Rare earth elements in mud volcano sediments from the Gulf of Cadiz, South Iberian Peninsula.

Eight gravity cores (GC) were retrieved from the deep mud volcanoes Sagres, Bonjardim, Soloviev and Porto in the Gulf of Cadiz. Cores with 137 to 317 cm long were sliced in intervals of 15 to 20 cm thickness, and 46 samples were analyzed for grain size distribution, loss on ignition, Al, Fe, Ca, Mg, Mn, Sr, Ba and the rare earth elements (REE) La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu. REE profile normalized to Post-Archean Australian Average Shale (PAAS) was dominated by a mid-REE bulge with a pronounced Eu enhancement. Sediment reducing conditions resulting from the anaerobic oxidation of methane probably contributed to the positive-Eu anomaly (1.18-2.19, PAAS normalization). Most likely, reactions near the sulfate-methane transition zone such as the precipitation of barium sulfate found at layers around 50-cm depth explain the enhancement of Eu/Eu* ratios. The Ce anomalies (0.93-1.09) were almost absent.