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At least 721 records · Page 40Linked to original sources
USGS2022

Elevated levels of radiocarbon in methane dissolved in seawater reveal likely local contamination from nuclear powered vessels

Measurements of the natural radiocarbon content of methane ( 14 C-CH 4 ) dissolved in seawater and freshwater have been used to investigate sources and dynamics of methane. However, during investigations along the Atlantic, Pacific, and Arctic Ocean Margins of the United States, as well as in the North American Great Lakes, some samples revealed highly elevated 14 C-CH 4 values, as much as 4–5 times above contemporary atmospheric 14 C-CH 4 levels. Natural production of the 14 CH 4 isotopologue is too low to cause these observations nor can it explain the variations in location and depth. Numerous lab and field validation tests and blanks, as well as the relatively small number of samples that display these elevated values, all suggest that these signals are not derived from an unknown procedural issue. Here we suggest that the byproducts of nuclear power generation include localized discharges of the 14 CH 4 isotopologue into marine and aquatic environments, severely altering the measured 14 C-CH 4 isotopic signals. Since several of our sample sites are distant from on-land nuclear powerplants, we conduct further calculations concluding that the most elevated anomalies in 14 C-CH 4 likely originate with discharge from nuclear-powered vessels.

Maryland, North Carolina, Oregon, Virginia, Washin↗
USGS2008

Relating carbon monoxide photoproduction to dissolved organic matter functionality

Aqueous solutions of humic substances (HSs) and pure monomeric aromatics were irradiated to investigate the chemical controls upon carbon monoxide (CO) photoproduction from dissolved organic matter (DOM). HSs were isolated from lakes, rivers, marsh, and ocean. Inclusion of humic, fulvic, hydrophobic organic, and hydrophilic organic acid fractions from these environments provided samples diverse in source and isolation protocol. In spite of these major differences, HS absorption coefficients (a) and photoreactivities (a bleaching and CO production) were strongly dependent upon HS aromaticity ( r 2 >0.90; n = 11), implying aromatic moieties are the principal chromophores and photoreactants within HSs, and by extension, DOM. Carbonyl carbon and CO photoproduction were not correlated, implying that carbonyl moieties are not quantitatively important in CO photoproduction. CO photoproduction efficiency of aqueous solutions of monomeric aromatic compounds that are common constituents of organic matter varied with the nature of ring substituents. Specifically, electron donating groups increased, while electron withdrawing groups decreased CO photoproductivity, supporting our conclusion that carbonyl substituents are not quantitatively important in CO photoproduction. Significantly, aromatic CO photoproduction efficiency spanned 3 orders of magnitude, indicating that variations in the CO apparent quantum yields of natural DOM may be related to variations in aromatic DOM substituent group chemistry.

Environmental Science and Technology↗
USGS1996

Global distribution of beryllium isotopes in deep ocean water as derived from Fe-Mn crusts

