Laminated diatom ooze (LDO) has been recovered from several ODP Leg 138 sites and now is also recognized from several DSDP Leg 85 sites. These remarkable sediments are the result of massive and episodic flux of mats of the diatom Thalassiothrix longissima. By analogy with the Rhizosolenia diatom mat-forming events monitored by JGOFS (Joint Global Ocean Flux Study) in the equatorial Pacific Ocean during cooling conditions in late 1992, these episodes of massive flux of T. Iongissima mats may represent the 'fall out' from major frontal systems generated during La Nina (anti-El Nino) events. Laminations were preserved in the mat deposits because of the rapid mat deposition and high strength of the diatom mat meshwork, that subjugated benthic activity. This new mechanism of preservation of lamination in marine sediments has wide implications for other laminated sequences. The sustained periods of mat deposition documented in Neogene sediments of the eastern equatorial Pacific Ocean are part of the major cycles in the relative abundance of carbonate and silica in the region and, possibly, in the case of some intervals, also in the Atlantic Ocean.