The age and hydrological history of Blue Lake, South Australia

Three sediment cores from the Blue Lake, a groundwater fed lake of volcanic origin in South Australia, have been investigated using a range of chemical and isotopic parameters. The C-14 activity of both the inorganic and organic carbon fractions of the sediment decreases monotonically with depth. The rate of change with depth is greater for near-surface samples with an apparent hiatus in sedimentation rate at about 7000 yr B.P. Estimates of age for the precipitated authigenic carbonate, after correction for dilution with dead carbon from the groundwater, agree well with calculated ages from the organic carbon fraction of the sediment. We suggest the lake is much older than previously proposed using other dating techniques. Variations in the delta(13)C and delta(18)O composition of the authigenic carbonate reflect different residence times of dissolved inorganic carbon and water in the lake caused by changes in the lake level. During periods of hydrologic steady-state, it is suggested that relative changes in the temperature of the lake can be seen in delta(18)O changes in authigenic carbonate. Blue Lake has been undergoing sedimentation for at least 28,000 years, including two lengthy periods of hydrologic steady state. The lake, for a large proportion of its existence, was much shallower while for the last 7000 years has maintained a level close to the present one. The influence of pumping from the lake for urban water supply during this century is reflected in the isotopic composition of carbonate in the sediment

Astronomical calibration of the late Oligocene through early Miocene geomagnetic polarity time scale (abstract of paper presented at AGU Fall Meeting, San Francisco, 8-12 Dec 2003)

At Ocean Drilling Program Site 1090 (subantarctic South Atlantic) benthic foraminiferal stable isotope data (from Cibicidoides and Oridorsalis) span the late Oligocene through the early Miocene (24-16 Ma) at a temporal resolution of 5 kyr. In the same time interval, a magnetic polarity stratigraphy can be unequivocally correlated to the geomagnetic polarity timescale (GPTS), thereby providing direct correlation of the isotope record to the GPTS. In an initial age model we use the newly derived age of the Oligocene/Miocene boundary of 23.0 Ma (Shackleton et al., 2000) revised to the new astronomical calculation of Laskar (2001) to recalculate the spline ages of Cande and Kent (1995). We then tune the site 1090 oxygen isotope record to obliquity, assuming a 7.2 kyr phase lag, using the new astronomic solution of Laskar (2001). In this manner we are able to refine the ages of polarity chrons C7n through C5Cn.1n. The new age model is consistent, within one obliquity cycle, with previously tuned ages for polarity chrons C7n to C6Bn from Shackleton et al. (2000), rescaled to the new astronomical solution of Laskar (2001). For early Miocene polarity chrons C6AAr through C5Cn, our obliquity-scale age model is the first to allow a direct calibration to the GPTS. The new ages are also close to, within one obliquity cycle, to those obtained by rescaling the Cande and Kent (1995) interpolation using the new age of the O/M boundary (23.0 Ma), and the same middle Miocene control point (14.8 Ma) used by Cande and Kent (1992). Thus we have confidence in the orbitally tuned age model and the refined GPTS calibration for the late Oligocene through early Miocene

The climatic consequences of a rare orbital anomaly at the Oligocene/Miocene boundary (23Ma)

The late Oligocene to early Miocene (20-26Ma) is characterized by a complex climate history that includes a stepped transition toward a cooler climate, intermittent partial glaciations of Antarctica, and a transient glaciation, Mi-1, at the Oligocene/Miocene (O/M) boundary. The Mi-1 event is characterized by an anomalous positive oxygen isotope excursion, the magnitude of which suggests the brief appearance of a full-scale ice-sheet on east Antarctica coupled with a few degrees of deep sea cooling. A recent breakthrough in extending the astronomical calibration back to ~30 Ma has provided a unique opportunity to compare the climatic events of the O/M transition relative to Earth’s orbital variations. Here, we present an uninterrupted 5.5 My long high-fidelity chronology of late Oligocene-early Miocene climate and ocean carbon chemistry that is based on a composite in the western equatorial Atlantic. This unique isotope record provides a rare window into how the climate system responded to orbital forcing uncer boundary conditions significantly different from those of the recent past. Time-series analyses reveal climate variance concentrated at all Milankovitch frequencies, but with unusually strong power at the primary eccentricity band periods of 406, 125, and 95-ky. These cycles, which represent in part glacial advances and retreats of Antarctic ice-sheets, show significantly enhanced variability over a 1.6 my period (21.4-23.0 Ma) of suspected low greenhouse gas levels as inferred from the carbon isotope record. Perhaps the most unexpected finding is that of a rare orbital congruence between eccentricity and obliquity that precisely corresponds with the Mi-1 glaciation. This orbital anomaly involves ~four consecutive cycles of low amplitude variance in obliquity (a node) during a period of low eccentricity. The net result is an extended period (~200ky) of low seasonality orbits, which allows for a step-like expansion of an Antarctic ice-sheet

