Environmental Engineering Reference
In-Depth Information
during the LGM. This could have been driven by greater nitrogen fixation [10],
diminished denitrification [12, 13, 40, 60], greater NO
3
utilization in the present
high nutrients - low chlorophyll regions [37, 87]) or a combination of some or
all of the above possibilities. Although increased productivity driven by greater
relative nitrogen fixation (which causes an increase in the combined nitrogen
inventory) would enhance oceanic capacity to sequester atmospheric CO
2
,it
would also lead to greater oxygen demand at depth and, presumably, more
denitrification (which causes a decrease in the combined nitrogen inventory).
Clearly, the interactions between nitrogen fixation, primary production, deep-
water oxygen depletion and denitrification are far from resolved.
The high productivity and suboxic conditions in the Arabian Sea result in
preservation of excellent sedimentary records of past conditions in the oceans,
especially in margin areas where the ODZ impinges on the sediments. The lack
of oxygen in waters overlying the ODZ sediments also precludes large benthic
organisms and bioturbation such that sediments from these zones are frequently
laminated [85]. Furthermore, denitrification in suboxic waters produces
15
N-
enriched nitrate, which, upon being upwelled into the photic zone, becomes
incorporated into the plankton. Remains of these plankton imprint the denitri-
fication signal in the organic rain to the sediments where the isotopic signal of
denitrification becomes part of the sedimentary record [3, 82]. Consequently,
down-core records of δ
15
N have been treated as records of denitrification in the
past [2, 3, 39, 40].
In the Arabian Sea, contemporary sediments have an isotopic δ
15
Nof
8‰,
and this highly positive signal is prevalent throughout the Holocene; in con-
trast, lighter isotopic values (
∼
5-6‰) occur in sediments that accumulated
during the LGM and indeed during all glacial stages [1, 2, 3, 39, 40, 90].
These values are typical of non reducing environments in today's ocean. This
implies that water-column denitrification was either absent or it was much
weaker during glacial times. Spectral analysis of the sedimentary δ
∼
15
N records
showed a high degree of cyclicity associated with the three Milankowitch bands
of 100,000, 41,000 and 23,000 year frequencies [2]. The association with
the 23,000 year precessional band which dominantly regulates the monsoon
strength is particularly strong. Higher resolution records indicate variations on
an even shorter (millennial) time scale, closely linked with the climatic changes
recorded by the Greenland ice cores [1, 90]. For example, the δ
15
N records in
sediment cores from the Oman Margin (Fig. 10; [1]) show oscillations that are
remarkably similar in structure and timing to the Dansgaard/Oeschger (D/O)
events recorded in the Greenland (GISP2) ice cores (D/O events are warm
periods punctuating the last glacial with a frequency of about 1600 y; [24]).
Moreover, when smoothened with a 3000 yr running average - to account for
the dampening effect of the residence time of nitrogen in the sea - the Arabian
Sea denitrification records are incredibly similar to the Antarctic temperature
