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Journal of Paleolimnology 26: 283–292, 2001. © 2001 Kluwer Academic Publishers. Printed in the Netherlands.
Lacustrine organic matter and the Holocene paleoenvironmental record of Lake Albano (central Italy)* D. Ariztegui1,*, C. Chondrogianni1, A. Lami2, P. Guilizzoni2 & E. Lafargue3 1 Geologisches Institut, ETH Zentrum, 8092 Zürich, Switzerland 2 CNR-Istituto Italiano de Idrobiologia, 28922 Verbania-Pallanza, Italy 3 Institut Français du Pétrol, Rueil-Mailmaison, France *Present address: Institut Forel, University of Geneva, Geneva, Switzerland (E-mail:
[email protected]) Received 10 July 2000; accepted 16 October 2000
Key words: Holocene, Rock-Eval® Pyrolysis, pigments, trophic-state, paleolimnology, organic carbon
Abstract A combined bulk and detailed geochemical study of the sedimentary organic matter in Lake Albano, central Italy, provides critical data to track the response of this aquatic system to the environmental changes of variable amplitude that occurred during the Holocene. Rock-Eval pyrolysis of this predominantly laminated, organic carbon-rich sedimentary sequence shows changes in hydrogen and oxygen indices that are related to variations in the dominance of the primary producers. These variations are further confirmed by the pigments and the carbon isotopic composition of bulk organic matter showing that cyanobacteria dominated the lake waters during the early and late Holocene whereas diatoms have been the main producers during the middle Holocene. Sharp decreases in productivity, 2–3 centuries long, are identified at ca. 8.2, 6.4 and 3.8 ka. B.P. Changes in temperature and/or effective moisture are suggested as the most probable causes, although human impact cannot be ruled out for the latest part of the Holocene. Introduction Holocene climatic fluctuations have been recently identified in high-resolution paleoenvironmental records from different areas of the world. Multiproxy data from lake cores show comparable variability during the Holocene to isotope records from polar ice cores (Stager & Mayewski, 1997; Alley et al., 1998; Willemse & Törnqvist, 1999) and to an increasing number of marine cores (e.g., Lamb et al., 1995; Sirocko et al., 1996). These environmental changes have been particularly well studied in several terrestrial records in central Europe using a wide range of approaches (e.g., Niessen & Kelts, 1989; Niessen et al., 1992; Leemann & Niessen, 1994; Ariztegui et al., 1996; Ramrath et al., 1999; Wilkes *This is one of a series of papers to be published in Journal of Paleolimnology that were contributed from the keynote speakers at the 2nd International Congress of Limnogeology, held May, 1999, in Plouzane, France, and organized by Dr. Jean-Jacques Tiercelin.
et al., 1999). These sedimentary records have in common that they all provide the environmental sensitivity and the high-temporal resolution necessary to reveal the extent and effect of changes in the environment during the Holocene. Several indirect estimations of a given parameter, or ‘proxies’, such as the sedimentary organic fraction, have been used to reconstruct past environmental conditions from lake sediment archives. The composition and amount of organic matter delivered to the lake sediments change in response to the types and abundance of organisms in and around the lacustrine basin (Tyson, 1995 and references herein; Meyers & LalliesVergès, 1999). Thus, the sedimentary organic matter preserved in the sediments yields valuable paleoenvironmental information. A combined study of both sedimentological and organic matter compositional variations of the organic carbon-rich sediments contained in sedimentary cores of Lake Albano, Italy, provides new, continuous information on the development
284 of the lake throughout the Holocene. These results can be used to further constrain the nature and regional extent of Holocene environmental changes in the Mediterranean region.
Site location Lake Albano (ca. 293 m a.s.l.; 6 km2 surface; 175 m maximum water depth) is a closed crater basin located in the Albani Hills in Latium, central Italy, ca. 25 km southeast of Rome (Figure 1). This region of the Mediterranean is climatically sensitive to both the North Atlantic-driven climate patterns and the monsoonal climate (Chondrogianni et al., 1996a). It is, therefore, a key region to investigate the response of ecosystems to these different climate regimes and to anthropogen-
ically-induced environmental changes through time (e.g., Hoek, 2000). The sedimentary record of Lake Albano has been extensively studied in the framework of the EUproject PALICLAS (Palaeoenvironmental Analysis of Italian Crater Lake and Adriatic Sediments). The present paper discusses two cores recovered from 120-m water depth (PALB94-3A and B) that contain a complete Holocene sedimentary record of paleoenvironmental changes (Figure 1). Both cores were retrieved at the same location and are lithologically almost identical containing the same sedimentary sequence.
