Seafloor Temperature Research Articles

Temperature dependence of organic matter remineralization in deeply-buried marine sediments

Assuming that the temperature dependence of sediment organic matter remineralization can be described by the Arrhenius equation, organic matter that is highly refractory at seafloor temperatures (~ 2–3 °C) should become more reactive at sediment depths of several hundred meters due to burial and heating by the natural geothermal gradient. Results obtained using a coupled non-linear reactive-transport model support this suggestion. For deeply-buried marine sediments (i.e., those found 100s of meters below the seafloor) model results predict the occurrence of a deep zone of methanogenesis that is separated by a relatively thick region in which methane diffuses upwards to be oxidized by downward diffusing sulfate. This depth zonation of biogeochemical processes in deeply-buried sediments is in sharp contrast to that observed in nearshore marine sediments where sulfate reduction and methanogenesis generally occur in much closer vertical proximity. Model results presented here also provide a simple mechanistic explanation for the occurrence of linear pore water sulfate profiles that are common in many continental margin sediments. Linear sulfate gradients are often taken as an indirect indicator of gas hydrate occurrence although results presented here suggest that this does not necessarily always have to be the case. Application of the model to gas hydrate-containing sediments on the Blake Ridge yields results that are in good agreement with sediment and pore water data, and microbial rate studies in these sediments. Model results also suggest that in situ methanogenesis in and/or near the gas hydrate stability zone may play an important role in supplying the methane found in Blake Ridge gas hydrates.

Utility of a spatial habitat classification system as a surrogate of marine benthic community structure for the Australian margin

Abstract Przeslawski, R., Currie, D. R., Sorokin, S. J., Ward. T. M., Althaus, F., and Williams, A. 2011. Utility of a spatial habitat classification system as a surrogate of marine benthic community structure for the Australian margin. – ICES Journal of Marine Science, 68: 1954–1962. This study tests whether a continental-scale classification of Australian benthic habitats (termed “seascapes”) and the interpolated environmental data from which they are derived are useful as abiotic surrogates of biodiversity at a local [tens of kilometres, Great Australian Bight (GAB)] and regional scale [hundreds of kilometres, Western Australian (WA) margin]. Benthic invertebrate community structure is moderately associated with specific seascapes in both the GAB (R = 0.418) and WA margin (excluding hard substrata, R = 0.375; all substrata, R = 0.313). Mud content, seafloor slope, and seafloor temperature are significantly correlated with invertebrate communities at both scales, with disturbance and primary production correlated with GAB communities. Seascapes are not consistently useful surrogates because the strength and significance of relationships between seascapes and community structure differs among seascapes, regions, and spatial scales. Nevertheless, a national system of seascapes is an appropriate surrogate for broad-scale benthic invertebrate community patterns when biological data are limited, provided the uncertainty is acknowledged and, where possible, an assessment made of each seascape's ability to differentiate biological communities. Further refinement of seascape derivations may include updated and additional environmental data (particularly for hard vs. soft substrata) and validation among biological datasets from a range of habitats and scales.

Latitudinal variation of demersal fish assemblages in the western Ross Sea

AbstractDemersal fishes were sampled using a large fish trawl during two surveys carried out in February and March 2004 and 2008 in the Ross Sea, and around seamounts and islands just to the north at 66°S. The distribution and abundance of 65 species collected in these surveys were examined to determine if demersal fish communities varied throughout the area, and what environmental factors might influence this. Species accumulation with sample frequency did not reach an asymptote, but the rate of new species was low suggesting data were adequate for describing the main components of the communities. Three broad assemblages were identified, in the southern Ross Sea (south of 74°S), central–northern Ross Sea (between latitudes 71°–74°S), and the seamounts further north (65°–68°S) where some species more typical of sub-Antarctic latitudes were observed. Multivariate analyses indicated that environmental factors of seafloor rugosity (roughness), temperature, depth, and current speed were the main variables determining patterns in demersal fish communities.

