Accepted Manuscript

A new particulate Mn-Fe-P-shuttle at the redoxcline of anoxic basins

Olaf Dellwig, Thomas Leipe, Christian März, Michael Glockzin, Falk Pollehne,

Bernhard Schnetger, Evgeniy V. Yakushev, Michael E. Böttcher, Hans-Jürgen

Brumsack

PII: S0016-7037(10)00530-2

DOI: 10.1016/j.gca.2010.09.017

Reference: GCA 6929

To appear in: Geochimica et Cosmochimica Acta

Received Date: 8 March 2010

Accepted Date: 13 September 2010

Please cite this article as: Dellwig, O., Leipe, T., März, C., Glockzin, M., Pollehne, F., Schnetger, B., Yakushev,

E.V., Böttcher, M.E., Brumsack, H-J., A new particulate Mn-Fe-P-shuttle at the redoxcline of anoxic basins,

Geochimica et Cosmochimica Acta (2010), doi: 10.1016/j.gca.2010.09.017

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers

we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and

review of the resulting proof before it is published in its final form. Please note that during the production process

errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

1

A new particulate Mn-Fe-P-shuttle at the redoxcline of anoxic basins 1

2

Revised manuscript (September 4th 2010) for publication 3

in “Geochimica et Cosmochimica Acta” by 4

5

Olaf Dellwiga,*, Thomas Leipea, Christian Märzb, Michael Glockzina, Falk Pollehnea, 6

Bernhard Schnetgerb, Evgeniy V. Yakushevc, Michael E. Böttchera, Hans-Jürgen Brumsackb 7

8

a Leibniz Institute for Baltic Sea Research, IOW, 18119 Rostock, Germany 9

b Institute for Chemistry and Biology of the Marine Environment, ICBM, University of 10

Oldenburg, 26111 Oldenburg, Germany 11

c Norwegian Institute for Water Research, NIVA, 0349 Oslo, Norway 12

13

14

15

Abstract 16

Pelagic redoxclines of anoxic basins and deeps form the suboxic transition between 17

oxygenated surface and anoxic or even sulfidic bottom waters. Intense element cycling, 18

favoured by elevated microbial activity, causes steep gradients of physico-chemical 19

parameters, nutrients and redox-sensitive trace metals. This study presents a conceptual model 20

for authigenic particle formation at pelagic redoxclines, which is based on the tight coupling 21

of Mn, Fe, and P cycles. Besides the well-known occurrence of Mn-oxides, textural (SEM-22

EDX) and geochemical (ICP-OES, ICP-MS) analyses of particles from the redoxclines of the 23

* Corresponding author. Tel.: +49(0)381-5197-437. E-mail address: olaf.dellwig@io-warnemuende.de

2

Black Sea and the Baltic Sea (Gotland Basin, Landsort Deep) evidence the existence of earlier 24

postulated Fe-oxyhydroxo-phosphates and emphasize mixed phases consisting of Mn-oxides 25

and Fe-oxyhydroxo-phosphates as a new solid species. Most of the analyzed particles are star-26

shaped, of about 5 µm in size, and occur as single particles or aggregates without any 27

morphological differences between Mn-oxides, Fe-oxyhydroxo-phosphates, and mixed 28

phases. Throughout the redoxcline, these minerals show a general succession with maximum 29

abundance of Mn-oxides above the redoxcline followed by mixed phases and almost pure Fe-30

phosphates within and below the redoxcline, respectively. Molar Fe/P ratios of single particles 31

argue against the formation of known pure Fe-phosphates like vivianite or strengite at the 32

lower end of the redox transition zone, but are consistent with recent experimental findings 33

for colloidal P-bearing hydrous ferric oxides. Moreover, morphological similarities suggest 34

the formation of irregular Fe-oxyhydroxo coatings due to oxidation of upward diffusing Fe2+ 35

by oxygen and stepwise replacement of Mn(IV) by Fe(III) on sinking MnOx particles 36

followed by immediate adsorption or even co-precipitation of phosphate. Batch-type 37

experiments using biogenic MnOx particles demonstrate the efficient potential of Fe2+ 38

oxidation by sinking MnOx particles. When entering sulfidic waters MnOx particles are 39

progressively reduced leading to an increasing relative abundance of Fe- and P-rich particles. 40

In deeper parts of the water column these particles are also reductively dissolved, thereby 41

releasing Fe2+ and phosphate to the water column. This Mn-Fe-P-shuttle likely affects 42

phosphate transport throughout the water column and thus impacts primary production at least 43

over longer time scales. Furthermore, the particulate Mn-Fe-P-shuttle must have played an 44

important role for the cycling of P and certain trace metals in ancient ocean basins, e.g., 45

during certain periods of Cretaceous black shale formation and should be considered in future 46

mass balances and modelling approaches dealing with oxic/anoxic interfaces of aquatic 47

ecosystems. 48

49

3

1. INTRODUCTION 50

Phosphorus is an essential macronutrient and limits phytoplankton growth in the ocean at 51

least over longer time scales, thereby regulating modern and ancient atmospheric CO2-O2 52

equilibria (e.g. Van Cappellen and Ingall, 1996; Tyrrell, 1999). More intensive oceanic 53

phosphorus recycling and sedimentary reflux under anoxic conditions supposedly plays a key 54

role in this system, as riverine input of phosphorus alone is insufficient to sustain 55

phytoplankton demands (Ingall et al., 2005). Therefore, vertical transport of released 56

phosphate towards surface waters is an important driver of primary production. 57

A typical property of stratified water columns in anoxic basins are redoxclines, i.e., 58

suboxic transition zones separating oxygenated surface waters from anoxic or even sulfidic 59

(euxinic) deep waters. Such redoxclines are characterized by steep gradients of physico-60

chemical properties, nutrients, metabolites, and redox-sensitive trace metals caused by intense 61

element cycling and elevated microbial activity (e.g. Dyrssen and Kremling, 1991; Neretin et 62

al., 2003; Tebo et al., 2004; Taylor et al., 2001; Yakushev et al., 2007). Hence, transfer 63

processes between the dissolved and particulate phase are of particular interest to understand 64

transport modes within elemental cycles. A well known example is the so-called “Manganese-65

pump”, which is driven by bacterial oxidation of upward diffusing Mn2+ and re-dissolution of 66

MnOx particles in sulfidic waters (e.g. Burdige and Nealson, 1986; de Lange et al, 1990; 67

Huckriede and Meischner, 1996; Neretin et al., 2003). A similar cycle is also valid for Fe, 68

albeit with dissolved oxygen as oxidant, and has been intensively studied in the field and 69

laboratory (e.g. Davison et al., 1982; Millero et al., 1987; Buffle et al., 1989). Using birnessite 70

suspensions in solutions with different pH values, Postma (1985) demonstrated 71

experimentally that Mn-oxides can be important and fast oxidants for dissolved Fe2+, as well. 72

At pelagic redoxclines of anoxic basins dissolved phosphate reveals a vertical distribution 73

contrasting to a simple diffusion-like pattern and release from decomposition of organic 74

matter, respectively. Moreover, phosphate shows distinct intervals of depletion and 75

4

enrichment called the “phosphate dipole” (Yakushev et al., 2007). In a previous model, 76

Shaffer (1986) explained these anomalies for the Black Sea by postulating Fe-oxyhydroxo 77

and Mn-oxyhydroxo particles, which adsorb phosphate at the redoxcline. Since the early work 78

of Einsele (1938) the relation between Fe and phosphate was investigated by many scientists 79

and the formation of Fe-oxyhydroxo-phosphates due to adsorption and even co-precipitation 80

of phosphate was evidenced in many laboratory experiments and field studies of limnic and 81

marine sediments, hydrothermal systems, anoxic brines, stratified lakes and rivers (e.g. 82

Berner, 1973; Tessenow, 1974; Trefry et al., 1984; Buffle et al., 1989; Fox, 1993; Slomp et 83

al., 1996; Gunnars et al., 2002; Hyacinthe and Van Cappellen, 2004; Poulton and Canfield, 84

2006). However, field studies dealing with the formation of Fe-oxyhydroxo-phosphates in the 85

water column of anoxic basins are comparatively rare and often restricted to sporadic findings 86

