Distribution, source and chemical speciation of phosphorus in surface sediments of the central Pacific Ocean

Deep-Sea Research I 105 (2015) 74–82

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Distribution, source and chemical speciation of phosphorus in surface sediments of the central Pacific Ocean Jianyu Ni a,b, Peng Lin a, Yang Zhen b, Xuying Yao b, Laodong Guo a,n a b

School of Freshwater Sciences, University of Wisconsin-Milwaukee, 600 East Greenfield Avenue, Milwaukee, WI 53204, USA Key Laboratory of Submarine Geosciences, Second Institute of Oceanography, State Oceanic Administration, Hangzhou 310012, China

art ic l e i nf o

a b s t r a c t

Article history: Received 8 May 2015 Received in revised form 11 August 2015 Accepted 14 August 2015 Available online 28 August 2015

The abundance of five forms of phosphorus (P) in surface sediments from the central Pacific Ocean (4.5–15ºN, 154–143ºW) was determined using a sequential extraction procedure (SEDEX) to examine the distribution and source of different P species. Total P (TP) concentrations ranged from 13.2 to 119 mmol-P/g with an average of 48.6727.4 mmol-P/g. Within the TP pool, total inorganic P (TIP) concentrations varied from 11.1 to 121 mmol-P/g, while total organic P (TOP) concentrations ranged from undetectable to 4.8 mmol-P/g. Inorganic P was generally the predominant form in surface sediments, comprising on average up to 93% of sedimentary TP, leaving o16% as TOP. Among the five P species, the authigenic or CaCO3-bound P and detrital P were the two major P species (comprising on average 43.4713.5% and 45.7714.8% of TP, respectively), followed by the refractory organic P, representing 6.772.4% of TP. Fe-bound P accounted for 3.371.3% of TP, and exchangeable or adsorbed P made up less than 1% of TP. The spatial distribution of different sedimentary P species showed that higher concentrations of detrital P and Fe-bound P were both found at around 11°N, suggesting similar sources for these two P species. Much of the detrital P was derived from atmospheric sources in the study area, where heavy rainfall in the intertropical convergence zone between 3°N and 11°N has been widely reported. Compared with other marine environments, the central Pacific Ocean had relatively higher detrital P, but lower abundance of adsorbed-P and Fe-bound P. These unquine results suggested that most of the labile P could have been released into the water column during its settling from the surface to the seafloor, or that atmospheric inputs of refractory P were an important source for sedimentary P, accounting for an average of 63% of the TP, in the central Pacific Ocean. High proportions of authigenic P in deep-sea sediments, on the other hand, implied that oceanic sediments are an important sink for reactive P species. Relatively lower OC/Org-P and OC/Preact ratios suggested a higher sedimentary burial for Org-P and/or attested the importance of detrital P derived from atmospheric sources in the central Pacific Ocean. & 2015 Elsevier Ltd. All rights reserved.

Keywords: Phosphorus speciation SEDEX Sediments Central Pacific Ocean

1. Introduction Phosphorus (P) is an important macronutrient for the growth of marine phytoplankton and is thought to control marine productivity over both geological and shorter time scales (Van Cappellen and Ingall, 1996; Tyrrell, 1999; Benitez-Nelson, 2000; Ruttenberg, 2003). Riverine and atmospheric inputs are considered to be the main sources of P to the ocean, and sediments represent an important sink for P in its oceanic biogeochemical cycle (Filippelli, 1997; Delaney, 1998). Phosphorus can be removed from seawater by primary productivity in the euphotic zone and transported to the seafloor and buried in sediments. It can also be removed by various physicochemical processes including adsorption onto particle n

Corresponding author. E-mail address: [email protected] (L. Guo).

http://dx.doi.org/10.1016/j.dsr.2015.08.008 0967-0637/& 2015 Elsevier Ltd. All rights reserved.

surfaces such as clay minerals, iron oxyhydroxides, carbonate and the formation of authigenic minerals such as apatite (Eijsink et al., 2000; Zhang and Huang, 2007, 2011) due to the high particle reactivity of P (Lin et al., 2013). Over the last two decades, the SEDEX sequential extraction technique has been widely used to operationally classify sedimentary and suspended particulate P into different phases based on its binding phases and solubility properties (Ruttenberg, 1992; Zhang et al., 2004, 2010) and to study the marine P cycle in the water column and sediments (Ruttenberg and Berner, 1993; Berner and Rao, 1994; Vink et al., 1997; Küster-Heins et al., 2010; Lin et al., 2012, 2013; März et al., 2014). However, studies characterizing P in sediments from open oceans such as the central Pacific Ocean remain few. In addition, previous studies mainly focused on ODP cores and their paleoceanographic significance (Filippelli and Delaney, 1996; Delaney and Anderson, 1997, 2000), but less on spatial distributions and source terms. The open ocean is the main area for seabed mineral resources and

