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

Abstract

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. δ¹⁸O-H₂O and δD 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, δ¹⁸O, δD 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; δ¹⁸O = –4.2 to –6.8 per mil; δD ≈ –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; δ¹⁸O ≈ –1 to +2 per mil; δD ≈ –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; δ¹⁸O = –0.2 to +0.3 per mil; δD = –8.72 to –0.95 per mil. Amazon river 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.