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Laghi di Monticchio (Southern Italy, Region Basilicata):
genesis of sediments-a geochemical study
Georg Schettler, Patrick Albéric
To cite this version:
Georg Schettler, Patrick Albéric. Laghi di Monticchio (Southern Italy, Region Basilicata): genesis of
sediments-a geochemical study. Journal of Paleolimnology, Springer Verlag, 2008, 40 (1), pp.529-556.
�10.1007/s10933-007-9180-4�. �insu-00252989�
Laghi di Monticchio (Southern Italy, Region Basilicata): genesis
of sediments—a geochemical study
Georg Schettler
1
and Patrick Albéric
2
(1) GeoForschungsZentrum Potsdam, Section Climate Dynamics and Sediments,
Telegrafenberg C328, 14473 Potsdam, Germany
(2) Institut des Sciences de la Terre d’Orléans (ISTO), UMR 6113 CNRS-Université
d’Orléans, Bâtiment Géosciences, BP 6759, Rue de St Amand, 45067 Orléans Cedex 2,
France
Abstract The sedimentation record of Lago Grande di Monticchio (LGM) is one of the most
prominent paleoclimatic archives in the non-glaciated areas of Europe. However, the modern
lake system has never been the subject of intense limnological studies. On the basis of
hydrochemical water profiles, detailed investigations of sediment short cores and in situ pore
water profiles from the littoral to the profundal zone, we elucidate spatial variations of
sediment genesis within the lake basin and the importance of various depth sections for the
lake’s internal nutrient cycling. Sediments from the smaller meromictic Lago Piccolo di
Montichio are discussed as a reference. Our study demonstrates: (i) distinctly higher sediment
accumulation for the centre of the lake basin by focussing of the settling particle flux; (ii)
decline of carbonate from the littoral to the profundal zones; (iii) non-synchronous change of
calcite net-accumulation for various water depths; (iv) exceptionally high cation release from
sediments covering the steeply inclining sector of the lake basin; (v) relatively constant
dissolved silica concentrations in the pore waters (SiO
2
~42 mg/l) independent of water depth
and sediment composition; (vi) influx of oxygen-bearing groundwater into the anoxic
hypolimnion after heavy rainfall and the associated precipitation of Fe-oxihydroxides; (vii)
higher release of NH
4
by anaerobic degradation of organic matter at a water depth of 23 m
than for sediments at a maximum water depth of 32 m, whereby the latter reflects the
importance of seasonal sediment re-oxidation for anaerobic degradation of organic debris;
(viii) although seasonal re-oxidation of sediments from various water depths is quite different,
Oxygen Index values of LGM sediments fall in a small range, which reflects rapid microbial
consumption of seasonally re-generated easily bio-degradable organic molecules.
Keywords Lacustrine sediments - Geochemistry - Genesis
Introduction
Sediment from Lago Grande di Monticchio (LGM, Fig. 1a–c) represents one of the most
prominent European terrestrial paleoenvironmental archives with a continuous 100 kyr record
spanning the Eem interglacial to the modern industrial period (Allen et al. 1999). During the
last decade, a reliable chronology for the LGM sedimentation record has been developed
based on varve counting, AMS
14
C-dating, and tephrochronostratigraphy (Newton and
Dugmore 1993; Zolitschka and Negendank 1993, 1996; Narcisi 1996; Huntley et al. 1999;
Wulf et al. 2004). Contributions to local vegetation development as a mirror of paleoclimate
change have been given by Watts (1985), Watts et al. (1996a, b, 2000), Huntley et al. (1999)
and Allen et al. (2000, 2002). Diatom assemblages in LGM sediments as a proxy for
paleoenvironmental change were investigated by Nimmergut et al. (1999). A pioneering
geochemical study of a 50 m sedimentation record from LGM was done by Robinson (1994).
Further geochemical sediment profiles were published by Creer and Morris (1996), Ramrath
et al. (1999) and Brauer et al. (2000). Still missing is a deeper understanding of sediment
genesis in LGM, which constrains paleoclimatic interpretation of geochemical and
geomagnetic sediment profiles that are partially based on sediment cores from various water
depths (see contributions by Creer and Morris 1996; Brandt et al. 1999).
Here, a study of the modern LGM, based on snapshots of thermal and chemical lake
stratification, in situ pore water profiles and investigation of short sediment cores taken along
a transect from the littoral zone to the centre of the lake during summer stratification, is
presented. The study was carried out to assess: (1) principal differences in sediment
characteristics at various water depths, (2) differences in sediment accumulation between the
shallow water area and the centre of the lake basin, (3) geochemical implications for
groundwater inflow, (4) post-depositional redistribution of chemical elements and (5) the
importance of various depth sections for the nutrient and sulphur cycle in LGM.
