Carbon isotopes in pore water, calcite, and organic carbon from distal turbidites of the Madeira Abyssal Plain

Gmhmica ef Cwmx/tim;ca Am Copy&t 8 1989 Per@mon Rs

GfJl6-7037/89/53.00

Vol. 53, pp. 2997-3004 pk. Printed m U.S.A.

+ .w

Carbon isotopes in pore water, calcite, and organic carbon from distal turbidites of the Madeira Abyssal Plain J. M. MCARTHUR* Research School of Earth Science, Australian National University, Canberra, ACT 260 1, Australia

(Received October I 1, 1988; accepted in revisedform August 16. 1989)

Ahstmet--Carbon isotope data are given for organic matter, calcite, and pore water in carbonate turbidites from the Madeira Abyssal Plain. The carbon isotope composition of dissolved inorganic carbon (DIC) in pore water is between 0.3 and 1’50heavier than modelled values. The 6 I36 of DIC added to the system is -5.5 to -6.9L, whereas the predicted value is -9.0%. This difference does not result from isotopic fractionation of organic matter or calcite. During diagenesis, 3 1% of the organic matter is oxidized. The isotopic composition is - 19.4 f 0.1 k before and after oxidation. Depressurization artefacts affecting DIG occur at the sediment-water interface and at a lithologic discontinuity within the sediment.

CARBONATE-RICHTURBIDITESare common on the Madeira Abyssal Plain (WEAVER and KUIJPERS, 1983; SEARLEet al., 1985; WEAVER et al., 1986; THOMSON et al., 1986, 1987; JARV~Sand HIGGS, 1987 ) . They occur as homogenous units, each with a distinctive mineralogy, chemistry, and thickness (tens of centimeters to a few meters). Many turbidites have concentrations of labile organic carbon that are high for deep sea sediments (0.2 to 0.9%, i.e., up to 75% of their total organic carbon; JARVISand HIGGS, 1987). Shortly after deposition, oxidation of organic matter makes the pore water anoxic. Subsequently, oxygen and nitrate from bottom water diffuse into the sediment, localizing organic diagenesis at a reaction front that moves downward into the sediment with time (WILSON et al., 1985, 1986; THOMSONet al., 1986, 1987). Several interesting problems can be addressed with a study

of the carbon isotopic composition of pore water, organic matter, and calcite in such turbidites. A study of the carbon isotopic composition of calcite and organic matter can elucidate the effects of early burial diagenesis on 6 13Cin biogenic calcite and organic matter. Such knowledge is essential to the interpretation of isotope records in palaeo-oceanography (cf., SPIKER and HATCHER, 1984, 1987; DEAN et al., 1986; BERGER and VINCENT, 1986). In deep-sea turbidites the effects of early burial diagenesis can be studied with the certainty that the sediment above and below the diagenetic reaction front was originally identical. Sediment above the front represents the sediment state after diagenesis. The complicating effect of non-steady-state sedimentation, that affects most sediments, is absent. A study of the isotopic com~sition of d~sso~yed~~orgff~ic carbon (DIG) in pore water (d ‘3Crw) is important in palaeooceanography. Benthic foraminifera may be partly or wholly infaunal ( BERLANGERet al., I98 1; CORLISS, 1985; BERGER and VINCENT, 1986). The 6 13Cof calcite of such foraminijku will record the chemistry ofpore water rather than the chemistry of bottom water. It is therefore implant to understand what controls S’“Crw in order that &13Cvalues of benthic * Present nddress: Department of Geological Science, University College London, Gower Street, London WCIE 6BT, U.K. 2997

