Soil micromorphogenesis and Early Holocene paleoclimate at the desert margin of Southern Arabia

The Ramlat as-Sab’atayn desert margin near Ma’rib, Yemen, displays well-preserved Early Holocene paleosols that are documented by micromorphological and pedological data. The buried soils, which are represented by Ahb horizons, indicate soil formation mostly before 8.3 cal ka BP. In contrast, sandy cover sediments without signs of pedogenesis appeared between 8.3 and 6.6 cal ka BP due to increasing aridity. Characteristic micromorphological features of the cover sediments are a single grain microstructure, crystallitic b-fabric, predominant occurrence of fresh sideromelane, and remnants of microlayers. Micromorphological pedogenic features in the buried Ah horizons include a subangular blocky microstructure, undifferentiated b-fabric as a result of enrichment of organic matter and decalcification, and the predominant occurrence of completely altered sideromelane. Most of these horizons appeared to be nearly completely decalcified so that in parts a stipple speckled b-fabric and neoformed clay coatings could be detected as a result of stronger weathering and soil formation. Pedogenic data provide important information about Holocene climate fluctuations, including the amount of precipitation, which was calculated on the basis of geochemical data from buried A, AB and B horizons. The buried paleosols represent moist climate conditions with precipitation ranging from 400 to 600 mm a-1.


Introduction
Paleosols at desert margins are unique objects for the investigation of soil formation against the background of climate change.With respect to a pedogenic response to climate change in southern Arabia, two periods are crucial: (i) the Early Holocene, 12.5 to 6.5 cal ka BP with moist climate conditions and strong monsoons and (ii) the Middle Holocene, 6.5 to 3.5 cal ka BP with moderately moist climate conditions and increasing aridity (e.g.Sirocko et al. 1993;Neff et al. 2001;Mayewski et al. 2004;Fleitmann et al. 2003Fleitmann et al. , 2007)).Paleosols at the desert margin of Yemen were found in the Wadi Al-Jubah area, southeast of Ma'rib, dated at 9.5 to 7.5 cal ka BP (Ghaleb 1990;Brinkmann 1996) and 10.5 to 5.5 cal ka BP (Overstreet and Grolier 1988).In the context of archaeopedological research around the Ma'rib Oasis, located at the western margin of the Ramlat as-Sab'atayn desert, buried paleosols were dated to the Early Holocene, in particular between 8.3 and 6.6 cal ka BP (Pietsch et al. 2010;Pietsch and Kühn 2012).Only one less developed soil represented by Bwkb horizons was dated to the Middle Holocene from 5.8 to 5.5 cal ka BP (Kühn et al. 2010).
Within the same climate period Pooid grassland and wood existed between 8 and 6 cal ka BP in the northeastern Ar-Rub' al-Khali desert.Around 4 cal ka BP an open vegetation cover appeared, and concurrently eolian sedimentation increased due to higher aridity in the transition from the Middle to the Early Holocene (Parker et al. 2004).At the southwestern Ramlat as-Sab'atayn desert margin nearby Sirwah, a palaeolake appeared about 9.9 cal ka BP.This lake existed permanently for about 0.5 ka, and occasionally until the upper Middle Holocene (Weiß and Brunner 2010, cf. Figure 2).Lakes in the Yemen Highlands, southwest of Ma'rib and Sirwah, appeared occasionally even until 3.9 to 3.7 cal ka BP (Davies 2006).
The present contribution ties up with results on paleosols and climate change at desert margins by giving micromorphological analyses of well-preserved paleosols occurring in Southern Arabia.Previous studies indicate that the humic paleosols (Ahb horizons) were formed during a wet phase whereas the cover sediments, which are the result of mainly eolian deposition during increasing aridity, were already starting after 8.3 cal ka BP.The pedostratigraphic position, AMS ages of gastropod shells and of humic paleosol material, and the soil properties of the paleosols imply in situ soil formation during the Early Holocene (Pietsch et al 2010;Pietsch and Kühn 2012).

