Difference between revisions of "Past major events"
(New page: Behaviour of the Earth System during Major Events According to our understanding of Earth System history, as recorded in marine sedimentary rocks, it seems that there were long periods of...)
Revision as of 22:49, 28 March 2008
Behaviour of the Earth System during Major Events
According to our understanding of Earth System history, as recorded in marine sedimentary rocks, it seems that there were long periods of relative stasis interrupted by occasional sharp transitions to a new environmental state. The rock record may be somewhat misleading though, and our understanding may partly reflect the (lack of) resolution of the records. As scientists obtain more finely-resolved records with a closer spacing of data points in time then more variability is revealed. This is also the case for the last few hundred thousand years, during which time the closely-spaced points from the ice-core record reveal many more episodes of abrupt climate change than were apparent in the earlier records obtained from drilling through ancient seabeds.
Regardless of issues of how well the available data records can reveal variability, it is nevertheless apparent that there have been occasions when the Earth System has changed rather rapidly or has undergone a sudden large perturbation such as due to a comet impact. The behaviour of the Earth System during these perturbations or transitions or periods of anomalous behaviour is of great interest because it can potentially illuminate how the Earth System works.
Many hypotheses have been put forward to explain what happened to the Earth during these events and the mechanisms underpinning its subsequent recovery. These Jmodels have been used to quantitatively test competing hypotheses and in some cases disprove them. The Jmodels can be used to examine, among others, the following events:
(a) greenhouse-icehouse transition: at the Eocene-Oligocene boundary (ca. 34 Mya) new polar ice sheets formed, after many tens of millions of years during which the Earth had had none. There was also an unprecedented and permanent change to ocean carbon cycling. Several explanations have been put forward as to how the two might be linked.
(b) ocean acidification event: at the Paleocene-Eocene boundary (ca. 55 Mya) the world experienced several sharp changes including global warming and dissolution of calcium carbonate indicating ocean acidification. This event may be the closest analogue in the palaeo record for the ongoing human-associated changes.
(c) diatom event: an event christened the ‘Silica Burp’ occurred ca. ?? Mya. Greater numbers of diatom shells are seen in marine sediments dating from this time than from any other.
(d) Snowball Earth: the Earth became completely or almost completely covered in ice during several events during the period 600-580?? Mya. But in all the intervening time since, while large-scale glaciation has occurred many times there have never been any subsequent snowballs. Could this be due to the arrival of the carbonate compensation feedback?
(e) anoxic oceans: in contrast to today’s ocean in which oxygen is almost everywhere present, during past Oceanic Anoxic Events (ca. ?? Mya) the deep ocean became almost everywhere depleted in oxygen. Because low oxygen conditions promote fixed nitrogen destruction, nitrogen cycling must have been distinctly different from today.