Marine Geology 366 (2015) 62–68

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Marine Geology

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When deep diagenesis in Arctic Ocean sediments compromisesmanganese-based geochronology

Bjorn Sundby a,b,⁎, Pascal Lecroart c, Pierre Anschutz c, Sergei Katsev d, Alfonso Mucci b

a ISMER, Université du Québec à Rimouski, Rimouski, QC G5L 3A1, Canadab GEOTOP and Earth & Planetary Sciences, McGill Univ., Montreal, QC H3A 0E8, Canadac EPOC, Université de Bordeaux, 33405 Talence, Franced University of Minnesota Duluth, Duluth, MN 55812, USA

⁎ Corresponding author at: Earth & Planetary ScieUniversity Street, Montreal (Quebec) H3A 2A7, Canada.

E-mail addresses: bjorn.sundby@mcgill.ca (B. Sundby)(P. Lecroart), pierre.anschutz@u-bordeaux.fr (P. Anschutz(S. Katsev), alfonso.mucci@mcgill.ca (A. Mucci).

http://dx.doi.org/10.1016/j.margeo.2015.04.0050025-3227/© 2015 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 14 November 2014Received in revised form 11 April 2015Accepted 25 April 2015Available online 27 April 2015

Keywords:Arctic OceanSedimentsMethane oxidationManganese remobilizationGlacial–interglacial cyclicityModeling

We used a diagenetic model to test the hypothesis that manganese-rich layers in gas hydrate-bearing ArcticOcean sediments are reliable time markers for interglacial periods. In the model, diagenesis is fuelled by twosources of reactive carbon: particulate organic carbon settling to the sediment surface, and methane diffusingup from deep gas hydrate deposits. The model includes oxidation of organic carbon and soluble reduced manga-nese by oxygen supplied continuously from an invariant bottom-water oxygen reservoir; reduction of particulatemanganese by hydrogen sulfide generated through anaerobic methane oxidation; transport of dissolved oxygenand manganese by diffusion; and advective transport of particulate components by burial. Particulate organicmatter and particulate manganese are only supplied to the sediment during interglacials. Sulfate reduction isnot modeled explicitly; instead, the effect of anaerobic methane oxidation on Mn reduction is simulated at thelower boundary of the model by prescribing that particulate manganese is reduced there to soluble Mn(II).The soluble reduced Mn then diffuses upward and is oxidatively precipitated to Mn(IV) by downward diffusingoxygen. The upward flux of soluble Mn(II) is thus a function of the rate at which particulate manganese isadvected into the Mn-reduction layer at the bottom of the model; it is not synchronous with events at thesediment–water interface. Model runs reveal that, under idealized but realistic conditions for the Arctic Ocean,oxidation of upward-diffusing Mn(II) generates post-depositional manganese enrichments that cannot readilybe distinguished from the manganese-rich sediment layers that accumulate during interglacials. This compro-mises the use of manganese-rich layers as proxies for interglacial periods. In contrast, manganese-rich layersmay be used as first-order markers of interglacial periods in sediments where gas hydrates or other forms of re-active carbon are absent.

© 2015 Elsevier B.V. All rights reserved.

1. Introduction

Poor preservation of microfossil records in Arctic Ocean sedimentshas spurred the search for alternative proxies for the Quaternary chro-nology of these sediments (Jakobsson et al., 2000; and references there-in). Manganese-rich interbedded layers, found in numerous long coresrecovered from the Central Arctic Ocean (März et al., 2011; Löwemarket al., 2014;Macdonald andGobeil, 2012) are strong candidates as proxiesof interglacial periods. A time scale based on manganese content andsediment color in a core from the Lomonosov Ridge correlates closelywith an independent paleomagnetic chronology of the same core, basedon Brunhes-age estimates of geomagnetic excursions (Jakobsson et al.,

nces, McGill University, 3450

, pascal.lecroart@u-bordeaux.fr), skatsev@d.umn.edu

2000; Löwemark et al., 2008). Recent geochemical work has shownthat the manganese-rich layers are also rich in several trace metals aswell as in biogenic and ice-rafted carbonate and contain preservedtraces of bioturbation (März et al., 2011; Löwemark et al., 2014).

