text: Cu is strongly cycled in the surface and deep ocean. In the deep ocean, Cu concentrations are ~5 nM in the Pacific and ~1.5 nM in the Atlantic. The deep/surface ratio of Cu in the ocean is typically <10, and vertical concentration profiles for Cu are roughly linear due to biological recycling and scavenging processes as well as adsorption to particles.
  Due to equilibrium and biological processes that fractionate Cu isotopes in the marine environment, the bulk copper isotopic composition (δCu = +0.6 to +1.5‰) is different from the δCu values of the riverine input (δCu = +0.02 to +1.45‰, with discharge-weighted average δCu = +0.68‰) to the oceans. δCu values of the surface layers of FeMn-nodules are fairly homogenous throughout the oceans (average = 0.31‰), suggesting low biological demand for Cu in the marine environment compared to that of Fe or Zn. Additionally, δCu values in the Atlantic ocean do not markedly vary with depth, ranging from +0.56 to +0.72‰. However, Cu isotope compositions of material collected on sediment traps at depths of 1,000 and 2,500 m in the central Atlantic ocean show seasonal variation with heaviest δCu values in the spring and summer seasons suggesting seasonal preferential uptake of Cu by biological processes.
  Equilibrium processes that fractionate Cu isotopes include high temperature ion exchange and redox speciation between mineral phases, and low temperature ion exchange between aqueous species or redox speciation between inorganic species. In riverine and marine environments, Cu/Cu ratios are driven by preferential adsorption of Cu to particulate matter and preferential binding of Cu to organic complexes. As a net result, ocean sediments tend to be depleted in Cu relative to the bulk ocean. For example, the downcore δCu values of a 760 cm sedimentary core taken from the Central Pacific ocean varied from -0.94 to -2.83‰, significantly lighter than the bulk ocean.