Are the pelagic clay-rich sediments a major source of elements to bottom-waters?

Science Highlights

21 December 2023

Although pelagic red clay sediments (which represents almost 40% of the global ocean seafloor) are suspected to be a significant source of trace elements to the water column, very few data are available today to dimension this bottom source.

Steiner and colleagues (2023, see reference below) proposed to tackle this issue by analyzing porewater and bulk sediment concentrations of vanadium, chromium, cobalt, nickel, copper, arsenic, molybdenum, barium and uranium, as well as concentrations of the major oxidants nitrate, manganese, iron, and sulfate in the top 30 cm of 5 cores collected along a transect from Hawaii to Alaska. They establish that:

  • Porewater transition metal concentrations vary by up to an order of magnitude in aerobic pelagic clay sediments. Large concentration variability is not always requiring significant changes in redox conditions, because they can be due to the nature and quantity of material falling on the seabed, microbial activity, and different physical environment at the bottom of the sea.
  • Despite the large variability in porewater concentrations observed in the top 5 cm, calculated benthic fluxes of most transition metals agree with estimated fluxes based on water column concentrations. This likely reflects that the re-oxidation of reduced manganese and iron in the continental shelf top sediment and bottom water scavenges other trace metals and limits their transport.
Figure 1: Station location and bottom water depths. Map plotted using Ocean Data View 5.6.3 (Schlitzer, 2020), mean surface chlorophyll-a concentration for the period July 2002 to May 2023 are from Nasa Aqua-MODIS satellite data (https://oceancolor.gsfc.nasa.gov).
Figure 2: Cobalt, chromium and barium. (a–c) Porewater concentrations. Subplots (d) and (e) focus on the relevant concentration range of cobalt and chromium at four of the five stations. (f) Porewater saturation state with respect to barite (BaSO4), equilibrium saturation (ΩBarite = 1) is marked with a vertical dashed line. Error bars illustrate the analytic ±1σ SD. (g–h) Bulk sediment concentrations; the vertical dashed lines represent the average composition of the upper continental crust.

Reference:

Steiner, Z., Antler, G., Berelson, W. M., Crockford, P. W., Dunlea, A. G., Hou, Y., Adkins, J. F., Turchyn, A. V., & Achterberg, E. P. (2023). Trace Element Geochemistry in North Pacific Red Clay Sediment Porewaters and Implications for Water‐Column Studies. Global Biogeochemical Cycles, 37. Access the paper: 10.1029/2023gb007844

Latest highlights

Deep-sea mining, dewatering waste, accidental plumes and their potential consequences on trace metal fates in the North Pacific Ocean

Xiang and his colleagues conducted laboratory incubation experiments that simulate mining discharge into anoxic waters.

Biogeochemical behaviours of barium and radium-226 in the Pacific Ocean

Barium and radium-226 are not just proxies for nutrients and ocean circulation but are themselves marine biogeochemical tracers…

Intrigued by Rare Earth Elements and neodymium isotopes? A review for the curious

Vanessa Hatje and a group of Rare Earth Element (REE) specialists propose an exhaustive review on the behaviour of REE.

North-South section of bioactive cadmium, nickel, zinc, copper and iron along GEOTRACES transect GP19 in the Pacific Ocean

Zheng and his colleagues propose the first full sections of the simultaneous dissolved distributions of five nutrient-type trace metals in the western South Pacific Ocean.

Rechercher