McKenzie, J.A., Davies, P.J., Palmer-Julson, A., et al, 1993.Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 133

32. COMPARISONS BETWEEN THE OXYGEN ISOTOPIC COMPOSITION OF PORE WATERAND GLOBIGERINOIDES RUBER IN SEDIMENTS FROM HOLE 817C1

Peter K. Swart2

ABSTRACT

The oxygen isotopic composition of pore waters squeezed from sediments in Hole 817C co-varies with the oxygen isotopiccomposition of Globigerinoides ruber below 8 mbsf. The magnitude of the variation in the pore water δ O is approximately 30%of the variation in the foraminifers. Overall, the δ O of the pore waters increases down the core, a trend that is also present in theCl" concentrations. The variations in the δ 1 8 θ of pore waters may be the result of either of two phenomena. First, these may reflectoriginal variations in the waters, the magnitude of which has been subsequently reduced by process of diffusion. Second, these mayreflect recrystallization of the precursor sediment and isotopic exchange between the fluids and the recrystallized sediment. At themoment data are not available to ascertain which process is responsible although the correlation between the Cl" and the δ 1 8 θ datasuggests that these values reflect the original composition modified by diffusion.

INTRODUCTION

The oxygen and hydrogen isotopic compositions of water con-tained in the interparticle pore space of buried sediment are initiallysimilar to those of the bottom water at the time of deposition. Withincreasing age, the O and H isotopic compositions of the entrainedwater can be altered by (1) diagenetic interactions with the sediment,(2) mixing with waters deposited at different time periods, and/or (3)diffusion along concentration gradients. The importance of theseeffects has been studied by numerous researchers and was recentlyreviewed by Lawrence (1989). This study examines the δ 1 8 θ compo-sition of pore water contained in sediments from Hole 817C anddiscusses the potential importance of each of these processes foraltering the oxygen isotopic composition of the pore fluids.

SCIENTIFIC BACKGROUND

Hole 817C is situated on the northern side of the TownsvilleTrough in 1016.1 m of water (Fig. 1). Although no sedimentologicaldescriptions were performed for this core, it can be considered to beidentical to the upper portion of Holes 817A, 817B, and 817D.Drilling in Hole 817C penetrated 26.9 m of homogeneous micriticooze, which was squeezed at 10-cm intervals, yielding a total of 269samples that were processed for interstitial water analyses. The entirecore is latest Pleistocene in age. The boundary between Zones CN15and CN14b in Hole 817A was given as lying between the bottom ofCores 133-817A-1H and -2H, and the boundary between ZonesCN14b and CN-14a between Cores 133-817A-3H and -4H. Laterresearch by Gartner (this volume) refined these age boundaries to0.275 Ma at 25.08 mbsf and 0.465 Ma at 28.07 mbsf. Further agecontrol is presented here.

METHODS

Sediment samples were squeezed on board the JOIDES Resolu-tion using the conventional methods (Manheim and Sayles, 1972).After filtration, samples were analyzed for alkalinity and chloride. Aportion of the sample was subsequently sealed in a glass ampule for

McKenzie, J.A., Davies, P.J., Palmer-Julson, A., et al., 1993. Proc. ODP, Sci. Results,133: College Station, TX (Ocean Drilling Program).

2 Stable Isotope Laboratory, MGG/RSMAS, University of Miami, Miami, FL 33149,U.S.A.

later O isotopic analyses. The oxygen isotopic composition wasdetermined on CO2 equilibrated with 1 cm3 of sample in a water bathat 25 °C for 24 to 36 hr (after the method of Epstein and Mayeda,1954). Experiments using this technique showed that equilibrium wasattained in samples of normal salinity in less than 24 hr. As a result ofuncertainty regarding the precise correction factors to apply for thepresence of dissolved salts (Gonfiantini, 1986), I have not correctedour data for these effects. Reproducibility of oxygen isotopic analy-ses, determined by replicate analyses of 16 standards within a singlebatch of equilibrations, is ±O.l‰.