The direct measurement of the ratio of cosmogenic 10 Be ( T 1 2 = 1.5 Ma "> T12= 1.5 Ma ) to stable terrigenously sourced 9 Be in deep seawater or marine deposits can be used to trace water mass movements and to quantify the incorporation of trace metals into the deep sea. In this study a SIMS-based technique has been used to determine the 10 Be 9 Be "> 10Be9Be ratios of the outermost millimetre of hydrogenetic ferromanganese crusts from the worlds oceans. 10 Be 9 Be "> 10Be9Be ratios, time-corrected for radioactive decay of cosmogenic 10 Be using 234 U 238 U "> 234U238U , are in good agreement with AMS measurements of modern deep seawater. Ratios are relatively low in the North and equatorial Atlantic samples (0.4–0.5 × 10 −7 ). In the Southwest Atlantic ratios increase up to 1 × 10 −7 , they vary between 0.7 and 1.0 × 10 −7 in Indian Ocean samples, and have a near constant value of 1.1 ± 0.2 × 10 −7 for all Pacific samples. If the residence time of 10 Be ( τ 10 Be ) in deep water is constant globally, then the observed variations in 10 Be 9 Be "> 10Be9Be ratios could be caused by accumulation of 10 Be in deep water as it flows and ages along the conveyor, following a transient depletion upon its formation in the Northern Atlantic. In this view both 10 Be and 9 Be reach local steady-state concentration in Pacific deep water and the global τ10 Be ≌ 600 a ">≌ τ10Be≌ 600a . An alternative possibility is that the Be isotope abundances are controlled by local scavenging. For this scenario τ 10 Be would vary according to local particle concentration and would ≌ 600 a in the central Pacific, but τ10 Be ≌ 230 a ">≌ τ10Be≌ 230a in the Atlantic. Mass balance considerations indicate that hydrothermal additions of 9 Be to the oceans are negligible and that the dissolved riverine source is also small. Furthermore, aeolian dust input of 9 Be appears insufficient to provide the dissolved 9 Be inventory. The dissolution of only a small proportion (2%) of river-derived particulates could in principle supply the observed seawater 9 Be content. If true, ocean margins would be the sites for 9 Be addition. Due to the particle-reactive nature of Be, these would also be the primary sites of Be removal. A possible net result of horizontal water masses passing through these marginal areas might be a decrease in seawater 10 Be 9 Be "> 10Be9Be , and establishment of a relatively constant 9 Be concentration. As τ 10 Be (∼ 600 a) is less than the apparent age of deep water in the Pacific (∼ 1500 a), the Pacific record of 10 Be 9 Be "> 10Be9Be is not expected to show secular variations due to changes in deep-water flow, despite the large variations in 10 Be 9 Be "> 10Be9Be between different water masses. Because of this insensitivity to deep-water flow, however, it is suggested that the 10 Be 9 Be "> 10Be9Be ratio, determined in the authigenic phase of marine sediments or hydrogenetic precipitates, should be a suitable tool for monitoring changes in continental input or cosmic ray intensity on longer time scales.

Earth and Planetary Science Letters↗
USGS2023

Investigating spatio-temporal variability of initial 230Th/232Th in intertidal corals

One of the key factors in obtaining precise and accurate 230 Th ages of corals, especially for corals with ages less than a few thousand years, is the correction for non-radiogenic 230 Th based on an initial 230 Th/ 232 Th value ( 230 Th/ 232 Th 0 ). Studies that consider coral 230 Th/ 232 Th 0 values in intertidal environments are limited, and it is in these environments that corals have Th concentrations 100–1000 times greater than open-ocean corals. Here we present 66 reconstructed U–Th isochrons of modern and Holocene Porites lutea and lobata corals from throughout the Sumatran forearc islands (SFI). Mean square of weighted deviates ( MSWD ) is used to evaluate the variabilities of 28 location-specific 230 Th/ 232 Th0 values before the regional mean. Results show no significant spatial difference between the means of 3.1 ± 3.2 × 10 −6 for the northern SFI and 4.7 ± 1.3 × 10 −6 for the southern SFI. Using the weighted mean of 4.3 ± 2.5 × 10 −6 for all islands has the advantage that no data need be subjectively excluded in the calculation of reliable 230 Th ages, where a local initial value is unknown. There were no clear centennial to millennial time-scale changes in 230 Th/ 232 Th 0 in the past 2500 years. This study, therefore, suggests a reliable value of initial 230 Th/ 232 Th ratio in intertidal environments.

Sumatran forearc islands↗
USGS2006

Identification of marine hydrates in situ and their distribution off the Atlantic coast of the United States

Natural gas hydrates, mostly methane hydrates, occur within seafloor sediments almost everywhere in the world’s oceans where water depths exceed 300 to 500 m, and hydrates in this setting probably contain very large quantities of methane.’ Gas hydrates have been identified in marine sediments by coring and by the response that they create in seismic reflection profiles. Our research has endeavored to refine the criteria used to recognize hydrates in seismic reflection data and to use such data to map hydrates on the United States Atlantic continential rise. Gas hydrates in ocean floor sediments occur within a layer just below the sea floor, controlled by the pressure and temperature conditions. Actually, hydrates would be stable in deep ocean water (at depths greater than 300-500 m), but probably do not exist there due to lack of gas saturation. Furthermore, if they did form in the water the hydrates would float upward and melt at the lower pressure and higher temperature conditions found at shallower depths. However, gas is present in the sediments either as biogenic gas produced by bacteria or as thermogenic gas rising from deeper strata, and when hydrate forms in sediments it is trapped in the sedimentary matrix. Temperature increases downward through the sediments, and, although pressure also increases (which tends to make hydrate more stable), the temperature ultimately becomes too great for hydrate to exist at ambient pressure. Because the thermal gradient is fairly constant within a restricted geographic region, this stability limit will be reached at approximately the same subbottom depth everywhere in the region. The result is a zone of hydrate-cemented sediment that extends down from the sea floor; this zone can have a thickness of as much as 1000 m.