There are few measurements of dissolved manganese in long coresfrom the Arctic. Dickens et al. (2007) measured manganese in a longcore from about 1200 m depth on the Lomonosov Ridge and found in-creasing concentrations of dissolved Mn in the pore water beginning afew m below the sea floor and reaching maximum values at 20 mdepth. In a more detailed study, März et al. (2011) working with along core from1200mdepth on theMendeleev Ridge, found anupwarddirected gradient of dissolved manganese between 6 and 3 m, suggest-ing possible overprinting of diagenetically mobilized manganese onexisting Mn-rich layers.

A scenario whereby the fluxes of particulate organic carbon andmanganese to the sea floor in the Arctic Ocean varied dramatically dur-ing the glacial cycles of the Quaternary is consistent with observations.During glaciations, the absence of primary production virtually shuts

63B. Sundby et al. / Marine Geology 366 (2015) 62–68

off the flux of organic carbon, and the ice cover on the continents slowsdown the delivery of manganese-rich sedimentary material to the seafloor. During interglacials, primary production is turned on and deliversfresh organic carbon to the sea floorwhile rivers and the expanded con-tinental shelves deliver terrigenous organic carbon and manganese-bearing particulate matter (Macdonald and Gobeil, 2012). Upon burial,manganese-rich interglacial sediments enter the geological recordsandwiched between layers of manganese-poor glacial sediment. Ifthe manganese-rich layers remain intact, they mimic the glaciation-interglaciation cyclicity. Several plausible mechanisms for deliveringmanganese-rich sediment to the sea floor during interglacials havebeen proposed (e.g. Macdonald and Gobeil, 2012).

The hypothesis thatMn-rich sediment layers enter the geological re-cord intact rests on the assumption that diagenetic remobilization andredistribution ofmanganese is fuelled exclusively by organic carbon set-tling from thewater columnand thatmanganese diagenesis stopswhenthis carbon has been consumed. The objective of the present study is totest this hypothesis and verify if manganese-rich layers in deep ArcticOcean sediments can be used as a proxy for interglacial periods. It isnot our intention to reconstruct the past. Specifically, we test this hy-pothesis for sediments that host gas hydrates. Such sediments are abun-dant in the Arctic Ocean where vast amounts of methane hydrates arebelieved to be stored (Buffett and Archer, 2004; Klauda and Sandler,2005). The actual amounts are uncertain (Milkov, 2004).

2. Model description

2.1. The conceptual model

The choice of diageneticmodel for this studywas dictated by our ob-jective, which was to test the hypothesis that manganese-rich layers indeep Arctic Ocean sediments can be used as a proxy for interglacial pe-riods. To this end,we chose the simplest possiblemodel thatwould helpusmeet this objective and left out reactions that could give amore com-plete description of diagenesis such as oxidation of Fe(II) and ammoniaby oxygen and reduction of Fe(III) by hydrogen sulfide. The sedimentingreactive organic matter is assumed to be mineralized entirely throughoxic respiration (the mineralization process is carbon limited), so amore complex model that includes the products of anaerobic minerali-zation pathways would not be expected to change our conclusions.

Fig. 1. Spatial representation of the conceptual model. In the time evolution of this system, the p

Themodel we used is a one-dimensional non-steady state diagenet-ic transport-reaction model that captures the essential features of man-ganese diagenesis, i.e. reduction of Mn(IV) to Mn(II) (dissolution),reoxidation of Mn(II) to Mn(IV) (precipitation), transport of dissolvedmanganese by diffusion, and transport of precipitated manganese byadvection. Fig. 1 shows the conceptual version of the model. It includestwo sources of reactive organic carbon: fresh organic matter settlingthrough the water column and methane released from gas hydratesdeep in the sediment column. The reactive organic carbon depositedon the sediment surface is consumed in reactionswith dissolved oxygenas it is buried and advected downward. Oxygen diffuses into the sedi-ment from a permanently oxygenatedwater column. The second sourceof carbon,methane, is oxidized bymicro-organisms that use sulfate as aterminal electron acceptor. This process, anaerobic methane oxidation(AMO), is known to take place in the sulfate-methane transition layer(SMT) where upward diffusing methane and downward diffusingsulfate are consumed and become depleted. AMO produces sulfide,which reduces oxidized particulate Mn(IV) to soluble Mn(II) (Allerand Rude, 1988). Methanemay also reduce oxidizedmanganese direct-ly (Beal et al., 2009). The model domain is bounded at the top by thesediment surface and at the bottom by a depth that remains at a fixeddistance from the upper boundary. Thus, both boundaries move up-wards at the rate with which sediment accumulates, set at 1 cm Ky−1