J. Ni et al. / Deep-Sea Research I 105 (2015) 74–82

could be the potential mineral sources in future (Glover and Smith, 2003; Scott, 2011). Considering its vast area that covers approximately 54% of the earth's surface, and thus large reservoir and inventory for different elements and chemical species including P, the open ocean, both water column and sediments, has received increasing attention, especially on its significance in the global biogeochemical cycles and climatic change (Filippelli and Delaney, 1996; Delaney and Anderson, 1997, 2000; Benitez-Nelson, 2000; Glover and Smith, 2003; Ruttenberg, 2003; Filippelli, 2008). Furthermore, open ocean could also be vulnerable to anthropogenic influences. For example, deep-sea mining of mineral resources, such as polymetallic nodules, could result in the disruption of surface sediments (Glover and Smith, 2003), causing sediments resuspension into the overlying water column. Sequestered P in the sediments would then be released into bottom water, affecting the abundance, distribution, and speciation of P and the benthic ecosystem as a whole. P is associated with different chemical/physical phases in sediments. Different forms of sedimentary P have different biological and chemical reactivities and thus different geochemical behavior in marine environments (Delaney, 1998; Ruttenberg, 2003; Filippelli, 2008). Therefore, knowledge of chemical speciation of P, in addition to the abundance and distribution of total P, is important for a better understanding of environmental behavior and biogeochemical cycling pathways of P in the ocean. Major P species in sediments include organic P, P associated with hydrous ferric oxides, authigenic carbonate fluorapatite (francolite), hydroxyapatite of skeletal debris, P incorporated in CaCO3, and P adsorbed on other minerals such as clay minerals. Binding forms largely determine whether sedimentary P is reactive or refractory in the ocean (Zhang et al., 2004). In the present study, we investigated the chemical forms of sedimentary P and their distributions in surface sediments of the central Pacific Ocean. Our objective was to assess the abundance and distribution of sedimentary P and its chemical speciation and sources in the central Pacific Ocean for better understanding the biogeochemical cycle of P in marine environments.

2. Material and methods 2.1. Sampling Sediment samples were collected from the central Pacific Ocean (4.5–15°N, 154–143°W) during the DY29 cruise onboard the R/V HAIYANGLIUHAO using box-corer in 2013. Our sampling locations were at the west of polymetallic nodules enriched region in the Clarion–Clipperton fracture zone (Fig. 1, Table 1). The box-corer samples represent approximately the top 30–50 cm of the sediment, but only the top 2 cm was used and stored frozen until further processing in the laboratory. Sediment samples were freeze-dried and ground to a fine powder for sieving (o200 mesh, 74 mm) with an agate pestle and mortar prior to analysis. Aliquots of the ground sediment samples were taken for the analyses of P using SEDEX sequential extraction, as well as the measurement of total organic carbon (TOC) contents. 2.2. Sequential extraction and measurements Sedimentary P was chemically fractionated into five operationally defined P species: (1) P adsorbed onto grain surfaces (Adsorbed-P), (2) P associated with easily reducible iron and manganese oxides and/or oxyhydroxides (Fe–P), (3) authigenic carbonate fluorapatite, biogenic apatite and CaCO3-associated P (CFA-P), (4) detrital apatite P (Detr-P), and (5) refractory organic P (Org-P). The specific extraction procedures were largely based on SEDEX sequential extraction technique developed and revised by Ruttenberg (1992) and Zhang et al. (2004, 2010) for marine sediments (Fig. 2). Extractions were conducted in 50 ml polyethylene centrifuge

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Fig. 1. Sampling locations in the central Pacific Ocean (4.5–15°N, 154–143°W) during 2013.

