Palaeogeography, Palaeoclimatology, Palaeoecology 235 (2006) 321 – 329
www.elsevier.com/locate/palaeo
Occurrence of thick Ethmodiscus oozes associated with a terminal
Mid-Pleistocene Transition event in the oligotrophic subtropical
South Atlantic
Oscar Romero a,b,*, Frank Schmieder a
a
b
Department of Geosciences, University Bremen, PO Box 330440, 28334 Bremen, Germany
Instituto Andaluz de Ciencias de la Tierra (CSIC-UGR), Facultad de Ciencias, Campus Fuentenueva, 18002 Granada, Spain
Received 21 July 2005; received in revised form 10 October 2005; accepted 17 October 2005
Abstract
Within generally calcareous sediment sequences, layers of variable thickness of the giant diatom Ethmodiscus were found in
five cores recovered in the Subtropical South Atlantic between 238 and 338S from both sides of the Mid-Atlantic Ridge. Two types
of oozes occur: (almost) monospecific layers of Ethmodiscus and layers dominated by Ethmodiscus, with several accompanying
tropical/subtropical, oligotrophic-water diatoms. The two thickest Ethmodiscus layers occur in GeoB3801-6 around 298S, and
accumulated during late MIS 14 and MIS 12, respectively. Downcore concentrations of Ethmodiscus valves range between
3.4 * 104 and 2.3 * 107 valves g 1. We discuss the ooze formation in the context of migration of frontal systems and changes in the
thermohaline circulation. The occurrence of Ethmodiscus oozes in sediments underlying the present-day pelagic, low-nutrient
waters is associated with a terminal event of the Mid-Pleistocene Transition at around 530 ka, when the ocean circulation
rearranged after a period of reduced NADW production.
D 2005 Elsevier B.V. All rights reserved.
Keywords: Diatoms; Ethmodiscus; Mid-Pleistocene Transition; Subtropical South Atlantic; Thermohaline circulation
1. Introduction
Substantial fluctuations in the thermohaline circulation (THC) and deep-water characteristics occurred in
the South Atlantic between approximately 640 and 500
ka at the end of the Mid-Pleistocene Transition (MPT)
(Schmieder et al., 2000). The transitional period, the
Mid-Pleistocene interim state, is characterized by much
weaker production of North Atlantic Deep Water
* Corresponding author. Department of Geosciences, University
Bremen, PO Box 330440, 28334 Bremen, Germany. Tel.: +49 421
21865537; fax: +49 421 21865505.
E-mail address: [email protected] (O. Romero).
0031-0182/$ - see front matter D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.palaeo.2005.10.026
(NADW) (Raymo et al., 1997), and stronger carbonate
dissolution (Schmieder et al., 2000). The so-called
terminal MPT event was a response to major paleoceanographic and paleoclimatic changes at the end of this
period.
Surface waters of the Subtropical Atlantic Gyre are
presently characterized by low-silicate content, and
hence are less favorable for diatom growth (Romero
et al., 1999, 2005). Permanent oligotrophic conditions
keep the sedimentation rates below 1 cm kyr 1
(Schmieder et al., 2000). Since calcareous nanoplankton contributes the largest share to the phytoplankton
community (Romero et al., 1999), carbonate-rich
nannofossil oozes are the typical sediments deposited
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O. Romero, F. Schmieder / Palaeogeography, Palaeoclimatology, Palaeoecology 235 (2006) 321–329
beneath this bopen ocean desertQ. Among the highly
diversified diatom flora in the modern oligotrophic,
subtropical-tropical Atlantic plankton assemblages,
Ethmodiscus is a relatively rare component (Romero
et al., 1999, 2000; Romero, unpublished observations).
Ethmodiscus is representative of a group of oceanic
phytoplankton characterized by extremely large size
(108–109 Am3) and a vacuole that comprises over
99% of the cell’s volume (Villareal et al., 1999). The
life history of Ethmodiscus includes vertical migration
to the nutricline where nitrate is acquired for use during
photosynthesis at the surface (Villareal and Lipschultz,
1995). Whole Ethmodiscus frustules are rarely preserved
in the sediment, but fragmented material can build large
deposits in the sediments underlying tropical seas.
