The oceanic biogeochemical cycles of the transition metals have been eliciting considerable attention for some time. Many
of them have isotope systems that are fractionated by key biological and chemical processes so that significant information
about such processes may be gleaned from them. However, for many of these nascent isotopic systems we currently know too
little of their modern oceanic mass balance, making the application of such systems to the past speculative, at best. Here we
investigate the biogeochemical cycling of copper (Cu) and zinc (Zn) isotopes in the ocean. We present estimates for the isotopic
composition of Cu and Zn inputs to the oceans based on new data presented here and published data. The bulk isotopic
composition of dissolved Cu and Zn in the oceans (d65Cu +0.9&, d66Zn +0.5&) is in both cases heavier than their respective
inputs (at around d65Cu = +0.6& and d66Zn = +0.3&, respectively), implying a marine process that fractionates them
and a resulting isotopically light sedimentary output. For the better-known molybdenum isotope system this is achieved by
sorption to Fe–Mn oxides, and this light isotopic composition is recorded in Fe–Mn crusts. Hence, we present isotopic data
for Cu and Zn in three Fe–Mn crusts from the major ocean basins, which yield d65Cu = 0.44 ± 0.23& (mean and 2SD) and
d66Zn = 1.04 ± 0.21&. Thus for Cu isotopes output to particulate Fe–Mn oxides can explain the heavy isotopic composition
of the oceans, while for Zn it cannot. The heavy Zn in Fe–Mn crusts (and in all other authigenic marine sediments measured
so far) implies that a missing light sink is still to be located. These observations are some of the first to place constraints on the
modern oceanic mass balance of Cu and Zn isotopes.