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Orbital frequencies in radiolarian assemblages of the central Indian Ocean: implications on the Indian summer monsoon
Palaeogeography, Palaeoclimatology, Palaeoecology 197 (2003) 97^112 www.elsevier.com/locate/palaeo
Orbital frequencies in radiolarian assemblages of the central Indian Ocean: implications on the Indian summer monsoon Shyam M. Gupta National Institute of Oceanography (NIO), Dona Paula, Goa 403 004, India Received 3 September 2001; received in revised form 13 May 2002; accepted 1 February 2003
Abstract Radiolarian distribution in surface sediments and its relationships with overlying surface oceanography provided impetus for down-core faunal variation related to changes in monsoon intensity due to the Earth’s orbital eccentricity in the geologic past. In present study, a high resolution (Nt = 4.75 ka) radiolarian distribution for the last 485 ka in a core (AAS 2/3) from the central Indian Ocean Basin revealed orbital and sub-orbital cycles. Relationships between temporal distribution of: (1) high-salinity Pyloniids and Didymocyrtis; (2) low-salinity Spongodiscids and Spongaster; (3) transitional Euchitoniids; and (4) southern Anthocyrtidium assemblages, and the orbital forcing (ETP, i.e. the normalized sum of the Earth’s eccentricity, tilt and precession), and the May^July insolation were analyzed. Spectral analyses revealed significant cycles of Pyloniids (400-, 126-, 95-, 54-, 41-, 31-, 23-, 19-, 17-, and 15-ka), Spongaster (95-, 41-, 29-, 23-, 19-, 17-, and 15-ka), Euchitoniids (126-, 95-, 51-, 31-, 21-, 18-, and 17-ka), and Anthocyrtidium (400-, 69-, 41-, 31-, 23-, 19-, and 15-ka). Cross-spectral analysis between Pyloniids and ETP suggested coherent Pyloniid cycles lagging ETP by 9 ka at 100-ka eccentricity, while both were in-phase ( 6 2 ka) at 41-ka tilt and 23-ka precession cycles. Coherent Spongaster cycles led ETP by 38 ka at 100-ka eccentricity, 15 ka at 41-ka tilt, 5 ka at 23-ka precession cycles. Coherent Anthocyrtidium cycles led ETP by 14 ka at 100-ka eccentricity, 10 ka at 31-ka tilt, and were in-phase at 41-ka tilt as well as 23- and 19-ka precession cycles. Similarly, a radiolarian monsoon index defined as the normalized sum of Pyloniids, Didymocyrtis, Spongodiscids, Spongaster, Euchitoniids, and Anthocyrtidium (PDSSEA) led ETP at 54- and 23-ka cycles, while both were nearly in-phase at 41-, 31- and 15-ka cycles. Coherent PDSSEA cycles led insolation during May^July at 8‡S (core-site) at 23-ka precession, lagged 100-, 54- and 29-ka cycles and were in-phase at 41-ka tilt and 15-ka sub-orbital cycles. The results suggest that radiolarian assemblages exhibit proxy-monsoon cycles due to the Earth’s orbital eccentricity at 400-, 126- and 95-ka, tilt at 54-, 41- and 31-ka, next to precession at 23-, 19- and 17-ka as well as insolation forcings at the core-site during the Late Quaternary. A 2003 Elsevier Science B.V. All rights reserved. Keywords: orbital frequencies; radiolarian assemblages; Indian Summer Monsoon; Late Quaternary; Central Indian Ocean
1. Introduction
* Fax: +91-832-2456702/2456703. E-mail addresses: [email protected] (S.M. Gupta), [email protected] (S.M. Gupta).
On the Indian subcontinent the monsoon rains from June to September are the backbone of the agrarian economy, as the majority of conventional agriculture still depends on rain-fed irrigation.
0031-0182 / 03 / $ ^ see front matter A 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0031-0182(03)00388-2
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98 S.M. Gupta / Palaeogeography, Palaeoclimatology, Palaeoecology 197 (2003) 97^112
PALAEO 3112 17-7-03 Fig. 1. Location map generated by GMT map-site (http://aquarius.geomar.de) incorporating the surface sediment distribution of radiolarian Q-mode factors of Gupta et al. (1996) and Gupta and Fernandes (1997), and the drainage pattern of the Indian rivers into the Bay of Bengal (a). High-salinity ( s 35 ppt) at 50-m water depth towards the Arabian Sea during September (b), generated by NOAA salinity data site ingrid\\ldeo\\columbia\\edu\\SOURSE\\LEVITUS94, is characterized by a high abundance of Pyloniids, while low-salinity ( 6 34.5 ppt) towards the Bay of Bengal is dominated by Spongaster. Transitional salinity is characterized by a high abundance of Euchitoniids, while the southern watermass (10‡S) is characterized by the Anthocyrtidium assemblage. The rectangle forms the core-site of AAS 2/3. Filled circles are the surface sediments of Gupta et al. (1996) used in the Q-mode factor analysis of radiolarian groups. The faunal inserts are obtained by scanning electron microscopy and photomicrographs.
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Therefore, to ascertain the future monsoon intensities, it is required to know the monsoonal patterns and their forcing mechanism in the past. Much work has been done using foraminiferal percentage variations of Globigerina bulloides as the proxy of upwelling due to strong monsoonal wind forcing o¡ the Oman Margin (Prell, 1984; Clemens et al., 1991). However, another e¡ect of the monsoon are rains over the Indian subcontinent, and the majority ( s 80%) of rainwater drains as freshwater runo¡ by the Indian rivers, consequently lowering the surface salinity in the Bay of Bengal. Therefore, temporal changes in the salinity-sensitive proxies for the monsoon from the northern Indian Ocean may provide better insight on the past behavior of the monsoon rains, which may supplement the wind-resulted monsoonal upwelling index of G. bulloides o¡ Oman. Earlier, radiolarian studies by Gupta (1991, 1996, 1999), Gupta and Srinivasan (1992), and Gupta and Fernandes (1995, 1997, 1998) identi¢ed salinity-sensitive radiolarian assemblages in the central Indian Ocean, emphasizing that they could be used to decipher monsoonal changes in the geologic past. Here, temporal (down-core) variation of such salinity-sensitive radiolarian assemblages for the last 485 000 years is presented, and compared with the orbital and solar insolation forc-
99
ings over monsoon intensity during the Late Quaternary.
2. Radiolarians, surface oceanography and rationales A Q-mode factor analysis of radiolarians from the surface sediments revealed four statistically signi¢cant factors (Fig. 1a), characterized by: (1) Pyloniids and Didymocyrtis ; (2) Spongaster and Spongodiscids ; (3) transitional Euchitoniids; and (4) southern ( s 10‡S) Anthocyrtidium assemblages (Gupta et al., 1996). Taxonomic frame work of Pyloniids, Didymocyrtis, Spongaster, Spongodiscids, Euchitoniids, and Anthocyrtidium groups is based on the descriptions and illustrations in Riedel et al. (1985), Gupta and Srinivasan (1992), Gupta (1991, 1996), and Gupta and Fernandes (1995). The constituent species counted in the taxonomic groups are summarized in Table 1. As radiolarian plankton maximum coincides with chlorophyll-a maximum (Dworetzky and Morley, 1987) between 30 and 50 m water depth (Pollenhue et al., 1993), the surface salinity at 50 m at the end of monsoon (September) is presented generalizing regional dominance of the radiolarian fauna in the central Indian Ocean (Fig.
Table 1 Details of six radiolarian faunal groups, constituent species, and their taxonomic references SN Taxonomic group Constituent species 1
Pyloniids
2
Didymocyrtis
3
Spongodiscids
4
Spongaster
5
Euchitoniids
6
Anthocyrtidium
References
Tetrapyle octacantha, Octopyle stenozona, Hexapyle dodecantha
Riedel et al., 1985; Mullineaux and Westberg-Smith, 1986, p. 66, pl. 2, ¢g. 2; Gupta, 1991, pl. 10, ¢gs. 15^ 21; Gupta and Srinivasan, 1992, pl. 2, ¢gs. 15,16. Didymocyrtis tetrathalamus Riedel et al., 1985; Mullineaux and Westberg-Smith, 1986, p. 64, pl.2, ¢g. 1. Didymocyrtis group Gupta, 1991, pl. 8, ¢gs. 3^8. Spongotrochus glacialis, Spongodiscus resurgense, Riedel et al., 1985; Mullineaux and Westberg-Smith, Spongodiscus biconcavus, Spongurus spp., 1986, pl. 3, ¢gs. 11a,b; Gupta, 1991, pl. 16, ¢gs. 26^31; Spongopyle osculosa Gupta and Srinivasan, 1992, pl. 3, ¢gs. 26^28 and 52. Spongaster tetras, S. tetras irregularis, Riedel et al., 1985; Mullineaux and Westberg-Smith, Spongocore puella 1986, p. 65, pl. 1, ¢g. 13; Gupta, 1991, pl. 9, ¢gs. 23^ 24, pl. 10, ¢gs. 2^4. Euchitonia elegans, E. furcata, Euchitonia sp., Riedel et al., 1985, p. 505, pl. 2, ¢gs. 2a,b; Echitoniids Dictyocoryne profunda, D. truncata, group Gupta, 1991, pl. 8, ¢gs. 9^12; Gupta and Hymeniastrum euclidis Srinivasan, 1992, pl. 1, ¢gs. 15^18. A. ophirense, A. zanguebarcium Mullineaux and Westberg-Smith, 1986, pl. 3, ¢gs. 11a,b; Gupta, 1991, pl. 16, ¢gs. 26^31; Gupta and Srinivasan, 1992, pl. 3, ¢gs. 26^28 and 52.