(Bernard et al., 1989; Ingri et al., 1991; Leipe et al, 1996). 87

This communication focuses on the formation of Fe- and P-rich particles and their 88

potential relation to the Mn-cycle within the water column of anoxic deeps from the Baltic 89

and Black Seas. Geochemical analyses of the particulate phases and measurements of 90

dissolved elements (Fe, Mn, and phosphate) are complemented by SEM-EDX inspection of 91

solid particles. Our data reveal additional new insights into the formation of ‘Fe-oxyhydroxo-92

phosphates’ and emphasise a tight coupling of the Mn, Fe, and P cycles resulting in a new 93

conceptual model for authigenic particle formation at pelagic redoxclines. Recently, model 94

calculations by Tsandev and Slomp (2009) underlined the importance of P availability for 95

triggering past Oceanic Anoxic Events. In line with these results, the conceptual model 96

presented in this study is further applied to processes in ancient anoxic systems. 97

98

2. MATERIAL AND METHODS 99

2.1. Study area 100

5

This work is based on data from the following cruises: R/V “Professor Albrecht Penck” 101

(40PE/06/16, July 2006; 07PE/07/08, April 2007) in the Baltic Sea and R/V “Meteor” (M72-102

5, June 2007) in the Black Sea (Fig 1). 103

In the Baltic Sea, which represents one of the largest brackish water bodies in the world, 104

sampling was done in the Gotland Basin (57°35´N, 20°03´E; water depth 241 m) and in the 105

Landsort Deep (58°35´N, 18°14´E; water depth 456 m) during both cruises. The Gotland 106

Deep (max. depth 249 m) forms the deepest part of the eastern Gotland Basin, which 107

represents the basin with the largest average water depth in the Baltic. In contrast, the 108

Landsort Deep, which is the deepest site of the Baltic Proper (max. water depth 459 m), is 109

characterized by a comparatively small spatial extent. Currently, the water bodies of both 110

deeps are well stratified with a permanent pycnocline between 60 and 80 m depth. Water 111

samples were obtained using a CTD-bottle-rosette during the cruises in the Baltic Sea in 2006 112

and 2007 whereas a PUMP-CTD (Strady et al., 2008) was used in the Black Sea 2007. The 113

PUMP-CTD system is a modification of a previously developed system by Friedrich et al. 114

(1988). It enables profiling CTD measurement, besides O2, pH, fluorescence, and turbidity in 115

combination with continuous water sampling throughout the water column down to 350 m 116

water depth. Water is pumped through nylon tubing at a rate of about 2 L min-1 directly into 117

the ship-board laboratory. From the Black Sea cruise two PUMP-CTD casts were chosen for 118

this communication: 21-PCTD5, 42°45´N, 37°30´E, 2132 m and 27-PCTD4, 42°24´N, 119

36°16´E, 1920 m. These stations are located in the SE part of the Black Sea, an area without 120

any significant riverine input. 121

122

2.2. Particle microanalyses using SEM-EDX 123

On average 1 to 2 L of seawater were filtered through pre-weighed Millipore Isopore 124

membrane filters (0.4 µm polycarbonate PC) for SEM-EDX investigations and multi-element 125

6

analyses, respectively. Possible oxidation artefacts were excluded by parallel filtration of 126

selected samples under oxic and anoxic (inert atmosphere) conditions. 127

Scanning electron microscopy (SEM) and energy dispersive X-ray micro-analyses (EDX) 128

were performed on a FEI Quanta 400 microscope connected to an EDAX-Genesis system. 129

About 1 cm2 of PC filters was glued on Al stubs and covered by pure carbon (vacuum 130

sputtered) to assure electric conductivity. Measurement parameters of the microscope system 131

during analyses were: high vacuum; 15 kV electron beam; working distance: 10 mm; SE and 132

BSE detector; variable enlargement. The x-ray microanalyses were done by spot analyses on 133

selected particles taking EDX-spectra (EDAX-Econ 4 detector), identification and 134

quantification of the elements after ZAF-correction. Peak overlapping (e.g. Mn kβ and Fe kα 135

lines at 6.4 to 6.5 keV) was solved by holographic peak deconvolution (HPD). 136

Besides manual single particle analyses the system provides the opportunity of automated 137

particle analyses. The method is based on image processing, particle recognition and element 138

analyses of the particles on a series of different fields of the sample. On average 2000 single 139

particles of each sample were analyzed by this method. The resulting data set was processed 140

for mineral (or particle group) identification and quantification (counting). Minerals and 141

particle groups are defined by “border values” of the proven elements and calculated element 142

concentrations (or ratios). Analyses of standard mineral samples were used to verify this 143

method (Leipe et al., 1999). 144

145

2.3. Geochemical analyses 146

Samples for analysis of dissolved metals in seawater were taken with pre-cleaned PE-147

syringes and 0.45 µm SFCA syringe filters directly from CTD-bottle-rosette and PUMP-CTD 148

(Strady et al., 2008) and acidified to 1 vol.% HNO3 in pre-cleaned PE-bottles. 149

For analysis of suspended matter, PC filters were decomposed with 1 mL HClO4 in open 150

PTFE autoclaves. Residues were digested in the same closed vessels at 180°C after adding 2 151

7

mL concentrated HF. After digestion acids were evaporated at 180°C, residues were re-152

dissolved and fumed off three times with 1 mL semi-concentrated HCl and diluted with 2 153

vol.% HNO3 to a final volume of 5 mL. All acids used were of suprapure quality. 154

Particulate major elements (Fe, P, Mn), trace metals (As, Mo, V) and dissolved P were 155

analyzed by ICP-OES (Perkin Elmer Optima 3000XL; Thermo Scientific iCAP 6300Duo). 156

Particulate Cd, Sb, and U as well as dissolved Fe and Mn were measured by ICP-MS (Thermo 157

Scientific ELEMENT II). Parallel measurements of selected samples by spectrophotometry 158

(Grasshoff et al., 1983) revealed almost similar results (r2=0.97; standard deviation 0.06 µM) 159

indicating the reliability of ICP-OES for analyses of phosphate. Precision (1σ) and accuracy 160

of all measurements were checked by parallel analysis of international and in-house reference 161

materials (precision and accuracy were ≤7.4% and ≤4.3% for major elements and ≤6.8% and 162

≤10.7% for trace metals). During the cruises in the Baltic Sea, O2 and H2S were determined 163

onboard ship (Grasshoff et al., 1983). In the Black Sea, 2 mL seawater were fixed with 50 µL 164

of a 5 vol.% Zn-acetate solution for the subsequent spectrophotometric analysis of H2S 165

(Cline, 1969) using a Spekol 1100 (Analytik Jena) spectrophotometer. O2 data originate from 166

PUMP-CTD casts. 167

168

2.4. Laboratory experiments 169

Three batch-type experiments as well as one control run were carried out in a dark cold 170

room (8-10 °C) in order to investigate the importance of MnOx particles as a potential fast 171

oxidant for dissolved Fe2+. A gas tight reaction vessel was filled with 300 mL of Atlantic 172

seawater (OSIL) diluted to a salinity of 12 with deionised water, thus reflecting the salinity at 173

the redoxcline of the Gotland Basin. The vessel was equipped with four ports enabling the 174

inlet of Ar gas via a tube, the measurement of oxygen, temperature, and pH (Oxi 330I, pH 175

330I; WTW), and sampling of solution aliquots. The solution was permanently stirred to 176

prevent agglomeration of suspended particles. 177

8

In the control run, 2.5 mg iron(II) sulphate was directly transferred into the anoxic reaction 178

cell and the solution was stirred for 150 min under continuous Ar gas flow through the 179

solution. During the experiment six samples were taken every 30 min which showed no loss 180

of dissolved Fe. Therefore, anoxic conditions are assured and possible oxidation artefacts can 181

be excluded, respectively. 182

In experiment A the oxygen concentration was kept at around 25 µM (range 23.1-26.3 µM] 183

by adjusting the bubbling rate with Ar gas. Fe(II) sulphate (7 mg) was added directly and the 184

solution was stirred for 195 min. Experiments B (duration 260 min) and C (duration 220 min) 185

were carried out under oxygen-free conditions and biogenic MnOx particles were used (run B 186

ca 0.4 mg; run C ca 0.2 mg; synthezised by reaction of spores of Bacillus sp. strain SG-1 with 187

100 µM Mn2+; Tebo et al., 2004). The number of MnOx particles was determined by using a 188

Beckman Coulter Multisizer TA II (Beckham Coulter GmbH; Sheldon, 1967) revealed about 189