Landscape of the Ma'rib region
Ma'rib has a wide variety of landscapes with different elements such as a Jurassic limestone ridge, Quaternary volcanic fields, recent wadi systems and modern dunes or sand ramps (Kruck et al. 1996; Figure 1).In the volcanic fields of sands, silts and ashes, mainly Regosols and Andosols developed.Regosols and Cambisols with andic properties occur in pediments with colluvially mixed sediments.Fluvisols are formed on wadi terraces and on remains of plains influenced by sheet erosion (Pietsch et al. 2010).
Current vegetation belongs to the "Eastern semidesert and desert" type, with sparsely distributed herbs, grasses, shrubs, succulents and two kinds of Acacia sp.(Al-Hubaishi and Müller-Hohenstein 1984); large areas are free of vegetation.
Near open-water bodies and perennial wadis around the Ma'rib Oasis, shells of Pupoides coenopticus have been common in a more humid climate, as known from Oman (Neubert 1998), and very often they occur in association with Zootecus insularis.Numerous fossil shells of both species were found within sediments and paleosols (Pietsch et al. 2010).Both species tend to live in humid and semi-arid areas, but in the current arid climate of Eastern Yemen these land snails cannot be found.They are absent from wadi courses and from depressions, where fine sediments accumulate and produce small amounts of short-lived vegetation in short rainy seasons.[ KÜHN P. & PIETSCH D. ] 1.2.Modern regional climate and palaeoprecipitation of the study area Ma'rib is situated in the transition zone between the Yemen Highlands and the desert.The region is influenced by the Indian Summer Monsoon (ISM) and the North Westerly circulation in the winter months.The desert margin receives very limited moisture twice a year: in the form of ISM rains, reduced by lee effects caused by the Yemen Highlands, and in the form of minimal precipitation brought by the North Westerly in winter.The present climate in Ma'rib has a mean annual precipitation of < 100 mm, an evaporation of > 1800 mm and mean temperatures of 28 °C (Bruggemann 1997).
Holocene climate proxy data from the southwestern margin of the Ramlat as-Sab'atayn desert are not yet available.Based on the similarity of current regional climates in southern Oman (Kwarteng et al. 2009) and in the Ma'rib region (Bruggemann 1997), one could adapt the results of the ecosystem and climate reconstruction of the 'Wahiba Sand Sea' (IRSL dating, Radies et al. 2005) and Jebel Qara (AMS-14 C dating, Cremaschi and Negrino 2005), both in Oman, and assume for the southwestern desert margin of the Ramlat as-Sab'atayn a palaeoprecipitation of 250 to 500 mm a -1 between 10.6 to 8.4 ka BP.This also fits the calculations based on geochemistry data of the paleosols in the Ma'rib region yielding 400 to 600 mm a -1 (Pietsch and Kühn 2012).
Since it is known that on a local scale physical and chemical properties of soils vary spatially (e.g.Johnson et al. 1990), we decided to take bulk samples from the middle of each horizon/ layer of all profiles rather than to take high-resolution depth increments from one profile.Oriented, air dried soil samples were impregnated with Oldopal P80-21, cut and polished to 60 -90 mm and 26 -48 mm slices following the procedure of Beckmann (1997).Thin sections 30 µm thick were described under a polarizing microscope (Zeiss Imager.A2; Software AxioVision 4.7.2) mainly using the terminology of Stoops (2003).
Bulk samples were air dried and sieved to < 2 mm.The fine earth fraction < 2 mm was used for grain size analyses using a combined sieve and pipette method after removal of organic material by H 2 O 2 (Blume et al. 2000), for lactate extractable P (Rayment and Lyons 2011) measured with an Ionchromatograph (DX 120) as PO 4-, for gasvolumetric determination of CaCO 3 (Blume et al. 2000), and for pH [H 2 0] measurements with a glass electrode (Sentix 81, WTW) using a soil to solution ratio 1:2.5 (Rayment and Lyons 2011).
A portion of each sample < 2 mm was ground with a ball mill and used for determination of total carbon content (C t ) after heat combustion (1150 °C) with an element analyzer (Elementar Vario EL III): C org = C t -C CaCO 3 .The radiocarbon data presented in this study (cf. Figure 2) were obtained from soil organic matter (TOC) and mollusc shells (Pietsch and Kühn 2012).Pupoides coenopticus and Zootecus insularis have a size of 5-10 mm, i.e. they can be defined as so-called 'minute gastropods' which, in an environment of minimal hard water effects, yield reliable 14 C ages and are suitable for demarcating the ages of sediments in which they occur (Pigati et al. 2004).
[ SOIL MICROMORPHOGENESIS AND EARLY HOLOCENE PALEOCLIMATE AT THE DESERT MARGIN OF SOUTHERN ARABIA ] 3. Results

