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At least 343 records · Page 19Linked to original sources

USGS2009

Oblique Aerial Photography of the Arctic Coast of Alaska, Nulavik to Demarcation Point, August 7-10, 2006

The Arctic Coastal Plain of northern Alaska, an area of strategic economic importance to the United States, is home to remote Native American communities and encompasses unique habitats of global significance. Coastal erosion along the Arctic coast is chronic and widespread; recent evidence suggests that erosion rates are among the highest in the world (up to ~16 m/yr) and may be accelerating. Coastal erosion adversely impacts energy-related infrastructure, natural shoreline habitats, and Native American communities. Climate change is thought to be a key component of recent environmental changes in the Arctic. Reduced sea-ice cover in the Arctic Ocean is one of the probable mechanisms responsible for increasing coastal exposure to wave attack and the resulting increase in erosion. Extended periods of permafrost melting and associated decrease in bluff cohesion and stability are another possible source of the increase in erosion. Several studies of selected areas on the Alaska coast document past shoreline positions and coastal change, but none have examined the entire North coast systematically. Results from these studies indicate high rates of coastal retreat that vary spatially along the coast. To address the need for a comprehensive and regionally consistent evaluation of shoreline change along the North coast of Alaska, the U.S. Geological Survey (USGS), as part of their Coastal and Marine Geology Program's (CMGP) National Assessment of Shoreline Change Study, is evaluating shoreline change from Peard Bay to the United States/Canadian border, using historical maps and photography and a standardized methodology that is consistent with other shoreline-change studies along the Nation's coastlines (for example, URL http://coastal.er.usgs.gov/shoreline-change/ (last accessed March 2, 2009). This report contains photographs collected during an aerial-reconnaissance survey conducted in support of this study. An accompanying ESRI ArcGIS shape file (and plain-text copy) indicates the position of the aircraft and time when each photograph was taken. The USGS-CMGP Field Activity ID for the survey is A-1-06-AK, and more information on the survey and how to view the photographs using Google Earth software is available online at: URL http://walrus.wr.usgs.gov/infobank/a/a106ak/html/a-1-06-ak.meta.html (last accessed March 2, 2009).

Data Series↗

USGS1987

Mortalities of kelp-forest fishes associated with large oceanic waves off central California, 1982-1983

Observations of three incidents of the mass mortality of nearshore fishes are reported; each corresponded to periods of high-amplitude, long-period swells during the 1982-1983 El Niño event along the coast of central California. Members of the nearshore kelp forest fish assemblage, primarily of the genus Sebastes , accounted for 96% of the observed mortalities and S. mystinus (blue rockfish) alone accounted for 72%.

California↗

USGS2010

Oblique Aerial Photography of the Arctic Coast of Alaska, Cape Sabine to Milne Point, July 16-19, 2009

The Arctic Coastal Plain of northern Alaska, an area of strategic economic importance to the United States, is home to remote Native American communities and encompasses unique habitats of global significance. Coastal erosion along the Arctic coast is chronic and widespread; recent evidence suggests that erosion rates are among the highest in the world (as high as ~16 m/yr) and may be accelerating. Coastal erosion adversely impacts energy-related infrastructure, natural shoreline habitats, and Native American communities. Climate change is thought to be a key component of recent environmental changes in the Arctic. Reduced sea-ice cover in the Arctic Ocean is one of the probable mechanisms responsible for increasing coastal exposure to wave attack and the resulting increase in erosion. Extended periods of permafrost melting and associated decreases in bluff cohesion and stability are another possible source of the increase in erosion. Several studies of selected areas on the Alaska coast document past shoreline positions and coastal change, but none have examined the entire North coast systematically. Results from these studies indicate high rates of coastal retreat that vary spatially along the coast. To address the need for a comprehensive and regionally consistent evaluation of shoreline change along the North coast of Alaska, the U.S. Geological Survey (USGS), as part of their Coastal and Marine Geology Program's (CMGP) National Assessment of Shoreline Change Study, is evaluating shoreline change from Peard Bay to the United States/Canadian border, using historical maps and photography and a standardized methodology that is consistent with other shoreline-change studies along the Nation's coastlines (see, for example, http://coastal.er.usgs.gov/shoreline-change/, last accessed February 12, 2010). This is the second in a series of publications containing photographs collected during reconnaissance surveys conducted in support of the National Assessment of Shoreline Change Study. An accompanying ESRI ArcGIS shape file (and plaintext copy) indicates the position of the aircraft and time when each photograph was taken. The USGS-CMGP Field Activity ID for the survey is A-5-09-AK, and more information on the survey and how to view the photographs using Google Earth software is available online at http://walrus.wr.usgs.gov/infobank/a/a509ak/html/a-5-09-ak.photos.kmz (last accessed February 12, 2010). The initial report ?Oblique aerial photography of the Arctic coast of Alaska, Nulavik to Demarcation Point, August 7-10, 2006? is available online at http://pubs.usgs.gov/ds/436/, and the associated Google Earth .kmz file is available at http://walrus.wr.usgs.gov/infobank/a/a106ak/html/a-1-06-ak.photos.kmz (last accessed February 12, 2010).

