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At least 145 records · Page 8Linked to original sources
USGS2008

The release of dissolved actinium to the ocean: A global comparison of different end-members

The measurement of short-lived 223Ra often involves a second measurement for supported activities, which represents 227Ac in the sample. Here we exploit this fact, presenting a set of 284 values on the oceanic distribution of 227Ac, which was collected when analyzing water samples for short-lived radium isotopes by the radium delayed coincidence counting system. The present work compiles 227Ac data from coastal regions all over the northern hemisphere, including values from ground water, from estuaries and lagoons, and from marine end-members. Deep-sea samples from a continental slope off Puerto Rico and from an active vent site near Hawaii complete the overview of 227Ac near its potential sources. The average 227Ac activities of nearshore marine end-members range from 0.4??dpm m- 3 at the Gulf of Mexico to 3.0??dpm m- 3 in the coastal waters of the Korean Strait. In analogy to 228Ra, we find the extension of adjacent shelf regions to play a substantial role for 227Ac activities, although less pronounced than for radium, due to its weaker shelf source. Based on previously published values, we calculate an open ocean 227Ac inventory of 1.35 * 1018??dpm 227Acex in the ocean, which corresponds to 37??moles, or 8.4??kg. This implies a flux of 127??dpm m-2 y- 1 from the deep-sea floor. For the shelf regions, we obtain a global inventory of 227Ac of 4.5 * 1015??dpm, which cannot be converted directly into a flux value, as the regional loss term of 227Ac to the open ocean would have to be included. Ac has so far been considered to behave similarly to Ra in the marine environment, with the exception of a strong Ac source in the deep-sea due to 231Paex. Here, we present evidence of geochemical differences between Ac, which is retained in a warm vent system, and Ra, which is readily released [Moore, W.S., Ussler, W. and Paull, C.K., 2008-this issue. Short-lived radium isotopes in the Hawaiian margin: Evidence for large fluid fluxes through the Puna Ridge. Marine Chemistry]. Another potential mechanism of producing deviations in 227Ac/228Ra and daughter isotope ratios from the expected production value of lithogenic material is observed at reducing environments, where enrichment in uranium may occur. The presented data here may serve as a reference for including 227Ac in circulation models, and the overview provides values for some end-members that contribute to the global Ac distribution. ?? 2007 Elsevier B.V. All rights reserved.

Marine Chemistry↗
USGS2009

Environmental forcing on life history strategies: Evidence for multi-trophic level responses at ocean basin scales

Variation in life history traits of organisms is thought to reflect adaptations to environmental forcing occurring from bottom-up and top-down processes. Such variation occurs not only among, but also within species, indicating demographic plasticity in response to environmental conditions. From a broad literature review, we present evidence for ocean basin- and large marine ecosystem-scale variation in intra-specific life history traits, with similar responses occurring among trophic levels from relatively short-lived secondary producers to very long-lived apex predators. Between North Atlantic and North Pacific Ocean basins, for example, species in the Eastern Pacific exhibited either later maturation, lower fecundity, and/or greater annual survival than conspecifics in the Western Atlantic. Parallel variations in life histories among trophic levels also occur in adjacent seas and between eastern vs. western ocean boundaries. For example, zooplankton and seabird species in cooler Barents Sea waters exhibit lower fecundity or greater annual survival than conspecifics in the Northeast Atlantic. Sea turtles exhibit a larger size and a greater reproductive output in the Western Pacific vs. Eastern Pacific. These examples provide evidence for food-web-wide modifications in life history strategies in response to environmental forcing. We hypothesize that such dichotomies result from frequency and amplitude shifts in resource availability over varying temporal and spatial scales. We review data that supports three primary mechanisms by which environmental forcing affects life history strategies: (1) food-web structure; (2) climate variability affecting the quantity and seasonality of primary productivity; (3) bottom-up vs. top-down forcing. These proposed mechanisms provide a framework for comparisons of ecosystem function among oceanic regions (or regimes) and are essential in modeling ecosystem response to climate change, as well as for creating dynamic ecosystem-based marine conservation strategies.

