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High-resolution trace element profiles in shells of the mangrove bivalve Isognomon ephippium: a record of environmental spatio-temporal variations?
Estuarine, Coastal and Shelf Science 57 (2003) 1103–1114
High-resolution trace element profiles in shells of the mangrove bivalve Isognomon ephippium: a record of environmental spatio-temporal variations? C.E. Lazaretha,*, E. Vander Puttena, L. Andre´b, F. Dehairsa a Analytical Chemistry Department, Vrije Universiteit Brussel, B-1040 Brussels, Belgium Section of Petrography–Mineralogy–Geochemistry, Royal Museum for Central Africa, B-3080 Tervuren, Belgium
b
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 determine whether 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 analyses were made to evaluate inter-site differences. While some variability is observed for mean Mg concentrations, the mean Sr concentrations 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 LAICP-MS, displayed a close to regular sinusoidal pattern, depending on specimen and sample site. For the Tudor shells, an arbitrary fitting 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. With a time scale deduced from the Mg–SST relationship, the Ba and Mn maxima of the Tudor shells closely followed periods of maximal rainfall 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 and Mida specimens is thought to result from weaker seasonal variations in nutrient supply and reduced nutrients inputs. This study highlights the potential of I. ephippium as a recorder of spatio-temporal environmental variations in tropical coastal ecosystems. Ó 2003 Elsevier Ltd. 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 of various bivalves represents a record of environmental parameters (e.g. Dodd, 1965; Klein, Lohman, & Thayer, 1996a,b; Lorens & Bender, 1980). These parameters can either have a physico-chemical nature, like temperature and salinity, or a biological nature, like primary pro* Corresponding author. Present address: UR 055 IRD-Paleotropique, 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 Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0272-7714(03)00013-1
ductivity. For instance, variations in sea-surface temperature (SST) can lead to changes in the Mg content of bivalve shells (Dodd, 1965; Fuge, Palmer, Pearce, & Perkins, 1993; Klein et al., 1996a,b; Vander Putten, Dehairs, Keppens, & Baeyens, 2000), whereas phytoplankton blooms can be recorded by the shell as discrete Ba peaks (Stecher, Krantz, Lord, Luther, & Bock, 1996; Vander Putten et al., 2000). Because of their high growth rate, bivalve shells record these environmental changes on a seasonal scale that can be easily resolved using laserablation inductively-coupled-plasma mass spectrometry (LA-ICP-MS) (e.g. Fuge et al., 1993; Price & Pearce, 1997; Raith, Perkins, Pearce, & Jeffries, 1996).
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Tropical coastal ecosystems can undergo fast and important changes tuned by the monsoon regimes. For instance, freshwater (surface- and/or groundwater) inputs 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 cover most of the Kenyan coastline (about 53 000 ha; Doute, Ochanda, & Epp, 1981) and display various patterns of freshwater supply through groundwater and surface water (Talk & Polk, 1999; Woitchik & Dehairs, 1993). In this study, a common bivalve of the Kenyan mangroves, Isognomon ephippium, from three systems with distinct freshwater flow characteristics was analysed. This study focused on the shell chemistry of these bivalves to check their potential in: (1) differentiating between ecosystem environments characterised by different freshwater flow regimes; and (2) recording seasonal changes of the
environment. Therefore, the extent to which Mg, Sr, Ba and Mn variations could be related to ambient physicochemical and biological conditions was investigated.
2. Material and method 2.1. Samples Specimens of Isognomon ephippium, a relatively flat bivalve with a round shape that often lives attached to stilt roots of the mangrove Rhizophora mucronata, were collected 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 of Mombasa; and (3) the Tudor Estuary (T), adjacent to Mombasa city (Fig. 1). These three sites differ in freshwater input regime. In Mida, freshwater input occurs mainly 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.
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Tudor, the freshwater input occurs mainly through surface water flow from the River Kombeni (annual mean of 617 l s1 in 1975; Norconsult, 1975) and, to a lesser extent, from the River Tsalu (annual mean of 185 l s1 in 1975; Norconsult, 1975). Finally, in Gazi, the freshwater input occurs through the Kidogoweni River (mean of 1307 l s1 from January to July 1994; Kitheka, 1996) as well as through groundwater flow. Relative groundwater flow for these systems follows the pattern Gazi > Mida > Tudor, with flows of 0.164, 0.119 and 0.067 m day1, respectively. Maximal discharges (biannual 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 site differentiation, regarding trace element recording by the shells, the whole-shell composition of specimens from the three sites was compared. Subsequently, discrete point analyses was made, using LA-ICP-MS, in order to get temporal information over the entire lifetime of the molluscs. 2.2. Whole shell analyses—inductively coupled plasma-optical emission spectrometry Five specimens from each site (T1 to T5 for Tudor, G1 to G5 for Gazi and M1 to M5 for Mida) were selected for inductively coupled plasma-optical emission spectrometry (ICP-OES) analyses. One valve from each specimen
