h School of Cosmic Physics, Dublin Institute for Advanced Studies, 5 Merrion Square, Dublin, Ireland c Institute of Materials Science, National Centre for ...
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Earth and Planetary Science Letters, 102 (1991) 198-212
Elsevier Science Publishers B.V., Amsterdam [XLeP]
Palaeomagnetic and mineral magnetic studies of sediment from Ball's Cavern, Schoharie, U.S.A. S. P a p a m a r i n o p o u l o s
a,
P . W . R e a d m a n b, y . M a n i a t i s c a n d A . S i m o p o u l o s c
Laboratory of Geophysics and Seismology, Department of Geology, University of Patras, Patras, Greece h School of Cosmic Physics, Dublin Institute for Advanced Studies, 5 Merrion Square, Dublin, Ireland c Institute of Materials Science, National Centre for Scientific Research "'Demokritos'" A ghia Paraskevi, Athens, Greece
Received October 23, 1989; revised version accepted September 3, 1990
ABSTRACT Two sediment sequences from a cave near Schoharie, N.Y., have been sampled for palaeomagnetic measurements. The nature of the palaeomagnetic records obtained was found to be similar to other North American records. A correlation can be made with lake sediment records back to 13,000 yr B.P., and with a sediment record from Bessette Creek in Canada for the period between - 23,000 and 30,000 yr B.P. During both periods the dominant long-term trend of the VGP path has been that of anticlockwise motion, but during the last 13,000 years there has been more superimposed short-term clockwise motion than during the earlier period. Magnetic properties of the sediments suggest that magnetite is the dominant magnetic mineral and this is confirmed by the Mrssbauer spectra of both the sediments and magnetic extracts. M~ssbauer measurements on the magnetic extracts also detected the presence of a small amount of haematite.
1. Introduction The derivation of the secular variation of the geomagnetic d e c l i n a t i o n a n d i n c l i n a t i o n with time scales of h u n d r e d s to t h o u s a n d s of years can be best accomplished b y a n a l y s i n g the p a l a e o m a g n e tism of lake a n d other recent deposits (e.g. [1,2]). However apart from the recent work at Lac du Bouchet in F r a n c e [3], lake s e d i m e n t records are generally restricted to the Post Glacial period. Also because the core length is usually limited to 6 m, the records from cores taken with the s t a n d a r d version of the Mackereth corer [4], even outside glaciated regions, do n o t usually span more t h a n a b o u t 10,000 years. Ocean sediments, o n the other h a n d , show geomagnetic polarity changes b u t because they are deposited so slowly are not generally suitable as recorders of the secular variation, even on time scales of up to the order of 105 years. F u r t h e r m o r e , only i n c l i n a t i o n is usually available because of the difficulties in o r i e n t i n g such cores. I n this p a p e r we have looked for p a t t e r n s of the geomagnetic secular variations b y using sediments deposited inside a cave. A m a j o r a d v a n t a g e of 0012-821X/91/$03.50
© 1991 - Elsevier Science Publishers B.V.
cave deposits is the protected, u n d i s t u r b e d depositional e n v i r o n m e n t . Also the absolute azimuthal o r i e n t a t i o n of the samples can be determined. P a l a e o m a g n e t i c analysis of the sediments from three caves in the M e d i t e r r a n e a n area [5,6] indicated that it is feasible to use cave sediments as a p a l a e o m a g n e t i c recorder. Since these first investigations the p a l a e o m a g n e t i s m a n d a n i s o t r o p y of susceptibility of sediments has b e e n studied from caves in N o r w a y [7] a n d G r e a t Britain [8,9], a n d recently a p a l a e o m a g n e t i c secular variation record with a p r o b a b l e time s p a n of 10,000 years at a b o u t 120,000 yr B.P. has b e e n o b t a i n e d using sediments from a cave in N e w Z e a l a n d [10]. A p a r t from these studies, cave sediments have n o t b e e n widely exploited, possibly because of difficulties in d a t i n g them a n d the lack of general i n f o r m a t i o n a b o u t cave sediments, as well as the fact that sample collection can be difficult.
2. Description of the cave Bali's C a v e r n is situated at 4 2 ° 3 0 ' N , 7 4 ° 2 0 ' W (Fig. 1), a b o u t 7 k m n o r t h east of Schoharie, N.Y., U.S.A., in D e v o n i a n l i m e s t o n e a n d lies 13 m be-
PALAEOMAGNETIC AND MINERAL MAGNETIC STUDIES OF SEDIMENT FROM BALL'S CAVERN 120
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low a heavily karsted sinkhole plane. It is 650 m in length and, like most caves in the region, consists of a single-level linear solution tube of small cross section with occasional sinkholes connecting it with the surface. It is, however, unique in the immediate vicinity in containing several large chambers within its system. In one of these, the " R o t u n d a room", about 8 m of soft sediments have been deposited. Two sections of these sediments, each of length 2 m, named Omega and Alpha, banked at opposite sides of the chamber, were sampled for palaeomagnetic investigation. The sediments consist mainly of finely laminated silty clays with some sandy and gravel layers. The Omega sediments are predominantly tan in colour, and the Alpha sediments are reddish-brown. The sediments have either been brought into the room through small-diameter vertical openings communicating with the surface through a low-energy water system, or, in the case of the coarser detritus, originate from freeze-thaw mechanical weathering of the cave ceiling. The cave is similar to the previously studied Jeita Cave in the Lebanon [5,6] in that it contains
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very thick sediments and is presently accumulating deposits because of its location within the water table. The sediments have been deposited under conditions of abnormally high water flow, and thus represent flood episodes that probably occur at least several times every hundred years. For this reason, the strata represent specific climatic episodes rather than continuous deposition. The estimated average deposition rate is in the region of 200-250 mm per 1000 years [11]. The Omega section would therefore represent the time span stretching back from the present to 8000 or 10,000 yr B.P. The Alpha section represents an earlier sedimentation cycle, probably prior to the final Wisconsin and would therefore date from 20,000 or 25,000 yr B.P. back to about 30,000 or 35,000 yr B.P., at the above sedimentation rate. The lack of any archaeological or geochronological controls unfortunately preclude the definition of a better time scale. However, it is quite certain that the cave did not accumulate sediments during the Wisconsin stadials because it is located in a formerly glaciated area. 3. Palaeomagnetic measurements Palaeomagnetic samples were taken by pushing 2-cm cubical hollow sample boxes horizontally into the cleaned face of the sections. A compass was used to obtain the azimuthal orientation of the samples. The errors in the orientation of the samples is considered to be about _+2 ° in both azimuth and dip. The natural remanent magnetization (NRM) was measured using a Digico fluxgate spinner magnetometer, and the low field ( - 0 . 1 mT) susceptibility measured with a Digico susceptibility bridge. Alternating field (AF) demagnetization was carried out with an electronically ramped demagnetizer operating at 300 Hz [12].
3.1. Intensity, susceptibility and Q-ratio Plots of the N R M intensity, susceptibility and Q-ratio (i.e. intensity/susceptibility) are shown in Fig. 2. The intensity is on average about 2 - 3 times higher in the Alpha sediments than in the Omega sediments. Susceptibilities are, however, of comparable magnitude in the two sections and many of the changes in susceptibility, particularly in the Omega sediments, are reflected in the intensity.
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