tubes using about 0.1 g of freeze-dried sediment and 10 ml of the extraction solution. After each extraction, the sample was centrifuged and then the supernatant solution containing the extracted P was analyzed by the standard phosphomolybdenum blue method (Hansen and Koroleff, 1999) on an Agilent 8453 UV–vis spectrophotometer. The same medium as extraction solution for each P species was used to prepare the standard and blank samples. The modified chromogenic reagent was prepared to avoid the interference from extraction solution (e. g., bicarbonate–dithionite extracts of Fe–P) on the phosphomolybdenum blue method (Zhang et al., 2010). Acid extraction solution was neutralized and pre-treated prior to the analysis of phosphomolybdenum blue method. The detection limit for extracted P was 8–10 nM based on replicate blank sample measurements using 5 cm cuvettes, with a precision better than 2%. Reactive P was calculated as the sum of all non-detrital P-phases (Preact, Sutula et al., 2004; Lin et al., 2013). Independent analysis of total sedimentary phosphorus (TP) was also conducted using the high-temperature combustion method (Zhang et al., 2010; Lin et al., 2013) to compare with those from sequential extraction and to ensure data quality. In brief, 0.1 g of sediment sample was wetted with 0.5 M MgCl2 solution and heated in an oven at 95 ºC until dry, followed by ashing in a furnace at 550 ºC for 2 h to decompose organic P compounds. The residue was extracted using 1 M HCl solution at room temperature for about 24 h. Total inorganic phosphorus (TIP) in sediments was directly extracted from sediments with 1 M HCl solution at room temperature for 24 h (Aspila et al., 1976). Both extractions of TP and TIP were quantified after neutralization and dilution. Contents of total organic phosphorus (TOP) were then calculated based on the differences between TP and TIP. It should be noted that additional filtration was not conducted after sample centrifugation, which might result in potential carryover during the SEDEX extraction procedure. Data of organic fraction in the Adsorbed-P and CFA-P were not presented here due to their extremely low concentrations, typically below the detection limit. Regardless, excellent P recovery also indicated negligible sample loss during our SEDEX extraction procedures (see discussion below) and negligible organic P fraction in the Adsorbed-P and CFA-P phases in the study area. 2.3. Measurement of sedimentary organic carbon For TOC analysis, about 0.2 g sediment sample was decalcified

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Table 1 Sampling locations, water depths, sediment sample description, and contents (in mmol-P/g-dried-sediment) of total phosphorus (TP), total inorganic P (TIP) and total organic P (TOP).

Station

Longitude (°W)

Latitude (°N)

Depth (m)

Sediment type

BC01 BC03 BC02 BC17 BC16 BC10 BC09 BC04 BC12 BC07 BC14 BC13

153.295 150.606 154.801 154.045 154.793 147.227 145.354 149.425 145.053 142.556 149.422 142.547

14.772 11.901 10.580 10.032 9.188 8.408 8.358 8.248 7.491 6.490 6.448 4.728

5828 5363 5107 5166 5388 5219 5317 5006 5085 5000 4961 4417

Dark-gray mud Brown siliceouse ooze Brown siliceouse clay Brown siliceouse ooze Brown siliceouse ooze Pale-brown siliceous ooze Pale-brown siliceous ooze Pale-brown siliceous ooze Grayish brown siliceous ooze Pale-brown siliceous ooze Brown siliceous ooze Grayish-white biogenic calcareous ooze

a

TP

24.9 76.0 55.5 39.5 38.5 38.7 55.8 35.2 36.3 50.6 119.2 13.2

TIP (μmol-P/g) 23.2 72.8 51.9 36.9 33.8 35.2 56.2 31.3 33.7 49.0 121.1 11.1

TOPa

1.7 3.1 4.0 3.9 3.4 3.6 2.6 2.5 3.6 3.0 3.2 0.7

Values are derived from the Org-P based on SEDEX procedure.

( 20 mg) was measured for organic C on an elemental analyzer (Thermo NE1112). The precision of TOC analysis were 70.01% as determined by replicate analysis of standards and samples.

3. Results The total sedimentary phosphorus (TP) content and relative contributions of each P species to TP are shown in Fig. 3. The sum of the five P species derived from SEDEX agrees well with an independent determination of TP from the same samples by the hightemperature combustion method (Zhang et al., 2010; Lin et al., 2013), with a correlation coefficient of 0.9937 (Fig. 4), demonstrating an excellent P recovery of our sequential extraction procedures. 3.1. Total sedimentary phosphorus Concentrations of TP obtained by the high-temperature combustion method ranged from 13.2 to 119 μmol-P/g-dried-sediment, with an average of 48.6 727.4 μmol-P/g. Within the TP pool, concentrations of total inorganic phosphorus (TIP) ranged from 11.1 to 121 μmol-P/g with an average of 46.3 728.5 μmol-P/g. Total organic phosphorus (TOP) concentrations ranged from undetectable to 4.8 μmol-P/g with an average of 2.96 71.01 μmol-P/ g. Inorganic P was generally the predominant form of TP in the sediments of the study area, making up 92 74% of TP on average, while organic P was only a minor fraction of the TP pool. Higher contents of TP were found at station BC14 around 11°N. The lowest concentration was found at station BC13 where the sediment was mostly biogenic calcareous ooze (Fig. 5), similar to those reported by Sherwood et al. (1987). Total extracted sedimentary P concentrations from the SEDEX technique were in the range of 14.1– 114.4 μmol-P/g with an average of 49.4 726.5 μmol-P/g, and similar to those obtained by high-temperature combustion method. 3.2. Loosely adsorbed or exchangeable phosphorus

