The investigated FSPS (42∘02′17.7′′ N, 45∘21′18.7′′ E; 450 m a.s.l.) is located in the upper Alazani valley in eastern Georgia. The Alazani river originates at an altitude of ∼2800 m a.s.l. from the southern slopes of the Greater Caucasus Mountains (Figs. 1 and 2a). The Alazani flows from north to south for the first ∼40 km and then, after its confluence with the smaller Ilto River, follows the NW–SE-oriented Alazani thrust top basin for ∼160 km (Fig. 2a). Here the Alazani is paralleled by the southern foothills of the Greater Caucasus in the northeast (Adamia et al., 2010) and the southwesterly advancing Kura fold-and-thrust belt (Kura FTB) in the southwest (Forte et al., 2010) (Figs. 1 and 2a). Finally, the ∼240 km long Alazani drains via the Kura into the Caspian Sea (Fig. 1). The investigated FSPS is located in the upper part of the Alazani thrust top basin, ca. 10 km downstream of the confluence of the Ilto and upper Alazani (Fig. 2a). Upstream of this site both rivers show a braided character and are thus characterized by coarse gravelly river beds. The site-related subcatchment in the upper Alazani and Ilto valleys is mountainous with a relatively small size of ∼1100 km2. It shows quite steep slopes with an average slope angle of ∼20∘, and especially in the uppermost part slopes up to 50∘ are found (Bliedtner et al., 2018a; Fig. 2a and b). The uppermost part of the upper Alazani subcatchment in the central southern Greater Caucasus is formed by folded and metamorphosed Jurassic flysch and molasse deposits that consist of altered organic-rich black clay shales, sand- and siltstone, and volcanic rock (Fig. 2c). Following downstream, the middle part of the upper Alazani subcatchment is dominated by Cretaceous sand-, silt- and limestone. The southern part of the subcatchment is dominated by the Kura FTB, which reaches altitudes of 2000 m and consists of folded and overthrusted Cretaceous sand-, silt- and limestone and Paleogene to Quaternary sandstone, siltstone and conglomerates (Gamkrelidze, 2003; Fig. 2c). Brownish loamy slope deposits are found in parts of the lower Ilto subcatchment that probably originate from Late Pleistocene eolian loess (Bliedtner et al., 2018a; Fig. 2c).

At the studied site, the recent mean annual temperature and precipitation are about 12.0 ∘C and 720 mm a-1, respectively (http://de.climate-data.org/location/28480/, last access: 24 April 2020, station: Akhmeta). In the upper Alazani subcatchment, precipitation reaches up to 2000 mm a-1 because most parts are located in the central southern Greater Caucasus (unpublished precipitation map from the Vakhushti Bagrationi Institute of Geography in Tbilisi). Precipitation mainly falls in spring and early summer during convective events (Lydolph, 1977), and thus the Alazani reaches its maximal discharge between April and June due to both snowmelt in the Greater Caucasus and the concomitant precipitation maximum; i.e., there is a pluvionival runoff regime (von Suchodoletz et al., 2018).

The recent vegetation of eastern Georgia belongs to the Irano-Turanian group (Connor et al., 2004; Sagheb-Talebi et al., 2014). The natural vegetation of the floodplains in the upper Alazani valley, where the studied site is located, consists of deciduous elm–oak–vine forests (Connor and Kvavadze, 2008). However, agricultural fields and grasslands cover most of the recently inactive elevated valley floor that is outcropped by the investigated FSPS today, whereas the lower-lying active modern floodplain hosts scattered patches of deciduous riparian forests. The mid-mountain belt further upstream is characterized by mixed beech and in small parts also by fir–spruce forests. Alpine to subalpine meadows cover the highest parts of the catchment (Connor and Kvavadze, 2008). Today, ∼65 % of the upper Alazani subcatchment is covered by forests and ca. 35 % by grasslands and fields (Bliedtner et al., 2018a).

