Stable Isotope Tracers

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2 Outline Stable Isotopes - Introduction & Notation Isotope Fractionation Some Oceanographic Applications

3 Stable Isotope Tracers Determine sources/sinks and pathways of nutrients and chemicals in the ocean Tracers of biological, physical, geological ocean processes Record past changes in physical, chemical, and biological processes in the ocean

4 Uses of Stable Isotopes in Oceanography Most commonly studied: 2 H 3 He 13 C 15 N 18 O 34 S Food chain studies Horizontal mixing (water mass tracers) Paleooceanography Pulse/chase experiments to determine metabolic rates and pathways (labeled tracers)

5 Isotopes of Elements The chemical characteristic of an element is determined by the number of protons in its nucleus. Atomic Number (Z) = number of protons = defines the chemistry Atomic Mass (N) = protons + neutrons Isotopes = atoms with same Z but different N Abbreviated: 12 C, 13 C, 14 C or C-12, C-13, C-14

6 The chart of the nuclides (protons versus neutrons) for elements 1 (Hydrogen) through 12 (Magnesium) Valley of Stability Most elements have more than one stable isotope (shaded) 1:1 line

7 Examples: H, He, C, N and O Element Symbol Protons Neutrons % Abundance Half-life Hydrogen H D ( 2 H) T( 3 H) to τ1/2 = y Helium 3 He He Carbon 12 C C C τ1/2 = 5730 y Nitrogen 14 N N Oxygen 16 O O O % abundance is for the average Earth s crust, ocean and atmosphere All isotopes of a given element have the same chemical properties, yet there are small differences due to the fact that heavier isotopes typically form stronger bonds and diffuse slightly slower

8 The Delta Notation Absolute isotope ratios are measured for sample and standard, and the relative measure delta is calculated: ( C/ C) Sample ( C/ C) 13 Standard C ( C/ C) Standard = per mil = 10 3 o oo Negative delta values are depleted in 13 C ( light ) Positive delta values are enriched in 13 C ( heavy ) Example: δ 13 C = = -5

9 Each isotopic measurement is reported relative to a standard: Element δ value Ratio Standard Hydrogen δd 2 H/ 1 H Standard Mean Ocean Water (SMOW) Standard Light Antarctic Precipitation (SLAP) Helium δ 3 He 3 He/ 4 He Atmospheric He Boron δ 11 B 11 B/ 10 B NIST SRM 951 Carbon δ 13 C 13 C/ 12 C Pee Dee Belemnite (PDB) Nitrogen δ 15 N 15 N/ 14 N Atmospheric N2 Standard Mean Ocean Water (SMOW) Oxygen δ 18 O 18 O/ 16 O Standard Light Antarctic Precipitation (SLAP) Pee Dee Belemnite (PDB) δ 17 O 17 O/ 16 O Standard Mean Ocean Water (SMOW) Sulfur δ 34 S 34 S/ 32 S Canyon Diablo Troilite (CDT)

11 Isotopic Fractionation Fractionation occurs in reactions that do not go to completion: Lighter isotopes reacts faster, and to a greater extent Reaction products are enriched in the lighter isotopes Reservoir of reactants is depleted in the lighter isotopes Note: There is no fractionation if a reaction goes to completion (i.e., all of the reactants are consumed) Fractionation Factor (α): αa-b = RA / RB where RA and RB are isotope ratios in materials A and B

12 Kinetic Fractionation All isotope effects involving organic matter are kinetic. Example: 12 CO 2 + H 2 O 12 CH 2 O + O 2 faster 13 CO 2 + H 2 O 13 CH 2 O + O 2 slower Thus organic matter gets depleted in 13 C during photosynthesis (i.e., δ 13 C becomes more negative) Similarly, organic matter gets enriched in 13 C during respiration (i.e., δ 13 C becomes more positive) 12 CH 2 O + O 2 12 CO 2 + H 2 O faster 13 CH 2 O + O 2 13 CO 2 + H 2 O slower Terrestrial organic matter: ca. -19 (range -26 to -7 ) Marine organic matter: ca. -14 (range -22 to -8 )

13 Stable Isotope Applications in Oceanography 3 He to study deep ocean circulation in the Pacific 18 O to determine freshwater balance in the Arctic Ocean 18 O as an indicator of the ice ages

14 Stable Isotope Applications in Oceanography 3 He to study deep ocean circulation in the Pacific 18 O to determine freshwater balance in the Arctic Ocean 18 O as an indicator of the ice ages

