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Paleo-monsoon history of the last twomillennia from Southern Indian lake
sediment magnetism
R ShankarMangalore University
INDIA
Email: [email protected]
Contributors:
Anish K Warrier, Sandeep K & W Zhou
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IN THIS TALK:
• Introduction
• Study area
• Methodology
• Results and Discussion
• Conclusions
• Acknowledgements
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INTRODUCTION
• Monsoon and its importance
• Study of paleomonsoon data essential to forecastmonsoon
• Sediments from two Southern Indian lakes studied
• Multi-proxy approach
Objective of this study: To determine paleo-rainfall variations
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• Proxies used for paleomonsoon:Wind strengthPrimary productivityTerrigenous inputSalinity variations etc.
• Lacuna: All these proxies are indirect and NOT ameasure of rainfall per se.
• Any proxy to fill this lacuna?
• We explored magnetic susceptibility (χlf) as apotential candidate in this regard
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Climate:Temp: Rainfall: 17-28 0C (Nov-Feb) ~ 640 mm 36-41 0C (Mar-May) Received mainly during the
SW monsoon
1) Thimmannanayakanakere (TK)
Area of the lake:0.17 sq.km
Geology:> 50 % - GraniticGneiss.Greywacke, bandedferruginous chert, Fe-Mn formations,limestone etc.
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2) Pookot Lake (PK)
Area : 0.085 sq. km.Annual rainfall : ~ 4000 mmMain rock types: Hornblende-biotite, gneiss and charnockite.
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TK PK
Samples from: Pit wall Sediment cores PK1 and PK2
Sampling interval: 2 cm PK1 @ 0.5 cmPK2 @ 1 cm
Geochronology: C-14 dating onbulk sedimentsamples
AMS C-14 dating on bulksediment samples
Rock magneticstudies on:
185 samples PK1 = 441 samplesPK2 = 239 samples
SAMPLES AND METHODS
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What is Rock Magnetism?
Rock magnetism or Environmental magnetismdeals with the intrinsic magnetic properties ofnatural materials like soils, sediments, dusts andpeats.
The technique is simple, rapid, inexpensive, non-destructive and sensitive.
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Magnetic InstrumentationMS2 suscep*bility meter
ARM a5achment withshielded demagne*ser
Magnetometer
Pulse magne*ser
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Commonly Studied MagneticProperties and their Ratios
χ, IRM, SIRM
χfd
χARM
χARM/χ andχARM/ SIRM
IRM300 / SIRM
Concentration of magnetic minerals
Concentration of ultrafinesuperparamagnetic (<0.03 µm) grains
Concentration of magnetic minerals andbiased towards stable single domaingrains (0.02-0.4 µm range)
Magnetic grain size (higher ratiossuggest finer grain size)
Ratio of ferrimagnetic toantiferromagnetic minerals
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Is a Rock Magnetic approach suitablefor this study?
Source of sediments (and magnetic minerals):• Hills to the south of TK• Hills around PK
Tropical regions:• Chemical weathering dominant• High rainfall accentuates this
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Pedogenesis due to chemical weathering controlled by climate - mainly rainfall & temperature.
Iron in non-magnetic minerals is transformed intopedogenic magnetic minerals like magnetite andmaghemite.
High (Low) Suscep1bility
Higher (Lower) concentra1on of pedogenic magne1c minerals
High (Low) Rainfall
TEM of fine‐grained magne1te from asoil profile (Maher et al. 1999)
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Magnetic properties as proxy forpaleomonsoon / paleoclimate
Before this, the following have to be ruled out:•Presence of greigite•Bacterial magnetite•Dissolution of magnetic minerals•Anthropogenic magnetite
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• Greigite (Fe3S4) –a ferrimagnetic ironsulfide mineral
• SIRM / χlf ratio is anindicator of greigite inthe samples. Greigite ispresent if the ratio is >40 x 103 Am-1.