Despite its low abundance in tropical waters (usually
1–5 cells m 3, Belyayeva, 1970; Villareal, 1993), Ethmodiscus oozes have been reported from sediments
underlying equatorial regions of the world ocean (Gardner and Burckle, 1975; Mikkelsen, 1977; Stabell, 1986;
Broecker et al., 2000; Abrantes, 2001; De Deckker and
Gingele, 2002). Oozes in the Atlantic Ocean have been
observed by Stabell (1986) and Abrantes (2001). Stabell (1986) studied a core recovered in the south flank
of the Sierra Leone Rise at ca. 38N, and assigned the
occurrence of Ethmodiscus layers to glacial conditions.
Abrantes (2001) documents the occurrence of Ethmodiscus oozes in a southward location close to the Guinea Basin. In good temporal agreement with Stabell’s
record, higher contribution of Ethmodiscus fragments
in the eastern equatorial Atlantic appears during glacial
MIS 6 and 4. Records of Ethmodiscus oozes from
subtropical areas are scarce. Pike (2000) reports the
occurrence of an early Pliocene Ethmodiscus ooze recovered at the ODP Site 1010, located seaward of Baja
California in the NE Pacific Ocean. Gombos (1984)
found Ethmodiscus oozes in late Miocene/early Pliocene sediments at Site 520, located close to our GeoB
positions.
The presence of Ethmodiscus oozes is commonly
associated with the simultaneous occurrence of adequate conditions of production in the water column
and preservation in the sediment. According to the
bbloom hypothesisQ (Mikkelsen, 1977, and references
therein), Ethmodiscus oozes are formed by periods of
intense growth of the species followed by high settling
rates through the water column. After surviving both
the sinking process and grazing pressure, the siliceous
remains still have to survive dissolution at the sediment/
water interface. The bbloom hypothesisQ offers a physiological explanation for the occurrence of Ethmodiscus
oozes, but does not explain what might have caused the
bloom itself. Recent work has shown that Ethmodiscus
cells behave in a similar way to chains and mats of the
diatom Rhizosolenia (Villareal and Carpenter, 1989;
Villareal et al., 1999). Rhizosolenia and Ethmodiscus
cells have decoupled their light and nutrient utilization
mechanisms, taking up nutrients just below the nutricline and rising to surface to photosynthesize (Villareal,
1993; Villareal and Carpenter, 1994).
Fig. 1. Location of gravity cores below the Subtropical South Atlantic.
O. Romero, F. Schmieder / Palaeogeography, Palaeoclimatology, Palaeoecology 235 (2006) 321–329
Thick layers of specific diatoms are often found in
marine sediments (see Kemp et al., 2000, for a review).
The species may be different between different oceanic
regions, but the formation of the laminations probably
shares some important features. Since a bloom of Ethmodiscus represents tremendous uptake of silica from
normally silica-undersaturated surface waters, paleoceanographers have mostly interpreted Ethmodiscus
accumulation in the sediment as the result of biological
processes (e.g., Stabell, 1986; Broecker et al., 2000;
Abrantes, 2001). The current uncertainty over the depositional mechanisms of oozes makes important to
document each newly recovered occurrence of Ethmodiscus oozes, particularly because of their paleoceanographic implications. This paper adds to the growing
list of Ethmodiscus occurrences and describes oozes
found within generally calcareous sequences in five
cores from the Subtropical South Atlantic between
238 and 338S. Age estimations suggest that they were
deposited simultaneously. For the first time, downcore
concentrations of Ethmodiscus valves are presented.
We discuss the occurrence of Ethmodiscus oozes in
sediments underlying the present-day pelagic, low-nutrient waters from the South Atlantic in the context of
migration of frontal systems and the THC changes.