Data Series↗

USGS2011

Onshore and offshore geologic map of the Coal Oil Point area, southern California

Geologic maps that span the shoreline and include both onshore and offshore areas are potentially valuable tools that can lead to a more in depth understanding of coastal environments. Such maps can contribute to the understanding of shoreline change, geologic hazards, both offshore and along-shore sediment and pollutant transport. They are also useful in assessing geologic and biologic resources. Several intermediate-scale (1:100,000) geologic maps that include both onshore and offshore areas (herein called onshore-offshore geologic maps) have been produced of areas along the California coast (see Saucedo and others, 2003; Kennedy and others, 2007; Kennedy and Tan, 2008), but few large-scale (1:24,000) maps have been produced that can address local coastal issues. A cooperative project between Federal and State agencies and universities has produced an onshore-offshore geologic map at 1:24,000 scale of the Coal Oil Point area and part of the Santa Barbara Channel, southern California (fig. 1). As part of the project, the U.S. Geological Survey (USGS) and the California Geological Survey (CGS) hosted a workshop (May 2nd and 3rd, 2007) for producers and users of coastal map products (see list of participants) to develop a consensus on the content and format of onshore-offshore geologic maps (and accompanying GIS files) so that they have relevance for coastal-zone management. The USGS and CGS are working to develop coastal maps that combine geospatial information from offshore and onshore and serve as an important tool for addressing a broad range of coastal-zone management issues. The workshop was divided into sessions for presentations and discussion of bathymetry and topography, geology, and habitat products and needs of end users. During the workshop, participants reviewed existing maps and discussed their merits and shortcomings. This report addresses a number of items discussed in the workshop and details the onshore and offshore geologic map of the Coal Oil Point area. Results from this report directly address issues raised in the California Ocean Protection Act (COPA) Five Year Strategic Plan. For example, one of the guiding principles of the COPA five-year strategic plan is to 'Recognize the interconnectedness of the land and the sea, supporting sustainable uses of the coast and ensuring the health of ecosystems.' Results from this USGS report directly connect the land and sea with the creation of both a seamless onshore and offshore digital terrain model (DTM) and geologic map. One of the priority goals (and objectives) of the COPA plan is to 'monitor and map the ocean environment to provide data about conditions and trends.' Maps within this report provide land and sea geologic information for mapping and monitoring nearshore sediment processes, pollution transport, and sea-level rise and fall.

California↗

USGS2021

Geology of the Hawaiian Islands

Long known as the island chain farthest from any continental landmass, the Hawaiian Islands are the subaerial expression of volcanism above the relatively fixed Hawaiian hot spot as the Pacific plate drifts northwest above it. Each island is built by one or several overlapping shield volcanoes, some of the most voluminous on Earth. Plate translation creates the well-known age-progressive sequence of shield volcanoes from northwest to southeast. Total volume of magma produced along the Hawaiian chain has been irregular but generally increasing for the past 50 million years, a trend that has peaked in the last 3 million years. Hawaiian volcanoes grow through stages that have geologic expression and geochemical differences that reflect position relative to the underlying hot spot. Of these stages, the shield stage is the most productive when an estimated 80–95% of a volcano's ultimate volume is emplaced. The shield stage endures for about 1 million years. The burden of shield volcanoes depresses the ocean crust near the hot spot, creating the Hawaiian Moat. Greatest rate of subsidence today occurs at the Island of Hawai‘i, 2–3 mm per year along its coast. Flexural rebound occurs as volcanoes move away from the hot spot; the Island of O‘ahu shows the greatest uplift. Slow subsidence resumes downstream from the flexure, leading ultimately to submergence of each island in the chain. Large landslides, albeit infrequent, can occur at any stage of island evolution. Ground water is the principal source of potable and agricultural water on all islands; its distribution both reflects and influences island geology.

