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Providing context to the Homo naledi fossils: Constraints from flowstones on the age of sediment deposits in Rising Star Cave, South Africa
Abstract
Rising Star Cave in the Cradle of Humankind, South Africa, contains one of the richest hominin-bearing deposits in the world, and is the type locality for the Homo naledi fossils. This paper provides a stratigraphic and geochronological framework, within which published and future fossil finds from Rising Star Cave can be placed. Detailed mapping of flowstone horizons combined with new age constraints based on both U-Th disequilibrium and 234U/238U dates and one new OSL date help define seven periods of flowstone formation that punctuate episodes of clastic sedimentation and erosion. Clastic sediments entered the cave through an opening in the roof of the Postbox Chamber from about 600 ka onward, until the opening was choked by coarse breccia blocks, probably sometime after 180 ka. Depositional and erosional events changed the internal morphology of the cave chambers over time, and thereby changed the access route into the Dinaledi Chamber where the bulk of the H. naledi fossils were found.
Periods of pervasive flowstone formation at all levels of the cave occurred at >600 ka, ~500–400 ka, ~225–190 ka and ~110-90 ka. Additional periods of localised flowstone growth restricted to individual chambers (or parts thereof) occurred at ~300 ka, ~160 ka, ~70 ka, ~50 ka, ~30 ka, and ~10 ka. Flowstone horizons bracket sedimentary units that include a variety of sediment types that changed with time. The oldest flowstones overlie lithified mud clast breccias (LMCB), which were partly eroded before they were covered by externally derived laminated orange sands (LOS) and 500–400 ka flowstones. These flowstones and sands were removed between 290 ka and 225 ka, with sediment transported to deeper parts of the cave via erosion channels characterised by massive orange sands (MOS). In this period of time, the access route into the Dinaledi Chamber may have further changed due to the collapse of the Dragon's Back block. Deposition of laminated orange-red mud (LORM) from suspension occurred between 225 and 190 ka and temporally overlaps with widespread flowstone formation around ~225 ka and ~200 ka. The LORM deposits were largely removed from the upper chambers by ~110 ka, before the youngest group of flowstones formed in the cave; some of which are still growing today.
The U-Th ages from Rising Star Cave, combined with other dating constraints reveal age clusters of flowstone formation, which coincided with warmer interglacial or interstadial periods. The patterns recognised in Rising Star Cave overlap with periods of flowstone formation recognised in nearby caves in the Cradle of Humankind, thereby confirming a regional climatic control on flowstone growth in caves during the past 500 ky.
The new ages further constrain the minimum age of H. naledi to ~241 ka. Thus, H. naledi entered the cave between 241 ka and 335 ka, during a glacial period, at which time clastic sediments inside the cave were undergoing erosion. H. naledi would probably have entered the cave through an access point in the roof of the Postbox Chamber and made its way along a SW trending fracture towards the Dragon's Back and Dinaledi Chambers.
Introduction
The Rising Star Cave in the Cradle of Humankind (CoH) world heritage site, South Africa hosts the only known Homo naledi fossils, which are concentrated in two chambers: the Dinaledi and Lesedi Chambers (Fig. 1; Berger et al., 2015; Dirks et al., 2015; Hawks et al., 2017). H. naledi was recognised as a new species of hominin by Berger et al. (2015) and has been dated at 236–335 ka (Dirks et al., 2017), indicating for the first time that early Homo sapiens shared the African landscape with other hominin species well into the late Pleistocene (Berger and Hawks, 2017).
The context of the fossil remains in the Dinaledi Chamber, where >80% of all H. naledi fossils originate from, is unique in comparison to other hominin fossil sites in the CoH because: (1) the fossils are almost exclusively hominin; (2) they occur deep inside the dark zone in an area that is only accessible to hominins via a complex access route; and (3) they were recovered from mostly unlithified mud clast breccia (Dirks et al., 2015; Wiersma et al., 2020). To explain this unique setting, it was suggested that the bodies were deliberately disposed of in the Dinaledi Chamber, although an alternative, unknown death-trap mechanism could not be excluded (Dirks et al., 2015, Dirks et al., 2016; Berger and Hawks, 2017).
