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\begin{document}

\vspace*{2\baselineskip}

\title{Study of the Contourite Drift north of the Kane Gap (eastern equatorial Atlantic)}

\author{V. V. Sivkov$^{1,2}$, L. D. Bashirova$^{1,2}$, E.~V.~Dorokhova$^{1}$, M. V. Kapustina$^{1,2}$, E.~P.~Ponomarenko$^{2}$}

\affil{1}{Shirshov Institute of Oceanology RAS, Moscow, Russia}
\affil{2}{Immanuel Kant Baltic Federal University, Kaliningrad, Russia}

\abstract{The sediment drift north of the Kane Gap has been investigated by means of the seismoacoustic profiling.
The surface area of the drift is about 500 km$^2$ and maximal thickness is 70 m. The contourite drift was formed
 under the control of the Antarctic Bottom Water (AABW) flow in the Kane Gap. It is suggested naming this contourite
  fan as the ``Kane Drift''. Variations in the activity of the AABW current were registered in the sediment core
  ANS-33056, retrieved near the summit of the drift.
For this purpose, the mean size of sortable silt
($\overline{SS}$,
mineral fraction 10--63~$\mu$m) was used. It was shown that during the Mid-Pleistocene to
Holocene, the speed of the AABW flow north of the Kane Gap was not strictly determined by
the alternation of the climatic cycles. A strong relationship between the AABW flow and the
short-time climatic fluctuations was found only during the
marine\linebreak\newpage}

\noindent\centerline{\parbox{100mm}{ isotope stages (MIS) 4 and 5: intensification of the near-bottom current coincides with the cold intervals. Moreover, during the Terminations II and I (MIS~2/1 and MIS~6/5, respectively), the intensity of the current also increased. Maximal near-bottom current activity was registered at the end of the MIS~2.}}



\section{Introduction}

     During the Middle to Late Quaternary, significant variation in activity of the global
thermohaline circulation influenced by the glacial-interglacial alternations was registered. The
intensity of the near-bottom currents was also varying. This has determined the volume of the
Antarctic Bottom Water (AABW) entering the North Atlantic
[e.g. \itc{Johnson and Rasmussen,} \reflink{John84}{1984};
\itc{Ledbetter,} \reflink{Ledb79}{1979};
\itc{Lund et al.,} \reflink{Lund11}{2011}].
The main part of the AABW, the densest water mass
in the World Ocean, circulates from the Weddell Sea to the north along the bottom of the deep
ocean basins of the West Atlantic. Studies on the reconstruction of the past AABW flow regime
are focused on the main AABW routes in the West Atlantic. However, little is known about the
history of the AABW (and its modifications) circulation in the East Atlantic.\vspace*{.2pc}


     Relatively narrow deep channels and passages control the exchange of deep water between
the ocean basins. Here, the intensity of the near-bottom currents increases. In contrast, at the
downcurrent exit of the channels, they slow down and  unload. %``unload''.
 As a result, the sediment bodies
named the contourite fans are deposited there
[\itc{Rebesco et al.,} \reflink{Rebe14}{2014}].

     Sierra Leone Rise, which is situated between the Mid-Atlantic Ridge (MAR) and Africa, is
the main obstacle for the deep water spreading towards the East Atlantic. The Kane Gap (a
narrow deep-water channel), is the only possible gateway which allows the exchange of AABW
between the Sierra Leone and Gambia Basins
[\itc{Egloff,} \reflink{Eglo72}{1972}].


\begin{sloppypar}
     West of the Kane Gap and north of the St. Paul % fracture zone
     Fracture Zone  (near 3\deg N), a sedimentary
body was identified.
\itc{Jones and Okada} [\reflink{Jone06}{2006}]
suggested naming this Cenozoic sedimentary body
as ``the Guinea Drift''. The seismoacoustic profiling data obtained during the 32nd cruise of the
R/V~{\itshape Akademik Ioffe} (2010) allowed identifying another sedimentary body directly adjacent to
the Kane Gap from the north
[\itc{Levchenko and Murdmaa,} \reflink{Levc13}{2013}].
 We suggest that it could be a
%channel-related fan drift
contourite fan
which has been formed by the AABW flow. It is known that
sedimentation rates on the drifts are high, in comparison with adjacent pelagic sediments.
Therefore, like typical contourites, the sediments of the fan are of particular interest for studying
the ocean thermohaline paleocirculation
[\itc{Jones and Okada,} \reflink{Jone06}{2006};
\itc{Rebesco et al.,} \reflink{Rebe14}{2014}].
\end{sloppypar}


