Key Factors of Precipitation Stable Isotope Fractionation in Central-Eastern Africa and Central Mediterranean

: The processes of isotope fractionation in the hydrological cycle naturally occur during vapor formation, vapor condensation, and moisture transportation. These processes are therefore dependent on local and regional surface and atmospheric physical features such as temperature, pressure, wind speed, and land morphology, and hence on the climate. Because of the strong inﬂuence of climate on the isotope fractionation, latitudinal and altitudinal e ﬀ ects on the δ 18 O and δ 2 H values of precipitation at a global scale are observed. In this study, we present and compare the processes governing precipitation isotope fractionation from two contrasting climatic regions: Virunga in Central-Eastern Africa and the Central Mediterranean (Stromboli and Sicily, Italy). While Virunga is a forested rainy tropical region located between Central and Eastern Africa, the Mediterranean region is characterized by a rainy mild winter and a dry hot summer. The reported δ 18 O and δ 2 H dataset are from precipitation collected on rain gauges sampled either on a monthly or an approximately bimonthly basis and published in previous papers. Both regions show clearly deﬁned temporal and altitudinal variations of δ 18 O and δ 2 H, depending on precipitation amounts. The Central Mediterranean shows a clear contribution of local vapor forming at the sea–air interface, and Virunga shows a contribution from both local and regional vapor. The vapor of Virunga is from two competing sources: the ﬁrst is the continental recycled moisture from soil / plant evaporation that dominates during the rainy season, and the second is from the East African Great Lakes evaporation that dominates during the dry season.


Introduction
Oxygen and hydrogen stable isotopes (δ 18 O and δ 2 H) are among the tracers widely used to understand the processes involved in water circulation in the hydrological cycle. Besides the fact that oxygen and hydrogen isotopes are naturally occurring and quantitatively available, the changes in their concentration ratios are easily recorded during physical or chemical processes involving water, making them ideal to trace water mass movement [1][2][3][4][5]. The changes in the concentration ratios of isotopes during physical or chemical processes are referred to as isotopic fractionation. The nature of the processes that have occurred can then be determined from the observed differences in the isotopic compositions [1,4].
the Virunga lowlands (found below 2000 m a.s.l.; 1300-1700 mm yr -1 ) and highlands (> 2000 m; up to 2300 mm yr -1 ) [29,30]. Virunga has a bimodal precipitation regime: a dry season (July to August) and a wet season (September to June); with a short dry-like period observed from mid-January to late-February. The mean monthly air temperature in the lowlands is~20 • C with daily maxima of 32 • C, while in the highlands, the mean monthly temperature is~10 • C with daily minima of 7 • C [29]. Hence, the dominant temperature fluctuation occurs in the daily temperatures rather than in monthly or yearly mean temperatures. The hydrography of Virunga consists of Lake Kivu (1460 m a.s.l.; evaporation rates~1500 mm yr -1 ), Lake Edward (912 m a.s.l.), several springs, and middle to large rivers (see compilation in [22,30]). At the regional scale, Virunga is located at the border between the Central and Eastern Africa regions, where a humid rainforest tropical climate predominates. The Virunga mountains influence the regional wind circulation, particularly those of the lower atmosphere. Virunga is part of the African Great Lake Region (Figure 1), where Lake Victoria, Lake Tanganyika, Lake Malawi, Lake Albert, etc., large rivers, and wetlands impact the East African regional climate and moisture production [30]. These lakes, large rivers and wetlands are subject to important evaporation. The region abounds in large dense forests that mainly consist of national parks. The majority of these tropical rain forests are wet and clouded all year long, and are, thus, important sources of moisture through evapotranspiration, principally during the rainy season.
