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 Literature: Frank, 2002, Reviews in Geophysics Goldstein & Hemming, 2003, Treatise on Geochemistry http://www.ldeo.columbia.edu/~spk/Classes/G9802_PaleoProxies/paleo.html

Global thermohaline circulation The ocean is one of the main players of the climate system and affects climate in multiple ways. 71 % of the Earth surface are covered by the ocean and therefore most of the solar radiation received at the Earth’s surface goes into the ocean. The ocean does not only store this heat, but also redistributes it Before it is released again to the atmosphere. Additionally, the ocean can influence climate by its sea ice cover and by Being part of the biogeochemical cycles and exchanging gases with the atmosphere. Shown here is a simplified cartoon of the global thermohaline circulation, also called the “global conveyor belt” after Wally Broecker, which is responsible for the general overturning of the ocean, from the top to the bottom. Near surface waters (red lines) flow towards the main areas of deep water formation and recirculate at depth. While it is still argued , whether the thermohaline circulation can drive climate change, it is clear that it can act as an important amplifier. 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).

Deep water formation at high latitudes = two main sites of deep water formation = additional deep water formation The driving force for the thermohaline circulation is deep water formation, which happens at present day in the North Atlantic and in the Southern Ocean. From the North Atlantic, the newly formed North Atlantic deep water is transported southward, into the Southern Ocean, and the deep Pacific. In the Pacific upwelling happens and surface waters are then carried back to the Atlantic via the Indian Ocean (warm water route) and across the Drake Passage (cold water route) and the cycle begins again. Simplified on the basis of the two-layer conveyor belt model of Schmitz (1995). Modified after Frank (2002).

T-S curve at 9°S, Atlantic Ocean The temperature-salinity properties of Atlantic water masses can be nicely observed at a water profile at 9 degrees south. Plotted on the x-axis is the salinity of the water, and on the y-axis the temperature. North Atlantic water is very saline, and that is because a lot of salt is transported to the North Atlantic from the subtropics by the Gulf Stream. Antarctic Bottom water on the other hand is not that salty, but colder than North Atlantic deep water. Both water masses are dense enough to form the deepest waters in the area’s where they are build. Shown is also the field for Antarctic Intermediate Water and you can see that this water mass has lower salinity and temperature than the other two and hence the water is not dense enough to form a deep water mass. Open University Ocean Circulation Text Book

Circulation of the Atlantic Ocean The observations from the conservative water mass tracers salinity and temperature and others translate in a simplified circulation pattern in the Atlantic Ocean as outlined in this diagram, which is adapted from Arnold Gordon. What you can see is a profile through the Atlantic basin from 80°N to 60°S. Simplified we have surface and Antarctic intermediate waters moving northward, North Atlantic deep water being build in the North Atlantic and moving southwards, and Antarctic Bottom Water being build in the south and moving northwards. All these water masses are at present day easy to identify because they have very distinct temperatures, salinity and other physical properties. If we, however, want to look at ocean circulation patterns back in the past and in order to understand the interaction of ocean circulation and climate we need other proxy records, since conservative water mass properties such as temperature and salinity are not preserved in the geological record. NADW = North Atlantic Deep Water AAIW = Antarctic Intermediate Water AABW = Antarctic Bottom Water

a promising tracer for ocean circulation 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

Lanthanides The name neodymium is derived from the 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. Lanthanides

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 Before going into more detail, let’s do a small excursion to refresh your knowledge in radiogenic isotopes. What are radiogenic isotopes? Radiogenic isotopes are the stable products of the natural decay of radioactive parent nuclides. For example 147Sm decays to 143Nd by a decay with a half-life of 106 billion years. 143Nd is therefore a radiogenic isotope, the amount of which increases in a rock with time. the amount of 143Nd in a rock increases with time

