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2000 Annual Science Report

Harvard University Reporting  |  JUL 1999 – JUN 2000

The Planetary Context of Biological Evolution: Research on Terminal Proterozoic Evolution and the Coevolution of Life and Environments

4 Institutions
3 Teams
0 Publications
0 Field Sites
Field Sites

Project Progress

Research on terminal Proterozoic evolution and the coevolution of life and environments (dm)

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Pursuant to the Harvard NAI team’s articulated focus on the coevolution of Earth and life across the Proterozoic-Cambrian transition, we have completed two paleontological studies of early animal evolution. Well-skeletonized animals are generally thought to have arisen as part of the Cambrian “explosion” of animal diversity; however, slightly older reefs in the terminal Proterozoic Nama Group, Namibia, contain abundant and relatively diverse skeletonized fossils. We have completed computer-assisted three-dimensional reconstruction of the animals, showing them to be cnidarian-grade invertebrates not closely related to Cambrian shelly fauna. Coeval shales in South China contain another type of fossil assemblage previously unknown from Precambrian successions. Burgess Shale-type compressions from the terminal Proterozoic Doushantuo Formation preserve a remarkable window on biological diversity just before the Cambrian explosion, including diverse algae and two taxa of probable animals. Again, the animals are cnidarian-like and do not extend the record of crown group bilaterian bodyplans downward into the Proterozoic. In concert, these new windows on early evolution make it clear that whatever the antiquity of the animal kingdom, the characteristic body plans associated with bilaterian phyla arose only in the Cambrian Period.

We have been working to understand the stratigraphic coincidence between a major carbon cycle shift and the diversification of eukaryotes 1200-1000 million years ago. We propose that both are linked to trace metal abundance in Proterozoic oceans. Only in 1200-1000 Ma did oxygenation of deep oceans begin to eliminate sulfidic deep water, making Mo bioavailable for the first time (and allowing eukaryotic algae to incorporate nitrate).

With NAI colleagues from the Carnegie Institution, we are using the tools of analytical geochemistry to probe the tissue-level physiology of early land plants. We have examined the evolution of lignin chemistry using ion microprobe, NMR, and soft-X-ray analysis of Early Devonian fossils. Today, vascular plants conduct water in lignin-lined tracheids. Our results show that while stem land plants possessed both complex polyphenolic chemistry and elongated conducting cells, the polyphenolics were localized in surface tissues, only later to become associated with conducting tissues.

In lower-middle Cambrian of South China, we visited several new sites and collected about 12 samples of potential volcanic rocks interbedded with the spectacular soft-body fossils of the Chengjiang. This is a critical interval in animal evolution; however, at the present time we do not know the age of these spectacular rocks. They are commonly referred to as 530 Ma, however, they may be younger than 520 Ma, which would make a big difference for the interpretation of their evolutionary significance. We are currently processing the samples and more field work is planned.

We have made considerable progress on evaluating a prediction of the “Snowball Earth” hypothesis, that the global cover of sea-ice should have lasted at least 10 Ma and perhaps longer. We are attempting to constrain the duration of glaciations on the rock record by dating volcanic rocks both above and below the glacial deposits. One of the best places to do this is in Newfoundland. Here we have identified volcanic rocks below the glacial interval that are younger than 592 Ma and an ash which directly overlies Ediacaran fossils that is 575 Ma. It is noteworthy that there are > 5km of turbidite deposits between the top of the glacials and the 575 Ma ash. Thus we have constrained the glacial deposits to have been deposited in less than 15 Ma. Further work is ongoing to more tightly bracket this interval.

We are working to constrain the age of the putative oldest trace-fossils from India. Mark Martin spent 8 days collecting volcanic and clastic rocks in the Vindhyan Supergroup in central India. The purpose of collecting these samples is to better constrain the age of the Vindhyan supergroup and the age of purported trace fossils found in clastic rocks believed to be approximately 1.0 Ga. Volcanic rocks were only found stratigraphically below purported trace fossils. Preliminary U-Pb zircon analyses indicate that these volcanic rocks are approximately 1.6 Ga in age. U-Pb detrital zircon studies are in progress from sandstones collected at the same stratigraphic horizon as trace fossils and from above the trace fossile horizon in order to place a minimum depositional age on the these sandstones and trace fossils.