The second set of samples was collected from Sicily and Stromboli islands in the Central Mediterranean. From a geological point of view, Sicily represents part of the south-verging branch of the Apenninic-Maghrebian orogenic belt made up of a pile of nappes derived from the deformation of different Meso-Cenozoic domains resulting from the collision of the African and Eurasian tectonic plates [31]. The climate is a typically Mediterranean one: characterized by hot summer droughts and winter rain in the mid-latitudes, north of the subtropical climate zone. In summer, the subtropical high-pressure cells drift toward the Northern Hemisphere (from May to August). This coincides with substantially higher temperatures than in winter and little rainfall [32,33]. During the winter, the high-pressure cells drift back toward the equator, and the weather is dominated by cyclonic storms [34]. At ground level, the mean monthly relative humidity is always higher than 70% and close to 80% during the rainy season. The Mediterranean Sea represents an important source of energy and moisture for cyclone development [33]. Several authors have provided detailed descriptions of climatic features and cyclogenesis in the Mediterranean, and have related them to orographic effects [35,36] and to large-scale circulation [37,38]. In addition, climate change is believed to be responsible for the increased intensity of cyclones over the Mediterranean Sea [39]. The hydrological cycle in the Mediterranean has been described by [40]. These authors have shown that evaporation from the Mediterranean Sea is greatest during the winter and in its eastern part. Climatic features and the soil types are responsible for the poor vegetation coverage of the studied area. are also shown. The sampling sites were designed in order to capture any spatial and altitudinal variations of the stable isotope composition of oxygen and deuterium in precipitation.

Sampling and Analytical Techniques
Rain samples were collected from rain gauges that consisted of a plastic funnel fixed on a plastic container; the latter was filled with approximately 250 cm 3 of pure Vaseline oil to avoid evaporation. In Virunga, the samples were collected monthly from December 2013 to October 2015. After collecting the required aliquot for laboratory analysis and measuring the monthly precipitation amounts, the containers were washed with distilled water, and changed every 2-3 months during the study period. Water samples were filtered in the field through 0.45-mm-pore-size polysulfone syringe filters and stored at room temperature in 30-mL high-density polyethylene plastic bottles with double screwcaps. No additives were added to the samples. In the Mediterranean, rain samples were collected during different time periods. In Sicily, a rain gauge network was installed and sampled monthly in the period  [32,33,41,42].
The majority of the samples from Virunga were analyzed for hydrogen and oxygen stable isotopes in the Isotope Hydrology Laboratory of the International Atomic Energy Agency (IAEA) in Vienna, Austria. The analyses were performed using an off-axis integrated cavity output laser spectroscopy (OA-ICOS), following the analytical procedure of [43]. Samples were first run in duplicate on a different day and instrument. Samples for which repeatability of the two runs was not within 2 times the typical uncertainty were repeated on a dual-inlet isotope ratio mass spectrometer (Isoprime DI 100) following equilibration with H 2 or CO 2 gases respectively [44]. A few of the samples from Virunga were analyzed at the Istituto Nazionale di Geofisica e Vulcanologia (INGV) in Palermo using Analytical Precision AP 2003 and Finnigan MAT Delta Plus spectrometers to analyze δ 18 O and δ 2 H, respectively [30]. The majority of the samples from the Mediterranean was analyzed at the Istituto Nazionale di Geofisica e Vulcanologia (INGV) in Palermo. Details on the analytical methods are described in [32,33,41,42].
