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The influence of the SW monsoon on the deep-sea organic carbon cycle in the Holocene
Deep-Sea Research II 47 (2000) 2629}2651
The in#uence of the SW monsoon on the deep-sea organic carbon cycle in the Holocene T. Rixen*, V. Ittekkot, B. Haake-Gaye, P. SchaK fer Institut fu( r Biogeochemie und Meereschemie, Universita( t Hamburg, Bundesstr. 55, D-20146 Hamburg, Germany Received 5 July 1999; received in revised form 18 January 2000; accepted 19 January 2000
Abstract Results from long-term sediment trap experiments carried out since 1986 in the western, central, and eastern Arabian Sea are combined with satellite-derived wind "elds and paleoceanographic information to link the intensity of the SW monsoon to organic carbon #uxes and its preservation in sediments. The SW monsoon is characterized by the low-level jet (Findlater Jet) that crosses the Arabian Sea almost parallel to the Arabian coast. The intensity of the Findlater Jet mainly controls the velocities of upwelling that occurs to the northeast of the jet. Since up welling, in turn, mainly governs the organic carbon #uxes in the western Arabian Sea, variation in the strength of the Findlater Jet is the dominant factor determining the organic carbon #uxes on seasonal time scales. Changes in the subsurface nutrient concentrations due to variations in the surface ocean current systems seem to be another factor in#uencing the organic carbon #uxes, mostly on interannual time scales. The translation of sedimentary organic carbon burial rates into organic carbon #uxes according to Jahnke (1996). Global Biogeochemical Cycles 10, 71}88) allows us to extend our re#ections also to a millennium time scale. This indicates that changes in the SW monsoon intensity as observed during the last decade could almost account for the range of organic carbon #uxes deciphered from the Holocene record. 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction The Arabian Sea is strongly in#uenced by the monsoon, which is a seasonally changing climate system driven by the summer heating and winter cooling of the Asian landmass (e.g., Ramage, 1971; Tchernia, 1980). The summer heating forms
* Corresponding author. E-mail address: [email protected] (T. Rixen). 0967-0645/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 4 5 ( 0 0 ) 0 0 0 4 2 - 4
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Fig. 1. Sediment trap locations in the western (WAST), central (CAST) and eastern Arabian Sea (EAST) and the position of core 74 KL.
a strong atmospheric low that attracts the SE trade winds. These winds cross the equator and blow as SW winds (SW monsoon) over the Arabian Sea, where they form a low-level (Findlater) Jet extending almost parallel to the Arabian coast (Findlater, 1966, 1969; Legler et al., 1989). During winter, the sea-level pressure over Asia rises above that at the Equator, which results in moderate NE winds (NE monsoon). The monsoon winds reverse the surface ocean currents (Wyrtki, 1973) and produce a divergence extending from the Arabian coast to the centre of the Findlater Jet during the summer (Sastry and de Souza, 1972; Smith and Bottero, 1977; Luther et al., 1990; Brock et al., 1991). The upwelling of nutrient-rich subsurface water leads to a high biological production during the SW monsoon (Ryther and Menzel, 1965; Qasim, 1977). During the NE monsoon a second maximum in the biological production occurs (e.g., Caron and Dennett, 1999), due to winter cooling that drives convective mixing and thus the injection of nutrients into the euphotic zone (Banse and McClain, 1986; Madhupratap et al., 1996). The seasonality of the organic matter #ux collected by sediment traps in the deep western, central and eastern Arabian Sea (Fig. 1, Nair et al., 1989; Haake et al., 1993; Rixen et al., 1996) follows the monsoon-driven pattern of biological production, with organic carbon #ux maxima during the SW and the NE monsoons (Fig. 2). In contrast to the northern, central, and eastern Arabian Sea, organic carbon #uxes in the western Arabian Sea are much higher during the SW monsoon than during the NE monsoon. The high organic carbon #uxes during the SW monsoon into the deep western Arabian Sea have been shown to be related to the intensity of coastal upwelling occurring between Ras Fartak and Salalah (Fig. 1) and the upwelling established in the open ocean between the coast and the sediment trap site (Rixen et al., 2000a). Besides sediment trap experiments, deep-sea organic carbon #uxes have been derived globally by data on carbonate-corrected organic carbon burial rates and oxygen consumption (Jahnke, 1996). The latter, indicating benthic organic carbon
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Fig. 2. The mean annual organic carbon #ux record in the western, central, and eastern Arabian Sea (all data on organic carbon #uxes obtained during the period of observations between 1986 and 1995 are averaged and presented as biweekly means).
remineralization rates, so far have been measured only in the Paci"c and Atlantic Oceans. However, to validate these "ndings we used results from the Arabian Sea obtained by US JGOFS (Lee et al., 1998) as well as by our own sediment trap experiments. These results have been transferred to the geological record derived from sediment core 74 KL (Fig. 1). Subsequently, this information has been combined with our results concerning the link between organic carbon #uxes, upwelling velocities, and wind speeds. This allows us to investigate the role of the Findlater Jet for the deep-sea carbon cycle on seasonal, interannual, and geological time scales.
2. Methods 2.1. Wind xelds and wind stress Weekly averaged wind "elds on a 0.253 grid are derived from measurements of the special sensor microwave imager (SSM/I) for the period between July 1987 and December 1996. The algorithm used here is described in detail by SchluK ssel (1995). To obtain the wind speeds at the sediment trap site in the western Arabian Sea, wind speeds were averaged for an area of 1;1 degree square around the sediment trap location. Gaps in the wind speed record (e.g., between 1986 and July 1987) are "lled with wind speeds derived from ship observations (Rixen et al., 1996; Fig. 3) not included in the statistical analysis. The wind speeds at the trap site are used to de"ne the beginning and the end of the meteorological SW monsoon: Thus, the SW monsoon starts in May/June when the wind speeds at the trap site rise above their long-term average (5.1 m s\, calculated for the years 1987}1992; Rixen et al., 1996)
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Fig. 3. Time series of weekly averaged wind speeds and the measured organic carbon #uxes at 3000 m water depth at the sediment trap site in the western Arabian Sea (WAST). Filled circles indicate data points. Broken lines show wind speeds derived from ship observation and organic carbon #uxes measured at 2000 m water depth. both data sets are not included in the statistical analysis. Vertical thin, broken lines represent the time when the wind speeds rise above and drop below the long-term mean of 5.14 m s\. In contrast to our previous work, the beginning of the SW monsoon in the deep sea was predated to the initial increase in organic carbon #uxes which is also marked by the vertical lines. The dates which are represented by the vertical thin lines are given in Table 1.
and ends in September/October when the wind speeds drop below their average, respectively (Fig. 3; Table 1). Surface buoys deployed in the year 1995 close to our sediment trap site in the western Arabian Sea revealed persistent SW winds during the SW monsoon (Rudnik et al., 1997). Thus, this wind direction was used for calculating the wind stress (q"c o "u " ) "v ") during the SW monsoons. The drag coe$cient is c , the density of air is o , the x and y components of the wind stress are u and v . The drag coe$cient was assumed to be 0.0011 when u (7.7 m s\. When u *7.7 m s\, the c was calculated as follows: c "(0.61#0.063;u )/1000 (Smith, 1980; Gill, 1982). 2.2. Upwelling velocities Wind stress acting on the surface ocean causes up- and downwelling. The up- and downwelling velocities can be described as Ekman pumping where positive values
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Table 1 Beginning, end and duration of the meteorological SW monsoon over the western Arabian Sea and in the deep sea. De"nitions are given in the text. The dates listed in the table indicate the position of the vertical line in Fig. 3 and note that the wind speeds are measured weekly and the organic carbon #ux is measured in time intervals of 12}27 d (compare Table 2) SW monsoon
Meteo. start
Meteo. end
Duration (d)
Deep sea start
Deep sea end
Duration (d)
1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996
June June May May May May June May May June June
Sep. 17 Sep. 17 Sep. 21 Sep. 22 Oct. 03 Oct. 05 Sep. 20 Oct. 03 Sep. 14 Sep. 22 Oct. 03
105 107 114 122 134 129 100 132 107 109 123
June 22 June 14 June 27 No data June 22 June 31 June 18 June 13 June 14 July 09 No data
Sep. 07 Oct. 23 Oct. 29 No data Oct. 18 Oct. 17 Oct. 26 Oct. 16 Sep. 05 Oct. 21 No data
78 132 125
05 03 30 23 22 29 13 24 30 06 03
119 109 131 126 84 104 No data
reveal upwelling and negative values downwelling:
1 * q * q W ! V w " o *x f *y f
.
(1)
The meridional and zonal components of the wind stress are q and q , where x and W V y are the horizontal distances, f is the Coriolis parameter, and o is the density of sea water. To determine upwelling at the coast, Eq. (1) can be simpli"ed because of the restricted water mass circulation at the land}ocean boundary:
1 1 q u " o x f
.