     During the 33rd cruise of the R/V~{\itshape Akademik Nikolay Strakhov} (2016), north of the Kane
Gap, an additional seismoacoustic profiling was carried out
[\itc{Sivkov et al.,} \reflink{Sivk19}{2019}].   % , in press].         %%% ???+

     The present study aims to evaluate horizontal and vertical dimensions of the sedimentary
body formed by the AABW flow using the seismoacoustic data. It also aims to trace variations in
near-bottom current intensity during the Mid-Pleistoce\-ne to Holocene based on the grain size
data.

\begin{sidewaysfigure}                     % Fig 1
\figurewidth{140mm}
\setimage{}{}{145mm}{}{2019es000658-f01}
\fcaplabel{Study area north of the Kane Gap (A). The dotted line marks the traverse of the
seismoacoustic profiling and multibeam echo sounding. The position of the study area in the
Atlantic Ocean~(B). MAR -- Mid-Atlantic Ridge. The map was created using an Esri ArcGIS$^{\circledR}$
software; isobaths are given after
[\itc{Becker et al.,}  \reflink{Beck09}{2009}].}
\end{sidewaysfigure}

\section{Material and Methods}


The data are obtained using multibeam echo sounder RESON SeaBat 7150 (12 kHz) and seismoacoustic sub-bottom profiler
EdgeTech 3300-HM (2--6~kHz)
(\figref{1}).           %  (Fig. 1).
ANS-33056 marine sediment
core (09\deg 33.68$'$ N, 20\deg 00.80$'$ W, 4543 mbsl, 5.13 mbsf) was retrieved during the  33rd cruise of
the R/V~{\itshape Akademik Nikolay Strakhov} (2016),
 %on the seismic profile, near the summit of the sediment drift which was formed north of the Kane Gap.
 near the summit of the sediment drift.
  The core section was sampled onboard
with 1~cm step. Grain size and micropaleontological analyses, as well as
%the estimation of the relative abundances of CaCO$_3$ and the total organic carbon (TOC),
CaCO$_3$ and the total organic carbon (TOC) contents,
were carried out onshore, in
the AB IO~RAS laboratories. Lithological description of the core section was done according to
the
\itc{Myhre et al.} [\reflink{Myhr95}{1995}]
classification.
Wentworth grain size classification was applied
[\itc{Wentworth,} \reflink{Went22}{1922}].    %%% ???+
The boundary between sand and silt % was taken
is
at the 63~$\mu$m, while the boundary between silt  and
clay -- at the 4~$\mu$m.

\fitopen{1.053}
\begin{sloppypar}
     Sandy silts and clayey silts containing 10--25\%
 calcium carbonate were referred to
foraminifera-bearing mud; 25--50\%
calcium carbonate -- to foraminiferal mud; $> 50$\%
calcium
carbonate -- to foraminiferal ooze. We used the term non-calcareous (terrigenous) mud for low-carbonate silty clays ($<10$\%
CaCO$_3$). The grain size analysis was carried out on the laser
diffraction particle size analyzer \mbox{SALD-2300} (Shimadzu, Japan). The grain size of the bulk
sediments was used for the lithological description (every 10~cm; 1~cm thick slices).
 %Mineral part of the sediment was analyzed after standard chemical pretreatment (every 2--3~cm; 1~cm thick slices).
  Mineral part of the sediment was analyzed (every 2--3~cm; 1~cm thick slices).
 Organic matter and carbonates were removed by treatment with excess H$_2$O$_2$ and HCl,
respectively. To disaggregate component grains, sodium tripolyphosphate was added. Then each
sample was sonicated with an ultrasonic bath immediately before the analysis. Mean size of
sortable silt
($\overline{SS}$,  %%% ???
mineral fraction 10--63~$\mu$m), which is sensitive to the current sorting, was used
as a proxy for measuring a near-bottom paleocurrent intensity. An increase in the
$\overline{SS}$  indicates
the strengthening of the near-bottom current
[\itc{McCave et al.,} \reflink{McCa95}{1995}].
\end{sloppypar}


