Introduction
The south-western coast of Africa is currently characterised by cold
and nutrient-rich waters related to the occurrence of the Benguela
Upwelling System (BUS), favouring high primary production. It is one
of the major upwelling areas in the world. This system is supposed to
have initiated in the early Late Miocene (∼10–15 Ma)
(Siesser, 1980; Diester-Haass et al., 1990; Heinrich et al., 2011;
Rommerskirchen et al., 2011) during a phase of global cooling. This
cooling is part of the transition from a warm and humid variable
climate to cooler and drier conditions of the late Neogene driven by
changes of the Antarctic ice sheets (Wright et al., 1992; Zachos
et al., 2001; Billups and Schrag, 2002). Later, the BUS probably
played a decisive role in the climate development of the latest
Neogene (Etourneau et al., 2012).
Following the mid Miocene climatic optimum, the re-establishment of
major ice sheets in East Antarctica at around 10 Ma led to
a globally steeper meridional temperature gradient which would have
intensified pressure systems and ocean circulations (Flower and
Kennett, 1994; Zachos et al., 2001); the South Atlantic Anticyclone
(SAA) became stronger and with it the SE trade winds which led to
increased upwelling and marine productivity especially at around
6.5 Ma (Diester-Haass et al., 2002). These authors
hypothesized that the further growth of the Antarctic ice sheets led
to a northward migration of the Antarctic Polar Front causing also
a shift of the BUS and changing the composition of the water
(Diester-Haass, 1988; Diester-Haass et al., 1990, 1992). However,
recent modelling showed that the introduction of an ice-cap in
Antarctica resulted in strong downwelling in the Weddell Sea which
attracted warmer water from lower latitudes effectively reducing the
meridional sea-surface temperature (SST) gradient in the Atlantic
sector of the Southern Ocean. This resulted for the BUS region in
slightly reduced Ekman pumping, less upwelling, and warmer subsurface
temperatures (Goldner et al., 2013; Knorr and Lohmann, 2014; Jung
et al., 2014). On the other hand, the ongoing uplift of Southern
Africa during the late Miocene and Pliocene (Partridge, 1997; Roberts
and White, 2010) would have strengthened the low-level winds along the
southwest African coast (Benguela Jet) enhancing upwelling in the BUS,
especially in the central part. A comparison of different model
experiments estimates the effects of uplift of Southern Africa on the
surface winds driving the BUS far greater than the effects of far
field tectonic changes such as the closure of the Central American
Seaway (CAS) or the narrowing of the Indonesian Seaway. However,
a subsurface cooling increasing with depth was found in the case of
CAS closure, which indicates the possibility of changing ocean surface
conditions in the Benguela region even if upwelling remained constant
(Jung et al., 2014).
More important to the conditions in the BUS would be the effect of the
CAS closure on the deep water formation in the Atlantic. According to
Billups (2002), the closure of the CAS had already influenced the
ocean currents between 6.6 and 6 Ma by increasing the Atlantic
overturning circulation (AMOC). The data sets show a cooling of
Southern Ocean upper circumpolar and intermediate waters during the
latest Miocene. Poore et al. (2006) calculated the proportion of North
Atlantic Deepwater (Northern Component Water) and describe an increase
of Northern Component Water formation as well as a stronger AMOC for
the period between 6 and 2 Ma. The influence of the closing of
the CAS on the AMOC is furthermore underlined by models such as the
one used by Butzin et al. (2011) who concluded that the formation of
North Atlantic Deepwater (NADW) began when the CAS had shoaled to
a few hundred meters during the Late Miocene.
