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Physical properties of a porcellanite layer (Southwest Indian Ridge) constrained by geophysical logging
Marine Geology 140 ( 1997) 415–426
Short communication
Physical properties of a porcellanite layer (Southwest Indian Ridge) constrained by geophysical logging Sebastian Gerland a,*, Gerhard Kuhn a, Gerhard Bohrmann b a Alfred Wegener Institute for Polar and Marine Research, Postfach 120161, 27515 Bremerhaven, Germany b GEOMAR, Research Center for Marine Geosciences, Christian Albrechts University, Wischhofstr. 1–3, 24148 Kiel, Germany Received 7 December 1995; accepted 2 April 1997
Abstract A distinct porcellanite layer from the Southwest Indian Ridge intercalated in Pleistocene diatom ooze was studied using nondestructive physical property measurements and sedimentological data. This bed was sampled by two piston cores at a water depth of 2615 m. The 3–5 cm thick porcellanite layer appears in the cores at a depth of 6.03 m (Core PS2089-2) and 7.73 m (Core PS2089-1) below the seafloor. Due to its characteristic physical properties the porcellanite bed can be detected with core measurements, and its distribution and lateral extent mapped with echosounding. The physical index properties, wet bulk density and electrical resistivity, increase significantly across this bed. Magnetic susceptibility is used to compare the lithological units of both cores and to distinguish whether resistivity anomalies are caused by a higher amount of terrigenous components or by the presence of porcellanite. The porcellanite has the special characteristic to aect a positive anomaly in resistivity but not in susceptibility. Most marine sediments, in contrast, show a positive correlation of magnetic susceptibility versus electrical resistivity; therefore a combination of electrical resistivity and magnetic susceptibility logs yields a definite detection of the porcellanite bed. Images from the X-ray CT survey indicate that the porcellanite is lithified and brittle and fragmented when the piston corer penetrated the bed. © 1997 Elsevier Science B.V. Keywords: physical properties; porcellanite; marine sediments; nondestructive methods; high-resolution methods; cores
1. Introduction Porcellanite is a diagenetic rock that occurs in both unconsolidated and lithified sediments. Porcellanites have been recovered in Mesozoic to Tertiary sediment sections of all oceans (e.g., Riech * Corresponding author. Present address: Department of Geological Sciences, University College London, Gower Street, London WC1E 6BT, UK. Tel.: +44 ( 171) 387 7050 ext 2362; Fax: +44 ( 171) 388 7614; E-mail: [email protected] 0025-3227/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0 02 5- 3 22 7 ( 9 7 ) 00 04 6- 7
and von Rad, 1979). With a few exceptions, porcellanites of Plio–Pleistocene age have been described to occur only in deposits of the Southern Ocean (Bohrmann et al., 1990, 1994). Such porcellanites were found at the Kerguelen Plateau ( Weaver and Wise, 1973), Maud Rise (Barker et al., 1988; Bohrmann et al., 1992 ) and at the Southwest Indian Ridge (Bohrmann et al., 1990 ). Usually, porcellanite horizons in Antarctic marine sediments have been observed in unconsolidated diatom ooze sequences (biogenic opal-A-domi-
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nated), 5–15 m below the seafloor (Bohrmann et al., 1990). Such porcellanites have an almost entirely monomineralic composition of opal-CT, forming an extensively cemented rock. According to previous publications ( Heath, 1973; Wise and Weaver, 1973; von Rad et al., 1978; Riech and von Rad, 1979), porcellanite genesis over time requires biogenic silica (opal-A) as a source and the following environmental conditions: an increased but still low temperature (18–56°C; Pisciotto, 1981) and a specific host-rock facies. However, oxygen isotope data of Botz and Bohrmann (1991) showed that young Antarctic porcellanites were formed as very low-temperature precipitates (0–4°C ). Nondestructive, fast and high-resolution methods for investigating unsplit sediment cores in plastic liners are helpful and crucial for several reasons. They give the unique chance of getting an idea about the structure and properties of a core a short time after coring and before core splitting. Core splitting requires time, space and manpower and is therefore carried out on research ships only to a limited extent. Cores remaining unsplit during a cruise only support survey and coring planning in case they where investigated with nondestructive methods. Core splitting can influence and alter the original structure in sediment cores, especially in cores with consolidated or hard rock components (e.g., dropstones). Sampling and sample measurements after core splitting are usually performed with spacings of 5 to 10 cm. Small-scale features appearing in logs from spatial high resolving measurements are often invisible in results from sample measurements. Correlation between several cores along profiles becomes much easier using those high-resolution core logs. In this paper we present results from physical property measurements and sedimentological work on site PS2089 on the Southwest Indian Ridge, South Atlantic ( 53°11.3∞S, 5°19.8∞E; water depth 2615 m), northeast of Bouvet Island (Fig. 1 ). Two piston cores were recovered (core length of 8.46 m for PS2089-1 and 10.46 m for PS2089-2; diameter 90 mm) during cruise ANT-IX/4 of the R.V. Polarstern in 1991 ( Bathmann et al., 1992). The distance between the position ships at the moment
Fig. 1. Location of site PS2089 in the Southern Atlantic Ocean. The seafloor topography is indicated by dierent shadings as shown in the legend.
of coring for both sites was about 60 m. The porcellanite bed is intercalated in sediments of Pleistocene age ( Bohrmann et al., 1994 ) that vary from diatom ooze to terrigenous mud and sandy mud.
