Environmental Engineering Reference
In-Depth Information
measured in the 30L bottles to estimate a nominal sampling depth for the
particulate samples obtained from those bottles.
We deployed drifting Kiel-type sediment traps [63] at 100 m depth during
all three time-series studies. At 18
◦
N and 10
◦
N, a second drifting trap was
deployed at 500 m depth. The shallow (100 m) trap was recovered daily for
replacement of its single sample bottle. The deeper trap (500 m) was equipped
with a carousel of sample bottles programmed for daily sampling and was
deployed for the entire time-series study at each site. All sample bottles were
filled with a brine solution to preserve organic matter and to prevent washout
of the material collected. Immediately after trap recovery, the samples were
split with a rotating splitter [7] and filtered for later analysis of the particulate
matter.
Concentrations of nutrients and oxygen were measured using standard meth-
ods [24]. Samples of particulate organic carbon and nitrogen (POC, PON) were
collected by gentle vacuum filtration onto precombusted (12 hrs 450
◦
C) What-
man GF/F filters and kept frozen at -20
◦
C during the cruise and transport to the
laboratory.
All stable isotope and particulate organic matter concentration measurements
were carried out by continuous-flow isotope ratio mass spectrometry using an
elemental analyzer (CE/Fisons NA 1108) interfaced to a mass spectrometer
(Finnigan MAT Delta S). Prior to analysis, sample filters were decalcified by
fuming with HCl for 24 hours, then dried and formed into pellets in Sn cups.
All isotope data are reported as δ
13
C values using atmospheric N
2
and PDB, respectively, as isotopic reference materials. A sample of peptone
(Merck) was run after every fifth sample to provide a working standard for the
isotopic and elemental analyses. The standard deviation for replicate samples
is less than 0.2 ‰.
15
Norδ
2.2 General Distribution of Water Column Properties
The upper water column was well-stratified throughout our transect, with
the surface mixed layer extending through the upper 50 to 100m of the water
column (Fig. 3). All stations sampled showed a clear subsurface minimum in
O
2
concentration beginning at about 150 - 200 m depth and extending to more
than 1000 m depth. Oxygen concentrations in the upper 200 m of the water
column ranged from 0 to
>
267 µmol l
−
1
along our transect. The core of the
Arabian Sea OMZ occurred between 18˚N and 14˚N, but hypoxic conditions
extended well south of this region, forming a tongue of oxygen depleted water
extending southward and centered around 800m depth (data not shown).
Nitrate concentrations were at or near the limit of detection in the surface
mixed layer (Fig. 3). A sharp gradient in concentration extended through the
pycnocline with maximal concentrations occurring below 150 m (Fig. 3). In
