Importance of the Colloidal Pool for Trace Metal Cycling in Deep-Sea Sediments

Traditionally, dissolved metals have been determined in the 0.2 µm pool of seawater and pore water. A closer look into size fractionation reveals, however, that a significant fraction of oxide forming metals, such as Mn and Fe, are found in the colloidal pool (1 nm – 0.2 µm). Recently, more studies have focused on the distinction of the truly dissolved and colloidal pools.

Here we present pore water data for 0.2 µm and 0.02 µm filtered pore water samples from the Clarion Clipperton Zone (CCZ) in the central Pacific Ocean in an attempt to distinguish different parts of the classical dissolved metal pool. At some sites, up to 60% of Mn is found in the colloidal pool between 0.02 µm and 0.2 µm in oxic pore water and near-bottom seawater. This trend is also visible for Fe and Co. In the suboxic pore water, Mn is only found in the < 0.02 µm pool.

Less or not redox-sensitive elements such as V, U, As, Mo, and Cd are almost exclusively found in the < 0.02 µm pool, irrespective of redox zonation.

Interestingly, ca. 60% of Cu measured in the suboxic phase is found in the 0.02-0.2 µm pool. Copper is almost exclusively complexed by organic ligands, which are mostly <0.02 µm, in seawater and shallow, oxic pore water, with 75-95% Cu found in the <0.02 µm pool. Copper, however, seems to be decoupled from organics at several meters depth. Due to the decreasing availability of organic matter, colloidal Fe might be one of the controlling phases for Cu in the suboxic zone, leading to a shift of Cu from the <0.02 µm pool to the 0.02-0.2 µm pool. Sediments in the CCZ are not sufficiently reducing to dissolve Fe-oxides at depths shallower than 6 m, on which we focused in our study.

Consequently, the differentiation between the colloidal and truly dissolved pool is important to properly understand trace metal cycling in pore waters as well as the fluxes across the sediment-water interface and potential adsorption/desorption processes.