Tropical Pacific nutrient dynamics in the modern and pleistocene ocean : insights from the nitrogen isotope system

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 \[delta\]¹⁵N of nitrate is homogeneous 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 \[delta\]¹⁵N in eastern and western equatorial Pacific (WEP) sediments that provides a record of EEP nitrate consumption over the past 1,200,000 years. 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 \[delta\]¹⁵N records would seem to suggest an alteration of the original surface ocean signal, but additional \[delta\]¹⁵N measurements of sedimentary size fractions and components support the fidelity of bulk sedimentary \[delta\]¹⁵N as an archive of surface ocean nutrient cycling