LA-ICP-MS analysis of trace elements in biogenic minerals ...Trace element profiles The black smoker...

LA-ICP-MS analysis of trace elements in biogenic minerals: time series recording of environmental changes

Background and objectives of the project

Detailed description of results

Methodology

Sample preparationLA-ICP-MS analysis

The long-lived sclerosponge

Pb and δ13C resultsOther results

The mangrove bivalve Isognomon ephippium

Whole shellTrace element profiles

The black smoker mussel Bathymodiolus sp.

Mean trace element contentsTrace element profilesStable isotopes

Background and objectives of the project

The objectives of the project were to improve our understanding of trace element behaviour in biogenic minerals via Laser Ablation Inductively Coupled Plasma Mass Spectrometer (LA-ICP-MS) analysis. Indeed, it is well known that biogenic carbonates continuously record environmental conditions such as temperature, salinity and also pollutant level and thus represent potential archives of environmental changes that may have occurred during their lifetime. Focus was on carbonate secreting sponges from tropical seas and bivalves from coastal and deep-sea hydrothermal environments. The Analytical Chemistry department (ANCH) at Vrije Universiteit Brussel (VUB) and


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the Geochemistry department of "Musée Royal d'Afrique Centrale" (MRAC, Tervuren, Belgium) had already optimised a high-resolution methodology, based on discrete point analysis, to study the variations of trace elements like e.g. Ba, Sr, Mg, Pb, U at very low concentrations (down to sub ppm levels). The aim of the project was to further develop this method and to check the potential of new types of biocarbonates from different marine environments with focus on short- (bivalves) and long-time (sclerosponge) environmental changes. Detailed description of results

Methodology

All trace element analyses were done by using a Fisons-VG Microprobe frequency quadrupled Nd-YAG (Neodymium doped Yttrium Aluminum Garnet) operating at 266 nm (ultraviolet wavelength), coupled with a Fisons-VG PlasmaQuad II+ mass spectrometer. 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 range was between 10 and 20 s depending on the substrate whereas, the acquisition times were all set at 20 s.

Sample preparation

The specimens were cut with a Labcut low speed diamond saw to obtain a 5-mm-thick slab perpendicular to the surface for the sclerosponge and a 5-mm-thick slab along the maximum growth axis for the molluscs (Fig. 1). The bivalves LA-ICP-MS analyses were made in the middle of the calcite layer and from the umbo towards the edge, approximately every 200 µm (Fig. 1). For sclerosponges, further details on methodology are given below, in B.

Figure 1: Methodology (A) Isognomon ephippium specimen sawed for LA-ICP-MS analyses and (B) scanning electron microscope picture of part of a slab analysed by LA-ICP-MS. Craters are about 30 µm in diameter and about 200 µm spaced.B

A



LA-ICP-MS analysis

The most relevant elements analysed were 26Mg, 55Mn, 43Ca, 88Sr and 138Ba. The instrumental instability and drift were corrected using 43Ca as an internal standard. For quantification, the LA-ICP-MS data obtained on the samples must be compared with an external standard of known composition. For the sclerosponge, the LA-ICP-MS data were compared to a "sclerosponge standard" cut out from the ancient part of the specimen with elemental composition established by ICP-MS measurements on eight acid dissolved micro-drilled samples. Accuracy of the LA-ICP-MS sclerosponge analyses was controlled by measuring trace element distribution in the aragonitic skeleton by inductively-coupled-plasma mass spectrometry (ICP-MS). Two series of 42 and 44 evenly spaced samples (approximately every 1.5 mm) were collected next to the LA-ICP-MS profile with a 1-mm-diameter drill parallel to the major growth direction of the sponge. Both analytical techniques gave very similar results confirming the validity of the methodology applied. For the bivalves, analyses were calibrated using the NIST610 glass reference material as an external standard. 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 (Stix et al., 1995; Outridge et al., 1997; Vander Putten et al., 1999). In order to check whether the non-matrix matched standardisation produced accurate results, the concentrations determined with LA-ICP-MS were compared with ICP-OES and ICP-MS analyses of five successive samples taken from the calcite layer of each shell. These samples were prepared by carefully grinding the calcite surface layer of the shell part adjacent to the slab used for LA-ICP-MS with a 2 mm spherical drill (details in annex 4). The liquid results were compared with the average values of the corresponding series of laser ablation samples. Significant differences were observed between



LA-ICP-MS and ICP-OES/ICP-MS analysis on dissolved samples reflecting the difficulty of proper calibration in LA-ICP-MS in the absence of matrix matched standards. Consequently, mangrove bivalve LA-ICP-MS data are presented as a percentage of variation around the mean value (set to zero) of the transect for each profile. The black smoker data are however given in "ppm", what is valid for site by site comparison, but we must remember that they might not represent the exact trace element content.