foraminifera can be properly interpreted (see BERGER and VINCENT, 1986, for a review). Isotopic studies can also constrain parameters of the carbonate system. For example, the stoichiometry of diagenetic reactions, such as the ratio of calcite dissolution to organic carbon oxidation and of organic carbon oxidation to oxygen consumption, the rain-rate of organic carbon to the sediment, and coefficients of diffusion can be evaluated or constrained with isotopic data ( MCCORIUE et al., 1985; MCCORKLEand EMERSON, 1988; SAYLESand CURRY, 1988). Furthermore, the localization of diagenesis at the reaction front results in linear con~ntration profiles for many redox-sensitive elements. This fact permits the stoichiometry of redox reactions to be quantified without resorting to complex advection-diffusion-reaction models needed for normal pelagic regimes of slow (supposedly) continuous sedimentation. As a contribution to addressing the interests outlined above this paper reports isotopic data for calcite, organic carbon, and ZCOz in calcareous distal turbidites from the Northeast Atlantic. SAMPLE LOCATION, COLLECTION, AND PROCESSING Samples were recovered from the Madeira Abyssal Plain beneath the Northeast Atlantic Ocean 800 km WNW of the Canary Islands. Pore waters were obtained from two box cores and one in situsampler profile. The cores were also sampled for &ids. Box core BX 11327 wascollected from adepth of5380m at 31”18.3N, 25”23.4W. Box core BX 11328 was recovered in 5375 m of water at 31°29.9N, 24O28.1W. An in situpore-water sampler was deployed at 3 I”3 I .4’N, 24’27.OW in 5375 m of water, near the site of BX 11328. Box cores were sub-cored immediately after they arrived on deck and stored and processed in a cold van kept at 4°C. Cores and pore waters were processed entirely under nitrogen. Pore water was removed by squeezer within 6 hours of arrival on deck, filtered through 0.45 cc membrane filters into glass bottles in which 200 ~1 of saturated mercuric chloride had been evaporated to dryness.The bottles were sealed without airspace using rubber septa. The calcite content of the cores was obtained by using flame atomic absorption spectroscopy to measure the calcium soluble in dilute acetic acid. For the isotopic analysis of pore waters, samples (3-5 ml) of accurately known weiaht were transferred to a stripping line using a double-needle technic&e that avoided the sample contacting atmosohere. The CO, was extracted bv striuuine with He after addition of IO& phosphoric&id. The gas was pu%iedwith two traps of dryice/acetone and one of melting ethanol. Yields were measured by

J. M. McArthur

2998

electronic manometer cafibrated with pure dry CaCQ and solutions of NaHC03. The isotopic composition of organic carbon was determined on samples treated with 2% I-ICI, to remove calcite, and evap orated to dryness at room temperature. Organic carbon was oxidized to CO, by combustion with copper oxide and silver wire. The isotopic composition of calcite was determined on CO* produced by reaction with 100% phosphoric acid. All isotopic me~uremen~ were made on a modified MS 12 gassource mass spectrometer in the Environmental Geochemistry Unit of the Research School of Earth Science, ANU. The precision of the ZCOz analysis was assessed as better than *30 PM of C02: by replicate analysis of sea water and duplicate analysis of some samples. Precision of the carbon isotope analysis of pore water was ~~0.15%. About 50% of this error arose from the pore water extraction procedure, as six replicate analysis of bottom water gave S’T values of +0.63 t 0.06%. The precision of the isotopic analysis of organic carbon and calcite was 50.1 L for oxygen and carbon. SEDIMENT

CHARACTER

The sediments, and some aspects of the chemistry of their pore waters, are described elsewhere (WILSONet al., 1985; VOLLEY and THOMSON, 1985; JARV~S and HKGS, 1987; THOMON et al., 1987, and refs.therein ) so only a brief outline is given here. Turbidites are separated by thin units of pelagic sediment. On deposition, individual turbidites have an extraordinarily uniform composition ( JARVISand

HIGGS, 1987; THOMSONet al., 1987). After turbidite deposition, bioturbation and diagenesis introduce some chemical variation in their uppermost portion as pelagic units are mixed downwards and redox-sensitive elements are mobilized. The empIacement of each turbidite is followed by rapid consumption of oxygen and nitrate in the pore water owing to the presence of appreciable labile organic matter. Oxygen and nitrate from bottom water then diffuse into the turbidite creating a reaction front at which redox reactions are localized. This front moves downwards with time. Reaction fronts are deactivated and fossilized by sudden burial under later turbidites. Sulfate reduction has not been observed in any cores from the area (WILSONet al., 1985; COLLEYand THOMSON,1985; JARVISand HIGGS,1987; THOMSON et al., 1987, and refs. therein; THOMSON, pers. comm., 1988 ) . Box core BX 11327 contained 60 cm of turbidite a which was emplaced 600 + 100 years ago (WILSONet al., 1985, 1986; THOMSON et al., 1987; JARVIS and HICCS, 1987). The active reaction front occurs in the interval 2 1-22 cm and was identified by its associated color change. Box core BX 11328 was from a location where turbidite a is thinner than at site 11327. It contained 20 cm of turbidite Q overlying 3-4 cm of pelagic sediment that, in turn, overlay 40 cm of turbidite (I,. The upper 20 cm of uI is bioturbated. A fossil reaction front occurs at 52 cm, i.e., 28 cm from the top of the buried turbidite surface. Profiles of 02 suggest that the reaction front in this core is re-establishing itself at this fossil interface after breaking through the overlying sediment within the last 30 years. RESULTS Analytical data are shown in Table 1 and Figs. 1 and 2. The ZC02 content of bottom water is 2 172 + 17 PM (n = 8 ) and its 6°C value is +0.63 + 0.06% (n = 6). Values of ZCOZ in pore waters are affected by a decompression artefact the size of which must be asses&. In BX 11327, the ZKYO,profile is linear between 2 and 22 cm. Above 2 cm, curvature is likely to occur as the sediment-water interface is the most reactive part of most sediments, as far as respiration of organic matter is concerned. Linear regression of depth and ZC02 in the interval 2-22 cm shows that ZCO, extrapolates to 19 10 + 50 PM at the sediment-water interface. The artefact in ZCOz is therefore larger than 260 FM. In turbidite a 10% of the total change in oxygen concentration occurs in the O2 cm interval as a result of respiration near the sedimentwater interface (WILSON et al., 1985; THOMSONet al., 1987;

