Nitrogen Retention in Coastal Marine Sediments—a Field Study of the Relative Importance of Biological and Physical Removal in a Danish Estuary

The aim of this study was to elucidate the relative importance of physical versus biological loss processes for the removal of microphytobenthic (MPB) bound nitrogen in a coastal environment at different times of the year via a dual isotope labeling technique. We used ⁵¹Cr, binding to inorganic sediment particles but not participating in any biological processes, and ¹⁵N–NO₃ ^?, taken up by the MPB and turned over as part of the MPB nitrogen pool. Retention, down-mixing, and export of ¹⁵N were due to both biological and physical processes, so that by comparing retention of the two isotopes, we were able to discern the relative importance of physical and biological processes. The isotope marking was supplemented with measurements of sediment chlorophyll biomass and oxygen fluxes, allowing us to evaluate MPB biomass as well as primary production vs. respiration in the sediment. In spring/early summer, the system was characterized by tight N cycling and high N retention: any remineralized N was immediately taken up and retained in the MPB biomass. In late summer and autumn, the system was still physically stable, but high biological mediated N losses were observed. In early winter, the system was physically dominated due to low MPB biomasses and activity combined with a significant storm event. Our data support the hypothesis that the relative balance between physical and biological processes in determining retention and removal of MPB-bound nitrogen changes seasonally.     

Origin of July Antarctic precipitation and its influence on deuterium content: a GCM analysis

The NASA/GISS GCM is used to estimate the evaporative contributions of several oceanic regions (defined by temperature) to Antarctica's July precipitation. Tracer diagnostics in the GCM suggest that the weighted average evaporative source temperature for Antarctic precipitation as a whole is about 12°C. The average source temperature for local precipitation there varies from 9° C to 14° C. To examine the effect of evaporative source on water isotope concentration, the GCM also follows a global deuterium (HDO) tracer and deuterium tracers evaporating from each oceanic region. The results suggest that although evaporative source temperature does affect the concentrations of the individual HDO tracers, differences in evaporative source do not explain the scatter in the roughly linear relationship between condensation temperature and isotope concentration. Offprint requests to: R Koster     

Annual oceanic heat transports computed from an atmospheric model

Mean annual estimates of the oceanic poleward energy transport are obtained using a global atmospheric general circulation model. The computations are carried out by using the atmospheric model to determine the net annual heat flux into the ocean on an 8° × 10° grid. Assuming no net annual heat storage, the annual surface heat fluxes into any zonal band must be accompanied by a corresponding meridional heat transport in the ocean. Heat is transported northward at all latitudes in the Atlantic Ocean and is transported poleward in both hemispheres in the Pacific Ocean. To account for the net northward transport throughout the Atlantic, heat is transported into the Atlantic from the Indian and Pacific basins. The results are compared with several recent direct and indirect calculations of oceanic meridional heat transports.     

Precipitation changes due to the introduction of eddy-resolved sea surface temperatures into simulations of the “Pasha Bulker” Australian east coast low of June 2007

Stratocumulus is often present offshore of Peru and northern Chile and exists at the top of a cool, moist and well-mixed marine boundary layer (MBL) under a marked temperature inversion maintained by large-scale subsidence. The subtropical MBL and stratocumulus has been the focus of many recent studies, but mid-latitude systems can exert a strong influence. However, this connection is not well established due to debatable model results and few in situ measurements south of 20°S. During a 2-week field campaign in August 2011 at Robinson Crusoe Island (~700 km offshore at 33.6°S), radiosondes were launched to observe the response of the MBL to mid-latitude synoptic forcing. During the observation period a broad, slow-moving cutoff low (COL) passed over the region. Other observations include COSMIC GPS, infrared satellite imagery, TRMM radar reflectivity, and operational radiosondes from the Chilean weather service. A numerical simulation is included to diagnose the synoptic features. The inversion prior to the COL was maintained and lifted above 5 km as the COL passed over the island. Soon after the COL center passed the island, the MBL top did not descend or reform near the surface and then deepen, but rather an inversion reformed at ~2.7 km. Using a variety of datasets, the height of the reformation of the inversion is related to the cloud top height of the scattered shallow cumulus convection under the COL, which coincides with the level of maximum convergence of the vertical velocity.     

Using a global climate model to evaluate the influences of water vapor, snow cover and atmospheric aerosol on warming in the Tibetan Plateau during the twenty-first century

We examine trends in climate variables and their interrelationships over the Tibetan Plateau using global climate model simulations to elucidate the mechanisms for the pattern of warming observed over the plateau during the latter half of the twentieth century and to investigate the warming trend during the twenty-first century under the SRES A1B scenario. Our analysis suggests a 4°C warming over the plateau between 1950 and 2100. The largest warming rates occur during winter and spring. For the 1961–2000 period, the simulated warming is similar to the observed trend over the plateau. Moreover, the largest warming occurs at the highest elevation sites between 1950 and 2100. We find that increases in (1) downward longwave radiation (DLR) influenced by increases in surface specific humidity (q), and (2) absorbed solar radiation (ASR) influenced by decreases in snow cover extent are, in part, the reason for a large warming trend over the plateau, particularly during winter and spring. Furthermore, elevation-based increases in DLR (influenced by q) and ASR (influenced by snow cover and atmospheric aerosols) appear to affect the elevation dependent warming trend simulated in the model.     