A number of paleostudies focused on the upwelling history of either in
the southern and central parts of the BUS or on the ocean side of the
Walvis Ridge (Siesser, 1980; Diester-Haass et al., 1992; Marlow
et al., 2000; Berger et al., 2002b; Udeze and Obohikuenobe, 2005;
Heinrich et al., 2011; Rommerskirchen et al., 2011; Rosell-Melé
et al., 2014), whereby only ODP Site 1085 in the southern BUS covers
its history since the late Miocene (past 14 Ma). ODP Site 1084
in the central BUS area only covers the Pliocene down to ∼5 Ma. ODP Site 1081, which sediments cover the past
9 Ma, sits at the northern end of the modern BUS. Therefore,
the site is sensitive to changes at the northern boundary and to
movements of the Angola–Benguela Front (ABF). Furthermore, the site
is located on the Walvis Ridge (Wefer et al., 1998) which divides the
Cape Basin and the Benguela Current from the Angola Basin and the warm
waters of the Angola Current. We use a multi-proxy approach to
constrain changes in sea-surface conditions between the late Miocene
and Pliocene (9 to 2.7 Ma), with assemblages of organic-walled
dinoflagellate cysts as primary proxies, and more particularly to
describe changes on the northern edge of the BUS during times of
global cooling as well as discuss forcing mechanisms. We complement
our results on dinoflagellate cysts with total organic carbon
measurements (new and shipboard data) and compare them with
alkenone-derived SSTs (Hoetzel et al., 2013). Studies on recent
distribution of dinoflagellate cysts along the West African coast
(e.g. Dale et al., 2002; Marret and Zonneveld, 2003; Zonneveld et al.,
2013) enable us to associate the oceanographic context (upwelling
vs. oligotrophic conditions) with specific cyst assemblages.
The modern Benguela Upwelling System
The semi-permanent high pressure system, the SAA, over the South East
Atlantic is the dominating atmospheric feature located at around
32∘ S 5∘ W (austral summer) and 27∘ S
10∘ W (austral winter) (Petterson and Stramma, 1991)
(Fig. 1). The SAA is driving the Benguela Current (BC) which flows
northwards along the south-western coast of Africa. The BC contains
water from the eastward flowing cold Antarctic Circumpolar Current
(ACC), which is linked to the polar front. It also takes up water from
the warm Agulhas Current (AgC), which is flowing from the Indian Ocean
around South Africa. At around 28∘ S the BC divides into two
separate currents; one following the rotation of the SAA turning west
crossing the Atlantic and the other flowing further northwards along
the south-western African coast under the name of Benguela Coastal
Current (BCC). Additionally, the BCC is linked to the south-easterly
trade winds (Petterson and Stramma, 1991).
The coastal-parallel winds push the coastal surface waters via
Ekman-transport off shore inducing the upwelling of cold and
nutrient-rich subsurface waters (e.g. Lutjeharms and Meeuwis,
1987). The upwelled nutrients allow a vast production of phytoplankton
in the photic zone. Eight upwelling cells have been identified, of
which the one west of Lüderitz Bay is the strongest persisting all
year long resulting in the coldest surface waters of the BUS
(Lutjeharms and Meeuwis, 1987). Up to 600 km off shore cold
upwelled waters from the coastal upwelling areas mix with surface
waters and form nutrient-rich filaments (Lutjeharms and Meeuwis, 1987;
Lutjeharms and Stockton, 1987; Summerhayes et al., 1995). Water of
these filaments has clearly enhanced nutrient values, lower
temperatures and increased primary production.
The BCC meets the southward-flowing warm and nutrient-poor Angola
Current just north of the Walvis Ridge. Together they form the ABF
which today is situated between 14 and 16∘ S (Meeuwis and
Lutjeharms, 1990), dependent on the season. In detail, it is not
a single front but a couple of fronts arranged in two frontal zones,
a northern and a southern one, whereas the latter one has the larger
influence on the overall ABF's characteristics (Kostianoy and
Lutjeharms, 1999). Like the coastal upwelling, the position of the
front depends on the SAA; when the meridional pressure gradient is
high, the southern front of the ABF is located further north and the
ABF is narrower and sharper (Kostianoy and Lutjeharms, 1999). Under
weakened SAA conditions and resulting weakened winds, the ABF can be
located further to the south (Richter et al., 2010) so that warm
nutrient-poor water of the Angola current can penetrate further
southward as far as 24∘ S along the Namibian coast (Meeuwis
and Lutjeharms, 1990). As a result, the precipitation on the adjacent
coast is enhanced (Hirst and Hastenrath, 1983; Nicholson and
Entekhabi, 1987) and can be increased farther inland (Rouault,
2003). The phenomenon occurs on inter-annual timescales and is called
Benguela Niño for its similarity to the El Niño Southern
Oscillation (Shannon et al., 1986).
