Influence of Abyssal Circulation on Sedimentary Accumulations in Space and Time

216 Pages·1977·8.99 MB·English

by  Bruce C. Heezen (Eds.)

FURTHER TITLES IN THIS SERIES
L.M.J. U, VANS TRAATEN,E ditor
1.
DELTAIC AND SHALLOW MARINE DEPOSITS
2. G.C. AMSTUTZ,E ditor
SEDIMENTOLOGY AND ORE GENESIS
3. A.H. BOtWAa nd A. BROUWER, Editors
TURBIDITES
F.G. TICKELL
4.
THE TECHNIQUES OF SEDIMENTARY MINERALOGY
5. J.C. INGLE Jr.
THE MOVEMENT OF BEACH SAND
6. L. VAND ER PLAS
THE IDENTIFICATION OF DETRITAL FELDSPARS
I. S. DZVLYNSKIa nd E. K. WALTON
SEDIMENTARY FEATURES OF FLYSCH AND GREYWACKES
8. G. LARSENa nd G. V. CHILINGAR,E ditors
DIAGENESIS IN SEDIMENTS
9. G V. CHILINGAR,H .J. BISSELL and R.W . FAIRBRIDGE, Editors
CARBONATE ROCKS
P. McL. D. DUFF, A. HALLAMandE .K. WALTON
10.
CYCLIC SEDIMENTATION
11. C.C. REEVESJ r.
INTRODUCTION TO PALEOLIMNOLOGY
12. R.G.C. BATHURST
CARBONATE SEDIMENTS AND THEIR DIAGENESIS
13 A.A. MANTEN
SILURIAN REEFS OF GOTLAND
K. W. GLENNIE
14.
DESERT SEDIMENTARY ENVIRONMENTS
15. C.E. WEAVERa nd L.D. POLLARD
THE CHEMISTRY OF CLAY MINERALS
16. H.H. RIEKE III and G. V. CHILINGARIAN
COMPACTION OF ARGILLACEOUS SEDIMENTS
M.D. PICARD and L.R. HIGH Jr.
17.
SEDIMENTARY STRUCTURES OF EPHEMERAL STREAMS
G. V. CHILINGARIANa nd K.H. WOLF
18.
COMPACTION OF COARSE-GRAINED SEDIMENTS
19. W. SCHWARZACHER
SEDIMENTATION MODELS AND QUANTITATIVE STRATIGRAPHY
20. M.R. WALTER,E ditor
STROMATOLITES
B. VELDE
21.
CLAYS AND CLAY MINERALS IN NATURAL AND SYNTHETIC SYSTEMS
22. C.E. WEAVERa nd K.C. BECK
MIOCENE OF THE SOUTHEASTERN UNITED STATES
DEVELOPMENTS IN SEDIMENTOLOGY
23
INFLUENCE OF ABYSSAL
CIRCULATION ON SEDIMENTARY
ACCUMULATIONS IN SPACE AND TIME
EDITED
BY
BRUCE C. HEEZEN
Lamont-Doherty Geological Observatory of Columbia University, Palisades, N. Y. (U.S.A.)
Reprinted from Marine Geology Vol. 23 No. 1/2
ELSEVIER SCIENTIFIC PUBLISHING COMPANY
AMSTERDAM - OXFORD - NEW YORK 1977
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Copyright @ 1977 by Elsevier Scientific Publishing Company, Amsterdam
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PREFACE
The papers included in this special issue were given at a symposium entitled:
“Influence of Abyssal Circulation on Sedimentary Accumulations in Space
and Time” held August 27,1975 in Grenoble, France during the General As-
sembly of the International Association for the Physical Sciences of the Ocean
and the XVIth General Assembly of the International Union of Geodesy and
Geophysics. The symposium, sponsored by the IAPSO Commission on Marine
Geophysics was well attended and the discussions were spirited and
informative.
The nature and total thickness of sediment lying on oceanic basement in a
given area is largely determined by:
(1)t he date of commencement of sedimentation;
(2) the initial depth (below sea level) of the juvenile crust;
(3) the history of productivity of the overlying waters;
the presence of additional nonpelagic sources;
(4)
(5) the presence of processes of sediment redistribution.
