High-resolution trace element proﬁles in shells of the ... · Tudor, the freshwater input occurs...

Estuarine, Coastal and Shelf Science 57 (2003) 1103–1114

ARTICLE IN PRESS

High-resolution trace element profiles in shells of the mangrovebivalve Isognomon ephippium: a record of environmental

spatio-temporal variations?

C.E. Lazaretha,*, E. Vander Puttena, L. Andreb, F. Dehairsa

aAnalytical Chemistry Department, Vrije Universiteit Brussel, B-1040 Brussels, BelgiumbSection of Petrography–Mineralogy–Geochemistry, Royal Museum for Central Africa, B-3080 Tervuren, Belgium

Received 28 June 2002; accepted 14 January 2003

Abstract

The shell chemistry of Isognomon ephippium from three Kenyan sites (Tudor, Gazi and Mida) has been investigated to determinewhether these bivalves record environmental parameters. The Mg, Sr, Ba and Mn distributions in the calcite shell layer were

determined by using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). In addition, whole-shell analyseswere made to evaluate inter-site differences. While some variability is observed for mean Mg concentrations, the mean Srconcentrations were similar for the three sites. The decreasing mean Ba and Mn concentrations, following the order

Tudor>Gazi>Mida, are related to distinct regimes of freshwater and nutrient supply. The Mg profiles, determined by LA-ICP-MS, displayed a close to regular sinusoidal pattern, depending on specimen and sample site. For the Tudor shells, an arbitraryfitting of the Mg profiles to sea-surface temperature (SST) variations emphasised the good relationship between these two

parameters and allowed for the calculation of mean annual growth rates. In most of the shells, Sr partly co-varied with Mg and Ba,highlighting the complexity of Sr incorporation. The Ba and Mn profiles of the Tudor shells displayed several sharp maxima. Witha time scale deduced from the Mg–SST relationship, the Ba and Mn maxima of the Tudor shells closely followed periods of maximalrainfall associated with the southeast monsoon. These Ba and Mn maxima were tentatively associated with algal bloom events

known to succeed these periods of high rainfall. The less clearly marked seasonality of the Ba and Mn maxima for the Gazi andMida specimens is thought to result from weaker seasonal variations in nutrient supply and reduced nutrients inputs. This studyhighlights the potential of I. ephippium as a recorder of spatio-temporal environmental variations in tropical coastal ecosystems.

� 2003 Elsevier Science B.V. All rights reserved.

Keywords: mangrove; shell chemistry; trace elements; environmental change; bivalve; laser ablation-inductively coupled plasma-mass spectrometry;

productivity

1. Introduction

It has been demonstrated that the shell chemistry ofvarious bivalves represents a record of environmentalparameters (e.g. Dodd, 1965; Klein, Lohman, & Thayer,1996a,b; Lorens & Bender, 1980). These parameters caneither have a physico-chemical nature, like temperatureand salinity, or a biological nature, like primary pro-

* Corresponding author. Present address: UR 055 IRD-Paleotro-

pique, Centre IRD Ile de France, 32 Avenue Henri Varagnat, 93143

Bondy cedex, France.

E-mail address: [email protected] (C.E. Lazareth).

0272-7714/03/$ - see front matter � 2003 Elsevier Science B.V. All rights re

doi:10.1016/S0272-7714(03)00013-1

ductivity. For instance, variations in sea-surface temper-ature (SST) can lead to changes in the Mg content ofbivalve shells (Dodd, 1965; Fuge, Palmer, Pearce, &Perkins, 1993; Klein et al., 1996a,b; Vander Putten,Dehairs, Keppens, & Baeyens, 2000), whereas phyto-plankton blooms can be recorded by the shell as discreteBa peaks (Stecher, Krantz, Lord, Luther, & Bock, 1996;Vander Putten et al., 2000). Because of their high growthrate, bivalve shells record these environmental changeson a seasonal scale that can be easily resolved using laser-ablation inductively-coupled-plasma mass spectrometry(LA-ICP-MS) (e.g. Fuge et al., 1993; Price & Pearce,1997; Raith, Perkins, Pearce, & Jeffries, 1996).

served.

mailto:[email protected]

