Kromdraai Palaeomag 2025

Kromdraai is the type site of Paranthropus robustus, the first fossil of which was found in 1938 from what is now known as the Holotype Block. The site was more systematically excavated in the 1950s, 19702 and since the 1990s. Despite this, the age of the site remains unknown beyond estimates from biochronology and a preliminary palaeomagnetic study in the early 2000s. This palaeomagnetic study was only conducted on a small number of samples from a restricted area of the site in and around where the type specimen was recovered. Since this time extensive new deposits have been uncovered at Kromdraai, with a complete reanalysis of the stratigraphy of the classic southern wall deposits. A recent analysis of where the original palaeomagnetic samples have come from, along with a preliminary in situ analysis of the stratigraphy indicate that the results and conclusions of this original study maybe flawed. Firstly, it is clear that reversed polarity samples KBM7 and KBM8 come from a speleothem that was heavily eroded and truncated before the deposition of apparently overlying normal polarity sediments (KRM6). It is likely that a significant time gap occurs between these two units. Secondly, an apparent reversal from normal to reversed polarity in sample KBM2 that was used to suggest the occurrence of the end of the Olduvai SubChron at 1.78 Ma is only seen in a single sample from the Type site block area of the site. To confirm the presence of such a reversal more than a single sample is needed. Moreover, an analysis of the sample location suggests that this apparent reversal might be instead related to the core intersecting more than 1 deposit, one of which may actually be an eroded block within the sequence. Such reworked Brecia blocks are commonly seen throughout these deposits and have in the past been misinterpreted as a separate in situ unit.

To fully understand the nature of this reworked material and to confirm recent stratigraphic analysis we conducted combined palaeomagnetic and micromorphological analysis across the site to better characterize the sedimentary sequence and formational history and to make sure that similar misinterpretations of the sequence and palaeomagnetic analysis do not occur (export permit 4405, Feb 2025). The results of this analysis have confirmed that consistent magnetic polarity occurs across the site in a number of areas with at least 3-4 different polarity periods potentially identified. Moreover, micromorphological analysis has indicated the occurrence of major phases of erosion and reworking to explain depositional models at the site. However, to fully understand the extent of these polarity zones throughout the site we have conducted a second sampling of the deposits, the aim being to identify the extent of each polarity zone so that potential reversal boundaries can be identified for dating analysis. This involved denser sampling of units sampled in 2024. This palaeomagnetic analysis, along with other analysis being conducted (uranium series dating, cosmogenic nuclide burial dating) will provide the first comprehensive age for the Kromdraai B site.

Methodology (short):

Palaeomagnetic analysis: the sample blocks will be drilled to produce a minimum of 4 subsamples per block. One will be subjected to alternating field demagnetization, another to thermal demagnetization and the third and fourth to a to different hybrid strategies (alternating field followed by thermal demagnetisation and thermal demagnetisation followed by alternaitng field analysis). Alternating field sample will be subjected to mineral magnetic analysis to help understand what minerals are in the sample and which are holding the magnetic remanence that will be used to define reversed, intermediate or normal polarity and create a magnetostratigraphy for the site. Parts of this same block sample will be used for micromorphology to directly compare to the palaeomagnetism.

Damage/destructive analysis? (if yes, explain in detail) destructive. Part of the palaeomagnetic sediment blocks will be cut for thin section analysis. The other half will be drilled to provide palaeomagnetic samples that will then be heated up to 700oC. If any fossil material is identified during this process then it will be conserved and returned to South Africa but every effort has been made to avoid fossil material. Moreover, the analysis is concentrating on finer-grained sandstone and siltstone units that are fossil poor as this type of material is ideal for palaeomagnetic analysis. To save on destructive sampling all the analysis will be conducted on the same blocks taken for palaeomagnetic analysis.

Statement why this study cannot be done in South Africa: The facilities for undertaking the analysis are not available in South Africa.