Progressive dissolution of titanomagnetites at ODP Site 653 (Tyrrhenian Sea)

Earth and Planetary Science Letters, 96 (1990) 469-480 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

469

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Progressive dissolution of titanomagnetites at ODP Site 653 (Tyrrhenian Sea) J.E.T. Channell and T. Hawthorne Department of Geology, Unioersity of Florida, Gainesville, FL 32611 (U. S.A.) Received June 1, 1989; revised version received October 3, 1989 Magnetic properties, microscopy and ferrous/ferric iron ratio document increasing down-core alteration of titanomagnetite to iron sulfide at ODP Site 653 (Leg 107, Tyrrhenian Sea). Changes in magnetic properties are interpreted to indicate a grain size increase in titanomagnetite to 40 m below seafloor (bsf) and a grain size decrease from this level to about 150 m bsf, as dissolution affects first the fine grained and then the coarser grained magnetite. The availability of dissolved sulfate from underlying Messinian evaporites may account for the sulfide supply at depth and hence for the prolonged dissolution process. Dissolution at Leg 107 sites adjacent to Site 653 is less pronounced, possibly by the availability of organic matter that can be metabolised by sulfate-reducing bacteria.

I. Introduction

The fidelity of the magnetostratigraphic record in deep-sea sediments depends on the composition and grain size of primary magnetic minerals, and on diagenetic processes which cause dissolution of primary, and growth of secondary, magnetic minerals. Titanomagnetite is a common detrital mineral in deep-sea sediments, and low-Ti magnetite is now commonly observed as single-domain biogenic grains which are important carriers of remanent magnetization [1,2]. The alteration of primary magnetite during sediment diagenesis is a common cause for the loss of the magnetostratigraphic record in deep-sea sediments [3-8]. The understanding of this process will allow better appreciation of sediment types suitable for magnetostratigraphic study. In the mid-latitude Pacific "red clay" facies [9], where sedimentation rates are low at about 25 cm/My, the primary magnetization generally degrades at a few meters depth below the sediment-water interface [3-7]. At such low sedimentation rates, organic matter is oxidized close to the sediment-water interface. The resulting diagenetic conditions may result in the oxidation of magnetite to maghemite [4,5] a n d / o r the growth of secondary ferromanganese oxides [6]. By contrast, reducing diagenetic conditions in 0012-821X/90/$03.50

© 1990 Elsevier Science Publishers B.V.

organic-rich sediments from the Sea of Japan result in dissolution of primary magnetite and formation of iron sulfide below 6-8 m depth in the sediment cores [8]. Suttill et al. [10] demonstrated the progressive down-core growth of pyrite in rapidly depositing (up to 700 cm/ky) recent tidal flat sediments. In hemipelagic sediments from coastal Oregon and the Gulf of California, where sedimentation rates are about 120 cm/ky, Karlin and Levi [11] have documented the progressive reduction of ferrous iron in magnetite to ferric iron in sulfide, within the top 100 cm of sediment. In sediments with similar sedimentation rate from Long Island Sound, Canfield and Berner [12] showed that dissolution of magnetite and the formation of pyrite occurs within the top 100 cm of the sediment, and that magnetite dissolution rate depends on the concentration of iron phases (such as oxyhydroxides) which are more reactive than magnetite, the concentration and surface area of the magnetite, and on concentration of dissolved sulfide. This final condition will depend on the availability of sulfate, and on the activity of sulfate-reducing bacteria. This will, in turn, be controlled by availability and nature of organic matter [13-15]. This paper documents the record of magnetite dissolution in Plio-Pleistocene sediments at Site 653 in the Tyrrhenian Sea (Fig. 1), cored using the