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1b). High abundance of Pyloniids and Spongaster in surface sediments were reported from regions of higher salinity towards the Arabian Sea and lower salinity towards the distal Bay of Bengal, while Euchitoniids and Anthocyrtidium represented transitional and southern waters ( s 10‡S), respectively (Gupta and Fernandes, 1997), during the SW monsoon (Fig. 1b). Correlation analyses of the relationship between the surface salinity during July^August (Wyrtki, 1971) and the percentage of Pyloniids and Spongaster in surface sediments (Gupta, 1991, 1996) suggested positive and negative relationships (Table 2), respectively, with two distinct surface-salinity water masses during the July^August monsoonal months in the central Indian Ocean. The diagonal salinity contrast in the central Indian Ocean (Wyrtki, 1971) due to monsoon rains and freshwater runo¡ towards the Bay of Bengal (Gupta and Fernandes, 1997, see ¢gs. 4 and 5) is re£ected well in the radiolarian assemblages (Fig. 1). Similarly, the secondary constituents of Factor-1, Didymocyrtis, and Factor-2, Spongodiscids, also exhibited distributions similar to their primary constituents in the region (Gupta, 1996; Gupta and Fernandes, 1997). In present study, all these faunal groups were studied down the core (AAS 2/3) to infer paleomonsoonal changes during the Late Quaternary.
3. Sediment core and time control The core AAS 2/3 is located at 7‡48PS, 80‡E
(water depth 5463 m; core length 512 cm) near the 34.8 ppt iso-saline front (Fig. 1). It exhibited color laminations of dark and light brown layers of V1 cm thickness. As the core is from a depth below carbonate compensation depth, it is devoid of calcium carbonate, and hence N18 O stratigraphy and AMS C14 dating were not possible. With similar limitations, Morley and Hays (1981) used three datum levels based on: (1) Stylatractus universus ; (2) P. lacunosa; and (3) Brunhes/Matuyama magnetic reversal in addition to Cycladophora davisiana abundance stratigraphy for time series analysis. Similarly, C. davisiana abundance was compared with the Earth’s orbital tilt, insolation and N18 O, revealing climatic changes in subtropical radiolarian sediment o¡ Japan, where it lagged the tilt forcing by V9 ka (Morley and Heusser, 1997). In another example, Mommersteeg et al. (1995) analyzed variation in pollen dated by three magnetostratigraphic datums at V0.1, 128, and 783 ka for a 975-kalong time series, and found that the spectral results were acceptable as means to search for orbital cycles. The criterion used to date core AAS 2/3 is similar to these studies. Core AAS 2/3 is dated using the ¢rst appearance datum of Buccinosphaera invaginata at 185 cm (V180 ka), and the extinction of Stylatractus universus at 435 cm (425 ka; Gupta, 1999, see ¢g. 2), which are the radiolarian synchronous magnetobiostratigraphic datum levels (Fig. 2a) of Johnson et al. (1989). In addition, the Pyloniid abundance stratigraphy for tropical radiolarian ooze (Gupta, 2000,2002), which is similar to Cyclado-
Table 2 Spearman rank correlation (STATISTIX 3.1) between the six radiolarian faunal groups from surface sediments (Gupta, 1991, 1996) and surface salinity during July^August and SST during August (Wyrtki, 1971)
Anthocyrthidum Didymocyrtis Euchitoniids Pyloniids SAL July^August Spongaster Spongodiscids SST August
Anthocyrthidum
Didymocyrtis
Euchitoniids Pyloniids SAL July^August Spongaster
Spongodiscids
SST August
1.0000 0.2311 30.5020 0.0035 0.1755 30.2923 30.3190 30.1341
1.0000 30.3631 0.4033 0.7339 30.4640 30.5626 30.6904
1.0000 30.2597 30.3860 0.0997 30.0263 0.3031
1.0000 0.4687
1.0000
1.0000 0.5119 30.5511 30.5552 30.4001
1.0000 30.4649 30.5014 30.6692
1.0000 0.7718 0.3658
Note correlation values (r) s 0.48, s 0.39 and s 0.30 are signi¢cant at 0.001 (99.9%), 0.01 (99%) and 0.05 (95%) levels, respectively. (Cases: 42; missing cases: 0; 1 March 2002, 10:00 h.).
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Fig. 2. Time series of Pyloniids in core AAS 2/3 compared with (a) the Earth’s orbital forcing ETP, (b) insolation at 8‡S during May^July, (c) insolation at 65‡N during June, and (d) SPECMAP N18 O stack (Imbrie et al., 1984) for the last 485 ka. Note the striking similarities between the pairs of time series. The synchronous radiolarian biostratigraphic datum of ¢rst appearance of Buccinosphaera invaginata at V180 ka, and the last appearance of Stylatractus universus at V425 ka in core (Gupta, 1999) are also shown (a).
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Fig. 3. Radiolarian faunal variation of high-salinity fauna, Pyloniids and Didymocyrtis (a), low-salinity fauna, Spongodiscids and Spongaster (b), transitional fauna, Euchitoniids (c), southern 10‡S hydrographic front fauna, Anthocyrtidium (d), and a combined radiolarian monsoon index PDSSEA with May-July insolation at the core site (e) for the last 485 ka in core AAS 2/3.
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phora davisiana abundance stratigraphy (Morley and Hays, 1981), is used to de¢ne the age model of the core. Radiolarian biostratigraphy (Gupta, 1999, see ¢g. 2) and the Pyloniid abundance stratigraphy (Gupta, 2000,2002) based age model (Fig. 2a) yielded a sedimentation rate of 1.05 cm/ka with a time spacing of Nt = 4.75 ka, dating the core-base at V485 000 years. Besides, close correspondence of Pyloniids with ETP (the normalized sum of the Earth’s eccentricity, tilt and precession), insolations at 65‡N during June, and over the core-site 8‡S during May^July, and the SPECMAP-N18 O stack (Imbrie et al., 1984) provided additional con¢dence in the time control of core AAS 2/3 (Fig. 2a^d).
4. Down-core faunal variations The abundance variations of Pyloniids, Didymocyrtis, Spongodiscids, Spongaster, Euchitoniids, and Anthocyrtidium constitute down-core faunal time series (Fig. 3). The peaks of Pyloniids and Didymocyrtis characterize high-salinity Factor-1 and Spongaster and Spongodiscids characterize low-salinity Factor-2 of Gupta and Fernandes (1997), and suggest changes in monsoonal salinity in the central tropical Indian Ocean (Fig. 3a,b). Similarly, variations in Euchitoniids characterize transitional Factor-3, and Anthocyrtidium characterizes 10‡S hydrographic
103
front Factor-4 (Gupta and Fernandes, 1997), and indicate changes in the respective watermasses during the last 485 ka (Fig. 3c,d). In order to synthesize changes in these six faunal groups, a strategy similar to that used for ETP (involving normalization and summation of the eccentricity, tilt and precession; Imbrie et al., 1984) was applied to derive a radiolarian faunal index for the monsoon. Abundances of six faunal groups (Fig. 3a^d) were normalized by their means, and then summed to derive a faunal monsoon index named as PDSSEA: an acronym for Pyloniids+Didymocyrtis+Spongodiscids+Spongaster+Euchitoniids+ Anthocyrtidium. The PDSSEA time series compares well with solar insolation over the coresite (8‡S) during May^July (21 April^21 July), calculated in Analyseries (Paillard et al., 1996) using Berger’s (1978) astronomical equations (Fig. 3e), which suggest the in£uence of tropical insolation also, besides the insolation at 65‡N, over the Asian monsoon.
5. Spectral and cross-spectral analyses Pestiaux and Berger (1984) and Mommersteeg et al. (1995) suggested that no single method is ideal for detecting climatic cycles in a given time series. Hence, the climatic cycles in faunal data, listed in Table 3, were veri¢ed by using the multitaper (Thomson, 1990) method (MTM) by the
Table 3 List of cycles observed in radiolarian faunal groups, the PDSSEA monsoon index in core AAS 2/3, and insolation at the coresite during May^July SN
Groups/index
Cycles by multi-taper (Jtap.X) method (Lees and Park, 1995)
Cycles by SPECTRUM method (Schulz and Stattegger, 1997)
1
Pyloniids
400, 126, 95, 54, 41, 31, 23, 19, 17, and 15 ka.
2
Spongaster
5
Euchitoniids
3
Anthocyrtidium
4
PDSSEA index
5
Core-site insolation during May^July
s 95%: 31, 23, 19, 17, 15, and 13 ka; s 90%: 126, 95 and 41 ka. s 95%: 95, 27, 21, 17, and 15ka; s 90%: V51, 41, 19, and 13 ka. s 95%: 51, 33, 27, 17, 15, and 13 ka; s 90%: 400 and 126 ka. s 95%: V400, 69, 21, 17, 15, and 13 ka; s 90%: 33 and 19 ka. s 95%: 41, 29, 23, 19, and 13 ka; s 90%: V400, 95, 33, 17, and 15 ka. s 95%: 400, 100, 54, 41, 23, 19, 17, 15, and 13 ka.
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V95, 41, 29, 23, 19, 17, and 15 ka. 126, 95, 51, 33, 29, 19, 17, and 13 ka. 400, 69, 41, 31, 19, and 15 ka. 129, 95, 29, 23, 19, 15, and 13 ka. 23 and 19 ka.
