0
____________________________________o__
c_EA_N_O_L_O_G_I_C_A_A_C_T_A_1_9_8_1_-_v_o_L_._4_-_N__1__
Faunal and isotopie indices
of monsoonal upwelling:
Western Arabian Sea
~------Upwelling
1)180
o13 C
Arabian Sea
Foraminifera
Upwelling
1)180
0t3c
Mer d'Arabie
Foraminifères
W. L. Prell, W. B. Curry
Department of Geological Sciences, Brown University,
Providence, Rhode Island 02912, USA.
Received 29/l/80, in revised form 15/7/80, accepted 28/7/80.
ABSTRACT
The South west monsoon causes intense coastal upwelling along the coast of Arabia. This
upwelling causes distinct gradients of sea surface temperature and nutrient content across
the Ara bian Sea which are highly correlated with the relative abundance of planktonic
foraminifera Globigerina bulloides. We have measured the isotopie variation of
G. bulloides and three other species across the gradients of temperature and phosphate in
the Arabian Sea. The ô18 0 composition of surface-dwelling species is highly correlated
with sea surface temperature during the Southwest monsoon. Thus, the ~ô 18 0 gradient
records upwelling patterns, and is a useful tool to reconstruct the intensity of past
upwelling. The ô13 C composition of shallow-dwelling species is not related to upwelling
gradients during the Southwest monsoon. Renee, variations of ô13 C cannat be used to
reconstruct the intensity of upwelling. We suggest that the large stratigraphie variations of
ô13 C observed in ali oceans are not due to local variations in upwelling or nu trient supply.
These variations may reflect a more general phenomenon, such as changes in the global
carbon cycle.
Oceanol. Acta, 1981, 4, 1, 91-98.
RÉSUMÉ
Indices isotopiques et faunistiques de l'upwelling
provoqué par la mousson en Mer d'Arabie occidentale.
La mousson du Sud-Ouest provoque d'importantes remontées d'eaux froides
(upwellings) le long des côtes d'Arabie. Ce phénomène d'upwellings est à l'origine des
forts gradients de température de surface et de teneur en sels minéraux au travers de la
Mer d'Arabie, qui sont étroitement corrélés avec l'abondance relative du foraminifère
planctonique Globigerina bulloides. Les variations de composition isotopique de cette
espèce, ainsi que celles de Globigerinoides ruber, Globigerinoides sacculifer et Globorotalia
menardii, ont été mesurées en fonction des gradients de température et de teneur en
phosphore. La composition isotopique de l'oxygène (ô 18 0) des espèces d'habitat
superftciel est parfaitement corrélée avec la température de surface des eaux marines
pendant la mousson du Sud-Ouest. Par conséquent, le gradient des différences de
composition isotopique (~ô 18 0) est lié aux distributions d'upwelling, et peut servir d'outil
pour élucider l'intensité des upwellings passés. La composition isotopique du carbone
(ô 13 C) des espèces d'habitat superficiel ne présente pas de relation avec les gradients
d'upwelling au cours de la mousson du Sud-Ouest. Ainsi, les variations de ô13 C ne
peuvent pas être utilisées pour déterminer l'intensité des upwellings. Cette constatation·
suggère que les fortes variations de ô 13 C que l'on observe à certaines époques géologiques
dans tous les océans, ne peuvent être reliées à des variations locales d'upwelling ou de
teneurs en sels nutritifs. Ces variations doivent refléter un phénqmène plus général, tel
qu'un changement dans le cycle global du carbone.
Oceanol. Acta, 1981, 4, 1, 91-98.
0399-1784/1981191 1 $5.00/Cl Gauthier-Villars
91
W. L. PRELL, W. B. CURRY
Here, we ex~mine in detail the synoptic variations in
planktonic foraminifera populations and their carbon
and oxygen isotopie ratios across the well-established
gradient of upwelling in the Western Arabian Sea. Our
intent is to provide a modern analogue of the magnitude
and spatial extent of oxygen and carbon isotopie
variation related to upwelling. Renee, these patterns will
serve as a framework for the interpretation of down-core
isotopie variations.
INTRODUGTION
Coastal upwelling is a dynamic process caused by
interaction of atmospheric circulation with the surface
ocean and continental boundaries. With the exception of
major oceanic fronts, such as the subtropical convergence, coastal upwelling creates the most distinct
biological, chemical, and physical gradients observed in
the surface ocean. The cool, nutrient-rich upwelled
waters support distinctive planktonic floras and faunas.
The accumulation of these plankton on the sea floor
records the temporal variation of the overlying upwelling
system and therefore reflects variations in regional
climate and wind systems. Previous study of the deep-sea
stratigraphie record (see below) has shown that sorne
upwelling systems vary signiflcantly on glacialinterglacial time scales (10 4 -10 5 year). Thus, a detailed
understanding of the modern sedimentary record of
upwelling faunas is a prerequisite to their use for the
reconstruction and interpretation of past upwelling and
its climatic signiflcance.