Hawaii↗

USGS2002

Vulnerability assessment of a port and harbor community to earthquake and tsunami hazards: Integrating technical expert and stakeholder input

Research suggests that the Pacific Northwest could experience catastrophic earthquakes and tsunamis in the near future, posing a significant threat to the numerous ports and harbors along the coast. A collaborative, multiagency initiative is underway to increase the resiliency of Pacific Northwest ports and harbors to these hazards, involving Oregon Sea Grant, Washington Sea Grant, the National Oceanic and Atmospheric Administration Coastal Services Center, and the U.S. Geological Survey Center for Science Policy. One element of this research, planning, and outreach initiative is a natural hazard mitigation and emergency preparedness planning process that combines technical expertise with local stakeholder values and perceptions. This paper summarizes and examines one component of the process, the vulnerability assessment methodology, used in the pilot port and harbor community of Yaquina River, Oregon, as a case study of assessing vulnerability at the local level. In this community, stakeholders were most concerned with potential life loss and other nonstructural vulnerability issues, such as inadequate hazard awareness, communication, and response logistics, rather than structural issues, such as damage to specific buildings or infrastructure.

Oregon↗

USGS1961

Origin of the Gulf of California

The probable cumulative Late Cretaceous and Cenozoic right-lateral strike-slip displacement along the San Andreas fault in central California is 350 miles. The San Andreas and the allied faults into which it branches southward trend longitudinally into the Gulf of California , and the seismicity of the region indicates that the fault system follows the length of the Gulf and enters the Pacific basin south of Baja California . Crustal structure of most of the Gulf is of oceanic type, so that an origin by structural depression of continental rocks is not possible. Tectonic styles north and south of Los Angeles differ greatly. To the north, the Coast Ranges expose thick Upper Cretaceous and Cenozoic sedimentary rocks that were deposited in local basins and deformed tightly and repeatedly. To the south, in the Peninsular Ranges and Baja California , correlative rocks are thin and show little compressive deformation. The California batholith of mid-Cretaceous age and allied crystalline rocks form the basement of Baja California , southwestern Arizona, and northwestern Sonora and probably extend along the coast of mainland Mexico; the Gulf apparently bisects the crystalline belt longitudinally. These features suggest that Baja California initially lay 300 miles to the southeast, against the continental-margin bulge of Jalisco. The Gulf of California may be a pull-apart feature caused by strike-slip displacement plus up to 100 miles of cross-strike separation of the continental plate, subcontinental materials having welled up into the rift gap. The strike-slip motion has a tensional component across the continental margin south of Los Angeles but a compressional component to the north.

Gulf of California↗

USGS1995

Large-scale atmospheric forcing of recent trends toward early snowmelt runoff in California

Since the late 1940s, snowmelt and runoff have come increasingly early in the water year in many basins in northern and central California. This subtle trend is most pronounced in moderate-altitude basins, which are sensitive to changes in mean winter temperatures. Such basins have broad areas in which winter temperatures are near enough to freezing that small increases result initially in the formation of less snow and eventually in early snowmelt. In moderate-altitude basins of California, a declining fraction of the annual runoff has come in April–June. This decline has been compensated by increased fractions of runoff at other, mostly earlier, times in the water year. Weather stations in central California, including the central Sierra Nevada, have shown trends toward warmer winters since the 1940s. A series of regression analyses indicate that runoff timing responds equally to the observed decadal-scale trends in winter temperature and interannual temperature variations of the same magnitude, suggesting that the temperature trend is sufficient to explain the runoff-timing trends. The immediate cause of the trend toward warmer winters in California is a concurrent, long-term fluctuation in winter atmospheric circulations over the North Pacific Ocean and North America that is not immediately distinguishable from natural atmospheric variability. The fluctuation began to affect California in the 1940s, when the region of strongest low-frequency variation of winter circulations shifted to a part of the central North Pacific Ocean that is teleconnected to California temperatures. Since the late 1940s, winter wind fields have been displaced progressively southward over the central North Pacific and northward over the west coast of North America. These shifts in atmospheric circulations are associated with concurrent shifts in both West Coast air temperatures and North Pacific sea surface temperatures.

