Stable oxygen and
hydrogen isotopic tracers in Amazon shelf waters during Amasseds
Oceanologica Acta,
Volume 25, Issue 2,
Jonathan D. Karr and William J. Showers
Summary of Points:
·
A detailed hydrographic survey of the water column of the Amazon
shelf was performed using stable oxygen and hydrogen isotopes to determine the
source and fate of waters on the shelf.
·
18O-H2O and D measurements were made on water
column samples from approximately 60 stations (three depths per station) which
were each collected during four Amasseds (A
Multidisciplinary Amazon Shelf SEDiment Study)
cruises: I8909-August 1989, falling river discharge;
I9002-March 1990, rising river discharge; I9004-May 1990, peak discharge;
I9113-November 1991, minimum discharge.
·
Isotopes were compared with salinity and temperature measurements
in order to identify water masses and mixing. The characteristics (salinity,
temperature, 18O, D and d) are proposed for the following
end-number water masses: river water, open ocean surface water (0¯100 m)
and open ocean intermediate water (> 300 m). River water:
salinity = 0; temperature ~ 27¯29 °C; 18O = ¯4.2
to ¯6.8 per mil; D ~ ¯22.1 to ¯38.9
per mil; d = 9.4 to 17.0 per mil (compare to d = 10
for Meteoric Water Line). Open ocean surface water (~ 0¯100 m):
salinity ~ 35 to 37; temperature ~ 25¯29 °C;
18O ~ ¯1 to +2 per mil; D ~ ¯3.6 to +10.2
per mil. Open ocean intermediate water (300 m): salinity = 34.6
to 35.0; temperature = 4.8 to 9.6 °C; 18O = ¯0.2
to +0.3 per mil; D = ¯8.72 to ¯0.95
per mil.
·
Amazon water follows a seasonal isotopic cycle in response to
basin hydrologic processes. River water mixes with equatorial surface ocean
water and intermediate ocean waters originating in
mid-to-high southern latitudes. Near-surface waters in the region of North
Brazil Current retroflection were identical in isotope¯salinity space to waters on the outer
shelf during November 1991.
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