Sedimentary succesions in Namibia and Oman preserve exceptional records of life and environments just before the Cambrian explosion of animal life. Understanding the seminal diversification of animals in the Cambrian requires that antcedent states of life and environment be understood in detail. In Namibia, detailed field research has established the temporal and sequence stratigraphic framework for paleobiological and paleoenvironmental research. In Oman, this has been accomplished by field research, analysis of cores, and seismic data (provided by Petroleum Development Oman). Both regions preserve extensive development of carbonate platforms, and in both regions microbial reefs provided habitats for the earliest known skeleton-forming animals. Careful computer-assisted reconstruction of namibian fossils reveals a range of cnidarian-grade morphologies, each occupying a parituclar range of environments in and around the microbial reefs. Omani observations confirm and extend this ecological picture. Further, new geochronological and chemostratigraphic data from Oman provide insights into the timing and consequences of a major carbon-cycle perturbation at the Precambrian-Cambrian boundary.

In other research, we are developing a capability for remote field mapping, a capacity that will serve us well in future Mars exploration. We are also working on a mathematical model for stromatolite growth that enables one to quantify the relative effects of biology, physical environment, and chemical conditions on stromatolite growth form. In the absence of such an understanding, we cannot hope to gain maximal paleobiological information from these ubiquitous Precambrian sedimentary features; nor will we have confidence in assessing the possible role of biology in generating such laminated precipitates as may be found on Mars.

We continue tio work with trace elements in carbonaceous shales and with a section through the ca. 2.25 Ga Hekpoort paleosol near Garbarone in South Africa.We have shown that redox-sensitive trace elements are not enriched in several highly carbonaceous shales that are older than ca. 2.3 Ga, but that they are strongly enriched in the 1.5- 1.6 Ga carbonaceous shales which we have studied. These results are consistent with the proposed rapid rise of atmospheric oxygen between 2.3 and 2.0 Ga. Under low 02 conditions the redox-sensitive elements are not oxidized during weathering, are not transported in solution to the oceans, and are not concentrated in carbonaceous sediments. In contrast, the ratios of the concentration of the redox sensitive trace elements to organic carbon in the 1.5- 1.6 shales that we have analyzed is virtually identical to the ratios in Devonian black shales, suggesting that the intensity of oxidative weathering 1.5-1.6 Ga was quite similar to that during the Phanerozoic.

The ca. 2.25 Ga Hekpoort paleosol in South Africa is one of the most extensive and best studied Precambrian paleosols. The discovery of what appears to be a nearly complete section through this paleosol near Gabarone has made it possible to define rather precisely the conditions under which it was formed. The presence of a ferricrete horizon at the top of the paleosol proves that the atmosphere contained some 02 during soil formation. However, the relatively small fraction of FeO in the parent basalt that was oxidized to Fe2O3 indicates that O2 levels were much lower than today, probably between 10-4.0 and 10-3.0 atm. Interpretations of the paleosol as a ground water laterite formed under high atmospheric O2 conditions are difficult to reconcile with the distribution of the major and minor elements in the paleosol. The new paleosol section therefore supports the notion that the Hekpoort paleosol developed during the early stages of the Great Oxidation Event. The research of the last year has added important new data bearing on the evolution of atmospheric 02. The results are consistent with the model developed during the past decade, but a good deal more work is needed to define the course and the reasons for the Great Oxidation Event.

Field-based research continues on Neoproterozoic ice ages and their effects on biological evolution. The Snowball Earth hypothesis posits that the Earth was plunged into long periods of global glaciation repeatedly between 800 and 575 million years ago. Unusual geological and geochemical features found in association with glaciogenic rocks can be explained by ice that not only reached sealevel at the equator, but spread across the oceans to create a snowball Earth. How life survived and responded to these perturbations is of great interest to anyone interested in the long-term persustence of life on a planet
Detailed stratigraphic studies are currently being conducted in Namibia, West Africa, and Spitsbergen, with samples for geochemical anlysis collected in a sequences stratigraphic framework. New data from the current year’s research include geochemical and sedimentological evidence for pre-Varanger glaciation in Spitsbergen, as well as periglacial features of a type predicted by the snowball model in Mauritania.