\begin{sloppypar}
     The micropaleontological analysis was carried out using the MBS-10 microscope.
Carbonate shell preservation and presence of foraminifera in $>100$~$\mu$m fraction (every 5~cm; 1~cm thick slices) were studied. Some planktonic foraminiferal species were used as the
biostratigraphic markers.
%The CaCO$_3$ (every 2~cm; 1~cm thick slices) and TOC (every 10~cm; 1~cm thick slices) contents
The CaCO$_3$ (every 2~cm; 1~cm thick slices) and TOC (every 10 cm; 1~cm thick slices) contents
were obtained using an AN-7529 express analyzer by means of the
coulometric method. The carbonate content was calculated from the C$_{\mathrm{carb}}$ with a coefficient of~8.3.
\end{sloppypar}
\fitclose

     Marine isotope stages (MIS) were assigned using the CaCO$_3$ content that reflects the
glacial-interglacial alternation
[\itc{Barash,} \reflink{Bara88}{1988};
\itc{Helmke and Bauch,} \reflink{Helm01}{2001}].
The relative abundance
of CaCO$_3$ has the same trend when compared with a standard stable isotope curve LR04
[\itc{Lisiecki and Raymo,} \reflink{Lisi05}{2005}].
Additionally, the study uses the data on the TOC distribution in the
sediments. This is also one of the main indicators of upwelling
[\itc{Seibold,} \reflink{Seib82}{1982}].
It is well-known
that the Canary upwelling (the area near northwestern Africa) has strengthened during the cold
intervals of the Pleistocene. Thus, during MIS~2, its boundary was shifted to the south up to 14\deg N
[\itc{Barash,} \reflink{Bara88}{1988}].

\section{Results and Discussion}


\begin{sidewaysfigure}                    % Fig 2
\figurewidth{140mm}
\setimage{}{}{150mm}{110mm}{2019es000658-f02}
\fcaplabel{Echo-facies identified in the Kane Gap using the data of the seismoacoustic profiling
(see description in the text).}
\end{sidewaysfigure}


     Previously, three echo-facies were identified at the exit of the Vema Channel during the
study of the AABW flow fan
[\itc{Faug\`{e}res et al.,} \reflink{Faug02}{2002}].
We
have
 compared % the
 our data on the
seismoacoustic profiling with these echo-facies
(\figref{2}):           % (Fig. 2):
\begin{enumerate}
\item
     Prolonged echo, without sub-bottom reflectors,\linebreak which indicates the high speed of the
near-bottom current.
\item
 Distinct echo-facies with parallel, continuous sub-bottom reflectors with no evidence
of the high intensity of the near-bottom current.
\item
Wavy hyperbolic echo with sub-bottom reflectors related to sediment waves of
various morphology, which marks the moderate intensity of the near-bottom current.
\end{enumerate}

The multibeam echo sounder data
allowed building up a digital model of the bottom
topography
(\figref{3}).           % (Fig. 3).
This model provides a more detailed image of the bottom topography, in
comparison with existing maps
([\itc{Hobart et al.,} \reflink{Hoba75}{1975};
\itc{Smith and Sandwell,} \reflink{Smit97}{1997};
\itc{IOC, IHO, BODC,} \reflink{IOC03}{2003}],      %%% ???
%%IOC, I. (2003), BODC: Centenary Edition of the GEBCO Digital Atlas, published on CD-ROM
%%on behalf of the Intergovernmental Oceanographic Commission and the International
%%Hydrographic Organization as part of the General Bathymetric Chart of the Oceans, British Oceanographic Data Centre, Liverpool, UK).
%%
In particular, the coordinates of the sill in the Kane Gap (9\deg 24.5$'$ N, 19\deg 52$'$ W)
and its maximal depth (about 4525~m) were specified. Following the thalweg of the studied
Kane Gap, we have also specified the position of the axis of AABW flow.

