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Executive Summary

Introduction

The Harvard NAI team was constituted in 1998 as an interactive group of biogeochemists, paleontologists, sedimentary geologists, geochemists, and tectonic geologists assembled with the common goal of understanding the coevolution of life and environments in Earth history. The team originally proposed to focus multidisciplinary research on four critical intervals of planetary change: the early Archean (>3000 million years ago) when life began, the early Paleoproterozoic (2400-2200 Ma) when oxygen began to accumulate in the atmosphere and surface ocean, the terminal Proterozoic and Early Cambrian (750-525 Ma) when animal life radiated, and the Permo-Triassic boundary (251 Ma) when mass extinction removed some 90 percent of Earth’s species diversity, permanently altering the course of evolution. Given reduced funding levels in years 1 and 2, however, the team chose to focus on the latter three intervals; over the past five years, team members have made substantial contributions to each research area. Fortunately, increased funding in years 3 to 5 and strong interest by additional colleagues at Harvard and MIT enabled us to expand both our membership and intellectual purview. Thus, to the three projects funded from the outset (and approached in fresh ways by our newest co-investigators), we have added four additional research foci. John Hayes’ research at the interface of microbiology and biogeochemistry expanded as a result of incremental funding that enabled his group to undertake a substantial collaborative effort with the NAI Team at MBL; the addition of Dan Schrag (Harvard) in year 2 and Roger Summons (MIT) in year 4 further bolstered our team’s collaborative research in biogeochemistry. In association with the Spanish Center for Astrobiology, Andrew Knoll completed research on Neogene iron formations in southern Spain that will play a direct role in guiding exploration of Mars hematites by the recently launched Mars Explorer Rovers (MERs). Charles Marshall pursued statistical analyses of molecular sequence and biostratigraphic data as part of the NAI Focus Group on Evogenomics. Lastly, Dan Schrag initiated collaborative research with MIT’s Maria Zuber that addresses the enigma of Mars’ carbonate-poor surface and, hence, the history of water on the red planet.

Scope of Team Activities in 2003-2004

Research by the Harvard team is interdisciplinary, attracting broad participation by scientists within the five member institutions (Harvard, MIT, WHOI, Rochester, Smithsonian Institution). We have also been successful in promoting cross-team collaborations — research projects have been undertaken with colleagues from the Carnegie, MBL, Rhode Island, Penn State, NASA Ames, and JPL teams, as well as both the Spanish and Australian astrobiology centers. Moreover, our team participated actively in the Evogenomics Focus Group and took a leadership position in the Mission to Early Earth Focus Group (A. Anbar). Team members are active in research on novel biosignatures and digital mapping technologies that can be applied to solar system research; A. Knoll and J. Grotzinger are members of the 2003 Mars MER science team; and R. Summons serves on Mars Exploration Program Assessment Group (MEPAG) and the Astrobiology Science Strategy Group, committees charged with defining astrobiological research strategies for upcoming Mars missions. Equally important, research by Harvard team members on sedimentary and geochemical biosignatures as well as early states of Earth’s atmosphere will directly influence plans for continuing planetary exploration and the projected Terrestrial Planet Finder mission.

During the past year, Harvard team members conducted field research in Australia, southern Africa, Svalbard, the Canadian Rocky Mountains, China, and Oman. We also taught actively at three universities and in the International Geobiology course, a summer course in geobiology for graduate students and postdoctoral fellows, sponsored by the Agouron Institute. In year 5, much of our Education and Public Outreach (EPO) effort focused on university teaching, but we also helped to develop a traveling museum exhibit on biology and Earth history, in partnership with the Harvard Museum of Natural History, and individual team members lectured to K-12 and adult groups.

Research Accomplishments in 2003-2004

Subproject 1: The Proterozoic Oxidation of the Earth’s Surface

The history of oxygen in the oceans and atmospheres is thought to have played a key role in Earth’s long term biological evolution. Ongoing research by Harvard team members addresses the initial oxygenation of the atmosphere and surface ocean 2400-2200 million years ago (Ma), renewed oxygen influx near the end of the Proterozoic Eon, and life and environments between those two events.