in the model. One may reasonably expect that the sediment accumula-tion rate is higher during interglacials than during glacials and, there-fore, that the assumption of constant sedimentation rate may not berealistic. Nonetheless, since the differential rates are not known, andgiven that the spatial scale of Mn redistribution (~2 m) is greater thanthe thickness of the sediment layer deposited during one cycle, makingthis assumption is not expected to change the conclusions.

The reactions in the SMT that produce hydrogen sulfide are notincluded explicitly in the model, nor is the reduction of Mn(IV) toMn(II) by hydrogen sulfide. Instead, reducing conditions are prescribednear the bottom of the model domain. There, particulate oxidized man-ganese, advecteddownward by burial, is reduced to solubleMn(II),whichdiffuses upwards until reoxidized by downward diffusing oxygen. Thethickness of the prescribed reduction layer (a few cm in the numericalmodel implementation) is not critical because the production rate of dis-solved Mn(II) is transport controlled, i.e. the rate of reductive dissolutionoverwhelms the rate of supply of reduciblemanganese. The choice of a 5-m thick model domain was inspired by the data of März et al. (2011),

rocesses associatedwith step 4 only take place once step 5 has occurred for the first time.

Table 1Model description.

Dissolved and solid species included in the model

Primary and secondary redox reactions

Mass conservation equations

Boundary conditions

Initial conditions

Time functions

Species

Oxygen

Manganese

Organic matter

Manganese oxide

Variable

O2

Mn2+

OC

MnOx

Unit

Solutes

Solids

mmol/cm3

mmol/cm3

mmol/cm3

mmol/cm3

OC

z0

zb

FOM inter z0 × Ω(t) FMnOx inter z0 ×Ω(t)

No gradient No gradient

O2

O2 z0

O2 zb

Mn2+ z0

FMn zb

Mn2+ MnOx

Table 2Model input parameters.

Parameter Value Cited values Sources

Top of the calculationdomain

z0 = 0 cm

Bottom of the domain zb = 1000 (500) cmTemperature T = 2 °C 1Porosity φ= 0.6 1Time parameters t inter = 20 ka

t glac = 80 kaτ= 1.5 kan = 5

Simulation time Γ = 1000 kaSedimentation rate w = 1 cm/ka 0.1–1 1, 2Diffusion coefficients DO2 = 44 104 cm2/ka 3

DMn = 11 104 cm2/ka 3Tortuosity θ =φ1.14 Γ 1

Particle mixingcoefficient

Db = 70 cm2/ka 40–100 1, 2η = 0.5 cm

Mixing layer depth L = 3 cm 2–5 1, 2Boundary conditionsfor solutes

O2 z0 = 2.10-4 mmol/cm3 1Mn2+

z0 = 0 mmol/cm3

O2 zb = 0 mmol/cm3

FMn zb = 0 mmol/cm2/kaBoundary conditionsfor solids

FOM inter z0 = 150 mmol/cm2/ka 8–100 1FOM glac z0 = 0 mmol/cm2/kaFMnOx-inter z0 = 0.1 mmol/cm2/kaFMnOx glac z0 = 0 mmol/cm2/ka∂MnOx=∂zð Þzb ¼ 0 1

Initial condition MnOxinter = 0.24 mmol/cm3 0.04 0.24m 5

Rate constant k = 10 ka-1 4 10−2–100 1k1MnOx = 142·107 cm3/mmol/ka (35–250)107 1, 5k2MnOx = 60 ka-1 0–60 5

1. (Katsev et al., 2006); 2. (Clough et al., 1997); 3. (Boudreau, 1997); 4. (März et al., 2011);5. (Berg et al., 2003).

64 B. Sundby et al. / Marine Geology 366 (2015) 62–68

which suggest that dissolved manganese is produced at about this depthin their Lomonosov Ridge core.