6x104 particles mL-1 for run B and 2.3x104 particles mL-1 for run C. The average grain sizes 190

of particles were 3.2 µm (run B) and 3.7 µm (run C) ranging between 2 and 9 µm in both 191

runs. While run B was carried out only with Fe(II) sulphate (6 mg), run C (4 mg Fe(II) 192

sulphate) also contained potassium-dihydrogenphosphate (4 mg) as phosphate source. 193

Temperatures showed only minor variations during all experiments (8.9-10.5°C), and pH 194

values varied between 7.4 and 8.4. Such pH range is slightly above the natural level, however 195

is in fair agreement with the typical pH ranges across the redoxcline of the Gotland Basin 196

(pH: 7.1-7.8). Water samples (1 mL) for analysis of dissolved Mn, Fe, and P by ICP-OES 197

were taken every few minutes in the initial phase and every 30 to 45 min in the later stages of 198

the experiments. Samples (5 ml) for SEM-EDX were taken during run B at the beginning, 199

after 60 min and at the end of the run while for run C samples were retrieved after 30 min, 60 200

min, and the end of the run. 201

202

3. RESULTS AND DISCUSSION 203

9

3.1. Formation and fate of the Mn-Fe-P-shuttle 204

In two anoxic deeps of the central Baltic Sea (Gotland Basin GB, Landsort Deep LD) and 205

in the SE Black Sea we detected a new solid phase around the pelagic redoxclines, i.e. the 206

suboxic zone separating oxygenated waters from anoxic or even sulfidic conditions. Besides 207

well known Mn-oxides (Fig. 2 a) and iron-oxyhydroxo-phosphates (Figs. 2 e, f), which were 208

earlier postulated by Shaffer (1986), the occurrence of a transitional phase with variable 209

contents of Mn, Fe, and P (Figs. 2 b-d) was evidenced by SEM-EDX. These types of particles 210

will in the following be referred to as “MnOx”, “FeOOH-PO4”, and “MnOx-FeOOH-PO4” 211

phases. The results of SEM-EDX analyses of individual particles are summarised in Table 1. 212

SEM photographs shown in Figure 2 provide a representative overview about the 213

morphologies of Mn-, Fe-, and P-rich particles from the investigated sites in the Baltic and 214

Black Seas. The chosen depths represent samples above, within, and below the redoxcline. 215

Most of these particles reveal a branching vermicular morphology similar in structure to those 216

MnOx particles reported by Bernard et al. (1989) and Neretin et al. (2003), which will be 217

referred to as star-shaped structure in this work. Additionally, more globular structures were 218

observed especially in the samples from the Black Sea (Figs. 2b and d), which were also 219

reported by Schippers et al. (2005). Surprisingly, all three types of particles (MnOx, MnOx-220

FeOOH-PO4, FeOOH-PO4) exhibit the same star-shaped or globular texture, thus indicating a 221

tightly coupled mode of formation. 222

The star-shaped particles occur as single particles of about 5 µm in size (e.g. Fig. 2 a) but 223

also form larger aggregates (Fig. 2 b). Particles from deeper parts of the water column are 224

highly subjected to dissolution as seen in the almost pure FeOOH-PO4 particles from the 225

Gotland Basin (Fig. 2 f). In addition to the star-shaped structure, the occurrence of more 226

globular minerals was observed especially in samples from the Black Sea (Figs. 2 b, d). This 227

globular structure also consists of more or less pure MnOx and mixed MnOx-“FeOOH-PO4” 228

phases. In accordance with Neretin et al. (2003) no differences in the major chemical 229

10

composition of globular and star-shaped particles were found. Generally, the concentration of 230

detrital elements like Al, Ca, Mg, and Si is comparatively low for the investigated particles 231

(Table 1). Again, the different mineral phases and structures reveal no systematic differences 232

in these constituents. A few exceptions are observed in the centre of star-shaped MnOx 233

particles which partly contain elevated amounts of Si (max. 7.6%), probably reflecting 234

frustule fragments or detrital material which may have acted as a nucleus (Buffle et al., 1989). 235

Additionally, Si contents may also result from adsorption and /or co-precipitation of silica as 236

reported by Hartmann (1985) for anoxic brines of the Red Sea. These data from the water 237

column of the Atlantis-II Deep revealed a strong relation between Mn, Fe, and Si likely 238

initiated by extremely high silica levels (max. ca. 1 mM) exceeding the levels of this study by 239

factors of 50 to 100. Thus, Si contents of particles from the Black and Baltic Seas range only 240

between 1.1% and 1.8% with a higher affinity of phosphate towards solid Fe phases as seen in 241

increasing P/Si ratios (Table 1). 242

The vertical distribution of relative particle abundances along with selected geochemical 243

parameters from the Baltic and Black Seas are shown in Figs. 3-5. The profiles of SEM-EDX 244

analyses reflect the relative abundance of MnOx and FeOOH-PO4 particles as well as mixed 245

MnOx-FeOOH-PO4 phases normalised to the total amount of particles. The redoxcline, i.e. the 246

zone where O2 and H2S fall below the detection limit of our methods, is indicated by gray 247

bars. The extent of this zone is variable and ranges from about 4 m in the Landsort Deep in 248

2006 to app. 15 m in the Black Sea at site 21. The profiles show a pronounced variability, 249

however, a general succession of the aforementioned particles can be inferred. Thus, maxima 250

of MnOx are generally observed above and in the upper part of the redoxcline where oxygen 251

is still present followed by an increasing abundance of MnOx-FeOOH-PO4. These mixed 252

phases show their highest abundance slightly below the maximum occurrence of MnOx. 253

Almost pure FeOOH-PO4 phases increase below the maxima of the aforementioned solids and 254

reach values of almost 60% in the Gotland Deep in 2007 distinctly below the redoxcline. This 255

11

mineral sequence is compatible with the fact that oxidation kinetics of Fe2+ are much faster 256

than microbial oxidation of dissolved Mn2+ and Mn3+, with the latter probably forming an 257

important intermediate (Stumm and Morgan, 1996; Neretin et al., 2003; Trouwborst et al., 258

2006). 259

The vertical distribution of the total particulate Mn, Fe, and P concentration is well 260

correlated with the mineral abundances derived from SEM-EDX (Figs. 3-5) indicating only 261

marginal dilution by detrital material like clay minerals. Thus, maxima in MnOx abundance 262

are associated with prominent enrichments in the particulate Mn fraction. This is also true for 263

particulate P and Fe, which show enrichments in zones where the abundance of MnOx-264

FeOOH-PO4 and FeOOH-PO4 particles increases. When comparing the particulate and 265

dissolved phase, maximum values in the particulate phase are associated with depletions in 266

the dissolved species indicating a relation between phosphate anomalies and formation of P-267

rich particles as postulated earlier by Shaffer (1986). Dissolved Fe generally disappears 268

before dissolved Mn which is in accordance with the aforementioned faster oxidation kinetics 269

of Fe2+. However, there are large discrepancies in concentrations of dissolved and particulate 270

phases because sampling provides only a snapshot of the competition between sinking 271

particles and upward flux of dissolved species. 272

Average molar Fe/P ratios of all analyzed particles limited to Fe contents >5% are highly 273

variable ranging between 1.1 and 3.9 (av. 2.4) thus arguing against the formation of well 274

defined Fe-phosphate minerals (Table 1). This conclusion is in accordance with results from 275

thermodynamic modeling with Phreeq C. For instance, saturation indices of selected samples 276

from across the redoxcline are far away from equilibrium for vivianite (av. -16.9), whereas 277

supersaturation is indicated with respect to goethite (av. +5.2) or hematite (av. +12.3). 278

Therefore, we suggest that sinking MnOx particles act as nuclei for the precipitation of 279

amorphous Fe-oxyhydroxides (FeOOH) as pseudomorphs either by direct oxidation of 280

upward diffusing Fe2+ by either oxygen or MnOx (e.g. Hartmann, 1985) causing irregular 281

12

FeOOH coatings on MnOx particles. If Fe2+ is directly oxidized by MnOx, rather than by 282

oxygen or nitrate (e.g. van Cappellen and Wang, 1996; van der Zee et al., 2005), the textural 283

similarities may be explained by the stepwise replacement of Mn(IV) by Fe(III). A similar 284

process was proposed e.g. by Danielsson et al. (1980) and Hartmann (1985) for anoxic brines 285

of the Red Sea. Concomitant with the formation of solid Fe(III) phases, phosphate is adsorbed 286

or even co-precipitated (e.g. Berner, 1973; Slomp et al., 1996; Thibault et al., 2009) forming a 287

new solid phase. This contains variable amounts of Mn, Fe, and P (MnOx-FeOOH-PO4), and 288

preserves the morphology of original MnOx particles. It should be noted, however, that the 289

mean Fe/P ratios observed in the water column particulates (Table 1) are similar to the ratios 290

observed for colloidal P-bearing hydrous ferric oxides (Thibault et al., 2009). 291