Atlantic Coast↗
USGS2025

Submarine groundwater discharge creates cold‐water refugia that can mitigate exposure of heat stress in nearshore corals

Coral reef mortality around the world is accelerating due to human activities and rising sea temperatures that cause bleaching, which is expected to become more frequent. Our ability to predict which corals will be most resilient, however, remains limited due to insufficient information characterizing nearshore temperature and habitat conditions. In this study, we examine how submarine groundwater discharge (SGD) reduces nearshore water temperatures and exposure of corals to heat stress, complementing the understanding that SGD can adversely affect coral when it contains elevated nutrient concentrations. Data from fixed nearshore sensors and vertical depth profiles along ~100 km of the western shoreline of the Island of Hawai’i from 2003 to 2014 demonstrate that submarine groundwater discharge (SGD) can reduce nearshore water temperatures by 1 °C–5°C and create estuarine-like conditions with salinities as low as 20 PSU, where the prevalent coral species, Pocillopora meandrina, Porites lobata , and Montipora capitata , thrive. Time-series temperature records reveal that exposure to high ambient ocean temperatures, which are known to initiate bleaching events, are reduced up to 5%–46% of the time. Coral health surveys indicated coral bleaching in response to moderately high annual temperatures in 2010 and 2011, with more colonies affected farther from cold, SGD-fed waters. Synthesis of these results, along with coral response data following the more extreme marine heat wave of 2014–2015, demonstrates lower coral loss and greater coral recovery near groundwater seeps, particularly those with higher flux and influence on reducing nearshore water temperatures. Our results demonstrate that SGD may therefore provide a beneficial ecosystem service and enhance coral reef resilience, particularly where human-related nutrient additions to groundwater can be mitigated. The implications of our findings are relevant across tropical coasts where groundwater inputs can be substantial, such as the Caribbean and Indo-Pacific, and contribute to improving our understanding of coral sensitivity to gradients in temperature and nutrient stress. Improved management of groundwater resources could thus be vital to local–regional strategies for mitigating future heat stress.

Hawaii↗
USGS1981

Tonalites in crustal evolution

Tonalites, including trondhjemite as a variety, played three roles through geological time in the generation of Earth’s crust. Before about 2.9 Ga ago they were produced largely by simple partial melting of metabasalt to give the dominant part of Archaean grey gneiss terranes. These terranes are notably bimodal; andesitic rocks are rare. Tonalites played a crucial role in the generation of this protocontinental and oldest crust 3.7- 2.9 Ga ago in that they were the only low-density, high-SiO 2 rocks produced directly from basaltic crust. In the enormous event giving the greenstone-granite terranes, mostly 2.8-2.6 Ga ago, tonalites formed in lesser but still important proportions by partial melting of metabasalt in the lower regions of down-buckled greenstone belts and by remobilization of older grey gneisses. Tectonism in the Archaean (3.9- 2.5 Ga ago) perhaps was controlled by small-cell convection (McKenzie & Weiss 1975). Little or no ophiolite or eclogite formed, and only minor andesite. Plate tectonics of modern type (involving large, rigid plates) commenced in the early Proterozoic. Uniformitarianism thus goes back one-half of the age of the earth. Tonalites compose about 5-10 % of crust generated in Proterozoic and Phanerozoic time at convergent oceanic-continental margins. They occur here as minor to prominent members of the compositionally continuous continental-margin batholiths. A simple model of generation of these batholiths is offered: mantle-derived mafic magma pools in the lower crust above a subduction zone reacts with and incorporates wall-rock components (Bowen 1922), and breaches its roof rocks as an initial diapir. This mantle magma also develops a gradient of partial melting in its wall rocks. This wall-rock melt accretes in the collapsed chamber and moves up the conduit broached by the initial diapir, the higher, less siliceous fractions of melting first, the lower, more siliceous (and further removed) fractions of melting last. The process gives in the optimum case a mafic-to-siliceous sequence of diorite or quartz diorite through tonalite or quartz monzodiorite to granodiorite and granite. The model implies that great masses of cumulate phases and refractory wall rock form the roots of continentalmargin batholiths, and that migmatites overlie that residuum and underlie the batholiths.