Gas hydrate saturation in the Krishna–Godavari basin from P-wave velocity and electrical resistivity logs

During the Indian National Gas Hydrate Program (NGHP) Expedition 01, a series of well logs were acquired at several sites across the Krishna–Godavari (KG) Basin. Electrical resistivity logs were used for gas hydrate saturation estimates using Archie’s method. The measured in situ pore-water salinity, seafloor temperature and geothermal gradients were used to determine the baseline pore-water resistivity. In the absence of core data, Arp’s law was used to estimate in situ pore-water resistivity. Uncertainties in the Archie’s approach are related to the calibration of Archie coefficient ( a), cementation factor ( m) and saturation exponent ( n) values. We also have estimated gas hydrate saturation from sonic P-wave velocity logs considering the gas hydrate in-frame effective medium rock-physics model. Uncertainties in the effective medium modeling stem from the choice of mineral assemblage used in the model. In both methods we assume that gas hydrate forms in sediment pore space. Combined observations from these analyses show that gas hydrate saturations are relatively low (<5% of the pore space) at the sites of the KG Basin. However, several intervals of increased saturations were observed e.g. at Site NGHP-01-03 ( S h = 15–20%, in two zones between 168 and 198 mbsf), Site NGHP-01-05 ( S h = 35–38% in two discrete zone between 70 and 90 mbsf), and Site NGHP-01-07 shows the gas hydrate saturation more than 25% in two zones between 75 and 155 mbsf. A total of 10 drill sites and associated log data, regional occurrences of bottom-simulating reflectors from 2D and 3D seismic data, and thermal modeling of the gas hydrate stability zone, were used to estimate the total amount of gas hydrate within the KG Basin. Average gas hydrate saturations for the entire gas hydrate stability zone (seafloor to base of gas hydrate stability), sediment porosities, and statistically derived extreme values for these parameters were defined from the logs. The total area considered based on the BSR seismic data covers ∼720 km 2. Using the statistical ranges in all parameters involved in the calculation, the total amount of gas from gas hydrate in the KG Basin study area varies from a minimum of ∼5.7 trillion-cubic feet (TCF) to ∼32.1 TCF.

Observations of long‐duration episodic bottom currents in the Middle America Trench: Evidence for tidally initiated turbidity flows

Benthic flow in the Middle America Trench off the Pacific coast of Costa Rica is examined using time series from a single‐point acoustic current meter moored 21 m above bottom at 4386 m depth at the southern end of the trench from November 2005 to April 2007. In addition to significant (∼0.1 m s−1) tidal currents, the instrument recorded a series of 12 episodic northwestward along‐trench flow events of roughly monthly duration. Event velocities often exceeded 0.25 m s−1 and were contemporary with enhanced acoustic backscatter intensity. Events ended with a rapid (&lt;1 day) reversal to southeastward flow and reduced backscatter. Seafloor temperature records from two nearby Ocean Drilling Program (ODP) borehole observatory sites reveal that the flow events were accompanied by a steady rise in bottom water temperature. Temperatures dropped abruptly to background values at the end of each event. The event timing generally tracked the envelope of the tidal current modulation. On the basis of the November 2002 to February 2009 borehole observatory temperature records, the events had a mean duration of 40(±20) days and were separated by a between‐event interlude of 30(±25) days. Findings indicate that the episodic flows were likely rotationally modified, autosuspending turbidity currents initiated by tidal current resuspension of sediments above the shoaling trench floor to the southeast of the mooring site. Suspended particles in the turbidity currents are estimated to range from 0.0003% to 0.006% of the current by volume. Results suggest that tidally induced turbidity currents may be common to steep, well‐mixed regions of the deep ocean adjacent to sediment rich continental margins.

