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At least 559 records · Page 31Linked to original sources
USGS2011

Mg-spinel lithology: A new rock type on the lunar farside

High-resolution compositional data from Moon Mineralogy Mapper (M 3) for the Moscoviense region on the lunar farside reveal three unusual, but distinctive, rock types along the inner basin ring. These are designated "OOS" since they are dominated by high concentrations of orthopyroxene, olivine, and Mg-rich spinel, respectively. The OOS occur as small areas, each a few kilometers in size, that are widely separated within the highly feldspathic setting of the basin rim. Although the abundance of plagioclase is not well constrained within the OOS, the mafic mineral content is exceptionally high, and two of the rock types could approach pyroxenite and harzburgite in composition. The third is a new rock type identified on the Moon that is dominated by Mg-rich spinel with no other mafic minerals detectable (<5% pyroxene, olivine). All OOS surfaces are old and undisturbed since basin formation. They are effectively invisible in image data and are only recognized by their distinctive composition identified spectroscopically. The origin of these unusual lithologies appears to be linked to one or more magmatic intrusions into the lower crust, perhaps near the crust-mantle interface. Processes such as fractional crystallization and gravity settling within such intrusions may provide a mechanism for concentrating the mafic components within zones several kilometers in dimension. The OOS are embedded within highly anorthositic material from the lunar crust; they may thus be near contemporaneous with crustal products from the cooling magma ocean. Copyright ?? 2011 by the American Geophysical Union.

Journal of Geophysical Research E: Planets↗
USGS2020

Caldera collapse geometry revealed by near‐field GPS displacements at Kilauea Volcano in 2018

We employ near‐field GPS data to determine the subsurface geometry of a collapsing caldera during the 2018 Kīlauea eruption. Collapse occurred in 62 discrete events, with “inflationary” deformation external to the collapse, similar to previous basaltic collapses. We take advantage of GPS data from the collapsing block and independent constraints on the magma chamber geometry from inversion of deflation prior to collapse onset. This provides an unparalleled opportunity to constrain the collapse geometry. Employing an axisymmetric finite element model, the co‐collapse displacements are best explained by piston‐like subsidence along a high angle ( ∼ 85°) normal ring fault that may steepen to vertical with depth. Reservoir magma has compressibility of 2→15 × 10 −10 Pa −1 , indicating bubble volume fractions from 1% to 7% (lower if fault steepens with depth). Magma pressure increases during collapses are 1 to 3 MPa, depending on compressibility. Depressurization of a triaxial point source in a homogeneous half‐space fits the data well but provides a biased representation of the source depth and process.

Hawaii↗
USGS2021

Investigating vegetation responses to underground nuclear explosions through integrated analyses

Vegetation has the potential to respond to underground nuclear explosions, yet these links have not been fully explored. Given the lack of previously described signatures, the changes in vegetation are possibly subtle. The integration of multiple different data streams is potentially a useful approach to improve signal detection. Here, we investigate whether semi-arid vegetation growth patterns responded to eight legacy underground nuclear tests at the Nevada National Security Site in southern Nevada, USA. We tested for spatial and temporal changes in vegetation cover, tree growth patterns, and tree leaf spectral properties using ground-based measurements, including those from tree-rings and hyperspectral surface vegetation reflectance, as well as space-based measurements of Normalized Difference Vegetation Index (NDVI) from Landsat. Multiple data streams suggest a localized (<1.2 km) spatial pattern whereby tree growth is enhanced closer to the source of the underground test relative to sites further away. We also observed a more regional (>1.2–9 km) pattern whereby tree growth is suppressed coincident with a drought beginning 1 year before the 1989 tests, but continuing in the 5 years following the tests, which is anomalous relative to what is expected based on the response of tree growth to previous droughts. Quantification of the relative effects of the tests on vegetation remains a challenge due to the coincident drought and the potential for other disturbances to have impacted tree growth at this time, but the integration of these data reveals a more nuanced growth response than any other one data set indicates alone.

