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.
Highlights
1.1 Fragmentary data sets from the deep seaThe deep-sea floor below 1000 m covers approximately 60 % of our planet’s solid surface (Glover and Smith, 2003)
The analyses of seven GCs from the DISturbance and reCOLonization experiment (DISCOL) area show that a deep-sea basin can be highly heterogeneous even on small spatial scales
The variability is visible in organic matter content (POC) and related differences in NO−3, Mn, and Fe concentrations as well as for individual layers where redox sensitive elements such as U, Mo, V, and As are enriched
Summary
1.1 Fragmentary data sets from the deep seaThe deep-sea floor below 1000 m covers approximately 60 % of our planet’s solid surface (Glover and Smith, 2003). 1.1 Fragmentary data sets from the deep sea. The analyses, were based on one core per work area only, separated by hundreds of kilometers. Many studies in the past collected cores for porewater and solid-phase geochemical analyses based on sparse sampling distribution and spread over large areas (e.g., Froelich et al, 1979; Klinkhammer, 1980; Toyoda and Masuda, 1991; Drodt et al, 1997; König et al, 1997, 1999; Haley et al, 2004; Schacht et al, 2010; Kim et al, 2012; Soyol-Erdene and Huh, 2013; Kon et al, 2014; Deng et al, 2017; Volz et al, 2018; Abbott et al, 2019).
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