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JTAP.X program of Lees and Park (1995) as well as spectral and cross-spectral analyses by the SPECTRUM program of Schulz and Stattegger (1997). MTM analysis of Pyloniids exhibited Na|«ve-spectrum, degree of freedom and F-test suggesting s 95% signi¢cant cycles at 31-, 23-, 19-, 17-, 15-, and 13-ka; s 90% at 126-, 95-, and 41-ka, while SPECTRUM revealed 400-, 126-, 95-, 54-, 41-, 31-, 23-, 19-, 17-, and 15-ka cycles (Fig. 4a). These results re-validate the ¢rst reported triplet eccentricity cycles at 400-, 126-
and 95-ka in the tropical paleo-sea surface temperatures (Gupta et al., 1996), which were later found in foraminiferal sand (Muller and MacDonald, 1997), and the N18 O record (Clemens and Tiedmann, 1997). Muller and MacDonald (1997) discussed the 100-ka cycles, believed to be due to eccentricity (e) and argued to assign to inclination (i) of the Earth’s orbital plane, as the eccentricity should have triplet cycles at 400-, 126and 95-ka, according to Berger (1977). For an excellent review on the enigmatic problem of ec-
a
b
c
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Fig. 4. Multi-taper spectral analyses (Lees and Park, 1995) showing degrees of freedom at 90% and 95% statistical signi¢cance for the F-test and Na|«ve spectrum, and by SPECTRUM (Schulz and Stattegger, 1997) for Pyloniids (a), Spongaster (b), Euchitoniids (c), Anthocyrtidium (d), monsoon-index PDSSEA (e), and insolation at core-site for 21 April^21 July (f). Note statistically signi¢cant cycles at 90% and 95% in multi-taper and s 80% signi¢cant (error bars) cycles in SPECTRUM results.
centricity vs. inclination driving the 100-ka cycles may be referred to Kerr (1997). Therefore, the Pyloniid cycles at V400-, 126- and 95-ka are due to the Earth’s eccentricity (Berger, 1977; Gupta et al., 1996; Gupta, 1999; Clemens and Tiedmann, 1997; Muller and MacDonald, 1997), while the 54-, 41- and 31-ka cycles are due to axial-tilt as discussed by Gupta (1999) and Melice
et al. (2001). It is contrary to the prevailing notion that the 100- and 41-ka climatic cycles are related only to eccentricity and tilt forcing, respectively (Prell, 1984; Clemens et al., 1991, 1996), and provides evidence for the triplet cycles at eccentricity and tilt bands. Similarly, the triplet 23-, 19- and 17-ka Pyloniid cycles (Fig. 4a) are due to precession, while the 15- and 13-ka cycles are sub-orbit-
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0 +5 ka s 90% s 90% 0 +5 ka s 90% s 90% 0 31.1 ka s 90% s 90% +14 ka 0 31.1 ka 6 80% s 90% s 90% +40 ka 35.7 ka +5.7 ka s 90% s 90% s 90% +37 ka +15 ka +5 ka 6 80% s 90% s 90% 38 ka +18 ka 31.2 ka s 90% V90% s 90%
Blanks represent the non-deterministic coherence/phase. Phase angles were converted to ka (phase angle/360*cycle). Negative phases are lags, + leads, whereas 0 stands for in-phase relationships between the fauna and the ETP or insolation. SPECTRUM settings are ofac-8, hifac-1, segments 2, detrended, Welch window I, 6-dB bandwidth 0.0049, signi¢cance 0.2 (80%) (Schulz and Stattegger, 1997).
Phase
39 ka 32 ka 0 s 90% s 90% s 90% 100 ka 41 ka 23 ka
Phase Coh Phase Coh Phase Coh Phase Coh Phase Coh Phase Coh Coh Coh
Phase
Pyloniids vs. insolation 8‡S M^J Cycles
Pyloniids vs. ETP
Spongaster vs. ETP
Spongaster vs. insolation 8‡S M^J
Anthocyrtidium vs. ETP
Anthocyrtidium vs. insolation 8‡S M^J
PDSSEA vs. ETP
PDSSEA vs. insolation 8‡S M^J
S.M. Gupta / Palaeogeography, Palaeoclimatology, Palaeoecology 197 (2003) 97^112 Table 4 Coherency and phase at the 95-ka (eccentricity), 41-ka (tilt) and 23-ka (precession) cycles among Pyloniids, Spongaster, Anthocyrtidium, and PDSSEA monsoon index vs. ETP and insolation at core-site during May^July
106
al (Berger, 1977; Treut et al., 1988; Willes et al., 1999). MTM analysis of Spongaster revealed s 95% signi¢cance at the 95-, 27-, 21-, 17-, and 15-ka cycles, and s 90% signi¢cance at the V51-, 41-, 19-, and 13-ka cycles, while SPECTRUM suggested V95-, 41-, 29-, 23-, 19-, 17-, and 15-ka cycles (Fig. 4b). MTM of Euchitoniids revealed s 95% signi¢cance at the 51-, 33-, 27-, 17-, 15-, and 13-ka cycles, and s 90% signi¢cance at the 400- and 126-ka cycles, while SPECTRUM suggested 126-, 95-, 51-, 33-, 29-, 19-, 17-, and 13-ka cycles (Fig. 4c). MTM of Anthocyrtidium exhibited s 95% signi¢cance at the V400-, 69-, 21-, 17-, 15-, and 13-ka cycles, and s 90% signi¢cance at the 33- and 19-ka cycles, while SPECTRUM revealed cycles at V400, 69, 41, 31, 19, and 15 ka (Fig. 4d). MTM of PDSSEA monsoon index revealed s 95% signi¢cant cycles at 41, 29, 23, 19, and 13 ka, and s 90% signi¢cant cycles at V400, 95, 33, 17, and 15 ka, while SPECTRUM suggested signi¢cant cycles at 129, 95, 29, 23, 19, 15, and 13 ka (Fig. 4e). Similarly, MTM of insolation during May^July at core-site exhibited s 95% signi¢cant cycles at 400, 100, 54, 41, 23, 19, 17, 15, and 13 ka, while SPECTRUM revealed similar cycles of which, however, the most dominant ones are at 23 and 19 ka (Fig. 4f). Cross-spectrum analyses between fauna and ETP as well as insolation (Figs. 2 and 3) by SPECTRUM (Schulz and Stattegger, 1997) were performed using settings of over-sampling factor 8, highest frequency factor 1, segments 2, detrended, Welch window, 80% con¢dence, bandwidth 6-dB = 0.0049, and coherence and phase results (Table 4), and the results are presented in Fig. 5. Cross-spectrum analysis between Pyloniids and ETP suggested that coherent ( s 80%) Pyloniids lag ETP by V9 ka at 100-ka eccentricity, 2 ka at 41-ka tilt, 0 ka (inphase) at 23-ka precession cycles (Fig. 5a). Spongaster vs. ETP crossspectrum exhibited coherent cycles at 54, 41, 23, 17, and 15 ka, and Spongaster leads ETP by 38 ka at 126-ka eccentricity, 15 ka at 41-ka tilt, 5 ka at 23-ka precession cycles (Fig. 5b). The transitional Euchitoniids assemblage did not show reliable cross-spectral results. The Anthocyrtidium assemblage exhibited coherent cycles at 41, 31, 23, 19,
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and 17 ka, and it leads ETP by 14 ka at 126-ka eccentricity, in-phase (0 ka) at 41-ka tilt and 23-ka precession cycles, while it lags V10 ka at 31-ka tilt cycle (Fig. 5c). Cross-spectrum of PDSSEA monsoon index exhibited coherent cycles at 51, 41, 33, 23, and 15 ka, it leads ETP at 54 and 23 ka substantially, while both are nearly in-phase at the 41-, 31- and 15-ka cycles (Fig. 5d). Similarly, coherent cycles are to be found at 41, 23 and 15 ka for PDSSEA and summer insolation at 8‡S over the core-site, with a slight lead of PDSSEA over the insolation at 23 ka, and the in-phase relationship at 41- and 15-ka cycles (Fig. 5e). In order to illustrate the lead or lag relationship in the faunal time series with the ETP as discussed above, the representative pair of Pyloniids and ETP was selected, and their signi¢cant cycles were ¢ltered using Analyseries software. Gaussian ¢ltering of the Pyloniids and ETP time series at frequencies (cycles) 0.008 U 0.001 (126 ka), 0.0105 U 0.001 (95 ka), 0.0185 U 0.001 (54 ka), 0.024 U 0.001 (41 ka), 0.0325 U 0.0018 (31 ka), 0.043 U 0.003 (23 ka), 0.052 U 0.0018 (19 ka), and 0.065 U 0.003 (15 ka) were performed and compared (Fig. 6). The 400-ka cyclic component (frequency 0.0025 U 0.05) of eccentricity present in the Pyloniid time series was not ¢ltered and illustrated as it could be argued of being a longterm trend, because the length of the time series (485 ka) is slightly higher ( s 20%) than the 400ka period. Filtered Pyloniids and ETP cycles due to the Earth’s eccentricity at 126 and 95 ka; axialtilt at 54, 41, and 31 ka; precession at 23 and 19 ka (Fig. 6a^g), and a sub-orbital cycle at 15 ka (Fig. 6h) exhibited phase relationships shown in Fig. 5 in time domain. The 95- and 41-ka eccentricity and tilt related Pyloniid cycles marginally lag the ETP, while their 23-ka precession cycles exhibited an in-phase (zero phase) relationship (Fig. 6b,d,f) concurring to the SPETRUM results shown in Table 1. Similarly, the 126-ka eccentricity, 54- and 31-ka tilt componentrelated Pyloniid cycles lag the ETP (Fig. 6a,c,e) whereas the 19-ka precession and 15-ka cycles suggest a nearly in-phase relationship between the two (Fig. 6g,h). Similarly, cyclic components of other faunal time series may be ¢ltered and
107
may reveal phase relationships expressed in Table 4. Earlier, Prell (1984) suggested coherence between monsoon index Globigerina bulloides, the ETP and insolation at the 23-ka precession cycle, which lagged 4^6 ka depending on insolation considered at 21 July or 21 June, respectively, attributing that the lag of 4^6 ka is physically insignificant. As the insolation calculated herein is from 21 April to 21 July considering the length of summer heating of the equatorial Indian Ocean, and evaporation thereof resulting in monsoonal clouds, naturally the lag value may vary with that of Prell. Pyloniids lag insolation during May^July at 8‡S by 8 ka at 100-ka eccentricity, lead 18 ka at 41-ka tilt, and is in-phase at 23-ka precession cycles (Table 4). Clemens et al. (1991, table 1) reported a lag of 7.7^6.6 ka by G. bulloides, opal (radiolaria and diatom) and barium monsoon indices over the ETP at the coherent 23-ka precession cycles. Later, Clemens et al. (1996) suggested that the timing (phase) of strong monsoons changed by 5 ka at precession and 14 ka at tilt cycles relative to phase of the maximum global ice volume (N18 O). Morley and Heusser (1997) reported that ETP and Chinese monsoon are coherent (0.84 coherency), while ETP led Chinese rains by 1.4, 13 and 8.7 ka at the 100-, 41- and 23-ka cycles, respectively. In the present paper, six monsoon salinity-sensitive radiolarians and their PDSSEA suggest a phase range between 0 to 8 ka at precession and tilt cycles, respectively, which is closer to the phases reported by Prell (1984) and Morley and Heusser (1997). Besides, it also reveals triplet cycles due to eccentricity (400, 126 and 95 ka), tilt (54, 41 and 31 ka), and precession (23, 19 and 17 ka) as professed by Berger (1977), as well as sub-orbital 15-, 13- and 11-ka cycles which are not reported so far from a monsoon record. In brief, it may be surmised that salinity-sensitive radiolarian faunal assemblages and monsoon index PDSSEA revealed monsoonal cycles due to orbital forcing and insolation corresponding to multiple cycles of the eccentricity, tilt, precession, and sub-orbital bands during the Late Quaternary in the central Indian Ocean.