Nevada↗
USGS2021

Numerical simulations of the geospace response to the arrival of an idealized perfect interplanetary coronal mass ejection

Previously, Tsurutani and Lakhina (2014, https://doi.org/10.1002/2013GL058825 ) created estimates for a “perfect” interplanetary coronal mass ejection and performed simple calculations for the response of geospace, including . In this study, these estimates are used to drive a coupled magnetohydrodynamic-ring current-ionosphere model of geospace to obtain more physically accurate estimates of the geospace response to such an event. The sudden impulse phase is examined and compared to the estimations of Tsurutani and Lakhina (2014, https://doi.org/10.1002/2013GL058825 ). The physics-based simulation yields similar estimates for Dst rise, magnetopause compression, and equatorial values as the previous study. However, results diverge away from the equator. values in excess of 30 nT/s are found as low as magnetic latitude. Under southward interplanetary magnetic field conditions, magnetopause erosion combines with strong region one Birkeland currents to intensify the response. Values obtained here surpass those found in historically recorded events and set the upper threshold of extreme geomagnetically induced current activity at Earth.

Space Weather↗
USGS2021

Measuring and interpreting multilayer aquifer-system compactions for a sustainable groundwater-system development

Ever decreasing water resources and climate change have driven the increasing use of groundwater causing land subsidence in many countries. Geodetic sensors such as InSAR, GPS and leveling can detect surface deformation but cannot measure subsurface deformation. A single‐well, single‐depth extensometer can be used to measure subsurface deformation, but it cannot delineate the depths of major compaction and provide insight about the deformation mechanism throughout a complex aquifer system, unless man extensometers at different depths are used. We present a multilayer compaction well (MLCW), installed in a borehole, that uses magnetic rings to detect stratum compaction at 25 depths as deep as 300 m below land surface. Our laboratory and field assessments indicate 1 mm precision and accuracy for one single‐depth magnetic reading. We tested the performance of MLCW by measuring aquifer‐system compaction over the proximal, middle, and distal fans of the Choushui River Alluvial Fan (CRAF) that has long experienced severe land subsidence. The MLCW measurements were used to create time‐depth diagrams of compaction, showing different compaction rates at different layers of aquifers and aquitards to identify the depths of major compactions. The elastic (reversible) and inelastic (irreversible) compactions from MLCW were used in stress‐strain analyses to estimate skeletal specific storages and the safe groundwater levels, below which groundwater extractions have caused irreversible compactions. The hydrogeological parameters derived from MLCW measurements can help governmental agencies to determine effective land‐use and water‐use policies, and ascertain the best strategy for utilizing artificial recharge to prevent land subsidence and achieve sustainable groundwater management.

Water Resources Research↗
USGS2022

Earthquake-derived seismic velocity changes during the 2018 caldera collapse of Kīlauea volcano

The 2018 Kīlauea caldera collapse produced extraordinary sequences of seismicity and deformation, with 62 episodic collapse events which significantly altered the landscape of the summit region. Despite decades of focused scientific studies at Kīlauea, detailed information about the internal structure of the volcano is limited. Recently developed techniques in seismic interferometry can be used to monitor the internal structure of an active volcano more directly by detecting subtle spatiotemporal changes in seismic wave velocity, but their utility relies on accurate interpretations of the underlying phenomena causing those velocity changes. Here, we retrospectively apply repeating-earthquake-based seismic interferometry to the 2018 Kīlauea eruption sequence. We find that seismic velocities changed over two distinct time scales: a sudden increase followed by a slower decrease in velocity in the hours following each collapse event, and a gradual, long-term decrease in velocity over several weeks that ceased approximately 1 month prior to the end of the eruption. Modeling suggests that short-term changes can be explained by magma reservoir pressurization which specifically closed vertical ring fractures. Long-term changes are related to subsidence of the caldera and likely include the influence of inelastic strain from the formation of new fractures. These observations provide new insights into the evolution of Kīlauea during its progressive collapse and will inform future interpretations for near-real-time monitoring at hazardous volcanoes around the world using similar techniques, especially where a dominant fracture orientation is present.

