Introduction
Biogeochemical processes of nitrogen in the ocean are intimately related to
various elemental cycles (synergistically modulate atmospheric CO2 and
N2O concentrations), hence the feedback on the climate being on a millennial time
scale
(Gruber, 2004; Falkowski and Godfrey, 2008; Altabet et al., 2002). Though
oxygen deficient zones (ODZs) occupy only ∼ 4 % of ocean
volume, the denitrification process therein contributes remarkably to the
losses of nitrate, leaving excess P in the remaining water mass to stimulate
N2 fixation while entering the euphotic zone (Morrison et al., 1998;
Deutsch et al., 2007) and thus controlling the budget of bioavailable
nitrogen in ocean. Denitrification leaves 15NO3- in residual
nitrate (Sigman et al., 2001), whereas N2 fixation introduces new
bioavailable nitrogen with low δ15N values (Capone et al.,
1997) into ocean for compensation. The Arabian Sea (AS) – one of the three
largest ODZs in the world ocean with distinctive monsoon driven upwelling –
accounts for at least one-third of the loss of marine fixed nitrogen
(Codispoti and Christensen, 1985) playing an important role in the past
climate via regulating atmospheric N2O concentration (Agnihotri et al.,
2006) or nitrogen inventory to modulate CO2 sequestration through a
biological pump (Altabet, 2006).
Sedimentary nitrogen isotope, measured as standard δ notation, with
respect to standards of atmospheric nitrogen, is an important tool to study
the past marine nitrogen cycle. Nitrogen isotope compositions of sedimentary
organic matter potentially reflect biological processes in water columns,
such as denitrification (Altabet et al., 1995; Ganeshram et al., 1995,
2000), nitrogen fixation (Haug et al., 1998), and the degree of nitrate
utilization by algae (Altabet and Francois, 1994; Holmes et al., 1996;
Robinson et al., 2004). However, alteration may occur (through various ways
or processes; e.g., diagenesis) before the signal of δ15N of
exported production is buried.
Previous measurements of δ15Nbulk in various cores and
surface sediments in the AS showed the following points: (1) near-surface
NO3- in the AS is completely utilized in an annual cycle, resulting in
small isotopic fractionation between δ15N of exported sinking
particles and δ15N of NO3- supplied to the euphotic
zone (Altabet, 1988; Thunell et al., 2004); (2) monsoon-driven surface
productivity and associated oxidant demand were regarded as the main control
on water column denitrification in the past (Ganeshram et al., 2000;
Ivanochko et al., 2005); (3) sedimentary δ15Nbulk primarily
reflects the relative intensity of water column denitrification in this area
(Altabet et al., 1995, 1999); (4) oxygen supply at intermediate depth by
the Antarctic intermediate waters (AAIWs) can modulate the denitrification
intensity in the northern AS (Schulte et al., 1999; Schmittner et al., 2007;
Pichevin et al., 2007). Among previous research, the geographical features
in sedimentary δ15Nbulk between the north and south basins
of the AS have not been discussed, particularly on the basis of bottom-depth
effect, which might be different during glacial and interglacial periods.
In this study, a sediment core (SK177/11) collected from the slope of
the southeastern AS was measured for organic C and N contents and their stable
isotopes. We synthesized previous hydrographical and isotopic data, such as
dissolved oxygen (DO), N* (N* = NO3-16 × PO43-+2.9; Gruber and Sarmiento, 2002), and δ15N of
nitrate, as well as trapped material and surface/downcore sediments, among
which surface and downcore sediments may have experienced more intensified
diagenetic alteration. Based on the subsurface of a DO concentration of 25 µmol kg-1 isopleth at 150 m, the data sets in the AS were separated into
north and south basins by time span (glacial, Holocene and modern) for
comparison. We aim to (1) investigate the geographic and
glacial–interglacial differences in bottom-depth effect and to (2) retrieve
extra information from sedimentary δ15Nbulk by removing
basin/climate stage specific bottom-depth effects, thus better deciphering the
environmental history of the Arabian Sea.
Accelerator mass spectrometry (AMS) 14C dates of sediment core SK177/11. Radiocarbon ages
were calibrated using the CALIB 6.0 program
(http://calib.qub.ac.uk/calib/calib.html, Reimer et al., 2009).