2. Methods 2.1. Magnetic susceptibility Magnetic susceptibility measurements were carried out using a Bartington MS2C susceptibility meter, integrated in an automatic core logging system that was constructed at the Alfred Wegener Institute. The susceptibility values are not absolute volume or mass susceptibilities, because the data
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were not deconvolved and calibrated to the coil diameter. The measurements were completed aboard ship immediately after the cores were recovered and warmed up from in-situ temperature to +20°C. 2.2. Electrical resistivity Electrical resistivity (and derived porosity) was determined nondestructively using an inductive method (Gerland et al., 1993). The electrical measurements presented in this paper were also performed aboard ship with the automated core logging system (Gerland et al., 1993), which was also used for the susceptibility measurements. For both methods, susceptibility and resistivity, the vertical resolution of the measured values is limited by the dimensions of the coil. 2.3. Wet bulk density Density was determined after the expedition in a rapid and nondestructive way by measuring the attenuation of a gamma-ray beam that passed through the unsplit sediment core. This method is well known because it is used on DSDP/ODPcruises (GRAPE, Gamma-Ray Attenuation Porosity Evaluator, Evans, 1965). Following the same physical principle, a density measurement system, containing a Lo¨el Densitometer AW-2, was designed and manufactured at the Alfred Wegener Institute (Gerland and Villinger, 1995). The system uses a radioactive 137Cs isotope source (energy 662 keV, activity 0.7 Ci ). The gamma-ray beam collimation provides a spatial resolution of better than 1 cm, the accuracy is better than ±1%. Density measurements on Core PS2089-2 were performed using conventional determination (pycnometer). 2.4. Computer-aided X-ray tomography (CT) CT has predominantly been used for the nondestructive visualisation of the internal organs of the human body. CT images aid somewhat interpreting the physical properties measured. Some documented examples exist of X-ray CT being applied
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to rock and sediment samples (e.g., Vinegar, 1986; Holler and Ko¨gler, 1990 ). The image resulting from an X-ray CT survey is closely related to the density structure of the object being investigated. The systems works with a rotating X-ray tube (125 keV energy) and a pad of 704 detectors. 1440 rotation positions require 14 s measurement time. By using 106 attenuation values measured and an inversion algorithm, a CT image is gained. The CT system used here is a Siemens Somatom DR with a 125 keV X-ray tube, at a hospital in Bremerhaven. 2.5. Sediment echosounding The system ( Krupp Atlas Parasound, Grant and Schreiber, 1990) uses the parametric eect by first producing two acoustic waves of about 20 kHz. The frequency dierence between both waves can be adjusted from 2.5 kHz to 5.5 kHz. Waves are emitted and recorded at the ship, maximum penetration depth is in the range of 200 m below seafloor. The vertical resolution of the reproduced sediment structures can be better than 0.1 m ( Max et al., 1992), but varies according to the characteristics of the seafloor. The reflected signal strength depends on the impedance–depth–structure of the seafloor (product of wet bulk density and compressional wave velocity). However, constructive and destructive interference phenomena can lead to displacement and distortion of reflection signals (Rostek et al., 1991).
3. Measurement results 3.1. Sediment characteristics The porcellanite bed and the surrounding diatom ooze can be clearly distinguished after the cores were split (see Core PS2089-2, Fig. 2). The porcellanites from both cores are of Pleistocene age. The surrounding sediments belong to the lower part of the interglacial isotope stage 11 (Bohrmann et al., 1994 ). The porcellanite itself is characterized by pure opal-CT; a very high concen-
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Fig. 2. Photograph of the broken pieces of the porcellanite layer in Core PS2089-2, intercalated in diatom ooze sediments.
tration of biogenic opal-A ( 95–99%) and low amounts of detrital quartz, clay minerals, and carbonate are the components of the host diatom ooze layers (Botz and Bohrmann, 1991 ). Diatomaceous mud and diatom ooze are the characteristic sediment types of Core PS2089-1 and Core PS2089-2 (Figs. 3 and 4 ). Biogenic siliceous skeletal particle contents (mineralogical opal-A) determine these sediment types (Bohrmann and Petschick, 1992). High contents of diatom frustules within the ooze dilutes other sediment components, such as carbonate and quartz ( Bohrmann et al., 1994; Figs. 3 and 4 ). The sediment types of both cores ( Bohrmann and Petschick, 1992; Bohrmann, 1992 ) are similar, except for the upper three meters of Core PS2089-1
(0–2.31 m diatomaceous mud, 2.31–3.07 m diatom ooze), which were not recovered at PS2089-2. From top to bottom, diatomaceous (sandy) mud, diatom ooze that is twice as thick in Core PS2089-1 compared to Core PS2089-2, and diatomaceous sandy mud also thicker in Core PS2089-2, were recovered. The subjacent, nearly pure diatom ooze (7.62–7.80 m in Core PS2089-1, 5.85–6.28 m in Core PS2089-2) contains a 3–5 cm thick porcellanite horizon (7.71–7.74 m in Core PS2089-1, 6.02–6.07 m in Core PS2089-2) which is broken into numerous pieces. In Core PS2089-2, additional deeper layers of diatomaceous mud to sandy mud and diatom ooze were recovered. In general, dierences in layer thicknesses between both cores could be due to varying sedimentation because of
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Fig. 3. Logs of lithology, magnetic susceptibility, electrical resistivity, wet bulk density ( gamma-ray attenuation measurement), porosity, and content of carbonate for Core PS2089-1. The depth interval including the porcellanite layer is marked by the shaded area over all logs.
