Barron, J., Larsen, B., et a l , 1991Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 119

21. SEDIMENTATION RATE ESTIMATES FROM SULFATE AND AMMONIA GRADIENTS1

R. E. Cranston2

ABSTRACT

Data from the Ocean Drilling Program and from other marine studies are used to relate organic carbon flux pre-served in the seabed with the uptake of sulfate and production of ammonia in the pore water of the sediment column.Relationships are derived that can be used to estimate recent sedimentation rates to within one order of magnitude,spanning the range from 0.1 to 100,000 m/m.y. The relationships provide a simple rule of thumb to apply to initial ob-servations when an estimate of sedimentation rate is not readily available from biostratigraphic or isotope methods. Thesedimentation rates estimated for the Leg 119 sites agree to within a factor of 3 with the biostratigraphic results.

INTRODUCTION

The preservation of organic matter in marine sediments de-pends on the sedimentation rate, organic carbon concentration,and amount of carbon oxidized by biogeochemical processes(Reimers and Suess, 1983). The sediment becomes anoxic at adepth where the reactive organic carbon flux to the sediment ex-ceeds the oxygen flux into the sediment. In oxic sediments, themore reactive organic matter is efficiently oxidized by oxygen(Froelich et al., 1979). Below the oxic region, less reactive or-ganic matter remains and smaller amounts of oxidizing agents(nitrate, manganese, iron, and sulfate) are available, and thuscarbon preservation is more likely. Extensive efforts have beentaken to fit diffusion/reaction equations to observed pore-waterprofiles and in some cases to relate these to the sedimentationrate (e.g., Toth and Lerman, 1977; Berner, 1978; Bender andHeggie, 1984; Val Klump and Martens, 1989). Careful selectionof values for porosity, tortuosity, diffusion coefficient, sedimen-tation rate, and elemental ratio in organic matter allows one tofit pore-water profiles to equations; however, the equations maynot be applicable to other areas. This paper presents relation-ships that can be used to predict recent sedimentation rates towithin one order of magnitude.

The flux of carbon to an anoxic zone affects the flux of oxi-dant into the anoxic zone and the production of oxidation prod-ucts. A variety of oxidants and oxidation products can be mea-sured in the upper few decimeters to meters of the sediment col-umn (e.g., oxygen and nitrate); however, this region is notadequately studied during Ocean Drilling Program (ODP) oper-ations. Other dissolved chemical species are involved with pre-cipitation and adsorption processes that can provide confusingresults (e.g., manganese, iron, bicarbonate, and phosphate).Sulfate is the major oxidant in anoxic sediments and tends todecrease in concentration downcore. Ammonia is a by-productof organic matter oxidation (Henrichs and Reeburgh, 1987), in-creasing in concentration downcore.

A relationship between sedimentation rate and organic car-bon content has often been observed (Heath et al., 1977; Mullerand Suess, 1979; Stein, 1986; Henrichs and Reeburgh, 1987).However, this relationship may not apply to some environ-ments. Dilution by organic-poor terrigenous sediment in high-

1 Barren, J., Larsen, B., et al., 1991. Proc. ODP, Sci. Results, 119: CollegeStation, TX (Ocean Drilling Program).

2 Geological Survey of Canada, Bedford Institute of Oceanography, Box 1006,Dartmouth, Nova Scotia, B2Y 4A2, Canada.

latitude fjords (Syvitski et al., in press) or in deltas (Aller andMackin, 1984) can produce an inverse relationship between or-ganic matter content and the sedimentation rate.

A relationship between sedimentation rate and sulfate reduc-tion has been reported. The sedimentation rate controls the fluxof organic carbon, which controls the thickness of the anoxiclayer where dissolved sulfate is present (Berner, 1978; Canfield,1989). The rate of sulfate reduction can vary due to the qualityand quantity of organic matter, ambient temperature, and theamount of bioturbation (Edenborn et al., 1987). Specialized in-cubation experiments are necessary to determine sulfate reduc-tion rates (Canfield, 1989; Christensen, 1989).

Because such data are not available for ODP operations, asimpler method relating sulfate and ammonia gradients to car-bon fluxes will be used to estimate sedimentation rates. There is aconstant concentration of sulfate in overlying seawater, whereasammonia levels are generally below the detection limit. Oxida-tion of organic matter downcore consumes sulfate and producesammonia to a depth of hundreds of meters. The rate of sulfateconsumption and ammonia production relates to the amount oforganic carbon being oxidized, which in turn is related to theamount of organic carbon available. In areas where higher car-bon fluxes reach the anoxic sediments, more carbon is presentand more sulfate is consumed, and thus the flux of sulfate intothe anoxic region is larger. The following discussion will use thissteady-state balance to relate sulfate and ammonia gradients toorganic carbon concentrations and sedimentation rates.

