Core-top calibration of the alkenone index vs sea surface temperature in the Indian Ocean

DeepSea

Pergamon

PII: 60967-0645(!qwo10-6

Research II, Vol. 44, No. 6-7, pp. 144~1464, 1997 0 1997 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0967-0615/97 $17.00+0.00

Core-top calibration of the alkenone index vs sea surface temperature in the Indian Ocean CORINNE SONZOGNI,* EDOUARD BARD,* FRAUKE ROSTEK,* RAYMOND LAFONT,* ANTON1 ROSELL-MELEtS and GEOFFREY EGLINTONfg (Received 16 June 1995; accepted 3 1 December 1996)

Abstract-The alkenone index (UK 37) has been calibrated against sea surface temperature (SST) using a suite of 54 surface sediments from the Indian Ocean (20”N to 45”S), spanning temperatures between 6 and 29°C (0.2< Uxs7< 1.0). A computerized oceanographic atlas has been used to calculate modern SST for the coring sites. We have constructed two sets of l_lxs~_SST calibrations for various water depths between 10 and 50 m. The first set is based on the comparison of UK’s7with annual mean temperatures for all cores. In the second set, we assume that UK’s7 values in surface sediments are linked to seasonal productivity patterns, and UK37 is compared with mean temperatures at times of yearly production maximum. In both calibrations, the Ur“s, values are strongly correlated with SST, with a best fit obtained using temperatures from 10 m and seasonal productivity patterns. The regression line is similar to that found with laboratory cultures of E. huxleyi, and with other calibrations based on recent sediments from other oceans. Above 24°C the slope of our Uxs&ST relationship is smaller (approximately 0.020 Uxs7 unit ‘C-‘) than the general slope (approximately 0.033). 0 1997 Elsevier Science Ltd

INTRODUCTION A series of long chain C+& di-, tri- and tetraunsaturated methyl and ethyl ketones (alkenones) has been identified in some species of Prymnesiophyceae algae, in particular in the ubiquitous marine coccolithophorid Emiliuniu huxleyi (Volkman et al., 1980a, Volkman et al., 1980b). The structures of these alkenones have been determined by De Leeuw et al. (1980) and Rechka and Maxwell (1988). Initially, Brassell et al. (1986b) defined the UK 37 ratio as an index of C3, methyl-ketones unsaturation: UK

[C37:2Mel - [C37:4Me1 37 = [C37:2Mel + [C37:3Mel + [C37:4Mel

As the C37:4 is rarely present in sediments, the equation has been simplified and the new index Ur37 is usually employed (Brassell et al., 1986b):

* CEREGE, University d’Aix-Marseille III et CNRS-FR 6094 et UMR 6536, Europole de I’Arbois, BP 80, 13545 Aix-en-Provence cdx 4, France. t Organic Geochemistry Unit, University of Bristol, Bristol, BS8 ITS, U.K. $ Also at Biogeochemistry Centre, University of Bristol, Bristol, U.K. 4 Present address: Fossil Fuels and Environmental Geochemistry, University of Newcastle, Newcastle upon Tyne, NE1 7RU, U.K. 1445

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C. Sonzogni

et ul.

[C37:2Me1 UK’ s7 = [C37:2Me1 + [C37:3Me1 The unsaturation ratios for the Cs7 and Css alkenones are sensitive to growth temperature of the algae, as noted by Brassell et al. (1986a, Brassell et al., 1986b). These workers showed a correlation between planktonic ansO and UK’37 in a deep-sea core from Kane Gap, which suggested that UK’s7 could be used as an SST proxy. Laboratory cultures on E. huxleyi (Marlowe et al., 1984a, Marlowe et al., l984b; Prahl and Wakeham, 1987; Prahl et al., 1988) first showed that the UK’s7 index exhibits a linear response for water temperatures between 8 and 25°C. Some other coccolithophorids have similar properties, and recently Volkman et al. (1995) succeeded in culturing Gephyrocapsa oceanica, confirming the temperature dependence of their Cs7 and Css alkenones unsaturation ratios. However, the UK’37vs SST relationship deviates somewhat for mid- and low temperatures from the calibration obtained for E. huxleyi. Several workers have demonstrated that other alkenone ratios are not or less temperature dependent (Prahl et al., 1988; Conte et al., 1992; Rosell-Mel& et al., l994), which could be used to distinguish between different types of coccolithophorids (for example, the ratio of C37/C38) (Marlowe et al., 1990; Rosell-MelC et al., 1994; Volkman et al., 1995). To assess the validity of the culture calibration to the open ocean, different calibrations have been constructed by analysing particulate organic matter (POM) from the water column or sediment traps (Prahl and Wakeham, 1987; Conte et al., 1992; Conte and Eglinton, 1993; Sikes and Volkman, 1993) and by extracting organic material from recent surface sediments (Sikes et al., 1991; Rosell-Mel& et al., 1994; Rosell-Mel& et al., 1995a). These studies also have shown that the UK 37 index correlates with SST, but the scatter is often larger than obtained with E. huxleyi cultures. The differences and scatter found between the calibrations based on POM and surface sediments may be partly explained by the contribution of different Prymnesiophyte algae belonging to different genera, species or even strains. Degradation of the total stock of alkenones occurs during sedimentation through diagenetic processes (Prahl et al., 1989). However, studies have shown that the unsaturation index is not significantly affected by sedimentary diagenesis (Prahl et al., 1989; Poynter and Eglinton, 1991; Sikes et al., 1991; Conte et al., 1992, Conte et al., 1995; Madureira et al., 1995), although the effect of water column diagenesis may be important at low SST (RosellMele et al., 1995a). In this paper, we present new data on alkenone distributions in recent sediments from the Indian Ocean, which are then compared with modern temperatures. Our objective is to test the UK’37vs SST relationship in an oceanic area hitherto not considered in previous studies. Hence, 54 surface sediments (20°N-45”S) are analysed that cover a large range of SST (6 29°C). EXPERIMENTAL Lipid extraction and analysis procedures The alkenone data used in this paper have been generated by the same analyst (C. Sonzogni) in two laboratories, OGU in Bristol and CEREGE in Aix-en-Provence. A semiautomated analytical procedure was used in Bristol, and has been described elsewhere (Rosell-MelC et al., 1995b). A similar manual procedure was used in Aix-en-Provence. Briefly, a C& n-alkane was added to the freeze-dried surface sediment (1 g) which is

