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6.1. FORSCHUNGSSTELLE “RADIOMETRIE” 169
6.1.9 U-series dating and paleoclimatic interpretation of the stable isotope
profile of stalagmite ER76 from Grotta di Ernesto, Italy
Denis Scholz (Participating scientists: Christoph Spötl (Innsbruck), Silvia Frisia (Trento), Andrea
Borsato (Trento), Jens Fohlmeister)
Abstract Stalagmite ER76 from Grotta di Ernesto, Italy, was precisely dated by TIMS Th/Udating.
The age-depth-model was constructed using a Bayesian approach to improve the dating
uncertainty and applying a mixed effect regression model. Furthermore, high-resolution stable oxygen
and carbon isotope profiles were determined.
A g e [k y r]
1 0
9
8
7
6
5
4
3
2
1
A g e -d e p th m o d e l (H e e g a a rd e t a l., 2 0 0 5 )
9 5 % c o n fid e n c e lim its
N e w U -s e rie s a g e s
U -s e rie s a g e s (M c D e rm o tt e t a l., 1 9 9 9 )
0
0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0 4 0 0
D is ta n c e fro m to p [m m ]
Figure 6.10: Final age-depth-model calculated for stalagmite ER76
Background Stalagmite ER76 from Grotta di
Ernesto, Italy, was precisely dated by TIMS
Th/U-dating. The age-depth-model was constructed
using a Bayesian approach to improve the
dating uncertainty and applying a mixed effect regression
model. Furthermore, high-resolution stable
oxygen and carbon isotope profiles were determined.
Methods and results Nine TIMS U-series
ages as well as high-resolution stable isotope profiles
on stalagmite ER76 from Grotta di Ernesto,
Italy, were determined. Due to its low 238 U (20-
40 ng/g) content which is typical for stalagmites
from Grotta di Ernesto, U-series dating of ER76
is rather difficult. Nevertheless, we were able to
determine a 230 Th/U-age of 2.27 0.08 kyr in the
youngest part, at 30 mm distance from top (dft).
However, due to the low U content, the error bars
of some data points overlap significantly. By application
of a Bayesian approach which takes into
account both the results of the U-series dating
and the stratigraphic sequence of the individual
samples the uncertainty of some data points could
be reduced substantially. We suggest to generally
apply this Bayesian approach when the individual
data point errors overlap. Overall, 13 data points,
nine measured in Heidelberg and four by McDermott
et al. [1999], were used to construct the final
age-depth relationship for ER76. The age model
and its uncertainty (see Fig. 6.10) was estimated
using a mixed-effect regression model [Heegaard
et al. , 2005]. Based on the new age model, ER76
exhibits a rather constant growth rate of approximately
0.05 mm/yr between 8.5 and 2 kyr. It
was actively growing when it was collected, and
Frisia et al. [2003] have shown that ER76 exhibits
annual laminae in the period between 1650
and 1995 AD. The thickness of these annual laminae
lies between 0.02 and 0.1 mm. In combination
with the new age model obtained by U-series dating
this shows that there must be a substantial
hiatus between 2 and 0.5 kyr in ER76. Oxygen
and carbon stable isotope profiles were measured
at the University of Innsbruck at a resolution of
0.1 mm between 0 and 8 mm dft and 0.25 mm
below 8 mm dft.
Outlook/Future work An earlier study [Mc-
Dermott et al. , 1999] revealed that ER76 recorded
several Holocene warm/dry and cold/wet phases,
respectively. Because the resolution in that study
was approximately 2 mm, the new profiles have
a ten times higher resolution which should enable
to detect even rapid climate events.
Funding DFG Forschergruppe 668: ”Datierte
Speläotheme: Archive der Paläoumwelt”
Main publication Vollweiler et al. [2006],
Scholz & Mangini [2006], Scholz & Mangini [accepted
for publication], Mangini et al. [accepted
for publication]