Claude M. Laird

Did a gamma-ray burst initiate the late Ordovician mass extinction?

Gamma-ray bursts (GRBs) produce a flux of radiation detectable across the observable Universe. A GRB within our own galaxy could do considerable damage to the Earths biosphere; rate estimates suggest that a dangerously near GRB should occur on average two or more times per billion years. At least five times in the history of life, the Earth has experienced mass extinctions that eliminated a large percentage of the biota. Many possible causes have been documented, and GRBs may also have contributed. The late Ordovician mass extinction approximately 440 million years ago may be at least partly the result of a GRB. A special feature of GRBs in terms of terrestrial effects is a nearly impulsive energy input of the order of 10 s. Due to expected severe depletion of the ozone layer, intense solar ultraviolet radiation would result from a nearby GRB, and some of the patterns of extinction and survivorship at this time may be attributable to elevated levels of UV radiation reaching the Earth. In addition, a GRB could trigger the global cooling which occurs at the end of the Ordovician period that follows an interval of relatively warm climate. Intense rapid cooling and glaciation at that time, previously identified as the probable cause of this mass extinction, may have resulted from a GRB.

Gamma-Ray Bursts and the Earth: Exploration of Atmospheric, Biological, Climatic, and Biogeochemical Effects

Gamma-ray bursts (GRBs) are likely to have made a number of significant impacts on the Earth during the last billion years. The gamma radiation from a burst within a few kiloparsecs would quickly deplete much of the Earths protective ozone layer, allowing an increase in solar UVB radiation reaching the surface. This radiation is harmful to life, damaging DNA and causing sunburn. In addition, NO2 produced in the atmosphere would cause a decrease in visible sunlight reaching the surface and could cause global cooling. Nitric acid rain could stress portions of the biosphere, but the increased nitrate deposition could be helpful to land plants. We have used a two-dimensional atmospheric model to investigate the effects on the Earths atmosphere of GRBs delivering a range of fluences, at various latitudes, at the equinoxes and solstices, and at different times of day. We have estimated DNA damage levels caused by increased solar UVB radiation, reduction in solar visible light due to NO2 opacity, and deposition of nitrates through rainout of HNO3. For the typical nearest burst in the last billion years, we find globally averaged ozone depletion up to 38%. Localized depletion reaches as much as 74%. Significant global depletion (at least 10%) persists up to about 7 yr after the burst. Our results depend strongly on time of year and latitude over which the burst occurs. The impact scales with the total fluence of the GRB at the Earth but is insensitive to the time of day of the burst and its duration (1-1000 s). We find DNA damage of up to 16 times the normal annual global average, well above lethal levels for simple life forms such as phytoplankton. The greatest damage occurs at mid- to low latitudes. We find reductions in visible sunlight of a few percent, primarily in the polar regions. Nitrate deposition similar to or slightly greater than that currently caused by lightning is also observed, lasting several years. We discuss how these results support the hypothesis that the Late Ordovician mass extinction may have been initiated by a GRB.

Terrestrial Ozone Depletion Due to a Milky Way Gamma-Ray Burst

Based on cosmological rates, it is probable that at least once in the last gigayear the Earth has been irradiated by a gamma-ray burst (GRB) in our Galaxy from within 2 kpc. We have performed the first detailed computation of the effects on the Earths atmosphere of one such impulsive event: A 10 s 100 kJ m-2 burst penetrates to the stratosphere causing globally averaged ozone depletion of 35%, with depletion reaching 55% at some latitudes. Significant depletion persists for over 5 years after the burst. A 50% decrease in ozone column density leads to approximately 3 times the normal UVB (280-315 nm; a wavelength band that ozone significantly absorbs and that living organisms are sensitive to) flux, and widespread extinctions are likely, based on extrapolation from sensitivity of modern organisms. Additional effects include a shot of nitrate fertilizer and NO2 opacity in the visible, providing a cooling perturbation to the climate over a similar timescale. These results lend support to the hypothesis that a GRB may have initiated the late Ordovician mass extinction (Melott et al.).

Computed contributions to odd nitrogen concentrations in the Earth’s polar middle atmosphere by energetic charged particles

Abstract A two-dimensional photochemical transport model which has inputs that characterize the odd nitrogen production associated with galactic cosmic rays, solar particle events (SPEs), and lower thermospheric contributions (auroral electrons and solar EUV and soft X-rays) is used to compute odd nitrogen concentrations in the polar middle atmosphere from 1 January 1970 to 31 December 1994. We are able to separate out of the total odd nitrogen budget the contributions of the energetic charged particles according to type. The SPE contributions to annual average odd nitrogen concentrations in the polar stratosphere (latitudes > 50°) are computed to be significant (>10%) only for the larger events of August 1972 and October 1989. The SPE contributions to odd nitrogen concentrations in the polar middle atmosphere are found to be asymmetric with respect to hemispheres. The computed SPE contributions to odd nitrogen concentrations at 30 km are significant more often over the South Pole than the North Pole. The thermospheric contributions to odd nitrogen concentrations in the polar middle atmosphere are asymmetric with respect to hemispheres. A stronger thermospheric influence in the stratosphere is computed over the South Pole than the North Pole. An attempt has been made to compare the modeled odd nitrogen of the polar middle atmosphere to an ultra-high resolution polar ice cap nitrate sequence to examine the hypothesis that the nitrate sequences exhibit a signal associated with energetic particles. Variations of odd nitrogen production and modeled concentrations associated with energetic particles themselves cannot explain all of the huge variations observed in the fine structure present in nitrate data from the polar ice cap nitrates, but may be able to explain parts of some of them.