2. Material and methods
We have studied five gravity cores recovered below
the presently oligotrophic South Atlantic Gyre between
ca. 23–338S and 11–248W (Fig. 1, Table 1). For diatom
analyses, sediment samples were taken every 5 cm
using 10 cm3-plastic syringes and freeze-dried afterward. Dried samples were prepared following the method proposed by Schrader and Gersonde (1978). Only
fragmented valves of Ethmodiscus were found downcore at each site. Valve concentration was calculated
only at GeoB 3801-6 in the depth interval 5.10 to
Table 1
Name and location of GeoB gravity cores, water depth, thickness (cm)
of Ethmodiscus ooze in each core, and estimated age (ka) of ooze
occurrence
GeoB
Position
Water Ethmodiscus
Estimated
depth layer thickness age
S
W
m
cm
ka
3801-6
3813-3
5112-4
6425-2
6426-1
29851.2V
32826.8V
23849.5V
33849.5V
33830.0V
11855.7V
21896.7V
16815.5V
23835.2V
24801.5V
3294
4331
3842
4352
4385
124
38
14
31
(25)*
~537
~544–534
~524
~528
(~548–530)*
*: Ethmodiscus layer not clearly detectable.
323
7.92 m, where the thickest diatom ooze occurs (Table
1). We measured the surface area of at least 1400 valve
fragments of different sizes, when available, in each
sample in order to calculate the valve concentration. For
the final quantitative calculation of Ethmodiscus, all
fragments, independent of shape and size, found on
the counting transects were included, the number of
transects depending on the fragment abundance. We
add up the total area counted as based on the total
number of fragments, and then calculate the number
of diatom valves this represented with three estimates
based on assuming all the valves were small (340 Am
diameter), medium, or large (1900 Am diameter, following Wiseman and Hendey, 1953, and McHugh,
1954). We believe that this approach is more adequate
and better represents the incidence of disturbance by
valve fragmentation. We used the software AxioHOME
3.0, which allows accurate measurements of the area of
either whole valves or valve fragments on a computer
screen. Samples for fragment measurements were prepared following the method proposed by Schrader and
Gersonde (1978). Fragment area measurements were
carried out on permanent slides of acid-cleaned material
(Mountex mounting medium). Qualitative and quantitative diatom analyses were performed at 1000 magnification using a Zeiss–Axioscope with phase-contrast
illumination.
The sediment samples for bulk analyses were freezedried and ground in an agate mortar. Total carbon
contents (TC) were measured on untreated samples.
After decalcification of the samples by 6 N HCl, total
organic carbon contents (TOC) were obtained by combustion at 1050 8C using a Heraeus CHN-O-Rapid
elemental analyzer as described by Müller et al.
(1994). Carbonate was calculated from the difference
between TC and TOC, and expressed as calcite
(CaCO3 = TC–TOC) * 8.33). Magnetic susceptibility
was measured post-cruise on archive core halves
using a Bartington Instruments spot sensor at 1-cm
intervals (for detailed methodology see Schmieder et
al., 2000; Schmieder, 2004).
3. Results
Ethmodiscus rex layers of variable thickness were
found in five cores from the South Atlantic recovered
from both sides of the Mid-Atlantic Ridge (Table 1).
Although relative proportions vary from bottom to top
of the studied downcore section of core GeoB 3801-6,
Ethmodiscus clearly dominates the siliceous fraction
almost throughout the entire section (Fig. 2, righthand panel). Two types of oozes can be distinguished
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O. Romero, F. Schmieder / Palaeogeography, Palaeoclimatology, Palaeoecology 235 (2006) 321–329
Fig. 2. Left-hand panel (time scale): Low latitude d 18O-stack (Bassinot et al., 1994) indicating glacials (gray) and interglacials, SUSAS stack (Von Dobeneck and Schmieder, 1999) and magnetic
susceptibility of core GeoB 3801-6. Right-hand panel (depth scale): magnetic susceptibility, relative contribution of calcium carbonate (CaCO3, open circles) and organic carbon (Corg, closed circles)
expressed as percentage, and concentration of valves of Ethmodiscus rex, given as valves gr 1 (valves of minimum diameter, open circles; valves of maximum diameter, closed circles) in GeoB
3801-6 from the Subtropical South Atlantic. Turb.=Turbidite (no time assigned, therefore not shown in the left panel). Since no whole Ethmodiscus valves were found, we calculated the
concentration taking a diameter range of ~340–1900 Am (Wiseman and Hendey, 1953; McHugh, 1954). The mixed tropical–subtropical diatom assemblage is mainly composed of Alveus marinus,
Azpeitia neocrenulata, Fragilariopsis doliolus, Roperia tesselata, Thalassionema nitzschioides var. nitzschioides, Thalassiosira ferelineata, T. oestrupii var. oestrupii, T. oestrupii var. venrickae,
and an unidentified Thalassiosira species.