Geological setting and hydrological conditions
The Laghi Grande and Piccolo di Monticchio are maar lakes embedded in a collapsed
structure on the southwestern slopes of the Monte Vulture volcanic complex (Fig. 1c). The
suite of the local volcanic rocks is divided into two major units: the older Monte Vulture
complex and the younger Monticchio unit. The oldest volcanic products of the Monte Vulture
are the Foggianello sub-unit deposits of Fara d’Olivo ignimbrite (Crisci et al. 1983). The Fara
d’Olivo trachytic ignimbrite exhibits an age of 742 ± 11 kyr after Villa (1988). Buettner et al.
(2006) suggested an age of 740 ± 7 kyr for the lower and <740 kyr for the upper Fara d’Olivo
unit. The largest local volcanic rocks by volume are primary and epiclastic deposits of the
Barile unit. Volcanic products of this unit were ejected between ca. 673 and 610 kyr BP (see
Table 7-2 in Buettner et al. 2006 and references therein).
After a period of quiescence, the Monte Vulture volcano became dismembered by faulting
(Valle dei Greggi-Fosso del Corbo fault system). Volcanism resumed with volumetrically
subordinate eruptions assigned to the Monticchio unit (Laghi di Monticchio unit:
132 ± 12 kyr, Brocchini et al. 1994; 141 ± 11 kyr suggested by Buettner et al. 2006). Faulting
caused subsidence of the southern half of the Vulture complex and collapse of its
southwestern part. Eruptive activity of the Monticchio unit scattered along the active fault
systems and was dominantly linked to diametric structures, two of which are located under the
Monticchio Lakes (Giannandrea et al. 2006).
The igneous rocks of the Vulture complex comprise phonolites, tephri-phonolites, phonolitic-
foidites, and foidites (Principe et al. 2006 and references therein). Enhanced sulphur contents
associated with the occurrence of hauyne (Na,Ca)
4–8
[Al
6
Si
6
O
24
](SO
4
,S)
1–2
are peculiarities of
these rocks. High SO
4
contents of Vulture lavas are thought to be caused by magma
interaction with sedimentary SO
4
-rich brines from the basement of the volcano covering
Cretaceous to Pliocene sediments (La Volpe et al. 1984; De Fino et al. 1986). Substantial
hydrothermal calcite, gypsum, and anhydrite vein deposits were detected during explorations
in the hydrothermal fields of Tuscany and Latinum; Triassic marine evaporites are seen as the
major SO
4
source of these mineralizations (Marini and Chiodini 1994 and references therein)
and may also represent a major SO
4
source for hauyne-rich Vulture volcanites (see Table 1 in
De Fino et al. 1986 for hauyne contents). This interpretation is supported by relatively
positive δ
34
S values of metasomatic hauyne phenocrysts (+6.1, +6.6‰) from the Vulture
volcano (Cavarretta and Lombardi 1990). According to an alternative explanation, the
sulphate is primarily of magmatic origin: reduced sulphur became oxidized in the magma with
a high oxygen fugacity to SO
2
/SO
3
, and the escape of gaseous SO
2
/SO
3
left heavy
34
S behind
(see for detailed discussion Cavarretta and Lombardi 1990). The Laghi di Monticchio unit,
which is exposed in the western surroundings of LGM, involves a carbonatite-melilitite tuff
sequence abundant in mantle xenoliths (Stoppa and Principe 1998; Jones et al. 2000; Downes
et al. 2002). Formation of the maar lakes is associated with phreatomagmatic eruptions during
the final stage of volcanic activity.
The modern Lago Piccolo di Monticchio (LPM) has a maximum water depth of 38 m, a
surface area of 1.6 × 10
5
m
2
, and a water volume of 3.9 × 10
6
m
3
and receives sub-surface
inflow from a catchment area of 1.05 × 10
6
m
2
(Zolitschka and Negendank 1996). Lago
Piccolo di Monticchio is meromictic with a chemocline at about 13 m (Chiodini et al. 1997
and references therein). Monimolimnion waters in LPM are anoxic with high CO
2
content and
show a temperature increase with depth. Salinity increases to 1.8 g/l in the deepest part of the
lake basin (monitoring data 1995: Chiodini et al. 1997). The lake level of LPM is held
artificially 1 m above the lake level of LGM (~656 m asl).
The surface area of the dimictic LGM is 4.05 × 10
5
m
2
, and its water volume is 3.5 × 10
6
m
3
(Zolitschka and Negendank 1996). The lake receives sub-surface drainage from a catchment
of 3.04 × 10
6
m
2
, including that of LPM, and has one outlet. The morphology of the LGM
basin has been characterized by sonar measurements (Hansen 1993). Lago Grande di
Monticchio has an extended shallow area (Fig. 1b) with abundant submerged vegetation
(Ceratophyllum demersum) and a littoral fringe rich in Nympha alba and Polygonum
amphibum.
Mean annual precipitation (815 mm) is relatively high due to the elevation of the site (e.g.
