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USGS2014

Identifying hazards associated with lava deltas

Lava deltas, formed where lava enters the ocean and builds a shelf of new land extending from the coastline, represent a significant local hazard, especially on populated ocean island volcanoes. Such structures are unstable and prone to collapse—events that are often accompanied by small explosions that can deposit boulders and cobbles hundreds of meters inland. Explosions that coincide with collapses of the East Lae ‘Apuki lava delta at Kīlauea Volcano, Hawai‘i, during 2005–2007 followed an evolutionary progression mirroring that of the delta itself. A collapse that occurred when the lava–ocean entry was active was associated with a blast of lithic blocks and dispersal of spatter and fine, glassy tephra. Shortly after delta growth ceased, a collapse exposed hot rock to cold ocean water, resulting in an explosion composed entirely of lithic blocks and lapilli. Further collapse of the delta after several months of inactivity, by which time it had cooled significantly, resulted in no recognizable explosion deposit. Seaward displacement and subsidence of the coastline immediately inland of the delta was measured by both satellite and ground-based sensors and occurred at rates of several centimeters per month even after the lava–ocean entry had ceased. The anomalous deformation ended only after complete collapse of the delta. Monitoring of ground deformation may therefore provide an indication of the potential for delta collapse, while the hazard associated with collapse can be inferred from the level of activity, or the time since the last activity, on the delta.

Bulletin of Volcanology↗

USGS2026

Population structure and genetic stock identification in southeastern United States loggerhead sea turtles (Caretta caretta) using genome-wide SNPs

Characterizing the genetic structure and connectivity between populations of endangered species can be used to inform management actions. In vagile species with high gene flow or recently established populations, such characterizations can be difficult to undertake using traditional genetic markers, and genetic stock identification (GSI) may be confounded by allele-sharing between populations. Loggerhead sea turtles ( Caretta caretta ) in the southeastern United States comprise seven management units (MUs) based on female philopatry inferred via mitochondrial DNA sequences, yet nuclear microsatellite data do not reflect divergence. Further, loci for accurate GSI are not currently known. To address this, we generated genome-wide single nucleotide polymorphism (SNP) data from 146 females nesting at individual sites representative of each southeastern United States MU. We found weak (F ST =0.001–0.003) but significant divergence among all MUs, with more notable divergence between the Gulf Coast and Atlantic Ocean MUs, and amongst the Atlantic Ocean MUs. We then used an iterative leave-one-out approach to identify candidate loci for GSI. This approach identified loci that could assign individuals to natal ocean basins (i.e., to the Gulf Coast or to the Atlantic Ocean), and to individual MUs within the Atlantic Ocean, with high (≥90%) success and accuracy. Analyses of genome-wide SNPs refined our understanding of the magnitude and scale of population connectivity in loggerhead turtles in the southeastern United States, and provided a foundation for the development of SNP panels for accurate, fine-scale GSI in sea turtles.

Alabama, Florida, Georgia↗

USGS2020

Effects of elevated sea levels and waves on southern California estuaries during the 2015–2016 El Niño

The 2015–2016 El Niño provided insight into how low-inflow estuaries might respond to future climate regimes, including high sea levels and more intense waves. High waves and water levels coupled with low rainfall along the Southern California coastline provided the opportunity to examine how extreme ocean forcing impacts estuaries independently from fluvial events. From November 2015 to April 2016, water levels were measured in 13 Southern California estuaries, including both intermittently closed and perennially open estuaries with varying watershed size, urban development, and management practices. Elevated ocean water levels caused raised water levels and prolonged inundation in all of the estuaries studied. Water levels inside perennially open estuaries mirrored ocean water levels, while those inside intermittently closed estuaries (ICEs) exhibited enhanced higher-high water levels during large waves, and tides were truncated at low tides due to a wave-built sand sill at the mouth, resulting in elevated detided water levels. ICEs closed when sufficient wave-driven sand accretion formed a barrier berm across the mouth separating the estuary from the ocean, the height of which can be estimated using estuarine lower-low water levels. During the 2015–2016 El Niño, a greater number of Southern California ICEs closed than during a typical year and ICEs that close annually experienced longer than normal closures. Overall, sill accretion and wave exposure were important contributing factors to individual estuarine response to ocean conditions. Understanding how estuaries respond to increased sea levels and waves and the factors that influence closures will help managers develop appropriate adaptation strategies.