Presence of GreigiteSIRM/!
lf
(103 Am
-1)
3 4 5 6
Ag
e (c
al y
ears
B.P
.)
0
500
1000
1500
2000
2500
3000
3500
4000
TK
SIRM/!lf
(103 Am
-1)
0 10 20 30 40
Ag
e c
al y
ea
r B.P
.
0
200
400
600
800
1000
1200
1400
1600
1800
2000
2200
2400
PK1
0
200
400
600
800
1000
1200
1400
1600
1800
2000
2200
2400
2600
2800
3000
PK2
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Bullet-shaped magnetosomesof M. bavaricum
Biogenic (Bacterial) Magnetite
• Biogenic magnetitepresent ifχARM / χlf = 40 andχARM / χfd = 1000.
!ARM/!lf
2 4 6 8 10
Ag
e (c
al y
ea
rs B
.P.)
0
500
1000
1500
2000
2500
3000
3500
4000
!ARM/!fd
0 80 160
TK
!ARM/!lf
0 5 10 15 20 25 30
Ag
e c
al y
ea
r B.P
.
0
200
400
600
800
1000
1200
1400
1600
1800
2000
2200
2400
2600
0 200 400 600
!ARM/!fdPK1
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!ARM
/!lf
1 10 100
!ARM
/ !fd
10
100
1000
10000
TK sediment samples
TK sub-surficial soil samples
TK surficial soil samples
Bacterial magnetite derived from Adriatic sediments
and Irish Sea saltmarsh clays
Soil, palaeosol andcatchment derived
fine sediments
TK
Oldfield et al. (1994)
Biplot to distinguish the source of magnetic minerals present in lake sediments (Oldfield, 1994)
PK
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Dissolution of Magnetic Minerals
χARM/χlf & χARM/SIRMare good indicators ofdissolution /diagenesis.
!ARM
/SIRM
(10-5
mA-1
)
0 80 160 240
!ARM/!lf
2 4 6 8 10
Ag
e (c
al y
ea
rs B
.P.)
0
500
1000
1500
2000
2500
3000
3500
4000
PK1
!ARM/!lf
0 5 10 15 20 25 30
Ag
e c
al y
ea
r B.P
.
0
200
400
600
800
1000
1200
1400
1600
1800
2000
2200
2400
2600
!ARM/SIRM
(10-5
mA-1
)
0 100 200 300
TK
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Anthropogenic Magnetite• If present, it will be in the MD
grain size.
• No industries in the vicinity ofTK and PK.
• TK and PK located away fromurban areas and at a highelevation.
• TK and PK sediments – no MDor PSD grains present. Hence,no anthropogenic magnetite.
Abundantspherules in a
road-side top-soil
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Can Magnetic Susceptibility be used as aProxy for Paleomonsoon?
A case is made out for: χlf being principally derived from the catchment.
Next, see if χlf is correlated with Instrumental rainfall data Historical records Proxy rainfall data from other parts of India
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Chitradurga Stn. rainfall
(mm)
500 600 700 800 900
Yea
r (AD
)
1900
1920
1940
1960
1980
2000
Pen. India rainfall
(mm)
900 950 1000
1900
1920
1940
1960
1980
2000
r = 0.65
0 50 100 150 200
r = 0.45
!lf
(10-8
m3
kg-1
)
0 50 100 150 200
1900
1920
1940
1960
1980
2000Y
ear (A
D)
!lf
(10-8
m3
kg-1
)
COMPARISON WITH INSTRUMENTALRAINFALL RECORD
TK
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Total annual rainfall Kerala (mm)
16000 24000 32000 40000
!lf (x 10-8
m3/kg)
20 30 40 50 60
1880
1900
1920
1940
1960
1980
2000
Total annual rainfall- Vayittiri (mm)
2000 4000 6000
Peninsular india rainfall (mm)10000 12000 14000
Year A
.D
1880
1900
1920
1940
1960
1980
2000
r=0.39 r=0.44 r=0.38
PK
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It can be seen that χlf is enhanced by the presence ofpedogenic magne1te/maghemite.