The oxygen and hydrogen isotopic compositions of a sample are reported as the deviation of the isotopic ratio of the sample relative to that of an established reference material (standard), and calculated as following:

General Variabilities
The isotopic composition of the precipitation of both regions has monthly values that are spread over a wide range (Table 1) Table 1). As a general tendency for both regions, the higher δ values were obtained during the dry periods: the dry and warm season in Virunga and dry and hot summer months in the Central Mediterranean. These periods are characterized by very low precipitation, yielding the observed δ 18 O and δ 2 H enrichment. Furthermore, the most negative values were obtained during the heavy rainy season in Virunga and during the cold and wet winter period in the Central Mediterranean. extreme values are typically the result of extreme weather conditions (e.g., at the peak of the dry or wet periods, or during thunderstorms) or due to secondary processes that may affect raindrops during their fall and that modify the isotopic composition of the pristine rainwater. This point is further discussed in section 3.4. Hence, the mean precipitation-weighted values for the Central Mediterranean vary from −62.45 to −29.28‰ for δ 2 H and −10.47 to −5.45‰ for δ 18 O; and from −11.79 to 5.45‰ for δ 2 H and from −4.00 to −1.07‰ for δ 18 O in Virunga ( Figure 2B). Thus, using the precipitation-weighted values, the equations for the LMWL are δ 2 H = 5.84 δ 18 O + 12.34 (r 2 = 0.99; p < 0.0001; N = 13) for Virunga, and δ 2 H = 6.23 δ 18 O + 4.25 (r 2 = 0.93; p < 0.0001; N = 67) for the Central Mediterranean. These equations are referred to as the Mean Local Meteoric Water Lines (MLMW), MVLMW for Virunga and MCMLWL for the Central Mediterranean ( Figure 2B). The higher enrichment of Virunga precipitation clearly emerges when precipitation-weighted values are used ( Figure 2B).

Spatiotemporal Variabilities of δ 18 O, δ 2 H and D-Excess
As mentioned in section 3.1, clearly defined seasonal variations are observed for both δ 2 H and δ 18 O in the two regions ( Figure 3), with higher values found during the dry seasons in Virunga (July and August, and during the short dry season of mid-January to mid-February ( Figure 3A,B) and during the summer months in the Central Mediterranean ( Figure 3B,D). Conversely, the most depleted δ values were obtained during the rainy season in Virunga (principally from October to December and March to June) and during the cold and rainy winter period in the Central Mediterranean. In Virunga, samples from the lower altitudes generally showed the enriched δ 2 H and δ 18 O values during the sampling period; this is responsible for the alignment observed in the  Figure 2A). In such circumstances, precipitation-weighted values are commonly recommended for the calculation of the LMWL. These extreme values are typically the result of extreme weather conditions (e.g., at the peak of the dry or wet periods, or during thunderstorms) or due to secondary processes that may affect raindrops during their fall and that modify the isotopic composition of the pristine rainwater. This point is further discussed in Section 3.4. Hence, the mean precipitation-weighted values for the Central Mediterranean vary from −62.45 to −29.28% for δ 2 H and −10.47 to −5.45% for δ 18 O; and from −11.79 to 5.45% for δ 2 H and from −4.00 to −1.07% for δ 18 O in Virunga ( Figure 2B). Thus, using the precipitation-weighted values, the equations for the LMWL are δ 2 H = 5.84 δ 18 O + 12.34 (r 2 = 0.99; p < 0.0001; N = 13) for Virunga, and δ 2 H = 6.23 δ 18 O + 4.25 (r 2 = 0.93; p < 0.0001; N = 67) for the Central Mediterranean. These equations are referred to as the Mean Local Meteoric Water Lines (MLMW), MVLMW for Virunga and MCMLWL for the Central Mediterranean ( Figure 2B). The higher enrichment of Virunga precipitation clearly emerges when precipitation-weighted values are used ( Figure 2B).

Spatiotemporal Variabilities of δ 18 O, δ 2 H and D-Excess
As mentioned in Section 3.1, clearly defined seasonal variations are observed for both δ 2 H and δ 18 O in the two regions ( Figure 3), with higher values found during the dry seasons in Virunga (July and August, and during the short dry season of mid-January to mid-February ( Figure 3A,B) and during the summer months in the Central Mediterranean ( Figure 3B,D). Conversely, the most depleted δ values were obtained during the rainy season in Virunga (principally from October to December and March to June) and during the cold and rainy winter period in the Central Mediterranean. In Virunga, samples from the lower altitudes generally showed the enriched δ 2 H and δ 18 O values during the sampling period; this is responsible for the alignment observed in the mean data points shown in the δ 2 H versus δ 18 O chart in Figure 2B. Similar behaviors were observed in δ 2 H and δ 18 O data collected from the Central Mediterranean.  Therefore, strong negative relationships were found between both the mean precipitation-weighted δ 2 H and δ 18 O and the altitude, while, conversely, the deuterium excess showed a strong positive relationship with the altitude (Figure 4). Virunga shows slightly higher vertical oxygen (−0.15‰/100 m) and hydrogen (−0.86‰/100 m) gradients compared to the Central Mediterranean area, where the vertical oxygen and hydrogen gradients are −0.24‰/100 m and −1.00‰/100 m for Stromboli and −0.16‰/100 m and −1.07‰/100 m for Sicily. However, Stromboli showed the higher d-excess gradient of 0.89/100 m (Virunga has a value of 0.32/100 m), which is consistent with the fact that the d-excess gradient has an opposite trend to that of the vertical oxygen and hydrogen gradients. The oxygen and hydrogen gradients for the Central Mediterranean area consistent with those previously reported for Sicily (e.g., [45][46][47]).