Fig. 2. Procedures used for sequential extraction of different forms of sedimentary phosphorus (modified from Ruttenberg (1992) and Zhang et al. (2004, 2010)).

by reacting with an excess of 1 mol/L HCl (Guo et al., 2004; Zhang et al., 2010). After subsequent rinses with ultrapure water, the decalcified samples were freeze-dried. Aliquot of the samples

The concentrations of adsorbed-P in surface sediments varied from 0.19 to 0.48 μmol-P/g with an average of 0.347 0.08 μmol-P/ g (Table 2), which was relatively lower than those previously measured for different areas of the Pacific Ocean (e.g., Murray and Leinen, 1993; Filippelli and Delaney, 1996; Delaney and Anderson, 1997, 2000; Tamburini et al., 2002). Adsorbed-P only made up, on average, 0.9 70.8% of TP. In general, adsorbed-P showed a spatial gradient along the north–south transect with higher values in the region with lower TP contents (Fig. 5).

J. Ni et al. / Deep-Sea Research I 105 (2015) 74–82

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Fig. 3. Concentrations (left) and the partitioning (right) of phosphorus among five different sedimentary phases.

average, Detr-P comprised up to 45.7714.8% of TP in surface sediments of the central Pacific Ocean. Higher concentrations of Detr-P were observed in the region around 11°N, in addition to the highest detrital P concentrations at BC14 station around 6°N (Fig. 5). 3.6. Refractory organic phosphorus Concentrations of Org-P in surface sediments from the central Pacific Ocean ranged from 0.70 to 4.05 μmol-P/g, with an average of 2.94 70.97 μmol-P/g (Table 2). On average, the Org-P fraction constituted 6.7 72.4% of the TP pool in surface sediments. 3.7. Organic carbon Fig. 4. Comparisons in total phosphorus abundance between direct measurements using the high-temperature combustion method (Aspila et al., 1976; Lin et al., 2013) and the sum of five different phosphorus pools derived from the sequential extraction method (Fig. 2).

3.3. Iron-bound phosphorus The iron-bound P (Fe–P) content ranged from 0.79 to 2.09 μmolP/g with an average of 1.4070.45 μmol-P/g and comprised, on average, 3.471.3% of the TP in surface sediments (Table 2). It showed a similar spatial distribution pattern with that of detrital apatite (see below), with higher concentrations around the region at 11°N and lower concentrations in the southern part of the study area (Fig. 5), suggesting both Fe–P and detrital P might be from similar sources or preservation mechanisms in the central Pacific Ocean. Because of its high affinity for phosphate, ferric (oxy)hydroxides at the oxic environments were usually thought to be an important sink for P in the sediments, although mineralogy and crystallinity of Fe oxides could strongly influence the adsorption characteristics of P and their susceptibility to reduction (Ruttenberg, 1992). 3.4. Authigenic carbonate fluorapatite, biogenic apatite and CaCO3-associated phosphorus The concentrations of authigenic carbonate fluorapatite, biogenic apatite, and CaCO3-bound P (CFA-P) ranged from 11.2 to 35.8 μmol-P/g, with an average of 19.4 77.54 μmol-P/g (Table 2). As shown in Fig. 3, the CFA-P was the second largest pool of sedimentary P, making up, on average, 43.4 713.5% of TP in the sediment of the central Pacific Ocean. 3.5. Detrital apatite phosphorus Concentrations of detrital apatite (Detr-P) ranged from 1.0 to 74.1 μmol-P/g, with an average of 25.3719.3 μmol-P/g (Table 2), and represented the largest pool of sedimentary TP in the study area. On

Total organic carbon (TOC) contents in surface sediment samples varied from 0.07% to 1.07% (wt%), with an average of 0.4170.24%, showing an increasing trend from north to south (Table 2, Fig. 6). The TOC in the sediments was mainly derived from the primary productivity in the euphotic zone and the eventual sink from the water column to the sediment in the open ocean (Ingall and van Cappellen, 1990; Anderson et al., 2001; Algeo and Ingall, 2007). Lower TOC contents observed here are consistent with the oligotrophic condition in the study area.