WILSON, unpub. data, 1988). As a result, the d’3CPw in the O-l cm interval is 0.29% lighter than bottom water. Using the model of MCCORKLE et al. ( 1985 ) this offset in 6 ‘?ZPw can be used to infer a further offset in ZC02 of 60 PM. The bottom water offset of ZCOZ is therefore a minimum of 320 t 50 FM. In continuously accumulating sediments the a~umption of linearity near the sediment-water interface does not introduce significant errors in flux calculations (SAYLES and CURRY, 1988 ). As the Madeira profiles show much less curvature than is seen in continuously accumulating sediments, the true offset probably is not significantly different from 320 pM. The ZCOz offset for BX 1I328 is assumed to be the same at the sediment-water interface as both box cores have the same turbidite in their upper parts. Turbidite a is compositionally uniform with 60% CaC03 (salt corrected), except for a slight decrease in the topmost 2 cm where the influence of renewed pelagic sedimentation is seen (Table 1, Fig. 1). In BX I 1327 d 13C(calcite) is + 1.2%0, 6 13C(org. C) is -i9.4k, and 6 13Cpwis - 1.3 f 0.1 s at the reaction front and decreases to - 1.5 + 0.1 L at depth. In BX 11328, values of 6 13C(calcite) and 6 13C(org. C) in turbidite a are similar to values for BX 11327. Turbidite has values of -0.2!& for 613C (calcite) and -19.4% for 613C (org. C). In Bx 11328, turbidite a is underlain by 34 cm of pelagic sediment containing 52% calcite. This unit is bioturbated downwards into turbidite aJ. The decrease downwards of the CaCo3 content though the upper 20 cm of turbidite clearly shows the effect of this bioturbation. Across the pelagic interval an abrupt change occurs in ZCO2. A discontinuity in CaCO3 occurs at the top of uI but is obscured by biotur~tion. A further discontinuity occurs at the reaction boundary at 52 cm. This is the position of an earlier deactivated reaction front at which the current front is being re-established. Values of 6 13CPwplateau at - 1.4Ymin the upper oxidized part of Oxygen also plateaus at non-zero values in this zone. This suggests that in the last 30 years oxygen has broken through turbidite a and is in the process of re-establishing itself at the lower reaction boundary (WILSON, pers. comm., 1988). aJ

aJ

aJ.

DISCUSSION I. Eflect qfdiagenesis

on 6j3C c$ organic matter

The isotopic comp~ition of organic matter is an average of that of its constituents (e.g., lignins, lipids, carbohydrates, etc.). Individual components may have isotopic compositions up to 57~ different from others (DEINES, 1980). Carbohydrates in sapropelic organic matter may be 4Ymheavier than lignins and 5% heavier than lipids (SPIKER and HATCHER, 1984). Individu~ organic com~nents decay at different rates during oxidation, which may cause changes in the isotopic composition of organic matter during diagenesis (DEAN et al., 1986; SPIKER and HATCHER, 1987). In interpreting the carbon isotope record of sedimentary organic matter, a knowledge of such changes is vital to the separation of real signals from those induced by diagenesis. At the reaction boundary in BX I 1327, the totai organic carbon changes from 0.58 to 0.40%, so 31% of the total organic carbon is consumed by respiration ( JARVISand HIGGS, 1987).Valuesofd’“C(org.C)inBX 11327are-19.4+O.lYm

Isotope composition of carbon in turbidites Table 1.

lrotop~ d&u for pore wager and sediment ,n distal turSdites fmm the Madeira Abyssal Plain. NE Atlantic Ocean. All wolopic data relative f~ PDB. X02 110t wxrec:ted for offsel of 3mfiM.

