Growth reduction in the soft-shell clam Mya arenaria from a heavily oiled lagoon in Chedabucto Bay,Nova Scotia

Growth rates were determined in Mya arenaria collected from a heavily oiled lagoon lagoon shortly after the Arrow disaster (Nova Scotia, Canada). The clams, ranging in length from 30 to 50 mm, were compared with a sample collected from the same location 9 years later. Both samples from Janvrin Lagoon were compared with Mya arenaria collected in 1979 from a non-oiled neighbouring lagoon on Potato Island. Annual internal growth bands seen in shell cross-sections rather than shell external annuli were used for age determination. The 1979 Janvrin Lagoon sample, compared with the 1970 sample, suffered growth retardation in tissue and shell growth by 2–5 years.     

The modern distribution of chironomid sub-fossils (Insecta: Diptera) in the Sierra Nevada, California: Potential for paleoclimatic reconstructions

Surface lake sediment was recovered from 57 lakes along an elevation gradient in the central, eastern Sierra Nevada of California. The surface sediment was analysed for subfossil chironomid remains in order to assess the modern distribution of chironomids in the region. The lakes sampled for the calibration dataset were between 2.0 and 40.0 m in depth, spanned an altitudinal gradient of 1360 m and a surface water temperature gradient of approximately 14 °C. Redundancy analysis (RDA) identified that five of the measured environmental variables – surface water temperature, elevation, depth, strontium, particulate organic carbon – accounted for a statistically significant amount of the variance in chironomid community composition. Quantitative transfer functions, based on weighted-averaging (WA), partial least squares (PLS) and weighted-averaging partial least squares (WA-PLS), were developed to estimate surface water temperature from the chironomid assemblages. The best model was a WA model with classical deshrinking, which had a relatively high coefficient of determination (r² = 0.73), low root mean square error of prediction (RMSEP = 1.2 °C) and a low maximum bias (0.90 °C). The results from this study suggest that robust quantitative estimates of past surface water temperature can be derived from the application of these models to fossil chironomid assemblages preserved in late-Quaternary lake sediment in this region.     

Palaeomagnetic and K—Ar age studies of a 6 km-thick Cretaceous section from the Chilean Andes

Summary. Thirty-six palaeomagnetic sampling sites distributed within 6000 m of dominantly andesitic flows and tuffs of Cretaceous age from the La Serena area, Chile confirm the normal polarity bias of the Cretaceous period. Af, thermal and limited chemical demagnetization techniques have been used in testing the stability of the remanent magnetization isolated in samples from these sites. A positive fold test in the Quebrada Marquesa Formation, the second lowest in the stratigraphic pile, confirms that the magnetization isolated is pre-Tertiary in age. Ages calculated by the K–Ar whole rock method however, appear to have been variably up-dated probably due to argon loss caused by Cretaceous–Tertiary intrusives. Thermal and hydrothermal effects of these intrusions have probably reset the magnetization in the youngest formation of the volcanic pile. A composite palaeomagnetic pole calculated from the 30 site poles of the three lower formations (209° E, 81° S, A₉₅= 4½°), is in good agreement with mid to Late Cretaceous poles derived from rock units of the stable platform of South America. The use of Andean–Caribbean palaeomagnetic data however, to resolve small time-dependent polar shifts within the Cretaceous and thus to estimate the time of opening of the south Atlantic is questioned. Many of the Andean–Caribbean Cretaceous poles appear to have been affected by local tectonic rotation.     

A 2000 year midge-based paleotemperature reconstruction from the Canadian Arctic archipelago

A lake sediment core recovered from Lake V57 on Victoria Island, Nunavut, Canada, spanning the last 2000 years, was analyzed for sub-fossil midge remains and organic-matter content (estimated by loss-on-ignition (LOI)). Significant changes in midge community composition occurred during the last 2000 years, with a distinct midge community appearing after 1600 AD. The chironomid community between 0 and 1600 AD was dominated by Heterotrissocladius, Tanytarsus, Abiskomyia, and Paracladius. At approximately 1600 AD, Heterotrissocladius decreased in relative abundance and taxa such as Corynocera ambigua, C. oliveri, Psectrocladius sordidellus type, and Pentanneurini increased in relative abundance. Previously published midge-based inference models for average July air temperature (AJAT) and summer surface–water temperature (SSWT) were applied to the subfossil midge stratigraphy. The AJAT reconstruction indicates relatively cool conditions existed between 1100 and 1600 AD, with exceptional warming occuring after 1600 AD, as lake productivity inferred from organic-matter content increased concomitantly with midge-inferred AJAT and SSWT. The cooler conditions between 1200 and 1600 AD, and the pattern of warming over recent centuries inferred from Lake V57 is broadly consistent with temperature-sensitive biogenic silica records from other sites in the central Canadian Arctic and the treeline zone to the south suggesting a regionally synchronous response to climate forcing.

TRENDS IN THE VARIABILITY OF SURFACE AIR TEMPERATURE,CANADIAN ARCTIC

A measure of temperature variability was calculated using annual temperature records obtained from 40 stations in the Canadian Arctic, records beginning between 1929 and 1958, and concluding for this study in 1983. Long-term trends in the frequency of extreme temperature conditions were obtained for eight regions in the Arctic. The results indicate that temperatures in the Canadian Arctic, as a whole, have not become more variable. In the last 15 to 20 years of the period considered in this study, the occurrence of extreme temperature conditions has increased only in three of the eight regions.     