Hawaii↗
USGS2023

The severity of the 2014–2015 snow drought in the Oregon Cascades in a multicentury context

The western United States (US) is a hotspot for snow drought. The Oregon Cascade Range is highly sensitive to warming and as a result has experienced the largest mountain snowpack losses in the western US since the mid-20th century, including a record-breaking snow drought in 2014–2015 that culminated in a state of emergency. While Oregon Cascade snowpacks serve as the state's primary water supply, short instrumental records limit water managers' ability to fully constrain long-term natural snowpack variability prior to the influence of ongoing and projected anthropogenic climate change. Here, we use annually-resolved tree-ring records to develop the first multi-century reconstruction of Oregon Cascade April 1st Snow Water Equivalent (SWE). The model explains 58% of observed snowpack variability and extends back to 1688 AD, nearly quintupling the length of the existing snowpack record. Our reconstruction suggests that only one other multiyear event in the last three centuries was as severe as the 2014–2015 snow drought. The 2015 event alone was more severe than nearly any other year in over three centuries. Extreme low-to-high snowpack “whiplash” transitions are a consistent feature throughout the reconstructed record. Multi-decadal intervals of persistent below-the-mean peak SWE are prominent features of pre-instrumental snowpack variability, but are generally absent from the instrumental period and likely not fully accounted for in modern water management. In the face of projected snow drought intensification and warming, our findings motivate adaptive management strategies that address declining snowpack and increasingly variable precipitation regimes.

Oregon↗
USGS2024

Seasonal and species-level water-use strategies and groundwater dependence in dryland riparian woodlands during extreme drought

Drought-induced groundwater decline and warming associated with climate change are primary threats to dryland riparian woodlands. We used the extreme 2012–2019 drought in southern California as a natural experiment to assess how differences in water-use strategies and groundwater dependence may influence the drought susceptibility of dryland riparian tree species with overlapping distributions. We analyzed tree-ring stable carbon and oxygen isotopes collected from two cottonwood species ( Populus trichocarpa and P . fremontii ) along the semi-arid Santa Clara River. We also modeled tree source water δ 18 O composition to compare with observed source water δ 18 O within the floodplain to infer patterns of groundwater reliance. Our results suggest that both species functioned as facultative phreatophytes that used shallow soil moisture when available but ultimately relied on groundwater to maintain physiological function during drought. We also observed apparent species differences in water-use strategies and groundwater dependence related to their regional distributions. P . fremontii was constrained to more arid river segments and ostensibly used a greater proportion of groundwater to satisfy higher evaporative demand. P . fremontii maintained ∆ 13 C at pre-drought levels up until the peak of the drought, when trees experienced a precipitous decline in ∆ 13 C. This response pattern suggests that trees prioritized maintaining photosynthetic processes over hydraulic safety, until a critical point. In contrast, P . trichocarpa showed a more gradual and sustained reduction in ∆ 13 C, indicating that drought conditions induced stomatal closure and higher water use efficiency. This strategy may confer drought avoidance for P . trichocarpa while increasing its susceptibility to anticipated climate warming.

