Study of Recent Sediments in the City of Como (Northern Italy): Multidisciplinary Analysis and Implication for the Environmental Evolution of the Area
by
Sabrina Capelletti
Università degli Studi dell'Insubria, Como
Coauthors: Alessandro M. Michetti, Università degli Studi dell'Insubria, Como; Valerio Comerci, APAT (Italian Agency for Environment Protection and Technical Services), Rome; Sabina Rossi, Università degli Studi dell'Insubria, Como

The purpose of this work is to provide a reconstruction of the recent environmental evolution, after the Last Glacial Maximum (LGM, about 20000 years ago) throughout the Holocene, for the Lake Lario sector where the city of Como is located.

This aim is achieved through a multidisciplinary approach and analysis conducted on urban subsurface sediments. A more detailed knowledge of the Holocene stratigraphy of Como can lead to a more accurate territorial and urban planning, for the improvement of environmental management of a city like Como.

Como provides a privileged point of view for understanding the post glacial evolution because it conserves a continuous, datable and high resolution archive of environmental data. In fact this area was occupied by a closed branch of Lake Lario after the LGM, with a high sedimentation rate of deposits rich in organic matter and with limited erosion phenomena.

Since 2001 the University of Insubria, in collaboration with Como Administration, APAT (Italian Agency for Environmental Protection and Technical Services) and IMONT (National Institute for the Mountain) is carrying out a multidisciplinary research to investigate in detail the stratigraphy of the urban subsurface.

Firstly, we collected and interpretated about 100 boreholes stratigraphy located in the entire urban area. Then we conducted a detailed analysis in 3 zones of the city (S. Abbondio site, Ticosa's area and Valleggio street). In particular, we drilled three new shallow boreholes for the site of S. Abbondio in 2003. For these sites we collected samples for sedimentologic, stratigraphic, palynologic, mineralogic and radiocarbon dating analyses. Combining litostratigraphic, mineralogic and palynologic data we proposed a model of climatic and environmental evolution for the area after the LGM. Previous data (Comune di Como, 1980; Castelletti and Orombelli, 1986; Apuani et al., 2000) and our new analyses allowed us to work out a 3D geological model describing the spatial geometry of deposits and then to interpret the stratigraphic succession of Como subsurface from the palaeoenvironmental point of view.

A sandy, organic-rich level indicating a marsh environment (and so originally related to the lake level) has been used as "marker horizon" for the Late Glacial-Holocene transition. A wood sample found in the marker horizon from the S. Abbondio site was first dated at 11730 ± 180 yr 14C B.P. (Castelletti and Orombelli, 1986). A new wood sample collected during our 2003 drillings in the same deposits gave a 14C age of 13230 ± 120 yr B.P. (Centre for Isotopic Research, University of Gröningen). Therefore, during the period between about 13-11 kyr B.P. in the area of Como the lake surface level was stood at an elevation of 205-200 m a.s.l., very close to the present day one (198 m a.s.l.). The area was characterized by a transitional low lacustrine-swamp zone, at the air-water interface as corroborate by palynologic and mineralogic evidence.

In particular, while along the border of the basin (S. Abbondio and Ticosa sites) lacustrine shores developed, a marsh environment occupied the central part of the basin. A thick, sandy organic sequence is found in many boreholes at the centre of the basin (about 20-30 m of thickness in lake-front zone and 40 m in Cavour Square) while it becomes thinner towards the borders.

Our data show that the "marker horizon" is located at approximately 50 m depth in the Duomo area, while it outcrops in the S. Abbondio site. This difference is probably due to Holocene subsidence, that reaches an average maximum rate of 4 mm/yr during the last 12000 yr B.P. (see also the abstract by Comerci et al. in this volume).

New drillings planned for October 2005 would cross about 50 meters of Holocene sediments allowing us to reconstruct the recent history of physical environment with extreme detail and to calibrate realistic models of interaction between natural processes and human impact in this sector of Lake as well as in the whole Lario Basin.

References

APUANI T., CANCELLI A., CANCELLI P. (2000), Hydrogeological and Geotechnical Investigations along the Shoreline of the Town of Como, Italy. In: D.P. Moore and O. Hungr (eds.) "Engineering Geology and Environment" Proc. 8th. Cong. Intern. Assoc. Engineering Geology IAEG (21-25 Sept. 1998 Vancouver, B.C., Canada), Balkema, Rotterdam 2000 Vol. VI, pp. 4685-4692.

CASTELLETTI L., OROMBELLI G. (1986), Una nuova data 14C per la storia della deglaciazione del bacino del lago di Como. Geogr. Fis. Dinam. Quat., 9, pp. 56-58.

COMUNE DI COMO (1980), Relazione di sintesi della Commissione per lo Studio dei fenomeni di Subsidenza. Documenti e Ricerche, 34.

Date received: July 22, 2005

Copyright © 2005 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # caqy-48.