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At least 253 records · Page 14Linked to original sources
USGS2008

Mid-Cenozoic tectonic and paleoenvironmental setting of the central Arctic Ocean

Drilling results from the Integrated Ocean Drilling Program's Arctic Coring Expedition (ACEX) to the Lomonosov Ridge (LR) document a 26 million year hiatus that separates freshwater-influenced biosilica-rich deposits of the middle Eocene from fossil-poor glaciomarine silty clays of the early Miocene. Detailed micropaleontological and sedimentological data from sediments surrounding this mid-Cenozoic hiatus describe a shallow water setting for the LR, a finding that conflicts with predrilling seismic predictions and an initial postcruise assessment of its subsidence history that assumed smooth thermally controlled subsidence following rifting. A review of Cenozoic tectonic processes affecting the geodynamic evolution of the central Arctic Ocean highlights a prolonged phase of basin-wide compression that ended in the early Miocene. The coincidence in timing between the end of compression and the start of rapid early Miocene subsidence provides a compelling link between these observations and similarly accounts for the shallow water setting that persisted more than 30 million years after rifting ended. However, for much of the late Paleogene and early Neogene, tectonic reconstructions of the Arctic Ocean describe a landlocked basin, adding additional uncertainty to reconstructions of paleodepth estimates as the magnitude of regional sea level variations remains unknown.

Paleoceanography↗
USGS2009

Utilizing chromophoric dissolved organic matter measurements to derive export and reactivity of dissolved organic carbon exported to the Arctic Ocean: A case study of the Yukon River, Alaska

[1] The quality and quantity of dissolved organic matter (DOM) exported by Arctic rivers is known to vary with hydrology and this exported material plays a fundamental role in the biogeochemical cycling of carbon at high latitudes. We highlight the potential of optical measurements to examine DOM quality across the hydrograph in Arctic rivers. Furthermore, we establish chromophoric DOM (CDOM) relationships to dissolved organic carbon (DOC) and lignin phenols in the Yukon River and model DOC and lignin loads from CDOM measurements, the former in excellent agreement with long‐term DOC monitoring data. Intensive sampling across the historically under‐sampled spring flush period highlights the importance of this time for total export of DOC and particularly lignin. Calculated riverine DOC loads to the Arctic Ocean show an increase from previous estimates, especially when new higher discharge data are incorporated. Increased DOC loads indicate decreased residence times for terrigenous DOM in the Arctic Ocean with important implications for the reactivity and export of this material to the Atlantic Ocean.

Geophysical Research Letters↗
USGS2011

Western Arctic Ocean temperature variability during the last 8000 years

We reconstructed subsurface (∼200–400 m) ocean temperature and sea-ice cover in the Canada Basin, western Arctic Ocean from foraminiferal δ 18 O, ostracode Mg/Ca ratios, and dinocyst assemblages from two sediment core records covering the last 8000 years. Results show mean temperature varied from −1 to 0.5°C and −0.5 to 1.5°C at 203 and 369 m water depths, respectively. Centennial-scale warm periods in subsurface temperature records correspond to reductions in summer sea-ice cover inferred from dinocyst assemblages around 6.5 ka, 3.5 ka, 1.8 ka and during the 15th century Common Era. These changes may reflect centennial changes in the temperature and/or strength of inflowing Atlantic Layer water originating in the eastern Arctic Ocean. By comparison, the 0.5 to 0.7°C warm temperature anomaly identified in oceanographic records from the Atlantic Layer of the Canada Basin exceeded reconstructed Atlantic Layer temperatures for the last 1200 years by about 0.5°C.

Geophysical Research Letters↗
USGS2019

Heat flow in the Western Arctic Ocean (Amerasian Basin)

From 1963 to 1973 the U.S. Geological Survey (USGS) measured heat flow at 356 sites in the Amerasian Basin (Western Arctic Ocean) from a drifting ice island (T-3). The resulting measurements, which are unevenly distributed on Alpha-Mendeleev Ridge (AMR) and in Canada and Nautilus basins, greatly expand available heat flow data for the Arctic Ocean. Average T-3 heat flow is ~54.7 ± 11.3 mW m-2, and Nautilus Basin, including Mendeleev Plain, is the only well-surveyed area (~13% of data) with significantly higher average heat flow (63.8 mW m-2). Heat flow and bathymetry are not correlated at a large scale, and turbiditic surficial sediments (Canada and Nautilus basins) have higher heat flow than the sediments that blanket the AMR. Thermal gradients are mostly near-linear, implying that conductive heat transport dominates and that near-seafloor sediments are in thermal equilibrium with overlying bottom waters. Combining the heat flow data with modern seismic imagery suggests that some of the observed heat flow variability may be explained by local changes in sediment thickness or lithology or the presence of basement faults that channel circulating seawater. A thermal model that incorporates thermal conductivity variations along a profile from Canada Basin (thick sediment on mostly oceanic crust) to Alpha Ridge (thin sediment over thick magmatic units associated with the High Arctic Large Igneous Province) predicts heat flow lower than that observed on Alpha Ridge. This, along with other observations, implies that circulating fluids modulate conductive heat flow and contribute to high variability in the T-3 dataset. .