The high vertical oxygen and hydrogen gradients for Virunga are because of the high slope at Mt Nyiragongo, along which part of the samples were collected. Hence, in Virunga, a difference in gradients is observed between samples collected from the lowland sites found below 2000 m a.s.l. and those from the highland ones found above this altitude (see further discussions in [29,30]). Both the oxygen and hydrogen gradients at highland sites are 2-fold higher in comparison to those of lowland sites; these high values increase the overall gradient for Virunga. This difference is also attributed to two other important parameters: (1) different moistures affect highland and lowland areas in Virunga, and (2) the effect of precipitation amounts as they greatly vary between the low and highland sites (further discussed in Section 3.4). In the Central Mediterranean area, particularly Therefore, strong negative relationships were found between both the mean precipitation-weighted δ 2 H and δ 18 O and the altitude, while, conversely, the deuterium excess showed a strong positive relationship with the altitude (Figure 4). Virunga shows slightly higher vertical oxygen (−0.15% /100 m) and hydrogen (−0.86% /100 m) gradients compared to the Central Mediterranean area, where the vertical oxygen and hydrogen gradients are −0.24% /100 m and −1.00% /100 m for Stromboli and −0.16% /100 m and −1.07% /100 m for Sicily. However, Stromboli showed the higher d-excess gradient of 0.89/100 m (Virunga has a value of 0.32/100 m), which is consistent with the fact that the d-excess gradient has an opposite trend to that of the vertical oxygen and hydrogen gradients. The oxygen and hydrogen gradients for the Central Mediterranean area consistent with those previously reported for Sicily (e.g., [45][46][47]).

Meteorological Effects: Temperature and Precipitation Amount
The Virunga monthly air temperature means are nearly constant for a given station, and therefore show variations generally less than 2 °C at an annual time scale ( Figure 5A,B), because the region is located close to the equator. However, significant air temperature differences are observed between mean monthly values from stations found at different altitudes (e.g., compare temperature values of Figure 5A,B), or during diurnal air temperature variations at a given station. At a regional scale, in the Central and Eastern African regions nearing Virunga, precipitation is principally formed from the condensation of continental recycled moisture. The dominant evaporating and evapotranspiring surfaces are found in local lowlands [30,[48][49][50], and hence the surface air temperature at moisture source regions is comparable to that of the study area. Such a lack of spatiotemporal variation in the air temperature results in the noticed lack of dependency between The high vertical oxygen and hydrogen gradients for Virunga are because of the high slope at Mt Nyiragongo, along which part of the samples were collected. Hence, in Virunga, a difference in gradients is observed between samples collected from the lowland sites found below 2000 m a.s.l. and those from the highland ones found above this altitude (see further discussions in [29,30]). Both the oxygen and hydrogen gradients at highland sites are 2-fold higher in comparison to those of lowland sites; these high values increase the overall gradient for Virunga. This difference is also attributed to two other important parameters: (1) different moistures affect highland and lowland areas in Virunga, and (2) the effect of precipitation amounts as they greatly vary between the low and highland sites (further discussed in Section 3.4). In the Central Mediterranean area, particularly in Stromboli Island, the local topography does not allow to us evaluate for such an effect.

