Rare earth elements as new biogeochemical proxies in deep-sea mussels

We report on the abundances of REE in a comprehensive suite of shells of deep-sea chemosynthetic mussels from hydrothermal vents and cold seeps. Except for mussel shells from oceanic hydrothermal sites that often show extremely pronounced Eu anomalies ((Eu/Sm)sn = 2–200), and abundances for this element that can occasionally exceed 200 ng/g, REE concentrations are usually low and typically between 10−4 and 10−3 times the shale reference. In addition to exhibiting commonly high Eu anomalies, mussel shells from hydrothermal vents are depleted in light-REE and heavy-REE compared to reference shales (e.g., Pr/Sm)sn < 1, (Tb/Yb)sn = 1–6). These features are inherited from hydrothermal fluid. Mussel shells from cold seeps have very different REE concentrations, which also reflect the compositions of the waters they filter: their (Eu/Sm)sn ratios are much lower (<2) and are much less heavy-REE fractionated ((Tb/Yb)sn = 0.5–2.7). Furthermore, the REE distributions show a clear dichotomy between thiotrophic and methanotrophic mussels. The latter show marked enrichments in light-REE and even sometimes La enrichments much larger than those classically observed in deep-sea waters, leading to huge positive La anomalies (La/La* > 10). These light-REE enrichments are likely related to REE-dependent methanol dehydrogenase enzymes used by the symbionts that these mussels host in their gills. These data show that REE chemistry is a promising tool to study chemosynthetic faunas living near hydrothermal vents or cold seeps. Furthermore, REE chemistry, coupled with stable isotopes, should reveal the footprint of aerobic methanotrophy in carbonates formed in cold seeps, but also potentially in ancient sediments.