California↗

USGS2024

Enhancing assessments of coastal wetland migration potential with sea-level rise: Accounting for uncertainty in elevation data, tidal data, and future water levels

Sea-level rise rates are predicted to surpass rates of wetland vertical adjustment in the coming decades in many areas, increasing the potential for wetland submergence. Information on where wetland migration is possible can help natural resource managers for planning land acquisition or enhancing habitat connectivity to bolster adaptation of coastal wetlands to rising seas. Elevation-based models of wetland migration are often hampered with uncertainty associated with ground surface elevation, current water levels (i.e., tides and extreme water levels), and future water levels from sea-level rise. Here, we developed an approach that involved digital elevation model error reduction and the use of Monte Carlo simulations that utilize uncertainty assumptions regarding elevation error, contemporary water levels, and future sea levels to identify potential wetland migration areas. Our analyses were developed for Duvall and Nassau Counties in northeastern Florida (USA). We focus on the migration of regularly oceanic-flooded wetlands (i.e., flooded by oceanic water daily) and irregularly oceanic-flooded wetlands (i.e., flooded by oceanic water less frequently than daily). For two relative sea-level rise scenarios based on the 0.5 m and the 1.5 m global mean sea-level rise scenarios, we quantified migration by wetland flooding frequency class and identified land cover and land use types that are vulnerable to future exposure to oceanic waters. The variability in total coverage and relative coverage of wetland migration from our results highlights how topography and accelerated sea-level rise interact. Our wetland migration results communicate uncertainty by showing flooding frequency class as probabilistic outputs.

Florida↗

USGS1994

Seawater as the source of minor elements in black shales, phosphorites and other sedimentary rocks

Many of the minor elements in seawater today have a concentration-depth profile similar to that of the biologically essential nutrients, NO-3 and PO3-4. They show a relative depletion in the photic zone and enrichment in the deep ocean. The difference between their surface- and deep-ocean values, normalized to the change in PO3-4, approaches the average of measured minor-element: P ratios in marine plankton, although individual analyses of the latter show extreme scatter for a variety of reasons. Despite this scatter in the minor-element analyses of plankton, agreement between the two sets of data shows unequivocally that an important marine flux of many minor elements through the ocean is in the form of biogenic matter, with a composition approaching that of plankton. This interpretation is further supported by sediment studies, particularly of sediments which accumulate in shelf-slope environments where biological productivity in the photic zone is exceptionally high and organic carbon contents of the underlying sediment elevated. The interelement relations observed for some of these sediments approach the average values of plankton. These same interelement relations are observed in many marine sedimentary rocks such as metalliferous black shales and phosphorites, rocks which have a high content of marine fractions (e.g., organic matter, apatite, biogenic silica and carbonates). Many previous studies of the geochemistry of these rocks have concluded that local hydrothermal activity, and/or seawater with an elemental content different from that of the modern ocean, was required to account for their minor-element contents. However, the similarity in several of the minor-element ratios in many of these formations to minor-element ratios in modern plankton demonstrates that these sedimentary rocks accumulated in environments whose marine chemistry was virtually identical to that seen on continental shelf-slopes, or in marginal seas, of the ocean today. The accumulation of the marine fraction of minor elements on these ancient sea floors was determined largely by the accumulation of organic matter, settling from the photic zone and with a composition of average plankton. A second marine fraction of minor elements in these rocks accumulated through precipitation and adsorption from seawater. The suite of elements in this fraction reflects redox conditions in the bottom water, as determined by bacterial respiration. For example, high Mn, high Cr+V and high Mo concentrations, above those which can be attributed to the accumulation of planktonic matter, characterize accumulation under bottom-water oxidizing, denitrifying and sulfate-reducing conditions, respectively. ?? 1994.