Meteorological Effects: Temperature and Precipitation Amount
The Virunga monthly air temperature means are nearly constant for a given station, and therefore show variations generally less than 2 • C at an annual time scale ( Figure 5A,B), because the region is located close to the equator. However, significant air temperature differences are observed between mean monthly values from stations found at different altitudes (e.g., compare temperature values of Figure 5A,B), or during diurnal air temperature variations at a given station. At a regional scale, in the Central and Eastern African regions nearing Virunga, precipitation is principally formed from the condensation of continental recycled moisture. The dominant evaporating and evapotranspiring surfaces are found in local lowlands [30,[48][49][50], and hence the surface air temperature at moisture source regions is comparable to that of the study area. Such a lack of spatiotemporal variation in the air temperature results in the noticed lack of dependency between δ 18 O and the air temperature in Virunga ( Figure 5A,B), and is the common trend found in tropical regions [3,51,52].  Figure 5A,B), and is the common trend found in tropical regions [3,51,52].  Figure 1A), while in the Central Mediterranean the data used in C are from a station in the Spagnuola area (see map of Figure 1B2) and data for D are from the LBR station in Stromboli (see map of Figure 1B1). No clear relationships can be recognized between isotope composition and temperature.
Virunga monthly δ 18 O, δ 2 H and d-excess show weak correlation with monthly precipitation amounts (R 2 varies from 0.11 to 0.32 for δ 18 O; 0.13 to 0.34 for δ 2 H and 0.00 to 0.45 for d-excess). Particularly, the δ 18 O dependency averages 2.96 ‰/100 mm of monthly precipitation, but ranges from 2.10 to 4.51 ‰/100 mm. A combination of monthly precipitation amounts, and the altitudes show that the δ 18 O precipitation amounts dependency is higher in lowlands, which was likely favored by the influence from the heavier water vapor from the neighboring Lake Kivu [30]. The δ 18 O, δ 2 H and d-excess relationships with monthly precipitation amounts show higher correlations when mean precipitation-weighted and the average annual precipitation values are plotted, and give an R 2 of 0.58 for δ 18 O (Figure 6A), 0.49 for δ 2 H ( Figure 6B) and 0.76 for d-excess ( Figure 6C).
In the Central Mediterranean, monthly air temperature averages show a clear seasonality with Particularly, the δ 18 O dependency averages 2.96 % /100 mm of monthly precipitation, but ranges from 2.10 to 4.51 % /100 mm. A combination of monthly precipitation amounts, and the altitudes show that the δ 18 O precipitation amounts dependency is higher in lowlands, which was likely favored by the influence from the heavier water vapor from the neighboring Lake Kivu [30]. The δ 18 O, δ 2 H and d-excess relationships with monthly precipitation amounts show higher correlations when mean precipitation-weighted and the average annual precipitation values are plotted, and give an R 2 of 0.58 for δ 18 O (Figure 6A), 0.49 for δ 2 H ( Figure 6B) and 0.76 for d-excess ( Figure 6C).

