Transcript 投影片 1
11/3 papers review
Pleistocene geochronology radiocarbon dating chemical oceanography
Two hypotheses
1.
2.
Changes to reorganization of the ocean’s
thermohaline circulation: catastrophic input of fresh water to northern Atlantic. ocean modelers and paleoclimatologists
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. atmospheric physics and decadal variability studies
Heinrich events & Younger Dryas
Younger Dryas
Younger Dryas
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
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 → 11000 years BP Exception: New Zealand began as much as 500 years earlier than the onset of the YD in the Northern Hemisphere.
Younger Dryas
New Zealand
• Resolution • Interhemispheric synchroneity.
Younger Dryas
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
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.
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
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 http://bprc.osu.edu/foram/species/neogloboquadrina_pach.htm
A shutdown in conveyor circulation induced by the freshwater input rather than by the precursory cooling event itself.
Conclusion
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.
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
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.
r 2 =0.89
Our results give strength to the conclusion that CO important climate forcing on the modern Earth, on very long geologic timescales. 2 is an irrespective of whether other factors are more important Further, our results strengthen the hypothesis that the long-term synchrony of glacial them. ± interglacial cycling between Northern and Southern Hemispheres is due to greenhouse-gas variations, and feedbacks associated with 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.