Date of Award

2013

Degree Type

Thesis

Degree Name

Master of Science in Oceanography

Department

Oceanography

First Advisor

Rebecca Robinson

Abstract

Dissolved nitrogen in the water column is an essential nutrient and its distribution is linked to the cycling of carbon, phosphorus, and oxygen. The mean isotopic composition of nitrate (δ15Nglobal) is nearly uniform in the homogeneous deep ocean and appears to reflect the dominant nitrate removal processes. As a result, it is directly related to the absolute inventory of available nitrogen (Sigman et al., 1999; Brandes and Devol 2002). Constraining whether the isotopic composition of deep water nitrate has varied in time will allow examination of variations in the relative magnitude of removal processes and how climate may control them. A limitation to understanding these changes is the lack of direct or proxy measurements of δ15N of deep ocean nitrate in the past. This thesis represents an attempted reconstruction of the mean nitrogen isotopic composition of the deep ocean using nitrogen (N) and oxygen (O) isotope values measured on dissolved nitrate in sedimentary pore fluid from two sites in the North Pacific Gyre. The measured pore fluid profiles were corrected for in situ nitrate production. A >1.5 permil decrease in δ15N values and a >2 permil shift in δ18O values were observed. These are lower limit estimates because of the dampening effects of diffusion. The reconstructed profiles are compared with predicted diffusive profiles of a measured sedimentary d15N record and a modeled estimate of δ15Nglobal, output from a 1-D time dependent diffusion analysis. The comparison suggests that local processes are most likely responsible for such a large shift in δ15N. The pore fluid profiles record changes in deep ocean nitrate in the deep North Pacific Ocean since the LGM. A possible driving mechanism for this change is the occurrence of water column denitrification in the deep waters of the North Pacific Additional explanations include an enhanced transfer of surface/IW N isotopic signal via organic matter export since the LGM, or an elevated δ15N of preformed nitrate transferred from the surface ocean to the Pacific interior, although neither mechanism satisfactorily describes the concurrent δ18O shift. Understanding the changes observed in these pore fluid records could lead to a deepened understanding of the relationship between large scale climate change (glacial/interglacial transitions) and the nitrogen cycle, as well as the role they play in carbon storage in the deep ocean and sequestration at the surface. Improvements would come from a more tightly constrained, higher resolution data record, particularly in terms of the oxygen isotopes of relict nitrate. As well as from increased knowledge of isotopic changes in organic poor, deep, slow sedimentation rate sediment, and comparison with samples from similar sites.

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