Escanaba Trough Research Articles

Laboratory simulation of hydrothermal petroleum formation from sediment in Escanaba Trough, offshore from northern California

Petroleum associated with sulfide-rich sediment is present in Escanaba Trough at the southern end of the Gorda Ridge spreading axis offshore from northern California within the Exclusive Economic Zone (EEZ) of the U.S. This location and occurrence are important for evaluation of the mineral and energy resource potential of the seafloor under U.S. jurisdiction. In Escanaba Trough, petroleum is believed to be formed by hydrothermal processes acting on mainly terrigenous organic material in Quaternary, river-derived sediment. To attempt to simulate these processes in the laboratory, portions of a Pleistocene gray-green mud, obtained from ∼ 1.5 m below the seafloor at a water depth of ∼ 3250 m in Escanaba Trough, were heated in the presence of water in four hydrous-pyrolysis experiments conducted at temperatures ranging from 250 to 350°C and at a pressure of 350 bar for 1.0–4.5 days. Distributions of n-alkanes, isoprenoid hydrocarbons, triterpanes, and steranes in the heated samples were compared with those in a sample of hydrothermal petroleum from the same area. Mud samples heated for less than 4.5 days at less than 350°C show changes in some, but not all, molecular marker ratios of organic compounds that are consistent with those expected during hydrothermal petroleum formation. Our results suggest that the organic matter in this type of sediment serves as one possible source for some of the compounds found in the hydrothermal petroleum.

Comparison of 14C ages of hydrothermal petroleums

In order to set limits on the time frame of formation of hydrothermal petroleum, we have obtained 14C ages on samples from three diverse regions; Gulf of California (Guaymas Basin), Northeast Pacific Ocean (Escanaba Trough and Middle Valley), and the East African Rift (Tanganyika Trough). The results date the source of carbon and therefore provide maximum ages for the formation and emplacement of the hydrothermal petroleums. The youngest petroleum occurs iin the Souther Trough of Guaymas Basin (3200–6600 yr, mean 4692 yr); in the Northern Trough the petroleum is slightly older (7400 yr). Significantly older hydrothermal petroleum occurs in Escanaba Trough (17,000 yr) and Middle Valley (29,000 yr). A continental example from the East African Rift has an age of 25,000 yr, comparable to the ages observed in the oceanic samples from the Northeast Pacific Ocean. These ages affirm that hydrothermal petroleum formation is a very rapid process and took place some time between the latest Pleistocene and the present in these active hydrothermal systems.

Boron and halide systematics in submarine hydrothermal systems: Effects of phase separation and sedimentary contributions

Systematic studies of the distributions of B, δ 11B, NH 4, halides (Cl, Br, I) and trace alkalis (Li, Rb, Cs) in vent fluids, combined with experimental data on super- and subcritical phase separation, provide a method for separating the effects of interaction with basalts and/or sediments from those of phase separation. This allows a more general understanding of geochemical processes in submarine hydrothermal systems, especially where a connection with sediment is not otherwise obvious (e.g., Endeavour Segment, Juan de Fuca Ridge). Based on B and δ 11B corrected for wallrock reactions, all published boron and chloride data from mid-ocean ridge systems (MOR) (e.g., 11°N, 13°N and 21°N of the East Pacific Rise), except for the Endeavour Segment, Juan de Fuca Ridge, are consistent with experimental phase separation data, suggesting a dominant control by the latter process. Fluids from sedimented ridge (SR) (e.g., Escanaba Trough and Guaymas Basin), and from back-arc basins (BAB) (e.g., Mariana Trough, Lau Basin and Okinawa Trough), when compared with mid-ocean ridge data, show expected effects of organic matter and/or sediment contributions. This is particularly noticeable from enhanced levels of Br, I, NH 4, and trace alkali metal contents (such as Li, Rb and Cs). High B concentrations and elevated δ 11B in Endeavour Segment can be explained by a small, but distinguishable contribution from sediments, which is confirmed by slightly enhanced levels of Br, I and NH 4.

Genesis of massive sulfide deposits on a sediment-covered spreading center, Escanaba Trough, southern Gorda Ridge

Genesis of massive sulfide deposits on a sediment-covered spreading center, Escanaba Trough, southern Gorda Ridge

An association between organic matter and localised, prehnite-pumpellyite alteration, at Builth Wells, Wales, U.K.

