Search

The ANDRILL (Antarctic Drilling Project) McMurdo Ice Shelf (MIS) project drilled 1285 m of sediment in Hole AND-1B, representing the past 12 m.y. of glacial history. Downhole geophysical logs were acquired to a depth of 1018 mbsf (meters below seafloor), and are complementary to data acquired from the core. The natural gamma radiation (NGR) and magnetic susceptibility logs are particularly useful for understanding lithological and paleoenvironmental change at ANDRILL McMurdo Ice Shelf Hole AND-1B. NGR logs cover the entire interval from the seafloor to 1018 mbsf, and magnetic susceptibility and other logs covered the open hole intervals between 692 and 1018 and 237–342 mbsf. In the upper part of AND-1B, clear alternations between low and high NGR values distinguish between diatomite (lacking minerals containing naturally radioactive K, U, and Th) and diamictite (containing K-bearing clays, K-feldspar, mica, and heavy minerals). In the lower open hole logged section, NGR and magnetic susceptibility can also distinguish claystones (rich in K-bearing clay minerals, relatively low in magnetite) and diamictites (relatively high in magnetite). Sandstones can be distinguished by their high resistivity values in AND-1B. On the basis of these three downhole logs, diamictite, claystones, and sandstones can be predicted correctly for 74% of the 692–1018 mbsf interval. The logs were then used to predict facies for the 6% of this interval that was unrecovered by coring. Given the understanding of the physical property characteristics of different facies, it is also possible to identify subtle changes in lithology from the physical properties and help refine parts of the lithostratigraphy, for example, the varying terrigenous content of diatomites and the transitions from subice diamictite to open-water diatomite.

The Middle Cambrian Spence Shale Member (Langston Formation) and Wheeler and Marjum Formations of Utah are known to contain a diverse soft-bodied fauna, but important new paleontological material continues to be uncovered from these strata. New specimens of anomalocaridids include the largest and smallest near complete examples yet reported from Utah. New material of stem group arthropods includes two new genera and species of arachnomorphs: Nettapezoura basilika and Dicranocaris guntherorum. Other new arachnomorph material includes a new species of Leanchoilia comparable to L. protogonia Simonetta, 1970; Leanchoilia superlata? Walcott, 1912; Sidneyia Walcott, 1911a; and Mollisonia symmetrica Walcott, 1912. L. protogonia from the Burgess Shale is confirmed as a separate species and is not a composite fossil. The first example of the trilobite Elrathia kingii preserving traces of the appendages is described. In addition, new material of the bivalved arthropods Canadaspis Novozhilov in Orlov, 1960; Branchiocaris Briggs, 1976; Waptia Walcott, 1912; and Isoxys Walcott, 1890 is described.

A new metazoan, Skeemella clavula gen. et sp. nov., is described from the Middle Cambrian Pierson Cove Formation of the Drum Mountains, Utah, USA. Skeemella is similar to vetulicolians, but differs from other examples of this group in the relative proportions of the anterior and posterior sections, the large number of divisions, and the elongate bifid termination. The posterior section is arthropodan in character. The similarity of this fossil to vetulicolians throws hypotheses of their deuterostome affinity into question and highlights their problematic sta- tus.

Compressional-wave velocity in siliciclastic rocks is known to be a function mainly of porosity, and secondarily of mineralogy. We investigate the effect of cementation on velocity, using core-plug and whole-core measurements of velocity and porosity. The case study area is a drillhole at Cape Roberts, Antarctica, an environment with a rapid downhole gradient in cementation. After removal of porosity effect from velocity, the change from incipient to intense carbonate cementation is seen to account for up to 50% of total velocity. Increased intergrain coupling, particularly increased frame bulk modulus, is the cause of this velocity increase.

Ocean Drilling Program Leg 171A collected logging while drilling (LWD) data to investigate the porosity and velocity structure of Barbados accretionary complex. Porosity and velocity measurements were needed for interpretation of prism dynamics. However, the LWD tool string did not measure velocity, and previous velocity data from Barbados prism are scarce and generally of low quality. Furthermore, most density-based porosities are subject to errors associated with variations in smectite content. To determine porosities more accurately, we use smectite-corrected, density-based porosities to calibrate a porosity-formation factor (FF) transform, then calculate porosities from resistivity logs. We used cation exchange capacity (CEC) measurements and LWD logs to evaluate the possibility that resistivity is affected by clay conduction. The CEC data warn of potential clay conduction, especially in smectite-rich zones, but LWD data indicate that clay conduction is minor in these high-porosity sediments. Instead, tortuosity is the secondary control of FF, after porosity. By comparing resistivity-based porosities to velocities from vertical seismic profiles, we determine a porosity-velocity transform applicable to Barbados. We then use this transform to calculate velocity logs from resistivity-based porosity logs.