Chemical Geology↗

USGS1968

Variation of Nb-Ta, Zr-Hf, Th-U and K-Cs in two diabase-granophyre suites

Concentrations of Nb, Ta, Zr, Hf, Th, U and Cs have been determined in samples of igneous rocks representing the diabase-granophyre suites from Dillsburg, Pennsylvania, and Great Lake, Tasmania. Niobium and tantalum have a three to fourfold increase with differentiation in each of the suites. The chilled margin of the Great Lake intrusion contains half the niobium and tantalum content (5.3 ppm and 0.4 ppm, respectively) of the chilled basalt from Dillsburg (10 ppm and 0.9 ppm, respectively). The twofold difference between the suites is correlated with differences in their titanium content. The average Nb Ta "> NbTa ratios for each suite are similar: 13.5 for the Great Lake suite, and 14.4 for the Dillsburg suite. The zirconium content of the two suites is essentially the same and increases from 50 to 60 ppm in the chilled margins to 240–300 ppm in the granophyres. Hafnium is low in the early formed rocks (0.5 –1.5 ppm and achieves a maximum in the granophyres (5–8 ppm). The Zr Hf "> ZrHf ratio decreases from 68 to 33 with progressive differentiation. In the Dillsburg suite thorium and uranium increase from 2.6 ppm and 0.6 ppm, respectively, in the chilled samples to 11.8 ppm and 3.1 ppm in the granophyres. The chilled margin of the Great Lake suite contains 3.2 ppm thorium and 9.8 ppm uranium; the granophyre contains 11.2 ppm thorium and 2.8 ppm uranium. The average Th U "> ThU ratios of the Dillsburg and Great Lake suites are nearly the same—4.1 and 4.4, respectively. Within each suite the Th U "> ThU ratio remains quite constant. Cesium and the K Cs "> KCs ratio do not vary systematically in the Dillsburg suite possibly because of redistribution or loss of cesium by complex geologic processes. Except for the chilled margin of the Great Lake suite, the variation of Cs and the K Cs "> KCs ratio are in accord with theoretical considerations. Cesium increases from about 0.6 ppm in the lower zone to 3.5 ppm in the granophyre; the K Cs "> KCs ratio varies from 10 × 10 3 in the lower zone to 6 × 10 3 in the granophyre. A comparison of the abundance of some of these elements is made with those reported on oceanic tholeiites from the Atlantic and Pacific oceans. Trace elements with large ionic radii (Th, U, Cs) are present in significantly greater concentrations in the two continental tholeiitic series than in the oceanic tholeiites. However, this does not seem to be true for lithophilic elements of smaller ionic radii (Zr and Nb). These trace element distribution patterns, when considered with other minor element and isotopic studies, indicate that 1. crustal contamination does not entirely account for differences between continental and oceanic tholeiites, and 2. the oceanic tholeiites do not necessarily delimit the geochemical characteristics of the mantle.

Geochimica et Cosmochimica Acta↗

USGS1992

Interoceanic variation in the rare earth, major, and trace element depositional chemistry of chert: Perspectives gained from the DSDP and ODP record

Rare earth element (REE), major, and trace element abundances and relative fractionations in forty nodular cherts sampled by the Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) indicate that the REE composition of chert records the interplay between terrigenous sources and scavenging from the local seawater. Major and (non-REE) trace element ratios indicate that the aluminosilicate fraction within the chert is similar to NASC (North American Shale Composite), with average Pacific chert including ~7% NASC-like particles, Indian chert ~ 11% NASC, Atlantic chert ~ 17% NASC, and southern high latitude (SHL) chert 53% NASC. Using La as a proxy for ∑REE, approximations of La ex (the amount of La in excess of that supplied by the detrital aluminosilicate fraction) indicate that Pacific chert contains the greatest La ex (85% of La total ) and SHL chert the least (38% of La total ). As shown by interelement associations, this La ex is most likely an adsorbed component onto aluminosilicate and phosphatic phases. Accordingly, chert from the large Pacific Ocean, where deposition occurs relatively removed from significant terrigenous input, records a depositional REE signal dominated by adsorption of dissolved REEs from seawater. Pacific chert CeCe * ⪡ 1 and La n Yb n ~ 0.8-1, resulting from adsorption of local Ce-depleted seawater and preferential adsorption of LREEs from seawater (e.g., La n Yb n ~ 0.4), which increases the La n Yb n ratio recorded in chert. Chert from the Atlantic basin, a moderately sized ocean basin lined by passive margins and with more terrigenous input than the Pacific, records a mix of adsorptive and terrigenous REE signals, with moderately negative Ce anomalies and La n Yb n "> La n Yb n ratios intermediate to those of the Pacific and those of terrigenous input. Chert from the SHL region is dominated by the large terrigenous input on the Antarctic passive margin, with inherited Ce Ce * ~1 "> CeCe * ~1 and inherited La n Yb n "> La n Yb n values of ~1.2–1.4. Ce Ce * "> ~1.2–1.4.CeCe * does not vary with age, either throughout the entire data base or within a particular basin. Overall, Ce Ce * "> CeCe * does not correlate with P 2 O 5 concentrations, even though phosphatic phases may be an important REE carrier. This and previous studies of the large-scale controlling parameters of sedimentary REEs across ocean basins collectively indicate that REE indices of depositional regime (e.g., Ce Ce * "> CeCe * , La n Yb n "> La n Yb n , La ex ) are reproducible in a variety of sediment and rock lithologies, ages, and ocean basins, and present a coherent tool for paleoceanographic and tectonic basin reconstructions.

