GLACIAL-INTERGLACIAL CHANGES IN NUTRIENT UTILIZATION IN THE EQUATORIAL PACIFIC-OCEAN

THE eastern equatorial Pacific Ocean (EEP) today sustains up to 30% of global marine productivity(1), and the region is one of the largest a nd most variable marine sources of CO2 to the atmosphere(2). This vari ability is largely controlled by the balance between the physical inpu t of nutrients to the surface ocean and their removal by biological as similation--the relative nutrient utilization-but the spatial and temp oral variability of this balance are poorly understood. Here we use th e N-15/N-14 ratio in sedimentary marine organic matter to show strong spatial gradients in relative nitrate utilization throughout the moder n EEP. We interpret down-core decline in this ratio through the Last G lacial Maximum (12-24 kyr ago) as a decrease in relative nitrate utili zation; the increase in nitrate supply to surface waters due to upwell ing during this period was greater than the apparent increase in nitro gen removal by organic matter export out of surface waters. This inter pretation is consistent with cooler sea surface temperatures(3) and a higher CO2 flux to the atmospbere(4,5) during the Last Glacial Maximum , indicating that the EEP surface waters have remained enriched in nut rients, and have not acted as a net sink for CO2, for at least the pas t 30,000 years.