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Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.

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At least 451 records · Page 25Linked to original sources
USGS2008

Warming of the Indian Ocean threatens eastern and southern African food security but could be mitigated by agricultural development

Since 1980, the number of undernourished people in eastern and southern Africa has more than doubled. Rural development stalled and rural poverty expanded during the 1990s. Population growth remains very high, and declining per-capita agricultural capacity retards progress toward Millennium Development goals. Analyses of in situ station data and satellite observations of precipitation have identified another problematic trend: main growing-season rainfall receipts have diminished by ???15% in food-insecure countries clustered along the western rim of the Indian Ocean. Occurring during the main growing seasons in poor countries dependent on rain-fed agriculture, these declines are societally dangerous. Will they persist or intensify? Tracing moisture deficits upstream to an anthropogenically warming Indian Ocean leads us to conclude that further rainfall declines are likely. We present analyses suggesting that warming in the central Indian Ocean disrupts onshore moisture transports, reducing continental rainfall. Thus, late 20th-century anthropogenic Indian Ocean warming has probably already produced societally dangerous climate change by creating drought and social disruption in some of the world's most fragile food economies. We quantify the potential impacts of the observed precipitation and agricultural capacity trends by modeling 'millions of undernourished people' as a function of rainfall, population, cultivated area, seed, and fertilizer use. Persistence of current tendencies may result in a 50% increase in undernourished people by 2030. On the other hand, modest increases in per-capita agricultural productivity could more than offset the observed precipitation declines. Investing in agricultural development can help mitigate climate change while decreasing rural poverty and vulnerability. ?? 2008 by The National Academy of Sciences of the USA.

Proceedings of the National Academy of Sciences of↗
USGS1970

Strontium isotopes in some Cenozoic lavas from Oregon and Washington

The 87 Sr/ 86 Sr ratios have been determined in 22 volcanic rocks from Oregon and Washington. The emphasis of the study was on andesites and dacites, but several basalts were included. Most of the samples have a very limited range in strontium isotopic composition and all fall within variations previously defined for oceanic basalts. There are no systematic differences between lavas erupted through the pre-Cenozoic basement and those erupted in the Columbia River Embayment. It is concluded that the role played by crustal rocks in the generation of these andesites and dacites is minor or nonexistent, but assimiliation of crustal material may have been a factor for some of the basalts of the Columbia River Group.

Oregon, Washington↗
USGS2017

Terrestrial subaqueous seafloor dunes: Possible analogs for Venus

Dunes on Venus, first discovered with Magellan Synthetic Aperture Radar (SAR) in the early 1990s, have fueled discussions about the viability of Venusian dunes and aeolian grain transport. Confined to two locations on Venus, the existence of the interpreted dunes provides evidence that there could be transportable material being mobilized into aeolian bedforms at the surface. However, because of the high-pressure high-temperature surface conditions, laboratory analog studies are difficult to conduct and results are difficult to extrapolate to full-sized, aeolian bedforms. Field sites of desert dunes, which are well-studied on Earth and Mars, are not analogous to what is observed on Venus because of the differences in the fluid environments. One potentially underexplored possibility in planetary science for Venus-analog dune fields could be subaqueous, seafloor dune fields on Earth. Known to the marine geology communities since the early 1960s, seafloor dunes are rarely cited in planetary aeolian bedform literature, but could provide a necessary thick-atmosphere extension to the classically studied aeolian dune environment literature for thinner atmospheres. Through discussion of the similarity of the two environments, and examples of dunes and ripples cited in marine literature, we provide evidence that subaqueous seafloor dunes could serve as analogs for dunes on Venus. Furthermore, the evidence presented here demonstrates the usefulness of the marine literature for thick-atmosphere planetary environments and potentially for upcoming habitable worlds and oceanic environment research program opportunities. Such useful cross-disciplinary discussion of dune environments is applicable to many planetary environments (Earth, Mars, Venus, Titan, etc.) and potential future missions.

Aeolian Research↗
USGS2016

Building a subduction zone observatory

Subduction zones contain many of Earth’s most remarkable geologic structures, from the deepest oceanic trenches to glacier-covered mountains and steaming volcanoes. These environments formed through spectacular events: Nature’s largest earthquakes, tsunamis, and volcanic eruptions are born here.

