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\begin{document}

\vspace*{2\baselineskip}

\title{Late Quaternary lateral sedimentation in the S\~{ao Tom\'{e} Seamount area of the western South Atlantic}}

\author{Dmitrii G. Borisov$^{1}$, Ivar O. Murdmaa$^{1}$, Elena V. Ivanova$^{1}$, Evgenia V. Dorokhova$^{1}$}

\affil{}{Shirshov Institute of Oceanology RAS, Moscow,  Russia}

\abstract{This work is focused on a problem of distinguishing between contourites and turbidites in the southwestern Brazil Basin, which is crucial for better understanding of the Late Quaternary history of interplay between along slope and down slope sedimentation processes in the western South Atlantic. The study embraces an area between the Almirante Saldanha and S\~{a}o Tom\'{e} Seamounts, which is not linked to any known depositional system. This area was not previously studied in frame of the contourite paradigm. Very high-resolution seismic records and lithological data acquired during three cruises of the RV~{\itshape Akademik Ioffe} (2011--2013) revealed domination of lateral sedimentation in the area of the S\~{a}o Tom\'{e} Seamount. Interplay between along slope and gravity driven sedimentation processes interacting with the ocean floor topography resulted in the formation of complicated \linebreak\newpage}\noindent\centerline{\parbox{100mm}{  sediment structures and a wide range of seismic facies. Along slope sedimentation processes are controlled by the Antarctic bottom water current, which reworked coarse-grained material derived from the continental slope through turbidity flows. The study area is divided into two zones, which are characterized by domination of either gravity flows or bottom current related sedimentation processes. The S\~{a}o Tom\'{e} Seamount marks a boundary between these zones. The obtained results improved knowledges about the formation history of the Upper Quaternary sediment cover on the continental rise of the southwestern Brazil Basin and about interplay between gravity-driven and bottom current related sedimentation processes during the last glacial-interglacial cycle.}}


\section{1. Introduction}

Interplay between downslope and along slope sedimentation processes played a crucial
role in sediment cover formation along the South American oceanic margin during the
Quaternary. First, this sediment cover concerns an interaction between turbidity flows
and geostro\-phic (contour) currents, as their simultaneous action or alternation through
time in response to global climate changes, sea-level variations and tectonic
movements remains questionable. This is mainly due to a lack of unambiguous criteria
for distinguishing between contourites and fine-grained distal turbidites. The southern
part of the Brazil Basin is characterized by a number of mixed contourite-turbidite
systems, which represents a perfect area for investigation of both types of lateral
sedimentation processes. However, the sediment sections and paleoceanographic
records on the continental rise and in the contiguous abyssal part of the basin remain
poorly studied. Previous studies which were also focused on the lateral sedimentation
during the Quaternary embraced two depositional systems east and west of the study
area [\itc{Faug\`{e}res et al.,} \reflink{Faug02}{2002};
\itc{Gonthier et al.,} \reflink{Gont03}{2003};
\itc{Lima et al.,} \reflink{Lima09}{2009}].
These papers demonstrated the complexity and variety of interactions between gravity
driven and bottom current related processes. The low carbonate content in sediments,
frequent hiatuses and sediment reworking allowed neither establishing of reliable
stratigraphy for cores nor the detailed investigation of lateral sedimentation in relation to
global climate changes during the Late Quaternary. The available maps of echo facies
distribution in the south-western Brazil Basin have insufficient resolution for detailed
study of sedimentation processes in the area of the S\~{a}o Tom\'{e} seamount
[e.g. \itc{Damuth and Hayes,} \reflink{Damu77}{1977}],
so one of the main focuses of this paper is the detailed investigation of
echo character types on the base of very high resolution seismic records. The main aim
of this work is to cover (partly) the gap between the Columbia Channel depositional
system and the S\~{a}o Tom\'{e} deep-sea channel levee system and to study the interplay
between gravity driven and bottom current related processes, which occurred in this
area during the last glacial-interglacial cycle.

\begin{figure}                      % Fig 1a
\figurewidth{110mm}
\setimage{}{}{110mm}{}{2019es000689-f01a}
\fcaplabel{{\bfseries (A)} \footnotesize Bathymetric map of the South Brazilian continental margin with directions of AABW and NADW currents, location of DSDP 515 drilling site and locations of main contourite, turbidite and mixed sedimentary systems (1 -- S\~{a}o Tom\'{e} turbidite system  [\itc{Viana et al.,} \reflink{Vian03}{2003};
\itc{Gonthier et al.,} \reflink{Gont03}{2003}];
2 -- S\~{a}o Tom\'{e} Seamount mixed system
[\itc{Borisov et al.,} \reflink{Bori13}{2013}];
3 -- Columbia channel mixed contourite-turbidite system
[e.g., \itc{Lima et al.,} \reflink{Lima09}{2009}];
 4 -- Ioffe contourite drift
[\itc{Ivanova et al.,} \reflink{Ivan16}{2016}];
 5 -- Vema contourite fan
[\itc{M\'{e}zerais et al.,} \reflink{Meze93}{1993};
\itc{Mass\'{e} et al.,} \reflink{Mass94}{1994}];
AlSS -- Almirante Saldanha Seamount;
DB -- Dogaressa Bank;
CS -- Columbia Seamount;
M Ch -- Ma\c{c}ae channel;
STS -- S\~{a}o Tom\'{e} Seamount).}
\end{figure}


\addtocounter{figure}{-1}
\begin{figure}                      % Fig 1b
\figurewidth{110mm}
\setimage{}{}{65mm}{}{2019es000689-f01b}
\fcaplabel{{\bfseries (B)} Regional seafloor topography of the study area with direction of the AABW current and location of cores and seismoacoustic profiles. Bold purple lines
indicate fragments of seismic profile AI-35 shown in
\figref{4}.}           % Figure 4.
\end{figure}


\subsection{1.1. Seafloor Topography and Geological Setting}

In the southwestern Brazil Basin, the continental rise is confined by the Rio Grande Rise
in the south and the Vitoria-Trindade Seamounts in the north. The escarpment of the
S\~{a}o Paulo Plateau in the west is characterized by a smooth surface, which gently
slopes eastward. In addition to the W--E extending Vitoria-Trindade chain, there are two,
conically shaped seamo\-unts: the Almirante Saldanha and S\~{a}o Tom\'{e}
(\figref{1}A, \figref{1}B).           % (Fig. 1a, b).
The S\~{a}o Tom\'{e} Seamount is approximately 37 km $\times$ 42 km with a relative height of 2800~m
(minimum water depth of 1350 m). The height of the Almirante Saldanha Seamount
exceeds 3600 m and its dimensions are approximately $47 \times  58$~km.