Methodology The Rock-Eval® Pyrolysis method was initially developed to measure both the free hydrocarbon content and
Figure 1. Crater Lake Albano is a hydrologically closed basin that receives water mainly from atmospheric precipitation and underwater springs. Core PALB94-3, located at 120 m water depth, comprises the entire Holocene sequence as shown in seismic line Q-Z. Assuming a p-wave velocity of 1460 m/s in the sediments, 10 ms of two-way travel time in the 3.5 kHz. seismic profile corresponds to 7.3 m in the sediment cores. Black lines outline conspicuous seismo-stratigraphic units that correspond to the lithological units indicated by roman numerals (after Chondrogianni et al., 1996b).
285 the hydrocarbons released by thermal conversion of kerogen in rock and sediment samples (Espitalie et al., 1985). The method consists of progressive heating of sediment samples and measurements of the amounts of hydrocarbons that escape from the sediments at different temperatures. Three main signals are generated during the heating from ca. 200–600 °C: gaseous hydrocarbons (S0), volatile hydrocarbons (S1) and hydrocarbon components produced by the thermal degradation of humic substances (S2). Two parameters are obtained from the pyrolysis: The Hydrogen Index (HI), expressed in mg Hc × g–1 Corg, which gives an estimation of the amount of hydrocarbon contained in the sedimentary organic matter (i.e., chemical quality of the organic matter); and the Oxygen Index (OI), which represents the amount of oxygen in mg CO2 × g–1 Corg. These parameters are indicative of the H/C and O/C ratios of organic matter, respectively, and can be related to the origin of the organic matter. Talbot & Livingstone (1989) have summarised the application of these indices to the study of lacustrine sediments. The carbon stable isotope composition of the bulk organic matter, δ13C(OM), was measured from CO2 gas, generated by combustion of decarbonated samples, on an Optima mass spectrometer at the ETH-Zürich. The reproducibility of the measurements is ±0.2 ‰ and the results reported are per mil (‰) deviation with respect to the international standard V-PDB. Total pigments, chlorophyll plus chlorophyll derivatives (CD); and carotenoids (TC) were extracted with 90% acetone. For comparison with previous studies, chlorophyll derivatives are calculated as absorbance units × g–1 organic matter (Wetzel, 1970; Lami et al., 1994). Total carotenoids are expressed as mg g–1 organic matter (Züllig, 1982). Specific algal pigments were determined by ion pairing, reverse phase HPLC (Beckman), and are expressed as nMoles × g–1 organic matter (Lami et al., 1994). We defined the degree of bioturbation of the core sediments based on the Bioturbation Index (BI) scheme proposed by Behl & Kennett (1996). A value of 1 on the index represents distinct, continuous lamination; 2 represents diffuse, discontinuous or irregular lamination; 3 is bioturbated sediments with few patches of diffuse lamination; and 4 describes completely bioturbated sediments. The quality of the lamination was further evaluated using radiographs of 2 × 4 × 1 cm sub-samples at selected depth intervals. The cores were dated using a combination of AMS C-14 dating of terrestrial macrofossils, pollen stratigraphy and tephrochronology.