1104 C.E. Lazareth et al. / Estuarine, Coastal and Shelf Science 57 (2003) 1103–1114

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Tropical coastal ecosystems can undergo fast andimportant changes tuned by the monsoon regimes. Forinstance, freshwater (surface- and/or groundwater) in-puts can significantly modify environmental parameters,such as salinity (Kitheka, 1996; Osore, Tackx, & Daro,1997), nutrient concentrations (Kazungu, Dehairs, &Goeyens, 1989) and Ba availability (Carroll, Falkner,Brown, & Moore, 1993; Moore, 1997). Mangroves covermost of the Kenyan coastline (about 53 000 ha; Doute,Ochanda, & Epp, 1981) and display various patterns offreshwater supply through groundwater and surfacewater (Talk & Polk, 1999; Woitchik & Dehairs, 1993).In this study, a common bivalve of the Kenyan man-groves, Isognomon ephippium, from three systems withdistinct freshwater flow characteristics was analysed. Thisstudy focused on the shell chemistry of these bivalves tocheck their potential in: (1) differentiating between eco-system environments characterised by different freshwa-ter flow regimes; and (2) recording seasonal changes of the

environment. Therefore, the extent to which Mg, Sr, Baand Mn variations could be related to ambient physico-chemical and biological conditions was investigated.

2. Material and method

2.1. Samples

Specimens of Isognomon ephippium, a relatively flatbivalvewith a round shape that often lives attached to stiltroots of the mangrove Rhizophora mucronata, werecollected in August 1998 from three Kenyan sites: (1)Mida (M) and (2) Gazi (G), two mangrove ecosystems,respectively, 50 km south and about 100 km northeast ofMombasa; and (3) the Tudor Estuary (T), adjacent toMombasa city (Fig. 1). These three sites differ in fresh-water input regime. In Mida, freshwater input occursmainly through groundwater flow (Kitheka, 1998). In

Fig. 1. Sampling sites. (A) Position of Tudor, Gazi and Mida along the Kenyan coast. (B) Tudor (modified from Kazungu et al., 1989),

(C) Gazi (modified from Kitheka, 1996; s3 point from Osore et al., 1997) and (D) Mida (modified from Gang & Agatsiva, 1992). Grey, continent;

pale grey, mangrove; asterisks, shell sampling location; dots with labels, stations with information on salinity and nutrients.

1105C.E. Lazareth et al. / Estuarine, Coastal and Shelf Science 57 (2003) 1103–1114

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Tudor, the freshwater input occurs mainly throughsurface water flow from the River Kombeni (annualmean of 617 l s�1 in 1975; Norconsult, 1975) and, toa lesser extent, from the River Tsalu (annual mean of185 l s�1 in 1975; Norconsult, 1975). Finally, in Gazi, thefreshwater input occurs through the Kidogoweni River(mean of 1307 l s�1 from January to July 1994; Kitheka,1996) as well as through groundwater flow. Relativegroundwater flow for these systems follows the patternGazi>Mida>Tudor, with flows of 0.164, 0.119 and0.067mday�1, respectively. Maximal discharges (bi-annual flood) were observed in April–May (Norconsult,1975; Kitheka, 1996) and November (Norconsult, 1975),associated with rainfall.

In order to obtain a preliminary indication of inter sitedifferentiation, regarding trace element recording by theshells, the whole-shell composition of specimens from thethree sites was compared. Subsequently, discrete pointanalyses was made, using LA-ICP-MS, in order to gettemporal information over the entire lifetime of themolluscs.

2.2. Whole shell analyses—inductively coupledplasma-optical emission spectrometry

Five specimens from each site (T1 to T5 for Tudor, G1to G5 for Gazi andM1 toM5 forMida) were selected forinductively coupled plasma-optical emission spectrome-try (ICP-OES) analyses. One valve from each specimen

was treated overnight in warm (60 �C) diluted hydro-gen peroxide to remove organic matter. The shells werethen dried, weighed and dissolved in 10% Suprapur�HCl. Finally, these solutions were evaporated and re-dissolved into 10% Suprapur� HNO3, before analysiswith a Thermo Jarrell Ash Corporation IRIS spectro-meter (ICP-OES). The elements analysed were Mg(3838 nm), Mn (2605 nm), Sr (3464 nm) and Ba(4934 nm). The second valve was saved for LA-ICP-MS.