The material and methods
The sampled sediment core was retrieved at the Ocean Drilling Program
Site 1081 (19∘37′ S
11∘19′ E) in 794 m water depth. The site is
located on the Walvis Ridge 160 km off the Namibian coast and
is today influenced by filaments of the BUS. The sediment is composed
of olive-gray clayey nannofossil ooze and olive-gray to black clays
(Wefer et al., 1998). Sedimentation rates were calculated between 2
and 5 cmka-1 using an age model based on biostratigraphy,
magnetic reversals and magnetic susceptibility (Berger et al., 2002b).
Seventy-one samples were taken with ages between 9 and 2.7 Ma
and prepared for palynological investigation. The volume of each
sample was estimated by water displacement. For decalcification
diluted cold HCl (∼5%) was used and afterwards the material
was set in cold HF (∼20%) for two days to remove
silicates. Prior to the HCl-treatment Lycopodium tablets were
added. The residuals were sieved under ultrasonic treatment removing
particles smaller than 10–15 µm. The cleaned residues
were stored in water and mounted in glycerol for investigation under
a light microscope using magnifications of 400×,
600× and 1000×. For each sample at least 300
dinoflagellate cysts were identified and counted.
Identification of dinoflagellate cysts was done after De Verteuil and
Norris (1992), Marret and Zonneveld (2003), De Schepper and Head
(2009) and Schreck et al. (2012). Brigantedinium
spp. includes all round brown smooth dinoflagellate cysts. Because
Batiacasphaera micropapillata has a strong morphological
overlap with B. minuta and both occur in the samples we
adopted the nomenclature of De Schepper and Head (2008) and Schreck
et al. (2012) in treating both species as one complex.
Each species was ecologically sorted after its assumed metabolism
mechanism (autotroph and heterotroph, Table 1) and
a heterotroph/autotroph ratio (H/A) was calculated
expressed as ln(H/A). Percentages were calculated on
the total number of dinoflagellate cyst counted. For calculation of
the 95 % confidence intervals of the relative abundances the
equation of Maher (1971) was used. Accumulation rates of
dinoflagellate cysts were calculated by multiplying the sedimentation
rates (Berger et al., 2002b) with the cyst concentrations per ml,
which was calculated based on the known number of Lycopodium
spores added in the form of tablets.
For 21 samples the total organic carbon (TOC) contents has been
determined by decarbonating a sediment aliquot of ∼25 mg
using 6 N hydrochloric acid before combustion at 1050 ∘C in
a Heraeus CHN-O-rapid elemental analyzer (see Rommerskirchen et al.,
2011). This data set is complemented by TOC shipboard data published
in Wefer et al. (1998) and shown in Fig. 4.
Results
A total of 36 dinoflagellate cyst taxa were identified whereof the
Brigantedinium spp. group is the dominant one. In Figs. 2
and 3 the percentages and accumulation rates of the most abundant
species are shown. In general, both relative and absolute trends show
similarity for each species. The record has been visually divided into
5 zones.
Zone I runs from the start of the record at 9 to 7.8 Ma. This
zone is mainly characterized by cysts of the Batiacasphaera micropapillata complex with values mostly around 20 % reaching
a maximum of 50 %. Impagidinium paradoxum is well
represented (generally over 10 %) especially in the beginning with
a peak over 20 %. Lingulodinium machaerophorum is present
in all samples of Zone I, with two exceptions, reaches values up to
20 % at around 8.5 Ma, and decreases afterwards to a minor
representation in the assemblages (Fig. 2). Cysts of
Nematosphaeropsis labyrinthus reach up to maxima of
15 %. Impagidinium sp. 2 (De Schepper and Head, 2009)
cysts are also present and make up around 5 % of the assemblage
and have a maximum of 20 % at around
8.3 Ma. Selenopemphix quanta is only
marginally represented in Zone I. Brigantedinium spp. have
generally low values (around 5 %) that rise at the end of Zone I
to around 15 %. Almost completely lacking in this interval are
Operculodinium centrocarpum cysts.
