For the Instructor

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Holocene Sea Level Curves: A Closer Look

So, was sea level rise driven by increased solar radiation? Decreased albedo effects? Increased greenhouse gas concentrations in the atmosphere? or combinations thereof? What factor(s) led, which followed, etc.? These are all still up for debate.

Nevertheless, approximately 6,000 years ago, the rate of sea level rise slowed, leaving only significant ice sheets on Greenland, Iceland, and Antarctica. Why didn't the rate of ice-sheet decay continue? That's a very good question. You may recall from our Vostok core dataset example above and estimates of insolation, this time frame coincides with decreasing incoming solar radiation values from Milankovitch forcing models. We will explore this more in Figure 4.33 below.

Although it isn't yet clear, it is a hypothesis that is being tested. Did the slowdown in the rate of sea level rise to a near still stand correlate to decreasing insolation in the northern hemisphere? Despite the level greenhouse gas concentrations (~265 parts per million - based on the Vostok exercise above), the rate of rise could not be sustained because the vast ice sheets were already melted and decreased insolation values per unit area in the northern hemisphere may have contributed to development of stability in sea levels. In other words, greenhouse effects may have acted to continue to keep sea levels rising despite decreased insolation. Thus, sea levels achieved a much more stable condition as shown in Figure 4.32.

Holocene Seaâ€Level curve showing the most recent period of rise and warming

Figure 4.32: Holocene Sea Level curve showing the most recent period of rise and warming. Data is the same as in Figure 4.30, but at a higher resolution. Some of these data suggest that sea levels approached modern around 6,000 years ago, but may have actually exceeded modern sea levels in some regions (i.e., Malacca), but, on average, sea levels have been relatively slow to rise and have been fairly stable for at least the last few thousand years.

In the composite dataset (same dataset as Figure 4.30, just focused on the last 9,000 years after meltwater pulse 1A), you will notice the slowing in the rate of sea level rise. For the period from roughly 6,000 years ago to the last century, the amount of rise is estimated to have been just a few meters. Hence, rise rates fell almost to zero (~0.7 mm/year), a far cry from the 15 mm/year rise rates estimated for the immediately preceding interval. We have to be careful in looking at absolute values due to uncertainties (i.e., statistical error), but the integrated dataset shows consilience in the still-stand signal and indicates a relatively robust global signal.

Under these relatively stable conditions, many of the coastal features observed today developed and expanded. Coral reefs that were able to keep up with earlier rates of sea level rise began to expand laterally, building large reef systems including the Great Barrier Reef and others in the Pacific Ocean, and the barrier reef systems common off south Florida and in the Caribbean. Likewise, shoals were built by coastal currents together with wave activities that acted on sediments deposited by rivers along the eastern seaboard of the U.S., and near other large river systems around the globe. These formed numerous barrier islands that moved slowly up continental shelves (landward) as sea level rose.

Depending on many localized factors (subsidence, uplift, isostatic rebound, sedimentation, etc.) the signal of global or eustatic sea level may be somewhat different from local signals. So, scientists need to continue to integrate local datasets and work through complications in each local proxy record to better time and evaluate global signals. A good example of a local record comes from work in St. Croix coral reefs. This dataset helps illustrate the complexity involved in the process of integrating proxy datasets from various areas.

Figure 4.34 is from a Kansas Geological Survey publication and shows a sea level curve for St. Croix, U.S. Virgin Islands for the Holocene. Growth rates of coral reef systems are given relative to rates published in Bermuda that were calculated to be ~0.4 mm/year in the last few thousand years, compared to rates 4- to as much as 8-fold higher during earlier time intervals.

In the human timeline, the change from higher rates of sea level to lower-rates of sea level rise occurred near the onset of the Neolithic interval ~4,000 years ago. Coral reefs in St. Croix grew upward rapidly once sea level stabilized and conditions necessary for their growth became more optimal (warmer waters, less turbidity/sedimentation, etc.). From this example, coral growth rates would have lagged behind sea level rise. Water levels had to provide optimum growth conditions for the three coral species as shown here. By 2,000 years ago to at least 1,000 years ago, coral growth in St. Croix appears to have been 10-fold higher than it was prior (15 mm/year compared to 1.3 mm/year).

Although human beings began to influence Earth in interesting ways within the last few millennia, (i.e., you may recall from history classes that the Roman Empire began to expand about 2,000 years ago), anthropogenic impacts on sea levels may likely have occurred more recently. As is often the case, systems with feedback loops will often have significant lag times.

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