Philosophical Transactions of the Royal Society A:↗
USGS2011

Chemical ecology of red mangroves, Rhizophora mangle, in the Hawaiian Islands

The coastal red mangrove, Rhizophora mangle L., was introduced to the Hawaiian Islands from Florida 100 yr ago and has spread to cover many shallow intertidal shorelines that once were unvegetated mudflats. We used a field survey approach to test whether mangroves at the land-ocean interface could indicate watershed inputs, especially whether measurements of leaf chemistry could identify coasts with high nutrient inputs and high mangrove productivities. During 2001-2002, we sampled mangroves on dry leeward coasts of southern Moloka'i and O'ahu for 14 leaf variables including stable carbon and nitrogen isotopes (delta 13 C, delta 15 N), macronutrients (C, N, P), trace elements (B, Mn, Fe, Cu, Zn), and cations (Na, Mg, K, Ca). A new modeling approach using leaf Na, N, P, and delta 13 C indicated two times higher productivity for mangroves in urban versus rural settings, with rural mangroves more limited by low N and P nutrients and high-nutrient urban mangroves more limited by freshwater inputs and salt stress. Leaf chemistry also helped identify other aspects of mangrove dynamics: especially leaf delta 15 N values helped identify groundwater N inputs, and a combination of strongly correlated variables (C, N, P, B, Cu, Mg, K, Ca) tracked the mangrove growth response to nutrient loading. Overall, the chemical marker approach is an efficient way to survey watershed forcing of mangrove forest dynamics.

Hawai'i↗
USGS2007

A comparative analysis of pre-Silurian crustal building blocks of the northern and the southern Appalachian orogen

The New York promontory serves as the divide between the northern and southern segments of the Appalachian orogen. Antiquated subdivisions, distinct for each segment, implied that they had lithotectonic histories that were independent of each other. Using new lithotectonic subdivisions we compare first order features of the pre-Silurian orogenic 'building blocks' in order to test the validity of the implication of independent lithotectonic histories for the two segments. Three lithotectonic divisions, termed here the Laurentian, Iapetan, and the peri-Gondwanan realms, characterize the entire orogen. The Laurentian realm, composed of native North American rocks, is remarkably uniform for the length of the orogen. It records the multistage Neoproterozoic-early Paleozoic rift-drift history of the Appalachian passive margin, formation of a Taconic Seaway, and the ultimate demise of both in the Middle Ordovician. The Iapetan realm encompasses mainly oceanic and magmatic arc tracts that once lay within the Iapetus Ocean, between Laurentia and Gondwana. In the northern segment, the realm is divisible on the basis of stratigraphy and faunal provinciality into peri-Laurentian and peri-Gondwanan tracts that were amalgamated in the Late Ordovician. South of New York, stratigraphic and faunal controls decrease markedly; rock associations are not inconsistent with those of the northern Appalachians, although second-order differences exist. Exposed exotic crustal blocks of the peri-Gondwanan realm include Ganderia, Avalonia, and Meguma in the north, and Carolinia in the south. Carolinia most closely resembles Ganderia, both in early evolution and Late Ordovician-Silurian docking to Laurentia. Our comparison indicates that, to a first order, the pre-Silurian Appalachian orogen developed uniformly, starting with complex rifting and a subsequent drift phase to form the Appalachian margin, followed by the consolidation of Iapetan components and ending with accretion of the peri-Gonwanan Ganderia and Carolinia. This deduction implies that any first-order differences between northern and southern segments post-date Late Ordovician consolidation of a large portion of the orogen.