The Holocene record: Results and discussion Sedimentology The seismic section in Figure 1 shows that core PALB943 contains a complete Holocene sequence with the Pleistocene/Holocene transition securely dated at 11.48 ka. B.P. The identified sedimentological units could be assigned to equivalent seismic reflections. Chondrogianni et al. (1996b) distinguished six different lithological units in the Lake Albano sequence (Figure 2): (I) welllaminated coloured muds including whitish and reddish laminae; the white laminae representing authigenic carbonates; (II) massive dark olive brown muds interbedded with sections of coloured laminated muds and diatom laminae towards the bottom. This unit contains the distinctive ‘Avellino’ tephra dated at 4.1 cal. ka. B.P. (Calanchi et al., 1996); (III) diatom beds and laminae intercalated with coloured laminated muds; (IV) laminated coloured muds interbedded with diatom layers; (V) massive dark olive gray muds intercalated with vaguely laminated sections; and (VI) massive light olive gray spotted silts. Radiographs from different depths of the record (Figure 2) illustrate variable development of millimetre-scale lamination throughout the sequence with the exception of the middle Holocene which is characterised by thick diatom layers (up to 3.0 cm) and higher sedimentation rates. These fluctuations in the development of lamination have been semi-quantified using the BI (Figure 3). The absence of lamination characterises Pleistocene sedimentation in the lowermost part of the core (Unit VI). An incipient development of laminae can be observed during the early Holocene (Unit V) that is soon followed (Unit IV onwards) by increasing lamination occasionally masked by the presence of microlamination within the thick diatom mats (Units IV to II) during intervals of high primary productivity (Ryves et al., 1996). A robust chronology has been established showing a distinct change in sediment accumulation in Unit IV (Figure 2) separating the older part of the sequence (Units V and VI) with low rates (0.8 mm/a) from the younger sediments (Unit III onwards) with relatively higher rates (1.1 mm/a). Sedimentary organic matter Figure 3 shows the variations in total organic carbon content (TOC), hydrogen and oxygen indices (HI, OI) and in the bioturbation index (BI). Early Holocene sedi-
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Figure 2. Log of core PALB94-3 including lithological units and X-radiographs at selected sediment depths. Shaded sections within the lithology indicate diatom beds. Chronology is based upon combined AMS 14C age determinations; pollen data (well-dated rise of Olea and a peak in Castanea); and tephrochronology. Shaded intervals in the age/depth plot indicate lithological units. Notice that the most substantial change in accumulation rate throughout the Holocene occurred at ca. 880 cm.
mentation is marked by a two-step increase in TOC at the onset of units V and IV reaching highest values of 6% (dry mass). A distinctly continuous decrease follows through units III and II up to the tephra layer t1 (4.1 ka. B.P.). Late Holocene sedimentation (above t1) displays average values of 5% with increasing contents during the last century. A similar picture is shown by the HI variation. A gradual increase at the onset of the Holocene (Unit V) reaches a maximum (460 mg Hc × g–1 Corg) in Unit IV. The generally continuous decrease through Units III and the lowermost part of Unit II is terminated with a minimum at tephra t1. A second gradual increase during late Holocene peaks in a maxi-
mum (500 mg Hc × g–1 Corg) by the end of Unit II and displays averages values of 420 mg Hc × g–1 Corg up to the top of the core. OI ranges between 120 and ca. 200 mg CO2 × g–1. Unit III and parts of Units II and IV show higher OI and comparatively lower HI values at the very welllaminated, diatom-rich intervals. Furthermore, the diatom-rich part of the core is characterized by a mirror effect of HI to OI, whereas a general covariance is observed in the remainder of the core. Figure 4a shows a HI-OI plot with the entire set of data, which approximates the van Krevelen type plot of elemental H/C and O/C ratios. Although all samples plot within the Type
287
Figure 3. Percent TOC, HI (solid line) and OI (dashed line) were analysed in core PALB94-3A whereas BI was described in core PALB94-3B. Notice the different scales for HI and OI. Interpreted lithological units are shaded and labelled with roman numerals. The indicated unit time-boundaries are based on the age model.