California↗
USGS2026

Interplanetary electric fields for extreme magnetic storms

Using a list of sudden-commencement storms, the ring-current index, and 1-h near-Earth solar-wind measurements from solar cycles 20–25, we develop extreme-value statistical models relating storm intensity 𝐷 =max⁡{−𝐷⁢𝑠⁢𝑡} to the storm main-phase maximum duskward interplanetary electric field 𝐸 . The conditional relationship 𝐷|𝐸 is demonstrably sublinear—linear models are confidently rejected—indicating saturation of magnetospheric response under extreme solar-wind forcing. An event like that of July 2012 ( 𝐸 =69.6 mV/m), if Earth-directed, would be associated with a median storm intensity of 𝐷 =49⁢5 648 378 nT. Storms comparable to March 1989 ( 𝐷 =594 nT) correspond to electric fields of 𝐸 =5⁢4 76 39 mV/m, while Carrington-class storms ( 𝐷 =964 nT) correspond to 𝐸 =9⁢5 133 68 mV/m—substantially lower than several previous estimates. These results indicate that solar-wind conditions capable of driving extremely intense magnetic storms are less exceptional, and potentially more frequent, than previously thought.

Geophysical Research Letters↗
USGS2012

Galileo observations of Post-Imbrium lunar craters during the first Earth-Moon flyby

Copernican‐age craters are among the most conspicuous features seen on the far side and western limb of the Moon in the Galileo multispectral images acquired in December 1990. Among the new morphologic observations of far‐side craters are bright rays, continuous ejecta deposits, and dark rings associated with probable impact‐melt veneers. These observations suggest that the mapped age assignments of several large far‐side craters (Ohm, Robertson, and possibly Lowell and Lenz) need revision. New crater size‐frequency measurements on Lunar Orbiter images suggest the following age reassignments: Hausen (170 km diameter), Pythagoras (120 km), and Bullialdus (61 km) from Eratosthenian to Upper Imbrian, and Carpenter (60 km) and Harpalus (39 km) from Copernican to Eratosthenian. Colors and albedos of craters (away from impact‐melt veneers) are correlated with their geologic emplacement ages as determined from counts of superposed craters; these age‐color relations are used to estimate the emplacement age (time since impact event) for other Copernican‐age craters. These age‐color relations indicate a probable Copernican age for 27 far‐side or western limb craters larger than 10 km diameter that were not previously mapped as Copernican. The apparent deficiency of Copernican craters on the far side compared with the near side in published geologic maps is not present in our data. Age‐color trends differ between mare and highland regions and between the interiors and continuous ejecta of the craters. Similar trends are established for color and albedo versus soil‐maturity indices for the returned lunar samples, with distinct trends for mare and highland soils. However, the mare versus highland offsets are reversed in the two comparisons. These relations can be explained by variations in regolith thicknesses and rates of mixing with relatively fresh, crystalline ejecta. Therefore, the soil‐maturity trends represent longer geologic time periods in regions with thinner regoliths, such as the maria.

Journal of Geophysical Research E: Planets↗
USGS2012

Petrology and geochronology of metamorphosed volcanic rocks and a middle Cretaceous volcanic neck in the east-central Sierra Nevada, California