STRATEGY AND METHODS
The Arabian Sea is an ideal location to examine the
biologie and isotopie response to upwelling. First, the
upwelling is located along the coast of Arabia, and is not
downstream of an eastern boundary current as are most
other areas of major upwelling. This simplifies our faunal
interpretation, because cool water and its associated
fauna are not advected into the area. Second, the
upwelling produces distinct patterns and gradients of
temperature and nutrients across the Arabian Sea
(Wyrtki, 1971 ). Third, the upwelling is restricted to the
period of the South west monsoon (June to September).
and is thus an indicator of the intensity of the monsoon
circulation.
Information on the intensity and extent of paleoupwelling is contained in deep-sea sediments as
sedimentary, biologie, and isotopie signais. Sedimentarybiologic indices such as radiolaria/planktonic foraminiferal ratios, planktonic/benthonic foraminiferal ratios,
organic carbon content, and opal content have been used
to quantify the magnitude of paleo-upwelling (among
others : Diester-Haass, 1977, 1978; Molina-Cruz, 1977).
Assemblages of planktonic foraminifera, radiolaria, and
dia toms have also been correlated to upwelling and used
to estimate upwelling variation through time (Gardner,
Hays, 1976; Thiede, 1975, 1977; Molina-Cruz, 1977).
We examined an array of synoptic samples (core-top) to
establish the response of planktonic foraminifera and
their isotopie compositions to upwelling in the Arabian
Sea. The strategy of examining core-top {modern)
samples allows direct correlation of geologie indicators
to gradients of temperature and nutrients resulting from
upwelling. During the Southwest monsoon (JuneSeptember), sea surface temperature (SST) is low
( < 24°C) and nu trient content is high (P0 4 is 1.5 J.Lg-at/1)
along the coast of Arabia (see Fig. 1). In general, SST
and phosphate are highly correlated (r= -0.71 for 53
core locations in Fig. 1). The correlation is lower in the
Northeastern Arabian Sea, where high SST is associated
Recently, Berger et al. (1978a) have suggested that the
~ 13 C composition ofplanktonic foraminifera (Glohigeri-
noides ruber) is related to the intensity ofupwelling. They
reported stratigraphie variations of ~ 13 C ofup to 0.8°/ 00
in G. ruber off Northwestern Africa. They attributed
covariation of the light carbon isotopie composition
with sedimentologic indicators of upwelling to reflect
more intense upwelling which occurred during deglaciation. Sommer and Matthews (1976) reported a total
stratigraphie variation in ~ 13 C of 1.7°1 00 with ~ 13 C
minima in oxygen isotope stage 2 and the middle of
stage 5. They suggest that the variation in carbon
- isotopie ratios may reflect changes in the productivity of
the oceans or in the oxidation of terrestrial organic
matter during times oflower sea leve!. Shackleton {1977)
has reported down-core variations of ~ 13 C of 0. 7 to
0. 8°1 00 in G. ruber and G. sacculifer in the South
Atlantic. He has also reported a maximum range of~ 13 C
variation of 1. 95°1 00 in the benthic foraminifera
Uvigerina peregrina in Meteor core 12392 ofTNorthwestern Africa. On a longer time scale, Kroopnick et al.
(1977) have summarized evidence for a 2°1 00 shift of
~ 13 C in calcareous plankton during the last 75 M. Y.
Thus, planktonic and benthonic foraminifera record
large stratigraphie variations of ~ 13 C which have been
variously attributed to changes in upwelling, productivity, nutrient concentrations of the surface ocean, and to
the terrestrial biosphere.
.......... ...;
...
....... ê.
. : ~...... ·~ ..
. :-'t:··.
...
August SST ("C)
Figure 1
The distribution of temperature, phosphate, and G. bulloides during the
Southwest Monsoon. A) Sea surface temperature (°C) during August
ifrom Wyrtki, 1971 ). B) Phosphate content (~tg-at/ l) in the surface waters
during the monsoon season (May to Octoher). Data from Wyrtki (1971).
C) Relative frequency (~;,)of G. hulloides in 53 Arahian Sea core-top
samples. D) Regression of G. bulloides ubundunce (~~)and August sea
surface temperature gives a correlation coefficient of -0.80.
92
FORAMINIFERAL &18 0 AND &13 C: ARABIAN SEA UPWELLING
Arabian Sea (Bé, Hutson, 1977; Hutson, Prell, 1980).
G. sacculifer (300-355 J.liD) was selected because it
appears to fractionate near to carbon isotopie
equilibrium (Williams et al., 1977). G. menardii (500600 J.liD) was chosen to reflect conditions at 100-150 rn
depth. Ten to twenty individuals of each species were
cleaned in an ultrasonic bath to remove fme fraction
contamination. AH samples were roasted under vacuum
at 370°C for 1 hour. H 2 0 and C0 2 were extracted from
the carbonate reaction with orthophosphoric acid at
50°C, separated by a series of three freezing transfer
steps, and the C02 was analyzed in an on-line VG
Micromass 6020 mass spectrometer. All data are
referred to PDB by the standard ~ notation (Craig,
1957). Calibration to PDB is through three intermediate
laboratory standards. Agreement among all calibration
standards is ± 0.2°1 00 • The analytical precision from
working carbonate standards run before and after each
analytical session is ±0.12 (1 cr) for oxygen and ±0.12
( 1 cr) for carbon. Analytical precision based on
14 dup1icate analyses (8 blind) run on separate days is
±0.15 (average 1/2 ~~ 18 0) and ±0.13 (average
1/2 ~~ 13 C). A complete report of analytical results can
be obtained from the authors.