Geochimica et Cosmochimica Acta↗

USGS1988

Precambrian ophiolites of Arabia: Geologic settings, U Pb geochronology, Pb-isotope characteristics, and implications for continental accretion

Disrupted ophiolites occur in linear belts up to 900 km long between microplates that collided during the late Proterozoic to form the Arabian Shield. UPb zircon ages and Pb-isotope data from these ophiolitic rocks help constrain the history of accretion of the Arabian Shield and thereby contribute to the definition of its microplates and terranes. Terranes of the central and western Arabian Shield are generally thought to represent intraoceanic island arcs that range in age from about 900 to 640 Ma; however, a region of the eastern Arabian Shield contains rocks of Early Proterozoic age and may represent an exotic continental fragment entrained between the arc complexes. Ophiolites of the Yanbu suture (northwestern shield), dated by UPb (zircon) and SmNd (mineral isochron) methods, yield model ages of 740–780 Ma. These are among the oldest well-dated rocks in the northwestern Arabian Shield. Ages from the Jabal al Wask complex overlap with ages of adjacent arc rocks. This overlap in age supports geologic and geochemical evidence that the Wask complex represents a fragment of back-arc oceanic lithosphere formed during arc magmatism. Older ages of about 780 Ma for gabbro from the Jabal Ess ophiolite suggest that the ophiolite is either a fragment of fore-arc oceanic crust or oceanic basement on which an arc was built. Gabbro samples from ophiolites of the Bir Umq suture (west-central Arabian Shield) yield zircons with ages of 820–870 Ma and $ ̆ 1250 Ma "> 1250Ma . The 820–870 Ma dates overlap with ages of the oldest nearby arc rocks; this favors an intra-arc or near-arc paleotectonic setting. The older zircons suggest that middle or early Proterozoic crustal material, possibly derived from the Mozambique belt of Africa, was present during back- or intra-arc magmatism. Plagiogranite from the Bir Tuluhah ophiolitic complex at the northern end of the 900 km-long Nabitah mobile belt was dated by the zircon U Pb method at ∼ 830 Ma. This date is in the range of the oldest dated arc rocks along the northern and central parts of the Nabitah suture, but is ∼ 100 Ma older than the oldest arc plutons (tonalites) associated with the southern part of the belt. These age relations suggest that the northern part of the Nabitah belt contains an extension of the Bir Umq suture that was transposed parallel to the Nabitah trend during collision of the arc terranes of the northwest Arabian Shield with the Afif plate to the east. Feldspar lead-isotope data from the ophiolites are of three types: (1) lead from the ophiolitic rocks and arc tonalites of the northwestern Shield and ophiolitic rocks of the Nabitah suture is similar to lead in modern mid-ocean ridge basalt, (2) anomalous radiogenic data from the Thurwah ophiolite are from rocks that contain zircons from pre-late Proterozoic continental crust, and (3) feldspar from the Urd ophiolite shows retarded uranogenic lead growth and is related either to an anomalous and perhaps primitive oceanic mantle source, or in an unknown manner to ancient continental mantle or lower crust of the eastern Arabian Shield.

