The AD 79 eruption of Somma-Vesuvius completely buried the ancient landscape around Pompeii (Italy) to some extent conserving the pre-AD 79 Roman paleosols of the Sarno River plain. To estimate potential post-burial soil developments of these paleosols detailed soil liquid and solid phase analysis were carried out. Firstly, an in-situ soil hydrological monitoring was conducted within a pre-AD 79 paleosol in natural undisturbed stratification. The results show that soil water flow and nutrient transport from the overlying volcanic deposits into the pre-AD 79 paleosol take place. Secondly, to estimate their influence on the paleosol’s mineral soil properties, the solid phase of four pre-AD 79 paleosols and associated modern unburied soils were analysed and compared. By combining the data from the soil liquid and solid phase analysis, potential post-burial changes in the paleosols were estimated. Finally, a rise of the mean groundwater table was determined since AD 79. This distinguishes the Sarno River plain into two different zones of post-burial soil developments: 1) lower altitudes where formerly terrestrial paleosols are now influenced by groundwater dynamics and 2) higher altitudes where the paleosols are still part of the vadose zone and rather influenced by infiltration water or interflow. Thus, the mechanism of potential post-burial soil development being active in the pre-AD 79 paleosols is not uniform for the entire Sarno River plain but strongly depends on the paleotopographic situation.
Paleosols are geoarchives of a past soil formation since diverse mechanisms of burial can contribute to a preser- vation of original soil properties. Consequently, paleosols can yield important information on paleopedological and paleoenvironmental conditions before burial. However, a critical use of paleosol characteristics in interpret- ing paleoenvironments must be applied because pronounced diagenetic processes, i.e. post-burial soil develop- ments, can take place, such as (Crowther et al., 1996 [
1) Decomposition of soil organic matter (SOM)
Within well drained terrestrial soils SOM is decomposed after burial by aerobic microorganisms. That can be detected by a change in the Munsell soil colour towards brighter values (higher chroma). However, the general depth function of the SOM concentration of the paleosol stays mostly unaltered.
2) Leaching of nutrients from the overlying strata
In highly permeable substrate seepage water can lead to a dislocation of bases from the upper layers into the paleosol and hence to an alteration of the paleosol’s nutrient status.
3) Soil compaction after burial
The superimposed load of the overlying deposits can cause soil compaction and a reduction of pore space.
Due to the probability of post-burial soil developments detailed field descriptions and laboratory analyses are necessary to understand both past and present pedogenetic processes that lead to the present-day appearance and properties of a paleosol (Scudder et al., 1996 [
The explosive eruption of Somma-Vesuvius AD 79 almost completely buried the ancient landscape around Pompeii to some extent conserving the pre-AD 79 Roman paleosols. Foss, 1988 [
Inoue et al., 2009 [
As from today’s state of research little is known about the development of volcanic paleosols after their burial (Ze- hetner et al., 2003 [
The soil liquid and solid phase was studied in the vicinity of Pompeii. It is located in the Sarno River plain in the southern part of the Campanian plain and southwest of Somma-Vesuvius volcanic complex (
The climate of the study area is Mediterranean with almost 70% of the annual precipitation falling between October and March and a pronounced dry summer season. On a long term average the mean annual precipitation is 865 mm whereas November is the wettest month with 129 mm and July is dryest with 16 mm. The mean an- nual temperature is 17.4˚C. The hottest month is August with a mean temperature of 25.7˚C whereas January is coolest with 9.6˚C (Osservatorio Meteorologico, Università di Napoli Federico II, from 1870 to today).
The soil liquid phase was sampled within the archaeological excavation of Villa Regina (Boscoreale) around 1.3 km northwest of ancient Pompeii. Topographically, this Roman farm (villa rustica) is situated in a longitu- dinal depression between the western footslopes of the Pompeiian hill and the southern footslopes of Somma-
Vesuvius (
1) The post-AD 79 deposits at the footslopes of Somma-Vesuvius have an above-average thickness of 8 m compared to other parts of the plain. Consequently, in case soil water flow into the paleosol can be detected at Boscoreale it is likely to be occurring in most areas of the Sarno River plain where the post-AD 79 deposits are usually thinner.
2) At the archaeological excavation of Villa Regina the pre-AD 79 paleosol was easily accessible to install the soil hydrological monitoring system in natural and undisturbed stratigraphy. Furthermore, it was protected from vandalism over the study period of 20 months.
The soil solid phase was sampled at three different sites incorporating different topographic situations as well as slightly different stratigraphic conditions (
1) Scafati (Via della Resistenza (VdR)): This site is located in the river plain approximately 3 km east of an- cient Pompeii. The pre-AD 79 paleosol is situated in a depth of about 3.3 m. It is covered by 2 m of pumice fallout and 0.9 m of pyroclastic density current (PDC) deposits from the explosive AD 79 eruption of Somma- Vesuvius followed by a medieval paleosol and the modern soil.
