- Количество слайдов: 32
Greenhouse to Icehouse: The Last 50 Million Years
Since the Cretaceous the Climate has Grown Colder…. Why?
60 Myr Ago
50 Myr Ago
40 Myr Ago
30 Myr Ago
20 Myr Ago
Evidence of Cooling • Prior to 50 Myr ago we had palm trees and crocodiles north of the artic circle • 35 Myr ago the palms gave way to broadleaf evergreens • Cooling was first intense in the southern hemisphere
Evidence of Cooling • Until about 30 Myr ago Antarctica looked like this… • Now Antarctica looks like this…
Arctic Conditions • Breadfruit trees north of the arctic circle ~60 myr ago • Now tundra…
Oddly there is a relationship between leaf edges and temperature
Just from leaf shape it is clear that the climate is changing
Deep-Water O-18 Shows Strong Cooling • O-18 from deepwater forams. • Current temperatures are around 2 C • 50 Myr ago temperatures were ~16 C
More Evidence of Cooling • Turns out that the ratio of Mg to Ca in Foram shells depends on temperature. • Again, for deep water forams there is a very strong cooling signal
So what caused the cooling? • Geography? – Distribution of heat by ocean circulation – Gateway Hypothesis • Changes in seafloor spreading rates? • Uplift and Weathering? – Changes in uplift
The opening of the Mar de Hoces (Drake Passage) • Before the separation of South America and Antarctica warm currents would push heat south. • The development of the circum-polar current occurred about the same time as onset of major glaciation. • Climate models don’t support this idea. • I don’t really buy modeling as evidence…. modeling is not always a good guide to reality.
Mar de Hoces
• Closed about 4 -10 Myr ago. This redirects warm water north (gulf stream). • Suppresses sea ice and provides more moisture at high latitudes for snow buildup. • BUT, it is the gyres that drive the circulation. The seaway is too far south to strongly affect the North Atlantic gyre. The closing of the Panama Seaway
Spreading Rates? • The simplest idea about causes of cooling is that CO 2 dropped because of slower seafloor spreading. • But…. over the last 15 Myr spreading has been pretty constant while the climate became very cold.
Spreading Rates? • BUT…. not all subducted material is created equal – Remember it is subduction that actually delivers the carbon to volcanoes. – If the plate being subducted is carbon-poor, then there is less CO 2 available for eruption. • But…. that recycling process is VERY slow, so the current CO 2 is probably coming from plate subducted 10’s Myr ago. • Most current subduction may be on the carbon-poor side, but that would have no effect on CO 2 output for a long time. • I really doubt that changing spreading rates have anything to do with the current cooling.
Uplift and Weathering • The hypothesis is that we are currently experiencing unusually high rates of uplift and weathering – Unusually large amounts of high terrain – Unusual amounts of rock fragmentation – Unusually high rates of chemical weathering
Are today’s continental elevations normal? • Well…. no. • “Normal” is Andean style mountain building associated with subduction. • Continent to continent collision has been pretty sparse since the assembly of Pangaea (~200 Myr ago) • The Tibetan Plateau is a pretty unusual feature – 2. 5 million Km 2 an average of 5 km elevation • The Rocky Mountains are also pretty unusual
Is today’s physical weathering normal? • Well…. no. • Most of the suspended sediment load is associated with mountains (duh…) • But by far the largest loads are off of the Tibetan Plateau
Is today’s physical weathering normal? • And the sediments coming off the Himalayas have increased 10 X in the last 40 Myr.
Is today’s physical weathering normal? • Part of the reason for enhanced weathering comes from an enhanced Indian Monsoon • The higher the Himalayas rise, the more it pushes moist air into cool altitudes, creating more rainfall…. creating more weathering….
Is today’s physical weathering normal? • Probably not…. • But what is normal? • It is very hard to estimate CO 2 removal rates for past epochs. • It does seem that the rise of Tibet and other major uplifts do have a strong effect on climate.
Feedbacks • Changes in uplift could generate a negative feedback that moderates cooling. • The decrease in temperatures would decrease weathering in the rest of the world
Feedbacks • But, the overall effect of the feedback is not clear. • Uplift could produce a positive feedback. • Glaciation could increase fragmentation and create much more weathering • More weathering enhances cooling, creating more glaciers…. .
Feedbacks • Remember, what glaciers do is pulverize rock and push it around. • Just the thing to enhance weathering and make things colder.
Feedbacks • But this sort of weathering feedback would tend to moderate any increase in warming. • Weathering would rapidly increase as ice retreats. • That would reduce CO 2 and then reduce warming …. . not happening now.
Tectonics • Tectonic-scale processes operate very slowly. • Doesn’t really explain why we got four major ice advances in the last 500, 000 years. • Or how we got warm intervals (like now…).