Research by H.D. Holland and colleagues has sharply constrained the timing of Earth’s initial atmospheric oxygenation. Measurements of the isotopic composition of sulfur in pyrites from the Timeball Hill and Rooihogte formations, South Africa, show both a considerably larger range in δ³⁴S than older sulfides and the absence of a significant signal of mass independent fractionation of the sulfur isotopes. These observations provide strong evidence for the presence of oxygen in the atmosphere at the time these sediments were deposited. Re-Os dating of the pyrites yielded an age of 2,322 ± 15 Ma and an initial ¹⁸⁷Os/¹⁸⁸Os ratio of 0.1087 ± 0.0063. The former value shows that the loss of a major δ³³S signal in the atmosphere occurred between 2,450 and 2,322 ± 15 Ma, indicating that O₂ appeared in the atmosphere between these dates. The latter number is essentially equal to that of the mantle at 2,322 Ma, indicating that the input of riverine Os to the oceans was minor, an inference consistent with the absence of Re enrichment in highly carbonaceous shales from this period. This, in turn, suggests that 2322 million years ago atmopsheric oxygen levels were sufficiently high to eliminate the MIF signal in sulfur but sufficiently low to preclude significant oxidative weathering of Re and Os on the continents.

Research on mid-Proterozoic rocks by the Anbar and Knoll labs further contributes to our growing understanding of the redox development of the atmosphere and oceans. In 1998, Donald Canfield proposed that the cessation of iron formation deposition ca. 1850 Ma reflected the expansion of sulfidic deep oceans and not, as traditionally understood, the spread of oxygen throughout ocean basins. Research on the iron chemistry of the ca. 1500-1400 Ma Roper basin in northern Australia, completed by NRC postdoctoral fellow Yanan Shen, Knoll, and Australian Centre for Astrobiology colleague Malcolm Walter Research, demonstrated the presence of a strong redoxcline within this ancient marine basin. Indeed, Shen and colleagues have now shown that all northern Australian basins formed between 1730 and ca. 1400 Ma show evidence of basinal anoxia. The strong facies dependence of sulfur isotopic composition in Roper pyrites further supports the hypothesis that midway through recorded Earth history, marine sulfate levels remained well below their modern level. The Roper basin also contains abundant and exceptionally well preserved microfossils. Research in the Knoll lab showed that these fossils preserve complex wall ultrastructures, imaged by transmission electron microscopy (TEM), that document a moderate diversity of eukaryotic life in coastal environments of the mid-Proterozoic ocean. In complementary research, Ariel Anbar showed (1) that Mo isotopes can be used as paleotracers of redox conditions that integrate over the global ocean and (2) that Mo isotopic measurements of Roper samples support the hypothesis of relatively widespread anoxia in mid-Proterozoic oceans.

Subproject 2: Neoproterozoic-Cambrian environmental change and evolution

This subproject has enjoyed the broadest participation of Harvard team members, and for good reason. The Proterozoic-Cambrian transition witnessed remarkable changes in tectonics, climate, atmospheric composition, and especially life. This is the interval during which animal life — and, hence, the prospect of intelligence — radiated on Earth. Harvard team researchers are studying the paleontology (Knoll, Grotzinger, Erwin), geochronology (Bowring, Grotzinger), tectonics (Hoffman, Bowring), and environmental changes (Hoffman, Schrag, Bowring, Grotzinger) of this interval, with an eye to constructing models of integrated change in the Earth system.

Using new carbon isotope data from carbonate-rich Neoproterozoic and Early Cambrian sections in Svalbard, Namibia and Morocco, Paul Hoffman’s group compiled the first high-resolution C-isotopic curve for Neoproterozoic-Cambrian seawater. The curve highlights a series of high-amplitude biogeochemical anomalies that have no parallel in Phanerozoic (543 Ma to present) or earlier Proterozoic time. Two of these anomalies correspond to the Sturtian and Marinoan snowball Earth episodes, which were preceded uniquely by long intervals when fractional organic burial exceeded 0.4, or twice the modern value. Hoffman’s group also found low-temperature equilibrium fractionation of carbon and oxygen isotopes between coexisting dolomite and calcite in the Marinoan cap carbonate sequence in Namibia and northwestern Canada. This suggests that dolomite formed in contact with seawater, unlike the normal Phanerozoic and Neoproterozoic ocean in which dolomite precipitation was kinetically inhibited. As sulfate ion is a known inhibitor of dolomite above 2mM concentration (versus 28 mM in modern seawater), Hoffman and students proposed that low sulfate concentrations, evolved in an ice-covered ocean, led to sea-floor dolomite formation during post-glacial transgression. Unlike normal marine carbonate phases, dolomite is stable and should be a more faithful carrier of isotopic information.

