4.4. Protocol reproducibility assessment

4.6.2. Shell fractionation

Shells of articulated brachiopods consist of two opposing bilaterally symmetrical valves that are predominantly dissimilar in size with brachial (dorsal) valve normally smaller than the pedicle (ventral) valve (MacFarlan et al., 2009). The two valves are hinged posteriorly near the umbo (the most prominent and the oldest part of the valve) through a series of teeth and sockets, and simple opening and closing muscles (James et al., 1992).

The shell is secreted as multi-layers consisting of low-Mg calcite and a proteinaceous periostracum membrane succeeded by an outer granular calcite primary layer and an inner fibrous calcite secondary layer, and occasionally a stacked prismatic calcite tertiary layer

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in some species (Azmy et al., 1998; 2011; Auclair et al., 2003; Brand et al., 2003, 2013, 2015; Parkinson et al., 2005).

4.6.2.1. Umbo region

Specialized shell structures of modern brachiopods such as umbonal regions present anomalous Mg distributions and have their own isotopic compositions which have been attributed to their higher organic content (cf. Carpenter and Lohmann, 1995; Brand et al., 2003, 2013, 2015; Cusack et al., 2007; Pérez Huerta et al., 2008). In order to evaluate their REE fractionation, umbo areas of T. congregata and T. transversa from the Philippine Sea and Friday Harbor were separated, cleaned according to the Procedure-5 of Zaky et al.

(2015) and analysed for their REE contents.

The umbo regions seem to incorporate REEs compositions and distributions that are different from the rest of the shells (Fig. 4-5; Table 4-2). In addition, their partition coefficient values and pattern are also significantly different (Table 4-4; Fig. 4-8 A & B).

The inconsistency between REE contents within a brachiopod umbo-shell could be either species dependent during growth of the juvenile stage or a selective uptake of certain elements during their secretions. Consequently, REE contents of the umbo areas of brachiopods should not be considered for paleoenvironmental investigations.

158 Fig. 4-8. Distribution diagram of the ionic radii of the REEs Vs. the calculated log KD values for the shells of 1) the valve margin and the umbo of T. congregata species, 2) the valve margin and the umbo of T. transversa species, 3) primary and secondary layers of T. septentrionalis species, and 4) primary and secondary layers of M. cranium species.

4.6.2.2. Primary layer

The primary layer of modern articulated brachiopods is precipitated in oxygen and carbon isotope disequilibrium with their ambient seawater (cf. Carpenter and Lohmann, 1995; Parkinson et al., 2005; Brand et al., 2003, 2013, 2015). In a previous study, Zaky et al. (2015) demonstrated that the primary layer of M. cranium from deep water of the Irminger Basin is remarkable elevated in its Ce (Ce/Ce*= 0.99) and MREE contents (L:H,

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L:M and M:H= 0.69, 0.45 and 1.52: Table 4-5) relative to the secondary layer of deep-water counterparts of the Iceland Basin (Table 4-2).

For assessing the REE incorporation into the primary layer of shallow water articulated brachiopods, the P-3 segments of the T. septentrionalis and M. cranium from the Bay of Fundy and Irminger Sea were cleaned predominantly by abrasion of the external contaminants until they were deemed clean. The likelihood of success of the physical cleaning in removing adsorptive particulate matter of Mn, Fe and U oxides is reflected in the resemblance between the Mn, Fe and U concentrations in the P-3 (primary layer present) parts of the two species and their P-5 counterparts (primary layer removed; Table 4-2).

Higher MREE and HREE contents in P-3 segments from the Bay of Fundy relative to P-5 indicate their selective incorporation into the primary layer of the T. septentrionalis (Fig. 4-3; Table 4-2). On the other hand, the lower LREE, Sm, Eu and Gd contents in Irminger Sea P-5 specimens relative to those of the P-3 implies their preferentially incorporation into the primary layer of the shallow water M. cranium shells (Fig. 4-2; Table 4-2). The inconsistency of these results with those of the deep-water counterparts of Zaky et al. (2015) eliminates the “vital” effect on the REE incorporation of the primary layers of the modern articulated brachiopods. Instead, their partition coefficient values and pattern (Table 4-4; Fig. 4-8 C & D) suggest random, variable and incidental REE uptake during shell calcite secretion and implies that primary layers should be avoided for paleoenvironmental REE investigations.

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4.6.2.3. Dorsal and venteral valves

In contrast to the primary layer, secondary and tertiary layers in modern brachiopods are secreted in near isotopic near equilibrium with their ambient seawater (Carpenter and Lohman, 1995; Brand et al., 2003, 2011, 2013, 2015; Parkinson et al., 2005). The ventral (pedicle) valves of T. septentrionalis from Bonne Bay are slightly depleted in their LREE, MREE, HREE, ∑REE, Fe and U contents, but slightly enriched in their Mn and Ce/Ce* values in comparison to dorsal (brachial) valves (Table 4-6). In contrast, the LREE, MREE, HREE, ∑REE and U concentrations in the ventral valves of T.

transversa of Friday Harbor are slightly higher relative to those of the dorsal values, while their Fe, Mn and Ce/Ce* anomaly are lower (Table 4-6).

The dorsal and ventral valves of the M. cranium shells display variations from location to another. The ventral valves of those from Irminger Sea are enriched in their LREE, MREE, HREE, ∑REE, Mn, Fe and U contents relative to the dorsal valves. On the contrary, the dorsal valve of the Denmark Strait and Norwegian Sea shells are enriched relative to the ventral valve (Table 4-6). The ambivalence between elemental and REE compositions of the dorsal and ventral valves in the representatives of the different species and even within the same members discount the probability of selective REE uptake by a certain valve. Overall, the differences are small with most attributed to natural variation to the brachiopod’s lattice incorporation process rather than a major environmental factor.

161 Table 4-6. Mean Mn, Fe, U and the REE concentrations (in ppm), and L:H, L:M, M:H and Ce/Ce* values in pedicle and brachial valves of different brachiopods. Trace element results were normalized to 100 % carbonate basis (cf. Brand and Veizer, 1980).

Note: (N) number of samples.

LREEs= Sum of La:Nd concentrations.

MREEs= Sum of Sm:Dy concentrations.

HREEs= Sum of Ho:Lu concentrations.

In document Rare earth elements (REEs) in shallow and deep water articulated brachiopods: sensitive tracers of paleo-oceanography (Page 175-180)

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