(2)
q is the component of wind stress parallel to the coast. Since the error probability of satellite-derived wind speeds is higher at the land}ocean boundary than in the open ocean, we averaged the wind stress for an area extending roughly 83 km o!shore (three grid points) before it was used for Eq. (2). 2.3. Sediment trap and core samples Since 1986, Mark 6 and 7 sediment traps (Honjo and Doherty, 1988) have been deployed at water depths of approximately 3000 m in the western (16320 N, 60330 E), central (14331 N, 64346 E) and eastern Arabian Sea (15331 N 68343 E) (Fig. 1). Sampling intervals range between 14 and 26 d (Table 2). Prior to deployment the sampling cups are "lled with sea water collected at the mooring depth and enriched with sodium (35 g l\) and mercury (II)-chloride (3.3 g l\) to prevent degradation of
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Table 2 Station, position, trap depth, water depth, deployment periods, and mean sampling intervals. Station
Latitude
Longitude Trap depth (m)
Water depth (m)
Deployment start
Deployment end
Interval (d)
Wast-01 Wast-02 Wast-03 Wast-04 Wast-05 Wast-06 Wast-07 Wast-08 Wast-09 Wast-10 Wast-11
16323 N 16318 N 16319 N 16335 N 16324 N 16319 N 16320 N 16319 N 16320 N 16320 N 16313 N
60332 E 60328 E 60328 E 60328 E 60329 E 60331 E 60332 E 60330 E 60319 E 60318 E 60326 E
4020 4018 4010 4016 4016 4013 4027 4018 4018 4025 4230
May 10, 1986 Nov. 19, 1986 May 12, 1987 Nov. 22, 1987 Nov. 20, 1988 Jan. 15, 1990 Dec. 19, 1990 Jan. 16, 1992 Dec. 25, 1992 Oct. 18, 1993 Jan. 15, 1995
Oct. 26, 1986 May 1, 1987 Oct. 21, 1987 Oct. 31, 1988 Sept. 28, 1989 Oct. 15, 1990 Oct. 26, 1991 Dec. 16, 1992 Oct. 11, 1993 Oct. 05, 1994 Oct. 03, 1995
13.0 12.6 12.5 26.5 24.0 21.0 24.0 26.0 23.0 27.0 12.0
3024 3021 3033 3039 3029 3016 3039 3030 2002 3041 3246
trapped material (Lee et al., 1992). After recovery, the samples are sieved into '1 and (1 mm fractions. The wet sample material is "ltered on preweighed nuclepore "lter (0.4;10\ m) and dried at 403C. The dry weights of the (1 mm fraction are used for calculating the total #uxes, and the material itself is crushed prior to chemical analysis. Samples severely disturbed, e.g., by malfunctioning of the traps or the activity of swimmers, are ignored. The gravity core KL74 was taken in 1986 during the SONNE cruise 42 and subsampled in 0.5-cm intervals. Sedimentation rates, dry bulk density and the calibrated ages are obtained from Sirocko (1995, 1996) and Sirocko et al. (1993). 2.4. Chemical analysis Total carbon and nitrogen were measured on a Carlo Erba Nitrogen Analyzer 1500, a catalytic high-temperature combustion method with a subsequent thermal conductivity detector for measuring CO and N in a constant helium #ow. The reproducibility of this method is $0.05% for total carbon and $0.02% for nitrogen. Carbonate was measured with a WoK stho! Carmograph 6. With this method the conductivity change of a sodium hydroxide solution is measured while injecting CO released from the acid-treated sample. The mean standard deviation of this method is $1%. The di!erence between total and carbonate carbon is regarded as organic carbon. A more detailed description of the methods is given by Haake et al. (1993). 2.5. Mean deep-sea yuxes at the sediment trap site in the western Arabian Sea The initial increase of organic carbon #ux marks the beginning of the SW monsoon approximately 2}7 weeks after it is seen in wind speeds (Table 2, Fig. 3). Generally, the #uxes decrease 2}3 weeks after the end of the meteorological SW monsoon. The SW
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Fig. 4. The mean SW monsoon wind speeds over the Arabian Sea averaged for all SW monsoons between 1987 and 1996. The white circle represent the sediment trap sites WAST, CAST, and EAST and the white square the location of core 74 KL. The white line shows the mean position of the Findlater Jet axis.
monsoons 1986 and 1994 seem to have been exceptions, since the #uxes decreased 2 weeks prior to the end of the meteorological SW monsoon. To calculate the mean SW monsoon #uxes we integrated the #uxes over the period these #uxes were elevated (Fig. 3). The exact dates are given in Table 1. The calculation of annual #uxes is problematic when the observations do not cover the whole year. Therefore, observation gaps in the organic carbon #ux record (Fig. 3) were "lled with data extracted for the speci"c time from the mean annual organic carbon #ux curve (Fig. 2). Since organic carbon #uxes, wind speeds and, thus, upwelling velocities are obtained on di!erent time intervals all are interpolated on the same biweekly intervals for statistical analysis.
3. Results and discussion 3.1. The Findlater Jet The Findlater Jet is the dominant atmospheric feature of the SW monsoon (Fig. 4). The area of maximum wind speed, de"ned as the axis of the Findlater Jet, varies from year to year (Fig. 5a). The southernmost position was determined for the SW monsoon 1989 and the northernmost position could be observed during the SW
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Fig. 5. (a) Mean location of the Findlater Jet axis during the SW monsoons 1987}1996. The axis represents the meridional wind speed maxima which were zonally smoothed with a 23 moving average. The broken lines show the mean location of the axis during the SW monsoons 1989 (southernmost position) and 1987 (northernmost position). Bold lines represent the mean location of the axis, and dots the position of the sediment traps (compare with Fig. 4). (b) Mean distance between the sediment trap location (WAST) and the axis of the Findlater Jet versus the mean SW monsoon wind speeds at the trap site, (c) mean SW monsoon wind speed averaged for the Findlater Jet axis west of 653E (wind speed ) and those averaged at $ the sediment trap site (wind speed ). Open circles show the data obtained during the SW monsoons 5 12 1987 and 1989 (see text for futher explanations).
monsoon 1987. There is, however, a lack of satellite-derived wind "elds between July and September 1989, which covers the peak of the SW monsoon, and some data are missing at the beginning of the SW monsoon 1987 (Fig. 3). Ignoring the SW monsoons 1987 and 1989, the location of the axis varies interannually with a meridional extension of approximately 100 km at 603E (Fig. 5a). The 9-yr record on satellitederived wind speeds at the sediment trap site in the western Arabian Sea yields an interannual variation of SW monsoon wind speeds of !10 and #17%. These variations reveal no correlation with the distance between the trap site and the jet axis (Fig. 5b). Since the wind speeds at the trap site correlate with the wind speeds averaged
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Fig. 6. Mean SW monsoon Ekman pumping velocities. Negative values indicate downwelling and positive values upwelling. The white circle shows the trap site (WAST) and the white square the core site (74 KL). The lines delineate the area for calculation of average Ekman pumping (see Fig. 7 and text for explanation).
at the jet axis east of 653E (Figs. 5c), it is assumed that the wind speeds in the western Arabian Sea are mainly controlled by the strength of the Findlater Jet. 3.2. The link between the Findlater Jet and the upwelling velocities The satellite-derived wind"elds can be converted into up- and downwelling velocities assuming a wind direction of SW during the SW monsoon. This provides weekly distribution of up- and downwelling velocities in the western Arabian Sea, which show a downwelling region to the northeast of the Findlater Jet axis (Fig. 6). This downwelling region stretches almost parallel to the Arabian coast and is caused by the wind shadow produced by Socotra Island (compare Fig. 4). It separates two areas of enhanced upwelling, one extending along the Arabian coast and the other one occurring further o!shore in the open ocean. Open-ocean up- and downwelling velocities averaged for the area between Salalah and the sediment trap site as indicated in Fig. 6 correlate with the wind speeds at the trap site (Fig. 7a; r"0.82). The regression equation is w "0.115WS!0.613, (3) where w is the open-ocean upwelling velocity in 10\ m s\, and WS the wind speeds in m s\. This relationship shows that upwelling dominates downwelling in the open ocean when the wind speeds exceed 5.3 m s\. This explains our former observations
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Fig. 7. (a) Wind speeds at the sediment trap site in the western Arabian Sea versus Ekman pumping velociites averaged for the area marked in Fig. 6 and (b) versus Ekman pumping velocities averaged for the coastal area o! Salalah.
that the sea-surface temperature decreases, when the wind speed crosses its long-term average during the opening phase of the SW monsoon at the sediment trap site (Rixen et al., 1996). The coastal upwelling velocities o! Salalah correlate also with the wind speeds at the trap site (Fig. 7b), and the regression equation is u "0.242WS!0.955, (4) where u represents the coastal upwelling velocities in 10\ m s\. Although the resulting correlation coe$cient (r"0.66) is weaker than that obtained by the
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correlation between the wind speeds at the trap site and open-ocean upwelling (Fig. 7a), it implies that the strength of the Findlater Jet is an important factor controlling coastal upwelling velocities. 3.3. The link between upwelling velocities and organic carbon yuxes The organic carbon #uxes measured during the SW monsoons in the western Arabian Sea correlate with coastal upwelling velocities o! Salalah and the upwelling velocities averaged for the region between Salalah and the trap site (Fig. 8). The time-lags considered for the correlations between organic carbon #uxes and coastal as well as open-ocean upwelling are 8 and 2 weeks, respectively (see Rixen et al. (2000a) for further explanations). To combine the e!ects of coastal and open-ocean upwelling on organic carbon #uxes, a multiple regression analysis was carried out (Fig. 9a). The multiple regression equation is POC"(16.85w )#(3.53u )#0.67,
(5)
where POC represents the organic carbon #ux in mg m\ d\. The resulting correlation coe$cient of 0.82 implies that upwelling explains roughly 82% of the observed variation in the organic carbon #uxes during the SW monsoons. This does not hold for the SW monsoon 1991, which exhibited the highest mean SW monsoon wind speeds during the study period (10.3 m s\, compare Fig. 5b). The second highest mean SW monsoon wind speeds (9.5 m s\) were observed during the SW monsoon 1994. The organic carbon #uxes measured during this SW monsoon can be divided into two groups: the peak #uxes "t into the overall trend, and the other data match with the 1991 data. The latter and the 1991 data also correlate with the coastal and open-ocean upwelling velocities (Fig. 9b) but with the negative open-ocean upwelling term in the multiple regression equation: POC"(!1.65w )#(3.89u )#0.035.
(6)
Thus, higher open-ocean upwelling velocities seem to decrease the organic carbon #uxes during stronger SW monsoons. This appears to be accompanied by a reinforced Somali Current (Rixen et al., 1996), which carries Indo-Paci"c through #ow and South Indian Ocean water into the western Arabian Sea (You, 1997). These nutrientpoor and well-oxygenated water masses have been suggested to ventilate the Arabian Sea (Conkright et al., 1994; You, 1997). An associated reduction in the subsurface nutrient concentration has been proposed to be the reason for the reduced fertilization e!ect of upwelling and subsequent lower #uxes during stronger SW monsoons (Rixen et al., 1996). 3.4. The link between the Findlater Jet and the organic carbon yux The relation between upwelling velocities and organic carbon #uxes can be described by two equations: one valid for weak to moderate (Fig. 9a, Eq. (5)) and the
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other one for strong SW monsoons such as the SW monsoons 1991 and 1994 (Fig. 9b, Eq. (6)). Since upwelling velocities can be estimated from the wind speeds at the trap site (Fig. 7, Eqs. (3) and (4)), the latter could be used to estimate the organic carbon #uxes. To validate this "nding we converted the biweekly averaged, satellite-derived time series on SW monsoon wind speeds (Fig. 10, Table 1) at the trap site into upwelling velocities (Eqs. (3) and (4)) and further into organic carbon #uxes (Eqs. (5) and (6)). The calculated organic carbon #uxes correlate with the biweekly averaged organic carbon #uxes measured at the trap site (WAST). The correlation coe$cient of 0.63 (Fig. 11) indicates that changes in the strength of the Findlater Jet explain 63% of the observed variation in organic carbon #uxes.
