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11/3 papers review. Pleistocene geochronology radiocarbon dating chemical oceanography.

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Presentation on theme: "11/3 papers review. Pleistocene geochronology radiocarbon dating chemical oceanography."— Presentation transcript:

1 11/3 papers review

2 Pleistocene geochronology radiocarbon dating chemical oceanography

3 Two hypotheses 1. Changes to reorganization of the ocean’s thermohaline circulation: catastrophic input of fresh water to northern Atlantic. 2. Changes in the dynamics of the tropical atmosphere- ocean system : because tropical convective systems constitute the dominant element in the planet’s climate system, the trigger more likely resides in the region that houses the El Nin˜o–La Nin˜a cycle. ocean modelers and paleoclimatologists atmospheric physics and decadal variability studies

4 Heinrich events & Younger Dryas Younger Dryas

5 9500 km 3 of water was released

6 How to support?  The 14 C to 12 C ratio in the local surface water began to rise.  Sediment CaCO 3 ↑ 14 C ↑ Younger Dryas

7 Radiocarbon Dating  Make use of radiocarbon dating to determine whether the release of Agassiz water occurred at the time of the YD onset.  Tephra layer & poplar tree → years BP  Exception: New Zealand began as much as 500 years earlier than the onset of the YD in the Northern Hemisphere. Younger Dryas

8 New Zealand Younger Dryas Resolution Interhemispheric synchroneity.

9 In summary  With the exception of that for New Zealand, the chronological evidence is consistent with a sudden global onset of the YD impacts at about 11,000 14C years B.P.  If advocates of a tropical trigger discount the role of the Agassiz flood as the trigger for the YD, then they must attribute this apparent synchroneity either to coincidence or to a climate change initiated elsewhere as the cause of the flood. Younger Dryas

10 Heinrich events  Six layers dominated by ice-rafted debris have been identified in a series of cores extending from the Hudson Straits across the northern Atlantic to the coast of France. Reduce salinity of northern Atlantic surface water by a large enough amount to impact conveyor circulation.

11  Far-field impacts include times of the greatest glacial cooling in the Mediterranean Sea and in the Atlantic Ocean off the Iberian Margin, sediment- discharge events off eastern Brazil, pine events in central Florida, and sharp weakenings of the monsoons in the Chinese Hulu Cave record.  Thus, it is tempting to conclude that these impacts were triggered by disruptions of thermohaline circulation caused by freshwater inputs to the northern Atlantic. Heinrich events

12 Precursory Events  Distinguishing between the oceanic and tropical hypotheses The deposition of the Heinrich layers:  Existence of hematite-stained mineral grains, the lithic type originates far to the north of the core sites, a cold ocean favors their delivery by ice rafting.  Cold-loving Neogloboquadrina pachyderma reached a maximum during these precursory events

13  A shutdown in conveyor circulation induced by the freshwater input rather than by the precursory cooling event itself.  Conclusion

14

15  δD&CO 2 → r 2 =0.64  Mismatches  Variation of climate water vapor source regions  Correct this by a model →r 2 =0.89  This close relationship strongly supports the importance of carbon dioxide as a forcing factor of climate.

16  r 2 =0.64  If this δD change reflect a proportional temperature drop, then >1/2 temperature change occurred before CO 2 change. Minor contribution of CO 2

17 Why?  Source temperature (where water vapor comes from)  During glacial periods, source temperature was high, poles were still receiving moisture in response to feedbacks on insolation changes → cause a disproportionately large drop inδD of Antarctic precipitation at this time.

18 r 2 =0.89

19  Our results give strength to the conclusion that CO 2 is an important climate forcing on the modern Earth, irrespective of whether other factors are more important on very long geologic timescales.  Further, our results strengthen the hypothesis that the long-term synchrony of glacial±interglacial cycling between Northern and Southern Hemispheres is due to greenhouse-gas variations, and feedbacks associated with them.  In particular, we have shown that most Southern Hemisphere δT can be explained, in the correlative sense, without recourse to any Northern Hemisphere forcings, including insolation at 65N and ice-sheet albedo.


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