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was treated overnight in warm (60 C) diluted hydrogen peroxide to remove organic matter. The shells were then dried, weighed and dissolved in 10% SuprapurÒ HCl. Finally, these solutions were evaporated and redissolved into 10% SuprapurÒ HNO3, before analysis with a Thermo Jarrell Ash Corporation IRIS spectrometer (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 specimens from each site (T1 and T3 for Tudor, G4 for Gazi and M2 for Mida), a 5 mm wide slab was cut along the maximum growth axis using a Labcut low-speed diamond saw. The Isognomon ephippium shell is made up of two layers: an external calcite layer and an internal aragonite one. These layers can be distinguished (1) visually (Fig. 2A, B) and (2) from their chemical composition (Fig. 2C). The LA-ICP-MS analyses were made in the middle of the calcite shell layer, from near the umbo towards the edge, approximately every 200 lm (Fig. 2B). This way, successively formed layers were sampled. The chemical profiles were started around 10 mm from umbo, considering the small thickness and the fragility of the most ancient calcite layer (close to the umbo). In addition, crossing analyses, from the external towards 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 30 lm in diameter and 200 lm 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 calcitelayer homogeneity. The LA-ICP-MS equipment was a Fisons-VG Microprobe frequency quadrupled NdYAG (Neodymium doped Yttrium Aluminum Garnet) operating at 266 nm (ultraviolet wavelength), coupled with a Fisons-VG PlasmaQuad IIþMS. The laser was operated in the Q-switched mode with a power of 2 mJ and a repetition rate of 10 Hz. The preablation time was set at 10 s and the acquisition time at 20 s (all the conditions are summarised in Table 1). Under these conditions, the ablation craters that were obtained had a diameter between 30 and 45 lm. The elements analysed were 26Mg, 55Mn, 43Ca, 88Sr and 138Ba. The instrumental instability and drift were corrected using 43 Ca as an internal standard. Analyses were calibrated using the NIST610 glass reference material as an external standard (concentrations taken from Pearce et al., 1997). However, the use of a non-matrix-matched external standard (i.e. a glass standard for carbonate analysis) can lead to incoherent results due to matrix effects (Outridge, Doherty, & Gregoire, 1997; Stix, Gauthier, & Ludden, 1995; Vander Putten, Dehairs, Andre´, & Baeyens, 1999), although the use of an ultraviolet laser seems to reduce such effects (Geertsen et al., 1994; Gu¨nther, Longerich, Forsythe, & Jackson, 1995; Norman, Pearson, Sharma, & Griffin, 1996). As this study focused on trace elements variations through the shell, and on relative shell record differences between sites, the LA-ICP-MS data are presented in a semi-quantitative way as percentage of variation around the mean value (set to zero) of the transect for each profile. 3. Results 3.1. Shell trace element concentrations The Mg shell contents show some variability between sites (Fig. 3A), whereas for Sr, similar results are obtained for the three sites with an overall mean of 771 ppm (Fig. 3B). For Mn and Ba, the shells from the three sites show significant differences (one-way
Table 1 LA-ICP-MS operating conditions Laser probe Laser mode Laser power (mJ) Frequency (Hz) Preablation time (s)
ICP-MS Q-switched 2 10 10
Argon flow rate Carrier gas Auxiliary gas Cooling gas Acquisition mode Point per peak Dwell time (ms) Acquisition time (s)
l min1 1.01 1.28 13.75 Peak jumping 3 10.24 20
ANOVA). The Tudor shells show the highest concentrations for Mn and Ba (41 and 3.6 ppm, respectively) followed by the Gazi shells (11 and 1.2 ppm, respectively), which in turn are slightly richer in Mn and Ba concentrations than the shells from Mida (4 and 1.0 ppm, respectively; Fig. 3C, D). 3.2. LA-ICP-MS profiles 3.2.1. Mg and Sr The Mg profile of the T1 shell displays a sinusoidal variation with broad Mg maxima frequently consisting of two narrower peaks (Fig. 4A). In addition, there is a gradual increase in the Mg level, from 30 to þ10%. Except at the beginning of the profile, Sr shows a sinusoidal pattern similar to that of Mg, although with a slight phase shift towards the edge (Fig. 4A). For T3, Mg also displays a sinusoidal evolution with a clear Mg increase after 20 mm from umbo (Fig. 5A). The Sr pattern in T3 is less clear than that for T1, and does not show any co-evolution with Mg (Fig. 5A). For the two shells, some of the Sr maxima coincide with Ba and Mn maxima (Fig. 6). For G4, a sinusoidal-like pattern for Mg is also observed, although the profile is less regular than for the Tudor shells. Furthermore, while the Mg level in the Tudor shells increased gradually (T1) or more abruptly (T3), Mg in G4 sharply dropped to around 20 mm (Fig. 7A). For Sr, no clear sinusoidal-like pattern can be discerned. Finally, a sinusoidal pattern is also visible for Mg and Sr profiles in the M2 shell (Fig. 8A). The Mg and Sr profiles are similar, without a phase shift and, as for T1, show a slight gradual increase, from 20 to þ18. 3.2.2. Ba and Mn For T1, Ba and Mn are strongly correlated ðr2 ¼ 0:76Þ, and the profiles are characterised by sharp peaks separated by zone of low Ba and Mn (Fig. 4B). In the T3 shell, there is also a correlation between Ba and Mn, although less stronger than for T1 ðr2 ¼ 0:62Þ. Recurrent Ba and Mn peaks appear in the second part of the profile, after 20 mm from umbo (Fig. 5B). For these two Tudor shells, some of the Ba and Sr peaks are in phase (Fig. 6). For G4, Ba shows relatively little variation except at the end of the profile where a sharp peak appears and between 22 and 28 mm from umbo where smaller Ba peaks occur (Fig. 7B). The Mn profile shows even less variation with no noticeable characteristics. Finally, for M2, the Ba profile is characterised by peaks distributed all over the profile, with some of them, at 14, 21, 28 and 31 mm from the umbo, being more salient (Fig. 8B). Mn is not correlated with Ba in this shell and shows a plateau of higher values between 10 and 15 mm 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) 2rx indicated.
