P1250095217NvdPf - PowerPoint PPT Presentation

Title: P1250095217NvdPf


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Systematics of Neodymium Isotopes and their
Application to Paleoceanography
EESC G9802, Spring 2006 Toward an improved
understanding of paleoceanographic proxies
combining models with data
Samar Khatiwala, Tina van de Flierdt, Sidney
Hemming
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Global thermohaline circulation
Simplified on the basis of the two-layer conveyor
belt model of Schmitz (1995). Modified after
Frank (2002). Original global conveyor belt by
Brocker (1991).
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Deep water formation at high latitudes
two main sites of deep water formation
additional deep water formation
Simplified on the basis of the two-layer conveyor
belt model of Schmitz (1995). Modified after
Frank (2002).
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T-S curve at 9S, Atlantic Ocean
Open University Ocean Circulation Text Book
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Circulation of the Atlantic Ocean
NADW North Atlantic Deep Water AAIW Antarctic
Intermediate Water AABW Antarctic Bottom Water
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Reconstruction of past deep water circulation
patterns requires the use of proxy records, since
conservative water mass properties such as
temperature and salinity are not preserved.

• neodymium isotopes -
• a promising tracer for ocean circulation

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Neodymium was first separated in 1885 from a
material known as didymium.
The name neodymium is derived from the Greek
words neos new and didymos twin.
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What are Radiogenic Isotopes ?
stable products of the natural decay of
radioactive parent nuclides
?for example 147Sm decays to 143Nd by a decay
147Sm ? 143Nd a Q (half-life 106 billion
years)
?143Nd is therefore a radiogenic isotope
?the amount of 143Nd in a rock increases with
time
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?radiogenic isotopes are normalized to
non-radiogenic isotopes of the same element
(e.g. 143Nd to 144Nd) ?143Nd/ 144Nd varies in
natural rocks dependent on their age and their
composition (lithology)
geological tracer
parent
daughter
half-life
143Nd/144Nd
147Sm
143Nd
106 Ga
176Hf/177Hf
16Lu
176Hf
36 Ga
206Pb/204Pb
238U
206Pb
4.5 Ga
207Pb/204Pb
235U
207Pb
0.7 Ga
208Pb/204Pb
232Th
208Pb
14 Ga
(143Nd/144Nd)measured - (143Nd/144Nd)CHUR
e Nd
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(143Nd/144Nd)CHUR
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Why are Radiogenic Isotopes of Dissolved Trace
Metals Useful in Paleoceanography ?
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Boundary Exchange
Lacan and Jeandel, 2005, EPSL Tachikawa et al.,
2003, JGR
continental margins can be sources and sinks for
elements in the ocean by means of particulate /
dissolved exchange processes
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Input Pathways and Water Mass Mixing

• Nd isotopes
• trace provenance

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Two Water Mass Tracers
von Blanckenburg (1999)
Nd isotope profiles match the present-day
salinity distribution in the Atlantic Ocean.
(von Blanckenburg, 1999, Science)
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Nd Isotopes vs. Silicate in Atlantic Deep Waters
Goldstein and Hemming (2003)
? remarkable co-variation between dissolved eNd
and SiO2
? Nd isotopes quasi conservative water mass
tracer
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Global Seawater Stations for Nd Isotopes
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Archives
Ferromanganese Crusts

• precipitates of
• ambient seawater

• grow only a few
• mm per Myr

? archives of past seawater isotope composition
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Nd Isotope Composition of Seafloor Mn-Fe
Deposits (Deep and Bottom Water)
Albarède and Goldstein, 1992
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Ellen E. Martin/University of Florida Fossil fish
teeth, about one-fortieth of an inch long. New
York Times, January 22, 2002
Extraction of Nd From Fossil Fish Teeth
Fish teeth acquire their Nd signal post- mortem
(early diagenesis) while still in contact with
ambient bottom water. (e.g., Martin and Scher,
2004, EPSL 220)
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Extraction of Nd From Bulk Sediments
Extraction of dispersed FeMn fraction from bulk
sediments by sequential leaching allows
high-resolution records of Nd isotopes. (e.g.,
Rutberg et al., 2000 Bayon et al., 2002)
Deep-sea core repository at Lamont
Extraction of Nd From Deep-Sea Corals
Deep-sea corals offer the potential to achieve
past seawater Nd from absolutely (U-Th) dated
archives. (e.g., van de Flierdt et al.,
submitted)
Solitary coral Desmophyllum Dianthus from the New
England seamounts
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Case Studies

• tracing the closure of the
• Indonesian Gateway and
• associated changes in deep
• water circulation with Nd isotopes

Archive ferromanganese crusts (van de Flierdt
et al., 2003, Paleoceanography)
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Latitudinal Variability in the Pacific Ocean
eNd - 7

• Nd isotopes become progressively more radiogenic
  from the South to the North

• continuous dilution of
• Southern Ocean water
• with overlying Pacific
• Deep Water (PDW)

Neodymium isotopes monitor circulation patterns !
Equatorial Pacific data Ling et al. (1997) and
Lee et al. (1999)
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Major paleogeographic changes of the past 60 Myr
Closure of the Indonesian Gateway for deep
water 10 Ma (middle Miocene)
(Frank, 2002)
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Tasman Basin Record
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Case Studies
(2) The First High-Resolution Nd Record
Covering the Last Glacial Cycle
Archive Dispersed FeMn Fraction Extracted
From Bulk Sediments (Piotrowski et al., 2004,
EPSL 2005, Science)
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Study Location South Atlantic

• high
• sedimentation rate 20 cm/kyr

• high-resolution
• record possible

• water depth
• 3700 4700 m

• mixing front
• between NADW
• and AABW

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? weaker NADW during full glacials and
during cold stadial events
? stronger NADW during warm periods
Piotrowski et al., 2005, Science
Piotrowski et al., 2005, Science
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(No Transcript)
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Case Studies
(3) Quantifying and Identifying Nd Fluxes
in the North Pacific a Very Simple Box
Model
Archive ferromanganese crusts (van de Flierdt
et al., 2004, GCA)
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Dust Input From Asia Important for the Nd
budget ?
Importance of Circum Pacific Island Arcs ?
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Simple Box Model for the North Pacific (van de
Flierdt et al., 2004a)
North Pacific
Inventory 1.3 x 1012 g Nd
g Fdust edust Farc earc FAABW eAABW FNPDW
eNPDW Fscav eNPDW
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Results / Implications of the Box Model for the
Nd Budget in the North Pacific
? dust has been a minor component for the
dissolved Nd budget indicated by dissolution
rates of less than 3.4
? island arcs supply at least 4 x 108 g Nd / year
in the North Pacific
this is in the order of the global dissolved
riverine Nd flux
but no big rivers with an arc-like composition
discharge into the North Pacific ...
? supply mechanism of arc-like Nd small rivers
and / or particle seawater interaction
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Lacan and Jeandel, 2005, EPSL
... modeling studies should help better
understanding the precise processes involved
and the global significance ... of boundary
exchange.
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Questions ?