Bulk Geochemistry of the Sand Fraction from CRP-3 (Victoria Land Basin, Antarctica): Evidence for Provenance and Milankovitch Climatic Fluctuations
Received 9 January 200 1 : accepted in revised f i i t . ~ ~ ~ 0 November 200 1
Abstract - A total of 167 samples distubuted thioughout the CRP-3 clullhole fioni 5 77 to 787 68 mbsf and iepiesenting fine to coaise sandstones have been analysed by X-lay fluoiescence spectiometiy (XRF) Bulk sample geochcinistiy (major and trace e l e m e n t s ) indicates a d o m i n a n t p r o v e n a n c e of d e t r i t u s from the Ferrar S u p e r g r o u p in t h e u p p e r m o s t 200 m b s f o f t h e c o r e . A m a r k e d l y i n c r e a s e d contribution from the Beacon sandstones is recognized below 200 mbsf and down to 600 mbsf. I n the lower part of CRP-3. down to 7 8 7 . 6 8 mbsf, gcocheiiiical evidence for influxes of Ferrar materials is again recorded.
On the basis of preliminary magnetostratigraphic data reported for the lower 447
mbsf of the drillhole, we tentatively evaluated the main periodicities modulating the geochemical records.
Our results identify a possible influence of the precession. obliquity and long-eccentricity astronomical components (21, 41, and 400 ky frequency bands) on the deposition mechanisms of the studied glaciomarine sediments.
INTRODUCTION
This paper presents the results of major and trace e l e m e n t analyses performed on f i n e to c o a r s e sandstones from the CRP-3 drillhole. These results h a v e been discussed to assess t h e s e d i m e n t provenance and processed by spectral analysis to test t h e presence of cyclic patterns in the geochemical records.
When biostratigraphic and magnetostratigraphic constrains are poorly defined, the recognition of statistically significant periodicities in sedimentary records related to astronomical/orbital forcing may be useful to calibrate the time interval of the studied succession (House, 1995).
By analysing physical properties of mudstone and fine-grained sandstone intervals in C R P - 2 / 2 A drillhole, Niessen et al. (2000). Cape Roberts Science Team (1999), and Claps et al. (2000) identified high- frequency periodicities which the authors correlated to the Milankovitch periodic orbital forcing. Interesting cyclostratigraphic results have also been obtained for t h e upper 200 mbsf of the CRP-3 core. Based on c o m b i n e d spectral m e t h o d o l o g i e s , it h a s been s u g g e s t e d the p r e s e n c e of the t h r e e classic Milankovitch periodicities modulating the magnetic susceptibility in this part of the core (Cape Roberts Science Team, 2000).
T h e s e results s t i m u l a t e d us to u s e a similar approach for testing cyclicity in the geochemical
records from the lower part of the CRP-3. While a relatively reliable time framework for the upper part of the C R P - 3 core is provided by a good biostratigraphic and magnetostratigraphic control, only approximate temporal constrains are available for the 350 to 789.77 mbsf interval (Cape Roberts Science T e a m , 2000). Then, the recognition of selected periodicities in the geochemical signals could represent a useful tool to calibrate the time record of the lower part of the CRP-3 core, ascribed to the early Oligocene. Furthermore, the recognition of specific frequency bands related t o well-known climatic influences could give the opportunity t o study modes and times of possible response of the Oligocene East Antarctic Ice Sheet.
SAMPLES AND PROCESSING
Analyses were performed on 167 sand-grained samples scattered throughout the drillhole from 5.77 t o 7 8 7 . 6 8 mbsf, with a mean distribution of one sample every 5 m.
Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, P and CS, Ba, La, Ce, V. Zr, Nb, Y, Ss, Rb. Ni were determined by X-ray fluorescence spectrometry (XRF) on pressed, boric-acid backed pellets of bulk rock. Data reduction was achieved using the method described by Franzini et al. (1975). Certified reference materials were used as monitors of data quality. Analytical errors were
Corresponding author (rnarios@unipa.it)
lwlow 1% for Si, AI, Na; below 3 % for Ti, K , Fe, Moimtains (TAM) with yiartzose sandstones ol' ~ l u - C'a; a n d below 1 0 % f o r Mg, Mn. P a n d t r a c e I ~ e v o n i a i i - T r i a s s i c Beacon Supergroup, Jiir;issir elements. All samples were washed repeatedly in dolerites a n d Kirkpatrick basalts, coarse-yr:iiiir(l d e i o n i z e d water prior to analysis to avoid plutonic rocks (Cambro-Ordovician Granite H a r b o u r c o n t a m i n a t i o n r e s u l t i n g f r o m drilling m u d a n d Intrusive Complex), a n d minor metamorphic rocks
seawater from the Upper Proteroxoic bcisament.
RESULTS AND DISCUSSION
M a j o r a n d m i n o r e l e m e n t analyses of C R P - 3 samples are given in tables 1 and 2 and in figures 1 and 2. The data plotted in the figures are normalized to 100% L.O.I. (Loss On Ignition)-free. In order to d i s c u s s t h e g e o c h e m i c a l results in t e r m s o f provenance, w e assume that, as for the previously s t u d i e d C R P - l a n d CRP-212A s e q u e n c e s , m a i n sources for sediments in CRP-3 are the Transantarctic
T h e depth profile of S i O ( F i g . 1 ) e x h i b i t s :I marked increase of the element below approxima~ely
1 6 0 mbsf. Despite of w i d e f l u c t u a t i o n s , S i O , concentrations remain high, generally greater ~ h a n 80%, throughout a thick interval of the drillhole down to about 600 mbsf. Below 6 0 0 mbsf, S i O clearly decreases. The overall very high contents of S i O , arc 2
indicative of strong influxes of detritus f - . r o i t i n ' Beacon sandstones throughout most of CRP-3. Higher
m Diamictite , * * m * *
m Conglomerate
KEY . . . . . . . . . . . . . . . X Sandstone
- - - . - -
L - - - - - -
, - - - -
. Mudstone
-.. ...:,
Thin bed ofcoarser- - grained litho1ogy;length
indicates particle size
Fig. 1 - CRP-3 lithologic section with depth profiles for S i O , Ni, V, and Cr.
Bulk Geochemistry of the Saiul l:raction from ('RP-3
% / l . / Major element concentrations (wCX ) o f ('RI'.? samples. D;ua listed arc noni~iiliscd t o 1001/( (hydrous basis)
Depth
554
Tab. I - Continued
M. Sprovieri et al.
Depth 35 1 .85 351 .85 356.15 358.92 365.84 369.61 375.29 38 1.97 385.90 390.75 392.54 396.55 401.51 405.70 412.50 413.06 419.00 422.78 426.13 426.74 432.02 437.09 440.24 445.20 449.70 450.21 455.84 460.10 466.34 473.58 475.31 480.63 481.19 486.00 490.91 495.12 500.18 505.30 509.33 513.14 518.97 525.33 531.78 533.19 540.07 543.82 546.10 550.00 556.17 560.89 566.07 577.10 581.97 586.78 586.78 594.2 1 602.09 606.5 1 613.25 615.34 620.43 626.15 630.35 634.73 638.27 643.08 649.1 1 655.83 659.10 664.00
SiO, 87.20 83.61 84.46 83.88 80.23 8 1.98 82.83 88.46 83.77 86.1 1 71.12 79.98 85.62 73.45 79.1 1 8 1 .85 71.68 80.87 92.45 87.30 86.01 78.67 80.64 93.07 63.84 91.38 85.67 82.60 89.09 88.03 85.81 79.28 86.41 92.69 90.70 96.35 98.67 97.08 85.61 91.33 80.33 83.59 95.63 88.98 84.5 1 82.26 86.90 8 1.49 82.50 93.66 96.41 70.88 71.66 90.03 90.02 84.20 87.63 86.81 74.57 66.55 83.43 8 1.25 85.12 8 1.34 81.47 82.26 76.02 73.68 76.12
TiO, 0.29 0.27 0.21 0.24 0.15 0.12 0.13 0.18 0.19 0.19 0.22 0.18 0.17 0.36 0.32 0.29 0.09 0.12 0.10 0.11 0.16 0.27 0.28 0.09 0.07 0.08 0.19 0.19 0.17 0.12 0.17 0.25 0.18 0.11 0.09 0.05 0.04 0.04 0.12 0.11 0.14 0.08 0.06 0.07 0.22 0.21 0.16 0.18 0.29 0.13 0.09 0.12 0.52 0.18 0.18 0.16 0.17 0.08 0.16 0.64 0.20 0.1 1 0.17 0.23 0.22 0.2 1 0.27 0.31 0.24
MgO CaO
-
-
S i O values in the interval between approximately 200 a n d 6 0 0 mbsf account for increased proportions of quartz grains suggested on the basis of sand grain compositional modes (Cape Roberts Science Team, 2000).
