The relative contributions of weathering and aeolian inputs to postglacial formation of Mediterranean alpine loess

1 Between the southern margin of the European loess belt and Sahara Desert, thin and irregularly 2 distributed loess deposits occur in Mediterranean mountains. During the most recent deglaciation, along 3 the Pleistocene-Holocene boundary, the deposition of glacial, periglacial and outwash sediments, was 4 the main local source of Mediterranean alpine loess, whereas proximal alluvial planes comprised a 5 secondary source. The mid-Holocene termination of African Humid Period and subsequent aridification 6 of Sahara Desert occurred simultaneously with a change of the regional climate from Atlantic to 7 Mediterranean-dominated, characterized by frequent episodes of southerly winds. This resulted to a 8 change of the loess source, as deflation of quartz rich silts enriched in Zr during intense episodes of 9 Sahara dust transport became more dominant. Here, a 32cm loess profile from the Plateau of Muses 10 (PM), below the summit of Mount Olympus, Greece, is investigated on the basis of grain size, 11 mineralogy, environmental magnetism and geochemistry. Comparisons of loess samples with glacial 12 and periglacial deposits, enables us to differentiate relative contributions of local sources and 13 allochthonous aeolian inputs. Calcite sand rich in feldspars makes up the glacial and periglacial clast 14 free matrix. In contrast, PM loess is composed by clay and fine silt fractions with minor calcite sand 15 contributions. The mineralogical matrix of loess contains quartz, phyllosilicates and mixed layer clays, 16 while its geochemical composition contains high amounts of detrital Fe-Ti oxides and aeolian 17 transported Al and Zr. Based on the multi-proxy approach applied here, the loess profile is partitioned 18 in three layers. Holocene average deposition rates (~2.5 cm/ka) broadly agree with modern Sahara dust 19 deposition (~2.0 cm/ka) and long-term postglacial Mediterranean mountain denudation rates (~0.5 20 cm/ka). Such low rates provided ample time for post depositional modifications, such as decalcification, 21 deferrification and removal of K, evident from the trends of chemical weathering proxies Ca/Sr, Fe/Ti 22 and K/Rb, respectively. 23 27 28


INTRODUCTION 29
The most recent deglaciation of the Mediterranean mountains between 12 and 9.5 ka BP resulted 30 to deposition of large sequences of glacial, periglacial and outwash sediments that were mainly confined 31 in the highest valleys of the massifs (Hughes and Woodward, 2016;Oliva et al., 2018;Allard et al., 32 2020). Antecedent to glacial retreat was the deposition of loess and subsequent formation of alpine soils 33 on moraines, plateaus and outwash plains (e.g. Muhs 2007). Synergistic to the in-situ genesis of alpine 34 soils, is the deposition windblown dust, which results to the formation of alpine loess soils (Muhs and  determines the rate of geomorphic processes, such as landscape denudation. Furthermore, the study of 39 deflated sediments within alpine soils and loess can provide insights on the local and regional 40 atmospheric circulation patterns, reflected by the depositional dynamics of aeolian dust (e.g. Muhs et 41 al., 2007). 42 In the Mediterranean region, the formation of loess is influenced to a large extent by its proximity to 43 Sahara Desert (Pye, 1995 Mount Olympus is the highest mountain of Greece, rising 2918 meters above the Aegean Sea (Fig. 1). 74 In the lower part of the mountain, Mediterranean type climate prevails with wet winters and generally 75 dry summers. Wet winters are linked to cyclogenesis in the Aegean Sea basin that results from enhanced 76 mid-latitude westerlies ( Fig. 1 pattern B) and the influence of Atlantic climate (Xoplaki et al., 2000). 77 This pattern was dominant during the first part of the Holocene (Peyron et al., 2017). Dry winters are 78 associated with outbreaks of northerly continental cold and dry airflows ( Fig. 1 pattern B) funneling 79 through the large fluvial valleys exiting on the Aegean Sea (Rohling et al., 2002), which are connected 80 to the presence of high-pressure systems over the northern Balkans and/or Siberia (e.g. Xoplaki et al., 2000;Bartzokas et al., 2003). This pattern was persistent throughout the Holocene, when short periods 82 of cold and dry winters linked to the intensification of Siberian High (e.g. Rohling et al., 2002;Marino 83 et al., 2009) and resulted in Mediterranean rainfall minima associated Sahara dust transport episodes 84 (Zielhofer et al., 2017a). The transport of Sahara dust in the North Aegean occurs today under strong 85 southerly (Sirocco) winds ( Fig. 1 pattern C) during the winter and spring (Nastos, 2012), but there is 86 lack of evidence of how the southerly winds outbreaks evolved during the Holocene. However, the study 87 of Mediterranean alpine loess archives, where the Sahara dust signal is not blurred by erosion, reworking 88 and pedogenesis, can provide valuable information on the tempo of southerly warm and moist wind 89 outbreaks and their impacts on different ecosystems.