2) Pompeii (Villa dei Misteri (VdM)): It is situated at the southwestern slope of the Pompeiian hill near the archaeological site of Villa dei Misteri and about 350 m northwest of the city walls of ancient Pompeii. The pre- AD 79 paleosurface appears in a depth of 5.7 m. The thicknesses of overlying AD 79 pumice fallout and PDC deposits are about 2.3 m and 1.7 m, respectively. This is followed by about 1.7 m of a medieval paleosol and the modern soil (Rispoli et al., 2008 [
3) Boscoreale (Villa Regina (VR)): Five mechanical core drillings were carried out near the archaeological excavation of Villa Regina (VR) along a NW-SE transect crossing the longitudinal depression between the Pompeiian hill and Somma-Vesuvius. The drillings DAI18 and DAI19 are situated in the northwest at the foot- slope of Somma-Vesuvius whereas DAI17 is located at the footslope of the Pompeiian hill. DAI16 is situated at the bottom of the depression at the exact location of the soil liquid phase analysis whereas DAI20 is located on top of the Pompeiian hill in the southeast (Vogel and Märker, 2012 [
According to groundwater data of the “Autorità di Bacino del Sarno” the pre-AD 79 Roman layer of the three study sites is not influenced by groundwater. Thus, the pre-AD 79 paleosols are part of the vadose zone and soil water within the profile derives vertically from infiltration water or laterally from interflow.
To assess the post-burial soil developments of the pre-AD 79 Roman paleosols near Pompeii, we conducted soil liquid and solid phase analysis combined with an estimation of the post-AD 79 groundwater trend. Consequently, the following sections will be subdivided accordingly.
Between the eruption of Somma-Vesuvius in AD 79 and today a relative rise of the groundwater table can be determined for the Sarno River plain. Consequently, in many parts of the inner plain, although terrestrial before AD 79, the pre-AD 79 paleosol has come under the influence of groundwater. To calculate this approximate rise of the groundwater table since AD 79 and to confine the zone where the primal terrestrial pre-AD 79 paleosol is recently influenced by groundwater dynamics, present-day groundwater data were combined with the pre-AD 79 topography (Vogel and Märker, 2010 [
At first, the depth of the present-day mean annual groundwater table was modelled from more than 5600 groundwater observation points (by courtesy of the Autorità di Bacino del Sarno). Thereafter, this groundwater table was subtracted from the pre-AD 79 digital elevation model (DEM) of the Sarno River plain to determine the area where the groundwater table recently lies above the pre-AD 79 soil surface. Vogel et al., 2011 [
From these two groundwater levels (at the pre-AD 79 surface, today and before AD 79) the mean “altitude above channel network” index (AACN; SAGA terrain analysis module; 0.06% channel network density) was deduced from the pre-AD 79 DEM. The AACN calculates the vertical distance to the pre-AD 79 Sarno River and thus directly reflects the groundwater table. Finally, by subtracting the arithmetic means of the two indices the approximate rise of the groundwater table since AD 79 was determined.
At the archeological excavation of Villa Regina (Boscoreale) a soil hydrological monitoring system was in- stalled within the pre-AD 79 paleosol in natural stratification. It aims at determining if the pre-AD 79 paleosol is subject to soil water flow from the overlying volcanic deposits either vertically by percolation or laterally by interflow. The soil moisture and the soil liquid phase were determined in-situ by means of frequency domain re- flectometer (FDR) and tension lysimeters (suction cups), respectively. Tension lysimeters were used since this methodology predominantly samples the mobile fraction of soil water that moves through inter-aggregate pores or preferential flow channels and thus reflects the chemical transport within the stratigraphy (Wolt, 1994 [
Altogether four sampling systems were installed, two within the lower section of the AD 79 pumice fallout layer and two within the pre-AD 79 paleosol underneath. The composition of the soil liquid phase may not di- rectly refer to specific chemical processes active in the pre-AD 79 paleosol. Nevertheless, the net effect of recent
transport of nutrients into the paleosol is manifested in the concentration of substances in the soil water extract (Wolt, 1994 [
Due to the remarkable depth of burial and the absence of interactions with present-day plant roots the compo- sition of the soil liquid phase within the pre-AD 79 paleosol will be less heterogeneous compared to the rooting zone of the modern soil. This may also be favoured by the relative chemical homogeneity of the AD 79 pumice fallout layer overlying the pre-AD 79 paleosol. Consequently, four sampling systems are expected to yield qua- litative data on the soil liquid phase chemistry of the sampling area.
In the paleosol the suction cups were installed in slurry of the paleosol material to ensure an optimal contact between the instrument and the surrounding soil and a minimum of disturbance. Because of the coarseness of the AD 79 pumice lapilli layer above, the suction cups could not be installed in its own material. Hence, silica slurry of fine sand was used having a low adsorption capacity to minimise the influence of the slurry material on the soil liquid phase.
