Early diagenetic REE migration from Fe-Mn nodules to fish teeth in deep sea sediments

Abstract

In deep sea sediments, rare earth elements and yttrium (REYs) are predominantly carried by bioapatite and Fe-Mn (oxyhydr)oxides. According to recent studies, REYs can migrate into bioapatite after the dissolution of Fe-Mn (oxyhydr)oxides. However, the REY migration mechanism from Fe-Mn (oxyhydr)oxides to bioapatite are still unclear. Here, we integrated in situ trace element and Nd isotope analyses to elucidate the geochemical characteristics of fish tooth-bearing Fe-Mn nodules from the Northwest Pacific Ocean. Our Fe-Mn nodule samples can be classified into different morphological and geochemical types of the different periods in early diagenesis: (I) completely overgrown around bioapatite; II) overgrown around half of the bioapatite. Type I nodules grow around a bioapatite core, which has high LaN/SmN (0.41–0.50, avg. 0.46) and LaN/YbN (0.49–0.90, avg. 0.62). The ΣREY content is non-uniform in the two bioapatite types (WP-01 & WP-02) in Type I nodules, i.e., 475–2150 ppm and 272–431 ppm, respectively. The REY results show that the bioapatite is mainly adsorbed REY in early diagenesis. Type II nodules were covered both inside and at the tip of the tooth. The LaN/SmN (0.34–0.38, avg. 0.36) and LaN/YbN (0.26–0.61, avg. 0.37) ratios of the bioapatite samples are relatively low. Meanwhile, the ΣREY content is very uneven (229–3521 ppm). The REY data show that the bioapatite is mainly substituted during strong early diagenesis. The intercrystalline pores in some authigenic apatite may have closed upon recrystallization, causing REY redistribution and fractionation. The Y/Ho vs. ΣREE/Th plot shows a higher REY proportion from pore water in type II nodules than in type I ones. The widespread distribution of high Mn content and the gradual decrease of REY content from the nodule rim to core reflect the transport of REY from dissolved Fe-Mn (oxyhydr)oxides to bioapatite during early diagenesis under suboxic conditions. From type I to type II nodules, the mean εNd value of bioapatite increases from −6.69 to −5.56. The comparison of εNd values between REY-rich mud and bioapatite indicate that the REYs were derived from the bottom water and pore water in deep sea sediments. Our findings comprehensively document the transport and enrichment mechanisms of REY in early diagenetic processes from Fe-Mn (oxyhydr)oxides to bioapatite, which we argued to be relatively localized.