2nd Deep-Water Circulation Congress, 10-12 Sept. 2014, Ghent, Belgium
Short-term sediment dynamics on a contourite body (off NW Iberia), Part I:
Rapid changes of bottom-flow intensity during the past 50ka deduced from a
sediment-core transect
Till J.J. Hanebuth1,3, Antonia L. Hofmann1,3, Antje Lenhart1,4, Ludvig A. Löwemark2,
Tilmann Schwenk1 and Wenyan Zhang1
1
2
3
4
MARUM – Center for Marine Environmental Sciences, University of Bremen, 28369 Bremen, Germany. thanebuth@marum.de.
Dept. Geosciences, National Taiwan University, Taipei, Taiwan.
Currently at: Geology and Geophysics Dept, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02540, U.S.A.
Now at: Department of Earth Sciences & Engineering, Faculty of Engineering Imperial College, London, UK.
Abstract: The sediment dynamics in a confined contourite system off NW Spain are reconstructed using
high-res bathymetric mapping, seismic profiling, and sediment core transect. This depocenter and its
150m deep moat surround an 800m high obstacle. Since mid-Eocene, a pre-drift unit and two currentcontrolled contouritic units have formed. Fine-grained (10µm) basinwide current-influenced deposition
was episodically interrupted by short-lasting high-energy conditions during MIS-3. High glacial and later
Holocene periods show markedly calm conditions. The deglacial/early Holocene interval shows, in
contrast, a pronounced increase in bottom-flow energy (70µm) with a waxing-and-waning dynamic from
17 to 5 cal ka BP. Process-based simulation demonstrates that not a water-mass core or boundary
distributed those sands. Instead, pulse-like hydrographic fronts travelling inside the mixing zone of two
water masses led to sand mobilization. Compared to paleoceanographic reconstructions, the downwardupward migrating MOW/LSW mixing zone is suggested as driving mechanism. A conceptional model
shows how seafloor obstacles redirect and perturbate bottom flows with the special effect of
oceanographic-front pulses occurring in the mixing zone. Front-driven secondary eddies on the
contourite body itself provide an efficient mechanism for widespread sediment re-distribution.
Key words: Ocean bottom currents, current-obstacle interaction, Eastern Atlantic, Late Quaternary.
INTRODUCTION
The formation of bottom-current induced (i.e.
contouritic) depocenters is still a matter of debate with
regard to the detailed hydrographic control on the
sedimentation processes. This study uses a multidisciplinary approach to investigate the hydro- and
sediment dynamics in a comparably small-sized contourite system by using high-resolution bathymetric
data, 2D reflection seismic profiling, sediment-core
analyses along a transect, and numerical modelling of
ocean-current processes.
DATA AND RESULTS
A classically shaped 150m high contourite body off
NW Spain surrounds a pronounced 800m high structural
obstacle at the toe of the continental slope, separated
from each other by a 1.5km wide moat (Fig. 1). Seismic
profiles indicate that the basal sedimentary unit
overlying the acoustic basement represents normal
hemipelagic basin fill, without evidence for current
control (Fig. 2). In contrast, the following two
sedimentary units show a mounded climbing-upward
geometry in association with a successively filled
palaeo-moat.
The sediment cores across this contourite body illustrate
that MIS-3 was characterized by a rhythmic swap
between calm and high-energy conditions, and MIS-2
and the later Holocene were oceanographically quiet
FIGURE 1: Bathymetric map of the study area.
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2ndd Deep-Water Circulation C
Congress, 10-1
12 Sept. 2014,, Ghent, Belgiium
intervals. M
Most notably is the deg
glacial to eaarly
Holocene tim
me interval, neevertheless; with
w an episodee of
remarkable bottom-currrent intensiffication rapiidly
waxing-and-w
waning dynam
mics were sen
nsitively recorrded
across the coore transect (Fig. 3).
lasst deglacial to early Holocenne times. Witth the general,,
clim
mate-driven decrease inn salinity leeading to a
weeakening of th
he lower Medditerranean Outflow
O
Waterr
(M
MOW) core in the outflow rregion (the Gulf of Cadiz),,
thee upper MOW
W core has preesumably streengthened andd
duee to an increaase in its saliinity, temporaarily (17-5ka))
deeepened by th
he identified 300m in th
he study areaa
(Scchönfeld and Zahn,
Z
2000).
