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In 1841, a mark was cut into the rocks of the Isle of the Dead at Port Arthur, in an attempt to record the height of the sea in the area, and to provide a benchmark for future studies of the movements of the Earth's crust, relative to sea-level. It was made at the instigation of Captain Sir James Clark Ross, with the support of Thomas Lempriere (Deputy Assistant Commissary General at the Port Arthur penal settlement). Records indicate Lempriere had studied the tidal levels in the area for several years before the mark was made.
The mark at Port Arthur is among the earliest benchmarks in the world against which to scientifically measure changes in sea-level. Until recently, a lack of actual data from the time prevented a proper understanding of the site. However, some of the original data has been found, and together with some recent detailed monitoring at Port Arthur, a clearer understanding of sea-level changes in the area are now possible. It also aids in the understanding of global sea-level changes, as there are very few good long-term benchmarks in the southern hemisphere.
Setting the site in stone
Thomas Lempriere was a keen observer of his surroundings, and kept records of a variety of features, including meteorological conditions such as the wind, rainfall and temperature. He also operated a tide gauge, though the exact details of where and how the gauge worked are not known.
When Captain Ross visited Tasmania in 1841 during his voyages to Antarctica and the Southern Ocean, he visited Lempriere at Port Arthur. Ross was keen to have a sea-level benchmark installed in an area free from large wind and river flow influences, and Port Arthur appeared to be an ideal location. Lempriere's monitoring of the tides at the site added to its value, contributing several years' of important data. Together, they provide a baseline of great value to the contemporary science of sea-level and climate change.
On 1 July 1841, Ross and Lempriere had a standard survey mark cut into a sandstone cliff on the Isle of the Dead. The mark is a horizontal line with a broad arrow touching and pointing down at the horizontal line. A plaque was installed above the mark, but unfortunately it has not survived.
1841 Sea-level benchmark, Isle of the Dead, Port Arthur Source: Courtesy of Richard Coleman and John Hunter
1841 Sea-level benchmark, Isle of the Dead, Port Arthur
Source: Courtesy of Richard Coleman and John Hunter
Ross' journal of the event is confusing. It is not clear whether the mark was made of the mean sea-level, or high water. Ross did make two more marks on the Falkland Island on the same voyage, and these were both above mean sea-level.
A paper published in 1889 by Captain Shortt recorded the wording of the plaque, including the time the mark was struck and the height of the sea given by Lempriere's tide gauge. By taking a measurement of the height of the sea, and estimating what the tides were when the mark was made, Shortt determined that the mark was made near high water.
An article in The Australasian in 1892 also recorded the wording of the plaque. While almost the same as the version published in Shortt's paper, it differed in the time the mark was supposed to have been made, although both reports were consistent regarding the reading of Lempriere's tide gauge when the mark was struck. Taken on its own, the reported time of the striking would suggest that the mark was originally near mean sea-level.
Significant work has gone into determining which of the accounts is correct, including a current major study by a collaboration of international scientists, as knowing whether the mark was originally placed near mean sea-level or high water is crucial to being able to compare sea-levels of 1841 with today. This study has concluded that it is almost certain that the benchmark was originally placed near high water. The conclusion is based on other estimates of sea-level made later in the 19th century, and on the fact that, if the mark had originally been placed near mean sea-level, then the Penitentiary building would have suffered flooding every few years (there is no record of this having happened).
The old and the new
It was thought that Lempriere's original data had been entirely lost, making it very difficult to understand how the sea-level in 1841 related to the sea-level of today.
However in late 1995, Dr David Pugh of the Southampton Oceanography Centre, found Lempriere's data for 1841 and 1842 in the archives of the Royal Society in London. In mid-1998 Dr John Hunter, then of CSIRO's Division of Oceanography, found data for December 1839 and February 1840 to January 1841 in the Australian Archives at Rosny.
In 1999, the universities of Canberra and Tasmania, and CSIRO, set up a sea-level monitoring station at Port Arthur. Referencing this data with the benchmark on the Isle of the Dead, and utilising Lempriere's original data, has enabled the first comprehensive study that compares the Port Arthur sea-level of 1841 with that of today.
It is unfortunate that a continuous record of data has not been collected from Port Arthur, but having such an early benchmark to compare to is still of great importance.
Modern sea-level monitoring station, Port Arthur Source: Courtesy of Richard Coleman and John Hunter
Modern sea-level monitoring station, Port Arthur
Source: Courtesy of Richard Coleman and John Hunter
The Port Arthur site is particularly important because of the few long-term sea-level records in Australia and the southern hemisphere.
Records spanning many decades are considered necessary before long-term trends can actually be identified. Some highly accurate monitoring stations have been established in Australia during the last 20 years, but it will be some time before these show clear long-term trends. There are also a number of Australian ports that have reasonably long series of data, but still do not go back as far as 1841. Sydney, for example, has a good, reasonably continuous, data series from about 1884.
As the earliest sea-level benchmark in Australia, and possibly the southern hemisphere, the Port Arthur benchmark gives a great opportunity for assessing long-term sea-level change in this region and contribute to the understanding of global sea-level rise.
Dramatic changes in sea-level are possible during the next hundred years as the release of greenhouse gases enhance the Earth's natural greenhouse effect. While there may be some extra water added to the oceans by melting of ice caps and glaciers, much of the initial sea-level rise will be caused by the oceans expanding as they warm up (thermal expansion).
Global predictions are for the rate of change to increase, such that by 2100 sea-level will be between 9 and 88 cm above the 1990 global average sea-level. For the periods 1990 to 2025 and 1990 to 2050, the projected rises are 3 to 14 cm and 5 to 32 cm respectively (Houghton & Ding 2001). Already the Port Arthur benchmark is showing a rise in sea-levels of at least 13 cm since 1841, with an average annual rate of 0.8-1.0 mm/year (Pugh, Coleman & Hunter 2002). Hence the site will be an important benchmark to continue measuring the changes in the average level of the sea.
This Case Study was compiled from a number of studies and articles: Pugh, Coleman & Hunter 2002; Bowden, Hunter & Pugh 1997; and Houghton & Ding 2001. John Hunter (University of Tasmania) in particular provided valuable advice.
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Last Modified: 14 Dec 2006
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