The date of commencement can be estimated from magnetic anomaly
stripes. These ages have been calibrated by Deep Sea Drilling Project holes to
oceanic basement. The initial depth of the juvenile crust together with the
original and subsequent levels of the calcium carbonate compensation depth in
respect to the depositional surface determine the proportion of ooze and clay.
The history of productivity in a given area may include not only initial ooze
deposition followed by abyssal clay deposition on a subsiding crust but also a
crossing or recrossing of the equatorial or polar front productivity belts with
associated alternations of ooze and clay and episodes of higher and lower than
normal rates of deposition. Up-wind injection of sediments carried into the
atmosphere from volcanoes or desert areas can introduce significant variations
in sedimentation. Turbidity currents can have an overwhelming influence in
the areas they enter.
The first four factors all result in more or less sediment accumulation. The
last one can also result in the removal and redistribution on the sea floor of
previously deposited sediment. It is this later aspect which is the central theme
of the papers presented in the present volume.
In the following papers the effects of abyssal circulation on sedimentation
which are treated vary in magnitude from slight increases in suspended con-
centrations in sea water (Biscaye and Eittreim) to dissolution of planktonic
tests (Johnson et al., Mallet and Heezen) to scour of sediments from beds of
manganese nodules (Watkins and Kennett) to gentle scour observed from sub-
mersibles (Heezen and Rawson) to sharp-crested ripples and scour marks
photographed (Stanley and Taylor) on a seamount to huge sand waves and
scour channels revealed on deep-towed vehicle sideman records and photo-
graphs (Lonsdale and Spiess) to the creation of isthmian barriers (Holcombe
and Moore) and the stagnation of entire ocean basins (Ryan and Cita).
This convener wishes to thank both the speakers and the audience for their
contribution to this successful symposium. We also thank Dr. Eugene LaFond,
Secretary to IAPSO, and Professor Henry Lacombe, President of IAPSO, for
their invaluable assistance.
BRUCE C. HEEZEN
(President, IAPSO Commission
on Marine Geophysics)
Marine Geology, 23 (1977) 1-33
@Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
VEMA CHANNEL PALEO-OCEANOGRAPHY: PLEISTOCENE
DISSOLUTION CYCLES AND EPISODIC BOTTOM WATER FLOW*
DAVID A. JOHNSON’, MICHAEL LEDBETTER’ and LLOYD H. BURCKLE’
‘ Woods Hole Oceanographic Institution, Woods Hole, Mass. (U.S.A.)
‘Graduate School of Oceanography, University of Rhode Island, Kingston, R.I. (U.S.A.)
3Larnont-Doherty Geological Observatory, Palisades, N. Y. (U.S.A.)
(Received April 28, 1976)
ABSTRACT
Johnson, D.A., Ledbetter, M. and Burckle, L.H., 1977. Vema Channel paleo-oceanography:
Pleistocene dissolution cycles and episodic bottom water flow. Mar. Geol., 23: 1-33.
Investigations of piston cores from the Vema Channel and lower flanks of the Rio
Grande Rise suggest the presence of episodic flow of deep and bottom water d-ur ing the
Late Pleistocene. Cores from below the present-day foraminifera1 lysocline (at m)
4000
contain an incomplete depositional record consisting of Mn nodules and encrustations,
hemipelagic clay, displaced high-latitude diatoms, and poorly preserved heterogeneous
microfossil assemblages. Cores from the depth range between 2900 m and 4000 m
contain an essentially complete Late Pleistocene record, and consist of well-defined
-
carbonate dissolution cycles with periodicities of 100,000 years. Low carbonate
content and increased dissolution correspond to glacial episodes, interpreted by
as
oxygen isotopic analysis of bulk foraminiferal assemblages. The absence of diagnostic
high-latitude indicators (Antarctic diatoms) within the dissolution cycles, however,
suggests that AABW may not have extended to significantly shallower elevations on the
lower flanks of the Rio Grande Rise during the Late Pleistocene. Therefore episodic
AABW flow may not necessarily be the mechanism responsible for producing these
cyclic events. This interpretation is also supported by the presence of an apparently
complete Brunhes depositional record in the same cores, suggesting current velocities
insufficient for significant erosion. Fluctuations in the properties and flow characteristics
of another water mass, such as NADW, may be involved.