O. Romero, F. Schmieder / Palaeogeography, Palaeoclimatology, Palaeoecology 235 (2006) 321–329
according to the diatom composition: (a) almost
monospecific layers with the relative contribution of
Ethmodiscus ranging from 90–100%, and (b) layers
dominated by Ethmodiscus, but accompanied by several typically tropical/subtropical, oligotrophic-water
diatoms, such as Azpeitia barronii, A. nodulifera,
Hemidiscus cuneiformis, Roperia tesselata, and Thalassionema nitzschioides var. parva. The occurrence
of secondary diatom species is independent of Ethmodiscus concentration (Fig. 2). There is also no direct
relationship between Ethmodiscus ooze composition
and valve concentration. Monospecific oozes are associated with low valve concentrations (e.g., between
6.10 and 5.60 m), but also with relative maxima in
Ethmodiscus valve concentration (e.g., 7.25–7.15 m).
In all cores only fragmented valves of Ethmodiscus
appear and no whole frustule was observed. Initial age
estimations for all cores were based on magnetostratigraphies (Von Dobeneck and Schmieder, 1999; Schmieder, 2004). Subsequently, detailed chronostratigraphies
were constructed for GeoB 3801-6 and 3813-3 within
the stratigraphic network SUSAS by astronomical tuning of susceptibility records (Fig. 2, left-hand panel;
Von Dobeneck and Schmieder, 1999). For all other
cores susceptibilities were correlated to the SUSAS
stack (Schmieder, 2004).
A detailed study of two Ethmodiscus layers in
GeoB 3801-6 was performed (Fig. 2, Table 1). The
first layer, ca. 124 cm thick, is observed at late MIS 14
while the second one, only 2 cm thick, occurred during
MIS 12 (Fig. 2). The thickest Ethmodiscus-dominated
layer extends between 7.92 and 6.68 m, and contains a
highly variable diatom concentration (~537 ka, Diatom
Layer 1 in Fig. 2). The dominance of Ethmodiscus is
accompanied by the lowest susceptibilities and diminished contributions of calcium carbonate and organic
carbon. Like in the rest of the core, calcareous microfossils dominate between both Ethmodiscus layers
(also evidenced by increased values of calcium carbonate, Fig. 2). From ~6.68 up to ~5.10 m, valve
concentration decreases to minimum values but fragments of Ethmodiscus occur almost throughout this
section (late MIS 14 through late MIS 12). A secondary peak in valve concentration occurs at ~5.20 m
depth (~431 ka, Diatom Layer 2 in Fig. 2). The diatom
flora of the core section between ca. 6.50 and 5.30 m is
predominantly composed of E. rex, but two horizons
with several typically tropical/subtropical diatoms appear: a very narrow interval at ~6.16–6.13 m (early
MIS 13) and a second one between 5.60 and 5.45 m
depth (mid MIS 12). The diatom concentration remains
below 1 * 104 valves g 1 at both of these horizons.
325
Considering only the smallest valves (diameter = 340
Am), we obtain a concentration range of ~1.1 * 106 to
2.3 * 107 valves g 1 for the downcore section between
~7.90 and ~6.50 m depth.
The upper limit of the concentration range decreases by two orders of magnitude when considering
only the largest diatoms (diameter = 1900 Am): 7.4 * 105
valves g 1 (lowest range 3.4 * 104 valves g 1). Considering an average valve (~1120 Am), concentration
values range within two orders of magnitude from
6.6 * 104 to 1.4 * 106 valves g 1. Other siliceous components include several marine diatoms, radiolarians,
silicoflagellates, the dinoflagellate Actiniscus pentasterias, a few freshwater diatoms and some phytoliths
(silicified bodies of grass epidermal cells).