Metamorphosed Mesozoic volcanic rocks from the east-central Sierra Nevada range in composition from basalt to rhyolite and have ages, based on whole rock Rb-Sr and U-Pb zircon dating, of about 237–224, 185, 163, 134, and 100 m.y. The major plutons of the batholith in this area are of Triassic (215–200 m.y.) and Cretaceous (94–80 m.y.) ages. Initial 87 Sr/ 86 Sr values for the metamorphosed volcanic rocks of the area are in the range from 0.7042 to 0.7058 and are generally different from the values for the surrounding batholithic rocks (0.7056–0.7066). A circular, zoned granitic pluton, with an outcrop area of 2.5 km 2 , similar in appearance to a ring dike complex, was apparently a conduit for some or possibly all of the middle-Cretaceous metamorphosed volcanic rocks exposed about 5 km to the south in the western part of the Ritter Range. Samples from the metamorphosed volcanic rocks and the pluton yield a Rb-Sr whole rock isochron age of 99.9 ± 2.2 m.y. with an initial 87 Sr/ 86 Sr of 0.7048 ± 0.0001. Major element variation diagrams of the pluton and volcanic rocks define coincident compositional trends. The pluton, intruded into Early Jurassic metamorphosed volcanic rocks (Rb-Sr whole rock isochron age of 185 ± 6 m.y., initial 87 / 86 Sr = 0.70474 ± 0.00009), is composed of a central body of granite that is intruded into and almost completely surrounded by a crescent-shaped outer rim of quartz monzodiorite. Contact relations between the granitic rocks, intrusive in some places and completely gradational at other places, indicate that the quartz monzodiorite was not completely crystallized prior to the intrusion of the granite. Modal minerology within the pluton is also consistent with this inferred crystallization history. The granitic rock textures are cataclastic, characterized by a groundmass of feldspar and quartz that surrounds larger grains of feldspar, hornblende, and biotite. Widely spaced aplite dikes from the granite intrude the quartz monzodiorite along the contact between the granitic rocks. The aplite dikes and interstitial alkali feldspar associated with myrmekite in the pluton, being undeformed, indicate crystallization of a small amount of silicate liquid after cataclasis. Aplite compositions suggest a minimum crystallization pressure of 0.5 kb (1–2 km); this depth is consistent with crystallization near the base of a large stratovolcano. The pluton is characterized by steep, linear features, defined by mineral clots and inclusions, that probably formed during the upward movement of the nearly crystallized magma in the volcanic conduit. The ages of volcanic events relative to the ages of the major intrusive epochs and the major element and isotopic compositions of the volcanic rocks relative to the major plutons indicate that the volcanic rocks are not simply or directly related to the major plutons in the Sierra Nevada.

Journal of Geophysical Research Solid Earth↗
USGS2012

Strangways Crater, Northern Territory, Australia: Siderophile element enrichment and lithophile element fractionation

The Strangways Crater, Northern Territory, Australia (15°12′S, 133°35′ E ), has a central core, about 10 km in diameter, of shocked granitic gneiss and amphibolite, and some remnants of a melt rock sheet, surrounded by outer rings of quartzite and siltstone to a diameter of 20–25 km. Seven samples of melt rock (six granitic melts, one shale melt clast) and four samples of country rock (granitic gneiss, amphibolite, shale, quartzite) were analyzed by neutron-activation analysis: for Sc, Cr, Fe, Co, Zn, Rb, Zr, Sb, Cs, Ba, rare earth elements, Hf, Ta, Th, and U, the samples were analyzed instrumentally; and for Ni, Se, Pd, Ag, Cd, Re, Os, Ir, and Au, they were analyzed radiochemically. Siderophile elements are significantly enriched in the granitic melt rocks relative to country rocks; for example, the Ir enrichments range from 0.6 to 2.8 ppb. The low Ir/Ni ratio (∼0.16 relative to C1 chondrites) excludes a chondritic impacting body, and Cr enrichment argues against impact by an iron meteorite. The Strangways Crater may have been formed by the impact of an olivine-rich achondrite and melt rocks appear to contain about 3 wt.% of projectile material. The composition of the granitic melt rocks cannot be reproduced by any simple mixture of analyzed country rock types and chemical fractionation by selective shock melting appears to have taken place.

Journal of Geophysical Research Solid Earth↗
USGS2012

Active tectonic and magmatic processes beneath Long Valley Caldera, eastern California: An overview