χlf10-8m3kg-1
χfd10-8m3kg-1
χfd(%)
Pre-CBD
132 12.9 8.9
Post-CBD
64 2.0 3.0
CBD extrac*on studies:
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CBD extraction for TK Sediment samples
Samples depthwise (cm)
Ma
gn
etic
Su
sc
ep
tibility
(10
-8 m
3 k
g-1
)
0
50
100
150
200
Initial
After CBD 1
After CBD 2
After CBD 3
After CBD 4
0-2 2-4 4-6 6-8 8-10 10-12
• The samples from 8‐12 cm depth exhibit only a small reduc*on insuscep*bility.
These two samples relate to the drought period (1876; 1895) during which rainfall was very low and hence the low suscep*bility.
•The large reduc*on insuscep*bility occurredaTer the first step for thefirst four samples (0‐8cmdepth) high % ofpedogenic suscep*bility.
FOUR‐STEP PROCEDURE(Hunt et al. 1995)
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!lf
(10-8
m3 kg
-1)
0 50 100 150 200 250
Fe/A
l
0.2
0.4
0.6
0.8
1.0
1.2
r = 0.67
PositivecorrelationbetweenFe/Al&χlf
Becausemagneticmineralsaretransportedtothelakefromthecatchment.
0 100200300
0
1000
2000
3000
4000
!lf
(10-8 m3 kg-1)
Ag
e (c
al. k
a B
P)
Fe/Al
0.0 0.4 0.8 1.2
Age cal. yrs BP
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• Posi*ve correla*on between Ti/Aland χlf
• Ti ‐ a terrigenous indicator0 100200300
0
1000
2000
3000
4000
Ag
e (c
al. k
a B
P)
!lf
(10-8 m3 kg-1)
0.00 0.04 0.08
Ti/Al
0 50 100 150 200 250
Ti/A
l
0.02
0.03
0.04
0.05
0.06
0.07
r = 0.50
!lf
(10-8 m3 kg-1)
Age cal. yrs BP
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Chemical weathering proxies:
• K/Al, Ti/Al and K/Na – proxies for the intensity ofchemical weathering.
• Generally reflect higher rates of precipita1on(Temp. being nearly constant in the tropics).
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K/Al
0.05 0.10 0.15 0.20
Ag
e (c
al. k
a B
P)
0
1000
2000
3000
4000
0.02 0.04 0.06 0.08
Ti/Al
K/Na
0.0 0.1 0.2 0.3 0.4
0 50 100 150 200 250
0
1000
2000
3000
4000
Ag
e (c
al. k
a B
P)
!lf
(10-8 m3 kg-1)
CHEMICAL WEATHERING INTENSITY
CHEMICAL WEATHERING INTENSITY
Age cal. yrs BP
Age cal. yrs BP
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Component
Initial Eigen values
Total % of Variance Cumulative %1
8.21 63.12 63.12
21.53 11.77 74.89
31.26 9.69 84.58
PRINCIPAL COMPONENT ANALYSIS
Three major components explain the variance in the data.
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Principal Component Analysis1
TERRIGENOUS2
PEDOGENIC =RAINFALL
3SOIL CARBONATE
Xlf 0.34 0.89 0.17Xfd 0.04 0.93 0.08
Fe/Al 0.83 0.35 0.32Mn/Al 0.58 0.43 0.31Ti/Al 0.87 0.16 0.39Cu/Al 0.92 0.22 0.13Zn/Al 0.93 0.11 0.22Pb/Al 0.92 0.13 0.02K/Al 0.83 0.18 0.27Na/Al 0.66 0.28 -0.58Sr/Al 0.71 0.34 0.55Ca/Al 0.48 0.40 0.56Mg/Al 0.29 0.18 0.80
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A suggested model
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Thus, the physical basis for χlf – Rainfallcorrela1on is established.
