Processes controlling forms of phosphorus in surficial sediments from the eastern Arabian Sea impinged by varying bottom water oxygenation conditions

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Deep-Sea Research II 52 (2005) 1965–1980 www.elsevier.com/locate/dsr2

Processes controlling forms of phosphorus in surficial sediments from the eastern Arabian Sea impinged by varying bottom water oxygenation conditions C. Prakash Babu, B. Nagender Nath Geological Oceanography Division, National Institute of Oceanography, Dona Paula, Goa 403 004, India Received 17 April 2003 Available online 15 August 2005

Abstract The surficial sediments from the upper continental slope of the eastern Arabian Sea, impinged by the oxygen minimum zone (OMZ, 150–1200 m water depth), show varying concentrations of the biogenic element phosphorus (P, 0.1–0.2%) in the northern and southern areas even though total organic carbon concentrations are relatively constant (TOC, 2–5%; Prakash Babu et al., 1999). To understand this discordance, 17 surface sediment samples from shelf, slope and deep sea of the eastern Arabian Sea were investigated using a five-step sequential extraction scheme to delineate the process responsible for P enrichment in OMZ. High fractions of organic phosphorus (Porg 10–26%), biogenic phosphorus (Pbio 36–48%), relatively low molar Corg/Porg ratios (322–447), and Corg/Preactive ratios close to Redfield Ratio in OMZ sediments of the SE Arabian Sea suggest accumulation under high surface production and low residence time of labile forms of P due to high sedimentation rates. Despite higher productivity in surface waters, low fractions of Porg (8–13%; less than deep-sea sediments of the study area 12–13%), Pbio (25–33%), relatively high molar Corg/Porg ratios (341–508), and Corg/Preactive ratios less than Redfield Ratio in OMZ sediments from the NE Arabian Sea may indicate a higher degree of regeneration and diagenetic transformation of labile forms of P to other phases. Authigenic phosphorus (Paut) fraction varies by a factor of 2–8 in sediments from the OMZ when compared to well-oxygenated deep-sea sediments of the study area. While the Ptotal remains constant, significant P transformation seems to occur in NE Arabian Sea, which is suggested by high Paut fraction (
50%) compared to low Paut fraction (10–39%) in the SE Arabian Sea sediments. Supply rates of phosphorus, variable rates of P dissolution under varying dissolved oxygen contents in the bottom waters, and early diagenetic transformation of P within the sediments seem to influence P geochemistry in sediments overlain by the OMZ in the eastern Arabian Sea. r 2005 Elsevier Ltd. All rights reserved.

1. Introduction Corresponding author.

E-mail addresses: [email protected] (C.P. Babu), [email protected] (B.N. Nath).

Phosphorus (P) is a biolimiting element and plays a key role in oceanic biogeochemical cycles

0967-0645/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2005.06.004

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(Holland, 1978; Broecker, 1982; Smith, 1984; Codispoti, 1989). Phosphorus along with other nutrients vs. nitrate and silicate, besides light and other physical factors primarily control the oceanic productivity. It is believed that P is the ultimate limiting nutrient element over geologic time scales (Howrath et al., 1995; Tyrell, 1999). Thus a decrease in the availability of dissolved P may reduce the surface productivity, which may lead to reduction in export fluxes of carbon, eventually resulting in lower burial of organic carbon in sediments. Changes in the oceanic burial of organic carbon affect the sedimentary pyrite formation (Berner, 1984) and control the partial pressure of atmospheric oxygen and carbon dioxide (Holland, 1984; Berner, 1989; Berner and Canfield, 1989). Thus understanding the phosphorus cycle in the oceanic realm aids in understanding the geochemical cycling of P, C, S and oxygen on a geological time scale. A significant diagenetic reorganization of P has been observed in lacustrine, continental margin, and deep-sea sediments in various areas (Vink et al., 1997; Fang, 2000; Filippelli, 2001 and references therein). Based on P redistribution in various depositional environments, a conceptual diagram depicting P geochemistry in oceanic sediments was developed by Filippelli and Delaney (1996). Phosphorus is delivered to the sediments mainly in association with organic matter. During degradation of organic matter, P is released to interstitial waters. Phosphate in interstitial waters appears to be involved in various biogeochemical processes. The released-P may be adsorbed onto grain surfaces, bound to iron oxyhydroxides and diffused back to bottom waters, but P is ultimately buried as an authigenic carbonate fluorapatite (CFA). The primary delivery mechanism of P to the sediments is with organic carbon (Anderson et al., 2001). However the sedimentary fate of organic phosphorus (Porg) is less known, and current studies are devoted to its understanding (Ingall et al., 1993; Filippelli and Delaney, 1996; Anderson et al., 2001). The decoupling of P and C is observed with sediment depth and age (Ingall et al., 1993; Filippelli and Delaney, 1996; Filippelli, 2001; Anderson et al., 2001), and therefore we