The large S\~{a}o Paulo Plateau is located on the continental slope at a depth range of
2000--3200 m. The eastern boundary of the plateau is marked by a 200-m-high
escarpment with a N--S trend. The plateau is affected by numerous halokinetic
structures related to the Aptian salt deposits. The active diapiric structures control the
courses of the manifold channels that form a complex network on the plateau
[\itc{Viana et al.,} \reflink{Vian03}{2003}].
Some of these channels cross the escarpment extend to the rise, which
converge and form several major channels
[\itc{Gonthier et al.,} \reflink{Gont03}{2003}].
The most prominent
channel in the area is the Columbia (Trindade) channel, which has a depth of 200--400~m and width of up to 20~km. This channel runs in the WNW--ESE direction at a depth
range of 4200--5000~m
[\itc{Faug\`{e}res et al.,} \reflink{Faug02}{2002};
\itc{Lima et al.,} \reflink{Lima09}{2009}].
The
Rio de Janeiro channel has much smaller dimensions. This channel is oriented in the
W--E direction and linked to the Rio de Janeiro fracture zone
[\itc{Bassetto et al.,} \reflink{Bass00}{2000};
\itc{Gonthier et al.,} \reflink{Gont03}{2003}].
The largest tributaries are the  Ma\c{c}ae channel running along the
foot of the Almirante Saldanha Seamount and the Carioca channel, which is located
southward
(\figref{1}A).           % (Fig. 1a).


\begin{sloppypar}
In this paper, the area between the Almirante Saldanha and S\~{a}o Tom\'{e} Seamounts, and
the Vitoria-Trindade Seamounts and Ma\c{c}ae channel are studied
(\figref{1}B).           % (Fig. 1b).
According to the seismic data, the thickness of sediments deposited since the late
Cretaceous on the continental rise of the southwestern Brazil Basin is 1--1.8~s TWT (two
way time). The upper Pliocene--Quaternary seismic unit has a thickness of less than 0.1~s TWT
[\itc{Barker et al.,} \reflink{Bark83}{1983};
\itc{Lima et al.,} \reflink{Lima09}{2009};
\itc{Viana et al.,} \reflink{Vian03}{2003}].
The
Quaternary section recovered at DSDP site 515 located north of the northern Vema
channel outlet
(\figref{1}A)           % (Fig. 1a)
is approximately 40-m-thick
[\itc{Barker et al.,} \reflink{Bark83}{1983}].
It is composed
of grayish brown terrigenous mud and clay with occasional nannofossil-rich layers and
some foraminifer-rich layers
[\itc{Barker et al.,} \reflink{Bark83}{1983}].
\end{sloppypar}



East of the S\~{a}o Tom\'{e} Seamount (core V14-13, $21\deg 57'$ S, $36\deg 15'$ W, water depth 4259~m,
core length 6.56~m, 1957) sediments are presented by clay with thin laminas and wavy
lenses of fine grained sand in the upper 1.88 m and yellowish brown to gray sand below
(Lamont-Doherty Core Repository (LDCR)). South of the study area (core V24-254,
$22\deg 58'$ S, $36\deg 43'$ W, water depth 4029~m, core length 5.73~m, 1967; core \mbox{GeoB 2117-1},
\mbox{$23\deg 02.3'$ S}, $36\deg 39,1'$ W, water depth 4045~m, core length 4.66~m, 1993) sediments
consist of foraminiferal clay with sandy interbeds. Most of the sandy layers occur with
homogenous structure, whereas sandy foraminiferal ooze layers are characterized by
intense crossbedding (core V24-254)
[\itc{Bleil et al.,} \reflink{Blei93}{1993}],
 LDCR.


\subsection{1.2. Regional Oceanography}

Bottom circulation in the deepwater part of the southwestern Brazil Basin is generally
controlled by the Antarctic bottom water (AABW). The term AABW in this paper
comprises the Weddell Sea deep water and lower part of the circumpolar water
described by
\itc{Reid et al.} [\reflink{Reid77}{1977}].

The AABW propagates from the Argentine Basin to the Brazil Basin mainly through the
Vema channel and Hunter channel, located east of the Rio Grande Rise
(\figref{1}A).           % (Fig. 1a).
Northward flowing bottom currents of Antarctic water move to the east along the Vitoria-Trindade chain and pass through the deep passages between the Dogaressa Bank,
Columbia Seamount and Trindade Island
[\itc{Frey et al.,} \reflink{Frey19}{2019};
\itc{Morozov et al.,} \reflink{Moro10}{2010}].
Modern AABW current velocities do not exceed 10 cm/s in the open basin, while in the
channels and transform fault valleys, the current velocities exceed 20~cm/s
[e.g.
\itc{Morozov et al.,} \reflink{Moro18}{2018}].

The AABW is overlaid by highly saline, oxygen-rich and nutrient poor North Atlantic
deep water (NADW), which flows south at a velocity of less than 5~cm/s
[\itc{Reid,} \reflink{Reid89}{1989}, \reflink{Reid96}{1996}].
A boundary between the AABW and NADW in the southern Brazil Basin
corresponds to a depth of approximately 3600--3800~m and correlates with a neutral
density isosurface of $\gamma^n = 28.16$~g/cm$^3$
[\itc{Morozov et al.,} \reflink{Moro10}{2010}].