3.5. The link between organic carbon yux and sedimentary organic carbon burial rates The organic carbon burial rates derived from surface sediments beneath the US JGOFS sites and the sediment trap locations in the western, central, and eastern Arabian Sea have been published by Lee et al. (1998) and Haake et al. (1993). The SW monsoon organic carbon #ux measured in 1995 at the long-term sediment trap site in the western Arabian Sea (WAST) is 11% higher than mean SW monsoon #ux derived from the 8-yr record. Therefore, the annual #uxes given by Lee et al. (1998) are reduced by 11% to adjust it to the long-term mean before all annual organic carbon #uxes are compared with the sedimentary organic carbon burial rates (Table 3; Fig. 12). The sediment and sediment trap data obtained from the Arabian Sea reveal, similar to other areas (Hinrichs, 1992), generally increasing organic carbon burial and remineralization rates associated with increasing organic carbon #uxes (Fig. 13). A similar trend also was observed by Witte and Pfannkuche (2000) who measure the sediment in situ community oxygen consumption at di!erent sites in the western, northern, central, eastern, and southern Arabian Sea during the end of the SW monsoon 1995 as well as at the onset and the end of the late NE monsoon bloom in 1998 and 1997, respectively (Rixen et al., 2000b). To translate organic carbon remineralization into benthic oxygen consumption is generally problematic due to the often unknown contribution of organically bound oxygen to the decomposition of organic matter and the varying C/N ratios therein (Anderson and Sarmiento, 1994; Martin et al., 1987). The nitrogen remineralization rate was determined in a way similar to organic carbon remineralization rate at the sediment trap site in the western, central and eastern Arabian Sea (Table 3). Furthermore, it was assumed that the remineralized nitrogen was bound in proteins that have mean C/O and C/N ratios of 3.36 and 3.05, respectively (Libes, 1992). The carbon not bound in proteins was assumed to occur as carbohydrate with a C/O ratio of 1. According to the equation Cu(H O) (NH ) #yO PuCO #xNO the mean X V ratio of organic carbon to oxygen demand (C/O ) is 0.61 which is lower than suggested by Anderson and Sarmiento (1994; 0.69) and Red"eld et al. (1963; 0.77). Deep-sea oxygen consumption measured in the Paci"c and Atlantic Oceans correlate with the calcium carbonate corrected organic carbon burial rates (COB; Jahnke, 1996). The correction accounts for the in#uence of the bulk accumulation rates on the
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Fig. 8. Ekman pumping velocities averaged for the area marked in Fig. 6(a) and averaged o! Salalah (b) versus organic carbon #uxes measured at the sediment trap site in the western Arabian Sea. Ekman pumping velocities and carbon #uxes are interpolated on the same biweekly intervals. Time lags of 2 and 8 weeks are considered for open-ocean and coastal upwelling, respectively.
organic carbon burial rate: Acc !Acc POC, COB" 100
(7)
where Acc is the total accumulation rate, Acc represents carbonate accu mulation rate and POC indicates the contribution of organic carbon to the total sediment. The regression equation is O "0.143 COB ,
(8)
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Fig. 9. (a) Organic carbon #uxes measured at the sediment trap site in the western Arabian Sea during the SW monsoon 1987}1995 plotted against calculated organic carbon #ux. The multiple regression equation used for estimating organic carbon is given as Eq. (5) in the text. (b) Organic carbon #uxes measured at the trap site during the SW monsoon 1991 and the exceptional periods of the SW monsoon 1994 versus calculated organic carbon #uxes. The multiple regression equation used for calculating organic #ux is given as Eq. (6) in the text. Ekman pumping velocities and carbon #uxes are averaged on the same biweekly intervales. Time lags of 2 and 8 weeks are considered for open-ocean and coastal upwelling, respectively.
where O represents the oxygen consumption in mol m\ yr\, and COB is given in mmol C m\ yr\. Although change in the bottom-water content is also a factor in#uencing the organic carbon accumulation rates in shelf and slope sediments (e.g., Paropkari et al., 1993), this may not play a major role in the deep open western
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Fig. 10. Time series of biweekly averaged wind speed (broken line). Bold lines indicate the biweekly averaged SW monsoon wind speeds increased by 50% and decreased by 25% used for calculating the organic carbon #ux rates.
Fig. 11. Biweekly averaged organic carbon #uxes measured at 3000 m water depth in the western Arabian Sea versus calculated organic carbon #uxes. To calculate the organic carbon #uxes we used the biweekly averaged SW monsoon wind speeds at the trap site as shown in Fig. 10, and Eqs. (3)}(6).
Arabian Sea. Thus the organic carbon #ux in g m\ yr\ can be estimated as follows: POC "+([0.142COB ]0.61)#[COB/1000],12.01. (9) $ Eqs. (7) and (9) imply an increasing organic carbon burial and remineralization rate due to increasing organic carbon #uxes. The extent to which the burial and
3000 3000 2800 999 3150 2979 3484 3915
4000 3900 3700 1435 3647 3478 4004 4426
Water depth (m) 2.91 2.17 2.12 3.82 4.33 4.25 2.88 1.07
POC (g m\ yr\) 0.60 0.12 0.13 1.00 0.43 0.57 0.14 0.03
POC (g m\ yr\) 2.31 2.05 1.99 2.82 3.90 3.68 2.74 1.04
POC (g m\ yr\) 0.38 0.28 0.26 No data No data No data No data No data
PON (g m\ yr\)
0.07 0.02 0.02 No data No data No data No data No data
PON (g m\ yr\)
0.31 0.26 0.24 No data No data No data No data No data
PON (g m\ yr\)
The annual organic carbon #uxes measured at the US sites are reduced by 11% to adjust it to the long-term mean. The sedimentary burial rates at the locations WAST, CAST and EAST are derived from Haake et al. (1993).
WAST CAST EAST M1 M2 M3 M4 M5
Trap depth (m)
Table 3 Trap and water depth as well as annual organic carbon and nitrogen #uxes, sedimentary burial rate and remineralization rates at the sediment trap sites in the western (WAST), central (CAST) and eastern Arabian Sea (EAST) and at the US JGOFS locations M1}M4. The US JGOFS data obtained between 1994 and 1995 are from Lee et al. (1998)
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Fig. 12. (a) Location of the sediment trap sites, with grey dots representing the US JGOFS sites (Lee et al., 1998) and black dots the long-term stations (WAST, CAST, EAST) which have been under the umbrella of the German JGOFS since 1995. The small grey dot shows the location of the sediment core 74 KL. (b) Annual organic carbon #uxes versus the distance to the coast. The data from the US JGOFS site as listed in Table 2 are reduced by 11% (see text for explanation). The small grey dot represents the mean annual organic carbon #ux derived from the bulk accumulation rate, the carbonate and organic carbon content (COB) of the uppermost sampling interval of 74 KL.
remineralization rates increase depends on the carbonate content, which varies in the deep Arabian Sea roughly between 40 and 80% (Fig. 13; Kolla et al., 1981). Taking this into account, the data obtained at all sediment trap sites lie almost in the range given by the Eqs. (7) and (9), which suggests that Jahnke's "ndings are also valid for the Arabian Sea. Thus, Eqs. (7) and (9) have been used to convert organic carbon burial rates obtained from the Arabian Sea sediment core into organic carbon #uxes.
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Fig. 13. Mean annual organic carbon #uxes versus organic carbon remineralization (grey circles) and organic carbon burial rates (black circles). The data are given in Table 3. The lines show the relationship between organic carbon #ux and organic carbon remineralization (dotted), and organic carbon burial (bold) for carbonate contents of 40 and 80% (Eqs. (7) and (9)).
3.6. Holocene variations A sediment core (74 KL) was recovered slightly north to the axis of the Findlater Jet in the western Arabian Sea (Figs. 1, 4 and 5). Based on oxygen isotopes, varying contribution of calcium carbonate and dolomite to the bulk sediment of 74 KL, Sirocko et al. (1993 and references therein) have described variations in the monsoon strength over the last 24,000 yr. The most pronounced feature in this record is that organic carbon burial rates for the last glacial maximum (20}21 ka BP) were more than thrice as high as during the late Holocene (0.4 ka BP, Fig. 14). The latter is the organic carbon burial rate of the uppermost sampling interval (0.37 g C m\ yr\). However, due to the extreme environmental di!erences, e.g., lower sea level and associated changes in the coastline and surface current system etc., it might be questioned whether modern results could be transferred to glacial conditions. Therefore, we restricted the following discussion to the Holocene (11.6}0.4 ka BP). Sirocko et al. (1993) identi"ed a phase of strong SW monsoons occurring between 9.9 and 8.6 ka BP and a phase of weak SW monsoons between 5.6 and 5.2 ka BP. Our data show that the Holocene period of strong SW monsoons is characterized by organic carbon burial rates up to 75% higher than during the late Holocene, and 40% lower during the Holocene period of weak SW monsoons (Fig. 14). The organic carbon burial rates are transformed into COB and then into organic carbon #ux according to Eqs. (7) and (9), respectively. Since the decomposition of organic matter in the sediments is not considered, the derived organic carbon #ux rates are under- rather than overestimates. However, the resulting late Holocene organic carbon #ux of 3.5 g m\ yr\ is
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Fig. 14. Organic carbon burial rates and #uxes and their deviations from the late Holocene values in core 74 KL. The organic carbon #uxes have been determined according to Eqs. (7) and (9). Horizontal broken lines mark the periods of strong (8.6}9.9 ka BP) and weak SW monsoons (5.2}5.6 ka BP; Sirocko, 1995).
