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Used to measure rates of processes in the ocean: Rates of removal of reactive chemical species

Radionuclides (geochronometers and tracers). Used to measure rates of processes in the ocean: Rates of removal of reactive chemical species Air-sea exchange Particle scavenging Rate of sediment accumulation

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Used to measure rates of processes in the ocean: Rates of removal of reactive chemical species

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  1. Radionuclides (geochronometers and tracers) • Used to measure rates of processes in the ocean: • Rates of removal of reactive chemical species • Air-sea exchange • Particle scavenging • Rate of sediment accumulation • Growth rates of authigenic deposits and marine organisms (e.g. Mn nodules, coral skeletons, shells) • Rates of sediment mixing by benthic organisms • Mixing rates in water & water mass tracing • Aging of organic matter

  2. Types of natural radionuclides in the environment • Primordial - Present since Earth’s formation (long lived nuclides) • Cosmogenic - Formed by cosmic rays in the atmosphere • Anthropogenic - Man made (nuclear • reactors, bombs etc.)

  3. Less than 21 kg of 3H on the entire Earth – and this can be measured in a few liters of water!

  4. Nuclear decay • - results in change in the neutron/proton ratio • - decay results from thermodynamic instability of the nucleus and is an attempt to reach the most stable nuclear configuration

  5. Different modes of nuclear decay Alpha decay () of larger nuclides - loss of a helium nucleus (42He) to lower neutron/proton ratio. Mass and element changes. 23892U --> 23490Th + 42He + Q (radiation e.g. gamma rays) Beta decay (-) - converts a neutron to a proton with emission of a high energy electron (e-) - for atoms with extra neutrons 146C --> 147N + e-note increase in # protons, changes element, but not mass Electron capture - proton in nucleus grabs an electron from lowest orbital and combines to form a neutron. To fill empty orbital, another e- falls to lower energy level, emitting X-rays. 4019K --> 4018Ar note decrease in # protons, changes element but not mass Ion or mineral Inert gas

  6. Life is short - and then you decay 234Th 42He + Alpha decay t1/2 = 24.1 days Daughter nuclide I’m bored, I don’t want to be uranium any more 238U t1/2 = 4.5 billion years Parent nuclide

  7. α decay β decay All primordial series end with stable (non-radioactive) form of lead (Pb)

  8. Characteristics of Uranium and Thorium Series - Primordial Nuclides

  9. Each time a nucleus decays it is an “event” or disintegration. • Detection of radioactivity • ionization detector- energy windows (each nuclide decay emits a characteristic energy spectrum (e.g. photons of gamma radiation) and can be distinguished from another) • Fission tracks • Scintillation counting (uses chemicals to absorb radiation energy, leading to chain reactions that produce light. Light pulses are detected with high sensitivity. Again, different nuclides can be distinguished based on energy of emission. Radiation is the amount of energy emitted Radioactivityis a measure of nuclear disintegrations per unit time, often given as disintegrations per minute (dpm)

  10. Common units of radioactivity Curie = 2.22 x 1012 disintegrations per minute (dpm). A curie is defined by the amount of radioactivity in 1 gram of Radium. In practice we commonly work in millicuries (2.22 x 109 dpm), or microcuries (2.22 x 106 dpm) or just plain dpm. Becquerel - The SI unit for radioactivity 1Bq = 1 disintegration/sec (dps) So one Curie is = 3.7 x 1010 Becquerels (dps) Specific activity – the amount of radioactivity per mole of substance, e.g., mCi/mmol or dpm/pmol

  11. 238U is the most abundant radionuclide in seawater • ~3 g liter-1 mainlyas uranyl tricarbonate [UO2(CO3)3]-4 which has uranium in the oxidized form U(+VI). [Uranium] is conservative with salinity. • 238U dpm/liter = 0.07081 x salinity Uranium (+VI) can be reduced by microbes under anoxic conditions, adding 2 e- and producing U(+IV). This form is insoluble and precipitates. Iron reducing bacteria can carry out this reduction (much interest in this). At salinity = 35, 238U activity = 2.48 dpm/Liter of seawater • In seawater • 238U 99.28% of total U based on the # of atoms • 235U 0.72% • 234U 0.0055%

  12. For nuclides in solution, the chemical concentration (N/liter) is directly proportional to radioactivity per liter (N/liter)since: The absolute concentrations of many nuclides in seawater is very low, and not easily measured by chemical means. But, their radioactivity can be measured!