Eos, Earth and Space Science News↗
USGS2015

Coral 13C/12C records of vertical seafloor displacement during megathrust earthquakes west of Sumatra

The recent surge of megathrust earthquakes and tsunami disasters has highlighted the need for a comprehensive understanding of earthquake cycles along convergent plate boundaries. Space geodesy has been used to document recent crustal deformation patterns with unprecedented precision, however the production of long paleogeodetic records of vertical seafloor motion is still a major challenge. Here we show that carbon isotope ratios ( ) in the skeletons of massive Porites corals from west Sumatra record abrupt changes in light exposure resulting from coseismic seafloor displacements. Validation of the method is based on the coral response to uplift (and subsidence) produced by the March 2005 M w 8.6 Nias–Simeulue earthquake, and uplift further south around Sipora Island during a M∼8.4 megathrust earthquake in February 1797. At Nias, the average step-change in coral was 0.6±0.1‰/m for coseismic displacements of +1.8 m and −0.4 m in 2005. At Sipora, a distinct change in Porites microatoll growth morphology marks coseismic uplift of 0.7 m in 1797. In this shallow water setting, with a steep light attenuation gradient, the step-change in microatoll is 2.3‰/m , nearly four times greater than for the Nias Porites . Considering the natural variability in coral skeletal , we show that the lower detection limit of the method is around 0.2 m of vertical seafloor motion. Analysis of vertical displacement for well-documented earthquakes suggests this sensitivity equates to shallow events exceeding M w ∼7.2 in central megathrust and back-arc thrust fault settings. Our findings indicate that the coral paleogeodesy technique could be applied to convergent tectonic margins throughout the tropical western Pacific and eastern Indian oceans, which host prolific coral reefs, and some of the world's greatest earthquake catastrophes. While our focus here is the link between coral , light exposure and coseismic crustal deformation, the same principles could be used to characterize interseismic strain during earthquake cycles over the last several millennia.

Earth and Planetary Science Letters↗
USGS2026

Tracing mercury from land to river: Global sources, retention, and implications for sustainability

Mercury (Hg) pollution in river systems is a global sustainability challenge. Yet the transport, transformation, and retention of Hg within global rivers remain poorly quantified, particularly in regions with sparse observations such as Southeast Asia and Africa, hindering effective pollution mitigation and reinforcing geographic inequities in scientific knowledge and environmental governance. Here, we present the first global, high-resolution simulation of riverine Hg dynamics using a process-based model that traces Hg from land-based sources through river networks to the ocean. Under a realistic scenario, we estimate that ∼1900 megagrams per year (Mg/yr) of Hg enters global rivers, including 1500 Mg/yr from human-induced sources and 400 Mg/yr from soil erosion. Nearly half of this flux (∼1000 Mg/yr) is retained in reservoirs and dams, which act as major sinks. While such retention limits downstream delivery to the oceans, it also heightens in-reservoir Hg methylation risks. By bridging the gap between Hg releases and observed riverine exports, our framework offers a scalable tool for data-limited regions, promotes data access, and supports global freshwater and pollution-management strategies.

Environmental Science & Technology↗
USGS2008

On phytoplankton trends

P hytoplankton—unicellular algae in the surface layer of lakes and oceans—fuel the lacustrine and marine food chains and play a key role in regulating atmospheric carbon dioxide concentrations. How will rising carbon dioxide concentrations in the air and surface ocean in turn affect phytoplankton? Answering this question is crucial for projecting future climate change. However, because phytoplankton species populations appear and disappear within weeks, assessing change requires high-resolution monitoring of annual cycles over many years. Such long-term studies at coastal sites ranging from estuaries and harbors to open coastlines and islands are yielding bewildering variability, but also fundamental insights on the driving forces that underlie phytoplankton cycles.

Science↗
USGS2011

Global seabird responses to forage fish depletion - One-third for the birds

Determining the form of key predator-prey relationships is critical for understanding marine ecosystem dynamics. Using a comprehensive global database, we quantified the effect of fluctuations in food abundance on seabird breeding success. We identified a threshold in prey (fish and krill, termed “forage fish”) abundance below which seabirds experience consistently reduced and more variable productivity. This response was common to all seven ecosystems and 14 bird species examined within the Atlantic, Pacific, and Southern Oceans. The threshold approximated one-third of the maximum prey biomass observed in long-term studies. This provides an indicator of the minimal forage fish biomass needed to sustain seabird productivity over the long term.