Stable Isotope Fractionation in Virunga
Virunga LMWL has a higher intercept compared to the GMWL, and it shows a slope comparable to that of the GMWL (Figure 2A) and that from other Central African regions (e.g., [12,[53][54][55]. Such a slope suggests processes of rain formation under equilibrium conditions at the regional scale. In addition, the intercept indicates that Virunga precipitation originates from

Stable Isotope Fractionation in Virunga
Virunga LMWL has a higher intercept compared to the GMWL, and it shows a slope comparable to that of the GMWL (Figure 2A) and that from other Central African regions (e.g., [12,[53][54][55]. Such a slope suggests processes of rain formation under equilibrium conditions at the regional scale. In addition, the intercept indicates that Virunga precipitation originates from high-altitude recycled continental moisture [15,56]. The high d-excess values of precipitation are the consequence of vapor formation under non-equilibrium conditions at source, favored by both low RH and strong winds. A difference of~10% in the d-excess values is observed when comparing lowland to highland areas of Virunga, and is caused by the kinetic fractionation during vapor formation and moisture transportation, and by the difference in the composition of the moistures arriving in the low and highlands. A secondary process additionally affects the raindrops during their fall as they pass through the water vapor in the lower atmosphere [30]. In fact, the pristine falling raindrops interact with the isotopically-enriched water vapor present in the lower atmosphere, the vapor coming mostly from Lake Kivu. This lake has isotopically enriched surface waters, showing values up to 27.76% and 3.68% for δ 2 H and δ 18 O, respectively [30]. The lower atmosphere in the study area is, thus, saturated in isotopically enriched water vapor when considering the recorded high RH (monthly values range: 70 to 90%). According to [57], such high RH favors equilibrium between the falling raindrops and the water vapor, a process that likely contributes to the δ 2 H and δ 18 O enrichment of Virunga lowland precipitation. Therefore, Virunga highlands receive only precipitation from the upper atmosphere produced from remote moisture sources, whereas the lowland areas are additionally affected by local water vapor that mediates the enrichment of the falling raindrops.
The moisture source regions for Virunga abound in small to large open surface waters, the surface waters of the lakes are particularly isotopically enriched (e.g., [30,[58][59][60][61][62][63][64][65]). The enrichment of their surface waters in the heavy isotopes is due to the intense evaporation process that in some cases exceeds the precipitation they receive over their surfaces. These lakes represent the predominant source of water vapor for the atmosphere during the dry periods, while during the rainy season the soil/plants evapotranspiration is the major source (further discussed in [30]). Because the plants-transpired vapor tends to be isotopically similar to the liquid water drawn into its roots (e.g., [66,67]), there are few variations in the δ 2 H and δ 18 O during the rainy season when considering the abundant neighboring tropical forest and other isolated plants. Thus, the precipitation from the rainy season is isotopically depleted because it is derived from the condensation of vapor from (1) freshly precipitated rain, (2) plants-transpired vapor and (3) because of the high precipitation amount. Conversely, during the dry season, the residual soil-water that is then evaporating will be composed of heavier isotopes, and this vapor mixes with the permanently enriched vapor from the lakes and, thus, yields enriched precipitation.
The changes in wind direction also influence the precipitation δ 2 H and δ 18 O, as it brings moisture from different sources. In fact, the equatorial East African region experiences three major air streams: the northeast monsoon system, the southeast monsoon system from the Indian Ocean, and the southwesterly humid air from the Congo basin [68]. The NE, E and SE winds bring moisture evaporated from Lake Edward, Lake Albert and Lake Victoria surface waters, in addition to that evapotranspired in their moderately to densely forested catchments. Conversely, the S and SW winds carry moisture evapotranspired in the Tanganyika-Kivu basins, especially that from the neighboring Lake Kivu. Once in the study area, the NE, E and SE winds principally occupy the upper atmosphere as they originate from outside the rift, while the S and SW follow the low-lying channel of the rift and are consequently maintained in the lower atmosphere. Thus, the NE and E originating high-speed air mass moves upward as a result of topography changes and might yield orographic lifting at Virunga Mountains. The moisture they bring principally influences the δ 2 H and δ 18 O of the precipitation of highland sites, whereas the S and SW low-level moisture additionally influences that of lowland sites. The high speeds of the NE and E-SE winds yield the earlier discussed kinetic fractionation that lead to the higher d-excess values that characterize the precipitation of highland sites. Conversely, the S and SW winds in the lower atmosphere are less strong and favor a relatively equilibrium fractionation in conjunction with the high RH prevalent in lowland areas. Because of the topographic constraints of the rift structure and the prevailing weak winds, this S to SW originating moisture is essentially permanent in the lower atmosphere and drives the earlier noted isotopic enrichment of the falling rain drops.