To extract the soil liquid phase a transient tension (vacuum) of 550 mbar (pF 2.7) was established biweekly to the suction cups by evacuation with an external pump. Due to a slight decrease of tension during the two weeks between evacuations an applied tension of 550 mbar enables the sampling of the soil liquid phase near field ca- pacity. Even though the gradient generated by the suction cups will not exclusively act on macropores but to some extent also on smaller pores (Grossmann and Udluft, 1991 [
where r is the radius of the capillary tube,
Through the introduction of oxygen into the soil during installation of the suction cups the biological activity of the paleosol may be stimulated causing a mobilization of nutrients. This can influence the composition of the soil liquid phase directly after installation (Grossmann, 1988 [
The suction cups were installed 1.5 m inside the stratigraphy to prevent them from being influenced by water entering the soil profile from the open excavation side of Villa Regina. Furthermore, the installation was carried out on the upslope part of the excavation assuring that rain water directly entering the excavation is diverging. Additionally, the entire installation was covered by a roof over its whole length (
Suction cups were used that collect the solution in their interior. That has the advantage that the extracted wa- ter stays cool and protected from sunlight until the sampling takes place, minimising chemical transformations within the lysimeter solution. Due to its specific adsorption characteristics the filter material of the suction cup may have an influence on the composition of the sample. Consequently, the investigation of a certain analyte requires a particular filter material. As this study focusses on dissolved inorganic substances polyamide filters were used (Litaor, 1988 [
The composition of the soil liquid phase was monitored over a period of 20 months to cover the variability throughout the year. The autumn and winter period was studied twice. This period is believed to be most rele- vant because the precipitation maximum and relatively low evapotranspiration between October and March is most likely to cause soil water flow down to the pre-AD 79 paleosol. The soil liquid phase in the suction cups was controlled biweekly, sampled and analysed for: 1) pH value, 2) major cations (Ca2+, Mg2+, K+, Na+) and anions (
Since the composition and amount of the soil liquid phase sampled by the tension lysimeters varies with soil water content, additionally, three soil moisture sensors (SM200) where installed in the paleosol measuring the volumetric water content (θ) every six hours. The SM200 soil moisture sensor is a frequency domain reflecto- meter (FDR) measuring θ indirectly by determining the permittivity of the soil (Delta-T Devices Ltd., 2006 [
The soil liquid phase of the vadose zone strongly depends on water deriving from precipitation. To get a rough estimate of how the precipitation regime interacts with the soil moisture of the pre-AD 79 paleosol and its soil water chemistry throughout the year, meteorological data were collected (Campania Region). It may not be possible to relate a single precipitation event to the soil water sampled in the paleosol at a depth of 8 m. Never- theless, especially the time between the first precipitations in autumn and the first response in the paleosol’s soil moisture as well as the first soil water sampling will be of particular interest. Moreover, the first soil water flow after the dry summer season is expected to have a strong influence on the composition of the soil liquid phase in terms of higher element concentrations that are leaching into the pre-AD 79 paleosol (Arthur and Fahey, 1993 [
At Scafati, Pompeii and Boscoreale modern soils and associated pre-AD 79 paleosols were identified in four stratigraphies (DAI16, DAI18, VdM, VdR). They were described in the field and the soil solid phase was sam- pled for the subsequent soil physical and chemical laboratory analyses.
Following generally Retallack, 1998 [
To assess the effects of soil water flow and associated nutrient transport on the mineral soil properties the fol- lowing analyses were carried out: 1) soil pH measured in 0.01 M CaCl2 (DIN ISO 10390 [
To study the post-burial decomposition of soil organic matter in the paleosols total organic carbon (TOC) was determined by elementary analysis using the dry combustion reference method (DIN EN 13137 [
As mentioned earlier Foss, 1988 [
The particular chemical composition of the modern soils and the pre-AD 79 paleosols may differ due to a slightly different geology of the parent volcanic material as well as due to anthropogenic input in terms of fertil- izers or waste materials. This may falsify the comparison of the two soils. To characterise the chemical compo- sition of the soils, total amounts of Na, Ca, K, Mg, Mn, Al and Fe were determined by element analysis using aqua regia (nitrochloric acid) and AAS /ICP-AES. To eliminate the effect of a different geology of the soil sub- strate and anthropogenic input ratios were introduced correcting a particular measured element concentration by its total concentration (see Equations (2) and (3)).
The CEC ratio allows for a comparison of the nutrient status of the paleosols and the modern soils even if both soils exhibit different total element concentrations. A consistently higher CEC ratio in the paleosols may for instance indicate nutrient input via leaching. As CEC is the sum of exchangeable cations the CEC ratio was calculated by dividing the cation exchange capacity by the sum of the total concentrations of K, Na, Ca and Mg (Equation (2)).
The nutrient ratio is used for the comparison of a particular nutrient cation, hence dividing the amount of an exchangeable cation (Kexch, Naexch, Caexch, Mgexch) by the total amount of the respective element (Ktot, Natot, Catot, Mgtot) (Equation (3)). Thus, the nutrient ratio helps to identify which nutrients are predominantly leaching from the volcanic deposits into the paleosols.
