Abstract
An aperiodic collision of the Earth with extra-terrestrial icy-volatile bodies is proposed as a mechanism to produce rapid changes in the geologic record. Due to the volatile nature of these bodies, evidence for their impacts, particularly in the ocean, might be subtle and best seen as "spikes" in the geochemical or fossil record against normal background. Differing effects would result depending on the site of the major breakup of the object, in the atmosphere, on land, or in the ocean. This paper focusses on the effects of adding material to the seas, oceans, and atmosphere. The treatment is largely qualitative, however mass balance calculations were used to estimate the relative mass needed to affect changes in a variety of reservoirs. Although actual impactors probably have a variable composition, the effects of water-, C-, N-, and S-containing objects are discussed. In the atmosphere, effects could include increased rain acidity, increased levels of nutrients, and enhanced greenhouse warming/cooling. Oceanic effects might include increased oceanic productivity (nitrogen-containing objects). As a result of increased chemical weathering and/or greenhouse effects, increased temperatures coupled with enhanced productivity could result in wider-spread oceanic anoxia or altered calcite/aragonite stability. Possible examples of such impacts from the geologic record and potential biotic effects are given.Introduction
Interest in extra-terrestrial causes of events in the geologic record generally has focused on the introduction of solid material and its physical and chemical effects (e.g. Alvarez et al., 1980, 1992; Pollack et al., 1983, McLaren and Goodfellow, 1990; Smit et al., 1992 ). However, the presence of objects composed of icy volatiles in the solar system suggests that they also impact the Earth. Icy-volatile objects may be captured by the Earth's gravitational field via two mechanisms. Direct capture can result from the intersection of Earth's orbit with that of comets or that of dark bodies from the asteroid belt. Secondarily, capture might also result from the impact of an object (icy) with ice volatile objects in the outer solar system causing the ejection of an object from its previous orbit into an orbit that will lead to direct capture (McKinnon, 1989; Puckett and Miller, 1995; Yang and Ahrens, 1995). Evidence from analysis of cometary gas and dust tails now suggests that the nuclei of comets are formed primarily of water ice containing other volatiles and dust (e.g. Shoemaker, 1983; Krankowsky et al. 1986; Eberhardt et al., 1994; Meier et al., 1994). Pioneer and Voyager fly-bys have observed large volumes of frozen volatile material in various satellites of the outer planets in the solar system: Jupiter, Saturn, Uranus, and Neptune (Moore and Hunt, 1983; Clark et al., 1986; Stevenson, 1987; Jankowski and Squyres, 1988). Wilde (1987) suggested the high volatile content of such satellites could be analogues of volatile planetesimals potentially available during the accretionary formation of the Earth and, during the later stages, may have contributed significant volume to the formation of the primordial ocean. Such large inputs of volatiles to the Earth presumably ended roughly 4 billion years ago. However, similar impacts presumably continued with smaller bodies at a slower rate until the present.The earth is bombarded by a flux of comets possibly due to perturbations of the Oort cloud by passing stars (e.g. Shoemaker, 1983; Holser et al., 1988; Shoemaker et al., 1990; Weissman, 1990a) or interstellar gas clouds (Hut et al., 1987). Of the craters on Earth, most are probably due to impacts of Earth-crossing asteroids, however from 10 to 30% of the craters greater than 10 km in diameter are probably due to comet nuclei (Weissman, 1982, 1990b; Shoemaker, 1983; Shoemaker et al., 1990). The rate of collision probably has been highly variable over late geologic time due to the variability of these perturbations (Shoemaker et al., 1990). Terrestrial collisions with a comet equal to or larger than 10 km have been estimated as about one per 100 million years (Fegley and Prinn, 1989; Shoemaker et al., 1990). A surge of 3 - 30 times the mean background could occur at intervals of tens of millions of years (see Holser et al, 1988). This surge would be seen as cometary showers.
There has been much discussion concerning the geochemical, geophysical, paleontological, and environmental effects of asteroid impacts (e.g. Alvarez et al., 1980, 1992; Pollack et al., 1983; McLaren and Goodfellow, 1990; Tinus and Roddy, 1990; Smit et al., 1992; Hildebrand, 1993; Covey et al., 1994). In the case of asteroid impacts, the evidence of the impact has been associated with the platinum-group signature found at the Cretaceous-Tertiary boundary [K-T] (Alvarez et al., 1980; McLaren and Goodfellow, 1990; Evans et al., 1993). Sigurdsson et al. (1992) suggest that the K-T impact may be associated with a cometary impact. In this case, there had to be sufficient dust to contribute the Ir signature. While it is possible that some comets contain such dust (Geiss, 1987; Jessberger et al., 1988), it is not clear that all do. This issue is not resolved. It is logical to assume that a fair continuum exists. The arguments presented here explore the possibility of impacts of bodies composed primarily of icy-volatiles and of sufficient size to affect the inventory of the Earth or its seas and oceans.