  13. Emerson & Hedges, 2008 Chap 5

  14. Secular equilibrium (daughter-parent relationships) For a parent nuclide (P) with a long half life relative to its daughter nuclide (D), the activity of the Parent is given by: dP/dt = P[P] and this is the production rate of the Daughter (since daughter is short lived, its existence depends on its production from parent). The rate of change of the Daughter nuclide is determined by its production and loss dD/dt = P[P] - D[D] rate of change = Production - Loss(by radioactive decay) At steady state: dD/dt = 0 = P[P] - D[D] P[P] = D[D] orAp = AD or AD/ AP = 1

  15. Parent activity is constant with time since very few atoms decay (because of long half life) Thus, for nuclides with short-lived daughters and long lived parents, one predicts that the daughter/parent activity ratio ( AD/ AP)= 1. This situation is termed secular equilibrium. For a system starting out with parent nuclide, but no daughter, ADwill grow into the system. In other words, it takes time to reach secular equilibrium. 2 It takes about 6-8 daughter half-lives to reach secular equilibrium Total activity (parent+daughter) Parent activity 1 Activity Daughter activity becomes constant with time because Production = Loss In-growth of daughter activity 0 The activity of the daughter is supported by the parent Time

  16. Changing the decay constant for daughter will change ND but not λND (if λ goes up, ND goes down, and vice versa) The rate of flow into the daughter tank λNP is equal to the flow out λND Emerson & Hedges Chap 5

  17. Deviations from secular equilibrium If all nuclides were in secular equilibrium we couldn’t learn anything from them! The deviations from equilibrium are the basis for using the nuclides as tracers and chronometers! 234Th activity in the water column is often less than its parent 238U because of scavenging which removes the daughter

  18. Any process that adds or removes daughter nuclide will cause deviations from secular equilibrium. The deviations from secular equilibrium can be used to estimate the scavenging rate constant (particle removal rate constant) in the ocean water column (see steady state box model calculations as used in Coale and Bruland, 1987 L&O 32: 189). Production of daughter Loss of daughter Loss by rad decay Other first order loss (e.g. scavenging) d[D]/dt = P[P] - {D[D] + k[D]} The “k” here would be the scavenging rate constant (the fraction of particles exported from the surface ocean to depth per unit time. Something very useful to know.

  19. Which nuclide to use? Must use a nuclide with a half-life close to the rate of the process of interest. Nuclides with short half lifes can only be used to study fast processes. Long-lived nuclides cannot be used to study fast processes (too few decays over short time), and only are useful for slow processes. This matching of decay rate to process rate applies to radio-dating (aging) as well. oceanic coastal y axis 234Th (t1/2 = 24 d) is useful for water column particle scavenging rates x axis

  20. Coale & Bruland, L&O 1987 – Application of 234Th scavenging Expected Mixed layer pycnocline Euphotic Shaded area is deficit of 234Th due to scavening • Maximum scavenging near pigment maximum • Less scavenging in upper mixed layer (due to efficient recycling of particles & biomass) 234Th is particle reactive so most is rapidly adsorbed to particles. If particles sink quickly, then have deficit of 234Th

  21. 234Th-derived Moran et al., 2003. Limnol. Oceanogr. 48: 1018

  22. Radio-dating of materials with nuclides Useful for determining the age of a particular piece of matter (organism, fossil, rock etc). By obtaining an age for a piece of an accreting deposit (e.g. sediment, coral skeleton, clam shell, Mn-nodule) at some depth into the deposit, the accretion rates of deposit can be determined (assuming steady deposition). Depth z If you can put an age on the sediment in this layer, you therefore know how long it took to build up the sediment above it. From the depth of the layer and its age (t), you can determine the sediment accretion rate (z/t). Sediment core

  23. AD-excess (unsupported daughter) 0 Depth (cm) Exponential decay of excess activity with depth How to determine ages in deposits? Use unsupported nuclide activities If deposits are laid down with unsupported daughter activity, and no additional inputs (other than supported activity) occur within the deposit, then the unsupported (excess) activity will decay with time (=depth) into the deposit. Sediment is a good example. The sedimentation or accretion rate is given by: s= z/t Thus, t = z/s or t = z/s Sediment-water interface 20 Substitute z/s for t in decay law

  24. ADz = ADo e-λ(z/s) • Where ADz is the unsupported activity at depth z and ADo is the unsupported activity at the surface of the deposit. This can be rearranged to: • ADz/ADo = e-λ(z/s) • And linearized as: • ln(ADz) – ln(ADo) = -λ(z/s) which is the same as: • ln(ADz) = ln(ADo) - λ(z/s) and the same as: • ln(ADz) = ln(ADo) – (λ/s) z slope Y-intercept X-coordinate

  25. 210Pb (t1/2 = 22.3 y) is often used to estimate sediment accretion rates in coastal areas where sedimentation is high 210Pb is derived from decay of gaseous 222Rn (t1/2 = 3.8 days) which originates in rocks on land but goes into the atmosphere where it is carried over water. 210Pb produced in the atmosphere is rapidly rained out and it attaches to particles in the water which sink to the sediments. This leads to unsupported 210Pb activity at the surface of the sediment (activity ratio of 210Pb(daughter)/226Ra(parent) > 1). This is also referred to as excessactivity because it is in excess of what is supported by the secular equilibrium of the sedimentary 226Ra parent.