Lab code
Depth
Dating
pMC
Raw 14C age
Calibrated age
δ13C (‰)
cm
materials
(yrBP)
(yrBP) (1σ)
KIA24386
58
OM
65.58 ± 0.17
3390 ± 20
3186 ± 24
-18.55 ±0.04
KIA26327
125
OM
46.65 ± 0.20
6125 ± 35
6504 ± 26
-20.02 ±0.10
KIA24387
155
OM
31.38 ± 0.13
9310 ± 30
10054 ± 104
-19.50 ±0.08
KIA26328
175
OM
21.96 ± 0.12
12180 ± 45
13618 ± 104
-17.71 ±0.18
KIA24388
205
OM
13.94 ± 0.11
15830 ± 60
18646 ± 54
-21.65 ±0.15
KIA24389
275
OM
9.81 ± 0.12
18650 + 100(-90)
21774 ± 194
-18.02 ±0.10
KIA26329
355
OM
2.76 ± 0.06
28830 ± 180
32857 ± 207
-19.23 ±0.17
OM – organic matter; pMC – percent modern.
(a) Map of the Arabian Sea. Dissolved oxygen (DO) concentration at
150 m (World Ocean Atlas 09) was shown in color contour. Southern (⋆)
and northern (•) categories of available cores and SK177/11, in this
study, were defined by DO of 25 µmol kg-1 (see text; purple dash curve).
(b) Bathymetric map superimposed by core locations; (c), (d) and (e) are DO,
nitrate and N* transects (yellow dashed line in (a), online data originated
from cruises of JGOFS in 1995), respectively, for upper 2000 m. (f) N*
transect for the upper 300 m with arrows revealing the flow direction. In
(a), the northern cores include core MD-04-2876 (828 m; Pichevin et al.,
2007), core NIOP455 vs. NIOP464 (1002 m vs. 1470 m; Reichart et al., 1998),
SO90-111KL vs. ME33-NAST(775 m vs. 3170 m; Suthhof et al., 2001), ODP724C
vs. ME33-EAST (603 m vs. 3820 m; Möbius et al., 2011), RC27-24 vs.
RC27-61 (1416 m vs. 1893 m; Altabet et al., 1995), ODP723, ODP722(B) vs.
V34-101 (808 m, 2028 m vs. 3038 m; Altabet et al., 1999), RC27-14 vs. RC27-23
(596 m vs. 820 m; Altabet et al., 2002), GC08 (2500 m; Banakar et al.,
2005), MD-76-131 (1230 m; Ganeshram et al., 2000); the southern cores
include core SO42-74KL (3212 m; Suthhof et al., 2001), NIOP905 (1586 m;
Ivanochko et al., 2005) and SK177/11 (776 m; this study).
Continued.
Continued.
Study area
The Arabian Sea is characterized by seasonal reversal of monsoon winds,
resulting in large seasonal physical/hydrographic/biological/chemical
variations in water columns (Nair et al., 1989). Cold and dry northeasterly
winds blow during winter from a high-pressure cell of the Tibetan Plateau,
whereas heating of the Tibetan Plateau in summer (June to September)
reverses the pressure gradient leading to warm and moist southwesterly winds
and precipitationmaximum. In the present day, the SW monsoon is much stronger
than its northeastern counterpart.
The spatial distribution of DO at a depth of 150 m for the AS is shown in Fig. 1a (World Ocean Atlas 2009,
http://www.nodc.noaa.gov/OC5/WOA09/woa09data.html), which shows a clear
southwardly increasing pattern with DO having increased from ∼ 5 to
> 100 µmol kg-1, and the lowest DO value appears
northeast of the northern basin. As denitrification, the dominant nitrate
removal process generally occurs in the water column, where DO concentration
ranges from 0.7 to 20 µmol kg-1 (Paulmier et al., 2009). The intensity of denitrification was reported to descend gradually,
corresponding to the DO spatial pattern from the northern to the southern parts of
the AS, and did not become obvious at 11 or 12∘ N (Naqvi et al., 1982). As
indicated by the upper 2000 m N–S transect of DO (Fig. 1c), a southwardly
decrease in ODZ thickness can be observed and the contour line of 5 µmol kg-1 extends to around 13∘ N. Since the nitrate source
is mainly from the bottom of the euphotic zone at around 150 m, we postulate a
geographically distinctive sedimentary δ15Nbulk underneath
ODZs. Thus, an isopleth of 25 µmol kg-1 DO at 150 m is applied as a
geographic boundary to separate the northern from the southern part of the AS
basin. The interface where DO concentration changed from 20 to 30 µmol kg-1 was such a transition zone. On the other hand, the bottom layer of
the ODZ moves shallower toward the south, as shown previously by Gouretski and
Koltermann (2004). Accordingly, the bottom oxygen content may also be a
factor to influence the degree of alteration in sedimentary δ15Nbulk.