Since the model assumes that the entire sedimentation flux of reac-tive organic matter is consumed by oxygen, early diagenesis of manga-nese within the interglacial layer is not included, nor are fluctuations inthe flux of particulate manganese and organic carbon that could createmultiple peakswithin an interglacial layer (Katsev et al., 2006).Multiplepeaks in the manganese content of a layer being advected towards the

deep reduction zonewill affect the instantaneous production rate of sol-uble reduced manganese in the reduction zone, but it will not changethe total dissolved-manganese flux originating in that layer.

The oxygenflux and the penetration of oxygen into the sediment aregoverned by the oxidation of organic carbon and reduced manganese.At the end of an interglacial, themanganese-rich layer is covered by sed-iment free of organic carbon and manganese, and is progressively bur-ied. Consequently, oxygen diffuses further down into the sediment asit progressively oxidizes the reactive organic carbon that was suppliedand survived oxidation during the interglacial period.

The choice to keep the lower boundary of the model domain at afixed distance from the sediment surface throughout a glacial cyclewas pragmatic considering the complexity of the processes that actupon the depth of the SMT. On the one hand, interrupting the flux of or-ganic carbon to the sediment at the end of an interglacial period woulddeepen the oxygen penetration and the depth of the SMT (Contreraset al., 2013). On the other hand, the lowering of the sea level that char-acterizes a glacial period decreases the pressure on the sediment andcauses the stability field of gas hydrates, and thus the depth at whichgas hydrates decompose and release methane, to move closer to thesediment surface (Dickens et al., 1995; Paull et al., 1991). The combinedeffect of these two opposing processes on the location within the sedi-ment of the SMTwould be difficult to predict, butwewill attempt to ad-dress it in a subsequent paper.

2.2. Numerical implementation

The conceptual model was implemented numerically as a transienttransport-reaction model (e.g. Donnadieu et al., 2002; Berg et al.,2003), the details of which are described in Table 1. As outlined above,the model complexity was limited to the processes and species that re-late to manganese diagenesis on the time scale considered. The param-eter values we used are listed in Table 2. Dissolved species are Mn(II)

Mn

Ox

(mm

ol/c

m3 )

Mn2+

(µ

mo

l/L)

O2

(µm

ol/L

)

t = 5 ka t = 125 ka t = 140 ka t = 500 ka1.2

1.0

0.8

0.6

0.4

0.2

0

1.2

1.0

0.8

0.6

0.4

0.2

0

1.2

1.0

0.8

0.6

0.4

0.2

0

1.2

1.0

0.8

0.6

0.4

0.2

0

250

200

150

100

50

0

250

200

150

100

50

0

250

200

150

100

50

0

250

200

150

100

50

0

250

200

150

100

50

0

Depth (cm) Depth (cm) Depth (cm) Depth (cm)

Depth (cm) Depth (cm) Depth (cm) Depth (cm)

Depth (cm) Depth (cm) Depth (cm) Depth (cm)250

200

150

100

50

0

250

200

150

100

50

0

250

200

150

100

50

0

0 50 100 150 200 250 300 350 400 450 500 0 50 100 150 200 250 300 350 400 450 500 0 50 100 150 200 250 300 350 400 450 500 0 50 100 150 200 250 300 350 400 450 500

0 50 100 150 200 250 300 350 400 450 500 0 50 100 150 200 250 300 350 400 450 500 0 50 100 150 200 250 300 350 400 450 500 0 50 100 150 200 250 300 350 400 450 500

0 50 100 150 200 250 300 350 400 450 500 0 50 100 150 200 250 300 350 400 450 500 0 50 100 150 200 250 300 350 400 450 500 0 50 100 150 200 250 300 350 400 450 500

Fig. 2. Distribution of oxidized particulate manganese (upper panels), reduced dissolvedmanganese (middle panels), and dissolved oxygen (lower panels) in a model sediment at the beginning and during a run of five successive glacial–interglacialcycles. Each cycle lasted 100 ka. During each interglacial period, particulatemanganesewas added at the sediment surface, generating a Gaussian-shaped solid-phasemanganese peak. In the absence of reducing conditions at the bottom of themodeldomain, these peaks would remain intact, leading to the pattern of equally-spaced identically-shapedmanganese peaks propagating downward. This distributionwas then used as the initial state for a run of five glacial cycles, duringwhich reducingconditions were imposed at the bottom of the model domain. The resulting pattern (t = 500 ka) shows that new manganese-rich layers appeared below 175 cm sub-bottom depth. The background concentration and the inventory of manganeseincreased, but the increase is restricted to depths below 175 cm. Panels at t = 125 ka and t = 140 ka show two contrasting situations during glacial and interglacial intervals, respectively.

