Laghi di Monticchio (Southern Italy, Region Basilicata): genesis of sediments—a geochemical study
Summary (3 min read)
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).
- The oldest volcanic products of the Monte Vulture are the Foggianello sub-unit deposits of Fara d’Olivo ignimbrite (Crisci et al. 1983).
- The lake level of LGM shows seasonal fluctuations, with high levels in early spring (pers. comm., local residents 1994).
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.
- 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.
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 NH3 through a Teflon membrane (for details see Müller et al. 1992).
- Dissolved inorganic carbon (DIC) of lake water samples was measured coulometrically.
- 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 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).
- 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).
- Detector efficiency calibration for 46.539 keV was based on αspectrometric measurements using a 208Po recovery spike.
Lake water characteristics
- Increase of Ca and DIC in the hypolimnion of LGM towards the lake bottom reflects post-depositional dissolution of autochthonous carbonate.
- During summer stratification, the SO4 concentration had significantly decreased in the anoxic deep water, but increased again by the inflow of groundwater (Fig. 3i).
- The LGM lake water is moderately mineralized, so the authors could not obtain hints for subsurface inflow of highly saline waters that could trigger development of meromictic conditions.
Radiometric data
- Unsupported 210Pb in the upper 15 cm of the LPM core and in the LGM core from 4 m depth shows similar high activity values and a distinct decline in the upper sections of these cores (Fig. 4a–d, Table 1).
- The 137Cs profiles confirm that sediment accumulation at the 23 m site distinctly exceeds that of the profundal LPM sediments and that of LGM sediments from the shallow water (Fig. 4e–h).
- The initial unsupported 210Pb activity of 760 mBq/g calculated on the basis of CIC assumptions probably exceeds that of earlier sedimentation periods.
- The geochemical signatures of deeper sediments at the 8 m depth, showing distinctly lower Al contents (Fig. 5g), could reflect the presence of minerogenic debris with a geochemical composition that completely differs from those of the other LGM cores.
- They may also reflect in situ sulphide precipitation within discrete sulphate reduction zones and the diffusive flux of Fe dissolved in pore water towards these zones.
Pore water chemistry
- Pore water profiles can be used to assess microbial activity within sediments and the diffusive exchange between sediments and the overlying water.
- The pore water profile of the 12 m site is characterized by overall higher Ca and DIC concentrations and shows a distinct decline of the concentration gradients between 35 cm and the sediment water interface.
- The NH4 pore water profile recovered from the 12 m depth shows a different pattern with a maximum at 42 cm and a concentration gradient towards the sediment/water boundary.
- The SRP pore water profile reflects well the different phosphorus release at various depth levels.
- Sediments of the smaller LPM show higher TOC/N values than LGM sediments (Fig. 5a).
Synopsis
- (1) Sediments of LGM document gradually increasing contributions from dead planktonic matter in the course of the recent sedimentation history and show clear variations in their geochemical composition depending on water depth.
- Autochthonous calcite is characterized by a distinctly higher Ca/Sr ratio than the coexisting lake water.
- Relatively higher accumulation of macrophytic organic remnants and littoral CaCO3 does not counterbalance focussing of the particle flux towards the centre of the lake basin.
- Re-precipitation of pore water dissolved Fe as Fe-oxihydroxide is accompanied by substantial co-precipitation of U and Mo. Enhanced chemical alteration in the steeply inclining sector of LGM is documented in pore water profiles from the 12 m site that show distinctly elevated Na, K, Mg, Ca, Mn and DIC concentrations with strong gradients towards the sediment/water interface.
- (5) Silica pore water concentrations, which should largely originate from the post-depositional dissolution of biogenic opal, have a uniform value of ca. 42 mg/l SiO2 in LGM sediments.
Figures
- The basis of assumed values for porosity (ϕ) and density of the solid sediment (ρs).
- The depth estimate for the year 1963 in core 6 m (b) was calculated on the basis of CRS assumptions minerogenic sediment constituent on CaCO3-free base for an assumed Al2O3 content of 15.6 wt% of the bulk siliciclastic sediment fraction (Al2O3 value for mean continental crust composition after Taylor (1964), see (g) for Al concentration data).
- (h–l) Profiles of selected element ratios, characterizing the siliciclastic sediment fraction: LaN/TmN, ratio of Chondrite-normalized REE concentrations (used data for normalization from Taylor and McLennan (1985)).