DATA BASEDissolved sulfate and ammonia data, along with solid-phase

organic carbon data, are available for some ODP sites. Thequality of these results appears to vary for different legs, de-pending on priorities at each site. Data used in this discussionwere selected from ODP Initial Reports volumes 112 through121. Table 1 contains data for sulfate and ammonia concentra-tion-depth gradients for the upper three pore-water samples. Be-cause pore-water samples are normally collected at about 30-mintervals, the gradients represent the upper 100 m of the sedi-ment column. Organic carbon concentrations in dried sedimentare averaged for the upper 100 m of the sediment column. Esti-mates of average sedimentation rates in the upper 100 m at eachsite are taken from the biostratigraphic results.

Molecular diffusion allows pore-water profiles to re-adjustto present-day conditions at the sediment/water interface. Ob-served gradients in the upper few tens of meters of the sedimentcolumn represent responses to organic carbon fluxes over thepast 10,000 yr. Table 2 contains data from other study areaswhere conventional coring provided sulfate and/or ammonia

401

R. E. CRANSTON

Table 1. Redox and carbon flux data (ODP Initial

Reports volumes 112 through 121).

Site

682683684685686687688689690698699700701703704705707708709710711713714715717718719720721722723724725727728730731737738739740741742743744745748749751752753754756757758

Sulfategradient(mM/m)

0.600.680.941.41.90.752.00.0120.0270.0140.0200.0200.0280.0050.0250.0100.0020.0250.0030.0310.0100.0130.2000.0240.620.110.210.360.290.250.560.540.240.470.420.160.170.0330.0050.0900.060

———

0.0050.0500.0400.0090.0200.0030.0010.0070.0600.0600.008

Organiccarbon

(%)

3.03.04.02.52.53.03.00.050.080.050.100.050.300.050.200.100.050.200.100.050.050.050.500.501.01.00.500.301.00.801.01.00.702.02.02.00.500.300.050.501.00.200.300.300.050.140.050.050.100.050.050.050.050.050.05

Sedimentationrate

(m/m.y.)

262530

10016065

1009.0

1210131025

6.020

81115118.55.07.0

4015

26012081544347

17086

120110455038102.0

202020

7-30?5.0

305.01.03.02.00.53.05.0

1015

Ammoniagradient(mM/m)

0.100.200.170.200.560.120.30

0.0023

0.00010.00043

0.0078

0.0020.000020.005

0.0010.0040.0040.000060.002

—

gradients, organic carbon concentrations, and information onthe sedimentation rate.

DISCUSSION

Sulfate Gradients and Sedimentation RateSulfate concentrations in pore water begin at 28 mM at the

sediment/water interface and decrease with depth, because sul-fate is consumed as organic matter is oxidized. High sulfate gra-dients tend to occur where high organic carbon concentrationsand high sedimentation rates are found. A linear correlationanalysis for the 60 cases listed in Tables 1 and 2 is shown in Ta-ble 3. All correlation coefficients are significant at/? < .001. In-cluded in the analyses is a parameter directly related to the fluxof organic carbon to the seafloor. It is the product of the sedi-

mentation rate and organic carbon content. Porosity correctionsshould be applied; however, water content results were not de-termined for the samples. It is assumed that porosity and theapparent diffusion coefficient for sulfate in the upper portionof the sediment column are similar, to one significant figure, forall sites. The sulfate gradient correlates extremely well with thecarbon flux parameter (/• = 0.95) and the sedimentation rate (r= 0.93).

As mentioned in the "Introduction," there are cases wheresedimentation rates are very high but organic carbon content islow, and thus the flux of carbon to the seafloor is not exception-ally large. In such cases, the organic carbon flux estimate is amore reliable way to describe the sulfate gradient.

A linear regression equation was calculated using log-trans-formed data, where the sulfate gradient is the independent vari-able (– mM/m) and the flux parameter is the dependent varia-ble (organic carbon in percentage multiplied by the sedimenta-tion rate in m/m.y.; Fig. 1):

log (organic carbon × sedimentation rate)

= 1.2 × log (– sulfate gradient) + 2.2, (1)

r = -0.95, n = 60. A minus sign is used to convert the sulfategradient to a positive number in order to carry out the log trans-formation.