As mentioned in the introduction, nitrate is removed via denitrification in ODZs
resulting in excess P to stimulate N2 fixation. In Figs. 1d, e and f, we
presented the N–S transect of nitrate and N* (for both the upper 2000 m
and 300 m) in January. Even though there is nitrate in the surfacewater (Fig. 1d), as mentioned earlier, near-surface NO3- in the AS
is completely utilized in an annual cycle (Altabet, 1988; Thunell et al.,
2004). Furthermore, negative N* (P excess) throughout the water column
represents a nitrate deficit, and the lowest N* value appears at
∼ 300 m at 18–20∘ N, where DO is
< 1 µmol kg-1. Meanwhile, a gradual southwardly increase
in N* can be observed for upper 100 m (Fig. 1f) and the isopleth of N*
of -4 deepens southward with the highest N* (-2) appearing at
∼ 10–12∘ N. The volume expansion of high N* water, as well
as a simultaneous increase in N*, strongly indicate an addition of
bioavailable nitrogen when surface water is traveling southward.
(a) Plot of calendar age against depth; (b) Linear sedimentation
rate (indicates the 14C age controlling points).
Temporal variations of (a) stable isotopic compositions of bulk
nitrogen (δ15N); (b) stable isotopic compositions of total
organic carbon (TOC) (δ13C); (c) contents of total nitrogen; (d) total organic carbon; (e) TOC / TN ratio. Horizontal dashed lines are references for low
value periods.
Material and method
A sediment gravity core, SK177/11 (8.2∘ N and 76.47∘ E),
was collected at water depths of 776 m on the continental slope off the
southwest coast of India (Kerala) during the 177th cruise of ORV SagarKanya in October 2002. Although the core MD77-191 locates further south
in the AS (Bassinot et al., 2012), SK177/11 is, so far, the southernmost core
with reference to δ15N record. The 3.65 m long core was sub-sampled at interval of 2 cm for upper 1 m and of 5 cm for the rest (open circles in Fig. 2a). There is a distinct
boundary at ∼ 1.7 m, above which the core consists mainly of brownish
gray clayey sediments. Neither distinct laminations nor turbidities can be
observed by visual contact immediately after collection or at the time during
sub-sampling (Pandarinath et al., 2007). All sub-samples were freeze-dried
and ground into powder in an agate mortar with pestle. Sand was almost absent
(< 1 wt %) throughout the core.
The calendar chronology for core SK177/11 was based on seven accelerator mass
spectrometry (AMS) radiocarbon (14C) dates of bulk organic matter
(Fig. 2a). Calendar years were calculated using calibration CALIB 6.0 with a
reservoir age correction of 402 years (Stuiver et al., 1998; Reimer et al.,
2009). Details on the 14C age controlling points were presented in
Table 1. Given that the AMS 14C dates of SK177/11 were obtained on total
organic carbon (TOC), we may not be able to avoid the mixture of organics of
different ages during transport (Mollenhauer et al., 2005) or interference by
pre-aged organics sourced from land (Kao et al., 2008). However, besides the
reservoir age correction, due to higher TOC content (range: 2.2–5.5 %)
of sediments and their marine-sourced organic carbon, as confirmed by stable
C isotope data and C / N ratio, shown in Figs. 3b and e, we are confident that
our age model is reliable and less likely affected by age heterogeneity.
Bulk sedimentary nitrogen content and δ15N analyses were carried
out using a Carlo-Erba EA 2100 elemental analyzer connected to a Thermo
Finnigan Delta V Advantage isotope ratio mass spectrometer (EA-IRMS).
Sediments for TOC analyses were acid-treated with 1N
HCl for 16 h, and then centrifuged to remove carbonate. The acid-treated
sediments were further dried at 60 ∘C for TOC content and
δ13C. The nitrogen isotopic compositions of acidified samples were
obtained at the same time for comparison. Carbon and nitrogen isotopic data
were presented by standard δ notation with respect to PDB (Pee Dee Belemnite) carbon and
atmospheric nitrogen. USGS 40, which has certified δ13C of
-26.24 and δ15N of -4.52 ‰ and acetanilide
(Merck) with δ13C of -29.76 and δ15N of
-1.52 ‰ were used as working standards. The reproducibility of
carbon and nitrogen isotopic measurements is better than 0.15 ‰ .
The precision of nitrogen and carbon content measurements were better than
0.02 and 0.05 %, respectively. Meanwhile, the acidified and non-acidified
samples exhibited identical patterns in δ15N (not shown) with mean
deviation of 0.3 ‰.
