Paleoclimate reconstructions from speleothems from the De Hoop Nature Reserve, South Africa.

1. Rationale and objectives

The southern coast of South Africa is a key region to study the broader context of human evolution. In order to better understand the links between climate and human evolution we need to reconstruct past climate at high resolution near archeological sites. Speleothems (e.g. stalagmites) formed in caves are excellent archives recording past climate information such as temperature, hydroclimate (i.e. precipitation) and even vegetation.
Our research is tightly linked to Blombos cave and Klipdrift Shelter archeological records with the aim to provide environmental context during periods of significant cognitive changes (e.g. Henshilwood et al., 2011). We hence seek to expand the climate record further back to cover the full time interval of cave occupations at Blombos cave and Klipdrift shelter.

Previous and ongoing work
As part as of the SapienCE research centre, we are analyzing speleothems collected in 2020 in Bloukrantz cave (Fig. 1; 34°27.557’S, 20°46.697’E) in the De Hoop Nature Reserve (SAHRA permit ID 3125). These speleothem samples have proven extremely valuable not only because of the multitude of methods that can be applied, and hence the results that can be extracted (Maccali et al., 2023), but also because of the climate results themselves. The results from our pilot study, covering a short time window between 48 and 45 thousand years ago (ka), displayed a series of short dry events with no associated changes in temperature. Subsequent analyses on the three speleothem samples collected in 2020 in Bloukrantz cave allowed to expand the climate record from 90 to 45 ka (Fig. 2), and we are finding similar dry events in part of the record. We are still in the process of analyzing these speleothems for temperature reconstruction. Furthermore, we aim to (i) assess the spatial representativeness of our findings from Bloukrantz Cave by performing similar reconstructions in another, nearby cave and (ii) connect our climate reconstructions to reconstructions of ecosystem structure and landscape evolution.
Speleothems from other De Hoop caves

West cave (34°27.049’S, 20°41.883’E) and Morris cave (34°27.061’S, 20°42.016’E) in the De Hoop Nature Reserve were explored by Jane Noah during her master’s project (Wits University, 2011) and she reported the presence of speleothems in both caves. In West Cave, speleothems are furthermore witness to changes in landscape surrounding the cave; there, horizontal layers of carbonates have formed so-called “false floors” (Fig. 3) during times when the cave was filled with sand, which has later been eroded, leaving the carbonate layers behind. Dating both the speleothems and the false floors is therefore an elegant way to connect the paleoclimate reconstructions with the evolution of the surrounding landscape, both of key importance to the early humans inhabiting the area.
In order to find speleothems that cover a suitable time interval while avoiding surplus sampling and unnecessary impact on the cave environment, we drilled thin cores at the base and top of stalagmites in both West and Morris caves to assess their age. These samples will be used to select suitable speleothems for collection (as per Cape Nature permit No CN32-28-28178). When drilling was not possible, fragments of speleothem (including false floors) were carefully chipped off.

Biomarker reconstructions

Small amounts of organic matter are preserved in speleothems (Blythe et al., 2016). This organic matter originates either from the overlying environment (soil and vegetation) or from in situ microbial communities living in the cave. Some of the organic molecules found in speleothems can serve as proxies (biomarkers) to reconstruct the past ecosystem around and within the cave and will thus allow to greatly expand our environmental reconstructions. In order to do so, we will first analyze recent soils from within and outside the caves in order to understand which molecules can be used as indicators for the current fynbos and cave ecosystems. We will then analyze different types of recently formed calcite (formed on top of stalagmites, on the cave floor, or stalactites forming at the cave ceiling) before applying the approach to the old speleothems. For the speleothems, advances in instrumentation have allowed to overcome previous challenges posed by the low amount of organic matter preserved in speleothems. We will map the different biomarkers present in speleothems and assess their origin; in-situ vs inherited from the overlying vegetation.

Flowstones from archeological sites
Two flowstones were found in the stratigraphic sequence during excavation at Blombos cave. Flowstones can be formed in situ by degassing of drip-water and they can be dated by U-series. Dating these flowstones could provide an extra chronostratigraphic tie point as material above will be younger while material below will be older than the flowstone age.
The extremely high detrital content of some flowstones poses methodological challenges when applying Uranium-series dating. This is due to detrital contamination with non-authigenic 230Th which makes the age appear too old. Dating ‘dirty calcite’ can be achieved using regression technique (isochron plotting) where coeval subsamples with different degree of contamination are analyzed in order to calculate a ‘contamination-free’ end-member. We will attempt to date the two flowstones collected at Blombos cave.
2. Methods

Speleothems can be accurately and precisely dated by the U-Th radiometric technique that relies on the measurement of both the parent (Uranium) and daughter (Thorium) in samples. The method is destructive as the samples need to be dissolved in acid medium before being analyzed. U-Th dating requires radioactive material and trained staff to handle them. The samples will be prepared in the Uranium laboratory at the University of Bergen and analyzed in the ICP laboratory with a Nu™ Plasma MC-ICP-MS.
Because the age of speleothems is impossible to assess when in the cave, our approach is to take reconnaissance samples from tops and bottoms of promising speleothems in the cave, minimizing the amount of material taken out. These samples are then analyzed in Bergen and allow deciding for the best candidate speleothems for the paleoclimate research. A total of 15 speleothems have hence been subsampled to determine the age period of their growth.

Soil (and speleothem) samples (<50g each) will be prepared for biomarker analyses and analyzed at the Department of Earth Science, University of Bergen, using Gas Chromatography-Mass Spectrometry following established acid digestion and solvent extraction procedures (Blythe et al. 2016).

Both methods are destructive.

3. Duration and expected results

Results are expected within a year. Based on the age ranges found, speleothem(s) will be selected for collection for further paleoclimate investigations in the coming years.

4. People involved in the research

Prof. Simon Armitage (Royal Holloway University of London)
Dr. Jenny Maccali (University of Bergen)
Prof. Nele Meckler (University of Bergen)
Dr. Samuel Pereira (University of Bergen)
Ass. Prof. Eoghan Reeves (University of Bergen)
Dr. Karen van Niekerk (University of Bergen)
Dr. Ella Walsh (University of Bergen)