Encounters with icy-volatile objects might produce little or no physical record depending on size and point of impact or decay (atmospheric, oceanic or terrestrial). Weathering or other processes may erode or obliterate a crater, thereby removing any evidence of an impact of an icy object on land. Thus, the influence of icy-volatile impacts on geologic processes may be subtle and may only be observed as stable isotope or geochemical anomalies (Schidlowski et al., 1983; Holser et al., 1988), as geophysical markers in the stratigraphic record of short duration, or as relatively rapid changes in biota (bio-events in the sense of Walliser [1987] and McLaren [1988]). Holser et al. (1995) have identified a number of events in the carbon isotope record with 12 positive spikes and 11 negative spikes. Major isotopic d13 spikes have been reported for the Precambrian-Cambrian boundary event in China (Hsü et al., 1985), the K/T boundary event (Kump, 1991; Hollander et al., 1993), the early-late Maastrichtian transition in southern high latitude sites (Barerra, 1994), the Frasnian-Fammenian boundary in south China and Canada (Wang et al., 1991; Geldsetzer et al., 1993; Zheng et al., 1993) and the Permian-Triassic events (McLaren and Goodfellow, 1990; Wang et al., 1994). McLaren and Goodfellow (1990) argue that the Permian-Triassic extinctions, associated with negative del13C excursions and a spike in the del34S record, are probably caused by an extraterrestrial event, but note that little or no Ir anomaly has been found. They argue that the Ir might have been precipitated by sulfides due to developing anoxia. A much simpler explanation is that the impacting object was primarily composed of ice and volatiles, and therefore insufficient Ir was introduced to produce an Ir anomaly.
Such ephemeral excursions could be explained by impact-induced local changes in the composition and inventory of the various geochemical sources and sinks. A return to normal geochemistry would occur with the mixing of the sink, as the impactor contribution would be small relative to the total inventory of the sink. The environmental effects associated with impacts may also trigger aperiodic biologic extinctions (Walliser, 1987; McLaren, 1988) as many of the isotopic chemical events coincide with extinction events.
Composition of Ice/Volatile Bodies
The chemical composition of comets is not well-known, but is thought to be primarily water ice and dust (Shoemaker, 1983). Their bulk composition is inferred from dust and volatiles observed in their tails. From the 1986 encounter with comet P/Halley, Krankowsky et al. (1986) report the predominance of water vapor escaping from the comet, >80% by volume. Other volatiles have been detected. In comet P/Halley, the upper limit of production of carbon dioxide was 3.5% (by volume relative to water) and maximum concentrations of ammonia and methane were 10% and 7% (by volume relative to water) respectively. Subsequent analysis indicated NH3 production closer to 0.1-2% (Meier et al., 1994). Ammonia has also been detected in comet IRAS-Araki- Alcock 1983 VII. Sulfur species (including S2-, S0, and SO4=) are ubiquitous in solar system objects (Gibson, 1983). H2S has been detected in the coma of comets P/Halley (~0.2-0.5% production relative to water), Austin, Levy 1990 XX, and P/Swift-Tuttle (Eberhardt et al., 1994). The surfaces of the many satellites of planets in the outer solar system are predominately water or other volatiles (Clark et al., 1986). The surface of Io (satellite of Jupiter) contains H2S frost deposited on or co-condensed with SO2(Nash and Howell, 1989). Methane and ammonia ices or clathrates are reported on Uranus' moons Ariel and Miranda (Jankowski and Squyres, 1988). The evidence of the satellites suggests that the composition of icy objects may be either complex as in comet P/Halley or more uniform. It is interesting to note that dust particles previously assumed to be silicate particles in comet P/Halley have recently been shown to be complex mixtures of C, H, O, and N (CHON particles; Kissel et al., 1986a, b; Shoemaker et al, 1990, p. 167) Thus it is not unreasonable to hypothesize the presence of water, C, N, and S as volatiles in impacting objects.
