Robert A. Pockalny

Global distribution of microbial abundance and biomass in subseafloor sediment

The global geographic distribution of subseafloor sedimentary microbes and the cause(s) of that distribution are largely unexplored. Here, we show that total microbial cell abundance in subseafloor sediment varies between sites by ca. five orders of magnitude. This variation is strongly correlated with mean sedimentation rate and distance from land. Based on these correlations, we estimate global subseafloor sedimentary microbial abundance to be 2.9⋅1029 cells [corresponding to 4.1 petagram (Pg) C and ∼0.6% of Earth’s total living biomass]. This estimate of subseafloor sedimentary microbial abundance is roughly equal to previous estimates of total microbial abundance in seawater and total microbial abundance in soil. It is much lower than previous estimates of subseafloor sedimentary microbial abundance. In consequence, we estimate Earth’s total number of microbes and total living biomass to be, respectively, 50–78% and 10–45% lower than previous estimates.

Subseafloor sedimentary life in the South Pacific Gyre

The low-productivity South Pacific Gyre (SPG) is Earths largest oceanic province. Its sediment accumulates extraordinarily slowly (0.1–1 m per million years). This sediment contains a living community that is characterized by very low biomass and very low metabolic activity. At every depth in cored SPG sediment, mean cell abundances are 3 to 4 orders of magnitude lower than at the same depths in all previously explored subseafloor communities. The net rate of respiration by the subseafloor sedimentary community at each SPG site is 1 to 3 orders of magnitude lower than the rates at previously explored sites. Because of the low respiration rates and the thinness of the sediment, interstitial waters are oxic throughout the sediment column in most of this region. Consequently, the sedimentary community of the SPG is predominantly aerobic, unlike previously explored subseafloor communities. Generation of H2 by radiolysis of water is a significant electron-donor source for this community. The per-cell respiration rates of this community are about 2 orders of magnitude higher (in oxidation/reduction equivalents) than in previously explored anaerobic subseafloor communities. Respiration rates and cell concentrations in subseafloor sediment throughout almost half of the world ocean may approach those in SPG sediment.

Characteristics and evolution of the segmentation of the Mid-Atlantic Ridge between 20°N and 24°N during the last 10 million years

Abstract High-resolution bathymetry and geophysical data collected along the slow-spreading axis and flanks of the Mid-Atlantic Ridge between 20°N and 24°N reveal the characteristics and history of different wavelengths of segmentation during the last 10 m.y. The bathymetric data exhibit a morphotectonic pattern dominated by ridge-normal and oblique bathymetric lows that partition the ridge flanks into rhomb-shaped areas of relatively high elevation. At least four different types of oblique bathymetric lows have been identified which represent the off-axis traces of axial discontinuities and suggest a complex and ongoing evolution of ridge-axis segmentation. One group of oblique structures is represented by two deep ridge-normal depressions with typical fracture zone characteristics that are connected to the present active transform by oblique depressions near the ridge axis. These oblique traces correspond to the southward shift of axial discontinuities associated with the propagation of the ridge axis, while maintaining a constant offset of the latter. Two other types of oblique structures correspond to elongate bathymetric lows and oblique alignments of ridge-parallel bathymetric lows symmetric about the ridge axis. Both types of oblique structures frequently change their orientation (from normal to subparallel to the ridge axis) and appear to merge and diverge off-axis. These oblique depressions are characterized by positive filtered mantle Bouguer anomalies, high magnetizations, complex magnetic anomaly patterns, and possible exposure of mantle lithologies. The ridge segments defined by these oblique depressions lengthen or shorten along the ridge axis, with propagation rates varying from 0 to 25 km m.y. −1 . The last and smallest discontinuities observed in this area correspond to small ridge-axis offsets and off-axis traces identified by alignments of the terminations of abutting abyssal hills. The ridge-flank morphotectonic patterns produced by the evolution of these elementary segments of accretion may represent temporally variable upwelling volumes of melt. The centres of the rhomb-shaped areas correspond to maximum crust production and thin lithosphere, and the discontinuities correspond to a thick lithosphere with very thin crust and possible outcrops of peridotites. We propose a model which accounts for the punctuated injection of magma and the evolution of elementary segments of accretion over periods of several million years.

Aerobic Microbial Respiration in 86-Million-Year-Old Deep-Sea Red Clay

Deep Breathing Living microbes have been discovered many meters into marine sediments. On a cruise in the North Pacific Gyre, Røy et al. (p. 922) discovered that oxygen occurred for tens of meters into the sediment. The bacteria living in these sediments were respiring the oxygen but at a slower rate than the supply of organic material dropping out of the water column, allowing these ancient deep marine sediments to remain oxygenated. Modeling showed that the rate of respiration of specific carbon decreased as a function of sediment depth, that is, its age. Thus aerobic metabolism can persist in deep marine sediments. Microbes in Pacific sediments grow very, very slowly. Microbial communities can subsist at depth in marine sediments without fresh supply of organic matter for millions of years. At threshold sedimentation rates of 1 millimeter per 1000 years, the low rates of microbial community metabolism in the North Pacific Gyre allow sediments to remain oxygenated tens of meters below the sea floor. We found that the oxygen respiration rates dropped from 10 micromoles of O2 liter−1 year−1 near the sediment-water interface to 0.001 micromoles of O2 liter−1 year−1 at 30-meter depth within 86 million-year-old sediment. The cell-specific respiration rate decreased with depth but stabilized at around 10−3 femtomoles of O2 cell−1 day−1 10 meters below the seafloor. This result indicated that the community size is controlled by the rate of carbon oxidation and thereby by the low available energy flux.