Zone II (7.8–6.2 Ma) is marked by a decline of cysts
abundance of the B. micropapillata complex to less than
10 % (and less than 5 % around 7 Ma). Additionally the
absolute values are decreasing from around
3000 cystscm-2ka-1 to less than 1000. Values for
Brigantedinium spp. and S. quanta cysts (Fig. 3) are
increased in this zone but show decreasing trends towards the end of
the Zone II at 6.5 Ma. Also H. tectata decreases from
around 20 % to marginal occurrence at 6.5 Ma. However, an
increasing trend shows the representation of B. hirsuta from
low values of less than 10 % to more than 30 %. As in Zone I,
L. machaerophorum is mostly represented with less than
10 % but peaks at ∼7.5 Ma with 63 % of the
assemblage. Again O. centrocarpum is lacking.
Zone III (6.2–5.5 Ma) is characterized by H. tectata
cysts dominating the samples until 5.8 Ma and reaching values
between 40 and 50 %. Additionally, the S. quanta curve
increases again until 5 Ma. Spiniferites spp. and
B. hirsuta values are also increasing between 6 and
5 Ma.
Zone IV (5.5–4.4 Ma) is characterized by B. hirsuta
and Brigantedinium spp. Brigantedinium has low
relative abundances until 5.2 but reaches 40 % later in
Zone IV. Not represented is L. machaerophorum. The
B. micropapillata complex is better represented in the period
from 5.2 until 4.2 Ma with maxima around 25 % (Fig. 2). At
around 4.8 Ma Impagidinium sp. 2 is well represented
and peaks twice with values between 20 and 30 %.
Zone V (4.4–2.7 Ma) is defined by the representation of
O. centrocarpum which is rising from a few percent at the
start of the interval to 60 % at around 3.8 Ma. Towards
the end Brigantedinium spp. and Spiniferites
spp. cysts are increasing at the costs of O. centrocarpum.
In Fig. 4 the accumulation rates of summed dinoflagellate cysts are
plotted against age. The accumulation rates of cysts range between
7000 and 93 000 cystscm-2ka-1. One period with
lower accumulation rates occurred until 8 Ma and two periods
with generally higher accumulation rates were found between 8 and
6.4 Ma and from 5.4 to 4.2 Ma.
Discussion
Dinoflagellates as proxies for environmental conditions
Off the 2377 modern motile species listed worldwide (Gómez, 2012)
only between 10 and 20 % of them produce cysts during their
lifecycle that preserve well in the sediments (Head, 1996). The
organic-walled cysts found in recent sediments around the world show
a distribution that can be related to sea-surface gradients in
temperature, salinity, nutrients and sea-ice cover (e.g. De Vernal
et al., 1997; Dale et al., 2002; Marret and Zonneveld, 2003; Holzwarth
et al., 2007; Zonneveld et al., 2001, 2013). Therefore, it has been
possible to use them as a tool to reconstruct surface waters
conditions, e.g. nutrients and temperature, enabling past upwelling
intensity variations. The dissolution of cysts sensitive to oxidative
degradation (Zonneveld et al., 2001) such as Brigantedinium
cannot entirely be dismissed but the relative high abundance of
sensitive cysts throughout the record, as well as the high total
concentration values (Fig. 4) suggest a minor effect. Therefore, we
are confident to use the heterotrophic vs. autotrophic ratio
(H/A ratio). The H/A ratio indicates upwelling (Lewis
et al., 1990) based on the fact that the autotrophic dinoflagellates
are in strong competition to diatoms whereas the heterotrophic ones
feed on diatoms. Hence, high H/A ratio values reflect
higher production. In our record the ratio is in good concordance to
the total dinoflagellate cyst accumulation. Our results indicate five
successive regimes (discussed below); a weak upwelling regime with
strong influence of the warm waters of the Angola Current (Zone I,
9–7.8 Ma), an increase of upwelling linked to the
intensification of the meridional temperature gradient in combination
with Mio- to Pliocene uplift of south-western Africa (Zone II,
7.8–6.2 Ma), a period with exceptional conditions of
increasing TOC and warmer SST (Zone III, 6.2–5.5 Ma) during
the early stages of the Messinian Salinity Crisis
(5.96–5.33 Ma), a resumption of productivity and upwelling
after the Messinian (Zone IV, 5.5–4.4 Ma) and further
intensification of the upwelling together with the effect of the
Cunene River discharge (Zone V, 4.4–2.7 Ma).
