Understanding and predicting how global warming affects the structure and functioning of natural ecosystems is a key challenge of the 21st century. Isolated laboratory and field experiments testing global change hypotheses have been criticized for being too small-scale and overly simplistic, whereas surveys are inferential and often confound temperature with other drivers. Research that utilizes natural thermal gradients offers a more promising approach and geothermal ecosystems in particular, which span a range of temperatures within a single biogeographic area, allow us to take the laboratory into nature rather than vice versa. By isolating temperature from other drivers, its ecological effects can be quantified without any loss of realism, and transient and equilibrial responses can be measured in the same system across scales that are not feasible using other empirical methods. Embedding manipulative experiments within geothermal gradients is an especially powerful approach, informing us to what extent small-scale experiments can predict the future behaviour of real ecosystems. Geothermal areas also act as sentinel systems by tracking responses of ecological networks to warming and helping to maintain ecosystem functioning in a changing landscape by providing sources of organisms that are preadapted to different climatic conditions. Here, we highlight the emerging use of geothermal systems in climate change research, identify novel research avenues, and assess their roles for catalysing our understanding of ecological and evolutionary responses to global warming.

How ecological communities respond to predicted increases in temperature will determine the extent to which Earth's biodiversity and ecosystem functioning can be maintained into a warmer future. Warming is predicted to alter the structure of natural communities, but robust tests of such predictions require appropriate large-scale manipulations of intact, natural habitat that is open to dispersal processes via exchange with regional species pools. Here, we report results of a two-year whole-stream warming experiment that shifted invertebrate assemblage structure via unanticipated mechanisms, while still conforming to community-level metabolic theory. While warming by 3.8 °C decreased invertebrate abundance in the experimental stream by 60% relative to a reference stream, total invertebrate biomass was unchanged. Associated shifts in invertebrate assemblage structure were driven by the arrival of new taxa and a higher proportion of large, warm-adapted species (i.e., snails and predatory dipterans) relative to small-bodied, cold-adapted taxa (e.g., chironomids and oligochaetes). Experimental warming consequently shifted assemblage size spectra in ways that were unexpected, but consistent with thermal optima of taxa in the regional species pool. Higher temperatures increased community-level energy demand, which was presumably satisfied by higher primary production after warming. Our experiment demonstrates how warming reassembles communities within the constraints of energy supply via regional exchange of species that differ in thermal physiological traits. Similar responses will likely mediate impacts of anthropogenic warming on biodiversity and ecosystem function across all ecological communities.

Great thicknesses of eolian dune deposits of early Oligocene age crop out in the Chuska Mountains of northwestern NewMexico-Arizona (as much as 535 m thick) and in the Mogollon-Datil volcanic fi eld of western New Mexico-Arizona (as much as 300 m thick). 40 Ar/ 39 Ar ages of intercalated volcanic rocks indicate eolian deposition in these areas was approximately synchronous, with eolian accumulation beginning regionally at ca. 33.5 Ma and ending at ca. 27 Ma. Probable eolian sandstone of Oligocene age 483 m thick is also present in the subsurface of the Albuquerque Basin of the Rio Grande rift. The beginning of eolian deposition on the Colorado Plateau corresponds closely to the beginning of eolian (loessic) deposition in the White River Group of the Great Plains and major Oi-1 glaciation in Antarctica, suggesting possible global paleoclimatic control.Successions of Oligocene eolian sandstone on the Colorado Plateau are thicker than all of the better known Upper Paleozoic-Mesozoic eolianites in the region, except the Jurassic Navajo Sandstone. We suggest that the widely separated Oligocene eolianites in the Colorado Plateau region were probably originally continuous, and thus are erosional remnants of an extensive (~140,000 km 2 ), regional sand sea (the Chuska erg). This interpretation is based on: (1) comparison with thickness trends of older eolianites in the Colorado Plateau region, (2) evaluation of regional topographic gradients of modern ergs, and (3) hydrologic modeling of a 300-to 400-mthick zone of saturation that existed during eolian deposition in the Chuska Mountains.The Chuska erg represents the fi nal episode of Paleogene aggradation on the central and southern Colorado Plateau. Aggradation was driven primarily by trapping of fl uvial sediments on the plateau by development of major volcanic fi elds along the eastern plateau margin. These volcanic fi elds blocked earlier Laramide drainages that had previously transported sediments eastward off the plateau. Following a shift to widespread eolian deposition at ca. 33.5 Ma, constructional volcanic topography induced eolian accumulation upwind of developing volcanic fi elds. Stratal accumulation rates (not decompacted) of eolian deposits were ~28-82 m/m.y.The reconstructed top of the Chuska erg would lie at a present-day elevation of ~3000 m or more, and provides a datum for assessing subsequent erosion on the Colorado Plateau. Major exhumation (≥1230 m) occurred during the late Oligocene and early Miocene, following the end of Chuska deposition and prior to the onset of Bidahochi Formation deposition at ca. 16 Ma on the southcentral part of the plateau. The Bidahochi Formation attained a thickness of ~250 m by ca. 6 Ma, followed by ~520 m of late Miocene and younger erosion in the valley of the Little Colorado River. The depth of late Oligoceneearly Miocene (ca. 26-16 Ma) exhumation of the central and southern Colorado Plateau thus was more than twice that of the late Miocene-Holocene (ca. 6-0 Ma). The timing of initial deep erosion in the Colorado Pla...

