High-resolution palaeoceanography of extreme early Pliocene carbonate-opal cycles in the eastern equatorial Pacific

Although the modern eastern equatorial Pacific Ocean is a micronutrient-limited HNLC region, late Neogene sediments recovered from ODP Leg 138 indicate the occurrence of past high-productivity periods between the mid-Miocene and early Pliocene. These are represented by the presence of well-laminated diatom oozes (<75% opal) comprised of monospecific (95-100% Thalassiothrix longissma) and mixed diatom assemblage laminations interspersed with intervals of foram-nannofossil ooze. The monospecific laminations have been interpreted to represent episodic rapid deposition of large diatom mats from surface waters, associated with enhanced activity of tropical instability waves along the equatorial front. This study focuses on the youngest, early Pliocene laminated diatom mat interval present at ODP Site 847 and chronologically identical interval at Site 851, in which these diatom mats are absent. The aims of the project were to provide a timescale for the early Pliocene laminated diatom mat intervals, re-evaluate sedimentation rate and biogenic silica flux estimates; investigate surface hydrographic changes, organic carbon export and calcium carbonate dissolution associated with diatom mat deposition. Stable oxygen and carbon isotope records determined from benthic foraminifera were used together with existing wet bulk density records for Sites 847 and 851 to establish the lines with Site 846, which has an established time stratigraphic framework, and thus determined a new timescale for Sites 847 and 851. The onset of the early Pliocene laminated diatom mat interval was dated at 4.69 Ma and its end at 4.66 Ma. Within this interval, the average sedimentation rate during diatom mat deposition was 234 m/Ma, compared to 35 and 29 m/Ma during 'normal' sedimentation before and after the diatom mat deposits at Site 847. Diatom mat deposition resulted in average biogenic silica fluxes of 3.50-5.20 x 10-5 molSi/cm2/yr compared to fluxes of 3.53x4.13 x 10-6 molSi/cm2/yr during 'normal' sedimentation.