Geological Society of America Special Papers

doi: 10.1130/2009.2458(40) 2009;458;941-950Geological Society of America Special Papers

 Mihaela Glamoclija, Gregory S. Gohn, David S. Powars and J. Wright Horton, Jr.Charles S. Cockell, Aaron L. Gronstal, Mary A. Voytek, Julie D. Kirshtein, Kai Finster, Ward E. Sanford, crater: Relationship to lithology and impact processesMicrobial abundance in the deep subsurface of the Chesapeake Bay impact  

Email alerting servicescite this article

to receive free e-mail alerts when new articleswww.gsapubs.org/cgi/alertsclick

SubscribeAmerica Special Papers

to subscribe to Geological Society ofwww.gsapubs.org/subscriptions/click

Permission request to contact GSAhttp://www.geosociety.org/pubs/copyrt.htm#gsaclick

viewpoint. Opinions presented in this publication do not reflect official positions of the Society.positions by scientists worldwide, regardless of their race, citizenship, gender, religion, or politicalarticle's full citation. GSA provides this and other forums for the presentation of diverse opinions and articles on their own or their organization's Web site providing the posting includes a reference to thescience. This file may not be posted to any Web site, but authors may post the abstracts only of their unlimited copies of items in GSA's journals for noncommercial use in classrooms to further education andto use a single figure, a single table, and/or a brief paragraph of text in subsequent works and to make

GSA,employment. Individual scientists are hereby granted permission, without fees or further requests to Copyright not claimed on content prepared wholly by U.S. government employees within scope of their

Notes

© 2009 Geological Society of America

on January 15, 2010specialpapers.gsapubs.orgDownloaded from

941

The Geological Society of AmericaSpecial Paper 458

2009

Microbial abundance in the deep subsurface of the Chesapeake Bay impact crater: Relationship to lithology and impact processes

Charles S. CockellAaron L. Gronstal

Centre for Earth, Planetary Science and Astronomical Research, Open University, Milton Keynes, MK7 6AA, UK

Mary A. VoytekJulie D. Kirshtein

U.S. Geological Survey, MS 430, 12201 Sunrise Valley Drive, Reston, Virginia 20192, USA

Kai FinsterInstitute of Biological Sciences, Microbiology Section, Aarhus University, 8000 Aarhus C, Denmark

Ward E. SanfordU.S. Geological Survey, MS 430, 12201 Sunrise Valley Drive, Reston, Virginia 20192, USA

Mihaela GlamoclijaCarnegie Institute of Washington, 1530 P Street NW, Washington, DC 20005, USA

Gregory S. GohnDavid S. Powars

J. Wright Horton Jr.U.S. Geological Survey, MS 430, 12201 Sunrise Valley Drive, Reston, Virginia 20192, USA

ABSTRACT

Asteroid and comet impact events are known to cause profound disruption to surface ecosystems. The aseptic collection of samples throughout a 1.76-km-deep set of cores recovered from the deep subsurface of the Chesapeake Bay impact structure has allowed the study of the subsurface biosphere in a region disrupted by an impac-tor. Microbiological enumerations suggest the presence of three major microbiologi-cal zones. The upper zone (127–867 m) is characterized by a logarithmic decline in microbial abundance from the surface through the postimpact section of Miocene to Upper Eocene marine sediments and across the transition into the upper layers of the impact tsunami resurge sediments and sediment megablocks. In the middle zone (867–1397 m) microbial abundances are below detection. This zone is predomi-nantly quartz sand, primarily composed of boulders and blocks, and it may have been

Cockell, C.S., Gronstal, A.L., Voytek, M.A., Kirshtein, J.D., Finster, K., Sanford, W.E., Glamoclija, M., Gohn, G.S., Powars, D.S., and Horton, J.W., Jr., 2009, Microbial abundance in the deep subsurface of the Chesapeake Bay impact crater: Relationship to lithology and impact processes, in Gohn, G.S., Koeberl, C., Miller, K.G., and Reimold, W.U., eds., The ICDP-USGS Deep Drilling Project in the Chesapeake Bay Impact Structure: Results from the Eyreville Core Holes: Geological Society of America Special Paper 458, p. 941–950, doi: 10.1130/2009.2458(40). For permission to copy, contact editing@geosociety.org. ©2009 The Geological Society of America. All rights reserved.

on January 15, 2010specialpapers.gsapubs.orgDownloaded from

942 Cockell et al.

INTRODUCTION

Asteroid and comet impacts have had a profound infl uence on surface-dwelling organisms, and large impacts have been implicated in past mass extinctions (Alvarez et al., 1980; Toon et al., 1997). Because large impacts are rare, ecological alterations at the local level, caused by smaller impacts, are more common (Kring, 1997; Smelror et al., 2002). In addition to devastating a biota by direct effects of shock, such as by heat, and the infl u-ence on atmospheric chemistry, impacts can change the target lithology (e.g., Kieffer, 1971), which may alter the colonization potential of the material to microorganisms (Cockell et al., 2002).

Cavities formed by impacts form topographic depressions that can allow for the formation of a standing body of water. These lakes and the surrounding areas have been the subject of both paleomicrobiological investigations (Riding, 1979) and studies of the present-day biota (Schoeman and Ashton, 1982; Ashton and Schoeman, 1983; Gronlund et al., 1990; Cremer and Wagner, 2003). Biological observations that are more directly linked to impact include data showing that impact bulking and fracturing can change the microbial colonization potential of rocks (Cock-ell et al., 2002; Cockell and Osinski, 2007). The energy associ-ated with impact changes the permeability, porosity, and light- transmission properties of rocks, and, potentially, their chemical composition (Melosh, 1989). Changes in rock chemistry by impact can alter the long-term colonization potential of breccias and melt rocks formed in impact (Cockell et al., 2001; Cockell and Lee, 2002) as compared to outlying lithologies.