By means of division by the total element concentrations, the CEC ratio and the nutrient ratios also eliminate methodical defects of the CEC method with respect to CaCO3 contents > 10 g∙kg−1 which may cause dissolution of CaCO3 and increased values of CEC and exchangeable Ca2+ (Dohrmann and Kaufhold, 2009 [
For the comparison between the modern soils and the pre-AD 79 Roman paleosols only topsoils were consid- ered as in general they are characterised by high SOM and nutrient turnover rates as well as by major soil chemical reactions due to plant roots and microorganisms. Finally, considering the results of both the soil liquid and soil solid phase analysis major differences between the unburied modern soils and the buried paleosols are utilised to gain valuable insights into potential post-burial soil developments of the pre-AD 79 paleosols.
The combination of the present-day groundwater table with the pre-AD 79 topography reveals that a large area of the inner Sarno River plain is affected by a groundwater table lying at or above the pre-AD 79 paleosurface (
The phase of rewetting of the pre-AD 79 paleosol after the dry summer season lasted from early December 2008 until February 2009 when θ continuously increased to its maximum of 25.8%. The main rainy season ter- minated in mid-February with more sporadic rainfall events until the end of April. From mid March and again with a delayed response of about one month θ started to decrease, at first rather slightly and more explicitly from the end of April. Then θ decreased to its minimum of 13.6% in the end of September 2009. Finally, the soil moisture data reveal that the pre-AD 79 paleosol at Villa Regina was at no time of the year water saturated.
The first soil liquid phase was extracted by the suction cups at the end of December, another month after the first response of θ. By the time of the first soil water sampling θ was 24.3%. Henceforward, soil liquid phase could
be sampled biweekly for the next four months until the end of April 2009. By the end of the sample period θ had decreased to 24.5%.
In contrast to the pre-AD 79 paleosol no soil water could be extracted from the AD 79 pumice lapilli fallout directly above.
During the winter season of 2009/10 the first lysimeter sample was taken at the beginning of February 2010 about one month later than 2009. This coincides with the paleosol having a much lower initial soil moisture (θ) in the summer of 2009 (13.6%) in comparison to 2008 (22%) (
The soil water chemistry of the 2008/09 and 2009/10 seasons is summarised in Tables 1 and 2. The pH value of the soil liquid phase represents the active acidity of the pre-AD 79 paleosol (Mengel and Kirkby, 2001 [
Date of sampling | 29-Dec 2008 | 14-Jan 2009 | 26-Jan 2009 | 9-Feb 2009 | 24-Feb 2009 | 9-Mar 2009 | 23-Mar 2009 | 6-Apr 2009 | 15-Apr 2009 | 30-Apr 2009 | Arithmetic mean |
---|---|---|---|---|---|---|---|---|---|---|---|
θ [%] | 24.3 | 24.8 | 25.4 | 25.8 | 25.4 | 25.6 | 25.7 | 25.6 | 25.4 | 24.5 | 25.3 |
pH | 7.7 | 7.6 | 7.5 | 7.8 | 7.6 | 7.6 | 7.2 | 7.3 | 7.7 | 7.4 | 7.5 |
ECcalc [µS cm–1] at 25˚C (ECcalc/θ) | 526.2 (21.7) | 433.0 (17.5) | 388.1 (15.3) | 405.6 (15.7) | 398.2 (15.7) | 401.9 (15.7) | 463.7 (18.0) | 464.4 (18.1) | 465.0 (18.3) | 459.7 (18.8) | 440.6 (17.5) |
Cations [mg l–1] | |||||||||||
