Nitrogen isotopes in chlorophyll and the origin of Eastern Mediterranean sapropels
Nitrogen isotopes in chlorophyll and the origin of Eastern Mediterranean sapropels
Date
1997-02
Authors
Sachs, Julian P.
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Date Created
Location
Eastern Mediterranean
DOI
10.1575/1912/5707
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Keywords
Nitrogen
Isotopes
Nitrates
Chlorophyll
Stratigraphic geology
Geochemistry
Sapropel
Marine sediments
Le Suroit (Ship) Minos Cruise
Thomas G. Thompson (Ship) Cruise TN041
Knorr (Ship : 1970-) Cruise KN134-08
Moana Wave (Ship) Cruise MW87-08
Isotopes
Nitrates
Chlorophyll
Stratigraphic geology
Geochemistry
Sapropel
Marine sediments
Le Suroit (Ship) Minos Cruise
Thomas G. Thompson (Ship) Cruise TN041
Knorr (Ship : 1970-) Cruise KN134-08
Moana Wave (Ship) Cruise MW87-08
Abstract
The goals of this thesis were: (1) to establish methods for the determination of
nitrogen and carbon isotope ratios in marine particulate and sedimentary
chlorophyll derivatives; (2) to establish chlorophyll δ15N and δl3C as proxies for
the nitrogen and carbon isotopic composition of marine phytoplankton; and (3)
to use chlorophyll nitrogen isotopic ratios to understand the origin of Late
Quaternary Eastern Mediterranean sapropels.
Techniques are presented for the determination of chlorin nitrogen and carbon
isotopic ratios in marine particles and sediments with a precision greater than
0.15 per mil for both isotopes. The procedure can be performed in about 4 hours
for particulate and 8 hours for sediment samples, and relies on multiple
chromatographic purifications. About 20 g of a moderately organic-rich
sediment are required.
A technique is also presented for the determination of chlorin nitrogen and
carbon isotopic ratios by isotope-ratio monitoring gas chromatography-mass
spectrometry (irmGC-MS) by synthesizing bis-(tert.-butyldimethylsiloxy)Si(IV)
chlorin derivatives. However, yields for the 4-step synthesis were only about 5-6% and there was a net isotopic depletion of 1.2 (± 0.3) per mil in the derivative,
relative to the starting material.
These techniques are then used to show that the nitrogen isotopic difference
between chlorophyll and whole cells in six species of marine phytoplankton is
5.16 ± 2.40 per mil. For carbon, the isotopic difference between chlorophyll and
whole cells in five species of marine phytoplankton is -0.02 ± 2.12 per mil. A
model of the distribution of 15N in phytoplankton is constructed and it is
demonstrated that the interspecies variability observed for the nitrogen isotopic
difference between chlorophyll and whole cells can be attributed to differences in the partitioning of cellular nitrogen between non-protein biochemicals. In the
field, where mixed assemblages of phytoplankton prevail, the isotopic difference
beween chlorophyll and whole cells is expected to tend toward the average value
of 5.16 per mil.
Finally, the average nitrogen isotopic composition of chlorins from six Late
Quaternary Eastern Mediterranean sapropels (-5.01 + 0.38 per mil) was found to
be very similar to the δ15N of chlorophyll from the modem deep chlorophyll
maximum (-6.38 ± 1.80 per mil) in the Eastern Mediterranean. In addition,
sapropel photoautotrophic material, calculated from the chlorin δ15N, had the
same isotopic composition (0.15 per mil) as both bulk sapropel sediments (-0.08 ±
0.53 per mil) and deep water nitrate (-0.05 per mil). These data suggest (a) that
bottom waters were anoxic, (b) that organic matter burial efficiency was
enhanced, and (c) that oligotrophic conditions similar to today persisted, in the
Eastern Mediterranean during sapropel deposition. These results contradict
earlier interpretations of Late Quaternary bulk sedimentary δ15N in the Eastern
Mediterranean. The latter concluded that the pattern of high δ15N values in
intercalated marl oozes and low values in sapropels was the result of decreased
nutrient utilization, and hence, increased primary production, during sapropel
events. The low δ15N of deep water nitrate in the Eastern Mediterranean
suggests a significant source of new nitrogen from biological N2-fixation.
It is suggested that attempts to reconstruct the nitrogen isotopic composition of
marine organic matter in the past by measuring the δ15N of whole sediments
may be subject to misinterpretation due to the alteration of isotopic ratios during
diagenesis. The partial oxidation of marine organic matter can result in
significant isotopic enrichment of the preserved residual. The magnitude of this
enrichment appears to be large when bottom waters are well-oxygenated, and
small when bottom waters are anoxic. Environments where large temporal
reqox changes have occurred are expected to be the most problematic for the
interpretation of bulk sedimentary δ15N. In these environments, the diagenetic
signal can be at least as large as the primary isotopic signal being sought. The
Eastern Mediterranean Sea during the Late Quaternary appears to be one such
environment.
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 February 1997
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Citation
Sachs, J. P. (1997). Nitrogen isotopes in chlorophyll and the origin of Eastern Mediterranean sapropels [Doctoral thesis, Massachusetts Institute of Technology and Woods Hole Oceanographic Institution]. Woods Hole Open Access Server. https://doi.org/10.1575/1912/5707