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 and most variable marine sources of CO2 to the atmosphere(2). This variability is largely controlled by the balance between the physical input of nutrients to the surface ocean and their removal by biological assimilation--the relative nutrient utilization-but the spatial and temporal variability of this balance are poorly understood. Here we use the N-15/N-14 ratio in sedimentary marine organic matter to show strong spatial gradients in relative nitrate utilization throughout the modern EEP. We interpret down-core decline in this ratio through the Last Glacial Maximum (12-24 kyr ago) as a decrease in relative nitrate utilization; the increase in nitrate supply to surface waters due to upwelling during this period was greater than the apparent increase in nitrogen removal by organic matter export out of surface waters. This interpretation 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 nutrients, and have not acted as a net sink for CO2, for at least the past 30,000 years.