4. Discussion
Paleoceanographic interpretations of Ethmodiscus
oozes tend to differ among themselves and are mostly
different from the biologic explanations (Table 2). For
tropical areas of the oceans, the formation of Ethmodiscus oozes has been either related with stronger upwelling and intensified current circulation (Gardner and
Burckle, 1975; Stabell, 1986; Abrantes, 2001), strongly
stratified water column (De Deckker and Gingele,
2002), silica-enriched thermocline (Broecker et al.,
2000), or differential dissolution (Bukry, 1974; Schrader, 1974; Table 2). Ethmodiscus layers in sediments
underlying subtropical marine waters have been associated either with front zones (Pike, 2000; Gingele and
Schmieder, 2001) or bottom topography (Gombos,
1984; Table 2). The ecology, life history strategy, and
physiological characteristics of Ethmodiscus cells,
however, suggest that caution should be exercised in
its general use as paleo-upwelling indicator (Villareal et
al., 1999). Although Ethmodiscus has been reported as
living in equatorial upwelling zones (Villareal, 1993),
no evidence supports the assumption that it typically
occurs in greater abundance under upwelling conditions
(Villareal et al., 1999). No observations are available
about the present-day concentration of Ethmodiscus in
waters of the Subtropical South Atlantic; nevertheless,
very low frustule concentration has been reported from
ecologically similar areas of the world ocean (1–5
cells m 3, Belyayeva, 1970; Villareal, 1993).
The high-productivity, upwelling hypothesis additionally proposes that the rapid sinking of mass-produced valves minimizes dissolution effects (Mikkelsen,
1977). Arguments against the effect of differential dissolution of Ethmodiscus valves have been raised by
Abrantes (2001). She found no evidence of downcore
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O. Romero, F. Schmieder / Palaeogeography, Palaeoclimatology, Palaeoecology 235 (2006) 321–329
Table 2
Summary of known record of Ethmodiscus oozes
Atlantic
Core/site
Location and depth
Occurrence of
Ethmodiscus during. . .
Occurrence due to. . .
Reference
M16772
01821VS, 11858VW
3912 m
185–170 and 150–140 ka
(MIS 6.6 and 6.4) and
70–60 ka (MIS 4.2)
Abrantes (2001)
M13521
05840VN, 19851VW
2862 m
MIS 3, 4 and late MIS 6
14 cores
07813VN–08468S 24810V
W–04847VW
3330–5130 m
25831.40VS 11811.14VW
4207 m
Pleistocene glacial stages
Strong equatorial upwelling,
increased advection of water
from coastal upwelling areas
and important river run-off
Extremely high nutrient
conditions provided by
upwelling in periods of
strengthened trade winds
during glacials
Increased upwelling and
intensified equatorial
current circulation
Local topography: closed
depression on the
Mid-Atlantic Ridge with
poor bottom circulation and
reduced dissolved oxygen
levels
The abrupt strengthening
of the THC during the MPT
interim state made nutrients
from deep reservoirs
accessible
520
Pacific
Indic
Late Miocene and
early Pliocene
5 cores
23849.5V–33849.5VS
11855.7V–24801.5VW
MIS14 and MIS12
1010
29857.9VN, 11880.0VW
3475m
Early Pliocene
73
01854.58VS 137828.72VN
Pliocene
BAR9442
06804.56VS 102825.08VE
2542 m
Last Glacial Maximum
RC14-31
06804.56VS 102825.08VE
3900 m
11–30 ka
238
11809.21VS 70831.56VE
2844 m
10812.71VS 93853.77VE
5611 m
08807.30VS 86847.50VE
5319 m
Plio-Pleistocene
213
215
High concentration
(and subsequent deposition)
of Ethmodiscus along
convergence zones
High production and
differential dissolution
Water column permanently
stratified, high levels of
silica and nutrients from
a wider depth range
Thermocline waters moving
entering the Indic Ocean
from the Pacific
(via the Indonesian Straits)
provided the required silica
and other nutrients
Enrichment of Ethmodiscus
fragments in sediments due
to low phytoplankton
production in surface waters
Stabell (1986)
Gardner and Burckle
(1975)
Gombos (1984)
Gingele and
Schmieder (2001),
Schmieder et al.