%     In the area of the Kane Gap, the acoustic basement without sediments (echo-facies I)
%indicates the slow sedimentation or non-deposition, and even denudation.
According to
\itc{Faug\`{e}res et al.} [\reflink{Faug02}{2002}],
echo-facies I indicates the high speed of the near-bottom current.
However, in the studied area, the presence of this echo-facies
(acoustic basement without sediments) is linked to the slow sedimentation or non-deposition, and even denudation.
Based on the position
of the echo-facies II and III, the sediment drift north of the Kane Gap has been contoured on a
first approximation. The sediment thickness was estimated
(\figref{3}).           % (Fig. 3).

\begin{sidewaysfigure}                % Fig 3
\figurewidth{140mm}
\setimage{}{}{150mm}{105mm}{2019es000658-f03}
\fcaplabel{Digital model of the bottom topography created using a multibeam echo sounder data.
Variation in the sediment thickness related to the acoustic basement north of the Kane Gap is
also shown.}
\end{sidewaysfigure}

     The difference in the sound speed in the water column and the sediment % sequence
      was not
taken into account when estimating the drift thickness. A sound speed of 1500~m/s was % used.
applied.
 The
maximal drift thickness (the depth to Eocene acoustic basement
[\itc{Emelyanov et al.,} \reflink{Emel90}{1990},
\itc{Jones and Okada,} \reflink{Jone06}{2006}])
exceeds 70~m. The surface area of the drift is about 500~km$^2$. The drift has
been accumulated to the left of the axis of AABW flow. Near the axis, the
slope is relatively steep, especially at the exit of the Kane Gap. On the opposite side, the drift
slope flattens out.


In the Northern Hemisphere, influence of the Coriolis force determines the position of the drift to left of the contour current, as well as asymmetry of the drift [\itc{Rebesco et al.,} \reflink{Rebe14}{2014}].
Presence of these morphological features in the studied sediment drift, as well as the proximity of the AABW current indicates its contourite origin. We suggest naming this drift as ``the Kane Drift''.


It should be mentioned that the sediment supply from Africa and Guinea Plateau to the continental
slope could cause an increase in the sediment thickness outside the drift in the northeastern part
 of the study area, to the right of the axis of AABW flow
(\figref{3})           % (Fig. 3)
[\itc{Jones and Okada,} \reflink{Jone06}{2006}].
  This sediment transport is not influenced by AABW flow.

\begin{sloppypar}
According to lithological description,
     ANS-33056 core section is represented by the calcareous and non-calcareous muds of
different density and color
(\figref{4}).           % (Fig. 4).
Several thin greenish (glauconitic?) interlayers could indicate
the slowdown of sedimentation. Tracers of the bioturbation are pronounced within the interval
250--300~cm. The sand fraction is almost absent, excluding the upper 30 cm and intervals 305--330~cm and 355--380~cm. The coarse fraction is mainly represented by radiolarians, sponge
spicules, and agglutinated foraminifera. Poor preservation of calcareous foraminifera throughout
the core section indicates pronounced carbonate dissolution. However, the rate of carbonate
dissolution varies significantly.
 \end{sloppypar}

\begin{sloppypar}
     The rare tests of planktonic foraminiferal species
     {\itshape Globigerinoides ruber} pink occur
throughout the core section indicating the Mid-Pleistocene to Holocene age of the sediments
[\itc{Barash,} \reflink{Bara88}{1988}].
In the core section, seven marine isotope stages (MIS) are identified. The
concordance between distribution of CaCO$_3$ and standard stable isotope curve LR04
 [\itc{Lisiecki and Raymo,} \reflink{Lisi05}{2005}]
 indicates the climate-controlled sedimentation in the studied area
 (\figref{4}).           % (Fig. 4).
 \end{sloppypar}