INTERVAL

cIco3

d3c Cal

@O

c11

6’3COrg

Bottom Water CORE 11327:Turbidile a throughout to.84

1-2 2-3 3.5 s-7 7-9 9-11 II-13 13.15 IS-17 17.19 19-m m-21

55.4 56.2 58.3 58.3 58.5 58.7 58.6 58.4 59 1 58.9 59.3 59.3 59.7

21-22 22.23 2324 24-2.5 25.26 26-28 28-M 36-38 40-42 44.46 52-54 56.58 60.52

59.1 59.5 59.4 59.0 594 59.5 59.7 59 1 59.6 590 596 59 1 588

t1.18

528 55 9 55 7 566 55.3 563 58.0 560 576 573 55 7

+121

O-l

+1.54

-19.8

‘xo2

+0.63

1910 1780 2000

to.34 +0.11 0.11

m3o 2050 +1.28 +I.26

t1.2.5

tzm +204

t2.08 reacuon fmnl + 1.92

-19.4

2160 2270 z2Ea 2290 2390 2440 2540 2530

-19.5

2610

-19.3 -19.3

mm 25M 2fi60 2590 +I.18

+2.14

-19 3

6’3x02

2172

2670 2680 264G 2620 26cQ 2610 2640 2980

-0.54 -0.31 0.87 -1.13 -1.31 -1.16 -1.16 -1.24 -1.33 -1.36 -1.33 -1.25 -1.33 -1.40 -1.51 -1.66 -1.31

-1.53

CORE 11328 Turb,d,te 0 O-1 l-2 2-3 3-5 5-7 7-9 9.11 11.13 13.15 IS-17 17.19 Pelagic Interval 19.21 21.23

516 505

23-2.5 25.27 27.29 2931 31.33 33.35 35-37 37-39

31.2 32.5 33 8 31 9 300 27 4 22 0 197

(Harpoon, 40) 39.41 41.43 43.45 45.47 47-49 49.50 (Harpoon. SO) 50.51 51.52 52.54 54.56 56.58 58.60 (Harpoon. 60.62 62-54

184 173 170 20.9 24 8 284 30.5 24 8 24 8 24 6 257 25.5

60) 26 2 27 8

+, 47

-0.24

+I81

t225

to.59

-19.5

-19.6

1870 1920 1950 1880 1980

to.64 to.37 +0.31

2090

0.82

2210

0.99

2240

-1.14

2160

-1.32

-0.35

2290

-1.37

2350

-1.42 -1.42 (-1.76)

2570

-1.43

2630

-1.49

(3248) mm

(-1.76) -1.86

2680 2680 2720 2680

-1 61 -1.70 -1.n -1.74 -1.98 (-1.78) -2.17

(E, 2710 2780

above and below the reaction boundary. The oxidation of 3 I % of the organic carbon does not alter the carbon isotope composition of the organic matter. These data support the

2999

belief that early burial diagenesis has little affect on the isotopic record of organic carbon (cf., DEAN et al., 1986). 2. Modelling 6°C in pore water Arfefucts in X0,. Calcium carbonate precipitates during the recovery of cores from abyssal depths, and ZC02 and alkalinity of pore water decrease as a consequence (MURRAY et al., 1980; EMERSONet al., 1982; MCCORIUE et al., 1985). There appears to be no artefact in sediments that contain no calcium carbonate, presumably because no nucleation sites are present for rapid calcium carbonate precipitation. In cores containing more than a few percent calcium carbonate the size of the artefact is larger than thermodynamics predicts and may be explicable by precipitation at constant pH or may be related to the amount of calcium carbonate present (EMERSONet al., 1982; MCCORKLE et al., 1985). The real control on the precipitation of calcium carbonate, however, is probably more complex. At 20 cm within BX 11328 an additional offset of 220 PM occurs in ZC02. The offset is bounded by linear profiles of ZCO2. It is not the result of any sampling or handling procedure that can be identified. The additional offset is clearly controlled by a property of the pelagic unit beneath turbidite n . The pelagic unit is heavily bioturbated into the underlying turbidite, which explains why the additional offset is not confined wholly to the pelagic interval. A possible explanation of the additional offset is that the size of carbonate attefacts may be controlled by the degree of organic poisoning of precipitation sites on calcite surfaces. Different redox conditions are experienced by pelagic and turbidite sediments. Turbidites become anoxic shortly after emplacement because of their high content of labile organic carbon. Organic coatings on sediment grains have the opportunity to accumulate but have no chance to be removed by oxidation. Pelagic units form under oxic conditions so reactive sites for calcite precipitation may be burned free of blocking organic matter more effectively than in the turbidites. Arteficts in 6’3CPW. Artefacts in ZC02 might be mirrored by artefacts in 6 ‘3Cpw. The degree of kinetic fractionation during calcite precipitation depends upon the rate of the reaction and varies between nil and the equilibrium value (USDOWSKI, 1982; TURNER, 1982). Equilibrium fractionation should make d “Cpw more negative ( EMRICH et al., 1970; MCCORKLEet al., 1985 ). In accord with theory, MCCORKLE et al. ( 1985 ) found d i3Cpw in box cores was 0.3% more negative than in harpoon samples at MANOP Site C. The artefact was within error of the calculated artefact of 0.2 1‘%Ifor equilibrium precipitation. The additional offset of 220 PM of ZCO2, at 20 cm in BX 11328, may be accompanied by an offset in 6’3Cpw. If so, it is certainly smaller than 0.2’S (Table 1, Fig. 1). The size cannot be determined accurately, owing to the nonlinearity of the pore water profiles. Equilibrium fractionation would change ISi3Cpw by 0.1 ‘SOacross this interval. Thus, the resolution of the data does not permit an accurate assessment of the artefact in G’3Cpw associated with calcite precipitation. The data, however, do confirm the findings of MCCORKLE et al. ( 1985) that any artefact in 8’3Cpw is small in relation

