USGS Science⌕ Search
Home/Archive/USGS

USGS · 70019882

Evidence for deep mantle circulation from global tomography

R. D. Van Der Hilst·S. Widiyantoro·E.R. Engdahl
View original source ↗Export metadata ↓

Abstract

Seismic tomography based on P-wave travel times and improved earthquake locations provides further evidence for mantle-wide convective flow. The use of body waves makes it possible to resolve long, narrow structures in the lower mantle some of which can be followed to sites of present-day plate oonvergence at the Earth's surface. The transition from subduction-related linear structures in the mid-mantle to long-wavelength hetorogeneity near the core-mantle boundary remains enigmatic, but at least some slab segments seem to sink to the bottom of the mantle.

Explore related subjects

NatureArticle

Keep this discovery

Explore connections, maps & timelines

BibTeXRIS

R. D. Van Der Hilst, S. Widiyantoro, E.R. Engdahl. 1997. Evidence for deep mantle circulation from global tomography. https://doi.org/10.1038/386578a0

Cite the original work for its findings. Save a collection to share your selection of sources.

KEEP EXPLORING

Related USGS reports

USGS2026

Brown bullhead catfish melanoma represents a novel transmissible cancer

Since 2012, brown bullhead catfish ( Ameiurus nebulosus ) in a lake that spans Vermont, USA, and Quebec, Canada, have shown a high rate of melanomas, suggesting a causal contaminant or contagion 1 . We tested the hypothesis that this affliction represents a clonally transmissible cancer, a rare phenomenon in which cancer cells themselves spread between individuals, behaving more like parasites than conventional tumours 2 . Whole-genome sequencing of tumour and matched non-tumour host tissues revealed that tumour mitochondrial and nuclear genomes are more closely related to each other than to their hosts or unaffected fish. Hundreds of thousands of genetic variants are shared among tumour samples but are absent from host fish, vastly exceeding levels seen in conventional cancers 3 . These findings indicate that melanoma in these brown bullheads represents the fourth documented type of naturally occurring transmissible cancer in animals, after dogs 4 , Tasmanian devils 5 and several bivalve species 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 . This raises important questions about the cancer’s origin, the mode of transmission and the long-term impact on fish populations.

Quebec, Vermont↗
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↗
USGS2025

Diverging fish biodiversity trends in cold and warm rivers and streams

Worldwide, freshwater systems contain more than 18,000 fish species 1,2,3 , which are critical to the functioning of these ecosystems 4 and are vital cultural and economic resources to humans 5,6,7 ; despite this value, fish biodiversity is at risk globally 8,9 . In the USA, leading threats to fish communities in rivers and streams include climate change and invasive fish introductions and game fish stocking by humans 10,11,12,13,14 . Here we harmonized US federal biomonitoring datasets with 389 species spanning 27 years (1993–2019) and 2,992 sites to analyse trends in fish biodiversity. In cold streams (past summer stream temperatures below 15.4 °C), fish abundance and richness declined by 53.4% and 32% over 27 years, respectively, and uniqueness increased. Periodic (large-bodied, late-maturing) fishes increased, and opportunists (small-bodied, short generation time, ‘r-selected’) decreased, possibly due to proliferation of native or introduced game fishes. In warm streams (stream temperatures greater than 23.8 °C), fish abundance and richness increased by 70.5% and 15.6% over 27 years, respectively, and communities homogenized. Small opportunistic fishes replaced large periodic fishes. Intermediate streams (stream temperatures 15.4–23.8 °C), representing the average stream, had minimal changes in fish biodiversity through time. Interactions between warming and introduced fish were associated with increased rates of degradation to local fish biodiversity. Given the magnitude of these changes in a relatively short time span, there is an urgent need to curb degradation of fish biodiversity caused by fish introductions and warming water temperatures.

continental United States↗