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Magnetic properties of Argentine basin project MUDWAVE samples
Deep-Sea Research H, Vol. 40, No. 4/5, pp. 921-937, 1993.
0967~645/93 $6.00 + 0.00 © 1993 Pergamon Press Ltd
Printed in Great Britain.
Magnetic properties of Argentine Basin Project MUDWAVE samples BROOKS B . ELLWOOD*
(Received 19 February 1990; in revised form 13 October 1992; accepted 24 February 1993) Abstract--The magnetic properties of samples from nine box cores and three piston cores recovered from three giant mudwaves in the Argentine Basin, South Atlantic Ocean, have been measured. Measurements on mudwave samples include magnetic susceptibility, natural remanent moment and anhysteretic remanent moment. Anisotropy of magnetic susceptibility was also measured on selected box core samples. The results have been compared to magnetic data already available from similar measurements on Argentine Basin core-top samples. Magnetic characteristics for the core-top samples reflect broad changes in magnetic mineral concentration rather than substantial changes in magnetic grain size across the basin. Changes in this pattern are caused by multiple sources of terrigenous sediment influx from the South American continental margin. The mudwave fields in the center of the basin act as a sediment sink for these materials and are generally characterized by uniform magnetic-grain concentrations, compositions and small grain-sizes. Between and within the mudwaves sampled there are systematic differences as well as similarities in the magnetic properties that appear to be related, at least in part, to sample location with respect to depositional or erosional slopes within each mudwave. The magnetic results are interpreted to indicate that clear changes in bottom water flow are represented in samples recovered from the mudwaves.
INTRODUCTION
SEDIMENTSfrom the Argentine Basin area of the South Atlantic Ocean have been studied extensively to identify paleo-oceanographic changes in deep ocean currents that are flowing throughout the basin (e.g. EWING et al., 1971; LEDBETrER and JOHNSON, 1976; ELLWOOD and LEDBETTER, 1977; LEDBETrER, 1986; SACHS and ELLWOOD, 1988; and others). Studies have reported both the modern Antarctic Bottom Water (AABW) current flow direction in the basin (e.g. GEORGI, 1981; LEDBETrER, 1986) and temporal changes in bottom water production (LEDBETTER and JOHNSON, 1976; LEDBETrER and ELLWOOD, 1980). Especially interesting in the Argentine Basin are giant ripples or mudwaves first described by EWING et al. (1971). They suggested that these features were moving downcurrent in response to AABW flow in the deep Argentine Basin. It is clear that these mudwave fields are unusual, certainly in their size characteristics, because some of these mudwaves are >135 m in height and many have a wave length of 5 km or more (KLAtJS and LEDBETrER, 1988; FLOOD and SHOR, 1988). A summary of many of the Argentine Basin *Department of Geology, University of Texas at Arlington, P.O. Box 19049, Arlington, TX 76019, U.S.A. 921
922
B.B. ELLWOOD
studies as well as new data is represented by a recent series of papers dealing with the sedimentary, acoustic and magnetic properties observed in the basin (FLoor), 1988; FLoor) and SrmR, 1988; KLAUS and LEDBEaWER, 1988; MILLIMAN, 1988; SacHs and EkLWOOD, 1988) and in this volume. This work has delineated the mudwave fields in the basin (FLooD and SnoR, 1988) and dynamic models for these waves indicate that they are migrating upcurrent (FLooD, 1988). Accumulation of the large amount of sediment necessary to sustain the mudwaves appears to be in areas of the basin where A A B W is relatively weak (KLAUS and LEDBE~ER, 1988). The waves also appear to be very long-term features (FLooD and SnoR, 1988) that probably developed from sediment transported into the deep basin either from the South American continental margin (KLAus and LErgBETrER, 1988; SACrtS and ELLWOOD, 1988) or as the result of the accumulation of fine sediments winnowed from coarser near margin sediments and later redeposited as A A B W speed slows (MR, LIMAN, 1988). Due to the size of the mudwave fields and to questions concerning genesis of the mudwaves, source of sediment, controls by A A B W , and other factors, Cruise 28 of the R.V. Robert D. Conrad was planned, with the primary mission to study the topography, structure, sediment type and bottom water flow characteristics in the vicinity of the mudwaves. The purpose of this paper was to measure and interpret the magnetic characteristics of sediments recovered from each set of mudwaves. METHODS This study presents magnetic properties results for sediment samples taken from nine box cores and three piston cores recovered from three mudwaves at different localities in the central portion of the Argentine Basin (Fig. 1). Four of the box cores for which data are reported here sampled a mudwave at Site 5 (Fig. 2), and five box cores recovered sediment from a mudwave at Site 6 (Fig. 3). These box cores are well distributed within each mudwave and were chosen to represent both depositional and erosional mudwave settings (FLooD and SnOR, 1988). One piston core used in this study sampled the mudwave at Site 6 and two piston cores were chosen to sample a mudwave at Site 7 (Fig. 4), one from each flank of the mudwave. Plastic paleomagnetic sample boxes (8 cc) were used to recover a representative vertical cross section of the sediment from each box core and represent continuous samples from the sediment/water interface to the base of each core. Plastic boxes were inserted into the split half of piston cores: PC 21 ( - 1 7 m long); PC 28 ( - 1 7 m long); and PC 30 ( - 1 8 m long) at ~-0.t m average intervals. Closer spaced samples were also acquired between 4 and 5.3 m depth in PC 28 where the remanent magnetic properties were determined to be unusual. Measurements on all samples included initial magnetic susceptibility (Z), natural remanent moment (NRM), RM after alternating field (AF) demagnetization, and anhysteretic remanent moment (ARM). Anisotropy of magnetic susceptibility (AMS) also was measured for selected box core samples.
Rock magnetic studies A set of pilot samples was demagnetized in alternating fields to 50 millitesla (mT) magnetic induction. The RM was then remeasured and vector diagrams constructed from the data (Fig. 5). Based on these results, all other samples were A F demagnetized at 15 and 30 mT.
Magnetic properties of mudwavesamples RO"
55"
50"
45"
40'
35"
30"
25"
20"
923 15"
10"
5"
0o 30"
35"
40"
45"
50"
55"
60'
60"
55"
50"
45"
40"
35"
30"
25"
20"
15"
10"
,5
Fig. 1. Argentine Basin map, taken from SACHSand ELLWOOD(1988), with mudwave localities (Sites 5, 6 and 7) that were sampled and ARM contours for core-top samples taken from throughout the basin (indicated by dots). Higher ARM values are interpreted to represent the presence of finer magnetic grains in samples (SACHSand ELLWOOD,1988)
A R M was measured after subjecting samples to decreasing AF induction levels in the presence of a 0.1 mT steady (DC) magnetic induction. Such data have been used to estimate relative size of magnetic grains (for example see COLLINSON, 1983, for a discussion of the use of A R M data), with larger A R M values interpreted to indicate the presence of finer magnetic grains. ARM~Z ratios are used to increase the precision in magnetic grain-size estimates over ARM data alone (see for example the discussion by SACriS and ELLWOOD, 1988) by reducing the effects of magnetic grain concentrations. NRM/Z ratios illustrate RM intensity variations while the effect of varying magnetic grain concentrations on NRM values is reduced. Figures 6, 7 and 8 give the ARM~z, NRM/z, and Z data for box and piston cores recovered from mudwaves at Sites 5, 6 and 7, respectively.
Anisotropy of magnetic susceptibility (AMS) AMS is commonly expressed as an ellipsoid with principal K a (maximum), K2 (intermediate) and K3 (minimum) axes, which in general terms reflect the physical orientation
924
B . B . ELLWOOD
42"2~'
O o G
0 0
@
©
42-30 '
\
@
42"35' L 45ol0'
45"0'5'
45"00
ARGENTINE BASIN - R C 2 8 0 4 MUDWAVE SITE 5 Fig. 2. M u d w a v e Site 5: the four b o x - c o r e l o c a t i o n s are p l o t t e d for which data are r e p o r t e d . D e p o s i t i o n is i n f e r r e d to occur on the s t e e p e r , s o u t h e a s t e r n side of the m u d w a v e (FLooD and SttOR, 1988).
Magnetic properties of mudwave samples
925
4~'4o'
4 9 - I S'
49,10'
49'0 ~ '
ARGENTINE BASIN - RC2BO,4 MUDWAVE SITE 6 Fig. 3.
M u d w a v e Site 6: the five 5ox-corc and one piston core locations are plotted for which data
are reported. Deposition is inferred to occur on the steeper, northern side of the mudwave (FLOOD and SHOR, 1988).
of magnetic elements in a single sample [see KING and REES (1962) and NYE (1969) for a discussion of the method]. In sediment samples, the AMS ellipsoid has been shown to reflect the petrofabric orientation (e.g. TAIRAand LIENERT, 1979), which results from any process which physically orients magnetic constituents within the sample, such as flow, crystal growth, diagenesis or stress. Mass susceptibility in SI units is given as K = (K1 +/(2 +/(3)/3.