Glacial erosion 98
The geologic structure of Mount Olympus involves a stratigraphic upwards sequence of Triassic, and 99 Lower Cretaceous to Eocene metacarbonates, uplifted since the late Miocene along a major NW -SE 100 trending frontal fault ( Fig. 2A) (Nance, 2010). During uplift, deposition of erosional products along the 101 eastern (marine) and the western (continental) piedmonts occurred ( Fig. 2A). Their Quaternary counterparts 102 include thick sequences of glaciofluvial and alluvial fan deposits with intercalated soils, exposed along the 103 main river valleys and the frontal fault scarp (Fig. 3 in Smith et al., 2006). During the 104 Last Glacial Maximum (LGM), between 28 and 24 ka BP (Allard et al., 2020), an ice cap covered Mount 105 Olympus' highest cirques and upland plateaus extending to elevations of ~ 2000m (Kuhlemann et al., 106 2008). The post LGM retreat was followed by a Late Glacial (LG) glacier expansion at ~15 ka BP that 107 was confined in the highest cirques at elevations above 2200 m (Styllas et al., 2018).  (Fig 2E). The Plateau of Muses extents 0.8 km 2 and is covered by unconsolidated 143 glacial and periglacial sediments. Periglacial features such as solifluction beds are present below the exposed 144 bedrock of the surrounding peaks, while patterned grounds exist along its topographically lower surface 145 (Styllas et al., 2018). These features are tentatively considered to have formed during cold intervals over the 146 last ~12 ka BP, following the deglaciation of TZ cirque, but may be still active today as the permafrost 147 elevation of the region is placed at 2700 m (Dobiński, 2005). The formation of PM is the result of the 148 combined action of glacial scouring and karstic dissolution. The low relief in combination to the elliptical 149 to circular plan shape of the plateau, points to a doline type karstic depression filled with glacial and 150 periglacial sediments with a thickness between 4 to 10m (unpublished data from geophysical survey). The 151 surface layer (> 35cm) of PM sedimentary sequence is composed by a red to yellow homogenous fine-152 grained accumulation, with its basal part composed by glacial and/or snowmelt outwash limestone sand and 153 gravel, mixed with silty sediments (Fig. 2E, Fig. 3). 154 155 Fig. 3. Pictures of the PM 32cm soil loess profile with the respective discrete samples taken every 2cm.

HYPOTHESIS AND STUDY DESIGN 157
Based on the considerations regarding: the onset of deglaciation Mediterranean mountains at ~12 ka BP 158 and the termination of the African Humid Period at ~6 ka BP, this study, based on a suite of analytical 159 methods applied to samples from Mount Olympus, Plateau of Muses loess, is challenging the hypothesis 160 that the evolution of Mediterranean alpine loess, occurred along three distinct phases:  ii) The early to mid-Holocene phase between 10 and 6 ka BP when, under a warming and 166 seasonal Mediterranean climate (Peyron et al., 2017), the formation of loess was mainly 167 sourced by local glacial sediments and expanding alluvial planes in lower elevations. iii) 168 The mid to late Holocene from 6 to 0 ka BP, where following the termination of the African 169 Humid period and desiccation of Sahara Desert, along with a change of regional winter 170 climate from Atlantic to Mediterranean, increasing amounts of Sahara dust reached the 171 Mediterranean mountains during episodes of southerly advection. 172

Grain-size analyses 175
Grain-size analyses were performed on 21 samples. Five samples were retrieved from distinct clast free 176 horizons of the MK and TZ stratified scree deposits and sixteen samples from the PM loess sequence 177 (Fig. 2B). Samples were wet-sieved through a 350 μm sieve and were then analyzed with a Mastersizer 178 3000 laser diffraction particle size analyzer (Department of Earth Science, University of Bergen, 179 Norway), with a sensitivity of 0.01 -350 μm, to define the bulk grain-size distributions (GSD) of the 180 fine sand to clay fractions. GSD statistical analyses were performed with MATLAB Curve Fitting Lab (CFLab), which performs curve fitting on sediment grain size distributions using Weibull Probability 182 Distribution Functions (Wu et al., 2020). 183