J. M. McArthur

3000

1 co, pM 1800 I( 2000 I InBW g3cw

-2

I

I

-1

I

I 2400 t If 2600 1 2800 I

~c-,2pM

0 B&+1 I ITi

I

pc,

o-

Zoo0

J800 -2

BW

-1

l-l

-

I

2400

2600 8 , 2800 I , 3000 ‘

0 EW+l I ITI

+

t E +O+ ++

t

73 t =% _____-______ n-n= E +if t

= ++ lo-

+t

+0 +

zo-

:

cl

t

+cI

E

+

u 305 _c $ 0

t t3

+

t’

+

60-

t+

+

40-

50-

zo-

(”

t

:

+ +>______

.a_..

+

+

t

0

E

0 c c

30-

E 0

40-

Cl

t

0

t I

u

t t

-I

70-J

70lb0

_____..RF

cl

; ++

+ +

scac*3

++

t

lo-

=k-

cl

+

+

0

70 %CaCOS

8X11328

BX 11327

FIG. 1. Variation with depth of 613Cpw, L’COzand percentage of CaCOj in BX 11327 and BX 11328 from the Madeira Abyssal Plain. Stippled areas represent slowly accumulating pelagic intervals. a and a, are turbidites. Bottom water values for ZCOz and b “C are labeled with heavy arrows from their respective axes. Dashed horizontal lines show locations of current reaction fronts. + = CaC03, +

= 6 i3Cpw, D = ZCOz.

to the diagenetic signals caused by organic decay. By the same reasoning any kinetic fractionation produces an artefact in 6 ‘3Cpwat the additional offset that is too small to be seen. Stoichiometric models for 613Cpw. An understanding of the controls on 6 ‘3Cpwcan be obtained only when measured values match modelled values. Measured and predicted values of 6 13Cpware compared below and are shown to differ substantially. Reasons for the differences are evaluated; three models are used. First, the G”CPW is predicted from ZCOz using a simple two-endmember mixing model. The models of MCCORKLEet al. ( 1985 ) and SAYLESand CURRY ( 1988) are also evaluated. 1. Because ZCOa is added at a finite location in the sediment, from where it diffuses to bottom water, 6 13Cpwcan be predicted from a knowledge of BCOZ, 6 i3C (org. C), 6 “C (calcite), and (Y,the ratio of calcite dissolved to organic cabonoxidized during diagenesis. In making predictions, 1 focus on BX I 1327 because it consists of one hom~enous turbidite and has pore water profiles for 02, Mn, Fe, and NOa that depart little from linearity ( WlLsON et al. 1986; THOMSON et al., 1987; WILSON, pets. comm., 1988). Profiles in BX 1I328 are nonlinear because of the transient state of chemical development consequent on the re-establishment of the reaction front at the fossil location (52 cm) in turbidite a,. Values of 6 13C are treated as additive functions. Organic matter is oxidized at the reaction front by oxygen and nitrate. Iron and manganese, diffusing to the front from deeper in the core, also consume oxygen and nitrate, but their affect is negligible (see below). The metabolic products of carbon oxidation are expressed here as ZCO2, which is treated as diffusing from the reaction front to overlying sea water:

6’%&.l= G’3cBw*( I -S)

+ 6”C**j

(1)

f= the fraction of the Z;COr, that has been added by respiration and calcite dissolution such that AZC02 f=

XCOZBW

+

AZCOz

with

ZCOzew = 2 172 PM.

Subscripts PW and BW refer to pore water and bottom water. 6 13CA= the isotopic composition of the ZCO, added to the pore water at the reaction front. The value depends on the ratio of calcite dissolution to organic-CO* production (a), such that 6i3C = a*&“C(calcite) A

+ 6’3C(org.C) I+&

(2)

Application to box cores BX 11327 and BX 11328. At the reaction front in BX 11327, the uncorrected zlCOz is 2610 t 30 FM (Table I). The offset in ZCOt is 320 + 50 PM. Thus, BC02 is 2930 f 80 PM and AZCOz is 760 + 80 #M. For oxygen consumption and nitrate reduction, an average value for CYis 1.01 (SAYLES, 1981; EMERSONet al., 1982). Using these values, Eqn. f 2f gives 6 j3C, of -9.0% and Eqn. ( 1) predicts a S i3Cc, of - 1.9 + 0.3% at the reaction front. In BX 1 I327 the measured values of b”Cpw are - 1.3 + 0. I %O in the vicinity of the reaction front and go to - 1SL at depth. Measured values are very much heavier than modefled values. Back-calculation shows that the pore water data are consistent with a 6 i3CAof -6.8960, a value much heavier than the -9.0’%1 calculated above. Furthermore, at the reaction front at 52 cm in BX 11328, ZCOz is 3000 FM (correcting for an offset in XC02 of 320