Mechanism
Comets of varying sizes impinge on the earth's atmosphere. Smaller comets may break up and disperse in the atmosphere leaving the composite material in the atmosphere; others eventually impact the Earth. Calculations show that the minimum diameter of an icy object entering the Earth's atmosphere at 90o and retaining sufficient velocity to produce a hypervelocity impact crater is 150 m (Melosh, 1989). The energy released by the impact of such a meteoroid travelling at 11 km/s (approximately the Earth's minimum encounter velocity, Melosh, 1989) is roughly 23 megatons of TNT. Calculations have suggested that the K-T impact could have resulted from an icy, long-period comet, 14 km in radius with a mass of 1016 kg entering with a velocity of 65 km/sec at 75o(Prinn and Fegley, 1987), but probably would not have generated the Ir anomaly. (An ice-volatile object due to its gaseous nature is unlikely to contain any or enough Ir to produce an Ir anomaly.) It is not unreasonable to suggest that an icy object of some size penetrated the Earth's atmosphere and scattered its material on land, in the oceans, or in the atmosphere.The most simple mechanism of impact is the entry of a large icy body into the atmosphere and impacting either the land mass or the ocean (Figure 1). On land, such an impact would result in the formation of a crater and the ejection of large quantities of matter from both the impactor and the impacted region (Crook, 1967; Shoemaker, 1983; Melosh, 1989). As is noted above, terrestrial impacts have been discussed extensively elsewhere. If the object impacts the ocean, a crater may not be discernable; large quantities of matter would be ejected from both the impactor and impacted region into the atmosphere. Giant tidal as well as seismic waves would be generated. Impacts of large objects (> 4 km in diameter) might eject dust even if they occur in the ocean because their diameter is greater than the average depth of the ocean (Covey et al., 1994). The extent of impact craters predicted for large bodies is great: for the impact at the K-T boundary, Sharpton et al. (1993) and Pope et al. (1994) have suggested that the impact crater size may have been as great as 300 km and 500 km in diameter, respectively.
A comet or asteroid travelling at a highly oblique angle to the Earth's surface is rare and has a finite possibility of impact (Schultz and Gault, 1990; Schultz et al., 1994). A comet or asteroid impacting at an angle of 15o generates 10 times the vaporized mass as a vertical impact. Such an impact would eject vast quantities of both impacted and impactor material into the atmosphere (Schultz and Gault, 1990).
An icy object or small asteroid may not necessarily impact the Earth's surface (Melosh, 1989). Travelling through the atmosphere, it generates a bow wave composed of heated compressed gases. The pressure behind the object is nearly zero. The heated gases can cause an icy object to evaporate long before it reaches the earth leaving its component materials in the atmosphere. The differential pressure across the object can serve to slow it to the extent that its initial momentum is spent causing the object to fall to Earth or potentially melt or evaporate. The object is subjected to aerodynamic forces such that an object with low structural integrity will tend to fragment long before impact. In these cases, material released to the atmosphere may be at exceedingly high temperature and therefore result in the atomization of the component material. However as the result of drag on the initial object and subsequent loss of momentum, it is conceivable that some of the initial material could mix with the atmosphere or ocean. Material deposited or vaporized in the atmosphere or the seas and oceans would interact with the extant fluids through various chemical and photochemical reactions.
End-Member Primary Effects of Icy-Volatile Objects Containing Various Components
Icy objects probably have a mixed volatile content as noted above. For purposes of simplicity, however, it is useful to examine the effects of end-member volatiles. The effects of mixed volatiles would be more complex due to the results of cross reactions. Although other compounds have been detected in comets, the end-members (either as pure solids or in water ice) discussed here are bodies composed of water ice, carbon dioxide, methane, ammonia, NOx, and reduced- and oxidized-sulfur species. We examined the potential effects of bodies, 1, 2.5 and/or 10 km in radius, composed of a solid species (for example, water ice), a solid solution (ex.: 1% NH4OH), and where possible, a body composed of separate species, but in the concentrations found in the coma of comet P/Halley. The effects associated with a 10-km body have been examined with respect to the K-T extinction event (Alvarez et al., 1980; Prinn and Fegley, 1987).The mass of material added was compared to the resident mass in the oceans (total), and to the mass in oceans containing volumes of seawater comparable to those in the modern Atlantic, Pacific, Indian, Arctic Oceans and various marginal seas. These can be considered analogs for pre-existing oceans, such as Tethys and Iapetus, at various times throughout the geological record.
We are discussing only the primary effects of the addition of such material to the Earth's atmospheric and marine inventory. We are not discussing the physically-induced changes of an impact on a closed Earth system. For example, we avoid discussion of the effect of dust raised by such an impact, which has been extensively discussed elsewhere (e.g. Pollack et al., 1983; Covey et al., 1994). We recognize that it is often assumed that these objects completely vaporize (atomize) upon impact. However, for purposes of discussion, it is not unreasonable to examine the probability that such an object impacts in an ocean or sea and for some period of time the vaporized material affects the local composition. Clearly, with diffusion, the presence of the vaporized material would become undetectable. In fact, near extinction events, there are spikes in the isotopic record. We will suggest potential examples from the geologic record where addition of a spike of volatiles to the Earth's inventory would explain various anomalies and environmental changes (see below, Environmental Effects).
