John M. Brozena

Influence of subglacial geology on the onset of a West Antarctic ice stream from aerogeophysical observations

Marine ice-sheet collapse can contribute to rapid sea-level rise. Today, the West Antarctic Ice Sheet contains an amount of ice equivalent to approximately six metres of sea-level rise, but most of the ice is in the slowly moving interior reservoir. A relatively small fraction of the ice sheet comprises several rapidly flowing ice streams which drain the ice to the sea. The evolution of this drainage system almost certainly governs the process of ice-sheet collapse. The thick and slow-moving interior ice reservoir is generally fixed to the underlying bedrock while the ice streams glide over lubricated beds at velocities of up to several hundred metres per year. The source of the basal lubricant — a water-saturated till, overlain by a water system — may be linked to the underlying geology. The West Antarctic Ice Sheet rests over a geologically complex region characterized by thin crust, high heat flows, active volcanism and sedimentary basins. Here we use aerogeophysical measurements to constrain the geological setting of the onset of an active West Antarctic ice stream. The onset coincides with a sediment-filled basin incised by a steep-sided valley. This observation supports the suggestion, that ice-stream dynamics — and therefore the response of the West Antarctice Ice Sheet to changes in climate — are strongly modulated by the underlying geology.

New aerogeophysical study of the Eurasia Basin and Lomonosov Ridge: Implications for basin development

In 1998 and 1999, new aerogeophysical surveys of the Arctic Ocean9s Eurasia Basin produced the first collocated gravity and magnetic measurements over the western half of the basin. These data increase the density and extend the coverage of the U.S. Navy aeromagnetic data from the 1970s. The new data reveal prominent bends in the isochrons that provide solid geometrical constraints for plate reconstructions. Tentative identification of anomaly 25 in the Eurasia Basin links early basin opening to spreading in the Labrador Sea before the locus of spreading in the North Atlantic shifted to the Norwegian-Greenland Sea. With the opening of the Labrador Sea, Greenland began ∼200 km of northward movement relative to North America and eventually collided with Svalbard, Ellesmere Island, and the nascent Eurasia ocean basin. Both gravity and magnetic data sets reconstructed to times prior to chron 13 show a prominent linear anomaly oriented orthogonal to the spreading center and immediately north of the Yermak Plateau and Morris Jesup Rise. This anomaly may mark the locus of shortening and possibly subduction as Greenland collided with the nascent Eurasia Basin and impinged upon the southern Gakkel Ridge. This collision may have contributed to volcanism on the Morris Jesup Rise. By chron 13, Greenland had ended its northward motion and had become fixed to North America, and the plateau north of Greenland had rifted apart to become the Morris Jesup Rise and the Yermak Plateau.

Airborne gravity and precise positioning for geologic applications

Airborne gravimetry has become an important geophysical tool primarily because of advancements in methodology and instrumentation made in the past decade. Airborne gravity is especially useful when measured in conjunction with other geophysical data, such as magnetics, radar, and laser altimetry. The aerogeophysical survey over the West Antarctic ice sheet described in this paper is one such interdisciplinary study. This paper outlines in detail the instrumentation, survey and data processing methodology employed to perform airborne gravimetry from the multi- instrumented Twin Otter aircraft. Precise positioning from carrier-phase Global Positioning Sys- tem (GPS) observations are combined with measurements of acceleration made by the gravity me- ter in the aircraft to obtain the free-air gravity anomaly measurement at aircraft altitude. GPS data are processed using the Kinematic and Rapid Static (KARS) software program, and aircraft vertical acceleration and corrections for gravity data reduction are calculated from the GPS position solu- tion. Accuracies for the free-air anomaly are determined from crossover analysis after significant editing (2.98 mGal rms) and from a repeat track (1.39 mGal rms). The aerogeophysical survey covered a 300,000 km 2 region in West Antarctica over the course of five field seasons. The grav- ity data from the West Antarctic survey reveal the major geologic structures of the West Antarctic rift system, including the Whitmore Mountains, the Byrd Subglacial Basin, the Sinuous Ridge, the Ross Embayment, and Siple Dome. These measurements, in conjunction with magnetics and ice-penetrating radar, provide the information required to reveal the tectonic fabric and history of this important region.

Airborne gravimetry; an investigation of filtering

Low‐pass filtering in airborne gravimetry data processing plays a fundamental role in determining the spectral content and amplitude of the free‐air anomaly. Traditional filters used in airborne gravimetry, the 6 × 20-s resistor‐capacitor (RC) filter and the 300-s Gaussian filter, heavily attenuate the waveband of the gravity signal. As we strive to reduce the overall error budget to the sub-mGal level, an important step is to evaluate the choice and design of the low‐pass filter employed in airborne gravimetry to optimize gravity anomaly recovery and noise attenuation. This study evaluates low‐pass filtering options and presents a survey‐specific frequency domain filter that employs the fast Fourier transform (FFT) for airborne gravity data. This study recommends a new approach to low‐pass filtering airborne data. For a given survey, the filter is designed to maximize the target gravity signal based upon survey parameters and the character of measurement noise. This survey‐specific low‐pass filter approa...