the small-scale relief of the seafloor and/or core deformation from the piston corer. 3.2. Magnetic susceptibility The porcellanite horizon cannot be directly detected by susceptibility measurements, presumably because there is no significant dierence in abundance or type of magnetic minerals in the porcellanite and in the surrounding sediment. Nevertheless, the susceptibility logs give important hints about the porcellanite indirectly, which will be discussed later. Variations in the entire susceptibility logs of both cores (second panel in Figs. 3 and 4) delineate zones of dierent characteristics and allow for the correlation of the cores using significant anomaly peaks ( Kru¨ger and Petschick, 1992). The correlation shows that the upper 3 m of Core PS2089-1 are missing in Core PS2089-2
(Fig. 5 ). The dierent susceptibility characteristics also indicate several types of diatomaceous sediments. The higher the amount of opal, the lower the magnetic susceptibility (e.g., Core PS2089-2 from 3.20 m to 5.60 m). Diatomaceous sandy mud lithologies at the top and bottom of the cores are characterized by relatively high average susceptibility values ( 7×10−6 to 10×10−6 SI ) due to high amounts of terrigenous sediment components (e.g., Core PS2089-1 from 3.20 to 6.40 m), whereas the ooze is indicated by low susceptibility (average 10×10−5 SI ). 3.3. Electrical resistivity The porcellanite horizons in both cores appear in the electrical resistivity profiles as positive resistivity anomalies (third panel in Figs. 3 and 4 ). This reflects the large dierence in water contents
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Fig. 4. Logs of lithology (Bohrmann et al., 1994), magnetic susceptibility, electrical resistivity, wet bulk density measurements on samples, porosity, content of carbonate and opal-A ( Bohrmann et al., 1994 ), and isotope stratigraphy ( Bohrmann et al., 1994) for Core PS2089-2. The depth interval including the porcellanite layer is marked by the shaded area over all logs.
between the porcellanite ( low) and the surrounding diatom ooze (high). Diatom ooze is a sediment with extremely high porosity. This results in very low electrical resistivity (0.32–0.35 V m, Figs. 3 and 4 ). For the diatomaceous sediments, the electrical resistivity correlates with the amount of terrigenous components (increasing resistivity from ooze to mud and sandy mud ). The depth intervals with diatomaceous mud and sandy mud (relatively low porosity) can be indicated by resistivities in the range of 0.40±0.05 V m. The porcellanite horizon itself shows a measured resistivity of about 0.48 V m (Core PS2089-1) and 0.45 V m (Core PS2089-2). Due to the limited spatial resolution, the entire width of the porcellanite horizon is not represented by the resistivity log. Inspecting the resistivity log from 7.60 to 8.10 m of Core
PS2089-1 ( Fig. 6, middle column), this becomes obvious. 3.4. Wet bulk density The density log for Core PS2089-2 ( Fig. 4, forth panel) shows data from conventional density determination ( pycnometer). In contrast to Core PS2089-2 which was split aboard, Core PS2089-1 was split ashore after the expedition, so wet bulk density and CT surveys were carried out on the undisturbed core sections. The dierent diatomaceous sediment types can be well distinguished on the wet bulk density profile ( Fig. 3, fourth panel ). The high spatial resolution of the density measurements reveals both gradual and rapid changes in sediments. Focusing more in detail to the depth
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Fig. 6. Section of Core PS2089-1 where the porcellanite horizon occurs: magnetic susceptibility, electrical resistivity, and wet bulk density (gamma-ray attenuation measurement) profiles. The depth interval including the porcellanite layer is marked by the shaded area.
Fig. 5. Susceptibility logs for Cores PS2089-1 and PS2089-2 (after Kru¨ger and Petschick, 1992). It is obvious, that the upper part of Core PS2089-1 from 0 to about 3 m core depth is missing in Core PS2089-2. This may be caused by drifting of the ship between both stations or by dierent penetration processes during coring. Significant susceptibility signals in both logs, indicated by the connecting lines between them, were used for correlation. The isotope stages for Core PS2089-2 are mentioned on the log’s right side. The depth interval where the porcellanite occurs is marked by shadings.
where the porcellanite occurs in Core PS2089-1 ( Fig. 6), the density information marks the porcellanite layer between 7.70 m and 7.75 m more accurate than the resistivity log. However, the absolute density measured by gamma-ray attenuation ( 1.43 g/cm3) does not represent the real density of the porcellanite, which was determined by conventional measurement with a pycnometer after core splitting (1.70 g/cm3). This dierence is due to the
presence diatom ooze between the fragments of porcellanite. For comparison, grain density values measured on other Pliocene porcellanites from the Maud Rise and Kerguelen Plateau reveal values between 1.94 and 2.29 g/cm3 (G. Bohrmann, unpubl. data). As expected, the density log also mirrors the dierent sedimentological intervals apparent in the resistivity data. A direct comparison between wet bulk density data from gamma-ray attenuation measurements and echosounding records, using impedance and model calculations, would be dicult because no ultrasonic P-wave velocity measurements on the cores were performed for impedance calculations. Furthermore, as already mentioned, the absolute density of the pristine porcellanite horizon is not represented by the (uncorrected) wet bulk density profile from gamma-ray attenuation measurements. Additionally, there still exists uncertainty about the amount of loss of sediment and core compression during coring. 3.5. Computer-aided X-ray tomography A CT survey was performed on the unsplit Core PS2089-1. Fig. 7 shows an X-ray longitudinal section and two CT-scans (cross-sections) of the por-
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Fig. 7. X-ray longitudinal section of the upper part of the liner which contains the porcellanite of Core PS2089-1 and two tomographic cross-sections through the porcellanite (scan A and B). The depths where the cross-sections were taken are marked by the respective arrows. High densities of sediment are represented by bright shadings, low densities are represented by dark shadings. The scale bar on the right side of each cross-section scan represents 5 cm length. In scan B, an internal linear structure (see text) is marked by the dashed lines.