To assess the relative importance of both oxidants oxygen and MnOx for oxidation of Fe2+ 292

batch-type experiments were carried out using biogenic MnOx particles (Fig. 6). Although, 293

the MnOx particles used do not possess the identical star-shaped structure of natural particles 294

sampled from the water column, SEM photographs also show a fractal structure as well as 295

comparable sizes (Fig. 7a). Under dysoxic conditions (O2: ca 25 µM) run A shows an almost 296

linear decrease of Fediss during the experiment with an average loss of 13 µM h-1 (Fig. 6a). 297

This rate is slightly higher than in the anoxic run B containing biogenic MnOx particles with 298

an average loss rate of 10 µM h-1 (Fig. 6b). However, during the initial phase of run B a steep 299

but non-linear decrease of Fediss is apparent tending towards equilibrium until the end of the 300

experiment. Consequently, the pattern of Mndiss distinctly increases at the onset of the 301

experiment indicating a very fast reaction between MnOx and Fe2+, which is in accordance 302

with experiments carried out with Fe2+ and artificial birnessite by Postma (1985). After 4 303

hours, no significant changes in Fe and Mn concentrations were observed as the number of 304

MnOx particles and free reaction sites on mineral surfaces tends towards zero, respectively. 305

While the total loss in Fediss amounts to 43 µM, the gain in Mndiss reaches 25 µM, which is 306

almost compatible with the following stoichiometry: 307

13

MnO2 + 2 Fe2+ + 4 H2O => Mn2+ + 2 FeOOH + 2 H+ 308

As shown by Postma (1985) the stoichiometry of the reaction between MnOx and Fe2+ is 309

strongly pH dependent. While at pH <4 all reaction products remain in solution, precipitation 310

of FeOOH occurs only at pH levels >4. A pH dependence was also reported for the oxidation 311

of Fe2+ by oxygen (e.g. Roekens and van Grieken, 1983; Millero et al., 1987). Therefore, the 312

pH level of the present experiments is compatible with the precipitation of solid FeOOH on 313

the MnOx particles. SEM-EDX analyses clearly show a change from pure MnOx particles 314

towards mixed MnOx-FeOOH phases and even almost pure FeOOH phases containing only 315

minor amounts of Mn (Figs. 7b and c). This conclusion is supported by automated SEM-EDX 316

particle analysis at the end of run B where no remaining MnOx particles were identified when 317

compared with the onset of the experiment (87% of total particles). Moreover, the particle 318

composition after the experiment is dominated by mixed MnOx-FeOOH phases (85%) along 319

with a certain contribution of FeOOH (9%). 320

Going one step further, the addition of phosphate in run C suggests the formation of mixed 321

particles containing Mn, Fe, and P (Fig. 6c). Generally, the average loss of Fe is much smaller 322

(5 µM h-1) than in run B, which is apparently due to the lower amount of MnOx particles (see 323

section 2.4). Furthermore, a surface effect of phosphate lowering the reaction rates at the 324

solids` surfaces may also play a role. For instance, Yao and Millero (1993) reported a 325

significant inhibition of the oxidation of H2S by MnOx in the presence of phosphate. 326

Simultaneously decreasing concentrations of Fediss and dissolved phosphate imply the 327

formation of MnOx-FeOOH-PO4 and/or even almost pure FeOOH-PO4 phases which were 328

identified by SEM-EDX (Figs. 7d-f) and particle analysis reaching an abundance of almost 329

95% at the end of run C. Molar Fe/P ratios are on average 1.5 (range: 1.2-2.5; n=20), which is 330

distinctly lower when compared with the natural particles (Table 1). This difference is most 331

likely due to different concentration levels, element ratios and lacking complexation of 332

dissolved components in the experiments compared to the natural situation. Although, the 333

14

release of Mn2+ clearly indicates oxidation of Fe2+ by MnOx, the total loss of dissolved Fe (17 334

µM) and the total increase in dissolved Mn (4.2 µM) is not compatible with the above 335

mentioned stoichiometry. Such excess loss of Fe2+ was also observed by Postma (1985) who 336

explained this discrepancy by adsorption of Fe2+ on particles. A further possibility in the 337

present experiments may be the formation of certain amounts of Fe(II) or Fe(III) phosphates 338

(Table 1) leading to excess Fe loss and low Fe/P ratios, respectively. 339

In conclusion, the experiments demonstrate that both, the oxidation of Fe2+ by oxygen and 340

MnOx particles, play an important role in the investigated environments. When considering a 341

stable redoxcline with clearly defined gradients, oxidation by oxygen likely dominates the 342

upper redoxcline, whereas MnOx particles gain in importance in the lower part. In contrast, 343

under more dynamic conditions, as seen especially in the Gotland Basin, turbulence and 344

lateral currents frequently led to intrusions of slightly oxygenated waters into deeper parts of 345

the water column (e.g. Lass et al., 2010). Such conditions likely cause overlapping of both 346

oxidants`domains and higher oxidation rates as seen in steeply decreasing concentrations of 347

Fediss in the Gotland Basin (Fig. 3) when compared with the Landsort Deep (Fig. 4). 348

In comparison to MnOx and MnOx-FeOOH-PO4 particles, the vertical pattern of mineral 349

abundance implies that FeOOH-PO4 phases possess the highest stability in sulfidic waters 350

(e.g. Fig. 3), which can be attributed to the progressive reduction of MnOx mainly by H2S 351

(Böttcher and Thamdrup, 2001). According to experiments carried out by Burdige and 352

Nealson (1986) and Yao and Millero (1993) H2S is able to reduce MnOx within minutes. This 353

may therefore represent a fast enough process to produce the observed steep gradients. 354

Additionally, certain Fe-oxides are more stable under sulfidic conditions when compared with 355

MnOx. Experiments by Canfield (1989) identified ferrihydrite and leipidocrocite to be highly 356

reactive towards sulfide while goethite and hematite reacted much slower. Thus, FeOOH-PO4 357

coatings may also protect some MnOx against reduction to a certain degree until complete 358

reductive dissolution of particles may occur in greater depths either microbially or abiotically 359

15

(e.g. Canfield et al., 1992; Yao and Millero, 1996a, Poulton et al., 2004). Although FeOOH-360

PO4 phases account for almost 60% of all particles detected in the anoxic Gotland Deep at 361

165 m depth in 2007, SEM pictures reveal intense dissolution effects (Fig. 2 f). When 362

particles are able to reach deeper parts of the water column, released Fe2+ will partly be 363

removed from the cycle via formation of Fe-sulfides (e.g. Lyons, 1997; Suits and Wilkin, 364

1998). However, a direct linkage between the Mn-, Fe-, and P-containing particles and Fe-365

sulfides is not observed by SEM-EDX. Figure 8 presents the abundance of particulate FeSx 366

compounds at Black Sea site 21, which indicates a significant abundance only below 500 m 367

depth. Although, no data are available from the Black Sea between 200 and 500 m depth, 368

measurements from the Landsort Deep during a cruise with R/V “Prof. A. Penck” in 2008 369

(cruise No. 07PE/08/14) disclose maximum FeSx abundances of only 1.5% of total particles 370

down to 430 m depth. Textural similarities with Mn-Fe-P phases are not detectable as FeSx 371

appears as clusters of octahedral crystals close to the redoxcline and as typical framboids in 372

the deeper parts of the water column (Fig. 8). Neither the octahedral crystals nor the 373

framboids contain significant amounts of remaining Mn (below detection limit) and P (max. 374

1.5%), with the latter likely associated with organic matter coatings (Fig. 8). 375

376

3. 2. Conceptual model for anoxic basins 377

From the results discussed so far we suggest a conceptual model which tightly couples the 378

cycles of Mn, Fe, and P in the water column of anoxic basins (Fig. 9). Such coupling has been 379

also indicated by Hyacinthe and Van Cappellen (2004) for sediments of the Scheldt estuary. 380