Ca2+ (Ca2+/θ) | 41.1 (1.69) | 40.4 (1.63) | 38.2 (1.50) | 38.3 (1.48) | 38.1 (1.50) | 39.3 (1.54) | 48.0 (1.87) | 48.0 (1.88) | 48.6 (1.91) | 48.7 (1.99) | 42.9 (1.70) |
K+ (K+/θ) | 45.6 (1.88) | 39.3 (1.58) | 37.1 (1.46) | 33.8 (1.31) | 32.9 (1.30) | 32.8 (1.28) | 33.7 (1.31) | 33.7 (1.32) | 33.6 (1.32) | 33.8 (1.38) | 35.6 (1.41) |
Na+ (Na+/θ) | 24.5 (1.01) | 25.1 (1.01) | 23.1 (0.91) | 19.8 (0.77) | 20.5 (0.81) | 21.6 (0.84) | 24.1 (0.94) | 24.2 (0.95) | 24.8 (0.98) | 25.0 (1.02) | 23.3 (0.92) |
Mg2+ (Mg2+/θ) | 5.1 (0.21) | 4.8 (0.19) | 4.4 (0.17) | 5.3 (0.20) | 5.1 (0.20) | 5.2 (0.20) | 6.6 (0.26) | 6.6 (0.26) | 6.7 (0.26) | 6.8 (0.28) | 5.7 (0.22) |
Al3+ (Al3+/θ) | 0.3 (0.01) | 0.1 (0) | ND | ND | ND | ND | ND | ND | ND | ND | - |
Anions [mg∙l–1] | |||||||||||
Cl– (Cl–/θ) | 75.3 (3.10) | 53.5 (2.16) | 43.8 (1.72) | 58.8 (2.28) | 56.2 (2.21) | 55.2 (2.16) | 67.0 (2.61) | 67.1 (2.62) | 65.2 (2.57) | 63.4 (2.59) | 60.5 (2.4) |
44.3 (1.82) | 27.1 (1.09) | 20.9 (0.82) | 19.5 (0.76) | 19.3 (0.76) | 19.1 (0.75) | 20.8 (0.81) | 20.8 (0.81) | 21.7 (0.85) | 20.2 (0.82) | 23.4 (0.93) | |
33.2 (1.36) | 24.2 (0.98) | 23.2 (0.91) | 21.0 (0.81) | 20.8 (0.82) | 21.2 (0.83) | 20.9 (0.81) | 21.0 (0.82) | 21.4 (0.84) | 20.6 (0.84) | 22.7 (0.90) | |
ND | ND | ND | ND | ND | ND | ND | ND | ND | ND | - |
Date of sampling | 2-Feb 2010 | 18-Feb 2010 | 2-Mar 2010 | 18-Mar 2010 | 1-Apr 2010 | 15-Apr 2010 | 29-Apr 2010 | 13-May 2010 | Arithmetic mean |
---|---|---|---|---|---|---|---|---|---|
pH | 7.4 | 7.8 | 7.7 | 7.7 | 7.8 | 7.8 | 7.8 | 7.8 | 7.7 |
ECcalc [µS∙cm–1] at 25˚C | 528.2 | 495.3 | 465.0 | 470.1 | 478.2 | 498.9 | 489.3 | 502.1 | 490.9 |
Cations [mg∙l–1] | |||||||||
Ca2+ | 50.3 | 46.3 | 40.2 | 40.8 | 41.3 | 45.7 | 46.0 | 46.8 | 44.7 |
K+ | 46.7 | 42.3 | 39.7 | 40 | 41 | 40.7 | 40.5 | 40.8 | 41.5 |
Na+ | 18.3 | 20.4 | 21.4 | 21.9 | 22 | 23.8 | 22.5 | 24.6 | 21.9 |
Mg2+ | 6.9 | 7 | 6.7 | 6.72 | 6.75 | 6.8 | 6.7 | 6.8 | 6.8 |
Al3+ | 0.2 | 0.1 | ND | ND | ND | ND | ND | ND | - |
Anions [mg∙l–1] | |||||||||
Cl– | 70.3 | 60.2 | 58.3 | 58.7 | 60.2 | 62.3 | 61.5 | 61.9 | 61.7 |
38.7 | 39.6 | 37.5 | 37.9 | 38 | 38.4 | 39.0 | 38.7 | 38.5 | |
32.7 | 33.5 | 30.8 | 31.1 | 32 | 32.5 | 31.6 | 31.8 | 32.0 | |
1.1 | 0.7 | 0.5 | 0.5 | 0.6 | 0.7 | 0.6 | 0.7 | 0.68 |
be detected. Moreover, no
The amount of water available in the soil has a distinct influence on the liquid phase chemistry in terms of concentration or dilution effects during dry or wet conditions, respectively (Mengel and Kirkby, 2001 [
During the winter season of 2009/10 the above described characteristics of the soil water chemistry of the pre-AD 79 paleosol recurred. The ion concentration is increased at the beginning, then decreases to its minimum in the middle of the sample period and re-increases at its end (
Unfortunately, in autumn 2009 all three soil moisture sensors failed as they probably lost their attachment to the substrate possibly due to soil shrinkage during the dry summer season. Consequently, the ion concentrations could not be corrected for θ which would have even amplified the course of soil water chemistry to a small ex- tent as it did in 2008/09.
Tables 3 and 4 show the results of the soil solid phase analysis for the modern soils and the associated pre-AD 79 paleosols. Only when a soil parameter shows the same trend in all four pairs of soils, we assumed this to be the result of post-burial effects.