cellanite horizon inside the unsplit core. Nearly half (upper 0.5 m) of this 1-m-core section is shown. The cross-sections demonstrate the brittle nature of the porcellanite. Sharp-edged pieces of hard rock are clearly visible in both cross-sections, due to their strong density contrast compared to
the surrounding sediment. In the second crosssection (scan B), an internal linear structure is found on two large pieces of the porcellanite, indicating that these fragments were probably originally joined and later broken and separated by the coring process (see dashed lines on scan B of
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Fig. 7 ). The deformation of the horizon, especially at the margin ring of the core as seen in the longitudinal section, indicates a great deal of stress during the coring process, too. The porcellanite horizon is slightly less than 5 cm thick, although in the core it can span up to 8 cm due to its concave shape. The cross-sections clearly illustrate why the density determined by gamma-ray attenuation and by conventional measurements shows dierent results. Dependent on the core orientation, at least about 10% of the path of the gamma-ray beam runs through diatom ooze. This reduces the attenuation and apparent density returned by the system. 3.6. Sediment echosounding The character of the seafloor and the shallow subbottom at site PS2089 are shown on the echosounding records (Fig. 8 ). The porcellanite hori-
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zon is represented by a strong reflection at an apparent depth of about 9 m ( PS2089-1) and 8 m (PS2089-2) below seafloor. These values are calculated using an average wave velocity of 1500 m/s. The apparent echosounder depth of the porcellanite horizon diers from the respective depth in the cores ( 6.03 m and 7.73 m, see Figs. 3 and 4 ). The reason for this dierence is perhaps partially due to a true wave velocity smaller than 1500 m/s. However, deformation or loss of sediment during coring might be more probable. The piston cores may have been compressed to some degree relative to their in-situ state by the sampling procedure. By comparing echosounding records and core data, the corresponding core compression for both cores might have reduced the core lengths to about 67% and 97% of their in-situ situation, respectively. Core compression is a well known problem, but it is dicult to account for the absolute loss of core length quantitatively (e.g., Parker and Sills, 1990;
Fig. 8. Echograms from the Parasound survey on sites PS2089-1 and PS2089-2. The vertical depth scale is calculated using an average wave velocity of 1500 m/s. The depth where the porcellanite reflection occurs is marked with an arrow for each panel.
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Crusius and Anderson, 1991). However, the most likeliest reason for the dierence might be loss of the uppermost sediment section during penetration of the corer, which is supported by the fact that about 3 m core are missing in core PS2089-1 relative to PS2089-2. The seismic reflections in the upper 10 m of the sediment column, apart from the porcellanite, may correspond with lithologic changes: diatom oozes, muds, and sandy muds. The second strong subbottom reflection at a depth of about 15 m below the seafloor in both recordings, which was not reached by coring, could represent a second porcellanite horizon.
4. Conclusions
Fig. 9. (a) Magnetic susceptibility and electrical resistivity, plotted together versus core depth for Core PS2089-2. The depth, at which the porcellanite layer appears, is marked by the shaded area. ( b) Electrical resistivity versus magnetic susceptibility for Core PS2089-2. Data are plotted for depth values where resistivity measurements were undertaken ( 2 cm spacing). The shading indicates the area where all data from the porcellanite horizon can be found.
With a combination of both resistivity and susceptibility measurement the porcellanite can be identified clearly. Marine sediments which are characterized by relatively lower porosity, i.e., relatively higher resistivity, are composed of sediment material with high contents of terrigenous components relative to biogenic components (e.g., Gerland et al., 1993). This results usually in an increase in susceptibility, as is true for changes from diatom ooze to mud in both cores. The fact that porcellanite is not associated with an increase in magnetic susceptibility allows recognition of a
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porcellanite layer using susceptibility and resistivity data (Fig. 9 ). Most data show good positive correlation between both parameters, except the few points corresponding to the porcellanite (shaded area in each subfigure).
Acknowledgements We are grateful to H. Villinger of the University of Bremen for fruitful discussion. We also thank T. Kru¨ger of the AWI who carried out the resistivity and susceptibility measurements aboard R.V. Polarstern during the 1991 cruise ANT-IX/4 and the sta of the Reinkenheide hospital (Bremerhaven) for making the tomographic investigations possible. Helpful work by A. Abelmann, E. Dunker, R. Gersonde, H. Grobe, H. Hinze, W. Jokat, C. Mayer and H. Miller from the AWI is also acknowledged. Thanks also to the master and crew of the R.V. Polarstern for their support during the sediment coring. Constructive criticism by D. Fu¨tterer, P. Blum and two anonymous reviewers is gratefully acknowledged. Financial support was provided by the Deutsche Forschungsgemeinschaft ( Ku 683/1). This is Publication XXX of the Alfred Wegener Institute for Polar and Marine Research (Bremerhaven) and Contribution XXXX of Special Research Project SFB 261.
