Abstract:
Sediments recovered during an Ocean Drilling Program Leg on Shatsky Rise in the northwest Pacific hold clues to a number of abrupt climate events that took place during the Cretaceous and early Paleogene. These transient events caused major upheaval in marine communities and profoundly altered biogeochemical cycling. Shatsky Rise cores contain organic carbon-rich strata deposited during a brief interval of open ocean dysoxia or anoxia in the early Aptian (120 Ma). Analyses of exceptionally preserved organic compounds suggest that bacterial activity helped sequester organic carbon in these strata. Graphic evidence exists in Shatsky cores for the mid-Maastrichtian (ca. 69 Ma) extinction of the inoceramids, a long-ranging, widespread group of bottom-welling clams. This extinction is a global event that was likely related to a profound change in deep ocean circulation. Stratigraphically expanded records of the Cretaceous-Tertiary boundary extinction event (65 Ma) were recovered at four different sites. The cores contain evidence of the response of biogeochemical cycling and the recovery of oceanic plankton in the wake of this catastrophe. A new biotic event of major evolutionary significance was found in the early late Paleocene (ca. 58.4 Ma) associated with a change in deep-water circulation, possibly as a result of a brief pulse of warming. Abundant evidence of the Paleocene-Eocene thermal maximum (PETM; ca. 55 Ma), an abrupt warming event associated with major reorganization of benthic and planktonic communities, was recovered in cores from five sites along a depth transect. PETM warming is thought to have been induced by methane derived from dissociation of methane hydrates. The Shatsky Rise depth transect shows evidence of the predicted response of such methane input: pronounced, short-term shoaling of the lysocline and calcite compensation depth (CCD). Shatsky Rise cores record the response of the tropical Pacific to a rapid cooling event near the Eocene-Oligocene boundary (ca. 33.5 Ma) marking the transition to glacial climates that characterized the remainder of the Cenozoic. This event is reflected by a marked increase in carbonate content of the sediment preserved on Shatsky Rise, which signifies a profound drop in the CCD and markedly changed deep-sea circulation patterns.