4. Discussion 4.1. Distributions of phosphorus species Average concentrations of TP in sediments from different oceanic environments ranged from 7 to 40 μmol-P/g, depending on sedimentation settings and primary productivity in specific regions (e. g., Murray and Leinen, 1993; Filippelli and Delaney, 1996; Delaney and Anderson, 1997, 2000; Ruttenberg and Goñi, 1997; Tamburini, et al., 2002; Fang et al., 2007; Zhang et al., 2004, 2010). The TP contents observed in the central Pacific Ocean, ranging from 13.2 to 119 (average of 48.6 727.4) μmol-P/g-driedsediment, seemed to be relatively higher than those reported in some previous studies, even though our study area is in the oligotrophic region of the Pacific Ocean (Fig. 8). One possible reason for the higher TP contents might be that only the top 2 cm sediments were analyzed in the present study, which could be composed of mostly younger (500–5000 yr depending on sedimentation rates) and fresher materials. Usually, the sedimentary P concentration decreased with sediment depths and age since substantial reactive P may have been degraded and/or transported out of sediments and returned to the overlying water column during early diagenesis. On the other hand, higher TP contents could be related to the input of refractory P sources in addition to biogenic

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J. Ni et al. / Deep-Sea Research I 105 (2015) 74–82

Fig. 5. Spatial distributions of different sedimentary phosphorus species, including total phosphorus (TP), exchangeable or adsorbed P (Adsorbed-P), Fe-associated P (Fe–P), authigenic P (CFA-P), detrital P (Detr-P) and refractory organic P (Org-P), in the central Pacific Ocean (all in μmol-P/g-dried-sediment).

or autochthonous sources. Alternatively, the high TP contents might result from the adsorption and preservation of dissolved P onto particles and sediments, similar to the preservation mechanism of organic carbon in marine sediments (Keil et al., 1994). Our study area was located mainly at pelagic red clay region except one station in the south, and the major sediment type was finegrained siliceous ooze or pelagic clay in these areas. This type of sediment generally has high capability for the adsorption of phosphate (Paytan and McLaughlin, 2007). The high concentration of phosphate in the deep water column (up to 3 μmol-P/L) and low sedimentation rates (Murray and Leinen, 1993) would allow sediments to adsorb more P on grain surfaces, followed by the incorporation into the sediment. The relative proportions of different P species followed the order of Detr-P4CFA-P4Org-P4Fe–P4Adsorb-P in the central Pacific Ocean (Fig. 7). Detr-P was found to be the most important species in sedimentary P of the study area (Figs. 3 and 8). These results are similar to those observed in other studies showing higher detrital apatite and terrestrial inputs in continental margin sediments with 15% to over

40% of the total P as detrital apatite, such as the Changjiang River mouth (Rao and Berner, 1997), the Bohai and Yellow Seas (Liu et al., 2004), the East China Sea (ECS, Fang et al., 2007) and coastal Arctic Ocean (Zhang et al., 2010). However, the Detr-P abundance in the study area is much higher than that reported for the equatorial Pacific Ocean (Filippelli and Delaney, 1996). They found that Detr-P was a minor fraction of the TP pool (o1%) following the order of CFAP4Fe–P4Org-P4Adsorb-P4Detr-P in their abundance in the equatorial Pacific Ocean (Filippelli and Delaney, 1996). Higher contents of Detr-P observed in the central Pacific Ocean thus suggested a different source term compared to other open ocean environments. In general, P mainly enters the ocean via rivers and atmospheric transport, with the atmospheric/aerosol inputs predominating in open ocean environments, especially within the northern hemisphere “dust belt” between  10°N and 60°N (Prospero et al., 2002; Maher et al., 2010). Kyte et al. (1993) reported that bulk surface sediment at site LL44-GC3 in the North Central Pacific contained 95% eolian dust. In our study area, higher detrital P contents were observed mostly at around 11°N, consistent with the

J. Ni et al. / Deep-Sea Research I 105 (2015) 74–82

Table 2 Concentrations of different sedimentary P species, including exchangeable or adsorbed P (Ads-P), Fe-associated P (Fe–P), authigenic P (CFA-P), detrital P (Detr-P) and refractory organic P (Org-P) (all in μmol-P/g-dried-sediment), and total organic carbon (TOC in wt%) as well as the molar ratios of TOC over reactive P or organic P in the central Pacific Ocean. Station

Ads-P Fe–P CFAP

Detr-P Org-P TOC

BC01 BC03 BC02 BC17 BC16 BC10 BC09 BC04 BC12 BC07 BC14 BC13 Average SD

0.19 0.30 0.39 0.34 0.42 0.31 0.44 0.25 0.27 0.37 0.32 0.48 0.34 0.08

11.70 47.80 31.81 20.83 15.31 17.90 28.13 12.35 19.05 23.67 74.13 1.00 25.31 19.31

0.94 2.09 1.88 1.85 1.72 1.58 1.50 0.81 1.43 1.30 0.92 0.79 1.40 0.45

12.36 25.07 18.35 14.24 17.23 15.26 28.38 18.98 12.14 24.17 35.83 11.18 19.43 7.54

1.69 3.08 4.05 3.93 3.39 3.61 2.57 2.46 3.63 2.96 3.22 0.70 2.94 0.97

0.07 0.27 0.40 0.44 0.43 0.43 0.19 0.49 0.44 0.47 0.28 1.07 0.41 0.24

TOC/Preact ratio

Organic C/P ratio

4 7 14 18 16 17 5 18 21 14 6 69 13 4

33 73 82 93 105 99 62 164 101 131 72 1274 92 35

Fig. 6. Spatial distributions of total organic carbon (TOC) in the central Pacific Ocean, showing a general increasing trend from north to south of the study area.