American Journal of Science↗
USGS2004

Holocene thermal maximum in the western Arctic (0-180°W)

The spatio-temporal pattern of peak Holocene warmth (Holocene thermal maximum, HTM) is traced over 140 sites across the Western Hemisphere of the Arctic (0-180??W; north of ???60??N). Paleoclimate inferences based on a wide variety of proxy indicators provide clear evidence for warmer-than-present conditions at 120 of these sites. At the 16 terrestrial sites where quantitative estimates have been obtained, local HTM temperatures (primarily summer estimates) were on average 1.6??0.8??C higher than present (approximate average of the 20th century), but the warming was time-transgressive across the western Arctic. As the precession-driven summer insolation anomaly peaked 12-10ka (thousands of calendar years ago), warming was concentrated in northwest North America, while cool conditions lingered in the northeast. Alaska and northwest Canada experienced the HTM between ca 11 and 9ka, about 4000yr prior to the HTM in northeast Canada. The delayed warming in Quebec and Labrador was linked to the residual Laurentide Ice Sheet, which chilled the region through its impact on surface energy balance and ocean circulation. The lingering ice also attests to the inherent asymmetry of atmospheric and oceanic circulation that predisposes the region to glaciation and modulates the pattern of climatic change. The spatial asymmetry of warming during the HTM resembles the pattern of warming observed in the Arctic over the last several decades. Although the two warmings are described at different temporal scales, and the HTM was additionally affected by the residual Laurentide ice, the similarities suggest there might be a preferred mode of variability in the atmospheric circulation that generates a recurrent pattern of warming under positive radiative forcing. Unlike the HTM, however, future warming will not be counterbalanced by the cooling effect of a residual North American ice sheet. ?? 2003 Elsevier Ltd. All rights reserved.

Quaternary Science Reviews↗
USGS2016

Tracking millennial-scale Holocene glacial advance and retreat using osmium isotopes: Insights from the Greenland ice sheet

High-resolution Os isotope stratigraphy can aid in reconstructing Pleistocene ice sheet fluctuation and elucidating the role of local and regional weathering fluxes on the marine Os residence time. This paper presents new Os isotope data from ocean cores adjacent to the West Greenland ice sheet that have excellent chronological controls. Cores MSM-520 and DA00-06 represent distal to proximal sites adjacent to two West Greenland ice streams. Core MSM-520 has a steadily decreasing Os signal over the last 10 kyr ( 187 Os/ 188 Os = 1.35–0.81). In contrast, Os isotopes from core DA00-06 (proximal to the calving front of Jakobshavn Isbræ) highlight four stages of ice stream retreat and advance over the past 10 kyr ( 187 Os/ 188 Os = 2.31; 1.68; 2.09; 1.47). Our high-resolution chemostratigraphic records provide vital benchmarks for ice-sheet modelers as we attempt to better constrain the future response of major ice sheets to climate change. Variations in Os isotope composition from sediment and macro-algae (seaweed) sourced from regional and global settings serve to emphasize the overwhelming effect weathering sources have on seawater Os isotope composition. Further, these findings demonstrate that the residence time of Os is shorter than previous estimates of ∼10 4 yr.

Quaternary Science Reviews↗
USGS2004

The role of fire and fire management in the invasion of nonnative plants

Spacecraft imagery, especially from the National Atmospheric and Oceanic Administration's Improved TIROS (Television Infra-Red Observational Satellite) Operational Satellites, permits timely evaluations of snow and ice conditions encountered by arctic nesting geese. Imagery from the TIROS satellite for 5 wide]y scattered locations in arctic North America was obtained for three 3-day intervals in June 1973 and 1974. These pictures were used to expand fragmentary habitat data available from ground observations. Late disappearance of snow and ice may prevent or retard nesting effort and reproductive success. Our immediate aim is to recognize years of catastrophic or very good production; however, supporting information from ground studies, LANDSAT imagery, analyses of banding data, and studies of age ratios in popu]ations and harvests eventua]]y may permit assessment of relative reproductive success. Satellite images from both Multi-Spectral Scanner and Very High Resolution Radiomder sensors were first used as management tools for consideration in setting the U.S. and Canadian regulations for the 1975-76 goose hunting season. Preliminary age ratio data from field observations and harvests substantiated the prediction of good production during the 1975 breeding season.

