Response of the North Atlantic Ocean to climatic variation in the mid to late Pliocene
The response of the northeast Atlantic ocean to developing Late Pliocene glaciation has been investigated using a nearly complete hydraulically piston cored Pliocene section from the Deep Sea Drilling Project (DSDP), Site 548 of leg 80. Changing surface ocean conditions, responding to climatic forcing, are interpreted from planktonic foraminiferal assemblages in 158 samples. The North Atlantic ocean underwent a series of climatic transitions during the Pliocene, culminating in the initiation of glaciation at about 2.5 Myr ago. Evidence indicates that oceanic variability began as early as 3.4 Myr ago with considerable variation in the abundances of the foraminiferal species Globorotaloides puncticulata (tropical to subtropical species) and a Neoqloboquadrina acostaensis morphotype (A-P intergrade, a temperate to subpolar species). This may reflect an early variable displacement of North Atlantic current systems and water masses. Between 3.4 and 3.1 Myr, a marked, but temporary, cooling event is indicated by the complete disappearence of G. puncticulata which is replaced by colder water forms, indicating a possible southward shift of water masses and current systems. This interval may represent the initial growth of a Northern Hemisphere ice mass, implied by a positive shift in benthic oxygen isotopic records. This isotopic shift, however, occurs well after surface conditions had cooled, indicating that the surface water cooling and benthic isotopic shift are only partially linked. About 3.1 Myr G. puncticulata reappeared, once again dominating the assemblage, indicating a return of warmer water masses to the north. After this, assemblages exhibit greater variability and alternating dominance between G. puncticulata and N. acostaensis. This variation has a main cyclic component between 40,000 to 65,000 yrs: a characteristic high latitude signature, indicating a strong polar influence on conditions in the North Atlantic. Superimposed on this cyclicity is the gradual replacement of warmer water species by colder forms between 2.8 and 2.5 Myr. At 2.5 Myr the onset of the first major Pliocene glaciation is indicated by the strong dominance of cold water foraminifera, decreased species diversity, and a marked increase of ice-rafted materials. By 2.0 Myr conditions in the North Atlantic moderate to interglacial, but are not nearly as warm as in the mid-Pliocene, indicating a major restructuring of North Atlantic hydrography and climatology.