Science↗
USGS2002

The major-ion composition of Silurian seawater

One-hundred fluid inclusions in Silurian marine halite were analyzed in order to determine the major-ion composition of Silurian seawater. The samples analyzed were from three formations in the Late Silurian Michigan Basin, the A-1, A-2, and B Evaporites of the Salina Group, and one formation in the Early Silurian Canning Basin (Australia), the Mallowa Salt of the Carribuddy Group. The results indicate that the major-ion composition of Silurian seawater was not the same as present-day seawater. The Silurian ocean had lower concentrations of Mg2+, Na+, and SO2-4, and much higher concentrations of Ca2+ relative to the ocean's present-day composition. Furthermore, Silurian seawater had Ca2+ in excess of SO2-4. Evaporation of Silurian seawater of the composition determined in this study produces KC1-type potash minerals that lack the MgSO4-type late stage salts formed during the evaporation of present-day seawater. The relatively low Na+ concentrations in Silurian seawater support the hypothesis that oscillations in the major-ion composition of the oceans are primarily controlled by changes in the flux of mid-ocean ridge brine and riverine inputs and not global or basin-scale, seawater-driven dolomitization. The Mg2+/Ca2+ ratio of Silurian seawater was ~1.4, and the K+/Ca2+ ratio was ~0.3, both of which differ from the present-day counterparts of 5 and 1, respectively. Seawaters with Mg2+/Ca2+ <2 facilitate the precipitation of low-magnesian calcite (mol % Mg < 4) marine ooids and submarine carbonate cements whereas seawater with Mg2+/Ca2+ >2 (e.g., modern seawater) facilitate the precipitation of aragonite and high-magnesian calcite. Therefore, the early Paleozoic calcite seas were likely due to the low Mg2+/Ca2+ ratio of seawater, not the pCO2 of the Silurian atmosphere. Copyright ?? 2002 Elsevier Science Ltd.

Geochimica et Cosmochimica Acta↗
USGS2013

Topobathymetric model of Mobile Bay, Alabama

Topobathymetric Digital Elevation Models (DEMs) are a merged rendering of both topography (land elevation) and bathymetry (water depth) that provides a seamless elevation product useful for inundation mapping, as well as for other earth science applications, such as the development of sediment-transport, sea-level rise, and storm-surge models. This 1/9-arc-second (approximately 3 meters) resolution model of Mobile Bay, Alabama was developed using multiple topographic and bathymetric datasets, collected on different dates. The topographic data were obtained primarily from the U.S. Geological Survey (USGS) National Elevation Dataset (NED) (http://ned.usgs.gov/) at 1/9-arc-second resolution; USGS Experimental Advanced Airborne Research Lidar (EAARL) data (2 meters) (http://pubs.usgs.gov/ds/400/); and topographic lidar data (2 meters) and Compact Hydrographic Airborne Rapid Total Survey (CHARTS) lidar data (2 meters) from the U.S. Army Corps of Engineers (USACE) (http://www.csc.noaa.gov/digitalcoast/data/coastallidar/). Bathymetry was derived from digital soundings obtained from the National Oceanic and Atmospheric Administration’s (NOAA) National Geophysical Data Center (NGDC) (http://www.ngdc.noaa.gov/mgg/geodas/geodas.html) and from water-penetrating lidar sources, such as EAARL and CHARTS. Mobile Bay is ecologically important as it is the fourth largest estuary in the United States. The Mobile and Tensaw Rivers drain into the bay at the northern end with the bay emptying into the Gulf of Mexico at the southern end. Dauphin Island (a barrier island) and the Fort Morgan Peninsula form the mouth of Mobile Bay. Mobile Bay is 31 miles (50 kilometers) long by a maximum width of 24 miles (39 kilometers) with a total area of 413 square miles (1,070 square kilometers). The vertical datum of the Mobile Bay topobathymetric model is the North American Vertical Datum of 1988 (NAVD 88). All the topographic datasets were originally referenced to NAVD 88 and no transformations were made to these input data. The NGDC hydrographic, multibeam, and trackline surveys were transformed from mean low water (MLW) or mean lower low water (MLLW) to NAVD 88 using VDatum (http://vdatum.noaa.gov). VDatum is a tool developed by the National Geodetic Survey (NGS) that performs transformations among tidal, ellipsoid-based, geoid-based, and orthometric datums using calibrated hydrodynamic models. The vertical accuracy of the input topographic data varied depending on the input source. Because the input elevation data were derived primarily from lidar, the vertical accuracy ranges from 6 to 20 centimeters in root mean square error (RMSE). he horizontal datum of the Mobile Bay topobathymetric model is the North American Datum of 1983 (NAD 83), geographic coordinates. All the topographic and bathymetric datasets were originally referenced to NAD 83, and no transformations were made to the input data. The bathymetric surveys were downloaded referenced to NAD 83 geographic, and therefore no horizontal transformations were required. The topbathymetric model of Mobile Bay and detailed metadata can be obtained from the USGS Web sites: http://nationalmap.gov/.

