Monsoonal Fluctuations vs Marine Productivity during Past 10,000 Yrs 21

2C H A P T E R

Monsoonal Fluctuations vs MarineProductivity during Past 10,000

Years — A Study Based on SedimentCore Retrieved from Southeastern

Arabian Sea

V. Yoganandan, C. Krishnaiah1, K. Selvaraj2,G.V. Ravi Prasad and Koushik Dutta3

Department of Marine Science, Bharathidasan UniversityTiruchirappalli – 620024, India

1Ocean and Atmospheric Science & Technology CellDepartment of Marine Geology, Mangalore University

Mangalagangotri – 574199, India2Institute of Marine Geology and Chemistry

National Sun Yat-sen UniversityKaohsiung 804, Taiwan, R.O.C.

3Institute of Physics, Bhubaneswar – 751005, Indiayoganandan1@rediffmail.com

INTRODUCTION

The intensity of the Indian monsoon has varied greatly over the past glacial-interglacial cycles as well as on shorter time scales (Duplessy, 1982; VanCampo, 1986; Clemens et al., 1991; Sirocko et al., 1993; Reichart et al.,1998; Von Red et al., 1999). It is well known that seasonal variation in theheating of the southern Asian continent produces a semiannual reversal inwind direction over the northern Indian Ocean (Wyrtki, 1973). In summer(June-September), the intense SW monsoon winds cause strong mixing ofthe water column and upwelling which eventually promotes high productivity

J. Sundaresan et al. (eds.), Climate hange and Island and Coastal Vulnerability, DOI 10.1007/978-94-007-6016-5_2, © 2013 Capital Publishing Company

C 21

22 V. Yoganandan et al.

in the Arabian Sea. In contrast, during winter, reversal of weak and NWwind direction relatively lowers the productivity in the Arabian Sea (Quasim,1977). It was demonstrated that biological productivity and terrigenous supplyin the Arabian Sea is strongly linked to the intensity of the monsoon, althoughthe lowest biological productivity was noticed during inter-monsoon periodin the Arabian Sea. These strong seasonal contrasts during summer, winterand inter-monsoons also reflect in water column productivity in the ArabianSea and therefore, considered as an excellent natural laboratory to study thepaleomonsoon and associated productivity fluctuations. So carbonate andorganic matter percentage of the undisturbed marine sediment core can beused to unravel the paleomonsoon and paleoproductivity fluctuation of theparticular region.

Detailed studies have been carried out in the western and northeasternArabian Sea to understand the productivity variation and terrigenous supplyin relation to the strength of the SW monsoon during the Late Quaternary(Shimmield et al., 1990; Clemens et al., 1991; Anderson et al., 1993; Naiduet al., 1995). Nonetheless, limited studies have been made from the westerncontinental margin of India with a main focus on the solid phase productivityindicators, calcium carbonate and organic matter, especially in terms ofpaleomonsoon fluctuations (Naidu, 1991; Paropkari et al., 1991; Thambanet al., 2001; Sarkar et al., 2000; Bhushan et al., 2001; Pattan et al., 2003).

In this paper we present the paleproductivity and paleomonsoonfluctuations based on the sedimentary calcium carbonate and organic carbondown sediment core variations (4.2 m long gravity core raised from theupper continental slope of southeastern Arabian Sea). Down-core variationsof both parameters as well as textural parameters show productivity changesfrom late glacial through Holocene period.

MODERN CLIMATE AND OCEANOGRAPHY

The Arabian Sea experiences extremes in atmospheric forcing that lead tothe greatest seasonal variability. Modern surface circulation in the ArabianSea is modulated by the seasonal variation of the monsoonal wind system.This seasonal reversal of wind direction drives a strong southwest (SW)monsoon during the summer (June-September) and a moderate, dry northeast(NE) monsoon during the winter (December-February). Much of the intensityof the SW monsoon is derived from direct heating of the troposphere aboveAsia and through latent heat collected over southern subtropical Indian Ocean,which is transported across the equator and released by precipitation overSouth Asia (Clemens et al., 1991). The precipitation pattern over the Indianpeninsula is controlled by the Western Ghats, the major physiographic featurealong the west coast of India with elevations above 1000 m.