Field data and stratigraphy
Northwest of the Ma'rib Oasis, buried humusrich soils with Ahb and ABwkb horizons (for the latter cf.Pietsch and Kühn 2012) are widely distributed (Figure 2).The paleosols represent an old land surface that was later covered by silty-sandy sediments (C horizons on top of the profile).The buried marker horizon is characterized either by brown colours implying silicate weathering (Pietsch and Kühn 2012) or by dark colours showing humus enrichment due to dense vegetation in the Early Holocene (Table 1, Lézine 2009).Radicarbon ages of fossil molluscs and humic acid indicate that soil formation mostly took place before 8.3 cal ka BP, and that cover sediments were deposited between 8.3 and 6.7 cal ka BP.In the upper part of profile Mat03, mollusc shells are older then in the sediments below.This could have resulted from older shells becoming embedded in younger material.The stratified cover sediment above the paleosols, with an average thickness of 20 to 40 cm, usually shows no signs of soil formation i.e. no humification, silicate weathering, and de-calcification, except some slight secondary calcification or redistribution of calcium carbonate by weakly developed pseudomycelia.A desert pavement has developed on top of the cover sediment (cf.Al-Farraj and Harvey 2000).Stones of the desert pavement yield a black desert varnish on the upper side and are lighter coloured on the underside (cf.Thiagarajan and Aeolus Lee 2004).Therefore, disturbed sites are easily recognisable by such stones being turned upside down.
Rhizoliths frequently occur in the Ahb horizon.They are described as former root channels, slightly or completely calcified, with some organic remnants (Klappa 1980;Gocke et al. 2011).In our study area soft pressure causes them to fall apart.Some of the Ahb horizons reveal a structure after these root pseudomorphs, implying dense vegetative cover or a long period of vegetation (Pietsch et al. 2010).To date, similar root pseudomorphs have only been described in this region within irrigation sediments from the Sabaean period (Brunner 1983).
Colours also indicate sediment modifications due to soil forming processes: buried soils are darker and show colours of 10YR3/2, 3/3 or 4/3 (Table 1).The soils do not show signs of strong calcification, as is characteristic for other dry areas in Yemen (Pietsch and Kühn 2009), because precipitation is low and the groundwater level does not play a significant role.As in some cover sediments, only a few soft secondary carbonate precipitations (also pseudomycelia) were found in the Ahb horizons of MAT03 and LAS07, as evidenced by higher CaCO 3 contents (Table 1).