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66 B. Sundby et al. / Marine Geology 366 (2015) 62–68

and O2. Solid species are manganese oxide (Mn(IV)) and labile organiccarbon (OC). Particulate OC is deposited by sedimentation at the topof the domain.

As described above, the flux of particulate manganese to the sedi-mentwas kept constant during the interglacial period andwas null dur-ing the glacials. The abrupt transition in the manganese flux at thebeginning and the end of an interglacial period could cause numericalproblems for the model. To smoothen the transitions at the beginningand end of an interglacial and thus avoid this problem,we included par-ticle mixing in the 6 cm thick surface layer. The particle mixing wasmodeled as a diffusion analogous process, equivalent to biodiffusion.The mixing coefficient was kept constant from 0–3 cm, below which itdecayed exponentially to zero at 6 cm (e.g. Maire et al., 2008). Themixing process generates a layer of manganese-rich sediment inwhich the vertical manganese distribution has the shape of a Gaussiancurve. This layer is then buried.

Diffusion affects the distribution of soluble species and is describedin porous media by Fick's first law with their respective diffusivitiescorrected for tortuosity. Porosity is assumed constant, a fair assumptionbelow a few tens of centimeters. Burial is assumed to be an advectiveprocesswith constant burial velocity. The concentration of oxygen is as-sumed constant at the upper boundary and zero at the lower boundary.A zero dissolved manganese concentration is imposed at the top of themodel domain, and a zero flux of dissolved Mn is imposed at the baseof the domain. Imposing a zero Mn(II) flux across the lower modelboundary ignores the possible presence of a sink for Mn(II) deeper inthe sediment such as the coprecipitation of Mn(II) with calcium carbon-ate or the precipitation of a distinct Mn(II) carbonate. We have no infor-mation that can be used to establish the location of such a sink (depthbelow the lower model boundary) and can therefore not estimate adownward directed gradient and flux of Mn(II). In the absence of a fluxacross the lower boundary, the inventory of manganese within themodel domain builds up, and eventually—over several glacial–interglacialcycles—leads to a shallowing of the oxygen penetration depth during in-terglacials (see Fig. 3).

We applied a repeating pattern of high fluxes of OC andMn(IV) dur-ing interglacials and null fluxes of OC and Mn(IV) during glacials(Table 1). The model was tuned by varying the flux and reactivity ofthe OC and the duration of the interglacial period until the model pro-duced manganese oxide profiles comparable to profiles not subjectedto diagenesis. The equations for conservation of mass for the four spe-cies were solved using a finite element approach implemented withthe PDE module of COMSOL Multiphysics ®.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

0 100 200 300 400 5

Oxy

gen

pen

etra

tio

n d

epth

(m

)

Tim

Fig. 3.Model calculations of the temporal variations of the oxygen penetration depth into the sinterglacials. After an interglacial, oxygen penetration increases rapidly as it burns through resilivery of reactive OC to the sea floor rapidly draws it back closer to the surface (Katsev et al., 20encounters oxygen and, thus, the depthwhere dissolvedmanganese reprecipitates tracks the othe advection of manganese-rich sediment into the manganese reduction zone (SMT) at the botime in the model calculation reflects the progressive accumulation of manganese within the mboundary.