Water Ice. Locally, addition of a largely fresh-water impactor to saline waters would dilute them (Wilde et al., 1988). Lowered salinities could cause difficulties for stenohaline forms. While estuarine organisms are adapted to wide fluctuations in salinity, most true oceanic organisms are significantly less so (Sverdrup et al., 1942, pp. 839-843). If the object is sufficiently large, a rapid sea level rise unrelated to deglaciation might be experienced in small inland seas, but it is unlikely.
Table 1 shows the effect of adding icy water objects of radius 1, 2.5 and 10 km to various oceans and seas. It is clear that addition of 1 or 2.5 km objects would have no observable effect in terms of sea water dilution or depth. Addition of the 10 km object would have a significant effect in these terms only in small seas with volumes comparable to that of the modern Baltic. A dilution of 20% would be enough to alter the salinity sufficiently to impact stenohaline organisms and might alter sea level. However, the physical effects of a 10-km object impacting a small body of water as small as the Baltic would far outweigh any dilution or sea level effects.
* kg x 1016 based on a density of 1.025 x 1012 kg/km3 at 35 ppt salinity (Sverdrup et al., 1942, p. 14)
Table 1. Effect of adding the mass of water found in impacting objects with radii of 1, 2.5 and 10 km on variously sized sea and oceans Body of Water Mass of H2O* Effect of Addition as % of resident mass 1 km 2.5 km 10 km Total Ocean 137000 0 0 0 Pacific 70900 0 0 0 Atlantic 32400 0 0 0 Indian 29200 0 0 0 Arctic 1740 0 0 0 Caribbean 980 0 0 0 Mediterranean + Black 434 0 0 0.1 Red 22 0 0 2.09 Baltic 2.4 0 0.3 20.9 Carbon-containing Objects (Dry Ice, CO2 -hydrate or Solid Methane, CH4-hydrate Ice [Figure 2]).
The introduction of an object 10 km in radius that contains only C-compounds would only alter the atmospheric mass of C by roughly 1%. If diffusion were instantaneous, clearly such an impact would not be detectable, however, diffusion is not instantaneous. It is probable that the C species in any such object could be atomized. Atmospheric release of C. would result in combination with gases in the atmosphere. Given an oxygenated atmosphere much of the resultant substance would be CO2. It has been suggested that an impact of any significance into carbonate terrain would excavate 2 orders of magnitude more material than would be introduced by an incoming object (O'Keefe and Ahrens, 1989). This may be true, however, given the areal extent of the oceans, seas, and non-carbonate terrain (carbonate sedimates represent roughly less than 15% of the sedimentary rock mass [Garrels and Mackenzie, 1971, p. 40]; and less than half the oceanic sediments in the post-Jurassic [Sverdrup et al., 1942, p. 1006]), it is more likely that a non-carbonate terrain would be struck. In the pre-Cretaceous, before significant carbonate ooze deposition, it would be even less likely that a carbonate terrain would be hit. Thus it is of interest to consider the impact of a carbon-containing object into non-carbonate terrain, particularly the oceans.As noted above, a carbon-containing impactor might well be expected to atomize, ultimately forming CO2 in an oxygenated atmosphere. Given the density of CO2 at standard conditions, local chemical effects of such an impact might be experienced if sufficient mass were introduced. Such additions would lower the pH of rain, enhance greenhouse warming locally (Rasmussen and Khalil, 1986; Wuebbles and Edmonds, 1991), and somewhat enhance chemical weathering (Wilde et al., 1988). Increased atmospheric CO2 could also increase terrestrial productivity. (There is a vast body of literature on the effects of increased atmospheric CO2 - see for example IPCC, 1990, 1992, and Wuebbles and Edmonds, 1991 for an introduction to the literature.)
In the oceans, C. could react with dissolved oxygen, depleting oxygen and enhancing anoxicity, particularly below the photic zone (Barnes and Goldberg, 1976; Whitcar et al., 1986). Table 2 presents the effect of adding 2.5-, and 10-km radius carbon-containing objects into the present oceans. Each object is assumed to contain dry ice, CO2-hydrate (Sloan, 1990; Kvenvolden, 1993; Wadsley, 1993), or the combined percentage of C in CO2 and CH4 components observed in the coma of comet P/Halley (Krankowsky et al., 1986). The mass of C introduced is compared to the mass of C present in each ocean or sea (~28 mg/kg at 3.5% salinity, Sverdrup et al., 1942).
