Ge/Si as a tracer for Si in paired catchments of the Luquillo CZO
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Published version
Date
2018-04-02
DOI
Authors
Aguirre, Andres
Derry, Louis
Kurtz, Andrew
Version
Published version
OA Version
Citation
Andres Aguirre, Louis Derry, Andrew Kurtz. 2018. "Ge/Si as a tracer for Si in paired catchments of the Luquillo CZO." Goldschmidt Conference. Boston, MA, 2018-08-12 - 2018-08-17.
Abstract
Catchment lithology is a significant factor influencing
the generation and transport of solutes in the critical zone. In
the Luquillo Mountains of Puerto Rico, the Quebrada Guaba
and Bisley catchments are studied to understand how
lithology affects concentration-discharge (C-Q) relationships.
Ge/Si ratios in pore water and stream samples are used to
identify sources of Si to streams in the Bisley 1 watershed.
Quebrada Guaba is underlain by quartz diorite and is
characterized by strong Si dilution behavior (power law slope
= -0.47)^1. During baseflow, Ge/Si = 0.27-0.47 μmol mol^-1
due to weathering of plagioclase and precipitation of Ge
enriched kaolinite in the bedrock-saprolite interface2. During
storms, hydrologic pathways shift to shallower flow paths
with lower Si concentrations and higher Ge/Si (1.0-4.0 μmol
mol^-1)^3. The shift to saprolite-dominated flow paths carrying
dilute Si end-members drives the Si-Q pattern in this
catchment.
The volcaniclastic sub-catchment of Bisley 1 has a more
chemostatic Si-Q relationship (power law slope of = -0.30)^1.
In this study, lysemeters at the Bisley sites of B1S1, B1S2
and B1R show higher Si pore water concentrations than the
LG sites at Quebrada Guaba. Ge/Si ratios for Bisley are lower
than Guaba except for 200-300 cm depth were ratios increase
to 2.87 μmol mol^-1 (B1S1). Dissolved Si concentrations
increase markedly from 200 cm to the surface at B1S1 and
B1S2. Ge/Si shows the opposite trend with ratios decreasing
from 2.87 to 0.86 μmol mol^-1. This pattern of increased pore
water Si and low Ge/Si may be due to phytolith dissolution
also observed in Quebrada Guaba^2,4. Bisleys greater Si
depletion near the surface may result in more sensitivity to
phytolith inputs. Stream samples from Bisley 1 will be
analyzed for major cations and Ge/Si to understand how pore
water or other shallow surface reservoirs influence Si-Q
patterns in this catchment.