Currently, entry to the Dinaledi Chamber is complex (Fig. 1, Fig. 2). Human access is partly impeded by collapsed blocks and erosion remnants of a variety of flowstone and sediment deposits; and it is unclear what the cave may have looked like at the time H. naledi interacted with it. The purpose of this paper is to reconstruct the history of flowstone formation along the access route into the Dinaledi Chamber, focussing on the last 500 ky, i.e. the period of time that overlaps with the age estimates for H. naledi. By reconstructing a chronology for flowstones and associated sediment deposits in the cave, we intend to provide context to how and when H. naledi entered the Dinaledi Chamber.
Makhubela et al. (2019) used cosmogenic Al-Be dating of sediment and outcrops on the surface around Rising Star Cave to calculate an erosion rate of ~5 m/Ma for the land surface above the cave, and up to 13 m/Ma for the rocky escarpment that provides the entrance into the cave. These erosion rates indicate that in the last 500 ky the physical properties of the land surface external to the cave did not change by much beyond the opening of some new entry points, or the blocking of existing entry points as they became choked by sediment fill. What changed more dramatically during this period was the climate, which on the highveld of southern Africa swung sharply between drier glacial periods and wetter inter-glacials (Partridge et al., 1997; Hopley et al., 2007; Caley et al., 2011, Caley et al., 2018; Pickering et al., 2019) On the surface, climate variability affected vegetation patterns, soil thickness and accessibility of caves (e.g. Brain, 1995; Pickering et al., 2007). Inside caves such variability affected sedimentation and flowstone formation patterns, with flowstone formation during wetter periods and sedimentation of clastic sediments during drier periods (e.g. Pickering et al., 2007, Pickering et al., 2011, Pickering et al., 2019). For Rising Star Cave, with an entrance that is positioned close to the valley floor less than 10 m above the nearby Bloubank River, wet periods could have also meant episodic inundation during flood events with associated sediment influx.
Keeping these boundary conditions in mind, we aim to reconstruct the history of flowstone formation, and intervening periods of sedimentation and erosion in the cave. This will be achieved by building a chronostratigraphic framework for each of the connecting cave chambers between surface entry points into the cave and the Dinaledi Chamber. The sediment-flowstone stratigraphy for each of the chambers will be described, and chronological constraints will be presented for U-series dates (i.e. U-Th disequilibrium and 234U/238U dates; Edwards et al., 1987; Hellstrom, 2003, Hellstrom, 2006; Cheng et al., 2016) of flowstones. The U-series dates will be supplemented with additional OSL dating and age constraints available from literature (Dirks et al., 2017; Makhubela, 2019).
Flowstones in caves have long been utilised in an archaeological context to provide ages for interstratified fossils and artefacts (e.g. Blackwell et al., 1983; Marean et al., 2007; Pickering and Kramers, 2010; Pickering et al., 2010, Pickering et al., 2019). So far, dating of material from Rising Star Cave has focussed on obtaining an age for the H. naledi fossils and surrounding sediments and flowstones in the Dinaledi Chamber (Dirks et al., 2017). No stratigraphic or chronological information is available for the rest of the cave except for some cosmogenic Al-Be ages that have limited stratigraphic context, and some experimental (U,Th)-He dates (Makhubela, 2019). With this paper, we will build a chronology that provides a first order stratigraphic reference framework, within which published and future findings from Rising Star Cave can be placed.