The long-lived sclerosponge Sclerosponges are sponges that secrete an aragonitic skeleton in which information on environmental conditions such as e.g. ambient water temperatures can be recorded. These sclerosponges have a very low growth rate, ranging from 100 to 300 µm/yr depending on the species, implying that even relatively small specimens can be several centuries old. We studied the Ceratoporella nicholsoni species that secretes a massive skeleton allowing high-resolution analysis like LA-ICP-MS. The specimen studied (Fig. 2a) originates from the Bahamas (sampled 1985) and was obtained via the courtesy of Ph. Willenz (Royal Institute of Natural Sciences, Brussels). We assigned it a maximum age of 390 yr. based on a mean annual growth rate of 230 ± 45 µm/yr. obtained from in situ labelling of 10 specimens with calcein over a 10 yr. interval in Jamaica (Willenz and Hartman, 1999). The LA-ICP-MS ablation craters (Fig. 2b-c) were 30 µm or 50 µm in diameter, depending of instrumental settings, which means that, considering a mean annual growth rate of 230 µm/yr., the largest craters correspond to 3 months of skeletal accretion, whereas the smallest ones correspond to 1.5 months. The longest profile led to a time series extending from 1760 to 1983 with a measurement for every 1.6 yr. of growth. High resolution shorter profiles (one point analysis every 2.6 month), adjacent to the low resolution transect, confirmed the good reproducibility of the analysis whereas liquid analyses (dental-drill removal of CaCO3 powders analysed by ICP-MS) allowed an external control on elemental concentrations obtained by LA-ICP-MS. The micro-drilled CaCO3 samples for carbon isotope ratios determination were collected from a second slab of the C. nicholsoni studied, cut next to the one used for trace element determination.



C

A

B

Figure 2: Ceratoporella nicholsoni. (a) Section perpendicular to sample surface. (b) SEM view of framed area from a showing alignment of LA-ICP-MS craters (small dots running through rectangle) in most recent part of skeleton, below pseudocalicles (PC). (c) SEM view of framed area from B showing craters left by laser ablation.

Pb and δ13

C resultsIt appears that sclerosponges record major global environmental changes in their skeleton, like

atmospheric increase of lead and δ13

C decrease (Fig. 3). This result yielded to a meeting presentation (Lazareth et al., 1999, cf. annexe 1) and a publication (Lazareth et al. 2000, cf. annexe 2).

A

B

Figure 3: Ceratoporella nicholsoni record of global environmental changes.

(A) C. nicholsoni δ13

C record compared with CO2 content in ice-core air bubbles.

Thick continuous line—C. nicholsoni three-point running-average δ13

C profile in δ notation relative to Vienna PDB standard; open diamonds—CO2 records from Antarctic ice cores (data from Etheridge et al., 1998, Neftel et al., 1985, and Friedli et al., 1986).(B) Evolution of Pb concentration in C. nicholsoni from 1750 to 1990 compared



with other Pb records. Solid line - C. nicholsoni three-point running-average LA-ICP-MS profile; open circles - Greenland snow Pb concentration, data from Murozumi et al. (1969); open triangle - Florida Keys Montastrea annularis (scleractinian coral) Pb/Ca ratio, data from Shen and Boyle (1987). Bar - analytical reproducibility of LA-ICP-MS analyses, ±2σ .

The δ13

C decrease observed over the past two centuries (analytical details in annex 2) is related to the increase of fossil-fuel burning and deforestation since pre-industrial time. Indeed, the resulting dramatic increase of atmospheric CO2 has gone with a 13C depletion (cf. review in Trabalka and Reichle [1986] and in Siegenthaler and Sarmiento [1993]). The Ceratoporella nicholsoni Pb profile is very similar to the one observed for Greenland snows (Murozumi et al., 1969) and for a Florida Keys coral (Shen and Boyle, 1987) (Fig. 3). In particular, it records the threefold input of anthropogenic Pb in the atmosphere after 1930. This observation has three major implications. First, it confirms the applied growth-rate value, at least for long-term measurements. Second, the similarity between our Pb profile and the one observed in Greenland snows shows that most of the Pb found in surface waters around the Bahama Islands has been derived from the atmosphere. Third, the coincidence between C. nicholsoni and the coral record from Shen and Boyle (1987) suggests that Pb could be incorporated in the carbonate skeleton of these two reef-building animals, belonging to distinct phyla, following a similar biological process.

Other resultsSome other results are promising but need supplementary work to be conclusive. Among them is the similarity between most of the trace element profiles obtained (Fig. 4) that might indicate similar process of incorporation into the sclerosponge skeleton.