Chlorinities were measured in the waters at the time of collectionby titration with AgNO3 and standardized using IAPSO (InternationalAssociation of Physical Sciences Organization) seawater. The repro-ducibility of this method is approximately l‰.

A portion of the sediment cake was disaggregated and sieved intothree size fractions. The foraminifer Globigerinoides ruber was se-lected from the <120 and >63 µm size fractions. Between 5 to 10individuals were analyzed for their stable oxygen and carbon isotopiccomposition using an automated dissolution method at 90°C (Swartet al., 1991). The external reproducibility of this method determinedby replicate analysis of standards is ±0.02 ‰ for both C and O.

The isotopic ratios of CO2 for both the equilibrations and dissolu-tions were determined using a Finnigan-MAT 251 in the StableIsotope Laboratory (SIL) at the Rosenstiel School of Marine andAtmospheric Sciences, University of Miami. All data have beencorrected for the usual interferences and are quoted relative to PDBfor carbonate and SMOW for waters.

RESULTS

Chloride

The chloride concentration of the pore water varied between 540and 560 mM (Fig. 2). Although a gradual increase in Cl" can be seendown the core, the variations generally were within the analyticalerror of ±l‰ (Table 1) of the measurement method employed.

Alkalinity

The alkalinity data presented in the initial reports showed an initialrapid rise in alkalinity with depth from seawater values of 2.2 toapproximately 3.3 mM. This value remained fairly constant to a depthof a 8 mbsf, below which it increased rapidly to a value of 7 mM atthe bottom of the core (Fig. 3).

481

P.K. SWART

rt

Cf(mM)

530 540 550 560 570 580

Figure 1. Location map of Site 817.

Oxygen Isotopic Composition of Pore Water

The oxygen isotopic composition of the pore water varied betweenapproximately -0.6 and +0.7‰ SMOW (Fig. 4). These variationswere greater than the analytical error in our measurement technique(±O.l‰; see above) and snowed systematic changes down the core.The average oxygen isotopic composition of the pore waters increasestoward the base of the core.

Carbon and Oxygen IsotopicComposition of Foraminifers

The 818O of G. ruber varied between -2.0 and +0.6%o (PDB) andshowed variation, which might be interpreted as glacial-interglacialstages. The δ13C was variable and was not correlated with the δ 1 8 θ(Fig. 5).

Revised Age Assignments

Based on the oxygen isotope stratigraphy shown in Figure 5, ageshave been assigned to the various isotopic stages (Fig. 6). In thiscurve, I have identified most of the stages established by Imbrie et al.(1984). This chronology also agrees well with the FAD of Emilianiahuxleyi at 25.07 mbsf (Gartner, this volume). The sedimentation rateprofile calculated from these ages is shown in Figure 7.

Correlation Between G. ruber and Oxygen IsotopicComposition of Pore Fluids

The oxygen isotopic composition of the pore waters shows sys-tematic variations, which below 8 mbsf, are correctable with vari-ations in the oxygen isotopic composition of G. ruber. Although thiscorrelation is statistically significant, the significance increases sub-stantially if the data are plotted as five sample moving averages(Fig. 8). In the lower portion of the core, the range in the δ 1 8 θ of thepore water varies between approximately 0.0 to 0.5%e compared to analmost 1.5‰ variation in the isotopic composition of the foraminifers.In the upper 8 mbsf, there appears to be no correlation between theδ 1 8 θ of the pore fluids and foraminifers, and the magnitude of the

10

fI 15

α>D

20

25

30

Figure 2. Chloride concentration of pore waters from Hole 817C; error bar

indicates ×l‰. Regression line represents trend in Cl" concentration with depth.

variations in the oxygen isotopic composition of the pore fluids isgreater (-0.5-0.5%o). Overall, a tendency for the pore waters toincrease is seen in δ 1 8 θ values below 13 mbsf. This trend is similarto one seen in the Cl" data.