\subsection{1.3. Paleoceanographic Setting}

The modern regime of oceanic sedimentation in this part of the western South Atlantic
started during the Aptian/Albian
(113~Ma,
\itc{Chang et al.} [\reflink{Chan92}{1992}].
The Late Cretaceous--Eocene sedimentation was likely dominated by hemipelagic settling and turbidity
currents. The opening of the Drake and Tasman oceanic seaways in the Southern
Hemisphere during the Eocene--Oligocene
[e.g.,
\itc{Allen and  Armstrong,} \reflink{Alle08}{2008}]
caused
dramatic changes in the ocean circulation. Since that time, deposition has been
controlled by the interplay between geostrophic contour currents (along slope
processes), gravity flows and mass-wasting processes from the continental slope and
seamounts (downslope gravity driven processes). Global climate changes and tectonic
movements resulted in a dramatic increase in NADW and AABW production, which
caused episodes of drastic erosion and formation of several major regional
unconformities. These unconformities represent time lines of the period from the
Eocene--Oligocene to the late Pliocene
[\itc{Gamboa et al.,} \reflink{Gamb83}{1983};
\itc{Faug\`{e}res et al.,} \reflink{Faug02}{2002};
\itc{Lima et al.,} \reflink{Lima09}{2009};
\itc{Viana et al.,} \reflink{Vian03}{2003}].
Lateral sedimentation
processes are responsible for the formation of several depositional systems
(\figref{1}A):           % (Fig. 1a):
the S\~{a}o Tom\'{e} turbidite system at the foot of the S\~{a}o Paulo Plateau escarpment
[\itc{Gonthier et al.,} \reflink{Gont03}{2003};
\itc{Viana et al.,} \reflink{Vian03}{2003}],
Columbia channel mixed contourite-turbidite system
(e.g.,
\itc{Lima et al.,} \reflink{Lima09}{2009};
\itc{Murdmaa et al.,} \reflink{Murd12}{2012}],
S\~{a}o Tom\'{e} Seamount
mixed system
[\itc{Borisov et al.,} \reflink{Bori13}{2013}],
Vema contourite fan
[\itc{M\'{e}zerais et al.,} \reflink{Meze93}{1993};
\itc{Mass\'{e} et al.,} \reflink{Mass94}{1994}]
and Ioffe contourite drift
[\itc{Ivanova et al.,} \reflink{Ivan16}{2016}].
In this area, most of the
channels transported sands during the Late Quaternary and remained active until
recently
[\itc{Machado et al.,} \reflink{Mach98}{1998};
\itc{Viana et al.,} \reflink{Vian98}{1998}].

\section{2. Materials and Methods}

This study is based on seismic, lithological and micropaleontological data acquired
during cruises 33, 35, 37 and 43 of the RV~{\itshape Akademik Ioffe} (2010--2013)
[\itc{Lev\-chenko and Murdmaa,} \reflink{Levc13a}{2013a},
\reflink{Levc13b}{2013b};
\itc{Levchenko et al.,} \reflink{Levc14}{2014};
\itc{Skolotnev et al.,} \reflink{Skol18}{2018}]
as well as publicly
available bathymetric data.


\def\topfraction{.8}
\begin{figure}[t]                      % Fig 2
\figurewidth{90mm}
\setimage{}{}{115mm}{}{2019es000689-f02}\vspace*{\baselineskip}
\fcaplabel{Seismic facies types distinguished in seismic sections (based
on classification by
\itc{Damuth and Hayes} [\reflink{Damu77}{1977}]).}\vspace*{-1.2ex}
\end{figure}



\subsection{2.1. Bathymetry}

The physiographic map of the Brazil oceanic margin (scale $1:1,883,927$) is used as the
geomorphological background for the study area. This map was prepared by the Navy
Center of Hydrography of the Brazil Directorate of Hydrography and Navigation, and the
map comprises LEPLAC data (Brazilian Continental Shelf Survey Plan), nautical charts,
ETOPO global relief model and General Bathymetric Chart of the Oceans--GEBCO.


\subsection{2.2. Seismic Profiling}

High-resolution seismic lines running in the SW--NE direction east and west of the S\~{a}o
Tom\'{e} Seamount were collected using the SES 2000 deep (Innomar Technologie
GmbH, Germany) parametric sub-bottom echo-sounder (central frequency 4--5~kHz).
Several short intersecting profiles were collected at the foot of the northwestern slope of
the S\~{a}o Tom\'{e} Seamount. The acoustic penetration into the sediments reached 20--30~m with a vertical resolution of approximately 40~cm. Processing of the SES 2000 deep
data was conducted using Interactive Sediment Layer Editor (ISE) software. A sound
velocity of 1500~m/s was assumed for both the water column and sediments.


The profiles underwent seismic facies analysis, with the aim to determine the
geometries of studied depositional features and their internal acoustic structures.
A number of classifications of acoustic facies (also referred to as echofacies, echo
character types, echos, and seismic facies in some cases) was developed during the
last 45 years
[\itc{Damuth,} \reflink{Damu75}{1975}, \reflink{Damu80}{1980};
\itc{Damuth and Hayes,} \reflink{Damu77}{1977};
\itc{Loncke et al.,} \reflink{Lonc09}{2009};
\itc{Maestro et al.,} \reflink{Maes18}{2018};
\itc{Pratson and Laine,} \reflink{Prat89}{1989}].
In this study, the echo character types are
classified according to
\itc{Damuth} [\reflink{Damu75}{1975}, \reflink{Damu80}{1980}],
\itc{Damuth and Hayes} [\reflink{Damu77}{1977}]
 and
 \itc{Loncke et al.} [\reflink{Lonc09}{2009}],
%   mostly because these works are based on the data collected on the South
% American margin. , mostly because these works are based on the data % collected on the South American margin.         %%% ???+
mostly because these works are based on the data collected on the South American margin.