Section snippets
Geological setting
The Rising Star Cave is located in the valley of the Bloubank River, ~2 km west of Sterkfontein Cave, and it comprises several kilometres of mapped passageways connecting chambers, some of which contain sediment deposits with fossils (Fig. 1, Fig. 2). The cave system is largely confined to a relatively chert-poor, 15–20 m thick, stromatolitic dolomite interval in the lower parts of the Monte Christo Formation, which dips 15–20° to the west (Fig. 2; Eriksson and Truswell, 1974, Eriksson et al.,
Summary of previous age dating
Dating of cave sediments, flowstones and fossils from Rising Star Cave has focussed on the Dinaledi Chamber and Hill Antechamber with the aim of constraining the age of the H. naledi fossils (Dirks et al., 2017). Dirks et al. (2017) reported 17 U-Th disequilibrium ages for various flowstones in the subsystem (Fig. 4D, Table 1). These ages range from 9.05 ± 0.06 ka to 502 +181/−53 ka and fall into three broad groups. The first is a large group of late flowstones that cover older sediment
Flowstone samples for U-series dating
A total of 29 new flowstone samples (Fig. 5) from all chambers were dated with either U-Th disequilibrium or 234U/238U techniques (Table 1, Table 2). The spatial distribution of samples in the cave from proximal (near surface) to distal (deepest) are as follows (Fig. 4): five samples were dated from the Skylight Chamber (RS47, RS59, RS78, RS79, RS82); one from the Postbox Chamber (RS74); five from the Superman Chamber (RS04, RS28-31); ten from the Dragon's Back Chamber (RS00, RS03, RS34, RS37,
U-Th and 234U/238U ages for flowstones
A total of 29 new flowstone samples (Fig. 5) from five chambers were dated successfully via U-Th geochronology, to provide minimum age estimates for the sedimentary units they overlie. Table 1 summarizes the dating results from each of the flowstone groups and illustrates how they correlate to the flowstones described and dated in the Dinaledi Subsystem (Dirks et al., 2017). Detailed dating results used in age calculations are presented in Table 2. Results are presented on a chamber by chamber
Reliability of age estimates and comparisons with earlier results
The U-Th disequilibrium dating technique is well established and is likely to return highly reproducible (i.e., high precision) results (e.g. Edwards et al., 1987; Hellstrom, 2003, Hellstrom, 2006; Hellstrom and Pickering, 2015; Cheng et al., 2016). Dirks et al. (2017) tested this by analysing duplicate samples from the Dinaledi Chamber in independent laboratories (James Cook University and the University of Melbourne), and found that results were consistent and reproducible. In this study, we
Conclusions
Detailed mapping of flowstone horizons in Rising Star Cave together with 29 new U-series flowstone dates, one new OSL date and previously published ages allow us to define a chronostratigraphic framework for the cave. Seven distinct periods of flowstone formation, which have been colour coded for easy reference, punctuate periods of clastic sedimentation and erosion. Clastic sediments entered the cave through an opening in the roof of the Postbox Chamber from ~600 ka onward, until the opening
Abstract
Rising Star Cave in the Cradle of Humankind, South Africa, contains one of the richest hominin-bearing deposits in the world, and is the type locality for the Homo naledi fossils. This paper provides a stratigraphic and geochronological framework, within which published and future fossil finds from Rising Star Cave can be placed. Detailed mapping of flowstone horizons combined with new age constraints based on both U-Th disequilibrium and 234U/238U dates and one new OSL date help define seven periods of flowstone formation that punctuate episodes of clastic sedimentation and erosion. Clastic sediments entered the cave through an opening in the roof of the Postbox Chamber from about 600 ka onward, until the opening was choked by coarse breccia blocks, probably sometime after 180 ka. Depositional and erosional events changed the internal morphology of the cave chambers over time, and thereby changed the access route into the Dinaledi Chamber where the bulk of the H. naledi fossils were found.
Periods of pervasive flowstone formation at all levels of the cave occurred at >600 ka, ~500–400 ka, ~225–190 ka and ~110-90 ka. Additional periods of localised flowstone growth restricted to individual chambers (or parts thereof) occurred at ~300 ka, ~160 ka, ~70 ka, ~50 ka, ~30 ka, and ~10 ka. Flowstone horizons bracket sedimentary units that include a variety of sediment types that changed with time. The oldest flowstones overlie lithified mud clast breccias (LMCB), which were partly eroded before they were covered by externally derived laminated orange sands (LOS) and 500–400 ka flowstones. These flowstones and sands were removed between 290 ka and 225 ka, with sediment transported to deeper parts of the cave via erosion channels characterised by massive orange sands (MOS). In this period of time, the access route into the Dinaledi Chamber may have further changed due to the collapse of the Dragon's Back block. Deposition of laminated orange-red mud (LORM) from suspension occurred between 225 and 190 ka and temporally overlaps with widespread flowstone formation around ~225 ka and ~200 ka. The LORM deposits were largely removed from the upper chambers by ~110 ka, before the youngest group of flowstones formed in the cave; some of which are still growing today.
