nai

Project Progress

One of the major goals of Astrobiology is to understand how biological and geological processes operating on life-bearing, terrestrial planets interact with each other. Examples of this interaction include the mass extinction events that have punctuated the evolution of life on Earth, as well as understanding the geological, biological, geophysical, and geochemical triggers for major evolutionary events in the history of the biosphere, like the origin of atmospheric oxygen and the Cambrian Explosion. Research over the past decade has shown that such events are marked by large turnovers in faunal composition and major perturbations in the planetary carbon, sulfur, and oxygen cycles. Increasing our knowledge of the timing, paleogeography, and tectonic events associated with global-scale biological events will help lead to testable hypotheses about their causes, e.g., impacts, large volcanic eruptions, or rapid polar wander.

In collaboration with the Ward NAI node at the University of Washington, the Kirschvink group at Caltech has been using paleomagnetic techniques to constrain the timing of mass extinction events at the Permian/Triassic, Triassic/Jurassic, and Cretaceous/Tertiary boundaries at critical locations in Africa, Canada, and China. We have also used magnetic tools along with geological reasoning to resolve a major correlation problem between Canada and South Africa, which in turn places tight constraints on the origin of oxygenic photosynthesis and the triggering of what may be the largest Snowball Earth event in Earth history. Following up our earlier work on Inertial-Interchange True Polar Wander (IITPW) and the Cambrian Explosion, we now have data that suggests that the emplacement of large igneous provinces (plume-head style eruptions) may be the driving mechanism for these events, and can perhaps explain both the large carbon isotope anomalies and kilometer-scale rapid sea-level drawdowns associated with them. We have also continued, at a lesser pace, collaborative work on the biological precipitation of magnetite as a possible analogue for the origin of fine-grained crystals in carbonate blebs from Martian meteorite ALH84001.

Research Results:

Permian/Triassic Boundary:

Our major study on the stratigraphy and biological extinctions of land vertebrates was published early this year (Ward et al., 2005); we were able to resolve a primary, two-polarity magnetic reversal pattern in many of the sequences studied that allowed us to refine global correlations from the latest Permian through to the Greisbachian/Dienarian boundary. We were able to show that the P/T boundary in the Karoo Desert of South Africa is near the beginning of a distinctively long Normal magnetic polarity chron, similar to that from elsewhere in the world. Two weeks of additional field work are planned for September 2005, during which we will use a small, portable Winke drilling system to collect ~20-100 m of continuous core materials across the Permian/Triassic boundary in the Karoo sediments at the Carlton Flat locality, north of Middleberg, South Africa. The target sequence will cross the P/T “Event Beds” located by Peter Ward and Roger Smith in their earlier work in the area. In addition to the paleomagnetic work, these samples should also provide pristine materials for geochemical and micropaleontological analyses.

At the suggestion of Greg Retallack (of the University of Oregon), we collected a series of oriented samples for magnetostratigraphy and carbon isotope analyses at a locality near Heit Marina on Lake Powell, Utah. This locality contains a series of paleosols that he correlates to the Permian/Triassic boundary beds in Texas and South Africa; our samples should be able to test these correlations.

One of our Caltech students (R. Kopp) was able to attend the Triassic Chronostratigraphy and Biotic Recovery Conference in Chaohu, China, last May, and visited the now-classical Permian/Triassic exposures of the Grand Bank of Guizhou. Our intent was to collect a preliminary suite of samples for magnetostratigraphy and magnetofossil analyses, but we discovered that Paul Montgomery (formerly a student at the University of Kansas, now at Chevron/Texaco, and collaborating with the paleomag group at the Berkeley Geochronology Center) had already sampled extensively in the area and had preliminary results from some of the sections. However, a large number of his samples were sitting in the long paleomagnetics queue at the Berkeley Geochronology Center, with no prospects of completion. We are now formally collaborating with this group, and processing these materials at the Caltech paleomagnetics laboratory.

Triassic/Jurassic Boundary:

We are presently demagnetizing a suite of paleomagnetic samples collected from the Triassic/Jurassic boundary sediments at Kennecott Point in the Queen Charlotte Islands of Western Canada, collected on a field trip in July 2004 run by the UW NAI group. This section is being intensively studied for paleontological and isotopic analyses, in addition to the paleomagnetics. These samples are divided into three groups (not including the Karmutsen Basalts), 1KP, 2KP, and FI. The 1KP batch has been heated to 320˚C and the samples are dead or close to dead, strongly suggesting pyrrhotite as primary remanence carrier — rock magnetic analyses now underway should identify the magnetic phases properly. The 2KP samples are starting thermal demagnetization, and the FI samples are halfway to completion (if their demagnetization behavior parallels that of the 1KP samples). Visual inspection of the demagnetization data from all three sample groups suggest a dual-polarity, high inclination component. All three sample groups have magnetic moments between 10^-9 and 10^-11 Am².

Cretaceous/Tertiary Boundary:

Although magnetic polarity patterns across the K/T boundary played a critical role in testing the hypothesis that these extinctions were synchronous (which is a major implication of the impact hypothesis), no reliable magnetostratigraphy has yet been obtained from the international stratotype section (GSSP) in Tunisia. Kirschvink and Ward visited the sections about 5 years ago with Francis Robaszynski, collecting at El Kef and several adjacent localities that span the K/T boundary interval. NAI funds have been used to process the materials over the past year or so.

