A new Arctic Marine Gravity Field derived Using ICESat Altimetry: Tectonic Implications

Using altimeter data from SEASAT in the early 1980s, W.F. (Bill) Haxby and others pioneered the use of spaceborne radar altimetry for computation of accurate marine gravity (to several mGals) over the world's non- polar oceans. These altimetric gravity fields have been critical for filling in gaps between widely spaced surface marine geophysical profiles and hence the gaps in our knowledge of crustal structure as well as plate tectonic fabric beneath the oceans. However, because height trackers on board SEASAT and subsequent microwave satellite altimeters were optimized for open ocean, as opposed to ice-covered ocean operation, the computation of polar marine gravity proved essentially impossible. Starting in 1993, work on the reprocessing of microwave return echoes from the ERS-1 altimeter enabled computation of detailed marine gravity in ice- covered polar seas. These ERS fields revealed previously uncharted tectonic fabric in polar marine areas but these fields miss the northernmost Arctic Ocean because the ERS' orbital inclination confines observations to south of 81.5°N. However we have now used laser altimetry from the Ice, Cloud and land Elevation Satellite (ICESat) launched in January, 2003, to fill much of this polar hole and to map detailed gravity up to 86°N, ICESat's northern limit. Inasmuch as ICESat's mission is to measure sea ice as well as ice sheet elevations, reprocessing of ICESat waveforms is necessary to reduce the "noise" of sea ice freeboard before we can map sea surface topography and compute detailed marine gravity. We have reprocessed ICESat waveforms and then computed Arctic marine gravity using ICESat data from a 2-year time span beginning February, 2003. The resulting ICESat marine gravity field shows important new tectonic details in the high Arctic (81.5N to 86N) particularly in the Makarov Basin and other portions of the tectonically old Amerasian Basin where sea floor spreading appears to have ceased more than 100 million years ago. For example we can trace the southward extension of the Marvin Spur across the Makarov Basin to the Siberian continental margin. In addition we trace the site of a possible extinct spreading center in the Makarov as well as other constraints on the seafloor spreading history of the Makarov. Comparisons with ERS altimetric gravity as well as SCICEX submarine gravity show our ICESat gravity field to have an amplitude and spatial resolution which approaches that of ERS altimetric gravity, i.e., ~4 mGal and ~45 km respectively.