Although the consequences of impacts for surface biota have been investigated, their infl uence on the deep subsurface biosphere have not; however, in terms of biomass and role in biogeochemical cycling, the subsurface biota are important for global-scale earth-system processes (Horsfi eld et al., 2007). Impact events would be predicted to have a range of both del-eterious and benefi cial effects on the deep subsurface biosphere. Deleterious effects include the potentially sterilizing effects of the thermal pulse delivered by the impact and the biologically disruptive effects of the shock waves produced in impact (Kring, 1997). Benefi cial effects potentially include: (1) increase in permeability and porosity of target materials (Cockell et al., 2005); (2) increase in the fl ow of nutrients and redox couples to biota; (3) improvement of water availability caused by cavity formation (Ashton and Schoeman, 1983); (4) mixing of diverse lithologies, creating steep geochemical gradients and potentially

more favorable redox environments for life (Cockell, 2006); (5) formation of a biologically conducive hydrothermal system (Osinski, 2005; Osinski et al., 2001); and (6) introduction of new nutrients into the crater cavity, either from outside the impact zone (such as, for example, by tsunami) (Smelror and Dypvik, 2006) or from the impactor itself (e.g., glasses from the Henbury impact crater may have enriched iron concentrations that were contributed by the iron meteorite that formed the craters; Taylor and Kolbe, 1965).

The drilling of the Chesapeake Bay impact structure at Eyre-ville by the International Continental Scientifi c Drilling Program (ICDP)–U.S. Geological Survey (USGS) in 2005 (Gohn et al., 2006, 2008) offered an opportunity to apply robust contamina-tion controls to the acquisition of microbiological samples from the subsurface of an impact crater. The Chesapeake Bay impact structure is a buried late Eocene impact structure formed by the collision of an impactor into the continental shelf of eastern North America. It is the best preserved example of a large marine impact on Earth (Poag et al., 1992, 1999, 2004; Powars et al., 1993; Powars and Bruce, 1999; Koeberl et al., 1996). The impact rocks and their constituent microfossils contain evidence of the impact origin of the Chesapeake Bay impact structure (Edwards and Powars, 2003; Horton et al., 2005a; Lee et al., 2006; Jack-son et al., 2006). It was formed in a shallow-marine environment (water depth <350 m) underlain by a few hundred meters of Lower Tertiary and Cretaceous sediments on top of Neoproterozoic and Paleozoic basement. The crater has a diameter of ~85 km and now lies in the Atlantic margin of Virginia, USA (Fig. 1) (Horton et al., 2005b). It is composed of an ~900-m-high central uplift surrounded by a central crater (~35 km diameter), which is itself surrounded by an annular trough and an outer region of fractured rock containing irregular concentric and radial faults.

In 2005, two core holes (A and B) were drilled into the structure to a composite depth of 1.76 km at Eyreville Farm, in Northampton County, Virginia, USA (Fig. 1). A third shal-lower core hole (C) was drilled in spring of 2006 to a depth of 140 m. Samples were retrieved from deep part of the central cra-ter, approximately 9 km from the center of the structure. In this paper, we describe the microbial enumeration data retrieved from core sections and the relationships among microbial cell distribu-tion, select constituents of the pore water and the sediment, and lithological structuring of the Chesapeake Bay impact structure. The lithological information is from Edwards et al. (this volume, Chapters 3 and 4) and Horton et al. (this volume).

mostly sterilized by the thermal pulse delivered during impact. No samples were col-lected from the large granite block (1096–1371 m). The lowest zone (below 1397 m) of increasing microbial abundance coincides with a region of heavily impact-fractured, hydraulically conductive suevite and fractured schist. These zones correspond to lithologies infl uenced by impact processes. Our results yield insights into the infl u-ence of impacts on the deep subsurface biosphere.

on January 15, 2010specialpapers.gsapubs.orgDownloaded from

Microbial abundance in the deep subsurface of the Chesapeake Bay impact crater 943

METHODS

Contamination Assessment

Several methods for contamination assessment were used to select core subsamples for subsequent analyses. Some of these methods are similar to those previously applied to the recovery of marine deep subsurface cores (Gronstal et al., this volume). Briefl y, fi ve methods for contamination assessment were used. During the drilling process, drilling mud was infused with a perfl uorocarbon gas chemical tracer (halon) in order to moni-tor penetration of the drilling mud into cores. Bags containing fl uorescent microspheres (0.518 ± 0.01 µm) of a similar size to microorganisms were introduced into the core-capture device and used to mimic the ability of contaminant cells to enter the samples through fractures in the core material. Dissolved organic carbon (DOC) in pore waters from cores was examined using excitation emission matrix spectroscopy (EEMS). DOC signa-tures from core samples were then compared to the unique DOC signatures of drilling mud in order to detect contamination of core interiors. Direct-injection, negative-electrospray ionization mass spectrometry was used to analyze the acidic polar components in pore water collected from the core subsamples and samples of the drilling fl uid (Rostad and Sanford, 2007). Specifi c ions unique to the drilling fl uid were identifi ed in the mass spectral fi nger-prints and were used to determine whether infi ltration of drilling fl uid and contamination of the aqueous samples had occurred. Finally, microbial 16s rDNA (ribosomal deoxyribonucleic acid) clone libraries were obtained on samples of the drilling mud so that a set of potential microbial contaminants could be identifi ed

77°W38°N

37°

76°

Figure 1. Location of the Chesapeake Bay impact structure and Eyre-ville A and B cores (from Gohn et al., 2006).

and then compared to species cultured from core samples. The enumeration samples were evaluated for contamination based on the DOC drilling-fl uid signature and the presence or absence of fl uorescent microspheres (Gronstal et al., this volume).

Microbial Enumerations

The enumeration of microorganisms was undertaken from subcores obtained from retrieved core sections in a sterile hood using aseptic techniques. Eighty-eight samples were obtained at ~10–30 m intervals, depending upon core retrieval and the abil-ity to select consolidated sediment for enumeration. No samples were collected from the large granite block (Fig. 2). Samples of sediment were removed from the interior of core samples using sterile spatulas after cores were broken in two. For each depth, two separate samples were collected to improve statistical con-fi dence. Enumeration samples were stored in 4% formalin in phosphate-buffered saline (PBS) and refrigerated at 4 °C prior to counting. Enumerations were carried out on the suspended sediment samples fi ltered onto black 0.2 µm polycarbonate fi l-ters following mild sonication to remove organisms from sedi-ment particles. The DNA-binding dye DAPI (4′, 6′-diamidino-2-phenylindole 2HCl) was used to stain samples. Counts were also cross-calibrated on some sediment samples stained with Syto 9 DNA binding dye (Invitrogen, Paisley, UK). For each of the two samples acquired from each depth, two separate samples for enu-meration were prepared and enumerated. A minimum of 50 fi elds per sample were counted under epifl uorescence microscopy on a Leica DMRP microscope, so that a total of at least 200 fi elds of view were counted for each depth. The numbers reported here are the mean values. For DAPI staining, organisms were observed using an excitation fi lter with a bandpass of 340–380 nm (fi lter cube A) and an emission fi lter with long-band cut-off >425 nm. For Syto 9 staining, an excitation fi lter with a bandpass of 450–490 nm (fi lter cube I3) and an emission fi lter with long-band cut-off >515 nm were used. The number of organisms was calculated per gram of dry weight of sediment. Negative controls using sample bottles containing formalin without added sediment were examined. The detection limit of the method is relatively high because the volumetric conversion factor for calculating cells per gram is so high. For example, a count of 2–3 cells total in 200 fi elds of view leads to enumeration estimates ≤104 cells/g. We accept a detection limit here of 104 cells/g.