DISCUSSION

The changes in the O isotopic composition of the pore fluids resultfrom either a diagenetic signal (i.e., recrystallization of the carbonatesediment) or variations in the bottomwater signature at the time ofdeposition that have been subsequently modified by diffusion. Adefinitive method to test whether the oxygen isotopic compositionsof the pore fluids are driven by carbonate recrystallization will be tomeasure the hydrogen isotopic composition of the pore waters, as thisvalue is unaffected by diagenesis (Lawrence, 1989). If the δD of thepore water co-varies with that of the δ 1 8 θ, then the signal representsa modified original bottomwater signature. If no co-variation existsbetween the isotopic composition of these two elements, then in allprobability the variations in the δ 1 8 θ result from carbonate recrystal-lization. Certain aspects of the present data can support either of thesetwo hypotheses. The original pore-water idea is supported by theweak correlation between Cl" and δ I 8 0 down the core. The absenceof a stronger correlation is a result of scatter in the Cl" data caused bythe relatively high analytical errors involved in the technique relativeto the measured change.

Support for carbonate recrystallization is evident in data collectedfor the Sr2+ concentration from Hole 817A and the alkalinity from

482

OXYGEN ISOTOPIC COMPOSITION IN HOLE 817C SEDIMENTS

Alkalinity (mM)

2.5 3.5 4.5 5.5 6.5

Figure 3. Alkalinity of pore waters from Hole 817C.

ò O ( /oo) (SMOW)

-0.8 -0.6 -0.4 -0.2 0.2 0.4 0.6 0.8

30

Figure 4. Oxygen isotopic composition of pore waters from Hole 817C. Errorbar represents ×O. \%c.

- 3 - 2 - 1 0 1 2 3

Figure 5. Carbon and oxygen isotopic compositions of Globigerinoides ruberfrom Hole 817C.

Hole 817C (Davies, McKenzie, Palmer-Julson, et al., 1990), whichshow evidence of extensive recrystallization throughout. The fact thatthe alkalinity gradient is much stronger below 8 mbsf at Hole 817Csuggests that much more extensive carbonate recrystallization hasoccurred in this portion of the hole. This is coincidently the region ofthe hole in which the δ 1 8 θ of the pore fluids and the δ 1 8 θ of theforaminifers co-vary.

The absence of co-variation in the upper portion of the hole alsoposes an interesting problem. Either this discrepancy is also a conse-quence of diagenetic alteration, or it reflects waters of differingisotopic composition at the surface and at the bottom at this timeperiod. The latter idea can be tested by examining the isotopiccomposition of a benthic species (such as Cibicides wuellerstorfi)over this time interval. A difference in isotopic composition betweenthe bottom and surface waters also might imply that the stratigraphyestablished for the upper portion of the hole may not be correctbecause the surface waters are being influenced by fluids of unusualisotopic composition.

The notion of rapid depletion in the pore water δ 1 8 θ in the upperportion of the hole being a result of diagenesis is supported by modelsof carbonate recrystallization such as presented by Killingley (1983).In this model, the isotopic composition of the pore water after aspecified amount of recrystallization can be calculated from thefollowing equation:

δwα =103(l - α r

Mw + McaTR

In this equation, δwCa - δ 1 8 θ of interstitial water after recrystal-lization, δC, = δ 1 8 θ of calcium carbonate of initial sediment, δwα =

483

P.K. SWART

Table 1. Geochemical data from Hole 817C including stable O and C isotope data from G. ruber, porewater δ 1 8 θ and Cl measurements.

Core, section,interval (cm)

1H-1H-1H-

IH-1H-1H-1H-1H-1H-

,01,10,20,30,40,50,60,70,80

1H-1, 901H-1, 1001H-1,1101H-1,1201H-1,1301H-1,1401H-2, 01H-2, 101H-2, 201H-2, 301H-2, 401H-2, 501H-2, 601H-2, 701H-2, 801H-2, 901H-2, 1001H-2, 1101H-2, 1201H-2, 1301H-2, 1401H-3, 01H-3, 101H-3, 201H-3, 301H-3, 401H-3, 501H-3, 601H-3, 701H-3, 801H-3, 901H-3, 1001H-3, 1101H-3, 1201H-3, 1301H-3, 1401H-4, 01H-4, 101H-4, 201H-4, 301H-4, 401H-4, 501H-4, 601H-4, 701H-4, 801H-4, 901H-4, 1001H4,11O1H-4, 1201H-4, 1301H-4, 1401H-5, 01H-5, 101H-5, 201H-5, 301H-5, 401H-5, 501H-5, 601H-5, 701H-5, 801H-5, 901H-5, 1001H-5, 1101H-5, 1201H-5, 1301H-5, 1401H-6, 01H-6, 101H-6, 201H-6, 301H-6, 401H-6, 02H-1, 02H-1, 102H- , 20