Discussion
Downward transfer and transformation of N isotopic signal
As mentioned, the signal of sedimentary δ15N may be altered under
different burial conditions. Altabet and Francois (1994) reported little
diagenetic alteration of the near-surface δ15N in the equatorial
Pacific, while there was an apparent +5 ‰ enrichment relative to sinking
particles in the Southern Ocean, south of the polar front. In the Sargasso
Sea, sedimentary δ15N also enriched by 3–6 ‰
relative to sinking particles (Altabet et al., 2002; Gruber and Galloway,
2008). The degree of alteration was attributed to particle sinking rate and
organic matter (OM) preservation (Altabet, 1988). Gaye-Haake et al. (2005) also suggested that
low sedimentation rates benefit organic matter decomposition, resulting in a
positive shift in bulk sedimentary δ15N comparing to sinking
particles in the South China Sea. Finally, Robinson et al. (2012) concluded that
oxygen exposure time at the seafloor is the dominant factor controlling the
extent of N isotopic alteration. Thus, it is necessary to follow the track of
δ15N signal to clarify the occurrence of deviation during transfer.
The reported depth profiles of δ15NNO3 in the AS were
shown in Fig. 5, in which δ15NNO3 values of water
depth deeper than 1200 m range narrowly around 6–7 ‰, which
is slightly higher than the global average of the deep oceans
((4.8 ± 0.2) ‰ for > 2500 m, Sigman et al., 2000;
(5.7 ± 0.7) ‰ for > 1500 m, Liu and Kaplan, 1989). Below
the euphotic layer, δ15NNO3 increases, rapidly peaking
at around 200–400 m. The preferential removal of 14NO3 by
water column denitrification accounts for these subsurface δ15NNO3 highs (Brandes et al., 1998; Altabet et al., 1999;
Naqvi et al., 2006). The subsurface δ15NNO3 maximum
ranges from 10 to 18 ‰ for different stations, implying a great
spatial heterogeneity in water columns denitrification intensity. It is worth
mentioning that higher values, in general, appear in the northeastern AS
(15 ∼ 18 ‰) (Fig. 5), highlighting that the focal area of water
column denitrification is prone to the northeastern Arabian Sea (Naqvi et al.,
1994; Pichevin et al., 2007), also revealed by the DO spatial distribution
(Fig. 1a). Contrary to higher denitrification in the northeastern AS, the
export production is always higher in the northwestern AS throughout a year
(Rixen et al., 1996). Such decoupling between productivity and
denitrification was attributed to the oxygen supply by intermediate water
exchange besides primary productivity oxygen demand (Pichevin et al., 2007).
Note that the δ15NNO3 values at a water depth of
100–150 m, which correspond to the bottom depth of the euphotic zone
(Olson et al., 1993), from different stations fall within a narrow range of
7–9 ‰ despite wide denitrification intensity underneath.
The rapid addition of new nitrogen, as mentioned earlier, might account for the
relatively uniform δ15NNO3 at the bottom of the euphotic
layer. Unfortunately, there are no δ15NNO3 profiles
or sediment trap data from the southern basin for comparison.
Depth profiles of nitrogen isotope of nitrate (δ15NNO3) in water columns; data not marked are all from August; the location of Station January 1995 overlaps with Station SS3201 (data digitized from
Brandes et al., 1998; Altabet et al., 1999; Naqvi et al., 2006).
Interestingly, reported δ15N of sinking particles (δ15NSP) collected by five sedimentation traps deployed from
500 m throughout a depth of 3200 m ranged narrowly from
5.1 to 8.5 ‰ (Fig. 6), which is slightly lower but overlaps
largely with δ15NNO3 values at 100–150 m.
Such similarity in δ15NNO3 at 100–150 m and
sinking particles strongly indicated that (1) the NO3- source for sinking
particles was coming from a depth of around 100-150 m, instead of the ODZs (200-400 m) where the maximum δ15NNO3 value occurred (Schäfer and Ittekkot,
1993; Altabet et al., 1999), and (2) little alteration had occurred in
δ15NSP during sinking in the water column, as indicated
by Altabet (2006). Only these five trap stations with nitrogen
isotope information were available in the AS (Gaye-Haaake et al., 2005). The trap
locations were in the same area but not as far south compared to the δ15NNO3 stations (insert map in Fig. 6). The slightly lower
δ15N in sinking particles is attributable to their geographic
locations (see below), since incomplete relative utilization of surface
nitrate has been documented to have a very limited imprint on the δ15N signal in the AS (e.g., Schäfer and Ittekkot, 1993).
The uniformly low values of δ15NNO3 at the bottom of
the euphotic zone should be a consequence resulting from various processes in the
euphotic zone, such as remineralization, nitrification and N2 fixation.