California↗

USGS2022

New Idria serpentinite protrusion, Diablo Range, California: From upper mantle to the surface

The New Idria serpentinite body in the Coast Ranges of California is a diapir that resulted from the interaction of the migrating Mendocino trench-ridge-transform fault triple junction, transpression, metasomatic fluids, and previously subducted oceanic crust and mantle. Northward propagation of the San Andreas fault progressively eliminated the original subduction zone, allowing seawater to penetrate into the formerly subducting abyssal peridotite mantle, triggering serpentinization. The associated physical changes in density, volume, and strength yielded an expanding, buoyantly rising serpentinite protrusion, facilitated by transpression along the San Andreas fault. Sedimentary facies and intrusion of minor cross cutting syenite and alkali basalt dikes indicate that the serpentinization-driven diapir buoyantly rose and widely breached the surface by ca. 14 Ma, attending migration of the Mendocino Triple Junction past the latitude of New Idria.

California↗

USGS2006

Interpolation of reconnaissance multibeam bathymetry from north-central Long Island Sound

Introduction The U.S. Geological Survey (USGS), in cooperation with the National Oceanic and Atmospheric Administration (NOAA) and the Connecticut Department of Environmental Protection (CT DEP), has produced detailed maps of the sea floor in Long Island Sound, a major East Coast estuary surrounded by the most densely populated region of the United States (fig. 1). The current phase of this cooperative research program is directed toward studies of sea-floor topography and its effect on the distributions of sedimentary environments and benthic communities. Because anthropogenic wastes, toxic chemicals, and changes in land-use patterns resulting from residential, commercial, and recreational development have stressed the environment of the Sound, causing degradation and potential loss of benthic habitats (Koppelman and others, 1976; Long Island Sound Study, 1994), detailed maps of the sea floor are needed to help evaluate the extent of adverse impacts and to help manage resources wisely in the future.

Connecticut, New York↗

USGS2011

Surficial geology of the sea floor in Long Island Sound offshore of Orient Point, New York

The U.S. Geological Survey (USGS), the National Oceanic and Atmospheric Administration (NOAA), and the Connecticut Department of Environmental Protection (CT DEP) are working cooperatively to map and interpret features of the sea floor along the northeastern coast of the United States. This report presents multibeam bathymetry and sidescan-sonar data obtained during NOAA survey H11446, which was conducted in a 12-km2 area in Long Island Sound offshore of Orient Point, NY. In addition, sediment and photographic data from 26 stations obtained during a USGS verification cruise are presented. Overall, the sea floor slopes gently seaward, but topography is more complex in sand-wave and boulder areas, which are evident in the multibeam and sidescan-sonar data from the study area. Sand waves generally have north-south-oriented crests with 10- to 20-m wavelengths. Sand-wave asymmetry indicates eastward net sediment transport in the east and westward net sediment transport in the northern and western parts of the study area. Areas with boulders on the sea floor are typically hummocky and are part of a glacial moraine system. Boulders are typically encrusted with seaweed, sponges, and anemones as shown in the bottom photography.

Open-File Report↗

USGS2013

SAFRR (Science Application for Risk Reduction) Tsunami Scenario--Executive Summary and Introduction: Chapter A in The SAFRR (Science Application for Risk Reduction) Tsunami Scenario

The Science Application for Risk Reduction (SAFRR) tsunami scenario depicts a hypothetical but plausible tsunami created by an earthquake offshore from the Alaska Peninsula and its impacts on the California coast. The tsunami scenario is a collaboration between the U.S. Geological Survey (USGS), the California Geological Survey, the California Governor’s Office of Emergency Services (Cal OES), the National Oceanic and Atmospheric Administration (NOAA), other Federal, State, County, and local agencies, private companies, and academic and other institutions. This document presents evidence for past tsunamis, the scientific basis for the source, likely inundation areas, current velocities in key ports and harbors, physical damage and repair costs, economic consequences, environmental and ecological impacts, social vulnerability, emergency management and evacuation challenges, and policy implications for California associated with this hypothetical tsunami. We also discuss ongoing mitigation efforts by the State of California and new communication products. The intended users are those who need to make mitigation decisions before future tsunamis, and those who will need to make rapid decisions during tsunami events. The results of the tsunami scenario will help managers understand the context and consequences of their decisions and how they may improve preparedness and response. An evaluation component will assess the effectiveness of the scenario process for target stakeholders in a separate report to improve similar efforts in the future.

