『Abstract
Sr isotope and Ca/Mg/Sr chemical compositions of freshwater ostracode
tests separated from a sediment core represent the last 16 ka
of sedimentation in Lake Constance, Central Europe. The chemical
evolution of the paleowater's dissolved load of Lake Constance
was estimated by correcting the ostracode data for Ca/Mg/Sr fractionation
due to biogenetic calcification. Since the Lake Pleistocene deglaciation,
the Ca/Sr molar ratios of paleowaters increased systematically
from about 100 (a near marine signature) to about 200. Ca/Mg molar
ratios varied in the range of 1-25. The 87Sr/86Sr
ratios indicate Late Pleistocene paleowater compositions of 0.7086-0.7091,
significantly more radiogenic than present day waters (0.7085).
Sr isotopes and Ca/Mg/Sr chemical data together show that weathering
of Mesozoic evaporites consistently dominated the dissolved Sr
load (80-90％). Carbonate and silicate weathering were less important
(1-10％). Trends of Sr dissolved loads were therefore not related
to Mg which was mainly mobilized by carbonate weathering. Biotite
weathering was an important source of radiogenic Sr in the paleowaters.
The short-term release (duration about 600-8-- years) of radiogenic
Sr during glacier retreat started 15.2 ka ago and was due to enhanced
biotite weathering at the glacier base. Long-term release of radiogenic
Sr was due to biotite weathering in glacial soils and silicate
rocks, and has gradually declined since the Late Pleistocene/Holocene
transition.
Keywords: 87Sr/86Sr ratios; Ca/Mg/Sr chemistry;
Ostracode; Lake Constance; weathering; Alpine catchment』
1. Introduction
2. catchment geology and samples
3. Analytical procedures
4. Results and discussion
4.1. Data distributions
4.2. Ca/Sr and Ca/Mg trends
4.2.1. Ca/Sr trends
4.2.2. Ca/Mg trends
4.3. Sr isotope trends
4.4. Constraints on the Sr sources
4.5. Dissolved load budgets and paleoweathering relations
4.6. Importance of biotite weathering
5. Conclusions
Acknowledgements
References