Nevertheless, the distribution pattern of N* (Figs. 1e and f) illustrates
that there must be an addition of 14NO3 into the system to cancel
out the isotopic enrichment caused by denitrification. Note that the positive
offset in δ15NNO3 (Δδ15NNO3, 6∼ 12 ‰) in ODZs caused by various
degrees of denitrification was narrowed down significantly, while nitrate was
transported upward. This implies that a certain degree of addition processes,
most likely the N2 fixation, varied in concert with the intensity of
denitrification underneath. Since the upwelling zones distribute at the very
north and the west of the AS and the upwelled water travels southward (or
outward) on the surface, as shown in Fig. 1e, it is reasonable to see the
phenomenon of denitrification-induced N2 fixation to compensate the
nitrogen deficiency. Consistent to this notion, Deutsch et al. (2007)
discovered the spatial coupling between denitrification in eastern tropical
Pacific (upstream) and N2 fixation in western equatorial Pacific
(downstream). Such a horizontal nitrogen addition process can also be seen
clearly in our background information of N* (Fig. 1f). In fact, fixed N
had been proved to account for a significant part of surface nitrate in
the modern-day AS, where denitrification is exceptionally intense (Brandes et al.,
1998; Capone et al., 1998; Parab et al., 2012).
Vertical profiles for nitrogen isotope of nitrate (crosses in
inserted map), sinking particles (inverse triangles in map) and
trap-corresponding surface sediments. Data for sediment traps and surface
sediments are from Gaye-Haake et al. (2005). Depth profile of δ15NNO3 follows that in Fig. 5.
Compared with reported δ15N of surface sediments retrieved from
trap locations, a significant positive shift in δ15N can be seen at
the seafloor (Fig. 6). Such a positive deviation can be seen elsewhere in
previous reports (Altabet, 1988; Brummer et al., 2002; Kienast et al., 2005)
due to prolonged oxygen exposure after deposition (Robinsson et al., 2012)
associated with sedimentation rate (Pichevin et al., 2007). Although Cowie et
al. (2009) found an ambiguous relation between contents of sedimentary organic
carbon and oxygen in deep water, they also noticed the appearance of maximum
organic carbon contents at the lower boundary of ODZs, where oxygen content was relatively higher. Accordingly, they believed that other
factors controlling the preservation of organic carbon existed, such as the chemical
characteristics of organic matter, the interaction between organic matters
and minerals, the enrichment and activity of benthic organism or the
physical factor, including the screening and water dynamic effect.
Linear equations of bottom-depth effect during different climate
stages.
Location
Northern AS
Southern AS
Modern
δ15N = 0.55 (±0.08) × 10-3 × depth + 8.1 (±0.2)
δ15N = 0.76 (±0.14) × 10-3 × depth + 6.0 (±0.3)
(R2= 0.40, n = 78, P< 0.0001)
(R2=0.66, n = 18, P< 0.0001)
Holocene
δ15N = 0.70 (±0.20) × 10-3 × depth + 6.7 (±0.3)
δ15N = 0.93 (±0.06) × 10-3 × depth + 5.7 (±0.1)
(R2= 0.61, n = 16, P = 0.0067)
(R2=1.00, n = 3, P = 0.0152)
Glacial
δ15N = 0.64 (±0.20) × 10-3 × depth + 5.2 (±0.3)
δ15N = 1.01 (±0.31) × 10-3 × depth + 4.3 (±0.7)
(R2= 0.68, n = 16, P = 0.0013)
(R2=0.91, n = 3, P = 0.1899*)
* Insignificant by P value.
Geographically distinctive bottom-depth effects in the modern day
As classified by oxygen content of 25 µmol kg-1 at 150 m,
the documented surface sedimentary δ15Nbulk (Gaye-Haake et
al., 2005) was separated into northern and southern groups to examine the
geographic difference in bottom-depth effect. Both groups exhibit positive
linear relationships between δ15Nbulk and bottom depth
(deeper than 200 m) (Fig. 7a). The regression equations were shown in
Table 2. Interestingly, the regressions generally differ statistically from each other
in terms of slope and intercept. The slope represents the degree of
positive shift of sedimentary δ15N due to bottom-depth effect. For
the southern AS, the slope is (0.76
(±0.14) × 10-3 km-1), which is close to the
correction factor (0.75 × 10-3 km-1) for the world ocean,
proposed by Robinson et al. (2012) and further applied by Galbraith et
al. (2012). By contrast, the slope for the northern AS is significantly lower
(0.55 (±0.08) × 10-3 km-1), implying that the
depth-associated alteration in the northern AS is smaller. The correction
factor for bottom-depth effect was predicted to vary in different regions
such as that in the South China Sea (Gaye et al., 2009). Since the magnitude
of oxygen exposure is the primary control of depth effect (Gaye-Haake et al.,
2005; Mobius et al., 2011; Robinson et al., 2012), we attributed this lower
slope in the northern AS to relatively higher sedimentation rates (not shown)
and lower oxygen contents, as indicated by previous research (Olson et al.,
1993; Morrison et al., 1999; Brummer et al., 2002).
