Palaeomagnetic excursions recorded in mid-Weichselian cave sediments from Skjonghelleren, Valderøy, W. Norway

Physics of the Earth and Planetary Interiors, 45 (1987) 337-348 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

337

Palaeomagnetic excursions recorded in mid-Weichselian cave sediments from Skjonghelleren, Valderoy, W. Norway Reidar Lovlie Geophysical Institute, Allegt. 70, N-5000 Bergen (Norway)

Arne Sandnes STA TOIL, Postbox 2123, N-6501 Kristiansund N (Norway) (Revision accepted October 6, 1986)

Lovlie, R. and Sandnes, A., 1987. Palaeomagnetic excursions recorded in mid-Weichselian cave sediments from Skjonghelleren, Valderoy, W. Norway, Phys. Earth Planet. Inter., 45: 337-348. Anomalous single component palaeomagnetic directions in laminated clay-silt sediments in the non-karstic Skjonghelleren cave, W. Norway, are interpreted to record two high-resolution, but incomplete records of geomagnetic field excursions. A depositional, non-distorted origin of magnetization, carried by ultra-fine magnetite/maghemite grains, has been inferred from field evidence, redeposition experiments and magnetic fabric properties. The sediments deposited sub-glacially between 56 ka to 12 ka, determined by consistent isotope ages obtained from bones and travertine. The youngest excursion ( < 30 ka), defined by two equatorial anticlockwise VGP loops, are readily correlated with one of the synchrone Lake Mungo VGP positions, suggesting a phase of global instability of the geomagnetic field. Meridionally constrained, low-latitudinal VGP paths for the oldest excursion ( < 56 ka), have features consistent with transitional field geometries, suggesting an aborted reversal origin of this event.

I. Introduction

Excursions of the geomagnetic field are inferred from palaeomagnetic records as short-duration, high-amplitude directional fluctuations of regional to global extent (Verosub and Banerjee, 1977). These short-lived geomagnetic features are potential chronostratigraphic tools for high resolution dating/correlation of Quaternary sediments. However, although numerous excursions have been reported, only the Blake event (Denham et al., 1977) and the Mono Lake excursion (Liddicoat, 1979; Negrini et al., 1984) are described from more than two localities. In addition to seriously questioning the reality of excursions, the apparent lack of synchronicity may reflect either misinterpretations (Verosub, 1975; Marino and Ellwood, 0031-9201/87/$03.50

© 1987 Elsevier Science Publishers B.V.

1978), or improper dating. A more fundamental problem, however, is the apparent absence of geomagnetic excursions in nearby sedimentary sections presumably covering the time spans in question (Verosub et al., 1980; Doe and Steele, 1983). This may either be due to undetected stratigraphic hiati, or post-depositional processes (weathering/ deformation/bioturbation) affecting the ability of sediments to retain true depositional related palaeomagnetic records of short-duration, directional features of the geomagnetic field (Watkins, 1968). Several investigations have demonstrated the palaeomagnetic potentials for correlating/dating cave sediments (Creer and Kopper, 1974, 1976; Noel and St. Pierre, 1984). Cave sediments deposited in biologically sterile environments have

338

not been exposed to post-depositional mixing, and the constant temperature regime in deep caves is also likely to reduce the effect of surface weathering.

5KJONGHELLEREN UNITILITHOLOGY~

2. Lithology Skjonghelleren is a wedge-shaped, non-karstic cave situated on the island of Valderoy (62.28 o N, 6.11°E). The cave, which is 5-12 m wide and extends near east for some 70 m, was formed by marine abrasion in a coarse crystalline granodioritic gneiss, Fig. 1 (Larsen et al., 1987). The present cave floor is situated 64 m above sea level, well above the Late Weichselian marine limit. A 6 m deep trench in the inner section of the cave penetrates 10 distinct lithological beds A - J , Fig. 2. Coring has revealed another laminated sequence underlain by a block bed below K. The blocky beds B, G and K represent blocks falling from the roof during periods of frost-shattering in an airfilled cave. Precipitated calcium carbonate within the permeable blocky bed K, gave a possible U / T h age of c. 56 000 years B.P. Radiocarbon ages of bones (high-boreal to low arctic fauna) and pre-