The region off southwestern India is characterized by a weak upwellingsystem during the summer monsoon. It has been observed that the upwellingalong this coast begins as early as February, well before the onset of favourable

Monsoonal Fluctuations vs Marine Productivity during Past 10,000 Yrs 23

southwest monsoon winds (Shetye, 1984). Recent studies have demonstratedthat this early upwelling is a consequence of remote forcing by winds in theBay of Bengal and southwest coast of India via the poleward propagation ofKelvin waves along the west coast of India, which propagates rossby waves(Mc Mcreary et al., 1993; Shanker and Shetye, 1997). With the onset of theSW monsoon, the enhanced local winds intensify the upwelling effect.

MATERIALS AND METHODS

A 4.2 m long gravity core was collected from the southeastern part of theArabian Sea (10°30 N and 75°22 E; Fig. 2.1) at the water depth of 460 mduring the 215th Cruise of O.R.V. Sagar Kanya which has been used for thisstudy. The core was sub-sampled onboard at 2 cm intervals for the top 1 mand 5 cm interval for the rest of the core. All the sub-samples were ovendried at 55°C. Textural analysis of the sediments was carried out on 60representative sub-samples as per the standard procedure (Folk, 1968). Allthe sub-samples were finely powdered using an agate pestle and mortar fororganic carbon and carbonate determinations. Organic matter was determinedas readily oxidizable organic carbon by acid dichromate digestion andsubsequent titration with ferrous ammonium sulphate (Gaudette et al., 1974).Calcium carbonate (CaCO3) was determined as Ca by EDTA titration usingP&R as internal indicator (Shapero and Brannock, 1962). Specimens ofplanktonic foraminifera were picked from the >250 μm sediment fraction atfive selected depth intervals in core SK215/5 (Table 2.1) for 14C dating.

Fig. 2.1: Study area and location of the sediment core SK-215/5 and the Core GC5.

24 V. Yoganandan et al.

These specimens were dated with Accelerator Mass Spectrometry (AMS) atthe Institute of Physics, Bhubaneswar, India. Age model was then obtainedby applying a reservoir age correction of R138 ± 64 years to the 14C datesassumed for the core location (SE Arabian Sea off Malabar coast) (Southonet al., 2002). The conventional AMS 14C dates were calibrated into calendarages using the latest database of CALIB rev 5.0.2 program (modified fromthe version CALIB 5.0) (Stuiver and Braziunas., 1993) and linearlyinterpolated to provide a continuous age scale.

Table 2.1: Details of AMS 14C ages and calibrated calendar ages ofselected depth intervals of the core SK-215/5

Lab ID Sample ID Depth 14C age Cal14C Cal ageInterval (BP) age (BP) (kyr BP)(cm bsf)

693 SK-215/5 44 3392 ± 112 3068 ± 302 2.90694 SK-215/5 98 6029 ± 112 6299 ± 299 6.36695 SK-215/5 145 7530 ± 210 7885 ± 419 7.89696 SK-215/5 215 8924 ± 131 9468 ± 392 9.35697 SK-215/5 360 10,929 ± 122 12,244 ± 509 12.27

RESULTS

Texture Profile

Sand content of the core SK-215/5 fluctuates between 1% and 13% for thelast ~13.5 kyr. Depth profile of sand shows low contents; maximum ~2%was recorded in sediments deposited between ~13.5 and 8 kyr BP (Fig. 2.2),indicating less terrigenous input into the study area. An abrupt increasingtrend of sand was evident around 8 kyr BP and reaches its maximum (13%)at ~6.4 kyr BP, suggesting increased terrigenous input between these timeintervals. The profile reveals that terrigenous input was more or less constantduring the middle and latter part of the Holocene. Similar to sand, very lowclay contents between 4% and 10% suggest an unchanged chemical weatheringof continental rocks in the western part of India during the last 13.5 kyr. Inthe entire core, the dominant textural fraction of the sediment is silt, whichvaries between 80% and 96%. In general, minimum clay and maximum siltcharacterise the sediments of Late Glacial Maximum and early Holocene,whereas the maximum sand characterises the mid and late Holocene, indicatingenhanced detrital input.