On the other hand, the intercept for the northern AS regression
(8.1 ± 0.2) is significantly higher than that for the southern AS
(6.0 ± 0.3). As mentioned above, δ15N values of sinking
particle resembled the δ15N of nitrate sourced from a depth of
100–150 m. According to the depth-dependent correction factor,
we may convert sedimentary δ15Nbulk values at various
water depths into their initial condition when the digenetic alteration is
minimal to represent the δ15N of source nitrate. Higher intercept
suggests that a stronger denitrification had occurred in northern AS surface
sediments. The 2.1 ‰ lower intercept in the southern AS likely
reflects the addition of N2 fixation in the upper water column while it
travels southward. The progressive increase of N* toward the southern AS
supports our speculation, although no δ15NNO3
profiles had been published in the southern basin. Future works about δ15NNO3 and δ15NSP in the southern AS
are needed.
(a) Non-corrected δ15N values of modern surface
sediments against corresponding bottom depth in the northern and southern
Arabian Sea (see text for N–S boundary). Regression lines were shown in
dashed and solid lines, respectively, for the northern and southern AS. (b)
Corrected surface sedimentary δ15N values against water depth.
In Fig. 7b, we presented corrected δ15Nbulk values
along with bottom depth for the northern and southern AS surface sediments for
comparison. After removing site-specific bias caused by bottom-depth effect,
the values and distribution ranges of δ15Nbulk for both
the northern and southern AS became smaller and narrower. For the northern AS,
the distribution pattern skewed negatively, giving a standard deviation of
0.88 ‰, falling exactly in the range of 7–9 ‰ for
δ15NNO3 (7–9 ‰) at the bottom of
the euphotic zone. As a result, the corrected nitrogen isotopic signals in
sediments more truthfully represent the δ15NNO3 value
at the bottom depth of the euphotic zone. Meanwhile, the statistically
significant difference in δ15Nbulk distribution
between the northern and southern AS further confirms the feasibility of our
classification by using DO isopleth of 25 µmol kg-1 at
150 m.
(a) Temporal variations of non-corrected δ15Nbulk values of all reported cores in the AS. Data shown in
curves are for cores in the southern Arabian Sea (red for SK177/11, blue for
NIOP 905 and green for SO42-74KL), dots in gray are for the northern part
and pink dots are for core MD-04-2876. Mean values of δ15N for
fixed periods against corresponding water depths for cores in the (b) northern
and (c) southern Arabian Sea. Pink and indigo blue are for the Holocene and
glacial periods, respectively. Error bars represent the standard deviation
for mean δ15Nbulk.The dashed regression lines for modern
surface sediments are shown for reference.
Bottom-depth effect during different climate stages
In order to better decipher the history of δ15NNO3 in
the bottom the euphotic zone of the water column, we synthesized almost all
available δ15Nbulk of sediment cores reported for the
AS (see Figs. 1a and 1b for locations). Similar to modern surface sediments,
northern and southern groups were defined by the contour line of
25 µmol kg-1 DO. To keep data consistency in the temporal scale,
we focused on the last 35 ka (Fig. 8a). Unfortunately, data points were less
in 0–6 ka and there were only three sediment cores in the southern
AS: SK177/11 in this study, and NIOP 905 and SO42-74KL in previous studies.
As shown in Fig. 8a, the original δ15Nbulk from the
northern (gray dots) and southern AS (green, blue and red curves)
scatter in a wide range from 4.5 to 10.5 ‰ over the entire 35 ka.
The pink dots are for the data from core MD-04-2876, which is peculiar since
the relatively low δ15Nbulk values deviated from all
other reports in the northern AS. Pichevin et al. (2007) excluded the
influences from incomplete nitrate utilization and terrestrial input, thus
we still include this core in our statistical analyses. As for the southern
cores, the temporal variations of δ15Nbulk in core
SK177/11 and NIOP 905 (red and blue) had a very similar trend distributing at
the lower bound of the whole data set. The mean δ15Nbulk
values for SK177/11 and NIOP 905 during the glacial period were almost identical,
and the deviation in the Holocene was as small as 0.7 ‰. By
contrast, the temporal pattern for δ15Nbulk of core
SO42-74KL (green) resembles that of NIOP 905, yet with an enrichment in
15N by ∼ 2 ‰ for the entire period. The core SO42-74KL is
retrieved from a depth of 3212 m, which is the deepest of the three cores in
the southern AS; the positive offset is apparently caused by the bottom-depth
effect. Thus, inference should be made with caution when compare sediment
cores from different depths.