The U-Th ages from Rising Star Cave, combined with other dating constraints reveal age clusters of flowstone formation, which coincided with warmer interglacial or interstadial periods. The patterns recognised in Rising Star Cave overlap with periods of flowstone formation recognised in nearby caves in the Cradle of Humankind, thereby confirming a regional climatic control on flowstone growth in caves during the past 500 ky.
The new ages further constrain the minimum age of H. naledi to ~241 ka. Thus, H. naledi entered the cave between 241 ka and 335 ka, during a glacial period, at which time clastic sediments inside the cave were undergoing erosion. H. naledi would probably have entered the cave through an access point in the roof of the Postbox Chamber and made its way along a SW trending fracture towards the Dragon's Back and Dinaledi Chambers.
Introduction
The Rising Star Cave in the Cradle of Humankind (CoH) world heritage site, South Africa hosts the only known Homo naledi fossils, which are concentrated in two chambers: the Dinaledi and Lesedi Chambers (Fig. 1; Berger et al., 2015; Dirks et al., 2015; Hawks et al., 2017). H. naledi was recognised as a new species of hominin by Berger et al. (2015) and has been dated at 236–335 ka (Dirks et al., 2017), indicating for the first time that early Homo sapiens shared the African landscape with other hominin species well into the late Pleistocene (Berger and Hawks, 2017).
The context of the fossil remains in the Dinaledi Chamber, where >80% of all H. naledi fossils originate from, is unique in comparison to other hominin fossil sites in the CoH because: (1) the fossils are almost exclusively hominin; (2) they occur deep inside the dark zone in an area that is only accessible to hominins via a complex access route; and (3) they were recovered from mostly unlithified mud clast breccia (Dirks et al., 2015; Wiersma et al., 2020). To explain this unique setting, it was suggested that the bodies were deliberately disposed of in the Dinaledi Chamber, although an alternative, unknown death-trap mechanism could not be excluded (Dirks et al., 2015, Dirks et al., 2016; Berger and Hawks, 2017).
Currently, entry to the Dinaledi Chamber is complex (Fig. 1, Fig. 2). Human access is partly impeded by collapsed blocks and erosion remnants of a variety of flowstone and sediment deposits; and it is unclear what the cave may have looked like at the time H. naledi interacted with it. The purpose of this paper is to reconstruct the history of flowstone formation along the access route into the Dinaledi Chamber, focussing on the last 500 ky, i.e. the period of time that overlaps with the age estimates for H. naledi. By reconstructing a chronology for flowstones and associated sediment deposits in the cave, we intend to provide context to how and when H. naledi entered the Dinaledi Chamber.
Makhubela et al. (2019) used cosmogenic Al-Be dating of sediment and outcrops on the surface around Rising Star Cave to calculate an erosion rate of ~5 m/Ma for the land surface above the cave, and up to 13 m/Ma for the rocky escarpment that provides the entrance into the cave. These erosion rates indicate that in the last 500 ky the physical properties of the land surface external to the cave did not change by much beyond the opening of some new entry points, or the blocking of existing entry points as they became choked by sediment fill. What changed more dramatically during this period was the climate, which on the highveld of southern Africa swung sharply between drier glacial periods and wetter inter-glacials (Partridge et al., 1997; Hopley et al., 2007; Caley et al., 2011, Caley et al., 2018; Pickering et al., 2019) On the surface, climate variability affected vegetation patterns, soil thickness and accessibility of caves (e.g. Brain, 1995; Pickering et al., 2007). Inside caves such variability affected sedimentation and flowstone formation patterns, with flowstone formation during wetter periods and sedimentation of clastic sediments during drier periods (e.g. Pickering et al., 2007, Pickering et al., 2011, Pickering et al., 2019). For Rising Star Cave, with an entrance that is positioned close to the valley floor less than 10 m above the nearby Bloubank River, wet periods could have also meant episodic inundation during flood events with associated sediment influx.