Particle size distribution and pedochemistry
Soil layers can be easily identified by the vertical grain-size distribution.Sandy and siltysandy cover sediments are clearly distinguishable from the more silty-clay sediments of the paleosol.The grain-size quotient Q (sum 630-63 μm sand fraction/sum silt fraction) marks the boundaries between soil layers (Table 1): a tendency of higher clay contents can be noted in the paleosols, although volcanic ash may influence grain-size distribution as seen in Ahb horizons of MAT03 and LAS07.Weathering of volcanic glass to palagonite and halloysite may also contribute to higher clay contents.The organic carbon content of the buried soils usually ranges around 0.5%.
The CaCO 3 content ranges from 2% to 21% with lowest contents in Ahb horizons of MAT03 and LAS07.A few soft carbonate precipitations in the Ahb horizons imply secondary calcification, which followed a more or less complete decalcification.
The chemical index of alteration (CIA) has been widely used for identifying weathering processes in sediments (Nesbitt and Young 1982).The CIA estimates the transformation of feldspars to clay minerals: the higher the index the stronger the weathering.CIA values of the buried soil horizons range from 41 to 56 (Table 1).Higher CIA values in the cover sediment of LAS07 are most probably caused by the incorporation of pedosediments during cover sediment formation.Since the CIA ranges around 50 and CIA < 50 means unweathered material, not much silicate weathering can be detected geochemically.

Micromorphology
Essential micromorphological features (voids, microstructure, groundmass, micromass and pedofeatures) of the three soil profiles are given in Table 2 The micromorphological property is shown by its presence (cross) or absence (no cross).(x) = partly occurring.

LAS 07
The cover sediment (LAS07 22) with a single grain microstructure has a c/f 20 μm ratio of 6/1 and is much finer compared to the underlying sediments.The groundmass contains a lot of small (50-100 μm) brown-orange flakes (Figure 3), which are most probably fragments of palagonitelike altered sideromelane (isotropic with crossed polarizers).Those flakes do not occur in the buried soil horizons.Layering can easily be seen by remnants of microlayers, which are disturbed by passage features (Figure 4).
The 3Ahb1 horizon has a weakly developed subangular blocky microstructure and is much coarser (c/f 20 μm 9/1) than the cover sediment.
Micritic pendants occur -only in the upper centimeters of 3Ahb1 -at the lower side of coarse particles and fragments (Figure 5).An undifferentiated b-fabric is most prominent, crystallitic b-fabric is only partly present.
Coatings of neoformed clay could be detected in some of the completely altered volcanic glasses (Figure 6).The anisotropic behaviour of this clay shows that this is not allophane but probably smectite (Gérard et al. 2007;Drief and Schiffmann 2004).The yellow interference color of the coating after inserting a "red I plate" proves that the clay particles are oriented perpendicular to the walls (Figure 7; cf.Stoops 2003).Other vesicular sideromelane reveals the formation of ring-like structures (Figure 8) that could be attributed to allophane formation (Gérard et al. 2007).Silicate weathering can be shown by congruent dissolution of pyroxene (saw edges; Figure 9), alteration of volcanic glasses and clay neoformation.
A channel to partly spongy microstructure is characteristic for the 3Ahb2 horizon.Stipple speckled to weakly developed granostriated b-fabric indicates a stronger silicate weathering and decalcification of this horizon compared to the overlying and underlying horizons (Figure 10).The silicate weathering could not be detected by the CIA index (Table 1).There are many incomplete loose crumby and complete silt-sand infillings occur (Figure 11).Loose incomplete micritic infillings and weakly developed micritic hypocoatings are detectable along channels (Figure 12).Micritic and microsparitic carbonate in the groundmass occur only in silt-sand infillings.

MAT 03
The cover sediment has a single grain microstructure whereas the buried soil has a well developed subangular blocky microstructure (Figure 13).
Lenticular gypsum in rosette-like forms frequently occurs in planes and channels of the 4Ahb1 and 4Ahb2 horizons (Figure 14).Micritic nodules (up to 1500 μm length) occur in the cover sediment, whereas only few smaller (200 μm) micritic nodules could be detected in the 4Ahb1 and 4 Ahb2 horizons.
The subangular blocky microstructure is partly disturbed by numerous passage features in the 4Ahb2 horizon.A weakly developed crystallitic b-fabric is most prominent, undifferentiated b-fabric is only partly present.Altered to completely altered sideromelane fragments frequently occur in the groundmass (Figure 14).Only few fragments of mollusc shells, and few sparites were detectable in the groundmass.Some of the passage features have micritric precipitations (Figure 15) in the lower part of the 4Ahb2 horizon, which reveals the redistribution of carbonate to the boundaries.These features were only visible under fluorescence light (365 nm + 470 nm with a multibandpass filter).Most of the material is completely reworked by the burrowing activities of soil fauna.A crystallitic b-fabric, numerous micritic nodules (up to 3500 μm) and fragments of limestone are characteristic features of the 4Ck1 and 4Ck2 horizons.Fragments of completely altered sideromelane frequently occur; but less volcanic material is detectable compared to the overlying horizons and layers.Weakly developed micritic hypocoatings were found in 4Ck2 horizon (Figure 16).Many passage features can be found in both horizons.