2.3. High-spatial resolution analyses—LA-ICP-MS

From the remaining valve of one to two specimensfrom each site (T1 and T3 for Tudor, G4 for Gazi andM2 for Mida), a 5mm wide slab was cut along themaximum growth axis using a Labcut low-speed di-amond saw. The Isognomon ephippium shell is made upof two layers: an external calcite layer and an internalaragonite one. These layers can be distinguished (1)visually (Fig. 2A, B) and (2) from their chemicalcomposition (Fig. 2C). The LA-ICP-MS analyses weremade in the middle of the calcite shell layer, from nearthe umbo towards the edge, approximately every 200 lm(Fig. 2B). This way, successively formed layers weresampled. The chemical profiles were started around10mm from umbo, considering the small thickness andthe fragility of the most ancient calcite layer (close to theumbo). In addition, crossing analyses, from the externaltowards the inner face of the shell (Fig. 2B), were done

Fig. 2. Isognomon ephippium specimen. (A) View of a 5-mm wide slab sawed for LA-ICP-MS analyses. (B) A 30-lm thick section picture of part of

a slab analysed by LA-ICP-MS. Craters in the calcite layer are about 30lm in diameter and 200lm spaced out. White aligned circles represent

a crossing. (C) LA-ICP-MS crossings of Mg and Sr distributions, assessing the two layers structure and calcite homogeneity (three crossings

distributed along the same shell).


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in order to assess the two-layer structure and the calcite-layer homogeneity. The LA-ICP-MS equipment wasa Fisons-VG Microprobe frequency quadrupled Nd-YAG (Neodymium doped Yttrium Aluminum Garnet)operating at 266 nm (ultraviolet wavelength), coupledwith a Fisons-VG PlasmaQuad IIþMS. The laser wasoperated in the Q-switched mode with a power of 2mJand a repetition rate of 10Hz. The preablation time wasset at 10 s and the acquisition time at 20 s (all theconditions are summarised in Table 1). Under theseconditions, the ablation craters that were obtained hada diameter between 30 and 45 lm. The elementsanalysed were 26Mg, 55Mn, 43Ca, 88Sr and 138Ba. Theinstrumental instability and drift were corrected using43Ca as an internal standard.

Analyses were calibrated using the NIST610 glassreference material as an external standard (concentra-tions taken from Pearce et al., 1997). However, the useof a non-matrix-matched external standard (i.e. a glassstandard for carbonate analysis) can lead to incoherentresults due to matrix effects (Outridge, Doherty, &Gregoire, 1997; Stix, Gauthier, & Ludden, 1995; VanderPutten, Dehairs, Andre, & Baeyens, 1999), although theuse of an ultraviolet laser seems to reduce such effects(Geertsen et al., 1994; Gunther, Longerich, Forsythe, &Jackson, 1995; Norman, Pearson, Sharma, & Griffin,1996). As this study focused on trace elements variationsthrough the shell, and on relative shell record differencesbetween sites, the LA-ICP-MS data are presented ina semi-quantitative way as percentage of variationaround the mean value (set to zero) of the transect foreach profile.

3. Results

3.1. Shell trace element concentrations

The Mg shell contents show some variability betweensites (Fig. 3A), whereas for Sr, similar results areobtained for the three sites with an overall mean of771 ppm (Fig. 3B). For Mn and Ba, the shells fromthe three sites show significant differences (one-way

Table 1

LA-ICP-MS operating conditions

Laser probe ICP-MS

Laser mode Q-switched Argon flow rate lmin�1

Laser power (mJ) 2 Carrier gas 1.01

Frequency (Hz) 10 Auxiliary gas 1.28

Preablation time (s) 10 Cooling gas 13.75

Acquisition mode Peak

jumping

Point per peak 3

Dwell time (ms) 10.24

Acquisition time (s) 20

ANOVA). The Tudor shells show the highest concen-trations for Mn and Ba (41 and 3.6 ppm, respectively)followed by the Gazi shells (11 and 1.2 ppm, respec-tively), which in turn are slightly richer in Mn andBa concentrations than the shells from Mida (4 and1.0 ppm, respectively; Fig. 3C, D).