Park Science↗
USGS2005

The South China sea margins: Implications for rifting contrasts

Implications regarding spatially complex continental rifting, crustal extension, and the subsequent evolution to seafloor spreading are re-examined for the northern and southern-rifted margins of the South China Sea. Previous seismic studies have shown dramatic differences in the present-day crustal thicknesses as the manifestations of the strain experienced during the rifting of the margin of south China. Although the total crustal extension is presumed to be the same along the margin and adjacent ocean basin, the amount of continental crustal extension that occurred is much less along the east and central segments of the margin than along the western segment. This difference was accommodated by the early formation of oceanic crust (creating the present-day South China Sea basin) adjacent to the eastern margin segment while continued extension of continental crust was sustained to the west. Using the observed cross-sectional areas of extended continental crust derived from deep penetration seismics, two end-member models of varying rift zone widths and varying initial crustal thicknesses are qualitatively examined for three transects. Each model implies a time difference in the initiation of seafloor spreading inferred for different segments along the margin. The two models examined predict that the oceanic crust of the South China Sea basin toward the west did not begin forming until sometime between 6-12 my after its initial formation (???32 Ma) toward the east. These results are compatible with crustal age interpretations of marine magnetic anomalies. Assuming rifting symmetry with conjugate margin segments now residing along the southern portions of the South China Sea basin implies that the total width of the zone of rifting in the west was greater than in the east by about a factor of two. We suggest the most likely causes of the rifting differences were east-west variations in the rheology of the pre-rift crust and associated east-west variations in the thermal structure of the pre-rift lithosphere. The calculated widths of rifted continental crust for the northern and southern margins, when combined with the differential widths of seafloor generated during the seafloor spreading phase, indicate the total crustal extension that occurred is about 1100 km and is remarkably consistent to within ???10% for all three (eastern, central, western) segments examined. ?? 2005 Elsevier B.V. All rights reserved.

Earth and Planetary Science Letters↗
USGS2021

Long-term African dust delivery to the eastern Atlantic Ocean from the Sahara and Sahel regions: Evidence from Quaternary paleosols on the Canary Islands, Spain

Africa is the most important source of dust in the world today and dust storms from that continent frequently deposit sediment on the nearby Canary Islands. Many investigators have inferred African dust inputs to Canary Islands paleosols based only on the presence of quartz. However, some local rocks do contain this mineral, so quartz alone is insufficient proof of dust deposition. Further, it is not known whether the Sahara Desert or the Sahel region is more important as a dust source. We address these issues by study of sequences of Pleistocene aeolian sands on the islands of Lanzarote and Fuerteventura. Aeolian sands are composed mostly of marine carbonate minerals and locally derived volcanic minerals. They date from the early-middle Pleistocene to the Holocene. Trace element geochemistry shows that the soils formed from both locally derived basalt and African dust. Major element geochemistry and clay mineralogy indicate that dust additions to the Canary Islands likely come from both the Sahara and Sahel. Dust delivered from the Sahel indicates that droughts in that region have had a history extending through much of the Quaternary. Accretionary-inflationary profile development, from dust accretion, is evident in the upward growth of Canary Islands paleosols.

Quaternary Science Reviews↗
USGS2008

DDE in sediments of the Palos Verdes shelf, California: In situ transformation rates and geochemical fate

From 1947 to 1971 the world's largest manufacturer of DDT discharged process wastes into the sewers of Los Angeles County. Roughly 870-1450 t of DDT were released to the ocean off Palos Verdes, CA, a portion of which (???100 t) resides in sediments on the continental shelf and slope. The most abundant DDT compound in the sediments, p,p???-DDE, is degrading by reductive dechlorination, butthe rate of transformation and factors controlling it are not well understood. In order to estimate in situ transformation rates and predict the long-term fate of p,p???-DDE, box cores were collected in 1992 and 2003 from a single location on the Palos Verdes Shelf and analyzed for 8 DDT compounds and 84 polychlorinated biphenyl (PCB) congeners. The PCBs show no evidence of dechlorination, and inventories did not change between 1992 and 2003. By contrast, the inventory of p,p???-DDE decreased by 43%, whereas that of p,p???-DDMU, the putative reductive dechlorination product increased by 34%. The first-order transformation rate for p,p???-DDE at the study site is 0.051 ?? 0.006 yr-1. A multistep reaction model suggests that inventories of p,p???-DDE and p,p???-DDMU will continue to decline, whereas that of p,p???-DDNU will reach a maximum around 2014.