Ni and Cr curves closely mirror the S i O profile (Fig. 1). Above 160 mbsf and below 600 mbsf, values mainly fluctuate between 20 and 40 ppm and between 3 0 and 6 0 ppm f o r Ni and CS, respectively.
Concentrations of these elements are markedly lower through the interval between 160 and 6 0 0 mbsf.
G i v e n that modal investigations in C R P - 3 have revealed the absence of alkaline volcanic lithic grains and ferromagnesian minerals typical of the McMurdo Volcanic Group (Smellie, this volume). the higher contents of Ni and CS in the sediments above 160 mbsf and below 600 mbsf should be derived from the Ferrar dolerite. These elements average 6 1 and 96 p p m in the Ferrar dolerite, whereas they are very depleted (6 and 18 ppm) in the Beacon Sandstone (Roser & Pyne, 1989).
The vanadium profile (Fig. 1) is similar to those of N i and CS and supports the s u g g e s t i o n of an influence of Ferrar detritus in the upper and lower p a r t of the C R P - 3 c o r e and a d o m i n a n t input of Beacon materials in the rest of the drillhole. In fact, V i s tipically enriched in the Fen-as dolerite (283 p p m ) and depleted in the Lower Beacon (25 ppm;
Roser & Pyne, 1989).
Also RblSr ratios, that are lower (on average 0.42) a b o v e approximately 2 0 0 and below 6 0 0 mbsf
(Fig. 2) are compatible with a dominant Ferrar (Rb/Sr=0.34) provenance of the detritus in these stratigraphic intervals. Throughout the rest of the drillhole, the RbISr ratios widely fluctuate with averagely higher values (0.88) reflecting a striking influence of the Beacon sandstone (Rb/Sr=l . 17; Roser
& Pyne, 1989).
Our suggestions on the sediment provenance based on geochemical proxies from the CRP-3 drillhole are consistent with the results from sandstone detrital modes (Smellie, this volume) and clay mineral assemblages (Ehrmann, this volume).
Concentrations of zirconium (Fig. 2) exhibit high variability in CRP-3 sediments and the stratigraphic profile of this element does not correlate with those of S i O , Ni. CS, V, and RbISr. A reason for this could be the similar contents of Zr in the two main sources, Beacon and Ferrar (142 ppm for both sources; Roser
& Pyne, 1989).
Ca values in CRP-3 (Fig. 2) are mostly about 2%
but many anomalously high concentrations, measured between 200 and 600 mbsf, reveal the occurrence of diagenetic carbonate.