3.2. LA-ICP-MS profiles

3.2.1. Mg and SrThe Mg profile of the T1 shell displays a sinusoidal

variation with broad Mg maxima frequently consistingof two narrower peaks (Fig. 4A). In addition, there isa gradual increase in the Mg level, from �30 to þ10%.Except at the beginning of the profile, Sr shows a sinu-soidal pattern similar to that of Mg, although with aslight phase shift towards the edge (Fig. 4A). For T3,Mg also displays a sinusoidal evolution with a clear Mgincrease after 20mm from umbo (Fig. 5A). The Sr pat-tern in T3 is less clear than that for T1, and does notshow any co-evolution with Mg (Fig. 5A). For the twoshells, some of the Sr maxima coincide with Ba and Mnmaxima (Fig. 6).

For G4, a sinusoidal-like pattern for Mg is alsoobserved, although the profile is less regular than for theTudor shells. Furthermore, while the Mg level in theTudor shells increased gradually (T1) or more abruptly(T3), Mg in G4 sharply dropped to around 20mm (Fig.7A). For Sr, no clear sinusoidal-like pattern can bediscerned.

Finally, a sinusoidal pattern is also visible for Mg andSr profiles in the M2 shell (Fig. 8A). The Mg and Srprofiles are similar, without a phase shift and, as for T1,show a slight gradual increase, from �20 to þ18.

3.2.2. Ba and MnFor T1, Ba andMn are strongly correlated ðr2 ¼ 0:76Þ,

and the profiles are characterised by sharp peaksseparated by zone of low Ba and Mn (Fig. 4B). In theT3 shell, there is also a correlation between Ba and Mn,although less stronger than for T1 ðr2 ¼ 0:62Þ. RecurrentBa andMn peaks appear in the second part of the profile,after 20mm from umbo (Fig. 5B). For these two Tudorshells, some of the Ba and Sr peaks are in phase (Fig. 6).

For G4, Ba shows relatively little variation except atthe end of the profile where a sharp peak appears andbetween 22 and 28mm from umbo where smaller Bapeaks occur (Fig. 7B). The Mn profile shows even lessvariation with no noticeable characteristics.

Finally, for M2, the Ba profile is characterised bypeaks distributed all over the profile, with some of them,at 14, 21, 28 and 31mm from the umbo, being moresalient (Fig. 8B). Mn is not correlated with Ba in thisshell and shows a plateau of higher values between 10and 15mm from umbo.


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Fig. 3. Mg, Sr, Mn and Ba contents in the whole valve of Isognomon ephippium from Tudor, Gazi and Mida. Mean (ppm)� 2r�xx indicated.

4. Discussion

4.1. Mg profiles

The Mg LA-ICP-MS profiles in the calcite shell layerof Isognomon ephippium are characterised by a sinu-soidal-like pattern, which is less regular in the Gaziand Mida specimens. It has been observed that Mg/Cain bivalve shell calcite increases with the Mg/Ca ratioof seawater (Lorens & Bender, 1980). However, duringmixing of seawater with freshwater, Mg/Ca in solutionwill only be significantly different from that of the oceanfor salinity below 10 (Dodd & Crisp, 1982). For Tudor,at a site close to our sampling site (A4 station, Fig. 1B),the lowest salinity observed was 11.29 during the rainyseason (May 1986; Kazungu et al., 1989). During mostof the rest of the year, Tudor creek behaves simply asa fjord-like extension of the Indian Ocean (Kazunguet al., 1989) and has a salinity value similar to thatobserved in coastal waters. Thus, salinity variationsprobably did not have a significant influence on Tudorshell Mg variations. For Gazi, salinity at stations en-compassing the sampling site of G4 were reported torange from 5 to 37 (k3, Kitheka, 1996; s3, Osore et al.,1997), with the minimum value in May (1994) whenrainfalls are highest. However, a large gradient exists insalinity values between the point where river Kidogo-weni enters the lagoon and our G4 sampling site wasobserved. Indeed, salinity minima measured by Kitheka(1996) for stations upstream from our sampling site were2 and 5 for k2 and k3, respectively, whereas the salinityminimum at s3, slightly downstream of our sampling

site, was 20 (Osore et al., 1997). Therefore, it is believedthat G4 Isognomon specimen is likely to have experienceda minimum salinity environment between 10 and 15.Thus, it is unlikely that the Mg/Ca ratio in the waterambient to the site of G4 sampling underwent a sig-nificant decline linked to salinity decrease. Finally, atMida, mean salinity recorded between May 1996 andApril 1997 was >30 (Kitheka, 1998), excluding a possibleinfluence of dilution on the Mg/Ca ratio of the aquaticsystem and, consequently, on the shell Mg/Ca ratio.