Ammonia Gradients and Sedimentation RatesOnly 19 ODP sites with reliable ammonia profiles were found

(Table 1). Ammonia results from an additional nine coring siteswere found in the literature (Table 2). Ammonia concentrationstend to increase from detection limit values (<O.Ol mM) at thesediment/water interface to values that can exceed 1 mM atdepth. Correlation coefficients (p < .001) were calculated forthese 28 cases (Table 4). Ammonia gradient results correlatewith organic carbon (r = 0.87) and with sedimentation rate esti-mates (r = 0.90). However, ammonia results correlate moststrongly with the organic carbon flux estimate (r = 0.97), simi-lar to the previous discussion in which sulfate gradients corre-lated most strongly with the organic flux. A regression analysison log-transformed data provides a relationship between or-ganic carbon flux (% × m/m.y.) and the ammonia gradient(mM/m; Fig. 2):

log (organic carbon × sedimentation rate)

= 0.93 × log (ammonia gradient) + 3, (2)

r = 0.97, n = 28.

A strong negative correlation between sulfate and ammoniagradients (r = - 0.92; calculated from Table 1) reflects the closerelationship between sulfate reduction and ammonia releasefrom organic matter oxidation. Organic matter compositionwould suggest that 1 mole of sulfate oxidizes 2 moles of organiccarbon and releases 0.3 moles of ammonia (Froelich et al.,1979). Thus, in theory, the ratio between sulfate reduction andammonia production would b e - 1 / 0 . 3 = -3 .3 . Ammonia dif-fuses through the sediment column twice as fast as sulfate(Krom and Berner, 1980); thus, the observed gradient ratioshould be 2 × - 3.3 = - 6.6. A linear regression equation wasdetermined for sulfate and ammonia gradients in Table 1, where

sulfate gradient = -4.1 × (ammonia gradient) - 0.11, (3)

/•= -0.92, n = 19. The theoretical ratio and that observed ininterstitial pore-water concentrations varies for each area anddepends in part on core handling and analytical approaches.

402

SEDIMENTATION RATE FROM SULFATE AND AMMONIA GRADIENTS

Table 2. Redox and carbon flux data.

Area

NWCFOAMSachemBlack HoleNorth CarolinaNarragansett BaySanta BarbaraG10N48N56N60

Sulfategradient(mM/m)

22040

300300

4020

0.3——

—

Ammoniagradient(mM/m)

410—

801030.020.0020.020.01

Organiccarbon

(%)

11233330.40.10.40.3

Sedimentationrate

(m/m.y.)

5001,0005,000

50,000100,000

2,0002,000

100104030

Reference8

1,21, 21, 21, 23456,7, 8666

1 = Westrich (1983); 2 = Rosenfeld (1981); 3 = Val Klump and Martens (1989); 4 =Elderfield et al. (1981); 5 = Sholkovitz (1973); 6 = Buckley and Cranston (1988); 7 =G. DeLange (pers. comm., 1986); 8 = Nozaki et al. (1987).

Table 3. Sulfate correlation.

Sulfategradient

Organiccarbon

Sedimentationrate

Organicflux

Sulfate gradientOrganic carbonSedimentation rateOrganic flux

10.830.930.95

0.8310.740.91

0.930.7410.96

0.950.910.961

Note: n = 60, p < .001, Iog10-transformed data.

Table 4. Ammonia correlation.

Ammoniagradient

Organiccarbon

Sedimentationrate

Organicflux

Ammonia gradientOrganic carbonSedimentation rateOrganic flux

10.870.900.97

0.8710.650.86

0.900.6510.95

0.970.860.951

Note: n = 28, p < .001, Iogi0-transformed data.

> Λ 100000^

10000 =

1000-B

100-=

ol

ll

lI

I Il

lll

0.1 =

-<

•

•1

• * •• .! ..

1—1 1 1 Mill 1—1 1 1 llll

i

logY =r = - 0

—i—i i i mi 1—i Mini 1

•

1.2log× -95 n =I I I HIM 1

co

.α

ou

ucoen*̂

O

0.001 0.01 0.1 1 10 100

Sulfate gradient( — mM/m)Figure 1. Relationship between organic carbon flux and sulfate gradi-ent.

For example, Aller (1980) found gradient ratios of 7 to 10 inLong Island Sound while Elderfield et al. (1981) found ratios of4 to 11 in Narragansett Bay.