need to understand the influence of diagenesis on P and to resolve the fate of C and P after their burial (Ruttenberg and Berner, 1993; Filippelli and Delaney, 1996; Anderson et al., 2001). Setty and Rao (1972), Rao et al. (1978, 1987), and Paropkari (1990) have studied the distribution of Ptotal in the surficial sediments along the eastern Arabian Sea. These studies gave a general idea of P distribution and mode of incorporation into the sediments. Though acetic acid and HCl speciation experiments were carried out earlier (Rao et al., 1987), these studies were not able to distinguish between biogenic and authigenic apatite. Even though P distribution is studied in detail, organic P studies with respect to productivity have not been carried out so far in eastern Arabian Sea. But in the western Arabian Sea based on P speciation studies Tamburini et al. (2003) have given new evidence for enhanced productivity during glacial periods. While the organic carbon contents in sediments impinged by OMZ do not show significant geographical variations (2–5%; Prakash Babu et al., 1999) in the eastern Arabian Sea, total phosphorus (Ptotal) displays a distinct N–S variation (0.1–0.2%) in these sediments. Therefore, it is important to understand the processes controlling the delinking of organic carbon and P geochemistry in these sediments. Variable decomposition rates of organic carbon and P in oxic and anoxic setting may be one of the controlling factors (Delaney, 1998 and references therein). The upperslope sediments impinged by the OMZ in the eastern Arabian Sea offer an ideal site to understand the extent of P regeneration. The speciation studies of P when compared to bulk geochemistry have an additional advantage regarding the association of P with various phases, transformation from one phase to another and ultimate burial phase in marine sediments. In addition to the sediments from the OMZ, the sediments from shelf and deep sea of eastern Arabian Sea are also included in this study to compare the extent of P regeneration/accumulation in sediments under a well-oxygenated and oxygen-depleted water column. The dissolved oxygen concentrations in upper slope sediments (150–1200 m water depth)

ARTICLE IN PRESS C.P. Babu, B.N. Nath / Deep-Sea Research II 52 (2005) 1965–1980

impinged by OMZ in eastern Arabian Sea reach as low as 0 ml/l (Wyrtki, 1973; De Sousa et al., 1996), whereas the continental shelf (o150 m), lower slope, and deep-sea sediments (41200 m) are well oxygenated (Wyrtki, 1973). Based on barium geochemical studies, Prakash Babu et al. (2002) have recently shown an oxygenated interface in upper-slope sediments of the SW margin of India whereas the interface is suboxic/anoxic in the NW margin of India. Ingall et al., (1993) observed different molar C/P ratios in bioturbated sediments (average 150) overlain by oxic water column when compared to laminated sediments (average 3900) deposited under an anoxic water column and suggested that the regeneration of P is high in anoxic bottom water when compared to an oxic environment. The sedimentation rates in eastern Arabian Sea vary to a great extent between shelf, slope and deep sea. On shelf the sedimentation rates range from 0.56–19 mm/yr, 0.08–3.8 mm/yr on upper slope, 2–12 cm/kyr on lower slope, and 2.2– 3.84 cm/kyr in deep-sea sediments (see Rao and Wagle, 1997 and references therein for compilation of sedimentation rates; Bhushan et al., 2001). The productivity in eastern Arabian Sea is high during summer and winter monsoon/convection (Bhattathiri et al., 1996; Madhupratap et al., 1996). Marine fish production along Indian coast in 1997 is 2.7 M tonnes (Devaraj and Vivekanandan, 1999). Large amount of fish production in the Arabian Sea can probably be related to high productivity. Fish debris dissolution in sediments may also control benthic porewater phosphate fluxes, and burial of biogenic apaptite is recognized as an important mechanism for reactive P removal in sediments from upwelling regions (Suess, 1981; Froelich et al., 1988). The SEDEX method (Ruttenberg, 1992) does not distinguish between biogenic and authigenic apatite. In order to distinguish between biogenic and authigenic apatite leaching with 2 M NH4Cl (Schenau and De Lange, 2000) was combined with the updated SEDEX procedure of Anderson and Delaney (2000) to study the solid phase speciation of P in detail. NH4Cl technique extracts hard parts of fish debris and only small percentage of authigenic and

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detrital apatite and iron-bound P because of higher solubility of hydroxyapatite (fish bone apatite) compared to fluorapatite, lack of crystal perfection and large surface area of biogenic apatite (Schenau and De Lange, 2000). Schenau and De Lange (2000) have further observed that P concentrations start to increase once Ca concentration has dropped. This observation emphasizes that P extracted with NH4Cl probably represents biogenic apatite.

2. Material and methods The sediment samples used in the present investigation (Fig. 1, Table 1) were collected from the eastern Arabian Sea during cruises of RV Gaveshani and ORV Sagar Kanya using either a Petterson grab or gravity core. The grab samples approximately represent the top 10 cm of the sediment column whereas only the top 2 cm is used from the core samples. Sequential leaching extractions were undertaken to understand the association of phosphorus with various sedimentary components. Although leaching procedures are operationally defined, they provide an insight to understand the distribution of elements in different phases and the mode of fixation in the sediment column. The sequential leaching was carried out mainly using the updated SEDEX procedure (originally proposed by Ruttenberg, 1992) of Anderson and Delaney (2000) combined with NH4Cl leach to distinguish between biogenic and authigenic apatite (Schenau and De Lange, 2000). The details of sequential extraction are summarized below. 100 mg of sediment samples were leached in the first step with 2 M NH4Cl to separate exchangeable or loosely sorbed, carbonate-associated phosphorus and biogenic apatite (Schenau and De Lange, 2000). Ten ml of CDB buffer solution (0.22 M Na citrate, 0.033 M Na dithionite, 1 M Na bicarbonate pH (7.6), 25 ml of 2 M NH4Cl) and 10 ml of distilled water in step two isolate easily reducible or reactive iron-bound P (Ruttenberg 1992) and any less easily dissolvable fraction of fish bones that could not be separated in the first step. In the third step the solid residue

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Fig. 1. Station location map (see Table 1 for details) showing the percentage contribution of each leached phase to total P

was treated with 1 M Na-acetate, 1 M MgCl2 and distilled water to isolate authigenic apatite. In the fourth step 13 ml of 1 N HCl was added to the residual solid to separate detrital apatite. In the final step, which isolated organically bound P, the

residue from the previous step was dried with 1 ml of 50% (w/v) of MgNO3 solution in the oven at 80 1C, ash at 550 1C for 2 h, then treated with 13 ml 1 N HCl for 24 h. The details of the above procedure are given in Table 2.