well within the range measured at di!erent sites in the Arabian Sea (Table 3) and "ts the value we would expect due to the distance between the coast and the core site (Fig. 12b). This implies that the applied method produces reliable organic carbon #ux rates and suggests that the organic carbon #uxes had varied between !23 and 43% during Holocene (Fig. 14). Changes in the same order at the trap site in the western Arabian Sea would result in organic carbon #ux rates ranging between 2.2 and 4.2 g m\ yr\. This range is almost covered by the mean annual organic carbon #uxes, which vary between 2.3 and 3.6 g m\ yr\ during the 8 years of observations in the western Arabian Sea. 3.7. The link between the SW monsoon and the organic carbon burial rates In the following, we consider the extent to which Holocene variations of organic carbon #uxes from 2.2 to 4.2 g m\ yr\ could be related to past changes in the SW monsoon wind speeds at the trap site in the western Arabian Sea. Therefore, the established links between wind speeds at the trap site, upwelling velocities (Eqs. (3) and (4)) and organic carbon #uxes (Eq. (5)) were used. In contrast to our earlier approach (compare Fig. 11), all data on wind speeds from 1986 to 1996 have been integrated and decreased by not more than 25% (Fig. 10). This reduces the decadal mean SW monsoon wind speeds at the trap site from approximately 9 to 7 m s\
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Fig. 15. Relationship between the SW monsoon wind speed at the trap site in the western Arabian Sea and mean SW monsoon and annual organic carbon #uxes according to Eqs. (3)}(5) (bold line). Broken line indicates the same but Eq. (5) is replaced by Eq. (6) (see text for further explanation).
(Fig. 15). As monthly mean wind speeds exceeding 12 m s\ occur only in the Southern Ocean and during winter in the North Atlantic (SchluK ssel, 1995), higher wind speeds appear unrealistic for our model experiment. Therefore, we increased the wind speed by less than 50%, which leads to a mean SW monsoon wind speed of less than 12 m s\. Within our model experiments, the duration of the upwelling season during the SW monsoon is de"ned as the time between May and October, when the wind speeds are above 5.3 m s\. Thus, increasing and decreasing of the wind speeds could extend and reduce, respectively, the duration of the upwellling season by less than 6 weeks (compare Fig. 10). These changes are considered when the calculated biweekly organic carbon #uxes are summarized to average SW monsoon and further to a decadal mean SW monsoon #ux. Thus, a weakening of the SW monsoon wind speed by 25% corresponds to a decadal mean SW monsoon #ux of 0.8 g m\. Increasing SW monsoon wind speed raises the mean SW monsoon #ux up to 2.4 g m\. Adding to these SW monsoon #uxes the mean NE and intermonsoon organic carbon #ux obtained by our sediment trap experiments in the western Arabian Sea (1.56 g m\) results in annual organic carbon #uxes between 2.4 and 4 g m\ yr\. This range implies that the Holocene record of organic carbon #uxes almost could be explained by changes in the intensity of the SW monsoon wind speeds. Integrating changes in the subsurface nutrient regime as suggested for stronger SW monsoons into our re#ections (replacing Eq. (5) by Eq. (6)) means SW monsoon wind speed of 12 m s\ would be associated with a mean annual organic carbon #ux
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of 3.5 g m\ yr\, which, considering the uncertainties in the estimates, is similar to the deciphered Holocene values.
4. Conclusion The evaluation of the satellite-derived wind "elds and sediment trap data reveals that the strength of Findlater Jet is the dominant factor controlling organic carbon #uxes on seasonal time scales. Additionally, changes in the subsurface nutrient concentration due to changes in the surface current system seem to be another factor in#uencing the organic carbon #uxes. Its e!ect on organic carbon #uxes is most pronounced on an interannual time scale. Since the link between organic carbon #uxes and sedimentary burial based on data from the Atlantic and Paci"c oceans seems to be valid also for the Arabian Sea, we extended our study to the geological record. On this time scale changes in the intensity of the Findlater Jet appear to be su$cient to explain Holocene record on organic carbon #uxes.
Acknowledgements We thank H. Gra{l and P. SchluK ssel for their help in processing the satellite data and Dr. R. Seifert, Dr. A. Suthhof as well as the three anonymous reviewers for their valuable comments. Mrs. I. Jennerjahn is acknowledged for Secretarial assistance. P. Wessel and W.H.F. Smith (Wessel and Smith, 1991) are thanked for providing the generic mapping tools (GMT). The ongoing sediment trap program is being funded by the German Federal Ministry of Education, Science, Research and Technology (BMBF), the German Research Council (DFG), the Council of Scienti"c and Industrial Research (CSIR, New Delhi), and the Department of Ocean Development (DOD, New Delhi).
References Anderson, L.A., Sarmiento, J.L., 1994. Red"eld ratios of remineralization determined by nutrient data analysis. Global Biogeochemical Cycles 8, 65}80. Banse, K., McClain, C.R., 1986. Winter blooms of phytoplankton in the Arabian Sea as observed by the Coastal Zone Colour Scanner. Marine Ecology Progress Series 34, 201}211. Brock, J.C., McClain, C.R., Luther, M.E., Hay, W.W., 1991. The phytoplankton bloom in the northwest Arabian Sea during the southwest monsoon of 1979. Journal of Geophysical Research 96, 20613}20622. Caron, D.A., Dennett, M.R., 1999. Phytoplankton growth and mortality during the 1995 northeast monsoon and spring intermonsoon in the Arabian Sea. Deep-Sea Research II 46, 1665}1690. Conkright, M.E., Levitus, S., Boyer, T.P., Bartolacci, D.M., Luther, M.E., 1994. Atlas of the Northern Indian Ocean. NOAA/NODC, University of South Florida, Washington, DC. Findlater, J., 1966. Cross-equatorial jet stream at low level over Kenya. Meteorological Magazine 95, 353}364. Findlater, J., 1969. A major low-level air current near the Indian Ocean during the northern summer. Quarterly Journal of the Royal Meteorological Society 95, 362}380.
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Gill, A.E., 1982. Atmosphere}Ocean Dynamics. International Geophysics Series, Vol. 30. Academic Press, Inc., New York, 662pp. Haake, B., Ittekkot, V., Rixen, T., Ramaswamy, V., Nair, R.R., Curry, W.B., 1993. Seasonality and interannual variability of particle #uxes to the deep Arabian Sea. Deep-Sea Research I 40, 1323}1344. Hinrichs, S.M., 1992. Early diagenesis of organic matter in marine sediments: progress and perplexity. Marine Chemistry 39, 119}149. Honjo, S., Doherty, K.W., 1988. Large aperture time-series oceanic sediment traps: design objectives, construction, and application. Deep-Sea Research I 35, 133}149. Jahnke, R.A., 1996. The global ocean #ux of particulate organic carbon: areal distribution and magnitude. Global Biogeochemical Cycles 10, 71}88. Kolla, V., Ray, P.K., Kostecki, J.A., 1981. Sur"cial sediments of the Arabian Sea. Marine Geology 41, 183}204. Lee, C., Hedges, J.I., Wakeham, S.G., Zhu, N., 1992. E!ectiveness of various treatments in retarding microbial activity in sediment trap material and their e!ects on the collection of swimmers. Limnology and Oceanography 37, 117}130. Lee, C., Murray, D.W., Barber, R.T., Buesseler, K.O., Dymond, J.D., Hedges, J.I., Honjo, S., Manganini, S.J., Marra, J., Moser, C., Peterson, M.L., Prell, W.L., Wakeham, S.G., 1998. Particulate organic carbon #uxes: compilation of results from the 1995 US JGOFS Arabian Sea process study. Deep-Sea Research II 45, 2489}2501. Legler, D.M., Navon, I.M., O'Brien, J.J., 1989. Objective analysis of pseudowindstress over the Indian Ocean using a direct minimization approach. Monthly Weather Review 117, 709}720. Libes, S.M., 1992. An Introduction to Marine Biogeochemistry. Wiley, New York, 737 pp. Luther, M.E., O'Brien, J.J., Prell, W.L., 1990. Variability in upwelling "elds in the northwestern Indian Ocean, 1, Model experiments for the last 18,000 years. Paleoceanography 5, 433}445. Madhupratap, M., Prasanna Kumar, S., Bhattahiri, P.M.A., Dileep Kumar, M., Raghukumar, S., Nair, K.K.C., Ramaiah, N., 1996. Mechanisms of the biological response to winter cooling in the northeastern Arabian Sea. Nature 483, 549}552. Martin, J.H., Knauer, G.A., Karl, D.M., Broenkow, W.W., 1987. VERTEX: carbon cycling in the northeast Paci"c. Deep-Sea Research I 34, 267}285. Nair, R.R., Ittekkot, V., Manganini, S.J., Ramaswamy, V., Haake, B., Degens, E.T., Desai, B.N., Honjo, S., 1989. Increased particle #ux to the deep ocean related to monsoons. Nature 338, 749}751. Paropkari, A.L., Prakash Babu, C., Mascarenhas, A., 1993. New evidence for enhanced preservation of organic carbon in contact with oxygen minimum zone on the western continental slope of India. Marine Geology 111, 7}13. Qasim, S.Z., 1977. Biological productivity of the Indian Ocean. Indian Journal of Marine Science 6, 122}137. Ramage, C.S., 1971. Monsoon Meteorology. Academic Press, Inc., New York, 269 pp. Rixen, T., Haake, B., Ittekkot, V., 2000a. Sedimentation in the western Arabian Sea: the role of coastal and open-ocean upwelling. Deep-Sea Research II 47, 2155}2178. Rixen, T., Gaye-Haake, B., Ittekkot, V., SchaK fer, P., 2000b. Characteristics of the NE and SW monsoon blooms as indicated by results from long- and short-term sediment trap experiments in the Arabian Sea. Deep-Sea Research II, Submitted for publication. Rixen, T., Haake, B., Ittekkot, V., Guptha, M.V.S., Nair, R.R., SchluK ssel, P., 1996. Coupling between SW monsoon-related surface and deep ocean processes as discerned from continuous particle #ux measurements and correlated satellite data. Journal of Geophysical Research 101, 28569}28582. Red"eld, A.C., Ketchum, B.H., Richards, F.A., 1963. The in#uence of organisms on the composition of sea water. In: Hitt, M.N. (Ed.), The Sea. Wiley, New York, pp. 26}77. Rudnick, D.L., Weller, R.A., Eriksen, C.C., Dickey, T.D., Langdon, J.M., Langdon, C., 1997. Moored instruments weather Arabian Sea monsoons, yield data. EOS 78 117, 120}121. Ryther, J.H., Menzel, D.W., 1965. On the production, composition and distribution of organic matter in the western Arabian Sea. Deep-Sea Research I 12, 199}209. Sastry, J.S., D'Sourza, R.S., 1972. Upwelling and upward mixing in the Arabian Sea. Indian Journal of Marine Science 1, 17}27.
