Paleo-monsoon history of the last two millennia from ... · Paleo-monsoon history of the last two...

Paleo-monsoon history of the last twomillennia from Southern Indian lake

sediment magnetism

R ShankarMangalore University

INDIA

Email: rshankar_1@yahoo.com

Contributors:

Anish K Warrier, Sandeep K & W Zhou

IN THIS TALK:

• Introduction

• Study area

• Methodology

• Results and Discussion

• Conclusions

• Acknowledgements

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

• 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

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.

2) Pookot Lake (PK)

Area : 0.085 sq. km.Annual rainfall : ~ 4000 mmMain rock types: Hornblende-biotite, gneiss and charnockite.

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

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.

Magnetic InstrumentationMS2 suscep*bility meter

ARM a5achment withshielded demagne*ser

Magnetometer

Pulse magne*ser

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

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

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)

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

• 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

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

!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

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

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

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

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

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

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:

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)

!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

• 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

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).

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

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.

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

A suggested model

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).

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

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

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

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

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).

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:

Core correlation using magnetic susceptibility

~ 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

 ~ 2435‐2189 

~ 2143‐1486

~1406‐1069

~ 715‐647 

~ 559‐350: Lowest rainfall ‐ LiPle Ice Age?

Periods of lowbut steady rainfall

Low rainfall periods /events

TK SPECTRAL ANALYSIS

• 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

PK SPECTRAL ANALYSIS

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

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

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.

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

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

THANKS FORYOUR TIME ANDATTENTION