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cipitated calcium carbonate ( U / T h ) from bed G, gave consistent ages around 30 000 B.P. (Larsen et al., 1987). Laminated clay/silt sediments (J to H) overlying bed K and (F to C) G, Fig. 2, were probably deposited subglacially in a water-filled cave. N - N W dips of 20 o to 40 °, attributed to post-depositional compaction, increase towards north due to increasing sediment thickness. Small scale normal faults suggest the action of differential compaction during consolidation (Larsen et al., 1987). More details on sedimentology, datings and fabric are presented in Larsen et al. (1987).

339

3. Sampling

H and F were obtained with WNW push-directions. The remagnetizing potential of falling blocks was investigated by collecting samples 0.2 m below a boulder (1.5 to 0.7 m) which had fallen some 3 - 4 m (bed F, b in Fig. 1).

Palaeomagnetic samples were collected by pressing/hammering a stainless steel-enforced brass-tube (inner diameter: 20 mm) into the soft and freshly cleaned sediment surface. The bottom part of the sediment core enclosed in the tube (3-5 cm), was subsequently pushed into cylindrical plastic pots with a 1 mm larger inner diameter (volume: 7.2 cc) onto which the fiducial line was transferred. The pots were sealed with tight lids and stored at + 4 ° C, except during transport to the laboratory (2 days). Beds F to J cover a stratigraphic height of some 3 m in the sampling trench, which is situated some 60 m from the entrance, a in Fig. 1. A total of 124 samples were collected from beds F, H, I and J with a spacing of 2-2.5 cm. To determine if the sampling method used induced significant sub-sampling errors (Gravenor et al., 1984; Lovlie et al., 1986), bed J and 2 / 3 of bed I were sampled with push-directions almost due north, while the remainder of bed I, and beds

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The natural remanent magnetization (NRM) reflect rather anomalous palaeomagnetic directions, considering the age and undeformed appearance of the sediment, cf. Fig. 3. A systematic trend in declination from almost due west in bed J (bottom) through south and towards the east through bed I, is accompanied by quasi-periodic variations in inclination varying between subhorizontal to 70 ° . Almost due easterly declinations in bed F ( < 30 ka) are associated by inclinations increasing from sub-horizontal to intermediate values. Directional variations are not systematically related to intensity or susceptibility

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340

(k), which is significantly larger in silt beds J and H than in the clay beds (I and F). The susceptibility within each unit is rather constant, suggesting uniform compositions. The dominating southerly distribution of shallow to steep dipping NRM directions immediately roused the suspicion of a systematic orientation error by 180 o, but careful resampling of bed I at four levels confirmed the initial results.

5. Magnetic components Progressive alternating field (af) demagnetization of 28 equally spaced samples revealed high median destructive fields (MDF) ranging between 40 and 80 mT (mean: 6"1 + 11 roT). Af demagnetization to the maximum available field (80 mT) was not successful in reducing intensities by more than 75%, Fig. 4. Fourteen samples were thus subjected to progressive thermal demagnetization. The samples were dried in air (lids off, room temperature) in fixed positions for 115 days. NRM