Journal of Geophysical Research B: Solid Earth↗
USGS2024

Quantifying compound and nonlinear effects of hurricane-induced flooding using a dynamically coupled hydrological-ocean model

We recently developed a dynamically coupled hydrological-ocean modeling system that provides seamless coverage across the land-ocean continuum during hurricane-induced compound flooding. This study introduced a local inertial equation and a diagonal flow algorithm to the overland routing of the coupled system’s hydrology model (WRF-Hydro). Using Hurricane Florence (2018) as a test case, the performance of the coupled model was significantly improved, evidenced by its enhanced capability of capturing backwater and increased water level simulation accuracy and stability. With four model experiments, we present a framework to detangle, define, and quantify compound and nonlinear effects. The results revealed that the flood peaks in the lower Cape Fear River Basin and the coastal waters were contributed by inland flooding and storm surge, respectively. These two processes had comparable contributions to the flooding in the Cape Fear River Estuary. The compound effect was identified when the flood levels resulting from the combination of land and ocean processes surpassed those caused by an individual process alone. The compound effect during Hurricane Florence exhibited limited impact on flood peaks, primarily due to the time lag between the peaks of the storm surge and the inland flooding. In the period between the two peaks, the compound effect was salient and significantly impacted the magnitude and variation of the flood level. The nonlinear effect, defined as the difference between the compound flood level and the superposition of storm surge and inland flooding water levels, reduced flood levels in the river channels while increasing flood levels on the floodplain.

Water Resources Research↗
USGS2010

Late Quaternary paleoceanography of the Eurasian Basin, Arctic Ocean

We reconstructed late Quaternary deep (3000–4100 m) and intermediate depth (1000–2500 m) paleoceanographic history of the Eurasian Basin, Arctic Ocean from ostracode assemblages in cores from the Lomonosov Ridge, Gakkel Ridge, Yermak Plateau, Morris Jesup Rise, and Amundsen and Makarov Basins obtained during the 1991 Polarstern cruise. Modern assemblages on ridges and plateaus between 1000 and 1500 m are characterized by abundant, relatively species-rich benthic ostracode assemblages, in part, reflecting the influence of high organic productivity and inflowing Atlantic water. In contrast, deep Arctic Eurasian basin assemblages have low abundance and low diversity and are dominated by Krithe and Cytheropteron reflecting faunal exchange with the Greenland Sea via the Fram Strait. Major faunal changes occurred in the Arctic during the last glacial/interglacial transition and the Holocene. Low-abundance, low-diversity assemblages from the Lomonosov and Gakkel Ridges in the Eurasian Basin from the last glacial period have modern analogs in cold, low-salinity, low-nutrient Greenland Sea deep water; glacial assemblages from the deep Nansen and Amundsen Basins have modern analogs in the deep Canada Basin. During Termination 1 at intermediate depths, diversity and abundance increased coincident with increased biogenic sediment, reflecting increased organic productivity, reduced sea-ice, and enhanced inflowing North Atlantic water. During deglaciation deep Nansen Basin assemblages were similar to those living today in the deep Greenland Sea, perhaps reflecting deepwater exchange via the Fram Strait. In the central Arctic, early Holocene faunas indicate weaker North Atlantic water inflow at middepths immediately following Termination 1, about 8500–7000 year B.P., followed by a period of strong Canada Basin water overflow across the Lomonosov Ridge into the Morris Jesup Rise area and central Arctic Ocean. Modern perennial sea-ice cover evolved over the last 4000–5000 years. Late Quaternary faunal changes reflect benthic habitat changes most likely caused by changes in the import of cold, deepwater of Greenland Sea origin and warmer and middepth Atlantic water to the Eurasian Basin through the Fram Strait, and export of Arctic Ocean deepwater.

