Growth response of a deep-water ferromanganese crust to evolution of the Neogene Indian Ocean

A deep-water ferromanganese crust from a Central Indian Ocean seamount date d previously by Be-10 and Th-230(excess) was studied for compositional and textural variations that occurred throughout its growth history. The Be-10/ Be-9 dated interval (upper 32 mm) yields an uniform growth rate of 2.8 +/- 0.1 mm/Ma [Frank, M., O'Nions, R.K., 1998. Sources of Pb for Indian Ocean f erromanganese crusts: a record of Himalayan erosion. Earth Planet. Sci. Let t., 158, pp. 121-130.] which gives an extrapolated age of similar to 26 Ma for the base of the crust at 72 mm and is comparable to the maximum age der ived from the Go-model based growth rate estimates. This study shows that F e-Mn oxyhydroxide precipitation did not occur from the time of emplacement of the seamount during the Eocene (similar to 53 Ma) until the late Oligoce ne (similar to 26 Ma). This paucity probably was the result of a nearly ove rlapping palaeo-CCD and palaeo-depth of crust formation, increased early Eo cene productivity, instability and reworking of the surface rocks on the fl anks of the seamount, and lack of oxic deep-water in the nascent Indian Oce an. Crust accretion began (older zone) with the formation of isolated cusps of Fe-Mn oxide during a time of high detritus influx, probably due to the early-Miocene intense erosion associated with maximum exhumation of the Him alayas top. cit.). This cuspate textured zone extends from 72 mm to 42 mm r epresenting the early-Miocene period. Intense polar cooling and increased m ixing of deep and intermediate waters at the close of the Oligocene might h ave led to the increased oxygenation of the bottom-water in the basin. A co nsiderable expansion in the vertical distance between the seafloor depth an d the CCD during the early Miocene in addition to the influx of oxygenated bottom-water likely initiated Fe-Mn crust formation. Pillar structure chara cterises the younger zone, which extends from 40 mm to the surface of the c rust, i.e., similar to 15 Ma to Present. This zone is characterised by > 25 % higher content of oxide-bound elements than in the older zone, possibly c orresponding to further increased oxygenation of bottom-waters, increased s tability of the seamount slope, and gradually reduced input of continental detritus from the erosion of the Himalayas. Middle Miocene Antarctic glacia tion, which peaked similar to 12-13 Ma ago, increased the oxic bottom-water influx to the basin resulting in accretion of the crust with low detritus. Therefore, the younger crust started to accrete in response to a shift in bottom-water circulation towards the contemporary pattern, which produced a uniform growth rate and pillar structure up to the present. (C) 2000 Publi shed by Elsevier Science B.V. All rights reserved.