Progress in Oceanography↗
USGS2022

Ocean acidification in the Gulf of Mexico: Drivers, impacts, and unknowns

Ocean acidification (OA) has resulted in global-scale changes in ocean chemistry, which can disturb marine organisms and ecosystems. Despite its extensively populated coastline, many marine-dependent communities, and valuable economies, the Gulf of Mexico (GOM) remains a relatively understudied region with respect to acidification. In general, the warm waters of the GOM are better buffered from acidification compared to higher latitude seas, yet long-term acidification has been documented in several GOM regions. OA within the GOM is recognized as spatially variable, particularly within the coastal zone where numerous physical and biogeochemical processes contribute to carbonate chemistry dynamics. The historical progression of OA within the entire GOM is difficult to assess because only a few dedicated long-term monitoring sites have recently been established, and full-water column observations are limited. However, environmental drivers on smaller scales that affect GOM acidification were found to include freshwater, nutrient, and carbonate discharge from large rivers; ocean warming, circulation and residence times; and episodic extreme weather events. GOM marine ecosystems provide essential services, including coastline protection and carbon dioxide removal, and habitats for many marine species that are economically and ecologically important. However, organismal and ecosystem responses to OA are not well constrained for the GOM due to a lack of studies examining the specific effects of OA on regionally relevant species under contemporary and projected conditions. Tackling the vast number of remaining scientific unknowns in this region can be best coordinated through regional capacity networks, such as the Gulf of Mexico Coastal Acidification Network (GCAN), working to achieve a system-wide understanding of Gulf OA and its impacts. Here we synthesize the current peer-reviewed literature on GOM acidification across the ocean-estuarine continuum and identify critical knowledge, research, and monitoring gaps that require future attention.

Progress in Oceanography↗
USGS2003

Are high 3He/4He ratios in oceanic basalts an indicator of deep-mantle plume components?

The existence of a primordial, undegassed lower mantle reservoir characterized by high concentration of 3He and high 3He/4He ratios is a cornerstone assumption in modern geochemistry. It has become standard practice to interpret high 3He/4He ratios in oceanic basalts as a signature of deep-rooted plumes. The unfiltered He isotope data set for oceanic spreading centers displays a wide, nearly Gaussian, distribution qualitatively similar to the Os isotope (187Os/188 Os) distribution of mantle-derived Os-rich alloys. We propose that both distributions are produced by shallow mantle processes involving mixing between different proportions of recycled, variably aged radiogenic and unradiogenic domains under varying degrees of partial melting. In the case of the Re-Os isotopic system, radiogenic mid-ocean ridge basalt (MORB)-rich and unradiogenic (depleted mantle residue) endmembers are constantly produced during partial melting events. In the case of the (U+Th)-He isotope system, effective capture of He-rich bubbles during growth of phenocryst olivine in crystallizing magma chambers provides one mechanism for 'freezing in' unradiogenic (i.e. high 3He/4He) He isotope ratios, while the higher than chondritic (U+Th)/He elemental ratio in the evolving and partially degassed MORB melt provides the radiogenic (i.e. low 3He/4He) endmember. If this scenario is correct, the use of He isotopic signatures as a fingerprint of plume components in oceanic basalts is not justified. Published by Elsevier Science B.V.

Earth and Planetary Science Letters↗
USGS1997

Natural radionuclides and plutonium in sediments from the western Arctic Ocean: Sedimentation rates and pathways of radionuclides

