Download Quaternary paleo-oceanography from the geochemistry of sediment

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Sub-Project 3.3
Neogene-Quaternary palaeoceanography from the geochemistry
of successions on the South African margin
Participants
* Coordinator
Institution
Names
Email addresses
University of Cape Town
(UCT)
John Compton*
[email protected]
Alfred Wegener Institute
(AWI)
Gabriele UenzelmannNeben
[email protected]
Requested Funding:
Total for the 5-year duration project beginning in 2004: Euros 366000
UCT
2004
2005
2006
2007
2008
127000
58000
80000
61000
40000
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Summary
Multiple geochemical proxies will be used to infer the evolution
of the continental margin of South Africa from Neogene and
Quaternary successions. Strontium isotope stratigraphy will be
combined with biostratigraphy to develop an improved age
model of deposition and diagenesis on the margin. Nd isotopic
changes will be related to changes in oceanic circulation and
upwelled waters along the margin. Seismic stratigraphy will be
integrated with geochemistry, litho- and biostratigraphy in order
to determine the relation among climate, tectonic uplift,
upwelling intensity, ocean currents (erosional events) and sealevel fluctuations. The improved age model will allow us to
correlate events on the western margin with global tectonism
(e.g., opening of oceanic gateways), climate (glaciation of
Antarctica and the Northern Hemisphere), eustacy and ocean
circulation (e.g., intensity of North Atlantic Deep Water
formation). The age of Quaternary successions will be
determined using oxygen isotope stratigraphy and AMS
radiocarbon ages of the Holocene mud belt. High-resolution
Quaternary records will be compared to continental climate
proxy records and Northern Hemisphere (including ice core)
records.
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Scientific Motivation and State of the art
The southern tip of Africa occupies a unique and critical position relative to the global
circulation of the oceans as it interfaces with a major confluence of ocean currents. The warm,
Agulhas Current flows along the eastern continental margin leaking flow (“rings”) into the
South Atlantic as it retroflects back into the southern Indian Ocean. The cold, Benguela
Current flows along the western continental margin as part of the South Atlantic gyre. These
surface, as well as associated intermediate and deep water, currents form an important part of
the overall thermohaline circulation of the ocean in which cold salty waters that sink and
spread throughout the deep ocean are returned to the surface in a large scale, idealised
‘conveyor belt’ loop (Broecker and Denton, 1989). Of particular interest is the amount of
warm water that leaks into the South Atlantic via the Agulhas retroflection because it is
believed to control much of the heat transfer important to global climate. The present-day
thermohaline circulation is believed to have been established around 25 million years ago (25
Ma) when Antarctica became fully isolated by the opening of Drake Passage (Shackleton and
Kennett, 1975). The tectonic isolation of Antarctica allowed establishment of the Circum
Antarctic Current, build up of ice on the continent of Antarctica and a significant shift
towards colder climates and the current Ice Age (Kennett, 1982).
Palaeoceanographers are keen to determine past changes in oceanic circulation because of the
strong linkage to climate change, both on long, million-year and short, thousand-year time
scales. The ratio of the neodymium (Nd) isotopes 143 to 144 (expressed in shorthand relative
to chondritic meteorites as εNd) has proven to be a useful geochemical tracer of changes in
ocean circulation because of the short residence time of Nd in seawater. As a consequence
of the short residence time of Nd, the εNd values of seawater are distinct in each of the major
ocean basins and εNd values can be used to trace the mixing of these different ocean water
masses (e.g., Piepgras and Wasserburg, 1982; Jeandel, 1993). Changes in εNd of marine
precipitates have been used to document changes in Pacific - Atlantic exchange through the
Panama gateway (Frank et al., 1999) and through the Drake Passage (Piepgras and
Wasserburg, 1982; Jeandel, 1993) as well as to infer long-term (Stille, 1992; Stille et al.,