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6. Conclusions Ever since the orbital forcing on climate change was discovered (Hays et al., 1976), the 100-, 41-
and 23-ka cycles are invariably corroborated due to eccentricity, tilt, and precession, respectively. However, Berger (1977) cautioned, emphasized, and professed about the existence of multiple
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109
Fig. 5. Cross-spectrum results by SPECTRUM program showing coherence and phase spectrum between the ETP and Pyloniids (a), Spongaster (b), Anthocyrtidium (c), PDSSEA (d), and PDSSEA and insolation at core-site (e). Marginal deviation of phase spectrum from zero-phase line suggests that Pyloniid faunal variation (a) is as good as the ETP cycles with an accuracy of 6 U 8 ka at 100-ka and U 1 ka at 23-ka Milankovitch cycles.
cycles due to the Earth’s eccentricity at 400, 126 and 95 ka, tilt at 54, 41 and 31 ka, and precession at 23, 19 and 17 ka, besides 69- and 15-ka climatic cycles due to astronomical forcing. Radiolarian faunal assemblages and the PDSSEA monsoon index invariably resulted in multiple cycles due to eccentricity (400, 126 and 95 ka), tilt (54, 41 and 23 ka) and precession (23, 19 and 17 ka) besides the sub-orbital cycles at 15 and 13 ka in core AAS 2/3. On the contrary, Prell (1984) and Clemens et al. (1991, 1996) used Globigerina bulloides, opal £ux and barium as upwelling indices o¡
Oman due to monsoonal winds, and dealt with the 100-, 41- and 23-ka cycles in detail. It is for the ¢rst time that a monsoonal record revealed almost all the cycles professed by Berger (1977) not only in one species of G. bulloides (Prell, 1984), but several radiolarian faunal indices. Cross-spectral results between three radiolarian assemblages and the PDSSEA monsoon index with ETP as well as insolation at the core-site suggest a time lag ranging from 0 to 8 ka for the 23-ka precession cycle, which is within the range reported by earlier monsoonal studies
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a
e
b
f
c
g
d
h
Fig. 6. Signi¢cant cyclic components of Pyloniids and ETP time series (Fig. 2a) ¢ltered out by Analyseries showing phases in time domain for the 126- and 95-ka eccentricity (a,b), 54-, 41- and 31-ka tilt (c^e), 23- and 19-ka precession (f,g), and the 15-ka sub-orbital cycles.
(Prell, 1984; Clemens et al., 1991, 1996). The results suggest that the Indian monsoon, though dominated by the precession cycles (23 and 19 ka; Clemens et al., 1991), is also in£uenced by orbital eccentricity (400, 126 and 95 ka), axialtilt (41- and 31-ka cycles) forcing, and sub-orbital 15- and 13-ka cycles.
Acknowledgements I thank the Director of the NIO, Dr. Eherlich Desa, for use of the facilities and encouragement, and Professor C.N.R. Rao, Jawaharlal Nehru Centre of Advanced Scienti¢c Research, Bangalore, for awarding the Rajiv Gandhi Foundation
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Grant for innovative ideas in science and technology to support my research at NIO. Sincere thanks are expressed to A.L. Berger and an anonymous reviewer for constructive criticism and suggestions. I also thank Bjo«rn A. Malmgren for critical evaluation of the data, analyses, and suggesting improvements to the ¢nal manuscript. Thanks are also to D. Paillard and M. Schulz for providing the ANALYSERIES and SPECTRUM programs, respectively, and to B.N. Nath and V.N. Kodagali for assistance in various matters. The Council of Scienti¢c and Industrial Research and the Department of Ocean Development, New Delhi, are thankfully acknowledged for laboratory support and onboard ship-logistics. The paper is dedicated to my father late Shri Kanhaiya Lal Gupta. This is the National Institute of Oceanography, Goa, India, Contribution No. 3799.
References Berger, A.L., 1977. Support for the astronomical theory of climatic change. Nature 269, 44^45. Berger, A.L., 1978. Long term variations of daily insolations and Quaternary climatic changes. J. Atmos. Sci. 35, 2362^ 2367. Clemens, S.C., Tiedmann, R., 1997. Eccentricity forcing of Pliocene^Pleistocene climate revealed in a marine oxygenisotope record. Nature 385, 301^304. Clemens, S.C., Prell, W.L., Murray, D., Shimmield, G., Weedon, G., 1991. Forcing mechanisms of the Indian Ocean monsoon. Nature 353, 720^725. Clemens, S.C., Murray, D.W., Prell, W.L., 1996. Non-stationary phase of the Plio^Pleistocene Asian monsoon. Science 274, 943^948. Dworetzky, B., Morley, J.J., 1987. Vertical distribution of radiolarians in the eastern equatorial Atlantic: Analysis of multiple series of closely spaced plankton tows. Mar. Micropaleontol. 12, 1^19. Gupta, S.M., 1991. Radiolarian Distribution in Surface Sediments from the Central Indian Ocean Basin and its Paleoceanographic signi¢cance. Ph.D. Thesis, Goa University, 338 pp. Gupta, S.M., 1996. Quantitative radiolarian distribution in the surface sediment from the central Indian Ocean and their paleomonsoonal signi¢cance. J. Geol. Soc. India 47, 339^ 354. Gupta, S.M., 1999. Radiolarian monsoonal index Pyloniids respond to astronomical forcing in the last 500,000 years
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in the central Indian Ocean Basin. Man and Environment 24, pp. 99^105. Gupta, S.M., 2000. Pyloniid stratigraphy ^ a new tool to date tropical radiolarian oozes as the Cycladophora davisiana stratigraphy in high latitudes and SPECMAP N18 oxygen stack: Evidence from the central Indian Ocean. In: Carter, E.S., et al. (Eds.), Abstracts, Ninth Meeting of the International Association of Radiolarian Paleontologists, p. 33. Gupta, S.M., 2002. Pyloniid stratigraphy ^ a new tool to date tropical radiolarian ooze from the central Indian Ocean. Mar. Geol. 184, 85^93. Gupta, S.M., Fernandes, A.A., 1995. Supra-generic radiolarian transfer function based Late Miocene paleomonsoonal changes in the northeastern tropical Indian Ocean. Bull. Indian Geol. Assoc. 28, 29^51. Gupta, S.M., Fernandes, A.A., 1997. Quaternary radiolarian faunal changes in the central Indian Basin: Inferences to paleomonsoonal oscillation of the 10‡S hydrographic front. Curr. Sci. 72, 965^972. Gupta, S.M., Fernandes, A.A., 1998. Report-II: Radiolarian Neogene Climate Change Project. National Institute of Oceanography, Goa, 45 pp. Gupta, S.M., Fernandes, A.A., Mohan, R., 1996. Tropical sea surface temperatures and the Earth’s eccentricity cycles. Geophys. Res. Lett. 23, 3159^3162. Gupta, S.M., Srinivasan, M.S., 1992. Late Miocene radiolarian biostratigraphy and paleoceanography of Neill Island, Andaman, India. Micropaleontology 38, 209^236. Hays, J.D., Imbrie, J., Shackleton, N., 1976. Variations in the Earth’s orbit: Pacemaker of the Ice-Ages. Science 194, 1121^ 1132. Imbrie, J., Hays, J.D., Martinson, D.G., McIntyre, A., Mix, A.C, Morley, J., Pisias, N.G., Prell, W.L., Shackleton, N.J., 1984. Orbital theory of climate: Support from a revised chronology of the record. In: Berger, A.L., et al. (Eds.), Milankovitch and Climate, Part I. Riedel, Dordrecht, pp. 269^305. Johnson, D.A., Schneider, D.A., Nigrini, C., Caulet, J.P., Kent, D.V., 1989. Pliocene^Pleistocene radiolarian events and magneto-stratigraphic calibrations for the tropical Indian Ocean. Mar. Micropaleontol. 14, 33^66. Kerr, R.A., 1997. Upstart Ice age theory gets attentive but chilly hearing. Science 227, 183^184. Lees, J.M., Park, J., 1995. Multi-taper spectral analysis: A stand-alone C-subroutine. Comput. Geol. 21, 199^236. Melice, J.L., Coran, A., Berger, A.L., 2001. Amplitude and fequency modulations of the Earth’s oliquity for the last million years. J. Clim. 14, 1043^1054. Mommersteeg, H.J.P.M., Loutre, M.F., Young, R., Wijmstra, T.A., Hooghiemstra, H., 1995. Orbital forced frequencies in the 975000 year pollen record from Tenagi Philippon (Greece). Clim. Dynamics 11, 4^24. Morley, J.J., Hays, J.D., 1981. Towards a high resolution, global deep sea chronology for the last 750,000 years. Earth Planet. Sci. Lett. 53, 279^295. Morley, J.J., Heusser, L.E., 1997. Role of orbital forcing in East Asian monsoon climates during the last 350 kyr: Evi-
