Abstract
Within the framework of the RAIN project (Regional Archives for Integrated iNvestigations) which deals with interdisciplinary investigations of climate evolution in southern Africa, the coastal lake Groenvlei was investigated. The lake is located on the southern Cape coast within the year-round rainfall zone and is separated from the Indian Ocean by a 1.6 km wide dune cordon. Groenvlei is mainly fed by rainfall, runoff, and ground water and at present has no aboveground connection to the sea.
In order to reconstruct palaeoenvironmental changes around Groenvlei, a short sediment core (121 cm length) was recovered from the central part of the lake and investigated using geochemical (ICP-OES, XRF), mineralogical (XRD) and granulometric analyses. Moreover, micropalaeontological investigations, i.e., on distribution of ostracod and diatom associations were carried out.
The sediments consist mainly of carbonates and to a lesser degree of allochthonous dune material, especially quartz sand. Several sediment layers are characterized by the deposition of calcareous Chara fragments. Radiocarbon dating has revealed an age of 4,200 cal BP for the entire record with no age reversals, indicating a continuous sediment accumulation with an average rate of 0.3 mm.yr−1. A semi-quantitative evaluation of the XRD measurements shows a marked change in the mineralogical composition. Prior to 2,000 cal BP sediments are dominated by aragonite and additionally by the presence of dolomite. Sediments deposited thereafter mainly consist of Mg-calcites. This is supported by similar shifts in the chemical composition shown by the Mg and Sr data. The ability of Sr to be incorporated in the crystal lattice is much higher for aragonite than for calcite. The high Mg concentrations in the lower core section are related to the diagenetic formation of dolomite. Aragonite is the main calcium carbonate phase which is precipitated at high Mg concentrations and thus, high water salinity. In contrast, lower salinity probably led to the precipitation of calcite. Hence, the Groenvlei record enables the reconstruction of palaeosalinity and provides evidence for a distinct change from high to low lake water salinity at 2,000 cal BP. This is also reflected by a change from an ostracod association dominated by brackish water taxa to a halotolerant freshwater association and perishing marine-–brackish taxa at this time.
In order to reconstruct palaeoenvironmental changes around Groenvlei, a short sediment core (121 cm length) was recovered from the central part of the lake and investigated using geochemical (ICP-OES, XRF), mineralogical (XRD) and granulometric analyses. Moreover, micropalaeontological investigations, i.e., on distribution of ostracod and diatom associations were carried out.
The sediments consist mainly of carbonates and to a lesser degree of allochthonous dune material, especially quartz sand. Several sediment layers are characterized by the deposition of calcareous Chara fragments. Radiocarbon dating has revealed an age of 4,200 cal BP for the entire record with no age reversals, indicating a continuous sediment accumulation with an average rate of 0.3 mm.yr−1. A semi-quantitative evaluation of the XRD measurements shows a marked change in the mineralogical composition. Prior to 2,000 cal BP sediments are dominated by aragonite and additionally by the presence of dolomite. Sediments deposited thereafter mainly consist of Mg-calcites. This is supported by similar shifts in the chemical composition shown by the Mg and Sr data. The ability of Sr to be incorporated in the crystal lattice is much higher for aragonite than for calcite. The high Mg concentrations in the lower core section are related to the diagenetic formation of dolomite. Aragonite is the main calcium carbonate phase which is precipitated at high Mg concentrations and thus, high water salinity. In contrast, lower salinity probably led to the precipitation of calcite. Hence, the Groenvlei record enables the reconstruction of palaeosalinity and provides evidence for a distinct change from high to low lake water salinity at 2,000 cal BP. This is also reflected by a change from an ostracod association dominated by brackish water taxa to a halotolerant freshwater association and perishing marine-–brackish taxa at this time.
| Original language | English |
|---|---|
| Pages | 182-183 |
| Number of pages | 2 |
| DOIs | |
| Publication status | Published - Jun 2016 |
| Externally published | Yes |
| Event | The African Quaternary: environments, ecology and humans Inaugural AFQUA conference - Cape Town, South Africa Duration: 30 Jan 2015 → 7 Feb 2015 Conference number: 1 |
Conference
| Conference | The African Quaternary: environments, ecology and humans Inaugural AFQUA conference |
|---|---|
| Abbreviated title | AfQUA |
| Country/Territory | South Africa |
| City | Cape Town |
| Period | 30/01/15 → 7/02/15 |