Figure 4: Similarity between Sr, Mg and Ba profiles in Ceratoporella nicholsoni. Data are 3 point running averages.

Moreover, some cyclic evolution arises from those profiles (Fig. 5). The assumed periodicity have different frequencies, the longest ones being around 40-45 yr., followed by some around 25 yr, the shortest being around 10 yr.. At this stage, we don't have enough data to be conclusive about these cycles, which can not be interpreted for the moment.

A



B

Figure 5: Possible cycles of (A) Ba and (B) Sr profiles of the Ceratoporella nicholsoni studied. Data are 3 point running averages

Finally, as it is well known that Mg incorporation in carbonate is mostly temperature related, we compared our longest Mg profile with a sea-surface temperature (SST) curve from Mann et al. (1998, obtained from direct and indirect records (Fig. 6).

Figure 6: Mg vs SST anomaly of the Ceratoporella nicholsoni studied. SST data from Mann et al. (1998). Black arrows - direct correlation between Mg and SST; arc - Mg and SST shifted peaks; broken line and white arrow - break of relation between Mg and SST.

The comparison between Mg and SST revealed 3 features which are (i) some quite direct correlation (black arrows on Fig. 6) (ii) some time-shifted peaks (arc on Fig. 5) which might result from the use of a constant growth rate rather than a variable one and (iii) break of relation after 1920 which might be related to the fact that SST might not be the sole controlling factor for Mg incorporation.

We demonstrated the efficiency of LA-ICP-MS to obtain trace element profiles in sclerosponges. This research confirms that sclerosponges with a massive aragonitic skeleton represent a new promising archive for paleoclimate proxies. These may be studied in the future to complement data obtained from scleractinian corals with the advantage of accesses longer time scales. However, it is clear that further investigation is necessary to fully understand the potential of sclerosponges for reconstructing marine environmental changes. The mangrove bivalve Isognomon ephippium The work done on these mangrove bivalves has been presented during an international meeting (Lazareth et al. 2000) and is now published (Lazareth et al., 2003).


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It has been demonstrated that the shell chemistry of various bivalves represents a record of environmental parameters (e.g., Dodd, 1965; Lorens and Bender, 1980; Klein et al., 1996a,b). For instance, variations in sea-surface temperature (SST) can lead to changes in the Mg content of bivalve shells (Dodd, 1965; Fuge et al., 1993; Klein et al., 1996a,b; Vander Putten et al., 2000), whereas phytoplankton blooms and freshwater discharge can be recorded by the shell as discrete Ba peaks (Stecher et al., 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 laser-ablation inductively-coupled-plasma mass spectrometry (LA-ICP-MS) (e.g., Fuge et al., 1993; Raith et al., 1996; Price and Pearce, 1997). Here, we analysed Isognomon ephippium (Fig. 7), a common bivalve of the Kenyan mangroves, from three systems with distinct freshwater flow characteristics (Table 1): (i) the Tudor Estuary (T), adjacent to Mombasa city, (ii) Gazi (G) and (iii) Mida (M), two mangrove ecosystems, respectively 50 km south and about 100 km north-east of Mombasa, (Fig. 7).

Freshwater inputs Surface water flow Groundwater flow

Tudor River KombeniRiver Tsalu+++ /

Gazi River Kidogoweni ++ +Mida / ++

Table 1: Freshwater inputs characteristics of the three Kenyan sampling sites. ++ - major contribution; + normal contribution; / - no contribution.

A

B

5 cm

Figure 7: Sampling site localisation (A) and Isognomon ephippium specimen (B). In order to obtain a preliminary indication on site differentiation, regarding trace element recording by the shells, we decided to compare the whole shell composition of specimens from the three sites. Subsequently, we made discrete point analyses via laser ablation inductively coupled plasma spectrometry (LA-ICP-MS) to get temporal information over the lifetime of the



molluscs.

Whole shellOne shell of five specimens from each site was analysed by ICP-OES, after removing of organic matter (through H2O2 treatment) and dissolution (HNO3), for Mg, Mn, Sr and Ba (details in annex 4). The Mg shell contents show some variability between sites, whereas for Sr, the three sites give similar results with an overall mean of 771 ppm (Fig. 8a-b). For Mn and Ba on the other hand, the shells from the three sites show significant differences (one-way ANOVA). The Tudor shells show the highest concentrations for Mn and Ba followed by the Gazi shells which in turn are slightly richer in Mn and Ba than the shells from Mida (Fig. 8c-d). These lowest Ba and Mn content of the shells from Gazi and Mida could result from a poor nutrient availability of these sites.