Morphology and tectonics of the Mid‐Atlantic Ridge, 7°–12°S

[1] We present swath bathymetric, gravity, and magnetic data from the Mid-Atlantic Ridge between the Ascension and the Bode Verde fracture zones, where significant ridge–hot spot interaction has been inferred. The ridge axis in this region may be divided into four segments. The central two segments exhibit rifted axial highs, while the northernmost and southernmost segments have deep rift valleys typical of slow-spreading mid-ocean ridges. Bathymetric and magnetic data indicate that both central segments have experienced ridge jumps since � 1 Ma. Mantle Bouguer anomalies (MBAs) derived from shipboard free air gravity and swath bathymetric data show deep subcircular lows centered on the new ridge axes, suggesting that mantle flow has been established beneath the new spreading centers for at least � 1 Myr. Inversion of gravity data indicates that crustal thicknesses vary by � 4 km along axis, with the thickest crust occurring beneath a large axial volcanic edifice. Once the effects of lithospheric aging have been removed, a model in which gravity variations are attributed entirely to crustal thickness variations is more consistent with data from an axis-parallel seismic line than a model that includes additional along-axis variations in mantle temperature. Both geophysical and geochemical data from the region may be explained by the melting of small (<200 km) mantle chemical heterogeneities rather than elevated temperatures. Therefore, there may be no Ascension/Circe plume. INDEX TERMS: 1517 Geomagnetism and Paleomagnetism: Magnetic anomaly modeling; 3010 Marine Geology and Geophysics: Gravity; 3040 Marine Geology and Geophysics: Plate tectonics (8150, 8155, 8157, 8158);

An airborne gravity study of eastern North Carolina

The Naval Research Laboratory (NRL) has developed a prototype airborne gravity measurement system. The core of the system is a LaCoste and Romberg air‐sea gravity meter mounted on a three‐axis stable platform. Corrections to the gravimeter data for altitude and variations in altitude are determined from a combination of highly precise radar and pressure altimeters. The original prototype system was designed for use over oceanic areas. We recently incorporated the pressure measurement to extend use of the airborne system to terrestrial regions where occasional radar altitudes over points of known topographic height can be obtained. The radar heights are used to relate the pressure altitudes to absolute altitudes and to determine the slopes of the isobaric surfaces. Vertical accelerations due to horizontal velocity over a curved, rotating earth (the Eotvos correction) and precise two‐dimensional positions are determined from a Texas Instrument P-code global positioning system. The updated system was tested ...

Ridge-plume interactions or mantle heterogeneity near Ascension Island?

Between the Ascension and Bode Verde Fracture Zones, an ∼200 km length of the axis of the Mid-Atlantic Ridge has a large positive residual depth anomaly and an axial high morphology that suggest a plume influence. New wide-angle seismic and gravity data from this area show that (1) the crustal thickness increases from 6 km at the propagating rift marking the northern limit of the anomalous zone to 10 km at the center of the zone and (2) the residual depth anomalies may be largely explained by Airy isostatic support for these crustal-thickness variations. These observations, combined with the lack of long-wavelength gravity and residual depth anomalies associated with the proposed Ascension plume, suggest either the presence of a weak and intermittent plume, or melting of a series of small mantle heterogeneities.

A preliminary analysis of the NRL airborne gravimetry system

The Naval Research Laboratory has developed an airborne gravimetry system for marine use. The system utilizes global positioning system navigation and high‐resolution radar altimetry to overcome the major difficulties of Eotvos correction and vertical acceleration determination. Precise measurements of aircraft motions permit system operation in the noisy low‐altitude environment and allow averaging data over fairly short periods. This results in good system response to short‐wavelength gravity anomalies. Preliminary analysis indicates average errors of 4 mGals or less on three aircraft profiles over a gravity equipment evaluation range for a flight altitude of 150 m and air speed of 400 km/hr.

New gravity data in the Arctic Ocean : Comparison of airborne and ERS gravity

New gravity fields from airborne gravimetry and from ERS-1 and -2 satellite altimetry cover extensive portions of the Arctic Ocean. These two data sets may constitute as much as 60% of the data contributions to the Arctic Gravity Project compilation. Here we evaluate the accuracy and resolution of these data and quantify their impact on the compilation. Both gravity determinations compare favorably with Geological Survey of Canada surface measurements in the Beaufort Sea (airborne, 1.86-2.09 mGal rms; ERS, 2.64-3.11 mGal rms). Comparisons between the airborne and ERS data over the Chukchi Borderlands reveal a 4.38 mGal rms difference over the smoother region of the field and 7.36 mGal rms over the rugose field generated by the shallow ridges and deep troughs. Coherency between the two data sets in the Chukchi region implies a resolution of 19 km. Comparison with Science Ice Expedition submarine measurements over Chukchi Plateau suggests that the ERS field resolves even shorter-wavelength signal than the airborne data, whereas in the Beaufort Sea the airborne data showed better coherence to ground truth data. Long-wavelength differences exist between the two data sets, expressed as a 2-3 mGal offset over the Chukchi region. This study highlights the respective strengths of the two data sets. The ERS gravity field has the advantage of ubiquitous coverage of the ocean south of 81.5 degreesN, a denser sampling of the gravity field, and a recovery of signal down to similar to 15 km. The airborne data cover a significant portion of the polar hole in the satellite coverage, have lower measurement noise, and recover somewhat higher anomaly amplitudes in the 25-100 km wavelength range.

Accuracy of satellite altimeter elevations over the Greenland ice sheet

The accuracy of Geosat satellite altimetry over the Greenland ice sheet is evaluated by comparing the measured heights to radar elevations from the airborne Greenland Aerogeophysics Project. At the center of the ice sheet where the ice surface is nearly level, surface comparisons show a fit at the 1 to 3 m level as expected, but even at moderately sloping ice regions (0.3°–0.6°), satellite altimetry mean errors in the range of 10 to 35 m are observed. These errors are found for slope-corrected and waveform-retracked data, so most previous accuracy estimates of current satellite altimetry ice sheet elevations in regions of sloping or undulating ice appear to be too optimistic.