Stable oxygen and hydrogen isotopic tracers in Amazon shelf waters during Amasseds

Oceanologica Acta, Volume 25, Issue 2, March-April 2002, Pages 71-78
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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