Carbonate Profile

Carbonate content varies between 34% and 7% (Fig. 2.2). Sediments depositedbetween ~13.5 and 8 kyr BP show low CaCO3 content of around 7%.

Monsoonal Fluctuations vs Marine Productivity during Past 10,000 Yrs 25

Sediments of mid-Holocene show as high as 34% of CaCO3. Late Holocenesediments show an average carbonate content of 29% with significantvariations that is two highest percentage (34%) peaks at mid-Holocene at~6.3 kyr BP (98 cm of the core) and again at ~3 kyr BP (44 cm of the core).The reduced percentage (29%) at ~4.2 kyr BP (68 cm of the core) and againthe reduced values we could see after ~2 kyr BP. Clear increasing trend ofCaCO3 from 7% to 30% very consistent to sand increase and silt decreasebetween ~8 and 6 kyr BP indicating an enhanced supply of terrigenous inputinto the Seas.

Organic Carbon Profile

Down-core Corg variation of the core SK-215/5 fluctuates between 4% and2% (Fig. 2.2) with a Holocene average Corg content of 3.6%. The profileshows in general a decreasing trend since ~10 kyr BP with minimum valuesof Corg prior to the Holocene. This trend as well as a steady increase fromearly to mid-Holocene periods that reaches the highest percentage of Corg 4.6at ~3.8 kyr BP around 58 cm of the core and reduces to 3.6% at ~3 kyr BP(46 cm of the core) and again shows an increased value of 4% occuring at1.8 kyr BP.

Fig. 2.2: Down-core profiles of sand, silt, clay, CaCO3 and organic carbon in sedimentcore SK-215/5. All values are given in percentages. Gray bar highlightsmid to late Holocene increased productivity.

DISCUSSION

The core SK-215/5 extends up to the late Glacial period (13.5 kyr BP) andthis core has five age control points which reveal different sedimentationrates: late Holocene 15.4 cm/kyr, early Holocene 24.8 cm/kyr and late Glacialperiod records 31.16 cm/kyr. This record is comparable with sedimentation

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rate of shallow depth sediment core GC-5 (water depth: 280 m) studied byThamban et al. (2001). The major difference between these two records isSK-215/5 shows an abrupt increase of sedimentation rate from the late Glacialto the early Holocene period which is not evident in GC-5 records. Thisincreased sedimentation rate of SK-215/5 during late Glacial and earlyHolocene period may be due to high intensity monsoon/arid conditions inthe hinterlands that might have increased the sediment depositional rate ofSK-215/5 when compared to the shallow depth core GC-5.