15 3 He Plume from East Pacific Rise Broecker and Peng, 1982

16 3 He at 2500 m depth

17 3 He Plume from Loihi Seamount (Hawaii) Lupton, 1996

18 Stable Isotope Applications in Oceanography 3 He to study deep ocean circulation in the Pacific 18 O to determine freshwater balance in the Arctic Ocean 18 O as an indicator of the ice ages

19 Freshwater balance in the Arctic Ocean Balances of mass, salt, δ 18 O, and nutrients allow us to separate the contributions of the individual freshwater sources Schlosser et al., 1999 Ekwurzel et al., 2001

20 Freshwater balance in the Arctic Ocean δ18o Pacific Water River Runoff Sea-ice Meltwater from P. Schlosser

21 Freshwater balance in the Arctic Ocean from P. Schlosser

22 Freshwater balance in the Arctic Ocean Pacific Water from P. Schlosser

23 Freshwater balance in the Arctic Ocean from P. Schlosser

24 Freshwater balance in the Arctic Ocean from P. Schlosser

25 Stable Isotope Applications in Oceanography 3 He to study deep ocean circulation in the Pacific 18 O to determine freshwater balance in the Arctic Ocean 18 O as an indicator of the ice ages

26 Oceanic 18 O-H 2 O ( 18 O) 18 O follows the water molecules (not O 2!) Thus, excellent stable, conservative (SC) water mass tracer In seawater: 16 O (99.8%) 17 O (0.037%) 18 O (0.204%) H 2 16 O / H 2 18 O 500 / 1

27 Evaporation of H 2 16 O favored over heavier isotopes of water During ice ages (net evaporation) O isotope balance in ocean changes Can use 18 O to get ice volume

Rayleigh Distillation Fractionation occurs when water molecules evaporate from sea surface. Equilibrium effect when water molecules condense from vapor to liquid form (rain is heavier than vapor).

28 Rayleigh Distillation Fractionation occurs when water molecules evaporate from sea surface. Equilibrium effect when water molecules condense from vapor to liquid form (rain is heavier than vapor). Vapor becomes progressively lighter (i.e., δd and δ 18 O get lower) with distance from source. Evaporation from ocean creates depleted clouds. Air masses transported to higher latitude/altitude where it is cooler. Water lost due to rain; raindrops are enriched in 18 O relative to cloud. Cloud gets lighter

29 δ 18 O in Average Rain vs. Temperature

Gridded Surface Seawater δ 18 O Note the higher δ 18 O sw in the evaporation belts and the lower δ 18 O sw in the high latitudes, which are dominated by excess precipitation.

30 Gridded Surface Seawater δ 18 O Note the higher δ 18 O sw in the evaporation belts and the lower δ 18 O sw in the high latitudes, which are dominated by excess precipitation. 18 O Any δ 18 O- temperature relationship depends primarily on the δ 18 O of the water from which the carbonate is precipitated. Salinity

water lighter δ 18 O in CaCO 3 2) δ 18 O of seawater that

31 δ 18 O in Marine CaCO3 The δ 18 O of the CaCO 3 is a function of: 1) Temp of seawater that foraminifera are growing in: Warmer water lighter δ 18 O in CaCO 3 2) δ 18 O of seawater that foraminifera are growing in: Depends on latitude Depends on global ice volume

32 δ 18 O Paleothermometer 28 G. menardii less 18 O Temperature ( o C) N. pachyderma (d.) G. sacculifer O. universa G. ruber (pink) 12 more 18 O 8 1 G. bulloides e.g. Bemis et al., Paleoceanography, O shell water ( ) Temperature dependence of equilibrium fractionation between 16 O and 18 O during precipitation of CaCO 3

33 high 8 O low 18 O Changes in ice volume also influence δ 18 O of the ocean

34 δ 18 O in marine carbonates and paleotemperature records Vostok ice core Warmer/ less ice Colder/ more ice

35 IPCC Fourth Assessment Report, Working Group I Report "The Physical Science Basis", Chapter 6 Palaeoclimate

36 δ 18 O as Indicator of Climate for the Past 65 Million Years Stacked deep-sea benthic foraminiferal oxygen-isotope curve. The δ18o temperature scale, on the right axis, was computed on the assumption of an ice-free ocean; it therefore applies only to the time preceding the onset of large-scale glaciation on Antarctica (about 35 million years ago). Zachos et al., 2008

Stable Isotope Tracers OCN 623 Chemical Oceanography

Stable Isotope Tracers OCN 623 Chemical Oceanography 21 March 2017 Reading: Emerson and Hedges, Chapter 5, p.134-153 2017 Frank Sansone and David Ho Student Learning Outcomes At the completion of this