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Advanced Piston Corer (APC) during ODP Leg 107. Magnetic, geochemical and microscopic data indicate that titanomagnetite dissolution with the formation of iron sulfide increases downcore through at least 150 m of Plio-Pleistocene section. The question arises as to whether this increase is due to prolonged time-dependent dissolution or to variations in original sediment composition which control the rate of early diagenetic dissolution. The Plio-Pleistocene sediments at Hole 653A are 221 m thick and comprise foraminiferal-nannofossil oozes and nannofossil oozes with thin deposits of tephra, sapropel and foraminifer sands particularly in the upper 85 m [16]. At this site, the primary magnetization (and therefore the magnetostratigraphy) of the sediment could not be resolved. However, the primary magnetization and

a magnetostratigraphy was resolved from the Plio-Pleistocene sediments at Site 654 and Site 652 [17,18], located 100-150 km from Site 653 (Fig. 1). The mean sedimentation rates in the Plio-Pleistocene sediments at sites 652, 653 and 654 are 38, 45 and 49 m / M y , respectively.

2. Magnetic properties Orthogonal projections of thermal demagnetization data from Site 653 revealed the presence of a magnetization component with high blocking temperature (Fig. 2), but the inclination of this component was generally steep and downward, and did not define a magnetostratigraphy. The maximum blocking temperature did not exceed 600 o C. Several magnetic parameters measured at

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Site 653 indicate a change in magnetic properties down-core. Shipboard whole-core susceptibility measurements showed variable yet high values in the top 40 m (Fig. 3), which can be correlated with variations in volcanoclastic content. A distinct decrease in susceptibility occurs between 40 and 60 m below seafloor (bsf), with further decrease down to 150 m bsf (Fig. 3). A similar decrease at about 40 m bsf is seen in the intensities of natural remanent magnetization (NMR), saturation isothermal remanence (SIRM) and anhysteretic remanence (ARM) (Fig. 4. The A R M was acquired in a biasing d.c. field of 0.05 m T with a peak alternating field of 100 mT, and the SIRM in a magnetizing field of 0.9 T. Below 80 m bsf, the intensities of ARM become variable with some

high values (Fig. 4), whereas susceptibility and SIRM show a slight decrease to 150 m bsf. The median destructive fields (MDF) of the N M R (Fig. 5) show constant values in the top 40 m with a decreasing tendency between 30 and 40 m bsf, followed by an increasing trend further down-core. Isothermal remanence (IRM) acquisition curves from samples down to core 13 (110 m bsf) at Site 653 (Fig. 6) indicate saturation in low magnetizing fields, and therefore the apparent absence of high-coercivity magnetic minerals. The saturation magnetization intensity drops significantly below core 5 (40 m bsf) (Fig. 6). Both the M D F / d e p t h plot (Fig. 5) and the alternating field demagnetization of the N M R of individual samples (closed circles in Fig. 7) indicate a tendency

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(Fig, 6) and the thermal demagnetization data (Fig. 2) are consistent with the presence of magnetite and the absence of higher coercivity and higher blocking temperature minerals. It is therefore possible to interpret the variations in coercivity of ARM and SIRM in terms of a magnetite grain size coarsening at about 40 m bsf, underlain by a progressive grain size decrease down-core. The ARM intensity and the ARM/susceptibility ratio (Fig. 4), which are particularly sensitive to the fine magnetite fraction [20], and the M D F / depth plot (Fig. 5) are consistent with this interpretation. Susceptibility measurements taken between heating steps (Fig. 8) strongly suggest the presence of a mineral phase which alters to a higher susceptibility mineral at about 400 ° C. The initial phase is most probably an iron sulfide such as pyrite, which would be expected to convert to magnetite at this temperature when heated in air. Although an increase in susceptibility is often apparent after heating to temperatures above 400°C, the increase becomes more pronounced with depth, suggesting a down-core increase in concentration of iron sulfide (Fig. 8).

