2001 Annual Science Report
Harvard University Reporting | JUL 2000 – JUN 2001
The Planetary Context of Biological Evolution: Neoproterozoic-Cambrian Environmental Change and Evolution
Neoproterozoic-Cambrian Environmental Change (dm)
Our team effort on Neoproterozoic-Cambrian environmental change and evolution has enjoyed the broadest participation of Harvard team members, for good reason. The Proterozoic-Cambrian transition witnesses remarkable changes in tectonics, climate, atmospheric composition, and especially life. This is the interval during which animal life — and hence the propsect of intelligence — radiated on Earth. Harvard team researchers are studying the paleontology, geochronology, tectonics, and environmental changes of this interval, with an eye to inegrated change in the Earth system.
In conjunction with Jin Yugan from the Nanjing Institute of Palaeobiology, China, we have begun geochronological work on a series of volcanic ashes just above glacial deposits and as high as the lower middle Cambrian in an effort to constrain the ages of the spectacular fossil embryos found in terminal Proterozoic phosphorites from China, the age of Chinese glacial deposits, and the tempo of the Cambrian explosion. Continuing work on ash beds associated with Neoproterozoic glacials in Newfoundland has been problematic, but data indicate that the glacial rocks are ca 580 Ma and that the duration of their deposition is less than 10 Ma. Importantly, spectacular Ediacaran fossils first occur at 575 Ma, lending support to the idea that large animals appeared soon after the last glacial.
In 1998 Seilacher shocked the geological community by publishing a paper in which he described ca 1.0 Ga metazoan trace fossils from India. We initiated a project in conjunction with Jan Veizer at Ottawa University in Canada to investigate these findings. Rhyolitic volcanic rocks that belong to the Deonar Porcellanite Formation and which occur just below the unit containing trace fossils yield a conventional U—Pb zircon age of 1631 +/- 5 Ma. These results indicate that the Kajrahat limestone is of Paleoproterozoic age. The biological interpretation of Seilcher’s bedding features remains problematic.
We have obtained an age for the Precambrian/Cambrian boundary in Oman of ca 542 Ma, defined by a negative excursion in carbon isotopes. We have also collected many ashes from drill cores to further document tempo of changes in carbon and Sr isotopic signatures. This result raises questions regarding the biological significance of the Cambrian/Precambrian boundary as distinctive Neoproterozoic small-shelly fossils occur both above and below the boundary. In addition, a suite of “ashes” from within the glacial deposits exposed in Oman as well as a non-deformed granite that is unconformably overlain by the glacial deposits have been collected. Geochronological data indicate that the granite age is 727 Ma, and that of the overlying glacials,710 Ma, which may be the first robust age for the “Sturtian” glacial deposits.
In an attempt to fully understand the resolving power of U-Pb geochronology, we initiated a project to date one of the best know Ar-Ar standards, the Fish Canyon Tuff. This fully explores all sources of errors involved in U-Pb geochronology and allows us to have confidence in determining U-Pb dates in the age range of 2-550 Ma at the 20-500K year level.
We have investigated new findings from molecular development, with an eye for their application to understanding early animal evolution, concluding that developmental data have been overinterpreted, with ancestral bilaterians less morphologically complex than claimed.
The project in northern Namibia currently focuses on events directly preceding low-altitude glaciation. A decline in the d13C of seawater proxies of 10 per mil in ~0.5 million years is observed in virtually all sections of the Otavi Group of northern Namibia preceding the Ghaub glaciation. An analogous isotopic change is observed before the Elatina glaciation in South Australia.
We have completed the final year for our project in northeast Svalbard, which will be a major part of Halverson’s doctoral thesis at Harvard. We have discovered a sequence having the lithologic and isotopic signatures of “cap” carbonates (purportedly diagnostic of snowball Earth deglaciations) in the lower Akademikerbreen Group, predating any known glacial event in Svalbard. This indicates either that “cap” carbonates are not unique to glacial events, or that a third, previously unrecognized snowball event occurred. Preliminary paleomagnetic measurements indicate that the sequence retains a primary remnant magnetization. The data imply that the “phantom” glaciation occurred at low paleolatitude (<10â??) and that the sequence contains at least three, polarity reversals or excursions. If they prove to be true reversals, then the sequence was likely deposited more slowly than predicted by the snowball earth scenario. Alternatively, if they are excursions (attempted reversals), then unusually high sedimentation rates would be implied, consistent with the snowball scenario. Detailed isotopic profiles have also been obtained from carbonates bracketing the two known glacial events in the Polarisbreen Group. The older (Elbobreen) event has a preglacial isotopic deviation similar to that preceding the younger Ghaub glaciation in Namibia (see above), but this has not been found with the younger (Wilsonbreen) event in Svalbard.
