During the early Pliocene, two major tectonic events triggered a profound
reorganization of ocean and atmospheric circulation in the eastern equatorial
Pacific (EEP), in the Caribbean Sea, and on adjacent land masses: the
progressive closure of the Central American Seaway (CAS) and the uplift of
the Northern Andes. These affected, among other things, the mean latitudinal
position of the Intertropical Convergence Zone (ITCZ). The direction of an
ITCZ shift, however, is still debated, as numeric modeling results and
paleoceanographic data indicate shifts in opposite directions. To provide new
insights into this debate, an independent hydrological record of western
equatorial South America was generated. Vegetation and climate of this area
were reconstructed by pollen analysis of 46 samples from marine sediments of
Ocean Drilling Program (ODP) Hole 1239A from the EEP comprising the interval between 4.7 and 4.2 Ma.
The study site is sensitive to latitudinal ITCZ shifts insofar as a southward
(northward) shift would result in increased (decreased) precipitation over
Ecuador. The presented pollen record comprises representatives from five
ecological groups: lowland rainforest, lower montane forest, upper montane
forest, páramo, and broad range taxa. A broad tropical rainforest
coverage persisted in the study area throughout the early Pliocene, without
significant open vegetation beyond the páramo. Between 4.7 and 4.42 Ma,
humidity increases, reaching its peak around 4.42 Ma and slightly decreasing
again afterwards. The stable, permanently humid conditions are rather in
agreement with paleoceanographic data, indicating a southward shift of the
ITCZ, possibly in response to CAS closure. The presence of páramo
vegetation indicates that the Ecuadorian Andes had already reached
considerable elevation by the early Pliocene. Future studies could extend the
hydrological record of the region further back into the late Miocene to see
if a more profound atmospheric response to tectonic changes occurred earlier.</p