Award: OCE-1948716

Fluxes of metals to the deep seafloor are important both for our understanding of elemental cycling in the oceans as well as the evaluation of the deep seafloor as a potential mineral resource. Here, we focused on the elements thallium, molybdenum, zinc, and uranium that are all being used as isotope ratio tracers of past marine processes. Our study aimed to determine the modern fluxes and isotope compositions of these elements in the abyssal ocean in order to underpin the use of these isotope systems for application to past sediment archives. We also performed calculations of metal deposition rates for more than 20 other metals that have implications for the overallmetal resources hosted by the abyssal seafloor.

In our approach we assembled a set of ~80 abyssal sediment samples with relatively young depositional ages that have a wide geographic distribution throughout the oceans. We measured the concentrations of ~60 elements with ~25 of these identified as metals relevant for isotope studies or metal resources. We also determined the sedimentation rates of all the samples such that we could calculate the metal flux in each location. 

One of our major findings was that metal fluxes are much more variable than what has previously been assumed. Typically, metal deposition is broadly thought to vary less than a factor of 10 because precipitation of iron (Fe) and manganese (Mn) rich minerals that accommodate most metals on the abyssal plains should be relatively constant. However, we find that metal deposition rates vary by more than a factor of 100, much more than previously thought. We also find that samples within a 1000km distance of hydrothermal activity on mid ocean ridges have especially high metal deposition rates. Most likely, this result is explained by the great transportation distances of Fe and Mn from hydrothermal emanations on the seafloor that the GEOTRACES program over the last 2 decades have discovered.

Our second major finding is that some of the isotope ratio data we collected display strong correlation with metal deposition rates. Thus, the isotope cycling of especially thallium and zinc is also strongly influenced by hydrothermal supply of Fe and Mn at mid ocean ridges.

Last Modified: 07/06/2025
Modified by: Sune G Nielsen