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Geochemical tracers

Geochemical tracers Tracers – talking more about theory than application. Geochemical tracers trace: Circulation Climate Environmental conditions Geochemical tracers are proxies for past environmental conditions (and trace changes): Signals in sediments or other substrates

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Geochemical tracers

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  1. Geochemical tracers • Tracers – talking more about theory than application. • Geochemical tracers trace: • Circulation • Climate • Environmental conditions • Geochemical tracers are proxies for past environmental conditions (and trace changes): • Signals in sediments or other substrates • Isotopes (composition of the sediments) • Trace metals (in fossils) • Organic compounds

  2. The record of climate events and ocean change recorded chemically • The Tools: Tracers • Geochemical tracers….. • the application • How we really do it • what are the issues

  3. How do we trace the movement of components in the ocean? • Nutrient controls on productivity. • they are trace-able… • With major components like carbon, how can we track the cycling of that? • How can we trace changes in important properties in the past?

  4. The biological pump…. How do we know it changed in the past? How do we trace carbon movement through the system?

  5. The tools Geochemical tracers! Isotopes: 13C – circulation 18O – temperature Organic matter: TOC – productivity Biomarkers – temperature (relative productivity) Anthropogenic compounds - pollution tracers Trace metals- Trace metal ratios in calcite (CaCO3) Cd/Ca (phosphate), Ba/Ca (nutrients), Mg/Ca and Sr/Ca –temperature

  6. Trace metal inclusions in CaCO3 Trace metals that are of similar size and charge as Ca substitute into the matrix….. interpretation is based on the simple principle that if there is more in the water there is more in the calcite Empirically, we have found their chemistry is controlled by: Cd/Ca (phosphate), Ba/Ca (“productivity”) Mg/Ca and Sr/Ca –temperature

  7. CaCO3 Calcium carbonate … can have impurities

  8. CaCO3 impurities are controlled by: Many environmental parameters (thermodynamics, kinetics, biology) Temperature– inclusions are more common when temperatures are higher. Nutrients – many trace metal concentrations mimic nutrient distributions

  9. Organic markers or “molecular markers” • Total organic carbon (TOC) – productivity • Biomarkers (individual compounds) • Trace organisms • Different environments and conditions – temperature (relative productivity)

  10. EUCARYA m a n HO a n i m a l s R p l a n t s f l a g e l l a t e s f u n g i m i c r o s p o r i d i a c i l i a t e s s l i m e m o l d s d i p l o m o n a d s BACTERIA ARCHAEA green sulfur bacteria S u l f o l o b u s D e s u l f u r o c o c c u s T h e r m o f i l u m OH OH g r a m p o s i t i v e s T h e r m o p r o t e u s P y r o b a c u l u m OH OH Thermotoga p r o t c o b a c t e r i a P y r o d i c t i u m P y r o c o c c u s cyanobacteria M e t h a n o - M e t h a n o b a c t e r i u m t h e r m u s a v o b a c t e r i a A r c h a e o g l o b u s H a l o c o c c u s Thermocrinis 1 2 H a l o b a c t e r i u m 3 M e t h a n o p l a n u s Aquifex 4 M e t h a n o p y r u s M e t h a n o s p i r i l l u m M e t h a n o c o c c u s 1 j a n n a s c h i i M e t h a n o s a r c i n a >2.7 Ga 2 i g n e u s 3 t h e r m o l i t h o t r o p h i c u s 4 v a n n i e l l i i O O >2.7 Ga OH Biomarkers persist in the sediments and contain information about the presence of past organisms

  11. C29 C28 C30 C30 Algal Steroids • Encode a variety of age-diagnostic signatures – • C-isotopes + steroids from algae & plants • chlorophyceans • diatoms • chrysophytes • dinoflagellates ‘geo’ ‘bio’

  12. Alkenones as a tracer for sea surface temperature Alkenones are made only by Prymnesiophytes… Alkenones record SST…. in their saturation levels

  13. Alkenones record information about climate The ratio of alkenones in a sample records past temperature similar to foraminifera Southwest Pacific ~30,000 yr record

  14. Geochemical tracers Two isotopes of concern: Oxygen isotopes: 18O Carbon isotopes 13C

  15. Geochemical tracers Oxygen is: 16O=99.759% 17O=0.037% 18O=0.204% notation is simply the ratio of 16:18 in the sample relative to a standard …..

  16. “del” notation A= (RA -1) x1000 RSt which is the same as (18/16O)CaCO3sample –1) x1000 (18/16O)CaCO3 std The result is we use “per mil” ‰ as our units and we always speak about the data in reference to: the isotope of interest

  17. Emerson and Hedges 2008 Fractionation of oxygen isotopes

  18. oxygen isotopes in the climate record snow Ice -35‰

  19. Emerson and Hedges 2008 The record of  18O in the sediment record The distillation is controlled by temperature salinity and ice volume. The record is primarily a a function of temperature and ice volume

  20. Emerson and Hedges 2008 The marine isotopic record Is a climatic “standard”

  21. Geochemical tracer: carbon isotopes Carbon isotopes 12C=98.89% 13C=1.11% 14C=small and variable

  22. Emerson and Hedges (2008) Fractionation of carbon is primarily biologically mediated

  23. Fractionation of carbon is primarily biologically mediated Plants discriminate against 13C during photosynthesis. The the ultimate 13C amount varies with plant type and substrate. Libes chapt. 29

  24. Fractionation of carbon is primarily biologically mediated Plants discriminate against 13C during photosynthesis. The the ultimate 13C amount varies with plant type and substrate. Libes chapt. 29

  25. The biological pump moves organic carbon The biological pump moves organic matter with it’s depleted isotopic signal from the surface ocean to the deep water masses…..

  26. Emerson and Hedges (2008) The vertical movement of organic carbon can be seen in the isotopes of Carbonate

  27. The conveyor belt circulation And thermohaline circulation moves CO2 through the system…

  28. Water downwells: DIC enriched Low nutrients High O2 Controls on the fractionation of 13C 13C 13C 12C 13C 13C 12C 12C 13C 12C 13C 13C DIC (in Water) more depleted with increasing age 13C 12C 12C 12C 12C 12C Porewaters very sensitive to remineralization can be very depleted in 13C 12C 12C 12C 12C 12C 12C

  29. The vertical profile of 13C reflects the global thermohaline circulation

  30. How do we use these tracers? Evidence for the effects of… Short term perturbations (that we can verify other ways)

  31. Dissolved inorganic carbon change • DIC change in the ocean is due to increasing CO2 in the atmosphere • How can you trace increasing CO2 input when the ocean is full of CO2 ? Find another complimentary tracer: CFC: cloro-flouro-carbons

  32. Anthropogenic gases Tracers with a known history…..

  33. Atlantic Ocean DIC CFC as a tracer of ocean ventilation

  34. Pacific Ocean DIC

  35. Anthropogenic CO2 This is calculated via a proxy. (13C or CFC’s) using 13C we can also trace inputs directly in the DIC…..

  36. 13C can trace Fossil fuel input • Fossil fuels are organic molecules • They are derived from compounds that were originally produced by photosynthetic organisms…… • Therefore they have a depleted 13C signal (of about -20 to -30 ‰) We can use this to measure the change in  CO2 directly. And on long time scales.

  37. 13C traces the source of the DIC in the surface ocean Southern Ocean

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