Nedagolla,a remelted iron meteorite

The Nedagolla meteorite was recognized by Axon to be a rare example of an iron which has been preterrestrially reheated to the point of melting. The dendrite secondary arms are spaced 200 μm apart, implying that Nedagolla solidified and cooled at ~0·02°C/sec. The presence of (Fe, Cr)_1-xS inclusions precipitated during cooling in the interdendritic regions and evidence of solute redistribution of Ni, Cr, Co, Si and P are consistent with this cooling rate. Such a rate indicates that Nedagolla cooled very near the surface of its parent ‘body’. Secondary microstructural features including the presence of isothermal taenite and minute phosphide precipitates, which have formed from the dissolution of primary phosphide material, indicate a later reheating to about 750°C for a period of several hours.     

Chemical and isotopic investigations into the origin of clay minerals from the Galapagos hydrothermal mounds field

A dark green authigenic nontronite is the major component of the Galapagos hydrothermal mounds field sediments. Oxygen isotopic compositions of the chemically purified, <0.2-μm fraction of the nontronitic clays indicate formation temperatures of 25° to 47°C, in contrast with measured in situ mounds temperatures of up to 15°C. Assuming an authigenic origin, the Fe-rich montmorillonite that dominates in the noncarbonate clay fraction of the surrounding pelagic ooze has isotopic formation temperatures of 27° to 39°C, compared with measured in situ temperatures of ca. 3.5° to 6.5°C. The higher isotopic formation temperatures calculated for the hydrothermal nontronite suggest either complex patterns of fluid circulation and nontronite precipitation presently within the mounds or a higher thermal history associated with rapid and episodic periods of deposition during the Holocene-Pleistocene. The apparent high isotopic temperature of the Fe-rich montmorillonite may reflect: (1) formation under hydrothermal conditions at spreading centers with subsequent dispersal by bottom currents, (2) a detrital origin of the mineral, or (3) a mixture of authigenic Fe-montmorillonite and detrital Al-montmorillonite in this region.     

Source contributions to Devonian granite magmatism near the Laurentian border, New Hampshire and Western Maine, USA

Radiogenic isotope data (initial Nd, Pb) and elemental concentrations for the Mooselookmeguntic igneous complex, a suite of mainly granitic intrusions in New Hampshire and western Maine, are used to evaluate petrogenesis and crustal variations across a mid-Paleozoic suture zone. The complex comprises an areally subordinate monzodiorite suite [377±2 Ma; ε_Nd (at 370 Ma)=−2.7 to −0.7; initial ²⁰⁷Pb/²⁰⁴Pb=15.56–15.58] and an areally dominant granite [370±2 Ma; ε_Nd (at 370 Ma)=−7.0 to −0.6; initial ²⁰⁷Pb/²⁰⁴Pb=15.55–15.63]. The granite contains meter-scale enclaves of monzodiorite, petrographically similar to but older than that of the rest of the complex [389±2 Ma; ε_Nd (at 370 Ma)=−2.6 to +0.3; initial ²⁰⁷Pb/²⁰⁴Pb 15.58, with one exception]. Other granite complexes in western Maine and New Hampshire are 30 Ma older than the Mooselookmeguntic igneous complex granite, but possess similar isotopic signatures.

Derivation of the monzodioritic rocks of the Mooselookmeguntic igneous complex most likely occurred by melting of Bronson Hill belt crust of mafic to intermediate composition. The Mooselookmeguntic igneous complex granites show limited correlation of isotopic variations with elemental concentrations, precluding any significant presence of mafic source components. Given overlap of initial Nd and Pb isotopic compositions with data for Central Maine belt metasedimentary rocks, the isotopic heterogeneity of the granites may have been produced by melting of rocks in this crustal package or through a mixture of metasedimentary rocks with magmas derived from Bronson Hill belt crust.

New data from other granites in western Maine include Pb isotope data for the Phillips pluton, which permit a previous interpretation that leucogranites were derived from melting heterogeneous metasedimentary rocks of the Central Maine belt, but suggest that granodiorites were extracted from sources more similar to Bronson Hill belt crust. Data for the Redington pluton are best satisfied by generation from sources in either the Bronson Hill belt or Laurentian basement. Based on these data, we infer that Bronson Hill belt crust was more extensive beneath the Central Maine belt than previously recognized and that mafic melts from the mantle were not important to genesis of Devonian granite magma.     

Archaeological implications of late-holocene channel changes on the middle gila river,arizona

Archival and stratigraphic data from the middle Gila River, Arizona, provide insight into late-Holocene floods and channel changes that affected the Hohokam, prehistoric agriculturalists who practiced canal irrigation 2000-500 B.P. The relationship between floods and channel changes during the past 300 years is characterized by an increased frequency of wide, braided channel conditions during episodes of increased large flood frequency. Ten radiocarbon ages from overbank deposits from the middle Gila River flood plain indicate that the frequency of large floods was relatively low 4000-1000 B.P. (¹⁴C years) but increased after 1000 B.P., a pattern supported by paleoflood slackwater sites in the upper Gila River watershed. This suggests that channel instability (shifts between narrow, single-channel and wide, braided planforms) increased after 1000 B.P. The latter part of the 4000-1000 B.P. period of relative channel stability corresponds in time with the development of Hohokam irrigation agriculture and expansion of canal systems, whereas increased fluvial dynamics beginning 1000 B.P. correlate (in time) to changes in settlement patterns during the Sedentary-Classic period transition. Higher large flood frequency may be responsible for the consolidation of canal systems and relocation of villages along the middle Gila River after 1000 B.P. and during the Classic period. Increased flooding and concomitant channel changes alone, however, do not adequately explain the collapse of the Hohokam pattern.