Assessing Nitrogen Dynamics Throughout the Estuarine Landscape

Assessing nitrogen dynamics in the estuarine landscape is challenging given the unique effects of individual habitats on nitrogen dynamics. We measured net N₂ fluxes, sediment oxygen demand, and fluxes of ammonium and nitrate seasonally from five major estuarine habitats: salt marshes, seagrass beds (SAV), oyster reefs, and intertidal and subtidal flats. Net N₂ fluxes ranged from 332?±?116 μmol?N-N₂?m^?2?h^?1 from oyster reef sediments in the summer to ?67?±?4 μmol?N-N₂?m^?2?h^?1 from SAV in the winter. Oyster reef sediments had the highest rate of N₂ production of all habitats. Dissimilatory nitrate reduction to ammonium (DNRA) was measured during the summer and winter. DNRA was low during the winter and ranged from 4.5?±?3.0 in subtidal flats to 104?±?34 μmol?¹⁵NH ₄ ⁺ ?m^?2?h^?1 in oyster reefs during the summer. Annual denitrification, accounting for seasonal differences in inundation and light, ranged from 161.1?±?19.2 mmol?N-N₂?m^?2?year^?1 for marsh sediments to 509.9?±?122.7 mmol?N-N₂?m^?2?year^?1 for SAV sediments. Given the current habitat distribution in our study system, an estimated 28.3?×?10⁶?mol of N are removed per year or 76 % of estimated watershed nitrogen load. These results indicate that changes in the area and distribution of habitats in the estuarine landscape will impact ecosystem function and services.     

The Stanley unconformity in Lake Huron basin: evidence for a climate-driven closed lowstand about 7900 <Superscript>14</Superscript>C BP,with similar implications for the Chippewa lowstand in Lake Michigan basin

J.L. Hough in 1962 recognized an erosional unconformity in the upper section of early postglacial lake sediments in northwestern Lake Huron. Low-level Lake Stanley was defined at 70 m below present water surface on the basis of this observation, and was inferred to follow the Main Algonquin highstand and Post-Algonquin lake phases about 10 ¹⁴C ka, a seminal contribution to the understanding of Great Lakes history. Lake Stanley was thought to have overflowed from the Huron basin through the Georgian Bay basin and the glacio-isostatically depressed North Bay outlet to Ottawa and St. Lawrence rivers. For this overflow to have occurred, Hough assumed that post-Algonquin glacial rebound was delayed until after the Lake Stanley phase. A re-examination of sediment stratigraphy in northwestern Lake Huron using seismic reflection and new core data corroborates the sedimentological evidence of Hough’s Stanley unconformity, but not its inferred chronology or the level of the associated lowstand. Erosion of previously deposited sediment, causing the gap in the sediment sequence down to 70 m present depth, is attributed to wave erosion in the shoreface of the Lake Stanley lowstand. Allowing for non-deposition of muddy sediment in the upper 20 m approximately of water depth as occurs in the present Great Lakes, the inferred water level of the Stanley lowstand is repositioned at 50 m below present in northwestern Lake Huron. The age of this lowstand is about 7.9 ± 0.3¹⁴C ka, determined from the inferred ¹⁴C age of the unconformity by radiocarbon-dated geomagnetic secular variation in six new cores. This relatively young age shows that the lowstand defined by Hough’s Stanley unconformity is the late Lake Stanley phase of the northern Huron basin, youngest of three lowstands following the Algonquin lake phases. Reconstruction of uplift histories for lake level and outlets shows that late Lake Stanley was about 25–30 m below the North Bay outlet, and about 10 m below the sill of the Huron basin. The late Stanley lowstand was hydrologically closed, consistent with independent evidence for dry regional climate at this time. A similar analysis of the Chippewa unconformity shows that the Lake Michigan basin also hosted a hydrologically closed lowstand, late Lake Chippewa. This phase of closed lowstands is new to the geological history of the Great Lakes. This is the ninth in a series of ten papers published in this special issue of Journal of Paleolimnology. These papers were presented at the 47th Annual Meeting of the International Association for Great Lakes Research (2004), held at the University of Waterloo, Waterloo, Ontario, Canada. P.F. Karrow and C.F.M Lewis were guest editors of this special issue.     

Macroinvertebrate Production in the Submerged Aquatic Vegetation of the Mobile–Tensaw Delta: Effects of an Exotic Species at the Base of an Estuarine Food Web

This study, conducted in 1997, reports the first estimates of the impacts of the proliferation of an exotic submerged aquatic vegetation (SAV) species (Myriophyllum spicatum) on macroinvertebrate production via comparisons with two co-occurring native SAV species (Heteranthera dubia and Vallisneria americana) in the tide-influenced Mobile–Tensaw Delta (located in the north-central Gulf of Mexico, 30°40′ N, 87°55′ W). Production of macroinvertebrates was greatest on M. spicatum and H. dubia and least on V. americana. The key determinant of these differences was a greater abundance of amphipods (Gammarus mucronatus) found on the leaves of M. spicatum and H. dubia. Macroinvertebrate production on M. spicatum was three times greater (>1 kg m^?2 year^?1) than on either of the native SAV species. No-choice palatability tests showed that these differences could not be attributed to differences in invertebrate grazing on these plants. Instead, it is probable that the high production within the structurally complex M. spicatum and H. dubia was the result of reduced predator foraging efficiency. If true, then the presence of this exotic species probably renders this elevated production inaccessible to most high-order predators.     