Effect of cold temperatures on properties of unfrozen Troll clay

Oedometer and triaxial experiments have been performed on North Sea Troll clay previously warmed to room temperature and tested at both room temperature and 0 °C to assess the effect of cold temperature on undrained strength and consolidation properties. In general, both the undrained strength and preconsolidation pressure increase as the temperature decreases. Many subsea areas of development in extreme northern or southern latitudes experience seafloor temperatures continuously around 0 °C. These results suggest that conventional laboratory testing at room temperature of samples from these areas leads to an underestimation of strength and preconsolidation pressure. Therefore, additional testing is recommended to assess the effect of a warming and cooling cycle on strength and consolidation properties, as previous work suggests that some consolidation is induced by this temperature cycle and may be responsible for the ensuing strength gain.

Testing proposed mechanisms for seafloor weakening at the top of gas hydrate stability on an uplifted submarine ridge (Rock Garden), New Zealand

We evaluate different hypotheses concerning the formation of a peculiar, flat-topped ridge at Rock Garden, offshore of the North Island of New Zealand. The coincidence of the ridge bathymetry with the depth at which gas hydrate stability intersects the seafloor has been previously used to propose that processes at the top of gas hydrate stability may cause seafloor erosion, giving rise to the flat ridge morphology. Two mechanisms that lead to increased fluid pressure (and sediment weakening) have previously been proposed: (1) periodic formation (association) and dissociation of gas hydrates during seafloor temperature fluctuations; and (2) dissociation of gas hydrates at the base of gas hydrate stability during ridge uplift. We use numerical models to test these hypotheses, as well as to evaluate whether the ridge morphology can develop by tectonic deformation during subduction of a seamount, without any involvement from gas hydrates. We apply a commonly-used 1D approach to model gas hydrate formation and dissociation, and develop a 2D mechanical model to evaluate tectonic deformation. Our results indicate that: (1) Tectonics (subduction of a seamount) may cause a temporary flat ridge morphology to develop, but this evolves over time and is unlikely to provide the main explanation for the ridge morphology; (2) Where high methane flux overwhelms the anaerobic oxidation of methane via sulphate reduction near the seafloor, short-period temperature fluctuations (but on timescales of years, not months as proposed originally) in the bottom water can lead to periodic association and dissociation of a small percentage of gas hydrate in the top of the sediment column. However, the effect of this on sediment strength is likely to be small, as evidenced by the negligible change in computed effective pressure; (3) The most likely mechanism to cause sediment weakening, leading to seafloor erosion, results from the interaction of gas hydrate stability with tectonic uplift of the ridge, provided bulk permeability strongly decreases with increasing hydrate content. Rather than overpressure developing from dissociation of hydrates at the base of gas hydrate stability (as previously thought), we found that the weakening is caused by focusing of gas hydrate formation at shallow sediment levels. This creates large fluid pressures and can lead to negative effective pressures near the seafloor, reducing the sediment strength.

Comparative sclerochronology of modern and mid-Pliocene (c. 3.5 Ma) Aequipecten opercularis (Mollusca, Bivalvia): an insight into past and future climate change in the north-east Atlantic region

Records of environment contained within the accretionarily deposited tissues of fossil organisms afford a means of detailed reconstruction of past climates and hence of rigorous testing of numerical climate models. We identify the environmental factors controlling oxygen and carbon stable-isotopic composition, and microgrowth-increment size, in the shell of modern examples of the Queen Scallop, Aequipecten opercularis. This understanding is then applied in interpretation of data from mid-Pliocene A. opercularis from eastern England. On the basis of oxygen-isotope evidence we conclude that winter minimum seafloor temperature was similar to present values (typically 6–7 °C) in the adjacent southern North Sea and that summer maximum seafloor temperature was a few degrees lower than present values (typically 16–17 °C). This contrasts with evidence from other proxies that winter and summer temperatures were higher than present. The pattern of seasonal variation in microgrowth-increment size suggests the existence of intense thermal stratification in summer. We therefore conclude that summer surface temperatures were much higher (maxima well over 20 °C) than those recorded isotopically on the seafloor and that the annual range of surface temperature (probably over 14 °C) was greater than now at the times in the mid-Pliocene when the investigated A. opercularis were alive. Taken in conjunction with other proxy evidence of warmer winters as well as summers, the data point to substantial fluctuation (up to 10 °C) in winter minimum temperatures during the mid-Pliocene in the north-east Atlantic region. This fluctuation may be attributable to variation in the strength of the Gulf Stream/North Atlantic Drift. Since the Pliocene has been widely used as a test-bed for numerical models of a greenhouse Earth, the results have implications for prediction of future climate in the north-east Atlantic region under the influence of anthropogenic global warming.

