Insights on the events surrounding the final drainage of Lake Ojibway based on James Bay stratigraphic sequences

Deglaciation of the James Bay region was highly dynamic, with the occurrence of ice (Cochrane) readvances into glacial Lake Ojibway around final deglaciation time, which culminated with the drainage of Ojibway waters into Hudson Bay and subsequent incursion of the Tyrrell Sea at ～8 ka. Renewed interest on these events comes from the possible link between the drainage of the ice-dammed Lake Agassiz-Ojibway and a major climate deterioration known as the 8.2-ka cooling event. Recent glaciological modeling suggests that this drainage may have occurred subglacially, a mechanism that can accommodate more than one lake discharge, as suggested by marine records. The exact number and timing of drainage events, as well as location of the lake discharge pathway(s) remain, however, largely unconstrained. Here we focus on the events that led to the drainage of Lake Ojibway by documenting late-glacial sedimentary sequences located east of James Bay. Our investigations indicate that the deglacial sequence consists of a readvance till, extensive Ojibway rhythmites, and thick marine sediments. The glaciolacustrine and marine units are separated by a 60 cm-thick horizon composed of laminated silt beds containing rounded clay balls and disseminated clasts resulting from the abrupt drainage of the lake. Radiocarbon dating of marine fossils lying above the drainage horizon indicates that the glaciolacustrine episode ended around 8128–8282 cal yr BP. Micropaleontological analyses reveal that freshwater ostracods (Candona sp.) and marine microfossils (foraminifers, dinocysts) occur together in the upper part of the Ojibway sediments. Analysis of oxygen isotopes (δ¹⁸O) of ostracods and foraminifers originating from the same stratigraphic position show highly contrasting values that suggest possible subglacial exchanges between Lake Ojibway and Tyrrell Sea waters prior to the final drainage event. The complexity of the deglacial events is further indicated by radiocarbon dating of marine shells retrieved from a Cochrane till that suggests that the last ice readvance occurred almost simultaneously with the final lake discharge. These results bring additional constraints on the drainage mechanism of the coalesced Lake Agassiz-Ojibway and indicate that the James Bay region formed an important drainage pathway for meltwaters at the end of the last deglaciation.     

Diurnal timing of warmer air under climate change affects magnitude,timing and duration of stream temperature change

Stream temperature will be subject to changes because of atmospheric warming in the future. We investigated the effects of the diurnal timing of air temperature changes – daytime warming versus nighttime warming – on stream temperature. Using the physically based model, Heat Source, we performed a sensitivity analysis of summer stream temperatures to three diurnal air temperature distributions of +4 °C mean air temperature: i) uniform increase over the whole day, ii) warmer daytime and iii) warmer nighttime. The stream temperature model was applied to a 37‐km section of the Middle Fork John Day River in northeastern Oregon, USA. The three diurnal air temperature distributions generated 7‐day average daily maximum stream temperatures increases of approximately +1.8 °C ± 0.1 °C at the downstream end of the study section. The three air temperature distributions, with the same daily mean, generated different ranges of stream temperatures, different 7‐day average daily maximum temperatures, different durations of stream temperature changes and different average daily temperatures in most parts of the reach. The stream temperature changes were out of phase with air temperature changes, and therefore in many places, the greatest daytime increase in stream temperature was caused by nighttime warming of air temperatures. Stream temperature changes tended to be more extreme and of longer duration when driven by air temperatures concentrated in either daytime or nighttime instead of uniformly distributed across the diurnal cycle. Copyright © 2012 John Wiley & Sons, Ltd.     

Impact of road‐generated storm runoff on a small catchment response

The impact of road‐generated runoff on the hydrological response of a zero‐order basin was monitored for a sequence of 24 storm events. The study was conducted in a zero‐order basin (C1; 0·5ha) with an unpaved mountain road; an adjacent unroaded zero‐order basin (C2; 0·2 ha) with similar topography and lithology was used to evaluate the hydrological behaviour of the affected zero‐order basin prior to construction of the road. The impact of the road at the zero‐order basin scale was highly dependent on the antecedent soil‐moisture conditions, total storm precipitation, and to some extent rainfall intensity. At the beginning of the monitoring period, during dry antecedent conditions, road runoff contributed 50% of the total runoff and 70% of the peak flow from the affected catchment (C1). The response from the unroaded catchment was almost insignificant during dry antecedent conditions. As soil moisture increased, the road exerted less influence on the total runoff from the roaded catchment. For very wet conditions, the influence of road‐generated runoff on total outflow from the roaded catchment diminished to only 5·4%. Both catchments, roaded and unroaded, produced equivalent amount of outflow during very wet antecedent conditions on a unit area basis. The lag time between the rainfall and runoff peaks observed in the unroaded catchment during the monitoring period ranged from 0 to 4 h depending on the amount of precipitation and antecedent conditions, owing mainly to much slower subsurface flow pathways in the unroaded zero‐order basin. In contrast, the lag time in the roaded zero‐order basin was virtually nil during all storms. Copyright © 2009 John Wiley & Sons, Ltd.     

