## 2 Regional Setting and Archaeological Sequence  
The Sierra de Atapuerca is a gentle mountain range located in the north-central Iberian Peninsula, within the Duero Basin and between the Ubiquitin and Demanda mountains, to the north and south, respectively. From a geological point of view, it is a NNW-SSE overturned anticlinal ridge mostly made of Late Cretaceous carbonates. During the Quaternary, and in response to the evolution of the local phreatic condition, a multi-level endokarst system developed in these Cretaceous limestones [8]. The archaeological sites of the \"Trinchera del Ferrocarril\" (railroad trench) were exposed during the construction of a railway trench running southeast to northwest through the Atapuerca Mountain Range at the beginning of the 20 th century. Our study is centred in the Gran Dolina cave (Figure 1), which contains one of the most complete Quaternary stratigraphic sequences with human fossils in Spain. The sedimentary infill is 19 m thick and has traditionally been divided in 11 lithostragraphic units, TD1-TD11 from the base to the top [9; 10]. The formation of the karst and the sedimentary infill respond to the progressive incision of the main river, Arlanzon, starting at least around 1.4 Ma [11]. Shortly after the Jaramillo Subchron (1.07-0.99 Ma), the sedimentary input in the cave is mostly due to gravitational-alluvial sediments [12; 13; 14; 15]. The TD10.1 unit represents the last use of the cave as a central campsite before its final collapse [16]. The chronology of the sedimentary infill is due to paleomagnetism, ESR, U-series, and OSL ([17; 18; 19; 20] and references therein). For this study, it is important to highlight the  
Figure 1: Location of the archaeological site in the Sierra de Atapuerca. (**a**) Geomorphological map of the Sierra de Atapuerca, also respective to location in Spain. (**b**) Inner corner of the highest units of this cave, from which we have taken samples of TD9–TD10 units. (**c**) Front view of the studied profile analysed, from units TD6 to TD8.9. The photographs (**b,c**) were taken in summer 2021.  
presence of the Matuyama-Brunhes boundary in the upper part of the TD7 unit, and hence, the Lower-Upper Pleistocene limit. Therefore, units TD1 to TD7 belong to the Lower Pleistocene, whereas TD8 to TD11 belong to the Middle Pleistocene. In this paper, we will focus on the entrance facies of the cave--in particular, the interval between the upper part of TD6 and TD10 units. TD6, the oldest stratigraphic unit with hominid fossils in Gran Dolina [21], was deposited during a warm-temperate period, as revealed by the amphibian and squamate reptile assemblages [22; 23; 24; 25]. The passage to the overlying unit TD7 represents an abrupt climatic change as evidenced in our previous study [7]. TD8 is characterized by at least four units that overall show a decrease in grain size [13]. From an environmental point of view, the presence of thermophilous amphibian and squamates indicates relatively humid-temperate conditions and open environment with permanent water streams. A transition from dry open woodlands to a more open and humid environment has been suggested in the TD8.9 unit [26]. The TD9 unit is described as a phosphatic layer, sterile and composed of bat guano [13]. It has a lenticular shape with its maximum thickness of around one meter to the SE. TD10 unit is the last occupation level from Gran Dolina, dated as Marine Isotope Stage (MIS) 11-9 from geochronological studies [17; 27; 28; 29]. This unit is composed of four subunits (TD10.4 to TD10.1, from base to top) dated between 450 ka and 250 ka [27; 28; 29] where we find a succession of strata, 3 m thick, associated with breccias and fluvial facies. An environmental change has been detected within TD10 unit [30; 31], from the relatively closed-in cavity of TD10.4 to open cave of TD10.1, where we have high density of lithic and faunal remains with little evidences of secondary access and modification by carnivores.