With over 4,000 demagnetization experiments so far, it is clear that they are not anywhere near as bad as the Austin Chalk of Texas, they all seem to be rather weakly magnetized, with moments typically of a few 10^-11 Am² after the 200° C step,. Magnetic reversals are present, in what may be a coherent pattern. We are continuing the demagnetization experiments in small increments under pure N₂ gas to minimize the problem of the oxidation of the magnetic minerals.

Constraints on the Great Oxygenation Event:

The adaptation of life to the presence of free molecular oxygen in the environment was one of the major evolutionary events in the history of life on this planet and a prerequisite for the evolution of complex multicellular organisms. Using a combination of paleomagnetic, geochemical, and geological techniques, we have been able to constrain the correlation between the Transvaal Supergroup of South Africa and the Huronian Supergroup of Canada, two of the best-preserved stratigraphic sequences in the early Paleoproterozoic, the critical time interval that contains the first traces of the planetary oxygenation event.

We found that rocks from just above the final glacial unit failed a paleomagnetic fold test from just above the final Huronian glacial unit (Hilburn et al., 2005), which indicated that the Huronian glaciations were not necessarily low latitude and therefore removed the temptation to correlate any of them with the low-latitude Makganyene glaciation of South Africa. This result, coupled with newly published age constraints from the Hekpoort and Ongulek lavas in South Africa from the Beukes group at the University of Johannesberg, made clear that all three of the glacial units in the Huronian must pre-date the low-latitude Makganyene glaciation of South Africa. Critical re-evaluation of all of the published data on the geological constraints for the first appearance of oxygen indicate that significant environmental levels of O₂ likely first occurred in the time interval between the last of the Huronian glaciations (the Gowganda) and the Makganyene glaciation.

Kopp et al. (2005) develop the idea that this oxygen burst was due to the rather sudden evolution of oxygenic photosynthesis, which would have rapidly led to the destruction of a methane greenhouse and thereby triggered the Makganyene Snowball Earth event, which may have lasted for up to 100 million years (2.32 — 2.22 Ga). Kopp et al. argue that, driven by glacial weathering, the phosphate flux into the ocean during glacial intervals would have been high enough to remove nutrient limitations on any populations of oxygenic photosynthetic bacteria. Thus, the failure of pre-Makganyene glaciations (the three Huronian events and the ~2.9 Ga Pongola glaciation) to precipitate a greenhouse collapse constrains the timing of the evolution of oxygenic photosynthesis. We hypothesize that a singular evolutionary event (the deveolopment of oxygenic photosynthesis) triggered a global climate catastrophe, a Snowball Earth event that lasted tens of millions of years.

A related and long-standing problem in evolutionary biology concerns how any organism could evolve the ability to produce molecular oxygen without killing itself in the process — a hurdle that the cyanobacteria must have overcome before the final assembly of their photosynthetic apparatus. An inorganic source of strong oxidants must have been present somewhere in the environment to promote the evolution of oxygen-mediating enzymes like superoxide dismutase and catalase. Working in conjunction with Caltech professor Yuk Yung and his graduate student Danie Liang, we realized that, during a glacial event on an oxygen-poor planet, ultraviolet light acting on water vapor near the ice surface will produce H₂O₂ and H₂ gas; the peroxide will freeze out and be incorporated into the ice, while the hydrogen will diffuse away and either be utilized by the biosphere or (if present in high enough concentrations) escape into space. At the melting ice front the peroxide is released and decomposes into water and O₂. This process would have been active during both the Pongola and Huronian glacial episodes and might have driven the evolution of oxygen-mediating enzymes.

Inertial Interchange True Polar Wander and the Cambrian Explosion.

Eruption of a plume head is one possible geophysical mechanism that can perturb Earth’s moment of inertia tensor and possibly lead to inertial interchange events. For this reason we have focused an interdisciplinary study on the Sept-Iles mafic intrusive suite in Quebec, Canada, for which a previous paleomagnetic study suggested that both low and high-latitude magnetic directions were recorded during its emplacement. Our paleomagnetic re-study of this suite confirms these results, but in conjunction with new, high-resolution U/Pb data from zircons associated with the paleomagnetic sites our paleomagnetic data demonstrate that the TPW motions may have been exceedingly rapid, with equatorial to high latitude displacement on the order of only 1 My (Kirschvink et al., 2005c, h). It has not escaped our attention that such rapid motions, if real, could produce the km-scale regional sea-level variations deduced from incised canyons in both Australia and California. Sea-level drawdown would produce large perturbations in the global carbon cycle from the exposure and oxidation of organic-rich sediments exposed on the continental shelves and the pressure-destabilization of previously submerged methane clathrates. Thus, the mid-Ediacaran 'Shuram’ carbon isotope anomaly should be of the same age as the Sept-Iles intrusive suite, a testable prediction.

Biosynthesis of ribose and the origin of Terrestrial life on Mars:

Kirschvink & Weiss (2005) note that evaporite-rich environments capable of producing Ca-B minerals needed to stabolize 5-carbon sugars like ribose are more likely to be present on Early Mars, rather than on Early Earth.