The Builth Volcanic Group of the Builth Wells Ordovician Inlier, Wales, is a sequence of basic/intermediate lavas and volcaniclastics which shows a unique association between pyrobitumen and localised (domainal) calc-silicate alteration. The pyrobitumens occur in calcite veins and disseminated throughout alteration domains which form haloes to these veins. The pyrobitumens were emplaced by Ordovician hydrothermal fluids which were also responsible for the domainal alteration. Combined petrographic, gas chromatography and stable isotopic studies show that light organic gases, C 2-C 5, were liberated into the hydrothermal fluids by devolatilisation of liquid petroleum to form the pyrobitumens. These light organic gases mixed with methane, characterized by δ 13 C ≈ − 13‰ , which was already present in the fluids. This methane was most likely mantle-derived or produced from inorganic synthesis. The organic content of the fluids exerted an important control on the inorganic secondary assemblages. A similar control of inorganic assemblages by organic matter, might possibly occur deep within modern petroleum-bearing hydrothermal systems, such as the Guaymas Basin and Escanaba Trough hydrothermal systems.

Thermal effects of marine sedimentation in hydrothermally active areas

SUMMARY a hydrothermally active ocean basin, vigorous hydrothermal circulation in highly permeable basement rocks maintains a nearly constant temperature at the base of the overlying accumulating sediment section. To investigate the thermal effects of sedimentation in such geological settings, we have developed a simple onedimensional finite element model and applied it to cases in the northeast Pacific. The model accounts for differential motion of fluids and sediment grains during compaction, and can be used with any porosity-depth function. Results demonstrate clearly that the constant basal temperature of an accumulating sediment section, maintained by convective heat transfer in the basement, causes the section to remain thermally near steady state for even very high rates of accumulation, particularly when compared to conditions 'estimated for a section where heat is transported in the basement by conduction. A 10-kyr period of thermal recovery due to the highly diminished sediment supply during the post-Pleistocene further reduces the thermal effects of sedimentation by a significant amount. Only in rare cases where rates of accumulation exceed 10 mm yr-l and sediment thicknesses exceed 1 km are the sea-floor heat flow and temperatures at depth diminished significantly. An example is found in Middle Valley of the Juan de Fuca Ridge, in a part of which over 2 km of sediment has accumulated in the past 200 kyr. Even in this extreme case, the heat flow is estimated to be lower than that of the steady state by about only 15 per cent. While rates of accumulation are also high in other parts of Middle Valley and in many other hydrothermally active areas, such as Guayamas Basin, Escanaba Trough and the eastern flank of the Juan de Fuca Ridge, these rates and the accumulated sediment thicknesses are found to be insufficient to cause appreciable thermal anomalies.

Boron isotope systematics of hydrothermal fluids and tourmalines: A synthesis

Boron isotope and concentration data are presented for hydrothermal fluids from different tectonic settings that reflect derivation of boron from marine evaporites (Red Sea brines), non-marine evaporites (Salton Sea), clastic sediments (Escanaba Trough and Guaymas Basin), back-arc basalts (Mariana Trough) and mid-ocean ridge basalts (21° and 11–13°N East Pacific Rise; Juan de Fuca Ridge; and 23° and 26°N, Mid-Atlantic Ridge). Boron isotope systematics of these fluids are used to constrain the δ″B-values of tourmalines found in ancient massive sulfide deposits and tourmalinitcs that formed under conditions analogous to those of modern hydrothermal systems. The data also provide insights into the physical conditions required for the formation of tourmaline in certain geologic environments.

Boron isotope systematics of hydrothermal fluids and tourmalines: A synthesis

Boron isotope and concentration data are presented for hydrothermal fluids from different tectonic settings that reflect derivation of boron from marine evaporites (Red Sea brines), non-marine evaporites (Salton Sea), elastic sediments (Escanaba Trough and Guaymas Basin), back-arc basalts (Mariana Trough) and mid-ocean ridge basalts (21° and 11–13°N East Pacific Rise; Juan de Fuca Ridge; and 23° and 26°N, Mid-Atlantic Ridge). Boron isotope systematics of these fluids are used to constrain the S″B-values of tourmalines found in ancient massive sulfide deposits and tourmalinites that formed under conditions analogous to those of modern hydrothermal systems. The data also provide insights into the physical conditions required for the formation of tourmaline in certain geologic environments.

