The End of Monterey Submarine Canyon Incision and Potential River Source Areas-Os, Nd, and Pb Isotope Constraints from Hydrogenetic Fe-Mn Crusts

Abstract:

The Monterey Canyon off central California is the largest submarine canyon off North America and is comparable in scale to the Grand Canyon. The age and history of the Monterey Canyon are poorly constrained due to thick sediment cover and sediment disruption from turbidity currents. To address this deficit we analyzed isotopic proxies (Os, Pb, Nd) from hydrogenetic ferromanganese (Fe-Mn) crusts, which grow over millions of years on elevated rock surfaces by precipitation of metals from seawater. Fe-Mn crusts were studied from Davidson Seamount near the base of the Monterey submarine fan, the Taney Seamount Chain, and from Hoss Seamount, which serves as a regional control (Fig.). Fe-Mn crusts were dated using Os isotope ratios compared to those that define the Cenozoic Os isotope seawater curve. Four Fe-Mn crust samples from Davidson and Taney Seamounts deviate from the Os isotopic seawater curve towards radiogenic values after 4.5±1 Ma. Osmium is well mixed in the global ocean and is not subject to significant diffusive reequilibration in Fe-Mn crusts. We therefore attribute deviations from the Os isotope seawater curve to large-scale terrestrial input that ended about 4.5±1 Ma. The two Davidson samples also show more radiogenic Nd isotope values from about 4.5±1 Ma. Lead isotopes in one Davidson Seamount crust, measured by LA-ICPMS, deviate from regional values after 4.5±1 Ma for about 500 ka towards terrestrial sources. The Taney Seamount Fe-Mn crust does not deviate from regional Nd nor Pb isotope values due to its greater distance from Monterey Canyon and the shorter marine residence times of Nd and Pb. Isotope plots of our crust data and compiled data for potential source rocks indicate that the river that carved Monterey Canyon carried sediment with values closer to the Sierra Nevada than to a Colorado Plateau source, with cessation of major riverine input occurring approximately 4.5±1 Ma, an age that we interpret as the end of the Monterey Canyon incision.