Precambrian Research↗

USGS1981

Miocene benthic foraminiferal isotope records: A synthesis

18 O/ 16 0 and 13 C/ 12 C ratios of Miocene benthic foraminifera from a number of Atlantic, Pacific and Indian Ocean DSDP sites (71, 77B, 206, 208, 238, 279, 289, 296, 329, 357 and 366A) have been compiled. These provide a rather detailed history of Miocene deep water especially in the Pacific Ocean. Bottom-water temperatures rose during the early Miocene and then declined rapidly during the middle Miocene. This decline was accompanied by an increase in Antarctic glaciation. Late Miocene bottom temperatures and Antarctic ice volumes are inferred to be similar to today's, but exhibited some fluctuation. The early Miocene ocean was less thermally stratified at intermediate and abyssal depths while the late Miocene deep ocean had a thermal structure generally similar to the modern ocean. Foraminiferal carbon isotope ratios at most of the sites varied quasi-sympathetically throughout the Miocene. These variations must reflect comparable variations in the mean 13 C/ 12 C of marine HCO 3 . However, the causes of such variations are not yet clear.

Marine Micropaleontology↗

USGS2007

Crust and lithospheric structure – Global crustal structure

The Earth’s crust has played an important role in all aspects of this planet’s evolution. This chapter presents a review of our current understanding of the physical properties of the crust on a global basis. This understanding comes from extensive seismic measurements using many techniques, as well as nonseismic geophysics, including gravity, magnetic, geoelectric, and heat flow measurements. Seismic measurements include those that employ active (man-made) sources and those that use passive (naturally occurring) sources. Deep seismic reflection profiles provide a seismic image of the crust in twodimensions with a high (50–100m) resolution. Local earthquake tomography can provide three-dimensional (3-D) seismic images at moderate (500–1000m) resolution and higher, depending on the number and spacing of seismographs. Nonseismic methods provide estimates of crustal density, magnetic properties, conductivity and geotherms (temperature vs depth). The crust in deep ocean basins is 6–7km thick and has a relatively uniform seismic velocity structure, but there are numerous oceanic regions with anomalous crustal structure, including mid-ocean ridges, trenches, volcanic islands, and oceanic plateaux. Ocean–continent passive margins are also highly variable in structure, and may be classified as volcanic versus nonvolcanic margins. Continental crust ranges in thickness from 16 to 80km, and has a highly variable seismic velocity and density structure. The proportions of continental crust, by area, are 69% shield and platform (cratons), 15% old and young orogens, 9% extended (stretched) crust, 6 % magmatic arc, and 1% rifts. The weighted mean continental crustal thickness and average crustal velocity are 41km (SD 6.2km) and 6.45kms−1 (SD 0.21kms−1), respectively. A global geographic distribution of seismic data has made it possible to create global crustal models with cell sizes as small as 2°×2°. These models provide a complete description of seismic velocities and density within the crust and uppermost mantle, including, where present, ice, water, and sedimentary layers and the crystalline crust (parameterized in three layers, upper, middle and lower crust), and sub-Moho properties. The crust is the most intensely studied region of the Earth’s interior and consequently is the best understood in terms of its structure, composition, and evolution. © 2007 Elsevier B.V. All rights reserved.

Book chapter↗

USGS2013

Loess origin, transport, and deposition over the past 10,000 years, Wrangell-St. Elias National Park, Alaska

Contemporary glaciogenic dust has not received much attention, because most research has been on glaciogenic dust of the last glacial period or non-glaciogenic dust of the present interglacial period. Nevertheless, dust from modern glaciogenic sources may be important for Fe inputs to primary producers in the ocean. Adjacent to the subarctic Pacific Ocean, we studied a loess section near Chitina, Alaska along the Copper River in Wrangell-St. Elias National Park, where dust has been accumulating over the past ∼10,000 years. Mass accumulation rates for the fine-grained (<20 μm) fraction of this loess section are among the highest reported for the Holocene of high-latitude regions of the Northern Hemisphere. Based on mineralogy and geochemistry, loess at Chitina is derived from glacial sources in the Wrangell Mountains, the Chugach Mountains, and probably the Alaska Range. Concentrations of Fe in the silt-plus-clay fraction of the loess at Chitina are much higher than in all other loess bodies in North America and higher than most loess bodies on other continents. The very fine-grained (<2 μm) portion of this sediment, capable of long-range transport, is dominated by Fe-rich chlorite, which can yield Fe readily to primary producers in the ocean. Examination of satellite imagery shows that dust from the Copper River is transported by wind on a regular basis to the North Pacific Ocean. This Alaskan example shows that high-latitude glaciogenic dust needs to be considered as a significant Fe source to primary producers in the open ocean.