(1)
The AMS ellipsoid shape expresses either the preferred alignment of included magnetic grains, or the overall crystallographic mineral orientation in the sample. The parameter V, developed by GRAHAM(1966), is an excellent indicator of shape, where
926
B.B. ELLWOOD
ARGENTINE MUDWAVE
37" 00'~
BASIN - RC2804 SURVEY AREA 7
39o55'W
40°00 ' W
I
i
I
]
I
I
1
'
1
\
\
37°05'$
\ % J
1
Fig. 4. Mudwave Site 7: the two piston core locations are plotted for which data are reported. Deposition is inferred to occur on the northeastern side of the mudwave (MANLEY and F t o o n , 1993).
sin 2 V = ( K 2 - K 3 ) / ( K I - Ks)
(2)
and V <45 ° for prolate shapes and >45 ° for oblate shapes. Common usage of AMS parameters as well as standardization and calibration of the method has been discussed elsewhere (EccwooD et a l . , 1988). Initial mass susceptibility (Z) was measured using a susceptibility bridge (CoLLINSON et a l . , 1963) along the vertical axis of each sample. The data are given in Figs 6 and 7, while means for each box core are reported in Table 1. z d a t a are usually interpreted to represent magnetic grain concentrations within a sample. However, changes in magnetic grain composition within a core will be reflected in the Z values observed. For example, the reduction of magnetite or hematite to siderite or pyrite will greatly reduce Z. On the other hand, oxidation of siderite will result in the production of magnetite or maghemite (SEGUIN, 1966) which will strongly increase Z. Mineral studies
M6ssbauer effect spectra were obtained at room temperature and at 78°K for two samples from BC 9. The iron carbonate mineral siderite (FeCO3), along with pyrite and chlorite, is identified in these samples (Fig. 9). X-ray diffraction (XRD) data were also obtained for several piston and box core samples with a Philips diffractometer utilizing
Magnetic properties of mudwave samples
927
C u K - a radiation and a graphite crystal m o n o c h r o m e t e r . Each sample contained a small amount of siderite. RESULTS Susceptibility data
AMS m e a s u r e m e n t s were conducted on all samples from BC 9 and the axial data reported (Fig. 10), and for selected samples from each box core. A M S was not measured for all samples because neither box or piston cores were oriented. Because these m e a s u r e m e n t s on such weak samples are somewhat time consuming, only selected samples were measured and the data are given in Table 2. The close correspondence in mean K1 declinations for all box cores is probably due to soft sediment deformation during sampling. This effect does not destroy the overall fabric but may slightly modify the fabric parameters. Initial susceptibility for all box core samples is given in Figs 6-8 and Z means for individual box cores are reported in Table 1. R e m a n e n t m a g n e t i c data
All box core and PC 21 samples were normally magnetized suggesting that deposition and m o m e n t acquisition occurred during the Brunhes Normal Chron (<730,000 years; MANKINEN and DALRYMPLE, 1979). Most PC 28 and PC 30 samples were also normally magnetized. However, five reversed samples (one being a single-point reversal) were identified in PC 28 between 4.0 and 5.3 m depth in core (Fig. 11). Four single-sample, low
Table1. Mean )~ × lO-S in g mass Sl units and V (oblate >45°; prolate <45°) for all samples measured from each core (see text). Cores are reported as encountered across each mudwave, first from the steep (depositional) slope (BC 12 and BC 24), then to the shallow (erosional) slope (BC 11 and BC 26)
Mudwave Site 5 (AMS Data) CORE
Mean Z
Mean V
BC12 BC 9 BC10 BCll
11.7 12.0 12.4 14.4
45.4 46.9 46.9 45.2
Mudwave Site 6 (AMS Data) CORE
Mean Z
Mean V
BC26 BC25 BC15 BC23 BC24
16.7 16.1 15.0 16.5 20.5
42.6 49.0 40.0 54.3 51.4
928
B.B. ELLWOOO SITE 5 BC 9
SITE 6 BC 25
E, UP
N ::::
E,
NRM
:~:IIIISpoLARITyNORMALN I'1~
w,DOWN
W,DOWN
E,UP / 8 NRM 5
SITE 7 PC 28 5.15 m
f,o
SITE 7
PC 28 4.64 m
N ~-~4-,I-
S
N
E, UP
NRM
1,'
NORMAL
REVERSED POLAF~TY
POLARITY
W, DOWN
W,
DOWN
Fig. 5. Vector demagnetization plots (ZIJDERVELD, 1967) for typical samples from box cores from mudwaves at Site 5 and 6, and for normal and reversed polarity samples from PC 28 taken from the m u d w a v e at Site 7. Filled circles represent projections onto a horizontal plane. O p e n circles are plotted on a vertical plane. Total m o m e n t ranges from 1 × 10 2 to 3 × 10-4 A M 2 . Demagnetization levels in millitesla (mT).
inclination reversals were identified in PC 30, at 2,2.5, 4.4 and 4.7 m depth; inclination data only are given because the cores were not oriented. B o x core data
Magnetic properties measurements showed some unusual contrasts between the two mudwaves for which box core samples were available. For example, the large ARM/z values observed for BC 9 and 10 indicate that in general Site 5 sediment is composed of finer magnetic grains than are found at Site 6 (Table 3). Overall g is also lower at Site 5 than at Site 6 (Table 1). Within each mudwave there are also some interesting similarities. For example, the crest at Site 6 and the steep (depositional or upstream) slopes of both mudwaves exhibit relatively consistent Z values, with slight downcore increases in magnetic grain size (ARM/ Z values) and remanent intensity (NRM/X values). On the other hand, shallow (erosional, slow sedimentation or non-depositional) slopes in these mudwaves generally show a large downcore drop in ARM/;( ratios, indicating an increase in magnetic grain size (BC 10 and
929
Magnetic properties of mudwave samples
11 in Fig. 6, and B C 24 and the top of B C 23 in Fig. 7), with a decrease in Z and r e m a n e n t intensity. R e m a n e n t intensity m e a n values are lower on the erosional than on the depositional side of these m u d w a v e s (Table 3). T h e point in cores B C 10 (Site 5) and B C 24 (Site 6), where the largest decreases in magnetic properties are observed, c o r r e s p o n d s to a large jump in 14C ages (JONES, 1987; personal c o m m u n i c a t i o n ) , which is interpreted to reflect a hiatus in sedimentation in these cores. A M S azimuth data indicate g o o d d o w n c o r e consistency ( B C 9 in Fig. 10; N = 21) and the magnetic fabric generally is characteristic of p r i m a r y deposition, where the pole to magnetic foliation, r e p r e s e n t e d by the m e a n K 3 inclination, is near vertical. C u r r e n t flow direction, using K1 axes, could not be d e t e r m i n e d f r o m these data because the box cores were not oriented during recovery. T h e A M S results f r o m all o t h e r box cores (Table 2) indicate p r i m a r y depositional fabric. DISCUSSION T h e general magnetic characteristics in the A r g e n t i n e Basin indicate two m a j o r sources for magnetic particles contained within sediment deposited in the basin. T h e s e are f r o m BC 9 x
0
BC 12 NRM/x X 0
NRM/x
ARM/x BC 10 NRMIx x
ARMIx
0.1
BC 11 NRM/x X
ARM/× 0 O2
0.1 0.1 O.2 0,4
0.3
0.2
0.5 0.4 0.3 0.6
O,S
0.6
UPSTREAM
DEPOSITION
12
SITE 5 M U D W A V E
DOWNSTREAM EROSION
Fig. 6. Downcore variations in X (magnetic grain concentration or composition), NRM/X (magnetic grain alignment consistency in the Earth's magnetic field) and ARM/X (magnetic grain size) for box core samples from the mudwave at Site 5 (see Figs 1 and 2 for locations). Mudwave shape and core locations are presented schematically. Ages reported for BC 12 and BC 10 are 14C accelerator dates in thousands (K) of years determined on organic fractions (JONES,1987;personal communication). Dark pattern = %; stripped pattern = NRM/z; stippled pattern = ARM/X. All plots to the same scale. Z ranges from 0.34 × 10-4 to 3.59 x 10-4 in mass SI units; NRM/%ranges from 0.19 × 10-1 to 2.38 x 10-1 Am2 kg-1; ARM/X ranges from 0.01 x 102 to 1.46 × 102 AM2 kg -1.
ARM/
930
B.B. ELt.WOOD BC 15 •~ NRM/x ARM/ BC 25 BC 23 x
BC 24 NR~ X
NRM/~: ARM/
NRM/x A R M / x
ARM/~
•
BC 26 ~: NRM/x
0.~L
12.0K
ARM/~
15
26 23 24 SSW DOWNSTREAM EROSION
NNE UPSTREAM DEPOSmON
SITE 6 MUDWAVE
Fig. 7. Downcore variations in X (magnetic grain concentration), NRM/Z (magnetic grain alignment consistency in the Earth's magnetic field) and ARM/% (magnetic grain size) for box core samples from the mudwave at Site 6 (see Figs 1 and 3 for locations). Mudwave shape and core locations are presented schematically. Ages reported for BC 24 and BC 25 are C14 accelerator dates in thousands (K) of years determined on organic fractions (JONES,1987; personal communication). Dark pattern = Z; stripped pattern = NRM/x; stippled pattern = ARM Z. All plots to the same scale. Z ranges from 0.34 x 10-4 to 3.59 x 10 - 4 in mass SI units; N RM/Xranges from 0.19 x 10-1 to 2.38 x 10 1Am 2 kg i; ARM/z ranges from 0.01 x 102 to 1.46 × 102 Am 2 kg - ~.