Geological, chronological, and structural studies of the Long Valley-Mono/Inyo Craters area document a long history of related volcanic eruptions and earthquakes controlled by regional extensional tectonics of the Basin and Range province. This activity has persisted for hundreds of thousands of years and is likely to continue. The Long Valley magma chamber had a volume approaching 3000 km 3 prior to its climatic caldera-forming eruption 0.7 m.y. ago but has been reduced to less than a third of this volume by cooling, eruption, and crystallization. Seismic evidence indicates that the main mass of the present Long Valley magma chamber is about 10 km in diameter and that its roof is 8–10 km deep with smaller cupolas as shallow as 4–5 km. Although a chamber of this size is probably capable of producing an eruption approaching 30 km 3 of lava, the record over the past 0.5 m.y. suggests that eruptions of 1 km 3 or less are far more likely. Models proposed for the current ground uplift and seismicity within the caldera require inflation of 0.1–0.2 km 3 by additional magma since mid-1979, and some models suggest that inflation was accompanied by injection of a thin dike or dikes (probably of silicic magma) into the ring fracture zone beneath the south moat. Several of the M 5.8–6.2 earthquakes that occurred in the region beginning in 1978 had non-double-couple focal mechanisms. Whether these unusual mechanisms indicate injection of mafic (low-viscosity) magma at midcrustal depths in the Sierra Nevada block south of the caldera remains debatable. Studies of calderas of various ages throughout the world indicate that episodes of unrest are relatively common and do not invariably culminate in eruptions. Although current unrest is concentrated in the south moat of Long Valley caldera, the Inyo/Mono Craters probably hold a greater potential for producing an eruption in the foreseeable future. The Inyo/Mono Craters have erupted at 500-year intervals over the past 2000–3000 years, whereas the Long Valley magma chamber has erupted at about 200,000-year intervals over the past 700,000 years. In either case, a major earthquake near the caldera could strongly influence the course of volcanic activity.

Journal of Geophysical Research Solid Earth↗
USGS2012

Root zone of the late Proterozoic Salma caldera, northeastern Arabian Shield, Kingdom of Saudi Arabia

The eroded root of the late Proterozoic Salma caldera crops out in a striking, roughly elliptical feature, about 27 km long and 22 km wide, near the northeastern edge of the Arabian Shield. The caldera is genetically part of an elongate alkalic granitic massif (Jabal Salma) that extends 35 km from the caldera to the southwest. Comenditic ash flow tuff and lava(?) of the caldera fill, probably more than 1 km thick, are the oldest recognized rocks of the caldera complex. These rocks were erupted during caldera collapse associated with the rapid evacuation of the upper, mildly peralkalic part of a zoned magma reservoir. Within the caldera fill, a massive, lithic-rich intracaldera rhyolite, probably a lava in excess of 1 km thick, is overlain by a layered ash flow sequence. Numerous megabreccia blocks, probably derived from the caldera wall, occur in the massive rhyolite. Open folds in the layered volcanic rocks may be due to high-temperature slumping of the rocks toward the center of the caldera following collapse. Later peralkalic granite that intruded the caldera ring fracture zone occurs in an arcuate pattern outside the area of exposed caldera fill. After caldera collapse, metaluminous to peraluminous magma rose beneath the caldera at approximately 580 Ma and solidified as biotite alkali granite, rim syenogranite, and late, high-level granophyre. Rare earth element abundances indicate that the layered rhyolite tuff, peralkalic granite, and granophyre are chemically more evolved than the biotite alkali granite and rim syenogranite. The granophyre intruded the caldera fill as a dome-shaped body composed of numerous sheetlike masses. Granophyric texture resulted from rapid pressure release and quenching accompanying the intrusion of each sheet. Maximum penetration of the granophyre into overlying rocks occurred in the central region and along the west side of the caldera, where the caldera fill volcanic rocks have been removed by erosion. No apparent structural doming of the exposed volcanic rocks along the east side of the caldera took place; the layered ash flows commonly dip steeply toward the center of the caldera. Postemplacement deformation and metamorphism of the caldera are minimal. Small-displacement strike-slip faults cut the complex, which is tilted to the northeast by no more than about 2°.

Journal of Geophysical Research Solid Earth↗
USGS1984

Stable isotope compositions of gases and vegetation near naturally burning coal

Our measurements of stable isotope compositions of CO2 issuing from vents produced by naturally burning coal indicate that the coal is oxidized through a kinetic process in which groundwater is the oxidizing agent. The CO2 produced by the oxidation of the coal is extremely depleted in 13C relative to normal atmospheric CO2. The change in the ??13C value of atmospheric CO2 near the vents resulting from the burning coal was not recorded in tree rings from red cedars, but the ??13C values of some C3 and C4 type plants collected from within the area were greatly affected. Our results indicate that the ??13C values of some species of plants may be sensitive indicators of changes in the carbon isotopic composition of atmospheric CO2. ?? 1984 Nature Publishing Group.