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341

directions changed non-systematically by less than 15 ° of arc, and intensities decreased by less than 10%. The samples were transferred to cylindrical brass-containers (demagnetized to 700°C) with the same dimensions as the sample pots. A groove on the inside of the latter permits an accurate repositioning of the samples. All samples carry univectorial directions showing a conspicuous drop in intensity by more than 60% between 200 and 300 ° C, Fig. 4. Final thermal unblocking occurs below 580 ° C. A scattered distribution ( D / I / a g s : 343 ° / 48 ° / 12 ° ), Fig. 5, of components deviating significantly from a final straight movement towards origin in some samples is not consistent with viscous magnetizations acquired in situ. However, the components form an azimuthally tight group relative to the individual push-directions (D/I/or95: 2 ° / 2 7 ° / 9 ° ) , Fig. 5, which are thus attributed to directional sub-sampling errors acquired by the sampling procedure used. The components are removed below 40 mT 200 ° C, cf. Fig. 4. The remaining samples were demagnetized in 40 roT, resulting in a directional pattern not significantly different from the NRM. It is noted that the directional sub-sampling errors probably originated by deformation (shear strain, Games, 1977), and that exposure to multiple shocks during sampling (i.e., hammering) did not induce a shock induced magnetization as reported from other sediments (Symons et al., 1980).

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The shock generated by the falling boulder (b in Fig. 1), however, caused a complete remagnetization of the underlying bed F sediment. Ten samples carry univectorial directions (D/1/a95/: 78 ° / 7 8 ° / 6 o) with steep inclinations as opposed to the low-inclination directions in the synchronously deposited unit in the sampling trench (a in Fig. 1). Eight sub-samples from cores penetrating bed K (vibration corer, unknown azimuth), carry steep-dipping, univectorial magnetizations with significantly lower MDFs (12-25 mT) than the overlying sediments. An arithmetic mean inclination of 63 + 6 ° is compatible with the inclination for a normal configuration of the Quaternary geomagnetic field.

6. M a g n e t o m i n e r a l o g y

Thermomagnetic curves from the different units show the same concave feature on heating above 300 ° C, Fig. 7, probably reflecting the inversion of

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contributions than the upper section. Upon cooling from above 600 o C, the initial Curie-point has disappeared, attributed to the complete oxidation of very small magnetite grains to haematite. Isothermal remanent magnetization (IRM) acquisition curves from the different units exhibit identical features; saturation IRM (SIRM) is reached around 0.3 T, associated with remanent coercivities (He,) ranging between 50 and 75 mT (mean Hc,: 63 _+ 6 mT). This result is indicative of extremely fine grained magnetite. Grain-sizes of the magnetic minerals have been estimated according to empirical derived grain-size dependent relationships between MDF of SIRM (H1/21), Hc, and the remanent acquisition force (H'r) for magnetite and haematite (Dankers, 1981). Consistent results from 24 samples suggest the presence of almost pure magnetite (H~,/'Hc, = 1.5 +_ 0.1 and 111/21,/ He, = 0.48 + 0.04) with grain-sizes less than 1/~m. Normalized a.f. demagnetization curves of anhysteretic remanent magnetization ( D C / A C field: 0.05/100 mT) all fall well below the corresponding curves for SIRM, indicative of single domain magnetite grains less than 2.1 #m (Johnson et al., 1975; Bailey and Dunlop, 1983). These results indicate that all beds contain ultra-fine magnetite/maghemite grains (< 2/~m?). The relative proportions between ferri/paramagnetic constituents appear to increase upwards in the section.

7. Magnetic fabric maghemite into almost pure magnetite. The pronounced drop in intensity at this temperature during thermal demagnetization, cf. Fig. 4, is also attributed to this inversion. Since single component directions prevail above this temperature, this metastable maghemite phase is concluded to be of a pre-depositional origin. Repeated cooling from above 600 °C reduces both the initial Tc and Js, indicating high temperature oxidation of magnetite to less magnetic phases (haematite), cf. Skj6, Fig. 7. The latter sample is typical for beds F, H and the top 1/3 of I. The lower part of the sequence exhibit slightly different curves, cf. Skj33, Fig. 7; a weakly defined T~ around 580°C is associated by significantly higher paramagnetic

The anisotropy of magnetic susceptibility (AMS) in sediments reflects a spatial distribution of magnetic grains, the orientation of which is controlled by forces (gravitational/hydrodynamic/magnetic) acting during deposition. AMS and detrital remanent magnetization (DRM) in consolidated sediments are caused by some systematic ordering of, possibly, two different populations of magnetic grains. AMS which do not reflect a primary depositional magnetic fabric may hence indicate that any depositional related magnetization may be more or less distorted (Marino and EUwood, 1978). AMS is represented by a susceptibility ellipsoid