Chemical methods (XRF) and mineral analysis (XRD) 185
All samples were analyzed for their bulk mineralogy and geochemistry. The relative elemental 186 composition was determined by X-ray fluorescence using an ITRAX core scanner in the Department of 187 Earth Science of the University of Bergen in Norway. One cubic centimeter of the finer (<350 μm) 188 fraction of the samples was air-dried, filled into sample cups and compacted by hand. Four units of 21 189 sample cups were mounted on sample holders and measurements with the ITRAX XRF core-scanner 190 were performed using a Mo-tube, which can detect a wide range of elements from Al to U (Croudace et 191 al., 2006). Counting time was 10 s and power supply at 30 kV/55 mA. XRF spectra were translated into 192 element counts by mathematical peak fitting using Q-spec software (Croudace et al., 2006). Major and 10 -8 m /kg). During the measuring procedure, every sample was measured at least 3 times and the average 233 value was assigned as a measurement. Two air measurements before and after the samples' measurements were performed. Additionally, frequency dependent susceptibility (χFD%) was calculated according to 235 Dearing et al. (1996) [χFD% = 100(χLF-χHF)/χLF]. 236

RESULTS 238
The PM sediment profile (Fig. 2E)     Quartz grains display a rounded shape and variable grain sizes smaller than 15μm (Fig. 7F), which imply 338 their presence within M1, M2 and M3 grain size modes, respectively. The rounded shape of quartz grains 339 depicted from the SEM images, likely is a product of long-range aeolian transport. PM loess is additionally enriched in Zr and Cr, which appear in negligible quantities in MK and TZ 373 samples (Fig. 8E to H), so their origin from mechanical weathering of bedrock, is implausible. An 374 alternative mechanism for their transport and deposition in PM loess is deflation from distal or local 375 sources, such as the Sahara Desert, the proximal ophiolitic Pieria Mountains and Katerini alluvial plane.  for the upper layers, respectively (Fig. 9D). Due to its sensitivity to super paramagnetic (SP) particles, 392 χfd is often used to identify ultrafine-grained iron oxide formation e.g., magnetite, maghemite, and with an increase in M3 concentration (Fig. 9 D & E) and with a decrease of the K/Rb ratio (Fig. 9F),

Local weathering 409
The low correlation between M5 grain size and the Ca XRF counts among all samples (r = 0.45, p < 410 0.05, n = 21), contrasts the notion that the coarse rich sands, are only produced by physical weathering 411 of bedrock carbonate formations. The low correlation can be attributed to dissolution kinetics and 412 leaching of Ca during disintegration of carbonate bedrock to gravel and sand. Within PM loess sequence, 413 the positive correlation between M1 and M2 concentrations with M5 grain-size (r = 0.67, p < 0.05), 414 suggests that the production of coarser sandy debris is associated with higher concentrations of fine 415 particles. A physical mechanism that can explain this statistical relationship is the isovolumetric 416 replacement of Ca-rich sand to clay, as proposed by Merino  Within PM loess profile, the weight percent (wt%) concentration of mica and Zr XRF counts, display 466 high correlations (r > 0.70, p < 0.003) with M3 concentration. This relation argues that additionally to 467 mica (muscovite) presence in bedrock formations (TZ and MK samples) depicted from XRD spectra, 468 micaceous silt grains are also transported during Sahara dust episodes. The range of the M3 mean grain 469 size ranges between 14 and 28μm and is similar to modern values Sahara dust modal and median grain 470 sizes from Crete (Fig. 1), which range between 8 and 30 μm (Goudie and Middleton, 2001) and 4 to 16 471 μm (Mattson and Niéhlen, 1996), respectively. Thus, it is reasonable to support that M3 can be 472 considered a representative grain size mode of Sahara dust contribution to PM loess. 473 474 However, rounded quartz grains occur in a variety of grain sizes from 2 to 15 μm (Fig. 7F), which is 475 also supported by the correlation between the sum of M1, M2 and M3 modal concentrations with quartz 476 wt% (r = 0.74, p < 0.001). This suggests that transport of Sahara dust to Mount Olympus includes finer 477 particles in clayey silt range, assuming that all aeolian quartz comes from Sahara region. Since quartz is 478 traced in minor quantities in MK and TZ samples (Fig. 5A), the conclusion that the finer modes M1 and 479 M2 contain aeolian components, either from Sahara, or from local sources (Pieria Mountains and 480 Katerini alluvial plane), is valid, but the exact origin of quartz cannot be defined from the existing 481 analyses. Therefore, synergistic to the weathering of Mount Olympus carbonates and deposition of 482 detrital components with subsequent post depositional production of fine particles and aggregates rich 483 in Fe-Ti oxides, is the deposition of fine dust incorporated into M1 and M2. Background dust with grain 484 size similar to M1 and M2 (~3μm), is found in many European loess sequences and represents local, by the fact that decalcification of PM loess, largely occurs within the finer fractions, with subsequent 520 replacement of calcite to the formation of clay particles and mixed aggregates found in SEM images 521 (Fig. 7). 522 The observed Rb enrichment in PM, compared to MK and TZ samples (Fig. 6D), results from the 523 weathering of K-bearing minerals, such as mica (e.g. Anderson et al., 2000;Hošek et al., 2015;Zech et 524 al., 2008). In the previous section, it is argued that in addition to its detrital origin, mica is an inherent 525 component of Sahara dust transport to Mount Olympus and is identified in small concentrations (~6%) 526 in PM loess. The loss of mica to smectite cannot be quantified, but it appears that after its initial 527 deposition, mica is subjected to post depositional weathering with removal of K (Buggle et al., 2011, 528 Bosq et al., 2020. This is supported by the low values of K/Rb elemental ratio (Fig. 9F), used on many 529 occasions to describe the weathering intensity and removal of K from loess deposits (Profe et al., 2016). conditions under the cirque headwalls are low due to extensive snow cover, slope steepness, aspect, and 535 high production rates of coarse carbonate debris that enhances percolation of snowmelt. Translocation 536 of clay particles in the coarse matrix of glacial till and stratified scree deposits may also be responsible 537 for the minor contents of fine particles, but the assessment of these factors is beyond the scope of this 538 study. 539 540