While the percent change in C mass expected for both sizes in the smaller seas would be significant, any effect would probably be outweighed by the physical effects of an impact. However, in an ocean the size of the modern Arctic, the relative additional mass for a 10-km object is considerable - even with the C concentration as low as might be expected if the comet nucleus had a composition similar to comet P/Halley. If the comet were completely dry ice, sufficient C would be added by a 10-km comet to the oceanic mass to perturb even oceans the size of the Atlantic and Indian Oceans. Such additions might be observed in the geological record as rapid onset of anoxicity, apparent changes in local pH (altering sedimentary composition , Quinby-Hunt and Wilde, 1994), or possibly spikes in the isotopic record. * kg x 1011 based on a density of 1.025 x 1012 kg/km3 at 35 ppt salinity and 28 mg/kg C (Sverdrup et al., 1942, p. 220)
Table 2. Effect of adding the mass of C found in impacting objects with radii of 2.5 and 10 km to variously sized seas and oceans. Body of Water Mass of C * Effect of Addition of C as % of resident mass 2.5 km 10 km CO2 CO2-Hyd Halley CO2 CO2-Hyd Halley Total Ocean 390000 0 0 0 5 1 0 Pacific 200000 0 0 0 9 2 1 Atlantic 93000 0 0 0 19 4 1 Indian 84200 0 0 0 21 5 2 Arctic 4900 6 1 0 370 80 27 Caribbean 2700 10 2 1 650 140 47 Mediterranean + Black 1200 23 5 2 1460 320 107 Red 62 450 100 33 29000 6400 2100 Baltic 6.6 4200 930 307 270000 59000 20000
Nitrogen-Containing [Figure 3]
. Initially, the impact of a nitrogen-rich object in the atmosphere would cause increased concentrations of nitrogenated species or nitrogen atoms - the resultant species after recombination would depend upon the atmospheric composition. If the nitrogen-rich impactor dissipated in an oxygen-rich atmosphere, various NOx species would most likely form lowering rain pH, inhibiting photosynthesis due to solar extinction, and destroying ozone in the upper atmosphere (see e.g. Logan, 1983; Chamberlain and Hunten, 1987, pp.127-136; Prinn and Fegley, 1987; Wilde et al., 1988). Prinn and Fegley (1987) explored the possibility that the impact at the K-T boundary released massive quantities of matter that reacted to form NOx. As noted above, should instantaneous dissipation occur, the quantity of N2 in the atmosphere would swamp the quantity of N added to it. However, dissipation is not instantaneous and local concentrations of NOx would rise dramatically and might be expected to have the effects associated with any point source.Table 3 presents the effect of adding 2.5-, and 10-km radius nitrogen-containing objects into the present oceans. For both 2.5- and 10-km objects, two concentrations of N were considered - both conservative. The first was 1% NH4OH (as N) - a quantity less than originally proposed for comet P/Halley (upper limit 10% relative to water -Krankowsky et al., 1986), and within the limits of the revised estimates (0.1-2% NH3 production relative to water ) reported by Meier et al. (1994) . The second concentration was 0.1% as N - the lower limit of production for comet P/Halley reported by Meier et al. (1994). The mass of N introduced is compared to the mass of N present in each ocean. We used the high end estimate for N - 0.7 mg/kg at 1.9% chlorinity (Sverdrup et al., 1942, p. 176) - thus this is a conservative estimate. We have not considered stratification of the ocean - making the assumption that impact would cause at least local mixing.
It is clear that a 10-km object with even 1% NH4OH would dramatically increase the N levels in most of the Earth's oceans and seas. Even at concentrations similar to that minimum estimated in the coma of comet P/Halley, additions would be significant in oceans the size of the Arctic or smaller. * kg x 1016 based on a density of 1.025 x 1012 kg/km3 at 35 ppt salinity (Sverdrup et al., 1942, p. 14)
Table 3. Effect of adding the mass of N found in impacting objects with radii of 2.5 and 10 km on variously sized seas and oceans. Body of Water Mass of N* Effect of Addition as % of resident mass 2.5 km 10 km 1% NH4OH 0.1% NH3 1% NH4OH 0.1% NH3 Total Ocean 94000 0 0 4 0 Pacific 50800 0 0 7 0 Atlantic 23200 0 0 15 1 Indian 20900 0 0 17 1 Arctic 1220 4 0 280 17 Caribbean 690 8 0 500 30 Mediterranean + Black 300 18 1 1134 70 Red 15 350 21 2100 1400 Baltic 1.6 3300 200 210000 13000 NOx released in the ocean would at least locally reduce pH and, on dilution, act as a nutrient. The reduced pH might also release limiting nutrient species [a "limiting nutrient" is a nutrient that is present in insufficient concentrations for maximal biomass growth, thus its absence limits growth and its addition may stimulate growth.] from the sediments and particles in the water column (for example, Fe). In an oxidizing atmosphere it is highly unlikely that ammonia would survive. However, ammonia might be released locally in waters. Locally in marine waters, ammonia would initially form a toxic cloud, however dilution (and precipitation of hydroxides) would soon result in ammonia concentrations that were fertilizing rather than toxic. If ammonia were introduced into an oxygenated body of water, NH3 would oxidize to various oxygenated nitrogen species (nitrate, nitrite), consuming oxygen in the process and enhancing local anoxia. In any case, in the ocean, ammonia, nitrate, or nitrite would ultimately act as nutrients and would increase productivity (depending on the presence of other limiting reagents). Increased productivity would enhance anoxia at depth, which would be recorded in the geologic record.