A predictive numerical model for potential mapping of the gas hydrate stability zone in the Gulf of Cadiz

This paper presents a computational model for mapping the regional 3D distribution in which seafloor gas hydrates would be stable, that is carried out in a Geographical Information System (GIS) environment. The construction of the model is comprised of three primary steps, namely: (1) the construction of surfaces for the various variables based on available 3D data (seafloor temperature, geothermal gradient and depth-pressure); (2) the calculation of the gas function equilibrium functions for the various hydrocarbon compositions reported from hydrate and sediment samples; and (3) the calculation of the thickness of the hydrate stability zone. The solution is based on a transcendental function, which is solved iteratively in a GIS environment. The model has been applied in the northernmost continental slope of the Gulf of Cadiz, an area where an abundant supply for hydrate formation, such as extensive hydrocarbon seeps, diapirs and fault structures, is combined with deep undercurrents and a complex seafloor morphology. In the Gulf of Cadiz, the model depicts the distribution of the base of the gas hydrate stability zone for both biogenic and thermogenic gas compositions, and explains the geometry and distribution of geological structures derived from gas venting in the Tasyo Field (Gulf of Cadiz) and the generation of BSR levels on the upper continental slope.

Dynamic response of oceanic hydrate deposits to ocean temperature change

Vast quantities of methane are trapped in oceanic hydrate deposits. Because methane is a powerful greenhouse gas (about 26 times more effective than CO2), there is considerable concern that a rise in the temperature of the oceans will induce dissociation of oceanic hydrate accumulations, potentially releasing large amounts of carbon into the atmosphere. Such a release could have dramatic climatic consequences because it could amplify atmospheric and oceanic warming and possibly accelerate dissociation of the remaining hydrates. This study assesses the stability of three types of hydrates (case I, deep‐ocean deposits; case II, shallow, warm deposits; and case III, shallow, cold deposits) and simulates the dynamic behavior of these deposits under the influence of moderate ocean temperature increases. The results indicate that deep‐ocean hydrates are stable under the influence of moderate increases in ocean temperature; however, shallow deposits can be very unstable and release significant quantities of methane under the influence of as little as 1°C of seafloor temperature increase. Less permeable sediments, or burial underneath layers of hydrate‐free sediment, affect both the rate of hydrate dissociation and methane transport to the seafloor but may not prevent methane release. Higher‐saturation deposits can produce larger methane fluxes with the thermodynamics of hydrate dissociation retarding the rate of recession of the upper hydrate interface. These results suggest possible worst case scenarios for climate‐change‐induced methane release and point toward the need for detailed assessment of the hydrate hazard and the coupling of hydrate‐derived methane to regional and global ecosystems.

The short-term fate of organic carbon in marine sediments: Comparing the Pakistan margin to other regions