Table 1
Location of core-top samples with carbon and oxygen isotopie
determinations. Ali, Atlantis (WHO!); RC, Robert Conrad (LDGO); V,
V erna (LDGO). • core-top verijied by isotope stratigraphy. b core-top is
winnowed; see data discussion.
Longitude
Core
Latitude
("N)
("E)
Ail -15-585 HC
All-15-586
All-15-591
All-15-592
AII-15-596
AII-15-597
AII-15-612
RC9-161•
V14-103
V14-104
V19-178 2 cm PC"
V19-185•
V34-80
V34-83b
V34-85
V34-87
V34-88"
20"09'N
20"07'N
21"00'N
20"50'N
18"56'N
17"26'N
13"35'N
19"34'N
11"27'N
13"26'N
08"07'N
06"42'N
06"07'N
10"24'N
11"48'N
16"29'N
16"31'N
69"26'E
67"55'E
59"33'E
61"01'E
61"23'E
57"11'E
71'"34'E
59"36'E
56"14'E
53"27'E
73"15'E
59"20'E
59"26'E
57"58'E
57"37'E
59"46'E
59"32'E
Depth
(rn)
216
3047
1267
2628
3694
1805
1697
3 332
4232
2670
2188
2867
3292
1880
3190
2080
2120
with moderately high nutrient content. The lack of
correlation reflects the numerous mechanisms, such as
wind-induced upwelling, thermohaline upwelling, vertical diffusion, and deepening of the surface layer, which
may increase the nutrient content of the surface waters
(Ryther, Menzel, 1965; Sastry, D'Souza, 1972). The SST
pattern, however, is controlled by the coastal upwelling
off Arabia (Fig. 1). Thus, the SST pattern is the more
reliable indicator of coastal upwelling. Here we should
note that our usage of upwelling generally refers to the
distinct temperature and nutrient patterns (upwelling
effects) produced by coastal upwelling, and does not
imply vertical transport over the entire area of
"upwelling effects".
We determined the area where upwelling effects are
recorded in deep-sea sediments by mapping the relative
frequency of Globigerina bulloides in 53 core-top
samples. Previous works (Bé, Hutson, 1977; Hutson,
Prell, 1980) indicate that G. bulloides, a subpolar species,
occurs in the upwelling region off Somalia. However,
these studies included few samples from the Ara bian Sea.
Preliminary work by Prell ( 1978) shows that the
distribution of G. bulloides in the Arabian Sea is highly
correlated with upwelling indicators (sea surface
temperature, r = -0.80; phosphorous content, r = 0.67 for
the 53 samples in Fig. 1). Thus, we have used the
frequency of G. bulloides on the sea floor as a measure of
the gradient of upwelling (Fig. 1). The isotopie response
across this upwelling gradient was determined by .
measuring the ~ 13 C and ~ 18 0 ratios of four species of
planktonic foraminifera in 17 sea-bed samples (see
Table 1 for location of samples).
All samples were sieved to narrow size intervals to
minimize isotopie variation resulting from changing test
size (Curry, Matthews, 1977; Berger et al., 1978 b). Four
species were selected to represent various oceanographie
conditions and isotope fractionation patterns.
G. bu/laides (212-250 J.UTI) is most highly correlated with
upwelling conditions (see Fig. 1), whereas G. ruber (212250 J.liD) represents the normal tropical conditions in the
DATA
The carbon and oxygen isotopie data for 17 Ara bian Sea
sediment samples are summarized in Table 2 and shown
in Figure 2. Ali four species plot as distinct groups on the
carbon-oxygen diagram (Fig. 2). The ~ 18 0 values for
G. bullaides and G. ruber are similar, and consistent with
a shallow depth habitat. The values for G. sacculifer are
somewhat heavier which suggests a slightly deeper
habitat (Shackleton, Vincent, 1978). G. menardii has
~ 18 0 values approximately 1°1 00 heavier than the surface
dwellers, a result consistent with a deeper depth habitat
(Berger, 1969, 1971). The large variation in carbon
isotopie composition separates the species into distinct
groups. Strong disequilibrium ~ 13 C fractionation of
G. ruber and G. bulloides is apparent (Fig. 2), and is
consistent with other lndian Ocean data (Williams et al.,
1977). G. sacculifer and G. menardii, which have
enriched ~ 13 C and are apparently doser to equilibrium,
show somewhat less variation in carbon composition
than do G. bu/laides and G. ruber.