Exchangeable cations [mmolc∙kg–1] | ||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
Site | Soil | Soil texture | Munsell soil colour | pH (CaCl2) | CaCO3 [g∙kg–1] | TOC [g∙kg–1] | N [g∙kg–1] | CEC [mmolc∙kg–1] | K | Na | Ca | Mg |
DAI16 | modern soil | Loamy sand | 10YR3/1 | 6.0 | 7.1 | 11.6 | 1.1 | 139.0 | 20.7 | 48.7 | 52.2 | 17.1 |
pre-AD 79 paleosol | Sandy loam | 10YR4/2 | 7.1 | 7.9 | 8.9 | 0.8 | 140.2 | 20.3 | 13.2 | 94.7 | 11.7 | |
DAI18 | modern soil | Sandy loam | 10YR3/1 | 7.4 | 30.8 | 8.0 | 0.5 | 66.0 | 19.0 | 10.4 | 32.7 | 3.4 |
pre-AD 79 paleosol | Sandy loam | 10YR4/2 | 7.5 | 16.2 | 3.4 | 0.1 | 172.2 | 16.3 | 6.6 | 138.5 | 10.7 | |
VdM | modern soil | Loamy sand | 10YR3/2 | 6.8 | 2.7 | 14.3 | 1.4 | 216.9 | 33.6 | 5.6 | 164.0 | 14.3 |
pre-AD 79 paleosol | Sandy loam | 10YR4/3 | 7.7 | 4.0 | 3.1 | 0.3 | 222.9 | 22.1 | 8.3 | 177.1 | 15.1 | |
VdR | modern soil | Loamy sand | 10YR3/1 | 5.8 | 0.5 | 16.3 | 2.6 | 107.5 | 6.3 | 7.7 | 83.8 | 9.7 |
pre-AD 79 paleosol | Sandy loam | 10YR3/1 | 8.7 | 0.2 | 6.0 | 1.7 | 183.5 | 18.8 | 26.7 | 125.2 | 12.7 |
Nutrient ratio | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|
Site | Soil | Ktot [g∙kg–1] | Natot [g∙kg–1] | Catot [g∙kg–1] | Mgtot [g∙kg–1] | CECpot ratio | ||||
DAI16 | modern soil | 37.4 | 12.8 | 29.2 | 11.4 | 0.16 | 0.55 | 3.80 | 1.79 | 1.50 |
pre-AD 79 paleosol | 25.8 | 8.4 | 31.7 | 9.2 | 0.16 | 0.79 | 1.57 | 2.99 | 1.27 | |
DAI18 | modern soil | 38.6 | 13.5 | 38.2 | 13.1 | 0.07 | 0.49 | 0.77 | 0.86 | 0.26 |
pre-AD 79 paleosol | 33.2 | 16.7 | 30.3 | 8.3 | 0.15 | 0.49 | 0.40 | 4.57 | 1.29 | |
VdM | modern soil | 37.3 | 11.9 | 31.6 | 9.7 | 0.24 | 0.90 | 0.47 | 5.19 | 1.47 |
pre-AD 79 paleosol | 15.0 | 9.8 | 23.0 | 6.4 | 0.29 | 1.47 | 0.85 | 7.70 | 2.36 | |
VdR | modern soil | 35.4 | 10.7 | 32.5 | 10.5 | 1.21 | 0.18 | 0.72 | 2.58 | 0.92 |
pre-AD 79 paleosol | 21.1 | 6.6 | 23.2 | 7.8 | 3.13 | 0.89 | 4.05 | 5.40 | 1.63 |
The modern soils are well penetrated by roots and soil fauna and generally appear loosely structured. In con- trast, the paleosols have a massive structure with little recognizable macro porosity. The mean bulk density of the paleosols is 1.4 g/cm³ and that of the modern soils is 1.1 g/cm³. The Munsell soil colour (on moist samples) of the modern soils is very dark gray and that of the paleosols is dark grayish brown. The pH values of the mod- ern soils are between 5.8 and 7.4 and of the paleosols between 7.1 and 8.7. Furthermore, in contrast to the mod- ern soils, in the paleosols the pH value is not consistently increasing with increasing depth. The CEC ratios (Equation (2)) of the paleosols are higher compared to the modern soils. This coincides with higher nutrient ra- tios (Equation (3)) for Ca and K. The modern soils have low to moderate soil organic matter (SOM) contents whereas in the pre-AD 79 paleosols it is very low to low. This corresponds to amounts of total organic carbon (TOC) of 8 to 16.3 g∙kg−1 for the modern soils and 3.1 to 8.9 g∙kg−1 for the paleosols. Accordingly, nitrogen ranges between 0.5 and 2.6 in the modern soils and between 0.1 and 1.7 in the paleosols.
As mentioned above, after AD 79 the groundwater table increased by about 1.8 m. To some extent, this can be an indirect result of the AD 79 eruption. The burial of the Sarno River plain is believed to have left a bare land- scape free of vegetation and covered with unconsolidated volcanic material. Consequently, due to these unstable conditions especially the surrounding steep mountain slopes were vulnerable to severe erosion processes during intense rainfall events causing debris flows and landslides (Cinque et al., 2000 [
Considering this groundwater rise, the Sarno River plain must be divided into two different zones of potential post-burial soil developments:
1) the lower areas of the inner Sarno River plain where after AD 79 the originally terrestrial pre-AD 79 pa- leosol has come under the influence of a rising groundwater table; and
2) the higher areas of the Sarno River plain where the paleosol is still part of the vadose zone and more likely influenced by vertical or lateral soil water flow.
This demonstrates that the mechanism of post-burial soil developments being active in the pre-AD 79 paleo- sols is by no means uniform for the entire Sarno River plain but strongly depends on the paleotopography and the paleolandscape position.
Vogel and Märker, 2012 [
Soil water could be extracted from the paleosol in a four months period between the end of December 2008 and the end of April 2009. The paleosol required a approximate soil moisture of more than about 24.5% for mo- bile gravitational water to be available and to be extracted by the suction cups with the applied tension of 550 mbar. In contrast, when θ is below 24.5% the matric potential attracting the soil water to the mineral soil parti- cles is increased and exceeds the tension of the suction cups.