II organic matter there is a distinct differentiation between the unit specific values. The mineral matrix can produce artifacts on the calculation of both HI and OI (Espitale et al., 1985; Peters, 1986). In particular, clays, due to their highly reactive surface, can retain the hydrocarbon produced from the cracking of the organic matter through absorption leading to lower S2 values. This, in turn, will affect the HI (HI = S2 normalized to TOC content) which will show anomalously low values. This problem can be eliminated by classifying the organic matter using S2 vs. TOC diagrams (Langford & Blanc-Valleron, 1990; Ariztegui, 1993). Figure 4b shows a S2 vs. TOC diagram for the entire Holocene sequence where the different units are represented by distinct symbols. Boundaries between organic matter types (dashed lines in Figure 4b) have been drawn following Langford and Blanc-Valleron (1990). As in the van Krevelen type diagram (Figure 4a) all samples fall within Type II organic matter. A regression line can be fitted to the entire sequence and to each unit separately as shown in Table 1. A matrix effect is indicated by a positive ×-intercept of the regression line on the S2 vs. TOC diagram, and the position of the intercept is a measure of the amount of adsorption. The ×-intercept in Figure 4b is 1% TOC
which indicates the amount of organic material (with a given HI) that must be present before hydrocarbons are liberated from the sediments by pyrolysis. The overall high degree of correlation for all the units indicates a common origin for each individual group of samples. According to Langford et al. (1990) ten times the slope of the regression line gives a good estimate of the percent of pyrolizable hydrocarbons in the total organic carbon. When the entire sedimentary sequence is considered, this fraction is 45.1% (Table 1). The estimated percent of pyrolizable hydrocarbons increases at times of relatively high productivity, as during the deposition of Units I to IV, whereas times of decreasing productivity (Units V and VI) display substantially lower values. The ratio of chlorophyll derivatives (CD) to total carotenoids (TC) provides a good estimation of the balance between allochthonous and autochthonous sources of organic matter (Guilizzoni et al., 1982; Sanger, 1988). Figure 5a shows that the average value of this ratio for most of Lake Albano sedimentary organic matter has been produced in the lake, although higher than average values are observed in the uppermost part of Unit II. A few peaks reaching ~ 120, observed in Unit III, are associated with turbidites (i.e., higher contribution of allochthonous material). A diatom study of the Lake Albano sequence showed that Unit III, and to a lesser extend Unit IV and the lower part of Unit II (up to t1), contain mostly diatom-produced organic matter (Ryves et al., 1996). Previous studies have shown that diadinoxanthin is a characteristic carotenoid of diatoms, whereas echinenone characterizes cyanobacteria. Thus, the diadinoxanthin/ echinenone ratio, shown in Figure 5a, can be used as an indicator of the relative contribution of diatoms and cyanobacteria, respectively. While most of the sequence displays diadinoxanthin/echinenone ratio ≤ 1, the ratio in Unit III increases to values of 3 and higher (i.e., mostly diatom-produced organic matter) confirming previous observations. It has been shown, in both marine and lacustrine environments, that cyanobacteria have H/C ratios by 12% higher than diatoms (1.87 and 1.66, respectively; Pelet, 1981). Therefore, relative changes in the contribution of these organisms to the sedimentary organic matter should also imply variations in the HI values. Isorenieratene is a diagnostic pigment for a group of phototrophic bacteria (e.g., Phaeobium) that grows under strongly anoxic conditions (Züllig, 1985). Concentrations of isorenieratene in the Lake Albano core display the lowest values in Unit III and into Unit II
288
Figure 4. (a) Rock-Eval® van Krevelen-type diagram for the entire Lake Albano sedimentary organic matter. Symbols indicate the interpreted units and dashes lines trace maturity paths for organic matter types. (b) S2 vs. TOC diagram. Dashed lines define boundaries of organic matter types (see text), whereas the solid line indicates the regression line for the entire set of data.
(Figure 5a). This implies that the oxygen state of bottom waters was substantially different to that observed in the remaining of the Holocene sequence. High levels of productivity have been suggested as an alternative explanation for the low concentration of isorenieratene in the sediments due to limiting light availability (Ariztegui et al., 1996). Although distribution of other bacterial pigments in the sediments and the BI index indicates that anoxia during deposition of unit III was not as intense, the poor correlation observed between HI and isorenieratene concentrations (Figure 5b) suggests that changes in the oxygen-status of the water column have not been the most important factor for the preservation of the sedimentary organic matter. Fur-