MAT 09
Single grain microstructure and monic c/f related distribution are most prominent in all three thin sections.Weakly developed fine enaulic c/f related distribution partly occurs.
The micromass is nearly absent having a c/f 20 μm ratio of 9/1.Microlayers including vesicles build up vesicular crusts and show fining-upward sequences.These vesicular crusts are fragmented by passage features in the 4Cwk horizon (Figure 17).
Fresh tachylitic rock fragments, fresh sideromelane (up to 1000 μm) as well as partly altered fragments showing an orangeyellowish rim are present in all thin sections (Figure 18).The groundmass in thin section at 59 cm (4Cwk) contains lots of small (50-100 μm) brown-orange flakes -fragments of vitric glass-similar to the orange coloured flakes in LAS07 (2Cw).Smaller fragments of completely altered sideromelane (200-300 μm) were detectable only in 86-91 cm depth of the 6Cwk horizon (Figure 19).Micromass coatings (Stoops 2009) around volcanic coarse material occur particularly at this depth (Figure 20).
Passage features are predominant in the lower two thin sections.Some of the passage features have a higher portion of micrite compared to the surrounding groundmass (Figure 21).No decalcification features were detectable.Primary carbonate in form of rock fragments, micritic nodules and fragments of mollusc shells are frequently present in all thin sections (e.g.

Micromorphogenesis of cover sediments and paleosols
The cover sediments show no signs of decalcification or further pedogenic processes.Only passage features attributed to bioturbation could be detected.Characteristic micromorphological features of the cover sediments are (i) single grain microstructure, (ii) crystallitic b-fabric, (iii) predominant occurrence of fresh sideromelane, and (iv) remnants of microlayers.
Microlayers are broken by bioturbation in the 2Cw horizon of LAS07 as well as in the 4Cwk horizon of MAT 09.When the microlayers contain vesicles, these can be described as vesicular crusts and macroscopically designated as Av horizons.The vesicles may be attributed to former raindrop impact of rain drops in quickly drying sediment (Figueira and Stoops 1983).Av horizons -often related to desert pavementswith a high silt and clay content are common in dry regions and mark a land surface not used or disturbed for some time (Dietze et al. 2012;Mc-Fadden et al. 1998;Pietsch et al. 2010).Thus, these vesicular microcrusts represent former -in relation to Holocene short-time climate periodsstable land surfaces related to arid or semiarid climates (Pagliai and Stoops 2010).
Most of the Ahb horizons were most probably near or completely decalcified so that in parts stipple speckled b-fabric and neoformed clay coatings could be detected as a result of stronger weathering and soil formation.Subangular blocky microstructure can also be considered as a clear sign of soil development.Fresh, altered sideromelane together with completely altered sideromelane can be found in nearly all of the thin sections.Sideromelane with a yellowish weathering rim cannot be exclusively attributed to in situ weathering, because these rims can form very quickly (Gérard et al. 2007).
Since complete alteromorphs of sideromelane could hardly be found in the cover sediment (LAS 07), in the underlying horizons (MAT 03) or in the sedimentary profile MAT 09, but frequently occur in the Ahb horizons, the complete alteration of sideromelane can be attributed to in situ weathering processes.The humic acids in the Ahb horizons enhancing hydrolysis could possibly have promoted this process.The oc-currence of micritic nodules together with fresh and completely altered sideromelane within one horizon can be explained by burrowing activities of soil fauna.Secondary calcification of the Ahb horizons is only detectable within the upper centimeters by the occurrence of micritic pendants at the lower side of coarse fragments.Since no clear decalcification features could be detected, we propose a formation over shorter periods of sufficient infiltration-water availability.Thus these pendants may have formed during the period of formation of the cover sediment.
Characteristic pedogenic features in the buried A horizons are: the (i) the formation of a subangular blocky microstructure, (ii) an undifferentiated b-fabric as a result of enrichment of organic matter and decalcification, (iii) the predominant occurrence of completely altered sideromelane.