High alkalinity due to sulfate reduction in the CLAM hydrothermal field, Okinawa Trough

High alkalinity ( > 1.8 mmol kg −1), pH (4.7–5.3), and ammonium ( ∼ 5 mmol kg −1) in hydrothermal fluids from the JADE and CLAM sites in the Mid-Okinawa Trough back-arc basin demonstrate organic matter decomposition during high-temperature fluid-sediment interaction. The endmember fluid from the JADE site ( ∼ 320°C black smoker), which is free of Mg and SO 4, has similar chemical characteristics as those at the Escanaba Trough, one of the representative sediment-hosted ridge systems in the eastern Pacific. On the other hand, fluid samples from the CLAM site ( ∼ 100°C) show remarkably high alkalinity (10.3 mmol kg −1) comparable to the highest alkalinity of 10.6 mmol kg −1 (Guaymas Basin) yet observed in hydrothermal fluids, and significantly higher δ 34S of SO 4 ( > 25‰) than ambient seawater (21‰). It is suggested that sulfate reduction plays a dominant role in decomposing organic matter to increase alkalinity in the CLAM site. Variation of the δ 34S values of H 2S (from −0.2 to +3.0‰) in the CLAM fluids taken from different vents suggests that the CLAM site should consist of multiple local hydrothermal systems in different evolutionary stages with regard to sulfate reduction.

Beryllium isotope systematics of submarine hydrothermal systems

Both 10Be and 9Be concentrations have been measured in a hydrothermal fluid from the sediment-starved East Pacific Rise at 21°N as well as in fluids and associated sediments from the sediment covered Guaymas Basin and Escanaba Trough hydrothermal systems. The results show that while the 9Be concentrations in fluids from both types of systems are comparable, about three orders of magnitude higher than that of the seawater supplying the systems, their 10Be concentrations are significantly different. The 10Be concentration in the solution emanating from the studied sediment-starved hydrothermal system is at least two times lower than that of the Pacific bottom water. By contrast, the 10Be concentration in hydrothermal solutions from sediment-hosted environments are enriched over ambient seawater and are at least one order of magnitude higher than that measured in the 21°N fluid. The sedimentary cover being the only available source of 10Be in these systems, these data demonstrate that 10Be is an unambiguous tracer of interactions between hydrothermal fluids and recent sediments. As an example, this study examines hydrothermal solutions from the sediment-starved Mariana Trough back-arc basin which have compositions similar in many respects to those from sediment-hosted environments. However, these fluids are depleted in 10Be relative to ambient seawater, which precludes interaction with recent sediment as the cause of their peculiar chemical composition.

Hydrothermally Derived Petroleum: Examples from Guaymas Basin, Gulf of California, and Escanaba Trough, Northeast Pacific Ocean (1)

Petroleum is associated with sulfide-rich sediment at two active oceanic spreading axes in the northeast Pacific Ocean. In the Guaymas Basin, a spreading axis in the Gulf of California, petroleum having a wide range of compositions forms by hydrothermal alteration of organic matter in Quaternary sediment composed mainly of marine diatomaceous ooze and muddy turbidites. In Escanaba Trough, at the southern end of the Gorda Ridge spreading axis offshore northern California, petroleum is formed by hydrothermal processes acting on mainly terrigenous organic material in Quaternary turbiditic river-derived sediment. Comparisons of the distributions of hydrocarbons--n-alkanes, isoprenoids, terpanes, steranes, and aromatics--show that chemical differences among four petroleum samp es are such that two samples from Guaymas Basin can be distinguished from two samples from Escanaba Trough. Distinguishing characteristics resulting from differences in sources include n-alkane distributions and certain sterane ratios; distinguishing characteristics resulting from differences in thermal histories of the organic matter include hopane and sterane epimer ratios and various distributions of polycyclic aromatic hydrocarbons. These oils differ from conventionally derived petroleum in that they are admixtures of products generated over a wide range of thermal regimes, and their generation, expulsion, and migration occurred simultaneously over an instantaneous geological time period. The potential economic significance of hydrothermally derived petroleum is uncertain, but the act that petroleum can form at active oceanic spreading axes adds a new facet to understanding the processes of petroleum generation, expulsion, and migration.