青藏高原羌塘盆地东部鄂尔陇巴组火山岩的锆石SHRIMP U-Pb年龄及其地质意义

藏北羌塘盆地东部地区三叠系巴贡组之上沉积超覆了一套火山岩-火山碎屑岩. 该套地层下部玄武岩的锆石SHRIMP U-Pb年龄明显可以分为2组,一组年龄相对年轻 （220.4Ma±2.3Ma）,代表了羌塘东部地区鄂尔陇巴组火山岩的喷发年龄;另外一组 年龄相对较老（241.4Ma±3.6Ma）,可能与中三叠世羌塘地区普遍存在的碰撞隆升事 件有关。羌塘中生代（晚三叠世—早白垩世）盆地演化早期的沉积作用经历了由陆相 至海相的超覆过程,沉积超覆作用从冲洪积相开始,伴随着岩浆侵入、火山爆发及火 山碎屑沉积作用,总体上表现为一个向上变深的海侵序列,显示裂谷盆地的特征。     

The GTC: An Advanced 10m Telescope for the ORM

The Gran Telescopio CANARIAS (GTC) is a high performance 10-m class telescope whose construction has been promoted by the IAC (Instituto de Astrofísica de Canarias). It will be installed at the Roque de los Muchachos Observatory (ORM), in the island of La Palma. First light is planned for end-2002. The key science drivers for the project are image quality, operational efficiency and reliability, as emphasized in the Conceptual Design Document which was finished in mid-97. The Preliminary Design is now proceeding on all aspects of the project. The GTC Project is presently funded at the 70% level by Spain. The scientific drives behind the GTC project are described here, as well as the current technical, managerial, and operational baseline. This revised version was published online in July 2006 with corrections to the Cover Date.     

Spherical Slepian functions and the polar gap in geodesy

The estimation of potential fields such as the gravitational or magnetic potential at the surface of a spherical planet from noisy observations taken at an altitude over an incomplete portion of the globe is a classic example of an ill-posed inverse problem. We show that this potential-field estimation problem has deep-seated connections to Slepian's spatiospectral localization problem which seeks bandlimited spherical functions whose energy is optimally concentrated in some closed portion of the unit sphere. This allows us to formulate an alternative solution to the traditional damped least-squares spherical harmonic approach in geodesy, whereby the source field is now expanded in a truncated Slepian function basis set. We discuss the relative performance of both methods with regard to standard statistical measures such as bias, variance and mean squared error, and pay special attention to the algorithmic efficiency of computing the Slepian functions on the region complementary to the axisymmetric polar gap characteristic of satellite surveys. The ease, speed, and accuracy of our method make the use of spherical Slepian functions in earth and planetary geodesy practical.     

Vegetation changes in the Jornada Basin from 1858 to 1998

Notes made by land surveyors in 1858 were utilized to estimate cover of grasses and shrubs on the Jornada Experimental Range (JER) and the Chihuahuan Desert Range Research Center (CDRRC) in the northern Chihuahuan Desert in southern New Mexico, USA. Portions of these areas have been previously assessed for historical vegetation dynamics but the entire 84,271 ha assessed in the 19th century has not been examined in total. In 1858, fair to very good grass cover occurred on 98% and 67% of the JER and CDRRC, respectively. Shrubs were present throughout both properties but 45% of the JER and 18% of the CDRRC were shrub free. Reconnaissance surveys, made to determine carrying capacity for livestock were made in 1915–1916 and 1928–1929 on the JER and in 1938 on the CDRRC, show that shrubs had made large increases in area occupied at the time of the surveys. Vegetation type maps were made of both properties in 1998. Mesquite (Prosopis glandulosa) was the primary dominant on 59% of the JER in 1998 and creosotebush (Larrea tridentata) was the primary dominant on 27% of the area. On the CDRRC mesquite and creosotebush were primary dominants on 37% and 46% of the area, respectively. Grass cover has decreased greatly with the increase in shrubs and only shrub control efforts have maintained the once abundant black grama (Bouteloua eriopoda) as a primary dominant on 1% or less of the area on both properties.     

In Situ Observations of Solar Wind Stream Interface Evolution

The heliocentric orbits of the two STEREO satellites are similar in radius and ecliptic latitude, with separation in longitude increasing by about 45° per year. This arrangement provides a unique opportunity to study the evolution of stream interfaces near 1 AU over time scales of hours to a few days, much less than the period of a Carrington rotation. Assuming nonevolving solar wind sources that corotate with the Sun, we calculated the expected time and longitude of arrival of stream interfaces at the Ahead observatory based on the in situ solar wind speeds measured at the Behind observatory. We find agreement to within 5° between the expected and actual arrival longitude until the spacecraft are separated by more than 20° in heliocentric inertial longitude. This corresponds to about one day between the measurement times. Much larger deviations, up to 25° in longitude, are observed after 20° separation. Some of the deviations can be explained by a latitude difference between the spacecraft, but other deviations most likely result from evolution of the source region. Both remote and in situ measurements show that changes at the source boundary can occur on a time scale much shorter than one solar rotation. In 32 of 41 cases, the interface was observed earlier than expected at STEREO/Ahead.