Biogeochemistry of acetate in anoxic sediments of Skan Bay,Alaska

The role of acetate in the biogeochemical cycling of organic matter in contemporary marine anoxic sediments of Skan Bay, Alaska was investigated with inhibition and quasi in situ turnover experiments. The turnover time for acetate oxidation in the upper 30 cm of the sediment column is ca. 1 hr. A molybdate inhibition experiment indicated that sulfate reducing bacteria were responsible for more than 95% of acetate oxidation. However, measured acetate oxidation rates exceeded sulfate reduction rates indicating that acetate oxidation rates are overestimated. Values for acetate concentration calculated from sulfate reduction rates (0.3–3.4 μM) were considerably lower than directly measured acetate concentrations (3.1–10.8 μM). Much of the chemically measured acetate may be microbially unavailable, perhaps in the form of a soluble or colloidal complex. A sorption experiment indicates that 10% to 40% of added acetate associates with Skan Bay sediment particles. Production of methane from acetate was detected only at 2 m depth.     

Nd-Sr-Pb systematics and age of the Kings River ophiolite,California: implications for depleted mantle evolution

Sm-Nd whole-rock and mineral data for the Kings River ophiolite define two isochrons of 485±21 Ma and 285±45 Ma age with _Nd (483)= +10.7±0.5 and Nd (285)= +9.9±1.1, respectively. The 483 Ma isochron is defined by samples of the main igneous construct. Samples from crosscutting diabase dikes and flaser gabbro sheets within the peridotite unit yield the 285 Ma isochron. The 483 Ma data provide the first evidence of lower Paleozoic oceanic crust in the Sierran ophiolite belt. New U-Pb analyses of zircons from a plagiogranite lying on the 483 Ma Sm-Nd isochron yield upper and lower intercepts with the concordia of 430 _–60 ⁺²⁰⁰ and 183±15 Ma. Published zircon ages have underestimated the primary age of the ophiolite by 200–300 m.y. due to the effects of polymetamorphism. The 483 Ma samples have initial ⁸⁷Sr/⁸⁶Sr=0.7023–0.7030, ²⁰⁶Pb/²⁰⁴Pb=17.14–17.82, ²⁰⁷Pb/²⁰⁴Pb=15.37–15.52, ²⁰⁸Pb/²⁰⁴Pb=36.80–37.38. The 285 Ma samples have similar initial ⁸⁷Sr/⁸⁶Sr, but more radiogenic Pb. The range in Sr and Pb compositions is probably due to introduction of radiogenic Sr and Pb during multiple post-emplacement metamorphic events. The high _Nd, low ⁸⁷Sr/⁸⁶Sr, ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, ²⁰⁸Pb/²⁰⁴Pb of the least disturbed samples are clearly diagnostic of a midocean ridge origin for the 483 Ma portion of the ophiolite. Igneous activity at 285 Ma is thought to have occurred in an arc or back-arc setting, or perhaps along a leaky transform. The initial _Nd (483)=+10.7 is indistinguishable from that of the similar age Trinity Peridotite (Jacobsen et al. 1984). This value is the highest yet reported for the Mesozoic or Paleozoic depleted mantle and requires either a mantle source that was depleted 850 m.y. earlier than average or a source more highly depleted than average. Alternatively, if such values were more typical of the early Paleozoic mantle than is currently thought, then there has been little evolution of the depleted mantle over the last 500 m.y. This requires that the modern mantle has been refluxed by material with low _Nd, such as continental crust.Division Contribution # 4302 (530)     

Averaging procedures for flow within vegetation canopies

Most one-dimensional models of flow within vegetation canopies are based on horizontally averaged flow variables. This paper formalizes the horizontal averaging operation. Two averaging schemes are considered: pure horizontal averaging at a single instant, and time averaging followed by horizontal averaging. These schemes produce different forms for the mean and turbulent kinetic energy balances, and especially for the wake production term describing the transfer of energy from large-scale motion to wake turbulence by form drag. The differences are primarily due to the appearance, in the covariances produced by the second scheme, of dispersive components arising from the spatial correlation of time-averaged flow variables. The two schemes are shown to coincide if these dispersive fluxes vanish.     

Age and provenance of the target materials for tektites and possible impactites as inferred from Sm-Nd and Rb-Sr systematics