Estimating Sedimentation Rates

The relationships illustrated in Figures 1 and 2 can be used toprovide estimates of recent sedimentation rates, especially incases where biostratigraphic control was questionable, as forPrydz Bay during ODP Leg 119. Deviations can result from ap-

' V

E\E

^ y

flu

x i

'bo

n

σu

o

Org

an

100000 1

10000 =

,oooj

100 =

10-=

:

o

Min

i n

l

••

•* . "

•. log Y = 0.93logX + 3

r = 0.97 n = 28

• f m η — i i i H U M — i i i i n n ] — i i i i n i i | — i i i n n i | — i i i IMM| i i i n

0.0001 0.001 0.01 0.1 1 10

Ammonia gradient (mM/m)Figure 2. Relationship between organic carbon flux and ammonia gradi-ent.

proximations in profiles (e.g., gradients can change seasonally;Val Klump and Martens, 1989), from ammonia adsorption, andfrom changes in bottom water composition (e.g., anoxic overly-ing water may be depleted in sulfate and enhanced in ammonia,thus affecting gradients).

Table 5 contains biostratigraphic estimates of sedimentationrates for the Leg 119 sites compared to estimates from the rela-tionships derived in this paper. There is agreement to within a

403

R. E. CRANSTON

Table 5. Estimated and observed sedimentation rates for Leg 119 sites.

Site

736737738739740741742743744745

Sulfategradient(mM/M)

c0.10.0330.0050.090.06

c0.1c0.07c0.1

< 0.0050.05

Ammoniagradient(mM/m)

0.0080.0020.00020.005

c0.0080.0010.0040.004

< 0.000060.002

Organiccarbon

0.60.30.050.510.20.30.30.050.14

Sedimentation rate (m/m.y.)

Observedfrom dataa

50102

202020

7-30?5

30

EstimEquation (1)

2095

205

502030

< 530

* jb

lateαEquation (2)

2010

120108

2020

< 220

a Barron, Larsen, et al. (1989).b Equation (1): log (sedimentation rate) = 1.2 × log (sulfate gradient) + 2.2 - log (or-

ganic carbon). Equation (2): log (sedimentation rate) = 0.93 × log (ammonia gradient)+ 3 - log (organic carbon).

c Approximate value due to highly variable interstitial-water data.

factor of 3 in all cases. Biostratigraphic estimates were highlyapproximate or nonexistent for Prydz Bay Sites 742 and 743,whereas the estimates from the sulfate and ammonia gradientspredict that recent sedimentation rates for these two sites are inthe range of 20 to 30 m/m.y. This suggests that sulfate and am-monia gradients have adjusted over the last few thousand yearsto a bulk-sedimentation rate of tens of meters per million yearsfor sediments containing a few tenths of one percent organiccarbon. Kvenvolden et al. (this volume) suggested that much ofthe total organic carbon is actually terrigenous coal dust. If thereactive organic carbon is an order of magnitude lower than re-ported in Table 5, this would suggest that estimated sedimenta-tion rates for these two sites could be on the order of hundredsof meters per million years.

Both the sulfate and ammonia relationships can generally beused for ODP operations if reasonable quality data are availablefor the upper 100 m of the sediment column. However, in mostother geochemical field programs, recovered cores are not com-monly longer than 10 m. For these shorter cores from open-ocean regions, sulfate gradients may not be detected, whereasthe ammonia relationship is somewhat more sensitive and maybe more useful as a means for estimating sedimentation rates.In coastal systems where organic carbon fluxes are high, sulfatedepletion and ammonia production are observed, thus bothequations appear to be applicable.

CONCLUSIONS

1. Sulfate and ammonia concentration-depth gradients inpore water correlate closely with an estimate of the flux of or-ganic matter to the sediment column.

2. Relationships between sulfate and ammonia gradients withthe sedimentation rate cover a range of sedimentation rates from0.1 to 100,000 m/m.y.

3. Using simplified relationships derived from ODP sulfateand ammonia gradients, estimates of sedimentation rates forthe Leg 119 sites agree to within a factor of 3 in comparisonwith biostratigraphic estimates.

ACKNOWLEDGMENT

Geological Survey of Canada Contribution no. 45889.

REFERENCES

Aller, R. C , 1980. Diagenetic processes near the sediment-water inter-face of Long Island Sound. Adv. Geophys., 22:237-350.

Aller, R. C , and Mackin, J. E., 1984. Preservation of reactive organicmatter in marine sediments. Earth Planet. Sci. Lett., 70:260-266.

Barron, J., Larsen, B., et al., 1989. Proc. ODP, Init. Repts., 119: Col-lege Station, TX (Ocean Drilling Program).

Bender, M. L., and Heggie, D. T., 1984. Fate of organic carbon reach-ing the deep sea floor: a status report. Geochim. Cosmochim. Ada,48:977-986.