4. Discussion 4.1. Mg profiles The Mg LA-ICP-MS profiles in the calcite shell layer of Isognomon ephippium are characterised by a sinusoidal-like pattern, which is less regular in the Gazi and Mida specimens. It has been observed that Mg/Ca in bivalve shell calcite increases with the Mg/Ca ratio of seawater (Lorens & Bender, 1980). However, during mixing of seawater with freshwater, Mg/Ca in solution will only be significantly different from that of the ocean for 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 rainy season (May 1986; Kazungu et al., 1989). During most of the rest of the year, Tudor creek behaves simply as a fjord-like extension of the Indian Ocean (Kazungu et al., 1989) and has a salinity value similar to that observed in coastal waters. Thus, salinity variations probably did not have a significant influence on Tudor shell Mg variations. For Gazi, salinity at stations encompassing the sampling site of G4 were reported to range from 5 to 37 (k3, Kitheka, 1996; s3, Osore et al., 1997), with the minimum value in May (1994) when rainfalls are highest. However, a large gradient exists in salinity values between the point where river Kidogoweni enters the lagoon and our G4 sampling site was observed. Indeed, salinity minima measured by Kitheka (1996) for stations upstream from our sampling site were 2 and 5 for k2 and k3, respectively, whereas the salinity minimum at s3, slightly downstream of our sampling
site, was 20 (Osore et al., 1997). Therefore, it is believed that G4 Isognomon specimen is likely to have experienced a minimum salinity environment between 10 and 15. Thus, it is unlikely that the Mg/Ca ratio in the water ambient to the site of G4 sampling underwent a significant decline linked to salinity decrease. Finally, at Mida, mean salinity recorded between May 1996 and April 1997 was >30 (Kitheka, 1998), excluding a possible influence of dilution on the Mg/Ca ratio of the aquatic system and, consequently, on the shell Mg/Ca ratio. Another factor influencing the incorporation of Mg in bivalve shells is the seawater temperature. Indeed, it has been demonstrated that Mg variations in the shell of various types of bivalves show a positive correlation with seawater temperature (Dodd, 1965; Fuge et al., 1993; Klein et al., 1996a,b). Consequently, the more regular Mg profiles of the Tudor shells (i.e. the full T1 profile and the second half of the T3 profile, between 17 and 37 mm from umbo) were investigated more closely to deduce a time-dependent Mg distribution for these shells and to get a date for each laser crater, which can then be used for the other elements. Therefore, a fitting was performed by arbitrarily forcing the Mg minima to coincide with the SST minima (data from Reynolds & Smith, 1994), taking the date of collection as the reference time point (note that there is some uncertainty on this date since the LA-ICP-MS analysis had to be stopped before reaching the edge of the shell, because of poor ablation). The distance (in mm) between each Mg minima corresponds to 1 year of shell accretion, thus giving a year-averaged shell growth rate that was used to calculate 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 very good (Fig. 9) and can be regarded as reflecting a major control of SST on Mg incorporation. Moreover, the different year-averaged shell growth rates determined are decreasing with age, as expected, from 12.8 to 3.0 mm year1 for T1 and from 8.6 to 2.5 mm year1 for T3 (Fig. 9). Consequently, the protocol used to get temporally fitted profiles can be considered as a correct first approach. Nevertheless, considering part of the Mg variations (sudden or gradually increases), supplementary factor(s) might have an influence on Mg shell incorporation. Physiological factors could be part of the Mg increase around 20 mm of T3 (Fig. 5A), as well as of the less clear and not continuous sinusoidal Mg patterns of G4 and M2 (Figs. 7A and 8A). Finally, the overall gradual increase in the Mg content of the T1 and M2 shells might be related to the ageing of the specimen, as it 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 of Mg. Many authors have studied the incorporation of Sr into bivalve shells and the multiple results reveal the complexity of Sr-incorporation processes. Indeed, some authors found a relationship between shell Sr/Ca ratios and temperature (Dodd, 1965) or Sr/Ca ratios in solution (Lorens & Bender, 1980), whereas others explained shell Sr/Ca variations by kinetic factors like precipitation rate (Carpenter & Lohmann, 1992; Lorens, 1981), growth rate (Stecher et al., 1996) or mantle metabolic activity (Klein et al., 1996a). Moreover, a positive linear correlation between Mg and Sr contents has been observed both in artificial (Mucci & Morse, 1983) and natural calcite (Carpenter & Lohmann, 1992; Ohde & Kitano, 1984). This relationship is usually attributed to lattice distortion, related to the Mg2þ–Ca2þ substitution, that allows 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 these various invoked processes, it appears that the Sr incorporation is probably governed not by one process but rather by multiple factors. In the present study, the similarities between Mg and Sr in most shell profiles indicate that Sr incorporation might be Mg-dependent and/or, to a certain extent, temperature-dependent (Fig. 9). Nevertheless, the Sr profiles in T1 and in the second part of T3 are not perfectly in phase with Mg, and the G4 profile pattern even shows no sinusoidal evolution at all, nor a relationship with Mg. Consequently, additional factors affect the incorporation of Sr. As Lorens and Bender (1980) have shown, shell Sr/Ca ratios are influenced by the solution Sr/Ca ratio. However, Dodd and Crisp (1982) demonstrated that the Sr/Ca ratio of estuarine waters is only significantly different from that of the open ocean below a salinity value of 10. Such low salinity is generally not observed at the sites studied. Thus, it
seems unlikely that variations in the seawater Sr/Ca ratios are large enough to have an impact on the shell Sr profiles observed in the present study. For the Tudor shells, some of the Sr maxima coincide with Ba peaks (Fig. 6). This might indicate that the factor(s) explaining the 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 the Tudor Isognomon ephippium are characterised by several narrow maxima. These are regularly distributed along the profile of T1, but are only present in the second part of the T3 profile. For Gazi and Mida, the patterns differ from that of Tudor, showing less obvious high narrow peaks, and exhibiting less similarity between Ba and Mn. The presence of Ba peaks has already been reported for 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 (Vander Putten et al., 2000), in the latter case, together with Mn peaks. In both cases, the Ba (Mn) peaks were related to periods of high phytoplankton productivity. Indeed, it has been shown that Ba is incorporated and/or adsorbed onto phytoplankton, and may precipitate as barite in organic-rich microenvironments, mostly constituted of diatoms (Bishop, 1988; Dehairs et al., 1990; Stroobants, Dehairs, Goeyens, Vanderheijden, & Van Grieken, 1991). In coastal tropical systems, similar to the three studied Kenyan sites, the phytoplankton blooms