Monte Vulture: 1,262 m asl), although a pronounced dry period commonly occurs in the
summer. The hillside is densely forested and dominated by Beech (Fagus sylvatica) and
Turkey Oak (Quercus cerris) (Watts et al. 1996b). A high percentage of precipitation does not
drain but undergoes evapotranspiration. For an assumed evapotranspiration of 80% in the
catchment, the above hydrological data suggest a water residence time of ~7 years for LGM.
Loosely deposited pyroclastics in the catchment of LGM favour the seepage of precipitation.
Drainage by surface runoff into the lakes, however, may occur during the melting of snow
when the top-soils are frozen.
The lake level of LGM shows seasonal fluctuations, with high levels in early spring (pers.
comm., local residents 1994). During late spring and summer, the lake level commonly
decreases by one to two metres. Water level increase is limited by a canal built by monks of
the monastery San Ippolito that was founded within the Piano Comune tuff ring depression in
AD 1059 and discharges Lago Grande waters into the river Ofanto. During the winter of
1993/1994, both lakes were ice-covered. A clearly visible terrace, ca. 5 m above the modern
lake level, indicates that the lake level was higher in the past. The lands surrounding Monte
Vulture are cultivated, especially for cereals.
Sampling
On 22 August 1994 and 19 September 1994, temperature profiles of the water column were
taken at the deepest part of both lakes. Divers placed dialysis cells along a transect (Fig. 1b) in
the surface sediments of LGM at various water depths such that the uppermost dialysis
chamber fit with the sediment/water interface (see Schwedhelm et al. 1988 for cell
construction). The vertical distance between each chamber was 1 cm with 2 chambers at each
depth level. At each cell site, water samples were taken approximately 0.5 m above the
surface sediment. To determine the amount of cations, an aliquot of the water sample was
membrane-filtered (0.45 μm) and stabilized with nitric acid. Upon removal by divers on 19
September 1994, the single dialysis chambers were immediately sampled with syringes in
7 ml polypropylene tubes with a screw closure, manufactured by Sarstedt (Germany). One
sample from each depth was stabilized by addition of 20 μl HNO
3
.
Short gravity cores (70 cm) were taken from the centre of lake LPM and along a transect
(Fig. 1b) from LGM using a Niemistö gravity corer (Niemistö 1974). The cores were
continuously sampled at 3 cm slices by vertical extrusion; sample slices were immediately
stored in a refrigerated box. Sediments from the deepest part of LGM had very high gas
contents (methane), precluding taking a core from the deepest part of LGM.
Analytical methods
Water samples
The determination of fluoride, chloride, nitrate and sulphate was carried out by ion exchange
chromatography (DX 100, Dionex). Soluble Reactive Phosphorus (SRP) and ammonium were
determined colorimetrically (FIAS, Perkin Elmer) using the molybdenum-blue method for
SRP measurements and spectro-photometry of an indicator solution after separation of NH
3
through a Teflon membrane (for details see Müller et al. 1992). Dissolved inorganic carbon
(DIC) of lake water samples was measured coulometrically. The DIC of pore water samples
was determined by a commercial laboratory (ANTEUM, Berlin) by adding a small sample
volume to phosphoric acid and performing IR-spectrometry of the released carbon dioxide
(TOC 5000, Shimadzu). Dissolved silica and cations were measured sequentially by ICP-AES
(ARL 35000). Temperature profiles of the lakes were taken by means of a water-tight single
channel logger with an integrated thermistor temperature sensor (XL-100) manufactured by
Richard Brancker Research, Canada.
Sediment samples
Sediment samples were frozen on return to the laboratory (1–2 days after sampling) and later
freeze-dried. The <185 μm fraction was separated from the freeze-dried material by sieving. It
comprised almost 100% of the total sample. After a HNO
3
/HClO
4
/HF/HCl-decomposition of
0.25 g solid sample, the determination of major and minor elements, including phosphorus
and sulphur, was carried out by sequential ICP-AES (ARL 35000) and external calibration.
Selected trace elements were measured by ICP-MS using a VG Plasma Quad PQ2+.
Beryllium-9,
115
In and
187
Re were used for internal standardization. The total analytical errors
for ICP-MS measurements were below ±10%. Inorganic carbon was measured
coulometrically after decomposition with hot phosphorus acid (Coulomat 702, Ströhlein). The
total carbon and nitrogen were determined after thermal decomposition at 1,350°C in an
oxygen-gas-flow by IR-spectrometry and heat-conductivity detection, respectively (CNS
2000, LECO). Organic carbon was calculated by difference using the total and inorganic
carbon results. Elemental sulphur was determined after methanol-extraction (reflux, 7 min) by
reversed phase liquid chromatography (eluent: 80% methanol, column: C-18, UV-detection
254 nm, DX 100, Dionex, see Möckel 1984). The chromatographic method enables the
detection of polysulphides and sulphur of various chain- and ring-size, respectively.
Octagonal ring-sized sulphur (S
8
) was the dominant sulphur modification in the examined
methanol-extracts. Only its peak area was considered for quantitative analyses of total
elemental sulphur.