This year we began a project in the Anti-Atlas region of southern Morocco, which will be a major part of Maloof’s doctoral thesis at Harvard. Abundant dateable volcanic rocks occur in an extensively-exposed succession ranging from latest Neoproterozoic (Vendian) continental through Early Cambrian marine facies. Preliminary geochronology of samples collected during a reconnaissance of the area in 2000 yielded concordant U-Pb zircon ages ranging from 595 Ma (base of the carbonate platform sequence). Among the goals of this project are to improve the chronology of Vendian events and to investigate Vendian continental as well as marine environments and biota.
Two glacial diamictites (Rapitan and Icebrook), both with cap carbonates, are widely exposed in the Mackenzie Mountains of the northern Canadian Cordillera. The older glaciation is associated with major deposits of banded iron-formation (BIF), signifying prolonged ocean anoxia consistent with global ice cover. However, precious models imply that the BIF should form upon deglaciation, when the ocean is reventilated, but this seems inconsistent with the Rapitan BIF which is overlain by up to 800 m of glacial diamictite (Shezal Formation). One possibility is that the Shezal diamictite represents a second, younger glaciation distinct from the Rapitan event, a relation similar to that recently described in the Death Valley region, California. A second possibility is that the thick Shezal diamicitite was itself deposited rapidly during a single deglaciation. A third is that the BIF was deposited during, rather than at the end of, a snowball Earth event, but only where sea ice was <20m think, permitting oxygenic photosynthesis beneath the ice. A goal of our field work will be to investigate the timing of BIF sedimentation, relative to the history of glaciation. In addition, we will examine sections of the Icebrook cap carbonate, which has many of the unusual lithological features of the Ghaub cap carbonate in Namibia (e.g., reefs composed of pseudomorphosed aragonite cementstone), but which is less diagenetically altered.
We are continuing field studies of Proterozoic-Cambrian successions in Namibia and Oman. Sedimentologic, geochemical, paleontologic, and geochronological investigations of the Precambrian-Cambrian boundary in Namibia and Oman is designed to assess the rates of evolution and the taxonomic diversity of metazoans preserved below and above the boundary. Geochemical studies are directed toward establishing C, S, and O isotopic records which provide a paleoenvironmental baseline, as well as providing an independent basis for chronostratigraphy. Geochronologic studies involve U-Pb zircon dating and provide constraints on absolute rates. Recent paleontologic discoveries include the occurrence of Namacalathus and Cloudina in numerous cores in the subsurface of the South Oman salt basin, a new fossil in the Nama Group of Namibia, and several ash beds in the Oman cores.
Work is being conducted in Namibia and west Texas to test the application of differential GPS in the acquisition of high-resolution maps of geologic features. Other tested equipment includes laser-based range finders and theodolite total stations. The results are encouraging and in both regions this group has been able to collect over 50,000 measurements of stratigraphic features such as reefal bioherms. These bioherms, which are mapped in three dimensions, can be displayed in 3D using Silicon Graphics work stations which allow rapid display and visualization of the data.
Field work in Namibia also enabled us to determine how the superimposition of a taphonomic window of mat preservation on an ecological window of mat formation determine the distribution of diagnostic mat signatures in ancient siliciclastic rocks. The distinctive surface features imparted by microbial mats to sandstones are among the only diagnostic signatures of life that could be reliably identified by the 2003 Mars rover.
(i) Dan Schrag continues to explore models of coupled environmental and biological change at the end of the proterozoic Eon. This work has been collaborative with four team members: one manuscript is in preparation with Schrag, Bowring and Hoffman as co-authors, and another manuscript is in preparation with Schrag, Erwin and Knoll as co-authors. In addition, post-doc MaryLynn Musgrove is developing the ability to measure calcium isotopes with high precision to investigate biomineralization in Proterozoic sediments
PROJECT MEMBERS:Andrew Knoll
RELATED OBJECTIVES:Objective 5.0
Describe the sequences of causes and effects associated with the development of Earth's early biosphere and the global environment.
Define climatological and geological effects upon the limits of habitable zones around the Sun and other stars to help define the frequency of habitable planets in the universe.
Determine the resilience of local and global ecosystems through their response to natural and human-induced disturbances.