B"ci.,.,.)POB
·3.0
·3.0
"'
-2.0
0
0..
i
::;
-1.0
0
·2.0
·10
. .. .
·
.
........
0
+1.0
+2.0
•
•
·~·:
• • .,•
:,.
•
•
.~
•
eG.ruber
..t.G bullotdes
•G.socculifer
eG.menordil
•
Figure 2
&18 0 versus &13 C (0 / 00 PDB).for Arabian Sea planktonic foramin!f'era.
See Data section /ol' di."'l/,,,,;,111 ofralues. Ali ;,,otopic ch/la pre.~ented in
Table 2.
93
W. L. PRELL. W. B. CURRY
Table 2
0 xygen and carbon isotopie compositions of Arabian Sea foraminifera. The size range analyzed is given below
each species name. Values with asterisks are averages of duplicate analyses. Analytica/ precision based on 13
duplicate analyses is ±0.15for oxygen (average 1/2 f..ô 18 0) and ±0.13for carbon (average 1/2 t..ô 13 C). Al/
analytical results are available from the authors.
0
( / 00 ,
G. sacculifera
(300-355 11m)
PDB)
G. bu/loides
(212-250 llffi)
G. ruber
(212-250 llffi)
G. menardii
(500-600 llffi)
Core
r,ts 0
r,t3c
r,ts 0
ô 13 C
r,ts 0
513c
r,ts 0
AII-15-585
AII-15-586
AII-15-591
AII-15-592
AII-15-596
AII-15-597
AII-15-612
RC9-161
Vl4-103
V14-104
Vl9-l78
V19-185
V34-80
V34-83
V34-85
V34-87
V34-88
-1.55
+1.22
-1.56
-2.18
-1.55*
-0.72
-0.57
-0.28
+1.00
+1.03
+1.17
+ 1.24*
+1.35
-0.94
+0.89
-1.78
-1.86*
-1.95
-1.87*
-1.66
-1.43
-2.24
+0.81
+0.74*
+ 1.16*
+0.95
+0.89
+ 1.26
+1.26
+1.27
+ 1.13*
-1.70
-1.26*
-2.04
-1.65*
-1.34
-1.48
-2.59
-0.61
-0.46*
-0.93*
-0.45
-0.59
-1.93
-1.40
-1.64
-0.98*
+0.07*
-0.20
-0.02
-0.01
-0.01
+0.45*
+0.27*
-0.22
+0.25
+0.43
+0.16
+0.47*
+0.26
+0.49
+0.41
-1.67
-1.80
-1.36*
-1.03*
-1.51
-1.85*
-1.87
-1.48
-1.55
-1.80
-1.50*
-1.95*
-1.61
-1.39
-1.27
-2.58
-2.01*
-1.85
-1.32
-1.86
-1.06*
-0.51*
+0.92*
+ 1.03*
+ 1.16*
-0.01
-2.04
-1.43
-2.14
-2.21
+0.46*
-0.92
-1.16
-1.88
-1.32
-1.01
ô 13 C
The vertical profile of ô 13 C in the upper water column is
related to the stability of the water column, the nature of
organic particles, and the rate of oxidation-recycling by
the biologie community. The release of carbon and
nutrients by oxidation at depth is linearly related to
apparent oxygen utilization (AOU) (Redfteld, 1942).
Williams et al., (1977, Fig. 2) developed an empirical
relation between AOU and the ô13 C of ~COz. On the
basis of this relationship and data from Wyrtki (1971),
we calcula te that ô13 C of ~COz is approximately 0°1 00
(PDB) between 100 and 200 rn depth in the Arabian Sea.
If this 13 C-depleted water is upwelled to the surface, the
local surface carbon reservoir should be lighter than
average surface waters ( + 2°1 00 PDB). Therefore, the
maximum potential difference between the ô 13 C of
upwelled waters and average surface waters is ~ 2°1 00 •
This potential difference is damped by mixing, biologie
fixation, and atmospheric equilibration of the upwelled
C0 2 (ô 13 C). Likewise, because nutrients are ftxed and
released in association with carbon, the gradient of
phosphate in the surface waters should reflect upwelling
(see Fig. 1) and be inversely correlated with ô13 C. To
determine whether these patterns are recorded by the
planktonic foraminifera, we examine the isotopie
variation across gradients of temperature and phosphate
in the Arabian Sea.
DISCUSSION
The isotopie signature of upwelling
The expectation that upwelled waters should have a
distinct isotopie signature is based on the well-known
fractionation of oxygen with temperature (Epstein et al.,
1953; O'Neil et al., 1969) and the profile of ô13 C with
depth in the ocean (Deuser, Hunt, 1969; Kroopnick
et al., 1972; Kroopnick, 1974 a, 1974 b; Williams et al.,
1977). Because upwelled waters are cooler than
surrounding waters, the ô18 0 of carbonate precipitated
in equilibrium with those waters should be enriched by
approximately 0.25°1 00/°C. Thus, a 4°C gradient of SST
should produce a 1°/00 change in ô18 0. This effect of
. temperature gradient on the ô 18 0 of shallow-dwelling
foraminifera has been demonstrated by numerous
researchers (Emiliani, 1955; Savin, Douglas, 1973;
Williams, 1976; Curry, 1978; Shackleton, Vincent,
1978).