ARTICLE IN PRESS C.P. Babu, B.N. Nath / Deep-Sea Research II 52 (2005) 1965–1980

All the leached solutions (except CDB solutions) were measured for phosphorus by standard Table 1 Position details and water depth (m) of sediment samples investigated in the present study Stn. no. Sample number Water depth Lat. (N) Long. (E) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

RVG 84/1856 RVG 84/1857 RVG 84/1859 RVG 83/1824 RVG 83/1825 SK 5/23 SK 44/9 RVG 191/5 RVG 99/2303 RVG 191/23 RVG 30/299 RVG 163/3801 RVG 30/343 RVG 71/1432 RVG 167/3902 RVG 167/3910 RVG 167/3909

175 520 1350 750 1300 2896 3679 275 190 1110 609 2212 320 41 870 1110 1320

20.42 20.31 20.33 17.53 17.52 16.86 15.87 15.07 13.98 13.9 12.06 10.75 10.86 9.8 8.29 7.98 7.92

69.34 69.28 69.11 71.15 71.02 70.83 70.01 72.93 73.31 72.64 74.26 74.24 75.16 75.95 76.34 76.48 76.34

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ascorbic acid molybdate blue technique (Strickland and Parson, 1972). For citrate–dithionite buffer (CBD) leach, organic extraction technique developed by Watanabe and Olsen (1962) was used. The analytical precision based on duplicate analysis for total P is between 6% and 12%, Pbio 12–15%, PFe 10–17%, Paut and Porg o10%, and Pdet o5%. Phosphorus extraction results are provided in Table 3. Along with P speciation data, organic carbon and calcium carbonate values published earlier (Prakash Babu et al., 1999, 2002) are also included. The percentage contribution of each fraction of P to total P is provided in Fig. 1. 3. Results Ptotal concentrations in the study area range between 920 and 2496 ppm (Fig. 2A and Table 3, except one station (relict sediments) off Marmagao with a high concentration of 13,098 ppm). Ptotal concentrations are relatively low in deep-sea

Table 2 Details of sequential extraction scheme Step 1.

Extractant

 8  25 ml. of 2 M NH4Cl (adjusted to pH 7 with ammonia), 4 h

P Phase extracted

 Exchangeable or loosely sorbed P  Carbonate associated P  Easily reducible or reactive iron

Reference Schenau and De Lange (2000)

bound P 2.

 1  10 ml of 0.22 M Na-citrate,  

3.

0.033 M sodium dithionite, 1.0 M sodium bicarnoate (pH 7.6), 6 h 1  25 ml. 2 M NH4Cl, 2 h 1  10 ml. of distilled water, 2 h

 1  10 ml 1.0 M Na-acetate buffered  

 Easily reducible or reactive iron bound P

Ruttenberg (1992), Anderson and Delaney (2000), Schenau and De Lange (2000)

 Authigenic apatite

Ruttenberg (1992), Anderson and Delaney (2000)

(pH 4), 2 h 2  10 ml MgCl2 (pH 8), 1 h 1  10 ml of distilled water, 1h

4.

 1  13 ml of 1.0 N HCl

 Detrital apatite

Ruttenberg (1992), Anderson and Delaney (2000)

5.

 50% (w/v) MgNO3 1 ml dry at 80 1C,

 Organic P

Ruttenberg (1992), Anderson and Delaney (2000)

ash at 550 1C, 1 N HCl 13 ml, 24 h

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Table 3 Phosphorus concentrations (ppm) and molar TOC/Porg and TOC/Preactive ratios in various phases Water depth (m)

TOC (wt.%)a

CaCO3 (wt.%)a

Pbio (ppm)

PFe (ppm)

Paut (ppm)

Pdet (ppm)

Porg (ppm)

Total P (ppm)

TOC/Porg

TOC/Preac

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

RVG 84/1856 RVG 84/1857 RVG 84/1859 RVG 83/1824 RVG 83/1825 SK 5/23 SK 44/9 RVG 191/5 RVG 99/2303 RVG 191/23 RVG 30/299 RVG 163/3801 RVG 30/343 RVG 71/1432 RVG 167/3902 RVG 167/3910 RVG 167/3909

175 520 1350 750 1300 2896 3679 275 190 1110 609 2212 320 41 870 1110 1320

2.13 5.12 2.85 5.3 2.12 0.43 0.24 1.21 1.58 2.01 4.55 1.15 3.22 0.66 5.31 4.51 4.61

57.27 36.17 39.05 47.01 50.15 48.39 56.31 76.41 63.52 60.88 30.12 33.59 42.2 36.17 41.28 43.73 47.61

629 661 675 553 440 512 474 590 3362 577 595 517 464 944 577 333 365

b.d. 233 42 120 146 31 57 74 920 81 82 79 59 109 73 b.d. 80

894 1138 68 1016 686 58 123 139 2422 606 134 81 259 90 164 472 368

225 204 168 228 184 274 145 69 5906 163 207 121 184 384 84 110 60

160 260 197 288 160 122 121 97 488 161 288 163 240 105 308 286 197

1908 2496 1150 2205 1616 997 920 969 13098 1588 1306 961 1206 1632 1206 1201 1070

341 508 371 475 339 92 51 352 84 322 407 181 348 162 447 408 600

33 58 75 69 38 15 8 35 6 36 107 35 81 14 122 106 118

b.d. below detection. The stations impinged by oxygen minimum zone (OMZ) are highlighted. a Total organic carbon (TOC) and CaCO3 data from Prakash Babu et al. (1999, 2002).