Paleoceanography and Paleoclimatology↗
USGS1996

Along-axis variability in crustal accretion at the Mid-Atlantic Ridge: Results from the OCEAN study

The OCEAN experiment is an integrated geophysical study of a region of the Cape Verde abyssal plain that formed at 140 Ma. Deep seismic reflection and ocean bottom hydrophone (OBH) refraction data were acquired along lines parallel and perpendicular to the paleoridge axis trend identified from a detailed magnetic anomaly survey. The igneous basement is overlain by about 1.3 km of sediment which enables improved imaging of intracrustal structure beyond that possible near the Mid-Atlantic Ridge axis. We describe the results of a 150-km long profile oriented parallel to magnetic anomalies M15 and M16, along which deep seismic reflection data collected by the British Institutions Reflection Profiling Syndicate are complemented by refraction data constrained by four OBHs. The line spans an entire spreading segment between two fracture zones; the northern of which has an offset of 40 km and the other (central) has an offset of only 10 km. Away from the fracture zones, the mean igneous crustal thickness is 7.2 km; near both fracture zones, thinning of up to 4 km is observed, giving a mean igneous crustal thickness over the whole segment of approximately 6.5 km. Differences are seen between the two fracture zones in their seismic velocity structure, in the associated basement topography, and in the presence of a strong reflection extending into the mantle beneath the northern fracture zone. The boundary between oceanic layers 2 and 3 correlates with variably coherent normal incidence reflections and a change in the character of the reflectivity. A number of planar reflections up to 10 km in length are present within the middle and lower crust, dipping outward from beneath low-amplitude basement highs at ∼15°; these appear to be present only within layer 3. The Moho has several expressions in the reflection data, including isolated reflection events, a local increase in reflected amplitudes, and a downward decrease in coherent reflections. At the center of the segment there is a zone at the base of the crust within which both high- and low-velocity materials are present. This zone shows an enhanced level of discontinuous normal incidence reflectivity and may represent an initial fractionation event as melt was emplaced at the spreading ridge.

Journal of Geophysical Research B: Solid Earth↗
USGS2012

Hafnium isotope variations in oceanic basalts

Routine low-blank chemistry and 0.01-0.04% precision on the ratio 176 Hf/ 177 Hf allows study of Hf isotopic variations, generated by β − -decay of 176 Lu, in volcanic rocks derived from the suboceanic mantle. Normalized to 176 Hf/ 177 Hf = 0.7325, 176 Hf/ 177 Hf ranges from 0.2828 to 0.2835, based on 24 basalt samples. 176 Hf/ 177 Hf is positively correlated with 143 Nd/ 144 Nd, and negatively correlated with 87 Sr/ 86 Sr and 206 Pb/ 204 Pb. Along the Iceland-Reykjanes ridge traverse, 176 Hf/ 177 Hf increases southwards. The coherence of Hf, Nd and Sr isotopes in the oceanic mantle allows an approximate bulk Earth 176 Hf/ 177 Hf of 0.28295 to be inferred from the bulk Earth 143 Nd/ 144 . This requires the bulk Earth Lu/Hf to be 0.25, similar to that of the Juvinas eucrite. 60% of the Hf isotopic variation in oceanic basalts occurs among mid-ocean ridge samples. Lu-Hf fractionation probably decouples from Sm-Nd and Rb-Sr fractionation in very depleted source regions, with high Lu/Hf, and consequent high 176 Hf/ 177 Hf ratios developing in mantle residual from partial melting.

Geophysical Research Letters↗
USGS1974

Ophiolites and oceanic crust

OPHIOLITES consist of a pseudostratiform sequence, of harzburgite, tectonite, ultramafic and mafic cumulates sometimes including gabbro and quartz diorite (plagiogranite) intrusions, dolerite dyke swarms, pillow lava 1, and deep-sea sediments2-4. This assemblage occurs in all Phanerozoic mountain systems and is interpreted as fossil oceanic crust and uppermost mantle5-10. Outstanding problems include differences between the chemical properties of Ophiolites and rocks thought to represent present-day oceanic crust11,12, the lack in some complexes of recognised dyke swarms or cumulates, and the relative thinness of ophiolite mafic rocks compared with standard oceanic crustal sections5,8,13.

Nature↗
USGS1985

Segmentation of mid-ocean ridges

Studies of mid-ocean ridges in the Pacific and Atlantic oceans show that the volcanism that forms the oceanic crust along the spreading-plate boundaries is concentrated at regular intervals related to spreading rate. This observation and a new calculation for a Rayleigh-Taylor type of gravitational instability of a partially molten mantle region growing under spreading centres yield reasonable estimates of upper mantle viscosities.