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Short communication
Physical properties of a porcellanite layer (Southwest Indian Ridge) constrained by geophysical logging Sebastian Gerland a,*, Gerhard Kuhn a, Gerhard Bohrmann b a Alfred Wegener Institute for Polar and Marine Research, Postfach 120161, 27515 Bremerhaven, Germany b GEOMAR, Research Center for Marine Geosciences, Christian Albrechts University, Wischhofstr. 1–3, 24148 Kiel, Germany Received 7 December 1995; accepted 2 April 1997
Abstract A distinct porcellanite layer from the Southwest Indian Ridge intercalated in Pleistocene diatom ooze was studied using nondestructive physical property measurements and sedimentological data. This bed was sampled by two piston cores at a water depth of 2615 m. The 3–5 cm thick porcellanite layer appears in the cores at a depth of 6.03 m (Core PS2089-2) and 7.73 m (Core PS2089-1) below the seafloor. Due to its characteristic physical properties the porcellanite bed can be detected with core measurements, and its distribution and lateral extent mapped with echosounding. The physical index properties, wet bulk density and electrical resistivity, increase significantly across this bed. Magnetic susceptibility is used to compare the lithological units of both cores and to distinguish whether resistivity anomalies are caused by a higher amount of terrigenous components or by the presence of porcellanite. The porcellanite has the special characteristic to aect a positive anomaly in resistivity but not in susceptibility. Most marine sediments, in contrast, show a positive correlation of magnetic susceptibility versus electrical resistivity; therefore a combination of electrical resistivity and magnetic susceptibility logs yields a definite detection of the porcellanite bed. Images from the X-ray CT survey indicate that the porcellanite is lithified and brittle and fragmented when the piston corer penetrated the bed. © 1997 Elsevier Science B.V. Keywords: physical properties; porcellanite; marine sediments; nondestructive methods; high-resolution methods; cores
1. Introduction Porcellanite is a diagenetic rock that occurs in both unconsolidated and lithified sediments. Porcellanites have been recovered in Mesozoic to Tertiary sediment sections of all oceans (e.g., Riech * Corresponding author. Present address: Department of Geological Sciences, University College London, Gower Street, London WC1E 6BT, UK. Tel.: +44 ( 171) 387 7050 ext 2362; Fax: +44 ( 171) 388 7614; E-mail: [email protected] 0025-3227/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0 02 5- 3 22 7 ( 9 7 ) 00 04 6- 7
and von Rad, 1979). With a few exceptions, porcellanites of Plio–Pleistocene age have been described to occur only in deposits of the Southern Ocean (Bohrmann et al., 1990, 1994). Such porcellanites were found at the Kerguelen Plateau ( Weaver and Wise, 1973), Maud Rise (Barker et al., 1988; Bohrmann et al., 1992 ) and at the Southwest Indian Ridge (Bohrmann et al., 1990 ). Usually, porcellanite horizons in Antarctic marine sediments have been observed in unconsolidated diatom ooze sequences (biogenic opal-A-domi-
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nated), 5–15 m below the seafloor (Bohrmann et al., 1990). Such porcellanites have an almost entirely monomineralic composition of opal-CT, forming an extensively cemented rock. According to previous publications ( Heath, 1973; Wise and Weaver, 1973; von Rad et al., 1978; Riech and von Rad, 1979), porcellanite genesis over time requires biogenic silica (opal-A) as a source and the following environmental conditions: an increased but still low temperature (18–56°C; Pisciotto, 1981) and a specific host-rock facies. However, oxygen isotope data of Botz and Bohrmann (1991) showed that young Antarctic porcellanites were formed as very low-temperature precipitates (0–4°C ). Nondestructive, fast and high-resolution methods for investigating unsplit sediment cores in plastic liners are helpful and crucial for several reasons. They give the unique chance of getting an idea about the structure and properties of a core a short time after coring and before core splitting. Core splitting requires time, space and manpower and is therefore carried out on research ships only to a limited extent. Cores remaining unsplit during a cruise only support survey and coring planning in case they where investigated with nondestructive methods. Core splitting can influence and alter the original structure in sediment cores, especially in cores with consolidated or hard rock components (e.g., dropstones). Sampling and sample measurements after core splitting are usually performed with spacings of 5 to 10 cm. Small-scale features appearing in logs from spatial high resolving measurements are often invisible in results from sample measurements. Correlation between several cores along profiles becomes much easier using those high-resolution core logs. In this paper we present results from physical property measurements and sedimentological work on site PS2089 on the Southwest Indian Ridge, South Atlantic ( 53°11.3∞S, 5°19.8∞E; water depth 2615 m), northeast of Bouvet Island (Fig. 1 ). Two piston cores were recovered (core length of 8.46 m for PS2089-1 and 10.46 m for PS2089-2; diameter 90 mm) during cruise ANT-IX/4 of the R.V. Polarstern in 1991 ( Bathmann et al., 1992). The distance between the position ships at the moment
Fig. 1. Location of site PS2089 in the Southern Atlantic Ocean. The seafloor topography is indicated by dierent shadings as shown in the legend.
of coring for both sites was about 60 m. The porcellanite bed is intercalated in sediments of Pleistocene age ( Bohrmann et al., 1994 ) that vary from diatom ooze to terrigenous mud and sandy mud.
