TESTATE RHIZOPOD GROWTH AND MINERAL DEPOSITION ON EXPERIMENTAL SUBSTRATES FROM CROSS SEAMOUNT

We present the results of an experiment on Cross Seamount (18-degrees- 40'N, 158-degrees-17'W) in which basalt, ferromanganese-oxide and CaCO 3 substrates were deployed for 19 months. An experimental block design , of identical sets of well-characterized substrates arranged on three distinct panels, was used. Alterations resulting from exposure at 800 m water depth were documented by analytical Scanning Electron Microsc opy. Agglutinated rhizopods and irregularly shaped chambers occupied a n average of 37% of the basalt surfaces, and 13 and 20% of the ferroma nganese-oxide and CaCO3 substrates, respectively. This coverage is hig h, relative to that observed on dredged ferromanganese nodules and cru sts. High coverage can be attributed to the low abundance of other agg lutinated and calcareous foraminifera. Metal-rich deposits composed of Ba-S, Al-Si, Mn and Fe were common on substrate surfaces. Barite (Ba- S) particles, which originated in the water column, were observed atta ched to the substrate by agglutinated foraminiferal tests. Fe-enriched Al-Si deposits were often adhered to the substrate. These deposits ap peared to have been produced by benthic rhizopods, and may persist on the seafloor over time. Fe-coated bacteria-like colonies were numerica lly scarce, but covered large areas. Mn-oxide precipitates, also bacte ria-like, were numerically abundant on ferromanganese-oxide substrates , but never observed on other substrates. The occurrence and morpholog ical features of these Fe- and Mn-oxide coatings reinforce the idea th at bacteria play an important role in the accretion of ferromanganese- oxides on the seafloor.