Depth(mbsf)

0.10.20.30.40.50.60.70.80.91.01.11.21.31.41.51.61.71.81.92.02.12.22.32.42.52.62.72.82.93.03.13.23.33.43.53.63.73.83.94.04.14.24.34.44.54.64.74.84.95.05.15.25.35.45.55.65.75.85.96.06.16.26.36.46.56.66.76.86.97.07.17.27.37.47.57.67.77.87.98.08.18.28.38.4

Age(× k.y.)

01.22.43.54.75.97.18.39.510.611.813.016.419.923.326.830.233.737.140.644.047.450.954.357.861.264.768.171.675.076.077.178.179.280.281.282.383.384.485.486.487.588.589.590.691.692.793.794.795.896.897.998.999.9101.0102.0103.1104.1105.1106.2107.2108.3109.3110.3111.4112.4113.5114.5115.5116.6117.6118.6119.7120.7121.8122.8123.8124.9125.9127.0128.0131.7135.4139.2

δ1 3 C

‰

0.981.391.080.850.820.87

0.66

0.550.830.810.890.821.11

0.920.93

1.090.680.911.241.361.32

0.910.620.911.051.081.26

1.070.620.901.200.630.980.620.650.540.43

0.711.040.390.000.800.56

1.29

0.52

1.361.601.401.291.311.410.951.011.331.340.831.160.751.890.700.97

δ1 8 0

‰

-0.72-1.60-1.75-0.48-1.67-1.22

-1.00

-0.65-0.11-0.24-0.81-0.41-0.28

-0.46-0.90

-0.28-0.59-0.42-0.18-0.40-0.41

-0.62-1.12-0.73-0.56-0.93-0.87

-1.01-1.20-0.90-0.40-0.81-0.51-0.54-0.35-0.63-1.00-0.60-0.47-0.43-0.89-0.73-0.50-0.42

-1.65

-1.21

-0.54-0.65-0.93-1.03-1.09-0.71-1.26-2.04-1.85-1.73-0.63-1.60-1.45-0.48-0.45-0.06

δ18O‰

(SMOW)

0.30.4-0.6-0.60.40.10.20.60.50.10.1-0.3-0.1-0.2-0.2-0.3-0.1

-0.4-0.4

-0.3-0.4-0.50.1-0.5-0.2-0.3-0.3-0.20.00.20.10.00.20.40.3-0.4-0.5-0.20.2-0.5-0.5-0.30.60.10.20.10.20.40.20.40.50.1-0.10.1-0.1-0.3-0.2

-0.10.0-0.1-0.20.1-0.20.00.2-0.4-0.50.2-0.10.30.3-0.10.30.1-0.20.2-0.1-0.2-0.40.0-0.4

er (mM)

543541540551541543541543546546546545541547542543547

547547

544543543541543545542538538545546545548545544544547545546548547545540551540540544541556558548553549549542547543546

539538541544540542538538538538539543542542544542546541544543540540540542

Core, section,

interval (cm)