However, they reported distinctly smaller Fe/P ratios and suggest formation of a hydrous 381

ferric phosphate mineral (e.g. tinticite; Table 1), which most likely has to be attributed to the 382

different investigated settings, i.e. precipitation of mineral phases in phosphate-rich pore 383

waters. 384

16

Generally, the model is divided into two modes, a euxinic (left) and an anoxic non-sulfidic 385

(right) mode. Nevertheless, the initial point in both modes is the formation of MnOx particles 386

from upward diffusing Mn2+ released from the sediments and particles at depth. Once these 387

particles are formed they sink down, thereby acting as nuclei for precipitation of FeOOH 388

coatings resulting from oxidation of upward diffusing Fe2+ either by molecular oxygen or 389

MnOx. This Fe2+ originates from reduction of detrital Fe(III) compounds contributed by rivers 390

and aeolian input. Additionally, sedimentary reflux of Fe2+ is also feasible in the anoxic but 391

non-sulfidic mode as well as a contribution from oxic/suboxic shelf sediments (e.g. Wijsman 392

et al., 2001; Raiswell and Anderson, 2005; Lyons and Severmann, 2006). Simultaneously, 393

phosphate, which is preferentially released from the sediments under euxinic conditions as 394

indicated by the bold arrow in Figure 9 (Ingall et al., 2005), adsorbs on and/or even co-395

precipitates with FeOOH. Although, Yao and Millero (1996b) suggested Mn-oxides as 396

possible adsorbents for phosphate in natural waters, the MnOx particles of this study contain 397

only small amounts of P when compared with the Fe-containing phases (Fig. 2a, Table 1). 398

Therefore, the affinity of phosphate towards FeOOH appears to be much higher in this 399

environment than towards MnOx. This difference is possibly due to negative surface charges 400

of MnOx, which hamper adsorption of anions (e.g. Murray, 1974). On the other hand, the 401

content of phosphate adsorbed on the surface of MnOx is possibly underestimated by SEM-402

EDX measurements as the electron beam penetrates the particles to a depth of app. 1-2 µm. 403

During oxidation of Fe2+ by MnOx dissolved Mn2+ is released as seen especially in the 404

profile of the Gotland Basin in 2006 (Fig. 3). Regarding the remaining profiles in Figures 3 to 405

5 an immediate release of Mn2+ is possibly retarded by re-adsorption on MnOx-FeOOH-PO4 406

phases thereby contributing to the solid phase content (e.g. van der Zee et al., 2001). 407

Morphological similarities clearly indicate the mixed phases as a geochemically variable 408

transitional phase between almost pure MnOx and FeOOH-PO4 phases. Thus, phosphate is 409

17

transferred into the particulate pool within the redoxcline likely producing dissolved 410

phosphate anomalies seen in vertical profiles of Figures 3 to 5. 411

Such P-rich particles may also contribute significantly to the particulate inorganic P (PIP) 412

observed for instance in sediment traps from the Cariaco Basin (Benitez-Nelson et al., 2004, 413

2007). The authors identified PIP as a major component of the total particulate P reaching 414

maximum values in a sediment trap close to the redoxcline. In deeper waters this PIP 415

significantly decreases and shows no relation to the particulate organic P pool. Therefore, the 416

authors concluded that PIP forms an important source for dissolved P in the Cariaco Basin 417

whereas the origin of this PIP remained enigmatic. Considering a significant contribution of 418

the redoxcline-derived Mn-Fe-P-shuttle to the PIP pool, these particles would explain the high 419

PIP abundance close to the redoxcline as well as the decreasing amount of PIP with 420

increasing water depth due to dissolution of particles in sulfidic waters. Thus, our data 421

underline the trapping function of the Mn-Fe-P-shuttle for phosphate which will affect the 422

upward phosphate transport and consequently also primary production. We propose that this 423

shuttle exhibits a regulatory function within the complex element and nutrient cycles of 424

anoxic basins. As such systems are often characterized by elevated sedimentary fluxes of 425

dissolved phosphate (Ingall et al., 2005) unimpeded access of released phosphate to the photic 426

zone would significantly enhance primary production. The formation of MnOx-FeOOH-PO4 427

phases at the redoxclines, however, causes retention of phosphate and possibly keeps the 428

system balanced. Although, both anoxic systems investigated here operate on different time 429

scales, phosphate levels in the anoxic parts of the water column are similar in the Baltic and 430

Black Seas, which has to be attributed to such regulating mechanism. 431

While anoxic bottom water conditions were established in the Black Sea since about 7.500 432

years BP, the deeps of the Baltic Sea are frequently subjected to inflowing pulses of 433

oxygenated sea water from the North Sea (e.g. Huckriede and Meischner, 1996). Such inflow 434

events cause fundamental environmental shifts as the oxygenation of former anoxic deep 435

18

waters will lead to the pronounced precipitation of MnOx-FeOOH-PO4 phases. After 436

reduction of deposited particles at the still anoxic sediment/water interface elevated amounts 437

of phosphate are again released. This process will increase primary production and favours 438

the re-establishment of anoxic conditions after several months or a few years, thus forming a 439

kind of an “anoxic perpetuum mobile”. However, comparable phosphate levels in anoxic 440

waters of the Black and Baltic Seas point towards re-establishment of the redoxcline with its 441

regulating function during an early stage of this cycle. 442

443

3.3. Paleoenvironmental implications 444

The regulating function of the Mn-Fe-P shuttle is assumed to be relevant in Earth`s history, 445

as well. During the Phanerozoic for instance, when oceanic anoxic events resulted in 446

widespread black shale formation, large parts of the ancient ocean basins were affected by 447

anoxic or even sulfidic bottom water conditions (Brumsack, 2006). Although, euxinic 448

conditions are generally assumed to enhance sedimentary phosphate reflux (Ingall et al., 449

2005) certain P enrichments during oceanic anoxic events reflect periods of elevated P burial 450

(e.g. Föllmi, 1996; Handoh and Lenton, 2003). In particular, cyclic P enrichments within 451

certain intervals of Cretaceous black shales from the Demerara Rise (März et al., 2008), 452

which might be caused by fundamental changes in ancient ocean circulation, likely reflect the 453

deposition of such particles within a temporarily anoxic but non-sulfidic water column (Fig. 454

9, anoxic mode). Contrasting to the sulfidic mode, the particles have a higher chance of 455

sedimentary burial when H2S is absent. Independent sequential P and Fe extraction 456

procedures applied to one of such highly P-enriched layers indicate that about 50% of the 457

sedimentary P is bound to Fe minerals. Thus, coupled Fe and P cycles comparable to those in 458

the Black and Baltic Seas water columns are likely reflected in the sedimentary record and 459

may serve as a proxy over long time intervals (März et al., 2008). Molar Fe/P ratios of 1.0-1.5 460

(Table 1) found in black shale layers suggest that remaining Fe-P phases are composed of a 461

19

vivianite-strengite mixture. However, direct comparison of Fe/P ratios of pure mineral phases 462

and particles to those of sediments is limited by the potential transfer of particle-bound Fe and 463

P to form diagenetic Fe sulfides and apatite, respectively. In addition, transformation of fresh 464

Fe-P particles into well-crystallized Fe-P minerals might take place with progressive sediment 465

burial, and affect original Fe/P ratios. Similar recrystallisation processes are a well-known 466

phenomenon for other minerals (e.g. opal-quartz, ferrihydrite-hematite) as well. Still, under 467

certain conditions (low sedimentation rates, anoxic non-sulfidic bottom waters, a deep 468

redoxcline) Fe-P particles formed at the redoxcline of an anoxic basin may be preserved in 469

organic-rich sediments. Interestingly, P enrichments in these black shales are paralleled by 470

lowest TOC/Al ratios, which might be due to lower primary production resulting from the 471

suggested phosphate trapping at the redoxcline. We conclude that despite difficulties to 472

quantify the original contribution of Fe-P particles in ancient anoxic systems, traces of the 473

activity of a water column Mn-Fe-P-shuttle may be preserved in black shales, offering the 474

possibility for more detailed studies of similar deposits that experienced anoxic non-sulfidic 475

conditions. 476

The Mn-Fe-P-shuttle may also play an important role for the transfer of trace metals from 477

oxygenated waters into anoxic basins. Since seawater is considered to be an important source 478

for certain trace metal enrichments in sapropels and black shales, diffusion from oxygenated 479

waters into the anoxic zone forms a limiting factor for sustaining the trace metal inventory of 480

anoxic bottom waters (e.g. Brumsack, 2006). Therefore, settling of Mn- and Fe-rich particles, 481

which trap trace metals at the redoxcline via scavenging or co-precipitation, may provide an 482

additional mechanism for refreshing the trace metal reservoir of the anoxic bottom waters. For 483

instance, preliminary analyses of particles from the modern redoxclines of this study reveal 484

maximum values of 152 µg g-1 As, 21 µg g-1 Cd, 263 µg g-1 Mo, and 79 µg g-1 Sb. By 485

contrast, for U (5.2 µg g-1) and V (173 µg g-1) only slight enrichments were attained. Such 486

findings are in accordance with observations of Tribovillard et al. (2008) who reported 487

