4. Atmospheric and Geologic Constraints
63
describes natural waters of the world and whether Na+ in dilute waters is,
in fact, an exclusive tracer of atmospheric inputs. We have here enlarged
this envelope based on Eilers et al. (1992) and based on Gibbs' own
revisions (Gibbs, 1992) (Figure 4.1). The assumption that Na+ reflects
atmospheric inputs in dilute waters is generally valid for our analyses,
since we focus on near-coastal watersheds with strong inputs of marine
aerosols with high Na+ and Cl" (Galloway, Likens, Keene , & Moody,
1982).
It is relatively rare to find waters that, according to the Gibbs diagram
referred to in our Figure 4.1, are nearly exclusively dominated by atmospheric sources of elements (high Na+: [Na+ + Ca2+] and low TDS)
(Gibbs , 1970; Kilham, 1990). Such waters are most notably found draining
strongly weathered lateritic terranes such as in tropical Amazonia, where
soils are depleted of easily weathered Ca-rich minerals (Gibbs , 1970;
Kronberg & Melfi, 1987).
'
When plotted on the Gibbs diagram, the watersheds considered in this
study separate into three well-defined groups , as shown in Figure 4.1.
Values from inland North American watersheds (Figure 4.1, group D)
fall in the area of weathering dominance, with high Ca
2+
relative to Na+
and TDS values of approximately 100 to 400mgl1 . Values from coastal
North American temperate rainforests (group C in Figure 4.1) fall in
between weathering and precipitation dominance, with significant Ca
2+
relative to Na+ and low levels of TDS (1O-40mgl1 ) . In contrast , values
from South American temperate rainforests (groups A and B in Figure
4.1) fall in the bottom right-hand area of the graph , indicating a strong
dominance of atmospheric aerosol inputs relative to weathering inputs.
CP watersheds show particularly high values of Na+ : (Na+ + Ca
2+),
with
TDS ranging from 7.5 to 125mgl1 . CP watersheds with high TDS
(>100mgl1 ) were of the coastal forest type and located immediately
near the ocean, thus receiving direct sea spray.
Contributions from Sea-Salt Aerosols
We further evaluated the role of atmospheric versus geologic inputs of
elements to forest ecosystems by comparing abundances of Na+ versus
Cl" in watershed streams. Atmospheric inputs (wet and dry deposition)
from unpolluted marine air masses are dominated by sea-salt aerosols, in
which Na+ and Cl" are major elements and occur in a ratio approximating
0.56 by weight (Galloway et al., 1982; Keene, Pszenny, Galloway, &
Hawley, 1986; Warneck, 1988). Except for areas with halite deposits,
natural nonmarine sources of CI- are generally low and associated with
the weathering of sedimentary rocks (Berner & Berner, 1987). We thus
would expect a relationship between Na+ and Cl", near the ideal sea-salt
ratio, in streams that drain igneous or metamorphic terranes and where
63
describes natural waters of the world and whether Na+ in dilute waters is,
in fact, an exclusive tracer of atmospheric inputs. We have here enlarged
this envelope based on Eilers et al. (1992) and based on Gibbs' own
revisions (Gibbs, 1992) (Figure 4.1). The assumption that Na+ reflects
atmospheric inputs in dilute waters is generally valid for our analyses,
since we focus on near-coastal watersheds with strong inputs of marine
aerosols with high Na+ and Cl" (Galloway, Likens, Keene , & Moody,
1982).
It is relatively rare to find waters that, according to the Gibbs diagram
referred to in our Figure 4.1, are nearly exclusively dominated by atmospheric sources of elements (high Na+: [Na+ + Ca2+] and low TDS)
(Gibbs , 1970; Kilham, 1990). Such waters are most notably found draining
strongly weathered lateritic terranes such as in tropical Amazonia, where
soils are depleted of easily weathered Ca-rich minerals (Gibbs , 1970;
Kronberg & Melfi, 1987).
'
When plotted on the Gibbs diagram, the watersheds considered in this
study separate into three well-defined groups , as shown in Figure 4.1.
Values from inland North American watersheds (Figure 4.1, group D)
fall in the area of weathering dominance, with high Ca
2+
relative to Na+
and TDS values of approximately 100 to 400mgl1 . Values from coastal
North American temperate rainforests (group C in Figure 4.1) fall in
between weathering and precipitation dominance, with significant Ca
2+
relative to Na+ and low levels of TDS (1O-40mgl1 ) . In contrast , values
from South American temperate rainforests (groups A and B in Figure
4.1) fall in the bottom right-hand area of the graph , indicating a strong
dominance of atmospheric aerosol inputs relative to weathering inputs.
CP watersheds show particularly high values of Na+ : (Na+ + Ca
2+),
with
TDS ranging from 7.5 to 125mgl1 . CP watersheds with high TDS
(>100mgl1 ) were of the coastal forest type and located immediately
near the ocean, thus receiving direct sea spray.
Contributions from Sea-Salt Aerosols
We further evaluated the role of atmospheric versus geologic inputs of
elements to forest ecosystems by comparing abundances of Na+ versus
Cl" in watershed streams. Atmospheric inputs (wet and dry deposition)
from unpolluted marine air masses are dominated by sea-salt aerosols, in
which Na+ and Cl" are major elements and occur in a ratio approximating
0.56 by weight (Galloway et al., 1982; Keene, Pszenny, Galloway, &
Hawley, 1986; Warneck, 1988). Except for areas with halite deposits,
natural nonmarine sources of CI- are generally low and associated with
the weathering of sedimentary rocks (Berner & Berner, 1987). We thus
would expect a relationship between Na+ and Cl", near the ideal sea-salt
ratio, in streams that drain igneous or metamorphic terranes and where