are initiated essentially through an increase in nutrient inputs, linked with the monsoon regime (shown for Tudor by Kazungu et al., 1989). In addition to nutrients, freshwater is also enriched in Ba relative to the openocean water (Broecker & Peng, 1982). As a result, freshwater inputs into tropical coastal systems not only cause phytoplankton blooms through increased nutrient supplies, but also provide a significant input of Ba that can 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 with precipitation data, to which river outflows are, most probably, directly related. For this, the Ba profiles calibrated against time using the Mg–SST fitting were used. Apparently, the sharp Ba (and Mn) maxima generally occur at the end of the rainy period associated with the southeast monsoon (Fig. 9B). Thus, a delay of a few weeks seems to occur between the increased runoff of freshwater carrying high contents of nutrients and dissolved Ba and the appearance of high Ba (Mn) concentrations in the shell. This can be explained by the fact that phytoplankton blooms in Kenyan coastal lagoons generally occur after the surge of rainfall, when turbidity has decreased again but nutrient contents are still favourable (M.H. Daro, personal communication; Brunet et al., 1996). This also agrees with the hypothesis that Ba is probably deposited in the shell as a result of enhanced 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 dissolved Ba. For Tudor shells, the Ba–Mn co-variation indicates that: (1) freshwater inputs also bring a large amount of Mn to the environment; and (2) Mn incorporation occurs essentially through ingestion of enriched particulate matter such as phytoplankton. A similar situation has been observed for the blue mussel M. edulis in the temperate Scheldt Estuary (Vander Putten et al., 2000). The lack of a direct correlation between the magnitude of precipitation (and thus river run-off) and the amount of incorporated Ba (Mn) suggests, however, that the process is complex. The particularly well-expressed high narrow Ba and Mn peaks in the Tudor shells reflect the relatively important monsoon freshwater input to Tudor, resulting in a considerable increase in nutrient content in the Tudor creek (Kazungu et al., 1989). This nutrient increase is expected 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 the Gazi and Mida ones. These differences probably result from the differences in hydrodynamic regime between the three sites. The freshwater nutrient concentrations are significantly different between Tudor and Gazi. Indeed, at Tudor, the nitrate concentrations range between 0.28 and 17.8 lmoll during the rainy season (Kazungu et al., 1989), whereas at Gazi, the nitrate plus nitrite content ranges between 0.2 and maximum 4 lmoll during the rainy season (Kazungu, personal communication). Consequently, it can be expected that as a result of this poor nutrient availability, the phytoplankton blooms at Gazi are less important than at Tudor, resulting in less organic-rich microenvironments where Ba (Mn?) can precipitate before being incorporated by Isognomon ephippium. This also supports the lowest Ba and Mn contents observed in whole shells (Fig. 3). Also, in Mida, the freshwater input only occurs through 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 constant over time and show less seasonal variability, thus explaining the ÔflatterÕ Ba/Mn patterns in the Mida shell. The low Ba and Mn levels observed for the Mida shells are also probably directly linked to the absence of riverine freshwater input. 5. Conclusions The whole-shell Ba and Mn contents of the Isognomon ephippium, which were studied are directly related to the hydrodynamic properties and nutrient inputs of the sites. The good fitting between Mg in Tudor shells and SST is in accordance with a predominant control of SST on Mg incorporation. Although Sr profiles are often close to the Mg ones, some similarities with the Ba profiles underline the complexity of Sr incorporation into the shell. The
peaked profiles of Ba reflect the phytoplankton bloom successions as governed by monsoon regime. Also, the similarity between Ba and Mn confirms that incorporation of Mn is for a large part also governed by productivity. This study highlights the potential of Isognomon ephippium as environmental recorder. However, it appears that further investigations, on a larger specimen population associated with a site survey, are necessary to improve interpretation of data and to extend to fossil equivalents (Hendry, Perkins, & Bane, 2001) that might be the witnesses of ancient hydrodynamic/productivity characteristics.
Acknowledgements This work was supported by a Post Doctoral Grant to 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, contract FMBICT983440 and by EC-INCO research project IC18-CT96-0065 and FWO Flanders contract. We thank J. Cillis for assistance with the scanning electron microprobe and Th. Hubin for photographs of specimen. We thank Ph. Willenz for the bivalve species identification and for his help with the specimen preparation for LA-ICP-MS. We are also grateful to him for his helpful advice on the manuscript.
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and implications for palaeosalinity studies. Palaeogeography, Palaeoclimatology, Palaeoecology 38, 45–56. Doute, R., Ochanda, N., & Epp, H. (1981). Forest cover mapping using remote sensing techniques (72 pp.). Kenya Rangeland Ecological Monitoring Unit, Nairobi, 30. Fuge, R. F., Palmer, T. J., Pearce, N. J. G., & Perkins, W. T. (1993). Minor and trace element chemistry of modern shells: a laser ablation inductively coupled plasma mass spectrometry study. Applied Geochemistry 2, 111–116. Gang, P. O., & Agatsiva, J. L. (1992). The current status of mangroves along the Kenya coast: a case study of Mida Creek mangroves based on remote sensing. Hydrobiologia 247, 29–36. Geertsen, C., Briand, A., Chartier, F., Lacour, J.-L., Patrick, M., Sjo¨stro¨m, S., & Mermet, J.-M. (1994). Comparison between infrared and ultraviolet laser ablation at atmospheric pressure— implications for solid sampling inductively coupled plasma spectrometry. Journal of Analytical Atomic Spectrometry 9, 17–22. Gu¨nther, D., Longerich, H. P., Forsythe, L., & Jackson, S. E. (1995). Laser ablation microprobe-inductively coupled plasma-mass spectrometry. American Laboratory 9, 24–29. Hendry, J. P., Perkins, W. T., & Bane, T. (2001). Short-term environmental change in a Jurassic lagoon deduced from geochemical trends in aragonite bivalve shells. Geological Society of America Bulletin 113, 790–798. Kazungu, J. M., Dehairs, F., & Goeyens, L. (1989). Nutrients distribution patterns in Tudor estuary (Mombasa, Kenya) during rainy season. Kenya Journal of Sciences Series B 10, 47–61. Kitheka, J. U. (1996). Water circulation and coastal trapping of brackish water in a tropical mangrove-dominated bay in Kenya. Limnology and Oceanography 41, 169–176. Kitheka, J. U. (1998). Groundwater outflow and its linkage to coastal circulation in a mangrove-fringed creek in Kenya. Estuarine, Coastal and Shelf Sciences 47, 63–75. Klein, R. T., Lohman, K. C., & Thayer, C. W. (1996). Sr/Ca and 13 C/12C ratios in skeletal calcite of Mytilus trossulus: covariation with metabolic rate, salinity, and carbon isotopic