Oscillations of levels and cool phases of the Laurentian Great Lakes caused by inflows from glacial Lakes Agassiz and Barlow-Ojibway

Two distinct episodes of increased water flux imposed on the Great Lakes system by discharge from upstream proglacial lakes during the period from about 11.5 to 8 ka resulted in expanded outflows, raised lake levels and associated climate changes. The interpretation of these major hydrological and climatic effects, previously unrecognized, is mainly based on the evidence of former shorelines, radiocarbon-dated shallow-water sediment sequences, paleohydraulic estimates of discharge, and pollen diagrams of vegetation change within the basins of the present Lakes Superior, Michigan, Huron, Erie and Nipissing. The concept of inflow from glacial Lake Agassiz adjacent to the retreating Laurentide Ice Sheet about 11–10 and 9.5–8.5 ka is generally supported, with inflow possibly augmented during the second period by backflooding of discharge from glacial Lake Barlow-Ojibway.Although greater dating control is needed, six distinct phases can be recognized which characterize the hydrological history of the Upper Great Lakes from about 12 to 5 ka; 1) an early ice-dammed Kirkfield phase until 11.0 ka which drained directly to Ontario basin; 2) an ice-dammed Main Algonquin phase (11.0–10.5 ka) of relatively colder surface temperature with an associated climate reversal caused by greater water flux from glacial Lake Agassiz; 3) a short Post Algonquin phase (about 10.5–10.1 ka) encompassing ice retreat and drawdown of Lake Algonquin; 4) an Ottawa-Marquette low phase (about 10.1–9.6 ka) characterized by drainage via the then isostatically depressed Mattawa-Ottawa Valley and by reduction in Agassiz inflow by the Marquette glacial advance in Superior basin; 5) a Mattawa phase of high and variable levels (about 9.6–8.3 ka) which induced a second climatic cooling in the Upper Great Lakes area. Lakes of the Mattawa phase were supported by large inflows from both Lakes Agassiz and Barlow-Ojibway and were controlled by hydraulic resistance at a common outlet — the Rankin Constriction in Ottawa Valley — with an estimated base-flow discharge in the order of 200000 m³s^–1. 6) Lakes of the Nipissing phase (about 8.3–4.7 ka) existed below the base elevation of the previous Lake Mattawa, were nourished by local precipitation and runoff only, and drained by the classic North Bay outlet to Ottawa Valley.Geological Survey of Canada Contribution 42488.This is the twelfth of a series of papers to be published by this journal that was presented in the paleolimnology sessions organized by R. B. Davis and H. Löffler for the XIIth Congress of the International Union for Quaternary Research (INQUA), which took place in Ottawa, Canada in August 1987. Dr. Davis is serving as guest editor of this series.     

Regional-Scale Heat and Water Vapour Fluxes in an Agricultural Landscape: An Evaluation of CBL Budget Methods at OASIS

This paper evaluates convective boundary layer (CBL) budget methods as a tool for estimating regionally averaged sensible and latent heat fluxes for the study region used in OASIS (Observations at Several Interacting Scales). This is an agricultural region of mixed cropping and grazing extending about 100 km west of the town of Wagga Wagga, NSW, Australia.The analysis proceeds in three stages: first, a simpleone-dimensional model of the well-mixed layer (the CBL slab model), forced with measurements of the surface heat and evaporation fluxes, is evaluated by comparing measured and modelled CBL temperature, humidity and depths. A comparison of several entrainment schemes shows that a simple model, where the entrainment kinetic energy is parameterised as a fraction (₃) of the surface sensible heat flux, works well if is set to 0.5. Second, the slab model is coupled to a Penman–Monteith model of surface evaporation to predict regional scale evaporation and thence heat fluxes. Finally, the integral CBL budget approach, which is an inverse method using theone-dimensional slab model, is used to infer regional heat and evaporation fluxes from measured time series of CBL temperature and humidity.We find that the simple CBL slab model works reasonably well for predicting CBL depth and very well for CBL temperature, especially if approximate estimates of subsidence velocity and warming due to advection are included. Regional sensible heat fluxes estimated from the integral CBL method match those measured, although the method is very sensitive to measurement errors. Measurement-model differences were larger for short integration times, because the well-mixed assumptions are violated at particular times of the day. The corollary is that `whole-day' (0530–1530 h) estimates are in reasonable agreement with measured values. Integral methods could not be used to infer the regional evaporation flux directly because CBL humidity profiles were complex and often not well mixed until mid-afternoon. We recommend that regional evaporation fluxes be predicted either from a coupled Penman–Monteith – CBL slab model, or inferred as a residual term from estimates of the regionally averaged available energy and sensible heat flux. Furthermore, we show that inferring fluxes via integral methods will always be difficult when the scalar concentrations have either a large surface source and free atmosphere sink (in the case of water vapour and methane), or a large surface sink and upper level source (in the case of CO₂).     

CARBON AND NITROGEN ABUNDANCES IN SELECTED ACHONDRITES

Total carbon and nitrogen contents are reported for four howardites, three enstatite achondrites and one hypersthene achondrite. The mean value for eucrites is 650 ppm C and howardites 1440 ppm C. The Haraiya eucrite has an anomalously high carbon content The nitrogen contents range between 24 and 66 ppm N. The mean N value for eucrites is 36 ppm N and for the howardites 51 ppm N.     

Bedload sediment transport dynamics in a macrotidal embayment, and implications for export to the southern Great Barrier Reef shelf

Keppel Bay is a macrotidal embayment on a tectonically stable, tropical coast, which links the Fitzroy River with the Great Barrier Reef continental shelf. Estuaries and deltas act as conduits between catchments and inner shelf environments. Therefore, understanding sediment transport pathways in these complex systems is essential for the management of ecosystems such as coral-reefs that are potentially vulnerable to enhanced river sediment loads. Furthermore, the morphology and sediment dynamics of subtidal sand ridges and dunes are relatively poorly characterised in macrotidal estuaries, particularly in turbid, episodic systems such as the Fitzroy River and Keppel Bay. Our sedimentological analysis of seabed samples, shear-stress modelling and three-dimensional acoustic imaging reveals that Keppel Bay is a mixed wave- and tide-dominated estuarine system. Areas of sediment starvation and shoreward transport characterise the offshore zone, whereas a complex of both active and relict tidal sand ridges, and associated subaqueous dunes, dominate the relatively protected southern Keppel Bay. Transport within this region is highly dynamic and variable, with ebb-dominated sediment transport through tidal channels into the outer bay where there is a switch to wave-dominated shoreward transport. Ultimately, bedload sediments appear to be reworked back inshore and to the north, and are gradually infilling the bedrock-defined embayment. Our characterisation of the Keppel Bay system provides a detailed example of the physiography of the seaward portion of a tide-dominated system, and shows that sediment transport in these areas is influenced by a variable hydrodynamic regime as well as relict channels and bedrock topography.