The Angola–Benguela Front
ODP Site 1081 is located close to the modern position of the ABF,
which offers opportunities to record changes in its position since the
Miocene. The composition of the dinoflagellate cyst assemblages shows
affinity to upwelling conditions over the entire record through
nutrient indicating cyst species such as Brigantedinium spp.,
S. quanta, L. machaerophorum which is in accordance
to an onset of the BUS before 10 Ma (e.g., Siesser, 1980;
Diester-Haass et al., 1990; Heinrich et al., 2011; Rommerskirchen
et al., 2011). However, fluctuations in accumulation rates and
relative abundances, such as the presence or absence of
L. machaerophorum and O. centrocarpum, indicate
significant variability in the upwelling during the studied
period. Open oceanic species, Impagidinium paradoxum
(Dale et al., 2002; Zonneveld et al., 2013) and probably
Impagidinium sp. 2 are stronger represented in Zones I and II
(9–6.2 Ma) indicating warmer and less nutrient-rich
environments. Before 7.8 Ma the B. micropapillata
complex is well represented. Although the ecological background of the
B. micropapillata complex is uncertain, studies from Zegarra
and Helenes (2011) suggest this complex to be an indicator for warm
nutrient-poor waters because it is often recorded together with other
warm-water indicators. Hence, we consider B. micropapillata
as an indicator of relative warm nutrient-poor conditions. The
dinoflagellate cyst record corroborates the SST reconstruction of
around 26 to 27 ∘C for Zone I (Hoetzel et al., 2013). During
the early part of the record upwelling conditions were rather weak,
which is also indicated by low TOC values (Fig. 4) (Wefer et al.,
1998) and the influence of warm waters from the AC was still strong
compared to later periods.
Southward penetration of the AC is also suggested by the presence of
L. machaerophorum which today is not present in the BUS but
in the area of the AC (Holzwarth et al.,
2007). L. machaerophorum is an indicator for stratified warm
nutrient-rich waters, e.g. after upwelling relaxation (Marret and
Zonneveld, 2003; Zonneveld et al., 2013, and references therein)
suggesting that periods with stratified water conditions were more
frequent and/or longer than today. I. paradoxum and
N. labyrinthus which are more frequent in waters around the
ABF (Dale et al., 2002; Zonneveld et al., 2013) also may indicate
a southern position of the ABF at least 3∘ of latitude further
south than today. At present, the ABF shifts southwards on
inter-annual timescales during Benguela Niños (Shannon et al.,
1986). Under these special conditions, the waters over the Walvis
Ridge receive a contribution of the Angola Current (Petterson and
Stramma, 1991; Florenchie et al., 2004; Mohrholz et al., 2004; Richter
et al., 2010). It is possible that these Benguela Niño events were
more common during Miocene times when the SAA and the trade winds were
weaker.