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Fig. 2. Depth distribution of (A) Ptotal and (B) Paut. Notice the enrichment of Ptotal and Paut in the oxygen minimum zone (OMZ). The shaded area indicates the OMZ.

sediments (920–997 ppm; Fig. 2A and Table 3) when compared to sediments from upper slope (969–2496 ppm; Fig. 2A and Table 3), which are impinged by OMZ. An examination of Ptotal spatial distribution shows that its absolute concentrations are higher by a factor of 2 to 3 in the sediments overlain by OMZ when compared to deep-sea sediments of the study area (Fig. 2A). A spatial variation is noticed in sediments from the OMZ with a distinct north–south gradient. The upper-slope sediments of the NE Arabian Sea impinged by the OMZ are relatively enriched in Ptotal by a factor of 2 when compared to SE Arabian Sea sediments.

Water depth (m)

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Stn. No

ARTICLE IN PRESS C.P. Babu, B.N. Nath / Deep-Sea Research II 52 (2005) 1965–1980

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The phosphorus fraction associated with organic matter (Porg) is low in shelf (6%, Fig. 1, Table 3) and relatively high in deep-sea sediments (12–17%, Fig. 1, Table 3). In sediments overlain by OMZ, Porg proportion is relatively high in the SE Arabian Sea (10–26%, Fig. 1, Table 3) when compared to the NE Arabian Sea (8–13%, Fig. 1, Table 3), which have less Porg than the deep-sea sediments. The relative proportion of P associated with the biogenic fraction (Pbio) is generally high (26–61%) in carbonate-rich sediments and also in deep-sea sediments studied here (Fig. 1, Table 3). Among the sediments impinged by OMZ, Pbio proportion from the NE Arabian Sea is relatively low (25–33%, Fig. 1, Table 3) when compared to sediments from the SE Arabian Sea (28–61%, Fig. 1, Table 3). The spatial distribution of percentage contribution of reactive Fe-bound P (PFe) shows a marginal enrichment in sediments from OMZ (5–9%; Fig. 1) when compared to deep-sea sediments (3–8%; Fig. 1). But the sediments from OMZ show a N–S contrast with PFe percentage relatively enriched in NE Arabian Sea (5–9%; Fig. 1) when compared to SE Arabian Sea (5–7%; Fig. 1). Authigenic P (Paut) fraction is very high, by a factor of 2–8, in sediments from OMZ when compared to deep sea sediments (Figs. 2 and 3B, Table 3). A gradual decrease in Paut is observed from OMZ towards deep sea (Fig. 2B). Further, when the sediments from OMZ are exclusively considered the percentage contribution of Paut is relatively high in sediments from NE Arabian Sea (46–47%; Fig. 2B) when compared to SE Arabian Sea (10–39%; Fig. 2B). Detrital apatite (Pdet) fraction nearly reaches 50% of Ptotal in one upper-slope station (Table 3). In deep-sea sediments Pdet fraction is relatively high in sediments close to the Indus mouth in the north and decreases gradually towards south. In NE Arabian Sea, Pdet fraction increases from shelf towards deep sea as these deep-sea stations are close to the Indus river. In SE Arabian Sea, the sediments from OMZ between Goa and Kochi (Cochin) show relatively high Pdet when compared to sediments from SW coast of India.

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1000 1500 2000 2500 3000 NE Arabian Sea SE Arabian Sea

3500 4000

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Molar C/P reactive Fig. 3. Depth distribution of (A) molar C/Porg and (B) C/Preactive (Preactive ¼ Pbio+PFe+Paut+Porg) ratios. The shaded area indicates the OMZ. Note the change in the x-axis for C/Preactive ratios.

Molar C/Porg ratios are low in shelf and deepsea sediments (51–181; Fig. 3A, Table 3) when compared to high molar C/Porg ratios in sediments from OMZ (84–508, Fig. 3A, Table 3). Among the sediments from OMZ, C/Porg ratios are relatively higher in NE Arabian Sea (341–508, Fig. 4A, Table 3) than SE Arabian Sea (322–447, Fig. 4A, Table 3). The ratios are relatively low (322–408, Fig. 3A, Table 3) at greater depths of the OMZ (1110–1320 m) when compared to sediments from shallower depths of OMZ except at one station where molar C/Porg ratio reaches a value of 600 (Fig. 3A, Table 3), exceeding the maximum values even from shallower depths of the OMZ.

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1972 350

r 2 = 0.86

(ppm)

300

200

P

organic

250

150 100 50 0

1

2

3

4

5

6

Oraganic carbon (wt. %) Fig. 4. Scatter plot between organic carbon and Porg.

Molar Corg/Preactive (Preactive ¼ Pbio+PFe+Porg+ Paut) ratios are low in deep-sea sediments (8–35; Fig. 3B, Table 3). A distinct geographical contrast is noticed in Corg/Preactive ratios for the sediments from the OMZ between north and south. The Corg/ Preactive ratios are close to the Redfield ratio (81–122, Fig. 3B, Table 3) in the south, whereas ratios lower than Redfield Ratios are observed in the NE Arabian Sea (33–69, Fig. 3B, Table 3).