Table 1. The average HI for each sedimentary unit is given by the slope of the regression line × 10 representing the percent of pyrolizable hydrocarbons Unit no. Total I II III IV V VI
Fitted regression line
% pyrolizable Hc 2
y = 4.7562 + 4.5106× R = 0.766 y = 7.9535 + 5.1257× R2 = 0.890 y = 5.9455 + 5.1382× R2 = 0.627 y = 12.718 + 5.1356× R2 = 0.837 y = 5.9794 + 4.8077× R2 = 0.759 y = 3.2069 + 4.0849× R2 = 0.990 y = 5.6803 + 1.4516× R2 = 0.644
45.1 51.2 51.4 51.3 48.1 40.8 14.5
thermore, a careful analysis of the data indicates that variations in HI and OI in the Holocene sedimentary sequence of Lake Albano are most probably related to changes in the dominant primary producers within the lake. Figure 5c displays a HI vs. diadinoxanthin/ echinenone plot discriminating the different lithological units. Unit III displays relatively low HI (average 300) and the highest values of this pigment ratio in the whole sequence. The lowermost HI values within Units II and III are due to the higher contribution of terrestrial material during the deposition of the previously mentioned turbidites. The δ13C(OM) of selected samples shows that the diatom-rich units with comparatively low HI display more positive carbon isotopic compositions (Figure 5d). Laboratory experiments and phytoplankton studies in both marine and lacustrine systems have shown that bloomforming diatoms can be 6‰ more positive on average than other associated plankton and the bulk particulate organic matter (Fry & Wainwright, 1991; Tyson, 1995 and references herein). Paleoenvironmental reconstruction The collective sedimentary and organic matter evidence implies that most of the variations in HI values observed in Lake Albano Holocene sediments can be at-
289
Figure 5. (a) Diagnostic pigment curves for biological sources and degree of anoxia in Lake Albano sediments (core PALB94-3A). CD/TC = Chlorophyll derivatives/total carotenoids. Pigment data are expressed in nM gLOI–1. (b) The low regression line for the HI vs. isorenieratene distribution (dashed line) implies that the degree of anoxia in the water column did not play a major role increasing HI values. (c) A diadinoxanthin/echinenone vs. HI diagram for the different units reveals that comparatively low HI can be associated with the increasing contribution of diatoms (particularly Unit III). (d) A δ13C(OM) vs. HI plot of selected samples shows that higher contributions of diatoms-produced organic matter is reflected in comparatively lower HI values (see text).
tributed to changes of algal source between cyanobacteria or diatom dominance and, to a lesser extent, the fluctuating degree of water-column anoxia. Variations in these conditions are in turn related with environmental changes through time. High productivity levels characterise the entire Holocene sequence, as also estimated by reconstructed trophic status (Ryves et al., 1996). The early Holocene deposits are particularly organic-rich, as shown by high total organic content (TOC) as well as by relatively high HI values (Figure 3). During this interval (between ca. 9.8 and 6.6 cal. ka. B.P.) TOC values averaged 6%, and the laminated nature of the lake deposits as shown by low BI suggests that hypolimnetic anoxia developed in relatively deep waters. Cyanobacteria dominance indicates an increasing stabilisation of the water column in warming climates (Züllig, 1989) as well as low N to P ratios (Shapiro, 1990). In fact, stratification, reduced free CO2 concentrations during summer photosynthesis; and phosphorous accumulation are common
conditions triggering natural cyanobacteria blooms (McGowan et al., 1999). A similar scenario probably occurred in the early Holocene. Between 8.1 and 7.6 cal. ka. B.P., however, there is a decrease in TOC fluxes and HI values, a substantial decrease in pigment concentrations, a marked decrease in isorenieratene ratios and dominance of diatoms over other types of algae (Ryves et al., 1996), all of which suggests a short-lived episode of reduced anoxia contemporaneous with a decline in primary productivity and/or preservation. In addition, Manca et al. (1996) reported a drop in concentration and diversity of chydorids (type of Cladocera) during this short interval to values characteristic of the Würm late-glacial period, suggesting that a short-lived cold event affected the region during the mid-Holocene. Comparable evidence has been found in other terrestrial and marine records of the Mediterranean region and equated with the globally distributed Early-Middle Holocene transition (EHMT) cooling event (Ariztegui et al., 2000).