Seeking a global empirical relationship between compressional wave velocity and porosity for siliciclastic sediments, we have brought together an extensive suite of both new and published log- and core-based data. We undertook a detailed statistical analysis of Ocean Drilling Program data from Amazon Fan to examine variables affecting compressional velocity in shallow, unconsolidated sediments. We identify three dominant variables (porosity, shale fraction, and consolidation history) and present two empirically determined boundary curves (one for normally consolidated sediments and a second for highly consolidated environments (e.g., accretionary prisms)). These two empirical relationships predict the compressional velocity of siliciclastic sedimentary rocks with water-filled pores as a function of porosity and clay content for the full range of observed porosities. Velocities of siliciclastic sedimentary rocks decrease rapidly with both increasing porosity and increasing clay content. At fractional porosities higher than about 0.4, fluid dominates the elastic properties, and velocity exhibits a subtle dependence on porosity. Remarkably, the Amazon Fan data show that clay content has no direct influence on velocity at high porosities. Both clay content and sorting do indirectly affect velocity, through their control of porosity. Burial affects velocity not only by compaction-related porosity decrease but also by pressure-induced increase of intergrain coupling. Because of the sensitivity of velocity to consolidation history, particularly at intermediate fractional porosities of about 0.30–0.40, no single velocity-porosity relationship can apply to all high-porosity sediments. The two proposed relationships fit the majority of published and new data. They are applicable, however, only for normally pressured, in situ conditions and water-filled pores.

The electrical resistivity of siliciclastic rocks is a function both of pore-fluid resistivity and of formation factor (FF), an intrinsic rock property. For low-porosity rocks, FF depends on clay conduction and porosity. In contrast, we find that FF of high-porosity sediments (fractional porosities of 0.3–0.6) from Amazon Fan is controlled primarily by porosity and pore geometry; clay conduction is a minor effect. Porosity vs. formation factor (FF) plots for Amazon Fan well logs demonstrate two distinct trends which are dependent on the amount of shale present. Muds, with more than about 0.4 shale fraction, follow a trend that increases to much higher FF as porosity decreases, compared to sands and slightly muddy sands. This separation is of opposite sign to the separation between low-porosity sandstones and shales attributable to clay conduction. We conclude that in high-porosity muds, presence of clays reduces formation conductivity by increasing the tortuosity of pores.

At Cascadia accretionary prism, log-based porosities exhibit complex lateral changes in response to variations in cementation, horizontal compression, and fracturing. Porosities at the prism toe are lower than in reference sediments, whereas porosities landward of the toe are slightly higher than those of reference sediments. Ideally, a porosity/velocity relationship can be used to determine two-dimensional porosity structure from seismic velocities. Porosity is the major control on velocity at Cascadia, but variations in grain framework strength (frame bulk modulus) cause deformed and undeformed rocks to have quite different velocity/porosity relationships. Within the prism, cementation can raise velocities substantially above those for unconsolidated reference sediments of the same porosity; also, compression-induced compaction and fracturing may affect this velocity/porosity transform. Because these factors vary both within and between prisms, velocity/porosity transforms are local, not universal.

[2] The Eocene biosiliceous oozes of the tropical Pacific are an easily recognizable sediment type having a reddish brown color and sugary texture. These sediments are dominated by radiolarian tests, with traces of terrigenous debris, hydrothermal oxides, a few diatoms and little if any carbonate. The paucity of carbonate is a result of the very shallow Pacific calcite compensation depth (CCD) of the Eocene (3300 m) [Rea and Lyle, 2005, and references therein]. The dominance of radiolarian tests over those of diatoms is an enigma in that diatoms usually dominate siliceous-rich sediments of the Oligocene and Neogene. The opal-rich character of these Eocene sediments is commonly associated with the development of cherts within the sections. [3] These unusual characteristics of Eocene radiolarian oozes have prompted some authors to puzzle over how so much silica could have been delivered to the sea floor during

Cnidarians represent an early diverging animal group and thus insight into their origin and diversification is key to understanding metazoan evolution. Further, cnidarian jellyfish comprise an important component of modern marine planktonic ecosystems. Here we report on exceptionally preserved cnidarian jellyfish fossils from the Middle Cambrian (approximately 505 million years old) Marjum Formation of Utah. These are the first described Cambrian jellyfish fossils to display exquisite preservation of soft part anatomy including detailed features of structures interpreted as trailing tentacles and subumbrellar and exumbrellar surfaces. If the interpretation of these preserved characters is correct, their presence is diagnostic of modern jellyfish taxa. These new discoveries may provide insight into the scope of cnidarian diversity shortly after the Cambrian radiation, and would reinforce the notion that important taxonomic components of the modern planktonic realm were in place by the Cambrian period.

