OBTAINING LOWER AND UPPER BOUNDS ON THE VALUE OF SEASONAL CLIMATE FORECASTS AS A FUNCTION OF RISK PREFERENCES

A methodological approach to obtain bounds on the value of information based on an inexact representation of the decision makerÂ’s utility function is presented. Stochastic dominance procedures are used to derive the bounds. These bounds provide more information than the single point estimates associated with traditional decision analysis approach to valuing information, in that classes of utility functions can be considered instead of one specific utility function. Empirical results for valuing seasonal climate forecasts illustrate that the type of management strategy given by the decision makerÂ’s prior knowledge interacts with the decision makerÂ’s risk preferences to determine the bounds.Risk and Uncertainty,

Gravity evidence of very thin crust at the Gakkel Ridge (Arctic Ocean)

Gakkel Ridge, the active spreading center in the Arctic Ocean, is the slowest spreading portion of the global mid-ocean ridge system. Total spreading rates range from 0.6 cm/yr in the east where the ridge disappears beneath the Laptev shelf to 1.3 cm/yr in the west near Greenland. Bathymetry and gravity surveys of four sections of the Gakkel Ridge were carried out in 1996 by the U.S. Navy nuclear submarine USS POGY as part of SCICEX 96 in order to sample variations in seafloor morphology and gravity anomalies as a function of spreading rate. The ridge axis throughout the survey area is characterized by a continuous axial rift valley similar to that observed at other slow spreading ridges. The continuous rift axis suggests that well-organized seafloor spreading is occurring at total spreading rates of less than 1 cm/yr. In three faster spreading (1.13–1.24 cm/yr) western survey areas located between 7ºE and 54ºE, the Gakkel Ridge is deep compared with other ridge axes. Axial depths range between 4600 and 5100 m and ridge flanks at about 3200 m. The ridge flank morphology is very blocky and is characterized by large scarps and deep fault-bounded troughs. Very large amplitude free-water anomalies with peak-to-trough amplitudes of 85–150 mGal are observed centered on the axis of the Gakkel Ridge. Modeling of the free-water anomalies by varying the crustal thickness and average crustal density, including the gravity effect of the cooling of the mantle away from the axis, implies that if the average crustal density is less than 2900 kgm3, the crustal thickness must be less than 4 km. The axial rift valley at the fourth survey area, near 98ºE where the total spreading rate is 0.99 cm/yr, is buried by sediments. The axis in this region is associated with a continuous 70 mGal gravity minimum implying the presence of a large buried rift valley. The rift flanks at 95ºE are at a depth of greater than 3800 m, 600 m deeper than the average depth at the Gakkel Ridge axis west of 60ºE. Simple isostatic calculations suggest that the crust in this region may be vanishingly thin beneath the sediment cover. These observations indicate a relationship between melt production and seafloor spreading rate at very slow spreading rates, suggesting that ultra-slow spreading may suppress melt production or delivery at the Gakkel Ridge

Morphology and structure of the Lomonosov Ridge, Arctic Ocean

The Lomonosov Ridge is a band of continental crust that stretches across the Arctic Ocean and separates the Mesozoic Amerasian Basin from the Cenozoic Eurasian Basin. From about 87°N north of Greenland across the Pole to about 86°N, the Lomonosov Ridge is a single high-standing blocky ridge with minimum depths of ~ 950 - 1400 m. South of 86°N on the Siberian side, the ridge breaks up into a series of ridges spread over a width of about 200 km. In this region a high-standing blocky ridge with minimum depths of ~ 650 - 1400 m bounds the Eurasian Basin and continues to the Siberian continental margin. This ridge is continuous with the single ridge making up the Lomonosov Ridge toward North America and is the former outermost continental shelf of Eurasia bounding the Amerasian Basin. The Eurasian Basin margin of the Lomonosov Ridge consists of a series of rotated fault blocks stepping down to the basin that result from nearly orthogonal rifting to form the Eurasian Basin. No rotated fault blocks are observed on the Amerasian Basin margin of the Lomonosov Ridge. On the Amerasian Basin side, Marvin Spur, a linear ridge separated from Lomonosov Ridge by a deep basin, parallels Lomonosov Ridge on the North American side of the pole. At the bend in the Lomonosov Ridge near the North Pole, Marvin Spur continues along strike across the Makarov Basin. South of 86°N toward Siberia, a continuous outer ridge makes up the Amerasian Basin edge of the Lomonosov complex with a series of basins and ridges between it and the former Eurasian shelf. The outer ridge marks an abrupt boundary between the Lomonosov Ridge complex and the apparently oceanic crust of the Makarov Basin. The outer ridge and Marvin Spur very closely follow small circles about a pole located on the Mackenzie delta. The observed structure on the Amerasian Basin side of the Lomonosov Ridge is analogous to that observed at well-studied shear margins and supports rotational models for the development of the Amerasian Basin

