Climate changes and climate forcing in the Holocene: information from speleothem multi-proxy data.
by
Silvia Frisia
Museo Tridentino di Scienze Naturali, via Calepina 14, 38100 Trento, IT
Coauthors: Andrea Borsato (Museo Tridentino di Scienze Naturali, via Calepina 14, Trento, IT), Nereo Preto (Dip.Geol. Paleont. Geofisica, Università di Padova, via Giotto 1, 35137 Padova, IT), Frank McDermott (Dept. Geology, University College, Belfield, Dublin 4, Ireland), A. Longinelli (Dip. Scienze della Terra Università di Parma, Parco Area delle Scienze 157/A, Parma, IT), Ian Fairchild (School of Earth Sciences and Geography, Keele University, Staffs, ST5 5BG, UK)

Speleothems encode high-resolution paleoclimatic information in their mineralogy, fabrics, shape and geochemistry. Much attention has been focused on their oxygen isotope composition in the attempt to reconstruct temperature variability through time. It has become apparent, however, that temperature, rainfall seasonality, moisture track changes, local hydrology and even the geomorphology of the surface above the cave can influence delta¹⁸O values. Use of multiple parameters from different stalagmites from the same region is needed to discriminate local and global phenomena. Here we present a preliminary correlation between multiple parameters from 5 stalagmites from two climate settings (coastal and alpine) in the Northern Mediterranean region. Stalagmites were removed from Grotte de Clamouse, in France, 30 km north of Montpellier, and from two caves (Grotta di Ernesto and Bus de la Spia) of the Trento Province, in the Italian Southern Alps. Several different proxy data were tested for their potential of yielding reliable, regional-scale paleoclimate information. Preliminary results are as follows: 1) Mineralogy: Occurrence of coeval aragonite layers at 1.2 ka BP in the two stalagmites from Grotte de Clamouse correlate with the growth of dendritic fabric in those from Grotta di Ernesto. We can infer with confidence that fabric and mineralogy are reliable proxy for decrease in rainfall rate, below the current 750 mm/year (Clamouse) and 1100 mm/year (Ernesto), in the northern Mediterranean. 2) Growth lamina thickness: lamina thickness measured for the past ca. 470 years in three stalagmites from the Trento area were compared with the estimated northern hemisphere mean surface temperature (Mann et al., 1998), solar irradiance, and local instrumental temperature and precipitation time-series. Positive correlation with temperature and irradiance, rather than precipitation, is suggested. Correlation with atmospheric CO2 curve is doubtful. Spectral analyses suggest weak cyclicities at ca.1/0.2 cycles per 1.0 ka for the recent past and at ca. 1 cycle per 1.0 ka for the past 3.5 to 8.5 ka, which may be related to astronomical forcing (e.g. long term sunspot cycles). 3) delta¹⁸O : Short-term variability of the delta¹⁸O profiles of stalagmites from the same cave do not correlate, which indicate that local phenomena control the high-frequency delta¹⁸O signal rather than regional temperature or rainfall. Long-term trends of delta¹⁸O mark a shift from more negative to more positive values at 5.4 ka BP. This trend correlates with a shift to higher Mg/Ca ratio and a decrease of growth-band widths in both Alpine and Mediterranean speleothems. We infer that this delta¹⁸O shift is related to a possible large-scale change in storm tracks, which caused a general decrease in rainfall rate throughout the region. Additional delta¹⁸O data from radiocarbon dated mammal bones indicate that there was a change in meteoric delta¹⁸O signal between 6.0 and 5.3 ka BP. 4) delta¹³C : the carbon isotope profiles for 4 stalagmites (from S France and the Italian Alps) show a long-term positive correlation throughout the Holocene. All profiles are characterized by a gradual increase in delta¹³C starting 1.0 ka BP.

Date received: March 26, 2001

Copyright © 2001 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 # cagc-30.