[1] High-resolution mass accumulation rates (MAR) were determined from lithologic logs based on downhole log and continuous core data for six sites at four continental margins around the Southern Ocean. Total MAR was calculated at Ocean Drilling Program (ODP) Site 1095 on the margin of the Antarctic Peninsula and ODP Site 1165 on the margin of Prydz Bay, Antarctica. Carbonate and noncarbonate (terrigenous) MAR were calculated for ODP Sites 1123 and 1124 east of New Zealand and Sites 1168 and 1172 west and southeast of Tasmania. Shifts in carbonate MAR are seen around Tasmania and New Zealand at 23 and 14 Ma, suggesting changes in deep water circulation and surface carbonate productivity at these times. Carbonate MAR dropped at Sites 1124 and 1168 at 23 Ma and increased at 14 Ma at Sites 1123, 1168, and 1172. The overall character of total MAR at Antarctic Sites 1095 and 1165 is a continual and relatively constant decrease in total MAR during the Neogene, with periods of stepwise decrease. One such decrease is seen at 14 Ma at Site 1165, coincident with increased carbonate MAR in New Zealand and Tasmania (Sites 1123, 1124, 1168 and 1172) and likely related to mid-Miocene expansion of the East Antarctic Ice Sheet. Another step drop in total MAR is seen at ∼9 Ma at Site 1165, possibly reflecting diversion of sediment to a newly formed Prydz Channel fan.

[1] The alteration of upper oceanic crust entails growth of hydrous minerals and loss of macroporosity, with associated large-scale fluxes of H2O, CO2, Cl−, and K2O between seawater and crust. This age-dependent alteration can be quantified by combining a conceptual alteration model with observed age-dependent changes in crustal geophysical properties at DSDP/ODP sites, permitting estimation of crustal concentrations of H2O, CO2, Cl−, and K2O, given crustal age. Surprisingly, low-temperature alteration causes no net change in total water; pore water loss is nearly identical to bound water gain. Net change in total crustal K2O is also smaller than expected; the obvious low-temperature enrichment is partly offset by earlier high-temperature depletion, and most crustal K2O is primary rather than secondary. I calculate crustal concentrations of H2O, CO2, Cl−, and K2O for 41 modern subduction zones, thereby determining their modern mass fluxes both for individual subduction zones and globally. This data set is complemented by published flux determinations for subducting sediments at 26 of these subduction zones. Global mass fluxes among oceans, oceanic crust, continental crust, and mantle are calculated for H2O, Cl−, and K2O. Except for the present major imbalance between sedimentation and sediment subduction, most fluxes appear to be at or near steady state. I estimate that half to two thirds of subducted crustal water is later refluxed at the prism toe; most of the remaining water escapes at subarc depths, triggering partial melting. The flux of subducted volatiles, however, does not appear to correlate with either rate of arc magma generation or magnitude of interplate earthquakes.

[1] The hydrologic evolution of oceanic crust, from vigorous hydrothermal circulation in young, permeable volcanic crust to reduced circulation in old, cooler crust, causes a corresponding evolution of geophysical properties. Ocean Drilling Program (ODP) Hole 801C, which obtained the world's oldest section of in situ, normal oceanic crust, provides the opportunity to examine relationships among hydrologic properties (porosity, permeability, fluid flow), crustal alteration, and geophysical properties, at both core plug and downhole log scales. Within these upper crustal basalts, fluid flux in zones with high porosity and associated high permeability fosters alteration, particularly hydration. Consequently, porosity is correlated with both permeability and a variety of hydration indicators. Porosity-dependent alteration is also seen at the log scale: potassium enrichment is strongly proportional to porosity. We extend the crustal alteration patterns observed at Hole 801C to a global examination of how physical properties of upper oceanic crust change as a function of age based on global data sets of Deep Sea Drilling Project and ODP core physical properties and downhole logs. Increasing crustal age entails macroporosity reduction and large-scale velocity increase, despite intergranular velocity decrease with little microporosity change. The changes in macroporosity and velocity are significant for pillows but minor for flows. Matrix densities provide the strongest demonstration of systematic age-dependent alteration. On the basis of observed decreases in matrix density that are proportional to the logarithm of age, approximately half of all intergranular-scale crustal alteration occurs after the first 10–15 Myr. Apparently, crustal alteration continues, at a decreasing rate, throughout the lifetime of oceanic crust.

