The Holocene deglaciation, which began approximately 21,000 years ago, was modeled by Peltier (1994) using records of relative sea-level change. His calculations produced the data set ICE4G, consisting of 1-degree global grids of ice distribution and topography. We obtained ICE4G from the National Geophysical Data Center in Boulder, Colorado. We have rearranged ICE4G as images in which the topography is shown by colors ranging from dark blue through green to red, corresponding to bathymetry, sea-level, and higher elevations respectively. Grid-cells covered by ice are shown as gray pixels, grading to white as the elevation of the cell increases. The effect is to depict in a convenient manner the changes in ice distribution and topography shown by ICE4G.
In addition to these straightforward images, we have provided 1-degree grids in which the cells values indicate the proportion of the cell above sea-level. These were calculated by first creating high-resolution estimates of Late Quaternary topography. For each 1-degree grid of topography, we created a 1-degree grid containing the differences between ancient and modern topography. This grid was expanded to 5-minute resolution through bilinear interpolation, and the resulting difference cells were added to ETOPO5 and TerrainBase. Using this 5-minute gridded estimate of paleotopography, a 1-degree grid was created in which the cell values contain the percent of 5-minute cells that are land.
The middle Pliocene warm interval occurred approximately 3 million years ago. Diverse lines of evidence indicate that this was a period in which Earth's climate was significantly warmer than at present, with much smaller polar ice caps. Consequently global sea level was higher than present. There is some discussion among paleoclimate researchers as to how much higher than modern sea-level Pliocene eustatic sea-level was. We have chosen to depict a range of possible values (10, 20, 25, 30, 35, 40, 45, 50, and 60 meters above present sea level).
Because global topography was affected strongly by the Pleistocene glaciation, especially in North America and Europe, we have no better estimates of Pliocene topography than the modern world. Consequently our estimates of Pliocene land distribution are based on the modern topography shown in ETOPO5 and TerrainBase, modified by subtracting the constant corresponding to each estimate of eustatic sea-level. For each of the nine estimates of Pliocene sea level, we have created a 1-degree grid in which the cell values are the percent of land cells in the 5-minute grid. We have also created images at 5-minute and 20-minute resolution showing graphically the major changes in sea-level that have been proposed, including -120 meters representing coastlines in the Last Glacial Maximum, 0 representing modern coastlines, and +35 and +60 meters representing moderate and extreme estimates of Pliocene sea-level.
NGDC Key to Geophysical Records Documentation No. 27
Incorporated in Global Change Database, Volume 1 Documentation compiled by John J. Kineman and Mark A. Ohrenschall with additional contributions noted in the volume.
Bathymetric data shown in TerrainBase version 1.0 is clearly incorrect in the region just southeast of Halifax, Nova Scotia. These data were corrected somewhat by patching in that region data from ETOPO5. The pixels changed are listed in <URL:http://geochange.er.usgs.gov/pub/data/sea_level/Core/raw/ tbase/patched/patch.err>. The programs used to patch the data were <URL:http://geochange.er.usgs.gov/pub/data/sea_level/Core/raw/ tbase/patched/peek_topo.c> and <URL:http://geochange.er.usgs.gov/ pub/data/sea_level/Core/raw/tbase/patched/peek_topo.c>. The process used was to manually identify pixels that appeared to be incorrect using an image-display program (xv), then use peek_topo to find out what those values were in ETOPO5. The values from ETOPO5 were planted in TerrainBase using the program poke_topo. All uses of TerrainBase in this data set were based on the patched version, not the original.
Executed color_topo using default mapping of depths to colors, with both ETOPO5 and TerrainBase (patched) as input. This produced portable pixmaps (PPM) of the data. The programs ppmtogif and ppmtopict were used to produce, respectively, GIF and PICT versions of this image. The PPM was scaled down by a factor of four using ppmscale; the resulting image has grid-cell size of 20 arc-minutes. GIF and PICT versions of this version were produced using ppmtogif and ppmtopict. The key, showing the relationship between depth ranges and colors, was generated using the program key, which displays the key in an X window. The window was captured using xwd, the output of which was converted to a portable pixmap and from there to GIF and PICT formats.
Source_Code_Used: color_topo.c Source_Code_Used: key.c Data_File_Produced: mod05e.gif Data_File_Produced: mod05t.gif Data_File_Produced: mod05e.pict Data_File_Produced: mod05t.pict Data_File_Produced: mod20e.gif Data_File_Produced: mod20t.gif Data_File_Produced: mod20e.pict Data_File_Produced: mod20t.pict Data_File_Produced: modkey.gif Data_File_Produced: modkey.pict
The global topography data from W.R. Peltier show estimated elevation and bathymetry at 1,000-year intervals from 21,000 years ago to the present. In order to estimate the proportion of each 1-degree grid-cell that was covered by land, these data were combined with modern global topography as depicted in ETOPO5 and TerrainBase (patched).
This procedure was applied to each 1,000-year interval:
1. The estimate of topography from Dr. Peltier (a 1-degree grid) was subtracted from the modern topography given in the same source data, yielding a 1-degree grid of differences between modern and ancient topography. This was accomplished using the program subtract.c.
