Dating Speleothems: Palaeoclimate, Palaeoenvironment and Evolutionary Anthropology
- Speleothem records have been used by palaeoarchaeologists and palaeoanthropologists to provide climatic and environmental contexts for human evolution events such as morphological change and taxa disperal, to situate undated fossils in a geochronological time frame and to associate climatic episodes with cultural change.
- Three synopses of research outlining ways in which speleothems can be used in human evolutionary studies are provided below:
1. De Ruiter et al. (2009) provide a context for seven fossils from the site of Cooper’s D (South Africa) by examining uranium-lead (U-Pb) ages for speleothems associated with Australopithecus robustus. They place the fossils between 1,5 and 1,5 Ma. Although Au. robustus is well represented in African fossil-bearing sites, the geochronological age of the specimens has been unclear for those that derive from cave infills. Direct dating of cave infills in South Africa remains a significant problem for palaeoanthropologists because unlike in East Africa, there are no volcanic ash layers for K-Ar or Ar/Ar dating.
Dating has often been based on faunal assemblages. Unfortunately, these dates have large error margins and rely on the selection of an appropriate nuclide uptake model that also leaves samples more susceptible to diagenisis. Moreover, biostratigraphic dating rests on assumptions of continent-wide and contemporaneous evolutionary events and is further limited by problems regarding refugia and late surviving populations. For this reason, among others, speleothems provide more constraining dates as they record palaeomagnetic signals and can be compared to global magnetostratigraphy. However, only relative dates can be garnered from this method and direct dating of the fossils themselves is often impossible.
New methods have been developed for dating fossil-bearing deposits. “[C]osmogenic nuclides of 10Be and 26Al trapped in quartz grains record the time of burial of the crystals and can thus be used to date clastic cave sediments… the in situ decay of 238U trapped in speleothem calcite to 206Pb can be used to date the formation of these layers…and where flowstone layers are intercalated with fossil bearing clastic sediments, provide age brackets for the faunal material” (de Ruiter et al. 2009:502). Unfortunately, neither of these methods are without their problems. They have not produced complementary ages for a single deposit. Speleothems are not always sufficiently closely associated with fossil bearing sediments. Furthermore, there are issues with U-Pb dating of carbonates.
Despite these problems, the Cooper’s D West eastern and western extent deposits were sampled for U-Pb dating. They are part of a well-preserved stratigraphy of basal stalagmites overlain by clastic sediments with at least a single major flowstone layer. In order to obtain accurate dates, the concentration and distribution of uranium in the samples was ascertained. The U-Pb dates for the flowstones and the detailed stratigraphy together suggest a series of events that lead to the formation, infilling and erosion of the cave sediments. Before the opening of the cave to external sediments and fossils, drip water formed the stalagmites and flowstones. Thereafter aridity stopped speleothems growth and the surface began to weather creating fissures in the cave roof. Through these fissures sediments and fossils were introduced to the eastern and western sides of the cave.
It was determined that the Cooper’s deposits formed quickly during brief periods when the cave was open to the external environment. This occurs in other regions that have been dated more precisely and so the authors believe “that a specific set of environment conditions must have prevailed in order for clastic sediment deposition to occur” (de Ruiter et al. 2009:513). From these data, hypotheses concerning a more stable environment in a proposed South Africa refugium can be tested and would allow researchers to examine environmental factors behind hominin evolution.
2. Early anatomically modern humans (AMH) evolved in Africa ~200-150 ka and are believed to have left the continent in several waves, the first of which occurred 130-100 ka in a movement north toward the Levant. Researchers have debated the climatic conditions that prevailed during this migration event. Palaeoanthropological and archaeological evidence from northern Israel suggest that a migratory event occurred 130-110 ky. Vaks et al. (2007) dated speleothems from the central and southern Negev (Israel) at the margin of the Saharan-Arabian Desert using high precision multicollector-inductively coupled plasma-mass spectrometry U-Th methods.
Migration routes from Africa to early AMH archaeological sites in the Levant converge in the arid to hyper-arid Negev, Sinai and southern Jordan deserts. Vaks and colleagues found in their study of speleothems that formed from above ground wadi channels, that wet episodes in the location under discussion were brief but significant. Brief episodes of humidity occurred in the Negev 140-110 ka and are synchronous with increased monsoonal intensity in the southern Arabian Peninsula and Sahara. The retreat of the desert barrier between the Levant and central Africa, most notably in the Sinai-Negev land bridge, may have formed a climate-induced window for AMH to disperse from Africa.