Variation in resource supply can cause variation in temperature dependences of metabolic processes (e.g., photosynthesis and respiration). Understanding such divergence is particularly important when using metabolic theory to predict ecosystem responses to climate warming. Few studies, however, have assessed the effect of temperature-resource interactions on metabolic processes, particularly in cases where the supply of limiting resources exhibits temperature dependence. We investigated the responses of biomass accrual, gross primary production (GPP), community respiration (CR), and N2 fixation to warming during biofilm development in a streamside channel experiment. Areal rates of GPP, CR, biomass accrual, and N2 fixation scaled positively with temperature, showing a 32- to 71-fold range across the temperature gradient (approximately 7 degrees-24 degrees C). Areal N2-fixation rates exhibited apparent activation energies (1.5-2.0 eV; 1 eV = approximately 1.6 x 10(-19) J) approximating the activation energy of the nitrogenase reaction. In contrast, mean apparent activation energies for areal rates of GPP (2.1-2.2 eV) and CR (1.6-1.9 eV) were 6.5- and 2.7-fold higher than estimates based on metabolic theory predictions (i.e., 0.32 and 0.65 eV, respectively) and did not significantly differ from the apparent activation energy observed for N2 fixation. Mass-specific activation energies for N2 fixation (1.4-1.6 eV), GPP (0.3-0.5 eV), and CR (no observed temperature relationship) were near or lower than theoretical predictions. We attribute the divergence of areal activation energies from those predicted by metabolic theory to increases in N2 fixation with temperature, leading to amplified temperature dependences of biomass accrual and areal rates of GPP and R. Such interactions between temperature dependences must be incorporated into metabolic models to improve predictions of ecosystem responses to climate change.

Basalts from Hole 504B, Leg 83, exhibit remarkable uniformity in major and trace element composition throughout the 1075.5 m of basement drilled. The majority of the basalts, Group D', have unusual compositions relative to normal (Type I) mid-ocean ridge basalts (MORB). These basalts have relatively high mg values (0.60-0.70) and CaO abundances (11.7-13.7%; Ca/Al = 0.78-0.89), but exhibit a marked depletion in compatible trace elements (Cr and Ni); moderately incompatible trace elements (Zr, Y, Ti, etc.); and highly incompatible trace elements (Nb, LREE, etc.). Petrographic and compositional data indicate that most of these basalts are evolved, having fractionated significant amounts of plagioclase, olivine, and clinopyroxene. Melting experiments on similar basalt compositions from the upper portion of Hole 504B (Leg 70; Autio and Rhodes, 1983) indicate that the basalts are co-saturated with olivine and plagioclase and often clinopyroxene on the 1-atm. liquidus. Two rarely occurring groups, M' and T, are compositionally distinct from Group D' basalts. Group T is strongly depleted in all magmaphile elements except the highly incompatible ones (Nb, La, etc.), while Group M' has moderate concentrations of both moderately and highly incompatible trace elements and is similar to Type I MORB. Groups M' and T cannot be related to Group D' nor to each other by crystal fractionation, crystal accumulation, or magma mixing. The large differences in magmaphile element ratios (Zr/Nb, La/Yb) among these three chemical groups may be accounted for by complex melting models and/or local heterogeneity of the mantle beneath the Costa Rica Ridge. Xenocrysts and xenoliths of plagioclase and clinopyroxene similar in texture and mineral composition to crystals in coarse-grained basalts from the lower portion of the hole are common in Hole 504B basalts. These suggest that addition of solid components either from conduit or magma chamber walls has occurred and may be a common source of disequilibrium crystals in these basalts. However, mixing of plagioclase-laden depleted melts (similar to the Costa Rica Ridge Zone basalts) with normal MORB magmas could provide an alternate source for some refractory plagioclase crystals found out of equilibrium in many phyric MORB. The uniformity of major element compositions in Hole 504B basalts affords an ideal situation for investigating the effects of alteration on some major and trace elements in oceanic basalts. Alteration observed in whole-rock samples records primarily two events-a high-temperature and a low-temperature phase. High-temperature phases include: chlorite, talc, albite, actinolite, sphene, quartz, and pyrite. The low-temperature phases include smectite (saponite), epistilbite or laumontite, and minor calcite. Laumontite may actually straddle the gap between the low-and high-temperature mineral assemblages. Alteration is restricted primarily to partial replacement of primary phases. Metamorphic grade, in general, increases from the top to the bottom of Hole 504B (Legs 69, 70, and 83)...