Some of the greatest uncertainties in our understanding of Cenozoic global tectonics and climate can be traced back to our relatively meager knowledge about Antarctica9s continental lithosphere and its overlying continental glaciers. A trove of information about past tectonism and the behavior of the continental ice sheets lies buried along the submarine continental margins of Antarctica. In order to explore this area, there have been several international efforts over the last several decades to drill Cenozoic stratigraphic sequences within basins in the West Antarctic Rift system in the southern Ross Sea. The most recent of the Ross Sea drilling projects was part of the ANDRILL initiative (www.andrill.org) in which scientists from Germany, Italy, New Zealand, and the United States collaborated to acquire two high-resolution slim hole sedimentary cores from the southern Ross Sea. The papers within this Geosphere themed issue examine results associated with these projects. They are ultimately a result of significant efforts by a new generation of scientists who were willing and able to take the reins of leadership at the close of ANDRILL9s ancestor, the Cape Roberts Project. These papers speak to the importance of multidisciplinary science and international cooperation, and hopefully come at the dawn of further drilling and investigations of the scientific frontier represented by Antarctica9s continental margins.

Vertical seismic profile (VSP) data from two drill sites on the Cascadia margin show low-velocity zones, indicative of free gas, be. neath a bottom-simulating reflector (BSR). Offshore Oregon, at Ocean Drilling Program (ODP) Site 892, velocities drop from an average of 1750 m/s above the BSR to less than 1250 m/s below it. Sonic logs confirm that seismic velocity in the sediments adjacent to the borehole is less than that of water for at least 50 m beneath the depth of the BSR at this site. Similarly, at ODP Site 889 offshore Vancouver, velocities range from 1700 to 1900 m/s in the 100 m above the BSR and drop abruptly to 1520 m/s in the 15 m just beneath it. The low velocities observed beneath the BSR are strong evidence for the presence of 1%-5% free gas (by volume). The BSR at these two sites results from the contact between gas-free sediments containing a small quantity of hydrate above the BSR and a low-velocity free-gas zone beneath it. Although the BSR is associated with the base of the hydrate stability field, hydrate appears to account for relatively little of the velocity contrast that produces the BSR. Velocity above the BSR at Site 889 is only about 100 m/s greater than that expected for sediments of similar porosity. Sediments above the BSR at Site 892 appear to have normal velocity for their porosity and may, contain little hydrate.

Ocean Drilling Program (ODP) Sites 645 and 646, in Baffin Bay and the Labrador Sea, respectively, were logged fol­ lowing drilling during Leg 105. Geophysical logs in ODP drill sites yield long, continuous records of sedimentary se­ quences and thus provide the opportunity for examining evidence of periodicity in the Milankovitch band. Such logging records are extremely valuable where core recovery is incomplete and/or where cyclicity is on a scale larger than the length of a core (< 1 cycle/9.5 m), making recognition of cyclicity difficult on the basis of core description or carbonate data. Because these logs are continuous, they can be treated statistically with spectral techniques to enhance under­ standing of cyclic processes. Because these logs also indicate lithology, they can be readily interpreted in light of avail­ able sedimentary and geochemical analyses on any cores. The interval 288-455 mbsf of Site 645 was logged with sonic, resistivity, and gamma-ray tools. This zone corre­ sponds to an estimated time interval of 1.9-3.1 Ma. These logs exhibit strong cyclicity throughout, which is mainly con­ trolled by variations in porosity. The high sedimentation rate, which permits a vertical resolution of about 5 k.y. for sonic and 17 k.y. for resistivity logs, allows detection of cycle frequencies that correspond to a dominant 100-k.y. period and subordinate ~40-k.y. periodicity in the upper part of the logged interval and perhaps to an increasingly important ~ 20-k.y. period in the lower interval. The cycles most likely correspond to variations in flux of ice-rafted debris and/or to variations in activity of bottom currents. The patterns suggest waning influence of precession and increasing impor­ tance of obliquity as glaciation progressed in the Baffin Bay region. At Site 646, the openhole interval of 210-737 mbsf was logged with neutron, spectral-gamma-ray, gamma-spectroscopy, sonic, and resistivity tools. This interval corresponds in time to -8.5-2.1 Ma. Because of variations in sedimenta­ tion rate downhole (average 90 m/m.y.), the time resolution of logs varies somewhat. In general, variance is strongest in the eccentricity (—100 k.y.) and obliquity ( — 41 k.y.) bands, but vertical resolution limits our ability to resolve frequen­ cies corresponding to precession, if present. Again, the variation is largely in porosity, which apparently is directly re­ lated to variations in clay content. The cycles, which are alternations between clay-rich and coarser-grained sediment, most likely reflect orbital control on changes in intensity of bottom currents. Log responses indicate that significant in­ creases in the intensity of bottom-current activity on Eirik Ridge occurred at about 5.4-5.5 Ma and 4.1-4.2 Ma. A third sedimentary change occurred at about 3.4 Ma, causing a major shift in the porosity baseline and character of log cycles and corresponding to both the onset of biogenic silica sedimentation and a substantial increase in sedimentation rate.