Stable Isotope Fractionation in the Central Mediterranean
At a regional scale, in the Central Mediterranean, precipitation is principally formed from the condensation maritime vapor forming over the Atlantic Ocean as well as from the Mediterranean Sea [32,33]. The Mediterranean Sea is a classic example of a reservoir producing water vapor characterized by high deuterium excess [13,16]. Such an effect depends on kinetic fractionation occurring when dry air masses travel over the sea surface. This effect coupled with the kinetic condensation of local orographic clouds, forming on significant topographic features such as hills and mountains, is responsible for the high deuterium excess found in rain samples from mountains in the Central Mediterranean. The high seasonal variation in d-excess between summer and winter reflects the climatic features and circulation patterns of the Central Mediterranean. In fact, when cold air masses, characterized by low relative humidity, travel over seawater, kinetic fractionation produces high deuterium excess values in the vapor, whereas the movement of warm air masses, usually characterized by high relative humidity, leads to lower deuterium excess values [33]. At Stromboli Island, the production of volcanic aerosol close to the craters favors the non-equilibrium condensation of water droplets. Under these conditions, the diffusional growth of hydrometeors could cause a great deuterium excess as a result of the higher diffusivity of HD 16 O compared to that of H 2 18 O [69]. Therefore, the coalescence of raindrops and haze droplets in the orographic cloud generates the increase in the deuterium excess values in the rain. Consequently, the composition of the precipitation is not changed directly by volcanic activity. Nevertheless, the production of volcanic aerosols could indirectly enhance deuterium excess in rain near the craters.

Conclusions
Virunga precipitation originates from high-altitude recycled continental moisture. The slope of the LMWL further reveals that during the condensation, rainwater forms under equilibrium, conditions as is the case for central Africa forest regions, whereas the higher intercept and the δ 18 O values are closer to those from the East Africa region. Even though Virunga is generally a high-altitude region, a clear shift is observed between the isotope composition of the precipitation from the local lowland and highland sites, which is topographically driven. In fact, the lowland sites are characterized by higher δ 2 H and δ 18 O and lower d-excess values, while the highland areas have lower δ 2 H and δ 18 O and higher d-excess. The values δ 2 H, δ 18 O and d-excess at low-and highland areas are in line with the weather patterns such as the temperature (temperature is lower at highland and higher at lowland areas), wind speed and direction (high speed and of well-defined direction at highland, calm and generally lacking of unique-direction at lowland) or precipitation (higher and lower precipitation at highland and lowland areas, respectively). The monthly δ 2 H and δ 18 O showed a well-defined seasonality, with the heavier δ 2 H and δ 18 O obtained during the dry periods and the lighter, in the wet periods, with strong correlation to precipitation amount. The vapor from the East African Great Lakes has a strong influence on the isotopic composition of Virunga precipitation, particularly during the dry season, while in the rainy season, the precipitation is mainly formed from soil/plants evapotranspiration. No δ 18 O dependency on air temperature was observed as is generally the case for tropical regions, while the δ 18 O and δ 2 H are negatively correlated to precipitation amounts.
In the Central Mediterranean, the isotopic composition of precipitation clearly reflects the origin of the vapor feeding the raindrops. The climatic and morphological features of the Mediterranean represent a unique natural environment where air masses, mainly coming from the Atlantic Ocean, travel over the Mediterranean Sea and produce rain events from the condensation of water with different proportions of vapor from different areas. In addition, local orographic clouds induce a kinetic condensation process. With respect to Virunga, precipitation occurring in the Central Mediterranean does not show any clear contribution from the re-evaporated air masses, because of the poor vegetation cover and the regional air circulation systems.
The marked differences of the isotopic composition of precipitation in Virunga and in the Central Mediterranean reflect very different climatic and morphological features, where several fractionation processes and vapor sources play dissimilar roles. This study, thus, represents a key paper for researchers dealing with this topic.
Author Contributions: C.M.B. and M.L. drafted the manuscript, which they had latter amended and approved.
Funding: This research received no external funding and "the APC was funded by the Istituto Nazionale di Geofisica e Vulcanolgia -Sezione di Palermo.