RESULTS

Substantial differences were found between the DAPI and Syto 9 method of counting cells. The mean difference between DAPI and Syto 9 counts in the region corresponding to the logarithmic decline in cells (zone 1) was 128 times greater in counts using Syto 9. Counts using Syto 9 remained high (>107 cells) in the region at the bottom of zone 1, where nei-ther DAPI nor cell culturing revealed the presence of organ-isms (data not shown). The enumeration values reported here

on January 15, 2010specialpapers.gsapubs.orgDownloaded from

944 Cockell et al.

are based on DAPI counts. No microorganisms were observed in the negative control samples.

The large-scale trend of microbial abundance through the core exhibits three distinct microbiological zones (Fig. 2): zone 1 shows a logarithmic decline through the postimpact sediments (zone 1a: 127–444 m) into the diamicton interpreted as resurge deposits (zone 1b: 444–618 m) and sediment megablocks with diamicton intercalations (zone 1c: 618–867 m); zone 2 shows microbial abundances below the detection limit (800–1096 m); and zone 3 shows an increase in microbial abundance in the suevite and underlying schist and pegmatite (1397–1766 m).

The postimpact deposits (zone 1a) consist of marine, fi ne-grained siliciclastic sediments, commonly with biogenic com-ponents (shells and microfossils). The average porosity of these deposits is between 40% and 55%, average pore-water concentra-

tions of DOC and sulfate were 64 mg/L and 2809 mg/L, respec-tively, and sediments contained negligible Fe (III) and 1.6 µmol/g Fe (II) (Sanford et al., this volume). The interval of the impact material with detectable cells (zone 1b) is in the upper crater-fi ll unit. This interval (444–618 m) is composed of sediment-clast breccias consisting primarily of nonmarine-sediment clasts in a glauconitic matrix with sparse shocked rock, and mineral clasts, and melt particles. The abundant glauconite grains in this interval are explained by mixing of Cretaceous and Tertiary marine sedi-ments from the upper part of the impact target sediment layer with Cretaceous nonmarine sediments from the lower part. The region below this interval, from 618 to 867 m (zone 1c), consists pri-marily of clasts and matrix derived from the Cretaceous nonma-rine target sediments; here, microbial numbers are low but above detection. Within zones 1b and1c, the average concentration of

Postimpactsediments(lateEocenetoPleistocene)

Cra

ter

P

ostim

pact

(x 104 mg/L)

(°C)

Figure 2. Microbial enumerations (log abundance per gram dry weight) through the Chesapeake Bay impact structure. The stratigraphy, total dissolved solids (TDS) concentrations, and temperature profi le (to 1100 m) are shown.

on January 15, 2010specialpapers.gsapubs.orgDownloaded from

Microbial abundance in the deep subsurface of the Chesapeake Bay impact crater 945

DOC and sulfate were 27 mg/L and 1729 mg/L, Fe (III) was neg-ligible, and Fe (II) was 0.49 µmol/g (Sanford et al., this volume).

Zone 2, where microbial numbers are below the detection limit, corresponds to the lower part of the sediment breccias (867–1096 m). It consists primarily of Cretaceous nonmarine sands and clays from the lower section of the target sediment layer pres-ent as intact clasts, up to 17 m size, and as disaggregated matrix between the clasts. In these segments, salinity peaks at 60 g/L. Within zone 2, the average porosity is 20%–30%, the average pore-water concentrations of DOC and sulfate were 18 mg/L and 1758 mg/L, respectively, sediment Fe (III) was negligible, and Fe (II) was 1.33 µmol/g (Sanford et al., this volume).

Zone 3 (1397–1766 m), where microbial numbers increase below the granite megablock, corresponds to two distinct litho-logic units: (1) suevitic (melt-bearing) and impact lithic breccias corresponding to ground-surge and aerodynamic fallback mate-rial (1397–1551 m); and (2) cataclastically deformed schist and pegmatite (1766–1551 m). Within this zone, the average porosity is 5%–15%, the average pore-water concentrations of DOC and sulfate was 44 mg/L and 1574 mg/L, respectively, sediment Fe

(II) was 3.6 µmol/g, and Fe (III) was negligible (Sanford et al., this volume).

Comparisons of enumeration data against lithology (Fig. 3) show that lower abundances are associated with the lithologies deeper within the crater (e.g., oxidized clay, diamicton, and suevite), consistent with the lower cell counts in zones 1b–1c and 3 in Figure 2. Porosity measurements (Fig. 4) show that lower porosities (generally less than 35%) are associated with deeper clasts within the crater and lower cell numbers. This is in contrast to the postimpact sediments, where porosity is greater (>35%), and cell numbers are higher.

DISCUSSION

The drilling of the Chesapeake Bay impact structure has provided the fi rst opportunity to investigate the infl uence of an impact on the deep subsurface biosphere and the correlation between microbial abundance and the lithologic units deposited and/or disrupted by impact. Our data show three distinct zones of colonization.

