2 Pleistocene geochronology radiocarbon dating chemical oceanography
3 Two hypothesesChanges to reorganization of the ocean’s thermohaline circulation: catastrophic input of fresh water to northern Atlantic.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 paleoclimatologistsatmospheric physics and decadal variability studies
5 Younger Dryas <The cause of Younger Dryas> 9500 km3 of water was released
6 How to support?The 14C to 12C ratio in the local surface water began to rise.Sediment CaCO3 ↑ 14C ↑Younger Dryas
7 Radiocarbon DatingMake 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 BPException: New Zealand began as much as 500 years earlier than the onset of the YD in the Northern Hemisphere.Younger Dryas
8 New ZealandResolutionInterhemispheric synchroneity.Younger Dryas
9 In summaryWith 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 eventsSix 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 EventsDistinguishing between the oceanic and tropical hypothesesThe 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
15 δD&CO2 → r2=0.64MismatchesVariation of climate water vapor source regionsCorrect this by a model →r2=0.89This close relationship strongly supports the importance of carbon dioxide as a forcing factor of climate.
16 r2=0.64If this δD change reflect a proportional temperature drop, then >1/2 temperature change occurred before CO2 change.Minor contribution of CO2
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.
19 Our results give strength to the conclusion that CO2 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.