Applications: Carbon Isotopes in Soils
(w/ a digression on animals)
Grassland composition varies with climate
Shift in C4 abundance
%C4 = -0.9837 + 0.000594 (MAP) +
1.3528(JJA/MAP) + 0.2710 (lnMAT)
The modern prairie is a
C4-dominated grassland
Map from Tieszen et al. (1997)
Regression from Paruelo & Lauenroth (1996)
13C shifts with change in plants
Soil isotopes
Biomass (STATGO)
QuickTime™ and a
decompressor
are needed to see this picture.
13C of soils and roots vs. depth
QuickTime™ and a
decompressor
are needed to see this picture.
von Fischer et al. 2008
13C shifts with change in plants
QuickTime™ and a
decompressor
are needed to see this picture.
13C shifts with climate
QuickTime™ and a
decompressor
are needed to see this picture.
C3-to-C4 balance is also affected by pCO2
C3 plants
Crossover Temperature
Quantum
Yield
C4 plants
(moles C fixed per
photon absorbed)
Today (360 ppm)
LGM (180 ppm)
3
6
9
12
15
18
21
24
27
Temperature (°C)
30
Two questions about Great Plains ecosystems
At the LGM, was there less C4 biomass (because of lower
temperatures) or more C4 biomass (because of lower pCO2)?
When (and why) did C4 grass come to dominate Great Plains
ecosystems?
Use isotopes in animals and soils to track C3-to-C4 balance
Why Texus?
Climate means from 1931-1990
From New et al. (2000)
Archived at www.ipcc-ddc.cru.uea.ac.uk
N
High Plains
36¡N
OKLAHOMA
34¡N
Texas
vegetation
today
NEW MEXICO
High
Plains
Rolling
Plains
TEXAS
32¡N
Piney
Woods
Llano
Edw ards Uplift
Plateau
Trans Pecos
30¡N
28¡N
BP
S. Texas
Brushland
MEXICO
Gulf Coas t
Mars h&Prairie s
26¡N
From Diamond et al. 1987
106¡W
104¡W
102¡W
100¡W
98¡W
96¡W
94¡W
Another question: Were the plains and plateau regions covered by a
boreal forest at the LGM?
Proboscideans
Horses - Bison
Holocene Late Glacial
Holocene bison
Last Glacial
Maximum
Ingelside horses
Pre-LGM
Initial conclusions from isotope studies of Texas mammals
1) No changes in mean δ13C value through time (ANOVA).
2) Bison and mammoths are grazers. They can be used to monitor C3
to C4 balance on Pleistocene grasslands.
3) Mastodons are browsers. Their presence suggests tree cover.
4) Pleistocene horses ate lots of C3 vegetation, even when bison and
mammoths had ~100% C4 diets. Horses were mixed feeders.
What's next?
Compare %C4 from mammals to values simulated via modeling.
1) Use Quaternary climate model output, and estimate %C4 biomass
using the Regression Equation.
2) Use the same climate model output, but estimate %C4 biomass as
the percentage of growing season months that are above the
appropriate Crossover Temperature.
%C4 Grass from Regression Model
Holocene
0-10 Ka
Post-LGM
10-15 Ka
%C4 plants in grazer diets
Mammuthus
Bison
Mammut present
LGM
25-15 Ka
Holocene model driven by modern climate data from New
et al. (2000). LGM and Post-LGM models driven by GCM
output from Kutzbach et al. (1996)
(archived at www.ngdc.noaa.gov/paleo/paleo.html)
Quaternary Prairies from animals
1) Pleistocene horses were mixed feeders, not grazers.
2) Despite climate change, %C4 biomass is remarkably
constant through time.
3) Always lots of C4 biomass on plains and plateaus and
no mastodons. No LGM boreal forest in the region.
4) Only climate-vegetation models that account for
changes in pCO2 as well as temperature provide
reasonable %C4 estimates in parts of the Quaternary
with different atmospheric compositions.
Koch et al. (2004) P3
Quaternary Prairies from soils
QuickTime™ and a
decompressor
are needed to see this picture.
QuickTime™ and a
decompressor
are needed to see this picture.
Quaternary Prairies from soils
δ13C of paleosol carbonates from Pakistan
Quade et al. (1989)
Development of the
Asian monsoon
revealed by marked
ecological shift during
the latest Miocene in
northern Pakistan.
Nature 342: 163-166.
Diet shifts in Mio-Pliocene Mammals
Cerling et al. (1997)
Isotopic evidence for dietary change in
Great Plains horses
• Prior to the late
Miocene, all horses
ate mostly C3 plants
• 1st evidence for C4dominated diets is at
6.6 Ma
• After 6.6 Ma, horses
from Texas to
Nebraska had a wide
range of diets
Passey et al. (2002)
Isotopic evidence for dietary change in
Great Plains horses
• Prior to the late
Miocene, all horses
at mostly C3 plants
• 1st evidence for C4dominated diets is at
6.6 Ma
• After 6.6 Ma, horses
from Texas to
Nebraska had a wide
range of diets
Passey et al. (2002)
Paleosol localities
in the Central and
Southern Great
Plains
Modern C3 soils
with <100 mm precipitation
δ13C values of Great Plains
paleosol carbonates
n = 274
δ13C values of Great Plains
paleosol carbonates
n = 274
δ13C values of Great Plains
paleosol organics
n = 20
δ13C values of Great Plains
paleosol organics
n = 20
Isotopic evidence from new Great Plains faunas
CO2 drop or climate change?
Toto, I have a
feeling we're not in
Asia anymore.
Age (Ma)
Globally synchronous at 7 Ma?
Other sections in
East Africa
Levin et al. (2004)
13C (VPDB)
A late Miocene drop in pCO2?
Based on 13C
records in organic
biomarkers
Pagani et al. (1999)
Pearson & Palmer (2000)
Age
(Ma)
5
0
Plt.
18O (‰)
4
3
2
13C(‰)
1
0
-1
01
Decrease in MAT.
10
Increase in seasonality.
Decrease in length of
growing season.
20
Loss of winter growing
season.
30
40
Zachos et al. (2001)
2