Alabama↗
USGS1996

Recent volcanism in the Siqueiros transform fault: Picritic basalts and implications for MORB magma genesis

Small constructional volcanic landforms and very fresh-looking lava flows are present along one of the inferred active strike-slip faults that connect two small spreading centers (A and B) in the western portion of the Siqueiros transform domain. The most primitive lavas (picritic and olivine-phyric basalts), exclusively recovered from the young-looking flows within the A-B strike-slip fault, contain millimeter-sized olivine phenocrysts (up to 20 modal%) that have a limited compositional range (Fo91.5-Fo89.5) and complexly zoned Cr-Al spinels. High-MgO (9.5-10.6 wt%) glasses sampled from the young lava flows contain 1-7% olivine phenocrysts (Fo90.5-Fo89) that could have formed by equilibrium crystallization from basaltic melts with Mg# values between 71 and 74. These high MgO (and high Al2O3) glasses may be near-primary melts from incompatible-element depleted oceanic mantle and little modified by crustal mixing and/or fractionation processes. Phase chemistry and major element systematics indicate that the picritic basalts are not primary liquids and formed by the accumulation of olivine and minor spinel from high-MgO melts (10% < MgO < 14%). Compared to typical N-MORB from the East Pacific Rise, the Siqueiros lavas are more primitive and depleted in incompatible elements. Phase equilibria calculations and comparisons with experimental data and trace element modeling support this hypothesis. They indicate such primary mid-ocean ridge basalt magmas formed by 10-18% accumulative decompression melting in the spinel peridotite field (but small amounts of melting in the garnet peridotite field are not precluded). The compositional variations of the primitive magmas may result from the accumulation of different small batch melt fractions from a polybaric melting column.

Earth and Planetary Science Letters↗
USGS1978

Radioactive disequilibrium in altered mid-oceanic basalts

A young (<1 m.y.) tholeiitic basalt dredged from the Mid-Atlantic Ridge displays a 234 U excess and a 230 Th deficiency that have resulted from the addition of seawater uranium during weathering at seafloor temperatures. Two older samples, though they acquired substantial amounts of uranium from seawater, are depleted in 234 U, indicating preferential leaching of this isotope. Hydrothermally altered samples suggest that some uranium loss may have occurred. Possible isotopic effects of preferential 234 U leaching, however, are obscured by secondary addition of seawater uranium at lower temperatures.

Earth and Planetary Science Letters↗
USGS1991

Stratigraphy and paleoceanography of Pliocene deposits of Karaginsky Island, eastern Kamchatka, U.S.S.R.

The lithology and paleontology of Pliocene deposits from Karaginsky Island, off eastern Kamchatka Peninsula, U.S.S.R., are described in order to provide a paleoceanographic reconstruction of this region of the eastern North Pacific Ocean. The stratigraphic sequence is divided into three suites — the Limimtevayamian, the Ust-Limimtevayamian, and the Tusatuvayamian, which correspond to three standard Pacific diatom zones — the Thalassiosira oestrupii, Neodenticula koizumi-N. kamtschatica , and N. koizumi . The entire sequence is further subdivided into six members and 18 distinct beds on the basis of molluscan assemblages and three members and seven beds on the basis of benthic foraminiferal assemblages. Marine ostracodes are described from the Limimtevayamian suite. The marine invertebrate assemblages from Karaginsky Island provide evidence that: (1) ocean water temperatures were warmer than today about 4 Ma and again about 3.5-3.0 Ma; (2) several Arctic-Atlantic species of molluscs and ostracodes migrated to the eastern North Pacific between 4.2−3.0 Ma, presumably through the proto-Bering Strait; (3) many species found in the Karaginsky Island Pliocene represent Pacific taxa which migrated through the Arctic into the North Atlantic during the late Pliocene.

eastern Kamchatka, Karaginsky Island↗
USGS2002

The shaping of continental slopes by internal tides

The angles of energy propagation of semidiurnal internal tides may determine the average gradient of continental slopes in ocean basins (???2 to 4 degrees). Intensification of near-bottom water velocities and bottom shear stresses caused by reflection of semi-diurnal internal tides affects sedimentation patterns and bottom gradients, as indicated by recent studies of continental slopes off northern California and New Jersey. Estimates of bottom shear velocities caused by semi-diurnal internal tides are high enough to inhibit deposition of fine-grained sediment onto the slopes.