Pulse-chase experiments with isotopically labelled phytodetritus conducted across the Pakistan margin reveal that the impact of biological activities on benthic C-cycling varies markedly among sites exhibiting different seafloor conditions. In this study, patterns of biological C-processing across the Pakistan margin oxygen minimum zone (OMZ) are compared with those observed in previous tracer studies. Variations in site environmental conditions are proposed to explain the considerable variations in C-processing patterns among this and previous studies. Three categories of C-processing pattern are identified: (1) respiration dominated, where respiration accounts for >75% of biological C-processing, and uptake by metazoan macrofauna, foraminifera and bacteria are relatively minor processes. These sites tend to show several (although not necessarily all) of the properties of being cold and deep, and having low inputs of organic carbon to the sediment and relatively low-biomass metazoan macrofaunal communities; (2) active faunal uptake, where respiration accounts for <75%, and metazoan macrofaunal, foraminiferal and bacterial uptake each account for 10–25% of biological C-processing. This type is further split into metazoan macrofaunal- and foraminiferal-dominated situations, dictated by oxygen availability; and (3) metazoan macrofaunal uptake dominated, characterised by metazoan macrofaunal uptake accounting for ∼50% of biological C-processing, due to unusually large biomasses of the phytodetritus-consuming animals. Total respiration rates (of added C) on the Pakistan margin fell within the range of rates measured elsewhere in the deep sea (∼0.1–2.8 mg C m−2 h−1), and seem to be dominantly controlled by seafloor temperature. Rates of metazoan macrofaunal uptake of organic matter (OM) on the Pakistan margin are larger than those in most other studies, and this is attributed to the large and active metazoan macrofaunal communities in the lower OMZ, characteristic of OMZ boundaries. Finally, biological mixing of Pakistan margin sediments was reduced compared to that observed in comparable tracer studies on other margins. This probably reflects faunal feeding and burrowing strategies consistent with low oxygen concentrations and a relatively abundant supply of sedimentary OM.

On Using Carbon and Oxygen Isotope Data from Glendonites as Paleoenvironmental Proxies: A Case Study from the Permian System of Eastern Australia

Abstract Glendonites, calcite pseudomorphs after ikaite (CaCO3 • 6H2O), feature prominently in fine-grained, glacially influenced Permian marine strata of eastern Australia, which accumulated in a series of extensional basins that occupied polar to temperate latitudes along the southeastern margin of Gondwana. Because ikaite formation in the marine environment requires near-freezing temperatures, high alkalinity, and elevated concentrations of orthophosphate, the presence of glendonites in ancient strata implies a particular array of paleoenvironmental conditions. A petrographic and geochemical study was carried out to assess the veracity of isotopic data from glendonites as proxy indicators of ancient ocean chemistry, sea-floor temperature, and diagenesis. In thin section, glendonites consist of inclusion-zoned, equant to bladed calcite crystals, interpreted as the ikaite replacement phase, enclosed in a matrix of calcite and minor chalcedony and pyrite cements. Carbon and oxygen isotope data show a negative covariance, with δ18O values ranging from −22.5 to +0.3‰ and δ13C values from −27.7 to −4.5‰ V-PDB. Of the calcite phases examined, the ikaite replacement phase is the most enriched in 18O and most depleted in 13C; enclosing cements possess lower δ18O and higher δ13C values and are interpreted to have formed during later stages of burial diagenesis. The δ18O values of the ikaite replacement phase do not correspond to formation with Permian seawater at near-freezing temperatures unless isotopic disequilibrium is assumed. The δ13C values are similar to the carbon isotope composition of bulk organic matter in the host sediment, and suggest that the alkalinity and orthophosphate necessary for ikaite stability were generated in the zone of suboxic to anoxic diagenesis. Results indicate that although δ18O and δ13C values from glendonites are useful for understanding early to late diagenetic processes, they are not ideal proxies for seawater chemistry and temperature.