Figure 8: Mg, Sr, Mn and Ba contents in the whole valve of Isognomon ephippium from Tudor, Gazi and Mida. Mean (ppm) ± 2σ indicated.

Trace element profilesThe LA-ICP-MS analysis were made in the remaining valve of one to two specimens from each site (T1 and T3 for Tudor, G4 for Gazi and M2 for Mida). Only most relevant data are presented here, but further details are given in annex 3-4.

ƒ Mg profiles: The Mg LA-ICP-MS profiles in the calcite shell layer of Isognomon ephippium are characterised by a sinusoidal-like pattern (Fig. 9).



Figure 9: Mg and Sr profile of T1 and T3 (Tudor, Kenya), relative to the mean of all craters. Arrows indicate coinciding Sr and Ba peaks (Fig. 11).

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, we decided to investigate more closely the Tudor Mg profiles (the full T1 profile and the second half of the T3 profile), to deduce a time-dependent Mg distribution for these shells. Therefore, a fitting was performed by arbitrarily forcing the Mg minima to coincide

with the sea-surface temperature minima[1]

(SST), taking the date of collection as the reference time point. The fitting obtained between the Mg- and SST-profiles is very good (Fig. 10), pointing towards a dominant control of SST on Mg incorporation.

Figure 10: Tudor shells Mg fitting with monthly averaged sea-surface temperature (SST data from the internet site "http://ingrid.ldgo.columbia.edu/SOURCES/IGOSS/.nmc/.monthly/.sst/"; Reynolds and Smith, 1994). Red line: Mg profile of T1; Green line: Mg profile of T3; Blue line: SST.

ƒ Sr profiles: The Sr patterns are more complicated than those of Mg. 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. For the Tudor (and Mida shells), some of the Sr maxima coincide with Ba peaks (arrows in Fig. 9-11). This might indicate that the factor(s) explaining the Ba peaks also have an influence, direct or indirect, on the incorporation of Sr. ƒ Ba and Mn profiles: The Ba and Mn patterns in the calcite layer of the Tudor Isognomon ephippium are characterised by several narrow maxima (Fig. 11). These are regularly distributed


http://ingrid.ldgo.columbia.edu/SOURCES/.IGOSS/.nmc/.monthly/.sst/


along the profile of T1 but are only present in the second part of the T3 profile. Figure 11: Ba and Mn profile of T1 and T3 (Tudor, Kenya), relative to the mean of all craters. Arrows indicate coinciding Sr (Fig. 9) and Ba peaks. The presence of Ba peaks has already been reported for Mercenaria mercenaria and Spisula solidissima (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. In coastal tropical systems, like the Kenyan sites studied, the phytoplankton blooms are initiated essentially through an increase in nutrient inputs, linked with the monsoon regime. In addition to nutrients, freshwater is also enriched in barium relative to the open-ocean water (Broecker and 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 barium that can be incorporated by phytoplankton, which can subsequently be ingested by filtering bivalves. The Tudor Ba profiles were thus compared with

precipitation data[2]

, to which river outflows are, most probably, directly related. We used the Ba profiles calibrated against time via the Mg-SST fitting. Apparently, the sharp Ba (and Mn) maxima generally occur at the end of the rainy period associated with the Southeast monsoon (Fig. 12). Thus, a delay of a few weeks seems to occur between the increased run-off 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). This also fits with the hypothesis that Ba is probably deposited in the shell as a result of enhanced ingestion of Ba/barite associated with the filtered particles and not via direct uptake of dissolved Ba. 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 via ingestion of enriched particulate



matter such as phytoplankton. The lack of a direct correlation between the magnitude of precipitation (and thus river runoff) and the amount of incorporated Ba (Mn) suggests, however, that the process is complex.

Figure 12: Ba and precipitation in the Tudor shells (precipitation data from the internet site "http://ingrid.ldgo.columbia.edu/SOURCES/.NOAA/.NCEP/.CPC/.CAMS_OPI/.mean/.prcp/"). Red line: Mg profile of T1; Green line: Mg profile of T3; Blue line: mean precipitation.

The whole shell Ba and Mn contents of the Isognomon ephippium studied are directly correlated to the hydrodynamic properties and nutrient inputs of the sites. The strong correlation between Mg in Tudor shells and sea-surface temperatures confirms the dominant control of SST on Mg incorporation. Although Sr profiles are often close to the Mg ones, some similarities with the Ba profiles underlines the complexity of Sr incorporation into the shell. The peaked profiles of Ba reflect the phytoplankton bloom successions as governed by monsoon regime. 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 population associated with a site survey, are necessary to improve interpretation of data and to extend to fossil equivalents which might thus be witnesses of ancient hydrodynamic/productivity characteristics. The black smoker mussel Bathymodiolus sp.

