Hydrogenetic Ferromanganese Crusts of the California Continental Margin

• Main Author: Conrad, Tracey Ann
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: eScholarship, University of California 2017
• Subjects: Marine geology; Geochemistry; Chemical oceanography; Ferromanganese crust; Lead isotopes seawater; Marine mineral resources; Monterey Submarine Canyon; Neodymium isotopes seawater; Osmium isotopes seawater; Arctic; Arctic Ocean; Pacific; Western Basin
• Online Access: http://www.escholarship.org/uc/item/0c40c67x

Hydrogenetic Ferromanganese (Fe-Mn) crusts grow from seawater and in doing so sequester elements of economic interest and serve as archives of past seawater chemistry. Ferromanganese crusts have been extensively studied in open-ocean environments. However, few studies have examined continent-proximal Fe-Mn crusts especially from the northeast Pacific. This thesis addresses Fe-Mn crusts within the northeast Pacific California continental margin (CCM), which is a dynamic geological and oceanographic environment. In the first of three studies, I analyzed the chemical and mineralogical composition of Fe-Mn crusts and show that continental-proximal processes greatly influence the chemistry and mineralogy of CCM Fe-Mn crusts. When compared to global open-ocean Fe-Mn crusts, CCM crusts have higher concentrations of iron, silica, and thorium with lower concentrations of many elements of economic interest including manganese, cobalt, and tellurium, among other elements. The mineralogy of CCM Fe-Mn crusts is also unique with more birnessite and todorokite present than found in open-ocean samples. Unlike open-ocean Fe-Mn crusts, carbonate-fluorapatite is not present in CCM crusts. This lack of phosphatization makes CCM Fe-Mn crusts excellent candidates for robust paleoceanography records. The second and third studies in this thesis use isotope geochemistry on select CCM Fe-Mn crusts from four seamounts in the CCM to study past terrestrial inputs into the CCM and sources and behavior of Pb and Nd isotopes over the past 7 million years along the northeast Pacific margin. The second study focuses on riverine inputs into the Monterey Submarine Canyon System and sources of the continental material. Osmium isotopes in the crusts are compared to the Cenozoic Os seawater curve to develop an age model for the samples that show the crusts range in age of initiation of crust growth from approximately 20 to 6 Myr. Lead and neodymium isotopes measured in select Fe-Mn crusts show that large amounts of terrestrial material entered the CCM via the Monterey Canyon from prior to 6.8±0.5 until 4.5 ±0.5 Myr ago. These data combined with reconstructions of the paleo-coastline indicate that incision of the modern Monterey Canyon started around 7 Myr ago. Isotope plots of potential source regions indicate that the source of the material is the border of the southern Sierra Nevada and western Basin and Range. This answers a long-standing and fundamental question about the timing and formation of the Monterey Canyon, the dominant feature of the Monterey Bay. The third study presented here uses the differences in lead and neodymium isotopic values in CCM Fe-Mn crusts over time compared to open-ocean Pacific, North Pacific, and Arctic Ocean Fe-Mn crusts to identify regional time-series trends and sources for these important oceanographic tracers. I found that sediment fluxes and inputs of terrestrial material from North American rivers effects the lead and neodymium isotope composition of regional seawater.