t h e S o u t h A m e r i c a n c o n t i n e n t a l m a r g i n a n d f r o m b o t t o m c u r r e n t s e n t e r i n g the basin f r o m t h e s o u t h e a s t . T h e m u d w a v e sites in the b a s i n are c h a r a c t e r i z e d by o v e r a l l m a g n e t i c susceptibilities which a r e slightly h i g h e r t h a n a r e values r e p o r t e d by SACHS and ELLWOOD (1988) for t h e basin, i n d i c a t i v e o f h i g h e r m a g n e t i c - g r a i n c o n c e n t r a t i o n s in the c e n t e r of the basin. M a g n e t i c g r a i n sizes for t h o s e m u d w a v e s a m p l e s for which A R M / Z r a t i o s have b e e n d e t e r m i n e d a r e g e n e r a l l y l a r g e r (low A R M / Z r a t i o s ) in s a m p l e s f r o m m u d w a v e sites than in c o r e - t o p s a m p l e s f r o m c e n t r a l p o r t i o n s of the b a s i n a n d a r e similar to values for the S o u t h A m e r i c a n c o n t i n e n t a l m a r g i n (SACHS a n d ELLWOOD, 1988). T h e e x c e p t i o n is B C 9, w h e r e m a g n e t i c g r a i n sizes a p p e a r to b e s m a l l e r t h a n for d a t a e l s e w h e r e in t h e A r g e n t i n e Basin. H i g h e r c o n c e n t r a t i o n s a n d l a r g e r grains suggest a c c u m u l a t i o n from a c o n t i n e n t a l m a r g i n source. T h e r e a r e s o m e o b v i o u s similarities r e p r e s e n t e d b y s a m p l e s f r o m m u d w a v e Site 5 a n d Site 6. T h e N R M , A R M a n d Z for s e d i m e n t s a m p l e s r e c o v e r e d f r o m zones of e r o s i o n ,
931
Magnetic properties of mudwave samples
down-current within both mudwaves, exhibit values that decrease downcore (Tables 1 and 3; Figs 6 and 7). These surface sediments (sampled by BC 10 and 11 at Site 5, and BC 23 and 24 at Site 6) contain higher magnetic grain concentrations but have finer magnetic grains than do sediments lying deeper in the mudwaves. It follows that NRM/Z values are also high in upper portions of these cores due to the presence of finer, more stable magnetic carriers. Below 0.1 m in these cores, magnetic properties exhibit a large decrease. The depth at which this change occurs corresponds with a large increase in 14C ages (reported by JONES, 1987; personal communication) and is interpreted to represent a hiatus in sedimentation. The different characteristics of sediments at the top, as opposed to sediments from -0.15 m below the surface, suggest a major change in sediment deposition in the center of the Argentine Basin. This change may be due to a change in sediment source or concentration. Depositional slopes (up-current) within Sites 5 and 6 mudwaves exhibit down-core variations with lower but generally uniform magnetic grain concentrations (~), than on downcurrent slopes, and slight downcore increases in both magnetic grain size (low ARM/ Z ratios) and magnetic intensity (NRM/x) (Figs 6 and 7). The radiocarbon ages reported for box cores recovered from the depositional mudwave slopes (BC 12 at Site 5 and BC 25 at Site 6 [JONES, 1987; personal communication]) indicate that the - 0 . 5 m of the recovered sediment accumulated rapidly, in less than 4000 years (-0.01 m 80 yr-1). Such high sedimentation rates indicate that suspended sediment concentration was high in the depositing bottom waters. Slight downcore increases in NRM/X may be the result of very slight dewatering as the sediment thickens and motion of magnetic grains within the sediment is stabilized. Similar slight increases in ARM/Z ratios, indicative of smaller grain sizes deeper in the mudwaves, mean that more recent sediments contain slightly coarser magnetic particles. This suggests an influx of slightly coarser magnetic grains, perhaps tied up as particle aggregates and the result of very slight increase in bottom water activity. Susceptibility values for all but one core from the mudwaves at Sites 5 and 6 show higher values at the very top of each core. One possible explanation for these data is that the upper portions of the mudwaves are represented by iron oxide grains which are being chemically reduced to less magnetic phases after deposition and sediment accumulation. There are also some interesting differences between the mudwaves sampled at Sites 5 and 6. Magnetic grain sizes, as indicated by the ARM/Z data, are generally finer at Site 5
Table 2.
Core = box core number; 1 = (K1-K2)/K; f = (K2-K3)/K; 1 (lineation) and f (foliation) from KHAN (1962).
Mudwave AMS Means Core 10 11 12 15 23 24 25 26
K 1 Dec
K1 Inc
/(3 Dec
/(3 lnc
l
f
109.3 96.2 95.2 94.6 82.0 77.0 88.0 93.0
6.5 8.5 16.5 3.4 2.9 0.2 3.4 13.6
210.4 263.8 238.7 197.5 181.6 291.4 254.3 233.9
60.6 82.1 68.8 80.0 73.7 88.6 84.1 73.2
0.011 0.010 0.017 0.020 0.011 0.001 0.010 0.017
0.015 0.011 0.019 0.015 0.019 0.011 0.013 0.013
932
B . B . ELLWOOD
0
~
0
68K ,~,
1
/
%~
XARM/X NRM/x
167K
IXARM/X NRM/x
UPSTREAM LOWER~PEED
NE SW SITE 7 MUDWAVE
H ~ . e . ACC~LATm. RAreS
DOWNSTREAM HIQ~ItleRIWqlEED LOWERA C C U ~ l l O N RATES
Fig. 8. Downcore variations in Z (magnetic grain concentration), NRM/x (magnetic grain alignment consistency in the Earth's magnetic field) and ARM/g (magnetic grain size) for piston core samples from the mudwave at Site 7 (see Figs I and 4 for locations). Mudwave shape and core locations are presented schematically. Ages in thousands (K) of years are based on assumed constant sedimentation rates of 0.1 m 1000 yr- l for PC 28 and 0.03 m 1000 yr- l for PC 30, derived from diatom data (BuRCKLE, 1987). Surface and core top samples were not available. Dark pattern = Z; stripped pattern = ARM/z; stippled pattern = NRM/X. All plots to the same scale. X ranges from 0.38 x 10 -4 to 5.44 x 10 -4 in mass SI units; NRM/x ranges from 6.8 x 10-I to 105.7 × 10- ] A m 2 k g 1; ARM/x ranges from 0.18 x 102to3.22 × 102Am2kg -I.
.7 :: ~
99.5,
E
99.0'
~:.~
-
.
"
~
--
98.5 -4
-3
-2
-1
0
1
SOURCE VELOCITY (m m
2
3
"4
s "1 )
Fig. 9. M6ssbauer spectra obtained at 78°Kelvin (shown relative to natural iron foil) for a sample from BC 9. These data are consistent with the X R D data. c = chlorite; p = pyrite; s = siderite. Hyperfine parameters for these symmetric quadrupole doublet components have been constrained to their known values (ELLWOOD et al., 1986). Unlabeled component is believed to be a clay. Data were previously reported by ELLWOOO et al. (1988).
Magnetic properties of mudwave samples
933
than at Site 6 (Table 2). Since Site 5 is farther f r o m the South A m e r i c a n continental margin, the fine m a g n e t i c grain sizes o b s e r v e d at this site m a y r e p r e s e n t depletion of coarser m a g n e t i c grains away f r o m the continental margin. Susceptibility is slightly lower at Site 5 than at Site 6 (Table 1), representing decreasing magnetic grain concentrations away f r o m the continental margin. T h e fabric p a r a m e t e r V is very u n i f o r m at Site 5, exhibiting no unique fabric patterns and a nearly spherical shape (V - 45°; T a b l e 1). H o w e v e r , at Site 6, V ranges from m o d e r a t e l y oblate (foliated, w h e r e V > 4 5 °) on the d o w n - s t r e a m side of the m u d w a v e (BC 23 and 24; T a b l e 1) to marginally prolate (lineated, w h e r e V < 4 5 °) on the u p s t r e a m (depositional) side of the m u d w a v e ( B C 26 in T a b l e 1). Table 3. Mean NRM/g × 10-1 (Am 2 kg-l) and ARM/X (Am2 kg-1) for all samples rneasured frorn each core (see text). Cores are reported as encountered across each mudwave, first frorn the steep ( depositional) slope (BC 12 and BC 26), then to the shallow (erosional) slope (BC 11 and BC 24) see Figs 2 and 3
Mudwave Site 5 (RM Data) CORE
Mean NRM/g
Mean ARM/X
BC12 BC 9 BC10 BCll
1.62 1.53 0.54 0.53
15 170 45 4
Mudwave Site 6 (RM Data) CORE
Mean NRM/Z
Mean ARM/g
BC26 BC25 BC15 BC23 BC24
0.95 1.24 1.10 0.47 0.56
13 12 11 4 5
BC 9 Fig. 10. Equal area, lower hemisphere plot of AMS azimuths measured for all samples from box-core BC 9 recovered from Mudwave 5 (Fig. 2). KI = ;K2 = A; K 3 = 0. Consistency in lineations (KI) indicates constant current flow directions across the mudwave.
RM INCLINATION (Degrees) RC25-04 PC28
1.
I
RC28-04 PC30
1
I
2.
2
3,
3
I I
4
I
6
6,
7
7
8
8,
10
10,
11
11'
12
12,
13
13'
14
14.
15
15.
16
~
16 '
: +90"
~1
' -gO"
+90"
O"
-90"
Fig. 11. R e m a n e n t inclination (degrees) after demagnetization for samples from RC28-04, PC 28 and PC 30 recovered from the m u d w a v e at Site 7 (Fig. 4). N u m b e r s associated with reversed polarity peaks are ages in thousands of years and are based on a constant sedimentation rate for the core of 0.1 m 1000 yr - l determined from diatom data (BURCKLE, 1987). Cores were not oriented.