Analyzing Radar Meteor Trail Echoes using the Fresnel Transform Technique: A Signal Processing Viewpoint

Several techniques have been proposed for measuring speeds of meteoroids observed using radars. A recent technique involves the use of Fresnel transforms to accurately determine the speed of a meteoroid producing the trail. We follow a numerical modeling approach to analyze this technique in detail. Our studies indicate that high sensitivity to background noise levels might be a possible shortcoming of the Fresnel transform method. A matched filtering approach is presented as an alternative to alleviate this sensitivity to the noise problem. Performance of the two techniques is compared using numerical modeling and data from a 30 MHz radar.     

Some bianchi type II universes with special free-gravitational fields

The paper considers a homogeneous Bianchi type II universe. Under each of the conditionsC _hijk C ^hijk =0 and^* C _hijk C ^hijk =0 different types of models have been studied and their physical and kinematical properties have been discussed.     

Line-of-Sight Anisotropies in the Cosmic Dawn and Epoch of Reionization 21-cm Power Spectrum

The line-of-sight direction in the redshifted 21-cm signal coming from the cosmic dawn and the epoch of reionization is quite unique in many ways compared to any other cosmological signal. Different unique effects, such as the evolution history of the signal, non-linear peculiar velocities of the matter etc. will imprint their signature along the line-of-sight axis of the observed signal. One of the major goals of the future SKA-LOW radio interferometer is to observe the cosmic dawn and the epoch of reionization through this 21-cm signal. It is thus important to understand how these various effects affect the signal for its actual detection and proper interpretation. For more than one and half decades, various groups in India have been actively trying to understand and quantify the different line-of-sight effects that are present in this signal through analytical models and simulations. In many ways the importance of this sub-field under 21-cm cosmology have been identified, highlighted and pushed forward by the Indian community. In this article, we briefly describe their contribution and implication of these effects in the context of the future surveys of the cosmic dawn and the epoch of reionization that will be conducted by the SKA-LOW.     

Modelling the 21-cm Signal from the Epoch of Reionization and Cosmic Dawn

Studying the cosmic dawn and the epoch of reionization through the redshifted 21-cm line are among the major science goals of the SKA1. Their significance lies in the fact that they are closely related to the very first stars in the Universe. Interpreting the upcoming data would require detailed modelling of the relevant physical processes. In this article, we focus on the theoretical models of reionization that have been worked out by various groups working in India with the upcoming SKA in mind. These models include purely analytical and semi-numerical calculations as well as fully numerical radiative transfer simulations. The predictions of the 21-cm signal from these models would be useful in constraining the properties of the early galaxies using the SKA data.     

Probing Individual Sources during Reionization and Cosmic Dawn using Square Kilometre Array HI 21-cm Observations

Detection of individual luminous sources during the reionization epoch and cosmic dawn through their signatures in the HI 21-cm signal is one of the direct approaches to probe the epoch. Here, we summarize our previous works on this and present preliminary results on the prospects of detecting such sources using the SKA1-low experiment. We first discuss the expected HI 21-cm signal around luminous sources at different stages of reionization and cosmic dawn. We then introduce two visibility based estimators for detecting such signals: one based on the matched filtering technique and the other relies on simply combing the visibility signal from different baselines and frequency channels. We find that the SKA1-low should be able to detect ionized bubbles of radius \(R_{\mathrm {b}} \gtrsim 10\) Mpc with ～100 h of observations at redshift z～8 provided that the mean outside neutral hydrogen fraction \(\mathrm {x}_{\text {HI}} \gtrsim 0.5\). We also investigate the possibility of detecting HII regions around known bright QSOs such as around ULASJ1120+0641 discovered by Mortlock et al. (Nature 474, 7353 (2011)). We find that a 5σ detection is possible with 600 h of SKA1-low observations if the QSO age and the outside x_HI are at least ～2×10⁷ Myr and ～0.2 respectively. Finally, we investigate the possibility of detecting the very first X-ray and Ly- α sources during the cosmic dawn. We consider mini-QSOs like sources which emits in X-ray frequency band. We find that with a total ～ 1000 h of observations, SKA1-low should be able to detect those sources individually with a ～ 9σ significance at redshift z=15. We summarize how the SNR changes with various parameters related to the source properties.