Another factor influencing the incorporation of Mgin bivalve shells is the seawater temperature. Indeed, ithas been demonstrated that Mg variations in the shell ofvarious types of bivalves show a positive correlationwith seawater temperature (Dodd, 1965; Fuge et al.,1993; Klein et al., 1996a,b). Consequently, the moreregular Mg profiles of the Tudor shells (i.e. the full T1profile and the second half of the T3 profile, between 17and 37mm from umbo) were investigated more closelyto deduce a time-dependent Mg distribution for theseshells and to get a date for each laser crater, which canthen be used for the other elements. Therefore, a fittingwas performed by arbitrarily forcing the Mg minima tocoincide with the SST minima (data from Reynolds &Smith, 1994), taking the date of collection as thereference time point (note that there is some uncertaintyon this date since the LA-ICP-MS analysis had to bestopped before reaching the edge of the shell, because ofpoor ablation). The distance (in mm) between each Mgminima corresponds to �1 year of shell accretion, thusgiving a year-averaged shell growth rate that was used tocalculate a date for each laser crater. The fittings finally


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Fig. 4. Trace element profiles along the T1 shell (Tudor, Kenya), relative to the mean of all craters. (A) Mg and Sr, (B) Ba and Mn. The insert shows

Ba vs Mn with linear regression line (r2 noted).

obtained between the Mg- and SST-profiles are verygood (Fig. 9) and can be regarded as reflecting a majorcontrol of SST on Mg incorporation. Moreover, thedifferent year-averaged shell growth rates determinedare decreasing with age, as expected, from 12.8 to3.0mmyear�1 for T1 and from 8.6 to 2.5mmyear�1 forT3 (Fig. 9). Consequently, the protocol used to gettemporally fitted profiles can be considered as a correctfirst approach. Nevertheless, considering part of the Mgvariations (sudden or gradually increases), supplemen-tary factor(s) might have an influence on Mg shellincorporation. Physiological factors could be part of theMg increase around 20mm of T3 (Fig. 5A), as well as ofthe less clear and not continuous sinusoidal Mg patternsof G4 and M2 (Figs. 7A and 8A). Finally, the overallgradual increase in the Mg content of the T1 and M2shells might be related to the ageing of the specimen, asit has been shown for Sr/Ca in Mya arenaria (Palacios,Orensanz, & Armstrong, 1994).

4.2. Sr profiles

The Sr patterns are more complicated than those ofMg. Many authors have studied the incorporation of Srinto bivalve shells and the multiple results reveal thecomplexity of Sr-incorporation processes. Indeed, someauthors found a relationship between shell Sr/Ca ratiosand temperature (Dodd, 1965) or Sr/Ca ratios in solution(Lorens & Bender, 1980), whereas others explained shellSr/Ca variations by kinetic factors like precipitation rate(Carpenter & Lohmann, 1992; Lorens, 1981), growthrate (Stecher et al., 1996) or mantle metabolic activity(Klein et al., 1996a). Moreover, a positive linear cor-relation between Mg and Sr contents has been observedboth in artificial (Mucci & Morse, 1983) and naturalcalcite (Carpenter & Lohmann, 1992; Ohde & Kitano,1984). This relationship is usually attributed to latticedistortion, related to the Mg2þ–Ca2þ substitution, thatallows the incorporation of the larger Sr2þ cation in the


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Fig. 5. Trace element profiles along the T3 shell (Tudor, Kenya), relative to the mean of all craters. (A) Mg and Sr, (B) Ba and Mn. The insert shows

Ba vs Mn with linear regression line (r2 noted).

calcite network (Mucci & Morse, 1983). In view of thesevarious invoked processes, it appears that the Sr in-corporation is probably governed not by one processbut rather by multiple factors.