The geologic evidence in core-top samples near the present-day AABW/NADW
transition zone is consistent with either of two possible interpretations of the up-p er
limit of AABW on the east flank of the channel. The foraminiferal lysocline, at m,
4000
is near the top of the benthic thermocline and nepheloid layer, and may therefore
correspond to the upper limit of relatively corrosive AABW. On the other hand, the carbonate
compensation depth (CDD) at -4250 m, which corresponds to the maximum gradient
in the benthic thermocline, is characterized by rapid deposition of relatively fine-grained
sediment. Such a zone of convergence and preferential sediment accumulation would be
expected near the level no motion in the AABW/NADW transition zone as a con-
of
sequence of Ekman-layer veering of the mean velocity vector in the bottom boundary
layer. It is possible that both of these interpretations are in part correct. The “level of no
motion” may in fact correspond to the CCD, while at the same time relatively corrosive
*Contribution No. of the Woods Hole Oceanographic Institution.
3734
Contribution No. 2365 of the Lamont-Doherty Geological Observatory.
2
water of Antarctic origin may mix with overlying NADW and therefore elevate the
foraminifera] lysocline to depths above the level of no motion. Closely spaced obser-
vations of the hydrography and flow characteristics within the benthic thermocline will be
required in order to use sediment parameters as more precise indicators of paleo-
circulation.
INTRODUCTION
In recent years, the erosion and redeposition of sediment on the deep
ocean floor has come to be recognised a process which is important over
as
widespread geographic regions and throughout much of the geologic past.
Observations of differential sediment accumulation patterns in seismic
reflection profiling records, and the common occurrence of unconformities
in sediment cores and at DSDP sites, indicate that the depositional record
is generally incomplete. Numerous investigators have interpreted these
deep-ocean unconformities as indicative of episodic bottom current flow,
which in turn may result from changes in tectonic configurations and/or
climatic conditions (e.g., Jones et al., 1970; Watkins and Kennett, 1971;
Johnson, 1972; Berggren and Hollister, 1974). Particular attention has been
focused on interpreting paleocirculation patterns during the Late Tertiary
and Quaternary, inasmuch as major tectonic readjustments during this brief
span of time have been minimal, and consequently changing patterns of
sedimentation may be principally climatically controlled.
The various components of marine sediments represent a multiple of
source regions, and may therefore reflect climatic conditions in both
terrestrial, coastal, pelagic, and bathyal environments. Since most of the
sediment components in abyssal regions originate primarily at distant
terrestrial sources or in the photic zone in the overlying water, most studies
of the Pleistocene marine record have been directed toward interpreting
past terrestrial climates or paleocirculation patterns of the atmosphere and
surface waters of the oceans. The extensive geographic coverage of core
samples and the refinement of quantitative approaches to paleoclimatology
(e.g., CLIMAP, 1976) have demonstrated the value of such techniques in
interpretation, and perhaps in prediction as well.
By contrast, our understanding of the abyssal circulation of the world’s
oceans and its variations during the past is relatively poor. There are at least
two major factors which have contributed to our difficulty in attempts
at interpretation:
(1)O ne must first identify appropriate “base-level” conditions. In other
words, one must establish unequivocally a one-to-one correspondence
between parameters in the surface sediments and a characteristic property
of the overlying water.
(2) One must assume that the physical properties and flow characteristics
of the near-bottom water have been sufficiently stable over time such that
they remain identifiable in the geologic record when integrated over
thousands of years.
3
It is of course highly unlikely that the abyssal circulation has remained
perfectly uniform anywhere on a geologic time scale. Nevertheless,
meaningful paleo-oceanographic interpretations of the abyssal circulation
require that this condition be met as closely as possible.
The Vema Channel in the southwestern Atlantic is one of several topo-
graphic gaps which plays a major role in controlling the abyssal circulation
of the world's oceans. Other similarly important features include the
Samoan Passage (Hollister et al., 1974; Reid and Lonsdale, 1974; Johnson,
1974a), Kane Gap (Hobart et al., 1975), Walvis Gap (Connary, 1972), and
numerous fracture zones in the mid-oceanic ridge system (e.g., Gibbs
Fracture Zone, Romanche Trench). These passages are of particular
significance for paleocirculation studies in two principal respects:
(1)B ottom water flow through these passages may be relatively uniform
in comparison with that observed in more open regions of the abyssal ocean.
