Glacial climates in the eastern tropical South Pacific

Abstract

RECENT attempts to explain the glacial aridity at upper elevations in the Galapagos Islands have failed to take into account either concurrent events in neighbouring areas^1–6 or modern causes of climate. Yet, any reconstruction of the glacial climatology of the South Pacific must be consistent with all these data. Recognised glacial features of the region that must be taken into consideration and the empirical evidence which supports them include: increased upwelling along the equatorial divergence (enhanced plankton production⁷, simultaneous build up of guano⁸), upper elevation (650 m) aridity relative to the present in the Galapagos Islands (dated palaeomicrofloras¹); increased precipitation above 2,000 m along the western slopes of the Peruvian Andes (deposits of oxides^9,10, traces of humid cycle stream cutting and scouring of alluvium^9–11, remains of sheet flooding at the base of the mountains^9,10 a disproportionate lowering of the west facing glacial snowline¹², relics of formerly more continuous plant and animal distributions^13,14); continuous aridity throughout the Pleistocene along the Peruvian coast (unbroken sequence of arid cycle processes^9–11); and exposure of large areas of the western Indo–Australian region following the 100–120 m sea level drop¹⁶ (fossil terraces^16,17). It can be deduced from the position of this exposed land that the modern pattern of vernal mixing of water from the eastern and western Pacific could not have taken place¹⁷ and that the eastern Pacific anticyclone did not, therefore, weaken in the glacial summer¹⁷. Thus, the pressure gradients between the two sides of the ocean would have remained fixed throughout the year and the eastern anticyclone and the south-east trade winds would have retained high intensity in both summer and winter.