343 TABLE I Anisotropy of magnetic susceptibility results Unit

N

P1

/'2

P3

E

F H I J

28 8 72 12

1.01,4 ± 0.009 1.02 ± 0.01 1.016 ± 0.009 1.03 ±0.02

1.17 _ 0.06 1.07 ± 0.01 1.14 + 0.05 1.21+0.03

1.16 ± 0.06 1.05 ± 0.02 1.12 ± 0.05 1.18 +_0.02

1.14 ± 0.06 1.04 ± 0.03 1.11 ± 0.06 1.14±0.03

N:number of samples. See text for definition of parameters. w i t h t h r e e o r t h o g o n a l p r i n c i p a l axes: kmi . > kin t > k m ~ . L i n e a t i o n (P1), d e g r e e o f a n i s o t r o p y (P2), f o l i a t i o n ( / 3 ) a n d e l l i p t i c i t y ( E ) is d e f i n e d b y the

following

length-ratios:

km~,,/kmin, P3 = ki,t/k,~i,, The

anisotropy

P1 = kmax/kint,

P2 =

E = P3/P1.

of magnetic

susceptibility

in

BED F

/

I

z

eo:o

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344

120 samples, determined on an induction bridge (KLY-1), is characterized by oblate susceptibility ellipsoids with anisotropy factors (P2) ranging between 7 and 21%, Table I. Principal susceptibility axes define foliation planes dipping shallower than the laminated beds, Fig. 8, a feature compatible with a primary depositional fabric of sediments (sand-silt) accumulating on sloping surfaces (Rees, 1966). The AMS results tentatively suggest that the Skjonghelleren cave sediments have retained a depositional related magnetic fabric, implying that post-depositional distortions have not modified any fabric related remanent magnetization.

8. Origin of magnetization Fine-grained sediments acquire a detrital remanent magnetization (DRM) by preferred alignement of magnetic grains either during deposition ( d D R M ) or consolidation (post-depositional DRM, pDRM). Both processes may induce an 'inclination error' attributed to grain-size/shape (dDRM) and compactional effects (pDRM), respectively. The latter depend on the initial water content, and two redeposition experiments of reconstituted bed F sediments were performed to elucidate the properties of diluted/thick slurries: Diluted slurry: a 10% suspension was deposited daily for 28 consecutive days. Excess water was siphoned off after consolidation for five days. During subsequent drying in air, the sedimentary thickness decreased by c. 40%. The mean remanent direction of 10 sub-samples deviates by some 30 ° from the ambient field inclination ( I = 69 ° ), Table II, Fig. 9. This inclination error is attributed to the effect of compaction (Blow and Hamilton, 1978). Magnetic fabric is characterized by almost indeterminate ellipticities ( E - - 1 ) , Table II, vertical kmi~ axes and horizontal km~ axes confined within the meridian of the external magnetic field. Thick slurry: Sediment was mixed with a minimum of water to form a thick slurry and stirred in five sampling cups in the Earth's magnetic field for 2 mln. Drying in air caused only a slight reduction in volume. The acquired p D R M records true directions of the ambient field, Table

TABLE I1 Results redeposition experiments Slurry

Diluted

No of samples N R M (mA m ~) Declination Inclination O~95 P1 (lineation) /'2 (anisotropy) P3 (foliation) E (ellipticity)