(143Nd/144Nd)measured - (143Nd/144Nd)CHUR 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) parent daughter half-life geological tracer 147Sm 143Nd 106 Ga 143Nd/144Nd 16Lu 176Hf 36 Ga 176Hf/177Hf 238U 206Pb 4.5 Ga 206Pb/204Pb 235U 207Pb 0.7 Ga 207Pb/204Pb Instead of using this radioactive clock to date rocks we can also use it to identify certain rock types. If we normalize the radiogenic isotopes to another non-radiogenically build stable isotope of the same element, we end up with a ratio (e.g 143/144), which is very different for different rock types. To trace rock types and ultimately geological processes radiogenic isotopes are normalized to non-radiogenic isotopes of the same element. 144Nd is such a non-radiogenic isotope of Nd, the amount of which does not change with time. Therefore the ratios of 143Nd to 144Nd increases with time, since 143 is continuously build by the decay of Sm. Besides the age, however, the composition of the rock is important, means how much Sm there is initially (if there is not a lot, there is not a lot which can decay) and this varies between different rock types in the earth crust and mantle. In summary the ratios shown here for different isotope systems of Pb, Nd, and Hf are different for different rocks. In some of the following slides you will see Nd or Hf isotopes expressed in e units. This is just a way to make the numbers easier to read, because the 143/144 and 176/177 sometimes only show deviations in the 5th digit. 232Th 208Pb 14 Ga 208Pb/204Pb (143Nd/144Nd)measured - (143Nd/144Nd)CHUR e Nd = * 104 (143Nd/144Nd)CHUR

Why are Radiogenic Isotopes of Dissolved Trace Metals Useful in Paleoceanography ? Impact of hydrothermal inputs: • Nd isotopes: no effect Impact of eolian inputs: • distinct isotopic compo- sition of the source area is delivered to the ocean Impact of riverine inputs: • different lithologies and ages of source rocks result in different radiogenic isotope compositions

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

Input Pathways and Water Mass Mixing Nd isotopes trace water mass mixing Nd isotopes trace provenance Let‘s start with some basics on dissolved radiogenic isotopes in the ocean, and why we think they are useful tracers in paleoceanography. Well, first of all, we know that dissolved trace metals in the oceans are derived from three main sources, which are riverine inputs from the continents, eolian dust transported by winds and hydrothermal activity. For Nd, we can cancel out hydrothermal contribution, since Nd is scavenged very efficiently at hydrothermal vent sites. Instead, boundary exchange, meaning additon of Nd to the water column from interatction with the ocean‘s margin seems to be important in some areas. All of these inputs do implement a typical Nd isotopic signature to the water column, and therefore dissolved Nd isotopes can fingerprint the provenance of material. However, the Nd isotopic composition of a particular water mass is not only dependent on its input sources but also whether it has had exchange with another water mass, which may alter its isotopic composition. This is, however, only true if the residence time for the element of interest is in the order of or shorter than the global turn-over time of the ocean. The residence time of Nd is between 500 and 1000 years and is therefore perfectly suited to apply Nd isotopes as circulation and water mass tracer. Residence Time of Nd: 500 – 1000 yrs = the time for which dissolved species stay in the water column before they are scavenged / precipitated ocean inventory flux into the ocean t =

 Two Water Mass Tracers Antarctic Intermediate Water (AAIW) North Atlantic Deep Water (NADW)  Antarctic Intermediate Water (AAIW) Bottom Water (AABW) von Blanckenburg (1999) Nd isotope profiles match the present-day salinity distribution in the Atlantic Ocean. (von Blanckenburg, 1999, Science)

Goldstein and Hemming (2003) 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

Water Profile Stations Only (at least 3 points) Global Seawater Stations for Nd Isotopes

Archives Ferromanganese Crusts precipitates of ambient seawater grow only a few mm per Myr  archives of past seawater isotope composition

Nd Isotope Composition of Seafloor Mn-Fe Deposits (Deep and Bottom Water) -20 -8 -13 -11 -7 -4 -8 Albarède and Goldstein, 1992

Extraction of Nd From Foraminiferal Shells Extraction of Nd From The Nd isotopic composition of planktic foraminifera reflects the surface seawater composition. (e.g., Vance et al., 2004, Paleoceanography) 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)

Extraction of Nd From Bulk Sediments Extraction of Nd From 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

tracing the closure of the Indonesian Gateway and 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) The second case study is addressing what happens to the deep water isotope composition at the onset of the Northern Hemisphere glaciation. In other words: Is the associated change in the style of weathering from normal chemical weathering to predominantly physical weathering manifested in the deep water record.