composition of seawater. Geochimica et Cosmochimica Acta 60, 4207–4221. Klein, R. T., Lohmann, K. C., & Thayer, C. W. (1996). Bivalve skeletons record sea-surface temperature and d18O via Mg/Ca and 18 O/16O ratios. Geology 24, 415–418. Lorens, R. B. (1981). Sr, Cd, Mn and Co distribution coefficients in calcite as a function of calcite precipitation rate. Geochimica et Cosmochimica Acta 45, 553–561. Lorens, R. B., & Bender, M. L. (1980). The impact of solution chemistry on Mytilus edulis calcite and aragonite. Geochimica et Cosmochimica Acta 44, 1265–1278. Moore, W. S. (1997). High fluxes of radium and barium from the mouth of the Ganges–Brahmaputra River during low river discharge suggest a large groundwater source. Earth and Planetary Science Letters 150, 141–150. Mucci, A., & Morse, J. W. (1983). The incorporation of Mg2+ and Sr2+ into calcite overgrowths: influences of growth rate and solution composition. Geochimica et Cosmochimica Acta 47, 217–233. Norconsult, A. S., Consulting Engineers, Architects & Economists (1975). Mombasa water pollution and waste disposal study, VI. Marine investigations (256 pp.). Nairobi. Norman, M. D., Pearson, N. J., Sharma, A., & Griffin, W. L. (1996). Quantitative analysis of trace elements in geological materials by
laser ablation ICP-MS: instrumental operating conditions and calibration values of NIST glasses. Geostandards Newsletters 20, 247–261. Ohde, S., & Kitano, Y. (1984). Coprecipitation of strontium with marine Ca–Mg carbonates. Geochemical Journal 18, 143–146. Osore, M. K. W., Tackx, M. L. M., & Daro, M. H. (1997). The effect of rainfall and tidal rhythm on the community structure and abundance of the zooplankton of Gazi Bay, Kenya. Hydrobiologia 356, 117–126. Outridge, P. M., Doherty, W., & Gregoire, D. C. (1997). Ablative and transport fractionation of trace elements during laser sampling of glass and copper. Spectrochimica Acta Part B 52, 2093–2102. Palacios, R., Orensanz, J. M., & Armstrong, D. A. (1994). Seasonal and life-long variation of Sr/Ca ratio in shells of Mya arenaria from Grays Harbor (Washington)—an ancillary criterion in demographic studies. Estuarine, Coastal and Shelf Science 39, 313–327. Pearce, N. J. G., Perkins, W. T., Westgate, J. A., Gorton, M. P., Jackson, S. E., Neal, C. R., & Chenery, S. P. (1997). A compilation of new and published major and trace element data for NIST SRM 610 and NIST SRM 612 glass reference materials. Geostandards Newsletters 21, 115–144. Price, G. D., & Pearce, N. J. G. (1997). Biomonitoring of pollution by Cerastoderma edule from the British Isles: a laser ablation ICP-MS study. Marine Pollution Bulletin 34, 1025–1031. Raith, A., Perkins, W. T., Pearce, N. J. G., & Jeffries, T. E. (1996). Environmental monitoring on shellfish using UV laser ablation ICP-MS. Fresenius’ Journal of Analytical Chemistry 355, 789–792. Reynolds, R. W., & Smith, T. M. (1994). Improved global sea surface temperature analyses. Journal of Climate 7, 929–948. Stecher, H. A. I., & Kogut, M. B. (1999). Rapid barium removal in the Delaware estuary. Geochimica et Cosmochimica Acta 63, 1003–1012. Stecher, H. A., Krantz, D. E., Lord, C. J., Luther, G. W., & Bock, K. W. (1996). Profiles of strontium and barium in Mercenaria mercenaria and Spisula solidissima shells. Geochimica et Cosmochimica Acta 60, 3445–3456. Stix, J., Gauthier, G., & Ludden, J. N. (1995). A critical look at quantitative laser-ablation ICP-MS analysis of natural and synthetic glasses. The Canadian Mineralogist 33, 435–444. Stroobants, N., Dehairs, F., Goeyens, L., Vanderheijden, N., & Van Grieken, R. (1991). Barite formation in the Southern Ocean water column. Marine Chemistry 35, 411–421. Talk, J., & Polk, P. (1999). The influence of tropical catchments upon the coastal zone: modelling the links between groundwater and mangrove losses in Kenya, India/Bangladesh and Florida. In D. Harper, & T. Brown (Eds.), The sustainable management of tropical catchments (pp. 359–371). Chichester: Wiley. Vander Putten, E., Dehairs, F., Andre´, L., & Baeyens, W. (1999). Quantitative in situ microanalysis of minor and trace elements in biogenic calcite using infrared laser ablation-inductively coupled plasma mass spectrometry: a critical evaluation. Analytica Chimica Acta 378, 261–272. Vander Putten, E., Dehairs, F., Keppens, E., & Baeyens, W. (2000). High resolution distribution of trace elements in the calcite shell layer of modern Mytilus edulis: environmental and biological controls. Geochimica et Cosmochimica Acta 64, 997–1011. Woitchik, A. F., & Dehairs, F. (1993). Dynamics and assessment of Kenyan mangrove ecosystems TS2-0240-C (14 pp.). Brussels: Vrije Universiteit Brussel.
High-resolution trace element profiles in shells of the mangrove bivalve Isognomon ephippium: a record of environmental spatio-temporal variations? C.E. Lazaretha,*, E. Vander Puttena, L. Andre´b, F. Dehairsa a Analytical Chemistry Department, Vrije Universiteit Brussel, B-1040 Brussels, Belgium Section of Petrography–Mineralogy–Geochemistry, Royal Museum for Central Africa, B-3080 Tervuren, Belgium
b
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 determine whether 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 analyses were made to evaluate inter-site differences. While some variability is observed for mean Mg concentrations, the mean Sr concentrations 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 LAICP-MS, displayed a close to regular sinusoidal pattern, depending on specimen and sample site. For the Tudor shells, an arbitrary fitting 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. With a time scale deduced from the Mg–SST relationship, the Ba and Mn maxima of the Tudor shells closely followed periods of maximal rainfall 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 and Mida specimens is thought to result from weaker seasonal variations in nutrient supply and reduced nutrients inputs. This study highlights the potential of I. ephippium as a recorder of spatio-temporal environmental variations in tropical coastal ecosystems. Ó 2003 Elsevier Ltd. 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 of various bivalves represents a record of environmental parameters (e.g. Dodd, 1965; Klein, Lohman, & Thayer, 1996a,b; Lorens & Bender, 1980). These parameters can either have a physico-chemical nature, like temperature and salinity, or a biological nature, like primary pro* Corresponding author. Present address: UR 055 IRD-Paleotropique, 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 Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0272-7714(03)00013-1
ductivity. For instance, variations in sea-surface temperature (SST) can lead to changes in the Mg content of bivalve shells (Dodd, 1965; Fuge, Palmer, Pearce, & Perkins, 1993; Klein et al., 1996a,b; Vander Putten, Dehairs, Keppens, & Baeyens, 2000), whereas phytoplankton blooms can be recorded by the shell as discrete Ba peaks (Stecher, Krantz, Lord, Luther, & Bock, 1996; Vander Putten et al., 2000). Because of their high growth rate, bivalve shells record these environmental changes on a seasonal scale that can be easily resolved using laserablation inductively-coupled-plasma mass spectrometry (LA-ICP-MS) (e.g. Fuge et al., 1993; Price & Pearce, 1997; Raith, Perkins, Pearce, & Jeffries, 1996).
