Decadal-scale chemical variability in the subtropical North Atlantic deduced from nutrient and oxygen data

In August 1992 the R/V Hesperides reoccupied a transatlantic section nominally along 24.5 degrees N previously sampled in April 1957 and in August 1981. Observation of significant warming over the 35-year period has already been reported for the ocean layer between 800 and 2500 m depth (similar to 0.01 degrees C yr(-1)). We examine decadal-scale variability in the nutrient and O-2 measurements primarily comparing the 1981 and 1992 sections. The basin-scale water mass structure remained relatively unchanged from 1981 to 1992. Zonally averaged differences of the chemical data show that the layer of maximum increase in temperature and salinity at 1100 m is roughly coincident with an apparent decrease in O-2 concentration of about 7 mu mol kg(-1). The O-2 decrease is equivalent to an average rate of decrease of 0.6 mu mol kg(-1) yr(-1). The vertically integrated water column O-2 utilization rate is 0.9 mol m(-2) yr(-1). This is equivalent to 23-33% of the new production rate estimates in the Sargasso Sea. The O-2 decrease at 1100 m cannot be explained by using oxidative ratios relating regeneration of nutrients and O-2 consumption. The O-2 decrease is not highly correlated with zonally averaged changes in the nutrient measurements. The nutrients show small or insignificant changes near 1100 m over the 11-year period. Part of the O-2 decrease might be explained by measurement error (30%) and changes in solubility (20%) due to warming. The chemical changes are likely caused by changes in the characteristics of the source waters entering 24.5 degrees N near 700-1700 m depth.