The elusive blake: A record of plaeomagnetically reversed deposits in New Zealand post 120 ka

Quaternary Science Reviews (Quaternary Geochronology), Vol. 15, pp. 119-726,1996. Copyright 0 1996 Elsevier Science Ltd. Printed in Great Britain. All rights reserved. PII: SO2773791(96)00033-9 0277-3791196 $32.00

Pergamon

THE ELUSIVE BLAKE: A RECORD OF PALEOMAGNETICALLY REVERSED DEPOSITS IN NEW ZEALAND POST 120 ka

Department

of Physics,

R. G. LYONS University of Auckland, Private Bag, Auckland,

Abstract - As well as being valuable in its own right, paleomagnetic data can provide a control for other dating methods and indeed may sometimes be the only available age estimate for deposits. Paleomagnetically reversed deposits are generally taken as being beyond the Brunhes-Matuyama boundary, i.e. older than 780 ka. However, there is increasing evidence from several sites in the northern hemisphere for a period of reversed polarity from ca. 118-112 ka, known as the Blake event, but little evidence from the southern hemisphere. A cave sediment deposit in Waitomo, New Zealand, overlain by speleothem deposits dated by uranium series methods at 120 ka, proves to carry a measurable stable paleomagnetic record of secular variation. In the upper part of the sequence a reduction in inclination, from a mean consistent with the present geocentric axial dipole to zero, possibly indicates the beginning of the Blake event, stimulating further work on the overlying flowstone. The overlying deposits showed a trend back to normal inclination but strong evidence was found of a subsequent period of reversed inclination, possibly associated with a period of general climatic dryness. Caution must therefore be exercised in attributing any deposit, at least in New Zealand, to the Matuyama chron, solely on the basis of an observation of reversed polarity. Copyright 0 1996 Elsevier Science Ltd

right.

Correlation

of deposits

or rocks

value as a in their own

by sequences

of

for some time (Ninkovich et al., 1966) and, similarly, matching secular variation records has been used in younger sedimentary deposits (Turner, 1981). This technique is particularly valuable where only one of the correlated deposits may be associated with independently datable material, or where contemporaneity or otherwise is a key factor in interpreting a sequence of events. However, for paleomagnetic data to be effective in this way, it is essential that the paleomagnetic field be sufficiently well defined: if, for example, a period of reversed polarity had occurred, say at 120 ka, at a site but its existence was unknown, then equating a record of reversed polarity with the Matuyama chron and assigning an age of >780 ka would result in a grave error. Such a scenario is precisely that potentially generated by the observation of the period of reversed inclination of the earth’s magnetic field known as the Blake magnetic polarity event around 112118 ka, as has been documented in the Northern Hemisphere (Jacobs, 1984; Tucholka et al., 1987; Tric et al., 1991; Zhu et al., 1994). Its duration in the Mediterranean has been variously estimated at 6000 a (Tucholka et al., 1987) and less than 4000 a (Tric et al., 1991). Using interpolation and the assumption of constant deposition rates, Zhu et al. (1994) estimate its age and duration in Chinese loess to be 117 ka and 5300 a, paleomagnetic

reversals

QG

respectively. However, it has only been observed at one site in the southern hemisphere, at latitude 28 08’S in the Indian Ocean (Smith and Foster, 1969). It is not known whether these observations reflect a variation in the dipole component of the Earth’s magnetic field, and will therefore have occurred simultaneously in the southern and northern hemispheres, or represent a major variation in the non-dipole component and are much more localised. Data from the southern hemisphere are therefore important not only to validate the use of paleomagnetic reversal data for dating purposes but also for constructing models of the earth’s magnetic field. The purpose of this study was to locate, if possible, evidence of the Blake in New Zealand and hence provide an important southern hemisphere record. The two major difficulties in constructing a reliable record of the Earth’s paleomagnetic field are access to continuous well-preserved sequences of suitable age that carry a stable paleomagnetic signal and the availability of independent dates for such deposits. Cave deposits, including both sediments and secondarily deposited calcium carbonate or speleothems, are potentially valuable sources of such data. Sediment sequences are generally well-preserved, being in an atmosphere of constant humidity and temperature and with minimal bioturbation. While sampling may be restricted for ethical reasons, as speleothems and other cave deposits are irreplaceable, it can generally be carried out with a high degree of precision and close observation which is difficult to achieve in sites such as lake or sea beds where sampling is more remote. Replication of samples

INTRODUCTION Paleomagnetic data have demonstrated relative dating tool as well as being valuable

New Zealand

has been used routinely

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from the same stratigraphic layer may also be possible, allowing calculation of confidence limits on the measured parameters. While organic material for dating may be scarce, sedimentary deposits are frequently associated with speleothems which may be dated by radiocarbon (~35 ka) or uranium series dating (~350 ka). Both cave sediments (Foster et al., 1982) and speleothems (Latham et al., 1979) have been shown capable of carrying a welldefined, stable paleomagnetic signal.