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SchluK ssel, P., 1995. Passive Fernerkundung der unteren Atmosper und der Meeresober#aK che aus dem Weltraum. Habilitationsschrift Ber. A20, Zenturm fuK r Meeres- und Klimaforsch. der UniversitaK t Hamburg, Germany, 175pp. Sirocko, F., 1995. Century-scale events SW monsoon Data, Arabian Sea. IGBP PAGES/World Data Centre-A for Paleoclimatology Data Contribution Series C95-029. NOAA/NGDC Paleoclimatology Program, Boulder Co, USA. Sirocko, F., 1996. Past and present subtropical summer monsoons. Science 274, 937}938. Sirocko, F., Sarnthein, M., Erlenkeuser, H., Lange, H., Arnold, M., Duplessy, J.C., 1993. Century-scale events in monsoonal climate over the past 24,000 years. Nature 364, 322}364. Smith, R.L., Bottero, J.S., 1977. On upwelling in the Arabian Sea. In: Angel, M. (Ed.), A Voyage of Discovery. Pergamon, New York, pp. 291}304. Smith, S.D., 1980. Wind stress and heat #ux over the ocean in gale force winds. Journal of Physical Oceanography 10, 709}726. Tchernia, P., 1980. Descriptive Regional Oceanography, 2nd Edition, Pergamon, Tarrytown, NY, 253 pp. Wessel, P., Smith, W.H.F., 1991. Free software helps map and display data. EOS 72, 441, 445}446. Witte, U., Pfannkuche, O., 2000. High rates of benthic carbon remineralization in the abyssal Arabian Sea. Deep-Sea Research II 47, 2785}2804. Wyrtki, K., 1973. Physical oceanography of the Indian Ocean. In: Zeitzschel, B. (Ed.), The Biology of the Indian Ocean. Springer, New York, pp. 18}36. You, Y., 1997. Seasonal variations of thermocline circulation and ventilation in the Indian Ocean. Journal of Geophysical Research 102, 10391}10422.
The in#uence of the SW monsoon on the deep-sea organic carbon cycle in the Holocene T. Rixen*, V. Ittekkot, B. Haake-Gaye, P. SchaK fer Institut fu( r Biogeochemie und Meereschemie, Universita( t Hamburg, Bundesstr. 55, D-20146 Hamburg, Germany Received 5 July 1999; received in revised form 18 January 2000; accepted 19 January 2000
Abstract Results from long-term sediment trap experiments carried out since 1986 in the western, central, and eastern Arabian Sea are combined with satellite-derived wind "elds and paleoceanographic information to link the intensity of the SW monsoon to organic carbon #uxes and its preservation in sediments. The SW monsoon is characterized by the low-level jet (Findlater Jet) that crosses the Arabian Sea almost parallel to the Arabian coast. The intensity of the Findlater Jet mainly controls the velocities of upwelling that occurs to the northeast of the jet. Since up welling, in turn, mainly governs the organic carbon #uxes in the western Arabian Sea, variation in the strength of the Findlater Jet is the dominant factor determining the organic carbon #uxes on seasonal time scales. Changes in the subsurface nutrient concentrations due to variations in the surface ocean current systems seem to be another factor in#uencing the organic carbon #uxes, mostly on interannual time scales. The translation of sedimentary organic carbon burial rates into organic carbon #uxes according to Jahnke (1996). Global Biogeochemical Cycles 10, 71}88) allows us to extend our re#ections also to a millennium time scale. This indicates that changes in the SW monsoon intensity as observed during the last decade could almost account for the range of organic carbon #uxes deciphered from the Holocene record. 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction The Arabian Sea is strongly in#uenced by the monsoon, which is a seasonally changing climate system driven by the summer heating and winter cooling of the Asian landmass (e.g., Ramage, 1971; Tchernia, 1980). The summer heating forms
* Corresponding author. E-mail address: [email protected] (T. Rixen). 0967-0645/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 4 5 ( 0 0 ) 0 0 0 4 2 - 4
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Fig. 1. Sediment trap locations in the western (WAST), central (CAST) and eastern Arabian Sea (EAST) and the position of core 74 KL.
a strong atmospheric low that attracts the SE trade winds. These winds cross the equator and blow as SW winds (SW monsoon) over the Arabian Sea, where they form a low-level (Findlater) Jet extending almost parallel to the Arabian coast (Findlater, 1966, 1969; Legler et al., 1989). During winter, the sea-level pressure over Asia rises above that at the Equator, which results in moderate NE winds (NE monsoon). The monsoon winds reverse the surface ocean currents (Wyrtki, 1973) and produce a divergence extending from the Arabian coast to the centre of the Findlater Jet during the summer (Sastry and de Souza, 1972; Smith and Bottero, 1977; Luther et al., 1990; Brock et al., 1991). The upwelling of nutrient-rich subsurface water leads to a high biological production during the SW monsoon (Ryther and Menzel, 1965; Qasim, 1977). During the NE monsoon a second maximum in the biological production occurs (e.g., Caron and Dennett, 1999), due to winter cooling that drives convective mixing and thus the injection of nutrients into the euphotic zone (Banse and McClain, 1986; Madhupratap et al., 1996). The seasonality of the organic matter #ux collected by sediment traps in the deep western, central and eastern Arabian Sea (Fig. 1, Nair et al., 1989; Haake et al., 1993; Rixen et al., 1996) follows the monsoon-driven pattern of biological production, with organic carbon #ux maxima during the SW and the NE monsoons (Fig. 2). In contrast to the northern, central, and eastern Arabian Sea, organic carbon #uxes in the western Arabian Sea are much higher during the SW monsoon than during the NE monsoon. The high organic carbon #uxes during the SW monsoon into the deep western Arabian Sea have been shown to be related to the intensity of coastal upwelling occurring between Ras Fartak and Salalah (Fig. 1) and the upwelling established in the open ocean between the coast and the sediment trap site (Rixen et al., 2000a). Besides sediment trap experiments, deep-sea organic carbon #uxes have been derived globally by data on carbonate-corrected organic carbon burial rates and oxygen consumption (Jahnke, 1996). The latter, indicating benthic organic carbon
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Fig. 2. The mean annual organic carbon #ux record in the western, central, and eastern Arabian Sea (all data on organic carbon #uxes obtained during the period of observations between 1986 and 1995 are averaged and presented as biweekly means).
remineralization rates, so far have been measured only in the Paci"c and Atlantic Oceans. However, to validate these "ndings we used results from the Arabian Sea obtained by US JGOFS (Lee et al., 1998) as well as by our own sediment trap experiments. These results have been transferred to the geological record derived from sediment core 74 KL (Fig. 1). Subsequently, this information has been combined with our results concerning the link between organic carbon #uxes, upwelling velocities, and wind speeds. This allows us to investigate the role of the Findlater Jet for the deep-sea carbon cycle on seasonal, interannual, and geological time scales.
2. Methods 2.1. Wind xelds and wind stress Weekly averaged wind "elds on a 0.253 grid are derived from measurements of the special sensor microwave imager (SSM/I) for the period between July 1987 and December 1996. The algorithm used here is described in detail by SchluK ssel (1995). To obtain the wind speeds at the sediment trap site in the western Arabian Sea, wind speeds were averaged for an area of 1;1 degree square around the sediment trap location. Gaps in the wind speed record (e.g., between 1986 and July 1987) are "lled with wind speeds derived from ship observations (Rixen et al., 1996; Fig. 3) not included in the statistical analysis. The wind speeds at the trap site are used to de"ne the beginning and the end of the meteorological SW monsoon: Thus, the SW monsoon starts in May/June when the wind speeds at the trap site rise above their long-term average (5.1 m s\, calculated for the years 1987}1992; Rixen et al., 1996)
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Fig. 3. Time series of weekly averaged wind speeds and the measured organic carbon #uxes at 3000 m water depth at the sediment trap site in the western Arabian Sea (WAST). Filled circles indicate data points. Broken lines show wind speeds derived from ship observation and organic carbon #uxes measured at 2000 m water depth. both data sets are not included in the statistical analysis. Vertical thin, broken lines represent the time when the wind speeds rise above and drop below the long-term mean of 5.14 m s\. In contrast to our previous work, the beginning of the SW monsoon in the deep sea was predated to the initial increase in organic carbon #uxes which is also marked by the vertical lines. The dates which are represented by the vertical thin lines are given in Table 1.
and ends in September/October when the wind speeds drop below their average, respectively (Fig. 3; Table 1). Surface buoys deployed in the year 1995 close to our sediment trap site in the western Arabian Sea revealed persistent SW winds during the SW monsoon (Rudnik et al., 1997). Thus, this wind direction was used for calculating the wind stress (q"c o "u " ) "v ") during the SW monsoons. The drag coe$cient is c , the density of air is o , the x and y components of the wind stress are u and v . The drag coe$cient was assumed to be 0.0011 when u (7.7 m s\. When u *7.7 m s\, the c was calculated as follows: c "(0.61#0.063;u )/1000 (Smith, 1980; Gill, 1982). 2.2. Upwelling velocities Wind stress acting on the surface ocean causes up- and downwelling. The up- and downwelling velocities can be described as Ekman pumping where positive values
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Table 1 Beginning, end and duration of the meteorological SW monsoon over the western Arabian Sea and in the deep sea. De"nitions are given in the text. The dates listed in the table indicate the position of the vertical line in Fig. 3 and note that the wind speeds are measured weekly and the organic carbon #ux is measured in time intervals of 12}27 d (compare Table 2) SW monsoon
Meteo. start
Meteo. end
Duration (d)
Deep sea start
Deep sea end
Duration (d)
1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996
June June May May May May June May May June June
Sep. 17 Sep. 17 Sep. 21 Sep. 22 Oct. 03 Oct. 05 Sep. 20 Oct. 03 Sep. 14 Sep. 22 Oct. 03
105 107 114 122 134 129 100 132 107 109 123
June 22 June 14 June 27 No data June 22 June 31 June 18 June 13 June 14 July 09 No data
Sep. 07 Oct. 23 Oct. 29 No data Oct. 18 Oct. 17 Oct. 26 Oct. 16 Sep. 05 Oct. 21 No data
78 132 125
05 03 30 23 22 29 13 24 30 06 03
119 109 131 126 84 104 No data
reveal upwelling and negative values downwelling:
1 * q * q W ! V w " o *x f *y f
.