Magnetic properties of mudwavesamples
935
The only distinctly prolate shapes are found at Site 6; BC 15 on the crest of the mudwave, and BC 26 on the steep depositional side of the mudwave. Normally a foliated fabric would be expected due to settling during deposition. This can be modified by deposition from a high velocity water mass, where a more prolate fabric is usually produced. However, at Site 6, the prolate shapes appear where deposition is occurring and velocities are lower. This was not expected, and such unusual results may be due to the presence of slight amounts of the mineral siderite, present in many of the Argentine Basin samples (see below). Siderite exhibits a strong magnetocrystalline anisotropy, producing prolate magnetic fabrics. Variations in siderite concentration could easily explain the generally low values of V observed in these cores. These values can also be explained by high velocity bottom waters producing a lineated fabric that can modify the foliation sufficiently to reduce V values to the levels observed. Mineral studies
XRD and M6ssbauer effect analysis (for example see Fig. 9) identified the mineral siderite in a number of the box core samples. This siderite was probably formed authigenically by dissimilatory iron-reducing bacteria (BELL et al., 1987; ELLWOODet al., 1988) below the redox boundary. ELLWOODet al. (1986) have shown that siderite can cause unusual changes in magnetic behavior because siderite is unstable and readily oxidizes to form secondary, highly magnetic iron oxides. They also concluded that siderite was probably forming in Vema Channel sediments (located at the northern end of the Argentine Basin) at 0.4--0.8 m below the sediment water interface. Their conclusion was based on a distinct change in magnetic fabric patterns, exhibited by Vema Channel samples, from a primary depositional style to an anomalous, magnetocrystalline fabric pattern similar to that observed for siderite. The presence of siderite in small but varying amounts in the mudwaves may explain the few anomalous magnetic fabric patterns in some box core samples (Table 2). Of greater importance is the observation that siderite oxidation in the laboratory can produce abrupt remanent magnetic polarity changes (ELLWOOD et al., 1986, 1988). Therefore, we have made every effort to measure these samples rapidly, and demagnetize them to levels (30 mT) that have been shown to be sufficient to eliminate short-term, smallscale magnetic changes due to siderite oxidation (ELLWOOD et al., 1988). A stable remanent moment (RM) is exhibited by most samples from Sites 5 and 6 after AF demagnetization to 30 mT (Fig. 5). On the other hand, above 20 mT, demagnetization results for samples from the Site 7 mudwave exhibit unstable RM behavior (Fig. 5). The resulting magnetic polarities show most samples to be normally magnetized, consistent with an age of <730,000 years for these sediments (MANKINENand DALRYMPLE,1979). However, anomalous inclination values from PC 28 (Fig. 11) and PC 30 were not expected, because diatom data indicate that the base of cores PC 28 and PC 30 are <200,000 and <620,000 years, respectively (MANLEYand FLOOD, 1993; BURCKLE,1987). PC 30 anomalous samples are single point values and normally would not be used for determining polarities. However, between 4 and 5.3 m depth in PC 28, there are two sample clusters that exhibit reversed polarities (Fig. 11). If it is assumed that the core has a uniform sedimentation rate of 0.1 m 1000 yr- 1 (based on diatom ages), then the ages of the reversed samples fall in the range of 42,000-52,000 years. It is unlikely that these data represent recorded excursions in the geomagnetic field. Rather, the greater likelihood is that the
936
B . B . ELLWOOD
data are anomalous, possibly resulting from unusual polarities produced in sideritecontaining samples, like those occurring elsewhere (ELLWOOD et al., 1986). Furthermore, these "reversed" samples do not exhibit stable R M behavior with demagnetization (i.e. vector decay does not trend toward the origin; Fig. 5). A test of the diatom ages and estimated sedimentation rates determined for PC 28 and PC 30 is given in Fig. 8. Magnetic properties measurements for these cores show one unusually high susceptibility value in each core. Using the assumption of constant sedimentation and a rate of 0.1 m 1000 yr -1 for PC 28 and 0.03 m 1000 yr t for PC 30, the single high susceptibility peak in both cores give an age of - 6 8 , 0 0 0 years (labeled in Fig. 7). Only two other magnetic properties peaks are readily correlated to the same age in both cores; the ARM/X ratios labeled by 167K in Fig. 7 are assigned an age of 167,000 years based on sedimentation rates. These data support the conclusion that deposition is greater on the northeast than on the southwest side of the mudwave at Site 7. CONCLUSIONS Distinctive similarities and differences exist in magnetic properties between and within each Argentine Basin mudwaves sampled. Between mudwave similarities include a stable remanent m o m e n t to 20 or 30 m T A F demagnetization in samples and uniform downcore magnetic fabric patterns. Within mudwave similarities include magnetic grain size increases and downcore magnetic grain concentration decreases; prolate or nearly neutral magnetic fabrics in samples from portions of the mudwaves interpreted to represent erosional surfaces; downcore grain size and concentration uniformity; and oblate fabric patterns in portions of the mudwave representing depositional slopes. Differences include finer magnetic grain sizes at Site 5 as opposed to Site 6 but larger grain size variations at Site 5. The data from Sites 5 and 6 are consistent with the interpretation that b o t t o m water velocity, during deposition of the sediments analyzed, was either higher at Site 6 than at Site 5, or that Site 6 is closer to the source of the magnetic particles being deposited. Velocities over individual mudwave sites were higher over shallow slopes (inferred to be zones of erosion) than over steep slopes (inferred to be zones of deposition). Also, the magnetic data indicate clear changes in current flow and therefore deposition/erosion patterns in the mudwaves. Acknowledgements--Research was supported by ONR contract AF 33615-86-C-2721, to M. T. Ledbetter,
subcontract No. 399 from the San Jose State University Foundation to B. B. Ellwood. Gary Long, at the University of Missouri-Rolla, kindly provided the M6ssbauer effect data, research supported by the donors of the Petroleum Research Fund, administered by the American Chemical Societythrough grant # 18202-AC3 to Gary Long. Curation of cores at Lamont-Doherty is provided by NSF grant DES72-01568 and ONR contract N0001475-C-02t0. I thank M. T. Ledbetter and R. Lotti for help in obtaining samples, and M. T. Ledbetter for manuscript review. REFERENCES BURCKLEL. H. (1987) Diatom biostratigraphic data bearing on the formation of mudwaves in the central Argentine Basin, LOS (Transactions of the American Geophysical Union), 68, 1747. COLLINSONO. W., L. MOLYNEUXand D. B. STONE(1963) A total and anisotropic magnetic susceptibilitymeter. Journal of Scientific Instruments, 40, 310-312. ELLWOODB. B., W. BALSAM,B. BURKART,G. J. LONGand M. L. BUHL(1986) Anomalous magnetic properties in
Magnetic properties of mudwave samples
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rocks containing the mineral siderite: Paleomagnetic implications. Journal of Geophysical Research, 91, 12,779-12,790. ELLWOOD B. B., T. H. CHaZANOWSI~I,G. J. LONG and M. L. BUHL (1988) Siderite formation in anoxic deep-sea sediments: a synergetic bacterially controlled process with important implications in paleomagnetism. Geology, 16,980-982. ELLWOOD B. B., F. HROUDAand J.-J. WAGNER (1988) Symposia on magnetic fabrics: introductory comments, Phys. Earth Planet. Int., 51,249-252. ELLWOODB. B. and M. T. LEDBErrER (1977) Antarctic Bottom Water fluctuation in the Vema Channel: effects of velocity changes on particle alignment and size, Earth Planetary Science Letters, 35, 189-198. EWING M., S. L. EITTREIM, J. J. EWING and X. LE PICHON (1977) Sediment transport and distributions in the Argentine Basin. 3. Nepheloid layer and processes of sedimentation. In: Physics and Chemistry of the Earth, Vol. 8, L. H. AHRENS, F. PRESS, S. K. RUNCORNand H. C. UREV, editors, Pergamon Press, Oxford, pp. 49-77. FLOOD R. D. (1988) A lee-wave model for deep-sea mudwave activity. Deep-Sea Research, 35, 973-983. FLOOD R. D. and A. N. SHOR(1988) Mudwaves in the Argentine Basin and their relationship to regional bottom circulation patterns. Deep-Sea Research, 35,943-971. GEORGI D. T. Circulation of bottom water in the South Atlantic. Deep-Sea Research, 28a, 959-979. GRAHAM J. W. (1966) Significance of magnetic anisotropy in Appalachian sedimentary rocks. In: The earth beneath the continents, J. S. STEINHARDand T. J. SMITH,American Geophysical Union, Washington DC, pp. 627-648. JONES G. A. (1987) Project Mudwaves: Preliminary results of sediment analyses of selected box cores from the Argentine Basin, EOS (Transactions of the American Geophysical Union), 68, 1747. KHAN M. A. (1962) Anisotropy of magnetic susceptibility of some igneous and metamorphic rocks. Journal of Geophysical Research, 67, 2873-2885. KING R. F. and A. I. REES (1962) The measurement of the anisotropy of magnetic susceptibility of rocks by the torque method. Journal of Geophysical Research, 67, 1565-1572. KLAUS A. and M. T. LEDBETrER(1988) Deep-sea sedimentary processes in the Argentine Basin revealed by highresolution seismic records (3.5 kHz echograms). Deep-Sea Research, 35,899-917. LEDBETTER M. T. (1986) Bottom-current pathways in the Argentine Basin revealed by mean slit particle size. Nature, 321,423--425. LEDBETTER, i . T. (1993) Particle-size evidence of late Holocene paleocurrent activity on Argentine Basin mudwaves. Deep Sea Research H, 40, 911-920. LEDBETTERM. T. and B. B. ELLWOOD(1980) Spatial and temporal changes in bottom-water velocity and direction from analysis of particle size and alignment in deep-sea sediments. Marine Geology, 38,245-261. LEDBETTER M. T. and D. A. JOHNSON (1976) Increased transport of Antarctic Bottom Water in the Vema Channel during the last ice age. Science, 191,837-839. MANKINEN E. A. and G. B. DALRYMPLE(1979) Revised geomagnetic polarity time scale for the interval 0-5 m.y. BP. Journal of Geophysical Research, 84, 615-626. MANLEVP. L. and R. D. FLOOD(1993) Paleoflow history determined from mudwave migration: Argentine Basin. Deep Sea Research H, 40, 1033-1055. MILL1MAN J. D. (1988) Correlation of 3.5 kHz acoustic penetration and deposition/erosion in the Argentine Basin: a note. Deep-Sea Research, 35, 919-927. NVE J. R. (1969) Physical Properties of Crystals. Oxford University Press, New York, N.Y., 322 pp. SACHS S. D. and B. B. ELLWOOD (1988) Controls on magnetic grain-size variations and concentration in the Argentine Basin, South Atlantic Ocean. Deep-Sea Research, 35,929-942. SEGUIN M. (1966) Instability of FeCO3 in air. American Journal of Science, 264, 562-568. TAIRA A. and B. R. LIENERT (1979) The comparative reliability of magnetic photometric and microscopic methods of determining the orientations of sedimentary grains. Journal of Sedimentary Petrology, 49,759772. ZIJDERVELDJ. D. A. (1967)A.f. demagnetization of rocks: analysis of results. In: Methods in Paleomagnetism, D. W. COLLINSON et al., editors, Elsevier, Amsterdam, pp. 254-286.