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dence from terrestrial and marine climate proxies from core RC14-99. Paleoceanography 12, 483^493. Muller, R.A., MacDonald, G.J., 1997. Simultaneous presence of orbital inclination and eccentricity in the proxy climate records from Ocean Drilling Program site 806. Geology 25, 3^6. Mullineaux, L.S., Westberg-Smith, M.J., 1986. Radiolarians as paleoceanographic indicators in Miocene Monterey Formation, upper New Port Bay, California. Micropaleontology 32, 48^71. Paillard, D., Labeyrie, L., Yiou, P., 1996. Machintosh program performs time series analysis. EOS Trans. AGU. 77, 379. Pestiaux, P., Berger, A.L., 1984. An optimal approach to the spectral characteristics of deep sea climate record. In: Berger, A.L., et al. (Eds.), Milankovitch and Climate, Part I. Riedel, Dordrecht, pp. 417^445. Pollenhue, F., Klein, B., Zeitschel, B., 1993. Low light adaptation and export production in deep chlorophyll ^ a maximum layer in the northern Indian Ocean. Deep Sea Res. 40, 737^752. Prell, W.L., 1984. Monsoonal climate of the Arabian Sea during the Late Quaternary: A response to changing solar ra-
diation. In: Berger, A.L., et al. (Eds.), Milankovitch and Climate, Part I. Riedel, Dordrecht, pp. 349^366. Riedel, W.R., Westberg, M.J., Budai, A., 1985. Late Neogene Radiolaria and Mediterranean paleoenvironments. In: Stanley, D.J., Wetzel, F.C. (Eds.), Geological Evolution of the Mediterranean Basin. Springer, Berlin, pp. 487^523. Schulz, M., Stattegger, K., 1997. Spectral analysis of unevenly spaced paleoclimatic time series. Comput. Geosci. 23, 929^ 945. Thomson, D.J., 1990. Quadratic-inverse spectrum estimates: Application to paleoclimatology. Philos. Trans. R. Soc. Lond. A-332, 539^597. Treut, H.L., Portes, J., Jouzel, J., Gill, M., 1988. Isotopic modeling of climatic oscillations: Implications for a comparative study of marine and Ice core records. J. Geophys. Res. 93, 9365^9383. Willes, K.J., Kleczkowski, A., Briggs, K.M., Gilligen, C.A., 1999. Role of sub-Milankovitch climate forcing in initiation of northern hemisphere glaciation. Science 285, 568^571. Wyrtki, K., 1971. The Oceanographic Atlas of the International Indian Ocean Expedition. National Science Foundation, Washington, DC, 531 pp.
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Orbital frequencies in radiolarian assemblages of the central Indian Ocean: implications on the Indian summer monsoon Shyam M. Gupta National Institute of Oceanography (NIO), Dona Paula, Goa 403 004, India Received 3 September 2001; received in revised form 13 May 2002; accepted 1 February 2003
Abstract Radiolarian distribution in surface sediments and its relationships with overlying surface oceanography provided impetus for down-core faunal variation related to changes in monsoon intensity due to the Earth’s orbital eccentricity in the geologic past. In present study, a high resolution (Nt = 4.75 ka) radiolarian distribution for the last 485 ka in a core (AAS 2/3) from the central Indian Ocean Basin revealed orbital and sub-orbital cycles. Relationships between temporal distribution of: (1) high-salinity Pyloniids and Didymocyrtis; (2) low-salinity Spongodiscids and Spongaster; (3) transitional Euchitoniids; and (4) southern Anthocyrtidium assemblages, and the orbital forcing (ETP, i.e. the normalized sum of the Earth’s eccentricity, tilt and precession), and the May^July insolation were analyzed. Spectral analyses revealed significant cycles of Pyloniids (400-, 126-, 95-, 54-, 41-, 31-, 23-, 19-, 17-, and 15-ka), Spongaster (95-, 41-, 29-, 23-, 19-, 17-, and 15-ka), Euchitoniids (126-, 95-, 51-, 31-, 21-, 18-, and 17-ka), and Anthocyrtidium (400-, 69-, 41-, 31-, 23-, 19-, and 15-ka). Cross-spectral analysis between Pyloniids and ETP suggested coherent Pyloniid cycles lagging ETP by 9 ka at 100-ka eccentricity, while both were in-phase ( 6 2 ka) at 41-ka tilt and 23-ka precession cycles. Coherent Spongaster cycles led ETP by 38 ka at 100-ka eccentricity, 15 ka at 41-ka tilt, 5 ka at 23-ka precession cycles. Coherent Anthocyrtidium cycles led ETP by 14 ka at 100-ka eccentricity, 10 ka at 31-ka tilt, and were in-phase at 41-ka tilt as well as 23- and 19-ka precession cycles. Similarly, a radiolarian monsoon index defined as the normalized sum of Pyloniids, Didymocyrtis, Spongodiscids, Spongaster, Euchitoniids, and Anthocyrtidium (PDSSEA) led ETP at 54- and 23-ka cycles, while both were nearly in-phase at 41-, 31- and 15-ka cycles. Coherent PDSSEA cycles led insolation during May^July at 8‡S (core-site) at 23-ka precession, lagged 100-, 54- and 29-ka cycles and were in-phase at 41-ka tilt and 15-ka sub-orbital cycles. The results suggest that radiolarian assemblages exhibit proxy-monsoon cycles due to the Earth’s orbital eccentricity at 400-, 126- and 95-ka, tilt at 54-, 41- and 31-ka, next to precession at 23-, 19- and 17-ka as well as insolation forcings at the core-site during the Late Quaternary. A 2003 Elsevier Science B.V. All rights reserved. Keywords: orbital frequencies; radiolarian assemblages; Indian Summer Monsoon; Late Quaternary; Central Indian Ocean
1. Introduction
* Fax: +91-832-2456702/2456703. E-mail addresses: [email protected] (S.M. Gupta), [email protected] (S.M. Gupta).
On the Indian subcontinent the monsoon rains from June to September are the backbone of the agrarian economy, as the majority of conventional agriculture still depends on rain-fed irrigation.
0031-0182 / 03 / $ ^ see front matter A 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0031-0182(03)00388-2
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PALAEO 3112 17-7-03 Fig. 1. Location map generated by GMT map-site (http://aquarius.geomar.de) incorporating the surface sediment distribution of radiolarian Q-mode factors of Gupta et al. (1996) and Gupta and Fernandes (1997), and the drainage pattern of the Indian rivers into the Bay of Bengal (a). High-salinity ( s 35 ppt) at 50-m water depth towards the Arabian Sea during September (b), generated by NOAA salinity data site ingrid\\ldeo\\columbia\\edu\\SOURSE\\LEVITUS94, is characterized by a high abundance of Pyloniids, while low-salinity ( 6 34.5 ppt) towards the Bay of Bengal is dominated by Spongaster. Transitional salinity is characterized by a high abundance of Euchitoniids, while the southern watermass (10‡S) is characterized by the Anthocyrtidium assemblage. The rectangle forms the core-site of AAS 2/3. Filled circles are the surface sediments of Gupta et al. (1996) used in the Q-mode factor analysis of radiolarian groups. The faunal inserts are obtained by scanning electron microscopy and photomicrographs.
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Therefore, to ascertain the future monsoon intensities, it is required to know the monsoonal patterns and their forcing mechanism in the past. Much work has been done using foraminiferal percentage variations of Globigerina bulloides as the proxy of upwelling due to strong monsoonal wind forcing o¡ the Oman Margin (Prell, 1984; Clemens et al., 1991). However, another e¡ect of the monsoon are rains over the Indian subcontinent, and the majority ( s 80%) of rainwater drains as freshwater runo¡ by the Indian rivers, consequently lowering the surface salinity in the Bay of Bengal. Therefore, temporal changes in the salinity-sensitive proxies for the monsoon from the northern Indian Ocean may provide better insight on the past behavior of the monsoon rains, which may supplement the wind-resulted monsoonal upwelling index of G. bulloides o¡ Oman. Earlier, radiolarian studies by Gupta (1991, 1996, 1999), Gupta and Srinivasan (1992), and Gupta and Fernandes (1995, 1997, 1998) identi¢ed salinity-sensitive radiolarian assemblages in the central Indian Ocean, emphasizing that they could be used to decipher monsoonal changes in the geologic past. Here, temporal (down-core) variation of such salinity-sensitive radiolarian assemblages for the last 485 000 years is presented, and compared with the orbital and solar insolation forc-
99
ings over monsoon intensity during the Late Quaternary.