(1)
The meridional and zonal components of the wind stress are q and q , where x and W V y are the horizontal distances, f is the Coriolis parameter, and o is the density of sea water. To determine upwelling at the coast, Eq. (1) can be simpli"ed because of the restricted water mass circulation at the land}ocean boundary:
1 1 q u " o x f
.
(2)
q is the component of wind stress parallel to the coast. Since the error probability of satellite-derived wind speeds is higher at the land}ocean boundary than in the open ocean, we averaged the wind stress for an area extending roughly 83 km o!shore (three grid points) before it was used for Eq. (2). 2.3. Sediment trap and core samples Since 1986, Mark 6 and 7 sediment traps (Honjo and Doherty, 1988) have been deployed at water depths of approximately 3000 m in the western (16320 N, 60330 E), central (14331 N, 64346 E) and eastern Arabian Sea (15331 N 68343 E) (Fig. 1). Sampling intervals range between 14 and 26 d (Table 2). Prior to deployment the sampling cups are "lled with sea water collected at the mooring depth and enriched with sodium (35 g l\) and mercury (II)-chloride (3.3 g l\) to prevent degradation of
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Table 2 Station, position, trap depth, water depth, deployment periods, and mean sampling intervals. Station
Latitude
Longitude Trap depth (m)
Water depth (m)
Deployment start
Deployment end
Interval (d)
Wast-01 Wast-02 Wast-03 Wast-04 Wast-05 Wast-06 Wast-07 Wast-08 Wast-09 Wast-10 Wast-11
16323 N 16318 N 16319 N 16335 N 16324 N 16319 N 16320 N 16319 N 16320 N 16320 N 16313 N
60332 E 60328 E 60328 E 60328 E 60329 E 60331 E 60332 E 60330 E 60319 E 60318 E 60326 E
4020 4018 4010 4016 4016 4013 4027 4018 4018 4025 4230
May 10, 1986 Nov. 19, 1986 May 12, 1987 Nov. 22, 1987 Nov. 20, 1988 Jan. 15, 1990 Dec. 19, 1990 Jan. 16, 1992 Dec. 25, 1992 Oct. 18, 1993 Jan. 15, 1995
Oct. 26, 1986 May 1, 1987 Oct. 21, 1987 Oct. 31, 1988 Sept. 28, 1989 Oct. 15, 1990 Oct. 26, 1991 Dec. 16, 1992 Oct. 11, 1993 Oct. 05, 1994 Oct. 03, 1995
13.0 12.6 12.5 26.5 24.0 21.0 24.0 26.0 23.0 27.0 12.0
3024 3021 3033 3039 3029 3016 3039 3030 2002 3041 3246
trapped material (Lee et al., 1992). After recovery, the samples are sieved into '1 and (1 mm fractions. The wet sample material is "ltered on preweighed nuclepore "lter (0.4;10\ m) and dried at 403C. The dry weights of the (1 mm fraction are used for calculating the total #uxes, and the material itself is crushed prior to chemical analysis. Samples severely disturbed, e.g., by malfunctioning of the traps or the activity of swimmers, are ignored. The gravity core KL74 was taken in 1986 during the SONNE cruise 42 and subsampled in 0.5-cm intervals. Sedimentation rates, dry bulk density and the calibrated ages are obtained from Sirocko (1995, 1996) and Sirocko et al. (1993). 2.4. Chemical analysis Total carbon and nitrogen were measured on a Carlo Erba Nitrogen Analyzer 1500, a catalytic high-temperature combustion method with a subsequent thermal conductivity detector for measuring CO and N in a constant helium #ow. The reproducibility of this method is $0.05% for total carbon and $0.02% for nitrogen. Carbonate was measured with a WoK stho! Carmograph 6. With this method the conductivity change of a sodium hydroxide solution is measured while injecting CO released from the acid-treated sample. The mean standard deviation of this method is $1%. The di!erence between total and carbonate carbon is regarded as organic carbon. A more detailed description of the methods is given by Haake et al. (1993). 2.5. Mean deep-sea yuxes at the sediment trap site in the western Arabian Sea The initial increase of organic carbon #ux marks the beginning of the SW monsoon approximately 2}7 weeks after it is seen in wind speeds (Table 2, Fig. 3). Generally, the #uxes decrease 2}3 weeks after the end of the meteorological SW monsoon. The SW
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Fig. 4. The mean SW monsoon wind speeds over the Arabian Sea averaged for all SW monsoons between 1987 and 1996. The white circle represent the sediment trap sites WAST, CAST, and EAST and the white square the location of core 74 KL. The white line shows the mean position of the Findlater Jet axis.
monsoons 1986 and 1994 seem to have been exceptions, since the #uxes decreased 2 weeks prior to the end of the meteorological SW monsoon. To calculate the mean SW monsoon #uxes we integrated the #uxes over the period these #uxes were elevated (Fig. 3). The exact dates are given in Table 1. The calculation of annual #uxes is problematic when the observations do not cover the whole year. Therefore, observation gaps in the organic carbon #ux record (Fig. 3) were "lled with data extracted for the speci"c time from the mean annual organic carbon #ux curve (Fig. 2). Since organic carbon #uxes, wind speeds and, thus, upwelling velocities are obtained on di!erent time intervals all are interpolated on the same biweekly intervals for statistical analysis.
3. Results and discussion 3.1. The Findlater Jet The Findlater Jet is the dominant atmospheric feature of the SW monsoon (Fig. 4). The area of maximum wind speed, de"ned as the axis of the Findlater Jet, varies from year to year (Fig. 5a). The southernmost position was determined for the SW monsoon 1989 and the northernmost position could be observed during the SW
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Fig. 5. (a) Mean location of the Findlater Jet axis during the SW monsoons 1987}1996. The axis represents the meridional wind speed maxima which were zonally smoothed with a 23 moving average. The broken lines show the mean location of the axis during the SW monsoons 1989 (southernmost position) and 1987 (northernmost position). Bold lines represent the mean location of the axis, and dots the position of the sediment traps (compare with Fig. 4). (b) Mean distance between the sediment trap location (WAST) and the axis of the Findlater Jet versus the mean SW monsoon wind speeds at the trap site, (c) mean SW monsoon wind speed averaged for the Findlater Jet axis west of 653E (wind speed ) and those averaged at $ the sediment trap site (wind speed ). Open circles show the data obtained during the SW monsoons 5 12 1987 and 1989 (see text for futher explanations).
monsoon 1987. There is, however, a lack of satellite-derived wind "elds between July and September 1989, which covers the peak of the SW monsoon, and some data are missing at the beginning of the SW monsoon 1987 (Fig. 3). Ignoring the SW monsoons 1987 and 1989, the location of the axis varies interannually with a meridional extension of approximately 100 km at 603E (Fig. 5a). The 9-yr record on satellitederived wind speeds at the sediment trap site in the western Arabian Sea yields an interannual variation of SW monsoon wind speeds of !10 and #17%. These variations reveal no correlation with the distance between the trap site and the jet axis (Fig. 5b). Since the wind speeds at the trap site correlate with the wind speeds averaged
T. Rixen et al. / Deep-Sea Research II 47 (2000) 2629}2651
2637
Fig. 6. Mean SW monsoon Ekman pumping velocities. Negative values indicate downwelling and positive values upwelling. The white circle shows the trap site (WAST) and the white square the core site (74 KL). The lines delineate the area for calculation of average Ekman pumping (see Fig. 7 and text for explanation).
at the jet axis east of 653E (Figs. 5c), it is assumed that the wind speeds in the western Arabian Sea are mainly controlled by the strength of the Findlater Jet. 3.2. The link between the Findlater Jet and the upwelling velocities The satellite-derived wind"elds can be converted into up- and downwelling velocities assuming a wind direction of SW during the SW monsoon. This provides weekly distribution of up- and downwelling velocities in the western Arabian Sea, which show a downwelling region to the northeast of the Findlater Jet axis (Fig. 6). This downwelling region stretches almost parallel to the Arabian coast and is caused by the wind shadow produced by Socotra Island (compare Fig. 4). It separates two areas of enhanced upwelling, one extending along the Arabian coast and the other one occurring further o!shore in the open ocean. Open-ocean up- and downwelling velocities averaged for the area between Salalah and the sediment trap site as indicated in Fig. 6 correlate with the wind speeds at the trap site (Fig. 7a; r"0.82). The regression equation is w "0.115WS!0.613, (3) where w is the open-ocean upwelling velocity in 10\ m s\, and WS the wind speeds in m s\. This relationship shows that upwelling dominates downwelling in the open ocean when the wind speeds exceed 5.3 m s\. This explains our former observations
2638
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Fig. 7. (a) Wind speeds at the sediment trap site in the western Arabian Sea versus Ekman pumping velociites averaged for the area marked in Fig. 6 and (b) versus Ekman pumping velocities averaged for the coastal area o! Salalah.