(2000),
Schmieder (2004).
This study
Pike (2000)
Mikkelsen (1977)
De Deckker and
Gingele (2002)
Broecker et al. (2000)
Bukry (1974),
Schrader (1974)
We report records where Ethmodiscus makes at least 50% of the diatom assemblage.
increased dissolution in sediments from the eastern
equatorial Atlantic, hence concluding that selective
preservation did not affect Ethmodiscus concentration
(Abrantes, 2001). Considering that Ethmodiscus possesses large valves (diameter up to ~2 mm), the dissolution rate of its valves should be low due to rapid
sinking and the short exposure time in the water column
(Villareal, 1992); hence, we speculate that differential
dissolution cannot account by itself for the regional
extent of the oozes below the Subtropical Atlantic
Gyre.
Mikkelsen (1977) described two types of Ethmodiscus oozes: one where Ethmodiscus makes up 80–100%
of the diatom assemblage, and a second type where
O. Romero, F. Schmieder / Palaeogeography, Palaeoclimatology, Palaeoecology 235 (2006) 321–329
Ethmodiscus appears only as an accompanying species
with abundances up to 40%. At GeoB 3801-6 fragments of Ethmodiscus dominate throughout the two
ooze layers at the end of MIS 14 and 12 and in
between. In some horizons other warm-water diatoms
occur (Fig. 2; Ethmodiscus also dominates in the other
four GeoB cores, Table 1). The presence of smaller,
solution-resistant diatoms, such as species of Azpeitia,
Roperia, and Thalassiosira, typically characterizes the
diatom community occurring in oligotrophic areas of
the tropical/subtropical Atlantic Ocean (Romero and
Hensen, 2002; Romero et al., 1999). Observations
from pelagic areas of the equatorial Atlantic show
that well-silicified diatoms dominate in sediments underlying low-nutrient content waters (Romero et al.,
2000, 2005). It is well-known, however, that in pelagic
areas of the oceans, the effect of preservation removes
information from the most productive season thus leaving the sediment with less evidence of original variability (Romero et al., 1999).
Since only fragments of Ethmodiscus are preserved
in the sediments, quantifying Ethmodiscus concentrations have proved to be difficult. Stabell (1986) calculated the total number of E. rex valves at core M13521
from the Sierra Leone flank in the tropical Atlantic by
arbitrarily assuming an average of 250 fragments per
whole valve, independent of fragments size. Abrantes
(2001) counted fragments larger than 50 Am and calculated their accumulation rate on the core M16772
collected on the western edge of the Gulf of Guinea.
We have used a different approach. Measuring the area
327
of Ethmodiscus fragments of different sizes and shapes
preserved at GeoB 3806-1 allows us to minimize the
bias produced by fragmentation of entire valves in
calculating a theoretical valve concentration. Frequencies of measured fragments show predominantly an
asymmetrical unimodal distribution of fragment size
classes with the fragments smaller than 1000 Am2
dominating the class distribution (Fig. 3). The slight
variation in the dominant class of fragment size basically shows that the mechanical dissolution affecting
Ethmodiscus valves was similar throughout the production event which led to the ooze formation.
Together with unusual lithological features, the presence of Ethmodiscus oozes in the Subtropical South
Atlantic cores temporally coincides with the terminal
MPT event (Schmieder et al., 2000; Schmieder, 2004),
when the ocean circulation rearranged after a period of
reduced NADW production (Raymo et al., 1997). A
more stratified water column might have fostered deepwater accumulation of nutrients, mainly silicate, during
the MPT interim state (Gingele and Schmieder, 2001).