0967~645/93 $6.00 + 0.00 © 1993 Pergamon Press Ltd
Printed in Great Britain.
Magnetic properties of Argentine Basin Project MUDWAVE samples BROOKS B . ELLWOOD*
(Received 19 February 1990; in revised form 13 October 1992; accepted 24 February 1993) Abstract--The magnetic properties of samples from nine box cores and three piston cores recovered from three giant mudwaves in the Argentine Basin, South Atlantic Ocean, have been measured. Measurements on mudwave samples include magnetic susceptibility, natural remanent moment and anhysteretic remanent moment. Anisotropy of magnetic susceptibility was also measured on selected box core samples. The results have been compared to magnetic data already available from similar measurements on Argentine Basin core-top samples. Magnetic characteristics for the core-top samples reflect broad changes in magnetic mineral concentration rather than substantial changes in magnetic grain size across the basin. Changes in this pattern are caused by multiple sources of terrigenous sediment influx from the South American continental margin. The mudwave fields in the center of the basin act as a sediment sink for these materials and are generally characterized by uniform magnetic-grain concentrations, compositions and small grain-sizes. Between and within the mudwaves sampled there are systematic differences as well as similarities in the magnetic properties that appear to be related, at least in part, to sample location with respect to depositional or erosional slopes within each mudwave. The magnetic results are interpreted to indicate that clear changes in bottom water flow are represented in samples recovered from the mudwaves.
INTRODUCTION
SEDIMENTSfrom the Argentine Basin area of the South Atlantic Ocean have been studied extensively to identify paleo-oceanographic changes in deep ocean currents that are flowing throughout the basin (e.g. EWING et al., 1971; LEDBETrER and JOHNSON, 1976; ELLWOOD and LEDBETTER, 1977; LEDBETrER, 1986; SACHS and ELLWOOD, 1988; and others). Studies have reported both the modern Antarctic Bottom Water (AABW) current flow direction in the basin (e.g. GEORGI, 1981; LEDBETrER, 1986) and temporal changes in bottom water production (LEDBETTER and JOHNSON, 1976; LEDBETrER and ELLWOOD, 1980). Especially interesting in the Argentine Basin are giant ripples or mudwaves first described by EWING et al. (1971). They suggested that these features were moving downcurrent in response to AABW flow in the deep Argentine Basin. It is clear that these mudwave fields are unusual, certainly in their size characteristics, because some of these mudwaves are >135 m in height and many have a wave length of 5 km or more (KLAtJS and LEDBETrER, 1988; FLOOD and SHOR, 1988). A summary of many of the Argentine Basin *Department of Geology, University of Texas at Arlington, P.O. Box 19049, Arlington, TX 76019, U.S.A. 921
922
B.B. ELLWOOD
studies as well as new data is represented by a recent series of papers dealing with the sedimentary, acoustic and magnetic properties observed in the basin (FLoor), 1988; FLoor) and SrmR, 1988; KLAUS and LEDBEaWER, 1988; MILLIMAN, 1988; SacHs and EkLWOOD, 1988) and in this volume. This work has delineated the mudwave fields in the basin (FLooD and SnoR, 1988) and dynamic models for these waves indicate that they are migrating upcurrent (FLooD, 1988). Accumulation of the large amount of sediment necessary to sustain the mudwaves appears to be in areas of the basin where A A B W is relatively weak (KLAUS and LEDBE~ER, 1988). The waves also appear to be very long-term features (FLooD and SnoR, 1988) that probably developed from sediment transported into the deep basin either from the South American continental margin (KLAus and LErgBETrER, 1988; SACrtS and ELLWOOD, 1988) or as the result of the accumulation of fine sediments winnowed from coarser near margin sediments and later redeposited as A A B W speed slows (MR, LIMAN, 1988). Due to the size of the mudwave fields and to questions concerning genesis of the mudwaves, source of sediment, controls by A A B W , and other factors, Cruise 28 of the R.V. Robert D. Conrad was planned, with the primary mission to study the topography, structure, sediment type and bottom water flow characteristics in the vicinity of the mudwaves. The purpose of this paper was to measure and interpret the magnetic characteristics of sediments recovered from each set of mudwaves. METHODS This study presents magnetic properties results for sediment samples taken from nine box cores and three piston cores recovered from three mudwaves at different localities in the central portion of the Argentine Basin (Fig. 1). Four of the box cores for which data are reported here sampled a mudwave at Site 5 (Fig. 2), and five box cores recovered sediment from a mudwave at Site 6 (Fig. 3). These box cores are well distributed within each mudwave and were chosen to represent both depositional and erosional mudwave settings (FLooD and SnOR, 1988). One piston core used in this study sampled the mudwave at Site 6 and two piston cores were chosen to sample a mudwave at Site 7 (Fig. 4), one from each flank of the mudwave. Plastic paleomagnetic sample boxes (8 cc) were used to recover a representative vertical cross section of the sediment from each box core and represent continuous samples from the sediment/water interface to the base of each core. Plastic boxes were inserted into the split half of piston cores: PC 21 ( - 1 7 m long); PC 28 ( - 1 7 m long); and PC 30 ( - 1 8 m long) at ~-0.t m average intervals. Closer spaced samples were also acquired between 4 and 5.3 m depth in PC 28 where the remanent magnetic properties were determined to be unusual. Measurements on all samples included initial magnetic susceptibility (Z), natural remanent moment (NRM), RM after alternating field (AF) demagnetization, and anhysteretic remanent moment (ARM). Anisotropy of magnetic susceptibility (AMS) also was measured for selected box core samples.
Rock magnetic studies A set of pilot samples was demagnetized in alternating fields to 50 millitesla (mT) magnetic induction. The RM was then remeasured and vector diagrams constructed from the data (Fig. 5). Based on these results, all other samples were A F demagnetized at 15 and 30 mT.
Magnetic properties of mudwavesamples RO"
55"
50"
45"
40'
35"
30"
25"
20"
923 15"
10"
5"
0o 30"
35"
40"
45"
50"
55"
60'
60"
55"
50"
45"
40"
35"
30"
25"
20"
15"
10"
,5
Fig. 1. Argentine Basin map, taken from SACHSand ELLWOOD(1988), with mudwave localities (Sites 5, 6 and 7) that were sampled and ARM contours for core-top samples taken from throughout the basin (indicated by dots). Higher ARM values are interpreted to represent the presence of finer magnetic grains in samples (SACHSand ELLWOOD,1988)
A R M was measured after subjecting samples to decreasing AF induction levels in the presence of a 0.1 mT steady (DC) magnetic induction. Such data have been used to estimate relative size of magnetic grains (for example see COLLINSON, 1983, for a discussion of the use of A R M data), with larger A R M values interpreted to indicate the presence of finer magnetic grains. ARM~Z ratios are used to increase the precision in magnetic grain-size estimates over ARM data alone (see for example the discussion by SACriS and ELLWOOD, 1988) by reducing the effects of magnetic grain concentrations. NRM/Z ratios illustrate RM intensity variations while the effect of varying magnetic grain concentrations on NRM values is reduced. Figures 6, 7 and 8 give the ARM~z, NRM/z, and Z data for box and piston cores recovered from mudwaves at Sites 5, 6 and 7, respectively.
Anisotropy of magnetic susceptibility (AMS) AMS is commonly expressed as an ellipsoid with principal K a (maximum), K2 (intermediate) and K3 (minimum) axes, which in general terms reflect the physical orientation
924
B . B . ELLWOOD
42"2~'
O o G
0 0
@
©
42-30 '
\
@
42"35' L 45ol0'
45"0'5'
45"00
ARGENTINE BASIN - R C 2 8 0 4 MUDWAVE SITE 5 Fig. 2. M u d w a v e Site 5: the four b o x - c o r e l o c a t i o n s are p l o t t e d for which data are r e p o r t e d . D e p o s i t i o n is i n f e r r e d to occur on the s t e e p e r , s o u t h e a s t e r n side of the m u d w a v e (FLooD and SttOR, 1988).
Magnetic properties of mudwave samples
925
4~'4o'
4 9 - I S'
49,10'
49'0 ~ '
ARGENTINE BASIN - RC2BO,4 MUDWAVE SITE 6 Fig. 3.
M u d w a v e Site 6: the five 5ox-corc and one piston core locations are plotted for which data
are reported. Deposition is inferred to occur on the steeper, northern side of the mudwave (FLOOD and SHOR, 1988).
of magnetic elements in a single sample [see KING and REES (1962) and NYE (1969) for a discussion of the method]. In sediment samples, the AMS ellipsoid has been shown to reflect the petrofabric orientation (e.g. TAIRAand LIENERT, 1979), which results from any process which physically orients magnetic constituents within the sample, such as flow, crystal growth, diagenesis or stress. Mass susceptibility in SI units is given as K = (K1 +/(2 +/(3)/3.