The Gakkel Ridge: Bathymetry, gravity anomalies, and crustal accretion at extremely slow spreading rates

The Gakkel Ridge in the Arctic Ocean is the slowest spreading portion of the global mid-ocean ridge system. Total spreading rates range from 12.7 mm/yr near Greenland to 6.0 mm/yr where the ridge disappears beneath the Laptev Shelf. Swath bathymetry and gravity data for an 850 km long section of the Gakkel Ridge from 5°E to 97°E were obtained from the U.S. Navy submarine USS Hawkbill. The ridge axis is very deep, generally 4700–5300 m, within a well-developed rift valley. The topography is primarily tectonic in origin, characterized by linear rift-parallel ridges and fault-bounded troughs with up to 2 km of relief. Evidence of extrusive volcanic activity is limited and confined to specific locations. East of 32°E, isolated discrete volcanoes are observed at 25 - 95 km intervals along the axis. Abundant small-scale volcanism characteristic of the Mid-Atlantic Ridge (MAR) is absent. It appears that the amount of melt generated is insufficient to maintain a continuous magmatic spreading axis. Instead, melt is erupted on the seafloor at a set of distinct locations where multiple eruptions have built up central volcanoes and covered adjacent areas with low relief lava flows. Between 5°E and 32°E, almost no volcanic activity is observed except near 19°E. The ridge axis shoals rapidly by 1500 m over a 30 km wide area at 19°E, which coincides with a high-standing axis-perpendicular bathymetric high. Bathymetry and side scan data show the presence of numerous small volcanic features and flow fronts in the axial valley on the upper portions of the 19°E along-axis high. Gravity data imply up to 3 km of crustal thickening under the 19°E axis-perpendicular ridge. The 19°E magmatic center may result from interaction of the ridge with a passively imbedded mantle inhomogeneity. Away from 19°E, the crust appears thin and patchy and may consist of basalt directly over peridotite. The ridge axis is continuous with no transform offsets. However, sections of the ridge have distinctly different linear trends. Changes in ridge trend at 32°E and 63°E are associated with a set of bathymetric features that are very similar to each other and to inside/outside corner complexes observed at the MAR including high-standing ‘‘inside corner’’ ridges, which gravity data show to be of tectonic rather than magmatic origin

Abyssal Hill Segmentation: Quantitative Analysis of the East Pacific Rise Flanks 7°S-9°S

The recent RN Maurice Ewing EW9105 Hydrosweep survey of the East Pacific Rise (EPR) and adjacent flanks between 7°S and 9°S provides an excellent opportunity to explore the causal relationship between the ridge and the abyssal hills which form on its flanks. These data cover 100% of the flanking abyssal hills to 115 km on either side of the axis. We apply the methodology of Goff and Jordan (1988) for estimating statistical characteristics of abyssal hill morphology (rms height, characteristic lengths and widths, plan view aspect ratio, azimuthal orientation, and fractal dimension). Principal observations include the following: (I) the rms height of abyssal hill morphology is negatively correlated with the width of the 5- to 20-km-wide crestal high, consistent with the observations of Goff (1991) for northern EPR abyssal hill morphology; (2) the characteristic abyssal hill width displays no systematic variation with position relative to ridge segmentation within the EW9105 survey area, in contrast with observations of Goff (1991) for northern EPR abyssal hill morphology in which characteristic widths tend to be smallest al segment ends and largest toward the middle of segments; (3) abyssal hill rms heights and characteristic widths are very large just north of a counterclockwise rotating "nannoplate", suggesting that the overlap region is being pushed northward in response to microplate-style tectonics; and (4) within the 7°12'S-8°38'S segment, abyssal hill lineaments are generally parallel to the ridge axis, while south of this area, abyssal hill lineaments rotate with a larger "radius of curvature" than does the EPR axis approaching the EPR-Wilkes ridge-transform intersection