These above mentioned results verify that a soil water flow from the overlying volcanic deposits into the pre- AD 79 paleosol does take place as it was assumed by Foss, 1988 [
In contrast to the pre-AD 79 paleosol no soil water could be extracted from the AD 79 pumice lapilli fallout directly above. This could be due to the following two reasons. Firstly, the porosity of the pumice layer has a bimodal structure. On one hand the pumice lapilli has a very coarse texture with a lot of external macropores between the clasts. On the other hand each pumiceous particle consists of a very fine internal porous system. Thus, the water may not move homogeneously through the pumice layer. In the macropores between the clasts it randomly follows preferential flow paths until it is absorbed by the pumiceous particles showing a high water retention capacity (Sahin et al., 2005 [
The analysis of the soil water chemistry at Villa Regina showed that the dominant cation percolating into the pre-AD 79 paleosol is calcium. Despite the high concentration of Cl?, the relatively low ECcalc of the soil water extract reveals that the pre-AD 79 paleosol can be considered non-saline (Schoeneberger et al., 2002 [
The course of the soil water chemistry revealed increased ion concentrations at the beginning and at the end of the sample period. This can be explained by the “first flush effect” that can occur at the beginning of the rainy season when the first infiltration water reaches the pre-AD 79 paleosol after a dry summer season. An accumu- lation of ions during summer then results in higher element concentrations in the soil water extract. Arthur and Fahey, 1993 [
It is striking that the “first flush effect” is much stronger for the anions than for the cations. This may be due to the fact that volcanic soils have a high fraction of variable charge (Madeira et al., 2007 [
Beyond the described “first flush effect” no significant seasonal variation could be detected in the soil water chemistry. This may be due to the following reasons:
1) In the pre-AD 79 paleosol at about 8 m depth plants are absent. Thus, there is no influence on the soil water composition by nutrient uptake and ion secretion of plant roots;
2) The mobile soil water fraction predominantly sampled by tension lysimeters has much shorter contact times with the soil solid phase compared to capillary water. This results in lower ion concentrations and less seasonality (Wolt, 1994 [
3) Since soil water could only be extracted from the paleosol in a short period of four months, there was no seasonal variation in the soil water composition.
From the relative stratigraphic position of the pre-AD 79 paleosols and the modern soils and by comparison with the eruption history of Somma-Vesuvius an estimation of their approximate soil age was made. With respect to the paleosols, the total soil age has to be subdivided into two main soil development periods, i.e. the duration of pre-burial and post-burial soil development. The parent volcanic material at Boscoreale (VR) and Pompeii (VdM) can most likely be ascribed to the Avellino eruption or the following AP1/AP2 eruption (3450 - 3000 BP; Andronico and Cioni, 2002 [
The results of the soil solid phase analysis show distinct characteristics that distinguish the topsoils of the buried pre-AD 79 paleosols from the unburied modern soils at the same topographic location. The paleosols have a pH value that is more than one unit higher compared to the modern soils. This corresponds with the ob- servations of Foss, 1988 [
1) Active accumulation and decomposition of soil organic matter (SOM) producing organic acids;
2) Nutrient uptake and proton release of plant roots;
3) Acid deposition from the atmosphere (acid rain); and
4) Leaching of basic cations from the upper horizons by percolating soil water.
In contrast, the results of the soil liquid phase study verify that the pre-AD 79 paleosols are rather subject to post-burial nutrient input by leaching from the overlying deposits which increased their pH value. This confirms the earlier mentioned assumption of Foss, 1988 [
It was determined that the pH of the paleosols often does not consistently increase with depth as can be seen for most of the modern unburied soils. After burial, the former A-horizons of the paleosols are subject to ion accumulations from the overlying strata. Hence, the boundaries between zones of leaching and accumulation can gradually blur (Holliday, 2004 [
The post-burial nutrient accumulation in the paleosols also results in higher CEC ratios compared to the mod- ern soils. With regard to the nutrient ratios only Ca and K are clearly enriched in the paleosols. Hence, Ca and K seem to be the dominant base cations to be transported through the profile which is congruent with the results from the soil water analysis. This only partly corresponds to Foss, 1988 [
The soil organic matter (SOM) contents are low to moderate for the modern soils and very low to low for the paleosols. Thus, the amount of organic carbon (TOC) is approximately 7 g∙kg?1 lower in the paleosols. Macro- scopically, this agrees with a Munsell soil colour (on moist samples) having higher value and chroma, i.e. very dark gray compared to dark grayish brown. According to TOC, the paleosols also contain lower amounts of ni- trogen than the modern soils. The decreased concentrations of TOC and N in the paleosols may be due to post- burial decomposition of SOM. With burial in AD 79 the accumulation of SOM within the paleosols has stopped. As the AD 79 pumice fallout layer covering the pre-AD 79 paleosols is well aerated no air exclusion took place. Hence, microbial activity to gradually decompose SOM is not stopped even though it may have slowed down. This is supported by Crowther et al., 1996 [
In the Sarno River plain, a second reason for reduced amounts of TOC in the paleosols may be due to interac- tion with hot volcaniclastic fallout during the AD 79 eruption. To some extent, heating of the pre-AD 79 paleo- sols during contact with the AD 79 pumice fallout may have caused sublimation of SOM. Thomas and Sparks, 1992 [
Finally, soil structure and bulk density of the modern soils and the pre-AD 79 paleosols are used as an indica- tor of soil compaction. Already from the macroscopic comparison of the two soils it is evident that the pre-AD 79 paleosols show a massive soil structure with little recognizable secondary porosity, i.e. structure-dependent porosity. This may result from: 1) the superimposed load of 3.3 (VdR) to 8 m (VR) of overlying post-AD 79 deposits; 2) the absence of root penetration; and 3) the reduction of active soil biota such as earthworms. How- ever, as shown by the soil moisture and hydraulic conductivity measurements, no waterlogging occurred in the paleosols as the texture-dependent primary porosity of the sandy loam still permits soil water movement. In contrast to the paleosols, the unburied modern soils are well penetrated by roots and soil fauna and generally appear more loosely structured. This is in accordance with a slightly increased mean bulk density of 1.4 g/cm³ for the pre-AD 79 paleosols compared to 1.1 g/cm³ for the modern soils.