Late Miocene upwelling intensification
Between 7.8 and 6.2 Ma (Zone II) dinoflagellate cyst
accumulation rates are generally higher indicating higher productivity
as the result of stronger upwelling (Fig. 4). Further support of the
intensified upwelling is given by increased heterotrophic
dinoflagellate cyst occurrences, such as Selenopemphix and
Brigantedinium (Zonneveld et al., 2001), which is also shown
in the increased H/A ratio (Fig. 4). Slightly higher
TOC values as compared to the earlier period (Zone I; Fig. 4)
underline increased upwelling conditions. In general, a gradual
intensification of the BUS is suggested by other authors,
e.g. Rommerskirchen et al. (2011) who showed for ODP Site 1085
a strong sea-surface cooling until 6 Ma. Early during the
interval (prior to 7 Ma) periods of weaker upwelling and
stronger influence of warmer waters may have occurred over the Walvis
Ridge allowing the presence and even the dominance at 7.5 Ma
of L. machaerophorum. However, while it is likely that the
ABF was weaker and located further to the south and that the influence
of the AC was stronger due to more frequently occurring Benguela
Niño conditions, its influence seems to disappear towards the end
of Zone II, after 7 Ma. This is indicated by the low
occurrences of L. machaerophorum and the increase in
relative cyst abundance of oceanic species such as
I. paradoxum and the B. micropapillata complex. Our
data record a northward shift and intensification of the ABF implying
a restriction of the AC to latitudes north of 19∘ S as well
as open ocean conditions over the site. This interpretation is
corroborated by the SST record (Hoetzel et al., 2013) showing
a cooling trend between 7.8 and 6.2 Ma and by the TOC record
showing reduced values towards the end of the period between 6.7 and
6.5 Ma (Fig. 4).
Exceptional conditions during the Mediterranean Salinity Crisis
Zone III (6.2–5.5 Ma) coincides mostly with the Messinian
Salinity Crisis (5.96–5.33 Ma; Krijgsman et al.,
1999). Mediterranean deepwater formation was weakened since
7.2 Ma (Kouwenhoven and Van der Zwaan, 2006) although
a connection between the Atlantic Ocean and the Mediterranean Sea
remained until the later halite phase between 5.61 and 5.55 Ma
(Topper and Meijer, 2013). The Mediterranean begun to desiccate
shortly before 6.2 Ma, creating salty and dense waters flowing
into the Atlantic and contributing to NADW formation until
6.0 Ma (Pérez-Asensio et al., 2012). Further restriction
of the Mediterranean Outflow Water between 6.0 and 5.5 Ma
weakened the AMOC, although the effects of the Mediterranean Outflow
Water on the AMOC strongly depend on the salinity of the Outflow Water
(Ivanovic et al., 2014). An earlier salinity crisis in the
Mediterranean occurred during the Tortonian between 7.8 and
7.6 Ma (Kouwenhoven et al., 2003).
Between 6.2 and 5.8 Ma the record shows dominance of
H. tectata which is known to be a cool-water tolerant species
mostly of the Pliocene and Pleistocene of the North Atlantic region
(Head, 1994; Louwye et al., 2004; Versteegh, 1997) but also of the
Miocene (Head et al., 1989; Jimenez-Moreno et al., 2006). This maximum
of H. tectata is flanked by two maxima in the occurrence of
B. hirsuta (6.8–6.4 and 5.6–5.2 Ma) and an earlier
max of H. tectata occurred between 7.8 and
7.6 Ma. Both, H. tectata and B. hirsuta have
not yet been described from the BUS area but they alternatively
dominate the assemblages from 6.8 to 5.2 Ma, representing
unique conditions which did not exist before or after that period.
De Schepper et al. (2011) showed that H. tectata exceeded
30 % of the dinoflagellate cyst assemblages when the reconstructed
Mg/Ca sea-surface temperatures were between 10 and
15 ∘C. Although H. tectata has not yet been
described from the South Atlantic, it probably represents strong
surface water cooling. However, SST estimates of ODP Site 1081 only
show a moderate cooling (Hoetzel et al., 2013). According to Vidal
et al. (2002), the sedimentation rates at ODP Site 1085 increased
between 6.1 and 5.8 Ma which they correlate to the occurrences
of strong glacial events. S. quanta is also increased during
that period, indicating a higher nutrient supply suggesting increased
upwelling. On the other hand, other heterotrophic taxa
(e.g. Brigantedinium spp.) have low values and consequently
the H/A ratio is lower than before although some
upwelling is suggested by relative high TOC (Fig. 4).
For the period from 6.5–5.0 Ma, Rommerskirchen et al. (2011)
described TEX86 temperature estimates from ODP Site 1085 indicating
warming of subsurface waters around 6.3 Ma and between 5.6 and
5.2 Ma. They concluded that the subsurface water warming
resulted from a downward mixing of heat caused by a weakening of the
AMOC and reduced NADW formation related to reduced outflow of salty
Mediterranean water. The period of warm subsurface waters at Site 1085
coincides with the increased representations of B. hirsuta
suggesting that the downward mixing affected the quality of upwelled
waters at the ODP Site 1081 creating special and non-recurring
conditions.