2. Radiolarians, surface oceanography and rationales A Q-mode factor analysis of radiolarians from the surface sediments revealed four statistically signi¢cant factors (Fig. 1a), characterized by: (1) Pyloniids and Didymocyrtis ; (2) Spongaster and Spongodiscids ; (3) transitional Euchitoniids; and (4) southern ( s 10‡S) Anthocyrtidium assemblages (Gupta et al., 1996). Taxonomic frame work of Pyloniids, Didymocyrtis, Spongaster, Spongodiscids, Euchitoniids, and Anthocyrtidium groups is based on the descriptions and illustrations in Riedel et al. (1985), Gupta and Srinivasan (1992), Gupta (1991, 1996), and Gupta and Fernandes (1995). The constituent species counted in the taxonomic groups are summarized in Table 1. As radiolarian plankton maximum coincides with chlorophyll-a maximum (Dworetzky and Morley, 1987) between 30 and 50 m water depth (Pollenhue et al., 1993), the surface salinity at 50 m at the end of monsoon (September) is presented generalizing regional dominance of the radiolarian fauna in the central Indian Ocean (Fig.
Table 1 Details of six radiolarian faunal groups, constituent species, and their taxonomic references SN Taxonomic group Constituent species 1
Pyloniids
2
Didymocyrtis
3
Spongodiscids
4
Spongaster
5
Euchitoniids
6
Anthocyrtidium
References
Tetrapyle octacantha, Octopyle stenozona, Hexapyle dodecantha
Riedel et al., 1985; Mullineaux and Westberg-Smith, 1986, p. 66, pl. 2, ¢g. 2; Gupta, 1991, pl. 10, ¢gs. 15^ 21; Gupta and Srinivasan, 1992, pl. 2, ¢gs. 15,16. Didymocyrtis tetrathalamus Riedel et al., 1985; Mullineaux and Westberg-Smith, 1986, p. 64, pl.2, ¢g. 1. Didymocyrtis group Gupta, 1991, pl. 8, ¢gs. 3^8. Spongotrochus glacialis, Spongodiscus resurgense, Riedel et al., 1985; Mullineaux and Westberg-Smith, Spongodiscus biconcavus, Spongurus spp., 1986, pl. 3, ¢gs. 11a,b; Gupta, 1991, pl. 16, ¢gs. 26^31; Spongopyle osculosa Gupta and Srinivasan, 1992, pl. 3, ¢gs. 26^28 and 52. Spongaster tetras, S. tetras irregularis, Riedel et al., 1985; Mullineaux and Westberg-Smith, Spongocore puella 1986, p. 65, pl. 1, ¢g. 13; Gupta, 1991, pl. 9, ¢gs. 23^ 24, pl. 10, ¢gs. 2^4. Euchitonia elegans, E. furcata, Euchitonia sp., Riedel et al., 1985, p. 505, pl. 2, ¢gs. 2a,b; Echitoniids Dictyocoryne profunda, D. truncata, group Gupta, 1991, pl. 8, ¢gs. 9^12; Gupta and Hymeniastrum euclidis Srinivasan, 1992, pl. 1, ¢gs. 15^18. A. ophirense, A. zanguebarcium Mullineaux and Westberg-Smith, 1986, pl. 3, ¢gs. 11a,b; Gupta, 1991, pl. 16, ¢gs. 26^31; Gupta and Srinivasan, 1992, pl. 3, ¢gs. 26^28 and 52.
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1b). High abundance of Pyloniids and Spongaster in surface sediments were reported from regions of higher salinity towards the Arabian Sea and lower salinity towards the distal Bay of Bengal, while Euchitoniids and Anthocyrtidium represented transitional and southern waters ( s 10‡S), respectively (Gupta and Fernandes, 1997), during the SW monsoon (Fig. 1b). Correlation analyses of the relationship between the surface salinity during July^August (Wyrtki, 1971) and the percentage of Pyloniids and Spongaster in surface sediments (Gupta, 1991, 1996) suggested positive and negative relationships (Table 2), respectively, with two distinct surface-salinity water masses during the July^August monsoonal months in the central Indian Ocean. The diagonal salinity contrast in the central Indian Ocean (Wyrtki, 1971) due to monsoon rains and freshwater runo¡ towards the Bay of Bengal (Gupta and Fernandes, 1997, see ¢gs. 4 and 5) is re£ected well in the radiolarian assemblages (Fig. 1). Similarly, the secondary constituents of Factor-1, Didymocyrtis, and Factor-2, Spongodiscids, also exhibited distributions similar to their primary constituents in the region (Gupta, 1996; Gupta and Fernandes, 1997). In present study, all these faunal groups were studied down the core (AAS 2/3) to infer paleomonsoonal changes during the Late Quaternary.
3. Sediment core and time control The core AAS 2/3 is located at 7‡48PS, 80‡E
(water depth 5463 m; core length 512 cm) near the 34.8 ppt iso-saline front (Fig. 1). It exhibited color laminations of dark and light brown layers of V1 cm thickness. As the core is from a depth below carbonate compensation depth, it is devoid of calcium carbonate, and hence N18 O stratigraphy and AMS C14 dating were not possible. With similar limitations, Morley and Hays (1981) used three datum levels based on: (1) Stylatractus universus ; (2) P. lacunosa; and (3) Brunhes/Matuyama magnetic reversal in addition to Cycladophora davisiana abundance stratigraphy for time series analysis. Similarly, C. davisiana abundance was compared with the Earth’s orbital tilt, insolation and N18 O, revealing climatic changes in subtropical radiolarian sediment o¡ Japan, where it lagged the tilt forcing by V9 ka (Morley and Heusser, 1997). In another example, Mommersteeg et al. (1995) analyzed variation in pollen dated by three magnetostratigraphic datums at V0.1, 128, and 783 ka for a 975-kalong time series, and found that the spectral results were acceptable as means to search for orbital cycles. The criterion used to date core AAS 2/3 is similar to these studies. Core AAS 2/3 is dated using the ¢rst appearance datum of Buccinosphaera invaginata at 185 cm (V180 ka), and the extinction of Stylatractus universus at 435 cm (425 ka; Gupta, 1999, see ¢g. 2), which are the radiolarian synchronous magnetobiostratigraphic datum levels (Fig. 2a) of Johnson et al. (1989). In addition, the Pyloniid abundance stratigraphy for tropical radiolarian ooze (Gupta, 2000,2002), which is similar to Cyclado-
Table 2 Spearman rank correlation (STATISTIX 3.1) between the six radiolarian faunal groups from surface sediments (Gupta, 1991, 1996) and surface salinity during July^August and SST during August (Wyrtki, 1971)
Anthocyrthidum Didymocyrtis Euchitoniids Pyloniids SAL July^August Spongaster Spongodiscids SST August
Anthocyrthidum
Didymocyrtis
Euchitoniids Pyloniids SAL July^August Spongaster
Spongodiscids
SST August
1.0000 0.2311 30.5020 0.0035 0.1755 30.2923 30.3190 30.1341
1.0000 30.3631 0.4033 0.7339 30.4640 30.5626 30.6904
1.0000 30.2597 30.3860 0.0997 30.0263 0.3031
1.0000 0.4687
1.0000
1.0000 0.5119 30.5511 30.5552 30.4001
1.0000 30.4649 30.5014 30.6692
1.0000 0.7718 0.3658
Note correlation values (r) s 0.48, s 0.39 and s 0.30 are signi¢cant at 0.001 (99.9%), 0.01 (99%) and 0.05 (95%) levels, respectively. (Cases: 42; missing cases: 0; 1 March 2002, 10:00 h.).
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Fig. 2. Time series of Pyloniids in core AAS 2/3 compared with (a) the Earth’s orbital forcing ETP, (b) insolation at 8‡S during May^July, (c) insolation at 65‡N during June, and (d) SPECMAP N18 O stack (Imbrie et al., 1984) for the last 485 ka. Note the striking similarities between the pairs of time series. The synchronous radiolarian biostratigraphic datum of ¢rst appearance of Buccinosphaera invaginata at V180 ka, and the last appearance of Stylatractus universus at V425 ka in core (Gupta, 1999) are also shown (a).
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Fig. 3. Radiolarian faunal variation of high-salinity fauna, Pyloniids and Didymocyrtis (a), low-salinity fauna, Spongodiscids and Spongaster (b), transitional fauna, Euchitoniids (c), southern 10‡S hydrographic front fauna, Anthocyrtidium (d), and a combined radiolarian monsoon index PDSSEA with May-July insolation at the core site (e) for the last 485 ka in core AAS 2/3.
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phora davisiana abundance stratigraphy (Morley and Hays, 1981), is used to de¢ne the age model of the core. Radiolarian biostratigraphy (Gupta, 1999, see ¢g. 2) and the Pyloniid abundance stratigraphy (Gupta, 2000,2002) based age model (Fig. 2a) yielded a sedimentation rate of 1.05 cm/ka with a time spacing of Nt = 4.75 ka, dating the core-base at V485 000 years. Besides, close correspondence of Pyloniids with ETP (the normalized sum of the Earth’s eccentricity, tilt and precession), insolations at 65‡N during June, and over the core-site 8‡S during May^July, and the SPECMAP-N18 O stack (Imbrie et al., 1984) provided additional con¢dence in the time control of core AAS 2/3 (Fig. 2a^d).