Alkenone index vs sea surface temperature in Indian Ocean

1447

extracted with distilled CH$lz:CHsOH (2: 1, x 2) and CH&& ( x 1) by ultrasonication. The extracts were combined and washed with distilled water to remove sea salts and methanol, dried with preheated Na2S04 and rotary evaporated to dryness. The residue was redissolved in CH2C1z and cleaned by elution through a silica commercial cartridge (silica Bond Elut SPE column, Varian). The clean extract was concentrated under N2 and redissolved with CH2C12 (50 ~1) before gas chromatography (GC). Details on the analytical tests performed in Aix-en-Provence will be given elsewhere (Sonzogni, 1997). The chromatographic procedure used at CEREGE was performed on a 60 m x 0.25 mm x 0.1 urn non-polar fused silica capillary column DB-5-MS (J&W) fitted with a 2.5 m x 0.53 mm deactivated retention gap. A Fisons Instruments GC8065 gas chromatograph fitted with a cold on-column injector and flame ionization detector was used for routine analyses (HZ as carrier gas). The gas chromatograph oven was programmed from 45 to 200°C at 15°C min-‘, from 200 to 250°C at 50” min-’ and from 250 to 300°C at 3” min-’ . The final temperature was then maintained for 30 min. Alkenones and alkenoates typically elute between 39 and 44 min, and the C37:3Meand the C&M~ are well separated (approximately 20 s). The chromatograms of the sample extracts were compared with relative retention times of the C 37.3~eand the Cs7:zMestandards, synthesized by Rechka and Maxwell (1988). The alkenones and the alkenoates in the extracts were quantified using the ChromCard software (Fisons Instruments). The identification of compounds and verification of the absence of coelutions were performed by gas chromatography-mass spectrometry (GC-MS) on selected samples (Fisons Instruments MD800 GC-MS system). The accuracy and reproducibility of the above analytical procedures have been studied using mixtures of synthetic standards (Cse n-alkane, C16 and Cl9 methylketones commercially available and C 37.2~e and Cs,:s~~ alkenones provided by Professor J. R. Maxwell), algal cultures (Isochrysis galbana strain T. Iso.) used at the Division of Oceanography, CSIRO, Hobart, Australia, and provided by Dr J. Volkman), and homogenized sediment samples (South Atlantic sediments used as standards at the University of Bremen, Germany, and provided by Dr P. J. Mtiller). For the real precision is better than 0.01 UKs7 units sediment samples, the analytical (approximately 0.3”C), and the estimated accuracy is better than 0.02 UKs, unit (Sonzogni, 1997). Rosell-Mel& et al. (1995b) observed that the value of the UK37 changed with the alkenone concentration at extreme concentrations and suggested working within a restricted range of alkenone concentration (5-100 ng of Cg7 total). To assess the effect of the alkenone concentration on the unsaturation index, solutions of synthetic Cs7:s~~ and C37:ZMewith different ratios (1:25; 1:lO; 1:4; 2:5; 1:2; 1:1;4:3 and 4:l w/w Cs7:sr,.&37:2~e) and algal and sediment samples have been studied at different concentration (between 3 and 180 ng pl-’ of Cs, total alkenones). Figure l(a) and (b) shows the UK x7variations as a function of the Cs, total alkenones injected in the gas chromatograph. The entire interval of UKs7 is represented in these graphs. In the injection range between 10 and 200 ng of total Cs7, the Ups7 appears to be unaffected by the sample size (Fig. la and b). Indeed, the standard deviations on UKs7 vary between 0.0006 and 0.0016 for synthetic alkenones and 0.0020 and 0.0082 for algal and sediments samples. In addition, each solution of synthetic C 37.3~~and Cs7:zMewith different known ratios (1:25;1:10; 1:4; 1:2; 1:l; 4:3 and 4:l w/w C&s&C&& was injected five times in the gas chromatograph. Figure 2 represents the comparison between the measured value UKs7 and the theoretical UK’s7. The mean standard deviation of the five injection replicates is about

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(a) (.,:__,,,__________*_____________ ._-_ ~,wos 0.85-

_____

1:25 std

_____--++*++.+_i_,,a

c=o.cOo7 .~_~_~__~____~________~~__~~____~~?_~~_

0.75 -

0=0.0016 ,__e~~___-__*_________o-_____~~~~~ __~~0008_________-__*_*_.._--_~~~~t~_

fi r? 5

0.65 0..5.5-

l%LmM ____________-___-__,__,___-__!k?Cd_

FOOOlO 0.45-____-__________~__y_~.__-_-__~~~~~_ 0.35 -

us-

Synthetic

nlkenones

a=o.o015 ,___________-______f______,__4_:1”’6

0.15-r

I

I

0

20

40

I

I

I

I 4 120

Total C3?““alke%ned”(“,,,~g) 1.0, 0=0.0023 -&-_L-_---~_?_-_~_-_~_~_-~~_-&-_-&_-_-~ 0.9

U=O.C020

i

samples

Natural

0.8

0.7___r____*_p**_~2=0.0042____________ 0

1~0.0082 a________________________.

.____x_$ 0.6 -

a=o.c032 ~___----_______-____-_

___-m_+___*_ 0.5 -

0.4i

0

I

50

.

-I

I 100

150

200

Total C37 alkenones (ng) Fig. 1. The effect of the sample size on the U K’37values has been studied with (a) a series of synthetic alkenones at different proportions (1:25; 1:10; 1:4; 2:5; 1:2;1:1; 4:3; 4: 1 w/w C37:3~&37:2~~, and (b) algal and sediment samples (CSIRO, Isochrysis galbana strain T. Iso.; CC1025 and CC1027, homogenized South Atlantic sediment samples; 85681/l 5 cm and 85681/20 cm, Indian Ocean sediment samples). UK’37variations are shown as a function of the injected total C3, alkenones for an interval ranging between 3 and 180 ng. The mean UK’37(dashed lines) and the standard deviations (calculated for samples with total C37 higher than 10 ng) of the U K37values are also reported.