Sediment cores collected during R.V. Polar Sea AOS94 expedition from the Chukchi Shelf to the North Poke were analyzed for several decay-series natural radionuclides and Pu isotopes to study sedimentation rates and pathways of radionuclides in the western Arctic Ocean. The measured sedimentation rates vary by more than three orders of magnitude along the transect, from 210Pb based rates of 200-700 cm kyr-1 over the Chukchi Shelf and 89 cm kyr-1 at the Chukchi Slope to 230Th-based rates of 0.02-0.3 cm kyr-1 at various settings in the deep basin. 230Th(ex) profiles in the central western Arctic Basin are characterized by a cyclic pattern and a pronounced sub-surface maximum superimposed on an overall decrease with depth. Sediment inventories of excess 210Pb and 230Th in the deep basin as a whole cannot account for their in situ production and 2610Pb fall-out. The opposite is true at the slope and shallower waters. We contend that, as with other ocean basins, boundary scavenging also exists in the Arctic Ocean. The broad continental shelves and the slope region may have the potential of removing all or moat of the particle-reactive radionuclides unaccounted for in the deep basin. The Pu isotope data are consistent with the notion of boundary scavenging. Sediment inventories and concentrations of Pu decrease rapidly offshore. Isotopic composition of Pu suggests mixing of fall-out Pu, which decreases with increasing latitudes, and fuels reprocessing Pu derived from the Russian and Atlantic sides of the Arctic Ocean. Although fuel reprocessing Pu has impinged on the Chukchi Slope, its existence over the Chukchi Shelf is not evident and probably overshadowed by fall-out Pu.

Deep-Sea Research Part II: Topical Studies in Ocea↗
USGS2008

Pacific Ocean and Cenozoic evolution of climate

The Pacific Ocean has played a major role in climate evolution throughout the Cenozoic (65–0 Ma). It is a fundamental component of global heat transport and circulation, the dominant locus of primary productivity, and, consequently, the largest reservoir for carbon exchange between the oceans and the atmosphere. A satisfactory understanding of the Cenozoic evolutionary history of the Pacific and its impact on global climate is currently data-limited. Nevertheless, the large dynamic range of Cenozoic conditions sets the stage to greatly expand our understanding of global climate and biogeochemical cycles. Past Earth “experiments” are particularly useful to understand interactions between climate and geosystems under different greenhouse gas loads. We highlight in this review four important problems in which the Pacific played a major role: the effect of changing geographic boundary conditions on ocean circulation; interactions between the carbon cycle and climate; the Pacific Ocean's influence on North American climate and its water cycle; and the gradual evolution of climate systems.

Reviews of Geophysics↗
USGS2008

Low reservoir ages for the surface ocean from mid-Holocene Florida corals

The 14C reservoir age of the surface ocean was determined for two Holocene periods (4908-4955 and 3008-3066 calendar (cal) B.P.) using U/Th-dated corals from Biscayne National Park, Florida, United States. We found that the average reservoir ages for these two time periods (294 ?? 33 and 291 ?? 27 years, respectively) were lower than the average value between A.D. 1600 and 1900 (390 ?? 60 years) from corals. It appears that the surface ocean was closer to isotopic equilibrium with CO2 in the atmosphere during these two time periods than it was during recent times. Seasonal ??18O measurements from the younger coral are similar to modern values, suggesting that mixing with open ocean waters was indeed occurring during this coral's lifetime. Likely explanations for the lower reservoir age include increased stratification of the surface ocean or increased ??14C values of subsurface waters that mix into the surface. Our results imply that a more correct reservoir age correction for radiocarbon measurements of marine samples in this location from the time periods ???3040 and ???4930 cal years B.P. is ???292 ?? 30 years, less than the canonical value of 404 ?? 20 years. Copyright 2008 by the American Geophysical Union.

Paleoceanography↗
USGS2009

Mercury sources, distribution, and bioavailability in the North Pacific Ocean: Insights from data and models