  26. Linear slope (a) of the semi-log plot gives the sediment accretion rate. If slope not linear – steady state sedimentation model does not apply Concentrations of unsupported210Pb in sediments – can give estimate of sediment accretion rates Log scale Fig. 10.7 in Pilson

  27. This figure focuses on the longer lived nuclide 230Th (t1/2 = 75,200 y). Its chemistry (i.e. particle reactivity) is the same as 234Th, but its decay is too slow to be useful for particle scavenging rates in the surface waters. It is, however, useful for sediment accretion rates in the deep ocean where accretion rates are relatively slow. (Bioturbated) Excess Supported 230 Th activity (from 234U decay)

  28. Use of nuclides as event markers 137Cs Core taken in 1986 Picocuries per section Peak emissions of man-made 137Cs (t1/2 = ~30 y) into the atmosphere occurred in 1963. This particle reactive nuclide is scavenged to sediments, where profiles reflect time inputs. Depth above 137Cs peak has accreted since 1963. ~10 cm per 23 y 137Cs first appeared in atmosphere in ~1953 Wetland sediments DeLaune et al 1989

  29. Man-made 14C Produced from weapons testing – peak production in 1960’s Increased atmospheric 14C by over 2x – slowly taken up by ocean Natural 14C- a cosmogenic nuclide • Produced in the upper atmosphere by spallation of 14N • Becomes 14CO2 in atmosphere • Dissolves in ocean and taken up by plants • Diluted by fossil fuel burning of low 14C carbon (Suess effect) (Illustration by Jayne Doucette, Woods Hole Oceanographic Institution) http://www.whoi.edu/nosams/page.do?pid=40138

  30. Applications of 14C dating Much progress with introduction of accelerator mass spectrometer analysis – 14C content of micro- to milligram quantities of carbon can be determined. • Invasion of atmospheric CO2 into ocean can be observed • DIC of ocean water can be aged – giving estimate of deep residence time. • POC and DOC in seawater have been aged – DOC found to be old. • Bacteria in surface ocean use a mixture of old and new carbon 14C-content of natural materials (mg quantities) can be measured at the National Ocean Sciences Accelerator Mass Spectrometry (NOSAMS) Facility at Woods Hole - http://www.whoi.edu/nosams/

  31. 14C-ages for compounds containing carbon • At the time of carbon fixation (photosynthesis) some 14C will be incorporated into organic matter, based on the amount of 14C in the atmosphere (or seawater) at the time of fixation. • Once an organism dies, no replacement of 14C occurs, therefore the 14C radioactivity can only decrease, due to decay. • Since the decay rate of 14C is known (1.209 x 10-4 y-1), the deficit of 14C activity can tell us how much time has elapsed since that organic matter was alive.

  32. From Bauer & Bianchi, 2011. Dissolved organic carbon cycling and transformation. In: A treatise on Estuarine and Coastal Science. Vol 5: 7-67

  33. (14C/C)sample - (14C/C)std x 1000 - IF 14C = Fractionation factor (a small correction) (14C/C)std A zero value for Δ14C represents the 14C content of preindustrial atmosphere From Bauer & Bianchi, 2011. Dissolved organic carbon cycling and transformation. In: A treatise on Estuarine and Coastal Science. Vol 5: 7-67

  34. WOCE = World Ocean Circulation Experiment

  35. Deep DOC ~5900 years old Deep DOC ~4100 years old

  36. Belize and Florida corals Galapagos corals (upwelling area)

  37. Bacterioplankton use mainly recently fixed carbon – but in the open ocean, some older carbon from DOC is utilized also. Estuarine Site Oceanic Site From Cherrier et al, 2000

  38. End

  39. Fraction Modern S=14C/12C Sample B= 14C/12C Blank M = 14C/12C Modern reference Fm is corrected to that of -25 o/ooδ13C Where lambda is 1/(tru mean-life) of radiocarbon = 1/8267 = 0.00012097Yc is year of collection. Age = -8033 ln (Fm13C corr)

  40. Residence time =1/k Large uncertainty in residence time or k Large uncertainty in residence time or k From Coale & Bruland, 1987

  41. Primordial decay series (three major parent nuclides) • see Fig 10.2 in Pilson for decay chain and half lives of 238U series Stable end product Parent Daughters 238U -> 234Th ->->234U -> 230Th -> 226Ra -> 222Rn ->….210Pb->-> 206Pb 232Th -> 228Ra ->-> 228Th -> 224Ra -> 220Rn -> 216Po -> …208Pb 235U -> 231Th -> 231Pa ->-> 227Th -> 223Ra -> 219Rn -> …-> 207Pb All primordial series end with stable (non-radioactive) form of lead (Pb)

  42. In seawater • 238U 99.28% of total U based on the # of atoms • 235U 0.72% • 234U 0.0055% • Although the atom ratio of 238U/235U is 140, the activity ratio is only 21.7 because 235U has a much shorter half-life than 238U (so a greater fraction of the 238U atoms are undergoing decay at any time)

  43. This figure shows two shallow north-south sections from approximately the same area of the East Pacific depicting the C-14 concentrations, measured 20 years apart. Clearly visible is the evolution of the "bomb" C-14 signal (yellow-red) since the end of the nuclear bomb tests (top) to the present (top plot), especially at intermediate and high latitudes 1974 1992 http://www.nosams.whoi.edu/woce/wocegeos.html

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