that the sea-surface temperature decreases, when the wind speed crosses its long-term average during the opening phase of the SW monsoon at the sediment trap site (Rixen et al., 1996). The coastal upwelling velocities o! Salalah correlate also with the wind speeds at the trap site (Fig. 7b), and the regression equation is u "0.242WS!0.955, (4) where u represents the coastal upwelling velocities in 10\ m s\. Although the resulting correlation coe$cient (r"0.66) is weaker than that obtained by the
T. Rixen et al. / Deep-Sea Research II 47 (2000) 2629}2651
2639
correlation between the wind speeds at the trap site and open-ocean upwelling (Fig. 7a), it implies that the strength of the Findlater Jet is an important factor controlling coastal upwelling velocities. 3.3. The link between upwelling velocities and organic carbon yuxes The organic carbon #uxes measured during the SW monsoons in the western Arabian Sea correlate with coastal upwelling velocities o! Salalah and the upwelling velocities averaged for the region between Salalah and the trap site (Fig. 8). The time-lags considered for the correlations between organic carbon #uxes and coastal as well as open-ocean upwelling are 8 and 2 weeks, respectively (see Rixen et al. (2000a) for further explanations). To combine the e!ects of coastal and open-ocean upwelling on organic carbon #uxes, a multiple regression analysis was carried out (Fig. 9a). The multiple regression equation is POC"(16.85w )#(3.53u )#0.67,
(5)
where POC represents the organic carbon #ux in mg m\ d\. The resulting correlation coe$cient of 0.82 implies that upwelling explains roughly 82% of the observed variation in the organic carbon #uxes during the SW monsoons. This does not hold for the SW monsoon 1991, which exhibited the highest mean SW monsoon wind speeds during the study period (10.3 m s\, compare Fig. 5b). The second highest mean SW monsoon wind speeds (9.5 m s\) were observed during the SW monsoon 1994. The organic carbon #uxes measured during this SW monsoon can be divided into two groups: the peak #uxes "t into the overall trend, and the other data match with the 1991 data. The latter and the 1991 data also correlate with the coastal and open-ocean upwelling velocities (Fig. 9b) but with the negative open-ocean upwelling term in the multiple regression equation: POC"(!1.65w )#(3.89u )#0.035.
(6)
Thus, higher open-ocean upwelling velocities seem to decrease the organic carbon #uxes during stronger SW monsoons. This appears to be accompanied by a reinforced Somali Current (Rixen et al., 1996), which carries Indo-Paci"c through #ow and South Indian Ocean water into the western Arabian Sea (You, 1997). These nutrientpoor and well-oxygenated water masses have been suggested to ventilate the Arabian Sea (Conkright et al., 1994; You, 1997). An associated reduction in the subsurface nutrient concentration has been proposed to be the reason for the reduced fertilization e!ect of upwelling and subsequent lower #uxes during stronger SW monsoons (Rixen et al., 1996). 3.4. The link between the Findlater Jet and the organic carbon yux The relation between upwelling velocities and organic carbon #uxes can be described by two equations: one valid for weak to moderate (Fig. 9a, Eq. (5)) and the
2640
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other one for strong SW monsoons such as the SW monsoons 1991 and 1994 (Fig. 9b, Eq. (6)). Since upwelling velocities can be estimated from the wind speeds at the trap site (Fig. 7, Eqs. (3) and (4)), the latter could be used to estimate the organic carbon #uxes. To validate this "nding we converted the biweekly averaged, satellite-derived time series on SW monsoon wind speeds (Fig. 10, Table 1) at the trap site into upwelling velocities (Eqs. (3) and (4)) and further into organic carbon #uxes (Eqs. (5) and (6)). The calculated organic carbon #uxes correlate with the biweekly averaged organic carbon #uxes measured at the trap site (WAST). The correlation coe$cient of 0.63 (Fig. 11) indicates that changes in the strength of the Findlater Jet explain 63% of the observed variation in organic carbon #uxes.
3.5. The link between organic carbon yux and sedimentary organic carbon burial rates The organic carbon burial rates derived from surface sediments beneath the US JGOFS sites and the sediment trap locations in the western, central, and eastern Arabian Sea have been published by Lee et al. (1998) and Haake et al. (1993). The SW monsoon organic carbon #ux measured in 1995 at the long-term sediment trap site in the western Arabian Sea (WAST) is 11% higher than mean SW monsoon #ux derived from the 8-yr record. Therefore, the annual #uxes given by Lee et al. (1998) are reduced by 11% to adjust it to the long-term mean before all annual organic carbon #uxes are compared with the sedimentary organic carbon burial rates (Table 3; Fig. 12). The sediment and sediment trap data obtained from the Arabian Sea reveal, similar to other areas (Hinrichs, 1992), generally increasing organic carbon burial and remineralization rates associated with increasing organic carbon #uxes (Fig. 13). A similar trend also was observed by Witte and Pfannkuche (2000) who measure the sediment in situ community oxygen consumption at di!erent sites in the western, northern, central, eastern, and southern Arabian Sea during the end of the SW monsoon 1995 as well as at the onset and the end of the late NE monsoon bloom in 1998 and 1997, respectively (Rixen et al., 2000b). To translate organic carbon remineralization into benthic oxygen consumption is generally problematic due to the often unknown contribution of organically bound oxygen to the decomposition of organic matter and the varying C/N ratios therein (Anderson and Sarmiento, 1994; Martin et al., 1987). The nitrogen remineralization rate was determined in a way similar to organic carbon remineralization rate at the sediment trap site in the western, central and eastern Arabian Sea (Table 3). Furthermore, it was assumed that the remineralized nitrogen was bound in proteins that have mean C/O and C/N ratios of 3.36 and 3.05, respectively (Libes, 1992). The carbon not bound in proteins was assumed to occur as carbohydrate with a C/O ratio of 1. According to the equation Cu(H O) (NH ) #yO PuCO #xNO the mean X V ratio of organic carbon to oxygen demand (C/O ) is 0.61 which is lower than suggested by Anderson and Sarmiento (1994; 0.69) and Red"eld et al. (1963; 0.77). Deep-sea oxygen consumption measured in the Paci"c and Atlantic Oceans correlate with the calcium carbonate corrected organic carbon burial rates (COB; Jahnke, 1996). The correction accounts for the in#uence of the bulk accumulation rates on the
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2641
Fig. 8. Ekman pumping velocities averaged for the area marked in Fig. 6(a) and averaged o! Salalah (b) versus organic carbon #uxes measured at the sediment trap site in the western Arabian Sea. Ekman pumping velocities and carbon #uxes are interpolated on the same biweekly intervals. Time lags of 2 and 8 weeks are considered for open-ocean and coastal upwelling, respectively.
organic carbon burial rate: Acc !Acc POC, COB" 100
(7)
where Acc is the total accumulation rate, Acc represents carbonate accu mulation rate and POC indicates the contribution of organic carbon to the total sediment. The regression equation is O "0.143 COB ,
(8)
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Fig. 9. (a) Organic carbon #uxes measured at the sediment trap site in the western Arabian Sea during the SW monsoon 1987}1995 plotted against calculated organic carbon #ux. The multiple regression equation used for estimating organic carbon is given as Eq. (5) in the text. (b) Organic carbon #uxes measured at the trap site during the SW monsoon 1991 and the exceptional periods of the SW monsoon 1994 versus calculated organic carbon #uxes. The multiple regression equation used for calculating organic #ux is given as Eq. (6) in the text. Ekman pumping velocities and carbon #uxes are averaged on the same biweekly intervales. Time lags of 2 and 8 weeks are considered for open-ocean and coastal upwelling, respectively.
where O represents the oxygen consumption in mol m\ yr\, and COB is given in mmol C m\ yr\. Although change in the bottom-water content is also a factor in#uencing the organic carbon accumulation rates in shelf and slope sediments (e.g., Paropkari et al., 1993), this may not play a major role in the deep open western
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2643
Fig. 10. Time series of biweekly averaged wind speed (broken line). Bold lines indicate the biweekly averaged SW monsoon wind speeds increased by 50% and decreased by 25% used for calculating the organic carbon #ux rates.
Fig. 11. Biweekly averaged organic carbon #uxes measured at 3000 m water depth in the western Arabian Sea versus calculated organic carbon #uxes. To calculate the organic carbon #uxes we used the biweekly averaged SW monsoon wind speeds at the trap site as shown in Fig. 10, and Eqs. (3)}(6).
Arabian Sea. Thus the organic carbon #ux in g m\ yr\ can be estimated as follows: POC "+([0.142COB ]0.61)#[COB/1000],12.01. (9) $ Eqs. (7) and (9) imply an increasing organic carbon burial and remineralization rate due to increasing organic carbon #uxes. The extent to which the burial and
3000 3000 2800 999 3150 2979 3484 3915
4000 3900 3700 1435 3647 3478 4004 4426
Water depth (m) 2.91 2.17 2.12 3.82 4.33 4.25 2.88 1.07
POC (g m\ yr\) 0.60 0.12 0.13 1.00 0.43 0.57 0.14 0.03
POC (g m\ yr\) 2.31 2.05 1.99 2.82 3.90 3.68 2.74 1.04
POC (g m\ yr\) 0.38 0.28 0.26 No data No data No data No data No data
PON (g m\ yr\)
0.07 0.02 0.02 No data No data No data No data No data
PON (g m\ yr\)
0.31 0.26 0.24 No data No data No data No data No data
PON (g m\ yr\)
The annual organic carbon #uxes measured at the US sites are reduced by 11% to adjust it to the long-term mean. The sedimentary burial rates at the locations WAST, CAST and EAST are derived from Haake et al. (1993).