Relative chronology of PM loess 541
The main step in establishing the relative chronology of PM loess deposition is to constrain the transition 542 period between the upper and lower layers from 14 to 16 cm of profile depth that partition several 543 sedimentological and geochemical changes. The 15% increase of M3 concentration along the transition 544 layer (Fig. 4C), suggests a growth in Sahara dust availability that can be associated with the 545 midHolocene termination of the African Humid Period (AHP; 10-6 ka BP) and the regional climatic The curve similarity of the three profiles shown in Fig. 10, tentatively confirms the previous 573 consideration that the transition period between the lower and upper layers of PM loess broadly coincides with the termination of African Humid Period at ~6 ka BP. A subsequent peak in Sahara dust transport 575 around 4.5 ka BP marks the upper boundary of this transition layer. Of particular interest is the temporal 576 constrain of the profile base with the relative date of sample PM3 placed ~10 ka BP. This implies that 577 the calcite rich samples PM1 and PM2 were deposited during the initial stages of Mount Olympus 578 deglaciation phase, between 12 and 10 ka BP, in agreement with the stabilization of moraines in TZ 579 cirque ( Fig. 2A). During this phase, influx of meltwater from the retreating cirque glaciers provided 580 aggressive solutions that were reacting with the carbonate bedrock dissolving it in high rate.  westerlies. A mid-Holocene swift in the regional climate from Atlantic to Mediterranean type with drier 625 conditions and more frequent periods of Scirocco winds coincided with the termination of AHP and 626 increased deflation of Sahara dust grains from the desiccated areas. This regional climatic shift resulted 627 in prominent increases in the aeolian silt deposition (increase of M3 concentrations) and Zr/Al ratio 628 between 6 and 4.5 ka BP, with a concomitant decrease in the pedogenic modification of the deposited 629 dust and decreasing clay particle formation (decrease of M1 and M2 concentrations). Contrary to the 630 enhancement of Sahara dust transport on Mount Olympus since 6 ka BP, is the decrease of local dust 631 from the Pieria mountaintops and Katerini plain, as shown by the correlation of clay and fine silt with 632 Cr and Ni. The associated decrease of clay concentration with the heavy elements, can result either from 633 decreases either in summer convection and/or to northern winds outbreaks. 634 635 PM loess is decalcified and subjected to secondary syn or post depositional chemical weathering, which 636 include removal of Ca and K respectively. The upwards decreasing trends of Ca/Sr and K/Rb imply that 637 the elemental modification of PM loess has been gradual and independent of the aeolian deposition and regional climatic dynamics. The secondary mineralogical modification may be responsible for the high 639 amounts of smectite and kaolinite observed in the clay fraction, through weathering of mica to smectite 640 and plagioclase to kaolinite, but further conclusions on these processes cannot be achieved through the 641 analyses presented here. In addition, during deposition of the upper PM loess layer (6 -0 ka BP), wetter 642 than present summer conditions likely resulted in waterlogging and subsequent dissolution of Fe from 643 the Fe-Ti oxides (deferrification) and to pedogenic depletion of the magnetic signal. 644 645 Overall, the mechanisms responsible for the formation of PM loess are complex and involve several 646 convoluting processes, such as mechanical weathering of the glacial carbonate debris, chemical 647 dissolution of the weathered products, syn and post depositional alteration and formation of aggregates, 648 pedogenetic modification and aeolian dust deposition from local and regional sources. In the absence of 649 continuous reconstructions from Mediterranean alpine settings, future analyses of alpine loess deposits 650 in the sub cm scale can provide a powerful tool to study the local weathering dynamics and regional 651 atmospheric circulation patterns, focusing on periods of Sahara dust events and enhanced Sirocco winds 652 throughout the Holocene. 653 654 655