SEARCH · Geology Reports

Results for “Ocean Science”

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

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 667 records · Page 37Linked to original sources

USGS2022

The potential of wave energy conversion to mitigate coastal erosion from hurricanes

Wave energy conversion technologies have recently attracted more attention as part of global efforts to replace fossil fuels with renewable energy resources. While ocean waves can provide renewable energy, they can also be destructive to coastal areas that are often densely populated and vulnerable to coastal erosion. There have been a variety of efforts to mitigate the impacts of wave- and storm-induced erosion; however, they are either temporary solutions or approaches that are not able to adapt to a changing climate. This study explores a green and sustainable approach to mitigating coastal erosion from hurricanes through wave energy conversion. A barrier island, Dauphin Island, off the coast of Alabama, is used as a test case. The potential use of wave energy converter farms to mitigate erosion due to hurricane storm surges while simultaneously generating renewable energy is explored through simulations that are forced with storm data using the XBeach model. It is shown that wave farms can impact coastal morphodynamics and have the potential to reduce dune and beach erosion, predominantly in the western portion of the island. The capacity of wave farms to influence coastal morphodynamics varies with the storm intensity.

Alabama↗

USGS2014

Temperature data acquired from the DOI/GTN-P Deep Borehole Array on the Arctic Slope of Alaska, 1973-2013

A homogeneous set of temperature measurements obtained from the DOI/GTN-P Deep Borehole Array between 1973 and 2013 is presented; DOI/GTN-P is the US Department of the Interior contribution to the Global Terrestrial Network for Permafrost (GTN-P). The 23-element array is located on the Arctic Slope of Alaska, a region of cold continuous permafrost. Most of the monitoring wells are situated on the Arctic coastal plain between the Brooks Range and the Arctic Ocean, while others are in the foothills to the south. The data represent the true temperatures in the wellbores and surrounding rocks at the time of the measurements; they have not been corrected to remove the thermal disturbance caused by drilling the wells. With a few exceptions, the drilling disturbance is estimated to have been on the order of 0.1 K or less by 1989. Thus, most of the temperature measurements acquired during the last 25 yr are little affected by the drilling disturbance. The data contribute to ongoing efforts to monitor changes in the thermal state of permafrost in both hemispheres by the Global Terrestrial Network for Permafrost, one of the primary subnetworks of the Global Terrestrial Observing System (GTOS). The data will also be useful for refining our basic understanding of the physical conditions in permafrost in Arctic Alaska, as well as providing important information for validating predictive models used for climate impact assessments. The processed data are available from the Advanced Cooperative Arctic Data and Information Service (ACADIS) repository at doi:10.5065/D6N014HK.

Alaska↗

USGS2019

Spatial distribution of water level impact to back-barrier bays

Water level in semi-enclosed bays, landward of barrier islands, is mainly driven by offshore sea level fluctuations that are modulated by bay geometry and bathymetry, causing spatial variability in the ensuing response (transfer). Local wind setup can have a secondary role that depends on wind speed, fetch, and relative orientation of the wind direction and the bay. Inlet geometry and bathymetry primarily regulate the magnitude of the transfer between open ocean and bay. Tides and short-period offshore oscillations are more damped in the bays than longer-lasting offshore fluctuations, such as storm surge and sea level rise. We compare observed and modeled water levels at stations in a mid-Atlantic bay (Barnegat Bay) with offshore water level proxies. Observed water levels in Barnegat Bay are compared and combined with model results from the Coupled Ocean-Atmosphere-Wave-Sediment Transport (COAWST) modeling system to evaluate the spatial structure of the water level transfer. Analytical models based on the dimensional characteristics of the bay are used to combine the observed data and the numerical model results in a physically consistent approach. Model water level transfers match observed values at locations inside the Bay in the storm frequency band (transfers ranging from 70-100%) and tidal frequencies (10-55%). The contribution of frequency-dependent local setup caused by wind acting along the bay is also considered. The approach provides transfer estimates for locations inside the Bay where observations were not available resulting in a complete spatial characterization. The approach allows for the study of the Bay response to alternative forcing scenarios (landscape changes, future storms, and rising sea level). Detailed spatial estimates of water level transfer can inform decisions on inlet management and contribute to the assessment of current and future flooding hazard in back-barrier bays and along mainland shorelines.

Natural Hazards and Earth System Sciences↗

USGS1968

The geochronology of foraminiferal ooze deposits in the "Southern Ocean"

Many cores raised from the Drake Passage are characterized by alternating zones of foraminiferal ooze and sandysilt. Cores raised from the East Pacific Rise are foraminiferal ooze or alternating siliceous and carbonate ooze. The uranium and thorium concentrations and isotopic ratios in foraminifers separated from these cores were measured by alpha-spectroscopy. 230 Th in foraminiferal tests is found to be out of equilibrium with its parent ( 234 U), enabling the “excess 230 Th” method to be used in estimating sedimentation rates. The uranium concentration of the tests is constant, suggesting constant uranium concentrations in the sea and thus constant production of 230 Th during periods of foraminiferal production. 232 Th concentration is 2.1 ± 0.7 ppm in the Drake Passage and below the level of detection in tests from the East Pacific Rise. Within each core the 232 Th is constant, varying closely with distance from the source. Based on the degree of disequilibrium of 230 Th, correlations between cores can be made. Age estimates of the foraminiferal zones indicate deposition occurred between 8 000 and 120 000 and between 190 000 and 280 000 years before present. Reported dates of high stands of the sea fall between the ages estimated for the foraminiferal zones, suggesting these zones were deposited during northern hemisphere glacial maxima.