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Tropical coastal ecosystems can undergo fast and important changes tuned by the monsoon regimes. For instance, freshwater (surface- and/or groundwater) inputs 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 cover most of the Kenyan coastline (about 53 000 ha; Doute, Ochanda, & Epp, 1981) and display various patterns of freshwater supply through groundwater and surface water (Talk & Polk, 1999; Woitchik & Dehairs, 1993). In this study, a common bivalve of the Kenyan mangroves, Isognomon ephippium, from three systems with distinct freshwater flow characteristics was analysed. This study focused on the shell chemistry of these bivalves to check their potential in: (1) differentiating between ecosystem environments characterised by different freshwater flow regimes; and (2) recording seasonal changes of the
environment. Therefore, the extent to which Mg, Sr, Ba and Mn variations could be related to ambient physicochemical and biological conditions was investigated.
2. Material and method 2.1. Samples Specimens of Isognomon ephippium, a relatively flat bivalve with a round shape that often lives attached to stilt roots of the mangrove Rhizophora mucronata, were collected 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 of Mombasa; and (3) the Tudor Estuary (T), adjacent to Mombasa city (Fig. 1). These three sites differ in freshwater input regime. In Mida, freshwater input occurs mainly 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.
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Tudor, the freshwater input occurs mainly through surface water flow from the River Kombeni (annual mean of 617 l s1 in 1975; Norconsult, 1975) and, to a lesser extent, from the River Tsalu (annual mean of 185 l s1 in 1975; Norconsult, 1975). Finally, in Gazi, the freshwater input occurs through the Kidogoweni River (mean of 1307 l s1 from January to July 1994; Kitheka, 1996) as well as through groundwater flow. Relative groundwater flow for these systems follows the pattern Gazi > Mida > Tudor, with flows of 0.164, 0.119 and 0.067 m day1, respectively. Maximal discharges (biannual 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 site differentiation, regarding trace element recording by the shells, the whole-shell composition of specimens from the three sites was compared. Subsequently, discrete point analyses was made, using LA-ICP-MS, in order to get temporal information over the entire lifetime of the molluscs. 2.2. Whole shell analyses—inductively coupled plasma-optical emission spectrometry Five specimens from each site (T1 to T5 for Tudor, G1 to G5 for Gazi and M1 to M5 for Mida) were selected for inductively coupled plasma-optical emission spectrometry (ICP-OES) analyses. One valve from each specimen
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was treated overnight in warm (60 C) diluted hydrogen peroxide to remove organic matter. The shells were then dried, weighed and dissolved in 10% SuprapurÒ HCl. Finally, these solutions were evaporated and redissolved into 10% SuprapurÒ HNO3, before analysis with a Thermo Jarrell Ash Corporation IRIS spectrometer (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 specimens from each site (T1 and T3 for Tudor, G4 for Gazi and M2 for Mida), a 5 mm wide slab was cut along the maximum growth axis using a Labcut low-speed diamond saw. The Isognomon ephippium shell is made up of two layers: an external calcite layer and an internal aragonite one. These layers can be distinguished (1) visually (Fig. 2A, B) and (2) from their chemical composition (Fig. 2C). The LA-ICP-MS analyses were made in the middle of the calcite shell layer, from near the umbo towards the edge, approximately every 200 lm (Fig. 2B). This way, successively formed layers were sampled. The chemical profiles were started around 10 mm from umbo, considering the small thickness and the fragility of the most ancient calcite layer (close to the umbo). In addition, crossing analyses, from the external towards 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 30 lm in diameter and 200 lm 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 calcitelayer homogeneity. The LA-ICP-MS equipment was a Fisons-VG Microprobe frequency quadrupled NdYAG (Neodymium doped Yttrium Aluminum Garnet) operating at 266 nm (ultraviolet wavelength), coupled with a Fisons-VG PlasmaQuad IIþMS. The laser was operated in the Q-switched mode with a power of 2 mJ and a repetition rate of 10 Hz. The preablation time was set at 10 s and the acquisition time at 20 s (all the conditions are summarised in Table 1). Under these conditions, the ablation craters that were obtained had a diameter between 30 and 45 lm. The elements analysed were 26Mg, 55Mn, 43Ca, 88Sr and 138Ba. The instrumental instability and drift were corrected using 43 Ca as an internal standard. Analyses were calibrated using the NIST610 glass reference material as an external standard (concentrations taken from Pearce et al., 1997). However, the use of a non-matrix-matched external standard (i.e. a glass standard for carbonate analysis) can lead to incoherent results due to matrix effects (Outridge, Doherty, & Gregoire, 1997; Stix, Gauthier, & Ludden, 1995; Vander Putten, Dehairs, Andre´, & Baeyens, 1999), although the use of an ultraviolet laser seems to reduce such effects (Geertsen et al., 1994; Gu¨nther, Longerich, Forsythe, & Jackson, 1995; Norman, Pearson, Sharma, & Griffin, 1996). As this study focused on trace elements variations through the shell, and on relative shell record differences between sites, the LA-ICP-MS data are presented in a semi-quantitative way as percentage of variation around the mean value (set to zero) of the transect for each profile. 3. Results 3.1. Shell trace element concentrations The Mg shell contents show some variability between sites (Fig. 3A), whereas for Sr, similar results are obtained for the three sites with an overall mean of 771 ppm (Fig. 3B). For Mn and Ba, the shells from the three sites show significant differences (one-way
Table 1 LA-ICP-MS operating conditions Laser probe Laser mode Laser power (mJ) Frequency (Hz) Preablation time (s)
ICP-MS Q-switched 2 10 10
Argon flow rate Carrier gas Auxiliary gas Cooling gas Acquisition mode Point per peak Dwell time (ms) Acquisition time (s)
l min1 1.01 1.28 13.75 Peak jumping 3 10.24 20