Deep-accumulated silica might have become accessible
to Ethmodiscus cells with the sudden and abrupt
strengthening in the THC. Blooms of Ethmodiscus
require high quantities of silica supplied on a very
rapid basis (Gardner and Burckle, 1975; Villareal,
1993; Villareal et al., 1999). Due to the positive and
negative buoyancy capacity of Ethmodiscus cells (Villareal, 1992, 1993), they probably grew and reproduced
quickly and sank into deeper, silicate-richer waters
where they subsequently took up nutrients. Intensified
Fig. 3. Frequency distribution of Ethmodiscus fragment size classes at 12 selected depths in gravity core GeoB 3801-6.
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O. Romero, F. Schmieder / Palaeogeography, Palaeoclimatology, Palaeoecology 235 (2006) 321–329
circulation and the depletion of macronutrients probably caused the rapid sedimentation of Ethmodiscus
valves, enhanced by their comparatively large size,
which in turn may have impeded their consumption
by zooplankton. Recirculated nutrients again found
their way into the surface waters where they were
utilized by calcareous nanoplankton, promoting the
return to bnormalQ calcareous productivity of the Subtropical South Atlantic.
Since all our cores are close to the present-day
position of the Subtropical Front (Orsi et al., 1995), it
can be argued that northward-advected colder waters of
southern origin contributed to the formation of Ethmodiscus oozes. The buoyant Ethmodiscus cells might
have accumulated in relatively warm waters, where
downwelling velocities should have been low compared to colder waters on the opposite southern side
of the frontal boundary (Villareal and Carpenter, 1994).
Following the model proposed for species of Rhizosolenia (Yoder et al., 1994), positively buoyant Ethmodiscus might accumulate at the surface in convergence
zones, these accumulation being the result of physical,
not biological processes (Villareal et al., 1999, and
references therein). Ethmodiscus might have taken
nutrients up and grew more rapidly in the colder,
upwelled waters, although it occurred at higher abundance in the warmer waters of the convergence. While
it is possible that a similar front was present at 29–338S
in the Atlantic around 524–548 ka and 432 ka, this
hypothesis remains highly speculative at this point.
Arguments regarding the potential involvement of oceanic fronts in the genesis of Ethmodiscus oozes have
been rehearsed by Pike (2000). She speculates about the
occurrence of an Ethmodiscus layer of Miocene-toearly Pliocene age in the subtropical Eastern Pacific
at ODP Site 1010 (29857.9VN, 11880.0VW) as being
deposited on the outer edge of the productive waters
of the California Current as they moved shoreward
across Site 1010, hence suggesting a possible frontal
mechanism for ooze building (Pike, 2000).
The production and sinking events leading to the
ooze formation at the GeoB positions took place in the
binvariantQ oligotrophic South Atlantic gyre. Our
results, however, suggest that diatoms can be important
vectors in the vertical transport of biogenic silica and
organic matter to the deep ocean in nutrient-poor areas
of the world ocean. Lowered values of organic carbon
in the Ethmodiscus ooze between 7.92 and 6.68 m in
GeoB3801-6 reflect rather the decreased contribution of
calcareous organisms, as reflected by lowered contribution of CaCO3. In GeoB 3813-3 a prominent Ba/Al
peak associated with the Ethmodiscus ooze at the end
of MIS 14 indicates that the ooze was built during a
period of increased productivity (Gingele and Schmieder, 2001). The C : Chl ratios in Ethmodiscus cells are
higher than typically reported for phytoplankton while
Si : C and Si : N molar ratios are about six to seven times
higher than the average value known for diatoms (Villareal et al., 1999). Our records show that Ethmodiscus
has been adapted to survival in the open, poor-nutrient
ocean probably via vertical migrations during glacial
Pleistocene stages (e.g., Broecker et al., 2000; De
Deckker and Gingele, 2002). The detection of simultaneously deposited Ethmodiscus oozes in a wide area of
the Subtropical South Atlantic strengthens the hypothesis that a major paleoceanographic event took place at
the end of the MPT interim state.
Acknowledgements
Captain and crew of the RV Meteor are gratefully
acknowledged for their cooperation during the core
retrieval. W. Hale improved the English. Data are electronically available in the database www.pangaea.de\
Pangavista.
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