Environmental Science & Technology↗
USGS1984

Uplift and submarine formation of some Melanesian porphyry copper deposits: Stable isotope evidence

Hydrogen and oxygen isotope analyses of sericites and kaolinites from four young porphyry copper deposits (Ok Tedi (1.2 Ma) and Yandera (6.5 Ma), Papua New Guinea; Koloula (1.5 Ma), Solomon Islands; and Waisoi (<5 Ma), Fiji) indicate that the fluids from which these minerals precipitated were of mixed magmatic and non-magmatic sources. The non-magmatic component of the fluid from the island arc deposits (Koloula, Waisoi) was ocean water. For Ok Tedi, the non-magmatic component was a meteoric water with an isotopic composition different from that of the present meteoric water in the region. The isotopic signature of the former meteoric water is consistent with a surface elevation of 200 m a.s.l. or less at the time of mineralization. The deposit was later exposed and supergene kaolinitization commenced at approximately 1200 m a.s.l. Uplift and erosion has continued to the present at which time the elevation of the exposed deposit is 1800 m a.s.l. This rate of uplift is consistent with that known from other geological evidence. If the rate of uplift were approximately constant during the last 1.2 Ma, the age of supergene enrichment can be dated at approximately 0.4 Ma B.P. Similarly, influx of meteoric water at Yandera occurred when the ground surface above the deposit was at an elevation of approximately 600 m a.s.l. The deposit's present elevation is 1600 m a.s.l. In this case a total uplift of approximately 2.2 km is indicated, with removal of 1.2 km of overburden by erosion.

Earth and Planetary Science Letters↗
USGS1989

Emergence of burrowing urchins from California continental shelf sediments: A response to alongshore current reversals?

Two sequences of bottom photographs taken every two or four hours for two months during the Coastal Ocean Dynamics Experiment (CODE) off the Russian River, California, reveal the dynamic nature of interations between the water column, the sediments, and benthic organisms in the mid-shelf silt deposit. Time-lapse photographs taken between late spring and early summer in 1981 and 1982 show that the subsurface-dwelling urchin Brisaster latifrons (one of the largest invertebrates found in shelf-depth fine sediment off the U.S. Pacific coast) occasionally emerged from the sediment, plowed the sediment surface during the course of a few hours to several days, then buried themselves again. Frame-by-frame study of the film sequences shows that the urchins typically emerged following relaxation of coastal upwelling, periods characterized by current direction reversals and increases in bottom water turbidity. Among the possible causes of the emergence of urchins and the consequent bioturbation of the upper few cm of sediment, a response to an enhanced food supply seems most plausible. Circumstantial evidence suggests the possibility that phytoplankton sedimentation during periods of upwelling relaxation could provide a new source of food at the sediment surface.

California↗
USGS1989

Style of extensional tectonism during rifting, Red Sea and Gulf of Aden

Models describing the development of the Red Sea and the Gulf of Aden, prior to the present periods of sea-floor spreading, include those that use block faulting on steep normal faults, uniform diffuse shear in continental crust, simple shear on large detachment faults that cut the entire lithosphere, combinations involving detachment faults/ductile deformation/plutonic inflation, and ones that minimize the role of mechanical extension in favor of an earlier stage of sea-floor spreading. Geologic and geophysical studies from the Arabian continental margin in the southern Red Sea and LANDSAT analysis of the northern Somalia margin in the Gulf of Aden suggest that the early continental rifts were long narrow features that formed by extension on closely spaced normal faults above moderate- to shallow-dipping detachments with break-away zones defining one rift flank and root zones under the opposing rift flank. The rift flanks presently form the opposing continental margins across each ocean basin. The detachment on the Arabian margin dips gently to the west, with a breakaway zone now eroded above the deeply dissected terrain of the Arabian escarpment. The Arabian detachment projects westward to middle crustal levels beneath the sediment of the southern Red Sea coastal plain. Strata in the upper plate dip as steeply as 60° to the west, and the beds are repeated by numerous planar and listric normal faults that dip to the east. Most of the faults truncate downward at the detachment. Thus, the upper plate is highly extended and the rocks in its eastern part have been translated about 20 km westward and 21/2- to 5-km downward relative to the rest of Arabia. A prominent detachment surface, with a north dip, is evident in northernmost Somalia where it breaks away north of the Somalian escarpment in an otherwise undeformed section of cratonic strata of Jurassic to Eocene age. The upper plate of the Somalian detachment consists of a highly faulted collage of the cratonic strata. This fault projects to middle crustal levels in the opposing Arabian margin to the northeast.

Journal of African Earth Sciences↗