3. Results and discussion

3.1. Burial of manganese-rich layers in sediments where gas hydrates areabsent

In the absence of decomposing gas hydrates that can supply meth-ane to the sediment pore water, the Mn-rich interglacial sedimentlayers would remain intact once buried. Under these conditions, themodel does not falsify the hypothesis thatmanganese-rich layers in Arc-tic Ocean sediments can be used as a proxy for interglacial periods. Al-though this statement is self-evident, it is not trivial, because it offersthe means to locate a sedimentary manganese record that has notbeen compromised by post-depositional alterations. As pointed out byBorowski et al. (1996, 1999), the pore-water sulfate distribution is sen-sitive to the presence of sub-surfacemethane and, thus, the absence of asulfate gradient can be interpreted as an absence of decomposing gashydrates.

3.2. Burial of manganese-rich layers in gas hydrate containing sediments

Reducing conditions generated by the presence of methane at depthin the sediment column lead to the remobilization and redistribution ofmanganese. Fig. 2 illustrates this with a model run over five successiveglacial–interglacial cycles, each lasting 100 Ky. During the interglacialperiod of each cycle, manganese was added to the sea floor, wheresediment accumulation and particle mixing combined to generate aGaussian-shaped peak in the vertical manganese distribution. Whenthe run started, the 5-m thick sediment column contained five of theselayers, identical and equally spaced as the initial condition for theMn(IV) distribution. During the run, 5 m of new sediment — includingfive new manganese-rich layers — were added to the sediment columnwhile the top of the model domain kept track with the upward movingsea floor. By the end of the model run, the initial five interglacial layershad been advected into the reduction layer (SMT), and their manganesecontent had been remobilized and redistributed in the overlying sedi-ment column.Of thefive interglacial layers added to the sediment columnduring the course of the model run, the two most recent ones were notaltered, but the three older layers grew in size and changed shape becauseof the precipitation of remobilized manganese.

Fig. 2 demonstrates that evenwhen one starts outwith a simple pat-tern of solid-phasemanganese distributed across the sediment column,the distribution pattern that results from diagenesis can be complex. Inthis model run, most of the remobilized manganese is redistributed

00 600 700 800 900 1,000e (ka)

ediment (in meters below sea floor). The shallowest oxygen penetration occurs during thedual reactive OC and remains deep until the beginning of a new interglacial when the de-06). Dissolved manganese, originating in the deepest model domain, precipitates when itxygen penetration depth. The actual upward flux of dissolvedmanganese is determined byttom of the model domain. The gradual shallowing of the oxygen penetration depth withodel domain, a consequence of imposing zero flux of dissolved Mn(II) at the lower model

Mn

Ox

(mm

ol/c

m3 )

1.0

0.8

0.6

0.4

0.2

0

1.0

0.8

0.6

0.4

0.2

0

1.0

0.8

0.6

0.4

0.2

0

250

200

150

100

50

0

O2

(µm

ol/L

)

250

200

150

100

50

0

250

200

150

100

50

0

400

300

200

100

0

Mn

2+ (µ

mo

l/L)

t = 0 ka t = 20 ka t = 45 ka

Depth (cm) Depth (cm) Depth (cm)

Depth (cm) Depth (cm) Depth (cm)

Depth (cm) Depth (cm) Depth (cm)

400

300

200

100

0

400

300

200

100

0

0 50 100 150 200 250 300 350 400 450 500 0 50 100 150 200 250 300 350 400 450 500 0 50 100 150 200 250 300 350 400 450 500

0 50 100 150 200 250 300 350 400 450 500 0 50 100 150 200 250 300 350 400 450 500 0 50 100 150 200 250 300 350 400 450 500

0 50 100 150 200 250 300 350 400 450 5000 50 100 150 200 250 300 350 400 450 5000 50 100 150 200 250 300 350 400 450 500

Fig. 4. Distribution of oxidized particulate manganese (upper panels), reduced dissolved manganese (middle panels), and dissolved oxygen (lower panels) of manganese-rich sediment after a single glacial–interglacial cycle. The initial solid-phasemanganese distribution (upper right panel) consisted of three different manganese-rich layers un-equally spaced over a 5-m sediment column. Note the erosion of the deepest manganese-rich layer, the appearance of a newmanganese-rich layerbeginning at 2 m depth, and the increase in the background concentration of oxidized manganese below 2 m depth. Note also the shift in the oxygen penetration depth due to the presence/absence of a layer, rich in Mn and organic carbon, at thesurface.