Engineering condition assessment of cycling infrastructure: Cyclists’ perceptions of satisfaction and comfort

The UK National Cycle Network comprises 23,660 km of cycling and walking paths of which a significant percentage is dedicated off-road infrastructure. This represents a significant civil engineering infrastructure asset that currently contributes to the provision of asustainable transport mode option nationwide. Commuting and recreational cyclists have observed the often hazardous conditions on these paths. There are various simple measures that could be taken to improve the maintenance of such off-road paths. Reliance on walk-over surveys (direct visual inspection) and path users notifying the local authority may not be tackling maintenance in a resource efficient manner. The proposed inspectionmethod includes the use of an instrumented bicycle to examine cycle path condition through user perception of satisfaction and quality. A questionnaire was conducted toidentify the attributes of off-road cycling infrastructure people find most important in relation to their personal satisfaction. An exploratory factor analysis was undertaken on perception study data to elucidate the determination of the variables associated with perceived user satisfaction. The study has shown that people find maintenance issues to be of high importance, especially surface issues. From exploratory factor analysis of results, satisfaction has been found to load with comfort and safety. Field testing was then conducted using subjective user opinions and ojective vibration data. These results were then used to assist the creation of dedicated user perception based surface condition rating-scales

CHANGES IN THE DISTRIBUTION OF 513C OF DEEP WATER ZCO2 BETWEEN THE LAST GLACIATION AND THE HOLOCENE

International audienceCarbon isotopic measurements on the benthic foraminiferal genus Cibicidoides document that mean deep ocean 513C values were 0.46 ø/00 lower during the last glacial maximum than during the Late IIolocene. The geographic distribution of 5•3C was altered by changes in the production rate of nutrient-depleted deep water in the North Atlantic. During the Late Holocene, North Atlantic Deep Water, with high 513C values and low nutrient values, can be found throughout the Atlantic Ocean, and its effects can be traced into the southern ocean where it mixes with recirculated Pacific deep water. During the glaciation, decreased production of North Atlantic Deep Water allowed southern ocean deep water to penetrate farther into the North Atlantic and across low-latitude fracture zones into the eastern Atlantic. Mean southern ocean 5•3C values during the glaciation are lower than both North Atlantic and Pacific 5•3C values, suggesting that production of nutrient-depleted water occurred in both oceans during the glaciation. Enriched •3C values in shallow cores within the Atlantic Ocean indicate the existence of a nutrient-depleted water mass above 2000 m in this ocean

Climate response to orbital forcing across the Oligocene-Miocene boundary

Spectral analyses of an uninterrupted 5.5-million-year (My)-long chronology of late Oligocene-early Miocene climate and ocean carbon chemistry from two deep-sea cores recovered in the western equatorial Atlantic reveal variance concentrated at all Milankovitch frequencies. Exceptional spectral power in climate is recorded at the 406-thousand-year (ky) period eccentricity band over a 3.4-million-year period [20 to 23.4 My ago (Ma)] as well as in the 125- and 95-ky bands over a 1.3-million-year period (21.7 to 23.0 Ma) of suspected low greenhouse gas levels. Moreover, a major transient glaciation at the epoch boundary (~23 Ma), Mi-1, corresponds with a rare orbital congruence involving obliquity and eccentricity. The anomaly, which consists of low-amplitude variance in obliquity (a node) and a minimum in eccentricity, results in an extended period (~200 ky) of low seasonality orbits favorable to ice-sheet expansion on Antarctic

How fast did the ocean—atmosphere system run during the last deglaciation?

International audienc