Sulfur-Containing [Figure 4]
. An impacting sulfur-containing object could introduce H2S, SO2, or SO3 directly into the atmosphere which would have direct toxic effects. Assuming that the incoming object is atomized and is released into an oxygenated atmosphere, the sulfur species would eventually oxidize to SO2 causing acid rain (e.g. Lewis et al., 1982; Nguyen et al., 1983; Wilde et al., 1988; Chamberlain and Hunten, 1987). Researchers investigating effects of extraterrestrial impacts have suggested that sulfur species would be introduced into the atmosphere due to volatilization of sulfate from evaporite targets, (Sigurdsson et al., 1992), which would result in the formation of acid rain (D'Hondt et al., 1994). Others (Brett , 1992; Sigurdsson et al., 1992; Pope et al., 1994) note the potential darkness, cooling, and surface water acidification effects of SO2 volatilization. Direct volatilization from a sulfur-containing impactor is a more direct mechanism. Although evaporite deposits are common, only a small percentage (~ 5%) of the Earth's sedimentary rock is evaporite (Garrels and Mackenzie, 1971). Therefore the direct mechanism should be considered . An impact directly into evaporite or anhydrite terrain would eject from 1.3 x 1016 to 8.4 x 1018 g SO2 into the air (Brett, 1992, Sigurdsson et al., 1992; Pope et al., 1994) which is comparable to the quantity of material in H2SO4.H2O objects with radii from 1-11 km. If a 10-km comet containing the reported percentages of H2S in the coma of comet P/Halley (Krankowsky et al., 1986) were to release its S into the atmosphere as SO2, roughly 2 x 1016g would be released, certainly a significant quantity. Local effects could be quite pronounced. As the pH of rain is lowered, incidence of Mg2+ and Ca2+ could increase in runoff, in addition to increased levels of sulfate, which might result in the precipitation of CaSO4 evaporites in shallow shelf seas or alter the stability of calcite relative to aragonite (Railsback and Anderson, 1987).Introduction of such an object into the ocean would raise the concentration of sulfides, sulfates and sulfites directly. In an oxygenated body of water, sulfur species would eventually convert to sulfate. Because of the large reservoir of sulfate in the oceans, only an impact of a 10-km H2SO4 object into a very small body of water (e.g. the size of the Red or Baltic Seas) would result in significant addition to the reservoir. Local effects could be important, however, in this case, the physical effects of the impact would probably outweigh any chemical effect.
Environmental Effects
Differentiating between the impact of a primarily rocky object and one that is primarily composed of icy volatiles is problematic. Many of the physical effects of such an encounter are similar and have been discussed elsewhere (see e.g. Alvarez et al., 1980, 1992; Pollack et al., 1983; McLaren and Goodfellow, 1990; Tinus and Roddy, 1990; Wolbach et al., 1990; Smit et al., 1992; Covey et al., 1994 and the references therein). The principle differences between impacts of primarily rocky object and one that is primarily composed of icy volatiles would be seen in the chemistry. Certainly the object that impacted at the K/T boundary introduced a globally- distributed layer with very high concentrations of Ir (Alvarez et al., 1980, 1992; Smit et al., 1992). On the other hand, an icy-volatile impactor would not introduce such material (even one containing silicates), but would introduce locally large quantities of volatile species, which then disperse. A few of the important environmental effects of icy-volatile impacts are discussed below.Lowered pH in Rain, and Terrestrial and Marine Waters. Impacts in the terrestrial atmosphere of icy-volatile objects containing CO2, CH4, N or S species, all could decrease locally the pH of rain either directly on dissolution or after oxidation in the atmosphere. The extent to which the pH of rain, fresh, and marine waters alters is determined by the chemical introduced, the size of the impactor, and the volume and buffer capacity of each body of water. Nitric or sulfuric acid rain could cause massive loss in diversity of land plants as a result of direct toxicity and foliage destruction (Lewis et al., 1982; Johnson and Siccama, 1983; NRC, 1983; Prinn and Fegley, 1987; Zahnle, 1990; D'Hondt et al., 1994).
