Rare Earth Element Geochemistry of the Surficial Sediments from the Adriatic Sea

Tadej Dolenec University of Ljubljana, Department of Geology, As˜kerc˜eva 12, 61000 Ljubljana, Slovenia

and Joz˜ef Stefan Institute, Jamova 39, 61000 Ljubljana, Slovenia

Jadran Faganeli Marine Biology Station, Fornac˜e 41, 66330 Piran, Slovenia

The concentrations and distributions of REE in the
surficial sediments were studied in attempt to establish the occurrence of their enrichment in the Adriatic Sea. Results indicate that REE are tightly related to the catchment geology. Surficial sediments from the Adriatic appear to preserve the REE patterns of their source areas, indicating that these elements are not appreciably fractionated during processes of erosion, transportation and deposition. However, the homogenization effects of sedimentary processes resulted in only slightly variable REE patterns of surficial sediments. The REE distribution is thus essentially controlled by textural type of sediments and it reflects the upper continental crust abundances. The most remarkable feature of the surficial sediments from the Adriatic Sea is the similarity in chondrite-normalized REE patterns which are characterized by LREE enrichment, relatively flat HREE distribution (GdN/YbN ratio of 1.57 to 2.32) and pronounced Eu anomaly with Eu/Eu* between 0.54 and 0.74 (Fig. 1). Similar patterns are also observed in terrigenous sedimentary rocks from the widely separated geographical regions (PAAS - Australia, Nance and Taylor, 1976; Taylor and McLennan, 1985; ES - Europe Haskin and Haskin, 1966; and NASC - North America Haskin et al., 1968, Gromet et al., 1984). It is important to realise that the surficial sediments from the Adriatic are slightly depleted in total REE content relative to the PAAS, ES and NASC. The cause of this depletion is probably related to grain size. Investigated sediments are generally coarser-grained as compared to shales, and thus tend to have lower total content of REE than shales (Cullers et al., 1979).

The surficial sediments from the Southern Adriatic are characterized by high REE enrichment (average 127 ppm), less variable LREE and HREE concentrations, slightly higher La/Sm ratios of 5.0 to 5.7 (average 5.4) and lower Eu depletion with Eu/Eu* between 0.69 in 0.75 (average 0.71). In the Southern Adriatic the REE probably reflect a origin of the Albanian hinterland. In the drainage area of the Albanian rivers are exposed the mineralized mafic, ultramafic, and other igneous as well as metamorphic rocks which are also the main source of Co, Cr, Ni, Cu, Se, V and Zn for the Southern Adriatic (Dolenec and Faganeli, 1995). Their distribution in the SE part of the basin results from the Albanian riverine inflows and is further controlled by roughly counter- clockwise water circulation system of the Adriatic. Another area of higher REE content coincided with the dominantly pelitic silty sediments along the Italian coast. The high correlations (r > 0.93) of REE with Al and K probably indicate their close association with clay mineralogy. Statistical analysis also confirms a highly significant correlation (r > 0.80) of REE with Mg, Fe, P, Zr and lower association with Ca (r from 0.52 to 0.66).

In the Central Adriatic where REE are introduced from the Southern and Northern Adriatic as well as from the lithologies exposed on the coastal area, the REE content is considerably lower relative to the Northern and Southern Adriatic (average 108 ppm). Surficial sediments from this part of the Adriatic also show slightly variable LREE and HREE enrichment, a low La/Sm range of 4.5 to 5.5 (average 5.3) and a negative Eu anomaly with Eu/Eu* between 0.65 and 0.73 (average 0.70). Correlation analysis indicates that the REE are largely associated with clay mineralogy, Fe, P and Zr (r > 0.83), while negative correlation between Mg, Ca and REE indicates that minerals containing Mg and/or Ca in the Central Adriatic do not play any role as a carrier phase of REE.

In the Northern Adriatic the highest concentrations for several REE (La, Co, Pr, Nd, Sm, Gd, Dy, Ho, Er, Yb and Lu) were measured. The surficial sediments from this part of the Adriatic display the highest REE enrichment (average 127 ppm), variable LREE and HREE abundances, low La/Sm ratios of 4.9 to 5.6 (average 5.3) and the most pronounced Eu anomaly with Eu/Eu* ratios in the range between 0.54 and 0.71 (average 0.65). The peak anomalies of REE were found in surficial sediments along the Italian coastal area and offshore from the Italian river Po and Croatian river Rasa inflow. The drainage area of the river Po and other north Italian rivers is also the source of weathered material of metamorphic and predominately acid to intermediate igneous rocks, as well as of the sediments. The variety of the source area and thus the variety of the origin of the exposed rocks have resulted in various mineral and REE associations in the surficial sediments. Correlation analysis confirms the presence of relatively high correlation coefficient only between REE and P (r from 0.76 to 0.96) and rather low correlation (r from 0.33 to 0.62) of REE with Al, K, Fe, Mg, Ca and Zr.


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Fig. 1: Chondrite-normalized REE diagrams for surficial sediments from the Adriatic Sea (-) and shale composites (Squares - PASS, Nance and Taylor, 1976; Taylor and McLennan, 1985; Crosses - , Haskin et al., 1968, Gromet et al., 1984; Circles - ES, Haskin and Haskin, 1966).