FIELD

SITES

AND METHODS

The sections studied were located in Gardners Gut and Coincidence Cavern, two caves developed in Tertiary limestone at Waitomo in the North Island of New I km west of the well-known Zealand, approximately Glowworm Tourist cave (Fig. 1). It is highly probable that

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Geochronolog~):

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15

both caves were eroded by the Waitomo stream and that Coincidence Cavern is geomorphically equivalent to the upper levels in Gardners Gut, though evolution of the system and the relative sequence of deposition of sediments in each cave was unknown. Horizontally laminated sediment sections up to 5 m in depth represent decreasingly frequent overflow episodes as the stream erodes lower levels capable of taking higher proportions of flow. This is reflected in finer grainsizes and laminations towards the top of the sections. The sediments are principally of volcanic origin from the weathered tephras and ignimbrites unconformably deposited over the area by the eruptions from the Central Volcanic Plateau 100 km to the southeast, but also contain some finer sediments from remnants of overlying mudstone. In some areas the sections are overlain by flowstone speleothem, while in other areas layers of calcite, pool deposits or flowstone are incorporated within the sedimentary layers. There is therefore potential for dating by independent methods. Several sediment sections were checked for postdepositional deformation. before being sampled in triplicate at 10 cm intervals using 2 x 2 x 1.5 cm cuboid boxes pressed into the sediments by hand. The boxes were orientated in three dimensions to better than 1” accuracy and sealed to protect the contents from distortion due to post-sampling changes in moisture content. The overlying flowstone for the primary section, CA, was cored i~zsitu but subsequent speleothem samples of the same tlowstone were orientated in the field, removed in one piece and cut into cuboid subsamples in the laboratory. Samples were measured using a cryogenic magnetometer with two orthogonal SQUID detectors. Each sample was measured in eight different orientations and the results averaged to give the three orthogonal components, thus minimising the effect of noise and background signals. The overall noise level was less than IO-“’ Am-’ or 1.7 x IO-’ mAm_” for the 6.0 cm’ specimens used. Eight pilot sediment specimens and eight pilot speleothem samples were subjected to stepwise alternating demagnetisation, keeping the specimens stationary and demagnetising successively along three orthogonal axes. From these data a routine demagnetisation procedure to remove viscous remanent magnetisation was set at 10 mT for sediment samples and IS mT for speleothem samples. Although a sample size of 3 is minimal for statistical work, a Fisherian 9.5% cone of confidence (analogous to two standard deviations in two-dimensional statistics) has been calculated for each position to give an indication of the reliability of each mean value.

RESULTS Coincidence FIG. I. Location of study site in Waitomo, New Zealand, latitude 30”17’S, 175”8’E, corresponding to a geocentric axial declination D = 0. Present day dipole inclination I = -57.5”, magnetic field is I = -64”. D = 19”E.

Cavern Section CA

This is a 5 m section which has been shown to carry a stable paleomagnetic record showing secular variation; as this aspect has been reported in detail in Turner and Lyons (1986) it will not be discussed further here. Of

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R.G. Lyons: The Elusive Blake

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FIG. 2. NRM paleomagnetic demagn&sation

directions for CA sediment section. Samples have been cleaned using alternating to 10 mT. Error bars are 95% confidence limits using Fisherian statistics.

considerable interest, however, in the light of the reports of a period of reversed inclination in the Northern Hemisphere and a Uranium series date of 120f6 ka on the flowstone overlying this section, is the strong swing to low inclinations at the top of the section (Fig. 2). The sediments comprising these layers are finely laminated clays, horizontally bedded with a well-defined stable paleomagnetic signal after removal of an unstable VRM component by 5 mT a-f demagnetisation. The decrease is not due to depositional ‘inclination error’ from physical realignment of magnetised elongated grains in shallow water, as has sometimes been suggested (King, 1955), because this would affect the section as a whole resulting in an average inclination value less than that of the geocentric axial dipole. The mean value for the rest of the section is in fact slightly but not significantly steeper than the geocentric axial dipole. In addition, the demagnetisation characteristics of these horizons are similar to those found elsewhere in the section, and there is no

field

disturbance of the horizontal laminations. It appears therefore that this is a real shallowing of the magnetic field, which is, however, not large enough by itself to be classified as an excursion. The extreme virtual magnetic pole associated with this shallowing is 35.8“ from the geographic pole, only slightly less than the arbitrary 40” defined by Barbetti and McElhinny (1976) as constituting an excursion. The incentive to look for ways of extending this record and capturing the elusive Blake was high.