4. Down-core faunal variations The abundance variations of Pyloniids, Didymocyrtis, Spongodiscids, Spongaster, Euchitoniids, and Anthocyrtidium constitute down-core faunal time series (Fig. 3). The peaks of Pyloniids and Didymocyrtis characterize high-salinity Factor-1 and Spongaster and Spongodiscids characterize low-salinity Factor-2 of Gupta and Fernandes (1997), and suggest changes in monsoonal salinity in the central tropical Indian Ocean (Fig. 3a,b). Similarly, variations in Euchitoniids characterize transitional Factor-3, and Anthocyrtidium characterizes 10‡S hydrographic
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front Factor-4 (Gupta and Fernandes, 1997), and indicate changes in the respective watermasses during the last 485 ka (Fig. 3c,d). In order to synthesize changes in these six faunal groups, a strategy similar to that used for ETP (involving normalization and summation of the eccentricity, tilt and precession; Imbrie et al., 1984) was applied to derive a radiolarian faunal index for the monsoon. Abundances of six faunal groups (Fig. 3a^d) were normalized by their means, and then summed to derive a faunal monsoon index named as PDSSEA: an acronym for Pyloniids+Didymocyrtis+Spongodiscids+Spongaster+Euchitoniids+ Anthocyrtidium. The PDSSEA time series compares well with solar insolation over the coresite (8‡S) during May^July (21 April^21 July), calculated in Analyseries (Paillard et al., 1996) using Berger’s (1978) astronomical equations (Fig. 3e), which suggest the in£uence of tropical insolation also, besides the insolation at 65‡N, over the Asian monsoon.
5. Spectral and cross-spectral analyses Pestiaux and Berger (1984) and Mommersteeg et al. (1995) suggested that no single method is ideal for detecting climatic cycles in a given time series. Hence, the climatic cycles in faunal data, listed in Table 3, were veri¢ed by using the multitaper (Thomson, 1990) method (MTM) by the
Table 3 List of cycles observed in radiolarian faunal groups, the PDSSEA monsoon index in core AAS 2/3, and insolation at the coresite during May^July SN
Groups/index
Cycles by multi-taper (Jtap.X) method (Lees and Park, 1995)
Cycles by SPECTRUM method (Schulz and Stattegger, 1997)
1
Pyloniids
400, 126, 95, 54, 41, 31, 23, 19, 17, and 15 ka.
2
Spongaster
5
Euchitoniids
3
Anthocyrtidium
4
PDSSEA index
5
Core-site insolation during May^July
s 95%: 31, 23, 19, 17, 15, and 13 ka; s 90%: 126, 95 and 41 ka. s 95%: 95, 27, 21, 17, and 15ka; s 90%: V51, 41, 19, and 13 ka. s 95%: 51, 33, 27, 17, 15, and 13 ka; s 90%: 400 and 126 ka. s 95%: V400, 69, 21, 17, 15, and 13 ka; s 90%: 33 and 19 ka. s 95%: 41, 29, 23, 19, and 13 ka; s 90%: V400, 95, 33, 17, and 15 ka. s 95%: 400, 100, 54, 41, 23, 19, 17, 15, and 13 ka.
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V95, 41, 29, 23, 19, 17, and 15 ka. 126, 95, 51, 33, 29, 19, 17, and 13 ka. 400, 69, 41, 31, 19, and 15 ka. 129, 95, 29, 23, 19, 15, and 13 ka. 23 and 19 ka.
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JTAP.X program of Lees and Park (1995) as well as spectral and cross-spectral analyses by the SPECTRUM program of Schulz and Stattegger (1997). MTM analysis of Pyloniids exhibited Na|«ve-spectrum, degree of freedom and F-test suggesting s 95% signi¢cant cycles at 31-, 23-, 19-, 17-, 15-, and 13-ka; s 90% at 126-, 95-, and 41-ka, while SPECTRUM revealed 400-, 126-, 95-, 54-, 41-, 31-, 23-, 19-, 17-, and 15-ka cycles (Fig. 4a). These results re-validate the ¢rst reported triplet eccentricity cycles at 400-, 126-
and 95-ka in the tropical paleo-sea surface temperatures (Gupta et al., 1996), which were later found in foraminiferal sand (Muller and MacDonald, 1997), and the N18 O record (Clemens and Tiedmann, 1997). Muller and MacDonald (1997) discussed the 100-ka cycles, believed to be due to eccentricity (e) and argued to assign to inclination (i) of the Earth’s orbital plane, as the eccentricity should have triplet cycles at 400-, 126and 95-ka, according to Berger (1977). For an excellent review on the enigmatic problem of ec-
a
b
c
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105
Fig. 4. Multi-taper spectral analyses (Lees and Park, 1995) showing degrees of freedom at 90% and 95% statistical signi¢cance for the F-test and Na|«ve spectrum, and by SPECTRUM (Schulz and Stattegger, 1997) for Pyloniids (a), Spongaster (b), Euchitoniids (c), Anthocyrtidium (d), monsoon-index PDSSEA (e), and insolation at core-site for 21 April^21 July (f). Note statistically signi¢cant cycles at 90% and 95% in multi-taper and s 80% signi¢cant (error bars) cycles in SPECTRUM results.
centricity vs. inclination driving the 100-ka cycles may be referred to Kerr (1997). Therefore, the Pyloniid cycles at V400-, 126- and 95-ka are due to the Earth’s eccentricity (Berger, 1977; Gupta et al., 1996; Gupta, 1999; Clemens and Tiedmann, 1997; Muller and MacDonald, 1997), while the 54-, 41- and 31-ka cycles are due to axial-tilt as discussed by Gupta (1999) and Melice
et al. (2001). It is contrary to the prevailing notion that the 100- and 41-ka climatic cycles are related only to eccentricity and tilt forcing, respectively (Prell, 1984; Clemens et al., 1991, 1996), and provides evidence for the triplet cycles at eccentricity and tilt bands. Similarly, the triplet 23-, 19- and 17-ka Pyloniid cycles (Fig. 4a) are due to precession, while the 15- and 13-ka cycles are sub-orbit-
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0 +5 ka s 90% s 90% 0 +5 ka s 90% s 90% 0 31.1 ka s 90% s 90% +14 ka 0 31.1 ka 6 80% s 90% s 90% +40 ka 35.7 ka +5.7 ka s 90% s 90% s 90% +37 ka +15 ka +5 ka 6 80% s 90% s 90% 38 ka +18 ka 31.2 ka s 90% V90% s 90%
Blanks represent the non-deterministic coherence/phase. Phase angles were converted to ka (phase angle/360*cycle). Negative phases are lags, + leads, whereas 0 stands for in-phase relationships between the fauna and the ETP or insolation. SPECTRUM settings are ofac-8, hifac-1, segments 2, detrended, Welch window I, 6-dB bandwidth 0.0049, signi¢cance 0.2 (80%) (Schulz and Stattegger, 1997).
