Light-Dependant Biostabilisation of Sediments by Stromatolite Assemblages

For the first time we have investigated the natural ecosystem engineering capacity of stromatolitic microbial assemblages. Stromatolites are laminated sedimentary structures formed by microbial activity and are considered to have dominated the shallows of the Precambrian oceans. Their fossilised remains are the most ancient unambiguous record of early life on earth. Stromatolites can therefore be considered as the first recognisable ecosystems on the planet. However, while many discussions have taken place over their structure and form, we have very little information on their functional ecology and how such assemblages persisted despite strong eternal forcing from wind and waves. The capture and binding of sediment is clearly a critical feature for the formation and persistence of stromatolite assemblages. Here, we investigated the ecosystem engineering capacity of stromatolitic microbial assemblages with respect to their ability to stabilise sediment using material from one of the few remaining living stromatolite systems (Highborne Cay, Bahamas). It was shown that the most effective assemblages could produce a rapid (12–24 h) and significant increase in sediment stability that continued in a linear fashion over the period of the experimentation (228 h). Importantly, it was also found that light was required for the assemblages to produce this stabilisation effect and that removal of assemblage into darkness could lead to a partial reversal of the stabilisation. This was attributed to the breakdown of extracellular polymeric substances under anaerobic conditions. These data were supported by microelectrode profiling of oxygen and calcium. The structure of the assemblages as they formed was visualised by low-temperature scanning electron microscopy and confocal laser microscopy. These results have implications for the understanding of early stromatolite development and highlight the potential importance of the evolution of photosynthesis in the mat forming process. The evolution of photosynthesis may have provided an important advance for the niche construction activity of microbial systems and the formation and persistence of the stromatolites which came to dominate shallow coastal environments for 80% of the biotic history of the earth

Seasonality of volcanic eruptions

An analysis of volcanic activity during the last three hundred years reveals that volcanic eruptions exhibit seasonality to a statistically significant degree. This remarkable pattern is observed primarily along the Pacific "Ring of Fire" and locally at some individual volcanoes. Globally, seasonal fluctuations amount to 18% of the historical average monthly eruption rate. In some regions, seasonal fluctuations amount to as much as 50% of the average eruption rate. Seasonality principally reflects the temporal distribution of the smaller, dated eruptions (volcanic explosivity index of 0-2) that dominate the eruption catalog. We suggest that the pattern of seasonality correlates with the annual Earth surface deformation that accompanies the movement of surface water mass during the annual hydrological cycle and illustrate this with respect to global models of surface deformation and regional measurements of annual sea level change. For example, seasonal peaks in the eruption rate of volcanoes in Central America, the Alaskan Peninsula, and Kamchatka coincide with periods of falling regional sea level. In Melanesia, in contrast, peak numbers of volcanic eruptions occur during months of maximal regional sea level and falling regional atmospheric pressure. We suggest that the well-documented slow deformation of Earth's surface that accompanies the annual movements of water mass from oceans to continents acts to impose a fluctuating boundary condition on volcanoes, such that volcanic eruptions tend to be concentrated during periods of local or regional surface change rather than simply being distributed randomly throughout the year. Our findings have important ramifications for volcanic risk assessment and volcanoclimate feedback mechanisms. Copyright 2004 by the American Geophysical Union

A statistical model for the timing of earthquakes and volcanic eruptions influenced by periodic processes

Evidence of nonuniformity in the rate of seismicity and volcanicity has been sought on a variety of timescales ranging from ∼12.4 hours (tidal) to 103-104 years (climatic , but the results are mixed. Here, we propose a simple conceptual model for the influence of periodic processes on the frequency of geophysical "failure events" such as earthquakes and volcanic eruptions. In our model a failure event occurs at a "failure time" tF = tI + tR which is controlled by an "initiation event" at the "initiation time" tI and by the "response time" of the system tR. We treat each of the initiation time, the response time, and the failure time as random variables. In physical terms, we define the initiation time to be the time at which a "load function" exceeds a "strength function" and we imagine that the response time tR corresponds to a physical process such as crack propagation or the movement of magma. Assuming that the magnitude and frequency of the periodic process are known, we calculate the statistical distribution of the initiation times on the assumption that the load and strength functions are otherwise linear in time. This allows the distribution of the failure times to be calculated if the distribution of the response times is known also. The quantitative predictions of this simple theory are compared with some examples of observed periodicity in seismic and volcanic activity at tidal and annual timescales. Copyright 2004 by the American Geophysical Union

Spectral estimates of bed shear stress using suspended-sediment concentrations in a wave-current boundary layer

High-resolution time series of suspended-sediment profiles have been obtained using an acoustic backscatter system at an inner shelf site (North Carolina) where flows are dominated by wind-driven currents and waves. We analyzed the spatial and temporal structure of near-bed turbulence in particle-transporting flows and scalar-like fluctuations of suspended-sediment concentrations. An important element of our analysis is a new inertial dissipation method for passive tracers to estimate the shear stress acting on the seabed, using the spectral properties of suspended sediment concentrations observed by acoustic backscatter sensors. In flows that provide adequate separation of the scales of turbulence production and dissipation, a sufficiently thick constant stress wall layer, and significant sediment suspension, frequency (or associated wave number) spectra of near-bed sediment concentration exhibit a -5/3 slope in the inertial subrange that spans frequencies of order 1 Hz. This observation suggests that the suspended sediment is effectively a passive tracer of turbulent fluid motions. Inversion of the relevant, Kolmogorov scaling equations yields estimates of the shear velocity that agree reasonably well with other, independent and widely used measures. High- and low-frequency limits on application of the inertial dissipation method to sediment concentration are related to the inertial response time of sediment particles and the sediment settling timescale. We propose that, in future applications, the inertial dissipation method for passive tracers can be used to estimate either the shear velocity, effective settling velocity of suspended sediment (or equivalent particle size) or dynamic bed roughness if two of these three quantities are independently known