Fish harvested from the Pacific Ocean are a major contributor to human methylmercury (MeHg) exposure. Limited oceanic mercury (Hg) data, particularly MeHg, has confounded our understanding of linkages between sources, methylation sites, and concentrations in marine food webs. Here we present methylated (MeHg and dimethylmercury (Me 2 Hg)) and total Hg concentrations from 16 hydrographic stations in the eastern North Pacific Ocean. We use these data in combination with information from previous cruises and coupled atmospheric‐oceanic modeling results to better understand controls on Hg concentrations, distribution, and bioavailability. Total Hg concentrations (average 1.14 ± 0.38 pM) are elevated relative to previous cruises. Modeling results agree with observed increases and suggest that at present atmospheric Hg deposition rates, basin‐wide Hg concentrations will double relative to circa 1995 by 2050. Methylated Hg accounts for up to 29% of the total Hg in subsurface waters (average 260 ± 114 fM). We observed lower ambient methylated Hg concentrations in the euphotic zone and older, deeper water masses, which likely result from decay of MeHg and Me 2 Hg when net production is not occurring. We found a significant, positive linear relationship between methylated Hg concentrations and rates of organic carbon remineralization ( r 2 = 0.66, p < 0.001). These results provide evidence for the importance of particulate organic carbon (POC) transport and remineralization on the production and distribution of methylated Hg species in marine waters. Specifically, settling POC provides a source of inorganic Hg(II) to microbially active subsurface waters and can also provide a substrate for microbial activity facilitating water column methylation.

Global Biogeochemical Cycles↗
USGS2021

Miocene phosphatization of rocks from the summit of Rio Grande Rise, Southwest Atlantic Ocean

Marine phosphorites are an important part of the oceanic phosphorus cycle and are related to the effects of long-term global climate changes. We use petrography, mineralogy, rare earth elements contents, and 87 Sr/ 86 Sr-determined carbonate fluorapatite (CFA) and calcite ages to investigate the paragenesis and history of phosphatization of carbonate sediments, limestones, ferromanganese crusts, and ironstones from the summit of Rio Grande Rise (RGR), Southwest Atlantic Ocean. Phosphatization of all the rock types occurred throughout the Miocene from 20.2 to 6.8 million years ago (Ma), and occasionally during the Quaternary, mainly through the cementation of carbonate sediments by cryptocrystalline CFA, likely involving the dissolution of the smaller size fraction of foraminifera-nannofossil ooze. Porosity/permeability and abundance of fine calcite material were important factors determining the intensity of phosphatization of the various rock types. Phosphatization was initiated during a transition to a more dynamic circulation system in the South Atlantic Ocean, which remobilized phosphorus from deeper waters and increased primary productivity that culminated with the middle-Miocene Climatic Optimum between ∼17 and 14.8 Ma. The relatively shallow-water depth of RGR summit during the Miocene provided proximity to the oxygen minimum zone, a reservoir for reactive phosphorus, especially during periods of enhanced phosphorus cycling spurred by surface primary productivity. The cessation of phosphatization at RGR resulted from a rapidly cooling and dry climate that characterized the Miocene-Pliocene transition. Our results support previous observations that periods of broadly intensified ocean circulation and local hydrodynamic changes were the key paleoceanographic links to phosphorite formation.

Paleoceanography and Paleoclimatology↗
USGS2012

The Geoid: Effect of compensated topography and uncompensated oceanic trenches

The geoid is becoming increasingly important in interpretation of global tectonics. Most of the topography of the earth is isostatically compensated, so removal of its effect from the geoid is appropriate before tectonic modeling. The oceanic trenches, however, are dynamically depressed features and cannot be isostatically compensated in the classical way. Continental topography compensated at 35 km gives intracontinental geoidal undulations of up to 15 m over mountain ranges in a spherical harmonic expansion to order and degree 22. Oceanic topography compensated at 40 km, reasonable for the thermally supported long wavelengths, matches the +10 m difference between old continents and old oceans in a detailed NASA/GSFC geoid. Removing the assumed compensation for the oceanic trenches leaves negative anomalies of up to 9 m amplitude caused by their uncompensated mass deficit. This mass deficit acts as a partial "regional compensation" for the excess mass of the subducting slabs, and partly explains why geoidal (and gravity) anomalies over the cold slabs are less than thermal models predict.