Fig. 7. Phase partitioning of different sedimentary phosphorus species in the central Pacific Ocean, including exchangeable/adsorbed P (Adsorbed-P), Fe-associated P (Fe–P), authigenic P (CFA-P), detrital P (Detr-P) and refractory organic P (Org-P), based on average data.

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heavy rainfall in the intertropical convergence zone (ITCZ) between 3°N and 11°N (Xie and Marcantonio, 2012) where most of dust was removed from the atmosphere and deposited into the ocean. In this area, eolian dust mainly came from arid regions of Asia. Flaum (2008) and Guo et al. (2011) have reported that Detr-P and CFA-P are the dominant P species in the dust from Chinese Loess Plateau and Northern China desert and account for about 42.5% and 41.9% of TP, respectively. Interestingly, the percentage of detrital apatite and CFA-P in sedimentary TP of the study area (45.7714.8% for Detr-P and 43.4713.5% for CFA-P) is comparable to that of Asian eolian dust, suggesting that the important role of Asian eolian dust in the source of sedimentary P in the central Pacific Ocean and detrital P in the study area is perhaps largely derived from eolian inputs. High abundance of detrital P in sediment of the study area and similar detrital apatite percentages between sediment samples and eolian dust (45.7714.8% vs. 42.5%) are consistent with the most inert nature of detrital apatite among the five P species in the TP pool and support our hypothesis that the sedimentary detrital P is largely derived from eolian inputs in the central Pacific Ocean (see also discussion below). The CFA-P is the second largest pool of sedimentary TP in the study area (Fig. 3). In marine sediments, CFA-P precipitates in situ at the expense of other P pools, particularly from organic, loosely adsorbed, and iron oxide-bound P during early diagenesis (Ruttenberg and Berner, 1993), and is a permanent sink for reactive P in the ocean (Froelich et al., 1982; Ruttenberg and Berner, 1993; Filippelli, 2008). High organic matter flux with either low or high oxygen would favor the formation of CFA-P, while it would be limited by conditions with low organic matter flux and intermediate oxygen such as oligotrophic deep-sea settings (Kraal et al., 2012; Tsandev et al., 2012). Recent studies found that a significant portion of CFA-P in marine sediments could also be resulted from atmospheric inputs in addition to in situ authigenic CFA-P formation (Eijsink et al., 2000; Faul et al., 2005; Palastanga et al., 2011; März et al., 2014), and accounted for 16–30% of TP in suspended sediments of the Amazon River (Berner and Rao, 1994) and  13% of TP on the Arctic Ocean shelf (Zhang et al., 2010; März et al., 2014). Therefore, terrestrially derived CFA-P inputs from aerosol and riverine sediments would likely result in the observed P burial efficiency in marine sediment (Eijsink et al., 2000; Anderson et al., 2010; Lyons et al., 2011; Kraal et al., 2012; Sekula-Wood et al., 2012; März et al., 2014). Regarding the potential eolian input of CFA-P to the North Pacific Ocean, Flaum (2008) and Guo et al. (2011) found that the TP of eolian dust from the Chinese Loess Plateau and Inner Mongolia desert contained 40–74% of CFA-P. Therefore, it is likely that part of CFA-P in central Pacific Ocean sediments might also be derived from eolian inputs. Following the approach of Ruttenberg (2003), the average contribution of detrital CFA-P from atmospheric eolian was estimated to be as high as 41714% of CFA-P in surface sediments, assuming that the dust settled in the study area was mainly from Asia desert and loess, and the content of CFA-P in the dust did not change during its transportation (7 μmol-P/g) (Flaum, 2008; Guo et al., 2011). Overall, assuming all detrital P, including detrital apatite (45.7% of the TP) and 41714% of the CFA-P (43.4% of the TP), is from eolian dusts, on average, 63% of the sedimentary P could be from atmospheric sources in the central Pacific Ocean. In oceanic environments, P transport from the upper water column to the seafloor is mainly in organic form, with additional contributions from detrital and authigenic forms as well as P bound to Fe (oxyhydr) oxides and adsorbed onto particle surfaces (Föllmi, 1996; Paytan et al., 2003). Among different forms of sedimentary P, loosely adsorbed, Fe-bound, and organic P may be readily released to ambient water through desorption, reductive dissolution and degradation during their settling. In the pelagic clay environment, below the carbonate compensation depth and with low sedimentation rates, most of organic P and P-containing

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Fig. 8. Comparisons in the abundance (left) and partitioning (right) of phosphorus among five sedimentary phases in different oceanic environments (literature data are from Delaney and Anderson, 1997, 2000; Fang et al., 2007; Zhang et al., 2010).

