold periods
Besides periglacial sources of loess, what would be the other potential source?
Desert derived
What are the two factors of loess grains?
Usually silt sized, tend to be the same size – well sorted
What colour dust does goethite produce?
Orange
What colour dust does haematite produce?
Red
What mineral produces red dust?
Haematite
What mineral produces orangey dust?
Goethite
What are the most common magnetic minerals in loess?
Haematite/goethite, yet still not very common
What are the three main factors of loess source areas?
Extensive, Seasonally dry, Non-vegetated
Pedogenisis is a function of what 5 main things?
Time, Climate, Parent material, Organic activity, Relief
For the Chinese case study, soil formed from loess after how many years?
~100 years (quick for soil formation)
What are the three factors that make loess such a good paleoclimate archive?
Similar grain size, Well mixed by transport, Good for magnetic measurements
Why is well mixed loess advantageous for paleoclimate archives?
Reduces the variability of a major source
What is the most common domain sized magnetite grains found, and what proxy confirms this?
SD, Xarm
What are the seven soil condition controls which effect magnetic content?
(1) Soil moisture
(2) Temperature
(3) Organic matter input and consumption
(4) Bioturbation
(5) Oxygen availability
(6) Soil age
(7) Burning of vegetation
For the Russian Steppes example, the highest susceptibility (most magnetic) was found to have the highest ____.
Rainfall
Too much rainfall (>____mm) results in soil being too __ or __ for magnetic mineral formation
2000mm, acidic or wet
Insufficient drainage/too ___ rich (acidic) results in what?
Organic, Fe in solution (ferrous ions)
Too dry/insufficient ___ results in what?
Organic matter, Fe oxidized (haematite)
In alkaline and slightly reducing conditions, what occurs in soils?
Fe as magnetite (pedogenic enhancement)
What is the term for when a soil is too wet/organic rich?
Gleyed
Rainfall generally has a rough link with what parameter, however crude, and why?
Susceptibility, soil moisture
Pedogenic magnetite is generally produced during what intervals, associated with what?
Wetter intervals (interglacials)
Besides SD magnetite and haematite/goethite, what else enhances X in soils?
SP magnetite content
How can we determine rough estimates of rainfall of the past?
Using susceptibility of todays soil as a marker/proxy
In China during an interglacial, when does the most rainfall occur and why?
Summer – monsoon
In China during a glacial period, when does the most rainfall occur and why?
Winter – monsoon
Where does most of the loess come from in China during a glacial period?
Siberia/Mongolia
With the low susceptibility during loess deposition, what is expected of grain size and what does this show?
Larger grain sizes – stronger winds able to carry larger grains
What could be a magnetic effect of natural fires or burning crops?
Thermal transformation of weakly magnetic minerals in presence of organic matter
How was the dating sequence of Chinese loess deposits determined?
Magnetic reversals and the identification of chrons and sub-chrons
In Poland, there is examples of a ___ soil region sandwiched by loess layers, where susceptibility is lower than the loess.
gleyed