The hypothesis that upwelled waters should contain a
carbon isotopie signature is based on the observation
that 12 C is preferentially ftxed in the surface waters
(along with nutrients) and settles to depth. The carbon is
subsequently released by oxidation and recycled to the
surface. This hypothesis is deceptively simple. Because
carbon is intimately involved with biological cycles
within the ocean, the distribution of its isotopie ratio is
quite complex. Phytoplankton preferentially fix 12 C
from the total dissolved C02 ; the organic matter
produced is 19 to 26°/ 00 depleted in 13 C relative to
surface water (Sackett et al., 1965; Fontugne, Duplessy,
1978). This preferential extraction of light carbon by
primary producers enriches the ô 13 ~ co2 of surface
water to approximately +2°/ 00 PDB (Deuser, Hunt,
1969; Kroopnick et al., 1972; Kroopnick, 1974 a,
1974 b).
Oxygen isotopes
During the Southwest monsoon, waters upwelled off
Arabia are > 4°C cooler than surface waters in the
Central and Eastern Ara bian Sea (Fig. 1). Hence, a
range of ô 18 0 of about 1°100 should be observed in
the planktonic foraminifera across the Arabian Sea.
Regressions offoraminiferal ô18 0 and seasonal SST give
good correlations ( -0.59 to -0.92) for ali species
94
FORAMINIFE.RAL ii''O AND iiuc: ARABIAN SE.A UPWELLING
slopes. The slope of G. menardii is Jess than expected,
because it bas a deeper habitat and experiences Jess
temperature change across the Arabian Sea. Local and
seasonal variations ofSST and salinity may explain sorne
of the scatter and wh y the observed slopes are shallower
than the expected equilibrium slope (see below).
•
.~-··
·2
•
-1
·2
•
/ • .
A certain amou nt of scatter is inevitable in a core-top data
set. The major sources of scatter in our data are due to: 1)
seasonal production offoraminifera; 2) a possibility that
not all core-tops represent the most recent sediment;
and 3) differentiai carbonate dissolution (Berger,
Killingley, 1977). Carbonate content of Arabian Sea
sediments is high during isotope stage 1 and decreases
rapidly across the terminations, probably reflecting the
increased terrigenous sedimentation on the Indus Fan
during intervals of lower sea level. On the basis of high
carbonate content in isotope stage 1, we believe that all
core-tops are Holocene in age. Additionally, the late
Holocene age of four cores is verified by oxygen isotope
stratigraphy (Table 1). The core-top values in these cores
are at least 1% 0 lighter than glacial (stage 2) values.
Comparison of the 8 18 0 of verif1ed core-tops to
surrounding cores shows that most of our core-top data
falls within 0.2°100 of the verifled core-tops. One
exception is V34-83, which bas been winnowed to a
foram sand. The G. menardii value in this core is
especially heavy and was not included in our regressions.
We note that the lightest values (Table 2, Fig. 2) in our
data set (Vl9-178) agree weil with the values of Williams
( 1976) for G. sacculifer and G. ruber from the same core.
We have attempted to minimize the potential sources of
isotopie variability, and believe that the existing scatter is
a consequence of the real variation in Holocene coretops.
We have used the temperature and salinity data from
Wyrtki (1971) to estimate the equilibrium ù 18 0 of
foraminiferal calcite in the Arabian Sea surface waters
(Table 3 ). The calculation of equilibrium ù 18 0 incorpora tes the paleotemperature equation of Epstein et al.
(1953) and the empirical equation of Williams (1976),
which relates the salinity and ù 18 0 of sea water. The
•
. . .----.------:;
. .----·
-·--.
..
•
- •.
..
__,...-
_____....-.
••
•1
~---
o,t-,_,.......~,.~""-,.~-:':-'.......J
AUGUST SST i•c)
..
·
25
u
AUGUST SST 1-tl
Figure 3
1> 18 0 (0 / 00 PDB) versus August SST for Arabian Sea foraminifera.
Regression fine (dashed) statistics are given in Table 4. The starred value
ofG. menardii (V34-83) was not included in the re!(ression analysis (see
Data discussion). Note that the L'il> 18 0 of shallow-dwelling species
(calculated from the regression fine) is O. 7 to 0.8°/ 00 across the Arabian
Se a (4°C range), and compares weil to the predicted value of 1°100 •
(Table 4). The 818 0 of shallow-dwelling species
generally shows a higher correlation with August SST
than with February SST (see Fig. 3). The slopes of
8 18 0/°C approach -0.25% 0 /°C, which is the equilibrium slope for calcite (Epstein et al., 1953). The
regression equations {Table 4) for shallow-dwelling
species give a ~8 18 0 of0.7 to 0.8°/ 00 across the observed
SST range (24 to 28°C). Examination of Figure 3 reveals
the necessity of using the best-f1t line rather than
individual samples. The regression equation givcs the
correct ~ù 18 0 across the Arabian Sea, whereas
combinations of a few samples can give qui te misleading
Table 3
Oceanographie data and calculated ô18 0 equilibrium values for ali sample locations. Oceanographie data are from Wyrtki (1971).