\begin{sidewaystable}                                   % Table 1
\tablewidth{150mm}
\ftablabel{Core Location}
\vspace{5pt}
\small
\begin{tabular}
{@{}l@{\hspace{12pt}}
c@{\hspace{12pt}}
c@{\hspace{12pt}}
c@{\hspace{12pt}}
c@{}}
\hline
\\ [-7pt]
Core number &  Latitude          &  Longitude       & Water depth, m & Core length, m \\
  [7pt]  \hline   \\ [-4pt]
AI-2445     &  21\deg 38.3' S   &   36\deg 40.1$'$W  &    4108    &  2.2 \\
AI-2561     &  22\deg 18.8$'$ S   &   36\deg 35.8$'$ W &    4053    & 1.94 \\
AI-2562     &  22\deg 37.8$'$ S   &   36\deg 47.5$'$ W &    3950    &    3 \\
AI-3150     &  21\deg 40.237$'$ S &   36\deg 41.178$'$ W &  4090    & 2.61 \\
[4pt] \hline
\end{tabular}
\end{sidewaystable}


Since the acoustic images of the same deposits recor\-ded using the SES 2000 deep
profiler and echo-sounders in the 1960--70s differ due to their technical characteristics,
the classification of seismic facies given by by %%% ???
\itc{Damuth} [\reflink{Damu75}{1975}, \reflink{Damu80}{1980}],
\itc{Damuth and Hayes} [\reflink{Damu77}{1977}]
and
\itc{Loncke et al.} [\reflink{Lonc09}{2009}]
has been adapted for seismic facies
distinguished in the SES 2000 deep records
(\figref{2}).           % (Fig. 2).
This adaptation is based on a
correlation between the SES 2000 deep seismic data collected during cruises 32, 33,
35, 37, and 40 of the RV~{\itshape Akademik Ioffe} (2010--2013) in the Western Atlantic
[\itc{Levchenko and Murdmaa,} \reflink{Levc13a}{2013a},
\reflink{Levc13b}{2013b}; %%% ???  Levchenko, Murdmaa, 2013a, b;
% Levchenko, 2014;
\itc{Levchenko et al.,}\reflink{Levc14}{2014}]
and the
seismic facies distribution map of the east Brazilian oceanic margin, which was
published by
\itc{Damuth and Hayes} [\reflink{Damu77}{1977}].
In the seismic records collected using the SES
2000 deep profiler, distinguishing facies type IIA from IIB is difficult, and thus, these
types were combined into type IIAB
(\figref{2}C--\figref{2}E),           % (Fig. 2C-E),
which is described further in detail.

\subsection{2.3. Lithology}

Four gravity cores with recoveries from 1.94 to 3 m were retrieved to the northwest and
southeast of the STS at water depths from 3950 to 4108~m
(\figref{1}A, \figref{1}B).           % (Fig. 1a, b).
The
coordinates, water depths, and core lengths of each core are provided in
\tabref{1}.           % Table 1.
All
collected cores were subjected to visual macroscopic and smear-slide descriptions.
After opening, the hue and chroma attributes of the sediment color were determined
using the Munsell Geological Rock Color Chart
[\itc{Munsell Color Company,} \reflink{Muns95}{1995}].    %%% ???

Geochemical and grain-size analyses were carried out in the Atlantic Branch of the
Shirshov Institute of Ocea\-nology RAS. Calcium carbonate content (CaCO$_3$) were
measured using the AN-7529 express coulometric carbon analyzer (Gomel Plant of
Measuring Equipment, Republic of Belarus) on 94 dried sediment samples taken from
all cores. Sampling intervals in homogenous layers were 15--30~cm and 2--3~cm in
sand/silt rich or laminated layers.

Grain-size distribution analyses were performed with the SALD 2300 laser diffraction
particle size analyzer (Shimadzu, Japan) on 119 bulk sediment samples taken from
cores AI-2561, AI-2562, and AI-3150. This type of analysis was conducted every 10--30~cm in homogenous and bioturbated layers and every 1--2~cm in suggested gravitite or
contourite cycles. The grain-size distribution of the terrigenous fraction was studied in
samples from the same intervals after removal of CaCO$_3$ by an HCl N1 solution.
Samples for the analyses were dispersed by ultrasonation in sodium tripolyphosphate
solution.

Volume magnetic susceptibility was measured on split cores (AI-2561, AI-2562, and AI-3150)
using the Bartington MS3 meter and Bartington MS2E surface sensor.
Measurements were performed at 0.5~cm intervals. Core AI-2445 was not subjected to
magnetic susceptibility measurements due to a lack of an archive half of the core.

\subsection{2.4. Micropaleontology}

The pilot study of foraminiferal assemblages, index spe\-cies and fragmentation (the ratio
of fragments to the sum of fragments and whole tests) was performed under the MBI6
binocular at grain-size fractions of $> 0.1$~mm. The sampling interval ranged from 5 to 45~cm depending on the sediment lithology and CaCO$_3$ content.

\section{3. Results}

\subsection{3.1. Seismic Profiling}

The seismic facies analysis revealed two widespread types of seismic facies in the
study area: IB and IIAB. Type IB facies are characterized by continuous, sharp, parallel
moderate-amplitude reflectors persisting for tens of kilometers and an acoustic
penetration of approximately 15--30~m
(\figref{2}A, \figref{2}B).           % (Fig. 2a, b).
These facies are traced south of the
S\~{a}o Tom\'{e} Seamount and at the foot of its western slope. Seismic facies of type IIAB
show intermittent parallel moderate to high-amplitude reflectors with zones
characterized by the absence of sub-bottom reflectors. Acoustic penetration varies from
5 to 15~m
(\figref{2}C--\figref{2}E).           % (Fig. 2c-e).
These facies are widespread north of the S\~{a}o Tom\'{e} Seamount
and are found in the Ma\c{c}ae channel valley
(\figref{3}).           % (Fig. 3).
The widths of the Ma\c{c}ae channel in
the upper part reaches 18~km and its relative depths is approximately 40--45~m. The
other channels crossed by seismic lines are smaller and did not exceed 7~km in width
and 40~m in depth. Large, irregular, overlapping and single hyperbolae with widely
varying vertex elevations above the sea floor correspond to type IIIA facies
(\figref{2}F),           % (Fig. 2f),
which are related to the S\~{a}o Tom\'{e} Seamount and Vitoria-Trindade chain. Type IIIB is
characterized by regular, single or slightly overlapping hyperbolae with conformable
reflectors (acoustic penetration of 20--25~m). These facies are generally traced in areas
adjacent to channels. The height of the hyperbolae varies from 5 to 15~m and the
wavelength is approximately 0.2--1~km
(\figref{2}G--\figref{2}I).           % (Fig. 2g-i).
Most of these wave-like features
demonstrate migration toward the nearest channel. Overlapping hyperbolae with
varying vertex elevations above the sea floor and no sub-bottom reflectors related to
type IIIC were found in the Ma\c{c}ae channel valley or alternating with the IIIB facies
(\figref{2}J, \figref{2}K).           % (Fig. 2j, k).
Seismic facies IIIE represent zones of regular, intense, overlapping hyperbolae
with vertices tangent to the sea floor, which are interrupted by zones of distinct echoes
with parallel sub-bottom reflectors
(\figref{2}L).           % (Fig. 2l).
 These facies are related to the northern
slope of the Ma\c{c}ae channel. South of this channel and northwest of the S\~{a}o Tom\'{e}
Seamount, the seismic lines crossed broad, single, irregular hyperbolae with
disconformable reflectors and acoustic penetrations of 10--20~m (type IIIF facies)
(\figref{2}M, \figref{2}N).           % (Fig. 2m, n).

