Dork L. Sahagian

Anthropogenic Disturbance of the Terrestrial Water Cycle

climate, ecology, and biogeochemistry of the planet. Mounting historical evidence for the influence of greenhouse warming on recent climate, and modeling projections into the future, highlight changes to the landbased water cycle as a major global change issue (Houghton et al. 1995, Watson et al. 1996, SGCR 1999). Disturbance of the hydrologic cycle has received significant attention with respect to land–atmosphere exchanges, plant physiology, net primary production, and the cycling of major nutrients (Foley et al. 1996, Sellers et al. 1996, McGuire et al. 1997). Changes in land use are also recognized as critical factors governing the future availability of fresh water (Chase et al. 2000). Another important but seldom articulated global change issue is direct alteration of the continental water cycle for irrigation, hydroelectricity, and other human needs. Although the scope and magnitude of water engineering today are colossal in comparison with preindustrial times, most of the very same activities—irrigation, navigation enhancement, reservoir creation—can be traced back several thousand years in the Middle East and China. Stabilization of water supply has remained a fundamental preoccupation of human society and is a key security concern for most nations. Reducing flood hazard, enhancing food security, and redirecting runoff from water-rich to water-poor areas continue to provide a major challenge to our engineering infrastructure. In this article we address three issues. First, we document the nature and magnitude of direct human alteration of the terrestrial water cycle, specifically through construction of engineering works for water resource management. We focus on the redistribution of freshwater among major storage pools and the corresponding changes to continental runoff. Second, we explore some of the impacts of this disturbance on drainage basins, river systems, and land-to-ocean linkages. Finally, we review key uncertainties regarding our current understanding of human–water interactions at the global scale and make suggestions on potentially useful avenues for future research. Evidence for global-scale human impacts on the terrestrial water cycle Although an exact inventory of global water withdrawal has been difficult to assemble, the general features of anthropogenic water use are more or less known. Reviews of the recent literature (Shiklomanov 1996, Gleick 2000) show a range in estimated global water withdrawals for the year 2000 between approximately 4000 and 5000 km3/yr. Despite reductions in the annual rate of increase in withdrawals from 1970 (Shiklomanov 1996, 2000, Gleick 1998a), global water use has grown more or less exponentially with human population and economic development over the industrial era. By one account (L’vovich and White 1990), there was a 15-fold increase in aggregate

Eustatic Curve for the Middle Jurassic--Cretaceous Based on Russian Platform and Siberian Stratigraphy: Zonal Resolution

We have used the stratigraphy of the central part of the Russian platform and surrounding regions to construct a calibrated eustatic curve for the Bajocian through the Santonian. The study area is centrally located in the large Eurasian continental craton, and was covered by shallow seas during much of the Jurassic and Cretaceous. The geographic setting was a very low-gradient ramp that was repeatedly flooded and exposed. Analysis of stratal geometry of the region suggests tectonic stability throughout most of Mesozoic marine deposition. The paleogeography of the region led to extremely low rates of sediment influx. As a result, accommodation potential was limited and is interpreted to have been determined primarily by eustatic variations. The central part of the Russian latform thus provides a useful frame of reference for the quantification of eustatic variations throughout the Jurassic and Cretaceous. The biostratigraphy of the Russian platform provides the basis for reliably determining age and eustatic events. The Mesozoic section of the central part of the Russian platform is characterized by numerous hiatuses. In this study, we filled the sediment gaps left by unconformities in the central part of the Russian platform with data from stratigraphic information from the more continuous stratigraphy of the neighboring subsiding regions, such as northern Siberia. Although these sections reflect subsidence, the time scale of variations in subsidence rate is probably long relative to the duration of the stratigraphic gaps to be filled, so the subsidence rate can be calculated and filtered from the stratigraphic data. We thus have compiled a more complete eustatic curve than would be p ssible on the basis of Russian platform stratigraphy alone. Relative sea level curves were generated by backstripping stratigraphic data from well and outcrop sections distributed throughout the central part of the Russian platform. For determining paleowater depth, we developed a model specifically designed for this region based on paleoecology, sedimentology, geochemistry, and paleogeography. The curve describes a series of high-frequency eustatic events superimposed on longer term trends. Many of the events identified from our study can be correlated to those found by Haq et al. (1988) and other sea level studies from other parts of the world, but there are significant differences in the relative magnitudes of events. Because the eustatic curve resulting from this study is based on a stable reference frame, the curve can be used in sedimentary basin modeling and as a tool for quantifying subsidence history from the stratigraphy of passive margins, basins, and other active regions.