Science↗
USGS1992

Suspension freezing of bottom sediment and biota in the Northwest Passage and implications for Arctic Ocean sedimentation

Ice observations and sediment collected in a summer transit through the Northwest Passage provide insights on suspension freezing, the most important sediment entrainment mechanism for the Arctic Ocean. No evidence was seen for entrainment by bottom adfreezing, bluff slumping, river flooding, dragging ice keels, or significant eolian transport from land to sea. Lack of eolian sediment loading in the Northwest Passage, together with that already reported for northern Alaska, eliminates wind as an important source for fine sediment in the pack of the Beaufort Gyre and related parts of the Transpolar Drift. Muddy sediment with pebbles and cobbles, algae with holdfasts, ostracodes with appendages, and well-preserved mollusks and sea urchins were collected from two sites in a 50 km long stretch of turbid ice. These materials indicate that suspension freezing reaching to a water depth of 25–30 m during the previous fall was responsible for entrainment. This mechanism requires rapid ice formation in open, shallow water during a freezing storm, when the ocean becomes supercooled, and frazil and anchor ice attach to and ultimately lift sediment and living organisms to the sea surface. The mechanism, already known to be important in the Beaufort Sea, probably also affects wide, shallow Siberian shelves and leads to cross-shelf transport of shallow-water organisms and dropstones with "glacial striations" toward deep basins. This makes distinguishing glacial–interglacial cycles more difficult.

Nunavut↗
USGS1978

Strontium isotopes in basalts from the Pacific Ocean basin

Basalts being erupted in the Pacific Ocean Basin vary in Sr isotopic composition in a simple geographic pattern. 87 Sr/ 86 Sr increases away from the East Pacific Rise to very high values for islands in the south-central Pacific. The 87 Sr/ 86 Sr variations are almost certainly related to past segregation of Rb, K, and other large cations. The segregation process was probably incipient partial melting which resulted in various mantle zones being enriched or depleted.

Earth and Planetary Science Letters↗
USGS1987

15N/14N variations in Cretaceous Atlantic sedimentary sequences: Implication for past changes in marine nitrogen biogeochemistry

At two locations in the Atlantic Ocean (DSDP Sites 367 and 530) early to middle Cretaceous organic-carbon-rich beds (“black shales”) were found to have significantly lower δ 15 N values (lower 15 N/ 14 N ratios) than adjacent organic-carbon-poor beds (white limestones or green claystones). While these lithologies are of marine origin, the black strata in particular have ° 15 N values that are significantly lower than those previously found in the marine sediment record and most contemporary marine nitrogen pools. In contrast, black, organic-carbon-rich beds at a third site (DSDP Site 603) contain predominantly terrestrial organic matter and have C- and N-isotopic compositions similar to organic matter of modern terrestrial origin. The recurring 15 N depletion in the marine-derived Cretaceous sequences prove that the nitrogen they contain is the end result of an episodic and atypical biogeochemistry. Existing isotopic and other data indicate that the low 15 N relative abundance is the consequence of pelagic rather than post-depositional processes. Reduced ocean circulation, increased denitrification, and, hence, reduced euphotic zone nitrate availability may have led to Cretaceous phytoplankton assemblages that were periodically dominated by N 2 -fixing blue-green algae, a possible source of this sediment 15 N-depletion. Lack of parallel isotopic shifts in Cretaceous terrestrially-derived nitrogen (Site 603) argues that the above change in nitrogen cycling during this period did not extend beyond the marine environment.

Earth and Planetary Science Letters↗
USGS2003

Coupling ice-sheet and climate models for simulation of former ice sheets

This chapter explores the development of coupled climate and ice-sheet models over the past two decades, discusses the current technical and physical capabilities of models, and identifies future work for developing a better understanding of ice-climate events that have punctuated Earth history. The chapter also illustrates the complex behavior of the climate system and the modeling challenges posed by the observations. Climate and ice-sheet models continue to improve, both in terms of model physics and technical capabilities. Regional climate model simulations should be carried out for each temporal snapshot or matrix element for improved mass–balance calculation over the ice sheets. Fully coupled atmosphere-ocean-cryosphere-land surface models are required for addressing a number of paleoclimatic puzzles, particularly with respect to millennial climate variability. The concerted effort to better understand West Antarctic Ice Sheet dynamics, and the development of subglacial hydrological models should lead to improvements in the next generation of ice-sheet models and help to address millennial-scale variability in ice-sheet/climate models.

Developments in Quaternary Sciences↗