ANOVA). The Tudor shells show the highest concentrations for Mn and Ba (41 and 3.6 ppm, respectively) followed by the Gazi shells (11 and 1.2 ppm, respectively), which in turn are slightly richer in Mn and Ba concentrations than the shells from Mida (4 and 1.0 ppm, respectively; Fig. 3C, D). 3.2. LA-ICP-MS profiles 3.2.1. Mg and Sr The Mg profile of the T1 shell displays a sinusoidal variation with broad Mg maxima frequently consisting of two narrower peaks (Fig. 4A). In addition, there is a gradual increase in the Mg level, from 30 to þ10%. Except at the beginning of the profile, Sr shows a sinusoidal pattern similar to that of Mg, although with a slight phase shift towards the edge (Fig. 4A). For T3, Mg also displays a sinusoidal evolution with a clear Mg increase after 20 mm from umbo (Fig. 5A). The Sr pattern in T3 is less clear than that for T1, and does not show any co-evolution with Mg (Fig. 5A). For the two shells, some of the Sr maxima coincide with Ba and Mn maxima (Fig. 6). For G4, a sinusoidal-like pattern for Mg is also observed, although the profile is less regular than for the Tudor shells. Furthermore, while the Mg level in the Tudor shells increased gradually (T1) or more abruptly (T3), Mg in G4 sharply dropped to around 20 mm (Fig. 7A). For Sr, no clear sinusoidal-like pattern can be discerned. Finally, a sinusoidal pattern is also visible for Mg and Sr profiles in the M2 shell (Fig. 8A). The Mg and Sr profiles are similar, without a phase shift and, as for T1, show a slight gradual increase, from 20 to þ18. 3.2.2. Ba and Mn For T1, Ba and Mn are strongly correlated ðr2 ¼ 0:76Þ, and the profiles are characterised by sharp peaks separated by zone of low Ba and Mn (Fig. 4B). In the T3 shell, there is also a correlation between Ba and Mn, although less stronger than for T1 ðr2 ¼ 0:62Þ. Recurrent Ba and Mn peaks appear in the second part of the profile, after 20 mm from umbo (Fig. 5B). For these two Tudor shells, some of the Ba and Sr peaks are in phase (Fig. 6). For G4, Ba shows relatively little variation except at the end of the profile where a sharp peak appears and between 22 and 28 mm from umbo where smaller Ba peaks occur (Fig. 7B). The Mn profile shows even less variation with no noticeable characteristics. Finally, for M2, the Ba profile is characterised by peaks distributed all over the profile, with some of them, at 14, 21, 28 and 31 mm from the umbo, being more salient (Fig. 8B). Mn is not correlated with Ba in this shell and shows a plateau of higher values between 10 and 15 mm 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) 2rx indicated.
4. Discussion 4.1. Mg profiles The Mg LA-ICP-MS profiles in the calcite shell layer of Isognomon ephippium are characterised by a sinusoidal-like pattern, which is less regular in the Gazi and Mida specimens. It has been observed that Mg/Ca in bivalve shell calcite increases with the Mg/Ca ratio of seawater (Lorens & Bender, 1980). However, during mixing of seawater with freshwater, Mg/Ca in solution will only be significantly different from that of the ocean for 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 rainy season (May 1986; Kazungu et al., 1989). During most of the rest of the year, Tudor creek behaves simply as a fjord-like extension of the Indian Ocean (Kazungu et al., 1989) and has a salinity value similar to that observed in coastal waters. Thus, salinity variations probably did not have a significant influence on Tudor shell Mg variations. For Gazi, salinity at stations encompassing the sampling site of G4 were reported to range from 5 to 37 (k3, Kitheka, 1996; s3, Osore et al., 1997), with the minimum value in May (1994) when rainfalls are highest. However, a large gradient exists in salinity values between the point where river Kidogoweni enters the lagoon and our G4 sampling site was observed. Indeed, salinity minima measured by Kitheka (1996) for stations upstream from our sampling site were 2 and 5 for k2 and k3, respectively, whereas the salinity minimum at s3, slightly downstream of our sampling
site, was 20 (Osore et al., 1997). Therefore, it is believed that G4 Isognomon specimen is likely to have experienced a minimum salinity environment between 10 and 15. Thus, it is unlikely that the Mg/Ca ratio in the water ambient to the site of G4 sampling underwent a significant decline linked to salinity decrease. Finally, at Mida, mean salinity recorded between May 1996 and April 1997 was >30 (Kitheka, 1998), excluding a possible influence of dilution on the Mg/Ca ratio of the aquatic system and, consequently, on the shell Mg/Ca ratio. Another factor influencing the incorporation of Mg in bivalve shells is the seawater temperature. Indeed, it has been demonstrated that Mg variations in the shell of various types of bivalves show a positive correlation with seawater temperature (Dodd, 1965; Fuge et al., 1993; Klein et al., 1996a,b). Consequently, the more regular Mg profiles of the Tudor shells (i.e. the full T1 profile and the second half of the T3 profile, between 17 and 37 mm from umbo) were investigated more closely to deduce a time-dependent Mg distribution for these shells and to get a date for each laser crater, which can then be used for the other elements. Therefore, a fitting was performed by arbitrarily forcing the Mg minima to coincide with the SST minima (data from Reynolds & Smith, 1994), taking the date of collection as the reference time point (note that there is some uncertainty on this date since the LA-ICP-MS analysis had to be stopped before reaching the edge of the shell, because of poor ablation). The distance (in mm) between each Mg minima corresponds to 1 year of shell accretion, thus giving a year-averaged shell growth rate that was used to calculate 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 very good (Fig. 9) and can be regarded as reflecting a major control of SST on Mg incorporation. Moreover, the different year-averaged shell growth rates determined are decreasing with age, as expected, from 12.8 to 3.0 mm year1 for T1 and from 8.6 to 2.5 mm year1 for T3 (Fig. 9). Consequently, the protocol used to get temporally fitted profiles can be considered as a correct first approach. Nevertheless, considering part of the Mg variations (sudden or gradually increases), supplementary factor(s) might have an influence on Mg shell incorporation. Physiological factors could be part of the Mg increase around 20 mm of T3 (Fig. 5A), as well as of the less clear and not continuous sinusoidal Mg patterns of G4 and M2 (Figs. 7A and 8A). Finally, the overall gradual increase in the Mg content of the T1 and M2 shells might be related to the ageing of the specimen, as it 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 of Mg. Many authors have studied the incorporation of Sr into bivalve shells and the multiple results reveal the complexity of Sr-incorporation processes. Indeed, some authors found a relationship between shell Sr/Ca ratios and temperature (Dodd, 1965) or Sr/Ca ratios in solution (Lorens & Bender, 1980), whereas others explained shell Sr/Ca variations by kinetic factors like precipitation rate (Carpenter & Lohmann, 1992; Lorens, 1981), growth rate (Stecher et al., 1996) or mantle metabolic activity (Klein et al., 1996a). Moreover, a positive linear correlation between Mg and Sr contents has been observed both in artificial (Mucci & Morse, 1983) and natural calcite (Carpenter & Lohmann, 1992; Ohde & Kitano, 1984). This relationship is usually attributed to lattice distortion, related to the Mg2þ–Ca2þ substitution, that allows 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 these various invoked processes, it appears that the Sr incorporation is probably governed not by one process but rather by multiple factors. In the present study, the similarities between Mg and Sr in most shell profiles indicate that Sr incorporation might be Mg-dependent and/or, to a certain extent, temperature-dependent (Fig. 9). Nevertheless, the Sr profiles in T1 and in the second part of T3 are not perfectly in phase with Mg, and the G4 profile pattern even shows no sinusoidal evolution at all, nor a relationship with Mg. Consequently, additional factors affect the incorporation of Sr. As Lorens and Bender (1980) have shown, shell Sr/Ca ratios are influenced by the solution Sr/Ca ratio. However, Dodd and Crisp (1982) demonstrated that the Sr/Ca ratio of estuarine waters is only significantly different from that of the open ocean below a salinity value of 10. Such low salinity is generally not observed at the sites studied. Thus, it