Keeping these boundary conditions in mind, we aim to reconstruct the history of flowstone formation, and intervening periods of sedimentation and erosion in the cave. This will be achieved by building a chronostratigraphic framework for each of the connecting cave chambers between surface entry points into the cave and the Dinaledi Chamber. The sediment-flowstone stratigraphy for each of the chambers will be described, and chronological constraints will be presented for U-series dates (i.e. U-Th disequilibrium and 234U/238U dates; Edwards et al., 1987; Hellstrom, 2003, Hellstrom, 2006; Cheng et al., 2016) of flowstones. The U-series dates will be supplemented with additional OSL dating and age constraints available from literature (Dirks et al., 2017; Makhubela, 2019).
Flowstones in caves have long been utilised in an archaeological context to provide ages for interstratified fossils and artefacts (e.g. Blackwell et al., 1983; Marean et al., 2007; Pickering and Kramers, 2010; Pickering et al., 2010, Pickering et al., 2019). So far, dating of material from Rising Star Cave has focussed on obtaining an age for the H. naledi fossils and surrounding sediments and flowstones in the Dinaledi Chamber (Dirks et al., 2017). No stratigraphic or chronological information is available for the rest of the cave except for some cosmogenic Al-Be ages that have limited stratigraphic context, and some experimental (U,Th)-He dates (Makhubela, 2019). With this paper, we will build a chronology that provides a first order stratigraphic reference framework, within which published and future findings from Rising Star Cave can be placed.
Section snippets
Geological setting
The Rising Star Cave is located in the valley of the Bloubank River, ~2 km west of Sterkfontein Cave, and it comprises several kilometres of mapped passageways connecting chambers, some of which contain sediment deposits with fossils (Fig. 1, Fig. 2). The cave system is largely confined to a relatively chert-poor, 15–20 m thick, stromatolitic dolomite interval in the lower parts of the Monte Christo Formation, which dips 15–20° to the west (Fig. 2; Eriksson and Truswell, 1974, Eriksson et al.,
Summary of previous age dating
Dating of cave sediments, flowstones and fossils from Rising Star Cave has focussed on the Dinaledi Chamber and Hill Antechamber with the aim of constraining the age of the H. naledi fossils (Dirks et al., 2017). Dirks et al. (2017) reported 17 U-Th disequilibrium ages for various flowstones in the subsystem (Fig. 4D, Table 1). These ages range from 9.05 ± 0.06 ka to 502 +181/−53 ka and fall into three broad groups. The first is a large group of late flowstones that cover older sediment
Flowstone samples for U-series dating
A total of 29 new flowstone samples (Fig. 5) from all chambers were dated with either U-Th disequilibrium or 234U/238U techniques (Table 1, Table 2). The spatial distribution of samples in the cave from proximal (near surface) to distal (deepest) are as follows (Fig. 4): five samples were dated from the Skylight Chamber (RS47, RS59, RS78, RS79, RS82); one from the Postbox Chamber (RS74); five from the Superman Chamber (RS04, RS28-31); ten from the Dragon's Back Chamber (RS00, RS03, RS34, RS37,
U-Th and 234U/238U ages for flowstones
A total of 29 new flowstone samples (Fig. 5) from five chambers were dated successfully via U-Th geochronology, to provide minimum age estimates for the sedimentary units they overlie. Table 1 summarizes the dating results from each of the flowstone groups and illustrates how they correlate to the flowstones described and dated in the Dinaledi Subsystem (Dirks et al., 2017). Detailed dating results used in age calculations are presented in Table 2. Results are presented on a chamber by chamber
Reliability of age estimates and comparisons with earlier results
The U-Th disequilibrium dating technique is well established and is likely to return highly reproducible (i.e., high precision) results (e.g. Edwards et al., 1987; Hellstrom, 2003, Hellstrom, 2006; Hellstrom and Pickering, 2015; Cheng et al., 2016). Dirks et al. (2017) tested this by analysing duplicate samples from the Dinaledi Chamber in independent laboratories (James Cook University and the University of Melbourne), and found that results were consistent and reproducible. In this study, we
Conclusions
Detailed mapping of flowstone horizons in Rising Star Cave together with 29 new U-series flowstone dates, one new OSL date and previously published ages allow us to define a chronostratigraphic framework for the cave. Seven distinct periods of flowstone formation, which have been colour coded for easy reference, punctuate periods of clastic sedimentation and erosion. Clastic sediments entered the cave through an opening in the roof of the Postbox Chamber from ~600 ka onward, until the opening