2. Methods 2.1. Magnetic susceptibility Magnetic susceptibility measurements were carried out using a Bartington MS2C susceptibility meter, integrated in an automatic core logging system that was constructed at the Alfred Wegener Institute. The susceptibility values are not absolute volume or mass susceptibilities, because the data
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were not deconvolved and calibrated to the coil diameter. The measurements were completed aboard ship immediately after the cores were recovered and warmed up from in-situ temperature to +20°C. 2.2. Electrical resistivity Electrical resistivity (and derived porosity) was determined nondestructively using an inductive method (Gerland et al., 1993). The electrical measurements presented in this paper were also performed aboard ship with the automated core logging system (Gerland et al., 1993), which was also used for the susceptibility measurements. For both methods, susceptibility and resistivity, the vertical resolution of the measured values is limited by the dimensions of the coil. 2.3. Wet bulk density Density was determined after the expedition in a rapid and nondestructive way by measuring the attenuation of a gamma-ray beam that passed through the unsplit sediment core. This method is well known because it is used on DSDP/ODPcruises (GRAPE, Gamma-Ray Attenuation Porosity Evaluator, Evans, 1965). Following the same physical principle, a density measurement system, containing a Lo¨el Densitometer AW-2, was designed and manufactured at the Alfred Wegener Institute (Gerland and Villinger, 1995). The system uses a radioactive 137Cs isotope source (energy 662 keV, activity 0.7 Ci ). The gamma-ray beam collimation provides a spatial resolution of better than 1 cm, the accuracy is better than ±1%. Density measurements on Core PS2089-2 were performed using conventional determination (pycnometer). 2.4. Computer-aided X-ray tomography (CT) CT has predominantly been used for the nondestructive visualisation of the internal organs of the human body. CT images aid somewhat interpreting the physical properties measured. Some documented examples exist of X-ray CT being applied
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to rock and sediment samples (e.g., Vinegar, 1986; Holler and Ko¨gler, 1990 ). The image resulting from an X-ray CT survey is closely related to the density structure of the object being investigated. The systems works with a rotating X-ray tube (125 keV energy) and a pad of 704 detectors. 1440 rotation positions require 14 s measurement time. By using 106 attenuation values measured and an inversion algorithm, a CT image is gained. The CT system used here is a Siemens Somatom DR with a 125 keV X-ray tube, at a hospital in Bremerhaven. 2.5. Sediment echosounding The system ( Krupp Atlas Parasound, Grant and Schreiber, 1990) uses the parametric eect by first producing two acoustic waves of about 20 kHz. The frequency dierence between both waves can be adjusted from 2.5 kHz to 5.5 kHz. Waves are emitted and recorded at the ship, maximum penetration depth is in the range of 200 m below seafloor. The vertical resolution of the reproduced sediment structures can be better than 0.1 m ( Max et al., 1992), but varies according to the characteristics of the seafloor. The reflected signal strength depends on the impedance–depth–structure of the seafloor (product of wet bulk density and compressional wave velocity). However, constructive and destructive interference phenomena can lead to displacement and distortion of reflection signals (Rostek et al., 1991).
3. Measurement results 3.1. Sediment characteristics The porcellanite bed and the surrounding diatom ooze can be clearly distinguished after the cores were split (see Core PS2089-2, Fig. 2). The porcellanites from both cores are of Pleistocene age. The surrounding sediments belong to the lower part of the interglacial isotope stage 11 (Bohrmann et al., 1994 ). The porcellanite itself is characterized by pure opal-CT; a very high concen-
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Fig. 2. Photograph of the broken pieces of the porcellanite layer in Core PS2089-2, intercalated in diatom ooze sediments.
tration of biogenic opal-A ( 95–99%) and low amounts of detrital quartz, clay minerals, and carbonate are the components of the host diatom ooze layers (Botz and Bohrmann, 1991 ). Diatomaceous mud and diatom ooze are the characteristic sediment types of Core PS2089-1 and Core PS2089-2 (Figs. 3 and 4 ). Biogenic siliceous skeletal particle contents (mineralogical opal-A) determine these sediment types (Bohrmann and Petschick, 1992). High contents of diatom frustules within the ooze dilutes other sediment components, such as carbonate and quartz ( Bohrmann et al., 1994; Figs. 3 and 4 ). The sediment types of both cores ( Bohrmann and Petschick, 1992; Bohrmann, 1992 ) are similar, except for the upper three meters of Core PS2089-1
(0–2.31 m diatomaceous mud, 2.31–3.07 m diatom ooze), which were not recovered at PS2089-2. From top to bottom, diatomaceous (sandy) mud, diatom ooze that is twice as thick in Core PS2089-1 compared to Core PS2089-2, and diatomaceous sandy mud also thicker in Core PS2089-2, were recovered. The subjacent, nearly pure diatom ooze (7.62–7.80 m in Core PS2089-1, 5.85–6.28 m in Core PS2089-2) contains a 3–5 cm thick porcellanite horizon (7.71–7.74 m in Core PS2089-1, 6.02–6.07 m in Core PS2089-2) which is broken into numerous pieces. In Core PS2089-2, additional deeper layers of diatomaceous mud to sandy mud and diatom ooze were recovered. In general, dierences in layer thicknesses between both cores could be due to varying sedimentation because of
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Fig. 3. Logs of lithology, magnetic susceptibility, electrical resistivity, wet bulk density ( gamma-ray attenuation measurement), porosity, and content of carbonate for Core PS2089-1. The depth interval including the porcellanite layer is marked by the shaded area over all logs.