0.0010 UK37 unit (an order of magnitude better than the analytical precision reached on natural sediment samples). Figure 2 also shows the agreement between measured and predicted UK’3-,. Sampling

Samples were collected during several R.V. Marion Dufresne cruises covering a wide range of longitudes (40-95”E) and latitudes (20°N-45”S) (Fig. 3 and Appendix A). The sediments were taken using box corers, pilot corers and piston corers and stored in cool repositories at

Alkenone index vs sea surface temperature in Indian Ocean

0

0.10.20.30.40.50.60.70.80.9

1449

1

Calculated UK’37 Fig. 2. A series of synthetic alkenones with different relative proportions (0:l; 1:25; 1:lO; 1:4; 1:2; 1:l; 4:3; 4:l; 1:O w/w C37:3Mc/C37:.Me) were injected five times in the gas chromatograph. The mean standard deviation of the groups of five injection replicates is of the order of 0.001 UK’s7 unit (approximately 0.03”C). Comparison between the theoretical value UK’s, and the measured value UK’s, shows the equivalence of both with high coefficient of correlation (UK’s, ,.,,,,d=O.Ol2+0.992 UK’s,ca,cu,atcd,r=0.99).

the Museum National d’Histoire Naturelle in Paris, Centre Europeen de Recherche et d’Enseignement en Geosciences de I’Environnement in Aix-en-Provence and the Centre des Faibles Radioactivites in Gif-sur-Yvette. We have assumed that the surface layer of sediments taken with box and pilot corer is undisturbed and thus very recent. Highresolution S1*O stratigraphies (Duplessy, 1982; Fontugne and Duplessy, 1986) and 14C dating (Duplessy et al., 199 1) have confirmed that the core tops are representative of the late Holocene. In addition, sediments from the upwelling areas of Oman, Somalia and west of India are characterized by medium to high sedimentation rates during the Holocene period, which further validates the use of the core tops (between 3 and 51 cm kyr-‘; Fontugne and Duplessy, 1986; Sirocko and Ittekkot, 1992). Bioturbation continuously mixes the recently deposited sediments. The top 8-l 5 cm of the sediment column are thus usually homogeneous, as demonstrated for a wide range of sedimentological and geochemical tracers (Peng and Broecker, 1979; Bard er al., 1987; Lapicque et al., 1987). This has been recently confirmed by accelerator mass spectrometry (AMS) dating of box cores collected from the upwelling areas off Oman, Yemen and Somalia (Heier-Nielsen et al., 1995). However, the climate during the late Holocene has been relatively stable (Dansgaard et al., 1993) and the geochemical tracers controlled by climatic parameters should be stable in the core tops.

RESULTS

AND DISCUSSION

Modern oceanographic data

Indian Ocean core top UK37 values have been compared with modern water temperatures

C. Sonzogni

et al. 800E

600

400E

N 2oc

INDIAN

OCEAN

0 400 S

2' I\ 5. *' .\ -.'. 16 6' ,,/' ..._ --_ ,' *----____ .* -----_______--_

??

Fig. 3. Location map of the 54 Indian Ocean sediment cores used for this study (core names, locations and water depths are given in Appendix A). 0, Location of the sediment traps used to model the seasonal productivity patterns (Ittekkot et al., 1992) of the three areas of the Northern Indian Ocean [western Arabian Sea (C), eastern Arabian Sea (D) and Bay of Bengal (E)]. The areas representing the equatorial Indian Ocean (A) and the southern Indian Ocean (B) are also indicated.

a computerized oceanographic atlas (Levitus, 1982). This atlas allows monthly water temperature profiles to be read for a defined position (with a grid of f 1” of latitude and longitude). For each core site, seasonal temperature cycles were calculated for 10, 30 and 50 m water depths (Fig. 4). In the northern Indian Ocean, the sea surface circulation is strongly influenced by a seasonal wind system with a strong and humid SW monsoon from June to October and a moderate dry NE monsoon from December to March. During the SW monsoon, coastal upwelling and surface water mixing are observed off Somalia, Oman and west of India. These phenomena produce a marked temperature drop during summer in upwelling areas (see Fig. 4, areas C-E). To derive a calibration, UKs7 values should be compared with mean water temperatures at times of maximum alkenone production. As a first-order approach to calculate a mean production temperature (Flux-Weighted Annual Mean SST; FWAM-SST), we have constructed for each core a simplified annual productivity cycle, which has then been weighted with the seasonal SST cycle. The 54 cores selected for this calibration cover a wide from

1451

Alkenone index vs sea surface temperature in Indian Ocean I

33

I

I

I

I

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l

II

EQUATORIAL INDIAN OCEAN (A)

31 _-

U29 D $ z,:&==--

_----

.-*_

_7____._..-

25, 23 24

I

I

I

I

I

I

I

I

I

I

I

I

I

I

I

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11

I I

SOUTHERN INDIAN OCEAN (B)

22

Y

I

20

r

14 I

31 29 u a k 3

I

,

-

I

I

I

I

I

I

I

l

I

I

I

II

I

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4

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4

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I4

WESTERN ARABIAN SEA

27 25 23 21

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33

III

III

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III

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0

EASTERN ARABIAN SEA (D)

31 -

25 -

III

33 31

I

II

II

BAY OF BENGAL (E)

-

23

I Jan

II

I

I

Feb Mar Apr May Jun

I

I

I

Jul Aug Sep

0

Ott Nov Dee

Months Fig. 4. Representative examples of seasonal temperature cycles from Levitus (1982) for various water depths (10 m, thick continuous line; 30 m, thick dashed line; 50 m, thick dotted line). The areas A-E are described in Fig. 3. The seasonal productivity patterns obtained on representative sediment traps (Ittekkot et al., 1992) deployed in the western Arabian Sea (C), eastern Arabian Sea(D) and in the Bay of Bengal (E) are indicated with thin continuous lines. The differences between the SST from 0 m and 10 m are negligible.

