| The tropical Pacific is a region where nutrient delivery, upper ocean dynamics, and global climate variability are tightly coupled. For example, the depth of the eastern equatorial Pacific (EEP) thermocline, for the most part, determines the delivery of the essential nutrient nitrate, but it is also a key aspect of an ocean-atmosphere feedback system responsible for global climate variability---the El Nino Southern Oscillation (ENSO). To provide a unique view of the upper ocean processes directly responsible for nutrient and climate dynamics in this region, I apply a single tracer---the ratio of 15N to 14N (hereafter "delta 15N")---in modern seawater and sediment throughout the tropical Pacific. I find that the delta15N of nitrate is homogenous throughout the tropical Pacific and is elevated by 2‰ relative to the Southern and Atlantic Oceans by mixing with the denitrified waters of the eastern tropical Pacific. The results of this modern ocean survey are used to devise a new application of delta15N in eastern and western equatorial Pacific (WEP) sediments that provides a record of EEP nitrate consumption over the past 1,200,000years. This record strongly suggests that the depth of the EEP thermocline---and therefore the upper ocean conditions driving the tropical climate system---has little to no response to high-latitude processes such as icesheet dynamics. Instead, as has been suggested by coupled ocean-atmosphere models, the east/west thermocline tilt responded primarily to changes in local seasonal insolation over thousands of years (a product of planetary axial precession). Some of the long-term changes in these deep-sea sediment delta15N records would seem to suggest an alteration of the original surface ocean signal, but additional delta15N measurements of sedimentary size fractions and components support the fidelity of bulk sedimentary delta15N as an archive of surface ocean nutrient cycling. |
