Small-scale heterogeneity of trace metals including rare earth elements and yttrium in deep-sea sediments and porewaters of the Peru Basin, southeastern equatorial Pacific

Abstract

Abstract. Due to its remoteness, the deep-sea floor remains an\nunderstudied ecosystem of our planet. The patchiness of existing data sets\nmakes it difficult to draw conclusions about processes that apply to a wider\narea. In our study we show how different settings and processes determine\nsediment heterogeneity on small spatial scales. We sampled solid phase and\nporewater from the upper 10\u2009m of an approximately 7.4×13 \u2009km 2 \narea in the Peru Basin, in the southeastern equatorial Pacific Ocean, at 4100\u2009m water\ndepth. Samples were analyzed for trace metals, including rare earth elements\nand yttrium (REY), as well as for particulate organic carbon (POC),\n CaCO3 , and nitrate. The analyses revealed the surprisingly high spatial\nsmall-scale heterogeneity of the deep-sea sediment composition. While some\ncores have the typical green layer from Fe(II) in the clay minerals, this\nlayer is missing in other cores, i.e., showing a tan color associated with\nmore Fe(III) in the clay minerals. This is due to varying organic carbon\ncontents: nitrate is depleted at 2–3\u2009m depth in cores with higher total\norganic carbon contents but is present throughout cores with lower POC\ncontents, thus inhibiting the Fe(III)-to-Fe(II) reduction pathway in organic\nmatter degradation. REY show shale-normalized (SN) patterns similar to\nseawater, with a relative enrichment of heavy REY over light REY, positive\nLa SN anomaly, negative Ce SN anomaly, and positive\nY SN anomaly and correlate with the Fe-rich clay layer and, in some cores,\nalso correlate with P. We therefore propose that Fe-rich clay minerals, such as\nnontronite, as well as phosphates, are the REY-controlling phases in these\nsediments. Variability is also seen in dissolved Mn and Co concentrations\nbetween sites and within cores, which might be due to dissolving nodules in\nthe suboxic sediment, as well as in concentration peaks of U, Mo, As, V, and\nCu in two cores, which might be related to deposition of different material\nat lower-lying areas or precipitation due to shifting redox boundaries.