20

distinct Mo enrichments in sediments of the stratified Orca Basin while U and V reflect only 488

the geogenic background. This consistency possibly underlines the importance of authigenic 489

particles for trace metal budgets and emphasizes detailed investigations on trace metal 490

composition of particles as a future challenge. 491

492

4. CONCLUSIONS 493

A new solid phase consisting of Mn, Fe, and P was evidenced by SEM-EDX analyses at 494

pelagic redoxclines of the Baltic and Black Seas. Vertical particle distributions generally 495

show increasing abundances of this species between MnOx and earlier postulated Fe- 496

oxyhydroxo phosphates. Besides oxidation of upward diffusing Fe2+ by dissolved oxygen, 497

batch experiments also suggest formation of mixed phases by reaction of Fe2+ and sinking 498

MnOx particles thereby releasing Mn2+. The resultant FeOOH coatings on MnOx may adsorb 499

or even co-precipitate phosphate. When these particles encounter sulfidic waters, remaining 500

MnOx is progressively reduced leading to residual particles significantly enriched in Fe and P. 501

This Mn-Fe-P-shuttle traps dissolved phosphate at the redoxcline and contributes 502

significantly to the pool of particulate inorganic P. Furthermore, the shuttle likely hampers 503

phosphate transport throughout the water column, thereby increasing the importance of 504

upwelling for phosphate transport into surface waters. Over longer time scales, these particles 505

may also influence primary production and possess a regulating function in modern and 506

ancient anoxic basins because the trapping of phosphate at the redoxcline enhances its 507

residence time in anoxic waters (“anoxic perpetuum mobile”). 508

The Mn-Fe-P shuttle has to be taken into account in future modelling approaches dealing 509

with oxic/anoxic interfaces of aquatic ecosystems. Thus, the common consideration of only 510

two different mineral phases (MnOx and FeOOH), when modelling biogeochemical cycles at 511

pelagic redoxclines, does not adequately reflect our field and experimental observations. 512

513

21

ACKNOWLEDGEMENTS 514

The authors wish to thank Rainer Bahlo for his assistance during SEM-EDX analyses, 515

Vera Winde, Conny Lenz, and Frank Schoster for their assistance during the Baltic Sea cruise 516

in April 2007, Siggi Krüger for his PUMP-CTD support. OD wishes to thank Antje Wegwerth 517

for inspiring comments on an earlier version of this manuscript. In particular, we wish to 518

thank B. M. Tebo for providing biogenic MnOx particles from culture experiments. Further 519

thanks go to B. B. Jørgensen for his support within the Black Sea project, the captains and 520

crews of R/V “Professor Albrecht Penck” and R/V “Meteor”, and to the respective cruise 521

leaders G. Jost (Penck 2006) and C. Borowski (cruise M72-5). This study benefited from the 522

constructive comments of two anonymous reviewers which significantly helped us to improve 523

the original manuscript. The study was supported by the Leibniz Institute for Baltic Sea 524

Research (IOW). BS and HJB were supported by the Deutsche Forschungsgemeinschaft 525

(DFG) through grant BR 775/23-1. 526

527

REFERENCES 528

Benitez-Nelson C. R., O’Neill L., Kolowith L. C., Pellechia P. and Thunell R. (2004) 529

Phosphonates and particulate organic phosphorus cycling in an anoxic marine basin. 530

Limnol. Oceanogr. 49, 1593–1604. 531

Benitez-Nelson C. R., O`Neill Madden L. P., Styles R. M., Thunell R. C. and Astor Y. (2007) 532

Inorganic and organic sinking particulate phosphorus fluxes across the oxic/anoxic water 533

column of Cariaco Basin, Venezuela. Mar. Chem. 105, 90-100. 534

Bernard P. C, van Grieken R. and Brügmann L. (1989) Geochemistry of suspended matter 535

from the Baltic Sea. 1. Results of individual particle characterization by automated electron 536

microprobe. Mar. Chem. 26,155-177. 537

Berner R.A. (1973) Phosphate removal from sea water by adsorption on volcanogenic ferric 538

oxides. Earth Planet. Sci. Lett. 18, 77-86. 539

22

Böttcher M. E and Thamdrup B. (2001) Anaerobic sulfide oxidation and stable isotope 540

fractionation associated with bacterial sulfur disproportionation in the presence of MnO2. 541

Geochim. Cosmochim. Acta 65, 1573-1581. 542

Brumsack H.-J. (2006) The trace metal content of recent organic carbon-rich sediments: 543

Implications for Cretaceous black shale formation. Palaeogeogr. Palaeoclimat. 544

Palaeoecol. 232, 344-361. 545

Buffle J., De Vitre R. R., Perret D. and Leppard G. G. (1989) Physico-chemical 546

characteristics of a colloidal iron phosphate species formed at the oxic-anoxic interface of 547

a eutrophic lake. Geochim. Cosmochim. Acta 53, 399-408. 548

Burdige D. J. and Nealson K. H. (1986) Chemical and microbiological studies on sulfide-549

mediated manganese reduction. Geomircobiol. J. 4, 361-387. 550

Canfield D. E. (1989) Reactive iron in marine sediments. Geochim. Cosmochim. Acta 53, 551

619-632. 552

Canfield D. E., Raiswell R. and Bottrell S. (1992) The reactivity of sedimentary iron minerals 553

toward sulfide. Am. J. Sci. 292, 659-683. 554

Cline J. D. (1969) Spectrophotometric determination of hydrogen sulfide in natural waters. 555

Limnol. Oceanogr. 14, 454-458. 556

Danielsson L.-G., Dyrssen D. and Graneli A. (1980) Chemical investigations of Atlantis II 557

and Discovery brines in the Red Sea. Geochim. Cosmochim. Acta 44, 2051-2065. 558

Davison W., Woof C. and E. Rigg E. (1982) The dynamics of iron and manganese in a 559

seasonally anoxic lake; direct measurement of fluxes using sediment traps. Limnol. 560

Oceanogr. 27(6), 987-1003. 561

de Lange G. J., Catalano G., Klinkhammer G. P. and Luther III G. W. (1990) The interface 562

between oxic seawater and the anoxic Bannock brine; its sharpness and the consequences 563

for the redox-related cycling of Mn and Ba. Mar. Chem. 31, 205-217. 564

23

Dyrssen D. and Kremling K. (1990) Increasing hydrogen sulfide concentration and trace 565

metal behavior in the anoxic Baltic waters. Mar. Chem. 30, 193-204. 566

Einsele W. (1938) Über chemische und kolloidchemische Vorgänge in Eisen-Phosphat-567

Systemen unter limnochemischen und limnogeologischen Gesichtspunkten. Arch. 568

Hydrobiol. 33, 361-387. 569

Föllmi K. B. (1996) The phosphorus cycle, phosphogenesis and marine phosphate-rich 570

deposits. Earth-Sci. Rev. 40, 55-124. 571

Fox L. E. (1993) The chemistry of aquatic phosphate: inorganic processes in rivers. 572

Hydrobiol. 253, 1-16. 573

Friedrich G. E., Codispoti L. A., Sakamoto C. M. (1988) Bottle and pumpcast data from the 574

1988 Black Sea expedition. MBARI, Technical Report No. 90-3. Pacific Grove, Monterey, 575

CA 93950, USA. 576

Grasshoff K., Ehrhardt M. and Kremling K. (1983) Methods of seawater analysis. Verlag 577

Chemie, Weinheim. 578

Gunnars A., Blomqvist S., Johansson P. and Andersson C. (2002) Formation of Fe(III) 579

oxyhydroxide colloids in freshwater and brackish seawater with incorporation of phosphate 580

and calcium. Geochim. Cosmochim. Acta. 66, 745-758. 581

Handoh I. C. and Lenton T. M. (2003) Periodic mid-Cretaceous oceanic anoxic events linked 582

by oscillations of the phosphorus and oxygen biogeochemical cycles. Glob. Biogeochem. 583

