『Abstract
This paper presents an investigation into the degree and nature
of chemical weathering during soil formation on a volcanic (phonolite)
substrate on the southern slopes of Mt. Kilimanjaro in northern
Tanzania. The high field strength elements Nb and Ta were used
to estimate enrichments and depletions relative to the bedrock.
The degree of weathering was found to increase with depth in the
soil profile. At depths greater than 200 cm, Si, Na, K, Ca, and
Mg have been depleted by nearly 100% while Al has been enriched,
resulting in a highly aluminous soil residue (40-50 wt.% Al2O3). At depths shallower
than 200 cm, the soil is also depleted in Si, Na, K, Ca and Mg
though not to the extents seen at depths greater than 200 cm.
The lower degrees of weathering in the upper 200 cm are also evidenced
by the fact that layer above 200 cm is characterized by slight
positive Eu anomalies relative to other rare earth elements whereas
the deeper layer exhibits no Eu anomalies. The rare earth element
systematics are consistent with preferential weathering of the
glassy matrix in the upper 200 cm, leaving behind plagioclase
phenocrysts, which are enriched in Eu. In the deeper layer, weathering
appears to be so extensive that both Eu-rich plagioclase phenocrysts
and Eu-poor glass/ash have largely weathered away. These observations
collectively show that the upper and lower layers of the weathering
profile have undergone different weathering histories. Four scenarios
may explain the apparent inverted weathering profile: re-precipitation
followed by erosion, Aeolian deposition, a buried paleosol, and
enhanced weathering due to lateral subsurface water flow. The
first hypothesis fails to explain the massive loesses of Si, Na,
K, ca, and Mg below 200 cm. The Aeolian deposition hypothesis
is also untenable because it contradicts the trace element and
REE's behaviors. The latter two hypotheses are both reasonable;
however, the buried paleosol model is inconsistent with some physical
and geochemical observations and the subsurface flow model requires
the influence of hydraulic conditions not tested in this study.
It is concluded here that either the buried paleosol model or
the subsurface flow model can explain the formation of the Machame
soils with the latter having novel implications for the transport
of dissolved cations to the ocean.
Keywords: Soil formation; Subsurface water; Chemical weathering;
Tanzania』
1. Introduction
2. Study area and description of weathering profile
3. Methods
4. Results
5. Estimating mass changes during soil formation
5.1. Calculating chemical weathering and element mobility
5.2. Identification of an appropriate immobile element tracer
5.3. Estimating mass fluxes
6. Possible explanations for the inverted weathering profile
6.1. Re-precipitation followed by erosion
6.2. Aeolian dust deposition
6.3. Buried Paleosol
6.4. Lateral subsurface water flow
7. Implications of lateral subsurface water flow
8. Conclusions
Acknowledgments
References