Associated

Speleothems

(1) An initial coring of the flowstone directly overlying the sediment section produced a cylindrical sample for which the horizontal coordinates were lost due to fractioning of the sample on the horizontal axis.

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However, as the latitude of the site is 30 17’, giving rise to an expected inclination of -57.5 for a geocentric axial dipole, the demagnetised inclination of + 13 is clearly reversed. A small stalactite from the same area also proved to have a reversed inclination but as its chronological relationship to the sedimentary deposits was undefined, its reversed inclination was encouraging but not definitive. (2) The overlying flowstone was sectioned into sequential cubes, each 1 cm thick. Pilot samples were wellbehaved under demagnetisation (Fig. 3(a)), but the results failed to reveal the expected low or reversed inclination in the NRM values (Fig. 4). However, the VRM component removed by demagnetisation was reversed for all specimens, indicating the existence of a post-depositional reversed field. (3) In the hope of obtaining more detail than was possible from the flowstone, which was only 6 cm thick, a 60 cm stalagmite which also overlaid the sediments was sampled. A vertical cross-section of the stalagmite confirmed that it was deposited contemporaneously with the flowstone as several major growth lines could be identified in corresponding positions in both speleothems. However, there appeared to be extensive post-depositional recrystallisation in some parts

Genchmnology):

Volume I5

of the stalagmite, resulting in large crystals and lacunae and casting doubt on the chronological validity of the record. Demagnetisation supported this interpretation: in some cases it showed the presence of more than one underlying signal (Fig. 5) and, although all specimens were measured after routine demagnetisation, results were inconsistent and did not give a well-defined sequential record. (4) Sandwich flowstone sampling. Again in an attempt to obtain more detail of the paleomagnetic record subsequent to sediment deposition, the llowstone was cut horizontally into thin layers which were then amalgamated as in Fig. 6, using Araldite resin. The composite specimens should then be of measurable intensity but, because they were made up out of single layers, would represent a much shorter time span than those cut from 1 cm thick layers. The results proved disappointingly chaotic, particularly in declination (Fig. 4(c)). This was eventually shown to be because the specimens had acquired a large overprint aligned with the current magnetic field but interfering differently in different specimens with the original NRM vector, depending on the different orientation in which individual specimens were laid out during construction (Fig. 3(b)). This overprint was stable under demagne-

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R.G. Lyons: The Elusive Blake

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directions for flowstone overlying sediment section CA. (a) NRM for sequential cuboid specimens. (b) a-f cleaned cuboid specimens (15 mT). (c) composite ‘sandwich’ specimens, a-f cleaned. These erratic results are attributed to an overprint incorporated during the assembly process.

FIG. 4. Paleomagnetic

tisation and its demonstrated existence sounds a note of caution for such techniques. The laboratory was not noticeably contaminated with dust particles obviously the requirement for purity is higher than had been anticipated or the adhesive itself is capable of carrying a stable record. Such reconstruction would be best carried out in a null magnetic field such as that provided by tuned Helm-Holtz coils.

Extension

of Sediment

Other Related Sediment Sections in Coincidence

Cavern

Two other sections in Coincidence Cavern each contained a horizon with a shallow (2.5”) inclination, overlain by horizons with inclinations less than the GAD. They are topographically slightly lower than section CA and contain pool deposits of calcite crystals overlying the low inclination sediments. On geomorphic evidence it is probable that the low inclination sediment horizons correspond with the top of the CA sequence and the calcite deposits with the flowstone overlying CA.

Section CA

It proved possible to obtain a further small number of specimens above datum in CA by correlating depositional layers with a nearby extension of the section. These specimens did not show the hoped-for continuation of the swing to low inclinations seen in the top of the original CA section which inspired this search for the Blake, moving smoothly back towards the GAD.