\begin{sidewaysfigure}                     % Fig 4
\figurewidth{140mm}
\setimage{}{}{150mm}{97mm}{2019es000658-f04}
\fcaplabel{ANS-33056 core lithology, distribution of the fractions of the bulk sediment, CaCO$_3$
and TOC contents, as well as variability in the intensity of near-bottom paleocurrent
($\overline{SS}$)  during
the investigated period. Data of CaCO$_3$ are compared with the standard stable isotope curve
LR04  [\itc{Lisiecki and Raymo,} \reflink{Lisi05}{2005}].
 Black arrows indicate an increase in intensity of the near-bottom current.}\vspace*{1pc}
\end{sidewaysfigure}



     Relatively high calcium carbonate content (up to 50\%)
     is typical for warm MIS~1, 5 \mbox{(a, c, e)} and 7, with a maximum during the MIS~5e (56\%).
      Minimal values of CaCO$_3$ (6--9\%)
correspond to glacials (MIS~2, 4, 6) and stadials of MIS~5 (b, d). Relatively cold MIS~3 is also
marked by low CaCO$_3$ content (7--20\%).
 Increased values of the TOC during the MIS~2, 3 and 6
correspond to the strengthening of the Canary upwelling during the cold intervals. However,
MIS~4 doesn't show the high TOC content.

     It should be noted that all warm periods are marked by relatively high CaCO$_3$ content
despite the permanent presence of AABW in the study area. It allows us to suggest that
aggressiveness of the AABW to CaCO$_3$ was changing during the investigated period. The
previous studies
[\itc{Diester-Haass,} \reflink{Dies78}{1978};          %%% ???+
\itc{Dittert and Henrich,} \reflink{Ditt00}{2000}]
suggested the modern calcite
lysocline depth to be 5000~m in the study area.

The investigation of the grain size distributions of the mineral part of ANS-33056 core sediments allows distinguishing the sedimentary patterns.
The grain size distributions are bimodal. The 4-$\mu$m mode indicates the pelagic sedimentation
[\itc{McCave and Hall,} \reflink{McCa06}{2006}].
 The coarse silt mode at
 14--17~$\mu$m is also clearly pronounced. It is well-known that this mode is sensitive to the variations in near-bottom current activity and is typical for contourites
 [\itc{McCave et al.,} \reflink{McCa95}{1995}].

     The significant distance of the studied area from the continental slope ($\sim 500$~km far away)
and wide shelf (more than 250~km far away) almost completely isolates the investigated drift
from the continent. Thus, the coarse-grained sediment supply from the land to the Kane Gap is
prevented. Propagation of the turbidity currents % across
from
 the continental shelf to the deep basins is
quite active, especially during the glacial time when the sea level decreased. Here, sediment
transport occurs along the giant canyons developed on the continental slope. The closest to the
study area canyon is located on the Guinea Plateau slope (150~km north of the study area), at the
downcurrent of AABW
[\itc{Jones and Okada,} \reflink{Jone06}{2006}].
 Moreover, studied drift is situated on the sub-bottom slope of the Sierra Leone Rise but not on the continental slope. Such position of the
investigated drift, as well as the presence of the coarse silt mode, indicates the key role of the
near-bottom current in sediment sorting in the study area. Thus, it is possible to use the
$\overline{SS}$
parameter to evaluate the intensity of this current.

     There is a weak positive trend in a current activity %within      the
     from  MIS 7 to 1
(\figref{4}).           %      (Fig. 4).
     We did not find a
     %strong relationship
     clear link
     between % the latter
     the near-bottom flow intensity
     and the glacial-interglacial alternations. However,
there is a relationship with several short-time climatic fluctuations: strengthening of the near-bottom
current during the cold MIS~4, 5b and 5d, as well as during the Terminations II and I
(MIS~6/5 and MIS~2/1, respectively). Maximal current activity is registered at the end of the MIS~2.
During MIS~1, the current activity was relatively high in comparison with previous warm
periods. Thus, \mbox{Mid-Quaternary} to Holocene variations in the intensity of the AABW flow north
of the Kane Gap indicate the nontrivial relationship between its activity and the global climatic
cyclicity. This coincides with previous data on variations in the AABW flow at the exit of the
Vema channel
[\itc{Mass\'{e} et al.,} \reflink{Mass94}{1994}].