Equilibrium values u·ere calculated using the equation ofEpstein et al. (1953) and an estima te ofl> water [l> .. p 08 =0 .48 (S 0 100 ) -16. 73]
(Williams. 1976 ).
AII-15-585
AII-15-586
AII-15-591
AII-15-592
AII-15-596
AII-15-597
AII-15-612
RC9-161
V14-103
V14-104
V19-178
V19-185
V34-80
V34-83
V34-85
V34-87
V34-88
SST (0 C)
August
S (Dfoo)
July_
August
SST (0 C)
February
S (0 /oo)
January_
February
(P04
28.0
27.7
24.0
25.0
26.2
24.0
27.7
24.0
25.1
25.0
28.0
27.5
27.6
25.8
25.5
25.4
25.5
36.5
36.5
35.75
36.0
36.05
35.8
35.8
36.0
35.5
35.75
35.5
35.7
35.7
35.8
35.75
36.2
36.1
26.2
26.0
24.4
25.0
25.6
25.0
28.0
25.0
26.0
26.0
28.0
26.7
27.0
26.6
25.9
25.8
25.7
35.8
37.0
36.5
36.6
36.4
36.3
35.5
36.5
35.75
36.1
33.6
35.0
34.9
35.4
35.65
36.1
36.1
0.6
0.61
1.5
0.9
0.8
0.92
0.2
1.30
0.39
0.4
0.2
0.26
0.25
0.3
0.28
0.52
0.58
95
(~-tg-at/1)
May_
October
li'"o...h
calculated
-1.63
-0.49
-0.92
-0.99
-1.22
-1.14
-2.13
-1.04
-1.61
. -1.44
-3.04
-2.11
-2.22
-1.90
-1.64
-1.40
-1.38
018 ÜAug
calculated
-1.65
-1.59
-1.20
-1.28
-1.51
-1.18
-1.93
-1.08
-1.54
-1.40
-2.13
-1.93
-1.95
-1.54
-1.51
-1.27
-1.34
W. L. PRELL. W. B. CURRY
Table 4
Regression statisticsfor foraminiferal 11 18 0 and ô 13 C versus seasona/ SST, phosphate concentration,
and ca/culated seasonal equilibrium 11 1 "0. r, correlation coefficient; m, slope; /, intercept. Ali
correlation coefficients marked by and asterisk are signijicant at the 95% confidence levet.
1)180
G. sacculifer
{ ôt3c
1)180
G. ruber
{ ôtJC
1)180
G. bulloides
{ 11 •3c
{ 1)180
G. menardii
11 .3c
SST
August
SST
February
18
Ô 0Feb
Ô18 0Aug
P04
calculated
calculated
r = -0.87*
m =-0.20
1= +4.02
r =+0.54
m= +0.05
1= -0.12
r = -0.67*
m= -0.17
1= +2.76
r =+0.04
m=+0.01
1 = 0.00
r = -0.64*
m= -0.18
1=+3.02
r = -0.34
m= -0.07
1= -0.14
r = -0.73*
m= -0.10
1 = +1.90
r =+0.28
m=+0.02
1 = -t-0.40
-0.72*
-0.26
+5.46
+0.45
+0.06
-0.45
-0.59*
-0.22
+4.04
+0.37
+0.09
-2.07
-0.64*
-0.26
+5.18
-0.37
-0.10
+0.88
-0.92*
-0.18
+3.98
+0.21
+0.02
+0.33
+0.45
+0.64
-1.62
-0.10
-0.06
+ 1.23
+0.32
+0.31
-1.86
-0.60*
-0.36
+0.39
+0.32
+0.33
-1.80
+0.19
+0.14
-1.94
+0.77*
+0.39
-0.89
0.00
0.00
+0.97
+0.78*
+0.50
-0.39
+0.84*
+0.96
+0.30
+0.66*
+0.45
-0.94
+0.72*
+0.89
-0.30
+0.69*
+0.53
-0.75
+0.75*
+ 1.07
0.00
+0.86*
+0.31
-0.12
+0.81*
+0.54
+0.23
expected o18 0 for calcite precipitated in equilibrium with
sea surface temperature and salinity at each core location
• is given in Table 1. Regression of observed and
18
calculated
0 more closely reflects sea surface
conditions during August (the South west monsoon). Ail
shallow-dwellers have higher correlations with the
calculated August o18 0 values. Importantly, these
species also have slopes close to unity and intercepts close
to zero (Table 4). We note that the oxygen isotopie
composition of G. bulloides, whose relative· abundance
best correlates with upwelling, is most consistent with
calculated August equilibrium o18 0 (Tables 3 and 4). ln
contrast, the ftt of predicted versus observed o18 0 for
February is poor. Speciftcally, the slopes are not close to
unity, and the intercepts are -0.39 to -0.94°/ 00 lighter
than the expected values.