1300

OC/Org-P (molar)

Org-P ( molP/g)

4 3 2

1250 1200 150 100 50

1

0 0

0.2

0.4

0.6

0.8

1

1.2

TOC (wt%)

0

0.2

0.4

0.6

0.8

1

1.2

TOC (wt%)

Fig. 9. Relationship between total organic carbon (TOC) and organic phosphorus (Org-P) or molar TOC/Org-P ratio. Note that different y-scales are used in both plots to include the high TOC data point.

biogenic carbonate particles were regenerated and dispersed into the water column during settling to seafloor or surface sediments (Zhou and Kyte, 1992). Compared to other marine environments, the central Pacific Ocean seemed to have relatively lower adsorbed-P, Fe–P, and Org-P contents (Fig. 8), suggesting that most of the labile P could have been released into the water column during its long transport from the surface to seafloor and/or during early diagenesis in the sediment. 4.2. Sedimentary organic carbon to phosphorus ratio Phosphorus is a key element in global biogeochemical cycles due to its role as an essential nutrient for the growth of marine phytoplankton, and through the primary productivity in the euphotic zone and its linkage with global carbon cycles. The primary delivery mechanism of P to the sediments is through its association with organic matter. Therefore, the total organic carbon to organic P (OC/Org-P) ratio preserved in the sediments can be used to interpret the fate of sedimentary P (Ingall and Van Cappellen, 1990; Anderson et al., 2001; Ruttenberg, 2003; Algeo and Ingall, 2007; Kraal et al., 2010, 2012). In oxic oceanic environments, most of the labile OC and P can be regenerated in the water column before reaching seawater-sediment interface, resulting in a low Org-P content (6.7% of TP on average) and low TOC content (only 0.41% of dried sediment on average) in sediments of the study area. Indeed, previous studies have observed that burial of Org-P is not the largest P sink in the deep ocean (e. g., Filippelli and Delaney, 1996; Delaney, 1998). In the present study, TOC concentrations had an overall weak correlation

with Org-P contents (Fig. 9a), consistent with previous findings in regions with low TOC content and low sedimentation rates (e. g., Ingall and Van Cappellen, 1990; de Lange, 1992). However, there are two distinct correlations between Org-P and TOC, showing a positive correlation when the TOC content was o0.4% and a weak negative correlation between Org-P and TOC when the TOC content was 40.4% (Fig. 9a). These two distinct correlations between Org-P and TOC also had different OC/Org-P ratios, showing values lower than the Redfield Ratio when TOC was o0.4% and mostly higher than the Redfield Ratio when TOC was 40.4% (Table 2, Fig. 9b). The difference between these two correlations might reflect a combination of the variation in sources, initial concentrations and degradation status of organic matter (Anderson et al., 2001; Ruttenberg, 2003). Organic matter in marine sediments mainly comes from terrestrial and marine materials that may have different carbon isotopic composition. Based on available carbon isotope data from the same sediment samples, the δ13C values ranged from 20.32‰ to 24.35‰ (Ni, unpublished data), which are within the range of C3 plants and marine plankton, but cannot be used to exclusively differentiate terrestrial vs. marine sources. However, it is likely that samples with lower TOC and lower OC/Org-P ratios reflect the influence of terrestrial particles from aerosols with more refractory P, while sediments with higher TOC and higher OC/Org-P ratios contained mostly marine particles with labile or easily degradable P leading to higher OC/Org-P ratios. Anderson et al. (2001) and Algeo and Ingall (2007) found that the OC/Org-P ratios could reflect the redox condition in bottom waters and sediments since the OC/Org-P ratios were lower than or equal to the Redfield Ratio under oxic conditions, but higher

