Herries_Kromdraai_geo paleomag

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 aim to conduct micromorphological analysis across the site (separate export permit to Italy) 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. Palaeomagnetic analysis, along with other analysis being conducted (uranium series dating, cosmogenic nuclide burial dating) will provide the first comprehensive age for the southern wall deposits and Type specimen. In addition we will use the same blocks for the palaeomagnetic analysis to conduct thermal transferred optically stimulated luminescence and electron spin resonance resonance dating on quartz from within the blocks. It is hoped this will provide a validation test against the cosmogenic nuclide burial dating on quartz, which has produced dates that are often not in tandem with other dating methods in the region. If successful then a targeted dating program for these methods will be undertaken.

In addition to a focus on the classic southern wall deposits at Kromdraai, including the Holotype Block, we have also taken samples from more recently exposed northern deposits. This includes an extensive series of siltstone and sandstone deposits on the eastern side of the site and at a similar level to the Holotype Block. This also includes the newly discovered Units O and P from the centre of the site, including a large in situ stalagmite that may have formed on Unit O and been covered by Unit P. Again, all the above stated methods will be conducted on these deposits and blocks to establish the nature of the stratigraphic sequence and its age. In particular we aim to establish the age of the stalagmite itself as this is a key marked in the sequence as well as establish the relationship between calcified deposits under and overlying the stalagmite and decalcified bone bearing breccia that fills much of the centre of the site. Furthermore, we want to establish the age and relationship of a series of speleothems that are associated with a heavily calcified breccia deposit that has yielded a new hominin cranium KW11000.


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 a fourth to a hybrid alternating field followed by thermal demagnetisation. The fourth 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 and another part will be sued to undertake ESR and TT-OSL.
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TT-OSL: Sample blocks will be crushed and the quartz grains will be extracted from the internal part of the block samples that have not been exposed to light. These will then be etched in HF acid to remove the external layers. Measurements will be made using the single-aliquot regenerative-dose (SAR) procedures.

ESR (quartz): the same quartz prepared for TT-OSL analysis will then be used for ESR analysis. Quartz grains of the 100- to 200-μm size fraction will be dated using the multiple center approach and the multiple aliquot additive dose protocol. The ESR signals of both the Al and Ti centres will be measured.

Damage/destructive analysis? (if yes, explain in detail) destructive. Part of the palaeomagnetic sediment blocks will be crushed up to extract quartz for the electron spin resonance and thermal transferred optically stimulated luminescence 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 electron spin resonance and thermal transferred optically stimulated luminescence analysis on quartz do not exist in South Africa. Moreover, a cryogenic magnetometer for the measurement of weak speleothem samples does also not exist in South Africa.