Bathymetric and oceanic controls on Abbot Ice Shelf thickness and stability

Ice shelves play key roles in stabilizing Antarctica’s ice sheets, maintaining its high albedo and returning freshwater to the Southern Ocean. Improved data sets of ice shelf draft and underlying bathymetry are important for assessing ocean–ice interactions and modeling ice response to climate change. The long, narrow Abbot Ice Shelf south of Thurston Island produces a large volume of meltwater, but is close to being in overall mass balance. Here we invert NASA Operation IceBridge (OIB) airborne gravity data over the Abbot region to obtain sub-ice bathymetry, and combine OIB elevation and ice thickness measurements to estimate ice draft. A series of asymmetric fault-bounded basins formed during rifting of Zealandia from Antarctica underlie the Abbot Ice Shelf west of 94°W and the Cosgrove Ice Shelf to the south. Sub-ice water column depths along OIB flight lines are sufficiently deep to allow warm deep and thermocline waters observed near the western Abbot ice front to circulate through much of the ice shelf cavity. An average ice shelf draft of ~200m, 15% less than the Bedmap2 compilation, coincides with the summer transition between the ocean surface mixed layer and upper thermocline. Thick ice streams feeding the Abbot cross relatively stable grounding lines and are rapidly thinned by the warmest inflow. While the ice shelf is presently in equilibrium, the overall correspondence between draft distribution and thermocline depth indicates sensitivity to changes in characteristics of the ocean surface and deep waters

Bathymetry in Petermann fjord from Operation IceBridge aerogravity

Petermann Glacier is a major glacier in northern Greenland, maintaining one of the few remaining floating ice tongues in Greenland. Monitoring programs, such as NASA’s Operation IceBridge have surveyed Petermann Glacier over several decades and have found it to be stable in terms of mass balance, velocity and grounding-line position. The future vulnerability of this large glacier to changing ocean temperatures and climate depends on the ocean–ice interactions beneath its floating tongue. These cannot currently be predicted due to a lack of knowledge of the bathymetry underneath the ice tongue. Here we use aerogravity data from Operation IceBridge, together with airborne radar and laser data and shipborne bathymetry-soundings to model the bathymetry beneath the Petermann ice tongue. We find a basement-cored inner sill at 540–610 m depth that results in a water cavity with minimum thickness of 400 m about 25 km from the grounding line. The sill is coincident with the location of the melt rate minimum. Seaward of the sill the fjord is strongly asymmetric. The deepest point occurs on the eastern side of the fjord at 1150 m, 600 m deeper than on the western side. This asymmetry is due to a sedimentary deposit on the western side of the fjord. A 350–410 m-deep outer sill, also mapped by marine surveys, marks the seaward end of the fjord. This outer sill is aligned with the proposed Last Glacial Maximum (LGM) grounding-line position for Petermann Glacier. The inner sill likely provided a stable pinning point for the grounding line in the past, punctuating the retreat of Petermann Glacier since the LGM

Central anomaly magnetization high: constraints on the volcanic construction and architecture of seismic layer 2A at a fast-spreading mid-ocean ridge, the EPR at 9º30'–50'N