Below we consider two time spans – 0 ∼ 11 ka (Holocene) and
19∼ 35 ka (glacial) – to examine the bottom-depth effect at different
climate stages. We ignore the transgression period, which is shorter with more
variable in δ15Nbulk, to avoid bias caused by dating
uncertainties in different studies. Also, we will discuss the peculiar
patters for 0–6 ka later. The mean and standard deviation of reported
δ15Nbulk values for the specific time span were plotted
against the corresponding depth of the core. Accordingly, we obtained the
correction factors for glacial and early Holocene, respectively, for the northern
and southern AS (Fig. 8b and c). Since only 35 ka was applied in this
practice, the long-term alteration (Reichart et al., 1998; Altabet et al.,
1999) is ignored. The regression curves for the modern day (dashed lines) were
plotted for comparison.
The difference among regressions of three climate stages in the northern AS
(Table 2) is not significant (0.55 × 10-3 km-1 to
0.70 × 10-3 km-1). However, the regression slopes for
the northern AS are significantly lower compared with those obtained from the
southern AS for all climate states. This might indicate that the oxygen content in
the northern AS is always lower, resulting in a lower degree of alteration of
δ15Nbulk. On the other hand, we may not exclude the
effect by sedimentation rate changes over these two stages, which also affect
the oxygen exposure time; unfortunately, insufficient sedimentation rate data
in the northern AS in previous reports prevent us from implementing further
analysis.
As for the southern AS, correction factors are always higher than those in
the northern AS. The overall spatial–temporal patterns are consistent with the
oxygen distribution in the Arabian Sea (Olson et al., 1993; Morrison et al.,
1999; Pichevin et al., 2007) and agree with the view that DO concentration
was the dominant factor for organic matter preservation (Aller, 2001;
Zonneveld et al., 2010). Meanwhile, the regression slopes remained high from
0.76 × 10-3 to 1.01 × 10-3 km-1 over
different climate stages in the southern AS, suggesting that environmental
situations, and thus those correction factor, change less relatively to that in the
northern AS. For SK177/11, the sedimentation rate in Holocene is two-fold higher
compared to that in the glacial period. However, the influence caused by the
sedimentation rate changes is likely not significant enough to alter the
regression slopes for the southern AS, based on the small changes in the slope
(0.93 × 10-3 and 1.01 × 10-3 km-1).
Insights from temporal changes in geographic δ15Nbulk distribution
Based on the earlier comparison among δ15NNO3, sinking
particles and surface sediments, we recognized that the regression intercept is
representative of the nitrogen isotope of nitrate source at a depth of 100 m.
Therefore, the regression-derived intercepts given in Table 2 can be used to
infer the δ15NNO3 source at different climate stages,
while the slopes can be used as correction factors to eliminate the positive
shift in δ15Nbulk caused by bottom depth; by doing
this, we can get the original signal of δ15Nbulk prior
to alteration. We applied the correction factor to be equal to (bottom depth
-100 m) × slope, ignoring the sea level changes during the
different climate stages.
Noticeably, the regression intercepts for both the northern and southern AS are
higher in the Holocene compared to those in the glacial period, indicating the
intensified isotopic enrichment in δ15NNO3 in the entire
AS in Holocene. Such increment is almost the same to be
∼ 1.5 ‰, which is similar to the increase in the eastern tropical
North Pacific, but slightly smaller than that in the eastern tropical South
Pacific (Galbraith et al., 2012). The 120 m sea level increase, which may
induce only a 0.1 ‰ offset, cannot be the reason for such a
significant increase of average δ15Nbulk during the
Holocene. Moreover, deviations between the northern and southern AS at the respective
climate stage are almost identical (1.0 ‰ for Holocene and
0.9 ‰ for glacial), indicating a synchronous shift in the relative
intensity of denitrification and N2 fixation over the basin to keep such
a constant latitudinal gradient of subsurface δ15NNO3.