Tectonic reconstruction of the Clipperton and Siqueiros Fracture Zones: Evidence and consequences of plate motion change for the last 3 Myr

An electronic circuit is described that adapts the output of a position-sensing detector, designed to determine the position of an incident CW laser beam, to determine the position of a fast pulsing laser incident on the detectors surface.

Mid-Cretaceous tectonic evolution of the Tongareva triple junction in the southwestern Pacific Basin

The trace of the ridge-ridge-ridge triple junction that connected the Pacific, Farallon, and Phoenix plates during mid-Cretaceous time originates at the northeast corner of the Manihiki Plateau near the Tongareva atoll, for which the structure is named. The triple junction trace extends >3250 km south-southeast, to and beyond a magnetic anomaly 34 bight. It is identified by the intersection of nearly orthogonal abyssal hill fabrics, which mark the former intersections of the Pacific-Phoenix and Pacific-Farallon Ridges. A distinct trough is commonly present at the intersection. A volcanic episode from 125 to 120 Ma created the Manihiki Plateau with at least twice its present volume, and displaced the triple junction southeast from the Nova-Canton Trough to the newly formed Manihiki Plateau. Almost simultaneously, the plateau was rifted by the new triple junction system, and large fragments of the plateau were rafted away to the south and east. The Tongareva triple junction originated ca. 119 Ma, when carbonate sedimentation began atop the Manihiki Plateau. Subsequent spreading rates on the Pacific-Phoenix and Pacific-Farallon Ridges averaged 18–20 cm/yr until 84 Ma.

Spatial and temporal variability of crustal magnetization of a slowly spreading ridge: Mid-Atlantic Ridge (20°–24° N)

The results of the two- and three-dimensional magnetic inversions performed on data located between 20°–24° N on the Mid-Atlantic Ridge indicate the crustal magnetization has decayed exponentially for the last 10 Ma, and that this decay has been fairly symmetric about the ridge axis. After removal of the mean temporal decay, the residual field is characterized by more positive magnetizations at the second-order discontinuities, regardless of initial magnetization direction. A model that involves the preferential emplacement of serpentinized lithologies near the discontinuities is proposed to explain this correlation. The temporal detrending method also indicates that several ridge-parallel depressions located on the flanks of the ridge axis are regions of more positive magnetizations. These bathymetric depressions may mark the locations of detachment faulting that occurred during amagmatic periods of extension. The general symmetry of the crustal magnetization about the ridge axis does not support the occurrence of continuous detachment faulting proposed to correspond to the inner and outer corners of ridge axis discontinuities.

A Case Study of Ships Forming and Not Forming Tracks in Moderately Polluted Clouds

The effects of anthropogenic particulate emissions from ships on the radiative, microphysical, and chemical properties of moderately polluted marine stratiform clouds are examined. A case study of two ships in the same air mass is presented where one of the vessels caused a discernible ship track while the other did not. In situ measurements of cloud droplet size distributions, liquid water content, and cloud radiative properties, as well as aerosol size distributions (outside cloud, interstitial, and cloud droplet residual particles) and aerosol chemistry, are presented. These are related to measurements of cloud radiative properties. The differences between the aerosol in the two ship plumes are discussed; these indicate that combustion-derived particles in the size range of about 0.03‐0.3-mm radius were those that caused the microphysical changes in the clouds that were responsible for the ship track. The authors examine the processes behind ship track formation in a moderately polluted marine boundary layer as an example of the effects that anthropogenic particulate pollution can have in the albedo of marine stratiform clouds.

Evidence of transpression along the Clipperton Transform: Implications for processes of plate boundary reorganization

Abstract High-resolution bathymetry data of the Clipperton Fracture Zone and focal mechanisms of recent earthquakes along the active transform provide evidence for recent and ongoing transpression along the Clipperton Transform. Structural and morphological features observed in the bathymetry data include: normal and strike-slip faults oriented obliquely to the strike of the transform; curved trends of abyssal hills produced by simple shear of the lithosphere bordering the transform fault; abyssal hills offset by obliquely trending strike-slip faults on lithosphere bordering the transform; and lithospheric flexure and associated volcanism near the active transform. All of these features can be explained by a transpression scenario in which a counter-clockwise change in spreading direction of ∼ 5° began about 0.4–0.5 Ma and subjected the right-stepping Clipperton Transform to compression. A comparison of the median ridge volume to calculated estimates of the convergent volume predicted by the transpression scenario indicates that the median ridge is likely to be a product of the brittle deformation associated with transpression. The convergent volume estimates suggest that larger spreading direction changes (∼ 5°) do not occur instantaneously. Instead, spreading direction changes begin slowly and accelerate to the new spreading direction once compression across the transform has ceased. Many of the structures observed along the Clipperton Transform are very similar to features observed along other known convergent transforms, such as the San Andreas Fault in California, and may have important implications for understanding continental transforms. The transpression scenario also indicates that transforms are very important in constraining the rate and character of global plate motion changes.

Oceanic transverse ridges: A flexural response to fracture-zone–normal extension

One of the most puzzling characteristics of sea-floor morphology is the occurrence of anomalously shallow, fracture-zone–parallel, oceanic transverse ridges. A model is proposed for the formation of transverse ridges near lat 21° and 24°N on the Mid-Atlantic Ridge in which the differential responses of large-offset and small-offset fracture zones to recent changes in spreading direction result in the generation of normal faults that coincide with the off-axis traces of fracture zones. Numerical models of the flexural response of the lithosphere to normal faulting suggest that modest amounts of extension (<5 km) along low-angle faults (<45°) are responsible for the transverse ridges.