\begin{figure}                      % Fig 3
\figurewidth{90mm}
\setimage{}{}{115mm}{}{2019es000689-f03}
\fcaplabel{Seismic facies distribution along the SES 2000 deep seismic lines with
locations of sediment cores (black arrows). Location of seismic lines is shown in
\figref{1}B.}           % Figure 1b.
\end{figure}


\begin{sidewaysfigure}                      % Fig 4
\figurewidth{140mm}
\setimage{}{}{155mm}{}{2019es000689-f04}
\fcaplabel{Fragments of seismic profile AI-35 (with interpretation). Red solid line indicate
seismic reflector corresponding to coarse grained layer recovered in the lower parts of
the sediment cores. Location of seismic lines and core is shown is
\figref{1}B. Core lithology is presented in \figref{6}.} \vspace*{\baselineskip}
\end{sidewaysfigure}

Sediment cores AI-2561 and AI-2562 were collected in the distribution area of seismic
facies type IB
(\figref{3}C).           % (Fig. 3c).
 Acoustic penetration in this area is 25 m. The uppermost sub-
bottom reflector was identified at depths of approximately 2--3~m below sea floor (BSF)
(\figref{4}).           % (Fig. 4).
Cores were retrieved from sites AI-2445 and AI-3150 in the distribution area of
seismic facies type IIIF
(\figref{3}A, \figref{3}B).           % (Fig. 3a, b).
The cores were retrieved from mounded sediment
bodies with relative heights of 15--25~m separated by small channels. Depositional
bodies with evidence of migration toward the channels are related to their northern
gentle flanks, while bodies with evidence of erosion are adjacent to steep southern
flanks. Acoustic penetration in this area varies from 5 to 20~m. The two uppermost
seismic reflectors were identified at water depths of 1 and 2~m BSF, respectively
(\figref{5}).           % (Fig. 5).

\begin{figure}                      % Fig 5
\figurewidth{90mm}
\setimage{}{}{115mm}{}{2019es000689-f05}
\fcaplabel{Fragments of seismic profile AI-37 (with interpretation). Red solid line
indicates seismic reflector corresponding to coarse grained layer recovered in the lower
parts of the sediment cores. Green solid line marks seismic reflector corresponding to
the layer of foraminiferal sand in cores AI-3150 and AI-2445. Seismic evidence of
erosion is presented in the upper right insert. Suggested direction of bottom currents
near the S\~{a}o Tom\'{e} Seamount is shown in the lower right insert. Location of seismic
lines is shown is
\figref{1}B.           % Figure 1b.
Core lithology is presented in
\figref{6}.} \vspace*{2\baselineskip}
\end{figure}



\subsection{3.2. Lithology}

From core AI-2562, intercalations of bioturbated, foraminifera-bearing, clayey silt layers
and nearly pure terrigenous silt and clayey silt layers with sandy silt/silty sand interbeds
of several millimeters thickness were recovered
(\figref{6}, \figref{7}A, \figref{7}I).           % (Fig. 6, 7a, i).
The carbonate content
varies from 1.6 to 33\%.
In the lower part of the core, there is 5-cm-thick layer of well-sorted sand, which is generally composed of quartz and feldspar
(\figref{7}F).           % (Fig. 7f).
Numerous
burrows of benthic fauna are scattered throughout the interval, and upper and lower
contacts of the layer are distinct but disturbed by bioturbation. The coarse-grained layer
overlies a silt-sand-enriched layer with lenses of well-sorted sand. A grain-size analysis
showed reverse grading in the layer with sand lenses and normal grading with unimodal
distribution in the sand layer. The grain size distribution in the layer with sand lenses
demonstrate bimodal distribution with peaks related to sand and clay.


\begin{sidewaysfigure}                      % Fig 6
\figurewidth{150mm}
\setimage{}{}{150mm}{}{2019es000689-f06}
\fcaplabel{\footnotesize Lithology, calcium carbonate content, mean grain size (for bulk sediment and
terrigenous fraction after removal of CaCO$_3$), magnetic susceptibility and age of
sediments recovered from cores. Black solid lines mark correlation between sediment
cores. Orange circles indicate sampling intervals for foraminiferal study. PF -- planktic
foraminifera. Location of cores is shown in
\figref{1}B.}           % Figure 1b.}
\end{sidewaysfigure}


\begin{sidewaysfigure}                      % Fig 7
\figurewidth{140mm}
\setimage{}{}{155mm}{}{2019es000689-f07}
\fcaplabel{Photos of split core fragments with evidence of lateral sedimentation. Black
dashed lines on photos indicate variations in magnetic susceptibility. Black arrows mark
intervals with measured carbonate content. Grain-size distributions are shown right of
the photos. Color of sediments is changed due to oxidation. Location of cores is shown
in  \figref{1}B.}           % Figure 1b.}
\end{sidewaysfigure}


Core AI-2561 is represented by intercalation of yellowish-brown silt and clayey silt with
an admixture of nannofossils, foraminifera and their fragments. Carbonate content is
generally low, ranging from 3 to 30\%.
Thin lenses of sandy silt with a thickness of
approximately 5--8~mm were revealed at core depth intervals of 58--63~cm, 66--73~cm,
and 124--126~cm
(\figref{7}B, \figref{7}D, \figref{7}H).           % (Fig. 7b, d, h).
Sand was found in the core catcher.

