Description: Author Posting. © American Geophysical Union, 2017. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 18 (2017): 3946–3963, doi:10.1002/2017GC007071.

Description: The sources of terrestrial material delivered to the California margin over the past 7 Myr were assessed using 187Os/188Os, Nd, and Pb isotopes in hydrogenetic ferromanganese crusts from three seamounts along the central and southern California margin. From 6.8 to 4.5 (±0.5) Ma, all three isotope systems show more radiogenic values at Davidson Seamount, located near the base of the Monterey Canyon System, than in Fe-Mn crusts from the more remote Taney and Hoss Seamounts. At the Taney Seamounts, approximately 225 km farther offshore from Davidson Seamount, 187Os/188Os values, but not Pb and Nd isotope ratios, also deviate from the Cenozoic seawater curve toward more radiogenic values from 6.8 to 4.5 (±0.5) Ma. However, none of the isotope systems in Fe-Mn crusts deviate from seawater at Hoss Seamount located approximately 450 km to the south. The regional gradients in isotope ratios indicate that substantial input of dissolved and particulate terrestrial material into the Monterey Canyon System is responsible for the local deviations in the seawater Nd, Pb, and Os isotope compositions from 6.8 to 4.5 (±0.5) Ma. The isotope ratios recorded in Fe-Mn crusts are consistent with a southern Sierra Nevada or western Basin and Range provenance of the terrestrial material which was delivered by rivers to the canyon. The exhumation of the modern Monterey Canyon must have begun between 10 and 6.8 ± 0.5 Ma, as indicated by our data, the age of incised strata, and paleo-location of the Monterey Canyon relative to the paleo-coastline.

Description: Funding was provided by the United States Geological Survey Pacific Coastal and Marine Science Center Marine Minerals Group, the University of California Santa Cruz Scholarship for Re-Entry Women in Science, and the UCSC Earth and Planetary Science Department Waters Award.

Description: 2018-05-15

Description: Ferromanganese (Fe–Mn) crusts are strongly enriched relative to the Earth's lithosphere in many rare and critical metals, including Co, Te, Mo, Bi, Pt, W, Zr, Nb, Y, and rare-earth elements (REEs). Fe–Mn nodules are strongly enriched in Ni, Cu, Co, Mo, Zr, Li, Y, and REEs. Compared to Fe–Mn crusts, nodules are more enriched in Ni, Cu, and Li, with subequal amounts of Mo and crusts are more enriched in the other metals. The metal ions and complexes in seawater are sorbed onto the two major host phases, FeO(OH) with a positively charged surface and MnO2 with a negatively charged surface. Metals are also derived from diagenetically modified sediment pore fluids and incorporated into most nodules. Seafloor massive sulfides (SMS), especially those in arc and back-arc settings, can also be enriched in rare metals and metalloids, such as Cd, Ga, Ge, In, As, Sb, and Se. Metal grades for the elements of economic interest in SMS (Cu, Zn, Au, Ag) are much greater than those in land-based volcanogenic massive sulfides. However, their tonnage throughout the global ocean is poorly known and grade/tonnage comparisons with land-based deposits would be premature. The Clarion–Clipperton Fe–Mn Nodule Zone (CCZ) in the NE Pacific and the prime Fe–Mn crust zone (PCZ) in the central Pacific are the areas of greatest economic interest for nodules and crusts and grades and tonnages for those areas are moderately well known. We compare the grades and tonnages of nodules and crusts in those two areas with the global terrestrial reserves and resources. Nodules in the CCZ have more Tl (6000 times), Mn, Te, Ni, Co, and Y than the entire global terrestrial reserve base for those metals. The CCZ nodules also contain significant amounts of Cu, Mo, W, Li, Nb, and rare earth oxides (REO) compared to the global land-based reserves. Fe–Mn crusts in the PCZ have significantly more Tl (1700 times), Te (10 times more), Co, and Y than the entire terrestrial reserve base. Other metals of significance in the PCZ crusts relative to the total global land-based reserves are Bi, REO, Nb, and W. CCZ nodules and PCZ crusts are also compared with the two largest existing land-based REE mines, Bayan Obo in China and Mountain Pass in the USA. The land-based deposits are higher grade but lower tonnage deposits. Notably, both land-based deposits have 〈 1% heavy REEs (HREEs), whereas the CCZ has 26% HREEs and the PCZ, 18% HREEs; the HREEs have a much greater economic value. Radioactive Th concentrations are appreciably higher in the land-based deposits than in either type of marine deposit. A discussion of the differences between terrestrial and marine impacts and mine characteristics is also presented, including the potential for rare metals and REEs in marine deposits to be recovered as byproducts of mining the main metals of economic interest in nodules and crusts.