Universita di Milano, Italy

Periodic dissolution horizons signifying abrupt shoaling of the lysocline and CCD are characteristic features of deep-sea sections. These horizons are often attributed to Milankovitch forcing via their diagnostic frequencies. Prominent dissolution horizons also correspond to abrupt climate events, such as the Paleocene-Eocene thermal maximum, as a result of input of significant CH (sub 4) -CO (sub 2) into the ocean-atmosphere system. The question arises whether other significant dissolution horizons were formed as a result of greenhouse gas input, or whether they were related to cumulative effects of periodic changes in ocean chemistry and circulation. Here we showcase ongoing investigation of a widespread deep-sea dissolution horizon that occurred in the early late Paleocene at 58.4 Ma and corresponds to an important biotic event. The event was first discovered at ODP sites on Shatsky Rise in the western Pacific, but has subsequently been found at sites in the Atlantic. A prominent, 5-25 cm thick, clay-rich ooze was found at four Shatsky sites with a water depth range of 500 m. The layer contains crystals of phillipsite, fish teeth, and phosphatic micronodules suggesting slow sedimentation or intervals of seafloor exposure. Planktonic foraminifers in the clay-rich layer are characterized by a low diversity, largely dissolved assemblage, dominated by the genus Igorina. Even though microfossil assemblages are altered by dissolution, they appear to record a significant environmental perturbation in surface waters. Nannoplankton assemblage studies across the clay reveal a sharp increase in Ericsonia, a genus that is most abundant in low-latitude, warm-water sites. Surface-water warming is supported by a decrease in oxygen isotope values of planktonic foraminifers. We are currently evaluating mechanisms for the widespread change in deep-water chemistry, its connection to the surface-water response, and the relationship of the event with theoretical orbital solutions. Regardless of its cause, the early late Paleocene event had significant evolutionary implications coinciding with the first occurrences of Heliolithus kleinpellii and primitive discoasters, both of which are important, and often dominant, components of late Paleocene and younger nannoplankton assemblages.