4. Discussion The above results have shown marked geographical N–S differences in the distribution of Ptotal, Pbio, PFe, Paut and Porg, and molar C/P and Corg/ Preactive ratios in sediments from OMZ of eastern Arabian Sea. The possible mechanism for these variations and the causes for P enrichment in sediments from OMZ are discussed. 4.1. Organic phosphorus variations and its relation with Corg Phosphorus is delivered to the seafloor mainly associated with organic matter. The productivity along the SE Arabian Sea is high (440 mg C m2 d1 and 1760 mg C m2 d1, Bhattathiri et al., 1996) during SW (summer) monsoon due to upwelling of nutrient-rich waters. Relatively

high proportion of Porg in sediments impinged by OMZ (10–26%; Fig. 1) in SE Arabian Sea indicate Porg deposition due to high sedimentation rates in this region (17–51 cm ka1, Sirocko and Lange, 1991; Thamban et al., 2001). In the absence of Porg flux data that can be linked to primary productivity and which enhances the knowledge of Porg preservation, molar C/P and Corg/Preactive ratios are useful in understanding Porg burial (see discussion below). Similar to the SE Arabian Sea the relatively low fraction of Porg (8–12%, Fig. 1) in NE Arabian Sea sediments, may indicate low productivity. However, productivity is high in this region due to convection during winter (335 mg C m2 d1 and 807 mg C m2 d1, Madhupratap et al., 1996). Despite high productivity, the low Porg fraction in sediments from NE Arabian Sea sediments even when compared to deep-sea sediments, is surprising. A possible mechanism is that Porg released due to intense remineralization of organic matter is diagenetically incorporated in to other sedimentary phases. The preferential regeneration and transformation of Porg has probably reduced the Porg fraction in sediments from this area. Release of Porg during organic matter degradation and transformation to other inorganic phases in marine sediments is reported in many areas (Froelich et al., 1982; Ruttenberg and Berner, 1993; Filippelli et al., 1994; Ingall and Jahnke, 1994; Filippelli and Delaney, 1995, 1996; Filippelli, 2001; Tamburini et al., 2002, 2003). A scatter plot between organic carbon and organic phosphorus shows a good correlation (r2 ¼ 0:86, Fig. 4, sample no. 99/2303 is excluded because of relict nature). The non-zero intercept on y-axis at
90 ppm Porg is similar to that of 70 and 60 ppm found in other studies (Mach et al., 1987; De Lange, 1992). Mach et al. (1987) and de Lange (1992) have attributed to excess Porg the experimental error in estimating organic P. Part of inorganic-P may be extracted with the organic phase, which might have resulted in non-zero intercept. As the water column is oxygenated in the deep sea, most of the labile organic carbon and phosphorus are degraded in the water column, resulting in a low Porg fraction. This is consistent with the observation that burial of Porg is not the

ARTICLE IN PRESS C.P. Babu, B.N. Nath / Deep-Sea Research II 52 (2005) 1965–1980

foremost P sink in most marine environments (Filippelli and Delaney, 1996, Delaney, 1998, Schenau and De Lange, 2001). High molar C/Porg ratios are found in stations above the OMZ (close to the coast) and ratios lower than Redfield ratios (106, Redfield et al., 1963; or 117, Anderson and Sarmiento, 1994) occur in oxygenated deep-sea sediments (except stn. 163/3801, Fig. 3A) of the study region. While the stations proximal to the slope may have possible contribution of more refractory terrestrial Porg, the values in deeper sediments indicate extensive oxidation of organic carbon and accumulation of refractory P-rich organic phases. High molar C/Porg ratios in sediments from the OMZ (Fig. 3A) higher than the Redfield ratio of organic matter indicate preferential loss of P-rich compounds relative to carbon from these sediments during organic matter decomposition compared to deep-sea sediments. The ratios in sediments impinged by the OMZ range between 322 and 508 (Fig. 3A), which are close to the values reported from OMZ sediments of the northern Arabian Sea (Schenau and De Lange, 2001). Among the OMZ sediments, the C/Porg ratios are relatively high in NE Arabian Sea sediments (341–508; Fig. 3A), indicating that under intense OMZ the preferential release of P is relatively high when compared to sediments from less intense OMZ (322–447; Fig. 3A) in SE Arabian Sea. The C/Porg ratios observed in this study are in close agreement with C/Porg ratios of other continental margin sediments (Ingall and Jahnke, 1994; McManus et al., 1997; van der Zee et al., 2002) but lower than those observed in anoxic setting (Ingall et al., 1993, Filippelli, 2001). Low Porg fraction and high C/Porg ratios under the intense OMZ in the NE Arabian Sea is in concert with the studies of Ingall et al., (1993), who observed very high C/Porg ratios in laminated sediments when compared to very low ratios in bioturbated sediments, implying the dependence on the degree of oxygenation in the water column. In a similar way relatively low molar C/Porg ratios in the deeper OMZ compared to the shallower OMZ indicate less release of Porg due to variability in intensity of OMZ.

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As all the P fractions, except P held in detrital phase, have a tendency to participate in biogeochemical reactions, Preactive is calculated as a sum of Pbio, PFe Porg and Paut (see Anderson et al., 2001). C/Preactive ratios lower than Redfield Ratios are noticed for deep-sea sediments of the study area (8–35, Fig. 3B,Table 3), indicating the degradation of organic carbon. Anderson et al. (2001) also have observed C/Preactive ratios less than the Redfield ratio for sediments with low organic carbon content (p2 wt%) for any redox setting. The C/Preactive ratios close to the Redfield ratio for the sediments from the OMZ in the SE Arabian Sea (81–118, Fig. 3B,Table 3) infer the preservation of organic matter that is initially delivered with a C/P ratio that is close to the Redfield value. In contrast, high Preactive and relatively low C/Preactive ratios less than Redfield Ratios (33–69, Fig. 3B, Table 3) in OMZ sediments from the NE Arabian Sea indicate the diagenetic transformation of labile forms of P to the authigenic phase. On the California margin very high C/Preactive ratios (4400) in younger sediments compared to substantially low values in subsurface sediments have been attributed to age-dependent organic carbon variations (Anderson et al., 2001), but low values observed in the surficial sediments studied here are presumably due to controls of organic carbon by variations in productivity, bottomwater oxygenation, bottom topography, molecular concentrations, etc. (Pedersen and Calvert, 1990; Paropkari et al., 1992, Keil and Cowie, 1999; Prakash Babu et al., 1999; Rao and Veerayya, 2000). Traditionally Porg depletion and the resulting high C/Porg ratios are used to infer depositional environments (Ingall et al., 1993; Filippelli, 2001). In the present study, C/Porg ratios are higher by almost an order of magnitude when compared to C/Preactive ratios. The variations in organic carbon content in marine sediments can be related to its delivery rate, initial organic carbon concentrations and degradation of individual organic compounds with time (Lee, 1992). Unlike organic carbon, all the reactive forms of P are transformed to Paut with Ptotal remaining similar with age. Thus the