Pedosedimentary reconstruction and palaeoenvironmental implications
The Early Holocene soils at the Ramlat as-Sab'atayn desert margin are covered by sediments which contain gastropod shells yielding radiocarbon ages ranging from 8.3 to 6.6 cal ka BP (MAT03, LAS07, cf. Figure 2).In contrast, the layered cover sediments do not show pedogenic features.Since sufficient moisture over longer periods of time (e.g.some hundred to few thousand years) would have supported soil formation, particularly in the upper horizons, we can presume a predominantly arid climate during and since the formation of the cover sediment.Thus the formation of the cover sediment and sediment sequences as in MAT09 can be explained by occasional occurrences of strong precipitation in an arid period.A moist period of any sufficient duration for pedogenesis should be recognizable by significant pedogenic features in MAT09 and in other profiles with cover sediment over buried soil horizons.Despite the fact that the investigated soil profiles are situated on nearly flat areas the abrupt sedimentary change between the upper layer and the paleosol may also include an erosional event prior to the emplacement of the cover sediments.Another hiatus or erosional event could be also included within the uppermost shallow layer (C horizon).However, the occurrence of an undisturbed desert pavement, which started to develop in the Middle Holocene (Pietsch and Kühn 2012), and vesicular Av horizons on top of the profiles indicate a lasting aridity with occasional rainfall.
The AMS radiocarbon age of humic acid in the 4Ahb horizon in MAT03 yielded an age of 8.5 to 8.3 cal ka BP (Figure 2; cf.Table 3 in Pietsch and Kühn 2012).Ages of fossil molluscs in cover sediments and the relative ages of buried paleosols allow us to deduce that a major phase of soil formation took place before 6.7 cal ka BP.Our results and those of Weiß and Brunner (2010) do not support a moist climate and soil formation at the Ma'rib desert margin after 5.5 ka BP as it was proposed for Qa Jahran in the Yemen Highlands (Wilkinson 1997).This discrepancy could either be explained either by higher and constantly periodic rainfalls in the highlands during the Middle Holocene, or by the misinterpretation of the dark colluvial deposits (pedosediments), which are widely distributed in the highlands, as in situ developed soils.

Local pedogenic response to climate variations
After 6.6 ka BP only the plains and terraces along the Wadi Dhana were favoured by the periodic rains of the ISM in the Yemen Highlands, and this may have provided sufficient water for soil development or agriculture (Pietsch et al. 2010;Pietsch and Kühn 2012).The profiles MAT03, MAT 09 and LAS07, however, are situated at sites far from the Wadi Dhana (Figure 1) and, therefore, do not profit from the periodic sayl (flash flood) that follows rainfall in the Yemen Highlands.

