Bone diagenesis at the Florisbad spring site, Free State Province (South Africa): Implications for the taphonomy of the Middle and Late Pleistocene faunal assemblages
Résumé
TheMiddle to Late Pleistocene spring site of Florisbad, South Africa, is the namesite of the Florisian Land Mammal
Age (LMA), and it has produced a large collection of fossil bones fromdifferent sedimentary contexts. The largest
of these faunal assemblages derives fromvertically intrusive dormant spring vents and is taphonomically distinct
from specimens associated with a Middle Stone Age occupational horizon. The Florisbad faunal assemblages include
several extinct and extant animal species, mainly grazing ungulates that represent a highly productive
open grassland environment. The Florisian LMA was characterized by substantially wetter conditions compared
to the Holocene. Geological evidence at Florisbad and the characteristic occurrence of water-dependent species,
such as lechwe, waterbuck, hippopotamus and a variety ofwater birds, reflect the presence of a vast intermittent
paleolake forming during thewet phases of the Middle and Late Pleistocene. During dry phases, the area of spring
activity was not inundated and provided a suitable habitat for both humans and animals. Bones collected from
different sedimentary contexts exhibit a number of post-depositional alterations that reflect specific diagenetic
processes, which led to fossilization. Two contrasting hypotheses regarding these processes, based on morphological
studies of bones and groundwater chemistry, were proposed in the past. The first identified spring
water as a major fossilization agent, whereas the second saw it as a medium conducive to bone dissolution.
Using Fourier Transform infrared spectrometry (FTIR) and micro-spectrometry (μFTIR), we determined the
mineralogical composition of these bones. We show here that specimens from inactive paleo-spring vents are
characterized by the formation of carbonate fluorapatite on the outer surface of cortical bone, and locally exhibit
authigenic calcite crystals, as opposed to bones collected from non-spring contexts at the site. The nucleation of
carbonate fluorapatite is linked to the high fluoride content of the alkaline spring water, thus confirming its key
role in the fossilization process of bones recovered from paleo-spring vents. Therefore, our results support the
first of the two hypotheses advanced in the past.