2. The grid of differences was extended, using bilinear interpolation, to a grid mapping precisely onto ETOPO5 and TerrainBase. This was carried out by the program bilinear.c.
3. The resulting 5-minute grid of differences from modern topography was added to ETOPO5 or TerrainBase itself, producing an estimate of ancient topography on a 5-minute grid that accounted for ice loading. This was carried out by the program add_topo.c.
4. The program q1x1 was run using this estimate of ancient topography, yielding a 1-degree grid in which each cell estimates the percentage of the corresponding area of the earth's surface that was covered by land.
Source_Code_Used: subtract.c Source_Code_Used: bilinear.c Source_Code_Used: add_topo.c Source_Code_Used: q1x1.c Data_File_Produced: grid00ke.txt Data_File_Produced: grid01ke.txt Data_File_Produced: grid02ke.txt Data_File_Produced: grid03ke.txt Data_File_Produced: grid04ke.txt Data_File_Produced: grid05ke.txt Data_File_Produced: grid06ke.txt Data_File_Produced: grid07ke.txt Data_File_Produced: grid08ke.txt Data_File_Produced: grid09ke.txt Data_File_Produced: grid10ke.txt Data_File_Produced: grid11ke.txt Data_File_Produced: grid12ke.txt Data_File_Produced: grid13ke.txt Data_File_Produced: grid14ke.txt Data_File_Produced: grid15ke.txt Data_File_Produced: grid16ke.txt Data_File_Produced: grid17ke.txt Data_File_Produced: grid18ke.txt Data_File_Produced: grid19ke.txt Data_File_Produced: grid20ke.txt Data_File_Produced: grid21ke.txt Data_File_Produced: grid00kt.txt Data_File_Produced: grid01kt.txt Data_File_Produced: grid02kt.txt Data_File_Produced: grid03kt.txt Data_File_Produced: grid04kt.txt Data_File_Produced: grid05kt.txt Data_File_Produced: grid06kt.txt Data_File_Produced: grid07kt.txt Data_File_Produced: grid08kt.txt Data_File_Produced: grid09kt.txt Data_File_Produced: grid10kt.txt Data_File_Produced: grid11kt.txt Data_File_Produced: grid12kt.txt Data_File_Produced: grid13kt.txt Data_File_Produced: grid14kt.txt Data_File_Produced: grid15kt.txt Data_File_Produced: grid16kt.txt Data_File_Produced: grid17kt.txt Data_File_Produced: grid18kt.txt Data_File_Produced: grid19kt.txt Data_File_Produced: grid20kt.txt Data_File_Produced: grid21kt.txt
For each estimate of the difference between Pliocene and modern eustatic sea-level, the program subset_etopo5 was used to subtract the estimate from the cell values of ETOPO5 and TerrainBase (lowering the land surface is equivalent to raising the sea level). The file resulting from this operation had the same dimensions and units as ETOPO5 or TerrainBase, but purported to show the global topography assuming that estimate of Pliocene eustatic sea-level, and not accounting for changes in landforms occurring during the past three million years.
The program q1x1 was run using this estimate of Pliocene topography, yielding a 1-degree grid in which each cell estimates the percentage of the corresponding area of the earth's surface that was covered by land.
Source_Code_Used: subset_topo.c Source_Code_Used: q1x1.c Data_File_Produced: grid10me.txt Data_File_Produced: grid20me.txt Data_File_Produced: grid25me.txt Data_File_Produced: grid30me.txt Data_File_Produced: grid35me.txt Data_File_Produced: grid40me.txt Data_File_Produced: grid45me.txt Data_File_Produced: grid50me.txt Data_File_Produced: grid60me.txt Data_File_Produced: grid10mt.txt Data_File_Produced: grid20mt.txt Data_File_Produced: grid25mt.txt Data_File_Produced: grid30mt.txt Data_File_Produced: grid35mt.txt Data_File_Produced: grid40mt.txt Data_File_Produced: grid45mt.txt Data_File_Produced: grid50mt.txt Data_File_Produced: grid60mt.txt
Executed color_topo on ETOPO5 and TerrainBase (patched) using mapping of depths to colors found in file chg.bins. This produced a portable pixmap (PPM) of the image. The programs ppmtogif and ppmtopict were used to produce, respectively, GIF and PICT versions of this image. The PPM was scaled down by a factor of four using ppmscale; the resulting image has grid-cell size of 20 arc-minutes. GIF and PICT versions of this version were produced using ppmtogif and ppmtopict. The key, showing the relationship between depth ranges and colors, was generated using a modified version of the program key, which displays the key in an X window. The window was captured using xwd, the output of which was converted to a portable pixmap and from there to GIF and PICT formats.
Source_Code_Used: color_topo.c Source_Code_Used: chg.bins Data_File_Produced: chg05e.gif Data_File_Produced: chg05t.gif Data_File_Produced: chg05e.pict Data_File_Produced: chg05t.pict Data_File_Produced: chg20e.gif Data_File_Produced: chg20t.gif Data_File_Produced: chg20e.pict Data_File_Produced: chg20t.pict Data_File_Produced: chgkey.gif Data_File_Produced: chgkey.pict
Source_Code_Used: q1x1.c Data_File_Produced: grid00e.txt Data_File_Produced: grid00t.txt