3001

Isotope composition of carbon in turbidites 0.0

-

1

Referencelevel 2.5cm

from profiles of O2 and N03, asthese are available (THOMSON et al., 1985; WILSON, pets. comm., 1988). This permits an evahtation of the quality and internal consistency of the ZCOz data: for O2 consumption, AZCOp

= - AO2. Do,/D~cc,~ *( 1 + a)-P:

(3)

for NO3 utilization, -2.0

I 0.1

0.2 x0&4

4

.-bottom

AZCOro3 = -AN03.

\

1

0

/ICO,Fl)

0.3 -1

D = Diffusion

coefficient

D~&DHcQ~-( 1 + (Y)*/!% (4)

(Do,/Dncol

= 1216, DNO,/

DHCO,= 10/6).

(Y= The stoichiometric ratio of calcite dissolution to organic-CO* production (see text). 6 = The stoichiometric ratio of organic matter oxidized to oxidant consumed (see text). A02 = 256 PM and ANO = 21 PM (WILSON et al. 1985: THOMSONet al. 1987; WILSON, pers. comm., 1988 ).

water

ICO,lx)

/X0,(R)

-1

FIG. 2. CO&CO~(R) - 1 W~SUSb13C- ZC02cx,/ZC02cRj for BX 11327. Slope of the line = d “C (flux) = 6”C,, a) Reference depth 2.5 cm: slope of regression line = -5.5 + 1.8%~~ b) Reference depth 0 cm: slope of regression line = -6.6 + 1.4%0.0 represents data below 22 cm averaged to avoid skewing the regression analysis.

FM), and the predicted 6’3Cpw is -2.2% (using.an (Yof 1.01 and the 6 13C in calcite and organic carbon for BX 11328 given in Table 1). Again, this value is more negative than measured values of - 1.75 + 0.15’%0at this depth. Including in the calculation the additional offset at 20 cm of 220 PM merely increases the discrepancy. Finally, in pore water from depths of 40, 50, and 60 cm, taken near station 11328 by an in situ pore water sampler, ZC02 concentrations are constant at 3335 f 90 PM and Gt3Cpw are -1.76 & 0.15% (Table 1). The predicted value of b’3Cpw, based on the measured ZCOZ, is -3.0 + 0.30/wif the value of 613C (calcite) and 6°C (org. C) for BX 11328 are used. As with the box cores, the measured value is very much heavier than predicted. Similar discrepancies between measured and predicted values were reported by MCCORKLE et al. ( 1985) for pore water from MANOP Sites C and S and by SAYLESand CURRY ( 1988) for pore waters from Atlantic and Pacific sediments. 2. In the model of MCCORKLE et al. (1985), ZCO? is equated to HCO; . The model can be used to derive I;C02

For BX 11327, Eqns. (3) and (4)-with (Y = 1.06, fl = 106 / 138 for oxygen consumption, and (Y= 0.08, fl= 106 / 94.4 for nitrate reduction ( FROELICH et al., 1979: SAYLES, 1981; EMERSONet al., 1982; MCCORKLEet al., 1985)-prediet AZCOz of 850 PM, so ZCOZ is 3020 PM. Using these values in Eqn. ( 1) gives a 6’3Cpw at the reaction front of -2.2. The measured values (-1.3 + 0.1% at the reaction front, - 1.5%0at depth) are very much heavier than the values obtained from McCorkle’s model. The predicted ZCOZ of 3020 PM is a little larger than the measured value of 2930 f 80 PM. This may result from underestimating the offset in ZCOs at the sediment-water interface or from uncertainties in the values of N, & and D. Variations in (Y,p, and D cannot provide an explanation of the discrepancies between measured and modelled isotopic data. Such variations can explain the difference between model and measured ZCOz. They are worth brief examination in view of the central role they play in pore water models ( FROELICHet al.. 1979: SAYLES. 198 1; EMERSONet al., 1982; MCCORKLEet al.. 1985; MCCORKLEand EMERSON, 1988: BENDERet al., 1986; BOUDREAU, 1987 ). Stoichiometric reactions for organic respiration normally represent organic matter as CHzO (op. cit.). In marine organic matter. H/C ratios are 11.5. and O/C ratios are 0.2-0.5 (CORNELIUS,1978; PELET, 198 1; TISSOT and PELET, 198 1). Real organic matter ( CHl.000.2 to CH, sOO.s)has a lower oxygen content than model organic matter ( CH20), so it consumes more oxygen during respiration. The extra demand is partially offset by its lower H/C ratio. Compared to model organic matter ( CHzO ) , the formulas CH ,.sOO.Z and CH I.500.s create an additional demand for oxygen of 21 and 13%, respectively. The effect of this additional demand is to lower B to between 106 / 167 and 106 / 15 1. These values are at the high end of the range of 1061130 to 1061175 reported for respiration by TAKAHASHI et al. ( 1985 ). Balancing the opposing fluxes of ZC02 and (0, + NO,) in BX 11327, with (Y= 1.06 for oxic respiration and 0.08 for NO3 reduction, requires a 6 of 106/ 155. This value is consistent with the stoichiometry of real organic matter. Using the more extreme value for fl of 106117 1 for oxic respiration