1 x 108

1 x 107

1 x 106

1 x 105

1 x 104

1 x 103

Clay Clay

Oxidize

d clay

Suevite

Diamict

on

Pegm

atite

& sc

hist

Clayey

silt

Sandy s

iltSan

dSan

d

Lithology

Cel

l abu

ndan

ce

N = 5

N = 5

N = 5

N = 7

N = 7

N = 6

N = 6

N = 16

N = 13

N = 18

Figure 3. Cell abundances within specifi c clasts or sediment types. Solid lines represent the range of values; boxes are the mean and one standard deviation. Lithologic units within the crater are shown to the right of the vertical line and within the gray shading. Depth ranges for the enumeration samples within each category: clay 142–442 m and 700–1020 m; clayey silt 131–362 m; sandy silt 244–344 m; oxidized clay 537–940 m; sand 127–136 m and 588–1085 m; diamicton 446–1095 m; suevite 1397–1551 m; and pegmatite and schist, 1551–1750 m. Lithology is simplifi ed after Edwards et al. (this volume, Chapters 3 and 4) and Horton et al. (this volume). N equals the number of samples analyzed within each category.

on January 15, 2010specialpapers.gsapubs.orgDownloaded from

946 Cockell et al.70

60

40

50

30

20

10

0

Por

osity

(%

)M

icro

bial

cel

l abu

ndan

ce (

g-1)

A

B

Figure 4. (A) Average percent porosity based on laboratory estimates and (B) cell numbers per gram for lithologic units in the Chesapeake Bay impact structure (after Sanford et al., this volume). Solid line is the range of values, and the total height of each block equals one standard deviation. Yorktown Formation, Eastover Formation, St. Marys Formation, Calvert Formation, and Chickahominy Formation are part of the postimpact depositional sequence. The matrix, sand clasts, clay clasts, and sand blocks are in the resurge sediments and avalanche of the Chesapeake Bay impact structure. No enumerations were done above the St. Marys Formation.

on January 15, 2010specialpapers.gsapubs.orgDownloaded from

Microbial abundance in the deep subsurface of the Chesapeake Bay impact crater 947

From 127 to 867 m, the fi rst zone, microbial abundance data show a logarithmic decline in cell numbers, from almost 108 to 104 cells/g. The cause of this steep decline is not easily explained. The trend begins in the postimpact sediment and continues through the uppermost section of the resurge sediments, which is qualitatively consistent with previously studied marine environ-ments (e.g., Parkes et al., 1994; D’Hondt et al., 2004). Chapelle et al. (1987) observed a similar decline in cell abundances measured on samples from a series of noncontiguous cores of unconsoli-dated coastal-plain sediments collected from within the Patap sco aquifer at a site 32–40 km west of the outer rim of the crater. The aquifer contains fresh, relatively young water (<10,000 a) that is recharged in or near sediment outcropping or subcrops, and groundwater fl ow is to the south and east. The samples come from geologic provinces similar to Chesapeake Bay impact struc-ture, from recent to Lower Cretaceous, but they were retrieved from a shallower core hole (<200 m). These samples contained lignitic organic matter but had very low concentrations of DOC (0.4–1 mg/L). The general decline in cell number in these samples (both deep marine sediments and the coastal-plain sediments) could be related to a decrease in available carbon at depth. How-ever, a decline in carbon does not appear to explain the steep-ness in the Chesapeake Bay impact structure samples because the DOC concentrations in the Chesapeake Bay impact structure substantially exceed those found in both environments. DOC availability appears to be high throughout the core (>16 mg/L) (Sanford et al., this volume). However, total organic carbon (TOC) is only high in the interval 200–400 m, where it can exceed 1.36% by weight, and at 1671 m, where it is the highest in the core (2.22%; Sanford et al., this volume). Although there is no linear correlation between cell numbers and either DOC or TOC, the higher concentrations of TOC in the postimpact sediments may refl ect a greater supply of biologically available carbon and may account for some of the high cell counts in the upper region of the core compared to the sediment-clast breccias.

Alternatively, the gradient of increasing salt concentration may be an important factor. Salinity concentrations throughout the impact sections substantially exceed that of seawater (almost reaching two times that of seawater between 800 and 1100 m) (Poag, 1999; Sanford et al., 2004; McFarland and Bruce, 2006). High salt concentrations generally inhibit microbial growth, and although marine microorganisms may be tolerant, and some spe-cies even prefer high salinities, few organisms have adapted to such high salinities (Grant, 2004). Thus, the salinities reported here would be expected to limit the diversity and abundance of microorganisms. Although briny liquids might plausibly be pro-duced by hydrothermal boiling, the volume of brine suggests instead that it is a pre-impact relic that was never fl ushed from the crater due to the low hydraulic conductivity (Sanford, 2003, 2005; Gohn et al., 2008; Sanford et al., this volume).

The porosity of the materials may contribute to differences in abundance. Porosity refl ects the amount of “free” space poten-tially available for microbial colonization. In general, porosi-ties are higher in the postimpact sediments than in the impact

sediment-clast breccias; however, porosities throughout the post-impact sediments do not show a sequential decline with depth. This suggests that decreasing porosity is not exclusively respon-sible for establishing the logarithmic trend of decline. Although porosity is high, hydraulic conductivity is lower in sections of the postimpact sediments than in many of the regions within the crater. There may be space for cells but inadequate fl ow to deliver nutrients, and, thus, growth and cell abundances would be more limited. Beneath the postimpact sediment, the more sandy, glauconite-rich material has a lower porosity but is generally unsorted and unstratifi ed, which may contribute to the accessi-bility of this region of the core to microorganisms and nutrients.

Small-scale variations occur within the overall trend of loga-rithmic decline in microbial cell abundances. Variations through-out the top sections of the core may refl ect changes in mineral-ogy, sediment grain size, and/or in situ geochemistry associated with postimpact sedimentary deposition from 127 to 440 m. Cell abundances were not directly related to the sediment or clast type per se. Enumerations in samples from clays in the postimpact sediments were higher than samples from oxidized clays within the impactites fi lling the crater. The same was true for the sand clasts. However, within the postimpact sediments, lower values are associated with more clay-rich clasts, and the highest val-ues are found in the sandy silt, which also generally has higher values of TOC (Fig. 3; Sanford et al., this volume). From 444 m to the bottom of the fi rst microbiological zone (867 m), larger-scale variations in microbial abundance are observed, and these may be caused by the abrupt changes in lithologies and the jux-taposition of very different source impact material as a result of oscillations during ocean resurge into the crater cavity (Powars et al., 2007, this volume). This suggestion is supported by the different mean cell abundances within clasts shown in Figure 3. Variations probably occur at the small scale even within zones of apparently similar gross clast defi nition. In other subsurface environments, variability in cell abundances at depth have been observed and correlated with lithological changes (Inagaki et al., 2003) or shifts in interstitial water geochemistry (Parkes et al., 2005; D’Hondt et al., 2004; Biddle et al., 2005).