The thermal regime of South African continental margins

We present thermal data from 100 oil exploration boreholes on the continental margins of South Africa. Near surface temperature profiles are constrained with temperatures acquired during reservoir tests, corrected bottom-hole temperatures and seafloor temperatures. The thermal conductivity profiles are estimated from geophysical well logs using a neural network method. Available data allow us to determine 41 new heat flow values computed from depth ranges greater than 1000 m, located on the western and southern continental margins of South Africa. We believe that the variations in surface heat flow are not controlled by the contribution of the sediments or the basement, but rather by the mantle heat flow. An upper bound of ~30 mW m−2 for the mantle heat flow on the interior of the continent can be inferred from the lowest available estimates, located near the coastline on the southern continental margin. On both continental margins, heat flow values reach a maximum of about 60 mW m−2, 100 km away from the shore, and consistently decrease further away towards values typically encountered in old oceanic domains (~50 mW m−2). This indicates that (1) the mantle heat flow increases over a short distance at the edge of the continent, which is consistent with studies on the effect of continents on mantle convection, and (2) it may increase to a value greater than heat flow on the adjacent ocean, as a possible consequence of the effect of the transition between thin oceanic lithosphere and thick and cold continental lithosphere on mantle convection.

Oceanic gas hydrate instability and dissociation under climate change scenarios

Global oceanic deposits of methane gas hydrate (clathrate) have been implicated as the main culprit for a repeated, remarkably rapid sequence of global warming effects that occurred during the late Quaternary period. However, the behavior of contemporary oceanic methane hydrate deposits subjected to rapid temperature changes, like those predicted under future climate change scenarios, is poorly understood, and existing studies focus on deep hydrate deposits under equilibrium conditions. In this study, we simulate the dynamic response of several types of oceanic gas hydrate accumulations to temperature changes at the seafloor and assess the potential for methane release into the ecosystem. The results suggest that while many deep hydrate deposits are indeed stable under the influence of rapid seafloor temperature variations, shallow deposits, such as those found in arctic regions or in the Gulf of Mexico, can undergo rapid dissociation and produce significant carbon fluxes over a period of decades.

Free energies of carbon dioxide sequestration and methane recovery in clathrate hydrates

Classical molecular dynamics simulations are used to compare the stability of methane, carbon dioxide, nitrogen, and mixed CO(2)N(2) structure I (sI) clathrates under deep ocean seafloor temperature and pressure conditions (275 K and 30 MPa) which were considered suitable for CO(2) sequestration. Substitution of methane guests in both the small and large sI cages by CO(2) and N(2) fluids are considered separately to determine the separate contributions to the overall free energy of substitution. The structure I clathrate with methane in small cages and carbon dioxide in large cages is determined to be the most stable. Substitutions of methane in the small cages with CO(2) and N(2) have positive free energies. Substitution of methane with CO(2) in the large cages has a large negative free energy and substitution of the methane in the large cages with N(2) has a small positive free energy. The calculations show that under conditions where storage is being considered, carbon dioxide spontaneously replaces methane from sI clathrates, causing the release of methane. This process must be considered if there are methane clathrates present where CO(2) sequestration is to be attempted. The calculations also indicate that N(2) does not directly compete with CO(2) during methane substitution or clathrate formation and therefore can be used as a carrier gas or may be present as an impurity. Simulations further reveal that the replacement of methane with CO(2) in structure II (sII) cages also has a negative free energy. In cases where sII CO(2) clathrates are formed, only single occupancy of the large cages will be observed.

Generalization of gas hydrate distribution and saturation in marine sediments by scaling of thermodynamic and transport processes

Gas hydrates dominated by methane naturally occur in deep marine sediment along continental margins. These compounds form in pore space between the seafloor and a sub-bottom depth where appropriate stability conditions prevail. However, the amount and distribution of gas hydrate within this zone, and free gas below, can vary significantly at different locations. To understand this variability, we develop a one-dimensional numerical model that simulates the accumulation of gas hydrates in marine sediments due to upward and downward fluxes of methane over time. The model contains rigorous thermodynamic and component mass balance equations that are solved using expressions for fluid flow in compacting sediments. The effect of salinity on gas hydrate distribution is also included. The simulations delineate basic modes of gas hydrate distribution in marine sediment, including systems with no gas hydrate, gas hydrate without underlying free gas, and gas hydrate with underlying free gas below the gas hydrate stability zone, for various methane sources. The results are scaled using combinations of dimensionless variables, particularly the Peclet number and Damkohler number, such that the dependence of average hydrate saturation on numerous parameters can be summa- rized using two contour maps, one for a biogenic source and one for upward flux from a deeper source. Simulations also predict that for systems at steady state, large differences in parameters like seafloor depth, seafloor temperature and geothermal gradient cause only small differences in average hydrate saturation when examined with scaled variables, although important caveats exist. Our model presents a unified picture of hydrate accumulations that can be used to understand well-characterized gas hydrate systems or to predict steady-state average hydrate saturation and distribution at locations for which seismic or core data are not available.

