The Tswaing Impact Crater, South Africa: derivation of a long terrestrial rainfall record for the southern mid-latitudes T.C. PARTRIDGE Climatology Research Group, University of the Witwatersrand, Johannesburg, South Africa
Summary diatom diagram for the Saltpan core
µs cm -1 DIATOM DIATOM ZONES Ka BP µs cm-1 Saltpan EC Reconstructed elecrical conductivity (in µs cm -1 ) for the Saltpan using the CALIBRATE program and data from Gasse et al (1995)
WHAT WERE THE ENVIRONMENTAL CONDITIONS DURING THE EEMIAN AND HOW DID THEY DIFFER FROM WHAT WAS TO COME IN THE HOLOCENE? Very few dated terrestrial records span this period The best is Tswaing (Pretoria Saltpan), whose chronology in this interval relies on tuning to the solar precessional curve. The Tswaing evidence indicates that : the lake shallowed and became more concentrated between 140 and 125 ka, but was still relatively deep the Eemian coincided with a precessional low, during which proxy data suggest that rainfall was 0-10% higher than present (as compared to about 35% higher during the subsequent 5d interval)
` no pollen are preserved, but there is a good diatom flora and the sediments appear to be annually laminated, suggesting that highresolution proxies could be derived the interval apparently coincides with a d 18 O anomaly in lake carbonates of at least 5 the implications of which remain to be determined Micromammalian evidence from Border Cave (eastern hinterland) indicates elevated temperatures, with a southward shift of the Miombo boundary by about 3º and ~100% more rainfall than present. Fauna and pollen from Florisbad in the Free State are dated to ~125 ka and are associated with a Middle Stone Age industry; they point to high local moisture availability and a grassy regional environment.
Estuarine molluscan faunas from the SE (Agulhas) coast indicate significantly elevated inshore SSTs. The Eemian of the mid latitudes of southern Africa was strongly influenced by orbital precession, whereas the Holocene falls within an interval of significantly lower precessional amplitude; frequent medium and small amplitude oscillations during the latter period may relate to fluctuations in solar output or other forcings TCP January 2001
HOW DID DUST PATTERNS FLUCTUATE WITH TIME? DURING WHICH PERIODS WERE THE FOSSIL KALAHARI SAND DUNES ACTIVE AND WHEN WERE THEY STABLE? Intervals of dune mobility, and the corresponding deposition of dust in the oceans surrounding southern Africa, appear to have been short in relation to intervening more humid spells. With the exception of localized remodelling, periods of dune mobility have been restricted to the pre-holocene. The size of the Mega-Kalahari (2.5 x 10 6 km 2 ) implies the occurrence of major intervals of past aridity which extended into areas now covered by equatorial forest; it is unlikely, however, that dunes in any major part of this area were active simultaneously
Dune alignment can be used to reconstruct past wind regimes and patterns of atmospheric circulation: reactivation of dunes in the most arid SW area of the Kalahari would have necessitated a 50 % reduction in rainfall (at present temperatures) or a 20% increase in the time when the wind exceeded the transport threshold. Far greater changes would have been required to restore dune mobility in more humid areas to the north. Based on alignments, the NE Kalahari dunes formed when the semi-permanent South African anticyclone was 2º north of its present position. These dunes were most recently reactivated between 46 41 ka and 26 20 ka, based on OSL dates. Viewed together, the OSL data from the Kalahari indicate the following periods of dune mobility; 16-10 ka 26-20 ka 46-41 ka 119-95 ka Earlier intervals of activity undoubtedly occurred.
It is instructive to plot these intervals on the proxy rainfall record for Tswaing and to match both with the largest Heinrich events in the North Atlantic. The earliest (119 95 ka) corresponds to a major period of desiccation (linked to an insolation low in the precessional curve). [It is interesting to note that yet drier, precessionally correlated episodes appear to have been centred at 175 ka and 198 ka]. After the precessional signal weakened around 50 ka, pronounced intervals of drying at Tswaing coincide with H5 and H4; the former is contained within the 46 41 ka period of dune activity. H2 and H1 are centred in the 26 20 ka and 16 10 ka dune building intervals respectively and are associated with a sustained period of aridity at Tswaing. Of note is the fact that the start of desiccation at Tswaing appears to precede each Heinrich event by several thousand years, implying that these events were not a response to THC changes but to climatic events generated by some other mechanism. TCP January 2001
HOW DID SOUTHERN AFRICA RESPOND TO CHANGES IN THE EXTENT OF THE ANTARTIC ICE-SHEET AND SEA-ICE IN THE SOUTHERN OCEAN? Southern African climates have responded most obviously to: 1. Fluctuations in the extent of the circumpolar vortex 2. Changes in the heat budget of the western Indian Ocean (WIO)
Sea-ice extent controls the position of the Antarctic Convergence, which directly influences a), while also affecting the strength of the South Atlantic tropical easterlies (or trade winds). Strong trades increase Benguela upwelling as well as heat and moisture transfer into the northern hemisphere (enhancing the build-up of northern ice-sheets). An enlarged circumpolar vortex extends (northwards) the influence of the westerlies, simultaneously shifting (and probably enlarging) the zone of Hadley cell subsidence over the western hinterland. Northward movement of the ITCZ diminishes the influence of precessional forcing via the tropical easterlies, which convey moisture from the WIO. Such changes in latitudinal gradients influence the timing and synchronicity of climatic responses as well as the extent of zones of contrasting rainfall seasonality The break in synchronicity between events in the Antarctic and those in the WIO (and the North Atlantic) coincides with the Antarctic Convergence, to the north of which responses are linked to the THC. Phases of warming in Antarctica led those in the WIO (and the northern hemisphere) by between 2 and 5 ka
An antiphase relationship exists between abrupt climatic changes in the northern hemisphere and Antarctica over the past 50 ka and especially during the last deglaciation Reduction of the THC during N. Atlantic cooling, and its cession during strong Heinrich events (especially H2) diminished heat transfer via the Agulhas Current and northwards in the Atlantic, leading to warming in the WIO and the southern Ocean. WIO warming strengthened the NE (winter) monsoon and moderated LGM SST temperature decline in the tropics to 1.7 2.4ºC (cf. declines of 5 6ºC indicated by terrestrial proxies) TCP January 2001
Heat Content (10 22 J) Temperature Anomaly (ºC) Year Time series of yearly ocean heat content for the 0-700m layer From Levitus et al (2005 Science 287(5,461), 2,225-2,229.