Recovered from cores AI-3150 and AI-2445 were bioturbated, yellowish-brown clayey
silt and foraminifera-bearing clayey silt with three 5 to \mbox{7-cm-thick} sand-enriched
intervals
(\figref{7}C, \figref{7}E, \figref{7}G)           % (Fig. 7c, e, g)
and a layer of foraminiferal sand (21-cm-thick in core AI-3150
and 16-cm-thick in core AI-2445
[\itc{Murdmaa et al.,} \reflink{Murd12}{2012}]
 with a significant amount of
quartz
(\figref{7}J).           % (Fig. 7j).
Sand-enriched intervals demonstrate normal grading (slight decrease in
in mode value of the grain size distribution toward the core top) and strong evidence of
bioturbation, even in basal layers. A distinct, normal grading was distinguished for a
terrigenous fraction of the foraminiferal sand layer. A grain-size analysis of bulk
sediment samples showed moderately well-sorting in the upper part of the foraminiferal
sand layer (103--111~cm). As for the terrigenous fraction in the upper half of the layer,
quartz grains are poorly-sorted, whereas in the lower half, sediments are moderately to
well-sorted
(\figref{7}J).           % (Fig. 7j).


Silt and sand-enriched layers revealed in cores are generally darker than the over and
underlying sediment
(\figref{7}A, \figref{7}H, \figref{7}I).           % (Fig. 7a, h, i).
In some cases, these layers demonstrate parallel
lamination (distinct or partly destroyed by bioturbation). Thin layers and lenses of fine
sand were observed at the base of most laminated layers. Coarse-grained layers
correspond to the highest values of magnetic susceptibility
(\figref{7}C--\figref{7}G).           % (Fig. 7c-g).
The lowest values correspond to forami\-niferal sand layers
(\figref{7}J).           % (Fig. 7j).

\subsection{3.3. Foraminiferal Study}

In all four cores, planktic foraminiferal fauna of tropical-to-subtropical character
demonstrate downcore variations in preservation and diversity. The cores comprise
Late--Middle Quaternary foraminiferal assemblages with index species
{\itshape G. ruber}  pink and
{\itshape G. truncatulinoides}. The intervals with better preservation generally contain specimens
of the {\itshape G. menardii } group. However, there are rare and sporadic occurrences of
reworked Neogene species in all cores, including
{\itshape Globorotalia multicamerata},
{\itshape Dentoglobigerina altispira, Pulleniatina primalis, Globorotalia miocenica, G. exilis, G. margaritae}, and others.


Benthic foraminifera occur in low numbers in nearly all core samples. These
foraminifera are represented by several species including
{\itshape Cibicides wuellerstorfi,
Oridorsalis umbonatus, Pullenia bulloides, and Nuttalides umbonifer}. The former three
taxa are known to be associated with NADW, whereas the last taxa is considered to be
typical of AABW
[\itc{Harloff and  Mackensen,} \reflink{Harl97}{1997};
\itc{Mackensen et al.,} \reflink{Mack93}{1993}].
 A clear
relationship between glacial and interglacial stratigraphic intervals and the occurrence of
specific benthic assemblages or species was not found in any of the cores studied.

The upper interval (0--20 cm) of core AI-2561 contains a tropical planktic foraminiferal
assemblage of medium preservation (fragmentation 30--50\%)
with aforementioned
species of {\itshape Globigerinoides ruber} pink,
{\itshape Globorotalia trancatulinoides} and
{\itshape Globorotalia menardii}.
The underlying interval (20--160~cm) is characterized by a strong
fragmentation (up to 90--95\%),
decrease in species diversity, increase in faunal portion
of dissolution resistant species (such as
{\itshape Globorotalia inflata, Globigerinoides
conglobatus} and
{\itshape Pulleniatina finalis}) and an absence of
{\itshape G. menardii} group. The fauna in
the lowest analyzed sample (175~cm) is similar to those in the uppermost interval.

There is a similar foraminiferal fauna, notably in the upper 35 cm, that is generally
more abundant and better preserved in shallow-water core AI-2562 than in core AI-2561. However, foraminiferal dissolution is also significant in core AI-2562. The interval
(35--300~cm) demonstrates a downcore increase in fragmentation. The index species
{\itshape G. ruber} pink occurs only in the upper 75 cm. However, the absence of this species in the
lower part of the core is most likely the result of stronger dissolution. The {\itshape G. menardii}
group occurs in significant numbers in the warm water assemblages in the upper 35~cm
and at 252~cm, is the broken specimens of the same group are found at 145~cm and
from the intervals 35--50~cm and 200--300~cm. In the latter interval, several tests of
{\itshape G. menardii-tumida flexuosa} are found.

In cores AI-2445 and AI-3150, downcore changes in preservation and an
alternation between more and less warm water assemblages are inferred. The turbidite
layers in both cores are especially rich in the foraminiferal tests including the reworked
specimens of Neogene species.