Namely, pedogenic forma1on of magne1te inthe catchment which is related to rainfallintensity (Temp. being constant in tropics).
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1. AD 1876-77 drought inChitradurga: References madeto porridge centers set up bytwo people independantly(Lowest susceptibility in TKprofile)
2. AD 1741 abnormal rainfall inChitradurga: Reference made to“mad rains”. Also, coincides witha) Total Solar IrradianceMaximum between Dalton andMaunder Minima in the Little IceAge; and b) A high rainfall eventrecorded in Akalagavispeleothem.
Historical data support the proposition
0.0 0.5 1.0 1.5 2.0 2.5
!lf (10
-8 m
3 kg
-1)
AD 1876 Drought
AD 1741 High
Rainfall
AD 1617
AD 1640
AD 1890
AD 1845
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TK χlf – Pen. India rainfall correlation: 0.65 Comparable to δ18O of Akalagavi speleothem-
instrumental rainfall data: -0.62 TK χlf – Akalagavi δ18O correlation: -0.61
!lf
(10
-8 m
3 k
g-1
)
Year (AD)
1650 1700 1750 1800 1850 1900 1950 2000
-1.6
-1.2
-0.8
-0.4
0.0
0.4
"18
#
Year (AD)
r = -0.61
1650 1700 1750 1800 1850 1900 1950 2000
0
50
100
150
200
Proxy data also support the proposition
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Paleomonsoonal variations based on TK χlf
Chronology of TK sediments:
0 1 2 3 40.5 1.5 2.5 3.5
Average Sedimentation rate= 0.90 mm/yr
Mean Sedimentation rate = 0.99 mm/yr
Age (cal. ka BP)
Dep
th (c
m)
0
100
200
300
400
Average Sedimentation rate= 1.07 mm/yr Mean sedimentation rate
(0.99 mm/yr) used to assignages to different layers in thesediment column.
Depth (cm) CalibratedC-14 ages
364-366 3690 yr. BP
144-146 1620 yr. BP
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1876 AD Drought
1741 AD High Rainfall
! lf
(10-8
m3kg
-1)
0 100 200 300
Ag
e (cal. y
ears B
.P.)
0
500
1000
1500
2000
2500
3000
3500
4000
!fd
(10-8
m3kg
-1)
0 10 20 30
!fd
%
0 5 10 15
!ARM
(10-5
m3kg
-1)
0.0 0.5 1.0 1.5
SIRM
(10-5
Am2kg
-1)
0 500 1000
SIRM/ ! lf
(103 Am
-1)
3 4 5 6
!ARM/SIRM
(10-5
mA-1
)
0 80 160 240
!ARM
/! lf
2 4 6 8 10
!ARM
/!fd
0 80 160
S-ratio
0.8 0.9 1.0
HIRM
(10-5
Am2kg
-1)
0.00 0.05 0.10 0.15
Ag
e (cal. y
ears B
.P.)
0
500
1000
1500
2000
2500
3000
3500
4000
TK
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0 100 200 300
!lf
(10-8
m3
kg-1
)
3.11
2.41
2.26
2.13
1.96
1.64
1.55 1.49
1.40
1.221.13
AD 1260
AD 1890
AD 1741 High Rainfall event
AD 1325
AD 1612AD 1640
AD 1741AD 1845
AD 1505
AD 1876 Drought event
Paleorainfall / PaleoclimateReconstruction
Increase in rainfall during the last 100 years Rainfall was low during AD 1890-1845, 1617 and
1325, and during 1.13 and 1.55 cal. ka B.P. High-rainfall events occurred around AD 1741, 1640,
1505 and 1260, and during 1.22, 1.40 and 1.49 cal. kaB.P.
Rainfall was most deficient during AD 1890-1845 andmost copious during AD 1640 in the past 3,700 years.