2H-1, 302H-1, 402H-1, 502H-1, 602H-1, 702H-1, 802H-1, 902H-1, 1002H-1,1102H-2, 02H-2, 102H-2, 202H-2, 302H-2, 402H-2, 502H-2, 602H-2, 702H-2, 802H-2, 902H-2, 1002H-2, 1102H-2, 1202H-2, 1302H-2, 1402H-3, 02H-3, 102H-3, 202H-3, 302H-3, 402H-3, 502H-3, 602H-3, 702H-3, 802H-3, 902H-3, 1002H-3, 1102H-3, 1202H-3,1302H-3, 1402H-4, 02H-4, 102H-4, 202H-4, 302H-4, 402H-4, 502H-4, 602H-4, 702H-4, 802H-4, 902H-4, 1002H-4, 1102H-4, 1202H-4, 1402H-5, 02H-5, 102H-5, 202H-5, 302H-5, 402H-5, 502H-5, 602H-5, 702H-5, 802H-5, 902H-5, 1002H-5, 1102H-5, 1202H-5, 1302H-5, 1402H-6, 02H-6, 102H-6, 202H-6, 302H-6, 402H-6, 502H-6, 602H-6, 702H-6, 802H-6, 902H-6, 1002H-6, 1102H-6, 1202H-6,1302H-6,1402H-7, 0

Depth(mbsf)

8.58.68.78.88.99.09.19.29.39.79.89.910.010.110.210.310.410.510.610.710.810.911.011.111.211.311.411.511.611.711.811.912.012.112.212.312.412.512.612.712.812.913.013.113.213.313.413.513.613.713.813.914.114.214.314.414.514.614.714.814.915.015.115.215.315.415.515.615.715.815.916.016.116.216.316.416.516.616.716.816.917.017.117.2

Age

(× k.y.)

142.9146.6150.3154.1157.8161.5165.2168.9172.7176.4180.1183.8187.6195.0196.3197.5198.8200.1201.4202.6203.9205.2206.5207.7209.0210.3211.5212.8214.1215.4216.6217.9219.2220.5221.7223.0224.3225.5226.8228.1229.4230.6231.9233.2234.5235.7237.0238.3239.5240.8242.1243.4244.6245.9247.2248.5251.0251.5252.0252.5253.0253.5254.0254.5255.0255.5255.9256.4256.9257.4257.9258.4258.9259.4259.9260.4260.9261.4261.9262.4262.9263.4263.9264.4

δ1 3 C

‰

0.730.940.690.551.131.32

1.250.611.04

2.150.870.860.611.140.650.951.070.840.661.35

1.581.251.060.990.980.911.111.151.191.041.201.141.341.06

1.251.281.340.990.921.040.941.140.791.031.231.16

1.060.900.330.610.550.510.720.400.710.510.130.730.570.491.300.82

0.580.670.780.810.770.470.500.900.721.000.990.790.810.350.540.54

δ1 8 0

‰

-0.60-0.46-0.35-0.24-0.97-0.61

-0.700.12-0.67

-0.540.22-1.35-1.13-1.19-1.17-0.85-0.080.02-0.06-0.31

-0.24-0.45-0.48-0.53-0.57-0.61-0.76-0.99-0.66-0.84-0.99-0.58-0.51-1.01

-1.16-1.10-1.25-1.23-1.06-1.30-1.68-1.85-1.89-1.72-1.61-1.74

-1.05-1.15-1.19-1.27-1.15-0.98-0.71-1.14-0.36-0.53-0.46-0.070.07-0.03-0.75-0.74

-0.66-0.73-0.44-0.39-0.60-0.55-0.84-1.03-0.52-0.96-0.80-0.91-1.27-0.71-0.660.20

δ18O‰

(SMOW)

-0.20.10.10.1-0.10.10.50.30.3-0.1-0.10.10.30.30.20.0-0.2-0.1-0.30.30.10.10.00.20.30.1-0.20.1-0.1-0.30.10.20.30.20.30.10.3-0.20.0-0.3-0.2-0.3-0.40.00.00.00.00.00.0

0.0

0.20.20.30.00.20.20.20.30.30.10.10.10.10.20.10.20.10.1-0.1-0.1-0.10.2

0.50.40.3

er (mM)

543542542542542537541541539543546550547544549555547547548551547545541545548547545547547546545544554542555543550547546544549539540546549552546546547

543

544554550562551550555549550549549548545554550550552552556544548546

562546552

484

OXYGEN ISOTOPIC COMPOSITION IN HOLE 817C SEDIMENTS

Table 1 (continued).