the small-scale relief of the seafloor and/or core deformation from the piston corer. 3.2. Magnetic susceptibility The porcellanite horizon cannot be directly detected by susceptibility measurements, presumably because there is no significant dierence in abundance or type of magnetic minerals in the porcellanite and in the surrounding sediment. Nevertheless, the susceptibility logs give important hints about the porcellanite indirectly, which will be discussed later. Variations in the entire susceptibility logs of both cores (second panel in Figs. 3 and 4) delineate zones of dierent characteristics and allow for the correlation of the cores using significant anomaly peaks ( Kru¨ger and Petschick, 1992). The correlation shows that the upper 3 m of Core PS2089-1 are missing in Core PS2089-2
(Fig. 5 ). The dierent susceptibility characteristics also indicate several types of diatomaceous sediments. The higher the amount of opal, the lower the magnetic susceptibility (e.g., Core PS2089-2 from 3.20 m to 5.60 m). Diatomaceous sandy mud lithologies at the top and bottom of the cores are characterized by relatively high average susceptibility values ( 7×10−6 to 10×10−6 SI ) due to high amounts of terrigenous sediment components (e.g., Core PS2089-1 from 3.20 to 6.40 m), whereas the ooze is indicated by low susceptibility (average 10×10−5 SI ). 3.3. Electrical resistivity The porcellanite horizons in both cores appear in the electrical resistivity profiles as positive resistivity anomalies (third panel in Figs. 3 and 4 ). This reflects the large dierence in water contents
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Fig. 4. Logs of lithology (Bohrmann et al., 1994), magnetic susceptibility, electrical resistivity, wet bulk density measurements on samples, porosity, content of carbonate and opal-A ( Bohrmann et al., 1994 ), and isotope stratigraphy ( Bohrmann et al., 1994) for Core PS2089-2. The depth interval including the porcellanite layer is marked by the shaded area over all logs.
between the porcellanite ( low) and the surrounding diatom ooze (high). Diatom ooze is a sediment with extremely high porosity. This results in very low electrical resistivity (0.32–0.35 V m, Figs. 3 and 4 ). For the diatomaceous sediments, the electrical resistivity correlates with the amount of terrigenous components (increasing resistivity from ooze to mud and sandy mud ). The depth intervals with diatomaceous mud and sandy mud (relatively low porosity) can be indicated by resistivities in the range of 0.40±0.05 V m. The porcellanite horizon itself shows a measured resistivity of about 0.48 V m (Core PS2089-1) and 0.45 V m (Core PS2089-2). Due to the limited spatial resolution, the entire width of the porcellanite horizon is not represented by the resistivity log. Inspecting the resistivity log from 7.60 to 8.10 m of Core
PS2089-1 ( Fig. 6, middle column), this becomes obvious. 3.4. Wet bulk density The density log for Core PS2089-2 ( Fig. 4, forth panel) shows data from conventional density determination ( pycnometer). In contrast to Core PS2089-2 which was split aboard, Core PS2089-1 was split ashore after the expedition, so wet bulk density and CT surveys were carried out on the undisturbed core sections. The dierent diatomaceous sediment types can be well distinguished on the wet bulk density profile ( Fig. 3, fourth panel ). The high spatial resolution of the density measurements reveals both gradual and rapid changes in sediments. Focusing more in detail to the depth
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Fig. 6. Section of Core PS2089-1 where the porcellanite horizon occurs: magnetic susceptibility, electrical resistivity, and wet bulk density (gamma-ray attenuation measurement) profiles. The depth interval including the porcellanite layer is marked by the shaded area.
Fig. 5. Susceptibility logs for Cores PS2089-1 and PS2089-2 (after Kru¨ger and Petschick, 1992). It is obvious, that the upper part of Core PS2089-1 from 0 to about 3 m core depth is missing in Core PS2089-2. This may be caused by drifting of the ship between both stations or by dierent penetration processes during coring. Significant susceptibility signals in both logs, indicated by the connecting lines between them, were used for correlation. The isotope stages for Core PS2089-2 are mentioned on the log’s right side. The depth interval where the porcellanite occurs is marked by shadings.
where the porcellanite occurs in Core PS2089-1 ( Fig. 6), the density information marks the porcellanite layer between 7.70 m and 7.75 m more accurate than the resistivity log. However, the absolute density measured by gamma-ray attenuation ( 1.43 g/cm3) does not represent the real density of the porcellanite, which was determined by conventional measurement with a pycnometer after core splitting (1.70 g/cm3). This dierence is due to the
presence diatom ooze between the fragments of porcellanite. For comparison, grain density values measured on other Pliocene porcellanites from the Maud Rise and Kerguelen Plateau reveal values between 1.94 and 2.29 g/cm3 (G. Bohrmann, unpubl. data). As expected, the density log also mirrors the dierent sedimentological intervals apparent in the resistivity data. A direct comparison between wet bulk density data from gamma-ray attenuation measurements and echosounding records, using impedance and model calculations, would be dicult because no ultrasonic P-wave velocity measurements on the cores were performed for impedance calculations. Furthermore, as already mentioned, the absolute density of the pristine porcellanite horizon is not represented by the (uncorrected) wet bulk density profile from gamma-ray attenuation measurements. Additionally, there still exists uncertainty about the amount of loss of sediment and core compression during coring. 3.5. Computer-aided X-ray tomography A CT survey was performed on the unsplit Core PS2089-1. Fig. 7 shows an X-ray longitudinal section and two CT-scans (cross-sections) of the por-
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Fig. 7. X-ray longitudinal section of the upper part of the liner which contains the porcellanite of Core PS2089-1 and two tomographic cross-sections through the porcellanite (scan A and B). The depths where the cross-sections were taken are marked by the respective arrows. High densities of sediment are represented by bright shadings, low densities are represented by dark shadings. The scale bar on the right side of each cross-section scan represents 5 cm length. In scan B, an internal linear structure (see text) is marked by the dashed lines.