Sediment

Section in Gardner-s Gut

The most convincing evidence that a period of reversed inclination did occur subsequent to, and relatively soon after, the deposition of CA section was found in a 3 m section in Gardners Gut cave. The paleomagnetic record both before and after demagnetisation was of normal inclination, but if the vectors that were stripped off during

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Geochronology):

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Volume

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FIG. 5. Zjiderveld demagnetisation plots for typical stalagmite samples (symbols as in Fig. 3). Some samples were well behaved, with successive demagnetisation steps tending to the origin as exemplified in (a). However, many revealed the presence of underlying signals, as shown in the examples marked (b); as successive demagnetisation steps do not trend directly to the origin, the vector being stripped does not represent the most stable paleomagnetic signal. Sometimes the signal intensity was large enough for successive demagnetisations to identify this secondary component as in the examples marked (c), but frequently the signal intensity was too weak to strip this overprint totally. Hence the data, while revealing an interesting history of speleothem evolution and probable recrystallization, cannot provide a more detailed record of the original paleomagnetic field than the associated flowstone.

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R.G. Lyons: The Elusive Blake

FIG. 6. Subsampling of flowstone overlying section CA: construction of sandwich samples to represent shorter deposition intervals. It is recommended that adhesive be applied in a null magnetic field as it may not be possible to remove an overprint by the present-day

magnetic field.

demagnetisation are examined, it can be clearly seen that the removed vector stripped during demagnetisation is reversed, strongly so for the lower part of the section (Table 1). This is particularly prominent in the lower half of the section where the original horizontal laminations still clearly visible in the sediments are crossed by curved iron-stained banding. The overprint is therefore most probably due to chemical remanent magnetism, caused by post-depositional movement of water-soluble complexes within the sediments during a period of relatively dry conditions. This could have been when the passage was finally completely abandoned by its stream or during a period of more general climatic dryness.

DISCUSSION The period of low inclination at the top of the original sediment section has not been continued in immediately sequential sediment or speleothem deposits, although there is further evidence of a short-lived period of low inclination in related deposits in Coincidence Cavern. Technically, it therefore represents a pronounced variaTABLE 1. NRM and a-f cleaned data for two sediment sections. The vector stripped by cleaning from Heffalump Trap section has reversed polarity. It corresponds to a post-depositional chemical overprint associated with drying of the sediments, and is possibly contemporaneous with the low and reversed primary signals found overlying CA (horizons with @ >lO have been omitted from the calculations)

HT HT CA CA

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tion in the secular field rather than the beginning of a reversal. However, observations of positive inclination in other associated deposits cannot be discounted. The initial observation of a paleomagnetic reversal in the overlying flowstone from the cored specimen is in fact compatible with the non-reversed inclinations found in the cuboid flowstone specimens: the cored sample included a surface crusting removed from the cuboid samples during preparation, and this layer, having a greater magnetic intensity than the rest of the specimens, must also contain the reversed polarity signal. Such crusting or ‘coral’ is frequently observed on speleothems which are no longer growing actively by precipitation from flowing seepage water. It is common in entrance zones where evaporation occurs and is also found in drier parts of caves after the cessation of active deposition. In this case its formation could well correspond to the period of drier conditions which have caused the post-depositional chemical changes in the section from Gardners Gut. The sequence of events would then be as follows: (1) Deposition of sediment section in Gardners Gut during progressive abandonment of this upper level passage as the lower sections of the cave are eroded and become capable of carrying the full flood flow. (2) Deposition of sediment sections in Coincidence Cavern as erosion continues and this section of the cave is also abandoned by the stream. The top of section CA records a swing to low inclination as do some lower-lying sections in the same part of the cave. After the upper levels are completely abandoned by the (3) stream, flowstone and pool deposits are laid down around 120 6 ka (uranium series date on flowstone). These exhibit normal inclination, indicating that the initial swing to low inclination has been relatively brief. The cave becomes generally drier, seepage declines, (4) the flowstone above CA develops coral encrustation and the sediments in Gardners Gut dry out resulting in cross-lamination iron-banding. Both these phenomena carry a reversed paleomagnetic signal. A small stalactite which also carries a reversed inclination may or may not correspond to this episode but is encouraging evidence that at least one such reversal did occur. conditions ensure preservation (5) Stable environmental of the deposits and their paleomagnetic record. A small viscous remanent magnetisation reflects the present day field but is easily removable with standard demagnetisation techniques.

