Download Week 2 Discussion Questions

NAME--------------------------------------------------Week 2 Discussion Papers – This week, we will have 3 short papers that discuss the
Cretaceous Hot House;
a. Larson, R.L. Latest Pulse of Earth: Evidence for a min-Cretaceous Superplume
(Geology, v19, 547-550, 1991). b. Bralower, T.J. Volcanic Cause of a
Catastrophe, Nature vol. 454, 285-287, 2008. c. Turgeon, S.C. and R. A.
Creaser, Cretaceous Oceanic Anoxic Event 2 Triggered by a Massive Magmatic
Episode, Nature, 454, 323-326, 2008.
The first two papers are easy to read, and introduce the concept of a SuperPlume
volcanic source derived from the core/mantle boundary as being responsible for the midCretaceous hothouse climate. The Turgeon and Creaser paper is more difficult, but
introduces the critically important concept of using elemental isotopes as proxies for
geological processes. In the T&C paper, it is not important to understand many of the
details that they discuss in order to interpret their Figure 1. To help with this, I’ve
included some critical details in that section below.
LARSON SuperPlume Paper questions.
1. How do marine magnetic anomaly patterns support Larson’s hypothesis that there
was a faster ‘pulse’ of ocean crust formation during the Cretaceous period?
2. How did the mid-Cretaceous oceanic plateaus form in the Western Pacific? I.e., what
was the source of the excess heat and magma that caused these large submarine
plateaus to form?
3. What is the mid-Pacific ‘superswell’? and how is it related to the Cretaceous warming
event?
4.How can the behavior of the earth’s magnetic field be related to climate? Give a
short discussion of the ‘chain of events’ that would tie these non-intuitive processes
together.
Bralower Paper Questions.
1. How does LIP (large igneous province) formation impact ocean circulation?
2. How would the massive volcanic eruptions in the Cretaceous increased biological
productivity in the global oceans?
Turgeon and Creaser Paper (read below before tackling the paper).
Osmium has 7 stable isotopes. All we care about are 187Os and 186Os.
For the purposes of the T&C paper, all we need to know is that - Continental crust has a
lot of 187Os (compared to 186Os), and the Mantle does not have much 187Os. And so
ocean crust (which is formed directly from mantle material) also has relatively little 187Os.
So CONTINENTAL CRUST has a HIGH 187Os/186Os ratio. And river run-off from the
continents would also have a HIGH 187Os/186Os ratio.
OCEANIC CRUST has a LOW 187Os/186Os ratio. When seafloor forms volcanically,
hydrothermal circulation delivers the OCEAN CRUST (low 187Os/186Os) ratio to seawater.
Weathering delivers the CONTINENTAL CRUST (high 187Os/186Os ratio) to seawater.
SO, the 187Os/186Os ratio in seawater is a balance of input from weathering and
hydrothermal circulation (or the amount submarine volcanic activity).
The shift in 13C isotope ratio shown in T&C Figure 1 is due to the burial of 13C-depleted
material in the sediments during the period of anoxia (lots of biological activity going on;
we will discuss this at length later).
Questions.
1. What are oceanic ‘black shales’?
2. Why does the fact that there is little Osmium in seawater improve the ability to
observe changes in input sources that occur in short time intervals?
3. Why is it important that the changes in Osmium ratio occur before the changes in
carbon isotopes that define the OAE2 event?
4. Why did T&C decide that the observed osmium ratio anomaly associated with the
OAE2 event come from volcanic activity, rather than an extraterrestrial source?