Low time resolution analysis of polar ice cores cannot detect impulsive nitrate events

Ice cores are archives of climate change and possibly large solar proton events (SPEs). Wolff et al. (2012) used a single event, a nitrate peak in the GISP2-H core, which McCracken et al. (2001a) time associated with the poorly quantified 1859 Carrington event, to discredit SPE-produced, impulsive nitrate deposition in polar ice. This is not the ideal test case. We critique the Wolff et al. analysis and demonstrate that the data they used cannot detect impulsive nitrate events because of resolution limitations. We suggest reexamination of the top of the Greenland ice sheet at key intervals over the last two millennia with attention to fine resolution and replicate sampling of multiple species. This will allow further insight into polar depositional processes on a subseasonal scale, including atmospheric sources, transport mechanisms to the ice sheet, postdepositional interactions, and a potential SPE association.

Deep Ice Stratigraphy and Basal Conditions in Central West Antarctica Revealed by Coherent Radar

We discuss results from a high-sensitivity, multichannel, very high frequency, and surface-based radar depth sounder/imager. The instrument was used to map deep internal layers and characterize basal conditions over a 240- km2 grid in the vicinity of the West Antarctic Ice Sheet Divide ice core site. The ice thickness at the core site was found to be about 3470 m, and we detected internal layers to within 350 m of the ice/bed interface. Radar-detected layer stratigraphy does not show evidence of flow-induced disturbances that might complicate the depth-age relationship and the interpretation of climate history preserved in the ice. We also found that bed reflectivity over the region varies by more than 30 dB. Approximately 15 dB of this variability appears to be the result of transitions from a frozen to a thawed bed in a number of places. The remainder probably results from changes in bed roughness. Our data are important for planning drilling to the bed, as well as providing constraints and boundary conditions for regional ice-flow models.

Reply to comment by E. W. Wolff et al. on “Low time resolution analysis of polar ice cores cannot detect impulsive nitrate events”

Wolff et al. (2016) comment on Smart et al. (2014) and in doing so concentrate on issues other than the main point. They do not dispute our central assertion, the inadequate resolution of nearly all extant ice cores for detection of impulsive nitrate events (spikes) from any source, including past solar proton events (SPEs). We explain why comparing two short-length cores from other researchers and analyzed by different methods is insufficient for disputing subannual reproducibility, and call for a multiple, fine-resolution, replicate core study to resolve this issue. While acknowledging the creation of nitrate by SPEs and the existence of ice core nitrate spikes detected by others, they present several weak arguments, such as alleged scavenging of nitrate by some unnamed and unmeasured aerosol, and why no enhanced nitrate signal for documenting SPE statistics should be distinguishable in the ice. These are not derived from the main points in our Smart et al. (2014) paper. We address these briefly and show that ionization from the February 1956 SPE was sufficient to produce a winter, likely acidic, nitrate spike at Summit, Greenland. While noting some convergence of interpretation, we show why their claim that nitrate spikes cannot be used for deriving SPE statistics is unproven and why rejection of fine resolution core studies as unreliable is premature.

Has the Earth been exposed to numerous supernovae within the last 300 kyr

Firestone (2014) asserted evidence for numerous (23) nearby (d<300 pc) supernovae within the Middle and Late Pleistocene. If true, this would have strong implications for the irradiation of the Earth; at this rate, mass extinction level events due to supernovae would be more frequent than 100 Myr. However, there are numerous errors in the application of past research. The paper overestimates likely nitrate and 14C production from moderately nearby supernovae by about four orders of magnitude. Moreover, the results are based on wrongly selected (obsolete) nitrate and 14C datasets. The use of correct and up-to-date datasets does not confirm the claimed results. The claims in the paper are invalidated.

Ground based SAR survey of Basal interface at NEEM drill site

In August of 2008 a radar survey was conducted at the NEEM site in Greenland. An example echogram showing internal layers all the way to the bed, a digital elevation map around the drill site, and a side looking synthetic aperture radar image will be presented. The echogram appears to show a fairly continuous Eemian layer where predicted by modeling. Additionally the area around the drill site is very flat although some slope variation is observed. Finally side looking SAR images show reflected power variations that need more analysis to determine their source.

World grain yields, snow cover, solar activity and Quasi-Biennial Oscillation relationships.

Abstract Approximately 70% of the year-to-year variability in world grain yields for 1968–1987 can be explained by a simple model involving three variables: yields, winter northern hemisphere areal snow cover and annual solar activity in the preceding year. Eighty five percent of the world grain yield variability can be explained by this model, due to an enhanced solar signal, in years when the Quasi-Biennial Oscillation (QBO) is in its west phase. This result is bolstered by recent similar findings by others and supports the detectable impact of solar activity on weather in the stratosphere and troposphere when measured on a global scale.