290 From 7.6 to ca. 4.0 cal. ka. B.P. the progressive to almost complete dominance of diatoms indicated by comparatively low HI, enriched δ 13C(OM), high distinct pigment concentrations as well as the presence of microlamination imply substantial changes in the environmental conditions. These microlaminae often mask the macrolamination giving an unexpected massive aspect to the sediments (i.e., high BI values). The plankton cycle of modern seasonally stratifying lake waters demonstrate the interplay of turbulent mixing, light and nutrient distributions (Tyson, 1995). Cyanobacteria usually develop during stable, well lit but low nitrogen conditions – as in Lake Albano – whereas diatoms blooms in many temperate lakes occur preferentially during the season with comparatively more turbulent waters and higher silica concentrations. Pollen and magnetic evidence indicate increasing catchment disturbance and erosive input towards the younger sediments (Rolph et al., 1996). The latter would have triggered the observed change in the dominant plankton population and an overall reduction in aquatic productivity (Ryves et al., 1996). Whether the compositional changes in the organic matter are related to climate or human activities in the lake catchment cannot be answer conclusively from the existing evidence. Simulations with a synchronously coupled atmosphere-ocean-vegetation model as well as numerous paleodata in Europe and North Africa, however, imply substantial changes in the climate system around 6.0 ka. B.P. (Ganopolski et al., 1998 and references herein). At ca. 3.8 ka. B.P, a distinctive environmental alteration can be interpreted from both sedimentary organic matter and lithological features. Most often these changes have been interpreted as associated to deforestation and other human-induced disturbance of the environment for early-populated areas such as the Lazio region (Birks, 1986; Roberts, 1989). Pollen profiles and the magnetic record of lakes Albano and Nemi support this hypothesis (Rolph et al., 1996). However, well-constrained environmental changes, such as lake level regressions and glacial advances, have been documented in remote regions of Africa (Talbot et al., 1984, Halfman et al., 1994; Lamb et al., 1995) whereas increasing effective moisture have been described for the Atacama Altiplano in Northern Chile (Valero-Garcés et al., 1996). Moreover, results of paleolimnological studies in the Sahara area provide evidence of a major decline in precipitation at around 4.5 ka. B.P. (Ritchie et al., 1985). Thus, there is global evidence of substantial rapid moisture balance shifts during this last part
of the Holocene (Kelts, 1997) that might have influenced human migration and activity, rather than vice versa.
Conclusions Combined sedimentological, bulk and detailed analysis of the organic matter in Lake Albano indicates high levels of primary productivity during the entire Holocene. Changes in the dominant producers are recognised at different intervals using HI, OI and the isotopic composition of the bulk organic matter. These changes are further confirmed by more detailed analysis of the sedimentary organic fraction and can be used to infer former limnological conditions. During both the early Holocene and the last 4.0 ka. B.P. cyanobacteriaproduced organic matter was higher than the diatom contribution. Comparatively low HI and high OI values during the middle Holocene, in contrast, indicate intervals where diatoms are the predominant primary producers. Following the diatom-dominated productivity interval a sharp decrease in organic matter content is observed at around 4.0 ka. B.P. Approximately 300 years later, this drop in primary productivity is followed by the accumulation of organic-rich, cyanobacteriadominated sediments that display the highest HI values of the sequence. These variations are consistent with other records from the region (Magri, 1997; Ramrath et al., 1999). Although human activity in the catchment is evident, the global signal indicates that changes in climatic variables such as wind intensity, precipitation and temperature are the most probable factors affecting these environmental changes.
Acknowledgements We are indebted to all the participants of the EU-Environment Project PALICLAS (contract EV 5V CT930267) for stimulating discussions on the different aspects of the multiproxy data set as well as their open and helpful collaboration. We thank W. T. Anderson and C. Vasconcelos for their help in the Stable Isotopes Laboratory of the ETH-Zürich and I. Hajdas for the radiocarbon dating. The authors acknowledge the constructive and careful comments of J. Teranes and an anonymous reviewer. The Swiss participation in the EU-Project was supported by the Bundesamt für Bildung und Wissenschaft (BBW No. 93.0276).