Alaska↗

USGS2018

A numerical model investigation of the impacts of Hurricane Sandy on water level variability in Great South Bay, New York

Hurricane Sandy was a large and intense storm with high winds that caused total water levels from combined tides and storm surge to reach 4.0 m in the Atlantic Ocean and 2.5 m in Great South Bay (GSB), a back-barrier bay between Fire Island and Long Island, New York. In this study the impact of the hurricane winds and waves are examined in order to understand the flow of ocean water into the back-barrier bay and water level variations within the bay. To accomplish this goal, a high resolution hurricane wind field is used to drive the coupled Delft3D-SWAN hydrodynamic and wave models over a series of grids with the finest resolution in GSB. The processes that control water levels in the back-barrier bay are investigated by comparing the results of four cases that include: (i) tides only; (ii) tides, winds and waves with no overwash over Fire Island allowed; (iii) tides, winds, waves and limited overwash at the east end of the island; (iv) tides, winds, waves and extensive overwash along the island. The results indicate that strong local wind-driven storm surge along the bay axis had the largest influence on the total water level fluctuations during the hurricane. However, the simulations allowing for overwash have higher correlation with water level observations in GSB and suggest that island overwash provided a significant contribution of ocean water to eastern GSB during the storm. The computations indicate that overwash of 7500–10,000 m 3 s −1 was approximately the same as the inflow from the ocean through the major existing inlet. Overall, the model results indicate the complex variability in total water levels driven by tides, ocean storm surge, surge from local winds, and overwash that had a significant impact on the circulation in Great South Bay during Hurricane Sandy.

New York↗

USGS2009

A mosaic of diverse ideas: The ecological legacy of J. Frederick Grassle

During the 40 years (and counting) of his scientific career, J. Frederick Grassle has made fundamental contributions to our understanding of marine ecosystems from coral reefs to deep-sea sediments. His advocacy and passion for marine biodiversity in the form of myriad groundbreaking studies and influential reviews, his generosity of ideas and capacity to catalyze and inspire those working with him as well as the science community in general, his breakthroughs in improved ocean observation, his marine science infrastructure initiatives, together with his tireless persistence, have helped lead to major shifts in approaches to marine science and the shape of modern ocean studies to one that favours multidisciplinary research, teamwork, continuous, long-term observation, in situ experimentation, recognition of the importance of marine biodiversity, and global cooperation on research and data sharing. In shallow-water ecology, he co-discovered sibling species of Capitella spp., important not only because it is a key pollution indicator but also because the work helped to pave the way for the discovery of numerous sibling species in other taxa with major ramifications for ecological understanding. He was also a key player in the West Falmouth oil spill study which, along with complementary mesocosm experiments, remains one of the most important and detailed studies of its kind. He was also a lead player in the first biological expedition to hydrothermal vents and wrote the seminal articles that helped to inspire the flurry of vent research that followed. He is perhaps best known for his deep-sea work, where he brought submersibles to the forefront as a sampling tool, brought experimental manipulative studies to the primarily descriptive discipline of deep-sea benthic ecology, and generated tremendous excitement, debate, and rekindled interest in marine biodiversity with the first quantitative estimate of global deep-sea diversity. His efforts to document marine biodiversity resulted in the international Census of Marine Life, and his emphasis on the need for continuous, long-term ocean observation has led to breakthroughs in international cooperation in cabled observatories such as LEO-15. These efforts have also enhanced efforts to integrate ocean data on a global scale in platforms such as the Ocean Biogeographic Information System (OBIS). The diversity of his contributions to marine science mirror the immense marine diversity he has recognized, documented, and championed so effectively over the last four decades.

