Water depth-composition trends in ferromanganese crusts adjacent to the California margin compared to those in equatorial Pacific crusts

Ferromanganese (Fe-Mn) crusts have been used as proxies for paleo-seawater chemistry; however, element concentrations and growth rates in crusts can vary depending on the region, latitude, and water depth. Here we will look at 130 Fe-Mn crusts from seven seamounts adjacent to the California (CA) margin to explore trends in composition with water depth and latitude. Crusts were collected by ROV, resulting in a dataset with exact water depth and location coordinates. Water depth ranges from 570 to 3,934 m along a 700-km transect running roughly parallel to the CA margin. Crust samples used for comparison were collected by dredging along transects following the Gilbert Ridge and Tokelau Seamounts in the western equatorial Pacific, with water depths ranging from about 1,500 to 4,800 m. In addition to variations with latitude and water depth, element concentrations in CA margin crusts are influenced by high primary productivity in surface waters, terrestrial input, and upwelling along the continental margin. Elements associated with terrestrial input, including Na, Si, Al, K, Pb, and particularly Th, are enriched in CA margin crusts relative to crusts from the equatorial Pacific transects. Si is also associated with the biogenic phase, as are P, Ba, and Cu but these elements are lower in CA margin crusts. Ba is a proxy for primary productivity. CA margin crusts show Ba increasing with increasing water depth, while equatorial Pacific crusts show the inverse trend. In equatorial Pacific crusts, Ba correlates with decreasing latitude, which reflects increasing proximity to the high productivity zone of equatorial upwelling; additionally, local obstructional upwelling associated with primary productivity around seamounts and islands enhances the productivity signal. Cu, which is associated with the manganese oxide phase, in addition to the biogenic phase, also increases with water depth along the CA margin; this is consistent with the seawater profile for dissolved Cu. In both the CA margin and the equatorial Pacific crusts, select elements associated with the iron oxyhydroxide phase, Pb, Mo, As, Ca, P, and Th, have higher concentrations in crusts that formed in shallower water. In CA margin crusts aluminosilicate elements, Si, Al, and K increase with water depth; this is also true for Si and K in equatorial Pacific samples. Be, which can be associated with biogenic and iron oxyhydroxide phases, increases in shallower water in both data sets, however when the data are combined, Be appears to increase with water depth. This apparent increase in the combined data is due to lower Be concentrations in the CA margin crusts relative to the equatorial Pacific crusts, which may be due to an inverse correlation between Be and latitude. Both the CA margin and equatorial Pacific crust growth rates increase with increasing water depth and increasing latitude. However, these trends disappear when the data are combined. These relationships indicate that regional and geographic trends must be taken into account when using crusts for paleo-oceanographic studies.