SPECTRAL ANALYSIS AND TIME FRAMEWORK An ensemble of geochemical signals obtained f r o m the CRP-3 core w e r e processed by spectral analysis. T h e original adopted sampling rate (on average 1 sample15 m) did not allow us to extract from the geochemical signals of the upper part of the
Depth (mbsf)
5.77 9.90 13.26 21.16 24.32 24.88 28 80 33.14 47.61 4 9 4 0 67.0.7 69.97 73.40 78.35 83.39 89.47 94.47 99.47 103.58 110.16 1 15.87 123 42 130.57 135.15 139 88 I45 09 148.73 155 82 156.52 168.9.) 171.75 171.95 177.19 17747 183.66 189 16 18951 196.27 202.56 207 68 210.49 215.39 22639 22900 230.42 235.47 23986 244.83 250.24 256.63 259.30 270.94 273.85 279 65 283.21 286.19 289 17 291 40 298.96 30 L64 30571 312.35 31574 320.60 326.64 333 16 335.93 341.11 345.73 348 67 351.85 356.15 358 92 365.84 369.61 375.29 381 97 385.90 390.75 392 54 396.55 40151 405.70 4 12.50 413.06 419.00
Y Nil
0 l>,'(.
O S' 1 il 1,l.l 11 ¥ "
il<!+
11 z l i s l
11 5") I1 SW I1 S l l1 I1 111.1 0 -1-l 1 0 5 5 1 I1 6.''- 0 '51s 11 .l 1 .l 0 \w 0 ins I1 \.If.
0 I".
0 11'- 0 1.1'- 0 UK 0 171 0 51'- 0 015 0 ??i>
0, I 'Ill 0 IS";
0.17.' l1 ?<V il.O1il1 o..12i, 0 1 .X1 0 '1 10 0 10'' I H V I 0 (1'17 0 W>
I1 IS7 0.7'4'1 0.605 0 235 0 790 1.331 0.2 15 0.235 1 .U30 0 620 07SII (1.924 1191 0 260 0 4'io 0.452 0 263 0.580 0419 0472 0.489 0 743 0817 1 476 1339 I 460 1.169 0 439 0.646 1.123 0.4 14 0 5 3 1 0 330 0.796 1 038 0786 0.441 0.457 0.758 0.583 0.964 0.719 0.697 0.8 17 I .343 1.248 1.142 0.082
141 9s 72 12s 2-16 176 S 65 04 S 5 171 154 l l<) 204 116 l l 6 70 S6 70 . .
I>
62 l 5 2 74 114 76 71 47 121 114 94 l l7 1 07 99 .. l ! 100 KM 134 87 I 20 68 155 I 6 0 152 I l l 96 143 142 169 181 l 5 6 170 MO I 3 7 115 226 129 l l l
u S 190 134 33 I S7 146 175 1 85 179 176 150 235 l25 217 I13 167 S6 153 l 8 6 I ' M n o l 1 2 73 1-33
(I 5 ill l5 28
l l
>
Fig. 2 - CRP-3 lithologic section with depth profiles for RbISr. Zr. and Ca. Symbols as i n figure l
c o r e ( 0 - 3 4 0 m b s f ) t h e periodicity previously recognized in different sedimentary physical properties by C a p e R o b e r t s T e a m ( 2 0 0 0 ) . T h e s e scientists estimated that the short-eccentricity orbital component can be recorded in the magnetic susceptibility and porosity s i g n a l s of the upper part of C R P - 3 a n d calculated a wavelength of about 5.8-9.7m for this orbital periodicity band. Such a result highlights that the s a m p l i n g r a t e adopted f o r t h e g e o c h e m i c a l analysis of the whole core (average of 1 sample15 m) is lower than the Nyquist frequency and that all the classic Milankovitch periodicites are not identifiable in the geochemical records.
For the l o w e r p a r t of the C R P - 3 c o r e (below 340 mbsf), poorly constrained biostratigraphic and palaeomagnetic data, together with the presence of possible sedimentary hiatuses and abrupt variations of the sedimentation rate, limit the interpretation of the results of t h e s p e c t r a l a n a l y s e s . However, a preliminary s t u d y of t h e d o m i n a n t p e r i o d i c i t i e s characterizing selected geochemical signals is here proposed taking into account that the obtained results have to be considered only as pilot tools for further researches.