Core, section,interval (cm)

2H-7, 102H-7, 202H-7, 302H-7, 402H-7, 502H-7, 602H-7, 703H-1, 03H-1, 103H-1, 203H-1, 303H-1, 403H-1, 03H-1, 603H-1, 703H-1, 803H-1, 903H-1,1003H-1,1103H-1, 1203H-1, 1303H-1, 1403H-2, 03H-2, 103H-2, 203H-2, 303H-2, 403H-2, 503H-2, 603H-2, 703H-2, 803H-2, 903H-2, 1003H-2, 1103H-2, 1203H-2, 1303H-2,1403H-3, 03H-3, 103H-3, 203H-3, 303H-3, 403H-3, 503H-3, 603H-3, 703H-3, 803H-3, 903H-3, 1003H-3, 110

Depth(mbsf)

17.317.417.517.617.717.817.918.018.118.218.318.418.518.618.718.818.919.019.119.219.319.419.519.619.719.819.920.020.120.220.320.420.520.620.720.820.921.021.121.221.321.421.521.621.721.821.922.022.1

Age(×k.y.)

264.8265.3265.8266.3266.8267.3267.8268.3268.8269.3269.8270.3270.8271.3271.8272.3272.8273.3273.8274.2274.7275.2275.7276.2276.7277.2277.7278.2278.7279.2279.7280.2280.7281.2281.7282.2282.7283.2283.6284.1284.6285.1285.6286.1286.6287.1287.6288.1288.6

δ1 3 C

‰

0.66

0.470.310.500.911.07

1.651.70

1.190.910.67

0.730.400.610.470.650.560.43

0.630.851.301.030.910.860.751.010.951.531.161.20

δ1 8 0

‰

0.48

-0.34-0.37-0.51-0.35-1.09-0.48-0.49-0.22-0.29-0.430.29

0.220.240.23

-0.08-0.05-0.17-0.100.29-0.24-0.86

-1.12-1.18-0.97-0.89-1.24-0.960.11-0.01-0.40-0.49-0.69-0.62

818O‰

(SMOW)

0.50.20.30.50.30.20.50.20.20.30.50.60.40.30.40.10.50.40.10.40.40.40.10.50.40.2-0.10.20.1

0.20.2

0.2-0.10.00.10.20.00.30.10.40.20.10.20.10.40.2

Cl" (mM)

549553546546551540550553547540533552554550547563551550551545548551549552551551550545554

551554

551551545550563548545551540549544547548550554

Core, section,interval (cm)

3H-3,1203H-3,1303H-3, 1403H-3, 03H-4, 103H-4, 203H-4, 303H-4, 403H-4, 503H-4, 603H-4, 703H-4, 803H-4, 903H-4, 1003H-4, 1103H-4, 1203H-4, 1303H-4, 1403H-5, 03H-5, 103H-5, 203H-5, 303H-5, 403H-5, 503H-5, 603H-5, 703H-5, 803H-5, 903H-5,1003H-5,1103H-5, 1203H-5, 1303H-5, 1403H-6, 03H-6, 103H-6, 203H-6, 303H-6, 403H-6, 503H-6, 603H-6, 703H-6, 803H-6, 903H-6,1003H-6, 1103H-6, 1203H-6, 1303H-6,140

Depth(mbsf)

22.222.322.422.522.622.722.822.923.023.123.223.323.423.523.623.723.823.924.024.124.224.324.424.524.624.724.824.925.025.125.225.325.425.525.625.725.825.926.026.126.226.326.426.526.626.726.826.9

Age(×k.y.)