Above the granite megablock, in zone 2, microbial numbers between 867 and 1096 m are below the detection limits of the methods we used (~104 cells/gram dry weight sediment). In addi-tion, other microbiological methods (cell cultivation and molecu-lar biological studies) have not revealed the presence of cells (data not shown). The hydrologic data suggest that the permeability of this section is low and that water within the section is a relic of the immediate postimpact environment (see previous discussion). Our data are consistent with a scenario that includes sterilization of the section by impact-generated heat (either directly or as a result of vertically advected heat from the suevites and impact melt rocks) (Sanford, 2005; Malinconico et al., this volume) and high salinity that has limited microbial recolonization. Thus, this interval has remained biologically impoverished since impact.

Zone 2 (867–1096 m) corresponds to the lower part of the sediments that may have been deposited in an avalanche along

on January 15, 2010specialpapers.gsapubs.orgDownloaded from

948 Cockell et al.

with the granite megablock, since the succession is similar to the target stratigraphy, where Cretaceous nonmarine sediments over-lie pre-Mesozoic igneous and metamorphic rocks. However, the avalanche may have consisted of multiple slides that rode over one another. Microbial abundance below detection throughout this part of the sedimentary succession would be consistent with the section representing a single lithologic unit. This geologi-cal information suggests another hypothesis: that this section of the crater had low microbial abundance prior to impact and was emplaced as a single unit. It has remained impoverished despite an upward fl ux of water from beneath the suevite section that may not have displaced the pore water within the unit but may have moved through more hydraulically conductive material than the emplaced unit.

An important feature of the abundance data is a rise in cell numbers below the granite megablock in the region correspond-ing to suevite and fractured schist and pegmatite rock (zone 3). Although these materials were likely subjected to sterilizing temperatures in the impact-induced hydrothermal activity occur-ring after impact (Sanford, 2005) or by heating from melt rocks, microorganisms were reintroduced at some point after ambient temperatures within the structure dropped below the upper tem-perature limit for microbial growth (<113 °C; Böchl et al., 1997).

Cell numbers are around 106 per gram in the suevitic (melt-bearing) and impact lithic breccias (1397–1551 m). Although the lower part of the succession is melt-poor, the upper part of the section from 1397 to 1474 m contains 20%–30% impact-melt volume (Horton et al., this volume). Other minerals, e.g., quartz, also display shock features such as planar deformation features. The high degree of shock and melt in the upper part of the section suggests that the section, or its constituent minerals, were sub-jected to sterilizing temperatures during the impact. The quan-tity of melt phases suggests an average temperature at the time of deposition of >350 °C based on numerical calculations used for modeling thermal maturity (Malinconico et al., this volume). This exceeds the upper limit for growth and the accepted ster-ilization temperature for most microorganisms (>121 °C steam or >160 °C dry heat). Although there is no evidence to support regional lateral advection, microorganisms could have gradually diffused in from nearby clasts that were not sterilized or may have been carried in by compaction-driven vertical advection from the permeable schist and pegmatite region below (Sanford et al., this volume). Once ambient temperatures within the struc-ture dropped below the upper temperature limit for microbial growth, colonization could occur.

In the lower part of zone 3 (1551–1766 m), although the rocks do not exhibit shock features, breccia veins and dikes are locally present. The dikes contain shock-deformed clasts, and the suevites contain impact-melt particles. Fracture networks are associated with the dikes, and some narrow veins exhibit micro-fracture networks. These features indicate that brittle cataclastic deformation and fracture formation occurred, and probably con-tinued after the impact event. Because the rocks do not exhibit shock features (which is inconsistent with their location close

to the crater center), they are not likely in situ basement rock; instead, they are likely material slumped from the transient cav-ity in which fracture formation was induced by the attenuated shock wave. Although we do not have corresponding enumera-tion data for the unaltered material at this depth outside the crater, Chapelle et al. (1987) observed cell abundances of 103 cells per gram in sediment similar to the target material in this zone and at depths corresponding to Cretaceous sediments. Our data sug-gest that microorganisms would have migrated into this region of the crater and colonized this section as a result of the impact and postimpact processes that created new habitat and increased the permeability of this region of the crater with the formation of fracture networks. The presence of breccias dikes within the material suggests that dilatancy, or the opening of fractures, occurred during their emplacement before the introduction of a sedimentary cover, which would have contributed to increasing permeability for later biological recolonization.

In summary, we fi nd that present-day microbial abundances show patterns corresponding to the major distinct lithological transitions in the crater. The decline in microbial abundance in the upper part of the crater shows a logarithmic trend consistent with other deep subsurface sites. However, the gradient of this decline is steeper compared to that in marine settings. Increases in cell numbers in the deepest section of the core hole show that recolonization occurred after the impact in the deeper part of the crater and that this process was likely to have been facilitated by impact-induced fracturing of the target material and postimpact compaction-driven vertical advection.

ACKNOWLEDGMENTS

We thank the National Aeronautics and Space Administration (NASA) Astrobiology Institute for fi nancial assistance with the implementation of the contamination assessment procedures. The enumeration data presented here were obtained by Aaron Gronstal in fulfi llment of the degree of Doctor of Philosophy at the Open University. We thank Philipp Heidinger for provid-ing the core hole temperature data. We thank Brian Horsfi eld and Craig Moyer for valuable review comments. Kai Finster acknowledges support by the Carlsberg Foundation, grant 2005-1-275. Any use of trade, product, or fi rm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. government.

REFERENCES CITED

Alvarez, L.W., Alvarez, W., Asaro, F., and Michel, H.V., 1980, Extraterrestrial cause for the Cretaceous-Tertiary extinction—Experimental results and theoretical interpretation: Science, v. 208, p. 1095–1108, doi: 10.1126/science.208.4448.1095.

Ashton, P.J., and Schoeman, F.R., 1983, Limnological studies on the Pretoria Salt Pan, a hypersaline maar lake. 1. Morphometric, physical and chemi-cal features: Hydrobiologia, v. 99, p. 61–73, doi: 10.1007/BF00013718.