1996) and short-term (Rutberg et al., 2000) changes in ocean circulation. Most previous work
has focused on ferromanganese deposits that form at very slow rates of several mm per Myr
in the deep ocean. However, there have been several studies that have shown the usefulness of
phosphorite analyses (Shaw and Wasserburg, 1985; Stille, 1992). The advantages of
phosphorite samples are that they precipitate at much faster rates than Fe-Mn oxides, they are
generally widespread on continental margins and they take up abundant Sr from which the
age of the sample can be derived.
The age of marine minerals can be determined by their strontium (Sr) isotope composition. In
contrast to Nd, Sr has a long residence time in the oceans and is uniformly well mixed. In
addition, the ocean Sr isotope composition has steadily increased at variable rates from the
Oligocene to the present-day and allows mineral precipitates to be dated (DePaolo and
Ingram, 1985). One of the principal marine minerals on the South African margin is
phosphorite, a phosphorus-rich rock type that forms in highly productive upwelling areas
(Birch, 1990; Baturin, 2000). The utility of Sr isotope analysis is that it provides a means to
date phosphorite formation directly (Compton et al., 1993) and, depending on the steepness of
the marine Sr isotope record, can provide ages with uncertainties of less than 1 Myr (Hodell
et al., 1991; Farrell et al., 1995). The age of sediment deposition will be determined by
integrating Sr isotope analysis of biogenic grains and biostratigraphy (foraminifera and
pollen). In addition to biostratigraphy, the pollen record will provide important insights into
the evolution of the Cape Floral Kingdom. Improved age resolution is critical to our
understanding of the complex depositional and diagenetic history of these shelf successions
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and their relation to sea-level fluctuations and climate change. Recent results from the Orange
River Prodelta (Compton et al., 2002; in press) indicate that upwelling was initiated in the
earliest Miocene, significantly older than the late Miocene age indicated at the Walvis Ridge
(Siesser, 1980). Episodes of phosphogenesis on the margin can be linked to sea-level
fluctuations and periods of increased organic matter accumulation related to greater
productivity (upwelling) and preservation of organic matter (Mallinson and Compton, 1997).
Climate, upwelling and formation of phosphorite are connected and, therefore, it is important
to understand the age of phosphorite formation in order to infer past changes in climate.
Our understanding of present-day ocean circulation is far from complete. For example, it is
debated to what extent the return flow to the South Atlantic is from leakage of warm Agulhas
Current vs. cold waters passing through Drake Passage (Gordon, 1986; Rintoul, 1991;
Heywood et al., 2002). Intermediate water masses remain relatively poorly understood in
comparison to surface and deep water masses. What does appear clear is that significant ocean
mixing occurs in the southeast Atlantic at the confluence of the Benguela Current with the
Agulhas retroflection (J Lutjeharms, pers comm). Previous studies have looked at the deep
and intermediate water εNd signatures around southern Africa (Albarede and Goldstein, 1992;
Jeandel, 1993; Rutberg et al., 2000). These studies show the importance of Atlantic deep and
intermediate-depth water flow into the Indian Ocean near the Agulhas Plateau. To date, there
has been no attempt to look at the εNd values of intermediate and surface waters from outer
shelf and upper slope deposits of the western and southern margins.
Scientific goals
Neogene
The geological deposits on the continental margin of South Africa contain a number of marine
minerals whose εNd and Sr isotope values have the potential to document changes in ocean
circulation and source of upwelled waters at the critical and complex juncture of the Agulhas
and Benguela currents. The focus of this proposal is to determine the age of phosphogenic
episodes and their corresponding variations in εNd from the paired analysis of Sr and Nd
isotope compositions of specific mineral grain types from both the western margin (Benguela)
deposits and southern margin (Agulhas) deposits. It is believed that these analyses will