Cycl. 17, 1092. 584

Hartmann M. (1985) Atlantis-II Deep geothermal brine system. Chemical processes between 585

hydrothermal brines and Red Sea deep water. Mar. Geol. 64, 157-177. 586

Huckriede H. and Meischner D. (1996) Origin and environment of manganese-rich sediments 587

within black-shale basins. Geochim. Cosmochim. Acta 60,1399-1413. 588

Hyacinthe C. and Van Cappellen P. (2004) An authigenic iron phosphate phase in estuarine 589

sediments: composition, formation and chemical reactivity. Mar. Chem. 91, 227-251. 590

24

Ingall E. D., Kolowith L., Lyons T. W. and Hurtgen M. (2005) Sediment carbon nitrogen and 591

phosphorus cycling in an anoxic fjord Effingham Inlet British Columbia. Am. J. Sci. 305, 592

240-258. 593

Ingri J., Löfvendahl R. and Boström K. (1991) Chemistry of suspended particles in the 594

southern Baltic Sea. Mar. Chem. 32, 73-87. 595

Lass H.-U., Mohrholz V., Nausch G. and Siegel H. (2010) On phosphate pumping into the 596

surface layer of the eastern Gotland Basin by upwelling. J. Mar. Sys. 80, 71–89. 597

Leipe T., Löffler A. and Bahlo R. (1996) Untersuchungen an Schwebstoffen mittels 598

Rasterelektronenmikroskopie und automatisierter Röntgenmikroanalyse (EDX) - Beispiele 599

aus der Ostsee. Zentralbl. Geol. Paläont. Teil I H5/6, 485-501. 600

Leipe T., Löffler A., Bahlo R. and Zahn W. (1999) Automated particle analysis of water 601

samples by scanning electron microscopy and X-ray microanalysis. Vom Wasser 93, 21-37. 602

Lyons, T. W. (1997) Sulfur isotopic trends and pathways of iron sulphide formation in upper 603

Holocene sediments of the anoxic Black Sea. Geochim. Cosmochim. Acta 61, 3367-3382. 604

Lyons T. W. and Severmann S. (2006) A critical look at iron paleoredox proxies: New 605

insights from modern euxinic marine basins. Geochim. Cosmochim. Acta 70, 5698-5722. 606

März C., Poulton S. W., Beckmann B., Kuester K., Wagner T. and Kasten S. (2008) Redox 607

sensitivity of P cycling during marine black shale formation: Dynamics of sulfidic and 608

anoxic non-sulfidic bottom waters. Geochim. Cosmochim. Acta 72, 3703-3717. 609

Millero F. J., Sotolongo S. and Izaguirre M. (1987) The oxidation kinetics of Fe(II) in 610

seawater. Geochim. Cosmochim. Acta 51, 793-801. 611

Murray J. W. (1974) The surface chemistry of hydrous manganese dioxide. J. Colloid 612

Interlace Sci. 46, 357 37l. 613

Neretin L. N., Pohl C., Jost G., Leipe T. and Pollehne F. (2003) Manganese cycling in the 614

Gotland Deep Baltic Sea. Mar. Chem. 82, 125-143. 615

25

Postma D. (1985) Concentration of Mn and separation from Fe in sediments – I. Kinetics and 616

stoichiometry of the reaction between birnessite and dissolved Fe(II) at 10°C. Geochim. 617

Cosmochim. Acta 49, 1023-1033. 618

Poulton S.W., Krom M.D. and Raiswell R. (2004) A revised scheme for the reactivity of iron 619

(oxyhydr)oxide minerals towards dissolved sulfide. Geochim. Cosmochim. Acta 68, 3703-620

3715. 621

Poulton S. W. and Canfield D. E. (2006) Co-diagenisis of iron and phosphorus in 622

hydrothermal sediments from the southern East Pacific Rise: Implications for the 623

evaluation of palaeoseawater phosphate concentrations. Geochim. Cosmochim. Acta 70, 624

5883-5898. 625

Raiswell, R., and Anderson, T. F. (2005) Reactive iron enrichment in sediments deposited 626

beneath euxinic bottom waters: constraints on supply by shelf recycling. In Mineral 627

Deposits and Earth Evolution (eds I. McDonald, , A.J. Boyce, I.B. Butler, R.J. Herrington 628

and D.A. Polya), Geological Society Special Publication Vol. 248, pp. 179–194. 629

Roekens E. J. and van Grieken R. E. (1983) Kinetics of iron(II) oxidation in seawater of 630

various pH. Mar. Chem. 13, 195-202. 631

Schippers A., Neretin L. N., Lavik G., Leipe T. and Pollehne F. (2005) Manganese (II) 632

oxidation driven by lateral oxygen intrusions in the western Black Sea. Geochim. 633

Cosmochim. Acta 69, 2241-52. 634

Shaffer G. (1986) Phosphate pumps and shuttles in the Black Sea. Nature 321, 515-517. 635

Sheldon R. W. (1967) A practical manual on the use of the Coulter Counter in marine science. 636

Coulter Electronics, Toronto. Pp. 66. 637

Slomp C. P., Van der Gaast S. J. and Van Raaphorst W. (1996) Phosphorus binding by poorly 638

crystalline iron oxides in North Sea sediments. Mar. Chem. 52, 55-73. 639

26

Strady E., Pohl C., Yakushev E. V., Krüger S. and Hennings U. (2008) PUMP-CTD-System 640

for trace metal sampling with a high vertical resolution. A test in the Gotland Basin Baltic 641

Sea. Chemosphere 70, 1309-1319. 642

Stumm W., Morgan J. J. (1996) Aquatic chemistry. Wiley, New York. 643

Suits N. S. and Wilkin R. T. (1998) Pyrite formation in the water column and sediments of a 644

meromictic lake. Geology 26(12), 1099-1102. 645

Taylor G. T., Iabichella M., Ho T.-Y., Scranton M. I., Thunell R. C., Muller-Karger F. and 646

Varela R. (2001) Chemoautotrophy in the redox transition zone oft he Cariaco Basin: A 647

significant midwater source of organic carbon production. Limnol. Oceanogr. 46, 148-163. 648

Tebo B. M., Bargar J. R., Clement B. G., Dick G. J., Murray K. J., Parker D., Verity R. and 649

Webb S. M. (2004) Biogenic manganese oxides: Properties and mechanisms of formation. 650

Ann. Rev. Earth Planet. Sci. 32, 287-328. 651

Tessenow U. (1974) Lösungs-, Diffusions- und Sorptionsprozesse in der Oberschicht von 652

Seesedimenten: IV. Reaktionsmechanismen und Gleichgewichte im System Eisen-653

Mangan-Phosphat im Hinblick auf die Vivianitakkumulation im Ursee. Arch. Hydrobiol. 654

Suppl. 47, 1–79. 655

Thibault P.-J., Rancourt D. G., Evans J. and Dutrizac J. E. (2009) Mineralogical confirmation 656

of a near-P:Fe=1:2 limiting stoichiometric ratio in colloidal P-bearing ferrihydrite-like 657

hydrous ferric oxide. Geochim. Cosmochim. Acta 73, 364-376. 658

Trefry J. H., Presley B. J., Keeney-Kennicutt W. L. and Trocine R. P. (1984) Distribution and 659

chemistry of manganese iron ans suspended particulates in Orca Basin. Geo-Mar. Lett. 4, 660

125-130. 661

Tribovillard N., Bout-Roumazeilles V., Algeo T., Lyons T. W., Sionneau T., Montero-662

Serrano J. C., Riboulleau A. and Baudin F. (2008) Paleodepositional conditions in the Orca 663

Basin as inferred from organic matter and trace metal contents. Mar. Geol. 254, 62-72. 664

27

Trouwborst R. E., Clement B. G., Tebo B. M., Glazer B. T. and Luther III G. W. (2006) 665

Soluble Mn(III) in suboxic zones. Science 313, 1955-1957. 666

Tsandev I. and Slomp C. P. (2009) Modeling phosphorus cycling and carbon burial during 667

Cretaceous Oceanic Anoxic Events. Earth Planet. Sci. Lett. 286, 71-79. 668

Tyrrell T. (1999) The relative influences of nitrogen and phosphorus on oceanic primary 669

production. Nature 400, 525-531. 670

Van Cappellen P., Ingall E. D. (1996) Redox stabilization of the atmosphere and oceans by 671

phosphorus-limited marine productivity. Science 271, 493-496. 672

Van Cappellen P. and Wang Y. (1996) Cycling of iron and Manganese in surface sediments: 673