In the following we present preliminary results on the shell chemistry of Bathymodiolus sp. (black smoker mussel), the third biocarbonate investigated. Bathymodiolus sp. specimens were collected during the AMORES programme (MAST-3, CE), in 1997, in three hydrothermal vent fields, Menez Gwen (MG), Lucky Strike (LS) and Rainbow (R) (Table 2, Fig. 13), from the Mid-Atlantic Ridge (MAR). These mussels represent potential records of the hydrothermal activity, through trace element variations in their shell.

Localisation Depth Bathymodiolus SamplesMenez Gwen (MG) N37°50 800 MG2

Lucky Strike (LS) N37°17 1700LS-003 (Eiffel Tower)LS1-M5 (Bairro Alto)LS1-M9 (Bairro Alto)


http://ingrid.ldgo.columbia.edu/SOURCES/.NOAA/.NCEP/.CPC/.CAMS_OPI/http://ingrid.ldgo.columbia.edu/SOURCES/.NOAA/.NCEP/.CPC/.CAMS_OPI/


Rainbow (R) N36°15 2400 R9M7

Table 2: Bathymodiolus sp. sampling site characteristics. Bairro Alto and Eiffel Tower are two sites from the Lucky Strike vent field.

A

B

5 cm

Figure 13: Localisation of sample sites on the Mid-Atlantic Ridge (A) and Bathymodiolus sp. specimen prepared for LA-ICP-MS analyses (B).

Mean trace element contentsSince most shells were covered with a Fe-Mn oxy-hydroxide crust, we first checked whether or not the calcite layer was contaminated from this crust. Therefore, we realised transversal profiles in the shell, from the exterior to the interior (Fig. 14a). The results clearly demonstrate that no contamination arise as the trace element contents in the calcite layer are almost constant, but also that little chemical differences exist between the two layers of the shell (Fig. 14b). Thus, the trace element profiles in the calcite layer of the Bathymodiolus sp. studied represent a record of environmental changes (± physiological effects) during the life-time of the mussels.

A

B

Figure 14: Assessment of the potential external pollution of the Bathymodiolus sp. shells. (A) LA-ICP-MS transect from exterior to interior, (B) Mn, Mg and Cr behaviour.

The mean content of some trace elements in the calcite layer (mean of all LA-ICP-MS analyses, shell by shell) reflects the fluid chemistry of the sites. Indeed, the Cu and Co contents of the Bathymodiolus sp. shells increase in the order MG, LS and R, as observed for the Cu and Co fluid



contents (VonDamme, 1990; Douville et al., submitted). The same trend occurred for the shell Ce content, thus suggesting an increase in the Ce hydrothermal fluid content from MG to R (no fluid data) (Fig. 15).

Figure 15: Mean content of Cu, Co and Ce (ppm, mean of all crater) of the Bathymodiolus sp. studied. Yellow: MG2; dark and light blue: LS samples; green: R9M7.

For other elements, such as Ba and Mn for which the relative fluid contents are in the order R>LS>MG (Von Damme, 1990; Douville et al., submitted), the direct relation between shell and fluid composition is only apparent for the MG and LS specimens (Fig. 16).

Figure 16: Mean content of Ba and Ce (ppm, mean of all crater) of the Bathymodiolus sp. studied. Yellow: MG2; dark and light blue: LS samples; green: R9M7.

However, although the overall trace element contents of the calcite shell layer reflect the distinct fluid chemistry of the sites, the quantitative aspects are not respected. As an example, the Cu fluid content at Rainbow is globally five times that of Lucky Strike and 100 times that of Menez Gwen whereas the Rainbow Bathymodiolus sp. shell contains only one or two times more Cu than the shells from the other sites. This might indicate the effectiveness of the detoxification processes of these hydrothermal bivalves (Rousse et al., 1999).

Trace element profiles



The trace element contents along the shells show variations (Fig. 17-18) with different patterns depending on the site. This might be related to changes in the intensity of the hydrothermal activity and/or in the fluid chemistry but also to mussel movements.

Figure 17: Bathymodiolus sp. Mn (ppm) profiles.

Figure 18: Bathymodiolus sp. Pb (ppm) profiles. For the specimens from Menez Gwen and Rainbow, Ba and Ce profiles are very similar (Fig. 19), which might indicate a same hydrothermal control on these two elements at these two sites.

Menez Gwen Rainbow



Figure 19: Ba and Ce profiles in Bathymodiolus sp. from MG and R. Moreover, for the Rainbow Bathymodiolus sp. shell, most of the trace elements follow the same trend, with a significant peak between 30 and 35 mm from umbo, even if some of them (e.g. U and Mo) are close to zero in other parts of the shell (Fig. 20). This might also be interpreted as reflecting chemical changes in the surrounding mixed seawater / hydrothermal fluid environment.