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for natural remanence coercivity to increase downcore. In addition, although the response of SIRM to alternating field demagnetization shows no systematic change down-core, the coercivity of ARM appears to change (Fig. 8). Above core 6 (above 40 m bsf), the coercivity of ARM is high relative to that of N M R and SIRM; and in cores 7-10 (51-89 m bsf), the coercivity of ARM decreases, and the coercivity of N M R increases (Fig. 8). Below core 10, the coercivity of N R M increases further (Fig. 8). According to Johnson et al. [19], the changes in relative coercivity of ARM and SIRM can be used as an indicator of magnetite grain-size change, if magnetite is the only magnetic mineral present. The IRM acquisition curves

The extraction procedure described below has been tested on a variety of magnetite-bearing sediments. It is capable of extracting submicron-sized magnetite grains, and does not appear to be grain-size selective. For this study, 50 g of sample from core 3, sections 2-4 (19.1-23.6 m bsf) and the same weight of sample from core 13, sections 4-6 (116.6-121.1 m bsf) were added to flasks containing 650 ml of 1 M sodium acetate buffer adjusted with acetic acid to a pH of 5.0. No carbonate remained after 2.5 days. In order to determine whether any ferrous iron had been digested by the solution, approximately 10 mg of bathophenanthroline was added to 3 ml of the solution; no ferrous iron was detected in the solution. After dissolution of the carbonate, 100 ml of a 4% solution of sodium hexametaphosphate was added to disperse the clays. The solution was then agitated in an ultrasonicator for 10 minutes. This solution was introduced into the extraction line, to allow the solution to pass, at a very slow flow rate, close to a rare earth magnet. The magnet is placed

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on top of a removable section of tube, inclined at approximately 45 °. After the solution has passed the magnet, the extraction line was flushed with deionised water in order to remove the non-magnetic particles adhering to the removable tube. The magnet and the tube were then removed from the extraction line and the magnetic extract flushed out of the tube. Several drops of suspended extract were placed on an aluminum stub, dried and sputter coated with gold for scanning electron microscopy. Images were obtained from both back scattered and secondary electrons. Elemental determinations were made with an energy dispersive X-ray fluorescence unit (EDS). Titanomagnetite was the dominant magnetic mineral in both extracts, as determined by powder X-ray diffraction. Individual grains could then be identified with the SEM based on crystal form and elemental analysis from the EDS. The extract from core 13 differed from that from core 3 in several respects. Firstly, 40% more magnetic ex-

tract was recovered from core 3 than from core 13, although the same initial sample weight was used. Secondly, the core 13 extract was devoid of magnetite grains smaller than several microns in diameter. Thirdly, whereas the core 3 extract was characterized by pristine titanomagnetite grains, the core 13 extract was composed of abundant pitted titanomagnetites which sometimes had secondary sulfide overgrowths (Fig. 9). Elemental analysis by X-ray fluorescence indicated that the iron sulfide contains or coexists with significant amounts of titanium. This observation and the distorted crystal form of the iron sulfides together with the presence of pyrite overgrowths suggest that the reduction of ferric iron is occurring "in situ" without significant migration of reduced iron. X-ray diffraction of residues left after magnetic extraction indicate prominant iron sulfide peaks for the core 13 sample, and weaker iron sulfide peaks for the core 3 sample. Weak magnetite peaks were observed in both residues.

474

J.E.T. C H A N N E L L A N D T. H A W T H O R N E

4. Ferrous/ferric iron ratio The ferrous iron content of sediment samples was determined using the spectrophotometric technique described by Fritz and Popp [21]. In order to determine the ferrous/ferric iron ratio, the total iron was determined using inductively coupled plasma (ICP) spectroscopy. The ferrous/ ferric iron ratio of the bulk sediment increases down core (Fig. 10). This trend argues for progressive "in situ" sulfide formation with insignificant migration of reduced iron. 5. Interpretation A combination of magnetic, microscopic and chemical methods indicate partial dissolution of detrital titanomagnetites with the degree of dissolution increasing downcore. The dissolution has destroyed the fine-grained magnetite which we postulate carries a primary magnetization compo-

