Upper Tagus loess formation and the marine atmosphere off the Iberian margin 
23 
Reconstruction of environmental moisture based on soil 
forming intensities, n-alkane ratios and stable carbon 
isotopes 
As discussed above, periods characterized by loess deposition 
fepresent mainly arıd environmental conditions (e.g., Sima 
et al., 2009). However, different levels of aridity can be 
deduced based on information from various proxies. In gene- 
ral, lower aridity indicated by lower 8°C wax values correlates 
well with higher pedogenic formation of superparamagnetic 
particles (Fig. 9), which is likewise a hint to more moisture 
availability. The formation of SU-4 during middle MIS 5 is 
related to moderate aridity as indicated by lower 813Cyrax values 
and processes linked to surface runoff that require a certain 
amount of precipitation. Soil formation until the onset of 
MIS 4 is evidenced by field results, micromorphological indi- 
cations (Table 1), and environmental magnetic measurements 
‘Figs. 2, 9), and points to more humid conditions that were 
in line with higher North Atlantic SSTs and still-high percent- 
ages of Mediterranean forest over Iberia during GI-23 and 
GI-21 (Sänchez Gofi et al., 2008; Fig. 14). Even if no com- 
plete decalcification and mineral weathering took place in 
ihis soil, features of carbonate dissolution indicate infiltration 
of surface waters (Fig. 11). Considering the time of exposure 
of 10 ka to 20 ka together with only minor pedological alter- 
ation, the paleosol indicates prolonged surface stability at a 
somewhat reduced environmental humidity. 
During MIS 4, another dry spell initiated loess formation, 
with 8BC wax values still indicating a moderate level of arid- 
ity. This arid phase is in agreement with the coldest SSTs 
off the Iberian margin (Pailler and Bard, 2002; Sänchez 
Goni et al., 2008; Fig. 14), although a number of studies in 
inland Iberia likewise identified MIS 4 as a cold but less 
arid period. Lewis et al. (2009) reported a maximum of gla- 
cial and fluvial activity in the south-central Pyrenees coincid- 
ing with MIS 4 and HS®6 instead of the global LGM, and 
expected less arid conditions during MIS 4 as compared to 
MIS 2. Jimenez-Sänchez et al. (2013) described increased 
glacier growth that led to a local LGM due to cold but less 
arid conditions between 70 ka and 50 ka in the Pyrenees. Bur- 
jachs and Juliä (1994) likewise reported cold but slightly wet- 
ier conditions during MIS 4 in NE Spain based on pollen 
information from the Abric Romani sequence. 
The subsequent period of surface stability between 59.7 + 
4.7 and 43.0 + 3.8 ka (PS-2 coinciding with GI-12 to GI-17) 
was linked to weak pedogenic processes in terms of carbonate 
depletion and calcite accumulation. An increase in the mag- 
netic parameters is far less pronounced but still apparent 
«Fig. 9). Thus, climatic conditions during lower MIS 3 
prevented loess formation and provided enough moisture 
for initial soil-forming processes. This interpretation is 
consistent with higher North Atlantic SSTs and higher per- 
centages of Mediterranean forest over Iberia (Fig. 14). 
The loess deposition phase between 42.5 + 3.6 and 41.3 + 
4.0 ka (SU-6) came along with the hitherto most arid condi- 
tions during the last glacial period based on stable carbon- 
isotope results (Figs. 9, 14). 81°C ax increased to values 
higher than -30 %o, exceeding the values of all previous 
phases and crossing a threshold line that we defined for the 
.nhabitability of interior Iberia. This threshold line is based 
on the finding that phases with 8 Cwax values above the 
line correspond to lacking occupation by hominines, with 
neither Neanderthals nor anatomically modern humans 
‘AMMH) occupying the area (Fig. 14; see Wolf et al., 2018). 
As soon as 8°C wax dropped below the threshold line after 
HS3, the interior of Iberia again revealed traces of human set- 
Jement (Fig. 14). Accordingly, we assume that starting with 
loess deposition during GS-12, GS-11, or GS-10, a climatic 
deterioration affected central Iberia in terms of unprecedented 
aridity that strongly limited the availability of key resources 
for human occupation, such as game, plant foods, and 
wood. A rough calculation of sedimentation rates is based 
on OSL age estimates, and shows a strong increase compared 
:O previous phases of loess formation. 
During GS-5/HS3, the absolute maximum of aridity is 
.ndicated by the highest 5'°C,., values and the highest sed- 
‚mentation rates (Fig. 14). Strongly arid conditions in central 
[beria already prevailed prior to HS3, e.g., during HS4 as sug- 
zested by model simulations by Sepulchre et al. (2007) or 
Jetween 40 and 34 ka as documented by Sala et al. (2020), 
5ased on faunal remains in a cave site. Considering the 
short time span for the formation of SU-7, the detected red- 
dish layers that represent palaeo surfaces (PS-4 and PS-5; 
Figs. 2, 3, and 6) were linked to exposure periods of not 
nore than a few centuries, which may correspond to the dura- 
jon of Greenland interstadials such as GI-5.1 (30.8—-30.6 ka 
52k, Rasmussen et al., 2014). Micromorphological results 
do not show evidence of soil-forming processes during 
hese short periods. However, 8°C ax within the reddish 
layers shows the lowest values during HS3 that might indicate 
slightly wetter conditions during these potential interstadial 
ıhases. Decreased contents of total iron together with the 
lecrease in magnetic parameters (e.g., S-ratio) and certain 
peaks in sand content (Fig. 9) might indicate changed prove- 
niences during these short and perhaps more stable periods- 
ı question that needs to be addressed in future. 
After the strong decline of aridity in the course of GS-5/ 
HS3, GS-3/HS2 was also linked to less arid conditions as 
ndicated by lower 8'°C ax values and strongly reduced sed- 
mentation rates. No indications of loess formation were 
found for the period of the global LGM (23-19 ka), thus, 
we expect less dry and less cold conditions in central Iberia 
during that time. This is in line with mild North Atlantic 
5STs (Eynaud et al., 2009) and indications of recurrent 
auman occupation due to temperate phases within and around 
he LGM in the northern part of the upper Tagus Basin 
‘Alcaraz-Castafio, 2015; Yravedra et al., 2016; Alcaraz- 
Jastano et al., 2017). 
For the last loess deposition phase contemporaneous with 
GS-2.1a/HS1, we are not able to provide further hydrological 
ınformation because this phase was only dated in one section 
(Fuentiduena; Figs. 2, 3). However, loess formation during 
this period indicates arid conditions corresponding to cold 
North Atlantic SSTs (Bard et al., 2000: Sänchez Gofii 
Downloaded from https://www.cambridge.org/core. IP address: 77.191.167.9, on 05 Feb 2021 at 17:09:02. subject to the Cambridge Core terms of use. available at 
attns’/Aanana cambridage araf/carefterms https’ /Z/idal araf1C 1017/13 20720