Relict Soil Evidence for the Onset of
Hyperaridity in the Atacama Desert,
Chile
Ronald Amundson1, Angela Ebling2, Erik Oerter1, John
Valley3
1
Environmental Science, Policy and Management, University of California,
Berkeley
2 aWisconsin Lutheran
3
College, 8800 W Bluemound Rd, Milwaukee, WI
Department of Geosciences, University of Wisconsin, Madision
Motivation
•
Rise of the Andes, and
subsequence climate
change, one of most active
areas of earth science
research
•
Evidence for this is varied
and imperfect
• Ancient relict soils have
ability to retain climate
information over long time
spans
Climate information in relict soils
Ideally, utilize soils that
undergo aridification
(evidence from more humid
period is preserved)
Clay carbonate soluble
salts
Research site
• At southern periphery of
the desert (transition to
Mediterranean climate)
•
Present climate:
MAP =~ 22mm
MAT = 15C
•
Mio-Pliocene gravelly
alluvium
Sampling Site
Well preserved geomorphic surface typical of Atacama Desert
Calcic + some Ca SO4
Argillic
Sequence of Soil Development
1. Clay formation and accumulation (up to 35%) from
chemical weathering of gravelly alluvium
2. Long period of carbonate accumulation within previous
clay forming horizons
3. Carbonate accumulation phase isotopically shows long
period of further aridification
4. Present climate is dryer than any suggested by soil
characteristics
Clay formation and chemical
weathering
Ce and Nd (found in
most minerals) were
most well-behaved
index elements
Soil showed net losses of Na, K, and
Si with either Ce or Zr as index
elements
Carbonate isotopes
C isotopes in fine-grained
fraction suggested soil
respiration rates typical of
rainfall >> today
Recent study of C isotopes
in soils in nearby Argentina,
in much more pluvial
conditions, similar to relict
soil:
MAP= 130 to 320 mm/y
Peters et al., 2012. EPSL
Isotope record in carbonate laminations
Laser ablation via SIMS
130
mm
Hand drilling
Carbon isotopes by hand drilling
• continous increase
in 13C for all
transects
• indicative of a
decline in soil
respiration of ~ 46 g
C m-2 y-1
• pronounced
aridification
Interior
exterior
Oxygen isotopes by drilling and SIMS
•
both hand drilling and SIMS
show no discernable trend with
time
•
SIMS (and much less averaging)
allows us to see much more
structure in the O pattern.
Suggestive of recording some
climatic oscillations
•
CHALLENGE: dating
– too old for U series
– U-Pb challenging due to high
Pb
Interior
exterior
Results and Interpretations
•
Evidence of extensive chemical weathering (prePleistocene?)
•
Evidence of post-weathering carbonate accumulation
(carbonate accumulation mass ~ 1.5 My)
•
Stable isotope evidence for on-going aridification during
carbonate accumulation phase (need for dating
methods)
•
Combined interpretation: decreases in MAP of 50 to 100
cm y-1 over life of the soil
That’s It
Look for paper in
2015!