Sm-Nd and Rb-Sr analyses of tektites and other impactites can be used to place constraints on the age and provenance of target materials which were impact melted to form these objects. Tektites have large negative ε^Nd(0) values and are uniform within each tektite group while the ε^Sr(0) are large positive values and show considerable variation within each group. Chemical, trace element, and isotopic compositions of tektites are consistent with production by melting of sediments derived from old terrestrial continental crust. Each tektite group is characterized by a uniform Nd model age,T_CHUR^Nd, interpreted as the time of formation of the crustal segment which weathered to form the parent sediment for the tektites: (1) ～1.15 AE for Australasian tektites; (2) ～1.91 AE for Ivory Coast tektites; (3) ～0.9 AE for moldavites; (4) ～0.65 AE for North American tektites, and (5) ～0.9 AE for high-Si irghizites. Sr model ages,T_UR^Sr, are variable within each group reflecting Rb-Sr fractionation and in the favorable limit of very high Rb/Sr ratios, approach the time of sedimentation of the parent material which melted to form the tektites. Australasian tektites are derived from ～0.25 AE sediments, moldavites from ～0.0 AE sediments, Ivory Coast tektites from ～0.95 AE sediments. Possible parent sediments of other tektite groups have poorly constrained ages. Our data on moldavites and Ivory Coast tektites are consistent with derivation from the Ries and Bosumtwi craters, respectively. Irghizites are isotopically distinct from Australasian tektites and are probably not related. Sanidine spherules from a Cretaceous-Tertiary boundary clay have initial ε^Nd ～ +2; ε^Sr ～ +5 and are not derived from old continental crust or meteoritic feldspar. They may represent a mixture of basaltic oceanic crust and sediments, implying an oceanic impact. These isotopic results are also consistent with a volcanic origin for the spherules.     

Bio-controlled thermostasis involving the sulfur cycle

The Gaia hypothesis proposed by Lovelock and Margulis presumes the existence of an unspecified biological means of ameliorating climate that has operated since the emergence of life 3500 Myr ago: Recently it was suggested that the mechanism of thermostasis may involve biospheric cycling of atmospheric carbon dioxide. We suggest an alternative hypothesis of biothermostasis operating through the sulfur cycle, rather than the carbon cycle. The mechanism would operate by altering planetary albedo through the selective creation of biospheric organic sulfide gases which go on to metamorphize into submicron particles and introduce cooling. In contrast to the carbon-cycle mechanism, sulfur-based cooling would have the ability to ameliorate climate well into the future, in principle over stellar Main Sequence time intervals. The main feature of interest is that the S cycle represents a particularly favorable thermodynamic pathway, involving three to four orders of magnitude less mass of active material cycled through the biospheric-atmospheric system (in response to a given temperature-imposed stress) than would be the case for a greenhouse gas hypothesis. There is no evidence that the suggested biospheric controlled particle-albedo change mechanism is actually operating, but we speculate that the probability of its rising importance and perhaps eventual dominance will improve when the partial pressure of atmospheric CO₂ drops low enough to impose stress on metabolic processes. The intriguing thing about the process is its extremely high efficiency.     

The meltwater hypothesis for subglacial bedforms

This paper presents an historical and in places informal account of the meltwater hypothesis, which invokes enormous outburst floods for the formation of subglacial bedforms. It begins with a brief discussion of the difficulties of determining processes of formation for landforms, which are not seen in formation. Analogy provides a solution to these difficulties. Analogy between erosional marks at the bases of turbidites and drumlins, which were the starting point for this hypothesis, rests on the idea that inverted erosional marks at the ice bed are subsequently infilled to form drumlins. Field tests on the sedimentology, architecture, and landform associations of drumlins in the Livingstone Lake drumlin field are outlined before more extensive work on bedrock erosional forms and flood routes is introduced. Bedrock erosional forms played a central part in establishing the hypothesis since their form and ornamentation are confidently interpreted as fluvial. Their form and genesis are discussed mainly with reference to sites at French River and Wilton Creek, Ontario, though some remarkable bedrock erosional forms in Antarctica support their regional extent. Initially in the meltwater hypothesis, drumlins were thought to be cavity fills and erosional drumlins were recognized later. This development is shown to be central to the realization that drumlin composition may be inferred from drumlin form. The scale of drumlin fields, measured at about 10³ km², and the magnitude of the inferred floods require that the flood events were regional. Regional-scale flood tracts in Ontario, Quebec, Alberta and the Northwest Territories extending over 1000 km in length and several hundred kilometers in width, support this suggestion. Floods, had they occurred, would have caused rapid rates of sea level rise and may have changed climate through their effects on ocean stratification and sea surface temperatures. The meltwater hypothesis covers a range of bedforms besides drumlins and bedrock erosional marks—fluting, Rogen moraine, hummocky terrain, and transverse ridges. Recent work shows how these forms are best explained by the meltwater hypothesis. The roles of water storage and release, which underpin the theory of the meltwater hypothesis, remain poorly understood.     

A generalized concept of resultant gradient to interpret potential field maps

The existing concept of the gradient of the potential field anomaly over a 3D source has been generalized. An observed anomaly is modified through a filter based on an assumed source geometry. The first-order derivatives of this modified anomaly in three mutually orthogonal directions form the components of a vector termed the resultant gradient. The gravity anomaly over a point mass, a vertical line mass and the gravity/magnetic anomaly reduced-to-pole over a bottomless right rectangular prism have been suitably modified to yield a specific shape for the amplitude of the resultant gradient in order to decipher the depth of the source centroid/corner. The applicability of the proposed technique is demonstrated by the analyses of a simulated example over a composite source and a real example from published literature with drill-hole information.