The Middle Jurassic Camaraca Formation, Arica, Chile: palaeomagnetism, K—Ar age dating and tectonic implications

Summary. Palaeomagnetic and K–Ar whole rock age studies are reported from samples of the Camaraca Formation exposed near Arica (18.6° S, 70.3° W), Chile. The Camaraca Formation is composed of andesites and interbedded marine shales which yield a fauna placing the formation in the Bajocian–Callovian stages of the Jurassic. Our sampling sites span a strati-graphic thickness of 2 km and yield a reversal stratigraphy of: N–R–N. The K–Ar isochron method, applied to least altered samples from the formation, gives a sharply defined age of 157 Myr which is in agreement with the palaeontologically assigned age of the formation. Normal and reversed directions of remanent magnetization, isolated by of and thermal de-magnetization methods, are statistically antiparallel. The pole position, computed from these directions of magnetization, is at 010° E, 70%0 S ( A ₉₅= 6.0°). This pole position, when compared with the well-studied Chon Alice Formation of Argentina, suggests that the sampling region has under-gone a 28°± 28° counter-clockwise rotation about a local vertical axis. The large uncertainty (between 0° and 56°) in this estimate is due to the large scatter in the South American reference data. When compared with African Jurassic palaeopoles, with allowance made for the opening of the South Atlantic, a counter-clockwise rotation of 44°± 14° is indicated.     

Role of methane clathrates in past and future climates

Methane clathrates are stable at depths greater than about 200 m in permafrost regions and in ocean sediments at water depths greater than about 250 m, provided bottom waters are sufficiently cold. The thickness of the clathrate stability zone depends on surface temperature and geothermal gradient. Average stability zone thickness is about 400 m in cold regions where average surface temperatures are below freezing, 500 m in ocean sediments, and up to 1,500 m in regions of very cold surface temperature (<-15 °C) or in the deep ocean. The concentration of methane relative to water within the zone of stability determines whether or not clathrate will actually occur. The geologic setting of clathrate occurrences, the isotopic composition of the methane, and the methane to ethane plus propane ratio in both the clathrates and the associated pore fluids indicate that methane in clathrates is produced chiefly by anaerobic bacteria. Methane occurrences and the organic carbon content of sediments are the bases used to estimate the amount of carbon currently stored as clathrates. The estimate of about 11,000 Gt of carbon for ocean sediments, and about 400 Gt for sediments under permafrost regions is in rough accord with an independent estimate by Kvenvolden of 10,000 Gt.The shallowness of the clathrate zone of stability makes clathrates vulnerable to surface disturbances. Warming by ocean flooding of exposed continental shelf, and changes in pressure at depth, caused, for example, by sea-level drop, destabilize clathrates under the ocean, while ice-cap growth stabilizes clathrates under the ice cap. The time scale for thermal destabilization is set by the thermal properties of sediments and is on the order of thousands of years. The time required to fix methane in clathrates as a result of surface cooling is much longer, requiring several tens of thousands of years. The sensitivity of clathrates to surface change, the time scales involved, and the large quantities of carbon stored as clathrate indicate that clathrates may have played a significant role in modifying the composition of the atmosphere during the ice ages. The release of methane and its subsequent oxidation to carbon dioxide may be responsible for the observed swings in atmospheric methane and carbon dioxide concentrations during glacial times. Because methane and carbon dioxide are strong infrared absorbers, the release and trapping of methane by clathrates contribute strong feedback mechanisms to the radiative forcing of climate that results from earth's orbital variations.Gordon J. MacDonald is Vice President and Chief Scientist of The MITRE Corporation, 7525 Colshire Drive, McLean, VA 22102.     

Constraints on the evolution of the Taranaki volcanoes,New Zealand,based on aeromagnetic data

Aeromagnetic anomalies measured over the three relict andesite volcanoes (Paritutu, Kaitake and Pouakai) in Taranaki are largely dominated by topographic effects. Three-dimensional magnetic modelling, well constrained by both contrasting levels of exposure and previous gravity models, shows that the bulk magnetization of the andesite edifice and edifice core of both Kaitake and Pouakai is 2.5–3.5 A m^-1 in a direction close to that of the present earth's field. However, the large andesite dyke/stock complexes below all three edifices and a localized area of the Kaitake edifice directly above the centre of the subedifice complex have anomalously low bulk magnetizations of 0.3–1.0 A m^-1 in the same direction. These subsurface complexes represent dyke injection from magma chambers situated in the basement, probably below 6 km depth. Here, we deduce that heat from these magma chambers drove hydrothermal convection systems causing widespread demagnetization, especially in the subedifice complexes, but also locally within the edifices themselves. A lesser degree of demagnetization at Pouakai, the youngest of the three volcanoes, may indicate a shorter and consequently more intense period of activity at this centre.     

Dynamics of the gas flux from shallow gas hydrate deposits: interaction between oily hydrate bubbles and the oceanic environment

Decomposition of methane hydrates on the continental margins is a potentially significant source of atmospheric methane, but the input depends upon the poorly understood fate of the hydrocarbon bubbles rising from the sea floor. During a field trip to the Gulf of Mexico, three different seepages were imaged and analyzed. Three different imaging techniques were tried (side, front, and back illumination), of which back illumination produced the best results. The images were analyzed and the size-dependent bubble distribution, mass flux, and rise speeds determined. The total observed gas flux was 62.3×10⁻³ mol s⁻¹, primarily methane, of which a single vent produced seven times the next largest vent. Of this major vent, 50% of the bubble mass was contained in the largest bubbles, r>5500 μm. The vertical velocities demonstrated that these bubbles were heavily contaminated with oil, which was also corroborated by bubble shape and oscillation observations.