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.
Comparison of Heterotrophic Bioleaching and Ammonium Sulfate Ion Exchange Leaching of Rare Earth Elements from a Madagascan Ion-Adsorption Clay