Figure 20: Similarities in the B, Cu, Mo and U profiles of the Bathymodiolus sp. shell from the Rainbow site.

Finally, the Sr evolution is close to a cyclic one for all the Bathymodiolus sp. shells (Fig. 21a) and the Sr cycles from two different sites (Eiffel Tower and Bairro Alto) within the Lucky Strike system seem to be quite concordant (Fig. 21b). Finally, each cycle represents, at a first approximation, between 1.5 and 2.7 cm of shell accretion, depending of specimen and site, what is comparable to the estimated mean growth rate of such bivalves (~ 2 cm; Comtet, 1998). Thus, the Sr signature seems to be governed by an annual environmental signal.

B

A

Figure 21: Cyclic evolution of Sr for (A) Bathymodiolus sp. from each sampling site with supposed periodicity (arrows) and for (B) Bathymodiolus sp. from the Lucky Strike field with concordant points (green linking lines). In order to evaluate which environmental parameter could be responsible of these cycles, stable

isotope ratios, δ18

O and δ13

C, were determined in one shell.

Stable isotopesThe stable isotope ratios were determined in the Bathymodiolus sp. LS1-M5 (Bairro Alto, Lucky Strike) which presented two quite similar Sr cycles with a nice amplitude (Fig. 21b). Analyses were done at the Department of Earth Sciences at the Vrije Universiteit Amsterdam under the guidance of H.B. Vonhof. Very small amounts of calcite powder (approximately 20 - 50 µg each, representing 4 laser ablation craters on average) were obtained by using the Merchantek

microdrill system (Fig. 22). The δ18

O and δ13

C ratios of the microdrilled CaCO3 samples were determined on a Finnigan 252 mass spectrometer, equipped with an automated carbonate extraction line (called Kiel device). Samples were digested in concentrated orthophosphoric acid at a temperature of 80°C. The data are reported vs VPDB (Vienna Pee Dee Belemnite) with a

reproducibility (1 sd), determined by replicate analysis of NBS-19, within 0.09 ‰ for δ18

O and



0.06 ‰ for δ13

C.

Figure 22: Microdrilling system. (A) Beginning and (B) ending of the sampling operation.

The δ13

C ratios range between +3.46 and +4.14 ‰ and the δ18

O between +2.18 and +3.30 ‰.

Signals are mostly anticorrelated (Fig. 23). It appears that the Sr profile is close to both the δ13

C

profile (correlated) and the δ18

O one (anticorrelated) (Fig. 24).

δ13C

δ18O

Figure 23: Stable isotope ratios, δ18O and δ13C, in the calcite layer of the shell of a Bathymodiolus sp. (LS1M5) from the Bairro Alto site of the Lucky Strike field (MAR). The bars represent the extension of each microsampling.



A

δ18

O

B

δ13

C

Sr

Sr

Figure 24: Sr profile of LS1M5 compared with (A) δ18O (inverse scale) and (B) δ13C.

Mollusc shells are generally precipitated in 18O/16O isotopic equilibrium with the external medium. Moreover, the oxygen isotope fractionation between shell calcite and the surrounding water is temperature related as described by Epstein et al. (1953).

T = 16.5-4.3(δ18

Oc-δ18

Ow )+0.14(δ18

Oc-δ18

Ow)2²

with c: carbonate; w: water and δ18

O the oxygen ratio of the sample compared to an appropriate

isotopic reference standard ((δ18

O = 1000Rs-Rstd)/Rstd where Rs and Rstd are the 18O/16O-ratios

of the sample and the standard). δ18

Ow is generally given relative to the SMOW standard

whereas the δ18

Oc are given relative to PDB standard. To convert δ18

Ow values from SMOW to



PDB scale, 0.26 ‰ are subtracted from all δ18

Ow values (Coplen et al., 1983). To calculate the possible maximum range of temperature witnessed by the Bairro Alto Bathymodiolus sp., we

used as δ18

Ow (i) the Mean Ocean Water oxygen isotope ratio value (δ18

Ow = 0 ‰), as if the

mussel lived in a pure deep-seawater environment and (ii) the mean δ18

Ow of Lucky Strike fluid

endmember (δ18

Ow = 1.17 ‰; Jean-Baptiste et al., 1997), as if the mussel lived in a pure hydrothermal fluid environment (without any mixing with seawater). The maximum range obtained (Table 3) include mean discrete temperature data obtained at Bairro Alto site in location where Bathymodiolus sp. were observed (Desbruyères et al., submitted). Min. Max. Mean ± 1 sdCalculated temperature (°C)

from LS1M5 δ18

O data

pure seawater environ. 2.98 6.85 5.0 ± 0.91pure hydrothermal fluid environ. 7.04 11.27 9.2 ± 1.0

Bairro Alto (LS) temperature measurements (°C)

mussels + shrimps 9.5 ± 3.4mussels 6.7 ± 1.8

Table 3: Calculated and measured water temperature data for the Bairro Alto site (Lucky Strike, MAR).