The effect of altered pH on marine waters depends on the extent of change and the variation in pH to which the species are already adapted (EPA, 1973). Thus estuarine species may have adapted to large variations in pH, and could survive or even thrive after an impact. Marine species might be less affected, due to the large buffering capacity of the ocean, which may prevent any large variation in pH. However, if the buffer system were overwhelmed locally, organisms adapted to relatively constant pH could be adversely effected by a relatively small change in pH. A lower oceanic pH might inhibit the consumption of CO2 causing decreased production of calcareous tests, as is seen at the end of the Cretaceous. At the K-T boundary event, large massive reef corals were greatly reduced, as were the calcareous nannofossils (Wiedmann, 1986; Johnson and Kauffman, 1988; Perch-Nielsen, 1988).
Acid rain could also enhance chemical weathering, potentially increasing the ionic strength of the bodies of water affected (Prinn and Fegley, 1987). Salinity and nutrient content of waters would increase. The effect of impactor-induced acid rain would be influenced by the extent and diversity of vascular land plants (Knoll and James, 1987). Thus, prior to the development of vascular land plants in the late Wenlock [middle Silurian] (Edwards and Feehan, 1980), rain of lower pH would have increased weathering. After the development of vascular land plants, which themselves enhance weathering (Knoll and James, 1987), mechanisms would have been more complex. Increased CO2 could enhance both growth and weathering. More strongly acidic rain initially would be expected to kill vegetation (Johnson and Siccama, 1983; NRC, 1983; Prinn and Fegley, 1987; D'Hondt et al., 1994) and also enhance weathering, resulting in runoff with increased major and trace elements, but also increased organic content. Following enhanced weathering, massive amounts of evaporites might form due to precipitation of insoluble sulfates triggered by excessive amounts of dissolved cations. Such evaporites appear in the Permo-Triassic and throughout the rock record (Borchert, 1965). Prinn and Fegley (1987) suggest that such conditions caused increased levels of a variety of trace elements (depending on local conditions) including Be2+, Al3+, Hg2+, Cu++, Fe2+, Fe3+, Ti3+, Pb2+, Cd2+, Mn2+, and Sr2+. Hildebrand (1992) reports such anomalies at the K/T boundary.
Ions such as Ca2+, Mg2+, sulfates, nitrates, and phosphates may be released. If vegetation were killed, then runoff may contain a major component of dead biomass. Runoff could therefore introduce both toxic or nutrient species into receiving waters. Depending on the relative concentrations of each of these chemical species, geochemical conditions may develop that favor development of new species. In the Late Permian and Early Triassic, the d34S signature suggests high marine concentrations of sulfate due to enhanced weathering (Railsback and Anderson, 1987). Railsback and Anderson (1987) suggest that increased Mg++ and sulfate would have favored the development of aragonite-secreting organisms that prevailed in the Early Triassic over calcite-secreting organisms of the Late Permian (Mucci and Morse, 1983; Walter, 1986). If productivity is enhanced or runoff contains large quantities of deceased biomass, anoxic conditions may spread (see below).
Greenhouse Warming/Cooling
. The introduction of massive amounts of carbon dioxide and methane to the atmosphere could result in greenhouse warming. The import of CO2 in greenhouse warming is well-studied (for an introduction to the vast literature on this topic, see Davis, 1990; IPCC, 1990, 1992; Wuebbles and Edmonds, 1991); CH4 is an even stronger greenhouse gas than carbon dioxide with a radiative forcing some 20 times that of CO2 (Wuebbles and Edmonds, 1991). Such an impact would be documented by an increase in global temperature, deglaciation, and increased atmospheric levels of CO2 (increasing terrestrial productivity). The geological record is replete with abrupt increases in average global temperature and increased perceived levels of atmospheric CO2 (Budyko et al. 1987, Berner, 1990). Greenhouse warming is associated with enhanced vegetative growth, extreme atmospheric events, and forest fires. Evidence of fire at the K/T boundary has been reported (Hansen et al., 1987; Wolbach et al., 1990). In addition to the often-discussed effects of greenhouse warming, oceanic productivity and temperature might be expected to rise thereby enhancing the probability of anoxia at depth (Quinby-Hunt et al., 1989; Wilde and Quinby-Hunt, 1993).Similarly, as has been proposed regarding the effect of an impact into evaporite deposits, release of SO2 to the atmosphere would result in cooling (e.g. Brett , 1992; Sigurdsson et al., 1992; Pope et al., 1994). Evidence for such an effect could be decreased productivity and the onset of glaciation as is reported at the end of the Ordovician (e.g. Berry and Boucot, 1973; Frakes, 1979; Brenchley and Newall, 1984; Fortey, 1984; Frakes et al., 1992).
