『Abstract
This study presents U and Li isotope and major and trace element
data for the dissolved load, suspended particulates and bedload
for Icelandic rivers draining predominantly basaltic catchments.
Physical erosion rates range from 920 to 2084 t/km2/yr,
with the higher values associated with glacier-fed rivers. Chemical
erosion rates ranges from 45 to 91 t/km2/yr, with lower
rates being associated with glacier-fed rivers. Uranium activity
ratios, (234U/238U), are close to secular
equilibrium in the suspended and bedloads, but all dissolved load
samples show values greater than unity, ranging from 1.13 to 2.41.
The highest (234U/238U) values are found
in glacier-fed rivers, and can be attributed to α-recoil effects,
as grinding by glaciers locally enhances rates of physical weathering.
Activity ratios in glacial rivers decrease with distance from
the glacial source due to input from non-glacial tributaries which
have high levels of dissolved uranium and lower activity ratios.
In contrast, in non-glacial rivers, uranium activity ratios increase
with distance downstream due to continued weathering in soils
and of bedrock. The δ7Li value of the suspended load
is always lower than that of the bedload due to preferential retention
of 6Li in secondary minerals during weathering. In
turn, the δ7Li value of the dissolved load is always
greater than that of the bedload, ranging from 17.0 to 43.7‰.
δ7Li for the dissolved load decreases with increasing
levels of dissolved silicon, and the saturation index of secondary
minerals, which suggests that δ7Li decreases with increasing
chemical weathering. There is no correlation between δ7Li
values and uranium activity ratios for the dissolved load for
non-glacial rivers, but for glacier-fed rivers there is an increase
in δ7Li with increasing (234U/238U),
suggesting that where physical comminution of mineral grains by
glaciers is high, chemical weathering is suppressed, and vice-versa.
Keywords: uranium isotopes; lithium isotopes; basalt weathering;
glacial weathering; Iceland』
1. Introduction
2. Geology, climate and river setting
3. Field and analytical techniques
3.1. Sampling
3.2. Cations, anions and trace elements
3.3. U and Li isotopes
4. Results
4.1. Physical measurements
4.2. Major and trace elements
4.3. Uranium elemental and isotope data
4.4. Lithium elemental and isotope data
5. Discussion
5.1. Physical and chemical erosion rates
5.2. Mineral saturation states
5.3. Uranium isotope ratios
5.4. Li-isotopes
5.5. Glacial rivers
6. Conclusions
Acknowledgements
References