The central anomaly magnetization high (CAMH) is a zone of high crustal magnetization centered on the axis of the East Pacific Rise (EPR) and many other segments of the global mid-ocean ridge (MOR). The CAMH is thought to reflect the presence of recently emplaced and highly magnetic lavas. Forward models show that the complicated character of the near-bottom CAMH can be successfully reproduced by the convolution of a lava deposition distribution with a lava magnetization function that describes the variation in lava magnetization intensity with age. This lava magnetization function is the product of geomagnetic paleofield intensity, which has increased by a factor of 2 over the last 40 kyr, and low-temperature alteration, which decreases the remanence of lava with exposure to seawater. The success of the forward modeling justifies the inverse approach: deconvolution of the magnetic data for lava distribution and integration of that distribution for magnetic layer thickness. This approach is tested on two near-bottom magnetic profiles AL2767 and AL2771, collected using Alvin across the EPR axis at 9º31'N and 9º50'N. Our analysis of these data produces an estimate of the relative thickness of the magnetic lava layer, which is remarkably consistent with existing multichannel estimates of layer 2A thickness from lines CDP31 and CDP27. The similarity between magnetic layer and seismic layer 2A at the 9º–10ºN segment of the EPR crest provides independent support to the notion that seismic layer 2A in young oceanic crust represents the highly magnetic lava layer, and that the velocity gradient at the base of layer 2A is related to the increasing number of higher velocity dikes with depth in the lava–dike transition zone. The near-bottom magnetic anomaly character of the CAMH is a powerful indicator of the emplacement history of upper crust at MORs which allows prediction of the relative thickness and architecture of the extrusive lavas independent of other constraint

Continuous near-bottom gravity measurements made with a BGM-3 gravimeter in DSV Alvin on the East Pacific Rise crest near 9°31 'N and 9°50'N

A Bell BGM-3 gravimeter has been used to collect continuous, underway, near-bottom (3- to 10-m altitude) gravity measurements from the deep-diving submersible DSV Alvin during surveys on the East Pacific Rise (EPR) crest near 9° 31'N and 9° 50'N. Closely spaced (20- to 30-m) gravity measurements were made along transects up to 8 km-long in both regions. Repeatability of measurements made at the same location on different dives is ~ 0.3 mGal. Along-track spatial resolution of anomalies is ~130-160 m, with the limiting factors being precision and sampling rate of the pressure gauge depth data used to calculate vertical accelerations of the submersible. The average upper crustal density of the ridge crest determined from the relationship between depth and free-water gravity anomalies varies greatly between 9 °31 'N and 9° 50'N. Average upper crustal densities of2410 kg/m3 for the 9° 50'N area and 2690 kg/m3 for the 9° 31'N area were calculated. The different densities are not due to differing geometry of the Layer 2A-2B boundary or a regional cross-axis gravity gradient. Differences in porosity of the shallow crustal rocks, or a difference in the proportion of low-density extrusives to higher-density dikes and sills within Layer 2A in these two areas, are the likely causes of the different upper crustal densities. Bouguer gravity anomalies near the EPR axis are primarily small amplitude (0.5-2 mGal), are a few hundred meters across, and appear to be lineated parallel to the axis. Larger-amplitude Bouguer anomalies of up to 4 mGal were found at a few locations across the crestal plateau and are associated with pillow ridges composed of lavas which are clearly younger than the surrounding seafloor. These ridges have distinct chemical compositions compared to lavas from the axial summit collapse trough (ASCT) at the same latitude. Probable sources of the 0.5- to 2-mGal anomalies observed on the summit plateau include areas of collapsed and fissured terrain and dike swarms feeding melt through Layer 2A to the surface. A grid survey of the ridge axis near 9° 50'N shows Bouguer anomalies lineated along the axis, suggesting that dike swarms do contribute to the observed Bouguer anomalies. The along-axis continuity of the gravity anomalies is disrupted at a 75-m offset of the ASCT, suggesting that shallow feeders of lava to the surface may be segmented on a finer scale than the deeper crustal magmatic system. This initial study confirms the ability to conduct high-resolution, near-bottom, continuous gravity measurements from Alvin. It also provides important information on how the shallow crustal structure of a fast spreading mid-ocean ridge develops and how it varies with the surface morphology