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nent at other Leg 107 sites [17,18]. An abrupt decrease in NRM intensity, SIRM intensity and ARM intensity (Fig. 4) occurs close to 40 m bsf; this may coincide with the depth at which much of the remanence-carrying fine-grained magnetite fraction disappears. This is consistent with the decreased ARM coercivity in the core 7 to core 10 interval, relative to that above core 6 (Fig. 7). The microscopic observations (Fig. 9), X-ray diffraction of the residue after magnetic mineral extraction, and the observed increases in susceptibility during heating (Fig. 8) suggest that iron sulfide is being formed down-core. The MDF appears to increase downward throughout the core (Fig. 5), and the ferrous/ferric iron ratio has a similar trend (Fig. 10) reflecting the replacement of magnetite by iron sulfide. We suspect that the increasing MDF is due to a progressive etching that results in a decrease in the effective grain size of the magnetite as alteration proceeds, rather than any contribution to the remanence from iron sulfide or other authigenic minerals. This conclusion is based on the fact that we see no evidence for a sulfidic remanence carrier in the blocking temperature spectrum during the thermal demagnetization of the NRM (Fig. 2). This implies that the sulfide mineral is paramagnetic. Pyrite is the common paramagnetic iron sulfide whereas pyrrhotite, greigite and mackinawite can be viable carriers of magnetic remanence. The more unstable iron sulfides (greigite and mackinawite) may be the precursors to pyrite, and it is conceivable that conversion occurred post-coring, during core and sample storage. The X-ray diffraction patterns of residues after magnetic extraction indicate peaks which may be associated with several iron sulfide minerals (including pyrite), but the records are such that it is not possible to determine their relative importance. Karlin and Levi [11] have interpreted increasing down-core coercivity in Pacific hemipelagic sediments as also resulting from a progressive decrease in effective magnetite grain size. However, in that case, the dissolution is occurring in the top 100 cm, representing a duration for the process of about 1000 y. In the case of Site 653, we could not effectively sample the top few meters of sediment. However, if the dissolution process was/is confined to the top few meters of sediment, the observed down-core changes in mag-

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netic and geochemical parameters may be explained in terms of variations in reactive iron a n d / o r organic content in the original sediment, which would control the r a t e of magnetite dissolution. Although thin discrete tephra and sapropelic layers reflect variations in detritus and organic content of the sediments, these layers were avoided during sampling. Chromium, barium and calcium carbonate concentrations in the oozes do not indicate systematic variations in sediment composition which correspond to the magnetic or geochemical observations. We conclude that the observations can be best explained by postulating that dissolution has been a prolonged process, occurring progressively down to at least 150 m bsf, rather than a process that has occurred in the top few meters of the sediment pile at variable rates. At Site 653, 40 m bsf and 150 m bsf correspond to

sediment ages of about 0.7 Ma and 3.3 Ma, respectively. Reduction of magnetite to iron sulfide during marine diagenesis has been observed in a variety of sedimentary environments characterized by high organic content [8,10-15]. In the absence of more reactive iron phases, surface area of the magnetite grains and the concentration of dissolved sulfide are the principal controls on the rate of magnetite dissolution [12]. The concentration of dissolved sulfide is controlled in turn by the availability of sulfate and the activity of sulfate-reducing bacteria. The Plio-Pleistocene sediments at Site 653 and 654 are underlain by Messinian evaporites which may have provided a source for sulfate in addition to the seawater source. Sulfate profiles in pore waters at Hole 653B indicate increasing sulfate concentrations down-core (Fig. 11). For Hole 653A