Geophysical Research Letters↗
USGS1984

Amino acid epimerization implies rapid sedimentation rates in Arctic Ocean cores

The palaeooceanography of the Arctic Ocean is less well known than any other ocean basin, due to difficulties in obtaining cores and in providing a secure chronological framework for those cores that have been raised. Most recent investigators have suggested that low sedimentation rates (0.05-0.1 cm kyr-1) have characterized the deep basins over the past 5 Myr (refs 1,2) despite the glacial-marine character of the sediment and proximity to major centres of shelf glaciation. These calculations have been primarily based on the down-core pattern in the inclination of magnetic minerals, supported by uranium-series, 14C and micropalaeontological evidence. Here we analyse amino acid diagnesis in foraminifera from two gravity cores raised from the floor of the Arctic Ocean, our results suggest that these cores span <200 kyr., conflicting with the earlier estimate of 3 Myr based on palaeomagnetic data. The chronology of other Arctic Ocean cores and previous palaeoenvironmental interpretations need re-evaluation. ?? 1984 Nature Publishing Group.

Nature↗
USGS2016

Evidence for an ice shelf covering the central Arctic Ocean during the penultimate glaciation

The hypothesis of a km-thick ice shelf covering the entire Arctic Ocean during peak glacial conditions was proposed nearly half a century ago. Floating ice shelves preserve few direct traces after their disappearance, making reconstructions difficult. Seafloor imprints of ice shelves should, however, exist where ice grounded along their flow paths. Here we present new evidence of ice-shelf groundings on bathymetric highs in the central Arctic Ocean, resurrecting the concept of an ice shelf extending over the entire central Arctic Ocean during at least one previous ice age. New and previously mapped glacial landforms together reveal flow of a spatially coherent, in some regions >1-km thick, central Arctic Ocean ice shelf dated to marine isotope stage 6 (~140&thinsp;ka). Bathymetric highs were likely critical in the ice-shelf development by acting as pinning points where stabilizing ice rises formed, thereby providing sufficient back stress to allow ice shelf thickening.

Nature Communications↗
USGS2007

Decreased abundance of crustose coralline algae due to ocean acidification

Owing to anthropogenic emissions, atmospheric concentrations of carbon dioxide could almost double between 2006 and 2100 according to business-as-usual carbon dioxide emission scenarios 1 . Because the ocean absorbs carbon dioxide from the atmosphere 2, 3, 4 , increasing atmospheric carbon dioxide concentrations will lead to increasing dissolved inorganic carbon and carbon dioxide in surface ocean waters, and hence acidification and lower carbonate saturation states 2, 5 . As a consequence, it has been suggested that marine calcifying organisms, for example corals, coralline algae, molluscs and foraminifera, will have difficulties producing their skeletons and shells at current rates 6, 7 , with potentially severe implications for marine ecosystems, including coral reefs 6, 8, 9, 10, 11 . Here we report a seven-week experiment exploring the effects of ocean acidification on crustose coralline algae, a cosmopolitan group of calcifying algae that is ecologically important in most shallow-water habitats 12, 13, 14 . Six outdoor mesocosms were continuously supplied with sea water from the adjacent reef and manipulated to simulate conditions of either ambient or elevated seawater carbon dioxide concentrations. The recruitment rate and growth of crustose coralline algae were severely inhibited in the elevated carbon dioxide mesocosms. Our findings suggest that ocean acidification due to human activities could cause significant change to benthic community structure in shallow-warm-water carbonate ecosystems.

Nature Geoscience↗
USGS2018

Pronounced centennial-scale Atlantic Ocean climate variability correlated with Western Hemisphere hydroclimate

Surface-ocean circulation in the northern Atlantic Ocean influences Northern Hemisphere climate. Century-scale circulation variability in the Atlantic Ocean, however, is poorly constrained due to insufficiently-resolved paleoceanographic records. Here we present a replicated reconstruction of sea-surface temperature and salinity from a site sensitive to North Atlantic circulation in the Gulf of Mexico which reveals pronounced centennial-scale variability over the late Holocene. We find significant correlations on these timescales between salinity changes in the Atlantic, a diagnostic parameter of circulation, and widespread precipitation anomalies using three approaches: multiproxy synthesis, observational datasets, and a transient simulation. Our results demonstrate links between centennial changes in northern Atlantic surface-circulation and hydroclimate changes in the adjacent continents over the late Holocene. Notably, our findings reveal that weakened surface-circulation in the Atlantic Ocean was concomitant with well-documented rainfall anomalies in the Western Hemisphere during the Little Ice Age.