Dynamics of diffusive bubble growth in magmas: Isothermal case

We have conducted a parametric study and developed a new cell model describing diffusion-induced growth of closely spaced bubbles in magmatic systems. The model accounts for (1) the effects of advection of melt resulting from bubble growth, and its affect on the local concentration profile; (2) dynamic resistance of the viscous melt during diffusive growth; (3) diffusion of volatiles in response to evolving concentration gradients; (4) mass balance between dissolved volatiles and gas inside the bubble; (5) changes in the equilibrium saturation concentration at the bubble-melt interface; (6) total pressure within the bubble consisting of ambient, surface tension, and dynamic pressures. The results of this study reveal that bubble growth depends strongly on ambient pressure, volatile oversaturation in the melt, and diffusivity coefficients, but only weakly on bubble separation and initial bubble radius. Increased volatile oversaturation increases growth rate to the point at which it actually reduces time for complete bubble growth. This counterintuitive result is due to significant advective volatile flux toward the bubble interface during growth. Viscosity controls growth dynamics only for cases of high viscosity (>104 Pa s). The documentation of the evolution of gas fraction in the melt and bubble wall thickness as a function of time makes it possible to estimate bubble disruption thresholds which bear on volcanic eruption mechanisms. Model results can be applied to the larger-scale problem of magmatic degassing in terms of bubble coalescence, flotation and the development of foams in magma chambers and vent systems, and ultimately to the dynamics of eruption mechanisms.

Dynamics and energetics of bubble growth in magmas: Analytical formulation and numerical modeling

We have developed a model of diffusive and decompressive growth of a bubble in a finite region of melt which accounts for the energetics of volatile degassing and melt deformation as well as the interactions between magmatic system parameters such as viscosity, volatile concentration, and diffusivity. On the basis of our formulation we constructed a numerical model of bubble growth in volcanic systems. We conducted a parametric study in which a saturated magma is instantaneously decompressed to one bar and the sensitivity of the system to variations in various parameters is examined. Variations of each of seven parameters over practical ranges of magmatic conditions can change bubble growth rates by 2–4 orders of magnitude. Our numerical formulation allows determination of the relative importance of each parameter controlling bubble growth for a given or evolving set of magmatic conditions. An analysis of the modeling results reveals that the commonly invoked parabolic law for bubble growth dynamics R∼t1/2 is not applicable to magma degassing at low pressures or high water oversaturation but that a logarithmic relationship R∼log(t) is more appropriate during active bubble growth under certain conditions. A second aspect of our study involved a constant decompression bubble growth model in which an initially saturated magma was subjected to a constant rate of decompression. Model results for degassing of initially water-saturated rhyolitic magma with a constant decompression rate show that oversaturation at the vent depends on the initial depth of magma ascent. On the basis of decompression history, explosive eruptions of silicic magmas are expected for magmas rising from chambers deeper than 2 km for ascent rates >1–5 m s−1.