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Fig. 8. Initial susceptibilityafter heating for various samples from Hole 653A. and sites 652 and 654, very few sulfate analyses are available and there is no apparent downward increase. However, significant available sulfate appears to be present throughout the Plio-Pleistocene sediments in all these cores. Upward migration of sulfate-rich fluids is consistent with heat flow at Hole 653A which is highly variable with depth, decreasing from 141.7 m W / m 2 between the surface and 41.6 m bsf, to 66 m W / m 2 between 155.2 and 192.9 m bsf [16]. The critical difference between the diagenetic environment at Site 653 and Site 654 may be the concentration of organic matter that can be metabolized by sulfate-reducing bacteria, which are the primary control on the availability of dissolved sulfide in environments where there is abundant sulfate in the pore waters. At Site 654, apart from the sapropelic layers, the weight percent organic carbon in the Plio-Pleistocene sediments is usually less than 0.1, and organic

matter is probably the limiting agent in sulfide formation [22]. No detailed record of the organic carbon content at Site 653 is available, however, this site has a higher frequency of sapropelic layers relative to other Leg 107 sites, such as Site 654 [23]. As the total thicknesses, and sedimentation rates, of the Plio-Pleistocene sequences at sites 654 and 653 are similar, sedimentation rate is probably not controlling differences in preservation of organic matter at the two sites. We suggest that bottom water conditions at Site 653, which is located in the deeper part of the Cornagha Basin (Fig. 1), enhanced the accumulation and burial of organic matter. Whereas the availability of organic matter which can be metabolised by sulfate-reducing bacteria may have been the critical factor controlling the differences in magnetite dissolution at adjacent Leg 107 sites, the question arises as to why the

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p r o c e s s is n o t , a p p a r e n t l y , c o n f i n e d to t h e t o p f e w m e t e r s o f t h e s e d i m e n t a r y s e q u e n c e as o b s e r v e d in h e m i p e l a g i c a n d t i d a l f l a t s e d i m e n t s [11,12]. I n o n e o f t h e s e s t u d i e s [11], t h e p r i m a r y m a g n e t i c r e m a n e n c e c a r r i e d b y m a g n e t i t e s u r v i v e s t h e diss o l u t i o n p r o c e s s , w h e r e a s this m a g n e t i z a t i o n c o m -

Fig. 9. (a) Photomicrograph of a pitted titanomagnetite dipyramid from Hole 653A core 13. Scale bar corresponds to 10 /xm. (b) Photomicrograph of a pitted titanomagnetite dipyramid from Hole 653A core 13 which contains fine-grained iron sulfides within the void. Scale bar corresponds to 5 t~m. (c) Photomicrograph of an iron titanium sulfide (center) with abnormal crystal form, and a large titanomagnetite (upper left) coated with fine-grained iron sulfides, from Hole 653A Core 13. Scale bar corresponds to 5 vm.

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Acknowledgements This research was financially supported b y J O I / U S S A C t h r o u g h the Texas A & M Research F o u n d a t i o n . W e t h a n k R. K a r l i n , K. Emeis, A. G e h r i n g , D. Mueller a n d D. H o d e l l for discussions o n previous versions of this paper, M. Torii for help with the s h i p b o a r d analysis of Site 653 sediments, a n d R. Berggeron, T. H a w t h o r n e a n d F. G o d m a n for their advice o n a n a l y t i c a l techniques.

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(mmol/t) Fig. 11. Sulfate, chlorinity and salinity in interstitial water plotted against depth for Hole 653B [16].

p o n e n t at Site 653 is destroyed. W h e r e the p r i m a r y r e m a n e n c e survices, the availability of sulfate m a y be the controlling factor that confines the dissolution to the top few meters of the s e d i m e n t a r y sequence. I n the case of Site 653, sulfate conc e n t r a t i o n increases down-core (Fig. 11) i n d i c a t i n g sulfate c o n t r i b u t i o n from the u n d e r l y i n g Mess i n i a n evaporites. W e suppose that m a g n e t i t e dissolution c o n t i n u e s to at least 150 m bsf, i m p l y i n g active bacterial r e d u c t i o n of sulfate a n d p r o d u c tion of h y d r o g e n sulfide to this depth. Viable anaerobic bacterial activity, such as sulfate-reduction, has recently b e e n detected at depths u p to 135 m bsf i n D S D P Leg 95 sediments [24], a n d to 80 m bsf i n O D P Leg 112 sediments [25].

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