Earth and Planetary Science Letters↗

USGS2017

Conversing with Pelehonuamea: A workshop combining 1,000+ years of traditional Hawaiian knowledge with 200 years of scientific thought on Kīlauea volcanism

The events surrounding volcanic eruptions and damaging earthquakes in Hawai‘i have often been described in journals, letters, and newspapers articles in the English language; however, the Hawaiian nation was among the most literate of countries in the 19th century, and many Hawaiian-language newspapers were in circulation through all but the earliest decades of the 19th century. Any modern reconstruction of the history of Hawaiian eruptions or earthquakes should take advantage of all available sources, and so we seek to add the Hawaiian-language newspaper articles, journals, stories, and chants to the volcano and earthquake literature. These sources have been used in many recent volcanological studies. Another aspect to the mix of science and traditional Hawaiian values is that many of the volcanic summits in Hawaiʻi are considered sacred to Hawaiians. Hawaiian travelers brought the first Western missionary team to the summit of Kīlauea and advised them of the proper protocols and behaviors while in this sacred area. The missionaries dismissed this advice as native superstition and they began a campaign of aggressively stamping out customs and protocols related to the Hawaiian volcano goddess Pelehonuamea. What has survived as native knowledge of the volcanoes is a few phrases from native guides included in some of the missionaries’ journals, and a few stories. Pualani and Ku ʻ ulei Kanahele provide excellent introductions to the Pelehonuamea chants. In the 21st century, amid a reawakening of Hawaiian culture, modern Hawaiians are demanding protection of their sacred areas, and scientists must be aware of these interests. At the very least, scientists should show respect to Hawaiian values when working in these areas, and should try to minimize disruptions caused by their work. Kaeo Duarte, Peter Mills, and Scott Rowland describe taking this approach in their field work. Traditional knowledge is also contained in place names. It is important not only to preserve old place names and to recover those no longer used, but also to preserve the stories of those places. Bobby Camara talks about the joys and frustrations of getting information on and recovering Hawaiian place names. Finally, we hope that a broader interest in Hawaiian views about locations in Hawaiʻi where physical scientific work is done will be as beneficial to physical scientists as it has been to life scientists investigating Hawaiian lifeforms on land and in the ocean, and that both studies will continue to benefit the native peoples of Hawaiʻi. Note that these proceedings are transcripts of oral presentations illustrated with PowerPoint presentations or charts. Although every effort has been made to assure the accuracy of the oral presentations, there are some gaps where words are not discernible in the voice recordings and are so noted. In other places, bracketed words were added to clarify the speaker’s meaning.

Hawaii↗

USGS2019

A seascape-scale habitat model to support management of fishing impacts on benthic ecosystems

Minimizing fishing impacts on seafloor ecosystems is a growing focus of ocean management; however, few quantitative tools exist to guide seascape-scale habitat management. To meet these needs, we developed a model to assess benthic ecosystem impacts from fishing gear contact. The habitat impacts model is cast in discrete time and can accommodate overlapping fisheries as well as incorporate gear-specific contact dynamics. We implemented the model in the North Pacific using fishing data from 2003 to 2017, estimating that habitat in 3.1% of the 1.2 million km 2 study area was disturbed at the end of the simulation period. A marked decline in habitat disturbance was evident since 2010, attributable to a single regulatory gear change that lifted trawl gear components off the seafloor. Running scenarios without these gear modifications showed these policies might have contributed to a 24% reduction in habitat disturbance since their implementation. Ultimately, model outputs provide direct estimates of the spatial and temporal trends of habitat effects from fishing — a key component of regulatory policies for many of the world’s fisheries.

Canadian Journal of Fisheries and Aquatic Sciences↗

USGS2021

Understanding collaborative governance from a communication network perspective: A case study of the Atlantic Salmon recovery framework

Atlantic salmon populations in Maine remain critically low despite extensive hatchery supplementation and habitat improvement efforts. In 2000, the Gulf of Maine Distinct Population Segment was listed as Endangered under the ESA with joint listing authority shared by the National Oceanic and Atmospheric Administration (NOAA) and the United States Fish and Wildlife Service (USFWS). Because, regulators, and tribal, federal and state managers operate with independent authorities, recovery decisions depend upon effective communication and coordination among groups. Using a mixed-methods approach, we surveyed (n = 41) and interviewed (n = 28) members of the Atlantic Salmon Recovery Framework (ASRF), the joint governance structure responsible for Atlantic salmon management in Maine. We used survey results to examine the communication between members of ASRF through communication network analysis. While there is a relatively high network density for individual communication (56 %), connections are decentralized, a characteristic that can be incompatible with some organizational structures. Challenges reported by members fit into three general categories; i. slow and ineffective decision-making, ii. confusion surrounding leadership and accountability, and iii. low adaptive capacity. Despite these challenges, participants reported a commitment to maintaining a collaborative governance structure and a long history of inter-organizational relationships. This introspective effort has led to a reorganization effort to optimize communication pathways.

Maine↗

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 &sim;10 4 yr.

Quaternary Science Reviews↗