Biddle, J.F., House, C.H., and Brenchley, J.E., 2005, Microbial stratifi ca-tion in deeply buried marine sediment refl ects changes in sulfate/methane profi les: Geobiology, v. 3, p. 287–295, doi: 10.1111/j.1472-4669.2006.00062.x.

on January 15, 2010specialpapers.gsapubs.orgDownloaded from

Microbial abundance in the deep subsurface of the Chesapeake Bay impact crater 949

Böchl, E., Rachel, R., Burggraf, S., Hafenbradl, D., Jannasch, H.W., and Stet-ter, K.O., 1997, Pyrolobus fumarii, gen and sp. nov. represents a novel group of archaea, extending the upper temperature limit for life at 113°C: Extremophiles, v. 1, p. 14–21, doi: 10.1007/s007920050010.

Chapelle, F.H., Zelibor, J.L., Grimes, D.J., and Knobel, L.L., 1987, Bacteria in deep coastal plain sediments of Maryland: A possible source of CO

2 to

groundwater: Water Resources Research, 23, no. 8, p. 1625–1632.Cockell, C.S., 2006, The origin and emergence of life under impact bombard-

ment: Philosophical Transactions of the Royal Society of London, v. 361, p. 1845–1855, doi: 10.1098/rstb.2006.1908.

Cockell, C.S., and Lee, P., 2002, The biology of impact craters—A review: Bio-logical Reviews of the Cambridge Philosophical Society, v. 77, p. 279–310, doi: 10.1017/S146479310100584X.

Cockell, C.S., and Osinski, G., 2007, Impact-induced impoverishment and transformation of a sandstone habitat for lithophytic microorganisms: Meteoritics & Planetary Science, v. 42, p. 1985–1993.

Cockell, C.S., Lee, P., Schuerger, A., Hidalgo, L., Jones, J., and Stokes, D., 2001, Microbiology and vegetation of micro-oases and polar desert, Haughton impact crater, Devon Island, Canadian High Arctic: Arctic, Alpine and Antarctic Research, v. 33, p. 306–318, doi: 10.2307/1552238.

Cockell, C.S., Lee, P., Osinski, G., Horneck, G., and Broady, P., 2002, Impact-induced microbial endolithic habitats: Meteoritics & Planetary Science, v. 37, p. 1287–1298.

Cockell, C.S., Lee, P., Broady, P., Lim, D.S.S., Osinski, G.R., Parnell, J., Koe-berl, C., Pesonen, L., and Salminen, J., 2005, Effects of asteroid and comet impacts on habitats for lithophytic organisms—A synthesis: Mete-oritics & Planetary Science, v. 40, p. 1901–1914.

Cremer, H., and Wagner, B., 2003, The diatom fl ora in the ultra-oligotrophic Lake El’gygytgyn, Chukotka: Polar Biology, v. 26, p. 105–114.

D’Hondt, S., Jorgensen, B.B., Miller, D.J., Batzke, A., Blake, R., Cragg, B.A., Cypionka, H., Dickens, G.R., Ferdelman, T., Hinrichs, K.U., Holm, N.G., Mitterer, R., Spivack, A., Wang, G.Z., Bekins, B., Engelen, B., Ford, K., Gettemy, G., Rutherford, S.D., Sass, H., Skilbeck, C.G., Aiello, I.W., Guerin, G., House, C.H., Inagaki, F., Meister, P., Naehr, T., Niitsuma, S., Parkes, R.J., Schippers, A., Smith, D.C., Teske, A., Wiegel, J., Padilla, C.N., and Acosta, J.L.S., 2004, Distributions of microbial activities in deep subseafl oor sediments: Science, v. 306, p. 2216–2221, doi: 10.1126/science.1101155.

Edwards, L.E., and Powars, D.S., 2003, Impact damage to dinocysts from the late Eocene Chesapeake Bay impact event: Palaios, v. 18, p. 275–285, doi: 10.1669/0883-1351(2003)018<0275:IDTDFT>2.0.CO;2.

Edwards, L.E., Powars, D.S., Gohn, G.S., and Dypvik, H., 2009, this volume, Chapter 3, Geologic columns for the ICDP-USGS Eyreville A and B cores, Chesapeake Bay impact structure: Sediment-clast breccias, 1096 to 444 m depth, in Gohn, G.S., Koeberl, C., Miller, K.G., and Reimold, W.U., eds., The ICDP-USGS Deep Drilling Project in the Chesapeake Bay Impact Structure: Results from the Eyreville Core Holes: Geological Society of America Special Paper 458, doi: 10.1130/2009.2458(03).

Edwards, L.E., Powars, D.S., Browning, J.V., McLaughlin, P.P., Jr., Miller, K.G., Self-Trail, J.M., Kulpecz, A.A., and Elbra, T., 2009, this volume, Chapter 4, Geologic columns for the ICDP-USGS Eyreville A and C cores, Chesapeake Bay impact structure: Postimpact sediments, 444 to 0 m depth, in Gohn, G.S., Koeberl, C., Miller, K.G., and Reimold, W.U., eds., The ICDP-USGS Deep Drilling Project in the Chesapeake Bay Impact Structure: Results from the Eyreville Core Holes: Geological Society of America Special Paper 458, doi: 10.1130/2009.2458(04).

Gohn, G.S., Koeberl, C., Miller, K.G., Reimold, W.U., Cockell, C.S., Horton, J.W., Jr., Sanford, W.E., and Voytek, M.A., 2006, Chesapeake Bay impact structure drilled: Eos (Transactions, American Geophysical Union), v. 87, p. 349, 355.

Gohn, G.S., Koeberl, C., Miller, K.G., Reimold, W.U., Browning, J.V., Cockell, C.S., Horton, J.W., Jr., Kenkmann, T., Kulpecz, A.A., Powars, D.S., San-ford, W.E., and Voytek, M.A., 2008, Deep drilling of the Chesapeake Bay impact structure: Science, v. 320, p. 1740–1745.

Grant, W.D., 2004, Life at low water activity: Philosophical Transactions of the Royal Society of London, ser. B, v. 359, p. 1249–1267, doi: 10.1098/rstb.2004.1502.

Grönlund, T., Lortie, G., Guilbault, J.-P., Bouchard, M.A., and Saarnisto, M., 1990, Diatoms and arcellaceans from Lac du Cratère du Nouveau-Québec, Ungava, Québec, Canada: Canadian Journal of Botany, v. 68, p. 1187–1200, doi: 10.1139/b90-151.