289.1289.6290.1290.6291.1291.6292.1292.5293.0293.5294.0294.5295.0295.5296.0296.5297.0297.5298.0298.5299.0299.5300.0300.5301.0301.5301.9302.4302.9303.4303.9304.4304.9305.4305.9306.4306.9307.4307.9308.4308.9309.4309.9310.4310.8311.3311.8312.3

S13C n0 TOO

0.580.821.52

1.171.421.761.631.48

1.391.371.390.901.161.351.401.531.171.351.060.770.641.200.941.060.710.940.891.711.350.891.131.381.391.24

1.221.111.401.100.861.030.631.081.74

1.26

δ1 8 0

‰

-0.12-0.67-0.56

-0.38-0.08-0.25-0.37-0.58

-0.70-0.72-0.55-1.04-0.89-0.79-0.81-1.38-1.40-0.67-0.40-0.43-0.42-0.65-0.47-0.46-0.56-0.580.110.28-0.42-0.35-0.46-0.43-0.09-0.42

-0.36-0.66-0.41-0.91-0.81-1.12-1.09-1.05-0.45

-1.01

δ18O‰

(SMOW)

0.20.10.10.40.00.10.0-0.20.30.20.0-0.3-0.30.60.30.30.30.30.20.20.20.20.30.30.30.40.20.30.30.10.30.70.40.50.50.30.50.70.60.60.30.40.30.30.50.5

Cl" (mM)

547551549553552549547550549550540548543547543535554546548544557549549547544552575547546546557549557552

544548548552570557556551553558553549

549

δ 1 8 θ of interstitial waters before reaction, Mc = mole fraction ofoxygen in carbonate sediment, Mw = mole fraction of oxygen in porewaters, R = percentage of recrystallization, and α = fractionationfactor between calcite-water.

In the case for Hole 817C, I have used a bottom temperature of10°C, a geothermal gradient of 50°C/km (data from Davies, McKen-zie, Palmer-Julson, et al., 1990), an initial sediment isotopic compo-sition of-2%o (PDB), a water isotopic composition of 0.5%e (SMOW)(Fig. 9), a porosity of 50‰, and a carbonate content of 80‰. If in thiscase one assumes that the recrystallized calcium carbonate does notre-equilibrate with the pore fluids, then the isotopic composition ofthe pore waters will rapidly become depleted with depth. This issimilar to the case at Hole 817C. As may be observed from thissimplistic example the result of initial recrystallization is to producepore waters that are isotopically depleted. If the pore waters do notcontinue to react with the carbonate, then the isotopic composition ofthe fluid should in the absence of diffusion remain at approximately-l.O‰. However, continued recrystallization of the sediment maycause the isotopic composition of the fluids to co-vary with theforaminifers, as is observed in the lower portion of Hole 817C.

The origin of these δ 1 8 θ values will be further tested by (1)constructing a mathematical model that will account for not only

diffusion, but also advection, and carbonate recrystallization, (2)measurement of the δD of the pore waters, and (3) measurement ofthe isotopic composition of benthic foraminifers from the cores.

CONCLUSIONS

There is a co-variance between the oxygen isotopic composition ofG. ruber and the pore waters below 8 mbsf. The magnitude of thevariation in the δ 1 8 θ of the waters is approximately 30‰ of that seen inthe δ 1 8 θ of the foraminifers, suggesting that diffusion and perhapsrecrystallization have altered the isotopic compositions. Circumstantialevidence exists to support both of these hypotheses, and the eventualcause will only be revealed by (1) analyzing the hydrogen isotopiccomposition of the pore fluids, (2) mathematically modeling the proc-esses of carbonate recrystallization, diffusion, and advection, and (3)investigating the isotopic composition of the benthic foraminifers.

ACKNOWLEDGMENTS

The author would like to thank the technicians, scientists, and crewof Leg 133 for continued help throughout the cruise. Joe Powers, JoeDeMorett, and Scott Chaffey are especially thanked for their help in

485

P.K. SWART

the chemistry laboratory. Alex Isern, Danny Muller, and Judy McKen-zie provided friendship and discussion throughout. I would especiallylike to thank all those who helped with the marathon squeezing effort.In addition to those mentioned above, Christian Betzler, George Dix,David Feary, Craig Glenn, Jose Martin, Lucien Montaggioni, andChris Pigram manned the presses during the 36-hr squeezing effort.Help with the shore-based analyses was provided by Amel Saied, JeffAbell, and Jim Leder. Thanks to Michael Lopez and Yvette Alger formany hours of selecting foraminifers. Conversations with GeoffHaddad, Andre Droxler, Larry Peterson, and Pam Reid helped withthe interpretation of the oxygen isotope stratigraphy.