- Distinctly enhanced Fe/Al, U/Al and Mo/Al values in the profiles from the 8 m sampling site reflect intense chemical alteration and re-distribution of Fe, U and Mo. Increase of Fe/Al and Mo/Al in the LPM sediments coinciding with Stotal increase, (n) indicates sulphide-precipitation of Fe and Mo in the modern LPM, (m) excess.
- Fe balanced for an assumed Fe/Al mass ratio of 0.9 for the minerogenic debris.
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Citations
51 citations
Cites background from "Laghi di Monticchio (Southern Italy..."
...Lake Piccolo is the tributary of Lake Grande through an artificial channel (Cioni et al. 2006; Schettler and Alberic 2008)....
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...A channel built by monks of the nearby San Ippolito Monastery, to prevent water level increases, connects the lake to the Ofanto river (Schettler and Alberic 2008)....
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...A channel built by monks of the nearby San Ippolito Monastery, to prevent water level increases, connects the lake to the Ofanto river (Schettler and Alberic 2008)....
[...]
33 citations
Cites background from "Laghi di Monticchio (Southern Italy..."
...For comparison, in the deepest part of the lake basin where focusing of settling particle fluxes may occur (Schettler and Alb́eric, 2008), the chronology established by Schettler et al. (2007) based on varve counting (cf. Fig....
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...…et al., 1994; Siffedine et al., 1996; Oldfield, 1996; Brauer et al., 2008), but if some limnological studies describe the genesis of sediments (Schettler and Alb́eric, 2008), little is known in these volcanic environments about the triggering factors of gravity reworking phenomena and related…...
[...]
33 citations
22 citations
13 citations
Cites background or result from "Laghi di Monticchio (Southern Italy..."
...A previous investigation of the geochemistry of sediment found both pyrite and siderite in the sediment [Schettler and Alberic, 2008], and hence, the chemistry of deeper LPM waters is controlled by the stability of minerals and the inflow of CO2....
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..., 2000] and the genesis of its sediments [Schettler and Albéric, 2008]....
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...In contrast, investigations of LGM have mainly focused on paleoclimatic aspects [Brauer et al., 2000] and the genesis of its sediments [Schettler and Albéric, 2008]....
[...]
...In contrast to previous reports [Schettler and Albéric, 2008] which classified classifying LGM as a warm monomictic lake, we observed complete overturning of the water in winter 2009 and a subsequent new chemical stratification during spring 2009....
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References
83 citations
"Laghi di Monticchio (Southern Italy..." refers methods in this paper
...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)....
[...]
...During the last decade, a reliable chronology for the LGM sedimentation record has been developed based on varve counting, AMS 14C-dating, and tephrochronostratigraphy (Newton and Dugmore 1993; Zolitschka and Negendank 1993, 1996; Narcisi 1996; Huntley et al. 1999; Wulf et al. 2004)....
[...]
83 citations
83 citations
"Laghi di Monticchio (Southern Italy..." refers background in this paper
...…parameter that determines the molecular diffusive flux of SO4 into the sediments, where it is used as an electron acceptor for anaerobic bacterial respiration and microbial mediated anaerobic oxidation of methane (e.g. Furrer and Wehrli 1996; Iversen and Jørgensen 1985 and references therein)....
[...]
...Sulphate is utilized as an electron acceptor for the bio-degradation of organic matter in the absence of other preferentially used electron acceptors (O2, NO3, reactive Fe(III), e.g. Furrer and Wehrli 1996) and is consumed by anaerobic methane oxidation (e.g. Iversen and Jørgensen 1985)....
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77 citations
"Laghi di Monticchio (Southern Italy..." refers background or methods in this paper
...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)....
[...]
...A prominent geochemical signature of the igneous rocks of the Monte Vulture complex including the LGM carbonatite-melilitite formation is the distinct enrichment of light rare earth elements (LREEs), as seen in data presented by De Fino et al. (1986) and Stoppa and Principe (1998)....
[...]
74 citations
"Laghi di Monticchio (Southern Italy..." refers background in this paper
...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)....
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Frequently Asked Questions (21)
Q2. What can be used to assess microbial activity within sediments?
Pore water profiles can be used to assess microbial activity within sediments and the diffusive exchange between sediments and the overlying water.
Q3. What is the sulphur content of the sediments from different depths?
The diffusive penetration of oxygen into surface sediments, which depends on the oxygen concentration of the overlying water and the oxygen consumption rate in the sediment (mainly aerobic mineralization of organic matter and oxidation of reduced sulphur), determines the sediment depth where sulphate reduction can principally proceed from a thermodynamic viewpoint.