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

USGS2017

Formation of Fe-Mn crusts within a continental margin environment

This study examines Fe-Mn crusts that form on seamounts along the California continental-margin (CCM), within the United States 200 nautical mile exclusive economic zone. The study area extends from approximately 30° to 38° North latitudes and from 117° to 126° West longitudes. The area of study is a tectonically active northeast Pacific plate boundary region and is also part of the North Pacific Subtropical Gyre with currents dominated by the California Current System. Upwelling of nutrient-rich water results in high primary productivity that produces a pronounced oxygen minimum zone. Hydrogenetic Fe-Mn crusts forming along the CCM show distinct chemical and mineral compositions compared to open-ocean crusts. On average, CCM crusts contain more Fe relative to Mn than open-ocean Pacific crusts. The continental shelf and slope release both Fe and Mn under low-oxygen conditions. Silica is also enriched relative to Al compared to open-ocean crusts. This is due to the North Pacific silica plume and enrichment of Si along the path of deep-water circulation, resulting in Si enrichment in bottom and intermediate waters of the eastern Pacific. The CCM Fe-Mn crusts have a higher percentage of birnessite than open-ocean crusts, reflecting lower dissolved seawater oxygen that results from the intense coastal upwelling and proximity to zones of continental slope pore-water anoxia. Carbonate fluorapatite (CFA) is not present and CCM crusts do not show evidence of phosphatization, even in the older sections. The mineralogy indicates a suboxic environment under which birnessite forms, but in which pH is not high enough to facilitate CFA deposition. Growth rates of CCM crusts generally increase with increasing water depth, likely due to deep-water Fe sources mobilized from reduced shelf and slope sediments. Many elements of economic interest including Mn, Co, Ni, Cu, W, and Te have slightly or significantly lower concentrations in CCM crusts relative to crusts from the Pacific Prime Crust Zone and other open-ocean basins. However, concentrations of total rare earth elements and yttrium average only slightly lower contents and in the future may be a strategic resource for the U.S.

California↗

USGS2011

Bathymetric controls on Pliocene North Atlantic and Arctic sea surface temperature and deepwater production

The mid-Pliocene warm period (MPWP; ~ 3.3 to 3.0 Ma) is the most recent interval in Earth's history in which global temperatures reached and remained at levels similar to those projected for the near future. The distribution of global warmth, however, was different than today in that the high latitudes warmed more than the tropics. Multiple temperature proxies indicate significant sea surface warming in the North Atlantic and Arctic Oceans during the MPWP, but predictions from a fully coupled ocean–atmosphere model (HadCM3) have so far been unable to fully predict the large scale of sea surface warming in the high latitudes. If climate proxies accurately represent Pliocene conditions, and if no weakness exists in the physics of the model, then model boundary conditions may be in error. Here we alter a single boundary condition (bathymetry) to examine if Pliocene high latitude warming was aided by an increase in poleward heat transport due to changes in the subsidence of North Atlantic Ocean ridges. We find an increase in both Arctic sea surface temperature and deepwater production in model experiments that incorporate a deepened Greenland–Scotland Ridge. These results offer both a mechanism for the warming in the North Atlantic and Arctic Oceans indicated by numerous proxies and an explanation for the apparent disparity between proxy data and model simulations of Pliocene northern North Atlantic and Arctic Ocean conditions. Determining the causes of Pliocene warmth remains critical to fully understanding comparisons of the Pliocene warm period to possible future climate change scenarios.

Palaeogeography, Palaeoclimatology, Palaeoecology↗

USGS2003

Lead isotopes in North Pacific deep water - Implications for past changes in input sources and circulation patterns

The sources of non-anthropogenic Pb in seawater have been the subject of debate. Here we present Pb isotope time-series that indicate that the non-anthropogenic Pb budget of the northernmost Pacific Ocean has been governed by ocean circulation and riverine inputs, which in turn have ultimately been controlled by tectonic processes. Despite the fact that the investigated locations are situated within the Asian dust plume, and proximal to extensive arc volcanism, eolian contributions have had little impact. We have obtained the first high-resolution and high-precision Pb isotope time-series of North Pacific deep water from two ferromanganese crusts from the Gulf of Alaska in the NE Pacific Ocean, and from the Detroit Seamount in the NW Pacific Ocean. Both crusts were dated applying 10 Be/9Be ratios and yield continuous time-series for the past 13.5 and 9.6 Myr, respectively. Lead isotopes show a monotonic evolution in 206Pb/204Pb from low values in the Miocene (??? 18.57) to high values at present day (??? 18.84) in both crusts, even though they are separated by more than 3000 km along the Aleutian Arc. The variation exceeds the amplitude found in Equatorial Pacific deep water records by about three-fold. There also is a striking similarity in 207Pb/204Pb and 208Pb/ 204Pb ratios of the two crusts, indicating the existence of a local circulation cell in the sub-polar North Pacific, where efficient lateral mixing has taken place but only limited exchange (in terms of Pb) with deep water from the Equatorial Pacific has occurred. Both crusts display well-defined trends with age in Pb-Pb isotope mixing plots, which require the involvement of at least four distinct Pb sources for North Pacific deep water. The Pb isotope time-series reveal that eolian supplies (volcanic ash and continent-derived loess) have only been of minor importance for the dissolved Pb budget of marginal sites in the deep North Pacific over the past 6 Myr. The two predominant sources have been young volcanic arcs, one located in the northeastern part and one located in the northwestern part of the Pacific margin, from where material has been eroded and delivered to the ocean, most likely via riverine pathways. ?? 2003 Elsevier Science B.V. All rights reserved.