Gronstal, A., Voytek, M.A., Kirshtein, J.D., Bach, N., Lowit, M., and Cock-ell, C.S., 2009, this volume, Contamination assessment in microbiologi-

cal sampling of the Eyreville core, Chesapeake Bay impact structure, in Gohn, G.S., Koeberl, C., Miller, K.G., and Reimold, W.U., eds., The ICDP-USGS Deep Drilling Project in the Chesapeake Bay Impact Struc-ture: Results from the Eyreville Core Holes: Geological Society of Amer-ica Special Paper 458, doi: 10.1130/2009.2458(41).

Horsfi eld, B., Kieft, T.L., and the Geobiosphere Group, 2007, The Geobio-sphere, in Harms, U., Koeberl, C., and Zoback, M.D., eds., Continental Scientifi c Drilling: Heidelberg, Springer, p. 163–211.

Horton, J.W., Jr., Aleinikoff, J.N., Kunk, M.J., Gohn, G.S., Edwards, L.E., Self-Trail, J.M., Powars, D.S., and Izett, G.A., 2005a, Recent research on the Chesapeake Bay impact structure, USA—Impact debris and reworked ejecta, in Kenk-mann, T., Hörz, F., and Deutsch, A., eds., Large Meteorite Impacts III: Geo-logical Society of America Special Paper 384, p. 147–170.

Horton, J.W., Jr., Powars, D.S., and Gohn, G.S., eds., 2005b, Studies of the Chesapeake Bay Impact Structure—The USGS-NASA Langley Core-hole, Hampton, Virginia, and Related Coreholes and Geophysical Sur-veys: U.S. Geological Survey Professional Paper 1688, chapters pagi-nated separately.

Horton, J.W., Jr., Gibson, R.L., Reimold, W.U., Wittmann, A., Gohn, G.S., and Edwards, L.E., 2009, this volume, Geologic columns for the ICDP-USGS Eyreville B core, Chesapeake Bay impact structure: Impactites and crystalline rocks, 1766 to 1096 m depth, in Gohn, G.S., Koeberl, C., Miller, K.G., and Reimold, W.U., eds., The ICDP-USGS Deep Drilling Project in the Chesapeake Bay Impact Structure: Results from the Eyre-ville Core Holes: Geological Society of America Special Paper 458, doi: 10.1130/2009.2458(02).

Inagaki, F., Suzuki, M., Takai, K., Oida, H., Sakamoto, T., Aoki, K., Nealson, K.H., and Horikoshi, K., 2003, Microbial communities associated with geological horizons in coastal subseafl oor sediments from the Sea of Okhotsk: Applied and Environmental Microbiology, v. 69, p. 7224–7235, doi: 10.1128/AEM.69.12.7224-7235.2003.

Jackson, J.C., Horton, J.W., Chou, I-M., and Belkin, H.E., 2006, A shock-induced polymorph of anatase and rutile from the Chesapeake Bay impact structure, Virginia, USA: The American Mineralogist, v. 91, p. 604–608, doi: 10.2138/am.2006.2061.

Kieffer, S.W., 1971, Shock metamorphism of the Coconino sandstone at Meteor crater, Arizona: Journal of Geophysical Research, v. 76, p. 5449–5473, doi: 10.1029/JB076i023p05449.

Koeberl, C., Poag, C.W., Reimold, W.U., and Brandt, D., 1996, Impact origin of the Chesapeake Bay structure, and source of the North American tektites: Science, v. 271, p. 1263–1266, doi: 10.1126/science.271.5253.1263.

Kring, D.A., 1997, Air blast produced by the Meteor crater impact event and a reconstruction of the affected environment: Meteoritics & Planetary Sci-ence, v. 32, p. 517–530.

Lee, S.R., Horton, J.W., and Walker, R.J., 2006, Confi rmation of a meteor-itic component in melt-rocks of the Chesapeake Bay impact structure, Virginia, USA—Evidence from osmium isotopic and PGE systematics: Meteoritics & Planetary Science, v. 41, p. 819–833.

Malinconico, M.L., Sanford, W.E., and Horton, J.W., Jr., 2009, this volume, Postimpact heat conduction and compaction-driven fl uid fl ow in the Chesapeake Bay impact structure based on downhole vitrinite refl ec-tance data, ICDP-USGS Eyreville deep core holes and Cape Charles test holes, in Gohn, G.S., Koeberl, C., Miller, K.G., and Reimold, W.U., eds., The ICDP-USGS Deep Drilling Project in the Chesapeake Bay Impact Structure: Results from the Eyreville Core Holes: Geological Society of America Special Paper 458, doi: 10.1130/2009.2458(38).

McFarland, E.R., and Bruce, T.S., 2006, The Virginia Coastal Plain Hydrogeo-logic Framework: U.S. Geological Survey Professional Paper 1731, 118 p.

Melosh, H.J., 1989, Impact Cratering: A Geologic Process: Oxford University Geological Monograph 11, 245 p.

Osinski, G.R., 2005, Hydrothermal activity associated with the Ries impact event, Germany: Geofl uids, v. 5, p. 202–220, doi: 10.1111/j.1468-8123.2005.00119.x.

Osinski, G.R., Spray, J.G., and Lee, P., 2001, Impact-induced hydrothermal activity within the Haughton impact structure, Arctic Canada: Generation of a tran-sient, warm, wet oasis: Meteoritics & Planetary Sciences v. 36, p. 731–745.

Parkes, R.J., Cragg, B.A., Bale, S.J., Getliff, J.M., Goodman, K., Rochelle, P.A., Fry, J.C., Weightman, A.J., and Harvey, S.M., 1994, Deep bacterial biosphere in Pacifi c Ocean sediments: Nature, v. 371, p. 410–413, doi: 10.1038/371410a0.

Parkes, R.J., Webster, G., Cragg, B.A., Weightman, A.J., Newberry, C.J., Fer-delman, T.G., Kallmeyer, J., Jørgensen, B.B., Aiello, I.W., and Fry, J.C.,

on January 15, 2010specialpapers.gsapubs.orgDownloaded from

950 Cockell et al.

2005, Deep sub-seafl oor prokaryotes stimulated at interfaces over geo-logical time: Nature, v. 436, p. 390–394, doi: 10.1038/nature03796.