Conclusions
The paleosols at the desert margin of Yemen represent Early Holocene moist climate conditions with a precipitation ranging from 400 to 600 mm a -1 .The soils formed before 8.3 cal ka BP.Micromorphological pedogenic features in the buried Ah horizons include (i) the formation of a subangular blocky microstructure, (ii) an undifferentiated b-fabric as a result of enrichment of organic matter and decalcification, and (iii) the predominant occurrence of completely altered sideromelane.Most of these horizons were very probably nearly completely decalcified so that in parts stipple speckled b-fabric and neoformed clay coatings could be detected as a of stronger weathering and soil formation.
After 8.3 and at least until 6.6 cal ka BP the soils of the land surface (A, AB and B horizons) were covered by sandy-gravelly sediments (BC and C horizons).Single events with intense precipitation may have caused sheet wash and the development of these cover sediments.The overlay of various C layers and the different grain sizes (from fine particles to coarse gravels, see Figure 2) suggest a multiple-event depositional history for the cover sediments.The remnants of vesicular crusts, only disturbed by bioturbation, corroborate this deduction.Stones on top of the cover sediments developed a desert varnish and a desert pavement formed on flat areas.The cover sediments show no signs of pedogenesis, corroborating the presence of dry climate conditions after their formation.Characteristic micromorphological features are (i) a single grain microstructure, (ii) crystallitic b-fabric, (iii) the predominant occurrence of fresh sideromelane, and (iv) remnants of microlayers.
Destruction of sedimentary layering by pedogenic processes can be inferred from a homogeneous grain-size distribution even over boundaries of different Ahb horizons.From the abrupt boundary between the cover sediment and the buried paleosol, as well as the lack of pedogenic features in the cover sediments, we can deduce that an abrupt climate change occurred at the beginning of the formation of cover sediments, i.e. after around 8.3 cal ka BP and locally at the latest from 6.7 cal ka BP.From that period onwards, water was only periodically available in Wadi Dhana, and that coming from the Yemen Highlands.It should also be mentioned that effects of occasional river floods both from the mountain front and from the wide alluvial plain may have led to erosional events and associated hiatuses in the cover sedimentation after 6 to 8 ka.It can be concluded, however, that at least since the formation of the the still-undisturbed desert pavement, erosional events in the cover sediment are unlikely.The occurrence of this undisturbed desert pavement and vesicular Av horizons on top of the profiles indicate lasting aridity with occasional rainfall.

Acknowledgements
We thank the German Archaeological Institute (DAI) and the German Research Foundation (DFG, PI 452/2) for funding.We are very much indebted to the team of the German Archaeological Institute (DAI), Oriental Department, Sana'a branch, particularly to Iris Gerlach, for all imaginable support during fieldwork.We are grateful to our friends Christian Weiß (University of Erlangen) and Ueli Brunner (Zürich) for many fruitful days in the field.

Figure 1 .
Figure 1.Location of investigated soil profiles in the volcanic field (reddish-brown colours) at the desert margin near Ma'rib; NW-SE oriented ridge consists of Cretaceous limestone; green colours indicate irrigated fields around Wadi Dhana (source: Google Earth).

Figure 2 .
Figure 2. Selected sediment and soil profiles, AMS 14 C-data of gastropod shells and organic material, calculated palaeoprecipitation in mm a -1 (for further information see Pietsch and Kühn 2012).

Figure 7 .
Figure 7. (LAS07 40-50 cm, xpl+red I plate, same as Figure 6): SW-NE oriented portions of the clay coatings show in parts orange colour instead of expected green (white arrows); n of red I plate is oriented SW-NE.

Figure 13 .
Figure 13.(MAT03 46-56 cm, ppl): Transition from cover sediment (upper part with predominating single grain microstructure) to buried Ah horizon (subangular blocky microstructure).Note the slightly darker colour of peds and fragments from the Ahb horizon indicating a higher content of organic matter.

Figure 17 .
Figure 17.(MAT09 70 cm, ppl): Vesicular microcrust -with horizontal or slightly inclined layers-fragmented by passage features.Note the smaller dark fragments of the microcrust in the centre of the microphotograph.
[ SOIL MICROMORPHOGENESIS AND EARLY HOLOCENE PALEOCLIMATE AT THE DESERT MARGIN OFSOUTHERN ARABIA ]
a CFA: Coarse fragment abundance by % volume, according to FAO (2006).b Q: Quotient of sum middle+fine+finest sand/sum silt fractions.c

Table 1 .
Substrates, soil physical and soil chemical data (nd: not determined)