Berger et al. (2002a) argued that the quality of the coastal upwelling
waters at the BUS is linked to the deep water circulation and the
strength of the NADW. Increasing NADW formation brings, until
a critical point, more silica from the Northern Ocean into the
Southern Ocean and eventually changes the chemistry of the upwelled
waters at the BUS (enabling the Matuyama Diatom Maximum). Changes of
the silica content might have affected dinoflagellates indirectly via
ecological competition with algae or other microorganisms or via
changes in food supply and sources. The latter one is important for
the heterotrophic species since they feed on diatoms which are highly
depending on silica contents.
A further change of the quality of the upwelled waters could be caused
by a poleward undercurrent flowing south from the Angola dome along
the African margin transporting silica-rich, phosphate-rich and oxygen
depleted waters (Berger et al., 1998) and increasing the fertile
thermocline. A strengthening of this undercurrent and a higher silica
content of the waters representing a mixing of the intermediate water
and this poleward undercurrent is indicated by a radiolarian peak from
5.8 until 5.25 Ma at ODP Site 1085 (Diester-Haass et al.,
2002) coinciding with the younger B. hirsuta maximum. The
peak in radiolarian might explain lower H/A ratios
since radiolarian might have competed with heterotrophic
dinoflagellates.
Resumption of upwelling
Around 5.6 Ma at ODP Site 1081 (Zone IV) and between 5.6 and
5.3 Ma at Site 1085 (Rommerskirchen et al., 2011), maxima in
SST estimates are recorded after which SSTs fluctuated around
a cooling trend during the rest of the Pliocene. Resumption of
upwelling in the BUS area is also indicated by a sharp increase of
Brigantedinium spp. around 5.3 Ma corresponding to
increased sedimentation rates at ODP Site 1085 (Diester-Haass et al.,
2002). Additionally, high H/A ratios and increasing
TOC indicate increased primary production at Site 1081 (Fig. 4). After
5.33 Ma, the Gibraltar strait opened and again very salty and
dense Mediterranean Outflow Water could re-intensify NADW formation
and AMOC (Ivanovic et al., 2014). More important for the
intensification of the BUS Upwelling however, might have been the
uplift of south-western Africa during the Pliocene (Partridge, 1997;
Roberts and White, 2010). Uplift would have strengthened the coastal
low-level winds and increased Ekman pumping causing enhanced upwelling
(Jung et al., 2014).
Upwelling intensification and river supply
The presence and dominance of O. centrocarpum since
4.4 Ma (Zone V) indicate nutrient-rich and well mixed waters
representing conditions adjacent to strong upwelling and/or river
outflow (Dale et al., 2002). Although O. centrocarpum is
a cosmopolitan species, it occurs in the South East Atlantic in
vicinity to the upwelling area (Dale et al., 2002; Marret and
Zonneveld, 2003; Holzwarth et al., 2007; Zonneveld et al., 2013). It
is, additionally, represented in turbulent and well mixed waters at
the boundary of coastal and oceanic waters (Wall et al., 1977; Dale
et al., 2002). L. machaerophorum is, however, completely
absent, indicating that the partly warm stratified conditions of the
Miocene have been completely replaced by stronger upwelling, better
mixing, and cooler conditions. The increase of Brigantedinium
spp. abundance and the high TOC concentration (between 4 and
5 WT%; Fig. 4) and further decreasing SSTs (Hoetzel et al.,
2013) underline nutrient rich conditions of a strong upwelling.
Between 4.4 and 3.0 Ma, the H/A ratios show
minima during dinoflagellate cyst flux maxima indicating again
a change in the quality of upwelled waters corresponding to
a northward shift of diatom assemblages to the central BUS area
(Marlow et al., 2000). These changes can be linked to a stronger
influence of Antarctic Intermediate Water.
