The 20kyr Lake Record of Salton Basin:
Applications of AMS 14C, δ18O, δ13C and
δ87Sr on the Lake Tufa Deposits
Hong-Chun Li
• National Cheng-Kung University, Taiwan
• USC
OBJECTIVES
• To study jet stream shift and
monsoon variability, and their
influence on the regional climates
• To identify rapid climatic shifts
such as the LIA, MWP, HO, YD
• To understand the forcing
mechanisms of above climate
elements
Laguna Macuata
1. Salton Trough
2. Colorado Delta
3. Salton Basin
4. Salton Sea
Colorado Desert Climate
Two rainy seasons
Precipitation: ~8 cm/yr
Evaporation: ~180 cm/yr
Monthly temperature: 15 to 35oC
Salton Basin
Today, it’s the third-largest saltwater lake in N. America,
but the Salton Sea was created accidentally in 1905 when the
enormous flash floods of the C.R. went over the river bank
and broke through the temporary gates of irrigation canals.
1. Bad sedimentation
short lake history
2. C.R. switch in/out
-Regional climatic
representative
3. Non-climatic reason
of C.R.. switch
Climatic variability in Salton Basin should resemble
that of the C.R. drainage basin
500
450
400
1 AZ D3 north central
2 CO D2 drainage basin
3 Denver
4 AZ D5 southwestern
5 UT D4 south central
6 San Diego
7 UT D7 southeastern
1
2
3
350
4
5
300
6
7
250
200
150
1840
1860
1880
1990
1920
1940
Calendar Year (AD)
1960
1980
2000
Historic records show that the Salton Basin was flooded between 1700
and 1750 and in 1840, 1849, 1852, 1859, 1867, 1884, 1891 and 1905 AD
due to heavy winter storms in the upper C.R. drainage basin, rather
than tectonic movements or naturally meandering adjustment.
• 210Pb and 137Cs dating on sediment core.
• AMS 14C dating on tufa and fossil shell.
• 230Th/234U dating on tufa (not successful).
• Geochemical analyses on water samples.
• Analyses of δ18O and δ13C on total
carbonates in lake sediment, tufa, and
fossil shell.
• Analysis of 87Sr/86Sr in tufa carbonates
• Measurement of trace elements, i.e., Mg,
Sr, Mn, and Fe, in tufa samples.
1:750,000
3000
Salton Sea water
Colorado River
0 10 20km
33o
30‘
2000
33o
2000
200
3000
Creek Water
2000
Tufa LC and SST
200
Gravity core 64
32o
30‘
10
32o
12-m sill
for lake
Cahuilla
200
200
2000
3000
31o
30‘
Whitewater River
elevation (ft)
Gulf of
California
approximate limit
of the alluvial basin (~40 ft)
116o
115o30‘
115o
114o30‘
Salton Sea: 9.14
km3 and 960 km2,
Ave. depth ~6.5 m
and 16.5 m.
Turner and Reynolds’ tufa (1975)
Age
(yr BP)
VI
7,205±120
V
9,030±135
IV
9,180±135
III
10,670±155
II
11,500±165
I
15,970±235
Tufa deposits at Travertine Point
17,590±280
granitic boulder
0
1
(a) Barnacle shells
2 cm from modern shoreline, with
AMS 14 C age: 160±25 yr BP
δ18O: -1.80‰ (PDB)
δ13C: -0.57‰ (PDB)
(b) Gastropod shell from an ancient shoreline, with
1660±30 yr BP, δ18O: -8.14‰, δ13 C: -3.89‰
Gastropod shell inside tufa
(c) Gastropod shell from an ancient shorelin, with.
1390±30 yr BP, δ18O: -6.67‰, δ13C: -0.39‰
(d) Gastropod shell from an ancient shoreline, with
1405±30 yr BP, δ18O: -6.53‰, δ13C: 0.25‰
14C
dating on Corg
and fossil shells in
lake sed. indicates
the deposition of
the sed. took place
between 490 and
1,340 yr BP
Tufa SST
LC-1
0.716
Whitewater
input
0.714
0.712
Major
Local stream
0.710
Salton Sea
Colorado
input
0.708
-12
-10
Minor
Local stream
-8
-6
-4
-2
δ18O (permil, SMOW)
0
2
87Sr/86Sr
Sr/Ca
2
0
Salton Sea
-2
-4
-6
-8
-10
-12
Freshwater
0
2000
4000
6000
Na (ppm)
8000
10000
4
-2
δ13C
-3
Deglaciation
δ18O (PDB)
-4
-5
-6
-7
US trans wet
H-1
MWP
-8
-9
YD
HO
0
5000
10000
Age (yr BP)
15000
2
δ18O
1
0
20000
δ13C (PDB)
3
Oxygen and carbon isotopic
covariance indicates that the
lake was in closed conditions
during most periods of 20 ka
Model calculation shows that
the tufa grew in relatively
saline lake conditions
5
4
y = 4.35 + 0.38x
R = 0.77
3.5
0
3
2.5
-5
2
-10
Freshwater lake
1.5
1
-15
0.5
5
10
15 20 25 30
Temperature ( o C)
35
40
-9
-8
-7
-6
-5
-4
18
δ O (‰, PDB )
-3
-2
• The δ18O of Salton Sea is strongly
correlated with salinity, indicating the
feasible of using the lake δ18O to decipher
variations of lake hydrology during the
past.
• AMS 14C dating on tufas, barnacle Shells,
bi-valve shells and gastropod shells
indicates that the influence of dead
carbon and reservoir effect is negligible.
Lake Cahuilla existed since 40,000 yrs BP
in the Salton Basin.
•
87Sr/86Sr
(0.710169) of the C.R. input is identical
to that of Salton Sea (0.710105), and is much
lower than that of Whitewater River (0.715960).
These results enable us to trace past changes in
the CR input using 87Sr/86Sr in lake carbonates.
• 87Sr/86Sr ratios of tufas are relatively constant
throughout the past 20 ka, averaging 0.710060
± 0.000049 (n=30) which is very close to that of
the C.R. input. This means that the C.R. input
had been the dominant water source for Lake
Cahuilla during 800-20500 kyr BP when the tufa
grew in the Basin.
• Large shifts of δ18O and δ13C in LC-1 reflect
changes in the climatic conditions in the C.R.
drainage basin. The lake was under either closed or
overflowed lake conditions with relative humidity
ranging from 20% to 40%.
• The climate changes in the C.R. drainage basin
between 1,300 and 20,500 yr BP has been strongly
influenced by N. American monsoon, whose
strength was strong in early Holocene and
decreased toward to late Holocene, corresponding
to variations of solar insolation and shifts in ITCZ
and Westerlies. We have found climatic shifts such
as Heinrich-1 event and “Trans US wet period”,
and mega droughts around 8.2, 6.2, 4.2 and 2.5 kyr.
• Between 7000 and 18000 yr BP, Lake Cahuilla level
was above -24m a.s.l. which was at least 50m higher
than modern Salton Sea. Between 1300 and 7000 yr
BP, the lake stayed at the highest level -12m a.s.l.
which was 87m higher than Salton Sea. During these
periods, Colorado River was connected with the
Salton Basin. After 1300 yr BP, the lake dropped its
level rapidly probably due to both arid climates and
change in course of Colorado River. Salton Basin
became a part of Sonora Desert after 1300 yr BP.
Human occupation on the lake shore below 12m a.s.l.
should be after 1300 yr BP or before 7000 yr BP.