\section{4. Discussion}

\subsection{4.1. Stratigraphy}

The strong dominance of extant foraminiferal species, including {\itshape Globigerinoides ruber}
pink and
{\itshape Globorotalia truncatulinoides} in all examined samples of the four cores
suggests the recovered sediments are of Quaternary age, regardless of the rare
occurrence of reworked Neogene species. In the nearby area of the Rio Grande Rise
and the Ioffe Drift,
{\itshape G. ruber} pink is typical of the last 280~ka and
{\itshape G. truncatulinoides}
occurred during the last 1.8--2~Ma
[\itc{Barash et al.,} \reflink{Bara83}{1983};
\itc{Ivanova et al.,} \reflink{Ivan16}{2016},
and references therein).
The intervals with better preservation generally contain
specimens of the
{\itshape G. menardii} group, which are typical of interglacials in the Atlantic
[\itc{Ericson and  Wollin,} \reflink{Eric68}{1968}].
Hence, the presence/absence of the
{\itshape Globorotalia menardii}
group as well as changes in foraminiferal preservation and CaCO$_3$ content collectively
suggest that cores AI-2561 and AI-2562 most likely covered the last climatic cycle (MIS
5.5-1). In addition, an occurrence of
{\itshape G. menardii-tumida flexuosa} is indicative of the
last interglacial (MIS 5.5) in the Atlantic
[e.g.,
\itc{Barash,} \reflink{Bara88}{1988}].
If the assumption about
the Late Quaternary age of both cores is correct, a mean sedimentation rate of
approximately 1.6 and 2~cm/ka can be estimated for cores AI-2561 and \mbox{AI-2562},
respectively. These estimates are in line with previously obtained values from core
GeoB 2117-1
[\itc{Bleil et al.,} \reflink{Blei93}{1993}],
which is located southward of core AI-2562 (water
depth 4045 m). Unfortunately, the stratigraphic subdivision of cores \mbox{AI-2445} and AI-3150 is hampered by the occurrence of at least two turbidite layers.

Along with foraminiferal data the recovered sections were correlated based on magnetic
susceptibility records and lithological data. A correspondence found between the
coarse-grained sedimentary sequences found in each core suggests that the upper 198~cm of core AI-3150 also covered the last climatic cycle (MIS~5.5-1).


\subsection{4.2. Seismic and Lithological Evidence of Lateral Sedimentation}

The prevalence of seismic facies IIAB corresponding to coarse-grained deposits
confirms a previous suggestion
[\itc{Faug\`{e}res et al.,} \reflink{Faug02}{2002};
\itc{Lima et al.,} \reflink{Lima09}{2009}]
about the domination of gravity driven sedimentation processes north of the S\~{a}o
Tom\'{e} Seamount.   Gravity flows in this area transport coarse-grained material from the
continental slope and Vitoria-Trindade Seamounts to the continental rise.

A diversity of seismic facies south of the S\~{a}o Tom\'{e} Seamount suggests a complicated
interplay between different sedimentation processes
(\figref{3}).           % (Fig. 3).
Seismic facies type IIIB
usually corresponds to contourite features
[e.g.
\itc{Damuth and Hayes,} \reflink{Damu77}{1977}].
However, in
the case of the study area, it is difficult to define the sedimentation processes
responsible for formation of these wave-like depositional features. The study area is
affected by gravity flows moving from the continental slope through the widespread
network of canyons and channels.


As postulated by
\itc{Embley et al.} [\reflink{Embl80}{1980}],
the curvature of hyperbolic echoes depends on
the angle between the ship's heading and the actual bedforms: the hyperbolae are the
steepest when a seismic line runs normal to the sediment wave orientation. Based on
the configuration of the recorded, well-defined sediment waves, these features may be
oriented both normal or at a small angle to seismic lines (and to the general direction of
AABW current). The available data do not allow us to precisely distinguish the sediment
wave orientation.

Domination of seismic facies type IB points to rather low bottom current velocity on a
long-term scale out of prominent topographic obstacles
(\figref{3}).           % (Fig. 3).
Nevertheless, current
velocity can increase due to water flow overcoming obstacles. Internal waves may have
been generated when the flow of bottom water crossed the described channels, which
could also affect the formation of sediment waves. This mechanism was described by
[\itc{Hern\'{a}ndez-Molina et al.,} \reflink{Hern08}{2008}].
Furthermore, the simultaneous action of
bottom currents and gravity flow might also be responsible for sediment wave formation.
Alternation of seismic facies IIIC and IIIB as well as IIIC and IB could be considered
evidence of post-depositional deformation or mass-wasting processes. Seismic facies
IIIF south of the Ma\c{c}ae channel mark the embranchment of smaller channels running in
the ESE direction
(\figref{3}).           % (Fig. 3).
These facies at the foot of the northeastern slope of the S\~{a}o
Tom\'{e} Seamount are related to the erosional-depositional system briefly described by
[\itc{Borisov et al.,} \reflink{Bori13}{2013}].
This system consists of several mounded depositional bodies with
the sizes of approximately 1.9~km$ \:\times \: 2.9$~km, which are separated by small channels
(\figref{5}).           % (Fig. 5).
According to the configuration of erosional and depositional features and the
direction of the Coriolis force, the system was likely formed by the AABW bottom current
along the western slope of the S\~{a}o Tom\'{e} Seamount. As a large topographic obstacle,
the seamount increases the bottom current velocity. Due to the local bottom
topography, several high velocity jets most likely occurred in the flow structure. These
jets were rather stable in time and space and formed an erosional-depositional system
at the base of the seamount.


Coarse-grained layers observed in the sediment cores are undoubtedly of turbiditic
origin, but some of these layers might be affected (reworked) by bottom currents.
Bioturbation and specific sediment structures, such as wavy layers at the base of
laminated intervals and lenses of silt and sand, were considered to be criteria for
distinguishing between contourites and turbidites
[\itc{Stow and  Faug\`{e}res,} \reflink{Stow08}{2008};
\itc{Wetzel et al.,} \reflink{Wetz08}{2008}].
Evidence of bioturbation can also occur in turbidites, but in some
intervals of the studied cores, even basal sand-enriched layers were affected by
bioturbation
(\figref{7}F, \figref{7}E, \figref{7}G).           % (Fig. 7f, e, g).
The bi-gradational sedimentary sequence recovered in the
lower part of core AI-2562 contains strong evidence of bioturbation
(\figref{7}F)           % (Fig. 7f)
and can be
generally described by a standard contourite facies model
[\itc{H\"{u}neke and  Stow,} \reflink{Hune08}{2008};
\itc{Stow and  Faug\`{e}res,} \reflink{Stow08}{2008}].
The bi-gradational structure of the layer most likely resulted from
long-term fluctuations in bottom current velocity. The foraminiferal sand layer in cores
AI-2445 and AI-3150 corresponds to gravity flow (grain flow) deposits
(\figref{7}J).           % (Fig. 7j).
A considerable amount of quartz indicates that this material was derived from the
continental slope. The largest amount of coarse-grained layers in core AI-2562 indicates
that during the Late Quaternary, the gravity flow activity in the Ma\c{c}ae channel was
higher than in the channels located northward. Due to the overspill of gravity flows
resulting from a change in orientation of the Ma\c{c}ae channel, the transported sediment
material was deposited on the southern flank of the channel (in the area of core AI-2562). The same mechanism most likely took place northward of the S\~{a}o Tom\'{e}
Seamount in the area of cores AI-3150 and AI-2445.