cellanite horizon inside the unsplit core. Nearly half (upper 0.5 m) of this 1-m-core section is shown. The cross-sections demonstrate the brittle nature of the porcellanite. Sharp-edged pieces of hard rock are clearly visible in both cross-sections, due to their strong density contrast compared to
the surrounding sediment. In the second crosssection (scan B), an internal linear structure is found on two large pieces of the porcellanite, indicating that these fragments were probably originally joined and later broken and separated by the coring process (see dashed lines on scan B of
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Fig. 7 ). The deformation of the horizon, especially at the margin ring of the core as seen in the longitudinal section, indicates a great deal of stress during the coring process, too. The porcellanite horizon is slightly less than 5 cm thick, although in the core it can span up to 8 cm due to its concave shape. The cross-sections clearly illustrate why the density determined by gamma-ray attenuation and by conventional measurements shows dierent results. Dependent on the core orientation, at least about 10% of the path of the gamma-ray beam runs through diatom ooze. This reduces the attenuation and apparent density returned by the system. 3.6. Sediment echosounding The character of the seafloor and the shallow subbottom at site PS2089 are shown on the echosounding records (Fig. 8 ). The porcellanite hori-
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zon is represented by a strong reflection at an apparent depth of about 9 m ( PS2089-1) and 8 m (PS2089-2) below seafloor. These values are calculated using an average wave velocity of 1500 m/s. The apparent echosounder depth of the porcellanite horizon diers from the respective depth in the cores ( 6.03 m and 7.73 m, see Figs. 3 and 4 ). The reason for this dierence is perhaps partially due to a true wave velocity smaller than 1500 m/s. However, deformation or loss of sediment during coring might be more probable. The piston cores may have been compressed to some degree relative to their in-situ state by the sampling procedure. By comparing echosounding records and core data, the corresponding core compression for both cores might have reduced the core lengths to about 67% and 97% of their in-situ situation, respectively. Core compression is a well known problem, but it is dicult to account for the absolute loss of core length quantitatively (e.g., Parker and Sills, 1990;
Fig. 8. Echograms from the Parasound survey on sites PS2089-1 and PS2089-2. The vertical depth scale is calculated using an average wave velocity of 1500 m/s. The depth where the porcellanite reflection occurs is marked with an arrow for each panel.
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Crusius and Anderson, 1991). However, the most likeliest reason for the dierence might be loss of the uppermost sediment section during penetration of the corer, which is supported by the fact that about 3 m core are missing in core PS2089-1 relative to PS2089-2. The seismic reflections in the upper 10 m of the sediment column, apart from the porcellanite, may correspond with lithologic changes: diatom oozes, muds, and sandy muds. The second strong subbottom reflection at a depth of about 15 m below the seafloor in both recordings, which was not reached by coring, could represent a second porcellanite horizon.
4. Conclusions
Fig. 9. (a) Magnetic susceptibility and electrical resistivity, plotted together versus core depth for Core PS2089-2. The depth, at which the porcellanite layer appears, is marked by the shaded area. ( b) Electrical resistivity versus magnetic susceptibility for Core PS2089-2. Data are plotted for depth values where resistivity measurements were undertaken ( 2 cm spacing). The shading indicates the area where all data from the porcellanite horizon can be found.
With a combination of both resistivity and susceptibility measurement the porcellanite can be identified clearly. Marine sediments which are characterized by relatively lower porosity, i.e., relatively higher resistivity, are composed of sediment material with high contents of terrigenous components relative to biogenic components (e.g., Gerland et al., 1993). This results usually in an increase in susceptibility, as is true for changes from diatom ooze to mud in both cores. The fact that porcellanite is not associated with an increase in magnetic susceptibility allows recognition of a
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porcellanite layer using susceptibility and resistivity data (Fig. 9 ). Most data show good positive correlation between both parameters, except the few points corresponding to the porcellanite (shaded area in each subfigure).
Acknowledgements We are grateful to H. Villinger of the University of Bremen for fruitful discussion. We also thank T. Kru¨ger of the AWI who carried out the resistivity and susceptibility measurements aboard R.V. Polarstern during the 1991 cruise ANT-IX/4 and the sta of the Reinkenheide hospital (Bremerhaven) for making the tomographic investigations possible. Helpful work by A. Abelmann, E. Dunker, R. Gersonde, H. Grobe, H. Hinze, W. Jokat, C. Mayer and H. Miller from the AWI is also acknowledged. Thanks also to the master and crew of the R.V. Polarstern for their support during the sediment coring. Constructive criticism by D. Fu¨tterer, P. Blum and two anonymous reviewers is gratefully acknowledged. Financial support was provided by the Deutsche Forschungsgemeinschaft ( Ku 683/1). This is Publication XXX of the Alfred Wegener Institute for Polar and Marine Research (Bremerhaven) and Contribution XXXX of Special Research Project SFB 261.
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