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across the deeper part of the sediment column; none of it seems toreach above about 2 m below the sediment surface. This result can beunderstood in light of the cyclic pattern of the oxygen penetrationinto the sediment. Fig. 3 shows that duringmost of this run, oxygen pen-etrates nearly to the bottomof themodel domain.Only during (relative-ly) brief periods of time does the penetration depth rise towards theupper reaches of the sediment column. The timing of the shallowoxygen penetration events coincides with the interglacial part of thecycle, as could be expected since this is when fresh reactive organic car-bon is delivered to the sea floor. It follows that this is the timewhen dis-solved manganese — if present in the pore water — can be transportedclosest to the sea floor before being oxidized and reprecipitated. The rel-atively deep oxygen penetration in the model results during these pe-riods can be attributed to the relatively low value we used for theorganic carbon flux to the sediment during the interglacials.

The complexity of the diagenetic remobilization ofmanganese is fur-ther illustrated in Fig. 4, which shows the results of a model run over asingle glacial cycle, beginning when the bottom of a thick manganese-rich layer enters the reduction zone at the bottom of our sedimentcolumn. At the end of the 50 Ky cycle, the deepest Mn-rich layer(400–500 cm) had been reductively dissolved. The layer at 350 cmdepth had been buried and grown in intensity because of the increasedbackground concentration of Mn(IV). A newMn-rich layer had appearedat 200–250 cm, and the original Mn-rich layer at 50 cm had been buriedintact. Facedwith the complexities ofmanganese diagenesis in our simplemodel sediment, it cannot be assumed that manganese-rich layers foundin gas hydrate bearing sediments are primary features, laid down duringinterglacial periods.

3.3. The deep manganese cycle

The presence and distribution of dissolved manganese in the porewater depend on the advection of manganese-rich sediment acrossthe upper boundary of the reduction zone (SMT), the timing of whichis unlikely to be synchronous with the oxygen penetration pattern.Most of the time, as shown in Fig. 3, oxygen is present deep in the sed-iment, close to the bottom of the model domain. Therefore, the condi-tions in the pore water will be oxidizing most of the time and anydissolved manganese produced will only be able to diffuse a short dis-tance before it is oxidized and precipitated. Except during and shortlyafter interglacial periods when oxygen penetration is shallow, dissolvedmanganese will reprecipitate deep in the sediment, close to the SMT,and oxidized Mn may accumulate, perhaps to high levels, before it isburied again and enters a new cycle of dissolution, diffusion, oxidation,and burial. The deep recycling ofmanganese, the results of which can beseen just above the dissolution zone in Fig. 2, explains why the redistri-bution pattern of manganese is decoupled from the distribution patternin the sediment column that undergoes burial.

4. Conclusion

A one-dimensional non-steady state diagenetic transport-reactionmodel reveals that destabilization of deep-seated gas hydrates and sub-sequent oxidation via anaerobic sulfate oxidation of methane diffusingup in the sediment column may remobilize manganese-rich sedimentlayers and thus compromise the use of such layers in Arctic Ocean sed-iments as a primary record of glacial–interglacial transitions. Unfortu-nately, neither the redistribution of manganese-rich layers nor theoriginalmanganese record can be readily reconstructed because verticalexcursions of the sediment oxygen penetration depth (temporal varia-tions in the distribution of dissolved oxygen in the sediment column)and the production of dissolved Mn (burial of manganese-rich layersin the sulfate methane transition zone) are not likely to be in phase.

In contrast, where methane or other forms of secondary organicmatter are absent from sediments, manganese-rich layers depositedduring interglacials will be preserved, and their presence in the sedi-ment column can be used as a proxy of interglacial periods. The absenceof a sulfate gradient in the sediment pore-water is an indicator of thistype of sediment because the sulfate distribution is sensitive to the pres-ence of sub-surface methane.

Acknowledgments

Thework of Dr. ChristianMärz andhis colleagues inspired this study,and Dr. März gave us access to their geochemical data from the ArcticOcean. Comments and suggestions by Dr. David Burdige and an anony-mous reviewer helped improve the manuscript. The Natural Researchand Engineering Council of Canada RGPIN 124367 and the Universitéde Bordeaux provided financial support. Bacchus, the God ofWine, pro-vided spiritual guidance. We thank them all.

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