California↗

USGS1989

Hydrogeologic framework of the New Jersey Coastal Plain

This report presents the results of a water-resources, oriented subsurface mapping program within the Coastal Plain of New Jersey. The occurrence and configuration of 15 regional hydrogeologic units have been defined, primarily on the basis of an interpretation of borehole geophysical data. The nine aquifers and six confining beds are composed of unconsolidated clay, silt, sand, and gravel and range in age from Cretaceous to Quaternary. Electric and gamma-ray logs from more than 1,000 Coastal Plain wells were examined. Of these, interpretive data for 302 sites were selected, on the basis of logged depth, quality of data, and data distribution, to prepare structure contour and thickness maps for each aquifer and a thickness map for each confining bed. These maps, together with 14 hydrogeologic sections, show the geometry, lateral extent, and vertical and horizontal relationships among the 15 hydrogeologic units. The hydrogeologic maps and sections show that distinct lower, middle, and upper aquifers are present within the Potomac, Raritan-Magothy aquifer system near the Delaware River from Burlington County to Salem County. Although the lower aquifer is recognized only in this area, the middle aquifer extends into the northeastern Coastal Plain of New Jersey, where it is stratigraphically equivalent to the Farrington aquifer. The upper aquifer extends throughout most of the New Jersey Coastal Plain and is stratigraphically equivalent to the Old Bridge aquifer in the northeastern Coastal Plain. The overlying Merchantville-Woodbury confining bed is the most regionally extensive confining bed within the New Jersey Coastal Plain. Its thickness ranges from less than 100 feet near the outcrop to more than 450 feet along the coast. The Englishtown aquifer system acts as a single aquifer throughout most of its subsurface extent, but it contains two water-bearing sands in pars of Monmouth and Ocean Counties. The overlying Marshalltown-Wenonah confining bed is a thin, leaky unit ranging in thickness from approximately 20 to 80 feet. The Wenonah-Mount Laurel aquifer is identified in the subsurface throughout the New Jersey Coastal Plain southeast of its outcrop area. Sediments that overlie the Wenonah-Mount Lauren aquifer and that are subjacent to the major aquifers within the Kirkwood Formation and the Cohansey Sand are described hydrologically as a composite confining bed. These include the Navesink Formation, Red Bank Sand, Tinton Sand, Hornerstown Sand, Vincentown Formation, Manasquan Formation, Shark River Formation, and Piney Point Formation and the basal clay of the Kirkwood Formation.. The Vincentown Formation functions as n aquifer within 3 to 10 miles downdip of its outcrop area. In areas farther downdip the Vincentown Formation functions as a confining bed. The Piney Point aquifer is laterally persistent from the southern New Jersey Coastal Plain northward into parts of Burlington and Ocean Counties. The Atlantic City 800-foot sand of the Kirkwood Formation can be recognized in the subsurface along coastal areas of Cape May, Atlantic, and southern Ocean Counties, but inland only as far west as the extent of the overlying confining bed. In areas west of the extent of the overlying confining bed, the Kirkwood Formation is in hydraulic connection with the overlying Cohansey Sand and younger surficial deposits and functions as an unconfined aquifer.