1452

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Sonzogniet al

latitudinal transect between 20”N and 45’S (Fig. 3) and we have grouped them in five different oceanic regions that exhibit different productivity patterns (areas A-E in Figs 3 and 4). For the equatorial band (area A), the seasonal SST variations are small (less than 2°C) and the FWAM-SST is almost equal to the Annual Mean SST (AM-SST) (Banse and English, 1994; Brock et al., 1994). For the southern part of the Indian Ocean (area B), the productivity maximum occurs during the austral spring and summer (Banse and English, 1994). In the western Arabian Sea (area C), eastern Arabian Sea (area D) and in Bay of Bengal (area E), the productivity is mainly influenced by upwelling during the SW monsoon and surface water mixing during the NE monsoon (Nair et al., 1989; Ittekkot et al., 1992; see also Fig. 4, areas C-E). To model this productivity cycle, we have used the seasonal cycles averaged over 2 years of particle fluxes from sediment traps deployed in the Arabian Sea and in the Bay of Bengal (dots in Fig. 3) (Nair et al., 1989; Ittekkot et al., 1992). In the eastern Arabian Sea the flux reaches its maximum between August and October, whereas it peaks between July and September in the western Arabian Sea. In the Bay of Bengal, the flux maximum occurs between June and September. These productivity patterns are in agreement with pigment data (Banse, 1987; Brock et al., 1991; Banse and English, 1994) and with the data based on relative frequencies of species of foraminifers and coccolithophorids E. huxleyi and G. oceanica (Kleijne et al., 1988). Recently, Antoine and Morel (1996) and Antoine et al. (1996) presented monthly maps of primary productivity by using chlorophyll satellite imagery (Coastal Zone Color Scanner) and by taking into account ecological and physiological parameters. In general, the mean production temperatures calculated by using these productivity maps are very similar to the weighted temperatures computed for this study (usually the difference is smaller than 0.5X). The most abundant Prymnesiophyceae algae known to produce alkenones (E. huxleyi and G. oceanica) live in the upper water column (Okada and Honjo, 1973; Brown and Yoder, 1994). For our calibration it is necessary to select a water depth representative of the depth where maximum alkenone production occurs. To illustrate this problem, Fig. 4 shows typical seasonal cycles obtained at different depths (IO, 30,50 m) for selected sites of the five regions A-E. We have used a pragmatic approach and considered three water depths (10, 30,50 m) to calculate the AM-SST and FWAM-SST as described above. The living depth of the alkenone producers is probably not constant throughout the year. However, this effect should be negligible considering all other potential sources of errors in the above calculations. The main error is due to bioturbation through which core tops integrate centuries to millennia of productivity cycles. UK,, and temperature We have quantified the alkenones at several depths in the first 10 cm of each core (usually three samples: 0 cm, 5 cm and 10 cm), and we have calculated a mean value for constructing the Ur3,SST relationships. As expected and explained in the sampling section (see above), the maximum difference between the three measurements in the core top is usually less than 0.033 UK 37 unit (approximately 1°C). Most of our samples have been collected in mid- to high SST (6-29°C) regions and IJK3, values vary between 0.2 and 1 (see Appendix A). The samples are characterized by insignificant amounts of C 37,4Mealkenones, which are the basis of the UKa7 index (Brassell et al., 1986b). Indeed, Resell-Melt et al. (1994) showed that for SSTs higher than 5°C the two indices UK’s7 and UKs7 are equal.

Alkenone index vs sea surface temperature in Indian Ocean

5

10

25 F&l-s;;

Fig. 5. Uxs, vs AM-SST (a) and regression line and open circles). whereas the results for 30 m (thin linear best fits. The equations and

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30

(“C)

UK’s7vs FWAM-SST (b) from Levitus(1982) at 10 m depth (thick For clarity, UK 37 results are only shown for 10 m water depth continuous line) and 50 m (dashed line) are summarized by their the coefficients of the regression analysis are given in the Table 1.

Figure 5 summarizes the relationships observed between UK’s7 and water temperature. Figure 5(a) represents the AM-SST against UK 37, and Fig. 5(b) shows the seasonal productivity fluctuations or FWAM-SST (see Appendix A). On the graphs only the results obtained at 10 m water depth are plotted, whereas the data on 30 and 50 m are summarized by their linear best fits (see Table 1 and Appendix A). The UKs7 values linearly correlate with water temperatures whatever the water depth for both the AM-SST and FWAM-SST data sets. For the studied three depths, the slopes of the linear regressions are not statistically different (approximately 0.033 UKs7 unit “C-l; see Table 1). However, the linear equations have different y-intercepts, the SST estimated with the 10 m equation being l-2°C warmer than for 50 m water depth. Although it is barely significant (F= VarSo m sux/ Var ,e ,,.,sux = 1.53, close to the value 1.55 of the Fisher-Snedecor law for a significance level of 95%), the correlation coefficient decreases with water depth, which suggests that

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Table

C. Sonzogni

1. Linear regression coejkients AM FWAM FWAM AM FWAM AM FWAM

10

30 50 Same conventions

calculatedfor

0.043,0.052 0.013&0.063 0.392kO.143 0.065 kO.055 0.054 + 0.064 0.091 kO.063 0.099 &0.070

et al.

the relationships between UK’,, and SST in the Indian Ocean

0.033+0.002 0.034+0.002 0.02OIf:O.O05 0.033 iO.002 0.033 + 0.003 0.033+0.003 0.033 kO.003

54 54 44 54 54 54 54

0.948 0.936 0.710 0.945 0.928 0.924 0.905

5-30 5-30 2429 5-30 5-30 S-30 5-30

as in Table 2.

alkenone production at 10 m is a likely assumption. These observations are in agreement with a recent study of NE Atlantic surface sediments (Rosell-Mell et al., 1995a). Most of our data are for the high temperature range 24-29°C (Fig. 6) but an enlargement of the diagram still shows that U K’37 is linearly correlated with SST. The correlation coefficients calculated for this 5°C interval are smaller than those found for the entire temperature range, which is logical as the dynamic range is reduced by more than a factor of three. Taken at face value, the results also suggest that the slope of the calibration is slightly smaller than that found for the full range of temperatures (0.020 ) 0.005 vs 0.033 f 0.002; for a significance level of 95%). Comparison

with previous studies

In Fig. 7 the calibration from the Indian Ocean is compared with those obtained by using core tops from other oceanic environments: the Pacific and Atlantic oceans and Black Sea (Sikes et al., 1991) and North-East Atlantic (Rosell-Meli: et al., 1995a). These calibrations are similar, with a common slope of about 0.03 UK’37 unit “C-l (see Table 2). The standard deviations of the data points around the three regression lines are also similar for the three data sets (between 1” and 1.SOC).