J. M. McArthur

3002

(the value used by MCCORKLEand EMERSON,1988 ) requires a D,JD,,, of 2.2. Using B = 106 J 138 gives DsJDHco, = 1.8. These values are within error of the value of 2.0 in common use. The values of diffusion coefficients are not known to better than ? 15 to 20% ( LI and GREGORY, 1974; HIMMELBLAU,1964). In particular, values of Do, for temperatures < 15°C are not reported by HIMMELBLAU( 1964) and are obtained by extrapolation. The diffusion coefficient ratios could be refined by accurately measuring p using CJ H /O/N/P analysis of organic matter from above and below the reaction boundary. 3. SAYLESand CURRY ( 1988) have shown that the 6°C of the flux across any horizonta1 interface, at depth R within a sediment, is given by

6’T. zco2(x,Ixoq~) = 6’T (flux), ~co*(x,l~coz(R) - 1 equivalent to 6 13CAin Eqn. ( 1) where the subscripts refer to t;CO1 at depth X and any chosen reference depth, R. The appli~tion of this relation to BX 11327data is shown in Fig. 2. The reference depths chosen am 0 cm (the sedimentwater interface) and 2.5 cm (the 2-3 cm interval). The latter depth is included because the pore water profiles are strictly linear between this depth and the reaction front. This approach also minimizes uncertainties involved in estimating the artefact affecting I;COz and avoids complications arising from any input in the O-2 cm interval of COz with a Ei13C different from that produced at the reaction front. A d13C (flux) of -5.5 + 1.8%0is obtained for ZCOZ using a reference depth of 2.5 cm. This value represents the 613C of COZ added at the reaction front ( Si3C,). Using the sediment-water interface as the reference depth gives a 6 j3C of -6.6 t 1.4%. This latter value is close to the value of -6.8%0 for 6 13C, back-calculated using pore water data in Eqns. ( I ) and (2). 6 “CA is calculated to be -9.0%~ from the data in Table 1, using an (Y= 1.Ol. These calculations confirm the previous findings that the input of CO2 at the reaction front is much heavier than predicted. Measured versus modelled 6”Cpw. The three approaches to estimating 6’3Cpw show that real S’3Cpw values are much heavier than modelled values. A number of ways may be proposed to explain why:

A. Modelled values of d’3C~ are very sensitive to variations in model parameters. Values for LYand /3depend upon the stoichiometry of the reaction considered and whether competing reactions occur for oxygen or nitrate consumption. In seeking an explanation for the difference of modelled and measured 6 13Cpwin BX I 1327 there is, however, iittle latitude for variations of cyfor oxic respiration, which is the predominant oxidative mechanism. Values of (Yof 1.64 and 2.07 would be needed to generate fluxes of -6.6 and -5.5%0 in BX 11327. These values are far higher than any reported so far. Scope exists for substantial variation of @ and Do, Jr>,, but only in an antipathetic manner, because the flux of NO3 and O2 into the sediment is closely balanced by the flux out of the system of HC03 (here expressed as T;C02 ).

