 |
| Titel |
Have abrupt climate variations of the last glacial possibly been muted in the south-west African tropics by counteracting mechanisms? |
| VerfasserIn |
I. Hessler, L. Dupont, D. Handiani, S. Steinke, J. Groeneveld, U. Merkel, A. Paul |
| Konferenz |
EGU General Assembly 2012
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250065506
|
|
|
|
|
|
| Zusammenfassung |
| The last glacial period including the last deglaciation (73.5-10 ka BP) is characterised by
abrupt shifts between extreme climatic conditions. Millennial-scale climate variations
associated with North Atlantic Heinrich Stadials (HSs) are thought to be transmitted by
both the atmospheric and oceanic circulation resulting in a near-global footprint. It
is further thought that HSs are closely related to a reduction or shut-down of the
Atlantic Meridional Overturning Circulation, which, according to the bipolar-seesaw
hypothesis, leads to the accumulation of heat in the South Atlantic. In addition, it is
hypothesised that HSs result in a southward shift of the Intertropical Convergence
Zone which then would likely influence the vegetation composition in the African
tropics.
To investigate the impact of HSs on the terrestrial African realm and the south-east
Atlantic we reconstructed the vegetation development in Angola and the southern Congo
Basin as well as the sea surface temperatures (SST) of the south-east Atlantic using marine
sediments of ODP Site 1078 (11Ë 55’S, 13Ë 24’E, 427 m water depth).
Two species of planktonic foraminifera were selected to reconstruct variations in surface
water conditions in the south-east Atlantic. Due the ecological and seasonal preferences of
Globigerinoides ruber (pink) this species provides a good tool to estimate SST variations
during the southern hemisphere summer. In contrast, Globigerina bulloides is representing
the Benguela Upwelling System during the southern hemisphere winter. While Mg/Ca-based
SSTs of G. ruber (pink) were significantly higher by 1Ë -2Ë C during periods of
abrupt climate change, the impact of HSs during southern hemisphere winter is less
obvious.
However, although there are several vegetation records that show an impact of HSs in the
African tropics, our high-resolution pollen record from ODP Site 1078 reflects no vegetation
changes during periods of HSs. Model simulations conducted with an Earth System Model of
Intermediate Complexity (EMIC) provide one plausible explanation for the absence of HSs in
our vegetation record. Although both precipitation and evaporation are higher during HSs,
their contrasting nature leads to a net-freshwater flux of about zero. Consequently, the
resulting climatic response to HS might have been simply to weak in south-west Africa to
affect the vegetation composition in a distinct way. The reliability of the EMIC simulations is
supported by experiments conducted with the Community Climate System Model Version 3
(CCSM3) which is including a comprehensive atmospheric and land model component. |
|
|
|
|
|
|