Calibration UK3750.8

Calibration O.ZcUK37’41

24

28

2s F&G&

29

(“C)

Fig. 6. UK’&.ST calibrations for temperatures from 5 to 30°C (0.2 < UK’x7< 1; thin continuous line) and for temperatures higher than 24°C (UK 37 > 0.85; thick continuous line) using FWAM-SST at 10 m depth. The linear equation for these high temperatures is UK37 =0.020 SST+ 0.392 (r = 0.843). The f 1o temperature interval (SD OX’C) is given by dashed lines.

Alkenone index vs sea surface temperature in Indian Ocean

Resell-Mel6

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et al. 1

*o

0.6

0.5

Indian Ocean,

0.22 5

10

15

this work

25

:

SST p:) Fig. 7. Linear relationships between UK sr and water temperatures derived by analyses of core tops from the Pacific and Atlantic Oceans and the Black Sea (Sikes et al., 1991; thin continuous line), the NE Atlantic (Resell-MeK et al., 1995; dashed line) and the Indian Ocean (this work; thick continuous line; using FWAM-SST at 10 m depth). The regression lines obtained by Sikes et al. (1991) (Atlantic, Pacific and Black Sea) and by Resell-Mel& et al. (1995a) (Atlantic) are also shown. The equations and the coefficients corresponding to the regression analysis are listed in Table 2.

The dominant coccolithophorids in the northern Indian Ocean are E. huxleyi and G. oceanica, with high standing stocks in the western Arabian Sea and south of India (Kleijne et al., 1988). The three calibration equations (10 m, 30 m and 50 m) based on core tops are much closer to the E. huxleyi culture calibration than to that found for G. oceanica (Fig. 8 and Table 2). A first explanation could be that the subpolar G. oceanica strain cultured by Volkman et al. (1995) behaves differently from the algae living in the tropical Indian Ocean. As proposed by Rosell-Meli et al. (1994) and by Volkman et al. (1995), we used the ratio Table 2. Compilation of the regression coeficientsfor the relation between UK’,, and water temperatures established on cultures (Prahl et al., 1988; Volkman et al., 1995) and surface sediments (Sikes et al., 1991; Resell-Meli et al., 1995a; this paper)

Depth (m)

SST

(I

b

n

r2

TR (“C)

IJ

Temperature and depth used for calibration

TR (“C)

I ndex

Sample origin

a

b

n

References

Cultures

UK’3, E. huxleyi 0.040 f 0.066 UK’,, G. oceanica -0.524&0.124 Surface sediments tJr3, Atlantic, Pacific 0.082+0.090 Black Sea 0.093&0.024 UK3, NE Atlantic UK37 Indian Ocean 0.013+0.063

0.034 f 0.004 5 0.994 0.049+0.006 9 0.890

Culture growth Culture growth

8-25 Prahl et al., 1988 1l-29 Volkman et al., 1995

0.031kO.004 16 0.937 Warm seasons

9-29 Sikes et al., 1991

0.030+0.001 1090.958 Summer, 0 m 0.034kO.002 54 0.936 FWAM-SST, 10m

O-28 Resell-Meli et al., 1995a 5-30 This paper

The genera1 equation is UK’s7= a + bT, r’, correlation coefficient; n, number of samples; TR, temperature range. Confidence intervals have been calculated with a significance level of 95%.

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et al.

cultures calibration Prshl et al., 1988

0.8 0.7

G. oceanica cultures calibration, Volkman et al., 1995

0

5

10

15

20

25

30

SST (“C) Fig. 8. Linear relationships between UK’37 and water temperatures with data from E. huxleyi cultures (Prahl et al., 1988; dashed continuous line), G. oceanica cultures (Volkman et al., 1995; fine continuous line) and Indian Ocean surface sediments (the present study, open circles and thick line; using FWAM-SST at 10 m depth). The equations and the coefficients corresponding to the regression analysis are given in Table 2

of total Cj7 alkenones/total C 3s alkenones (X37/X38 alkenone ratio) to discriminate between the contribution of the different producers (Fig. 9 and Appendix A). In the Indian Ocean the mean X37/X38 is 1.20 (range 0.87-l .40; (3= 0.12; n = 39), which agrees with data obtained in the NE Atlantic at mid- to high latitudes (Rosell-Melt et al., 1994). The vast majority of our analyses are thus compatible with the ratios observed by Prahl et al. (1988) on cultures of E. huxleyi (mean I .46, CI= 0.21, range 1.18-l .71; n = 5) and by Conte et al. 20’

0.8

Fig. 9.

I

I

0.9

1

I

I

1

I

I

1.1 1.2 1.3 1.4 1.5 C C37/c C38

Histogram showing the spread of the total C37 alkenones/total C3s alkenones ratio (X37/ 238). The mean X37/X38 for all samples is 1.20 (SD 0.12; range 0.87-1.40; n = 39).

Alkenone index vs sea surface temperature in Indian Ocean

1457

(1994) for strains of E. huxleyi grown at 15°C [mean 1.07, o = 0.16, range 0.86-l .40; cited by Volkman et al. (1995)]. The ratios found for G. oceanica by Volkman et al. (1995) are all significantly different (mean 0.70, cr = 0.08, range 0.59-0.81, n = 9) from that observed in the Indian Ocean. A limited influence of G. oceanica on the Indian Ocean sediments (both for the UK3, and C37/Z38) also could be explained by a difference in total alkenones biosynthesis between the two algae. Indeed, culture studies (Prahl et al., 1988; Volkman et al., 1995) have shown that E. huxleyi biosynthesizes about three times more alkenones than G. oceanica (about 1.2 pg per cell for E. huxleyi vs 0.4 pg per cell for G. oceanica). CONCLUSIONS UK37 values (0.2-l) from surface sediments from the Indian Ocean (2O”N--45”S) are linearly correlated with SST (5-30°C). The best fit is obtained using SST from 10 m at times of highest seasonal productivity. The regression line is similar to those found with laboratory cultures of E. huxleyi and with other sets of core tops from other oceans (Sikes et al., 1991; Rose&MelC et al., 1995a). Above 24°C the slope of the UK37-SST relationship may be smaller (x0.020) than the general slope of about 0.033 UK’37 unit/C. Based on measurements of X37/X38 ratios and on the Up37-SST relationship we suggest that G. oceanica has a limited influence on the distribution of alkenones in the Indian Ocean sediments or that the strain living in the Northern Indian Ocean behaves similarly to E. huxleyi. Acknowledgements-We are grateful to Dr J.C. Duplessy for giving access to cores stored at CFR, Gif-sur-Yvette, Dr J. P. Caulet for cores stored at MNHN, Paris, and Dr Y. Lancelot for cores stored at CEREGE. We thank Professor J. R. Maxwell for providing synthetic alkenones, Dr J. Volkman for giving an aliquot of the Isochrysis galbana strain T. Iso. sample, and Dr P. J. Miiller for providing samples of the University of Bremen’s sediment standards. We also thank Dr J. Guiot for useful discussions. This research is supported by Tempus of EC and PNEDC of INSU-CNRS.