10 120 3o 39 o

Thick 515

5 °

1.0345:0.003 1.072 5:0.003 1.0365:0.003 1.001 + 0.003

5 171 5:13 357 o 67 o 2o 1.0045:0.003 1.025 + 0.003 1.0205:0.005 1.015 5:0.007

II. The magnetic fabric is characterized by oblate susceptibility ellipsoids, vertical kmi~ axes and sub-horizontal, randomly distributed kint/krnax axes. These features are typical for sediments deposited on a flat surface, Fig. 9 and Table II. The results are in general agreement with the natural sediments, although the latter have significantly higher values for the ratios between the principal susceptibility axes, Table I. This inconsistency may be accounted for by considering that the cave sediments in effect consolidated on sloping surfaces (Rees, 1966). Although short-term redeposition experiments only simulates (sensu stricto) natural conditions, the atypical magnetic fabric in the diluted slurry sediments tentatively suggests that the present sediments did not accumulate as a comparable thin suspension. N - N W bedding developed by differential compaction during consolidation. Deposition on sloping (a) surfaces may cause a 'bedding error' in inclination (/3) according to the expression 13 = 1.42 a (Hamilton and King, 1964) probably valid for silt-sediments dipping up to 29 ° (Noel, 1983). A recent model, invoking post-depositional rotation of silt-sized grains about axes normal to the bed (Noel, 1986), appear to be valid for beds dipping up to 60 °. However, this model is probably not applicable to the present very fine-grained sediments (clay) which consolidated on surfaces dipping in the same direction as the ambient field direction. In addition, existing models for bedding errors can neither explain the observed anomalous declinations, nor the fact that while the dips vary

345

DILUTED SLURRY N

w/

"'"

THICK SLURRY N

Fig. 9. Results of redeposition experiments of reconstituted bed F sediments. Stereographic projections of directions of depositional magnetization (left), and principal axes of magnetic susceptibility (right). Symbols: cf. Fig. 8. All directions lower hemisphere. Ambient field inclination: 69 ° .

between 20 ° and 40 ° , inclinations range between 0 ° and 70 °. N o n e of the above evidences can conclusively show that the palaeomagnetic results reflect real records of the geomagnetic field at the time of, or shortly after deposition. However, inclinations compatible with a normal field configuration are

present in sediments deposited before (bed K) and after (bed B) the investigated sequence, indicate that sediments accumulating in this cave environment may record true ambient field directions. The strongest evidence for the palaeomagnetic fidelity of the obtained results, however, is the high-amplitude variations in inclination (sub-

346

horizontal to 70 ° ) residing within and between different lithological units, cf. Fig. 5.

9. Virtual geomagnetic pole positions Virtual geomagnetic poles (VGP) are presented as 5-point running mean directions (univectorial) in Mercator projection in Fig. 10. Statistical estimates of the precision/scatter of mean directions (K, a95 , Fisher, 1953) do not show any correlation with either declination or inclination. Unit J record meridionally confined (270 °E) VGPs moving systematically from 3 4 ° N to 1 4 ° N and towards the equatorially situated VGPs for the base of unit I. The break in the VGP-path probably represents a short period of non-deposition reflected by the change in lithology between unit J and I. The latter unit describe three anticlockwise loops, a small, almost circular one at 70 ° E is succeeded by two latitude-elongated loops confined within +15 ° longitude and 17°S and 45 ° N. Bed H, deposited during a draining phase of the cave (Larsen et al., 1987), defines high latitude (63°N) VGPs distributed between 3 0 ° W and 70°E. The overlying blocky bed G introduces some ambiguity regarding the mode (deposi-

SKJONGHELLEREN VGP POSITIONS

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90

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0

~5

90

1350E

LONGITUDE

Fig. 10. Mercator projection of VGP positions from beds J, I and F based on 5 point running mean (univectorial). Beds J and I (bed F) accumulated between 56 and 30 ka (30-12 ka).

tional/shock) and hence time of remanence acquisition of this bed. However, since both frostshattering and draining of the cave probably occurred during the same climatic phase (ice-free period), it is concluded that the geomagnetic field had a normal configuration just prior to 30 ka

B.P. Unit F ( < 30 ka) defines two equatorially situated, anticlockwise VGP loops between 60 ° to 9 0 ° E and 15°S to 10°N. Considering that this VGP path probably represents a high resolution record of a geomagnetic excursion, there is a notably good correlation with one of the VGP determinations (spot-reading) for the Lake Mungo excursion (fireplace, F12, 47 ° E, 6 ° S, 28 000 + 410 B.P., Barbetti and McElhinny, 1976).