New Jersey Coastal Plain↗

USGS1974

Origin of Franciscan melanges in Northern California

In northern California, chaotic Franciscan melange occurs beneath the overlying ophiolite and Great Valley Sequence. Identical melanges occur to the west, separating well-bedded, coherent Franciscan units that differ markedly in age. Detailed studies in several places indicate that these melanges mark the boundaries of imbricate thrust sheets, and they appear to occur at several discrete structural horizons. The melange comprises blocks of graywacke, greenstone, chert, serpentinite, and isolated so-called knockers of high-grade blueschist and eclogite set in a matrix of sheared and quartz-veined mudstone and minor sandstone. Except for the blocks of high-grade schist, these rocks are similar to, but more deformed than, the orderly sedimentary, volcanic, and other rocks that occur immediately above the Coast Range thrust at the base of the Great Valley Sequence. Unlike the other Franciscan units, the melanges contain relatively abundant fossils, mainly Buchia, radiolarians, and dinoflagellates. Significantly, all of these fossils are of Tithonian to Valanginian age. We suggest, on the basis of similarity of lithology and fossil content, that the matrix of the melanges represents a distal, or seaward, portion of the basal sediments of the Great Valley Sequence and that the abundant greenstone, chert, and serpentinite found as tectonic blocks within the melanges were derived from the underlying oceanic crust and upper mantle. Formation of the melanges must be related to multiple subduction of separate plates, the melange beinggenerated repeatedly from the ultramafic-mafic-chert and Buchia-hearing shale and minor graywacke sequence that constitutes the oldest rocks of the Coast Ranges. This process of subduction probably began in the Early Cretaceous and continued into the Tertiary, as Eocene fossils have been found recently in deformed Franciscan (coastal belt) rocks structurally below the melanges. The tectonic blocks of high-grade blueschist and eclogite were formed during an earlier period of subduction, then embedded in serpentinite and carried westward by flow in the upper mantle. During subsequent subduction, the serpentinite and embedded blocks of schist were tectonically mixed with the overlying volcanic rocks, chert, graywacke, and fossiliferous shale.

SEPM Special Publication↗

USGS1968

Basalts dredged from the Amirante ridge, western Indian ocean

Oceanic tholeiitic basalts were dredged from 2500 to 3000 m depth on each flank of the Amirante Ridge, 1200 km southeast of Somalia in the western Indian Ocean, by R.V. Argo in 1964. One sample, probably shed from a flow or dike in basement beneath the coralline cap, gave a wholerock K-Ar age of 82±16×10 6 years. The age is similar to those reported by others for agglomerate from Providence Reef, nearer Madagascar, and for gabbro from Chain Ridge, the southwest member of Owen Fracture Zone, nearer the Somali coast. The Amirante Cretaceous-Early Tertiary occurrence lies between the “continental” 650 × 10 6 years granites of Seychelles Archipelago and the large Precambrian “continental” block of Madagascar. Trends of major structures and distribution of the related topographic and magnetic-anomaly lineations in 7–8 × 10 6 km 2 of the surrounding Indian Ocean suggest that in addition to spreading of the seafloor from the seismically-active Mid-Indian Ocean Ridge-Carlsberg Ridge complex there has been, since mid-Mesozoic time, distributed left-lateral shear along 52°–54°E that has moved Madagascar at least 700 km south relative to Seychelles Bank. Measurements by other indicate the absolute movement of Madagascar has been southward as well. The emplacement of oceanic tholeiitic basalts at shallow depth, the development of volcanic topography between the sedimented Somali and Mascarene basins, and the existence of the faulted Amirante Trench and Ridge are consequences of the displacement.

Deep-Sea Research and Oceanographic Abstracts↗

USGS2021

The tide turns: Episodic and localized cross-contamination of a California coastline with cyanotoxins

The contamination of coastal ecosystems from a variety of toxins of marine algal origin is a common and well-documented situation along the coasts of the United States and globally. The occurrence of toxins originating from cyanobacteria along marine coastlines is much less studied, and little information exists on whether toxins from marine and freshwater sources co-occur regularly. The current study focused on the discharge of cyanotoxins from a coastal lagoon (Santa Clara River Estuary) as a consequence of an extreme tide event (King Tides; December 3–5, 2017) resulting in a breach of the berm separating the lagoon from the ocean. Monthly monitoring in the lagoon throughout 2017 documented more than a dozen co-occurring cyanobacterial genera, as well as multiple algal and cyanobacterial toxins. Biotoxin monitoring before and following the King Tide event using Solid Phase Adsorption Toxin Tracking (SPATT) in the lagoon and along the coast revealed the co-occurrence of microcystins, anatoxin, domoic acid, and other toxins on multiple dates and locations. Domoic acid was ubiquitously present in SPATT deployed in the lagoon and along the coast. Microcystins were also commonly detected in both locations, although the beach berm retained the lagoonal water for much of the year. Mussels collected along the coast contained microcystins in approximately half the samples, particularly following the King Tide event. Anatoxin was observed in SPATT only in late December, following the breach of the berm. Our findings indicate both episodic and persistent occurrence of both cyanotoxins and marine toxins may commonly contaminate coastlines in proximity to cyanobacteria-laden creeks and lagoons.