The investigated FSPS is naturally exposed up to ∼7 m along the upper Alazani and consists mostly of fine-grained overbank sediments with intercalated paleosols. The sequence was previously investigated by von Suchodoletz et al. (2018), where a more detailed description can be found. Throughout the sequence, six blackish-grayish to reddish paleosols were developed in the fine-grained silty to clayey overbank sediments (Fig. 3). Three intensively developed paleosols are characterized by distinct upper but gradual lower limits (Ahb1, Ahb5, Ahb6), whereas three weakly developed paleosols (Ahb2, Ahb3, Ahb4) only showed gradual upper and lower limits. A well-developed recent soil (Ah) covers the surface of the sequence. The sediments of the FSPS were previously analyzed for carbonate content, total organic carbon (TOC), pH and mass-specific magnetic susceptibility (χ) to differentiate paleosols formed on-site from partly similar-looking fluvial sediments. Based on a relative soil development index (SDI), the differences in these measured values from the uppermost sample of a paleo(soil) to the underlying parent material were averaged to calculate soil development intensities (von Suchodoletz et al., 2018). All paleosols are characterized by systematically decreasing carbonate content and pH values and increasing TOC content and χ values from bottom to top.

The chronology of the FSPS is based on nine charcoal pieces that were dated with 14C (Fig. 3). The 14C ages are in stratigraphic order except two samples that are older than stratigraphically lower samples. Therefore, they must overestimate their true burial age; i.e., they might have been reworked. Apart from these two samples, the charcoal chronology of the FSPS represents the timing of sediment deposition and soil development and serves as an independent age control for comparison with leaf wax n-alkane 14C ages (Fig. 3). The chronology ranges between 8.0–8.2 ka cal BP (95.4 %) in the upper part of Ahb6 and 1.6–1.8 ka cal BP (95.4 %) in the sediments below the recent soil Ah. The oldest 14C age of 8.0–8.2 ka cal BP (95.4 %) was obtained from a charcoal piece that was found together with archeological artifacts (potsherds, bones, obsidian tools) in the upper part of Ahb6. This documents anthropogenic activity at this site at least since the Neolithic period. Based on the 14C age of 1.6–1.8 ka cal BP (95.4 %) from the nonpedogenic fluvial sediments below the recent soil Ah, the time to form the Ah was estimated to be ca. 1.6 kyr. This age was used as a reference to calculate the approximate soil forming durations for the paleosols based on the soil development index (SDI). For more detailed information how the SDI was derived, the reader is referred to von Suchodoletz et al. (2018).

Figure 4a shows the homolog distribution of petrogenic n-alkanes derived from a Jurassic black clay shale sample from the upper Alazani subcatchment (Fig. 2c). These are present with similar amounts at all chain lengths (C21–C35) and do not show a distinct odd-over-even predominance (OEP). These 14C-dead n-alkanes also contribute to the sedimentary leaf wax n-alkanes in our FSPS and are exemplarily shown as the assumed maximal petrogenic contribution for the n-alkane homolog distribution of nonheated sample Ala 425 I in Fig. 4b. During the pretest, this sample gave a 14C age of 12.4–13.3 ka cal BP (95.4 %), which is much older than the timing of sedimentation obtained from the charcoal 14C ages (∼5 to 4 ka cal BP). To at least partly reduce the contamination with petrogenic n-alkanes, we re-extracted this sample and applied the heat treatment of 120 ∘C for 8 h before 14C measurement. The n-alkane homolog distribution of the re-extracted and heated sample Ala 425 II (Fig. 4c) demonstrates that the short-chain n-alkanes and thus a significant amount of the petrogenic n-alkanes were effectively removed by this procedure. Consequently, the resulting 14C age of 6.4–7.9 ka cal BP (95.4 %) was much younger than the age obtained without preheating (Figs. 4c and 5).

