CHANGES IN RAINFALL AND WATER AVAILABILITY ON
ISLA ISABELA, GULF OF CALIFORNIA
AMY ENGLEBRECHT (1), B. LYNN INGRAM (1), ROGER BYRNE (2), ULRIKE KIENEL (3), HARALD BOEHNEL (4), AND GERALD HAUG (3)
(1) Dept. of Earth and Planetary Science, University of California, Berkeley CA
(2) Dept. of Geography, University of California, Berkeley CA
(3) Dept. of Climate Dynamics and Sediments, GeoForschungsZentrum, Potsdam Germany
(4) Centro de Geociencias, Universidad Nacional Autonoma de Mexico, Querétaro Mexico
RESULTS
LOCATION AND BACKGROUND
Isla Isabela (Fig. 1) is located in
the precipitation region of the
Mexican Monsoon (also called
the North American Monsoon),
a pronounced maximum in
rainfall that occurs during the
warm season over northwestern
Mexico and the southwestern
United States (Fig. 2). Changes
in the strength and timing of the
monsoon are frequently
associated with floods and
droughts, and at the most arid
sites in this region, the
interannual variability of
summer climate can be larger
than the mean summer rainfall
itself (Higgins et al. 1998).
Fig. 2. The region around Isla Isabela is
characterized as mostly arid to semi-arid, and
Isabela receives approximately 75% of its annual
precipitation in July, August, and September.
Fig. 3. Sediments are comprised of alternating organic- and clay-rich
intervals (core-top shown here). The organic-rich layers are olive
green to brown in color and contain abundant diatoms and
phytoplankton. The clay-rich intervals are dark gray to black in
color. In addition, discrete layers of calcite (2-5 mm thick) are
present, centered at 11, 20, 37, 41, 52, and 56 cm.
From Alcocer et al., 1998
Fig. 1. Isla Isabela (21°52'N, 105°54'W) is a small volcanic island located
approximately 30 km off the Mexican state of Nayarit.
Closed basin lakes, like the crater lake found
on Isla Isabela, are sensitive to changes in
precipitation/evaporation ratios, and the
properties of their sediments record changes
in effective moisture through time. The
analyses of stable isotopes of oxygen (O-18
and O-16) and carbon (C-13 and C-12) in
authigenic carbonate are well-established
proxy techniques and the ratios of the stable
isotopes can be very responsive to those
changes in effective moisture.
Fig. 4. Short cores from Isabela show coherent
periodicities in the carbon and oxygen isotopic
composition of bulk inorganic carbonate. Most
notably, strong excursions in carbon isotopes and
calcite deposition (*) occur with some frequency
throughout the record.
Fig. 5. An initial varve count
indicates that the carbonate layers
coincide with periods of drought
found in other paleoclimate records
in northern Mexico (see Table 1).
Table 1. Some paleo and historical records of relative dry (shaded) and wet periods in
northern Mexico.
Fig. 6. Carbon and oxygen isotopes appear to alternate between in phase (shaded) and out of phase variation.
These periods are roughly coincident with dry and wet periods recorded in other paleoclimate proxies from
northern Mexico (see Table 1).
A
Fig. 7. During in phase
periods (shaded in Fig. 6),
carbon and oxygen
isotopes correlate with an
r2 = 0.33 (A). Out of phase
periods have r2 < 0.01 (B).
DISCUSSION
δ18O (‰)
METHODS
Winter
Sediment cores were taken from the center of Isabela crater lake by U. Kienel and H. Boehnel. The coring
was accomplished with a modified Livingston Corer equipped with plastic (butyrate) liners. Two adjacent
cores were taken, allowing overlap between them. The longer of the two cores spans 0-66 cm, the other
covers 0-52 cm. Cores were sampled at intervals of 1 cm, and samples were wet sieved with a 125 µm
screen and washed onto filter paper. Samples were then dried at 70°C overnight and bulk inorganic
carbonate was analyzed for stable isotopic (oxygen and carbon) composition. Samples were analyzed on a
GV Isoprime irMS located in the Ingram Lab of UC Berkeley. The internal precision of both δ 13C and δ18O
measurements is 0.03 and 0.05 per mil, respectively. The external precision is < 0.1%.
REFERENCES
Alcocer, J. et al., 1998. Isabela crater-lake: a Mexican insular saline lake. Hydrobiologia 381: 1–7.
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Higgins, R.W. et al., 1998. Interannual variability of the U.S. summer precipitation regime with emphasis on the southwestern
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B
In this lake it appears that the ratio of the stable isotopes of
carbon is controlled by the biogenic calcium carbonate
contribution, leading to an increase in the heavier isotope
(C-13) during drier periods, and vice versa. The ratio of
the stable isotopes of oxygen appears to be primarily
controlled by the source of the precipitation as well as
evaporation occurring within the basin. Generally, shifts to
drier climate and increased evaporation lead to a relative
increase in the heavier isotope (O-18), and vice versa.
During dry periods, evaporation may dominate the system,
with an annual average oxygen isotope value that is
enriched relative to rainfall, and abiogenic calcium
carbonate deposition. Both would lead to more positive
isotopic values (Fig. 6, shaded).
Summer
From http://isohis.iaea.org
From Metcalfe et al., 2000
Fig. 8. Oxygen isotopes in precipitation vary by up to ~5 per
mil between winter and summer in northern Mexico. A
particularly strong gradient exists during winter from
northwest to southeast Mexico. So during the 1982-83 El
Niño year, for example, when more abundant precipitation
occurred later in the calendar year, the δ18O lake potentially
reflected the winter circulation and a northwesterly source of
winter precipitation.
Conversely, during normal/wet periods, the system may be
source dominated, leading to an oxygen isotope value that
more closely reflects that of rainfall and biogenic calcium
carbonate deposition. Because the oxygen isotopes of
precipitation in this region vary quite widely over the
seasons (~5 per mil), the source effect on the oxygen
isotopes would depend on the prevailing seasonal
circulation and storm track (Fig. 8).