The increase of Spiniferites at the end of the record might
be linked to more coastal conditions and river input (Dale et al.,
2002). Increased river discharge could also explain relative high
occurrences of O. centrocarpum. The desiccation and outflow
of the Cunene lake through the Cunene river is supposed to have
started in the early Pliocene (Hipondoka, 2005).
Links to oceanic and global climate change
The intensification of the BUS and the steepening of the ABF imply
strengthening of SE trade winds and the regional wind maximum
(Benguela Jet) along the coast (Nicholson, 2010). It should be kept in
mind that the period under discussion concerns the early stages of
upwelling of the northern BUS and that upwelling further intensified
during the Pleistocene (Berger et al., 2002a, b); in this region SSTs
dropped by 10 ∘C during the past 2.5 Ma
(Rosell-Melé et al., 2014).
Prior to 8 Ma, sub-Antarctic stable oxygen isotope values
indicate slightly warmer temperatures and less intense glaciations in
Antarctica (Billups, 2002). The polar front was probably weaker and
situated more southwards and so was the SAA. Additionally, glaciations
in the Northern Hemisphere were not yet extensive so that the
meteorological equator might have been located further north than
today creating a weak meridional temperature and pressure gradient in
the Southern Hemisphere (Flohn, 1981). A weak meridional temperature
gradient would shift the ABF southwards and allow the AC to penetrate
southwards over the Walvis Ridge creating Benguela Niño like
conditions (sensu Shannon et al., 1986). Conversely, increase of the
meridional gradient (Billups, 2002) would have resulted in a northward
shift of the ABF, northward migration of the BUS (Diester-Haass
et al., 1992), stronger currents and vigorous trade
winds. Additionally, Mio- to Pliocene uplift of Southern Africa
intensified the near-coastal low-level Benguela Jet and consequently
increased upwelling of the BUS (Jung et al., 2014).
The strengthened winds in turn, especially the stronger Benguela Jet,
led to aridification of the south-western coast of Africa since warm
and humid air from the Atlantic would be hindered to reach large areas
of Namibia. Moreover, a stronger Benguela Jet led to intensified
upwelling and lower SSTs, which would also act to reduce
rainfall. Dupont et al. (2013) discuss, based on pollen data and
deuterium values of plant waxes from ODP Site 1085, that the
aridification is caused by a decrease of precipitation derived from
the Atlantic and that the main source area of precipitation changed to
the Indian Ocean, as is the case today (Gimeno et al., 2010). Mio- to
Pliocene mountain uplift would have helped to both strengthen the
Benguela Jet as well as to block South Atlantic air from reaching far
into the continent (Jung et al., 2014). The expansion of the grass
savanna between 8 and 3 Ma and subsequent expansion of the
desert, is linked to this aridification (Hoetzel et al., 2013).
The period from 6.8 until 5.2 Ma when the representations of
H. tectata and B. hirsuta dominate forms an
interlude in the development of upwelling. It is a period of global
oceanic and climate changes contemporaneously to the initiation of the
NADW formation (Billups, 2002), the desiccation of the Mediterranean
Sea (Krijgsman et al., 1999) and the loss of contact between the deep
waters of the Pacific and Atlantic Oceans by the decreasing sill depth
of the CAS. Changes in the AMOC and the NADW formation would have
resulted in changes of the subsurface waters and thus in the quality
of upwelled waters (Berger et al., 2002a), which resulted in a unique
flora of the BUS.
From 4.4 Ma on, final closure of the CAS and the emergence of
the Panama Isthmus (Steph et al., 2010) proceeded, leading to the
intensification of the AMOC and deep water formation (Haug and
Tiedemann, 1998) changing the intermediate waters of the BUS which
again changed the dinoflagellate assemblages. Since 3.2 Ma the
intensification of the Northern Hemisphere glaciations and the
introduction to a bi-polar icehouse started the Inter-Tropical
Convergence Zone shifted southwards to its average modern position
(Billups et al., 1999). The southward shift probably caused an
intensified meridional temperature gradient and a further
intensification of the low-level winds driving upwelling, which is
recorded by the dinoflagellate cyst assemblages.