Latitudinal Variability in the Pacific Ocean What happened in the SW Pacific around 10 Ma ? 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)

Major paleogeographic changes of the past 60 Myr Acutally around that time the Indonesian Gateway, which was allowing deep water communication between the Pacific and Indian Ocean closed for deep waters. This is because the Australian plate continuously drifts northwards as indicated by this red arrow, and hence leads to a merging of the plate fragements in the Indonesian area. One effect is for example the large orogeny on Papua New Guinea. Closure of the Indonesian Gateway for deep water: ~ 10 Ma (middle Miocene) (Frank, 2002)

(open in the early Miocene)  from 10 Ma onwards: increased „Pacific-like“ signature due to increased southward deflection of water Indonesian Gateway between the Pacific and Indian Oceans closed around 10 Ma (closed around 10Ma) Indonesian Gateway (open in the early Miocene) Tasman Basin Record

(2) The First High-Resolution Nd Record 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) The second case study is addressing what happens to the deep water isotope composition at the onset of the Northern Hemisphere glaciation. In other words: Is the associated change in the style of weathering from normal chemical weathering to predominantly physical weathering manifested in the deep water record.

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

Nd isotopes NADW AABW  weaker NADW during full glacials and during cold stadial events  stronger NADW during warm periods The motivation for this study is as old as is the interest in Nd isotopes as a tracer for past ocean circulation. This diagram shows the very first high resolution record of authigenic Nd isotopes over the past glacial cycle produced by Alex Piotrowski, and co-workers here at Lamont. The record is based on the extraction of seawater-derived authigenic Nd from bulk sediments from the Cape Basin. Nd isotopes in red and carbon isotopes in blue and black show a remarkable close correspondence, which has been interpreted to reflect waxing and waning amounts of North Atlantic Deep Water in the South Atlantic. For example, the low Nd isotopic compositions in the Holocene are interpreted to reflect large contributions of NADW, and high values at the Last Glacial Maximum are interpreted to reflect reduced amounts of NADW in the Cape Basin. The validity of this interpretation however heavily depends on the assumption of constant end-member compositions. Therefore one of the key concerns in applying Nd isotopes to unravel the intensity of global overturning circulation is whether the Nd isotopic composition of NADW has changed on glacial-interglacial time-scale and this is the question I will try to answer today. Piotrowski et al., 2005, Science Piotrowski et al., 2005, Science

Deglacial Record Large deglacial changes with Piotrowski et al., 2004, EPSL intensification of NADW from 17-18 kyr onwards millennial excursions seem to be linked to changes in sea ice cover in the North Atlantic Large deglacial changes with millennial-scale variability are superimposed on the long-term trend.

(3) Quantifying and Identifying Nd Fluxes in the North Pacific – 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) The second case study is addressing what happens to the deep water isotope composition at the onset of the Northern Hemisphere glaciation. In other words: Is the associated change in the style of weathering from normal chemical weathering to predominantly physical weathering manifested in the deep water record.

Dust Input From Asia – Important for the Nd budget ? Importance of North Pacific Island Arcs modified AABW Importance of Circum Pacific Island Arcs ?

Simple Box Model for the North Pacific (van de Flierdt et al., 2004a) OUTPUTS: (1) FNPDW = flux of Nd in deep water leaving the N-Pacific eNPDW = Nd isotope compo- sition of North Pacific Deep Water (NPDW) (1) INPUTS: (3) FAABW* = advected flux of Nd eAABW* = Nd isotope composition of modified Antarctic Bottom Water (AABW*) (3) g x Fdust x e dust Farc x e arc INPUTS: (1) = fraction of dust that dissolves in seawater Fdust = dust-derived Nd-flux edust = Nd isotope composition of dust (1) OUTPUTS (2) Fscav = flux of Nd scavenged out of the water column eNPDW = Nd isotope composition of NPDW (2) INPUTS: (2) Farc = arc-derived Nd-flux earc = Nd isotope composition of arcs (2) Assuming steady state Farc can be modelled as a function of the dust dissolution rate g . g Fdust edust + Farc earc + FAABW* eAABW* = FNPDW eNPDW + Fscav eNPDW FAABW* x e AABW* North Pacific FNPDW x e NPDW Inventory: ~ 1.3 x 1012 g Nd Fscav x e NPDW g Fdust edust + Farc earc + FAABW* eAABW* = FNPDW eNPDW + Fscav eNPDW g Fdust + Farc + FAABW* = FNPDW + Fscav Fscav = g Fdust + Farc

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

Lacan and Jeandel, 2005, EPSL „[...] modeling studies should help better understanding the precise processes involved and the global significance [...] of boundary exchange.“

Questions ? I could give you many more examples about how we can use radiogenic isotopes to trace different processes in the past ocean, but I guess I better stop here to allow some time for questions…