A two steps strategy has been adopted to interpret
the results of spectral analysis.
Power spectra have been estimated in t h e space domain on selected geochemical signals. Hierarchical p a t t e r n s a m o n g t h e d o m i n a n t p e r i o d i c i t y bands recorded in the spectra have been compared with the ratios m e a s u r e d a m o n g t h e three M i l a n k o v i t c h frequencies (related to the precession, obliquity and eccentricity cycles) during the Cenozoic in order to preliminarily verify a hypothetical interaction between t h e s t u d i e d cyclical sedimentary s y s t e m a n d the orbital forcing.
T h e n , an average sedimentation rate h a s been e s t i m a t e d f o r t h e interval 3 4 0 . 8 - 6 2 7 . 3 m b s f , characterized by the palaeomagnetic c h r o n C 1311 (Florindo et al., this volume) which covers the time interval between 33.058 and 33.545 M a (Cande and Kent, 1995). The estimated average sedimentation rate allowed the transformation of the main frequency peaks present in the spectra (space domain) in time periodicities (time domain).
NUMERICAL METHODOLOGY
Data have been re-sampled with a 5 in constant rate using a cubic spline interpolation method to
avoid high frequency bias o n the original data.
Loii;:-term trends have been previously subtracted t o t i n ; o r i g i n a l record by means of a 5 points Ga~issi;tn filter
Welch's averaged perioiiogram inethod (Welch.
1967) w;is used for the estimation of the Fourier Power Spectra.
POWHR S P E C T R A , N U M E R I C A L F I L T E R I N G AND OR I3 ITALLY-CONTROLLED SEDIMENTARY PROCESSES
Data o f S i O ? , RbISr, and Ni f o r t h e C R P - 3 drillhole have been processed by spectral analyses.
These three geochemical signals have been chosen because they appear to be good tracers of sediment p r o v e n a n c e f o r t h e studied area ( s e e a b o v e ) . I n particular, higher S i 0 7 values and Rb/Sr ratios are thought to indicate a dominant Beacon Supergroup i n f l u e n c e , w h e r e a s h i g h e r Ni c o n t e n t s s h o u l d represent a more significant detrital input from the Fenas Supergroup.
In the power spectra (Fig. 3), the y-axis indicates the variance associated to each harmonic component, while the x-axis refers to frequencies in cycles/meter.
Peaks that are statistically significant (passing the 9 5 % confidence level) are labelled with length of periodicity in meters.
The power spectra estimated for the three signals, in the 340-789.77 mbsf interval, show a very similar frequency structure and the presence of three main peaks at about 0.003, 0.029, and 0.052 cycles/meter ( F i g . 3 ) . T h e h i g h v a r i a n c e and q u i t e n a r r o w frequency bands present in the spectra suggest a clear cyclic forcing process, which might have modulated the sedimentary record and the relative geochemical records. Moreover, the spectral results demonstrate the existence of a regular cyclic pattern in the studied
si";i;ils a n d allow us to discard a s t o c h a s t i c mechanism as :I c o n t r o l l i ~ i ~ f a c t o r of t h e i r
f l l l ~ t l l ~ .l 1 ' ions.
l we nori~i;ili/.c the iibove-~~icntionecl t h r e e frcqncncy h;nnls to thc highest one. we obtain a liicrardiiccil pattern (0'.05X-0,047. 0.36-0.55, 1) similar to th;il c;ilciilatetI I'or (lie c l a s s i c M i l a n k o v i t c h frequency bands (1/400 cyclelky for the long-term eccentricity, 1/41 - 1/54 cyclelky for the obliquity and 111 9-1/23 cyclc/ky for the precession) as indicated by Bcrger and 1,outre (1994) for the Cenozoic.