Earth and Planetary Science Letters↗

USGS2003

Serpentinization of abyssal peridotites from the MARK area, Mid-Atlantic Ridge: Sulfur geochemistry and reaction modeling

The opaque mineralogy and the contents and isotope compositions of sulfur in serpentinized peridotites from the MARK (Mid-Atlantic Ridge, Kane Fracture Zone) area were examined to understand the conditions of serpentinization and evaluate this process as a sink for seawater sulfur. The serpentinites contain a sulfur-rich secondary mineral assemblage and have high sulfur contents (up to 1 wt.%) and elevated ??34Ssulfide (3.7 to 12.7???). Geochemical reaction modeling indicates that seawater-peridotite interaction at 300 to 400??C alone cannot account for both the high sulfur contents and high ??34Ssulfide. These require a multistage reaction with leaching of sulfide from subjacent gabbro during higher temperature (???400??C) reactions with seawater and subsequent deposition of sulfide during serpentinization of peridotite at ???300??C. Serpentinization produces highly reducing conditions and significant amounts of H2 and results in the partial reduction of seawater carbonate to methane. The latter is documented by formation of carbonate veins enriched in 13C (up to 4.5???) at temperatures above 250??C. Although different processes produce variable sulfur isotope effects in other oceanic serpentinites, sulfur is consistently added to abyssal peridotites during serpentinization. Data for serpentinites drilled and dredged from oceanic crust and from ophiolites indicate that oceanic peridotites are a sink for up to 0.4 to 6.0 ?? 1012 g seawater S yr-1. This is comparable to sulfur exchange that occurs in hydrothermal systems in mafic oceanic crust at midocean ridges and on ridge flanks and amounts to 2 to 30% of the riverine sulfate source and sedimentary sulfide sink in the oceans. The high concentrations and modified isotope compositions of sulfur in serpentinites could be important for mantle metasomatism during subduction of crust generated at slow spreading rates. ?? 2003 Elsevier Science Ltd.

Geochimica et Cosmochimica Acta↗

USGS2002

The nature of the crust under Cayman Trough from gravity

Considerable crustal thickness variations are inferred along Cayman Trough, a slow-spreading ocean basin in the Caribbean Sea, from modeling of the gravity field. The crust to a distance of 50 km from the spreading center is only 2–3 km thick in agreement with dredge and dive results. Crustal thickness increases to ∼5.5 km at distances between 100 and 430 km west of the spreading center and to 3.5–6 km at distances between 60 and 370 km east of the spreading center. The increase in thickness is interpreted to represent serpentinization of the uppermost mantle lithosphere, rather than a true increase in the volume of accreted ocean crust. Serpentinized peridotite rocks have indeed been dredged from the base of escarpments of oceanic crust rocks in Cayman Trough. Laboratory-measured density and P-wave speed of peridotite with 40–50% serpentine are similar to the observed speed in published refraction results and to the inferred density from the model. Crustal thickness gradually increases to 7–8 km at the far ends of the trough partially in areas where sea floor magnetic anomalies were identified. Basement depth becomes gradually shallower starting 250 km west of the rise and 340 km east of the rise, in contrast to the predicted trend of increasing depth to basement from cooling models of the oceanic lithosphere. The gradual increase in apparent crustal thickness and the shallowing trend of basement depth are interpreted to indicate that the deep distal parts of Cayman Trough are underlain by highly attenuated crust, not by a continuously accreted oceanic crust.

Marine and Petroleum Geology↗