The Effect of Potential Future Climate Change on the Marine Methane Hydrate Stability Zone

Abstract The marine gas hydrate stability zone (GHSZ) is sensitive to temperature changes at the seafloor, which likely affected the GHSZ in the past and may do so in the future in response to anthropogenic greenhouse gas emissions. A series of climate sensitivity and potential future climate change experiments are undertaken using the University of Victoria Earth System Climate Model (UVic ESCM) with resulting seafloor temperature changes applied to a simple time-dependent methane hydrate stability model. The global GHSZ responds significantly to elevated atmospheric CO2 over time scales of 103 yr with initial decreases of the GHSZ occurring after 200 yr in shallow high-latitude seafloor areas that underlie regions of sea ice loss. The magnitude and rate of GHSZ change is dependent primarily upon the thermal diffusivity of the seafloor and the magnitude and duration of the seafloor temperature increase. Using a simple approximation of the amount of carbon stored as hydrate in the GHSZ, estimates of carbon mobilized due to hydrate dissociation are made for several potential climate change scenarios.

New insights into the carrier phase(s) of extraterrestrial 3He in geologically old sediments

To better understand the composition, characteristics of helium diffusion, and size distribution of interplanetary dust particles (IDPs) responsible for the long-term retention of extraterrestrial 3He, we carried out leaching, stepped heating, and sieving experiments on pelagic clays that varied in age from 0.5 Ma to ∼90 Myr. The leaching experiments suggest that the host phase(s) of 3He in geologically old sediments are neither organic matter nor refractory phases, such as diamond, graphite, Al 2O 3, and SiC, but are consistent with extraterrestrial silicates, Fe–Ni sulfides, and possibly magnetite. Stepped heating experiments demonstrate that the 3He release profiles from the magnetic and non-magnetic components of the pelagic clays are remarkably similar. Because helium diffusion is likely to be controlled by mineral chemistry and structure, the stepped heating results suggest a single carrier that may be magnetite, or more probably a phase associated with magnetite. Furthermore, the stepped outgassing experiments indicate that about 20% of the 3He will be lost through diffusion at seafloor temperatures after 50 Myrs, while sedimentary rocks exposed on the Earth’s surface for the same amount of time would lose up to 60%. The absolute magnitude of the 3He loss is, however, likely to depend upon the 3He concentration profile within the IDPs, which is not well known. Contrary to previous suggestions that micrometeorites in the size range of 50–100 μm in diameter are responsible for the extraterrestrial 3He in geologically old sediments [Stuart, F.M., Harrop, P.J., Knott, S., Turner, G., 1999. Laser extraction of helium isotopes from Antarctic micrometeorites: source of He and implications for the flux of extraterrestrial 3He flux to earth. Geochim. Cosmochim. Acta 63, 2653–2665], our sieving experiment demonstrates that at most 20% of the 3He is carried by particles greater than 50 μm in diameter. The size-distribution of the 3He-bearing particles implies that extraterrestrial 3He in sediments record the IDP flux rather than the micrometeorite flux.

Designing the Perfect Drilling-Fluid Additive: Can It Be Done?