variation of o13 C might be expected across the same
upwelling gradient. o13 C should be positively correlated
with temperature and inversely correlated with phosphate content. Surprisingly, our observations (Table 2,
Fig. 4) reveallittle to no systematic variation of ù 13 C
with temperature, phosphate, or the abundance of
G. bulloides. Correlation coefficients between o13 C and
August SST range from -0. 34 to + 0. 54 none are
signiftcant at the 95% confidence leve!. Likewise,
correlations of o13 C with phosphate content of surface
waters are low (Table 4), with the exception of G. ruber
(r= -0.60). The moderately high correlation of o13 C in
G. ruber with phosphate concentration, but low
correlation of o13 C to temperature and o18 0 to
phosphate is somewhat puzzling. Because SST is a more
relia ble indicator of the coastal upwelling of Arabia than
o
In summary, the o18 0 of shallow-dwelling foraminifera
in the Ara bian Sea is highly correlated to the gradient of
SST during August (Fig. 2, Table 4). The slope of o18 0
across the Arabian Sea records the SST gradient caused
by upwelling during the Southwest monsoon. This
pattern implies that a higher .1o 18 0 across the Arabian
Sea would indicate increased upwelling and a more
intense South west monsoon; e. g., cooler SST's off
Arabia and higher SST gradients across the Ara bian Sea.
Conversely, a lower .1o 18 0 implies less upwelling. We
propose that the synoptic mapping of o18 0 in shallowdwelling species across potential upwelling gradients is a
useful tool for estima ting the past variability ofupwelling
systems.
..
ti.611CCII/If•l'
•
~ •1.0
"
•
~-
•1.!5
•2'!;.
24
20
"
~ -·~
_, 0z3
Carbon isotopes
24
•
•
• •• •
• • ••
26
27
28
23
24
.. ..• .•
-·• •• • .
6.bli/lllltl••
-2.&
q '"'"'
-·-. .. ---·
~
.."
-·
G.,,.nllrd/1
,
2&
•1.0
26
AUGUST SST (•CJ
27
28 •Z.023
.
26
...
•·-
1
27
28
.
•
--.---·--~--~"=1
24
2&
26
Z7
28
AUGUST SST (•C)
Figure 4
On the basis that the .1o 18 0 of shallow-dwelling
foraminifera closely reflects the upwelling gradients
produced by the Southwest monsoonal upwelling, a
ô13 C (0 / 00 PDB) versus August SST for Arabian Seaforaminifera. Al/
isotopie data presented in Table 2. Note that none of the regressions are
significant.
96
FORAMINIFERAL 1)1BQ AND 1) 13(: ARABIAN SEA UPWELLING
is phosphate (see above discussion), we conclude that
Ll 13 C-phosphate relationship observed in G. ruber is not
uniquely related to that upwelling system. The ô13 C of
G. ruber does, however, reflect the generally high
phosphate content of the Northern Indian Ocean.
Thus, G. ruber may be a useful tool for measuring
concentration of the P0 4 through time, but in this case
the high phosphate content is not unique to the Arabian
coastal upwelling. However, we note that the total
variation (- 0. 5°1 00 , Table 4) of ô 13 C in G. ruber is Jow
across a large phosphate range (1. 3 11g-at/l ), and is not
great enough to explain much of the ô 13 C variation
observed in stratigraphie records. Down-core variations
of up to 1°1 00 would require much grea ter phosphate
ranges (up to 2. 6!1g-at/l), which are unlikely in the open
ocean or another mechanism in addition to upwelling.
Our results imply that the stratigraphie variations of
ô13 C in planktonic forams (up to 1°/00 ) (see
Introduction) are unlikely to be caused by local
variations of upwelling intensity, nutrient supply, or
productivity. This result is especially relevant to stable
areas, e. g., gyres, where the long-term variations of
upwelling, nutrient content, and productivity are likely
to be much Jess than the gradients observed across the
modern Arabian Sea. These past ô13 C variations may be
related to changes in the isotopie composition of
atmospheric COz which, when equilibrated with the
surface ocean, transmits its ô13 C signal to the shallowdwelling planktonic foraminifera. For example, the ô13 C
of atmospheric COz could be changed by oxidation of
terrestrial carbon (Shackleton, 1977). If the Jack of ô13 C
variation across other high-nutrient, high-productivity
gradients can be verified in core-top sediments, the role
oflocal phenomena can be limited in the interpretation of
stratigraphie ô13 C variations in shallow-dwelling foraminifera. Potentially, they record past global variations
of the isotopie composition of atmospheric COz.