CO
ONCLUSION
NS
This study refines
r
the cooncept how medium-sized
m
d
seaafloor obstaccles redirect and perturrbate bottom
m
currrents on loccal scale. Shharply defined
d water-masss
bou
undaries aree not necesssarily the hydrographicc
eleements to prov
vide high-enerrgy conditions but, instead,,
a rather
r
transitiional zone beetween two water
w
massess
might provide a powerful bottom-flow
w and, thus,,
sed
diment transport regime. T
The well-sorted sands seem
m
to originate fro
om the moatt itself, insteead of beingg
dellivered from a remote souurce and tran
nsported overr
lon
ng distances. The migratiting eddy sy
ystem on thee
con
ntourite body itself providdes an efficien
nt mechanism
m
to distribute these sediments in suspension
n equally overr
thee area.
FIGURE 2: Seissmic profile acrosss the contourite body.
A 3-D pprocess-based sediment-traansport modell is
applied baseed on differennt scenarios in
n which the ttwo
major waterr masses (M
MOW and LSW) potentiaally
responsible for sediment dynamics on
n the contouurite
body are reggulated, mimiicking the mo
odern and palleooceanographhic conditions.. Simulation results
r
excludde a
water-mass ccore as forcee driving an evolution of the
system. It cclearly indicaates, instead,, that pulse-llike
appearing froonts within thhe approximaately 300m thhick
water-mass mixing zonne have th
he potential to
remobilize annd distribute fine sands. Th
hese fronts caause
local bottom
m currents insside the moatt as well as kkmscaled eddiess on the gentlle seaward-directed contouurite
flank. Such m
migrating fronnts are well kn
nown as comm
mon
elements in tthe modern hyydrographic sy
ystem, but apppear
on the steepp and bare-off-sediments middle
m
slope off
Galicia at present (Fig. 4).
4 Thus, thee resulting sanndy
contouritic sediment recorrd is not main
nly controlledd by
a net increasse in front ennergy within this
t
mixing zzone
but by the teemporary clim
mate-related deepening
d
of this
zone of abouut 300m.
FIG
GURE 4: Conceptional scheme coomparing the mod
dern (yellow) andd
deg
glacial/early Holo
ocene (orange) LLSW/MOW mixin
ng zones. Whilstt
the modern mixing zone
z
interacts witth the middle slop
pe and the top off
the obstacle (possib
bly producing m
morphological/ero
osional terraces),,
the 300m lower palaeo-mixing zone with its hydrogrraphic fronts andd
ociated eddies had strong imppact on the co
ontouritic system
m
asso
sed
dimentation.
RE
EFERENCES
S
Haanebuth, T.J.J., A.L. Hofmaann, A.L., Löw
wemark, L.A.,,
Schwenk, T., Zhang, W., subm. Hydrog
graphic frontss
controlling bottom-curreent sedimen
nt dynamicss
deduced from
m a 2D contoourite sedimen
nt record andd
3D process-b
based numericcal simulation
n.
Sch
hönfeld, J., Zahn, R., 20000. Late Glacial to Holocenee
history of thee Mediterraneean outflow. Evidence
E
from
m
benthic forraminiferal aassemblages and stablee
isotopes at th
he Portuguesee margin. Palaaeogeography,,
Palaeoclimattology, Palaeooecology 159, 85-111.
Vö
ölker, A.H.L., Lebreiro, S.M
M., Schönfeld,, J., Cacho, I.,,
Erlenkeuser, H., Abrantess, F., 2006. Mediterranean
M
n
outflow strengthening duuring northern
n hemispheree
coolings: a salt source forr the glacial Atlantic?
A
Earthh
Planetary Science Letters 2245, 39-55.
FIGURE 3: Graain-size distributtion during the peeriod 20-0 cal kaa BP
illustrating thee offshore graddients as well as duration off the
coarsening duriing the deglacial//early Holocene in
nterval.
The com
mparison withh existing pallaeoceanograpphic
data suggessts that finee-grained dep
position on the
contourite boody was driveen by the Lab
brador Sea Waater
(LSW) over m
major parts off the past 50kaa. A short-lastting
increase in bbottom velociity during Daansgard-Oeschhger
intervals (V
Völker et all., 2006) caan be obserrved
throughout thhe basin. Thee most remark
kable intervall of
sediment traansport intenssification occu
urred during the
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