The δ18

O of the calcite layer of the Bathymodiolus sp. seems to be a good temperature proxy and might thus be used to reconstruct temperature changes of the mussel's environment. The temperature variations in such deep-sea hydrothermal environment are most probably related to variable inputs of hydrothermal fluids but we might also consider a seasonal-like control on this temperature as indicated by the Sr cycle pattern. Indeed, the close relationship between Sr and

δ18

O ratios of the shell (Fig. 24) might indicate a major temperature control on Sr incorporation. However, a physiological and/or a primary productivity control must not be excluded as indicated

by the δ13

C - Sr similarity (Perrier et al., 1994; Halley et Roulier, 1999; Vander Putten, 2000).

The chemistry of the Bathymodiolus sp. shells from three hydrothermal field of the Mid-Atlantic Ridge seems to reflect the relative chemical differences of the hydrothermal fluids emitted in each site. The trace element variations observed might be interpreted as changes in the surrounding mixed see-water / hydrothermal fluid environment. The (annual?) Sr cycles observed

might be related to a (seasonal?) temperature control as showed by its similarities with the δ18

O

signature. The temperature calculated from the δ18

O data are in agreement with those measured



on the site. This confirm the validity of Bathymodiolus sp. δ18

O ratios as temperature proxy. The

δ18

O of water samples taken close to the mussel sampling sites will be soon completed and will

be used to further validate the δ18

O-T°C pair. This study showed the great potential of the Bathymodiolus sp. shell as recorder of hydrothermal vent characteristics.

References

Comtet, T. (1998) - Structure des populations, reproduction, croissance et phylogéographie des Mytilidae des champs hydrothermaux Lucky Strike et Menez Gwenn (37°17'N et 37°50'N sur la dorsale médio-atlantique). Doctorat Thesis, Bretagne Occidentale, Brest, 244 pp.Coplen, T.B., Kendall, C. and Hopple, J. (1983) - Comparison of stable isotope reference samples. Nature, 302, n°5905, 236-238.Desbruyères, D., Biscoito, M., Caprais, J.-C., Comtet, T., Colaço, A., Crassous, P., Fouquet, Y., Khripounoff, A., Le Bris, N., Olu, K., Riso, R., Sarradin, P.-M. and Vangriesheim, A. (submitted) - Variations in deep-sea hydrothermal vent communities on the Mid-Atlantic Ridge when approaching the Azores plateau. submitted to Deep-Sea Research, Part I.Douville, E., Charlou, J.-L., Donval, J.-P., Fouquet, Y., Oelkers, E.H., Jove Colon, C.F., Bienvenu, P., Prieur, D. and Appriou, P. (submitted) - Trace metals in hot acidic fluids from a deep-sea hydrothermal system in an ultra-mafic environment: Rainbow vent field (36°14'N, MAR). Geology.Epstein, S., Buchsbaum, R., Lowenstam, H.A. and Urey, H.C. (1953) - Revised carbonate-water isotopic temperature scale. Bull. Geol. Soc. Am., 64, 1315-1326.Etheridge, D.M., Steele, L.P., Langenfelds, R.L., Francey, R.J., Barnola, J.-M. and Morgan, V.I. (1998) - Historical CO2 records from the Law Dome DE08, DE08-2, and DSS ice cores, Trends: A compendium of Data on Global Change. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, Oak Ridge, Tennessee.Friedli, H., Lötscher, H., Oeschger, H., Siegenthaler, U. and Stauffer, B. (1986) - Ice core record of the 13C/12C ratio of atmospheric CO2 in the past two centuries. Nature, 324, 237-238.Halley, R.B., Swart, P.K., Didge, R.E. and Hudson, J.H. (1994) - Decade-scale trend in sea water salinity revealed through δ18Ο analysis of Montastraea annualris annual growth bands. Bull. Mar. Sci., 54, n°3, 670-678.Jean-Baptiste, P., Charlou, J.L. and Stievenard, M. (1997) - Oxygen isotope study of mid-ocean ridge hydrothermal fluids: Implication for the oxygen-18 budget of the oceans. Geochim. Cosmochim. Acta, 61, n°13, 2669-2677.Lazareth, C., Vander Putten, E., André, L. and Dehairs, F. (2000) - On the Potential of Bivalve Shells to Record Environmental Conditions: A LA-ICP-MS Study of Trace Element Distributions Along a Growth/Time Axis. Goldschmidt 2000, September 3rd-8th, Oxford (UK). J. Conf. Abs., 5, n°2, 626. TéléchargerLazareth, C.E., Vander Putten, E., André, L. and Dehairs, F. - High-resolution trace element profiles in shells of the mangrove bivalve Isognomon ephippium: a record of environmental spatiotemporal variations? Accepted to Estuarine, Coastal and Shelf Sciences. TéléchargerLazareth, C.E., Willenz, P., Navez, J., Keppens, E., Dehairs, F. and André, L. (2000) - Sclerosponges as a new potential recorder of environmental changes: Lead in Ceratoporella nicholsoni. Geology, 28, n°6, 515-518. TéléchargerMann, M.E., Bradley, R.S. and Hughes, M.K. (1998) - Global-scale temperature patterns and climate forcing over