REFERENCES Antoine, D. and Morel, A. (1996) Organic primary production. 1. Adaptation of light-photosynthesis model in view of application to satellite chlorophyll observations. Global Biogeochemical Cycles, 10, 43-55. Antoine, D., Andre, J.-M. and Morel, A. (1996) Organic primary production. 2. Estimation at global scale from satellite (coastal zone color scanner) chlorophyll. Global Biogeochemical Cycles, 10, 5769. Banse, K. (1987) Seasonality of phytoplankton chlorophyll in the central and northern Arabian Sea. Deep-Sea Research, 34, 713-723.

Banse, K. and English, D. C. (1994) Seasonality of coastal zone color scanner phytoplankton pigment in the offshore oceans. Journal of Geophysical Research, 99, 7323-7345. Bard, E., Arnold, M., Duprat, J., Moyes, J. and Duplessy, J.-C. (1987) Reconstruction of the last deglaciation: deconvolved records of 8180 profiles, micropaleontological variations and accelerator mass spectrometric 14C dating. Climate Dynamics, 1, 101-l 12. Brassell, S. C., Brereton, R. G., Eglinton, G., Grimalt, J., Liebezeit, G., Marlowe, I. T., Pflaumann, U. and Sarnthein, M. (1986a) Paleoclimatic signals recognized by chemometric treatment of molecular stratigraphic data. Organic Geochemistry, 10, 649-660. Brassell, S. C., Eghnton, G., Marlowe, L. T., Pflaumann, U. and Sarnthein, M. (1986b) Molecular stratigraphy: a new tool for climatic assessment. Nature, 320, 129-133. Brock, J., McClain, C. R., Luther, M. E. and Hay, W. W. (1991) The phytoplankton bloom in the Northwestern Arabian Sea during the Southwest Monsoon of 1979. Journal of Geophysical Research, %, 2062320642. Brock, J., Sathyendranath, S. and Platt, T. (1994) A model study of seasonal mixed-layer primary production in the Arabian Sea. In Biogeochemistry of the Arabian Sea, D. La1 (ed.), pp. 6_%78. Indian Academy of Sciences, New Delhi.

1458

C. Sonzogni

et al.

Brown, C. W. and Yoder, J. A. (1994) Coccolithophorid blooms in the global ocean. Journal of Geophysical Research, 99, 746777482. Conte, M. H., Eglinton, G. and Madureira, L. A. S. (1992) Long-chain alkenones and alkyl aikenoates as paleotemperature indicators: their production, flux and early sedimentary diagenesis in the Eastern North Atlantic. Advances in Organic Geochemistry, 19, 287-298 Conte, M. H. and Eglinton, G. (1993) Alkenone and alkenoate distributions within the euphotic zone of the eastern North Atlantic: correlation with production temperature. Deep-Sea Research, 40, 1935-1962. Conte, M. H., Volkman, J. K. and Eglinton, G. (1994) Lipid biomarkers of the Prymnesiophyceae. In The Haptophyte Algae, B. S. C. Leadbeater and J. M. Green (eds), pp. 351-377. Oxford University Press, Oxford. Conte, M. H., Eglinton, G. and Madureira, L. A. S. (1995) Origin and fate of organic biomarker compounds in the water column and sediments of the eastern North Atlantic. Philosophical Transactions of the Royal Society of London, Series B, 340, 169-178. Dansgaard, W., Johnsen, S. J., Clausen, H. B., Dahl-Jensen, D.. Gundestrup, N. S., Hammer, C. U., Hvidberg, C. S., Steffensen, J. P., Sveinbjornsdottir, A. E., Jouzel, J. and Bond, G. (1993) Evidence for general instability of past climate from a 250-kyr ice-core record. Nature, 364, 218-220. De Leeuw, J. W., van der Meer, F. W.. Rijpstra, W. I. C. and Schenk, P. A. (1980) On the occurrence and structural identification of long chain ketones and hydrocarbons in recent and subrecent sediments. Advances in Organic Geochemistry, 2 1 l-2 I 7 Duplessy, J. C. (1982) Glacial to interglacial contrasts in the northern Indian Ocean. Nature, 295, 494498. Duplessy, J. C., Bard, E., Arnold, M., Shackleton, N. J., Duprat, J. and Labeyrie, L. (1991) How fast did the ocean-atmosphere system run during the last deglaciation? Earth and Planetary Science Letters, 103, 27-40. Fontugne, M. R. and Duplessy, J. C. (1986) Variations of the monsoon regime during the upper Quaternary: evidence from carbon isotopic record of organic matter in North Indian Ocean sediment cores. Palaeogeography. Palaeoclimatology. Palaeoecology, 56, 69988. Heier-Nielsen, S., Kuijpers, A. and Laier, T. (1995) Holocene sediment deposition and organic matter burial in the upwelling zone off Yemen, Northwest Indian Ocean. Netherlands Indian Ocean Programme Cruise Reports, 4, 111-l 19. Ittekkot, V., Haake, B., Bartsch, M., Nair, R. R. and Ramaswamy, V. (1992) Organic carbon removal in the sea: Geological Society, London, Special Publication, 64, 167-176. the continental connection. Kleijne, A., Kroon, D. and Zevenboom. W. (1988) Phytoplankton and foraminiferal frequencies in northern Indian Ocean and Red Sea surface waters. In Planktonic Foraminifers as Tracers of the Ocean-Climate History, G. J. A. Brummer and D. Kroon (eds), pp. 271&283. Free University Press, Amsterdam. Lapicque, G., Livingston, H. D., Lambert, C. E., Bard, E. and Labeyrie, L. (1987) Interpretation of 239,240Pu in Atlantic sediments with a non-steady state input model. Deep-Sea Research, 34, 1841-1850. Levitus, 8. (1982) Climatological Atlas of the World Ocean. NOAA Professional Paper, 13. US Government Printing Office, Washington, DC, 173 pp. Madureira, L. A. S., Conte, M. H. and Eglinton, G. (1995) The early diagenesis of lipid biomarker compounds in North Atlantic sediments. Paleoceanography, 10, 627-642. Marlowe, I. T.. Brassell, S. C., Eglinton, G. and Green, J. C. (1984) Long chain unsaturated ketones and esters in living algae and marine sediments. Organic Geochemistry, 6, 135-141. Marlowe, I. T., Green, J. C., Neal, A. C., Brassell, S. C., Eglinton, G. and Course, P. A. (1984) Long chain (nC37-C39) alkenones in the Prymnesiophyceae. Distribution of alkenones and other lipids and their taxonomic significance. Journal of British Phycological Society, 19, 203-216. Marlowe, I. T., Brassell, S. C., Eglinton, G. and Green, J. C. (1990) Long-chain alkenones and alkyl alkenoates and the fossil coccolith record of marine sediments. Chemical Geology, 88, 349-375. Nair, R. R., Ittekkot, V., Manganini, S. J., Ramaswamy, V., Haake, B., Degens, E. T., Desai, N. B. and Honjo, S. (1989) Increased particle flux to the deep ocean related to monsoons. Nature, 338, 749-751. Okada, H. and Honjo, S. (1973) The distribution of oceanic coccolithophorids in the Pacific. Deep-Sea Research, 20, 355-374. Peng, T. H. and Broecker, W. S. (1979) Rates of benthics mixing in deep-sea sediment as determined by radioactive tracers. Quaternary Research, 11, 141-149. Poynter, J. and Eglinton, G. (1991) The Biomarker concept-strengths and weaknesses. Journal of Analytical Chemistry, 339, 125-73 1. Prahl, F. G. and Wakeham, S. G. (1987) Calibration of unsaturation patterns in long-chain ketone compositions for palaeotemperature assessment. Nature, 330, 367-369.