(1)
The AMS ellipsoid shape expresses either the preferred alignment of included magnetic grains, or the overall crystallographic mineral orientation in the sample. The parameter V, developed by GRAHAM(1966), is an excellent indicator of shape, where
926
B.B. ELLWOOD
ARGENTINE MUDWAVE
37" 00'~
BASIN - RC2804 SURVEY AREA 7
39o55'W
40°00 ' W
I
i
I
]
I
I
1
'
1
\
\
37°05'$
\ % J
1
Fig. 4. Mudwave Site 7: the two piston core locations are plotted for which data are reported. Deposition is inferred to occur on the northeastern side of the mudwave (MANLEY and F t o o n , 1993).
sin 2 V = ( K 2 - K 3 ) / ( K I - Ks)
(2)
and V <45 ° for prolate shapes and >45 ° for oblate shapes. Common usage of AMS parameters as well as standardization and calibration of the method has been discussed elsewhere (EccwooD et a l . , 1988). Initial mass susceptibility (Z) was measured using a susceptibility bridge (CoLLINSON et a l . , 1963) along the vertical axis of each sample. The data are given in Figs 6 and 7, while means for each box core are reported in Table 1. z d a t a are usually interpreted to represent magnetic grain concentrations within a sample. However, changes in magnetic grain composition within a core will be reflected in the Z values observed. For example, the reduction of magnetite or hematite to siderite or pyrite will greatly reduce Z. On the other hand, oxidation of siderite will result in the production of magnetite or maghemite (SEGUIN, 1966) which will strongly increase Z. Mineral studies
M6ssbauer effect spectra were obtained at room temperature and at 78°K for two samples from BC 9. The iron carbonate mineral siderite (FeCO3), along with pyrite and chlorite, is identified in these samples (Fig. 9). X-ray diffraction (XRD) data were also obtained for several piston and box core samples with a Philips diffractometer utilizing
Magnetic properties of mudwave samples
927
C u K - a radiation and a graphite crystal m o n o c h r o m e t e r . Each sample contained a small amount of siderite. RESULTS Susceptibility data
AMS m e a s u r e m e n t s were conducted on all samples from BC 9 and the axial data reported (Fig. 10), and for selected samples from each box core. A M S was not measured for all samples because neither box or piston cores were oriented. Because these m e a s u r e m e n t s on such weak samples are somewhat time consuming, only selected samples were measured and the data are given in Table 2. The close correspondence in mean K1 declinations for all box cores is probably due to soft sediment deformation during sampling. This effect does not destroy the overall fabric but may slightly modify the fabric parameters. Initial susceptibility for all box core samples is given in Figs 6-8 and Z means for individual box cores are reported in Table 1. R e m a n e n t m a g n e t i c data
All box core and PC 21 samples were normally magnetized suggesting that deposition and m o m e n t acquisition occurred during the Brunhes Normal Chron (<730,000 years; MANKINEN and DALRYMPLE, 1979). Most PC 28 and PC 30 samples were also normally magnetized. However, five reversed samples (one being a single-point reversal) were identified in PC 28 between 4.0 and 5.3 m depth in core (Fig. 11). Four single-sample, low
Table1. Mean )~ × lO-S in g mass Sl units and V (oblate >45°; prolate <45°) for all samples measured from each core (see text). Cores are reported as encountered across each mudwave, first from the steep (depositional) slope (BC 12 and BC 24), then to the shallow (erosional) slope (BC 11 and BC 26)
Mudwave Site 5 (AMS Data) CORE
Mean Z
Mean V
BC12 BC 9 BC10 BCll
11.7 12.0 12.4 14.4
45.4 46.9 46.9 45.2
Mudwave Site 6 (AMS Data) CORE
Mean Z
Mean V
BC26 BC25 BC15 BC23 BC24
16.7 16.1 15.0 16.5 20.5
42.6 49.0 40.0 54.3 51.4
928
B.B. ELLWOOO SITE 5 BC 9
SITE 6 BC 25
E, UP
N ::::
E,
NRM
:~:IIIISpoLARITyNORMALN I'1~
w,DOWN
W,DOWN
E,UP / 8 NRM 5
SITE 7 PC 28 5.15 m
f,o
SITE 7
PC 28 4.64 m
N ~-~4-,I-
S
N
E, UP
NRM
1,'
NORMAL
REVERSED POLAF~TY
POLARITY
W, DOWN
W,
DOWN
Fig. 5. Vector demagnetization plots (ZIJDERVELD, 1967) for typical samples from box cores from mudwaves at Site 5 and 6, and for normal and reversed polarity samples from PC 28 taken from the m u d w a v e at Site 7. Filled circles represent projections onto a horizontal plane. O p e n circles are plotted on a vertical plane. Total m o m e n t ranges from 1 × 10 2 to 3 × 10-4 A M 2 . Demagnetization levels in millitesla (mT).
inclination reversals were identified in PC 30, at 2,2.5, 4.4 and 4.7 m depth; inclination data only are given because the cores were not oriented. B o x core data
Magnetic properties measurements showed some unusual contrasts between the two mudwaves for which box core samples were available. For example, the large ARM/z values observed for BC 9 and 10 indicate that in general Site 5 sediment is composed of finer magnetic grains than are found at Site 6 (Table 3). Overall g is also lower at Site 5 than at Site 6 (Table 1). Within each mudwave there are also some interesting similarities. For example, the crest at Site 6 and the steep (depositional or upstream) slopes of both mudwaves exhibit relatively consistent Z values, with slight downcore increases in magnetic grain size (ARM/ Z values) and remanent intensity (NRM/X values). On the other hand, shallow (erosional, slow sedimentation or non-depositional) slopes in these mudwaves generally show a large downcore drop in ARM/;( ratios, indicating an increase in magnetic grain size (BC 10 and
929
Magnetic properties of mudwave samples
11 in Fig. 6, and B C 24 and the top of B C 23 in Fig. 7), with a decrease in Z and r e m a n e n t intensity. R e m a n e n t intensity m e a n values are lower on the erosional than on the depositional side of these m u d w a v e s (Table 3). T h e point in cores B C 10 (Site 5) and B C 24 (Site 6), where the largest decreases in magnetic properties are observed, c o r r e s p o n d s to a large jump in 14C ages (JONES, 1987; personal c o m m u n i c a t i o n ) , which is interpreted to reflect a hiatus in sedimentation in these cores. A M S azimuth data indicate g o o d d o w n c o r e consistency ( B C 9 in Fig. 10; N = 21) and the magnetic fabric generally is characteristic of p r i m a r y deposition, where the pole to magnetic foliation, r e p r e s e n t e d by the m e a n K 3 inclination, is near vertical. C u r r e n t flow direction, using K1 axes, could not be d e t e r m i n e d f r o m these data because the box cores were not oriented during recovery. T h e A M S results f r o m all o t h e r box cores (Table 2) indicate p r i m a r y depositional fabric. DISCUSSION T h e general magnetic characteristics in the A r g e n t i n e Basin indicate two m a j o r sources for magnetic particles contained within sediment deposited in the basin. T h e s e are f r o m BC 9 x
0
BC 12 NRM/x X 0
NRM/x
ARM/x BC 10 NRMIx x
ARMIx
0.1
BC 11 NRM/x X
ARM/× 0 O2
0.1 0.1 O.2 0,4
0.3
0.2
0.5 0.4 0.3 0.6
O,S
0.6
UPSTREAM
DEPOSITION
12
SITE 5 M U D W A V E
DOWNSTREAM EROSION
Fig. 6. Downcore variations in X (magnetic grain concentration or composition), NRM/X (magnetic grain alignment consistency in the Earth's magnetic field) and ARM/X (magnetic grain size) for box core samples from the mudwave at Site 5 (see Figs 1 and 2 for locations). Mudwave shape and core locations are presented schematically. Ages reported for BC 12 and BC 10 are 14C accelerator dates in thousands (K) of years determined on organic fractions (JONES,1987;personal communication). Dark pattern = %; stripped pattern = NRM/z; stippled pattern = ARM/X. All plots to the same scale. Z ranges from 0.34 × 10-4 to 3.59 x 10-4 in mass SI units; NRM/%ranges from 0.19 × 10-1 to 2.38 x 10-1 Am2 kg-1; ARM/X ranges from 0.01 x 102 to 1.46 × 102 AM2 kg -1.
ARM/
930
B.B. ELt.WOOD BC 15 •~ NRM/x ARM/ BC 25 BC 23 x
BC 24 NR~ X
NRM/~: ARM/
NRM/x A R M / x
ARM/~
•
BC 26 ~: NRM/x
0.~L
12.0K
ARM/~
15
26 23 24 SSW DOWNSTREAM EROSION
NNE UPSTREAM DEPOSmON
SITE 6 MUDWAVE
Fig. 7. Downcore variations in X (magnetic grain concentration), NRM/Z (magnetic grain alignment consistency in the Earth's magnetic field) and ARM/% (magnetic grain size) for box core samples from the mudwave at Site 6 (see Figs 1 and 3 for locations). Mudwave shape and core locations are presented schematically. Ages reported for BC 24 and BC 25 are C14 accelerator dates in thousands (K) of years determined on organic fractions (JONES,1987; personal communication). Dark pattern = Z; stripped pattern = NRM/x; stippled pattern = ARM Z. All plots to the same scale. Z ranges from 0.34 x 10-4 to 3.59 x 10 - 4 in mass SI units; N RM/Xranges from 0.19 x 10-1 to 2.38 x 10 1Am 2 kg i; ARM/z ranges from 0.01 x 102 to 1.46 × 102 Am 2 kg - ~.