seems unlikely that variations in the seawater Sr/Ca ratios are large enough to have an impact on the shell Sr profiles observed in the present study. For the Tudor shells, some of the Sr maxima coincide with Ba peaks (Fig. 6). This might indicate that the factor(s) explaining the 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 the Tudor Isognomon ephippium are characterised by several narrow maxima. These are regularly distributed along the profile of T1, but are only present in the second part of the T3 profile. For Gazi and Mida, the patterns differ from that of Tudor, showing less obvious high narrow peaks, and exhibiting less similarity between Ba and Mn. The presence of Ba peaks has already been reported for 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 (Vander Putten et al., 2000), in the latter case, together with Mn peaks. In both cases, the Ba (Mn) peaks were related to periods of high phytoplankton productivity. Indeed, it has been shown that Ba is incorporated and/or adsorbed onto phytoplankton, and may precipitate as barite in organic-rich microenvironments, mostly constituted of diatoms (Bishop, 1988; Dehairs et al., 1990; Stroobants, Dehairs, Goeyens, Vanderheijden, & Van Grieken, 1991). In coastal tropical systems, similar to the three studied Kenyan sites, the phytoplankton blooms are initiated essentially through an increase in nutrient inputs, linked with the monsoon regime (shown for Tudor by Kazungu et al., 1989). In addition to nutrients, freshwater is also enriched in Ba relative to the openocean water (Broecker & Peng, 1982). As a result, freshwater inputs into tropical coastal systems not only cause phytoplankton blooms through increased nutrient supplies, but also provide a significant input of Ba that can 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 with precipitation data, to which river outflows are, most probably, directly related. For this, the Ba profiles calibrated against time using the Mg–SST fitting were used. Apparently, the sharp Ba (and Mn) maxima generally occur at the end of the rainy period associated with the southeast monsoon (Fig. 9B). Thus, a delay of a few weeks seems to occur between the increased runoff of freshwater carrying high contents of nutrients and dissolved Ba and the appearance of high Ba (Mn) concentrations in the shell. This can be explained by the fact that phytoplankton blooms in Kenyan coastal lagoons generally occur after the surge of rainfall, when turbidity has decreased again but nutrient contents are still favourable (M.H. Daro, personal communication; Brunet et al., 1996). This also agrees with the hypothesis that Ba is probably deposited in the shell as a result of enhanced 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 dissolved Ba. For Tudor shells, the Ba–Mn co-variation indicates that: (1) freshwater inputs also bring a large amount of Mn to the environment; and (2) Mn incorporation occurs essentially through ingestion of enriched particulate matter such as phytoplankton. A similar situation has been observed for the blue mussel M. edulis in the temperate Scheldt Estuary (Vander Putten et al., 2000). The lack of a direct correlation between the magnitude of precipitation (and thus river run-off) and the amount of incorporated Ba (Mn) suggests, however, that the process is complex. The particularly well-expressed high narrow Ba and Mn peaks in the Tudor shells reflect the relatively important monsoon freshwater input to Tudor, resulting in a considerable increase in nutrient content in the Tudor creek (Kazungu et al., 1989). This nutrient increase is expected 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 the Gazi and Mida ones. These differences probably result from the differences in hydrodynamic regime between the three sites. The freshwater nutrient concentrations are significantly different between Tudor and Gazi. Indeed, at Tudor, the nitrate concentrations range between 0.28 and 17.8 lmoll during the rainy season (Kazungu et al., 1989), whereas at Gazi, the nitrate plus nitrite content ranges between 0.2 and maximum 4 lmoll during the rainy season (Kazungu, personal communication). Consequently, it can be expected that as a result of this poor nutrient availability, the phytoplankton blooms at Gazi are less important than at Tudor, resulting in less organic-rich microenvironments where Ba (Mn?) can precipitate before being incorporated by Isognomon ephippium. This also supports the lowest Ba and Mn contents observed in whole shells (Fig. 3). Also, in Mida, the freshwater input only occurs through 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 constant over time and show less seasonal variability, thus explaining the ÔflatterÕ Ba/Mn patterns in the Mida shell. The low Ba and Mn levels observed for the Mida shells are also probably directly linked to the absence of riverine freshwater input. 5. Conclusions The whole-shell Ba and Mn contents of the Isognomon ephippium, which were studied are directly related to the hydrodynamic properties and nutrient inputs of the sites. The good fitting between Mg in Tudor shells and SST is in accordance with a predominant control of SST on Mg incorporation. Although Sr profiles are often close to the Mg ones, some similarities with the Ba profiles underline the complexity of Sr incorporation into the shell. The
peaked profiles of Ba reflect the phytoplankton bloom successions as governed by monsoon regime. Also, the similarity between Ba and Mn confirms that incorporation of Mn is for a large part also governed by productivity. This study highlights the potential of Isognomon ephippium as environmental recorder. However, it appears that further investigations, on a larger specimen population associated with a site survey, are necessary to improve interpretation of data and to extend to fossil equivalents (Hendry, Perkins, & Bane, 2001) that might be the witnesses of ancient hydrodynamic/productivity characteristics.
Acknowledgements This work was supported by a Post Doctoral Grant to 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, contract FMBICT983440 and by EC-INCO research project IC18-CT96-0065 and FWO Flanders contract. We thank J. Cillis for assistance with the scanning electron microprobe and Th. Hubin for photographs of specimen. We thank Ph. Willenz for the bivalve species identification and for his help with the specimen preparation for LA-ICP-MS. We are also grateful to him for his helpful advice on the manuscript.
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