Poag, C.W., 1999, The Chesapeake Invader: Princeton, New Jersey, Princeton University Press, 183 p.

Poag, C.W., Powars, D.S., Poppe, L.J., Mixon, R.B., Edwards, L.E., Fol-ger, D.W., and Bruce, S., 1992, Deep Sea Drilling Project Site 612 bolide event: New evidence of a late Eocene impact-wave deposit and a possible impact site, U.S. East Coast: Geology, v. 20, p. 771–774, doi: 10.1130/0091-7613(1992)020<0771:DSDPSB>2.3.CO;2.

Poag, C.W., Hutchinson, D.R., Colman, S.M., and Lee, M.W., 1999, Seismic expression of the Chesapeake Bay impact crater: Structural and mor-phologic refi nements based on new seismic data, in Dressler, B.O., and Sharpton, V.L., eds., Large Meteorite Impacts and Planetary Evolution II: Geological Society of America Special Paper 339, p. 149–164.

Poag, C.W., Koeberl, C., and Reimold, W.U., 2004, The Chesapeake Bay Crater—Geology and Geophysics of a Late Eocene Submarine Impact Structure: Impact Studies Series: Heidelberg, Springer, 522 p.

Powars, D.S., and Bruce, T.S., 1999, The Effects of the Chesapeake Bay Impact Crater on the Geological Framework and Correlation of Hydrogeologic Units of the Lower York–James Peninsula, Virginia: U.S. Geological Sur-vey Professional Paper 1612, 82 p.

Powars, D.S., Poag, C.W., and Mixon, R.B., 1993, The Chesapeake Bay “impact crater”: Seismic and stratigraphic evidence: Geological Society of America Abstracts with Programs, v. 25, no. 6, p. A-378.

Powars, D.S., Edwards, L.E., Gohn, G.S., and Catchings, R.D., 2007, Above the avalanches, below the waves: The ICDP-USGS Eyreville cores and adjacent seismic refl ections profi le within the moat of the Chesapeake Bay impact structure: Geological Society of America Abstracts with Pro-grams, v. 39, no. 6, p. 533.

Powars, D.S., Catchings, R.D., Goldman, M.R., Gohn, G.S., Horton, J.W., Jr., Edwards, L.E., Rymer, M.J., and Gandhok, G., 2009, this volume, High-resolution seismic-refl ection images across the ICDP-USGS Eyreville deep drilling site, Chesapeake Bay impact structure, in Gohn, G.S., Koe-berl, C., Miller, K.G., and Reimold, W.U., eds., The ICDP-USGS Deep Drilling Project in the Chesapeake Bay Impact Structure: Results from the Eyreville Core Holes: Geological Society of America Special Paper 458, doi: 10.1130/2009.2458(11).

Riding, R., 1979, Origin and diagenesis of lacustrine algal bioherms at the mar-gin of the Ries crater, Upper Miocene, southern Germany: Sedimentol-ogy, v. 26, p. 645–680, doi: 10.1111/j.1365-3091.1979.tb00936.x.

Rostad, C.E., and Sanford, W.E., 2007, Polar organic compounds in pore waters from the Chesapeake Bay impact crater: Geological Society of America Abstracts with Programs, v. 39, no. 6, p. 317.

Sanford, W.E., 2003, Heat fl ow and brine generation following the Chesapeake Bay bolide impact: Journal of Geochemical Exploration, v. 78/79, p. 243–247, doi: 10.1016/S0375-6742(03)00108-0.

Sanford, W.E., 2005, A simulation of the hydrothermal response to the Chesa-peake Bay bolide impact: Geofl uids, v. 5, p. 185–201, doi: 10.1111/j.1468-8123.2005.00110.x.

Sanford, W.E., Gohn, G.S., Powars, D.S., Horton, J.W., Jr., Edwards, L.E., Self-Trail, J.M., and Morin, R.H., 2004, Drilling the central crater of the Chesapeake Bay impact structure: A fi rst look: Eos (Transactions, Ameri-can Geophysical Union), v. 85, p. 369–377, doi: 10.1029/2004EO390001.

Sanford, W.E., Voytek, M.A., Powars, D.S., Jones, B.F., Cozzarelli, I.M., Cock-ell, C.S., and Eganhouse, R.P., 2009, this volume, Pore-water chemistry from the ICDP-USGS core hole in the Chesapeake Bay impact structure—Implications for paleohydrology, microbial habitat, and water resources, in Gohn, G.S., Koeberl, C., Miller, K.G., and Reimold, W.U., eds., The ICDP-USGS Deep Drilling Project in the Chesapeake Bay Impact Struc-ture: Results from the Eyreville Core Holes: Geological Society of Amer-ica Special Paper 458, doi: 10.1130/2009.2458(36).

Schoeman, F.R., and Ashton, P.J., 1982, The diatom fl ora of the Pretoria Salt Pan, Transvaal, Republic of South Africa: Bacillaria, v. 5, p. 63–99.

Smelror, M., and Dypvik, H., 2006, The sweet aftermath: Environmental changes and biotic restoration following the marine Mjølnir impact (Volgian-Ryazanian Boundary, Barents Shelf), in Cockell, C., Koeberl, C., and Gilmour, I., eds., Biological Processes Associated with Impact Events: Berlin, Springer-Verlag, p. 143–178.

Smelror, M., Dypvik, H., and Mørk, A., 2002, Phytoplankton blooms in the Jurassic-Cretaceous boundary beds of the Barents Sea possibly induced by the Mjølnir impact, in Buffetaut, E., and Koeberl, C., eds., Geologi-cal and Biological Effects of Impact Events: New York, Springer-Verlag, p. 69–83.

Taylor, S.R., and Kolbe, P., 1965, Geochemistry of Henbury impact glass: Geo-chimica et Cosmochimica Acta, v. 29, p. 741–745, doi: 10.1016/0016-7037(65)90028-1.

Toon, O.W., Zahnle, K., Morrison, D., Turco, R.P., and Covey, C., 1997, Envi-ronmental perturbations caused by the impacts of asteroids and comets: Reviews of Geophysics, v. 35, p. 41–78, doi: 10.1029/96RG03038.

MANUSCRIPT ACCEPTED BY THE SOCIETY 3 MARCH 2009

Printed in the USA

on January 15, 2010specialpapers.gsapubs.orgDownloaded from