Nutrient enhancement and Enhanced Anoxia.
Introduced into the oceans or seas, nitrogen species would act as a fertilizer. The effects of a collision with a nitrogen-containing impactor might initially cause dramatic losses in species diversity followed by an increase in those species to whom increased nitrogenated nutrient concentrations are advantageous, such as nitrogen-limited plants, algae etc. Further nutrient enhancement could occur as a result of enhanced weathering associated with acid rain (above) or dissolution of limiting metals (e.g. Fe). The presence of higher concentrations of nutrient species would result in increased productivity in as much as such enhancement could lead to increased food resources for marine organisms. Addition of ammonia or carbon dioxide to the atmosphere may have provided fertilization for terrestrial plants.Increased productivity in the surface mixed layer of the ocean would result in greater consumption of oxygen at depth as deceased organisms decay. Enhanced intensity and extent of hypoxic and anoxic zones in coastal zones will potentially kill large numbers of organisms including fish and shellfish, cause dramatic changes in species composition and diversity in coastal marine ecosystems, and alter the local chemistry precipitating some compounds and releasing others. Effects of anoxic events in modern time have been well-documented (Stachowitsch, 1984, 1989): migration of motile organisms from the anoxic region and death of aerobic organisms unable to migrate. An examination of del13C at Frasnian-Famennian boundary sections in Europe suggests two phases of enhanced burial and recycling of organic carbon (Joachimski, 1993). Joachimski (1993) associates enhanced burial and recycling with variations in atmospheric CO2, sea level fluctuations, and anoxic conditions.
Conclusions
Icy-volatile impactors enter the Earth's atmosphere. Many decay, break up, vaporize, or melt in the atmosphere. Others impact the Earth's surface. In addition to physical effects of such an impact, the encounter may also result in the release of chemicals whose effect will depend on their nature. Additions of products from C-containing objects to the atmosphere could increase global temperatures. The pH of rain will decrease somewhat. Nitrogen-containing objects may initially release materials that result in fertilization (NH3, NOx) or acid rain (NOx). Sulfur-containing impactors may release toxic gases (H2S, SOx) that may promote global cooling. In an oxic atmosphere, these gases will eventually result in acid rain. The gases may cause respiratory damage. Acid rain in turn is expected to damage foliage and increase weathering thereby releasing some major, trace, toxic, and fertilizing species.An oceanic impact will result in tidal, seismic, and shock waves, and in destabilization of ocean stratification, but may not leave a discernable crater. Thus the evidence of such an impact may only be found as geochemical markers in the geologic record. Environmental effects will depend on the size, shape, density, speed, and direction of the impactor and the chemical composition of the underlying waters. The physical and chemical effects of an impact may result initially in massive mortality, possibly followed by evolution of a new fauna and may be recorded as a del13C spike in the geologic record. Enhanced weathering may alter calcite/aragonite stability or increase oceanic productivity thereby spreading oceanic anoxia at depth. Addition of nitrogenated species, particularly in conjunction with enhanced weathering, may result in fertilization, followed by the spread of anoxia at depth. Addition of sulfates would tend to cause cooling resulting in glaciations and decreased productivity.
The oceans covered 60 to 70% of the earth's surface during the Phanerozoic. Accordingly, the vast majority of incoming objects not breaking up in the atmosphere would have been in the ocean. The effect of major impacts, particularly of ice-volatile objects, on oceanic chemistry and biota and thus on marine sediments and the fossil record would be profound. The physical effects of impacts on land and in the atmosphere are generally well-studied. However, the chemical and chemical-biological implications for oceanic impacts are far less understood. The study of the effects of such aperiodic impacts on ocean chemistry provides an important additional tool to interpret the marine record.
Acknowledgements
An initial version of this paper was given as a poster session at the Snowbird II Conference, 1988. This paper was given orally at the Geochemical Event Markers in the Phanerozoic Conference held at Erlangen, Germany, 1994. The authors thank Dr. Geldsetzer and Dr. Joachimski for their kind invitation to the Erlangen conference and their support. Conversations with Dr. Goodfellow and Dr. A.J. Hunt were exceedingly helpful. We thank also Dr. A. Hildebrand and anonymous reviewers for their constructive remarks. This is contribution 95-001 of the Marine Sciences Group.
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