Work at Woods Hole has focused on methane biogeochemistry and on hydrogen-isotopic biogeochemistry. Available funds supported a six-month visit by Dr. Roger E. Summons, Chief Research Scientist of the Australian Geological Survey Organisation and a co-investigator in the Harvard astrobiology group, and contributed to salary and research expenses for Sean P. Sylva, a research associate on the WHOI staff, for Alex L. Sessions, a doctoral student working on hydrogen-isotopic studies, and for Dr. Kai-Uwe Hinrichs, a postdoctoral scholar working on methane biogeochemistry.
Hinrichs, with assistance from Sylva, has continued to study molecular biomarkers from sediments in which methane is being oxidized anaerobically. Our first publication in the present series (Hinrichs et al., 1999a) reported the discovery of archaeal lipids that were strongly depleted in 13C and which, therefore, apparently derived from methanotrophs rather than methanogens. A 16S rRNA gene library from the same sediments (from the Eel River Basin, offshore northern California) yielded numerous phylotypes that were related to but distinct from known methanogens. The most prominent group branched deeply enough that it could represent a new order within the methane-related archaea. Accordingly, we proposed that the 13C-depleted biomarkers were products of obligately methanotrophic archaea and that the archaeal domain contains organisms capable of consuming methane as well as producing it.
Earlier studies of the anaerobic oxidation of methane have shown that the overall process involves transfer of electrons from methane to sulfate. A microbial consortium involving both methane-consumers (now shown to be archaeal) and sulfate-reducers has, therefore, been postulated. In work during the present year, Hinrichs, Summons, and Sylva have extended our earlier investigations and shown that a large group of 13C-depleted biomarkers that co-occurs with the archaeal products must be of bacterial (= eubacterial) origin. The definitive molecular structures within this group are n-alkyl and ether-linked (Hinrichs et al., 1999b, 2000a). Glycerol ethers are extremely rare among bacterial products but are not unique. They have been reported previously in thermophilic sulfate-reducing bacteria. We believe, therefore, that we have now identified products from organisms of both types within the hypothesized consortia: 13C-depleted isoprenoidal glycerol ethers from the archaea and 13C-depleted n-alkyl glycerol ethers from the bacteria. The efficient transfer of methane-derived (isotopically depleted) carbon from the methane consumer to the sulfate reducer indicates that CO2 is not the main product of the methane consumers. Our evidence is being exploited by microbial ecologists dealing with that issue.
Follow-up studies are in progress. On the archaeal side, we have found that the molecular structures deduced and described in our earlier report (Hinrichs et al., 1999a) were correct but that the chemical derivatives formed in the course of our analyses, and for which the mass spectra are shown, actually contain only one trimethylsilyl ether moiety rather than two. To prevent confusion among subsequent investigators, we have published a note clarifying this point (Hinrichs et al., 2000b). On the bacterial side, further work is showing that n-alkyl glycerol ethers are widely distributed in anaerobic sediments, apparently indicating that the sulfate reducer involved in the methane-consuming consortium, or its relatives, is a previously unrecognized but important member of sedimentary microbial communities. Hinrichs and Summons are drafting a further report for submission to The Proceedings of the National Academy of Sciences.
The process in which methane is oxidized anaerobically is notable for its low yield of energy. In separate work, Hayes and coworkers (Hayes et al., 1999) were able to calculate in situ yields of energy rather precisely for the process in sediments from the Kattegat (the waterway between Denmark and Sweden). They showed that the yield was barely adequate to sustain two trophic levels and suggested that the example was pertinent to considerations of chemotrophic systems on the early earth, in the deep bacterial biosphere, and on other planets. A detailed version of this report is in preparation for G3 (Geochemistry, Geophysics, Geosystems), the on-line journal now published by the American Geophysical Union.
Our work on processes of methane oxidation was initially motivated by hypotheses that the carbon-isotopic anomaly recorded in marine carbonates of late Paleocene age (55 Ma) reflected a globally significant release of methane from sub-sea-floor hydrates. Seeking to confirm this hypothesis, we searched related sediments for any 13C-depleted biomarkers deriving from the methanotrophic bacteria that must have flourished if such releases had occurred. Our results have been consistently negative, even as further evidence (e. g., results indicating that the isotopic anomaly reaches its maximum strength in only two to four thousand years) points increasingly to sudden decomposition of hydrates. One report of these negative findings is now in press (Bolle et al., 2000).
At present, Hinrichs is preparing a review of methane cycling for presentation at the Gordon Research Conference on the Origin of Life. He will, in particular, consider the possibility that the dramatic carbon-isotopic-depletion signal in the late Archean derives from anaerobic, rather than aerobic, processes and will provide estimated mass and redox balances for known sedimentary basins.

1. The environmental and biological context of the Cambrian metazoan radiation. Efforts over the past year have involved consideration of the biological effects of the proposed Neoproterozoic รข??Snowball Earth’; collaborative work with Sam Bowring (MIT ) on the rate and timing of the evolutionary events; search for new Ediacaran fossils in sub-Nama group sediments in southern Namibia; and a continuing effort to use molecular and developmental data to understand the nature of the protostome/deuterostome ancestor. In particular, this effort is focused on the consideration of highly conserved developmental control genes between protostomes (largely Drosophila) and vertebrates. Sequence conservation is a poor guide to functional conservation, making it difficult to infer the nature of the PD ancestor unequivocally.
2. Patterns and processes of post-extinction recovery. Focus has been on the nature of biotic recovery after mass extinctions. I have begun developing models of these processes with colleagues at the Santa Fe Institute.
3. Causes of the end-Permian mass extinction. Continuing effort to understand the rate and likely causes of the greatest mass extinction in the past 540 million years.