Phase
39 ka 32 ka 0 s 90% s 90% s 90% 100 ka 41 ka 23 ka
Phase Coh Phase Coh Phase Coh Phase Coh Phase Coh Phase Coh Coh Coh
Phase
Pyloniids vs. insolation 8‡S M^J Cycles
Pyloniids vs. ETP
Spongaster vs. ETP
Spongaster vs. insolation 8‡S M^J
Anthocyrtidium vs. ETP
Anthocyrtidium vs. insolation 8‡S M^J
PDSSEA vs. ETP
PDSSEA vs. insolation 8‡S M^J
S.M. Gupta / Palaeogeography, Palaeoclimatology, Palaeoecology 197 (2003) 97^112 Table 4 Coherency and phase at the 95-ka (eccentricity), 41-ka (tilt) and 23-ka (precession) cycles among Pyloniids, Spongaster, Anthocyrtidium, and PDSSEA monsoon index vs. ETP and insolation at core-site during May^July
106
al (Berger, 1977; Treut et al., 1988; Willes et al., 1999). MTM analysis of Spongaster revealed s 95% signi¢cance at the 95-, 27-, 21-, 17-, and 15-ka cycles, and s 90% signi¢cance at the V51-, 41-, 19-, and 13-ka cycles, while SPECTRUM suggested V95-, 41-, 29-, 23-, 19-, 17-, and 15-ka cycles (Fig. 4b). MTM of Euchitoniids revealed s 95% signi¢cance at the 51-, 33-, 27-, 17-, 15-, and 13-ka cycles, and s 90% signi¢cance at the 400- and 126-ka cycles, while SPECTRUM suggested 126-, 95-, 51-, 33-, 29-, 19-, 17-, and 13-ka cycles (Fig. 4c). MTM of Anthocyrtidium exhibited s 95% signi¢cance at the V400-, 69-, 21-, 17-, 15-, and 13-ka cycles, and s 90% signi¢cance at the 33- and 19-ka cycles, while SPECTRUM revealed cycles at V400, 69, 41, 31, 19, and 15 ka (Fig. 4d). MTM of PDSSEA monsoon index revealed s 95% signi¢cant cycles at 41, 29, 23, 19, and 13 ka, and s 90% signi¢cant cycles at V400, 95, 33, 17, and 15 ka, while SPECTRUM suggested signi¢cant cycles at 129, 95, 29, 23, 19, 15, and 13 ka (Fig. 4e). Similarly, MTM of insolation during May^July at core-site exhibited s 95% signi¢cant cycles at 400, 100, 54, 41, 23, 19, 17, 15, and 13 ka, while SPECTRUM revealed similar cycles of which, however, the most dominant ones are at 23 and 19 ka (Fig. 4f). Cross-spectrum analyses between fauna and ETP as well as insolation (Figs. 2 and 3) by SPECTRUM (Schulz and Stattegger, 1997) were performed using settings of over-sampling factor 8, highest frequency factor 1, segments 2, detrended, Welch window, 80% con¢dence, bandwidth 6-dB = 0.0049, and coherence and phase results (Table 4), and the results are presented in Fig. 5. Cross-spectrum analysis between Pyloniids and ETP suggested that coherent ( s 80%) Pyloniids lag ETP by V9 ka at 100-ka eccentricity, 2 ka at 41-ka tilt, 0 ka (inphase) at 23-ka precession cycles (Fig. 5a). Spongaster vs. ETP crossspectrum exhibited coherent cycles at 54, 41, 23, 17, and 15 ka, and Spongaster leads ETP by 38 ka at 126-ka eccentricity, 15 ka at 41-ka tilt, 5 ka at 23-ka precession cycles (Fig. 5b). The transitional Euchitoniids assemblage did not show reliable cross-spectral results. The Anthocyrtidium assemblage exhibited coherent cycles at 41, 31, 23, 19,
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and 17 ka, and it leads ETP by 14 ka at 126-ka eccentricity, in-phase (0 ka) at 41-ka tilt and 23-ka precession cycles, while it lags V10 ka at 31-ka tilt cycle (Fig. 5c). Cross-spectrum of PDSSEA monsoon index exhibited coherent cycles at 51, 41, 33, 23, and 15 ka, it leads ETP at 54 and 23 ka substantially, while both are nearly in-phase at the 41-, 31- and 15-ka cycles (Fig. 5d). Similarly, coherent cycles are to be found at 41, 23 and 15 ka for PDSSEA and summer insolation at 8‡S over the core-site, with a slight lead of PDSSEA over the insolation at 23 ka, and the in-phase relationship at 41- and 15-ka cycles (Fig. 5e). In order to illustrate the lead or lag relationship in the faunal time series with the ETP as discussed above, the representative pair of Pyloniids and ETP was selected, and their signi¢cant cycles were ¢ltered using Analyseries software. Gaussian ¢ltering of the Pyloniids and ETP time series at frequencies (cycles) 0.008 U 0.001 (126 ka), 0.0105 U 0.001 (95 ka), 0.0185 U 0.001 (54 ka), 0.024 U 0.001 (41 ka), 0.0325 U 0.0018 (31 ka), 0.043 U 0.003 (23 ka), 0.052 U 0.0018 (19 ka), and 0.065 U 0.003 (15 ka) were performed and compared (Fig. 6). The 400-ka cyclic component (frequency 0.0025 U 0.05) of eccentricity present in the Pyloniid time series was not ¢ltered and illustrated as it could be argued of being a longterm trend, because the length of the time series (485 ka) is slightly higher ( s 20%) than the 400ka period. Filtered Pyloniids and ETP cycles due to the Earth’s eccentricity at 126 and 95 ka; axialtilt at 54, 41, and 31 ka; precession at 23 and 19 ka (Fig. 6a^g), and a sub-orbital cycle at 15 ka (Fig. 6h) exhibited phase relationships shown in Fig. 5 in time domain. The 95- and 41-ka eccentricity and tilt related Pyloniid cycles marginally lag the ETP, while their 23-ka precession cycles exhibited an in-phase (zero phase) relationship (Fig. 6b,d,f) concurring to the SPETRUM results shown in Table 1. Similarly, the 126-ka eccentricity, 54- and 31-ka tilt componentrelated Pyloniid cycles lag the ETP (Fig. 6a,c,e) whereas the 19-ka precession and 15-ka cycles suggest a nearly in-phase relationship between the two (Fig. 6g,h). Similarly, cyclic components of other faunal time series may be ¢ltered and
107
may reveal phase relationships expressed in Table 4. Earlier, Prell (1984) suggested coherence between monsoon index Globigerina bulloides, the ETP and insolation at the 23-ka precession cycle, which lagged 4^6 ka depending on insolation considered at 21 July or 21 June, respectively, attributing that the lag of 4^6 ka is physically insignificant. As the insolation calculated herein is from 21 April to 21 July considering the length of summer heating of the equatorial Indian Ocean, and evaporation thereof resulting in monsoonal clouds, naturally the lag value may vary with that of Prell. Pyloniids lag insolation during May^July at 8‡S by 8 ka at 100-ka eccentricity, lead 18 ka at 41-ka tilt, and is in-phase at 23-ka precession cycles (Table 4). Clemens et al. (1991, table 1) reported a lag of 7.7^6.6 ka by G. bulloides, opal (radiolaria and diatom) and barium monsoon indices over the ETP at the coherent 23-ka precession cycles. Later, Clemens et al. (1996) suggested that the timing (phase) of strong monsoons changed by 5 ka at precession and 14 ka at tilt cycles relative to phase of the maximum global ice volume (N18 O). Morley and Heusser (1997) reported that ETP and Chinese monsoon are coherent (0.84 coherency), while ETP led Chinese rains by 1.4, 13 and 8.7 ka at the 100-, 41- and 23-ka cycles, respectively. In the present paper, six monsoon salinity-sensitive radiolarians and their PDSSEA suggest a phase range between 0 to 8 ka at precession and tilt cycles, respectively, which is closer to the phases reported by Prell (1984) and Morley and Heusser (1997). Besides, it also reveals triplet cycles due to eccentricity (400, 126 and 95 ka), tilt (54, 41 and 31 ka), and precession (23, 19 and 17 ka) as professed by Berger (1977), as well as sub-orbital 15-, 13- and 11-ka cycles which are not reported so far from a monsoon record. In brief, it may be surmised that salinity-sensitive radiolarian faunal assemblages and monsoon index PDSSEA revealed monsoonal cycles due to orbital forcing and insolation corresponding to multiple cycles of the eccentricity, tilt, precession, and sub-orbital bands during the Late Quaternary in the central Indian Ocean.
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6. Conclusions Ever since the orbital forcing on climate change was discovered (Hays et al., 1976), the 100-, 41-
and 23-ka cycles are invariably corroborated due to eccentricity, tilt, and precession, respectively. However, Berger (1977) cautioned, emphasized, and professed about the existence of multiple
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Fig. 5. Cross-spectrum results by SPECTRUM program showing coherence and phase spectrum between the ETP and Pyloniids (a), Spongaster (b), Anthocyrtidium (c), PDSSEA (d), and PDSSEA and insolation at core-site (e). Marginal deviation of phase spectrum from zero-phase line suggests that Pyloniid faunal variation (a) is as good as the ETP cycles with an accuracy of 6 U 8 ka at 100-ka and U 1 ka at 23-ka Milankovitch cycles.
cycles due to the Earth’s eccentricity at 400, 126 and 95 ka, tilt at 54, 41 and 31 ka, and precession at 23, 19 and 17 ka, besides 69- and 15-ka climatic cycles due to astronomical forcing. Radiolarian faunal assemblages and the PDSSEA monsoon index invariably resulted in multiple cycles due to eccentricity (400, 126 and 95 ka), tilt (54, 41 and 23 ka) and precession (23, 19 and 17 ka) besides the sub-orbital cycles at 15 and 13 ka in core AAS 2/3. On the contrary, Prell (1984) and Clemens et al. (1991, 1996) used Globigerina bulloides, opal £ux and barium as upwelling indices o¡
Oman due to monsoonal winds, and dealt with the 100-, 41- and 23-ka cycles in detail. It is for the ¢rst time that a monsoonal record revealed almost all the cycles professed by Berger (1977) not only in one species of G. bulloides (Prell, 1984), but several radiolarian faunal indices. Cross-spectral results between three radiolarian assemblages and the PDSSEA monsoon index with ETP as well as insolation at the core-site suggest a time lag ranging from 0 to 8 ka for the 23-ka precession cycle, which is within the range reported by earlier monsoonal studies
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a
e
b
f
c
g
d
h
Fig. 6. Signi¢cant cyclic components of Pyloniids and ETP time series (Fig. 2a) ¢ltered out by Analyseries showing phases in time domain for the 126- and 95-ka eccentricity (a,b), 54-, 41- and 31-ka tilt (c^e), 23- and 19-ka precession (f,g), and the 15-ka sub-orbital cycles.
(Prell, 1984; Clemens et al., 1991, 1996). The results suggest that the Indian monsoon, though dominated by the precession cycles (23 and 19 ka; Clemens et al., 1991), is also in£uenced by orbital eccentricity (400, 126 and 95 ka), axialtilt (41- and 31-ka cycles) forcing, and sub-orbital 15- and 13-ka cycles.
Acknowledgements I thank the Director of the NIO, Dr. Eherlich Desa, for use of the facilities and encouragement, and Professor C.N.R. Rao, Jawaharlal Nehru Centre of Advanced Scienti¢c Research, Bangalore, for awarding the Rajiv Gandhi Foundation
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Grant for innovative ideas in science and technology to support my research at NIO. Sincere thanks are expressed to A.L. Berger and an anonymous reviewer for constructive criticism and suggestions. I also thank Bjo«rn A. Malmgren for critical evaluation of the data, analyses, and suggesting improvements to the ¢nal manuscript. Thanks are also to D. Paillard and M. Schulz for providing the ANALYSERIES and SPECTRUM programs, respectively, and to B.N. Nath and V.N. Kodagali for assistance in various matters. The Council of Scienti¢c and Industrial Research and the Department of Ocean Development, New Delhi, are thankfully acknowledged for laboratory support and onboard ship-logistics. The paper is dedicated to my father late Shri Kanhaiya Lal Gupta. This is the National Institute of Oceanography, Goa, India, Contribution No. 3799.
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