REFERENCES*

Davies, PJ, McKenzie, J.A., Palmer-Julson, A., et al., 1991. Proc. ODP., Init.Repts., 133: College Station, TX (Ocean Drilling Program).

Epstein, S., and Mayeda, T, 1953. Variation of 18O content of waters fromnatural sources. Geochim. Cosmochim. Acta, 42:213-224.

Gonfiantini, R., 1986. Environmental isotopes in lake studies. In Fritz, P., andFontes, J.Ch. (Eds.) Handbook of Environmental Isotope Geochemistry,Vol 2., (Elsevier), 113-168.

Imbrie, J., Hays, J.D., Martinson, D.G., Mclntyre, A., Mix, A., Morley, J.J,Pisias, N.G, Prell, W.L, and Schackleton, NJ, 1984. The orbital theory of

Pleistocene climate: support from a revised chronology of the marine δ1 8Orecord. In Berger, A.L., et al. (Eds.), Milankovitch and Climate, Pt. I: NewYork (Reidel Publ. Co.), 269-305.

Killingley, J.S., 1983. Effects of diagenetic recrystallization on 18O/16O valuesof deep-sea sediments. Nature, 301:594-597.

Lawrence, J.R., 1989. The stable isotope geochemistry of deep-sea pore water.In Fritz, P., and Fontes, J.Ch. (Eds.), Handbook of Environmental IsotopeGeochemistry, Vol. 3: New York (Elsevier), 317-356.

Manheim, ET, and Sayles, FL., 1974. Composition and origin of interstitialwaters of marine sediments based on deep sea drill cores. In Goldberg,E.D., The Sea, Vol. 5: New York (Wiley), 527-568.

Swart, P.K., Burns, SJ., and Leder, J.J., 1991. Fractionation of the stableisotopes of O and C in CO2 during the reaction of calcite with phosphoricacid as a function of temperature and technique. Chem. Geol. (IsotopeGeosci. Sect.), 86:89-96.

* Abbreviations for names of organizations and publication titles in ODP reference listsfollow the style given in Chemical Abstracts Service Source Index (published byAmerican Chemical Society).

Date of initial receipt: 21 April 1992Date of acceptance: 10 December 1992Ms 133SR-260

.18

I O(%o)0.6 0.2 -0.2 -0.6 -1 -1.4 -1.8 -2.2

Age (x 1000) yr

4 -

8 -

I i

4.2

Figure 6. Oxygen isotopic stratigraphy of Hole 817C. The diamonds and thesolid lines represent replicated data. Stage numbers are from Imbrie et al. (1984).

100 200 300 400 500

f

25

30

83 m.y.

100 m.y.

77 m.y.

200 m.y.

Dlsconformity

CN15

CN14a

Figure 7. Sedimentation rate for Hole 817C. Nannofossil zones are fromGartner (this volume).

486

18

-2

J O(%o) (PDB)

-1.5 -1 -0.5 0 0.5

-0.4 -0.2

18i 0 ( % 0 ) (SMOW)

Figure 8. Smoothed oxygen isotopic composition of the pore waters comparedto G. ruber from Hole 817C.

OXYGEN ISOTOPIC COMPOSITION IN HOLE 817C SEDIMENTS

18

-1.5 -1

O (°/θθ) (SMOW)

-0.5 0 0.5 1 1.5

35

Figure 9. Theoretical effect of carbonate recrystallization on the δ 1 8 θ of porewaters using the following parameters (δ1 8θj= -2‰ (PDB); δ 1 8 θw = +0.5%c(SMOW); dt/dz = 50°C/km; and bottom temperature = 10°C). Line (A)represents pore water which initially experiences a great deal of recrystalliza-tion and little subsequent isotopic exchange with the sediments. Line (B) showspore water which continues to isotopically exchange with the sediments.

487