Distribution and controls on ocean  13 C with implications for water mass tracing

1. Distribution and controls on ocean 13C with implications for water mass tracing “Hey! Take your sediment cores somewhere else!” Peter Almasi Chemical Oceanography Spring 2002

2. What is the biological effect on 13C? • Phytoplankton productivity and • regeneration are primary controls on 13C in surface and deep ocean. • Total observed range in 13C is • ~3 ‰. • 13C increases ~ 1.1 ‰ per mol PO4 • Data >500 m depth Kroopnick (1985)

3. Air-sea exchange isotopic equilibrium • Always results in higher 13C • Strongly temperature • dependent • Over whole ocean temp range • 13C may vary ~3.5 ‰ Lynch-Stieglitz et al. (1995)

4. Isotopic equilibrium in air-sea exchange • Rapid 13C drop with CO2 invasion • Slow isotopic equilibrium follows Lynch-Stieglitz et al. (1995)

5. Invasion-Evasion of CO2 effect on 13C • Atmospheric CO2 has a 13C • of ~ -10 ‰. • Invasion can result in surface • water depletion ~ 0.5 ‰ • (typical N. Atlantic value) • Isotopic equilibrium slowly • increases 13C Lynch-Stieglitz et al. (1995)

6. Air-sea exchange + CO2 invasion/evasion 13C signature in the world ocean Charles et al. (1993)

7. Western Atlantic GEOSECS 13C data • Note NADW, AAIW, AABW Kroopnick (1985)

8. Eastern Atlantic GEOSECS 13C data • Note Circumpolar Water (CPW) at ~40 S. • Smaller influence of NADW Kroopnick (1985)

9. Western Pacific GEOSECS 13C data • Note influence of CPW to 20°S • North Pacific intermediate depth 13C minimum due to continued • input of organic matter and no deep convection Kroopnick (1985)

10. North Pacific zonal GEOSECS 13C data • North Pacific zonal transect reveals extent of intermediate 13C • minimum Kroopnick (1985)

11. Greenland Iceland Norwegian Sea 13C • On the west, a section across East Greenland Current • In the east, a section across North Atlantic inflow to GIN Sea Johannessen et al.

12. Modern water mass 13C values Endmembers: Vema Passage Water (VPW) = 0.44 Northern Component Water (NCW) = 1.08 Total range in South-North Atlantic Waters ~ 0.5 Oppo and Fairbanks (1987)

13. Glacial - interglacial carbon isotope variation as a proxy of thermohaline circulation variability Western Atlantic 42N Boyle and Keigwin (1982)

14. Holocene - Last Interglacial 13C Duplessy et al. (1984)

15. Water mass endmember variability on long time scales Oppo and Fairbanks (1987)

16. Eastern Atlantic Glacial and Holocene variability Oppo and Fairbanks (1987)

17. Major rapid climate/isotope events are clearly resolved • Carbon isotopes used • to demonstrate THC • events during Younger • Dryas and Heinrich • events • This core seems • particularly sensitive. Bond et al. (1997) figure from Boyle (2000)

18. High frequency North Atlantic Holocene NADW variability? • Carbon isotopes • -P. wuellerstorfi • -N. umbonifera • Trace metals • Sortable silt • Ice core proxies Keigwin and Boyle (2000)

19. Greenland Iceland Norwegian Sea 13C • On the west, a section across East Greenland Current • In the east, a section across North Atlantic inflow to GIN Sea Johannessen et al.

20. Processes which may contribute to the large 13C gradient in the GIN Sea • Mixing between Polar Surface Water and North Atlantic Water • Changes in biological productivity • Temperature influenced air-sea exchange enhanced by winds • Currently PO4 and 13C appear to be de-coupled • These may vary with climatic change on all time scales

21. Conclusions • Benthic carbon isotopes are useful for study of long-term • carbon cycle variations and major changes in ocean circulation. • Only on long timescales does 13C reflect nutrient effects primarily. • Millennial-scale variability of 13C has been identified in the North • Atlantic, varying as THC is expected to respond to climate change. • The extreme variation in 13C of mixing water masses in NADW • formation regions precludes interpretation of Holocene records in • terms of thermohaline circulation variability.

22. Aside #1 Why does everyone use P. wuellerstorfi for benthic 13C? • Not all benthic foraminifera record 13C reliably • Only two species 13C are correlated with 13C of bottom water • Uvigerina species 13C slope may change with organic matter • Even epifaunal species may be unreliable with high rain rate Duplessy et al. (1984)

23. Aside #2Deviation from modern planktonic 13C in core top sediments Kohfeld et al. (2000)

24. Planktic foram corrections Kohfeld et al. (2000)

25. References Bond G. et al. (1997) Science278, 1257-1266. Boyle E. A. and Keigwin L. D. (1982) Science218, 784-787. Duplessy J. C. et al. (1988) Paleoceanography3, 343-360. Johannessen T. and Ravelo A. C., (submitted). Keigwin L. D. and Boyle E. A. (2000) PNAS97, 1343-1346. Kohfeld K. E., Anderson R. F., and Lynch-Stieglitz J. (2000) Paleoceanography15, 53-64. Kroopnick P. M. (1985) Deep Sea Research32, 57-84. Lynch-Stieglitz J. et al. (1995) Global Biogeochemical Cycles9, 653-665. Oppo D. W. and Fairbanks R. G. (1987) Earth and Planetary Science Letters86, 1-15.

26. North Atlantic Deep Water variability -sensitivity to rapid freshwater perturbation -sensitivity to surface ocean coolings Glacial Intermediate Water variability Terrestrial biosphere productivity on orbital time scales Applications of 13C as water mass tracers

27. Major high frequency THC events are clearly resolved • Carbon isotopes used to demonstrate • Thermohaline circulation decreases during • Younger Dryas and Heinrich events • This sub-polar eastern North Atlantic location • is particularly sensitive. Bond et al. (1997)

28. Mid-depth tropical Atlantic 13C Charles et al. (1993)