The 14C ages of the other samples that were also measured both before and after the removal of the short-chain n-alkanes by heating were as follows: 5.8–6.3 ka cal BP (95.4 %) (Ala 225 I) and 3.1–4.2 ka cal BP (95.4 %) (Ala 225 II) for sample Ala 225, 7.6–8.2 ka cal BP (95.4 %) (Ala 290 I) and 5.8–7.2 ka cal BP (95.4 %) (Ala 290 II) for sample Ala 290, and 15.7–16.8 ka cal BP (95.4 %) (Ala 505 I) and 6.5–8.6 ka cal BP (95.4 %) (Ala 505 II) for sample Ala 505, respectively (Table 1; Fig. 5).

The results of our heating experiments show that here a simple heat treatment effectively removes the short-chain n-alkanes that contain a significant petrogenic component. However, we have to mention that no extensive heating experiments were carried out for the removal of short-chain n-alkanes prior to this study and that we only tested at three different temperatures (100, 110 and 120 ∘C) for 8 h. We found that most of the short-chain n-alkanes could best be removed at 120 ∘C for 8 h, although a slight proportion of the longer chains was also removed. We also have to mention that possible fractionation during heating could potentially lead to an enrichment of the heavier 14C isotope, but such a fractionation effect should be negligible because the F14C results were generally corrected for mass-dependent fractionation by the 13C isotopes. Therefore, the applied heat treatment is an effective pretreatment to derive a more homogeneous leaf wax n-alkane signal for compound-class 14C dating in general and especially in environmental settings where petrogenic OC and n-alkanes occur. Because of the partly removed petrogenic contribution, the 14C age of a heated sample is generally closer to its leaf-wax-derived n-alkane 14C age. However, since petrogenic n-alkanes are also present at the longer chains ≥C25 that mostly originate from leaf waxes and serve as paleoenvironmental proxies (see Fig. 4), this pretreatment cannot completely remove the petrogenic contribution.

Furthermore, we have to mention that beside n-alkane contributions from petrogenic sources, short-chain n-alkane contributions from aquatic and microbial sources were also reported from lake sediments (Ficken et al., 2000; Makou et al., 2018), and microbial utilization was reported to alter the leaf wax signal of the longer chains (Li et al., 2018). While we cannot completely rule out possible aquatic and microbial contributions to our leaf-wax-derived n-alkanes, we suggest that such contributions are negligible in our fluvial sediment sequence. This is because n-alkanes from aquatic and microbial production would show a clear OEP on the shorter chains with younger 14C ages, whereas our short-chain n-alkanes show no OEP and the 14C ages become significantly younger after the removal of the shorter chains by heating.

In the following, we will only focus on the 11 n-alkane samples from the investigated FSPS that were preheated with 120 ∘C for 8 h. n-Alkanes were present in all these samples after heating, with values ranging between 27 and 60 µg carbon per n-alkane fraction, which was enough carbon for robust 14C measurements. Their F14C values range from 0.7865±0.0110 to 0.3544±0.0174, what corresponds to calibrated calendar ages from 1.6–2. ka cal BP (95.4 %) to 8.4–10.7 ka cal BP (95.4 %) (Table 1).

Calibrated calendar ages for the n-alkanes from (paleo)soils and fluvial sediment layers are shown in Fig. 6. Compared to the independent charcoal-based 14C chronology of the sequence (von Suchodoletz et al., 2018), the n-alkane 14C ages show variable age offsets over the FSPS; i.e., they are generally older. The n-alkane 14C ages of intensively developed paleosols and the recent soil show generally lower age offsets (between 0 and 1.3 ka) than those from less intensively developed paleosols (up to 3.3 ka) (Fig. 6). The age offsets of the n-alkanes from fluvial sediment layers are generally older than those from all (paleo)soils and increase in the upper part of the FSPS (Fig. 6). Age offsets in the lower part range between ∼2.3 and 3.5 ka and increase to up to ∼6 ka in the upper part.