Stalagmite stable isotope record of recent tropical cyclone events

We present a 23 yr stalagmite record (1977-2000) of oxygen isotope variation, associated with 11 tropical cyclones (TCs), from Actun Tunichil Muknal cave in central Belize. High-resolution microsampling yielded a record of monthly to weekly temporal resolution that contains abrupt decreases (negative excursions) in calcite δ 18 O values that correspond with recent TC rain events nearby. A logistic discriminant model reliably identifi ed TC proxy signals using the measurable parameters δ 18 O and δ 13 C values, and single point changes in δ 18 O value. The logistic model cor- rectly identifi ed 80% of excursions as TC events and incorrectly classifi ed only 1 of nearly 1200 nonstorm sampling points. In addition to enabling high-resolution TC frequency reconstruction, this geologic proxy also provides information about the intensity of individual TCs. A multiple regression predicted TC intensity (R 2 = 0.465, p = 0.034) using sampling frequency and excursion amplitude. Consistent with previous low-resolution studies, we found that the decadal average δ 18 O value was lower during the 1990s when several TCs produced rainfall in the area, but higher during the 1980s when only one TC struck. Longer, accurately dated, high-resolution speleo- them stable isotope records may be a useful new tool for paleotempestology, to clarify associa- tions between highly variable TC activity and the dynamic range of Quaternary climate.

Dynamics of coupled diffusive and decompressive bubble growth in magmatic systems

Bubble growth in an ascending parcel of magma is controlled both by diffusion of oversaturated volatiles and decompression as the magma rises. We have developed a numerical model which explores the processes involved in water exsolution from basaltic and rhyolitic melts rising at a constant rate from magma chamber depths of 4 and 1 km. While the model does not attempt to simulate natural eruptions, it sheds light on the processes which control eruptive behavior under various conditions. Ascent rates are defined such that a constant rate of decompression dP/dt is maintained. A variety of initial ascent rates are considered in the model, from 1 m/s to 100 m/s for basalts, and from a few centimeters per second to 10 m/s for rhyolite, at the base of the conduit. The model results indicate that for any reasonable ascent rate, basaltic melt degasses at a rate sufficient to keep the dissolved volatile concentration at equilibrium with the decreasing ambient pressure. Rhyolitic melt reaches the surface at equilibrium if its ascent rate is less than 1 m/s, but it can erupt with high oversaturation at greater ascent rates. The latter may lead to explosive eruptions. If the ascent rate of rhyolite is 10 m/s or more, then melt barely degasses at all in the conduit and erupts with the highest oversaturation possible. For the case of slow magma rise, bubble growth is limited by decompression. For the case of rapid magma rise, bubble growth is limited by diffusion. The results of our simple model do not accurately simulate natural volcanic eruptions, but suggest that subsequent, more complex models may be able to simulate eruptions using the insights regarding diffusive and decompressive bubble growth processes explored in this study. Numerical modeling of volcanic degassing may eventually lead to better prediction of eruption timing, energetics and hazards of active volcanoes.

Stability of foams in silicate melts

Bubble coalescence and the spontaneous disruption of high-porosity foams in silicate melts are the result of physical expulsion of interpore melt (syneresis) leading to bubble coalescence, and diffusive gas exchange between bubbles. Melt expulsion can be achieved either along films between pairs of bubbles, or along Plateau borders which represent the contacts between 3 or more bubbles. Theoretical evaluation of these mechanisms is confirmed by experimental results, enabling us to quantify the relevant parameters and determine stable bubble size and critical film thickness in a foam as a function of melt viscosity, surface tension, and time. Foam stability is controlled primarily by melt viscosity and time. Melt transport leading to coalescence of bubbles proceeds along inter-bubble films for smaller bubbles, and along Plateau borders for larger bubbles. Thus the average bubble size accelerates with time. In silicate melts, the diffusive gas expulsion out of a region of foam is effective only for water (and even then, only at small length scales), as the diffusion of CO2 is negligible. The results of our analyses are applicable to studies of vesicularity of lavas, melt degassing, and eruption mechanisms.