The dramatic diversification of animal phyla during early Cambrian time has fueled debate regarding the mechanisms of early animal evolution for over a century. What is now clear is that intrinsic catalysts, such as the innovation of developmental genetic mechanisms, as well as extrinsic processes involving environmental change, are both critically important in accounting for this major event in the history of life. Research by John Grotzinger and Sam Bowring on biostratigraphic, geochemical and geochronometric data from Oman lends strong support to the hypothesis that environmental perturbation near the Proterozoic-Cambrian boundary facilitated the Cambrian radiation of bilaterian animal phyla. Biostratigraphic, carbon isotopic, and uranium-lead zircon geochronological data from Ara Group of Oman indicate an abrupt last appearance of latest Proterozoic calcified metazoans coincident with a large-magnitude, but short-lived negative C-isotopic excursion. U-Pb zircon age data from an intercalated ash bed directly constrain this negative excursion to be 542 Ma, consistent with previous constraints from Siberia and Namibia. The absence of calcified fossils in carbonate units above this negative excursion contrasts strongly with their great abundance in carbonate units below the negative excursion. Combined with the global biostratigraphic record, these new data strengthen hypotheses invoking mass extinction within terminal Proterozoic ecosystems at or near the Precambrian-Cambrian boundary.

Subproject 3: Permo-Triassic mass extinction and its consequences

At the end of the Permian Period, 251 million years ago, more than ninety percent of marine species disappeared; land ecosystems were similarly devastated. Harvard team members seek to understand the causes and evolutionary consequences of this greatest of all mass extinctions. During the past year, D. Erwin and S. Bowring have continued field and laboratory research on the timing of P-Tr mass extinction. During the same period, team member John Marshall and colleagues examined old models and developed new models of the oxygen cycle in the past and present oceans. In an earlier publication from this project, they demonstrated how it is difficult to achieve a widespread, sustained anoxia in models of the Late Permian Ocean. This has resulted in a dialog with a group at Penn State on the representation of the oxygen cycle in ocean models. Marshall and colleagues have used a model of the modern ocean circulation and biogeochemistry to explore the implications of the simplified parameterizations that are under scrutiny, finding that allowing negative oxygen concentrations is misleading, though it might be interpreted as “total oxidant,” since it leads to large and erroneously identified regions of anoxia in modern ocean simulations. In addition to better parameterization, models of oxygen distribution in ancient oceans will require more detailed representations of oceanic nutrient and oxygen cycles. Marshall’s team is developing such representations in the context of an atmosphere-ocean box model.

Subproject 4: Molecular and isotopic approaches to microbial ecology and Biogeochemistry

Ancient organic matter provides a rich source of biological and environmental information about the early Earth, as well as a potentially informative source of insights about Mars and other planets. John Hayes’ group continued their investigations of the biogeochemistry of hydrogen isotopes, and this year completed meticulous analyses that demonstrate the quantitative nature of hydrogen isotopic “exchange” between organic matter and water. Hayes also completed field research on the Lost City hydrothermal system, using the research submarine Alvin to gather samples of lipid-rich fluids for biomarker and isotopic analysis. Team member Roger Summons continued his exploration of lipids in living microorganisms and ancient sedimentary rocks, characterizing the lipids of new isolates of extremophilic bacteria and archaeans and discovering molecular biomarker evidence for basinal anoxia in mid-Proterozoic basins from northern Australia (see above). Working together, Hayes, Summons, and MIT colleague Dan Rothman have proposed a novel explanation of the unusual record of C-isotopes in Neoproterozoic marine basins (see above). Their model suggests that high concentrations of dissolved organic matter in anoxic deep waters governed the isotopic composition of Neoproterozoic oceans.

