Tephras T7 and T8 were dated previously to ~13–11 ka based on 14C ages for charcoal obtained from beneath T7 in Sector VII and above T8 in Sector IV (Morwood et al., 2004, 2005; Roberts et al., 2009). Recalibration of these 14C ages using the latest available datasets (Hogg et al., 2013) yields age ranges (95% CI) of 13.4–12.7 and 11.3–10.4 ka for these immediately pre-T7 and post-T8 samples, respectively. Two recently reported 14C age ranges (95% CI) of 12.8–12.7 and 12.9–12.7 ka for charcoal samples from just beneath T7 in Sector XXII (Sutikna et al., 2016) are statistically indistinguishable and consistent with this chronology.
To further refine the terminal Pleistocene and Holocene tephra sequence at Liang Bua, we selected charcoal samples recovered from sediments directly underlying T7 in Sector XVI, from the thin layer of sediments that separate T7 and T8 in Sector XXVI, and from immediately above T8 in Sector XXVI (Fig. 3.4). These samples yielded 14C age ranges (95% CI) of 13.1–12.8 ka beneath T7, 12.0–11.7 ka between T7 and T8, and 12.3–11.9 ka above T8. The latter two ages (both from Sector XXVI) are statistically indistinguishable. Taken together with the two ages from Sector XXII mentioned above, these results constrain the time of deposition of T7 to 13.1–11.7 ka and of T8 to 12.3– 11.7 ka (upper and lower 95% CIs for the oldest and youngest ages, respectively). These are modest, but important, refinements of the previously published constraints of 13.4– 10.2 ka for both tephras (Roberts et al., 2009), halving the time interval during which both tephras were deposited, from ~3 millennia to 1.4 ka or less.
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Previous 14C dating of the Holocene sequence at Liang Bua was carried out primarily in the cave centre (Sectors IV and III), with additional evidence from along the eastern wall (Sector XI) (Roberts et al., 2009). In this study, we explored in more detail the ages of the Holocene deposits near the eastern wall as well as from the cave centre. For the five charcoal samples from Sector XI, four of the ages appear systematically older than those collected from similar depths in Sectors XVI and XXVI (Fig. 3.4). A similar pattern is observed between the southern and northern parts of Sector XVI, and between both parts of Sectors XVI and XXVI. An ANCOVA for these four age/depth datasets results in significant pairwise differences between all but the data for Sector XI and the southern part of Sector XVI (Fig. 3.10). The least-squares mean slopes for the fitted regression lines to the ages for Sectors XI, XVI south, XVI north and XXVI are 5.16, 4.60, 4.08 and 3.53, respectively–that is, the regression slope decreases and the sedimentation rate increases towards the mouth (north) of the cave. These results support the stratigraphic interpretation of these deposits that suggests the sediments above T8, to a depth of ~75 cm, have been deposited on a downward sloping surface (Fig. 3.4), which has provided more space for sediments to accumulate in the more northerly Sectors.
Including Sector IV in an ANCOVA with the data for Sectors near the eastern wall yields similar results to the previous analysis, with significant pairwise differences between Sectors IV and XI and the southern part of XVI (p < 0.001). The fact that the linear relationship between age and depth in Sector IV is not significantly different from that in Sectors XXVI and the northern part of XVI is important (Fig. 3.10). Although only three charcoal samples have been dated from Sector XIX, immediately south of Sector IV, these ages are systematically older than those collected from similar depths in Sectors IV and XVI and they more closely follow the pattern observed in Sector XI.
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Overall, these results suggest that the Holocene sequence is essentially similar from the eastern wall to the cave centre, providing an important framework for reconstructing the depositional history of Liang Bua not only during the Holocene, but also during the Late Pleistocene (Fig. 3.11).
Figure 3.10 Linear regression analysis of the 14C ages by depth for the Holocene and terminal Pleistocene deposits in Sectors aligned south to north as follows: XI (blue), XVI south (orange), XVI south (grey) and XXVI (red). Sector IV (yellow) lies further to the west (Fig. 3.2). ANCOVA results indicate that the regression slopes of Sectors XI and XVI south are significantly different from the others (p < 0.05), essentially tracking the stratigraphic observations that the deposits slope downward towards the north. The regression slopes of Sectors XVI north, XXVI and IV are not significantly different from each other.
0 2,000 4,000 6,000 8,000 10,000 12,000 14,000 0 50 100 150 200 250 300 350 400 450 Med ian C alibr at ed Ag e (yr B P) Depth (cm)
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Figure 3.11 Composite section of the Liang Bua depositional sequence, showing the tephras and other key stratigraphic markers with approximate ages. The modern soil and basal sediments are shaded grey at the top and bottom of the section, respectively. Note that the previous gap between 46 and 20 ka (Fig. 3.1) now contains sediments dated to this interval in Sectors XXIV and XXVII as a result of this study. The depositional age of T8 is now more tightly constrained to ~12 ka, based on new 14C ages for charcoal samples collected from immediately below and above this tephra.
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The general depositional pattern emerging from Liang Bua is sediment deposition followed by erosional processes that have planed off the upper portions of the pre- existing deposits in the more northerly Sectors. As a result, the erosional surfaces consistently slope downwards to the north. The late Holocene – represented by the uppermost 75 cm of deposit – is the only period during which this pattern does not appear to dominate the stratigraphic sequence at Liang Bua. This suggests that, from ~1 ka ago, the previously recurring erosional periods became reduced in frequency and/or magnitude relative to the rate of sediment deposition, such that the most recent deposits at the site more closely preserve their original horizontal placement. Exactly how this pattern relates to the geomorphological evolution of the cave is still poorly understood, but a likely possibility is that the base of the cave also slopes downward to the north.