Drilling results and seismic-reflection records at and across Ocean Drilling Program (ODP) Sites 645 (western Baffin Bay), 646, and 647 (Labrador Sea) provide important constraints on the history of deep-water circulation and sedimentation in response to Cenozoic climatic change, as well as the tectonic evolution of the region. Sites 646 and 647 were drilled on the flanks of two sediment drift deposits—the Eirik Ridge and Gloria Drift, respectively. Age control at Site 645 was poor because of the restricted biotas there, but the drill site provides a continuous sequence from the lower Miocene to the present. Sediment at Site 646 was deposited at high rates, providing a high resolution record of the last 8.5 Ma. At Site 647 sedimentation was variable and discontinuous, but a complete upper-lower Eocene through lower Oligocene sequence was recovered, whereas the upper Oligocene to Holocene sequence was interrupted by several hiatuses. The drift sequence at Site 646 was constructed after the middle to early Pliocene (ca. 4.5 Ma). Before that time, evidence exists for variable bottom-current activity, with events at about 7.5 Ma (a change in water-mass characteristics and decreasing velocities) and 5.6 Ma (an increase in current velocity preceding the major 4.5-Ma event; R2 regional reflector). The 7.5-Ma event produced a major regional reflector (R3/R4), which was originally thought to be Eocene/ Oligocene in age. A major water-mass change also occurred at the onset of ice-rafting at about 2.5 Ma in the late Pliocene. In seismic records no evidence exists of drift building before the early Pliocene, but a probable late-middle Miocene erosional event occurred on the south flank of Eirik Ridge and along the West Greenland margin. Sediment supply from the Imarssuak mid-ocean canyon (IMOC) increased concurrently with the advent of drift construction. Gloria Drift also was built largely after the late Miocene. A major increase in sediment supply occurred in the early Pliocene, following a major hiatus (5.6 to 2.5 Ma; equivalent to the youngest possible age for the R2 reflector underlying Gloria Drift), and most seismic records exhibit sediment waves above this horizon. This increased sediment supply is the result of hemipelagic deposition from encroaching deposits of the North Atlantic mid-ocean canyon, as well as to supply of ice-rafted detritus in the late Pliocene. A hiatus encompasses the interval from approximately 17.5 to 8,2 Ma, and the interval between the two major hiatuses is extremely condensed. A deeper reflector (R3) corresponds to a change from calcareous (below) to opal-rich hemipelagic strata in the lower Oligocene, not to a regional unconformity reflecting increased bottom-water activity, as previously thought. However, some evidence exists to support a latest Eocene-earliest Oligocene increase in bottom-current activity on Gloria Drift. In Baffin Bay, there is evidence for bottom-water activity from textural studies of cores and from apparent drift features exhibited in multichannel lines along the western margin. Probable contour-currents have been active since at least the late middle Miocene, with episodes of decreasing intensity that apparently occurred in the late Miocene and Quaternary. The record from Site 645 and in seismic lines may indicate that formation of bottom water occurred in the late Neogene in Baffin Bay in conjunction with climatic deterioration, but Baffin Bay was not an important source of deep-water masses to the Labrador Sea after the late Pliocene. Not surprisingly, many of the Labrador Sea deep-circulation events correspond closely to major North Atlantic events and to important global climatic and paleoceanographic events, but a major drift-building episode may have occurred later in the Labrador Sea than it did in either the eastern North Atlantic or the western North Atlantic.

Forearc slivers, bounded by a trench and an active strike-slip fault, occur in about 50% of modern subduction zones. Analysis of earthquake slip vectors indicates that modern sliver terranes typically migrate at rates of 1–2 cm/yr. Tertiary transport of some forearc slivers by 1000 km or more is therefore expected as a consequence of normal subduction. Active arc-parallel strike-slip faulting occurs whenever interplate coupling is strong and convergence is somewhat oblique; strike-slip rate increases with greater convergence obliquity.