Directional deviations in the flow are generally minimal due to topographic
constraints, and the high-frequency tidal components to the flow are
sometimes (though not always) overshadowed by a strong unidirectional
component representing net transport between larger basins via these passages
(e.g., Reid and Lonsdale, 1974). If in fact the bottom boundary layer in
these regions is relatively time-independent, then sediment-current inter-
actions may also be relatively stable over time. Consequently, there is
relatively high assurance that sea floor properties as observed in bottom
photographs and sediment samples are in fact in equilibrium with the
existing flow conditions.
(2) Interpretations of paleocirculation must make the additional
assumption that flow characteristics were sufficiently stable such that they
remained identifiable in the geologic record when integrated over thousands
of years. Passages such as the Vema Channel may provide the most suitable
type of region for satisfying this condition, assuming that the first-order
basin/basin transport of bottom water is uniform over geologically significant
time periods.
REGIONAL SETTING 0" VEMA CHANNEL
Previous investigations have shown that Antarctic Bottom Water (AABW)
dominates the abyssal circulation of the southwestern Atlantic (Wust, 1957;
Gordon, 1972; Reid et al., 1973). Upon formation in the Weddell Sea, a
major portion of this water mass flows northward around the Scotia Arc
and through a topographic gap in the Falkland Fracture Zone, near 49" S
and 36"W (Le Pichon et al., 1971a). Upon leaving this gap, AABW flows
to the west along the northern edge of the Falkland Plateau. As it approaches
the continental rise in the extreme southwestern portion of the Argentine
Basin, the bottom current is deflected toward the north and continues as a
deep western boundary current (Fig.1).
At the northern margin of the Argentine Basin the flow of AABW is
restricted by the Rio Grande Rise, which forms a topographic barrier
4
10
3"
'0
O
TO0
70a
50
O
" i
50
Fig. 1. Regional bathymetry of the southwestern Atlantic. Shaded region designates areas
shallower than 2000 fathoms. Arrows designate the direction of flow of Antarctic Bottom
Water. The Vema Channel, whose bathymetry is shown in Fig. 3, separates the Rio Grande
Rise from the continental margin of South America.
5
between the Brazil and Argentine Basins (Fig.1). The principal deep passage
(greater than 4500 m) through the Rise is the narrow, sinuous gap referred
to as the Vema Channel (Le Pichon et al., 1971b). Numerous investigations
(Wright, 1970; Le Pichon et al., 1971b; Reid et al., 1973; Johnson et al.,
1976) have documented the presence of a significant northward transport
of AABW through the Vema Channel and into the Brazil Basin, with
maximum current velocities on the order of 20-25 cm/sec (Johnson et al.,
1976).
The Vema Channel is centered near 39" 30'W, with a sill depth of approxi-
mately 4600 m (Lonardi and Ewing, 1971; this report, Figs.2 and 3). Seismic
reflection profiling (LePichon et al., 1971b) has demonstrated that the
basement relief in the vicinity of the channel is highly irregular, and that
these irregularities may have been influential in controlling the orientation
and morphology of the channel during its development. In cross-section the
main branch of the channel is notably asymmetrical (Fig.4, profile AB).
The deepest part of the channel is generally adjacent to the western wall, a
high and steep slope which on many crossings appears to represent outcrops
or near-outcrops of acoustic basement (Le Pichon et al., 1971b; Figs.2, 3).
On the eastern side of the channel the walls rise steeply to a depth of -4200 m
and then level out onto a broad terrace up to -100 km in width (Fig.3).
>
The terrace is underlain by 1k m of acoustically transparent sediment of
unknown lithology and age. The eastern margin of the terrace merges imper-
ceptibly with the lower flanks of the Rio Grande Rise.
3O"OO'S
3 I POO'S
)O
Fig.2. Ship tracks in the vicinity of the Vema Channel, from which bathymetric data were
used in constructing a revised bathymetric chart (Fig.3).