Southern Juan De Fuca Ridge and Escanaba Trough, Gorda Ridge: Comparison of Hydrothermal Deposits in Sediment-Free and Sediment-Covered Ridge Settings: ABSTRACT

Southern Juan De Fuca Ridge (SJDF) is a low-relief, sediment-starved ridge axis that has a total opening rate of 6 cm/year and is characterized by lobate and brecciated sheet flows and pillows of MORB composition. Sulfide deposits form at {approximately}2,200 m water depth and are concentrated within a narrow graben centered within the ridge axial valley. Solitary and coalesced chimneys (0.25 to 12 m high) rise directly from the basalt basement and are composed predominantly of Zn sulfide accompanied by Fe and Cu-Fe sulfide and traces of Pb sulfide. Anhydrite occurs as a minor phase in some chimneys. Sulfide chimneys were formed by rapid-venting of high temperature ({approximately}285{degree}C) fluids. These acidic fluids (pH {approximately}3.5) are enriched in Cl, Na, and Ca and are depleted in Cu and Zn. In contrast to SJDF, Escanaba Trough (ET) is spreading at {approximately}2.3 cm/year and has the high relief and axial graben morphology typical of the slow-spreading Mid-Atlantic Ridge. The floor of the axial valley is buried by up to 500 m of clay and terrigenous silt. Rare basement exposures reveal unbrecciated sheet flows and pillow basalts of MORB composition. Large chimney-topped sulfide mounds up to 20 m high and hundreds of meters inmore » extent occur at {approximately}3,250 m water depth at the base of sediment hills; sulfide veins, small chimneys, and clastic deposits occur on, and within, the sediment between hills. Two distinct sulfide types occur at ET. Pyrrhotite-rich sulfide is enriched in Fe, Cu, and As and is associated with low-velocity venting of warm (<220{degree} C) alkaline (pH {approximately}5.4) fluids that are currently depositing anhydrite and barite sinter deposits on top of the sulfide mounds.« less

Petroleum generation, an easy and widespread process in hydrothermal systems: an overview

The conversion of organic matter to petroleum products by hydrothermal activity is an easy process, occurring in nature in many types of environments. Geologically immature organic matter of marine sediments has been shown to be altered to petroleum products by this process in the Guaymas Basin, Escanaba Trough, Bransfield Strait and Atlantis II Deep and this process probably occurs elsewhere in active sedimented rift systems. Contemporary organic detritus and/or viable microorganisms are also converted in part to petroleum-like products when they become entrained by turbulent mixing into hydrothermal vent waters, resulting in their instantaneous hydrous pyrolysis. This latter case occurs in hydrothermal vent fields without a sedimentary cover emanating directly from oceanic ridges, as for example on the East Pacific Rise at 13°N and 21°N and on the Mid-Atlantic Ridge at 26°N. This process is probably also ubiquitous for all actively venting sediment-starved oceanic ridges. The hydrocarbon products generated in these areas have been compared in terms of composition, organic matter sources and analogy to reservoir petroleum. They represent a major carbon source for chemosynthetic assimilation by vent biota and are compositionally similar to normal petroleum.

Hydrocarbon geochemistry of hydrothermally generated petroleum from Escanaba trough, offshore Californi U.S.A.

In 1986, three samples of sulfide-rich sediments, impregnated with hydrothermally derived, asphaltic petroleum, were recovered in a dredge and by submersible from Escanaba Trough, the sediment-covered, southern end of the Gorda Ridge spreading axis, offshore northern California. The molecular distributions of hydrocarbons in the two pyrrhotite-rich samples recovered by submersible are similar and compare well the hydrocarbon composition of the first pyrrhotite-rich samples containing petroleum discovered at a 1985 dredge site about 30 km to the south of the site of the submersible dive. In contrast, the 1986 dredge sample, composed of a polymetallic assemblage of sulfides, containes petroleum in which the distribution of hydrocarbons indicates a slightly higher of maturity relative to the other samples. The observation that petroleum of variable composition occurs with metallic sulfides at two and probably more distinct site indicates that petroleum generation may be a common process in the hydrothermally active Escanaba Trough.

Retrograde oxidation of hydrothermal precipitates and petroleum in Escanaba Trough sediments

Three push cores obtained near a hydrothermal vent in the Escanaba Trough, Gorda Ridge, have been analyzed for interstitial water chemistry, inorganic solids geochemistry, and organic geochemistry. The inorganic analyses indicate the study area has no present hydrothermal fluid flow but the presence of a high temperature mineral phase (calcite precipitate representing a temperature range of 168°–199°C) indicates past hydrothermal activity. The presence of weathered sulfides and an extensively weathered and oxidized hydrothemal petroleum is evidence for a retrograde oxidative weathering process occurring in the sediments.