Depth-dependent stretching: A different approach

Uplift of the crust during rifting is a major problem that has been left unresolved by uniform stretching models. Various depth-dependent stretching models have been proposed to account for this, but these require decoupling of the crust and mantle and result in space problems within the mantle. A significant modification of these discontinuous nonuniform or depth-dependent stretching models is proposed here that accounts for synrift uplift and doming but does not result in these problems. The model involves continuous nonuniform stretching within a polygonal region bounded by symmetrical, outward-sloping boundaries within the mantle. This geometry results in thinning of the lithosphere below crustal regions that are less stretched or unstretched, crustal uplift resulting from asthenospheric upwelling below these regions. This in turn results in stretching factors for the mantle that are less than those for the crust, a relationship opposite to relationships derived by discontinuous nonuniform stretching models. Corollary predictions for mantle-derived heat flow within and outside regions of stretching of these two different depth-dependent models may allow them to be distinguished from each other and from regions where uniform stretching predominates.

Timing of Colorado Plateau uplift: Initial constraints from vesicular basalt-derived paleoelevations

We have developed a technique for measuring paleoelevation on the basis of the vesicularity of basaltic lava flows. We are currently applying this technique to determine the timing and extent of Cenozoic uplift of the Colorado Plateau. Because the technique measures paleoatmospheric pressure, it is not subject to uncertainties stemming from the use of proxies that depend on environmental factors other than elevation alone. Vesicular lavas preserve a record of paleopressure at the time and place of lava emplacement because the difference in internal pressure in bubbles at the base and top of a lava flow depends on atmospheric pressure and lava flow thickness. The modal size of the vesicle (bubble) population is larger at the top than at the bottom. This leads directly to paleoatmospheric pressure and thus elevation because the thickness of the flow can easily be measured in the field, and the vesicle sizes can be measured in the laboratory. On the Colorado Plateau, volcanic fields are generally found around the margins of the plateau. Basaltic lavas range in age from 0 to 23 Ma; present elevations range from 1.7 km to 3.3 km. Samples were collected from lava flows in the areas of Marysvale, Springerville, Grand Mesa, and the San Juan Mountains and analyzed to determine the timing and extent of uplift. The analysis of the samples using X-ray computed tomography imaging and our numerical techniques for determining vesicle population statistics indicate uplift of 350-2200 m, depending on age. There is a clear relationship between age and amount of uplift (original elevation subtracted from present elevation). The results indicate slow uplift of ~40 m/m.y. between 25 Ma and 5 Ma (only 800 m of uplift during that time), and rapid uplift of 220 m/m.y. since 5 Ma (1100 m during that time). These results reconcile the long-standing controversy between interpretations of ancient versus recent uplift by providing an uplift history curve for the Colorado Plateau.

Long-period earthquakes and co-eruptive dome inflation seen with particle image velocimetry.

Dome growth and explosive degassing are fundamental processes in the cycle of continental arc volcanism. Because both processes generate seismic energy, geophysical field studies of volcanic processes are often grounded in the interpretation of volcanic earthquakes. Although previous seismic studies have provided important constraints on volcano dynamics, such inversion results do not uniquely constrain magma source dimension and material properties. Here we report combined optical geodetic and seismic observations that robustly constrain the sources of long-period volcanic earthquakes coincident with frequent explosive eruptions at the volcano Santiaguito, in Guatemala. The acceleration of dome deformation, extracted from high-resolution optical image processing, is shown to be associated with recorded long-period seismic sources and the frequency content of seismic signals measured across a broadband network. These earthquake sources are observed as abrupt subvertical surface displacements of the dome, in which 20–50-cm uplift originates at the central vent and propagates at ∼50 m s-1 towards the 200-m-diameter periphery. Episodic shifts of the 20–80-m thick dome induce peak forces greater than 109 N and reflect surface manifestations of the volcanic long-period earthquakes, a broad class of volcano seismic activity that is poorly understood and observed at many volcanic centres worldwide. On the basis of these observations, the abrupt mass shift of solidified domes, conduit magma or magma pads may play a part in generating long-period earthquakes at silicic volcanic systems.