Q4. What is the effect of sulphate on the DSC of the lake water?
Besides groundwater inflow, post-depositional ion release from this debris, in particular release of mono- and divalent cations and silica may significantly contribute to the DSC of the lake water.
Q5. What is the main reason for the higher Si consumption in the lake water of LGM?
Presuming that LGM and LPM are fed by groundwater inflow of similar composition, an inhibited P-cycle in LPM due to meromixis should be the major reason for the overall higher Si consumption in the lake water of LGM.
Q6. What is the role of sulphate in the bio-degradation of organic matter?
Sulphate is utilized as an electron acceptor for the bio-degradation of organic matter in the absence of other preferentially used electron acceptors (O2, NO3, reactive Fe(III), e.g. Furrer and Wehrli 1996) and is consumed by anaerobic methane oxidation (e.g. Iversen and Jørgensen 1985).
Q7. What mechanism can transfer dissolved oxy-anions of U(VI) and Mo(?
Co-precipitation with FeOOH is an important mechanism that can transfer dissolved oxy-anions of U(VI) and Mo(VI) from the water column into the sediments (e.g. Bruno et al. 1995; Gustafsson 2003).
Q8. What is the significance of the sulphate reduction in the lake?
In the nearly non-calcareous sediments from deeper parts of the lake basin, seasonal production of alkalinity associated with sulphate reduction is important.
Q9. What is the role of elemental sulphur in the formation of pyrite?
Elemental sulphur seems to play a key role in the formation of pyrite, which is less sensitive to oxidation (unpublished personal experimental results).
Q10. What is the Ca/Sr signature of the autochthonous calcite?
Assuming a substantial post-depositional dissolution of autochthonous calcite, pore water in the surface sediments should reflect the Ca/Sr signature of the autochthonous calcite.
Q11. What are the main factors that influence the diffusive nutrient exchange across the lake interface?
Diffusive nutrient exchange across the sediment/water interface is further influenced by a complex of interacting parameters including, e.g. temperature, oxygen availability, vertical and temporal variations in H2S production by SO4 reduction, abundance of bio-degradable organic matter, PO4 retention or release associated with authigenic mineral formation.
Q12. What was the method used for the determination of phosphorus and sulphur?
After a HNO3/HClO4/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.
Q13. What is the effect of the hypolimnion of LGM on the lake bottom?
Increase of Ca and DIC in the hypolimnion of LGM towards the lake bottom reflects post-depositional dissolution of autochthonous carbonate.
Q14. What is the way to obtain an integral picture of pore water chemistry?
During the exposure of the dialysis cell, changes in interstitial water chemistry can occur, which is why in situ dialysis pore water profiles detected in this manner give an integral picture over the exposure time of the dialysis cells.
Q15. What is the effect of the inflow of groundwater on the P-cycle of LGM?
Hydrogen sulphide produced by SO4 reduction releases PO4 from its precipitates with Fe. Inflow of SO4-bearing groundwater may therefore indirectly influence the P-cycle of LGM.
Q16. How does the SRP pore water profile at 12 m show the phosphate release?
Lowering of the pH seems to be sufficient to prevent precipitation of Ca and Mn, which show elevated concentrations in the pore water at the 12 m site.
Q17. What is the trend of 210Pb in the lower section of the 4 m profile?
In the lower core section (31.5–55.5 cm) of the 4 m profile, however, unsupported 210Pb values vary between 38 and 69 mBq/g without showing a declining trend versus depth.
Q18. How much biogenic opal is in the sediments from the 23 m site?
If the authors consider TOC and N contents, biogenic opal accounts for approximately 50 wt% in the nearly non-calcareous sediments from the 23 m site.
Q19. What mechanism may explain the lower Feexc of the profundal LGM sediments?
Three mechanisms may explain the lower Feexc of the profundalLGM sediments: (i) The dissolved Fe influx into the deep water and possible focussing of the settling FeOOH flux towards the centre of the lake basin does not counterbalance the Feexc loss by deposition of FeOOH particles in the shallow water area during overturn.
Q20. Why do LPM sediments have a higher OI than LGM?
Because of the permanent absence of oxygen in the LPM deep water, it appears astonishing that OI values of LPM sediments exceed the average OI of LGM sediments by a factor of ~1.5.
Q21. What is the underlying geochemical signature of the LGM sediments?
The geochemical signatures of deeper sediments at the 8 m depth, showing distinctly lower Al contents (Fig. 5g), could reflect the presence of minerogenic debris with a geochemical composition that completely differs from those of the other LGM cores.