Down-core profiles (Fig. 2.2) show reduced % of CaCO3, Corg andreduced sand content and subsequent increased % of silt and clay between~13.5 and ~8 kyr BP. This observation indicates reduced productivity duringlate Glacial to early Holocene period. This reduced productivity may be dueto dilution of water column by fresh water input when the monsoon wasintense, that might have altered the physico-chemical properties of the watercolumn that favours the growth of the marine organisms. This interpretationis comparable with modern conditions along the southeastern Arabian Seawhich gives supporting evidence to this argument. During intense monsoonlow saline water column was observed in southeastern Arabian Sea (Naqvi,1991; Stramma et al., 1996). Previous paleomonsoonal studies from theeastern Arabian Sea suggesting that intense precipitation had occurred duringlate Glacial to early Holocene period on the Indian subcontinent, which iscoinciding with the Northern Hemisphere summer insolation maxima (Prell,1984; Van Campo, 1986; Sirocko et al., 1993). Several workers have reportedthe major climatic, hydrographic and circulation change in the Indian monsoonregime immediately after ~16 kyr BP (Van Campo, 1986; Naqvi andFairbanks, 1996; Overpeck et al., 1996; Thamban et al., 2001). The 13Crecord of peat from the Nilgiri Hills of the Western Ghats (South India)revealed that following dry LGM, moist conditions started at ~16 kyr BP(Sukumar et al., 1993). The recent multi proxy studies from Arabian Sea hasproved that the summer monsoon, in general, was strongest in the earlyHolocene marked by high amplitude shifts between dry and wet phases(Guptha et al., 2005; Thamban et al., 2007; Tiwari and Ramesh, 2007;Yoganandan et al., 2009). The increased content of silt and clay and relativelyhigh sedimentation rate are characteristic of late Glacial to early Holocenesediments from this region which was same as recorded in the earlier study(Thamban et al., 1997).

After this reduced productivity period all these proxies of the presentstudied core show distinct shift from low to high values between ~8.1 and6.3 kyr BP. This shift indicates a major change in monsoonal rains fromstrong to weak Indian summer monsoon. The recent study from Arabian Seaalso suggests gradual weakening of summer monsoon starting from ~8.2 kyrBP and this climate shift event is recorded in even other parts of the world(Guptha et al., 2005). Our data further shows that a significant shift (abrupt

Monsoonal Fluctuations vs Marine Productivity during Past 10,000 Yrs 27

increasing trend) in carbonate % occurs between ~7 and 6.3 kyr BP, correlatingwith strong to weak summer monsoon transition during the mid-Holocene inIndia. A combination of archaeological and other land records in the Indiansubcontinent also supports a substantial weakening of the summer monsoonat ~7 kyr BP (Gupta et al., 2006). After this major shift the increased % ofCaCO3, Corg during the entire late Holocene period shows an increasedproductivity. This increased productivity may be due to weakening of summermonsoon which might have reduced fresh water input to the core site whichmight have favoured the marine organism to get suitable water column. Thismade the water column to become highly productive. This similar productivitytrend was recorded by Thamban et al. (2001) and Pratima et al. (2010). Theobserved textural variation i.e. increased percentages of sand and silt after~8.1 kyr BP may be due to rapid sea level rise which might have deliveredthe coarser grains to the core site due to coastal terrain erosion to maintaina dynamic equilibrium with static sea level.

The interesting observation from late Holocene section is the reductionof CaCO3 and increase of Corg at ~4.2 kyr BP and ~2 kyr BP. This reducedproductivity during these periods is due to the high intensity fresh waterinput resulting in dilution effect. This interpretation is supported by Indusriver increased discharge events at ~4 kyr BP, revealed from the varvesediments record (Von Red et al., 1999). High intensity monsoon event at ~2kyr BP is also recorded in an earlier study from the present study area(Thamban et al., 2001; Yoganandan et al., 2009). This reduced productivityevents recorded from the present studied sediment core revealed thatproductivity of the southeastern Arabian Sea is highly influenced bypaleomonsoon intensity of this region.

CONCLUSION

Texture, CaCO3 and OC data of the present studied core recorded late Glacialto early Holocene reduced productivity and gradual increasing trend after~8 kyr BP and reaches maximum ~6 kyr BP. Thereafter the increasedproductivity is continued till present except two major reduced productivityevents. The interesting observation from this study shows that the watercolumn productivity of the southeastern Arabian Sea, particularly southwestcontinental margin of India, is influenced by the paleoclimatic/paleomonsooncondition of the region, which is proved by the reduced productivity recordof late Glacial to early Holocene; and ~4.2 kyr BP and ~2 kyr BP werethe periods which injected large amount of fresh water to the southeasternArabian sea due to high intensity of the monsoon. These high intensitymonsoon periods were recorded very well in paleoclimatic studies fromIndian continent.

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