1459

Alkenone index vs sea surface temperature in Indian Ocean

Prahl, F. G., Muehlhausen, L. A. and Zahnle, D. (1988) Further evaluation of long-chain alkenones as indicators of paleoceanographic conditions. Geochimica et Cosmochimica Acta, 52, 2303-2310. Prahl, F. G., de Lange, G. J., Lyle, M. and Sparrow, M. A. (1989) Post-depositional stability of long-chain alkenones under contrasting redox conditions. Nature, 341, 434-437. Rechka, J. A. and Maxwell, J. R. (1988) Characterisation of alkenone temperature indicators in sediments and organisms. Organic Geochemistry, 13, 727-734. Rosell-Melt, A., Carter, J. and Eglinton, G. (1994) Distribution of long-chain alkenones and alkyl alkenoates in marine sediments from the North East Atlantic. Organic Geochemistry, 22, 501-509. Rosell-Melt, A., Eglinton, G., Pflaumann, U. and Sarnthein, M. (1995a) Atlantic core-top calibration of the Uk37 index as sea-surface paleotemperature indicator. Geochimica et Cosmochimica Acta, 59, 3099-3107. Resell-Melt, A., Carter, J., Parry, A. T. and Eglinton, G. (1995b) Determination of the Uk37 index in geological samples. Analytical Chemistry, 67, 1283-1289. Sikes, E. L. and Volkman, J. K. (1993) Calibration of long-chain alkenone unsaturation ratios for palaeotemperature estimation in cold polar waters. Geochimica et Cosmochimica Acta, 57, 1883-1889. Sikes, E. L., Farrington, J. W. and Keigwin, L. D. (1991) Use of alkenone unsaturation ratio Uk37 to determine past sea surface. temperatures: core-top SST calibrations and methodology considerations. Earth and Planetary Science Letters, 104, 3447. Sirocko, F. and Ittekkot, V. (1992) Organic carbon accumulation rates in the Holocene and glacial Arabian Sea: implications for Oz-consumption in the deep-sea and atmospheric CO2 variations. Climate Dynamics, 7, 167-172.

Sonzogni, C. (1997) Calibration et application de la methode paliothermometrique des alkenones aux sediments quaternaires de 1’Octan Indien. Ph.D. thesis, Univ. Aix-Marseille, France. Volkman, J. K., Eglinton, G., Comer, E. D. S. and Forsberg, T. E. V. (1980) Long chain alkenes and alkenones in the marine coccolithophorid Emiliania huxleyi. Phytochemistry, 19, 2619-2622. Volkman, J. K., Eglinton, G., Corner, E. D. S. and Sargent, J. R. (1980) Novel unsaturated straight-chain C37C39 methyl and ethyl ketones in marine sediments and a coccolithophore Emiliania huxleyi. Advances in Organic Geochemistry, 219-227

Volkman, J. K., Barrett, S. M., Blackburn, S. I. and Sikes, E. L. (1995) Alkenones in Gephyrocapsa oceanica; implications for studies of paleoclimate. Geochimica et Cosmochimica Acta, 59, 513-520.

APPENDIX COX

COE

Latitude

Longitude

Depth

lJr3,

AM-SST

no.

name

(“N

V)

(ml

mean

(“C) IOm

(“C) IOm

9

FWAM-SST

Z37/238

AM-SST

mean

(“C)30m

FWAM (“C)30m

SST

AM-SST

FWAM-SST

(“C) 50m

(“C) 50m

MD79254

- 17.530

38.400

1934

0.931

26.7

27.6

1.22

26.3

27.0

25.2

25.5

MD79257

-20.240

36.200

1262

0.953

25.9

26.9

1.17

25.6

26.4

24.6

25.0

MD79260

-20.270

36.490

2061

0323

25.9

26.9

1.38

25.6

26.4

24.6

25.0

MD79261

-29.310

33.150

2060

0.905

23.5

24.5

n.d.