Volcanic and Sedimentary Controls on Hydrothermal Mineralization at Escanaba Trough, Southern Gorda Ridge: ABSTRACT

Volcanic and Sedimentary Controls on Hydrothermal Mineralization at Escanaba Trough, Southern Gorda Ridge: ABSTRACT

Benthos of the Gorda Ridge axial valley (NE Pacific Ocean): Taxonomic composition and trends in distribution

Distribution and relative abundance of invertebrate mega-epifauna and benthic fishes were studied in the Gorda Ridge central rift valley off southern Oregon and northern California, USA. Faunal distribution and relative abundance were correlated with location, geological setting, substrate type, and depth. Bottom photographs and videotapes were from 1985–1986 cruises by the US Geological Survey and the US National Oceanic and Atmospheric Administration. Voucher specimens were collected by rock dredge, fish trap, and the Deep Submergence Vehicle (DSV) Sea Cliff. Location rather than substrate appears to have more effect on the overall taxonomic composition of the mega-epifauna in the northern and southern parts of the rift valley. Within each location, substrate type, i.e. soft sediments and rock outcrops, and percentage cover of these substrates appear to influence the existing faunal composition. Characteristic fauna are associated with each substrate type, e.g. crinoids, gorgonians and sponges (Demospongiae) on rocky surfaces. In the southern sediment-filled Escanaba Trough, deposit-feeding organisms, particularly ophiuroids, asteroids and holothuroids, are interspersed with stalked suspension feeders, such as hexactinellid sponges and pennatulids. Epifaunal community structure in the northern and southern sectors differs, even on similar substrate combinations. Except for the ubiquitous macrourids, fish species distributions may be correlated with substrate type. Abundant particulate material in the bottom water layer probably accounts for large concentrations of suspension and detritus feeding epibenthos.

Organic geothermometry of petroleum from Escanaba Trough, offshore northern California

We have measured the extent of hopane and sterane isomerization and of monoaromatic-steroid-hydrocarbon aromatization in a sample of hydrothermally derived petroleum from the Escanaba Trough, a sediment-covered, volcanically active ridge axis. The results, along with kinetic parameters, predict the possible time-temperature history of the petrolum-forming process for this sample. The extent of these reactions is consistent with petroleum formation by intense heating (about 300–350°C) if the time period of this heating was as short as about 100 yr. Such a time scale is reasonable for hydrothermal-discharge events associated with ridge-crest volcanism.

Pb isotopes in sulfides from mid-ocean ridge hydrothermal sites

The authors report Pb isotope ratios of sulfides deposited at seven recently active mid-ocean ridge (MOR) hydrothermal vents. Sulfides from three sediment-starved sites on the Juan de Fuca Ridge contain Pb with isotope ratios identical to their local basaltic sources. Lead in two deposits from the sediment-covered Escanaba Trough, Gorda Ridge, is derived from the sediments and does not appear to contain any basaltic component. There is a range of isotope ratios in a Guaymas Basin deposit, consistent with a mixture of sediment and MOR basalt Pb. Lead in a Galapagos deposit differs slightly from known Galapagos basalt Pb isotope values. The faithful record of Pb isotope signatures of local sources in MOR sulfides indicates that isotope ratios from ancient analogues ca be used as accurate reflections of ancient oceanic crustal values in ophiolite-hosted deposits and continental crustal averages in sediment-hosted deposits. The preservation of primary ophiolitic or continental crustal Pb isotope signatures in ancient MOR sulfides provides a powerful tool for investigation of crustal evolution and for fingerprinting ancient terranes.

Volcanism and massive sulfide formation at a sedimented spreading center, Escanaba Trough, Gorda Ridge, northeast Pacific Ocean

Seismic‐reflection profiles over the sediment‐filled Escanaba Trough at the southern Gorda Ridge reveal a series of volcanic centers that pierce the sediment. The volcanic edifices are 3 to 6 km in diameter and are spaced at 15 to 20 km intervals along the axis of the trough. The volcanic intrusions have uplifted the sediment as much as 100 m, and in places fresh basaltic flows are exposed at the sea floor. Unoxidized pyrrhotite‐rich massive‐sulfide samples were obtained from the flanks of one of these volcanic centers. The composition and form of the sulfide samples suggest significant interaction between hydrothermal fluids and sediment at depth, and deposition of sulfide within the sediment pile.