t h e S o u t h A m e r i c a n c o n t i n e n t a l m a r g i n a n d f r o m b o t t o m c u r r e n t s e n t e r i n g the basin f r o m t h e s o u t h e a s t . T h e m u d w a v e sites in the b a s i n are c h a r a c t e r i z e d by o v e r a l l m a g n e t i c susceptibilities which a r e slightly h i g h e r t h a n a r e values r e p o r t e d by SACHS and ELLWOOD (1988) for t h e basin, i n d i c a t i v e o f h i g h e r m a g n e t i c - g r a i n c o n c e n t r a t i o n s in the c e n t e r of the basin. M a g n e t i c g r a i n sizes for t h o s e m u d w a v e s a m p l e s for which A R M / Z r a t i o s have b e e n d e t e r m i n e d a r e g e n e r a l l y l a r g e r (low A R M / Z r a t i o s ) in s a m p l e s f r o m m u d w a v e sites than in c o r e - t o p s a m p l e s f r o m c e n t r a l p o r t i o n s of the b a s i n a n d a r e similar to values for the S o u t h A m e r i c a n c o n t i n e n t a l m a r g i n (SACHS a n d ELLWOOD, 1988). T h e e x c e p t i o n is B C 9, w h e r e m a g n e t i c g r a i n sizes a p p e a r to b e s m a l l e r t h a n for d a t a e l s e w h e r e in t h e A r g e n t i n e Basin. H i g h e r c o n c e n t r a t i o n s a n d l a r g e r grains suggest a c c u m u l a t i o n from a c o n t i n e n t a l m a r g i n source. T h e r e a r e s o m e o b v i o u s similarities r e p r e s e n t e d b y s a m p l e s f r o m m u d w a v e Site 5 a n d Site 6. T h e N R M , A R M a n d Z for s e d i m e n t s a m p l e s r e c o v e r e d f r o m zones of e r o s i o n ,
931
Magnetic properties of mudwave samples
down-current within both mudwaves, exhibit values that decrease downcore (Tables 1 and 3; Figs 6 and 7). These surface sediments (sampled by BC 10 and 11 at Site 5, and BC 23 and 24 at Site 6) contain higher magnetic grain concentrations but have finer magnetic grains than do sediments lying deeper in the mudwaves. It follows that NRM/Z values are also high in upper portions of these cores due to the presence of finer, more stable magnetic carriers. Below 0.1 m in these cores, magnetic properties exhibit a large decrease. The depth at which this change occurs corresponds with a large increase in 14C ages (reported by JONES, 1987; personal communication) and is interpreted to represent a hiatus in sedimentation. The different characteristics of sediments at the top, as opposed to sediments from -0.15 m below the surface, suggest a major change in sediment deposition in the center of the Argentine Basin. This change may be due to a change in sediment source or concentration. Depositional slopes (up-current) within Sites 5 and 6 mudwaves exhibit down-core variations with lower but generally uniform magnetic grain concentrations (~), than on downcurrent slopes, and slight downcore increases in both magnetic grain size (low ARM/ Z ratios) and magnetic intensity (NRM/x) (Figs 6 and 7). The radiocarbon ages reported for box cores recovered from the depositional mudwave slopes (BC 12 at Site 5 and BC 25 at Site 6 [JONES, 1987; personal communication]) indicate that the - 0 . 5 m of the recovered sediment accumulated rapidly, in less than 4000 years (-0.01 m 80 yr-1). Such high sedimentation rates indicate that suspended sediment concentration was high in the depositing bottom waters. Slight downcore increases in NRM/X may be the result of very slight dewatering as the sediment thickens and motion of magnetic grains within the sediment is stabilized. Similar slight increases in ARM/Z ratios, indicative of smaller grain sizes deeper in the mudwaves, mean that more recent sediments contain slightly coarser magnetic particles. This suggests an influx of slightly coarser magnetic grains, perhaps tied up as particle aggregates and the result of very slight increase in bottom water activity. Susceptibility values for all but one core from the mudwaves at Sites 5 and 6 show higher values at the very top of each core. One possible explanation for these data is that the upper portions of the mudwaves are represented by iron oxide grains which are being chemically reduced to less magnetic phases after deposition and sediment accumulation. There are also some interesting differences between the mudwaves sampled at Sites 5 and 6. Magnetic grain sizes, as indicated by the ARM/Z data, are generally finer at Site 5
Table 2.
Core = box core number; 1 = (K1-K2)/K; f = (K2-K3)/K; 1 (lineation) and f (foliation) from KHAN (1962).
Mudwave AMS Means Core 10 11 12 15 23 24 25 26
K 1 Dec
K1 Inc
/(3 Dec
/(3 lnc
l
f
109.3 96.2 95.2 94.6 82.0 77.0 88.0 93.0
6.5 8.5 16.5 3.4 2.9 0.2 3.4 13.6
210.4 263.8 238.7 197.5 181.6 291.4 254.3 233.9
60.6 82.1 68.8 80.0 73.7 88.6 84.1 73.2
0.011 0.010 0.017 0.020 0.011 0.001 0.010 0.017
0.015 0.011 0.019 0.015 0.019 0.011 0.013 0.013
932
B . B . ELLWOOD
0
~
0
68K ,~,
1
/
%~
XARM/X NRM/x
167K
IXARM/X NRM/x
UPSTREAM LOWER~PEED
NE SW SITE 7 MUDWAVE
H ~ . e . ACC~LATm. RAreS
DOWNSTREAM HIQ~ItleRIWqlEED LOWERA C C U ~ l l O N RATES
Fig. 8. Downcore variations in Z (magnetic grain concentration), NRM/x (magnetic grain alignment consistency in the Earth's magnetic field) and ARM/g (magnetic grain size) for piston core samples from the mudwave at Site 7 (see Figs I and 4 for locations). Mudwave shape and core locations are presented schematically. Ages in thousands (K) of years are based on assumed constant sedimentation rates of 0.1 m 1000 yr- l for PC 28 and 0.03 m 1000 yr- l for PC 30, derived from diatom data (BuRCKLE, 1987). Surface and core top samples were not available. Dark pattern = Z; stripped pattern = ARM/z; stippled pattern = NRM/X. All plots to the same scale. X ranges from 0.38 x 10 -4 to 5.44 x 10 -4 in mass SI units; NRM/x ranges from 6.8 x 10-I to 105.7 × 10- ] A m 2 k g 1; ARM/x ranges from 0.18 x 102to3.22 × 102Am2kg -I.
.7 :: ~
99.5,
E
99.0'
~:.~
-
.
"
~
--
98.5 -4
-3
-2
-1
0
1
SOURCE VELOCITY (m m
2
3
"4
s "1 )
Fig. 9. M6ssbauer spectra obtained at 78°Kelvin (shown relative to natural iron foil) for a sample from BC 9. These data are consistent with the X R D data. c = chlorite; p = pyrite; s = siderite. Hyperfine parameters for these symmetric quadrupole doublet components have been constrained to their known values (ELLWOOD et al., 1986). Unlabeled component is believed to be a clay. Data were previously reported by ELLWOOO et al. (1988).
Magnetic properties of mudwave samples
933
than at Site 6 (Table 2). Since Site 5 is farther f r o m the South A m e r i c a n continental margin, the fine m a g n e t i c grain sizes o b s e r v e d at this site m a y r e p r e s e n t depletion of coarser m a g n e t i c grains away f r o m the continental margin. Susceptibility is slightly lower at Site 5 than at Site 6 (Table 1), representing decreasing magnetic grain concentrations away f r o m the continental margin. T h e fabric p a r a m e t e r V is very u n i f o r m at Site 5, exhibiting no unique fabric patterns and a nearly spherical shape (V - 45°; T a b l e 1). H o w e v e r , at Site 6, V ranges from m o d e r a t e l y oblate (foliated, w h e r e V > 4 5 °) on the d o w n - s t r e a m side of the m u d w a v e (BC 23 and 24; T a b l e 1) to marginally prolate (lineated, w h e r e V < 4 5 °) on the u p s t r e a m (depositional) side of the m u d w a v e ( B C 26 in T a b l e 1). Table 3. Mean NRM/g × 10-1 (Am 2 kg-l) and ARM/X (Am2 kg-1) for all samples rneasured frorn each core (see text). Cores are reported as encountered across each mudwave, first frorn the steep ( depositional) slope (BC 12 and BC 26), then to the shallow (erosional) slope (BC 11 and BC 24) see Figs 2 and 3
Mudwave Site 5 (RM Data) CORE
Mean NRM/g
Mean ARM/X
BC12 BC 9 BC10 BCll
1.62 1.53 0.54 0.53
15 170 45 4
Mudwave Site 6 (RM Data) CORE
Mean NRM/Z
Mean ARM/g
BC26 BC25 BC15 BC23 BC24
0.95 1.24 1.10 0.47 0.56
13 12 11 4 5
BC 9 Fig. 10. Equal area, lower hemisphere plot of AMS azimuths measured for all samples from box-core BC 9 recovered from Mudwave 5 (Fig. 2). KI = ;K2 = A; K 3 = 0. Consistency in lineations (KI) indicates constant current flow directions across the mudwave.
RM INCLINATION (Degrees) RC25-04 PC28
1.
I
RC28-04 PC30
1
I
2.
2
3,
3
I I
4
I
6
6,
7
7
8
8,
10
10,
11
11'
12
12,
13
13'
14
14.
15
15.
16
~
16 '
: +90"
~1
' -gO"
+90"
O"
-90"
Fig. 11. R e m a n e n t inclination (degrees) after demagnetization for samples from RC28-04, PC 28 and PC 30 recovered from the m u d w a v e at Site 7 (Fig. 4). N u m b e r s associated with reversed polarity peaks are ages in thousands of years and are based on a constant sedimentation rate for the core of 0.1 m 1000 yr - l determined from diatom data (BURCKLE, 1987). Cores were not oriented.