Nature↗
USGS2012

Temperature as a potent driver of regional forest drought stress and tree mortality

s the climate changes, drought may reduce tree productivity and survival across many forest ecosystems; however, the relative influence of specific climate parameters on forest decline is poorly understood. We derive a forest drought-stress index (FDSI) for the southwestern United States using a comprehensive tree-ring data set representing AD 1000-2007. The FDSI is approximately equally influenced by the warm-season vapour-pressure deficit (largely controlled by temperature) and cold-season precipitation, together explaining 82% of the FDSI variability. Correspondence between the FDSI and measures of forest productivity, mortality, bark-beetle outbreak and wildfire validate the FDSI as a holistic forest-vigour indicator. If the vapour-pressure deficit continues increasing as projected by climate models, the mean forest drought-stress by the 2050s will exceed that of the most severe droughts in the past 1,000 years. Collectively, the results foreshadow twenty-first-century changes in forest structures and compositions, with transition of forests in the southwestern United States, and perhaps water-limited forests globally, towards distributions unfamiliar to modern civilization.

Nature Climate Change↗
USGS2025

A fire deficit persists across diverse North American forests despite recent increases in area burned

Rapid increases in wildfire area burned across North American forests pose novel challenges for managers and society. Increasing area burned raises questions about whether, and to what degree, contemporary fire regimes (1984–2022) are still departed from historical fire regimes (pre-1880). We use the North American tree-ring fire-scar network (NAFSN), a multi-century record comprising >1800 fire-scar sites spanning diverse forest types, and contemporary fire perimeters to ask whether there is a contemporary fire surplus or fire deficit, and whether recent fire years are unprecedented relative to historical fire regimes. Our results indicate, despite increasing area burned in recent decades, that a widespread fire deficit persists across a range of forest types and recent years with exceptionally high area burned are not unprecedented when considering the multi-century perspective offered by fire-scarred trees. For example, ‘record’ contemporary fire years such as 2020 burned 6% of NAFSN sites—the historical average—well below the historical maximum of 29% sites that burned in 1748. Although contemporary fire extent is not unprecedented across many North American forests, there is abundant evidence that unprecedented contemporary fire severity is driving forest loss in many ecosystems and adversely impacting human lives, infrastructure, and water supplies.

Nature Communications↗
USGS2026

Rising atmospheric CO2 reduces nitrogen availability in boreal forests

Anthropogenic nitrogen (N) pollution has been emphasized as a cause of eutrophication globally. However, several recent datasets have suggested widespread oligotrophication may be occurring in some ecosystems, which is suggested to be a response to rising atmospheric carbon dioxide (eCO 2 ). Plant δ 15 N chronologies have served as primary evidence for oligotrophication, however, there has been wide disagreement whether eCO 2 or temporal changes in N deposition explain these patterns. We constructed δ 15 N tree ring chronologies across Sweden’s 23.5 million hectare productive forest area from the 1950s to 2010s. The study area spans a 1500 km latitudinal distance where N deposition varies four-fold, but where eCO 2 is spatially uniform. Our data revealed negative δ 15 N chronologies throughout Sweden, including forests in the far north where atmospheric N deposition rates are very low. Linear mixed effects models showed that eCO 2 was by far the strongest predictor of δ 15 N values, whereas N deposition variables, temperature, and forest basal area had much lower explanatory power. Our results clarify debates on the interpretation of previous δ 15 N chronologies, and provide clear evidence that eCO 2 is causing oligotrophication in boreal forests, which has implications for predicting their future role as sinks in the global carbon cycle.

Nature↗