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than the Redfield Ratio under anoxic conditions. Higher organic C/ P ratios suggested the preferential loss of P during early diagenesis and degradation of organic matter. On the other hand, Ingall and Van Cappellen (1990) had related the OC/Org-P ratios to sediment accumulation rate. It was found that the OC/Org-P ratios were close to the Redfield Ratio in areas with the lowest sedimentation rates due to higher refractory Org-P in residual organic matter, and/or in situ produced organic matter from bacterial biomass. In our sediment samples, the OC/Org-P ratios varied from 33 to 164 with an average of 92735 (station BC13 was excluded because of its abnormal high value), which was similar to the Redfield Ratio of 106. The low TOC content and oxic conditions in the study area (Ni et al., 2001) suggest that the preserved organic matter has undergone degradation and contains mostly refractory compounds due to low sedimentation rates and long exposure to oxygen. Additionally, the OC/Org-P ratios show an exponential increase with increasing TOC contents (Fig. 9b), suggesting preferential loss of organic P during organic matter decomposition. During degradation of settling particles, P is released to ambient seawater. The released-P may be adsorbed on ferric oxyhydroxides or transformed to authigenic CFA-P with relatively constant TP contents during early diagenesis (Ruttenberg and Berner, 1993; Babu and Nath, 2005). Since all P species, except inert detrital apatite, have a tendency to participate in biogeochemical cycling processes, reactive P (Preact) could be estimated as the sum of Adsorb-P, Fe–P, CFA-P, and Org-P, and the TOC/Preact ratios may serve as a better proxy for the fate of buried sedimentary P (Anderson et al., 2001; Lin et al., 2012). As listed in Table 2, the TOC/Preact ratios varied from 4 to 21 with an average of 1376. If we correct for detrital CFA-P, the TOC/Preact ratios increase and range from 6 to 35 with an average of 1879. Both values are much lower than the Redfield Ratio. Similar results were also observed in the eastern Arabian Sea (Babu and Nath, 2005). Lower organic C/P ratios in the sediment suggest that buried sedimentary P was mostly refractory P, likely derived from atmospheric deposition in the central Pacific Ocean. In addition, low sedimentation rates or atmospheric deposition rates and oxic conditions may result in effective degradation of TOC and transformation of P during particle transport and early diagenesis in the sediment. For example, Org-P can be transformed to authigenic CFA-P while TOC is degraded and consequently released into the water column as CO2, making the TOC/Preact ratios less than the Redfield Ratio (Anderson et al., 2001) and suggesting efficient sedimentary sequestration of P.

5. Conclusions The abundance of various P phases in surface sediments from the central Pacific Ocean was quantified using the SEDEX sequential extraction technique to examine sources and distribution of P species. Concentrations of TP ranged from 13.2 to 119 μmol-P/ g in the central Pacific Ocean. Within the TP pool, TIP concentrations varied from 11.1 to 121 μmol-P/g, while TOP concentrations changed from undetectable to 4.8 μmol-P/g. In general, inorganic P was the predominant P species in surface sediments of the central Pacific Ocean, comprising up to 93% of the sedimentary P. Among the five sedimentary P species, the authigenic or CaCO3-bound P and detrital P were the two predominant P species, comprising 43.4% and 45.7% of TP, respectively, followed by the refractory organic P (Org-P), representing 6.7% of TP. Fe-bound P accounted for about 3.3% of TP, and less than 1% of TP was readily exchangeable or adsorbed P. The spatial distribution of different sedimentary P species showed that higher concentrations of detrital P and Fe-bound P were found at the same stations around 11°N, suggesting similar sources for these two P species in the central Pacific Ocean. On the

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other hand, the detrital P seemed mostly derived from atmospheric inputs of terrestrial particles, consistent with observations showing heavy rainfall in the ITCZ between 3°N and 11°N. On average, as high as 63% of the total sedimentary P could be derived from atmospheric sources in the central Pacific Ocean. Compared to other marine environments, the central Pacific Ocean had relatively higher detrital P, but lower adsorbed-P and Fe-bound P abundance. These results suggest the importance of atmospheric inputs for sedimentary P in open ocean environments. Furthermore, the high abundance of authigenic P (e. g., CFA-P) implies that sediments are important sink for reactive P species in the central Pacific Ocean. Overall, atmospheric inputs seemed to play an important role in contributing additional P to deep-sea sediments and in affecting P burial efficiency in the study area. There seems to have two depositional realms in the central Pacific Ocean: one with significant atmospheric inputs of refractory P, and the other with mostly autochthonous signatures of marine organic matter. The former realm is characterized with lower TOC contents ( o0.4%) but higher TP abundance and thus lower organic C/P ratio. The later is characterized with higher OC contents ( 40.4%) but lower Org-P and thus higher organic C/P ratios. Further studies are needed to reconstruct changes in the deposition and preservation history of different P species in sediments.

Acknowledgments We wish to thank the crew and science party of the R/V HAIYANGLIUHAO for their skillful support during sediment sampling in the Pacific Ocean. We also thank three anonymous reviewers for their constructive comments that improved the manuscript. This work was supported in part by China Ocean Mineral Resources Exploration and Development Special Foundation under Contract no. DY125-14-E-01, the National Natural Science Foundation of China (No. 41106050), the China Scholarship Council (CSC-2011418006 to J.N), and the University of WisconsinMilwaukee. (RGI-101X318).

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