A general theory for the coupled transport and reaction of carbon, oxygen, nitrogen, sulfur, 674

Iron, and Manganese. Am. J. Sci. 296, 197-243. 675

van der Zee C., van Raaphorst W. and Epping E. (2001) adsorbed Mn2+ and Mn redox 676

cycling in Iberian continental margin sediments (northeast Atlantic Ocean). J. Mar. Res. 677

59, 133-166. 678

van der Zee C., Slomp C. P., Rancourt D. G., de Lange G. J. and van Raaphorst W. (2005) A 679

Mössbauer spectroscopic study of the iron redox transition in eastern Mediterranean 680

sediments. Geochim. Cosmochim. Acta 69, 441-453. 681

Wijsman J. W. M., Middelburg J. J., Heip C.H.R. (2001) Reactive iron in Black Sea 682

sediments: implications for iron cycling. Mar. Geol. 172, 167-180. 683

Yakushev E. V., Pollehne F., Jost G., Kuznetsov I., Schneider B. and Umlauf L. (2007) 684

Analysis of the water column oxic/anoxic interface in the Black and Baltic seas with a 685

numerical model. Mar. Chem. 107, 388-410. 686

Yao W. and Millero F. J. (1993) The rate of sulfide oxidation by δMnO2. Geochim. 687

Cosmochim. Acta 57, 3359-3365. 688

Yao W. and Millero F. J. (1996a) Oxidation of hydrogen sulfide by hydrous Fe(III) oxides in 689

seawater. Mar. Chem. 52, 1-16. 690

28

Yao W. and Millero F. J. (1996b) Adsorption of phosphate on manganese dioxide in seawater. 691

Environ. Sci Techno. 30(2), 536-541. 692

29

693

Figure Captions: 694

695

Fig. 1. Maps of the study sites in the Baltic (a) and Black Seas (b). 696

697

Fig. 2. Textural and elemental properties of authigenic particles around pelagic redoxclines 698

from the Baltic and Black Seas. SEM photographs show typical star-shaped particles and 699

globular examples (b and d) along with results from EDX-analyses. Positions of EDX 700

measurements are marked by white arrows and contents of Mn, Fe, and P in weight% are 701

given below. 702

703

Fig. 3. Geochemical gradients around the pelagic redoxcline of the Gotland Basin (Baltic Sea) 704

in July 2006 and April 2007. Relative particle abundances originate from SEM-EDX 705

measurements. The gray bars denote the approximate position of the redoxcline. 706

707

Fig. 4. Geochemical gradients around the pelagic redoxclines of the Landsort Deep in July 708

2006 and April 2007. Relative particle abundances originate from SEM-EDX measurements. 709

The gray bars denote the approximate position of the redoxcline. 710

711

Fig. 5. Geochemical gradients around the pelagic redoxclines of the Black Sea (site 21 and 712

27, June 2007). Relative particle abundances originate from SEM-EDX measurements. The 713

gray bars denote the approximate position of the redoxcline. 714

715

Fig. 6. Concentrations of dissolved Mn, Fe, P, and O2 during dysoxic (a) and anoxic (b and c) 716

batch-type experiments in diluted seawater (salinity 12) versus the time. Iron(II)sulfate was 717

30

added to all runs. The anoxic runs B and C also contained biogenic MnOx particles and 718

phosphate was added to the solution in run C. 719

720

Fig. 7. Textural and elemental properties of particles sampled during the batch-type 721

experiments of run B and C. Areas of SEM-EDX measurements are marked by white zones. 722

The contents of Mn, Fe, and P are given in weight%. 723

724

Fig. 8. Depth profile of FeSx abundance from the Black Sea site 21 derived from automated 725

particle analyses by SEM-EDX. The upper profile shows results from across the redoxcline 726

and the lower plot displays the entire profile until 2.400 m depth. The gray bars denote the 727

approximate position of the redoxcline. Textural and elemental properties of FeSx particles 728

are shown on the right (b.d.l.=below detection limit). 729

730

Fig. 9. Conceptual model of the Mn-Fe-P shuttle for pelagic redoxclines of anoxic basins. The 731

left part describes the “euxinic mode” of H2S containing basins like the Black Sea. The right 732

part illustrates an anoxic, non-sulfidic basin, a situation presumably relevant for certain time 733

periods during Cretaceous black shale formation. The distinct difference between both 734

settings forms the stability of mineral phases during sinking and the possibility of sedimentary 735

burial. 736

737

738

31

739

740

741

742

Fig 1 743

744

33

746

34

747

748

749

Fig 3 750

751

35

752

753

754

Fig 4 755

756

36

757

758

Fig 5 759

760

37

761

762

763

Fig 6 764

765

38

766

767

768

Fig 7 769

770

39

771

772

773

Fig 8 774

775

40

776

777

778

779

Fig 9 780

781

42

782

Table 1. SEM-EDX analyses of particles from the Baltic and Black Seas showing average weight% of Al, Ca, Mg, Si, Mn, Fe, and P along with 783

molar P/Si and Fe/P ratios (n denotes the number of particles; standard deviations are given in brackets). Fe/P ratios are presented for particles with 784

Fe contents >5%. Ratios for Cretaceous black shales were calculated from sequential Fe and P extractions (März et al., 2008). 785

Location Depth [m] Particle type n Al Ca Mg Si Mn Fe P P/Si Fe/P

Gotland Basin 2007 Landsort Deep 2007 Black Sea 2007 (site 27)

Cretaceous Black Shales Vivianitea Barbosalitea

Tinticitea Strengitea

120 134 145 165 65 70 91 105 115 135

MnOx MnOx-FeOOH-PO4 FeOOH-PO4 FeOOH-PO4 MnOx MnOx-FeOOH-PO4 FeOOH-PO4 MnOx MnOx-FeOOH-PO4 FeOOH-PO4

FeII

3(PO4)2*8 H2O FeIIFeIII

2(OH,PO4)2 FeIII

4(PO4,OH)3*5 H2OFeIIIPO4*2 H2O

15 9 9 7 4 15 9 6 10 8

0.3 (0.2) 0.4 (0.3) 0.7 (0.6) 0.7 (0.7) 0.4 (0.2) 0.4 (0.4) 0.6 (0.2) 1.0 (1.0) 0.3 (0.2) 0.8 (0.5)

2.6 (0.6) 2.3 (0.2) 1.8 (0.4) 1.2 (0.5) 2.6 (0.7) 1.9 (0.5) 2.5 (0.4) 2.1 (0.8) 1.9 (0.6) 2.8 (0.9)

2.0 (0.4) 0.9 (0.5) 2.3 (0.6) 2.0 (0.4) 2.2 (0.5) 0.9 (0.4) 2.2 (0.3) 2.7 (0.3) 1.1 (0.3) 2.4 (0.6)

1.6 (1.7) 1.5 (0.6) 1.8 (1.1) 1.2 (0.5) 1.1 (0.4) 1.4 (0.5) 1.1 (0.5) 1.2 (1.1) 1.6 (1.0) 1.6 (0.8)

50 (5.0) 21 (10.0) 3.7 (1.7) 1.3 (0.8) 49 (0.9) 19 (14.4) 1.2 (0.3) 42 (5.6) 27 (9.2) 4.6 (1.1)

2.9 (1.5) 19 (4.9) 30 (8.1) 37 (8.7) 3.4 (1.6) 24 (9.0) 33 (3.2) 2.4 (1.0) 23 (6.6) 25 (6.6)

0.8 (0.4) 4.8 (1.0) 8.1 (2.7) 7.0 (1.1) 1.3 (1.6) 7.0 (2.7) 8.2 (0.8) 1.6 (0.8) 6.0 (1.4) 4.9 (0.9)

1.0 (0.6) 3.3 (1.3) 6.0 (4.3) 7.3 (5.2) 1.1 (0.3) 5.1 (2.2) 7.2 (3.7) 1.6 (0.9) 4.4 (2.1) 3.7 (2.2)

2.2 (0.4) 2.1 (0.6) 2.9 (0.5) 2.0 (0.4) 2.2 (0.2) 2.1 (0.5) 2.9 (0.6)

1.0-1.5 1.5 1.5 1.3 1.0

aknown Fe and P minerals 786 787

788