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Reconstruction of the variability of SW monsoon during the past 3 Ka from the continental margin of the South Eastern Arabian Sea
| Content Provider | Semantic Scholar |
|---|---|
| Author | Chauhan, Onkar S. Vogelsang, Elke Basavaiah, Nathani Kader, U. Syed Abdul |
| Copyright Year | 2010 |
| Abstract | From temporal variation in δO in G. ruber and G. sacculifer and geochemical indices of weathering/erosion (CIA, Al and Ti), we infer rapid Southwest Monsoon deterioration with dwindled fluvial and detrital fluxes during ~450-650, 1000, and 1800-2200 cal y BP during the Late Holocene. We have evaluated the role of solar influx (reconstructed) and high latitude climate variability (archived in GRIP and GISP-2 cores) on the Southwest monsoon precipitation (SWM). Broadly, our δO climate reconstruction is concordant with GRIP and GISP-2, and supports a teleconnection through atmospheric connection between the SWM and the North Atlantic climate -albeit temporal extents of LIA and MWP from high latitude are not entirely coeval. Moreover, there is humid climate and enhanced precipitation during terminal stages of the Little Ice Age. The medieval warming (~800-1300 AD) is not synchronous also, and is punctured by an arid event centered at 1000y BP. Though the delineation of the specific influence of solar influx on the SWM precipitation is elusive, we surmise that the SWM precipitation is a complex phenomenon and local orography along the SW India may have a role on the entrapment of moisture from the SW trade winds, while these brace land. Introduction Long term variations in the Asian monsoon, the major source of moisture for the Indian Subcontinent, have been attributed to Milankovich orbital parameters; mountain plateau orography; atmospheric CO2; and glacial surface boundary conditions etc. (Prell and Kutzbach, 1992). In alternative thesis, solar forcing has been postulated to cause rapid climatic instabilities in the monsoonal regime during past two millennia (Eddy, 1976; Bond et al., 2001; Haigh, 2001; Agnihotri et al., 2002; Rind, 2002; Gupta et al., 2003; Sinha et al., 2007). Influence of ENSO though Walker circulation is also suggested (Hasternath and Greichar, 1993; Tiwari et al., 2006; Saraswat et al., 2007). It is postulated further that the monsoon regime has a teleconnection with that of the climate of the North Atlantic (Sirocko et al., 1996; Gupta et al., 2003). However, documentation and linkage of abrupt, high frequency instabilities in the SWM precipitation with that of internal as well as external forcing is still in the state of infancy. One of the reasons contributing to debilitate in such studies is scanty, high resolution, continuous records of climate variability with decadal – centenary scale resolution from the Arabian Sea. Some few millennia precipitation history is available (Sinha et al., 2007; Yadava et al., 2004; Tiwari et al., 2005), though these records are either short mostly for less than a millennium or are not continuous (Staubwasser et al., 2003; Tiwari et al., 2005). A five ka history of fluvial influx of the River Indus has been reconstructed (von Rad et al., 1999), Staubwasser et al. (2003) have also provided some insight in the climate variability since the Mid Holocene. The Indus has large catchment and hinterland in semi-arid region. Considerable flux of the Indus is known to advect along-shelf and sinks inland (Chauhan et al., 2006). Being fed from the Himalayan ice melt as well as precipitation in the hinterland, its discharge is governed by a complex phenomenon. Conventionally, the Western Arabian Sea, having dominant evaporation over precipitation even though does not receive precipitation, has been a host of studies for estimating SWM intensity from upwelling or wind strength (Overpeck et al., 1996; Sirocko et al., 1996; Schulz et al., 1998; Anderson et al., 2002; Gupta et al., 2003). However, there exists no clue whether such proxies of climate reconstruction are coeval with the SWM precipitation in the other climatological regime with dominant precipitation over evaporation. We have attempted to decipher variations in the SWM precipitation during past three millennia from orographically enhanced precipitation (interception of the southwest trade wind by the hilly region, elevated about 2000-3000 m above MSL, known as the Western Ghat) along the SW continental margin of India, carried as fluvial flux into the sea influencing regional sea surface salinity (SSS). Even though our core is not laminated, a high sedimentation rate and sub-century scale resolution of our data have helped us to assess if there exists any link among the SWM precipitation, the multi-decadal the North Atlantic climate instabilities (GRIP and GISP-2 cores) and reconstructed variability in solar irradiance. Modern climatology and Hydrography The South East (SE) Arabian Sea, in contrast to its western counter part, has seasonal sea surface salinity (SSS) variation regulated by the SWM precipitation (Fig. 1). Inflowing into the Indian subcontinent, the SWM trade winds braces landmass here. Except during SWM, the region receives insignificant and episodic rainfall. During SWM (June-September) in the coastal region precipitation is moderate (150 cm)., Because of enhanced moisture condensation by orographic influence, inland precipitation enhances to ~500 cm in the Western Ghats, the hilly region adjacent to coastal plains (Chauhan and Gujar, 1996; Fig. 1). The SE Arabian Sea, therefore, receives high freshwater influx through several short, high gradient rivers; important among these are the Kallada, the Karmana, the Neyyar and the Tamraparni (Fig. 1). Except for the Kallada, these rivers do not have an estuarine mouth (Table 1; Aswathanarayana, 1964; Mallik et al., 1989; Nagam Aiya, 1989) and the fluvial flux is directly carried into the sea. Lower salinity has been observed in the vicinity of the core site (JuneJanuary; values 34-33‰; Levitus and Boyer, 1994; Levitus et al., 1994; Fig. 1). In contrast to the wind driven upwelling regions of the Western Arabian Sea, our study area has dominace of precipitation over evaporation. The SSS is significantly modified by the SWM precipitation, though there is no distinct SST instability associated with the SWM winds (Levitus and Boyer, 1994; Levitus et al., 1994). The hydrography of the area is seasonal. Polewards surface circulation during the North Eastern Monsoon (NEM) reverses and turns equatorwards during SWM. Materials and Methods From the low SSS regions off the precipitation dominated the SW continental margin of India (JuneJanuary; values 33-34‰ Fig. 1; Levitus and Boyer, 1994; Levitus et al., 1994), during various cruises of the R.V. Gaveshni and ORV Sagar Kanya, 23 cores were collected. Among these, Core 3904 only (Latitude 08 07.9’ N; Longitude 76 01.8’ E; water depth 1400 m; Fig. 1) was found to be: (i) suitable for deciphering the sub-century scale events on account of the highest rate of sedimentation, (ii) located in the area in which, as determined from the available data, the overlying waters are anoxic and bioturbation is expected to be minimal due to high productivity (Sarkar et al., 2000; Qasim, 1977), (iii) entirely turbidity free, and (iv) regional SSS in this area is influenced by SWM. The upper 58 cm of the core was sub-sampled (44 levels). For δO (18O/16O) sample/(18O/16O)standard – 1) measurements, about 25-30 tests (size 315-400 μm, crushed, ultrasonic cleaned in ethanol) of G. ruber ( s.s. white), G. sacculifer (without sac), and G. menardii were analyzed on a VG Micromass against the Peedee Belemenite (PDB) standard at the University of Kiel. The results have a standard deviation of 0.04‰ for oxygen and 0.05‰ for carbon isotopic analyses. The samples were wet sieved to separate out +63 μm fractions, while other fractions (+ 45, 25, 15, and 4 μm) were separated using standard pipette methods (Folk, 1966). Associated with high productivity in the region (Qasim, 1977), we have found high shell contents in +63 and +45 μm fractions, which are likely to influence estimation of continental Ca and terrigenous flux. We have treated bulk samples with 10% acetic acid to remove the biogenic carbonate. Chemical analyses are made on treated samples on a Philips XRF (model PW 1480) at UCSI, Delhi. The precision is obtained against standard SDO-1-3, SCO-1 and MAG-1, and it is better that +5%. Chemical Index of Alter (CIA) is obtained from CIA = molar ratio of Al2O3/(Al2O3+CaO+Na2O+K2O)x100 (Nesbitt and Young, 1982). Al and Ti are viewed as resistant elements, and an indicator of detrital flux (Juyal et al., 2009). Besides detrital contribution, Aluminum may be contributed from scavenging, volcanic dust and aeolian sources in the marine environment (Murray et al., 1993; Murray and Leinen, 1996; Pattan and Shane, 1999). In order to nullify any effect of these, we have estimated structurally unsupported detrital terrigenous fraction from following relation (Pattan and Shane, 1999): Alexc = Altotal –(Tisample x (Al/Tishale) Where Tishale = 0.6 The excess Al, even though is found to be very negligible, has been subtracted from the total Al content to obtain detrital Al. Chronology For most of the studies in the Arabian Sea, ages are measured at irregular intervals of several tens of cm (Anderson et al., 2002; Gupta et al., 2003; Tiwari et al., 2005, 2006; and references cited therein). The events are then assigned ages by extrapolation, assuming a constant rate of sedimentation between these levels. However, due to variations in the environment of deposition, variable productivity and terrigenous and biogenic flux, sedimentation is unlikely to be linear. The points of measurements, though presumed, may not be actual levels on which the sedimentation rates actually altered in a sediment column. Therefore, for most of the paleoclimatic reconstruction from this region, specifically in the areas of non-linear sedimentation, the age models by and large have poor chronometric control. In order to have better chronometirc control, four AMS ages (Poznan radio carbon laboratory, Poland) were obtained on G. ruber (250-415 μm size) to specifically date the pronounced instabilities (Table 2). In addition 8 bulk C ages obtained at each 4-6 cm interval were als |
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| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