It seems unlikely that variation of LY,/3, or D can explain the isotopic data. B. The differences are not due to isotopic f~ctionation of organic matter during oxidation (see above). Preferential oxidation of organic matter with 613C of - 14.57~ (4.9% heavier than the bulk organic matter) would generate a d “CA of -6.6%0 in BX 11327 (from Eqn. 2). After consumption of 3 1%of the organic matter at the reaction front, the residual organic matter would have an isotopic composition of -21.6%0. The reai value is -19.4%~ (Table 1). C. Iron and manganese are being oxidized at the reaction front and must be consuming part of the flux of NO3 and 02, effectively decreasing /3. Accompanying H+ preduction would dissolve calcite, thus increasing 0~.Both these effects would shift 6 13Cpv,to heavier values. In BX 11327, the combined flux to the reaction front of Fe and Mn is less than 2.5% of the oxygen flux (from pore water data in WILSONet al., 1986; THOMSONet al., 1987; and diffusion coefficients, corrected to appropriate viscosity and temperature, from LI and GREGORY, 1974; and HIMMELBLAU,1964). These oxidations are clearly not important in influencing 6 13Cpw. D. The isotopic composition of the calcite being dissolved may be heavier than that of the bulk calcite. BERGER and VINCENT( 1986) report variations of up to l-5%0 in 6°C as a function of size in individual species of planktonic foraminifera, so selective dissolution might fractionate calcite. If the reacting calcite is 4.9% heavier than the bulk calcite (i.e., 6 13C for the reactive calcite is +6.1 %o, rather than + 1.2%) the differences between model and measurement would be reconciled. This values seems extreme. Furthermore, the 6 13C (calcite) in BX 11327 is fractionally heavier above the reaction front than below it (Table I), the reverse of what would be expected if this mechanism operated. Optical and SEM examination of sediments from above and below the reaction boundary in BX 11327, and at several depths in BX 11328, failed to show evidence of selective dissolution so this explanation is discounted. E. Diffusion of SO4 in sediments may fractionate 34Sand “S with preferential concentration of the heavy isotope in solution ( NRIAGU. 1974; CORTECCI, 1976). These authors attributed the fractionation to repeated adsorptiondesorption reactions of SO4 with mineral surfaces. Similar processes may fractionate dissolved inorganic carbon. Laboratory and field studies (TAKAHASHI and BROECKER, 1977; EMERSONand BENDER, 198 1; BOUDREAU, 1987 ) show that calcite and dissolved inorganic carbon are in dynamic ~uilib~um. This tenet of equilibrium underpins solution the~~ynamics and all calculations of calcite saturation indices. Repeated adsorption/desorption may retard the diffusion of 13Cfrom the sediment more than that of 12Cas lighter species react more rapidly and heavier species form more stable bonds. Pore waters may be isotopically heavier than predicted as a result of this process. F. The isotopic composition of dissolved inorganic carbon is unquestionably buffered by calcite. Dissolution of metastable calcite is followed by equilibrium precipitation of a more stable form (e.g., coarser in grain size). In fresh water environments the alteration of the isotopic composition of DIG occurs over timescales of 103-lo4 years (WIGLEY, 1976; SMITH et al., 1976; EDMUNDSet al., 1987). In marine en-

Isotope composition of carbon in turbidites

vironments, carbonate diagenesis is much slower, and the isotopic signature of organically derived CO2 persists in marine waters, in the presence of calcite, for long periods (PRESLEY and KAPLAN, 1968; NISSENBAUMet al. 197 1; CLAYPOOL and KAPLAN, 1974; WHELAN et al., 1976). In Madeira turbidites, the bicarbonate produced at the reaction fronts diffuses to the sediment water interface in a time of the order of three years. This is probably too short a time for any appreciable isotopic exchange to occur. G. SAYLES and CURRY ( 1988) show that a significant proportion of the metabolic CO1 produced in sediments may be neutralized by CO:- diffusing into the sediment from bottom water. This explanation cannot account for the isotopic data. Neutralization by CO:- would increase the discrepancy between model and real 6 ‘3Cpw as CO:- is some 2-3%0 lighter than CaC03 under equilibrium conditions (TURNER, 1982; USDOWSIU, 1982). SUMMARY In carbonate turbidites on the Madeira Abyssal Plain, carbon isotope composition of ZC02 in pore water is heavier than predicted by modelling whilst concentrations of ZCOZ are close to the values predicted from O2 and NO, consumption. The differences are not the result of any known process. They do not result from isotopic fractionation of organic matter or calcite during diagenesis. Consumption of 3 1% of the organic carbon produces no change in its isotopic composition. The profile of ZC02 in BX 11328 shows a discontinuity within the sediment. It suggests that a major control on the carbonate depressurization artefact may be the degree to which reactive precipitation sites are poisoned by organic matter. Acknowledgments-The samples were collected during the cruise of RRS Shackleton to the North Atlantic in July 1986. The author thanks the crew and scientific party for their help with sample collection and processing. Particular thanks go to John Thomson, for the cruise invitation, and to him and Roger Wilson for stimulating discussion of the results. Tony Osbom provided the CaCOr analysis. Isotope measurements were made during a sabbatical leave in Australia. Sincere thanks go to Eleanor Laing and Jo Kali for invaluable assistance with the isotopic work. Allan Chivas is thanked for permission to use the laboratory at ANU and for assistance with the installation of the pore water stripping line. The author expresses his appreciation of the support provided during this period by ANU and the Royal Society. Reviews by Dan McCorkle and Sam Savin were of considerable benefit to the paper. Editorial handling: S. E. Calvert

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