Pollen records corroborate the speleothem data by providing evidence that a significant vegetation cover developed in this region at the time of the dispersal. Such conditions would have provided a habitable environment for early modern humans across the desert belt and would also have facilitated expansion from the Nile Delta. Supporting this hypothesis are early AMH remains dating to this period that have been found at the Israeli sites of Qafzeh and Skhul.
3. Bar-Matthews and colleagues (2010) analysed the first climate record from the south coast of South Africa that is sufficiently continuous and which has a high enough resolve to compare archaeological evidence of AMH behavioural modernity with periods of climatic and environmental fluctuation. The authors found correlations with short and rapid climatic events that weren’t recorded by lake cores in Africa. Their analysis provides evidence that during a particularly dynamic period of AMH cultural evolution there also occurred a rapidly fluctuating environment.
South Africa is the location of much early human archaeology and palaeoanthropology. Unfortunately, long and continuous climate and environmental records have not been available for this region. Bar-Matthews et al. (2010) endeavoured on a multi-proxy study 90-50 ka, which includes a significant portion of the Interglacial to glacial transition, in order to develop a record to compare the local environmental and climatic history with modern human behaviour.
The research location, at Crevice Cave, Pinnacle Point, is a particularly useful place because links with Antarctic ice cores and deep sea cores can be examined as can comparisons with regional climate, environment and biodiversity in relation to sea surface temperatures. Pinnacle Point is also situated at an intersection of rainfall and vegetation systems that differ in isotopic character which is ideal for studying temporal changes in rainful and vegetation as speleothems record such isotopic shifts.
Bar-Matthews et al. (2010) sought to compare modern human technological advancement in order to identify if climatic and environmental contexts are correlated with behavioural modernity. By noting that failure to modify technology in the face of climate change has been suggested to reflect non-modernity, they look at the Still Bay and Howieson’s Poort assemblages, which are believed to be early evidence of modernity and correlate them with climatic and environmental change. These assemblages provide evidence of the adoption of fine bone tool manufacture, refined bifaces, backed blades, bladelets, laceolates and pigments—usually ochre and sometimes engraved. Both the Still Bay and Howieson’s Poort assemblages record a change in raw material procurement choices and a shift to a preference for fine-grained materials such as silcrete that some suggest is evidence for modern exchange systems.
The researchers noted several coincidences between the archaeological record and climate and environment change as recorded by the speleothems they analysed. At Pinnacle Point site 5-6, during the span of Still Bay (~72-71 ka) and Howieson’s Poort (~65-60 ka) there is evidence for a shift from quartzite to silcrete raw material that corresponds with a shift from winter rain-C3 grass to stronger summer rain-C4 grasses. From ~72-71 ka they document short and abrupt climate change that correlates with an anomaly in the EDML ice core.
There is climatic and environmental instability from ~72 to 63 ka, a period in which the Still Bay is found and in which Howieson’s Poort begins. They found what they refer to as an intriguing record that documents a high amplitude climatic and environmental event ~72 ka. “The short span of the Still Bay and its overlap with a high amplitude environmental change suggests a rapid technological innovation in response to a punctuated shift in climate and environment, a response highly consistent with the abilities of behaviorally modern humans” (Bar-Matthews et al. 2010:2143).
Selected references:
- Bar-Matthews, M., Marean, C.W., Jacobs, Z., Karkanas, P., Fisher, E.C., Herries, A.I.R., Brown, K., Williams, H.M., Bernatchez, J., Ayalon, A. and Nilssen, P.J. 2010. ”A high resolution and continuous isotopic speleothem record of paleoclimate and paleoenvironment from 90 to 53 ka from Pinnacle Point on the south coast of South Africa,” Quaternary Science Reviews 29(17/18): 2131-2145.
- Frumkin, A., Bar-Yosef, O. and Scharcz, H.P. 2011. “Possible paleohydrologic and paleoclimatic effects on hominin migration and occupation of the Levantine Middle Paleolithic,” Journal of Human Evolution 60: 437-451.
- Vaks, A., Bar-Matthews, M., Mattews, A., Halicz, L. and Frumkin, A. 2007. “Desert speleothems reveal climatic window for African exodus of early modern humans,” Geology 35(9): 831-834.
- Woodhead, J. and Pickering, R. 2012. “Beyond 500 ka: Progress and prospects in the U-Pb chronology of speleothems, and their application to studies in palaeoclimate, human evolution, biodiversity and tectonics,” Chemical Geology 322/323(5): 290-299.
- Top and third images: Woodhead and Pickering, 2012.
- Centre image: Bar-Matthews et al., 2010.
- Bottom image: Maslin and Trauth, 2009.