Magnetic properties of mudwavesamples
935
The only distinctly prolate shapes are found at Site 6; BC 15 on the crest of the mudwave, and BC 26 on the steep depositional side of the mudwave. Normally a foliated fabric would be expected due to settling during deposition. This can be modified by deposition from a high velocity water mass, where a more prolate fabric is usually produced. However, at Site 6, the prolate shapes appear where deposition is occurring and velocities are lower. This was not expected, and such unusual results may be due to the presence of slight amounts of the mineral siderite, present in many of the Argentine Basin samples (see below). Siderite exhibits a strong magnetocrystalline anisotropy, producing prolate magnetic fabrics. Variations in siderite concentration could easily explain the generally low values of V observed in these cores. These values can also be explained by high velocity bottom waters producing a lineated fabric that can modify the foliation sufficiently to reduce V values to the levels observed. Mineral studies
XRD and M6ssbauer effect analysis (for example see Fig. 9) identified the mineral siderite in a number of the box core samples. This siderite was probably formed authigenically by dissimilatory iron-reducing bacteria (BELL et al., 1987; ELLWOODet al., 1988) below the redox boundary. ELLWOODet al. (1986) have shown that siderite can cause unusual changes in magnetic behavior because siderite is unstable and readily oxidizes to form secondary, highly magnetic iron oxides. They also concluded that siderite was probably forming in Vema Channel sediments (located at the northern end of the Argentine Basin) at 0.4--0.8 m below the sediment water interface. Their conclusion was based on a distinct change in magnetic fabric patterns, exhibited by Vema Channel samples, from a primary depositional style to an anomalous, magnetocrystalline fabric pattern similar to that observed for siderite. The presence of siderite in small but varying amounts in the mudwaves may explain the few anomalous magnetic fabric patterns in some box core samples (Table 2). Of greater importance is the observation that siderite oxidation in the laboratory can produce abrupt remanent magnetic polarity changes (ELLWOOD et al., 1986, 1988). Therefore, we have made every effort to measure these samples rapidly, and demagnetize them to levels (30 mT) that have been shown to be sufficient to eliminate short-term, smallscale magnetic changes due to siderite oxidation (ELLWOOD et al., 1988). A stable remanent moment (RM) is exhibited by most samples from Sites 5 and 6 after AF demagnetization to 30 mT (Fig. 5). On the other hand, above 20 mT, demagnetization results for samples from the Site 7 mudwave exhibit unstable RM behavior (Fig. 5). The resulting magnetic polarities show most samples to be normally magnetized, consistent with an age of <730,000 years for these sediments (MANKINENand DALRYMPLE,1979). However, anomalous inclination values from PC 28 (Fig. 11) and PC 30 were not expected, because diatom data indicate that the base of cores PC 28 and PC 30 are <200,000 and <620,000 years, respectively (MANLEYand FLOOD, 1993; BURCKLE,1987). PC 30 anomalous samples are single point values and normally would not be used for determining polarities. However, between 4 and 5.3 m depth in PC 28, there are two sample clusters that exhibit reversed polarities (Fig. 11). If it is assumed that the core has a uniform sedimentation rate of 0.1 m 1000 yr- 1 (based on diatom ages), then the ages of the reversed samples fall in the range of 42,000-52,000 years. It is unlikely that these data represent recorded excursions in the geomagnetic field. Rather, the greater likelihood is that the
936
B . B . ELLWOOD
data are anomalous, possibly resulting from unusual polarities produced in sideritecontaining samples, like those occurring elsewhere (ELLWOOD et al., 1986). Furthermore, these "reversed" samples do not exhibit stable R M behavior with demagnetization (i.e. vector decay does not trend toward the origin; Fig. 5). A test of the diatom ages and estimated sedimentation rates determined for PC 28 and PC 30 is given in Fig. 8. Magnetic properties measurements for these cores show one unusually high susceptibility value in each core. Using the assumption of constant sedimentation and a rate of 0.1 m 1000 yr -1 for PC 28 and 0.03 m 1000 yr t for PC 30, the single high susceptibility peak in both cores give an age of - 6 8 , 0 0 0 years (labeled in Fig. 7). Only two other magnetic properties peaks are readily correlated to the same age in both cores; the ARM/X ratios labeled by 167K in Fig. 7 are assigned an age of 167,000 years based on sedimentation rates. These data support the conclusion that deposition is greater on the northeast than on the southwest side of the mudwave at Site 7. CONCLUSIONS Distinctive similarities and differences exist in magnetic properties between and within each Argentine Basin mudwaves sampled. Between mudwave similarities include a stable remanent m o m e n t to 20 or 30 m T A F demagnetization in samples and uniform downcore magnetic fabric patterns. Within mudwave similarities include magnetic grain size increases and downcore magnetic grain concentration decreases; prolate or nearly neutral magnetic fabrics in samples from portions of the mudwaves interpreted to represent erosional surfaces; downcore grain size and concentration uniformity; and oblate fabric patterns in portions of the mudwave representing depositional slopes. Differences include finer magnetic grain sizes at Site 5 as opposed to Site 6 but larger grain size variations at Site 5. The data from Sites 5 and 6 are consistent with the interpretation that b o t t o m water velocity, during deposition of the sediments analyzed, was either higher at Site 6 than at Site 5, or that Site 6 is closer to the source of the magnetic particles being deposited. Velocities over individual mudwave sites were higher over shallow slopes (inferred to be zones of erosion) than over steep slopes (inferred to be zones of deposition). Also, the magnetic data indicate clear changes in current flow and therefore deposition/erosion patterns in the mudwaves. Acknowledgements--Research was supported by ONR contract AF 33615-86-C-2721, to M. T. Ledbetter,
subcontract No. 399 from the San Jose State University Foundation to B. B. Ellwood. Gary Long, at the University of Missouri-Rolla, kindly provided the M6ssbauer effect data, research supported by the donors of the Petroleum Research Fund, administered by the American Chemical Societythrough grant # 18202-AC3 to Gary Long. Curation of cores at Lamont-Doherty is provided by NSF grant DES72-01568 and ONR contract N0001475-C-02t0. I thank M. T. Ledbetter and R. Lotti for help in obtaining samples, and M. T. Ledbetter for manuscript review. REFERENCES BURCKLEL. H. (1987) Diatom biostratigraphic data bearing on the formation of mudwaves in the central Argentine Basin, LOS (Transactions of the American Geophysical Union), 68, 1747. COLLINSONO. W., L. MOLYNEUXand D. B. STONE(1963) A total and anisotropic magnetic susceptibilitymeter. Journal of Scientific Instruments, 40, 310-312. ELLWOODB. B., W. BALSAM,B. BURKART,G. J. LONGand M. L. BUHL(1986) Anomalous magnetic properties in
Magnetic properties of mudwave samples
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rocks containing the mineral siderite: Paleomagnetic implications. Journal of Geophysical Research, 91, 12,779-12,790. ELLWOOD B. B., T. H. CHaZANOWSI~I,G. J. LONG and M. L. BUHL (1988) Siderite formation in anoxic deep-sea sediments: a synergetic bacterially controlled process with important implications in paleomagnetism. Geology, 16,980-982. ELLWOOD B. B., F. HROUDAand J.-J. WAGNER (1988) Symposia on magnetic fabrics: introductory comments, Phys. Earth Planet. Int., 51,249-252. ELLWOODB. B. and M. T. LEDBErrER (1977) Antarctic Bottom Water fluctuation in the Vema Channel: effects of velocity changes on particle alignment and size, Earth Planetary Science Letters, 35, 189-198. EWING M., S. L. EITTREIM, J. J. EWING and X. LE PICHON (1977) Sediment transport and distributions in the Argentine Basin. 3. Nepheloid layer and processes of sedimentation. In: Physics and Chemistry of the Earth, Vol. 8, L. H. AHRENS, F. PRESS, S. K. RUNCORNand H. C. UREV, editors, Pergamon Press, Oxford, pp. 49-77. FLOOD R. D. (1988) A lee-wave model for deep-sea mudwave activity. Deep-Sea Research, 35, 973-983. FLOOD R. D. and A. N. SHOR(1988) Mudwaves in the Argentine Basin and their relationship to regional bottom circulation patterns. Deep-Sea Research, 35,943-971. GEORGI D. T. Circulation of bottom water in the South Atlantic. Deep-Sea Research, 28a, 959-979. GRAHAM J. W. (1966) Significance of magnetic anisotropy in Appalachian sedimentary rocks. In: The earth beneath the continents, J. S. STEINHARDand T. J. SMITH,American Geophysical Union, Washington DC, pp. 627-648. JONES G. A. (1987) Project Mudwaves: Preliminary results of sediment analyses of selected box cores from the Argentine Basin, EOS (Transactions of the American Geophysical Union), 68, 1747. KHAN M. A. (1962) Anisotropy of magnetic susceptibility of some igneous and metamorphic rocks. Journal of Geophysical Research, 67, 2873-2885. KING R. F. and A. I. REES (1962) The measurement of the anisotropy of magnetic susceptibility of rocks by the torque method. Journal of Geophysical Research, 67, 1565-1572. KLAUS A. and M. T. LEDBETrER(1988) Deep-sea sedimentary processes in the Argentine Basin revealed by highresolution seismic records (3.5 kHz echograms). Deep-Sea Research, 35,899-917. LEDBETTER M. T. (1986) Bottom-current pathways in the Argentine Basin revealed by mean slit particle size. Nature, 321,423--425. LEDBETTER, i . T. (1993) Particle-size evidence of late Holocene paleocurrent activity on Argentine Basin mudwaves. Deep Sea Research H, 40, 911-920. LEDBETTERM. T. and B. B. ELLWOOD(1980) Spatial and temporal changes in bottom-water velocity and direction from analysis of particle size and alignment in deep-sea sediments. Marine Geology, 38,245-261. LEDBETTER M. T. and D. A. JOHNSON (1976) Increased transport of Antarctic Bottom Water in the Vema Channel during the last ice age. Science, 191,837-839. MANKINEN E. A. and G. B. DALRYMPLE(1979) Revised geomagnetic polarity time scale for the interval 0-5 m.y. BP. Journal of Geophysical Research, 84, 615-626. MANLEVP. L. and R. D. FLOOD(1993) Paleoflow history determined from mudwave migration: Argentine Basin. Deep Sea Research H, 40, 1033-1055. MILL1MAN J. D. (1988) Correlation of 3.5 kHz acoustic penetration and deposition/erosion in the Argentine Basin: a note. Deep-Sea Research, 35, 919-927. NVE J. R. (1969) Physical Properties of Crystals. Oxford University Press, New York, N.Y., 322 pp. SACHS S. D. and B. B. ELLWOOD (1988) Controls on magnetic grain-size variations and concentration in the Argentine Basin, South Atlantic Ocean. Deep-Sea Research, 35,929-942. SEGUIN M. (1966) Instability of FeCO3 in air. American Journal of Science, 264, 562-568. TAIRA A. and B. R. LIENERT (1979) The comparative reliability of magnetic photometric and microscopic methods of determining the orientations of sedimentary grains. Journal of Sedimentary Petrology, 49,759772. ZIJDERVELDJ. D. A. (1967)A.f. demagnetization of rocks: analysis of results. In: Methods in Paleomagnetism, D. W. COLLINSON et al., editors, Elsevier, Amsterdam, pp. 254-286.