Our preheating of the n-alkane fractions with 120 ∘C for 8 h effectively removed the short-chained petrogenic n-alkanes (<C25) derived from Jurassic black clay shales in the upper Alazani subcatchment. However, such petrogenic n-alkanes are also present at the longer chains (C25–C35), where they underlie the leaf-wax-derived n-alkanes and do not show a distinct OEP (see Fig. 4c). Our preheating can therefore not fully remove all petrogenic n-alkanes. To correct for this underlying petrogenic contribution, we propose a simple correction procedure that uses a constant correction factor: a measured Jurassic black clay shale sample from the upper Alazani subcatchment yielded an average concentration of 0.007 µg g-1 per single n-alkane compound (see Fig. 4a). Assuming that this is the maximal possible concentration of petrogenic n-alkanes in the sediment samples of our FSPS and that this equally underlies both the even and odd leaf wax compounds from fluvial sediments and (paleo)soils, the proportion of petrogenic n-alkanes for each sample can be quantified and calculated as follows: 1petrogeniccontribution(%)=0.007µgg-1⋅sumofchainsn-alkaneconcentrationµgg-1⋅100. The sum of chains is the number of n-alkane chains from C25 to C35, and the n-alkane concentration is the sum of concentrations from C25 to C35. In cases where not all shorter chains (<C25) could completely be removed from the sample by heating, they will be included in the calculation.

The calculated petrogenic contribution (%) that is assumed to be 14C dead (i.e., has an F14C value of 0) can then be subtracted from the measured F14C value to derive the petrogenically corrected F14C value (F14Cpetrocorr): 2F14Cpetrocorr=F14Cmeasured+(F14Cmeasured⋅petrogeniccontribution⋅0.01). The obtained petrogenically corrected F14C values were then calibrated with IntCal13 to yield calendar ages again. We have to note that error propagation of the calibrated petrogenically corrected ages is difficult, and we simply used the measured 14C error of the respective sample before petrogenic correction. This might hold some uncertainty, but when corrected the error should basically become smaller and fall within the measured 14C error. We also have to note that calibrated petrogenically corrected ages are based on the maximal possible petrogenic contribution in the sediment samples from our FSPS. However, since the proportion of petrogenic n-alkanes is not necessarily constant over our FSPS and can become diluted in the sediment samples by sediment material that was not derived from black clay shales and/or the slight loss of long-chain n-alkanes during heating (see discussion in Sect. 3.1), the leaf-wax-derived n-alkane ages lie in between the heated and petrogenically corrected ages.

As illustrated in Fig. 7, the maximal possible petrogenic contribution from petrogenic n-alkanes can be quantified and corrected for by this approach. Maximal possible petrogenic contributions are relatively high with ∼0.8 to 2.0 ka throughout the investigated FSPS. Compared to the fluvial sediments, the (paleo)soils show lower contributions of petrogenic n-alkanes. Due to generally higher total n-alkane concentrations in the upper part of the FSPS, the relative proportion of the maximal possible petrogenic contribution generally decreases from bottom to top (Fig. 7).

Since the petrogenic n-alkanes could effectively be removed by heating and correction for maximal petrogenic contributions, the remaining n-alkanes should mostly derive from leaf waxes. Accordingly, their age offsets to the sediment layers in the FSPS in which they are buried became smaller. However, the age offsets still reach up to several millennia and vary over the FSPS. These have to be regarded as minimal age offsets since the correction for petrogenic contributions yielded maximal possible values. These offsets generally differ between (paleo)soils and fluvial sediment layers (Fig. 7).

The vegetation distribution of deciduous trees and shrubs as well as grasses and herbs can be derived from the leaf wax n-alkane distribution pattern and was previously described by Bliedtner et al. (2018a) for the investigated FSPS. There, the vegetation distribution is expressed in a two-component mixing equation as the grass / herb ratio based on regional end-members and corrected for possible degradation effects (Fig. 7). We have to mention that absolute grass and herb percentages can exceed 100 %, but those should not be overinterpreted and rather be considered as semiquantitative estimates.