291 References Alley, R. B., P. A. Mayewski, T. Sowers, M. Stuiver, K. C. Taylor & P. U. Clark, 1997. Holocene climate instability: A prominent, widespread event 8200 yrs ago. Geology 25: 483–486. Ariztegui, D., 1993. Palaeoenvironmental and palaeoclimatic implications of sedimented organic matter variations in lacustrine systems: Lake St. Moritz, a case study Unpublished PhD Thesis. ETH-Zürich, Switzerland, 188 pp. Ariztegui, D., P. Farrimond & J. A. McKenzie, 1996. Compositional variations in sedimentary lacustrine organic matter and their implications for high Alpine Holocene environmental changes: Lake St. Moritz, Switzerland. Org. Geochem. 24: 453–461. Ariztegui, D., A. Assioli, J. J. Lowe, F. Trincardi, L. Vigliotti, F. Tamburini, C. Chondrogianni, C. A. Accorsi, M. Bandini Mazzanti, A. M.Mercuri, S. van der Kaars, J. A. McKenzie & F. Oldfield, 2000. Palaeoclimatic reconstructions and formation of sapropel S1: Inferences from Late Quaternary lacustrine and marine sequences in the Central Mediterranean region. Palaeogeogr. Palaeoclimatol. Palaeoecol. 158: 215–240. Behl, R. J. & J. P. Kennett, 1996. Brief interstadial events in the Santa Barbara basin, NE Pacific, during the past 60 kyr. Nature 379: 243–246. Birks, H. J. B., 1986. Late-Quaternary biotic changes in terrestrial and lacustrine environments, with particular reference to NorthWest Europe. In Berglund, B. E. (ed.), Handbook of Holocene Palaecology and Palaeohydrology, 3–65. Calanchi, N., E. Dinelli, F. Lucchini &. A. Mordenti, 1996. Chemiostratigraphy of late Quaternary sediments from Lake Albano and central Adriatic Sea cores (PALICLAS Project). Memorie Istituto Italiano di Idrobiologia 55: 247–264. Chondrogianni, C., D. Ariztegui, P. Guilizzoni & A. Lami, 1996a. Lakes Albano and Nemi (central Italy): an overview. Memorie Istituto Italiano di Idrobiologia 55: 17–22. Chondrogianni, C., D. Ariztegui, F. Niessen, C. Ohlendorf & G. S. Lister, 1996b. Late Pleistocene and Holocene sedimentation in Lake Albano and Lake Nemi (central Italy). Memorie Istituto italiano di Idrobiologia 55: 23–38. Espitalie, J., J. Deroo & F. Marquis, 1985. Rock-Eval Pyrolysis and its Applications. Institut Français du Pétrole. Report #33578. Fry, B. & S. C. Wainwright, 1991. Diatom sources of 13C-rich carbon in marine food webs. Marine Ecology Progress Series 76: 149–157. Ganopolski, A., C. Kubatzki, M. Claussen, V. Brovkin & V. Petoukhov, 1998. The influence of vegetation-atmosphere-ocean interaction on climate during the mid-Holocene. Science 280:1916–1919. Guilizzoni, P., G. Bonomi, G. Galanti & D. Ruggiu, 1982. Basic trophic status and recent development of some Italian lakes as revealed by plant pigments and other chemical components in sediment cores. Memorie Istituto Italiano di Idrobiologia 40: 79–98. Hoek, W. Z., 2000. Integration of Ice-Core, Marine and Terrestrial Records (INTIMATE): A Core Project of the INQUA Commission on Paleoclimate. PAGES Newsletter 8/2: 21. Kelts, K., 1997. Aquatic response signatures in lake core sequences as global evidence of rapid moisture balance shifts around 4000 years ago. Terra-Nova 9: 626. Lamb, H. F., F. Gasse, A. Benkaddour, N. El Hamouti, S. van der Kaars, W. T. Perkins, N. J. Pearce & C. N. Roberts, 1995. Relation between century-scale Holocene arid intervals in tropical and temperate zones. Nature 373: 134–137.
Lami, A., F. Niessen, P. Guillizzoni, J. Masaferro & C. A. Belis, 1994. Palaeolimnological studies of the eutrophication of volcanic Lake Albano (central Italy). J. Paleolim. 10: 181–197. Langford, F. F. & M. M. Blanc-Valleron, 1990. Interpreting RockEval Pyrolisis data using graphs of pyrolizable hydrocarbons vs. total organic carbon. AAPG Bulletin 74: 799–804. Leemann, A. & F. Niessen, 1994. Holocene glacial activity and climatic variations in the Swiss Alps: reconstructing a continuous record from proglacial lake sediments. The Holocene 4/3: 259–268. Magri, D., 1997. Middle and late Holocene vegetation and climate changes in peninsular Italy. In Nüzhet Dalfes, H., G. Kukla & H. Weiss (eds.), Third Millennium BC Climate Change and Old World Collapse, NATO ASI series, I 49. Springer-Verlag, Heidelberg, 517–530. Manca M., A. M. Nocentini, C. A. Belis, P. Comoli & L. Corbella, 1996. Invertebrate fossil remains as indicators of late Quaternary environmental changes in Latium crater lakes (L. Albano and L. Nemi). Memorie dell’Istituto Italiano di Idrobiologia 55: 149–176. McGowan, S., G. Britton, E. Haworth & B. Moss, 1999. Ancient blue-green blooms. Limnol. Oceanogr. 