Increasing seasonal upwelling in the subtropical South Atlantic over the past 700,000 yrs: Evidence from deep-living planktonic foraminifera

Abstract

The distribution and character of deep-living planktonic foraminifera (such as Globorotalia truncatulinoides and G. hirsuta) are particularly sensitive to the structure of the upper ocean because of the wide range of water depths over which their life cycles extend. Like other planktonic foraminifera, the deep-living species begin growing in shallow surface waters, but unlike the other species they continue growing while sinking into much deeper water. Their reproduction is synchronized with the seasonal deep-mixing and upwelling of nutrient-rich waters that precedes spring bloom. Where deep-mixing is curtailed the abundances of the deeper-dwelling species ( G. hirsuta), the deeper-dwelling varieties (e.g., left-coiling G. truncatulinoides), and the larger (presumably faster-sinking) shells are reduced (leading to an apparent acceleration in shell development). These changes are observed today in transects from the seasonally deep-mixed subtropical gyres to the seasonally-stable surface waters equatorward of the subtropical convergences. Past changes in the abundance and character of these deep-living planktonic foraminifera have been measured in a Late Pleistocene record from the subtropical western South Atlantic Ocean. Over the past 700 kyrs, abundances of these species have increased, G. truncatulinoides has changed from the predominantly shallower-dwelling right-coiling to the deeper-dwelling left-coiling variety, and the accelerated shell development characteristic of shallow mixing has shifted toward the rates normally associated with deep mixing. All this indicates that the structure of the upper ocean in the subtropical South Atlantic before 700 kyrs ago was more like that nearer the equator today where mixing is limited to the upper 100–200 m. Since that time the scale of mixing has gradually increased to more than 400 m. This implies a gradual increase through the Late Pleistocene in the seasonal upwelling of nutrients in the subtropical South Atlantic and, consequently, in the intensity of the seasonal maximum in organic productivity.