In the present study, the similarities between Mg andSr in most shell profiles indicate that Sr incorporationmight be Mg-dependent and/or, to a certain extent,temperature-dependent (Fig. 9). Nevertheless, the Srprofiles in T1 and in the second part of T3 are notperfectly in phase with Mg, and the G4 profile patterneven shows no sinusoidal evolution at all, nor a relation-ship with Mg. Consequently, additional factors affectthe incorporation of Sr. As Lorens and Bender (1980)have shown, shell Sr/Ca ratios are influenced by thesolution Sr/Ca ratio. However, Dodd and Crisp (1982)demonstrated that the Sr/Ca ratio of estuarine waters isonly significantly different from that of the open oceanbelow a salinity value of 10. Such low salinity isgenerally not observed at the sites studied. Thus, it

seems unlikely that variations in the seawater Sr/Caratios are large enough to have an impact on the shell Srprofiles observed in the present study. For the Tudorshells, some of the Sr maxima coincide with Ba peaks(Fig. 6). This might indicate that the factor(s) explainingthe Ba peaks also have an influence, direct or indirect,on the incorporation of Sr.

4.3. Ba and Mn profiles

The Ba and Mn patterns in the calcite layer of theTudor Isognomon ephippium are characterised by severalnarrow maxima. These are regularly distributed alongthe profile of T1, but are only present in the second partof the T3 profile. For Gazi and Mida, the patterns differfrom that of Tudor, showing less obvious high narrowpeaks, and exhibiting less similarity between Ba and Mn.

The presence of Ba peaks has already been reportedfor Mercenaria mercenaria and Spisula solidissima


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Fig. 6. Ba vs Sr profiles of Tudor shells, relative to the mean of all craters. Arrows indicate coinciding Sr and Ba peaks. (A) T1, (B) T3.

(Stecher et al., 1996) and for Mytilus edulis (VanderPutten et al., 2000), in the latter case, together with Mnpeaks. In both cases, the Ba (Mn) peaks were related toperiods of high phytoplankton productivity. Indeed, ithas been shown that Ba is incorporated and/or adsorbedonto phytoplankton, and may precipitate as barite inorganic-rich microenvironments, mostly constituted ofdiatoms (Bishop, 1988; Dehairs et al., 1990; Stroobants,Dehairs, Goeyens, Vanderheijden, & Van Grieken,1991). In coastal tropical systems, similar to the threestudied Kenyan sites, the phytoplankton blooms areinitiated essentially through an increase in nutrientinputs, linked with the monsoon regime (shown forTudor by Kazungu et al., 1989). In addition to nutrients,freshwater is also enriched in Ba relative to the open-ocean water (Broecker & Peng, 1982). As a result,freshwater inputs into tropical coastal systems not onlycause phytoplankton blooms through increased nutrientsupplies, but also provide a significant input of Ba thatcan be incorporated by phytoplankton, which can

subsequently be ingested by filtering bivalves (Stecher& Kogut, 1999; Stecher et al., 1996; Vander Putten et al.,2000). The Tudor Ba profiles were thus compared withprecipitation data, to which river outflows are, mostprobably, directly related. For this, the Ba profilescalibrated against time using the Mg–SST fitting wereused. Apparently, the sharp Ba (and Mn) maximagenerally occur at the end of the rainy period associatedwith the southeast monsoon (Fig. 9B). Thus, a delay ofa few weeks seems to occur between the increased run-off of freshwater carrying high contents of nutrients anddissolved Ba and the appearance of high Ba (Mn)concentrations in the shell. This can be explained by thefact that phytoplankton blooms in Kenyan coastallagoons generally occur after the surge of rainfall, whenturbidity has decreased again but nutrient contents arestill favourable (M.H. Daro, personal communication;Brunet et al., 1996). This also agrees with the hypothesisthat Ba is probably deposited in the shell as a result ofenhanced ingestion of Ba/barite associated with the


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Fig. 7. Trace element profile along the G4 shell (Gazi, Kenya), relative to the mean of all craters. Arrow with number indicates high Ba value. (A)

Mg and Sr, (B) Ba and Mn.

filtered particles and not by direct uptake of dissolvedBa. For Tudor shells, the Ba–Mn co-variation indicatesthat: (1) freshwater inputs also bring a large amount ofMn to the environment; and (2) Mn incorporationoccurs essentially through ingestion of enriched partic-ulate matter such as phytoplankton. A similar situationhas been observed for the blue mussel M. edulis in thetemperate Scheldt Estuary (Vander Putten et al., 2000).The lack of a direct correlation between the magnitudeof precipitation (and thus river run-off) and the amountof incorporated Ba (Mn) suggests, however, that theprocess is complex.