44: 436–439. Meyers, P. & E. Lallier-Vergès, 1999. Lacustrine sedimentary organic matter records of Late Quaternary Paleoclimates. J. Paleolim. 21: 345–372. Niessen, F. & K. Kelts, 1989. The deglaciation and Holocene sedimentary evolution of southern perialpine Lake Lugano – Implications for Alpine paleoclimate. Ecolog. Geol. Helv. 82: 235–263. Niessen, F., L. Wick, G. Bonani, C. Chondrogianni & C. Siegenthaler, 1992. Aquatic system response to climatic and human changes: productivity, bottom water oxygen status, and sapropel formation in Lake Lugano after 10,000 years BP. Aquat. Sci. 54: 256– 276. Pelet, R., 1981. Preservation and alteration of present day organic matter. In Bjoroy, M. (ed.), Advances in Organic Geochemistry. John Wiley & Sons Ltd, 241–250. Peters, K. E.,1986. Guidelines for evaluating petroleum source rock using programmed pyrolisis. AAPG Bull. 70: 318–329. Ramrath, A., N. R. Nowaczyk & J. F. W. Negendank, 1999. Sedimentological evidence for environmental changes since 34,000 years BP from Lago di Mezzano, central Italy. J. Paleolim. 21: 423–435. Ritchie, J. C., C. H. Eyles & C. V. Haynes, 1985. Sediment and pollen evidence for an early to mid-Holocene humid period in the eastern Sahara. Nature 314: 352–355. Roberts, N., 1989. The Holocene: An Environmental History. Blackwell, New York, 277. Rolph, T. C., F. Oldfield & H. D. van der Post, 1996. Palaeomagnetism and rock-magnetism results from Lake Albano and the central Adriatic Sea (Italy). Memorie Istituto italiano di Idrobiologia 55: 265–283. Ryves, D. B., V. J. Jones, P. Guilizzoni, A. Lami, A. Marchetto, R. W. Battarbee, R. Bettinetti & E. C. Devoy, 1996. Late Pleistocene and Holocene environmental changes at Lake Albano and Lake Nemi (central Italy) as indicated by algal remains. Memorie Istituto italiano di Idrobiologia 55: 119–148. Sanger, J. E., 1988. Fossil pigments in palaeoecology and palaeolimnology. Palaeogeogr., Palaeoclim., Palaeoecol. 62: 343–359. Sirocko, F., D. Garbe-Schoenberg, A. McIntyre & B. Molfino, 1996. Teleconnections between the subtropical monsoons and high-latitude climates during the last deglaciation. Science 272: 526–529.
292 Shapiro, J., 1990. Current beliefs regarding dominance by bluegreens: The case for the importance of CO2 and pH. Verh. Int. Verein. Theor. Angew. Limnol. 26: 38–54. Stager, J. C. & P. A. Mayewski, 1997. Abrupt early to mid-Holocene climatic transition registered at the equator and the poles. Science 276: 1834–1836. Talbot, M. R. & D. A. Livingstone, 1989. Hydrogen index and carbon isotopes of lacustrine organic matter as lake level indicators. Palaeogeogr., Palaeoclim., Palaeoecol. 70: 121–137. Tyson, R., 1995. Sedimentary Organic Matter. Chapman & Hall, London, 615. Valero-Garcés, B. L., M. Grosjean, A. Schwalb, M. Geyh, B. Messerli & K. Kelts, 1996. Limnogeology of Laguna Miscanti: evidence
for mid to late Holocene moisture changes in the Atacama Altiplano (Northern Chile). J. Paleolim. 16: 1–21. Wetzel, R. G., 1970. Recent and postglacial production rates of a marl lake. Limnol. Oceanogr. 15: 491–503. Wilkes, H., A. Ramrath & J. F. W. Negendank, 1999. Organic geochemical evidence for environmental changes since 34,000 yrs BP from Lago di Mezzano, central Italy. J. Paleolim. 22: 349–365. Willemse, N. W. & T. E. Törnqvist, 1999. Holocene century-scale temperature variability from West Greenland lake records. Geology 27: 580–584. Züllig, H., 1985. Pigmente phototropher Bakterien in Seesedimenten und ihre Bedeutung für die Seenforschung. Sch. Z. Hyd. 47: 87–126.
