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 understudied ecosystem of our planet. The patchiness of existing data sets makes it difficult to draw conclusions about processes that apply to a wider area. In our study we show how different settings and processes determine sediment heterogeneity on small spatial scales. We sampled solid phase and porewater from the upper 10 m of an approximately 7.4×13 km2 area in the Peru Basin, in the southeastern equatorial Pacific Ocean, at 4100 m water depth. Samples were analyzed for trace metals, including rare earth elements and yttrium (REY), as well as for particulate organic carbon (POC), CaCO3, and nitrate. The analyses revealed the surprisingly high spatial small-scale heterogeneity of the deep-sea sediment composition. While some cores have the typical green layer from Fe(II) in the clay minerals, this layer is missing in other cores, i.e., showing a tan color associated with more Fe(III) in the clay minerals. This is due to varying organic carbon contents: nitrate is depleted at 2–3 m depth in cores with higher total organic carbon contents but is present throughout cores with lower POC contents, thus inhibiting the Fe(III)-to-Fe(II) reduction pathway in organic matter degradation. REY show shale-normalized (SN) patterns similar to seawater, with a relative enrichment of heavy REY over light REY, positive LaSN anomaly, negative CeSN anomaly, and positive YSN anomaly and correlate with the Fe-rich clay layer and, in some cores, also correlate with P. We therefore propose that Fe-rich clay minerals, such as nontronite, as well as phosphates, are the REY-controlling phases in these sediments. Variability is also seen in dissolved Mn and Co concentrations between sites and within cores, which might be due to dissolving nodules in the suboxic sediment, as well as in concentration peaks of U, Mo, As, V, and Cu in two cores, which might be related to deposition of different material at lower-lying areas or precipitation due to shifting redox boundaries.

Fractionation Trends and Variability of Rare Earth Elements and Selected Critical Metals in Pelagic Sediment from Abyssal Basin of NE Pacific (Clarion-Clipperton Fracture Zone)

“…Local heterogeneity of geochemical parameters is related to small scale local variability of depositional environment parameters, such as topography, biological productivity, hydrothermal effects, dissolution of nodules, or redox zonation [97]. In some parts, the elevated metal contents in CCFZ may also be related to buried hydrothermal microchimneys [98].…”

Importance of Spatial Autocorrelation in Machine Learning Modeling of Polymetallic Nodules, Model Uncertainty and Transferability at Local Scale

“…Inside the DEA (Figure 1), disturbance experiments were conducted to assess the environmental impact produced by a plough harrow [66,67]. The DEA is spatially heterogeneous regarding the seafloor morphology, geochemical properties, and PMN distribution, with many authors highlighting the need for further research [62][63][64]68,69]. The mapped area extends north and south of the DEA (in the N-S direction) and towards the northeast (hereafter DEA-NE).…”

“…A recent gravity core inside a black patch in DEA (Figure 1) revealed that its geochemical conditions differ significantly from the surroundings, having increased organic carbon content (0.5 wt%-0.8 wt%) [69]. The increased organic carbon content might have shifted the Mn-redox closer to the seafloor, where the diagenetically mobilized Mn is released into the bottom water and thus not supporting the PMN formation [68,69,78,79]. Photos inside the black patch confirmed the absence of surficial PMN (Figure A1).…”

“…PMN were found in the base of the crater, mixed with talus debris [67,77]. Geochemical analyses of the GC sediments revealed depth-profile variations for several chemical compounds, which exceed the area variability of other stations, highlighting the need for further research [69].…”

Assessment of Chronological Records of Rare Earth Elements in Sidi Moussa Lagoon Sediment (North-Western Morocco)

“…The reason of such different behaviour among REEs is unclear. The decoupling of Y and Ce from the LREEs is suggestive of fractionation likely due to preferential scavenging of individual REEs over the others [26]. The enrichment factor (EF) is a parameter widely used to asses qualitatively the inputs intensity of elements to the sediment.…”