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differences in transformation rate of organic carbon and Porg which affect C/Porg ratios show different values compared to C/Preactive ratios (Fig. 3A and B). C/Preactive ratios show geographic variations and point to P transformation in NE Arabian Sea sediments. Such a distinction is less visible when only C/Porg ratios are considered. 4.2. Biogenic (apatite) phase 4.2.1. Sediments from oxygen minimum zone The NH4Cl leach is able to distinguish between biogenic (fish debris) and authigenic apatite (Schenau and De Lange, 2000). In addition NH4Cl also leaches P associated with other phases such as exchangeable or loosely sorbed and with carbonates (Schenau and De Lange, 2000). The calcium carbonate content of the sediment samples ranges between 34% and 76% (Table 3). Moreover, as discussed below, P from terrigenous material is also traceable on slope and deep-sea sediments. The productivity in the Arabian Sea is high during summer and winter monsoon/convection (Bhattathiri et al., 1996; Madhupratap et al., 1996). The fish catch is also high during these seasons (Madhupratap et al., 2001). Hard parts of marine fish consist mainly of hydroxyapatite (Ca10(PO4)6(OH)2)8 crystals, which are more soluble when compared to fluorapatite (Posner et al., 1984). As the seawater is undersaturated with respect to biogenic apatite (Atlas and Pytkowics, 1977), dissolution of fish debris takes place in the water column and in the upper part of the sediment column. High Pbio proportion in sediments from the OMZ, in particular the SE Arabian Sea (28–48%; Table 3), when compared to oxygenated sediments, show preservation of fish debris under oxygen-depleted conditions, an observation in accordance with Schenau and De Lange (2000). High density, relatively large area of fish particles, and high sedimentation rates (as in the region off Kochi, 17–51 cm ka1, Sirocko and Lange, 1991, Thamban et al., 2001) appear to be responsible for preservation of Pbio. Earlier studies have shown that the laminated sediments and sediments impinged by the OMZ in other areas are sites for Pbio accumulation (Soutar and Isaacs, 1974; DeVries and Pearcy, 1982;

Schenau and De Lange, 2000). Therefore high Pbio accumulation is also expected in the NE Arabian Sea, as this region is characterized by the presence of an intense oxygen minimum zone with denitrification in the water column (Naqvi, 1991, De Sousa et al., 1996). Nevertheless, relatively low Pbio fraction is surprising (25–33%; Table 3), and sediments from this area deviate from the general trend of Pbio accumulation in reducing sediments. Sarma et al. (1996) and Hupe et al. (2001) have reported intense remineralization of organic matter in the northern Arabian Sea water column. Moreover Sarma et al. (1996) have observed an increasing trend of production of regenerated CO2 towards the northern Arabian Sea. This suggests Pbio degradation either in the water column and/or at sediment–sea-water interface and less Pbio accumulation in the sediments. This observation demonstrates the drastic differences in Pbio accumulation/regeneration for different areas of the Arabian Sea. Schenau and de Lange, (2000, 2001) also have attributed the drastic differences in Pbio depositional rates to regeneration processes. Phosphate concentrations in pore-water are maintained by microbial degradation of organic matter, desorption from iron oxides, and fish debris dissolution. Suess (1981) and Froelich et al. (1988) have observed an increase in pore-water phosphate concentrations in sediments from Peru continental margin overlain by OMZ due to dissolution of fish debris. But on the Mexican continental margin Jahnke et al. (1983) have shown less importance of fish debris dissolution as a source for pore-water PO3 4 . The phosphorus released during fish debris dissolution seems to be finally converted to authigenic phosphate. High authigenic phosphate concentrations (discussed below) give a supporting evidence for transformation of phosphorus released from dissolution of fish debris to authigenic phosphorus. At present, we do not have any pore-water data in the present investigation. But the pore-water phosphate data in Pakistan margin sediments (Schenau et al., 2000), which are exposed to similar oceanographic settings as seen here, show a drastic increase close to the sediment–sea-water interface and a decrease down-core. Similarly fluoride also shows a downcore decrease (Schenau et al., 2000). The decrease