A soil liquid and solid phase analysis was carried out in the pre-AD 79 Roman paleosols around Pompeii to es- timate potential post-burial soil development. The combination of the present-day mean groundwater table of the Sarno River plain (Autorità di Bacino del Sarno) with the pre-AD 79 topography (Vogel et al., 2011 [
The soil moisture and soil liquid phase study at the archaeological excavation of Villa Regina demonstrated that recently soil water flow down to the pre-AD 79 paleosol in a depth of 8 m takes place. Thus, since AD 79, this soil water can dissolve ions from the overlying volcanic deposits, incorporate them into the paleosol and in- fluence its solid phase properties. However, mobile soil water was only extracted from the paleosol in a four months period when the soil moisture exceeded 24.5%. This restricts the potential influence of post-burial nu- trient input to the rainy winter season. The soil liquid phase of the pre-AD 79 paleosol shows a distinct chemical composition. At the beginning and the end of the sample period increased ion concentrations were determined that can be explained by the “first flush effect” and its inversion, respectively.
Through the combination of the soil liquid phase study with the soil solid phase study an estimate of potential post-burial soil changes was made:
1) Leaching of nutrients from overlying deposits and accumulation in the paleosols led to a) higher CEC ra- tios, b) higher nutrient ratios for Ca2+ and K+, and c) higher pH values in the paleosols. However, the results from the soil water and soil moisture study restrict the soil water flow to the rainy winter season.
2) Since the buried A-horizon of the paleosols is recently subject to ion accumulations from the overlying strata, former topsoil eluvial horizons gradually turn into subsoil illuvial horizons. This results in a reversion of the depth function of the pH value.
3) The paleosols show a massive soil structure with little recognizable macro porosity and have a slightly in- creased bulk density due to burial by several meters of post-AD 79 deposits.
4) Considering the particular conditions during and after burial in AD 79, decreased organic carbon and ni- trogen contents and increased C/N ratios in the paleosols may derive from decomposition of soil organic matter or sublimation during contact with hot AD 79 pumice fallout.
It can be summarized that deeper insights were gained into the occurrence of mobile soil water within the pre- AD 79 paleosols and into the nature of potential post-burial soil developments. In the future, they should be taken into account when pre-AD 79 paleosols are characterised and interpreted with respect to paleoenviron- mental conditions. Additionally, the presented results may provide hints regarding the susceptibility and hazard of the groundwater body to contamination by agricultural and industrial pollutants.
This study is part of the interdisciplinary SALVE-research project undertaken by the German Archaeological Institute (DAI) in cooperation with the Heidelberg Academy of Sciences and Humanities (HAW) and the Uni- versity of Tübingen (www.salve-research.org). Project directors are Florian Seiler (DAI) and Michael Märker (HAW) and it was funded by the Deutsche Forschungsgemeinschaft (German Research Foundation). We would like to thank our local project partners and all their collaborators for their cooperation, particularly the Autorità di Bacino del Sarno and the Soprintendenza Speciale per i Beni Archaeologici di Napoli e Pompei. Particularly we thank Grete Stefani and her team at the archaeological excavation of Villa Regina for their support to install the soil water monitoring system. Special gratitude goes to Maria Teresa Pappalardo for sampling and mainte- nance as well as to Giovanni Di Maio for conducting the stratigraphic drillings and for his valuable local geo- logical knowledge. Furthermore we would like to thank Peter Kühn, Philipp Hoelzmann, Stefan Wessel-Bothe, Silke Schweighoefer, Jörn Breuer, Yvonne Oelmann and Marc Ruppenthal, ECOLAB G.M. ’65 S.R.L. and UABG GmbH for their various technical supports.