provide new insights into the sources and dynamics of South African margin water masses
and their relation to major tectonic and climatic events over the last 25 Ma. These
geochemical indicators of phosphogenesis and ocean circulation will be linked to the litho-,
bio- and seismic stratigraphy of the margin. The architecture of sediment deposits and major
erosional unconformities revealed by the seismic profiles proposed by G Uenzelmann-Neben
will allow us to develop an integrated model of margin evolution.
Critical questions are:
1. What is the age of phosphogenic episodes on the western and southern South African
margin and how do they relate to major palaeoceanographic events, such as upwelling,
changes in ocean currents, the opening of Drake Passage, sea-level fluctuations, global
cooling, aridity and the evolution of the Cape Floral Kingdom?
Preliminary results (Compton et al., 2002; in press) indicate that much of the phosphorite
formed during periods of rising or highstands of sea level, but in some areas phosphorites
formed during overall marine regressions. Major erosional unconformities on the margin
suggest that tectonic uplift of the margin has also played a significant role along with eustatic
sea-level fluctuations. A major phosphogenic episode represented by a 23.5-25.4 Ma
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phosphate-rich gravel lag deposit on the western margin corresponds to the Oligocene to
Miocene marine transgression and highstand and to the final, deep-water opening of Drake
Passage, a major oceanic gateway that initiated modern, thermohaline oceanic circulation.
The apparent paucity of phosphorites on the margin older than this initial major phosphogenic
episode suggests a major change in palaeoceanography on the margin across the
Oligocene/Miocene boundary. Factors other than rising sea level may have included a
significant increase in upwelling related to establishment of thermohaline circulation and the
Benguela Current. These phosphorites may mark the transition from the greenhouse to the
icehouse Earth where increased oceanic circulation resulted in a removal of deep-water
phosphorus.
2. How does the Nd isotope composition of the phosphorite and other marine minerals vary
over the last 25 Ma and what can we infer from these data on changes in the exchange of
Pacific, Indian and Atlantic water masses?
Phosphorite εNd values are expected to reflect the εNd values of upwelled waters with much
of the Nd scavenged from the water column and remobilised during early diagenesis to be
taken up by phosphate minerals. On the western margin upwelling waters are primarily
coming from water depths of less than 300 m (V Shannon, pers com.) and are associated with
nutrient-rich Central South Atlantic Water. Upwelled waters on the Agulhas Bank are most
likely Antarctic Intermediate Water which is nutrient-rich and believed to flow equatorward
and counter to the overlying Agulhas Current (A Meyer, pers. com.). The results of this study
should allow us to determine how similar Central South Atlantic Water was to Antarctic
Intermediate Water and how these water masses changed over the last 25 Myr. The Agulhas
Current is poor in nutrients and is not expected to be associated with increased productivity
and phosphorite formation; however, ferromanganese deposits on the Agulhas Bank/Plateau
may provide εNd signatures of the Agulhas Current that could be compared to the εNd of
upwelled phosphorite samples. In addition, we plan to look at Mn nodules from the western
continental rise to document changes in the εNd signatures of North Atlantic Deep Water. The
age of the Mn nodules probably only extends back to the latest Miocene, but will provide data
on oceanic circulation changes over the last 5 million years, a critical time period that
includes onset of Northern Hemisphere glaciation.
Quaternary
Quaternary successions will be studied in order to understand short-term (<1 Myr) changes on
the margin and will provide a useful comparison to the long-term (> 1 Myr) timescales of the
Neogene successions. The age model of the Quaternary will be based on oxygen isotope
stratigraphy and AMS radiocarbon ages of Holocene sediments. The Quaternary is
characterised by high-frequency and high-amplitude variations in climate and sea level that
make it significantly different from the Neogene.
Key scientific objectives for the Quaternary include:

Are phosphorites forming today on the western shelf mud belt deposits? Modern
phosphorites have been documented to occur in diatomaceous ooze off Namibia (Veeh et
al. 1973; Baturin, 2000) as well as perhaps in the Holocene mud belt of the Orange River
Prodelta (Compton et al., 2002).
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How are terrigenous sediments delivered by rivers and wind distributed on the western
margin and how does this sediment distribution relate to changes in ocean currents and sea
level?
The high sedimentation rates that result from sediment focussing allow for high organic
carbon preservation and promote pyrite and phosphorite formation in the Holocene mud
belt. Variations in the terrigenous flux from the continent should reflect changes in
terrestrial erosion rates that are linked to tectonism (relief) and climate (rainfall). Marine
records of continental erosion rates can be compared to the climate proxy records of the
last 200,000 yr at the Tswaing impact crater site.

How do high-resolution (100-500 yr) climate proxy records over glacial/interglacial
cycles and the Holocene relate to terrestrial climate change records, Northern Hemisphere
climate records and ice core records?
Moderate to high sedimentation rates in the Quaternary successions will allow us to look
at a number of different proxies of climate, tectonic and ocean circulation changes.
Similar records are well known from the polar ice caps and from marine and terrestrial
Northern Hemisphere sites, but there are relatively few from the Southern Hemisphere,
particularly on the southern African margin whose ocean circulation may play a critical
role in controlling climates of the North Atlantic. These records will provide increased
coverage of climate change records and allow the timing of events to be compared and to
evaluate lead or lag times between hemispheres. The Holocene ice records indicate that,
although far more subtle than during the glacial periods, there were significant variations
in Holocene climate. Holocene climate variations are of particular interest in their
application to our understanding of possible future climate change.
Work Schedule
TASK
YEAR 1
YEARS 2-3
YEAR 4
YEAR 5
2 MSc
1 PhD
1 post-doc
1 MSc
1MSc
1PhD
1post-doc
Co-ordinate
Projects
Collect and
analyse samples
Write-up theses;
papers
Sample material
The UCT core repository houses a large collection of offshore material recovered over the
years by various ships dredging the Agulhas Bank and there are a large number of vibracores
from the western margin donated by diamond exploration companies. In addition, there have
been a number of recent scientific expeditions along the southern African margin that have
recovered a large number of piston, gravity and surface cores. Some of this material is housed
in the UCT core repository, but the majority of it is available from core repositories in Europe
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(Germany and France). Future research expeditions are planned for over the next 5 years
(IMAGES) that will augment these collections, including cores from the Agulhas Plateau. The
location of existing samples is well documented and their coverage is sufficient to address the
objectives of this proposal.
Analytical methods
The mineralogy of the samples will be determined by petrography (including scanning
electron microscopy, SEM) and x-ray diffraction (XRD). Grains will be selected for Sr and
Nd analyses on the basis of texture. Previous results have shown how critical it is to analyse
different grain types from the same sample to reinforce interpretations. The different grain
types will include phosphorite (CFA-cemented) peloidal grains with no internal structure,
CFA replaced limestone, fossil fish bones (skeletal phosphorite grains), fish otoliths, benthic
foraminifera, echinoid spines, mollusc shell and any ferromanganese deposits. Samples will
be dissolved at room temperature in twice-distilled 5M glacial acetic acid for Sr and Nd
analyses to minimise dissolution of other minerals. The amount of Sr and Nd contributed by
minerals other than CFA in the phosphorite grain types is considered insignificant because
these samples are predominantly composed of CFA that generally has a high Sr and Nd
content (Shaw and Wasserburg, 1985). Major and trace element composition will be
determined using ICP-MS and XRF. Stable isotopes (oxygen and carbon) will be measured
on the mass spectrometer in Archaeology at UCT. AMS radiocarbon analyses will be
measured in an overseas lab.
Funds Requested
Personnel
This project is anticipated to involve at least two post-doc, two PhD and four MSc post
graduate students. An inflation/cost of living increase of approximately 10% is included. The
proposed study will result in the training of new postgraduate students at the MSc and PhD
level in the area of marine geochemistry and palaeoceanography. These fields are generally
under represented in South Africa and there is a need for well qualified specialists.
Year 1
Year 2
Year 3
Year 4
Year 5
1 PhD @ R60,000
1 PhD @ R65,000
2 postdoc2 @ R120,000
1 postdoc @ R135,000
1 postdoc @ R150,000
1 MSc @ R40,000
2 MSc @ R45,000
2 PhD @ R70,000)
1 PhD @ R80,000
1 PhD @ R90,000);
1 postdoc @ R100,000
1 postdoc @ R110,000
2 MSc @ R50,000
2 MSc @ R55,000)
1 MSc @ R60,000
= R200,000
= R265,000
= R480,000
= R325,000
= R300,000
5 year total = R1,570,000 (2 postdocs, 2 PhD, 4 MSc) (Euros 187,000)
Analytical costs (years 1-4)
The following analyses would be required (most facilities are available at UCT, unless stated
otherwise)
 mineralogy by X-ray diffraction: R5000/yr = R25,000.
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
elemental oxide composition by X-ray fluorescence: R8000/yr = R40,000.