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in pore-water phosphate and fluoride concentrations confirms removal of phosphate from pore waters and fixation in the sediments from OMZ as authigenic apatite. In general, increase in the exchangeable and authigenic P fraction must remove phosphate from the pore-water by adsorption and authigenic mineral precipitation, respectively (Sundby et al., 1992; Ruttenberg and Berner, 1993; Vink et al., 1997). While one carbonate-rich sample (191/5, 76% CaCO3 ) in the southern part of the study area overlain by the OMZ has high Pbio (61%, Table 3), another sample in the same area (99/2303) close to the outer shelf with high carbonate content (64%, Table 3) has low Pbio (26%) indicating that Pbio contents may not be dominantly carbonate-bound. Moreover, Pbio shows poor correlation with CaCO3 in the present investigation (figure not shown). This is consistent with the observation that biogenic CaCO3 does not generally bear large quantities of P (o3.2  106 mg/g; Sherwood et al., 1987; Delaney, 1998; Tamburini et al., 2002); P values are low in foraminiferal calcite (o3 ppm; Palmer, 1985; 18 ppm Sherwood et al., 1987), and calcite is not a significant source of P to the sediments (Tamburini et al., 2002). Oxyhydroxide coatings on carbonate shells appear to scavenge P from the water column. Alternatively the outer-shelf sediments of station 99/2303 are relict, iron stained, coarse sand and shells associated with angular basalt, laterite pebbles (Nair and Pylee, 1968). Fine-grained sediments are lacking in these sediments due to bottom water current activities. Lack of clay-sized particles perhaps reduce the adsorption for phosphorus. Siddiquie and Chowdhury (1968) noticed phosphorus variation to some extent with size. Alternatively, the nature of iron oxides (detrital?) appears to have reduced the adsorption capacity. 4.2.2. Sediments from oxygenated region The Pbio concentration in deep-sea sediments overlain by oxygenated water column is greater than 50%. Schenau and De Lange (2000) and Schenau et al. (2000) have observed low Pbio in deep-sea sediments, as much dissolution takes place in the oxygenated water column. As NH4Cl leach would extract P associated to fish debris,

1975

carbonate, and clay minerals (loosely sorbed), it can be assumed that high Pbio in deep-sea sediments may be due to sorption of P to clay minerals. 4.3. P associated with Fe-oxides (PFe) Fe-oxides with large sorption sites for P can act as a temporary or permanent trap for pore-water HPO2 diffusing upwards (Krom and Berner, 4 1980; Slomp et al., 1996; Filippelli, 2001). As discussed earlier low Pbio and Porg infer the presence of intense reducing conditions close to the interface in the NE Arabian Sea, which is also supported by Ba studies (Prakash Babu et al., 2002). Under these reducing conditions, relatively high PFe proportions in the NE Arabian Sea suggest (re)adsorption of HPO2 released from 4 either organic matter or Fe reduction at the interface and/or from depth onto reducible Fe–Mn phases, which act as a temporary sink. Fe–Mn oxyhydroxides also adsorb P while settling through the water column in addition to P adsorption from pore waters. Filippelli (2001) also has observed an increase in PFe along with Fe and Mn released from reductant treatment in Saanich Inlet shallower depth sediments of younger age. Due to oxygenated interface and under less intense reducing conditions shown by the accumulation of Pbio and Porg, the reduction of Fe-oxides may be occurring at depth in the SE Arabian Sea, resulting in relatively low PFe proportion in this region. 4.4. Authigenic phase (apatite) The fraction of phosphorus associated with the authigenic phase (Paut) is high in sediments from the OMZ by a factor of 2–8 when compared to well-oxygenated deep-sea sediments (Fig. 2B; Table 3). Among the sediments impinged by the OMZ, the percentage contribution of Paut is relatively high in NE Arabian Sea (46–47%; Fig. 2B) when compared to the SE Arabian Sea (10–39%, Fig. 2B). Phosphorus is released to porewater during organic matter degradation and Feoxide reduction. A part of the released phosphorus is (re)adsorbed on to Fe–Mn oxyhydroxides, released to the overlying water column, or

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transformed to an authigenic mineral phase. Though the mechanism of enhanced mineralization of P from organic matter under anoxia is still not understood completely (Colman and Holland, 2000; Slomp et al., 2002 and references therein), it is generally believed that the enhanced release of P from organic matter and Fe oxides in anoxic environments may be compensated by an increase of other forms of P such as authigenic (Ingall et al., 1993; Filippelli, 2001). The sediments with high Paut fraction coincide precisely with the presence of intense reducing conditions as their depositional environment revealed by cadmium (Cd) contents in the sediments, as Cd accumulates even under trace levels of H2S (Rosenthal et al., 1995; Morford and Emerson, 1999). Sediments with high Paut have elevated concentrations of Cd (up to 1.5 ppm; Prakash Babu et al., 2002). The combined effect of intense remineralization of organic matter, as revealed by low Porg and Pbio, and reduction of Fe–Mn oxyhydroxides, enhances the release and accumulation of phosphorus as an authigenic mineral. The enhanced contribution from PFe in upper-slope sediments (Table 3) overlain by the OMZ indicate the importance of easily reducible or reactive iron-bound P, which acts as a temporary shuttle in transferring P to authigenic mineral (Slomp et al., 1996, Filippeli, 2001). High Paut fraction in the OMZ sediments clearly demonstrates the process of phosphogenesis close to the interface in the eastern Arabian Sea. Similarly, Froelich et al. (1988) have observed that CFA formation linked to an interface precipitation process rather to organic diagenesis of phosphorus deeper in the sediment on the Peru continental margin. The inverse relations between Porg, Pbio and Paut give an ample evidence for P-sink switch during early diagenesis. The Paut contribution in the NE and SE Arabian Sea upper-slope sediments is in accordance with the intensity of the OMZ. Variations of Paut in conjunction with PFe, Porg and Pbio demonstrate extensive benthic regenerative mechanisms of reactive P under an intense OMZ in the NE Arabian Sea when compared to the SE margin sediments. The authigenic formation of CFA in upper-slope sediments as observed in the present investigation is in conformity with the studies of

