Stable isotope geochemistry of nitrogen in marine particulates
Citable URI
https://hdl.handle.net/1912/2603Location
PeruGulf of Maine
DOI
10.1575/1912/2603Keyword
Nitrogen; Isotopes; Marine sediments; Chemical oceanography; Isotope geology; Geochemistry; Atlantis II (Ship : 1963-) Cruise AII108-3; Atlantis II (Ship : 1963-) Cruise AII108-4Abstract
Isotope studies of nitrogen and carbon were undertaken to
investigate the fate of particulate organic matter (POM) during its
residence in the water column and after deposition on the seafloor. The
processes focused on were water-column transformations and sedimentary
diagenesis. Sampling sites were chosen to provide POM subject to
different specific mineralization processes (nitrification,
denitrification, and sulfate reduction), different lengths of water
column (duration of the mineralization process), and differences in the
size of the organic-matter flux. The δl5N and δ13C of plankton, POM,
and sediments from several oceanic sites were related to biological and
hydrographic processes identified from nutrient, temperature, and
salinity profiles. This was done to determine what effect these
processes have on the δ15N of POM. Four stations were studied in the
upwelling area off the coast of Peru and one station was studied in the
Gulf of Maine.
Important factors controlling the δ15N of plankton appear to be
the concentration and δl5N of nitrate in the surface waters, and the
relative zooplankton and phytoplankton abundances. Plankton from the
Peru Upwelling Area are enriched in 15N as compared to plankton from
other parts of the world's oceans where denitrification is absent. This
enrichment may be due to the assimilation of 15N-enriched nitrate,
produced by the selective reduction of 14N during denitrification.
Zooplankton are 3 to 4% enriched in 15N as compared with
phytoplankton. Production of 14N -enriched fecal pellets is suggested
as a mechanism for this trophic enrichment.
In the surface waters, the δl5N of POM is similar to that of
plankton. In the Peru Upwelling Area, the δ15N of POM from the
oxygen-deficient waters decreases with increasing depth. In the Gulf of
Maine, below the euphotic zone in the oxic deep waters, the δ15N of POM
increases with increasing depth. The difference in isotopic alteration
may be due to the effect of different redox conditions on the mechanism
and sequence by which specific organic nitrogen compounds, variably
enriched in 15M, undergo degradation. Furthermore, bacterial growth on
nitrogen-poor particles in the deep waters of the Peru Upwelling Area may
contribute to the low δ15N of POM.
In contrast to the large range in δ15N (-2 to +17%) of the
POM, the range of δ15N in the sediments is small (+5 to +9%).
Within a core, the average variation in δ15N was only 1.8%.
Temporal variability in the δ15N of sedimenting POM and benthic
activity appear to be important in determining the δ15N of the
sediments. The large changes in POM concentration and isotope content at
the sediment/water interface as compared with the more constant values
found down-core, suggest that processes occuring at the sediment/water
interface are critical, although bioturbation may also be important in
determining the δ15N of oxic sediments. If diagenesis causes a
significant loss of organic matter, profiles of organic carbon and
nitrogen contents should show decreases with increasing depth and C/N
ratios should increase with increasing depth (Reimers, 1981). Since none
of the sedimentary profiles exhibited such trends, it is concluded that
diagenesis was insufficient to erase the percent carbon, nitrogen and C/N
ratio signatures generated by the POM flux and alterations at the
sediment/water interface. Temporal variability in the δ15N of
bottom-water POM may be caused by changes in deep-water currents which
transport POM horizontally and to changes in bacterial and possibly other
biological activity in the water column.
This thesis work suggests that δ15N may be a useful tool in
studying the geochemistry of POM in the marine environment. In addition,
this research has shown that interpretation of the sedimentary 15N
record must include consideration of isotopic alteration associated with
bacterial remineralization of POM and benthic activity.
Description
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution January 1983
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Suggested Citation
Thesis: Libes, Susan M., "Stable isotope geochemistry of nitrogen in marine particulates", 1983-01, DOI:10.1575/1912/2603, https://hdl.handle.net/1912/2603Related items
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