Nature↗
USGS1987

Migration of mid-ocean-ridge volcanic segments

The propagation of small-offset volcanic spreading-centre segments along mid-ocean ridge crests may reflect absolute motion of the plate boundary relative to the underlying mesospheric frame. Such a relationship could be caused by a purely vertical flow of the mantle under spreading centres and would have value in constraining past plate motions from non-transform trends generated during along-ridge propagation and in linking the major-element variability of oceanic crust and upper mantle to the bulk composition and temperatures of mantle ascending under mid-ocean ridges.

Nature↗
USGS2008

Deformation and rupture of the oceanic crust may control growth of Hawaiian volcanoes

Hawaiian volcanoes are formed by the eruption of large quantities of basaltic magma related to hot-spot activity below the Pacific Plate. Despite the apparent simplicity of the parent process - emission of magma onto the oceanic crust - the resulting edifices display some topographic complexity. Certain features, such as rift zones and large flank slides, are common to all Hawaiian volcanoes, indicating similarities in their genesis; however, the underlying mechanism controlling this process remains unknown. Here we use seismological investigations and finite-element mechanical modelling to show that the load exerted by large Hawaiian volcanoes can be sufficient to rupture the oceanic crust. This intense deformation, combined with the accelerated subsidence of the oceanic crust and the weakness of the volcanic edifice/oceanic crust interface, may control the surface morphology of Hawaiian volcanoes, especially the existence of their giant flank instabilities. Further studies are needed to determine whether such processes occur in other active intraplate volcanoes. ??2008 Nature Publishing Group.

Nature↗
USGS2012

Deep Arctic Ocean warming during the last glacial cycle

In the Arctic Ocean, the cold and relatively fresh water beneath the sea ice is separated from the underlying warmer and saltier Atlantic Layer by a halocline. Ongoing sea ice loss and warming in the Arctic Ocean have demonstrated the instability of the halocline, with implications for further sea ice loss. The stability of the halocline through past climate variations is unclear. Here we estimate intermediate water temperatures over the past 50,000 years from the Mg/Ca and Sr/Ca values of ostracods from 31 Arctic sediment cores. From about 50 to 11 kyr ago, the central Arctic Basin from 1,000 to 2,500 m was occupied by a water mass we call Glacial Arctic Intermediate Water. This water mass was 1–2 °C warmer than modern Arctic Intermediate Water, with temperatures peaking during or just before millennial-scale Heinrich cold events and the Younger Dryas cold interval. We use numerical modelling to show that the intermediate depth warming could result from the expected decrease in the flux of fresh water to the Arctic Ocean during glacial conditions, which would cause the halocline to deepen and push the warm Atlantic Layer into intermediate depths. Although not modelled, the reduced formation of cold, deep waters due to the exposure of the Arctic continental shelf could also contribute to the intermediate depth warming.

Nature Geoscience↗
USGS2015

Robust global ocean cooling trend for the pre-industrial Common Era

The oceans mediate the response of global climate to natural and anthropogenic forcings. Yet for the past 2,000 years — a key interval for understanding the present and future climate response to these forcings — global sea surface temperature changes and the underlying driving mechanisms are poorly constrained. Here we present a global synthesis of sea surface temperatures for the Common Era ( CE ) derived from 57 individual marine reconstructions that meet strict quality control criteria. We observe a cooling trend from 1 to 1800 CE that is robust against explicit tests for potential biases in the reconstructions. Between 801 and 1800 CE , the surface cooling trend is qualitatively consistent with an independent synthesis of terrestrial temperature reconstructions, and with a sea surface temperature composite derived from an ensemble of climate model simulations using best estimates of past external radiative forcings. Climate simulations using single and cumulative forcings suggest that the ocean surface cooling trend from 801 to 1800 CE is not primarily a response to orbital forcing but arises from a high frequency of explosive volcanism. Our results show that repeated clusters of volcanic eruptions can induce a net negative radiative forcing that results in a centennial and global scale cooling trend via a decline in mixed-layer oceanic heat content.

