4. Atmospheric and Geologic Constraints
69
However, outcrops of soft, strongly weathered, saprolite parent material
indicate a lack of glaciation also at CPE .
Inland North American watersheds showed evidence of stronger contributions from weathering processes, with relatively high TDS in conjunction with low Na+: (Na" + Ca
2+)
ratios (see group D in Figure 4.1).
We interpret this to reflect two factors: 'I) a diminished influence of
marine aerosol inputs due to the inland location of these watersheds; and
2) a stronger contribution of geologic sources from the more easily
weathered parent materials of sandstone and shale. In fact, except for
Marmot Creek, these watersheds showed Na": 0- ratios that were substantially increased relative to sea-salt expectations (Figure 4.2), indicating
a significant weathering source for Na+.
Sea-salt corrections also differed between CP forests and forests of
CPE or North America. Levels of divalent cations (Ca2+ and Mg
2+)
were
particularly low in CP streams (Figure 4.3). Except for Ca
2+
, major
cations and anions could be almost exclusively (75%-100%) explained by
sea-salt aerosol inputs (sections F and G in Figure 4.3; Hedin et aI.,
1995). Thus, atmospheric sources appear to dominate over geologic
sources for these elements at CP. In contrast, while levels of Ca
2+
were
exceedingly low (median ca. 0.4mgl1
) at CP as compared to other
areas, only approximately 25% of the observed concentrations could be
explained by atmospheric inputs. When compared to other cations the
low contribution of atmospheric sources to Ca
2+
in CP watersheds correlates with the observation that Ca
2+
is the least abundant base cation in
sea-salt aerosol (Keene et aI., 1986). Thus, even very low geologicsupplies
of Ca
2+
can be significant relative to atmospheric supplies at CP. In fact,
soils in unglaciated silicated areas tend to contribute little Ca
2+,
because
Ca-rich feldspars are unstable and weather relatively rapidly on a
geological time scale (Berner & Berner, 1987; Stauffer, 1990). The overall
lack of weathering sources for most elements at CP is consistent with old
soils, lack of glaciation, and highly weathered parent materials.
In contrast, the relatively recently glaciated North American watersheds
showed markedly higher Ca
2+
and Mg2+ in stream waters, and corrections
for sea-salt aerosols were not as effective as for CP. For example, only
1% to 2% of Ca2+, 14% to 34% of Mg
2+
, and 4% to 52% of K+ could be
explained by atmospheric inputs of sea-salt aerosols in North American
forests (sections A-D, Figure 4.3). Similarly, low fractions of Ca2+,
Mg
2+,
and K+ could be explained by sea-salt aerosol at CPE (section E,
Figure 4.3). Yet , ratios of Na+:CI- in North American coastal forests
and at CPE did not differ substantially from expectations based on seasalt aerosol (Figure 4.2). In contrast to CP, our results from temperate
rainforests in North America and at CPE thus indicate that weathering
can be a particularly important source of divalent base cations (refer to
sections A-D in Figure 4.4 and A and B in Figure 4.5). As for the divalent base cations, sea-salt corrections had limited but varying (6%-64%
69
However, outcrops of soft, strongly weathered, saprolite parent material
indicate a lack of glaciation also at CPE .
Inland North American watersheds showed evidence of stronger contributions from weathering processes, with relatively high TDS in conjunction with low Na+: (Na" + Ca
2+)
ratios (see group D in Figure 4.1).
We interpret this to reflect two factors: 'I) a diminished influence of
marine aerosol inputs due to the inland location of these watersheds; and
2) a stronger contribution of geologic sources from the more easily
weathered parent materials of sandstone and shale. In fact, except for
Marmot Creek, these watersheds showed Na": 0- ratios that were substantially increased relative to sea-salt expectations (Figure 4.2), indicating
a significant weathering source for Na+.
Sea-salt corrections also differed between CP forests and forests of
CPE or North America. Levels of divalent cations (Ca2+ and Mg
2+)
were
particularly low in CP streams (Figure 4.3). Except for Ca
2+
, major
cations and anions could be almost exclusively (75%-100%) explained by
sea-salt aerosol inputs (sections F and G in Figure 4.3; Hedin et aI.,
1995). Thus, atmospheric sources appear to dominate over geologic
sources for these elements at CP. In contrast, while levels of Ca
2+
were
exceedingly low (median ca. 0.4mgl1
) at CP as compared to other
areas, only approximately 25% of the observed concentrations could be
explained by atmospheric inputs. When compared to other cations the
low contribution of atmospheric sources to Ca
2+
in CP watersheds correlates with the observation that Ca
2+
is the least abundant base cation in
sea-salt aerosol (Keene et aI., 1986). Thus, even very low geologicsupplies
of Ca
2+
can be significant relative to atmospheric supplies at CP. In fact,
soils in unglaciated silicated areas tend to contribute little Ca
2+,
because
Ca-rich feldspars are unstable and weather relatively rapidly on a
geological time scale (Berner & Berner, 1987; Stauffer, 1990). The overall
lack of weathering sources for most elements at CP is consistent with old
soils, lack of glaciation, and highly weathered parent materials.
In contrast, the relatively recently glaciated North American watersheds
showed markedly higher Ca
2+
and Mg2+ in stream waters, and corrections
for sea-salt aerosols were not as effective as for CP. For example, only
1% to 2% of Ca2+, 14% to 34% of Mg
2+
, and 4% to 52% of K+ could be
explained by atmospheric inputs of sea-salt aerosols in North American
forests (sections A-D, Figure 4.3). Similarly, low fractions of Ca2+,
Mg
2+,
and K+ could be explained by sea-salt aerosol at CPE (section E,
Figure 4.3). Yet , ratios of Na+:CI- in North American coastal forests
and at CPE did not differ substantially from expectations based on seasalt aerosol (Figure 4.2). In contrast to CP, our results from temperate
rainforests in North America and at CPE thus indicate that weathering
can be a particularly important source of divalent base cations (refer to
sections A-D in Figure 4.4 and A and B in Figure 4.5). As for the divalent base cations, sea-salt corrections had limited but varying (6%-64%