California↗

USGS2015

High mercury wet deposition at a “clean Air” site in Puerto Rico

Atmospheric mercury deposition measurements are rare in tropical latitudes. Here we report on seven years (April 2005 to April 2012, with gaps) of wet Hg deposition measurements at a tropical wet forest in the Luquillo Mountains, northeastern Puerto Rico, U.S. Despite receiving unpolluted air off the Atlantic Ocean from northeasterly trade winds, during two complete years the site averaged 27.9 μg m –2 yr –1 wet Hg deposition, or about 30% more than Florida and the Gulf Coast, the highest deposition areas within the U.S. These high Hg deposition rates are driven in part by high rainfall, which averaged 2855 mm yr –1 . The volume-weighted mean Hg concentration was 9.8 ng L –1 , and was highest during summer and lowest during the winter dry season. Rainout of Hg (decreasing concentration with increasing rainfall depth) was minimal. The high Hg deposition was not supported by gaseous oxidized mercury (GOM) at ground level, which remained near global background concentrations (<10 pg m –3 ). Rather, a strong positive correlation between Hg concentrations and the maximum height of rain detected within clouds (echo tops) suggests that droplets in high convective cloud tops scavenge GOM from above the mixing layer. The high wet Hg deposition at this “clean air” site suggests that other tropical areas may be hotspots for Hg deposition as well.

Puerto Rico↗

USGS2001

Tectonic erosion and consequent collapse of the Pacific margin of Costa Rica: Combined implications from ODP leg 170, seismic offshore data, and regional geology of the Nicoya Peninsula

The convergent margin off the Pacific coast of the Nicoya Peninsula of Costa Rica exhibits evidence for subduction erosion caused by the underthrusting Cocos plate. Critical evidence for efficacy of this process was recovered at the Ocean Drilling Program (ODP) drilling Site 1042 (Leg 170), positioned ???7 km landward of the Middle America trench axis off the Nicoya Peninsula. The primary drilling objective at this site was to identify the age and origin of a regionally extensive and prominent seismic discontinuity, the so-called base-of-slope sediment (BOSS) horizon or surface. The BOSS horizon, which can be traced landward from near the trench to the Nicoya coastal area and parallel to it for hundreds of kilometers, separates a low-velocity (??? 2.0-2.5 km s-1) sequence of slope sediment, from an underlying sequence of much higher-velocity (> 4-4.5 km s-1) rock. Site 1042 reached the acoustically defined BOSS horizon at a below sea level depth of ??? 3900 m and yielded a carbonate-cemented calcarenitic breccia of early-middle Miocene age. Sedimentological, geochemical, paleontological, and cement paragenesis data document that the breccia accumulated in a shallow water depositional environment. On the basis of coastal exposures, the BOSS horizon, as a margin-wide geologic interface, can be temporally and lithostratigraphically correlated to a regional angular unconformity. This unconformity, known as the Mal Pais unconformity, separates Neogene and younger shelf-to-littoral beds from the underlying mafic units of the Mesozoic Nicoya Complex and Cretaceous and early Tertiary sedimentary sequences. At Site 1042 it is inferred that tectonism caused the vertical subsidence of the early Neogene breccia from a shallow to a deep water setting. The Mal Pais unconformity of the BOSS horizon thus connects the rock fabric of the outermost part of margin to that of coastal Nicoya and implies that in the early Neogene the Nicoya shelf extended seaward to near the present trench axis. This circumstance requires that the early Neogene trench axis was at least 50 km seaward of where it is now located. The long-term effects of subduction erosion, similar to those described for the scientifically drilled Japan, Tonga, and Peru margins, best account for offshore and onshore evidence for a post-Paleogene history of crustal thinning and landward trench migration of Costa Rica's Pacific margin. During the past 16-17 Myr the calculated mass removal and landward migration rates are 34-36 km3 Myr-1 km-1 of margin, and 3 km Myr-1, respectively. These values are similar to those found for other Pacific margins dominated by nonaccretionary subduction zone processes.

Tectonics↗