Exploring the Pliocene-Pleistocene Interval
In order to answer this question, let’s explore sea level curves for the past five million years (our best and most complete interval of well-calibrated geologic time) in high resolution to see what temporal patterns might exist. Again, we have already explored high-frequency periodic (daily/monthly/seasonal) sea level changes, but what other scales are evident in the sea level change record?
This figure, produced by Dr. James Hansen at Columbia University's Earth Institute, shows sea levels based on proxy datasets for the last 5 million years, spanning the Pliocene-Pleistocene interval up to about 11,000 years ago at the end of the last glacial maximum (LGM).
Proxy datasets use various observations (typically geochemical measurements of radioisotopes derived from ice cores, sediment/rock cores, coral growth rings, tree rings, etc.) to establish global changes in atmospheric gas concentrations and associated temperature changes and hence changes in sea level positions. In this sea level dataset (albedo proxy; see this Climate Data Information webpage for more info), albedo is the proxy that is used to link climate change to lower and lower sea levels. The legend embedded in Figure 4.24 indicates that climate change (e.g., cooling), changes the location of deep ocean water formation. This results in changes to ocean circulation that may be integral in sea level change through this 5 million year (Mya) interval.
Any one of you should be able to observe the patterns and trends demonstrated in the dataset and arrive at a general consensus. That is:
In the transition out of the Pliocene and into the Pleistocene around 2 Mya, there is a pronounced change in behavior of the curve. The interval to the right of the 2 Mya (Pleistocene Epoch) is increasingly punctuated by more extreme volatility and oscillation of sea levels, especially in the terminal Pleistocene from about 600 kya to the LGM or Last Glacial Maximum.
You might ask, then, what changed? What happened to cause sea levels to not only change, but also to change so rapidly and with such pronounced shifts of up to 100 meters or more? In this case (the Hansen Curve), you will see that sea levels throughout the Neogene (Pliocene, Pleistocene, and Holocene) have been dynamic and volatile, but with a general long-term trend toward lower sea levels from sea level positions higher than modern about 3 million years ago. In contrast to the Pliocene, sea levels dropped to levels at least 100 meters below modern sea level on at least 4 occasions during the last 600,000 years. These sea level lowstands concur with glacial maxima (LGM - Last Glacial Maximum), the last of which demarcates the boundary between the Pleistocene and Holocene Epoch.