10. Discussion T h e p a l a e o m a g n e t i c directions in the Skjonghelleren cave sediments are interpreted to reflect incomplete records of two excursions of the geomagnetic field. Excursions have been observed to occur before and after reversal transitions, and are ascribed to initial or final phases of dipol instability. Near-sided, meridionally constrained low-latitude VGP-paths have been proposed to be characteristic features of transitional field geometries during geomagnetic reversals (Hoffman, 1982; Liddicoat, 1982). The remarkable agreement between synchronous VGP positions from two almost antipodal localitites, suggests that the Lake Mungo excursion may be viewed from global distances, and hence does not represent a local or regional feature of the geomagnetic field. Recent evidence (Valet et al., 1986) suggests that transitional fields are characterized by short term fluctuations, of the order of hundreds of years, superposed on persisting long-term, non-dipole field variations. The present high-resolution records of inferred transitional fields, cf. bed I, Fig. 10, may thus be attributed to short-term fluctuations of the order of hundreds of years. This implies that beds J and I, which were probably deposited sub-glacially when advancing ice-fronts were in the vicinity of the cave (Larsen et al., 1987), accumulated in less

347 than 10 ~ years. The present fine-grained (clay) sediments accumulated in a freshwater environment in which minimal particle flocculation occurs, The time lag between deposition and pDRM-acquisition (Lovlie, 1974) can thus be assumed to be maximal compared to marine environments. The present high-resolution record of rapidly moving VGPs signifies a very short ( < 1 year?) acquisition time for p D R M , suggesting negligible effects in natural depositional environments. It is proposed that part of the directional scatter may be attributed to non-uniform 'blocking' of magnetic particles due to the inferred high deposition rate. Several excursions have been reported to occur within the time interval in question, indicating periods of high instability of the geomagnetic field. On a regional scale, attention is put to the L a s h a m p / O l b y excursion (France, 36-42 ka, Gillot et al., 1979), low inclination records in Abreda cave sediments (Spain, 35-50 ka, Creer and Kopper, 1976), the Rubjerg low inclination excursion within an Older Yoldia clay sequence in Denmark (23-40 ka, Abrahamsen and Readman, 1980), and two inferred polarity reversals in three Arctic Ocean sedimentary cores (20-60 ka, Lovlie et al., 1986).

11. Conclusions (1) The r e m a n e n t magnetization in the Skjonghelleren cave sediments is of a depositional origin and carried by ultra-fine, detrital maghemite/magnetite grains. (2) Significant differential compaction during consolidation probably did not cause serious errors in inclination, in contrast with results of reported redeposition experiments. (3) The sampling procedure used induced a directional sub-sampling error which was successfully removed by partial a f / t h e r m a l demagnetization to 20 mT 200 ° C. (4) VGP positions of the youngest excursion ( < 30 ka) coincide remarkably well with one of the VGP determinations for the Lake Mungo excursion, suggesting a global, rather than l o c a l / regional extent of this geomagnetic event.

(5) The inferred excursion occurring < 56 ka have longitudinally constrained VGP paths (90 ° W and at the site longitude) in accordance with proposed transitional geomagnetic field geometries, suggesting that this excursion may represent an incomplete record of an aborted reversal of the geomagnetic field.

Acknowledgements The palaeomagnetic potential of the Skjonghelleren sediments was put to our attention by Eiliv Larsen and Jan Mangerud during an excavation financially supported by NAVF, Elf Aquitaine and Sunnmorsbanken. The assistance of E. Larsen in the field is greatly appreciated. Constructive comments to the manuscript by J. Mangerud, E. Larsen and T. Torsvik are greatly appreciated.

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