Berner, R. A., 1978. Sulfate reduction and the rate of deposition of ma-rine sediments. Earth Planet. Sci. Lett., 37:492-498.

Buckley, D. E., and Cranston, R. E., 1988. Early diagenesis in deep seaturbidites: the imprint of paleo-oxidation zones. Geochim. Cosmo-chim. Ada, 52:2925-2939.

Canfield, D. E., 1989. Sulfate reduction and oxic respiration in marinesediments: implications for organic carbon preservation in euxinicenvironments. Deep-Sea Res., Part A, 36:121-138.

Christensen, J. P., 1989. Sulfate reduction and carbon oxidation rates incontinental shelf sediments, an examination of offshelf carbon trans-port. Cont. Shelf Res., 9:223-246.

Edenborn, H. M., Silverberg, N., Mucci, A., and Sundby, B., 1987.Sulfate reduction in deep coastal marine sediments. Mar. Chem., 21:329-345.

Elderfield, H., McCaffrey, R.J.H., Luedtke, N., Bender, M., and Trues-dale, V. W., 1981. Chemical diagenesis in Narragansett Bay sedi-ments. Am. J. Sci., 281:1021-1055.

Froelich, P. N., Klinkhammer, G. P., Bender, M. L., Luedtke, N. A.,Heath, G. R., Cullen, D., Dauphin, P., Hammond, D., Hartman,B., and Maynard, V., 1979. Early oxidation of organic matter in pe-lagic sediments of the eastern equatorial Atlantic: suboxic diagene-sis. Geochim. Cosmochim. Ada, 43:1075-1090.

Heath, G. R., Moore, T. C , Jr., and Dauphin, J. P., 1977. Organic car-bon in deep-sea sediments. In Andersen, N. R., and Malahoff, A.(Eds.), The Fate of Fossil Fuel CO2 in the Oceans: New York (Ple-num), 605-625.

Henrichs, S. M., and Reeburgh, W. S., 1987. Anaerobic mineralizationof marine sediment organic matter: rates and the role of anaerobicprocesses in the oceanic carbon economy. Geomicrobiol. J., 5:191—237.

Krom, M. D., and Berner, R. A., 1980. The diffusion coefficients ofsulfate, ammonia and phosphate ions in anoxic marine sediments.Limnol. Oceanog., 25:327-338.

Muller, P. J., and Suess, E., 1979. Productivity, sedimentation rate andsedimentary organic matter in the oceans—I. Organic carbon pres-ervation. Deep-Sea Res., Part A, 26:1347-1362.

Nozaki, Y., Sato, M., and Shimooka, K., 1987. Th-230 dating of 30 mlong piston cores taken from two North Atlantic abyssal plains dur-ing the ESOPE cruise. Geochem. J., 21:125-131.

Reimers, C. E., and Suess, E., 1983. The partitioning of organic carbonfluxes and sedimentary organic matter decomposition rates in theocean. Mar. Chem., 13:141-168.

Rosenfeld, J. K., 1981. Nitrogen diagenesis in Long Island Sound sedi-ments. Am. J. Sci., 281:436-462.

Sholkovitz, E., 1973. Interstitial water chemistry of the Santa BarbaraBasin sediments. Geochim. Cosmochim. Acta, 37:2043-2073.

404

SEDIMENTATION RATE FROM SULFATE AND AMMONIA GRADIENTS

Stein, R., 1986. Organic carbon and sedimentation rate—further evi- changing pore water nutrient and sulfate distributions. Limnol.dence for anoxic deep water conditions in the Cenomanian/Türo- Oceanogr., 34:559-577.nian Atlantic Ocean. Mar. GeoL, 72:199-209. Westrich, J. T., 1983. The consequences and controls of bacterial sulfate

Syvitski, J.P.M, LeBlanc, K.W.G., and Cranston, R. E., in press. The reduction in marine sediments [Ph.D. dissert.]. Yale Univ., New Ha-flux and preservation of organic carbon in Baffin Island Fjords. In ven.Doweswell, J. A. and Scourse, J. D. (Eds.), Glacial Marine Environ-ments, Processes and Sediments. Spec. Publ. Geol. Soc. (London).

Toth, D. J., and Lerman, A., 1977. Organic matter reactivity and sedi-mentation rates in the ocean. Am. J. Sci., 277:465-485. Date of initial receipt: 6 December 1989

Val Klump, J., and Martens, C. S., 1989. The seasonality of nutrient re- Date of acceptance: 1 August 1990generation in an organic-rich coastal sediment: kinetic modelling of Ms 119B-172

405