A correlation between seismic lines and sediment layers showed that a prolonged
reflector at 2--3~m BSF likely corresponds to the coarse-grained sedimentary sequence
recovered in the lower parts of the cores
(\figref{4}, \figref{5}).           % (Fig. 4, 5).
The foraminiferal sand layer
corresponds to a reflector at a depth of approximately 1~m BSF. Thus, a previously
described correlation between the seismic reflector and foraminiferal sand layer
[\itc{Borisov et al.,} \reflink{Bori13}{2013};
\itc{Murdmaa et al.,} \reflink{Murd12}{2012}]
is now confirmed for the depositional bodies
at the foot of the northeastern slope of the S\~{a}o Tom\'{e} Seamount.

Most likely, hemipelagic settling through the water column did not play a significant role
in sedimentation in the study area during the Late Quaternary. The sediments over and
underlying the coarse-grained intervals resulting from turbidity flows and bottom current
activity are generally composed of fine and medium silt (clay amount does not exceed
35\%).
The southwestern part of the Brazil Basin is characterized by a lack of large
rivers, which would be able to produce large plumes and provide enormous amounts of
silt material to the study area, as the nearest large river is more than 400~km from the
shore. Additionally, this area is very distant from the La Plata River ($\sim 2500$~km) and
Amazon River ($\sim 3000$~km), which are the main sources of fluvial material in the region.
Thus, silt and sand material were transported to the continental rise by gravity flows. Silt
enrichment in the major part of the recovered sediments together with an absence of
turbidite evidence might be explained by bottom current activity, which washed out the
clay material and reworked the gravity flow deposits.

According to the mentioned criteria for contourite identification, the coarse-grained
sedimentary sequences corresponding to higher values of magnetic susceptibility and
correlated with each other were interpreted as turbidites reworked by bottom currents.
Notably, the sedimentary sequence shown in
\figref{7}E           % Figure 7e
may represent a turbiditic
sequence (only proposed bioturbation evidence in the basal layer suggests its
contourite origin). The upper two sequences are related to low carbonate intervals
corresponding to \mbox{MIS2--MIS4}. Sedimentary sequences in the lower parts of the cores
correspond to \mbox{MIS 5} (complete sequences were found only in cores \mbox{AI-2562} and \mbox{AI-3150}, whereas in core \mbox{AI-2561}, sand material was found in the core catcher). The
formation of such coarse-grained layers should be linked to increased gravity flow
activity and an intensification of bottom currents, which reworked the turbidite deposits.
Variations in sea-level in response to glacial-interglacial cycles controlled the sediment
supply in this area
[e.g.,
\itc{Lima et al.,} \reflink{Lima09}{2009}].
A relationship between
glacial-interglacial cycles and changes in the AABW velocity during the Quaternary is
still a subject of discussion among researchers. Some authors suggest intensification of
the AABW current during the transition from interglacial to glacial periods
[\itc{Ledbetter,} \reflink{Ledb86}{1986};
\itc{Mass\'{e} et al.,} \reflink{Mass94}{1994}].
Alternatively, some researchers
[e.g.,
\itc{Ledbetter,} \reflink{Ledb84}{1984};
\itc{Pudsey and  Howe,} \reflink{Puds98}{1998}]
argue that the AABW current in the Atlantic reached its
maximum velocity during the interglacial periods. Recent publications suggest an
increase in the AABW intensity during the Last Glacial Maximum compared to the
Holocene
[\itc{Spooner et al.,} \reflink{Spoo18}{2018}]
The data presented in this paper do not
distinctly solve this issue.


Downcore planktic foraminiferal preservation varies significantly in cores AI-2561 and
\mbox{AI-2562}, which mainly reflects the well-known glacial-interglacial variations in lysocline
depth associated with variations in AABW production. Low sedimentation rates in the
study area might be explained by both low sediment supply typical of an arid climatic
zone and by short-term hiatuses in the sediment record resulting from bottom current
activity.


\section{Conclusions}

The S\~{a}o Tom\'{e} Seamount marks a boundary between two zones characterized by
different sedimentation regi\-mes. North of the seamount sedimentation is controlled by
gravity flows, which transport material from the continental slope and Vitoria-Trindade
Seamounts downslope to the Columbia channel. The S\~{a}o Tom\'{e} Seamount also plays
an important role in the formation of erosional and depositional features at the base of
the northwestern slope of the seamount. As a large topographic obstacle, the seamount
causes an increase in the AABW current velocity and thus changes the AABW flow
structure. The area south of the S\~{a}o Tom\'{e} Seamount is characterized by the resulting
strong interaction between the gravity flows and AABW bottom current. This interplay
resulted in the formation of bottom current reworked turbidites, which were recovered in
the sediment cores. Seismic and lithological data indicate a domination of lateral
sedimentation over hemipelagic settling in the study area during the last glacial-intergla\-cial cycle at minimum. During this period, gravity flows were the most active in
the Ma\c{c}ae channel. Intensification of gravity driven processes in the study area is most
likely linked to sea-level low-stands during glacial periods. Variations in the AABW
current velocity in response to glacial-interglacial climate changes during the Late
Quaternary remain a subject for discussion.


\acknowledgements{The authors are grateful to scientific party, master and crew of RV{\itshape Akademik Ioffe}
cruises 33,  35 and 37 for their professional assistance with seismic profiling and coring.
The study was partially supported by the Russian Science Foundation grants 14-50-00095 (magnetic susceptibility measurements) and 18-17-00227 (seismic, sedimentological and micropaleontological studies and interpretations). The advanced
English language editing was provided by the Wiley Editing Services.}


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\end{document}