We have assumed that the ô 13 C and ô 18 0 ofplanktonic
foraminifera, although modified by vital effects, do
reflect the external oxygen and carbon reservoirs
(Williams et al., 1977). To assess the foraminiferal vital
effects on the carbon isotopie composition, we selected
three shallow-dwelling species with different disequi1ibrium fractionations (Williams et al., 1977). The ô13 C of
these species (G. sacculifer, G. ruber, and G. bulloides)
does not correlate with the upwelling temperature
gradient; only one species (G. ruber) correlates with the
phosphate gradient. However, ali of these species have
significant correlations of ô 18 0 to the upwelling SST
gradient. Hence, we conclude that the foraminifera do
not record a ô13 C gradient associated with the coastal
upwelling or that the gradient is small enough to be
obscured by the scatter in our data. This conclusion is
opposite the positive covariation of carbon isotopie
composition in G. ruber and down-core sedimentologic
indicators of increased upwelling reported by Berger
et al. (1978 a).
CONCLUSIONS
1) The Southwest monsoon produces distinct gradients
of sea surface temperature, nutrient content, and primary
productivity within the Arabian Sea due to upwelling
along the coast of Arabia. Sea surface temperature is the
most reliable indicator of this coastal upwelling signal,
and is highly correlated with relative abundance of
G. bulloides (Fig. 1 ).
2) The ô 18 0 composition of surface-dwelling planktonic foraminifera is highly correlated with gradients of sea
surface temperature and salinity during the Southwest
monsoon (see Fig. 3 and Table 4). The ilô 18 0 in
shallow-dwelling species is -0.7 to 0.8°/ 00 across the
Arabian Sea and is consistent with the modern SST
gradient caused by coastal upwelling off Arabia. Th us, a
proper geographie array ofsynoptic samples can be used
to reconstruct the ô18 0 gradient and bence thermal
gradients which are the result of paleo-upwelling. We
stress that Jinear regression of a large geographie array of
samples minimizes local variations, and is the best
estimate of ilô 18 0 across basins or water masses.
3) The ô 13 C composition of planktonic foraminifera
shows no significant correlation with sea surface
temperature for any species. Only G. ruber ô13 C is
significantly correlated with phosphate content. Since
our samples span one of the strongest upwellingnutrient-productivity gradients in the world ocean, we
conclude that only a small portion of the ô13 C variation
observed in planktonic foraminifera in deep-sea
sediments reflects local upwelling, productivity, and
nutrient variations.
Implications
The Jack of a foraminiferal ô13 C gradient within the
surface waters of the Arabian Sea suggests severa!
possibilities concerning the relationship between carbon
isotopie composition, associated foraminifera, and
upwelling. First, the waters upwelled with depleted
carbon signais ( < 2°1 00 ) are small in volume compared
with the surrounding surface waters and are quickly
diluted, minimizing their effect on the ô13 C content of
surface waters. Equilibration with atmospheric COz
would further dam pen the potential ô13 C signal. Second,
the potential ô13 C signal of2°/ 00 cannot be transmitted
to the surface waters because of mixing and biological
fixation. Since the phosphate content at 100 to 200 rn is
2 11g-at/l and the surface values within the upwelling
zone are approximately 1 11g-at/l (Wyrtki, 1971), at least
half of the potential ô13 C signal is removed by biologica1
fixation as water upwells from the oxygen minima zone.
This would result in a potential near-surface signal of
ô 13 C of only 1°1 00 across the Ara bian Sea. Third, the
large carbon disequilibrium vital effects in sorne species
may mask the externat carbon reservoir or that the vital
effects are not constant within species.
4) Similar glacial-interglacial differences of ô13 C (0.7 to
1.0°100 ) have been reported for shallow-dwelling
foraminifera from widely separated locations: the
Eastern Atlantic (G. ruher, Berger et al., 1978 a), the
Western equatorial Pacifie (G. sacculifer. Berger et al.,
97
W. L. PRELL. W. B. CURRY
Acknowledgements
1978 b), the South Atlantic (G. sacculifer and G. ruber,
Shackleton, 1977), and numerous Indian Ocean cores
(our unpub1ished data). We interpret the pattern of
depleted glacial ô 13 C and enriched Holocene ô 13 C to be a
global phenomenon in planktonic foraminifera as weil as
in benthics (Shackleton, 1977). The similarities in
amplitude and timing of this event over a wide area are
unlikely to result from variations of local upwelling
intensity. A more general mechanism is required to
explain the amplitude and timing of stratigraphie ô13 C
variation. This other factor may be variation in the ô13 C
composition of atmospheric co2 caused by variations in
terrestrial biomass. Such variations would equilibrate
with the surface ocean, and thereby be transmitted to
shallow-dwelling planktonic foraminifera.
We thank R. K. Matthews for his interest and
cooperation in this project, and for his review of the
manuscript. Ail isotopie determinations were made in the
Benedum Stable Isotope Laboratory at Brown University. This research was supported by National Science
Foundation grant ATM77-07755. Core-top samples
were curated and provided by Lamont-Doherty
Geological Observatory (NSF grant OCE78-25448;
ONR contract N00014-75-C-0210) and Woods Hole
Oceanographie Institution [NSF 20/25231 (OCE 79);
ONR contract N00014-C-0262].
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98