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 95196, “Designing the Perfect Drilling-Fluid Additive: Can It Be Done?,” by J. Hall, SPE, Halliburton, prepared for the 2005 SPE Asia Pacific Health, Safety, and Environment Conference and Exhibition, Kuala Lumpur, 19–20 September. The environmental effects and technical performance of drilling-fluid additives are important characteristics of these products, which at times seem to be at odds with each other. For example, materials that show good stability at high temperatures frequently do not biodegrade well. In addition to the paradox of performance vs. environmental acceptability, international drilling-fluid companies face diverse environmental regulations when operating in geographically distinct areas. The full-length paper discusses real-life examples of drilling chemicals that were designed to fit stringent environmental criteria, according to the areas where they were used. Introduction Use and discharge of drilling fluids and cut-tings during well-construction activities have occurred since the beginning of off-shore drilling. However, it was not until relatively recently that these discharges have been regulated. Some of the most active drilling regions in the world also contain important commercial fisheries. Where regulations have been established, government agencies have been charged with establishing a control system to ensure that long-term effects do not occur in areas where drilling and well-construction discharges take place. These control systems rely on a range of tools to predict, rank, or otherwise categorize the expected effect of discharged materials. There may be a heavy reliance on laboratory-based toxicity and biodegradation tests, or attempts to predict the potential for materials to bioaccumulate. For example, it is easy to demonstrate that many organic materials will almost completely biodegrade when tested by a commonly used test to assess biodegradation in seawater. However, this is not much help in predicting when a base fluid discharged on cuttings will no longer be detected on the seafloor, or when the effects of a drilling-fluid spill will no longer be detected. The reasons for this are two-fold. First, the marine environment is a dynamic place, and materials dispersal can be rapid. Also, to obtain test results in a useful time frame, the conditions of testing may be idealized. For example, in the biodegradation test, the water oxygen content, test temperature, and nutrient mixture available to the bacteria present are optimized to produce a result in 28 days. In the natural environment, seafloor temperature may be as low as 2°C, and if the material is buried in a pile of drill cuttings, influx of nutrient-containing, oxygenated seawater may be severely limited so that degradation of the same materials showing good laboratory results may be reduced. However, the laboratory test provides the regulator an indication of the relative performance of a range of oilfield chemicals.

Temperature reconstructions for SW and N Iceland waters over the last 10 cal ka based on δ18O records from planktic and benthic Foraminifera

We have obtained δ 18O carb data from planktic ( N. pachyderma (s.)) and benthic foraminifera (C. lobatulus, M. barleeanus) from eight cores between 40 and 500 m water depth along the SW (core B997–347) and N Iceland margins (cores B997-321, -324, -327, -328, -329, -330, and -332) that span the last 10 cal ka. Sampling resolution varies between centuries to millennial scales. Over this time range, changes in δ 18O carb in foraminifera are the product of changes in the global ice volume, and the temperature and salinity of the water at the level in which the foraminifera live. In order to evaluate the relative roles of these variables we investigate: (1) the salinity ( S‰) and δ 18O water values for present-day waters off Iceland; and (2) the present-day relationships between δ 18O carb (or calcite) values of foraminifera and water column temperature and salinity data. We estimate the average error in the temperature estimate to be ∼±0.3 °C based on a Monto Carlo simulation of the various error terms. We argue that around Iceland the variations in δ 18O carb of both benthic and planktic species over the last 10 cal ka are principally controlled by temperature variations while salinity changes prove less important. Spatial and temporal changes are examined for both bottom water and 50 m water temperature reconstructions based on six and four cores, respectively. Reconstructed temperatures for sites around SW-N Iceland indicate that surface and seafloor water temperatures were warmest in the early Holocene (10–6 cal ka). Beginning at 6 cal ka, surface and seafloor temperatures began to cool in N Iceland and to become less stratified in SW Iceland. Over the last 5000 yrs, sea surface temperature variations have been ⩽1 °C for 50% of the observations with a maximum range of ∼2.5 °C. A profound decrease in temperature of ∼1.5 °C is recorded in our fjord site (B997-328) during the Little Ice Age. An abrupt, earlier cold event is well expressed in at least three N Iceland cores around 5 cal ka.