Such a preliminary result, highlighting a good correspondence of the hierarchical patterns between the two sets o f periodicity bands, suggests a possible direct link between astronomical forcing a n d t h e sedimentary response in the glaciomarine environment at the margin of the Antarctic Ice Sheet during t h e early Oligoccne.
Considering the average 0.59 m/ky sedimentation rate estimated within the interval characterized by the p r e s e n c e of t h e chron C 1 3 n a n d assuming i t approximately constant along all the lower part of t h e core (from 340.8 to 789.77 ~nbsf), we could transform t h e t h r e e frequency peaks ( c o r r e s p o n d i n g to t h e wavelengths of 330, 34, and 19 meters reported i n Fig. 3) in periodicities of 560 ky, 57 ky, and 32 ky, respectively.We can hypothesize that the recorded cyclicities are essentially governed by external forcing not strictly related to the Milankovitch astronomical modulation. However, such a hypothesis seems to b e discarded considering the good match between t h e hierarchical patterns calculated among the orbital parameters and the set of periodicities modulating the studied sedimentary record. Alternatively, we could consider the main frequency peaks present in t h e power spectra as truly corresponding to the orbital p e r i o d i c i t i e s of p r e c e s s i o n , o b l i q u i t y a n d l o n g - e c c e n t r i c i t y , respectively. T h u s , t h e o b s e r v e d
Frequency (cycleslmeter)
Fig. 3 - Power Spectral Density of three selected geochemical signals (SiO,, RbISr and Ni) in the 350 to 787 mbsf interval of the CRP-3 core. Main peaks are labelled with the calculated periodicities in metres. B.W. and C.I. represent the Band-Width and the Confidence Interval of the power spectra.
500 M . Sprovicri et ill.
discrepancies between the couples of periodicity bands fin the orbital and sedimentary record) could be explained by hypothesizing that short sedimentary intervals (and. then, time records) are missing. T h c 'iiult recognized at 539 mbsf (Cape Roberts Science
, >
Ieam. 2000) represents one of the points. along the lower CRP-3 core. where the spaceltime record could be lost. However. abrupt variations in t h e sedimentation rate andlor other short intervals of interruption in t h e sedimentation (not evident by macroscopic analysis of the core) could justify the lack of the records.
In particular, considering the periodicity band of the obliquity and precession cycles and the time span of the palaeomagnetic ehroti C 1311, we can estimate that about 3 obliquity cycles and 8 precession cycles are missing in this segment. The deficiency of other stratigraphic constrains in the lowermost part of the core limits the possibility to evaluate the amount of time missingiin this part of the CRP-3 record.
CONCLUSIONS
Distribution of major and trace elements suggests c h a n g e s in t h e p r e d o m i n a n t source of d e t r i t u s throughout the CRP-3 sequence. The upper 200 mbsf of sediments are characterized by a strong influence of Ferrar Supergroup whereas, between 200 and 600 mbsf, provenance from Devonian Beacon Supergroup detritus becomes dominant. From -600 down to -788 mbsf, geochemistry of CRP-3 sediments appears to be again controlled by Ferrar detritus input.
Preliminary results obtained by spectral analyses of three selected geochemical signals (SiO,, RbISr and N i j f r o m t h e lower 447 mbsf of C R P - 3 c o r e suggest that the studied sedimentary system has been m a i n l y f o r c e d by t h e well-known M i l a n k o v i t c h astronomical components of long-term eccentricity, obliquity, and precession. Sediments appear to b e influenced by cyclic alternation of Devonian Beacon Supergroup detritus and Ferrar Supergroup materials with c o n s e q u e n t m o d u l a t i o n of t h e t h r e e l o n g intervals characterized by dominant inputs of t h e different source rocks. Further detailed researches, supported by a multidisciplinary approach, will enable the identification of the sedimentary mechanisms, which drove the deposition of sediments with periodic alternations of geochemical tracers,
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