The particularly well-expressed high narrow Ba andMn peaks in the Tudor shells reflect the relatively im-portant monsoon freshwater input to Tudor, resulting ina considerable increase in nutrient content in the Tudorcreek (Kazungu et al., 1989). This nutrient increase isexpected to coincide with high Ba and Mn inputs.However, whereas the Ba- and Mn-signals are well

expressed for the Tudor shells, this is not the case for theGazi and Mida ones. These differences probably resultfrom the differences in hydrodynamic regime betweenthe three sites. The freshwater nutrient concentrationsare significantly different between Tudor and Gazi.Indeed, at Tudor, the nitrate concentrations rangebetween 0.28 and 17.8 lmol�l during the rainy season(Kazungu et al., 1989), whereas at Gazi, the nitrate plusnitrite content ranges between 0.2 and maximum4 lmol�l during the rainy season (Kazungu, personalcommunication). Consequently, it can be expected thatas a result of this poor nutrient availability, the phyto-plankton blooms at Gazi are less important than atTudor, resulting in less organic-rich microenvironmentswhere Ba (Mn?) can precipitate before being incorpo-rated by Isognomon ephippium. This also supports thelowest Ba and Mn contents observed in whole shells(Fig. 3). Also, in Mida, the freshwater input only occursthrough groundwater and rainfall (no river input). In


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Fig. 8. Trace element profile along the M2 shell (Mida, Kenya), relative to the mean of all craters. (A) Mg and Sr, (B) Ba and Mn.

such systems with a predominance of groundwater flow,freshwater input can be expected to be more constantover time and show less seasonal variability, thusexplaining the �flatter� Ba/Mn patterns in the Mida shell.The low Ba and Mn levels observed for the Mida shellsare also probably directly linked to the absence ofriverine freshwater input.

5. Conclusions

The whole-shell Ba and Mn contents of the Iso-gnomon ephippium, which were studied are directlyrelated to the hydrodynamic properties and nutrientinputs of the sites. The good fitting between Mg inTudor shells and SST is in accordance with a pre-dominant control of SST on Mg incorporation.Although Sr profiles are often close to the Mg ones,some similarities with the Ba profiles underline thecomplexity of Sr incorporation into the shell. The

peaked profiles of Ba reflect the phytoplankton bloomsuccessions as governed by monsoon regime. Also, thesimilarity between Ba and Mn confirms that incorpora-tion of Mn is for a large part also governed byproductivity.

This study highlights the potential of Isognomonephippium as environmental recorder. However, itappears that further investigations, on a larger specimenpopulation associated with a site survey, are necessaryto improve interpretation of data and to extend to fossilequivalents (Hendry, Perkins, & Bane, 2001) that mightbe the witnesses of ancient hydrodynamic/productivitycharacteristics.

Acknowledgements

This work was supported by a Post Doctoral Grantto C.E.L. under the European Union Training and


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Fig. 9. Tudor shells fitting with environmental parameters. (A) Mg and monthly averaged SST (data from Reynolds & Smith, 1994). Thick line, Mg

profile of T1; thin line, Mg profile of T3; dotted line, SST. (B) Ba and precipitation (precipitation data from the internet site http://

ingrid.ldgo.columbia.edu/SOURCES/.NOAA/.NCEP/.CPC/.CAMS_OPI/.mean/.prcp/ for a geographical setting, 41.25�E 3.75�S, encompassing

the Mombasa area with its coastal waters). Thick line, Ba profile of T1; thin line, Ba profile of T3; dotted line, mean precipitation.

Mobility of Researchers (TMR) Program, contractFMBICT983440 and by EC-INCO research projectIC18-CT96-0065 and FWO Flanders contract. Wethank J. Cillis for assistance with the scanning electronmicroprobe and Th. Hubin for photographs of speci-men. We thank Ph. Willenz for the bivalve speciesidentification and for his help with the specimenpreparation for LA-ICP-MS. We are also grateful tohim for his helpful advice on the manuscript.

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