Nature Geoscience↗
USGS2023

Global assessment of marine plastic exposure for oceanic birds

Plastic pollution is distributed patchily around the world’s oceans. Likewise, marine organisms that are vulnerable to plastic ingestion or entanglement have uneven distributions. Understanding where wildlife encounters plastic is crucial for targeting research and mitigation. Oceanic seabirds, particularly petrels, frequently ingest plastic, are highly threatened, and cover vast distances during foraging and migration. However, the spatial overlap between petrels and plastics is poorly understood. Here we combine marine plastic density estimates with individual movement data for 7137 birds of 77 petrel species to estimate relative exposure risk. We identify high exposure risk areas in the Mediterranean and Black seas, and the northeast Pacific, northwest Pacific, South Atlantic and southwest Indian oceans. Plastic exposure risk varies greatly among species and populations, and between breeding and non-breeding seasons. Exposure risk is disproportionately high for Threatened species. Outside the Mediterranean and Black seas, exposure risk is highest in the high seas and Exclusive Economic Zones (EEZs) of the USA, Japan, and the UK. Birds generally had higher plastic exposure risk outside the EEZ of the country where they breed. We identify conservation and research priorities, and highlight that international collaboration is key to addressing the impacts of marine plastic on wide-ranging species.

Nature Communications↗
USGS2023

Increasing ocean wave energy observed in Earth’s seismic wavefield since the late 20th century

Ocean waves excite continuous globally observable seismic signals. We use data from 52 globally distributed seismographs to analyze the vertical component primary microseism wavefield at 14–20 s period between the late 1980s and August 2022. This signal is principally composed of Rayleigh waves generated by ocean wave seafloor tractions at less than several hundred meters depth, and is thus a proxy for near-coastal swell activity. Here we show that increasing seismic amplitudes at 3 σ significance occur at 41 (79%) and negative trends occur at 3 σ significance at eight (15%) sites. The greatest absolute increase occurs for the Antarctic Peninsula with respective acceleration amplitude and energy trends ( ± 3 σ ) of 0.037 ± 0.008 nm s −2 y −1 (0.36 ± 0.08% y −1 ) and 4.16 ± 1.07 nm 2 s −2 y −1 (0.58 ± 0.15% y −1 ), where percentage trends are relative to historical medians. The inferred global mean near-coastal ocean wave energy increase rate is 0.27 ± 0.03% y −1 for all data and is 0.35 ± 0.04% y −1 since 1 January 2000. Strongly correlated seismic amplitude station histories occur to beyond 50 ∘ of separation and show regional-to-global associations with El Niño and La Niña events.

Nature Communications↗
USGS2020

Paleoclimate ocean conditions shaped the evolution of corals and their skeletal composition through deep time

Identifying how past environmental conditions shaped the evolution of corals and their skeletal traits provides a framework for predicting their persistence and that of their non-calcifying relatives under impending global warming and ocean acidification. Here we show that ocean geochemistry, particularly aragonite–calcite seas, drives patterns of morphological evolution in anthozoans (corals, sea anemones) by examining skeletal traits in the context of a robust, time-calibrated phylogeny. The lability of skeletal composition among octocorals suggests a greater ability to adapt to changes in ocean chemistry compared with the homogeneity of the aragonitic skeleton of scleractinian corals. Pulses of diversification in anthozoans follow mass extinctions and reef crises, with sea anemones and proteinaceous corals filling empty niches as tropical reef builders went extinct. Changing environmental conditions will likely diminish aragonitic reef-building scleractinians, but the evolutionary history of the Anthozoa suggests other groups will persist and diversify in their wake.

Nature Ecology & Evolution↗
USGS2021

Arctic Ocean stratification set by sea level and freshwater inputs since the last ice age

Salinity-driven density stratification of the upper Arctic Ocean isolates sea-ice cover and cold, nutrient-poor surface waters from underlying warmer, nutrient-rich waters. Recently, stratification has strengthened in the western Arctic but has weakened in the eastern Arctic; it is unknown if these trends will continue. Here we present foraminifera-bound nitrogen isotopes from Arctic Ocean sediments since 35,000 years ago to reconstruct past changes in nutrient sources and the degree of nutrient consumption in surface waters, the latter reflecting stratification. During the last ice age and early deglaciation, the Arctic was dominated by Atlantic-sourced nitrate and incomplete nitrate consumption, indicating weaker stratification. Starting at 11,000 years ago in the western Arctic, there is a clear isotopic signal of Pacific-sourced nitrate and complete nitrate consumption associated with the flooding of the Bering Strait. These changes reveal that the strong stratification of the western Arctic relies on low-salinity inflow through the Bering Strait. In the central Arctic, nitrate consumption was complete during the early Holocene, then declined after 5,000 years ago as summer insolation decreased. This sequence suggests that precipitation and riverine freshwater fluxes control the stratification of the central Arctic Ocean. Based on these findings, ongoing warming will cause strong stratification to expand into the central Arctic, slowing the nutrient supply to surface waters and thus limiting future phytoplankton productivity.

Nature Geoscience↗