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

Through time, oceans have experienced physicochemical variations in a response to the evolution of the earth and its atmosphere. Geochemical proxies are employed to define these variations, evaluate their magnitudes and identify the responsible factors. Because of the unique behavior of the rare earth elements (REEs) within the oceanic system, they have been proven to be a leading proxy for tracing changes in the marine geochemical cycle (Elderfield et al., 1988; Holser, 1997). However, obtaining a reliable rare earth element (REE) proxy seems to be very difficult. Several archives were investigated for their REE proxy potential but with limited success. The biggest difficulty which complicates our understanding is perhaps the ambiguous REE compositions acquired from both modern and fossil archives.
\nREEs are incorporated into the crystal lattice of the marine components in very small fractions. Investigating their REE contents and fractionations in the different modern and ancient archive requires complete elimination of the internal and the external contaminants. Without their removal, inputs from the bound oxides, detritus and/or organic matter overprint the archives’ REE compositions. Despite the importance of the cleaning procedures, reviewing the literature reveals the lack of a rigorous methods or steps that should be followed to acquire uncontaminated REE signals. Therefore, the establishment of a reliable sample processing protocol that satisfies the most stringent demands for procuring reliable and robust REEs from marine materials is among the objectives of the thesis. To achieve this goal, five different cleaning procedures (P) were applied on Liothyrella neozelanica valves dredged from the deep water of the South Pacific Ocean (north of New Zealand). P-1 consisted only of rinsing the specimens extensively with distilled water. P-2 was the immersion of the specimens in hydrogen peroxide for 3 continuous days. Physically cleaning the shells under binocular microscope using a sharp stainless-steel blade comprised the P-3 procedure. In P-4 the specimens were physically cleaned, then leached with 10% hydrochloric acid until they deemed white. In P-5 the specimens were processed using all above cleaning procedures (water washing, physical cleaning, H2O2 treatment, and HCl leaching).
\nThe P-1 specimens yielded high concentrations of Mn, Fe, U and ΣREEs particularly Ce resulting from the primary layer, periostracum and the adsorptive particulates and detritus. P-2 specimens yielded similar trend as P-1, but with partial depletion in the Ce/Ce* anomalies. Specimens of P-3 (physical cleaning only) yielded depleted ΣREEs and lower Mn, Fe, and U concentrations with slightly negative Ce anomaly. P-4 and P-5 fragments yielded remarkable reductions if the ΣREE, Mn, Fe, U concentrations and ‘normal’ Ce anomaly, however, the P-4 fragments are slightly enriched in their ΣREE contents. The P-5 seems to be the ultimate protocol for modern shells but P4 should be applied to fossil shells due to the absence of the organic remnants adhering to the outermost shell.
\nThe reproducibility of the clearing protocol on individuals of different water masses was evaluated by applying the P-3 and P-5 procedures on deep water shells recovered from the abyssal depths of the Caribbean Sea. Its validity for archives of different settings, on the other hand, was assessed by applying the P-1, P-2, P-3 and P-5 on shallow water shells from the Irminger Sea, west of Iceland. The results were uniform and consistent supporting the reliability and robustness of the proposed protocol. However, it is infallible and its completion must be verified in the light of the archives’ elemental composition.
\nThe P-4 was applied to Ordovician, Silurian, Pennsylvanian and Permian brachiopods and near-micritc whole rock to test the efficiency of the suggested protocols on the ancient archives. The fossils yielded ΣREE, Mn, Fe, U concentrations and Ce anomalies comparable to their modern counterparts, while the whole rock are slightly to extensively enriched. Those with higher Fe and Mn contents than 300 and 100 ppm, respectively, their Ce/Ce* values should be adjusted by −13% to be considered for the paleoredox investigations.
\nOne of the thesis main objectives is to evaluate the REE proxy potential of the brachiopods archive for paleooceanographic and paleoenvironmental investigations. This evaluation required minimizing the impact of variations in the environmental conditions on seawater REE compositions. It was, consequently, conducted by assessing the REE uptake of the deep water shells (>500 m) of the orders Rhynchonellids and Terebratulids obtained from the Caribbean Sea, North Atlantic, South Pacific and Southern Oceans. The shells display gradual enrichment REESN patterns with increasing atomic numbers that are punctuated consistently with the negative CeSN excursions. The REE contents of their secondary fibrous calcite layers are elevated relative to the ambient water masses by an average factor of 0.81x10⁵. The REE distribution coefficients between the shells’ crystal lattice and the ambient seawater vary consistently within the series in response to changes in ionic radii relative to that of the Ca²⁺ cation. The calculated log KD values for the shells of two orders are remarkably similar and consistent with those of foraminiferal calcite.
\nThe deep water shells yielded L:H and L:M ratios similar to those of their water masses, while their M:H ratios are notably different, defining a secondary trend that is directly proportional to the shells’ depths. Although their Ce/Ce* anomalies are consistently negative, they vary significantly but in a manner reflecting the variations in the prevalent oceanographic conditions. The open water shells display a vertical Ce/Ce* profile relatively similar to the Ce vertical curve in seawater.
\nTo assess the REE incorporation into different parts/layers of the brachiopod shell structure, and the impacts of changes in the seawater physicochemical parameters on the REE fractionations and distributions of the brachiopod archives, articulated shells of the orders Terebratulida and Thecideidina were obtained from the shallow waters (<500 m) of the Norwegian, the Irminger and the Ross Seas, the Denmark Strait, Bonne Bay and Bay of Fundy and Friday Harbor. Their REE compositions were correlated to those of the deep water settings. The umbo region is secreted with either selective REE uptake or species dependency during the growth of the juvenile stage, whereas the primary layer’s REE incorporation is variable and incidental. Precipitation of the pedicle and brachial valves’ secondary layers occur with a small range of differences that falls within the natural variation of the brachiopod’s lattice. The Thecideidinid shells yielded remarkable variations in their elemental, REE compositions and the REESN patterns due to the domination of the primary layers over their structures. Seawater parameters of temperature, salinity and depth seem to have controls on the shells’ REE composition. Depth plays a major rule on the shell REE fractionations, while temperature and salinity have moderate to minor impacts on the REE contents.
\nThe shallow water shells have been divided into six depth intervals, and their L:H ratios were utilized to estimate the paleobathymetry of several Paleozoic formations. The evaluation was successful in placing the Permian Gyanyima, the Pennsylvanian Naco, Boggy and the Silurian Chicotte, Becscie and Jupiter Formations in shallow depths (<15-50 m).

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