     The values of integral speeds of the near-bottom geostrophic currents are obtained using
the long-term hydrophysical measurements. Comparison of the quantitative grain size parameters
with these values is a sophisticated process. However, according to
\itc{McCave et al.} [\reflink{McCa17}{2017}],
 it is
possible to convert our
$\overline{SS}$ data (15--21~$\mu$m,
\figref{4})           % Fig. 4)
to the current speed values. Using such an
approach we can suggest that during the investigated period the AABW flow speed varied in the
5--10 to 15--18~cm/s range. Herewith, it should be noted that in
\itc{McCave et al.} [\reflink{McCa17}{2017}]
the data of
Coulter Counter (Beckman Multisizer III) and Sedigraph 5100 were used when calculating the
current speeds. Still there are no such calculations for the laser diffraction particle size analyzer.
For this reason, our estimates of AABW flow speeds are very rough. It should be noted that these
estimated values have been obtained at the downcurrent exit of the channel where current is
slowing down and unloads. Directly in the Kane Gap, the flow was more intensive.

     Two sources of AABW, circulating in the Kane Gap, are known
     [\itc{Hobart et al.,} \reflink{Hoba75}{1975}]:
     the
southern one -- from the Romanche fracture zone and Sierra Leone Basin, and the northern one --
from the other fracture zones of the MAR and Gambia Basin. The modern studies have shown
that AABW circulates along the Kane Gap both southwards and northwards. An alternation in
AABW directions occurs several times per year. The northward transport of AABW prevails
[\itc{Morozov et al.,} \reflink{Moro10}{2010}, \reflink{Moro13}{2013}].
Our data allow tracing the variations in the activity
of the northward flow of AABW. It can be assumed that there is a similar %channel-related fan drift
contourite fan
to the south of the Kane Gap. The sediments of this drift could provide information about
southward AABW transport and its activity in the past.

\enlargethispage{-.9\baselineskip}

\section{Conclusions}

      The data on the seismoacoustic profiling confirm our suggestion about the contourite % nature
       origin of the sediment drift north of the Kane Gap. The surface area of the drift is about 500~km$^2$
and maximal thickness is 70~m. The drift was formed by the contour current of the AABW. It
has relatively steep slopes close to the axis of the AABW flow and flattens out with distance
increasing from the flow axis. We suggest naming this % channel-related fan drift
contourite fan
as the ``the Kane Drift''.

     It was shown that within the MIS 1--7, the speed of the AABW flow north of the Kane Gap
was not strictly determined by the glacial-interglacial alternation. Variations in the current
activity within the marine isotope stages were also registered. Intensification of the near-bottom
current coincides with MIS~4 and stadials of the MIS~5 demonstrating the strong relationship
between the AABW %flow and the short-time climatic
flow and the climatic
fluctuations. Moreover, during the
Terminations II and I (MIS~2/1 and MIS~6/5, respectively), the intensity of the current was also
increased. Maximal near-bottom current activity was registered at the end of the MIS~2. Despite
the subsequent decrease, the near-bottom current activity during the MIS 1 was still relatively
high, in comparison with previous maxima.

\acknowledgement{The authors are very grateful to V.~A.~Che\-ch\-ko for conducting of the lithological description of
the core section, to V. N. Lukashin and the anonymous reviewer for their valuable advice and
constructive comments. The field research, grain size analysis, and geoacoustic study were
supported by the state assignment of IO RAS (Theme No. 0149-2019-0013);
micropaleontological study and measurements of CaCO$_3$ content were done with the support of
RSF project \mbox{No. 14-50-00095}; stratigraphic subdivision of the core was supported by the IKBFU
``5-100'' Russian academic excellence Project; data processing of the multibeam echo sounder
was partly supported by RFBR project \mbox{No. 17-08-00085-a.}}

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\end{document}