The intermediate water formation near the polar region controls the oxygen
supply to the intermediate water and thus the extent of denitrification on
global scale and the stoichiometry of nutrient source to the euphotic zone
(Galbraith et al., 2004). Lower glacial-stage sea surface temperature may
increase oxygen solubility, while stronger winds in high-latitude regions
enhance the rate of thermocline ventilation. The resultant colder and rapidly flushed thermocline thus lessened the spatial extent of denitrification and, consequently, N fixation (Galbraith et al., 2004). Therefore, such a basin of
wide synchronous increase in δ15Nbulk is likely a
global control. The lower intercepts in glacial time (4.3 ‰ for the
south and 5.3 ‰ for the north), which are similar to the global mean
δ15NNO3 (4.5–5 ‰, Sigman et al.,
1997), illustrate a better ventilation of intermediate water during glacial
time in the Arabian Sea (Pichevin et al., 2007). In fact, the AAIWs penetrate
further northward over 5∘ N in the present day and even during the late
Holocene (You, 1998; Pichevin et al., 2007). Since the δ13C of
autochthonous particulate organic carbon is negatively correlated to
[CO2 (aq)] in the euphotic zone (Rau et al., 1991), the sharp decrease of
δ13CTOC in SK177/11 at the start of deglaciation
(Fig. 3b) may infer the timing of a rapid accumulation of dissolved inorganic
carbon driven by the shrinking of oxygenated intermediate water (Pichevin et
al., 2007) or enhanced monsoon-driven upwelling (Ganeshram et al., 2000);
both facilitate the promotion of denitrification. Nevertheless, the mirror
image between δ15N and δ13CTOC pro?les
revealed their intimate relation, of which the variability was attributable
to the change of physical processes.
The intercepts of the northern AS increase continuously from 5.2 to 8.1 from
glacial through to modern day, indicating the strengthened intensity of
denitrification relative to nitrogen fixation in the northern AS (Altabet,
2007). When we take a close look at the temporal pattern of corrected
δ15Nbulk for long cores (Fig. 9), we can see an amplified
deviation since 6 ka, during which δ15Nbulk increases
continuously in the northern AS, whereas it decreases in the southern AS.
Note that the northern most core, MD-04-2876, also followed the increasing
trend in recent 6 ka even though its δ15Nbulk values
deviated from all other cores. Such opposite trends indicate that the
controlling factors on the nitrogen cycle in the northern AS were different from
that in the southern AS, which means that localized enhancement in specific
process had occurred.
Temporal variations of corrected δ15Nbulk values
of all reported cores in the AS. Gray and black dots are for the northern and
southern AS, respectively. Pink dots are specifically for core MD-04-2876.
The deglacial period is in shadow because non proper equations for
bottom-depth effect correction. The upper panel is the blow-up for the
Holocene period. The intensified deviation trends since 6 ka were marked by
bold dashed lines.
Besides the oxygen supply to the intermediate water, the intensity of water
column denitrification varies with primary productivity (Altabet, 2006; Naqvi
et al., 2006). Strong summer monsoon and winter monsoon drive upwelling or
convective mixing enhances primary productivity, which, in turn,
intensifies denitrification (Altabet et al., 2002; Ganeshram et al., 2002).
However, it was also reported that primary productivity did not correlate
well with water column denitrification underneath during the Holocene in some
parts of the northern AS (Banakar et al., 2005, and references therein).
Regardless of the declining summer monsoon strength since 5500 ka (Hong et
al., 2003), primary productivity in the northern AS seemed to have increased.
Similar to the patterns observed for TOC and TN in this study, productivity
indicators (TOC and Ba / Al ratios), reported by Rao et al. (2010) in the core
SK148/4 located near our SK177/11, also increased gradually since the
Holocene. Incomplete nitrate consumption can hardly explain the decreasing
pattern for all three cores in the southern AS, where upwelling intensity is
much less relative to that in the north. Moreover, lower TOC / TN ratios
observed in Holocene in SK177/11, as mentioned earlier, rule out the influence
of terrestrial organic input. Therefore, a spatial coupling of
denitrification-dependent N2 fixation is the more plausible cause of the
decreasing δ15Nbulk pattern (Deutsch et al., 2007).
We suggested that the intensified supply of excess phosphorous (phosphorus in
stoichiometric excess of fixed nitrogen) toward the southern AS to stimulate
N2 fixation, subsequently responsible for the decreasing δ15Nbulk pattern in the southern basin. The intensification in
excess phosphorous supply can be driven by enhanced upwelling or intensified
subsurface water column denitrification or both. According to the increasing
pattern in δ15Nbulk and primary productivity in the
northern AS, synergetic processes are suggested. The upwelled water in
the northern AS basin brings up low N / P water to the surface for non-diazotrophs
to uptake. If we assume complete consumption, the remaining excess
phosphorous after complete consumption will be transported toward the south by
clockwise surface circulation and advection. Therefore, N2 fixation in
the southern AS acts as feedback to balance denitrification changes in the
northern AS. This phenomenon is similar to the illustration for the spatial
coupling of nitrogen inputs and losses in the Pacific Ocean, proposed by
Deutsch et al. (2007). The question as to why such forcing to expand the N–S deviation had not
occurred before 6 ka warrants more studies.