Source and characterization of the silt of Gran Dolina site (Sierra de Atapuerca, Spain)

Abstract

The Gran Dolina site is situated in the south of the Sierra de Atapuerca in north-central Spain. It is a cavity infilled by 25 m of Early and Middle Pleistocene sediments which have different fossil-rich layers, including a new hominin species, Homo antecessor (Carbonell et al. 1999). The sequence has been divided into 12 lithostratigraphic units, named form bottom to top a TD1 to TD11 (Trinchera Dolina), including the TD8-9 unit (Pérez-González et al. 2001; Campaña et al. 2017). Terra rossa of the slope of the Sierra de Atapuerca was postulated as the source of the sediments inside of Gran Dolina (Pérez-González et al. 2001). This kind of soil can be formed by differences processes as weathering of limestone or aeolian inputs (Yaalon 1997; Delgado et al. 2003; Berger et al. 2008). The aim of this work is to analyze and identify the sources of silt sediment that are found inside of Gran Dolina cave. 87 samples were taken from TD1 to TD11 lithostratigraphic units of Gran Dolina and terra rossa. The particle size separation was carried out first using a sieve to remove the greater than 63 µm fraction, and then settling times using Stoke's law was used to extract the silt fraction (2 – 63 µm). Granulometry of this fraction was analyzed using Laser diffraction Coulter LS13 320 with an ALM module. Each particle size distribution was fitted with Weibull distribution and normal distribution to identify the contribution of each mode (Sun et al. 2002). The most of the particle size distribution of silt of Gran Dolina sediment can be explain by two components: one distribution with mode about 42 µm and a second distribution with mode about 10 µm. A third component of 5 µm is observed in few samples of TD10 unit. Normal distribution seems to be more accuracy, especially in TD1 sediments, but Weibull distribution has best fitting in terra rossa samples. Weibull distribution gives more importance to the biggest distribution (about 42 µm) meanwhile normal distribution gives more significance to the smallest distribution (about 10 µm).

This bimodal distribution means that two sedimentary processes worked to accumulate this sediment in the terra rossa of the Sierra de Atapuerca. These sources can be aeolian transport, as suggested the biggest distribution (Vandenberghe 2013; Sun et al. 2004; Kovács 2008), fluvial transport as indicated by clay fraction presence (Vandenberghe 2013; Kovács 2008) and weathering detritus from limestone (Delgado et al. 2003).