Less humid (i.e., slightly arid) conditions prevailed during 1.55-2.5 cal. ka B.P.
Stronger aridity in the pre-2.5 cal. ka B.P. period. Rainfall during the pre-2.5 cal. ka B.P. period was
very low and is comparable to what was received during the most rainfall-deficient period (AD 1890 and 1845).
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Paleomonsoonal variations based on PK χlf
Age-depth model created using P_Sequence model of Oxcal v.4.1 (Ramsey, 2009)
Chronology of PK sediments:
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Core correlation using magnetic susceptibility
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~ 2435‐2189: Highest rainfall during thepast ~3,200 years.
~ 2189‐2143
~ 1486‐1406
~1069‐715: Overall higher rainfall (except for a brief arid period) ‐‐ Medieval warm period?
~ 647‐559
~ 350‐200: Increasing trend of rainfall
~ 200‐present: Steady rainfall
High rainfall periods /events
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~ 2435‐2189
~ 2143‐1486
~1406‐1069
~ 715‐647
~ 559‐350: Lowest rainfall ‐ LiPle Ice Age?
Periods of lowbut steady rainfall
Low rainfall periods /events
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TK SPECTRAL ANALYSIS
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• The raw χlf data revealed sta1s1cally significantperiodici1es of 906, 232, 147, 128, 96, 61, 54 and 44years
Frequency (1/year)
0.000 0.005 0.010 0.015 0.020
Decib
el (d
B)
10
20
30
40
50
60
70
Bias corrected spectrum (!lf)
Theoretical red noise spectrum
95 % Significance
906
232147 128
9661
54 44
6-db Bandwidth = 0.0006 a
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PK SPECTRAL ANALYSIS
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Periodicity in Magne*c suscep*bility (χlf) data plo5ed using REDFIT 3.8
Frequency (1 / year)
0.00 0.02 0.04 0.06 0.08
De
cib
el(d
B)
0
8
16
24
32
40
48
56
Bias corrected spectrum ( !lf )
Theoretical Red noise spectrum
90% Chi
95% Chi
1256
383129-28 25-23
15 14.4 13.5 12.2
11.5
6 dB bandwidth=4.82x10-4
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TSI (Wm-2)
1360 1362 1364 1366 1368
Year A
.D
800
900
1000
1100
1200
1300
1400
1500
1600
1700
1800
1900
! lf (x 10-8 m3/kg)
0 20 40 60 80
200
400
600
800
1000
1200
Dalton min.
Maunder min.
Sporer minimum
Wolf minimum
Oort minimum
Littile
Ice
ag
eM
ed
iev
al W
arm
p
erio
d
Years
Cal B
.P
COMPARISON OF χlf WITH TOTAL SOLAR IRRADIANCE
χlf TSI
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CONCLUDING REMARKS The χlf of TK and PK principally controlled by
catchment rainfall. Positive correlation between instrumental rainfall
record and χlf of TK and PK. The χlf-rainfall correlation bolstered by historical
records and proxy rainfall data for different parts ofthe country.
Aridity up to 2.5 cal ka B.P. recorded in the TKprofile supported by paleoclimate records fromRajasthan lakes, Nilgiri peat deposits, and westernArabian Sea sediment cores.
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First investigation to propose magneticsusceptibility as a proxy for rainfall in tropicalregions.
Opens up the prospect of obtaining paleorainfalldata from thousands of lakes in Southern India thathave not been studied so far.
These future investigations may provide ageographically widespread data-set onpaleorainfall that would help construct an All India“Paleo” Summer Rainfall Time Series and to betterpredict rainfall Aim of ASIA 2K PROJECT
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Acknowledgements
• Indian Space Research Organization (GBP)for financial assistance.
• Council of Scientific & Industrial Research(CSIR) and University Grants Commission(UGC), Government of India for researchfellowships
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THANKS FORYOUR TIME ANDATTENTION