CONCLUSION A swing to low inclination has been observed prior to, and most probably shortly before, 120 6 ka. It did not persist, but an episode of reversed polarity occurred subsequently and most probably from geomorphic evidence, shortly thereafter. It was associated with a drier regime in the cave, which could possibly be related to generally drier climatic conditions. If this were so, it is ironic that a change in climate occurring simultaneously

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with a paleomagnetic excursion should result in a change of conditions such that deposits capable of recording much sought-after details of the duration, extent and nature of the onset of this episode of reversed polarity were no longer formed. With the development of uranium series dating techniques requiring only small samples, it should be possible to date the coral encrustations on the flowstone and determine whether this reversal does indeed correspond to the period of reversal known as the Blake in the northern hemisphere, and hence its universality or otherwise, with consequences for models of the paleomagnetic field. (It is worth noting that if the estimate of the date for the short reversed interval in loess overlying Mamaku Ignimbrite, reported by Froggatt (1988) is revised according to the new date for the ignimbrite (Shane et al., 1994), the revised date for the reversed interval would be consistent with the data obtained here.) A note of caution must be sounded in any correlation of reversed samples with the Matuyama chron in New Zealand and possibly also in Australia, on the basis of reversed polarity alone. These particular deposits, containing as they do discontinuities, cannot establish the fine structure of the event; however, the paleomagnetic data, which is corroborated by the speleogenetic relationship between the deposits, has provided definite evidence that at least one such reversed episode has occurred since 120 6 ka. In addition, a younger excursion (in the range 25-50 ka) has been observed in New Zealand basalts by Shibuya et al. (1992) and an older one by Shane et al. (1994). Independent corroborative evidence is therefore highly desirable before assuming any other reversed samples to be of the Matuyama chron.

REFERENCES Barbetti, M.F. and McElhinny, M.W. (1976). The Lake Mungo geomagnetic excursion. Philosophical Transactions of the Royal Society of London, A281, 5 15-542.

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Foster, J.H., Noel, M. and Bull, P.A. (1982). The paleomagnetism of sediments from Clearwater Cave, Mulu, Sarawak. Transactions of the British Cave Research Association, 9, 134-141. Froggatt, P. C. (1988). Paleomagnetism of Last Glacial loess from two sections in New Zealand. In: Eden and Furkert (eds), Loess, Its Distribution, Geology and Soils. Balkema, Rotterdam. Jacobs, J. A. (1984). Reversals of the Earths Magnetic Field. Adam Hilger, Bristol. King, R.F. (1955). Remanent magnetism of artificially deposited sediments. Monagraphs of the Not. Royal Astronomical Society (Geophysical Supplement), 7, 115-l 34. Latham, A.G., Schwartz, H.P. and Ford, D.C. (1979). Palaeomagnetism of stalagmite deposits. Nature, 280, 383385. Ninkovich, D., Opdyke, N.D. and Heezen, B.C. (1966). Palaeomagnetic stratigraphy, rates of deposition and tephra chronology in North Pacific deep-sea sediments. Earth and Planetav Science Letters. 1, 4766492. Shane, P., Black, T. and Westgate, I.E.M. (1994). Isothermal plateau fission-track age for a late Quatemary stratigraphy and paleomagnetic implications. Geophysical Research Letters, 21, 1695-1698. Shibuya, H., Cassidy, J., Itaya, T. and Smith, I. (1992). A geomagnetic excursion in the Brunhes epoch recorded in New Zealand basalts. Earth and Planetary Science Letters, 111,41~8. Smith, T. and Foster, J.H. (1969). Geomagnetic reversal in Brunhes normal polarity epoch. Science, 163, 565. Tric, E., Laj, C., Valet, J.-P., Tucholka, P., Paterne, M. and Guichard, F. (1991). The Blake geomagnetic event: transition geometry, dynamical characteristics and geomagnetic and significance. Earth and Planetary Science Letter.r, 102, 1-13. Tucholka, P., Fontugne, M., Guichard, F. and Paterne, M. (1987). The Blake geomagnetic polarity episode in cores from the Mediterranean Sea. Earth and Planetary Science Letters, 86, 320-326. Turner, G.M. (1981). Lake sediment record of the geomagnetic secular variation in Britain during Holocene times. Geophysical Journal of the Royal Astronomical Society, 71, 159171. Turner, G.M. and Lyons, R.G. (I 986). A paleomagnetic secular variation record ca. 120,000 B.P. from New Zealand cave sediments. Geophysical Journal of the Royal Astronomical Society, 87, 1181-l 192. Zhu, R.X., Zhou, L.P., Laj, C., Mazaud, A. and Ding, Z.L. (1994). The Blake geomagnetic episode recorded in Chinese loess. Geophysical Research Letters, 21, 697-700.