Nature Communications↗
USGS2020

Machine learning identifies a strong association between warming and reduced primary productivity in an oligotrophic ocean gyre

Phytoplankton play key roles in the oceans by regulating global biogeochemical cycles and production in marine food webs. Global warming is thought to affect phytoplankton production both directly, by impacting their photosynthetic metabolism, and indirectly by modifying the physical environment in which they grow. In this respect, the Bermuda Atlantic Time-series Study (BATS) in the Sargasso Sea (North Atlantic gyre) provides a unique opportunity to explore effects of warming on phytoplankton production across the vast oligotrophic ocean regions because it is one of the few multidecadal records of measured net primary productivity (NPP). We analysed the time series of phytoplankton primary productivity at BATS site using machine learning techniques (ML) to show that increased water temperature over a 27-year period (1990–2016), and the consequent weakening of vertical mixing in the upper ocean, induced a negative feedback on phytoplankton productivity by reducing the availability of essential resources, nitrogen and light. The unbalanced availability of these resources with warming, coupled with ecological changes at the community level, is expected to intensify the oligotrophic state of open-ocean regions that are far from land-based nutrient sources.

Scientific Reports↗
USGS2008

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

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

Proceedings of the National Academy of Sciences of↗
USGS2019

An introduction to the “Oceans and Society: Blue Planet” Initiative

We live on a blue planet, and Earth’s waters benefit many sectors of society. The future of our blue planet is increasingly reliant on the services delivered by marine, coastal and inland waters and on the advancement of effective, evidence-based decisions on sustainable development. “Oceans and Society: Blue Planet” (hereafter denoted as “GEO Blue Planet”) is an initiative of the Group on Earth Observations (GEO) that aims to ensure the sustained development and use of ocean and coastal observations for the benefit of society. GEO Blue Planet works to advance and exploit synergies among the many observational programmes devoted to ocean and coastal waters; to improve engagement with a variety of stakeholders for enhancing the timeliness, quality and range of information delivered; and to raise awareness of the societal benefits of ocean observations at the public and policy levels. This paper summarizes the role of GEO Blue Planet, current activities and considerations for future directions.

Journal of Operational Oceanography↗
USGS2017

Retrospective analysis of seasonal ocean growth rates of two sea winter Atlantic Salmon in eastern Maine using historic scales

Substantial declines of anadromous Atlantic Salmon Salmo salar have occurred throughout its range, with many populations at the southern extent of the distribution currently extirpated or endangered. While both one sea winter (1SW) and two sea winter (2SW) spawner numbers for the North American stocks have declined since the 1950s, the decline has been most severe in 2SW spawners. The first months at sea are considered a period of high mortality. However, early ocean mortality alone cannot explain the more pronounced decline of 2SW spawners, suggesting that the second year at sea may be more critical than previously thought. Atlantic Salmon scales collected by anglers and the state agency from 1946 to 2013 from five rivers in eastern Maine were used to estimate smolt age and ocean age of returning adults. Additionally, seasonal growth rates of maiden 2SW spawners were estimated using intercirculi measurements and linear back-calculation methods. Generalized linear mixed models (Gaussian family, log link function) were used to investigate the influence of average sea surface temperature, accumulated thermal units, the Atlantic Multidecadal Oscillation (AMO) and North Atlantic Oscillation indices, smolt age, smolt length, postsmolt growth, and river of origin on growth rate during the oceanic migration of North American Atlantic Salmon. Results suggest that different factors influence salmon growth throughout their oceanic migration, and previous growth can be a strong predictor of future size. Growth was negatively impacted by the phase of the AMO, which has been linked to salmon abundance trends, in early spring following the postsmolt period. This is likely when the 1SW and 2SW stock components separate, and our results suggest that this period may be of interest in future work examining the disproportionate decline in 2SW spawners.

Maine↗