Cobalt in ferromanganese crusts as a monitor of hydrothermal discharge on the Pacific sea floor

Abstract

Ferromanganese oxide crusts, which accumulate on unsedimented surfaces in the open ocean 1–6 , derive most of their metal content from dissolved and particulate matter in ambient bottom water 7,8 , in proportions modified by the variable scavenging efficiency of the oxide phase for susceptible ions 9 . They differ in this respect from abyssal nodules, much of whose metals are remobilized from host sediments. Here we present maps of cobalt concentration and inferred accumulation rate of ferromanganese crusts from the Pacific Ocean. We propose that depletion of cobalt in Pacific crusts measures the location and intensity of submarine hydrothermal discharge. Use of the 'cobalt chronometer', an algorithm inversely relating cobalt content and crust growth rate, permits mapping of the accumulation rate of ferromanganese crusts with only indirect recourse to radioactivity-based dating methods. These maps show that crusts in hydrothermal areas grow from two to more than four orders of magnitude faster than in the Central Pacific Ocean. Cobalt-enriched crusts are found where water masses are most isolated from continental-coastal and hydrothermal sources of metals, now and in the past. This relationship can resolve the problem of cobalt enrichment in crusts without recourse to hypotheses invoking special cobalt sources or enrichment mechanisms.