trace element composition by inductively coupled plasma mass spectrometer (ICP-MS):
R12,000/yr = R60,000.

scanning electron microscopy (SEM): R6000/yr = R30,000.

stable isotopes by mass spectrometer: R6,000/yr = R30,000.

Sr and Nd isotopes by thermal ionisation mass spectrometer (TIMS): R30,000/yr =
R150,000.

radiocarbon analyses by accelerator mass spectrometry (AMS): R50,000/yr = R250,000
(overseas lab).
Total analytical costs = R585,000 (Euros 70,000)
Laboratory equipment (purchased in year 1)




Sedimentology/processing lab consumables (chemicals, glassware, etc.):
= R40,000
Carbon analyser (for organic carbon and carbonate carbon determination):
Binocular microscope/ light source (2): R300,000.
Computer workstations (3): R60,000.
R8,000/yr
R250,000
Total laboratory equipment = R650,000 (Euros 77,000)
Travel


Core repository sampling overseas: R60,000
(required for years 1 and 2).
Local and international meetings/conferences/workshops: R40,000/yr = R200,000.
Total travel = R260,000 (Euros 31,000)
Total funds requested:
R3,065,000 (equivalent to approximately 366,000€).
Time frame
Year 1: filling of post graduate and post doc position (advertisements/interviews; Co-ordinate
research project assignments to individuals, organise samples, procedures, academic contract
details with Petroleum Agency, etc.
Years 2-4: Data collection, assimilation, integration and interpretation. Cross pollination
between German and South African people and facilities.
Years 3-5: Abstract presentations, workshops, manuscripts submitted for publication
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References
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Geology 20, 761-763.
Baturin, G.N., 2000. Formation and evolution of phosphorite grains and nodules on the Namibian
Shelf, from Recent to Pleistocene. In: Glenn, C. R., Prévôt-Lucas, L. and Lucas, J. (Eds.),
Marine authigenesis: from global to microbial. SEPM Spec. Publ. No. 66, 185-199.
Birch, G.F., 1990, Phosphorite deposits on the South African continental margin and coastal terrace. In:
Burnett, W.C. and Riggs, S.R. (Eds.), Phosphate Deposits of the World: Vol. 3, Genesis of Neogene to
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Compton, J.S., Hodell, D.A., Garrido, J.R., Mallinson, D.J., 1993. Origin and age of phosphorite from the
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Compton, J.S., Mulabisana, J. and McMillan, I. 2002. Origin and age of phosphorite from the Last Glacial
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Rutberg R., Hemming S. and Goldstein S., 2000. Reduced North Atlantic Deep Water flux to the
glacial Southern Ocean inferred from neodymium isotope ratios. Nature 405, 935-938.
Shackleton, N and Kennett, J, 1975. Paleotemperature history of the Cenozoic and the initiation of
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Initial Reports of the Deep Sea Drilling Project, Vol. 29, Washington D.C., US Government
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during the past 80 m.y. Geology 20, 3877-390.
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