the past six centuries. Nature, 392, 779-787.Murozumi, M., Chow, T.J. and Patterson, C. (1969) - Chemical concentrations of pollutant lead aerosols, terrestrial dusts and sea salts in Greenland and Antarctic snow strata. Geochim. Cosmochim. Acta, 33, 1247-1294.Neftel, A., Moor, E., Oeschger, H. and Stauffer, B. (1985) - Evidence from polar ice cores for the increase in atmospheric CO2 in the past two centuries. Nature, 315, 45-47.Outridge, P.M., Doherty, W. and Gregoire, D.C. (1997) - Ablative and transport fractionation of trace elements during laser sampling of glass and copper. Spectrochim. Acta, 52, 2093-2102.Perrier, C., Hillaire-Marcel, C. and Ortlieb, C. (1994) - Paléogéographie littorale et enregistrement isotopique (13C, 18O) d'évènements de type El Niño par les mollusques holocènes et récents du Nord-Ouest péruvien. Géographie Physique et Quaternaire, 48, n°1, 23-38.Rousse, N., Boulegue, J., Cosson, R.P. and Fiala-Medioni, A. (1999) - Bioaccumulation des métaux chez les mytilidae hydrothermal Bathymodiolus sp. de la ride médio-atlantique. Oceanologica Acta, 21, n°4, 597-607.Shen, G.T. and Boyle, E.A. (1987) - Lead in corals: reconstruction of historical industrial fluxes to the surface ocean. Earth Planet. Sci. Lett., 82, 289-304.Stix, J., Gauthier, G. and Ludden, J.N. (1995) - A critical look at quantitative laser-ablation ICP-MS analysis of natural and synthetic glasses. Can. Mineral., 33, 435-44.Vander Putten, E., Dehairs, F., André, L. and 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. Anal. Chim. Acta, 378, 261-272.Vander Putten, E., Dehairs, F., Keppens, E. and Baeyens, W. (2000) - High resolution distribution of trace elements in the calcite shell layer of modern Mytilus edulis: Environmental and biological controls. Geochim. Cosmochim. Acta, 64, n°6, 997-1011.Von Damm, K.L., Bray, A.M., Buttermore, L.G. and Oosting, S.E. (1998) - The geochemical controls on vent fluids from the Lucky Strike vent field, Mid-Atlantic Ridge. Earth Planet. Sci. Lett., 160, n°3-4, 521-536.

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[1]

Monthly averaged values of sea surface temperature were obtained from the web site "http://ingrid.ldgo .columbia.edu/ SOURCES/.IGOSS/.nmc/.monthly/.sst/" as blended from ship, buoy and bias-corrected satellite data (Reynolds and Smith, 1994).[2]

Precipitation data from the web site "http://ingrid.ldgo.columbia.edu/SOURCES/.NOAA/.NCEP/ .CPC/.CAMS_OPI/ .mean/.prcp/" (as blended from data of NOAA, National Centers for Environmental Prediction, Climate Prediction Center) for a geographical setting (41.25E 3.75S) encompassing the Mombasa area with its coastal waters and for a time-period including the life-time of the bivalves studied (1990-1998).


http://www.wanadoo.fr/Pages_Perso/estat/bin/pwpestat_stat.cgihttp://ingrid.ldgo.columbia.edu/SOURCES/.IGOSS/.nmc/.monthly/.sst/http://ingrid.ldgo.columbia.edu/SOURCES/.IGOSS/.nmc/.monthly/.sst/http://ingrid.ldgo.columbia.edu/SOURCES/.NOAA/.NCEP/.CPC/.CAMS_OPI/http://ingrid.ldgo.columbia.edu/SOURCES/.NOAA/.NCEP/.CPC/.CAMS_OPI/

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