Team member Dan Schrag focused on rates of methanogenesis in sediments from the South American continental margin. Vigorous upwelling off the west coast of South America leads to the deposition of organic rich sediments, creating chemical potential gradients that are host to diverse microbial communities. Ocean Drilling Program Leg 201 drilled several sites from this region last year. Degradation of organic matter generates extremely high alkalinity as well as large concentrations of dissolved ammonium and phosphate. High alkalinity drives the shallow precipitation of authigenic carbonate, resulting in depletions of dissolved Ca that mask that released by calcite dissolution (as indicated by increases with depth in dissolved Sr). Dolomite precipitation causes significant negative excursions of dissolved Mg from background down-core trends that are due to uptake during clay alteration. These excursions are coincident with sharply defined minima in Ca. Isotopic analysis of dissolved inorganic carbon carried out by our laboratory shows a pattern characteristic of sites dominated by methanogenesis. δ¹³C values decrease sharply with depth in the first 10 to 50 meters, reaching values as low as -30 per mil, indicating organic matter oxidation as well as methanotrophy. Values increase below the zone where sulfate disappears, reaching a plateau at values between 0 and +15 per mil. In some cases, the sulfate content of the pore fluid increases again, because sulfate rich brine flows through the basaltic basement rock. Schrag is currently working with numerical models to calculate the rates of methanogenesis and methanotrophy to begin to ask questions about what limits the biological activity in these environments.

Subproject 5: Geobiology of Neogene hematitic sedimentary rocks

In 2003, NASA launched two rover missions to Mars. One of the rovers is slated to touch down in Meridini Planum, a region marked by aqueous hematite deposition. If we are to maximize the scientific opportunities of this mission, we must first complete careful studies of analogous systems on Earth, where biological and physical processes can be tied directly to paleobiological and geochemical patterns in deposited iron-rich sediments. The Rio Tinto drainage area of southern Spain offers just such an opportunity. During the past funding year, team member Andrew Knoll and colleagues from the Spanish Center for Astrobiology and the Johnson Space Center completed laboratory research on previously collected samples of present day Rio Tinto sediments and diagenetically stabilized iron rocks formed over the past two million years. The samples show how coarse grained hematites can arise during diagenesis of originally goethitic and jarositic sediments, and detail the macroscopic and microscopic textures that record biosignatures in hematitic rocks.

Subproject 6: Evogenomics (Collaborative focus group research)

Research by team member Charles Marshall focused on improving confidence intervals in paleontological estimates of evolutionary first and last appearances of taxa and on reconciling molecular clock and paleontological estimates of evolutionary divergence times. Research has also continued on the construction of a global database for the fossil record that will enable paleontologists to understand the history of biological diversity and facilitate the differentiation of the biological from geological signals in Phanerozoic marine diversity studies.

Subproject 7: Water and carbonates on the early surface of Mars

Although liquid water is not stable at the surface today, a large number of observations suggest that liquid water existed at least episodically at various times throughout Martian history. Although most morphological evidence for liquid water on Mars is consistent either with episodic and rapid release of water at the surface, or else with liquid water in the subsurface that results in chemical weathering reactions, the presence of valley networks and the degradation of impact craters on ancient surfaces of late Noachian age imply weathering and erosion by liquid water at the surface for substantial amounts of time. One particularly interesting aspect of Martian surface geology is the apparent discrepancy between the geological evidence for water on Mars during the Noachian and the lack of calcium carbonate on the surface. If a large body of liquid water on Mars persisted for millions of years or longer, the high CO₂ atmosphere would form carbonic acid, and react with the silicate crust, producing calcium carbonate. However, carbonate minerals have not yet been detected on the surface in sufficient quantities to be consistent with this hypothesis.

Team member Dan Schrag and MIT colleague Maria Zuber have begun to address this problem, hypothesizing that climate episodes warm enough to maintain an active hydrologic cycle endured only long enough to produce the erosional features, not long enough for calcium carbonate to reach saturation. They are currently modeling this scenario for different ocean volumes and different atmospheric CO₂ concentrations.