Rao and Rao (1996), Nath et al. (2000), and Schenau and De Lange (2001). While Schenau and de Lange (2001) identified phosphogenesis from pore-water fluxes in recent sediments from the Pakistan margin, Rao and Rao (1996) and Nath et al. (2000) reported the occurrence of authigenic CFA-rich phosphatic concretions on the marginal highs off Goa. Rare-earth elemental patterns, trace element concentrations, and mineralogy have unequivocally suggested the formation of phosphotic concretions in this region from authigenic deposition from pore waters (Nath et al., 2000). From the foregoing discussion it appears that there is a coherence between a high fraction of Paut, high concentrations of Cd, and low contributions from Porg and Pbio in NE Arabian Sea sediments where the OMZ is intense, and vice versa in the SE Arabian Sea sediments overlain by a less intense OMZ, wherein the contribution from Paut is low, concentrations of Cd are low whereas the proportion from Porg and Pbio is high. 4.5. Detrital phosphorus Pdet fraction is low (range between 6% and 16%) in all the samples except at two coastal stations in the southern Arabian Sea (24–45%, Table 3) and one deep station in the NE Arabian Sea (27%). This is consistent with the earlier observations that non-reactive P, primarily consisting of detrital apatite, usually represents only a small portion relative to the total accumulating P flux (Ruttenburg and Berner, 1993; Filippelli and Delaney, 1996; Schenau and de Lange, 2001). High Pdet contribution (24–45%) in upper-slope sediment of Karwar (RVG 99/2303) and in inner shelf off Kochi (RVG 71/1432) indicate high terrigenous influence in this region. The sediments in the Karwar region are relict in nature and were deposited during lowered sea levels (Nair and Pylee, 1968). As the sampling sites were close to the coast during lowered sea levels, the detrital apatite material might have been delivered directly to the sampling site. High Pdet in sediments off Kochi may be due to the deposition through the bar mouth, where detrital rare-earth element signatures are reported on the inner shelf off Kochi (Nath et al., 2000), and terrestrially derived

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clay minerals on outer shelf (Rao et al., 1983). In addition to these two samples, the Pdet contribution is relatively high (10–16%) in slope sediments off Marmagao–Mangalore region compared to slope sediments of other regions. Rao and Veerayya (2000) have shown the influence of winnowing in transporting the material from the shelf to the deeper basin in this region. The transportation of shelf material has probably enhanced the Pdet in this area. A latitudinal decrease of Pdet in deep-sea sediments from north to south is interesting. Relatively high Pdet contributions in the north appears to be due to terrigenous material from the Indus River. The decreasing Pdet contribution towards south infers waning influence of the Indus material. Decreasing influence of the Indusderived material from north to south was also observed earlier by Divakar Naidu (1991) and Rao and Rao (1995) based on variations in calcium carbonate and clay minerals.

5. Conclusions Authigenic phosphorus is the dominant phase of P and constitutes up to 50% of total phosphorus in the eastern Arabian Sea surficial sediments overlain by an oxygen-depleted water column. The early diagenetic enrichment of phosphorus through sink switching appears to be more significant under intense reducing conditions close to the interface in the NE Arabian Sea when compared to SE Arabian Sea. In the NE Arabian Sea Porg and Pbio are released to pore waters due to intense remineralization of organic matter and finally transformed to an authigenic P-mineral phase. The release of Pbio under intense OMZ conditions is not in line with the general notion that the sediments from OMZ and laminated sediments are sites for Pbio preservation. PFe appears to act as a temporary shuttle in transferring regenerated Porg and Pbio to Paut. The NH4Cl leach in addition to Pbio appears to extract P either in loosely adsorbed or in exchangeable position. A cautious approach is warranted before ascribing the P extracted in this leach entirely to P-bound to the biogenic phase.

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The formation of Paut in surface sediments from the OMZ of the eastern Arabian Sea is in conformity with the occurrence of phosphogenesis in Pakistan and Oman margin sediments and the authigenic P concretions in the Indian margin. Variation of Paut with the changes in the intensity of OMZ suggests that Paut may be used as a proxy for paleoxygenation levels. Further it appears that Pdet can be used as a tracer for delineating terrigenous influence, as the variations of Pdet in the present study are in good agreement with other proxies used to trace terrigenous sedimentation.

Acknowledgments The authors thank the Director, NIO for encouragement and permission to publish this paper. The paper benefited with valuable comments from Drs. G.M. Filippelli, Peggy Delaney, and an anonymous reviewer. Discussions with Dr P.V. Bhaskar also helped us improve the manuscript. The authors also thank Mr. A. Mahale, P.J. Pawaskar and Shyam Akerkar for their kind help while making figures. This is NIO’s contribution no. 3998. References Anderson, L.A., Sarmiento, J.L., 1994. Redfield ratios of remineralization determined by nutrient data analysis. Global Biogeochemical Cycles 8, 65–80. Anderson, L.D., Delaney, M.L., 2000. Sequential extraction and analysis of phosphorus in marine sediments: streamlining of the SEDEX procedure. Limnology and Oceanography 45, 509–515. Anderson, L.D., Delaney, M.L., Faul, K.L., 2001. Carbon to phosphorus ratios in sediments: implications for nutrient cycling. Global Biogeochemical Cycles 15, 65–79. Atlas, E., Pytkowics, R.M., 1977. Solubility behavior of apatites in sea water. Limnology and Oceanography 22, 290–300. Berner, R.A., 1984. Sedimentary pyrite formation: an update. Geochimica et Cosmochimica Acta 48, 605–615. Berner, R.A., 1989. Biogeochemical cycles of carbon and sulfur and their effect on atmospheric oxygen over Phanerozoic time. Palaeogeography, Palaeoclimatology, Palaeoecology 75, 97–122. Berner, R.A., Canfield, D.E., 1989. A new model for atmospheric oxygen over Phanerozoic time. American Journal of Science 289, 333–361.

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