23.2

24.1

22.4

22.9

MD73023

-42.129

51.191

3925

0.421

10.7

II.7

0.87

9.1

9.5

7.8

7.9

MD73029

-45.175

72.510

3825

0.372

8.2

7.9

1.03

8.1

7.8

8.1

7.7

MD76005

-32.383

50.471

4395

0.747

20.3

21.6

n.d.

19.9

20.8

19.1

19.5

MD76009

-34.365

63.304

4425

0.650

18.2

19.4

n.d.

17.6

18.2

16.7

16.7

MD76010

-31.227

61.553

4245

0.630

20.0

21.1

ad.

19.5

20.2

18.6

18.6

IO

MD76011

-24.057

59.553

4425

0.878

23.9

24.6

n.d.

23.7

24.2

23.3

23.4

II

MD77022

-24.121

64.052

1590

0.930

23.7

24.6

1.08

23.5

24.2

23.0

23.2

12

MD79275

- 30.450

48.153

4150

0.740

21.5

22.4

n.d.

21.2

22.0

20.5

21.0

13

MD79277

-30.472

48.144

4092

0.762

21.5

22.4

n.d.

21.2

22.0

20.5

21.0

14

MD79284

-32.395

50.462

4342

0.638

20.3

21.6

n.d.

19.9

20.8

19.1

19 5

I5

MD79294

-28.535

68.147

4386

0.610

21.7

22.8

1.15

21.3

22.1

20.6

20.8

I6

MD84527

-43.49

51.19

3262

0.214

6.6

6.3

1.20

6.6

6.3

6.6

6.3

I7

MD85663

-1.104

44.013

3405

0.961

26.8

26.8

1.20

26.6

26.6

25.9

25.9 25.8

I8

MD85665

-0.292

43.268

2203

0.959

26.6

26.6

n.d.

26.4

26.4

25.8

19

MD85668

-0.010

46.023

4020

0.946

26.7

26.7

1.30

26.6

26.6

25.7

25.7

20

MD85669

46.553

2590

0.955

26.6

26.6

I .22

26.4

26.4

25.7

25.7

2.291

C. Sonzogni

APPENDIX COX

Chre

Lataude

no

name

(N)

21

MD85670

2.417

46.370

803

22

MD85674

3.112

50 267

4875

23

MD65675

2.271

52.056

5077

24

MD85676

I 499

53.208

25

MD85661

7445

26

MD85682

10.535

27

MD90936

7 099

28

MD90939

5.341

29

MD90948

2.087

30

MD90950

2.012

Longitude

et al.

(continued)

Depth

UK37

AM-SST

(m)

mean

(C) IOm

(C) IOm

0941

26.6

26.h

26.4

0 945

27 0

26 3

26.7

0 Y4Y

27.3

26 k

27.0

26.5

26 2

25.9

5080

0 949

27 5

27 I

27.2

26.X

26.3

26.2

53.110

5062

0 928

26 5

256

26.3

25 3

25 7

24.7

52.235

3092

0 902

25.9

24 s

25.3

24.0

24.4

23 0

58 056

2360

0 970

27 4

27 4

26.6

26.6

24 I

24. I

61.385

3860

0 963

27 9

27 Y

27 0

27.0

24.5

24.5

76.218

3954

0 955

28.6

28 6

28.5

28 5

28 I

28. I

75 303

3315

0.9R9

28.6

28.6

28.5

28.5

28.2

28.2

(E)

FWAM-SST

37138

AM-SST

mean

(C) 30m

FWAM

SST

(C) 30m

AM-SST

FWAM-SST

(C) 50m

(C) 5om

26 4

28 7

25.7

26.0

2h 0

25.4

31

MD90956

5.261

73.114

778

0 Ynr

28 8

28 c

28.4

28.4

27.9

27 9

32

MD90957

5 285

73.130

I017

0 978

28.5

28 5

28.4

28.4

27 9

27 9

33

MD90960

5.049

73.470

1906

0.953

28.5

28 5

28.4

28 4

27.9

27 9

34

MD81338

6 550

78.450

2595

0991

28 2

2x 2

28 0

28 0

27 0

27.0 26.9

3s

MD77179

18.220

91.010

1986

0 947

27.8

28 0

27 7

27 8

26 8

36

MD77177

16250

93.240

2284

0 959

27 9

28 0

27.9

28.0

27. I

27.2

37

MD7717h

14.310

93.080

1375

0960

2x 0

2x 0

27.9

28 0

27.2

27.4

38

MD77169

IO.130

95.030

2360

0.971

2x. I

2n !

27.9

28.0

27.0

27.3

39

MD77164

6 060

93 270

2140

O.97h

28 5

28.5

28.3

2R 4

27.8

27 9

40

MD76132

17000

71.310

1430

0 966

27.9

2X.2

27 5

27 6

26.5

26.2

41

MD76131

I5 320

72.340

1230

0 972

28 ?

2x 1

27 8

27 5

26.9

26.2

42

MD76129

15000

72.200

1954

0.967

28 3

28 7

27.8

27.5

26 9

26.2

43

MD77197

13.110

73.260

1260

0.968

2x 5

28 2

27.9

27 I

26.9

25.8

44

MB76127

12050

75 540

1610

0.9hS

28 4

2: x

27 6

26 4

26.6

25.0

45

MD77195

I I 300

74.220

l42h

0 96X

2x.5

27 9

27 7

26.6

26.7

25 I

46

MD77194

10280

75 140

1222

0 974

2x 4

27 7

27 6

26 4

26 3

24.6

47

MD77191

7 300

76.430

,254

0 975

28 2

28 ,J

28 0

27 7

26.')

26. I

48

MD77203

20.420

59.240

2442

0.897

25.x

25 q

24.8

24 4

23 2

22.7

49

MD77204

19.180

58 260

I430

0.x70

2’ 6

2s 2

24 7

24 2

23.2

22 6

50

MD77202

19 I30

60410

2421

0.924

25 Y

25

25 3

25 [

23.9

23.7

51

MD77205

17.440

57.400

989

n 877

25 Y

?5 (

24 9

24.2

23.s

22 8

52

MD76135

14.270

50310

1895

0 925

26.8

Ih 4

24 b

23 3

22 Y

21.3

53

MD77200

16.330

h7.540

2910

0 9hR

27 I

2,

27 3

27 6

26 6

26.9

54

MD76136

12520

46.490

1649

0 930

27 6

27 h

26 I

26. I

23.X

23.8