WAST CAST EAST M1 M2 M3 M4 M5
Trap depth (m)
Table 3 Trap and water depth as well as annual organic carbon and nitrogen #uxes, sedimentary burial rate and remineralization rates at the sediment trap sites in the western (WAST), central (CAST) and eastern Arabian Sea (EAST) and at the US JGOFS locations M1}M4. The US JGOFS data obtained between 1994 and 1995 are from Lee et al. (1998)
2644 T. Rixen et al. / Deep-Sea Research II 47 (2000) 2629}2651
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2645
Fig. 12. (a) Location of the sediment trap sites, with grey dots representing the US JGOFS sites (Lee et al., 1998) and black dots the long-term stations (WAST, CAST, EAST) which have been under the umbrella of the German JGOFS since 1995. The small grey dot shows the location of the sediment core 74 KL. (b) Annual organic carbon #uxes versus the distance to the coast. The data from the US JGOFS site as listed in Table 2 are reduced by 11% (see text for explanation). The small grey dot represents the mean annual organic carbon #ux derived from the bulk accumulation rate, the carbonate and organic carbon content (COB) of the uppermost sampling interval of 74 KL.
remineralization rates increase depends on the carbonate content, which varies in the deep Arabian Sea roughly between 40 and 80% (Fig. 13; Kolla et al., 1981). Taking this into account, the data obtained at all sediment trap sites lie almost in the range given by the Eqs. (7) and (9), which suggests that Jahnke's "ndings are also valid for the Arabian Sea. Thus, Eqs. (7) and (9) have been used to convert organic carbon burial rates obtained from the Arabian Sea sediment core into organic carbon #uxes.
2646
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Fig. 13. Mean annual organic carbon #uxes versus organic carbon remineralization (grey circles) and organic carbon burial rates (black circles). The data are given in Table 3. The lines show the relationship between organic carbon #ux and organic carbon remineralization (dotted), and organic carbon burial (bold) for carbonate contents of 40 and 80% (Eqs. (7) and (9)).
3.6. Holocene variations A sediment core (74 KL) was recovered slightly north to the axis of the Findlater Jet in the western Arabian Sea (Figs. 1, 4 and 5). Based on oxygen isotopes, varying contribution of calcium carbonate and dolomite to the bulk sediment of 74 KL, Sirocko et al. (1993 and references therein) have described variations in the monsoon strength over the last 24,000 yr. The most pronounced feature in this record is that organic carbon burial rates for the last glacial maximum (20}21 ka BP) were more than thrice as high as during the late Holocene (0.4 ka BP, Fig. 14). The latter is the organic carbon burial rate of the uppermost sampling interval (0.37 g C m\ yr\). However, due to the extreme environmental di!erences, e.g., lower sea level and associated changes in the coastline and surface current system etc., it might be questioned whether modern results could be transferred to glacial conditions. Therefore, we restricted the following discussion to the Holocene (11.6}0.4 ka BP). Sirocko et al. (1993) identi"ed a phase of strong SW monsoons occurring between 9.9 and 8.6 ka BP and a phase of weak SW monsoons between 5.6 and 5.2 ka BP. Our data show that the Holocene period of strong SW monsoons is characterized by organic carbon burial rates up to 75% higher than during the late Holocene, and 40% lower during the Holocene period of weak SW monsoons (Fig. 14). The organic carbon burial rates are transformed into COB and then into organic carbon #ux according to Eqs. (7) and (9), respectively. Since the decomposition of organic matter in the sediments is not considered, the derived organic carbon #ux rates are under- rather than overestimates. However, the resulting late Holocene organic carbon #ux of 3.5 g m\ yr\ is
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2647
Fig. 14. Organic carbon burial rates and #uxes and their deviations from the late Holocene values in core 74 KL. The organic carbon #uxes have been determined according to Eqs. (7) and (9). Horizontal broken lines mark the periods of strong (8.6}9.9 ka BP) and weak SW monsoons (5.2}5.6 ka BP; Sirocko, 1995).
well within the range measured at di!erent sites in the Arabian Sea (Table 3) and "ts the value we would expect due to the distance between the coast and the core site (Fig. 12b). This implies that the applied method produces reliable organic carbon #ux rates and suggests that the organic carbon #uxes had varied between !23 and 43% during Holocene (Fig. 14). Changes in the same order at the trap site in the western Arabian Sea would result in organic carbon #ux rates ranging between 2.2 and 4.2 g m\ yr\. This range is almost covered by the mean annual organic carbon #uxes, which vary between 2.3 and 3.6 g m\ yr\ during the 8 years of observations in the western Arabian Sea. 3.7. The link between the SW monsoon and the organic carbon burial rates In the following, we consider the extent to which Holocene variations of organic carbon #uxes from 2.2 to 4.2 g m\ yr\ could be related to past changes in the SW monsoon wind speeds at the trap site in the western Arabian Sea. Therefore, the established links between wind speeds at the trap site, upwelling velocities (Eqs. (3) and (4)) and organic carbon #uxes (Eq. (5)) were used. In contrast to our earlier approach (compare Fig. 11), all data on wind speeds from 1986 to 1996 have been integrated and decreased by not more than 25% (Fig. 10). This reduces the decadal mean SW monsoon wind speeds at the trap site from approximately 9 to 7 m s\
2648
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Fig. 15. Relationship between the SW monsoon wind speed at the trap site in the western Arabian Sea and mean SW monsoon and annual organic carbon #uxes according to Eqs. (3)}(5) (bold line). Broken line indicates the same but Eq. (5) is replaced by Eq. (6) (see text for further explanation).
(Fig. 15). As monthly mean wind speeds exceeding 12 m s\ occur only in the Southern Ocean and during winter in the North Atlantic (SchluK ssel, 1995), higher wind speeds appear unrealistic for our model experiment. Therefore, we increased the wind speed by less than 50%, which leads to a mean SW monsoon wind speed of less than 12 m s\. Within our model experiments, the duration of the upwelling season during the SW monsoon is de"ned as the time between May and October, when the wind speeds are above 5.3 m s\. Thus, increasing and decreasing of the wind speeds could extend and reduce, respectively, the duration of the upwellling season by less than 6 weeks (compare Fig. 10). These changes are considered when the calculated biweekly organic carbon #uxes are summarized to average SW monsoon and further to a decadal mean SW monsoon #ux. Thus, a weakening of the SW monsoon wind speed by 25% corresponds to a decadal mean SW monsoon #ux of 0.8 g m\. Increasing SW monsoon wind speed raises the mean SW monsoon #ux up to 2.4 g m\. Adding to these SW monsoon #uxes the mean NE and intermonsoon organic carbon #ux obtained by our sediment trap experiments in the western Arabian Sea (1.56 g m\) results in annual organic carbon #uxes between 2.4 and 4 g m\ yr\. This range implies that the Holocene record of organic carbon #uxes almost could be explained by changes in the intensity of the SW monsoon wind speeds. Integrating changes in the subsurface nutrient regime as suggested for stronger SW monsoons into our re#ections (replacing Eq. (5) by Eq. (6)) means SW monsoon wind speed of 12 m s\ would be associated with a mean annual organic carbon #ux
T. Rixen et al. / Deep-Sea Research II 47 (2000) 2629}2651
2649
of 3.5 g m\ yr\, which, considering the uncertainties in the estimates, is similar to the deciphered Holocene values.
4. Conclusion The evaluation of the satellite-derived wind "elds and sediment trap data reveals that the strength of Findlater Jet is the dominant factor controlling organic carbon #uxes on seasonal time scales. Additionally, changes in the subsurface nutrient concentration due to changes in the surface current system seem to be another factor in#uencing the organic carbon #uxes. Its e!ect on organic carbon #uxes is most pronounced on an interannual time scale. Since the link between organic carbon #uxes and sedimentary burial based on data from the Atlantic and Paci"c oceans seems to be valid also for the Arabian Sea, we extended our study to the geological record. On this time scale changes in the intensity of the Findlater Jet appear to be su$cient to explain Holocene record on organic carbon #uxes.
Acknowledgements We thank H. Gra{l and P. SchluK ssel for their help in processing the satellite data and Dr. R. Seifert, Dr. A. Suthhof as well as the three anonymous reviewers for their valuable comments. Mrs. I. Jennerjahn is acknowledged for Secretarial assistance. P. Wessel and W.H.F. Smith (Wessel and Smith, 1991) are thanked for providing the generic mapping tools (GMT). The ongoing sediment trap program is being funded by the German Federal Ministry of Education, Science, Research and Technology (BMBF), the German Research Council (DFG), the Council of Scienti"c and Industrial Research (CSIR, New Delhi), and the Department of Ocean Development (DOD, New Delhi).
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SchluK ssel, P., 1995. Passive Fernerkundung der unteren Atmosper und der Meeresober#aK che aus dem Weltraum. Habilitationsschrift Ber. A20, Zenturm fuK r Meeres- und Klimaforsch. der UniversitaK t Hamburg, Germany, 175pp. Sirocko, F., 1995. Century-scale events SW monsoon Data, Arabian Sea. IGBP PAGES/World Data Centre-A for Paleoclimatology Data Contribution Series C95-029. NOAA/NGDC Paleoclimatology Program, Boulder Co, USA. Sirocko, F., 1996. Past and present subtropical summer monsoons. Science 274, 937}938. Sirocko, F., Sarnthein, M., Erlenkeuser, H., Lange, H., Arnold, M., Duplessy, J.C., 1993. Century-scale events in monsoonal climate over the past 24,000 years. Nature 364, 322}364. Smith, R.L., Bottero, J.S., 1977. On upwelling in the Arabian Sea. In: Angel, M. (Ed.), A Voyage of Discovery. Pergamon, New York, pp. 291}304. Smith, S.D., 1980. Wind stress and heat #ux over the ocean in gale force winds. Journal of Physical Oceanography 10, 709}726. Tchernia, P., 1980. Descriptive Regional Oceanography, 2nd Edition, Pergamon, Tarrytown, NY, 253 pp. Wessel, P., Smith, W.H.F., 1991. Free software helps map and display data. EOS 72, 441, 445}446. Witte, U., Pfannkuche, O., 2000. High rates of benthic carbon remineralization in the abyssal Arabian Sea. Deep-Sea Research II 47, 2785}2804. Wyrtki, K., 1973. Physical oceanography of the Indian Ocean. In: Zeitzschel, B. (Ed.), The Biology of the Indian Ocean. Springer, New York, pp. 18}36. You, Y., 1997. Seasonal variations of thermocline circulation and ventilation in the Indian Ocean. Journal of Geophysical Research 102, 10391}10422.