Mark T. Harris

Microbialite resurgence after the Late Ordovician extinction.

Microbialites, including biogenic stromatolites, thrombolites and dendrolites, were formed by various microbial mats that trapped and bound sediments or formed the locus of mineral precipitation. Microbialites were common and diverse during the Proterozoic, but declined in abundance and morphological diversity when multicellular life diversified during the Cambrian Radiation. A second decline occurred during the Ordovician Radiation of marine animals, and from then until the present microbialites have been confined largely to high-stress environments where multicellular organisms are rare. The microbialite declines in the Phanerozoic are attributed to disruption of the mats by animals. A resurgence of stromatolite abundance and size during reduced animal diversity after the Permian extinction has been documented anecdotally. Here we show, with statistical support, that a microbialite resurgence also occurred after the Late Ordovician extinction event in western North America. The resurgences were associated with loss of mat-inhibiting animals, providing insights into shallow-water community structures after extinction events.

The Foreslope and Toe-of-Slope Facies of the Middle Triassic Latemar Buildup (Dolomites, Northern Italy)

ABSTRACT The Latemar buildup was a circular carbonate buildup (4 km wide) with a central platform core (flat-bedded interior platform and massive reef margin) flanked on all sides by slope facies. Steeply dipping (30-35°) foreslope breccias are present adjacent to the margin and flat-lying graded grainstones at the toe of slope. Slope facies relate directly to depositional profile and slope angle. The steeply dipping foreslopes consist of lobate breccia beds that are 25 m thick and a few tens of meters across, and extend tens to hundreds of meters downslope. Some beds are conformably overlain by thinner (< 1 m thick) beds of finer-grained carbonate sediment. The basal surfaces of the breccias are erosional and are anastamosing in both strike and dip views. The breccia rains was derived principally from margin boundstones and deposited by rockfalls and avalanches. Planar clinoforms extend the entire height of the foreslope (hundreds of meters) and bracket depositional units (clinothems) tens of meters thick. Clinoforms appear to be shear surfaces formed during large slope failures (avalanches?). Graded grainstone beds less than 1 m thick are present at the nearly flat-lying toe of slope. These consist of redeposited shallow-water (platform-interior and reef-margin) carbonate sands, some with nodular limestone caps, that are interpreted as proximal carbonate turbidites. Toe-of-slope breccias are the downdip extensions of foreslope breccias and pinch out abruptly basinward. Some slope depositional processes are related to high-frequency (fourth-order and fifth-order) sea-level changes: toe-of-slope graded grainstones correspond to times of platform submergence. In contrast, foreslope breccia was deposited during both platform submergence and exposure. The slope deposits do not record the high-frequency cyclic rhythms identified in shallow-water platform sections. This is attributed to the nature of slope deposition. Downslope talus transport was episodic and localized; graded grainstone beds reflect storm redeposition.

TAKING THE FRUSTRATION OUT OF ICE

The magnetic spins of pyrochlore magnets are constrained to point either directly into or away from the centre of each structural building block. This is analogous to constraints on hydrogen ions in water ice. The discovery that the entropy of magnetic ‘spin ice’ and water ice are the same at low temperatures highlights a long-standing contradiction between the entropy of ice and the third law of thermodynamics.

A Volume-Based Approach to Reef Productivity and Submarine Erosion Rates: A Case Study of a Middle Triassic Reef Margin (Latemar Buildup, Northern Italy)

Reef productivity and submarine erosion rates for the Middle Triassic Latemar reef margin are estimated from debris volumes produced over time intervals of 1.5 to 6 Ma. This direct approach is applicable because the buildup geometry is simple, the sediment source area is limited (>95% of talus blocks are reef boundstone), and the platform section provides temporal control. The buildup geometry is simulated as a truncated cone with corrections for toe-of-slope geometry and 40% depositional porosity of slope breccias. Reef productivity per meter of perimeter was 5.0-18.9 m³/ m/ka; corresponding erosion rates were 4.4-18.2 m³/m/ka. Rate estimates of productivity and erosion demonstrate redeposition of approximately 85% of the reef. Productivity and erosion rates averaged across the 30 m wide preserved reef are 166-629 Bubnoffs and 146-606 Bubnoffs, respectively. Vertical accumulation rates (20-125 Bubnoffs) strongly underestimate reef productivity. Alternative time scales increase calculated rates such that Triassic reef productivity matches Holocene carbonate accumulation rates.

STRATEGIES FOR IMPLEMENTING PEDAGOGICAL CHANGES BY FACULTY AT A RESEARCH UNIVERSITY

Faculty desiring to implement pedagogical change face disincentives related to finding appropriate teaching models and concerns about content coverage as well as those related to institutional and departmental support. Strategies for getting started on changing teaching methods include identifying individuals and programs that promote pedagogical change to develop a support network, making changes incrementally, focusing on student learning, utilizing assessment techniques that address student learning styles and comprehension, starting with non-major courses, and making pedagogy part of ones research program. At the level of individual course development, incremental change allows incorporation of exercises that use relatively small amounts of lecture time (focused discussions and problem sessions). The use of cooperative learning strategies and reading notes expand on these activities. As instructors develop and gain experience with new teaching approaches, they can incorporate long-term projects and development of group analyses (which require investment of more classroom time). The important concern about content coverage can be addressed in two ways. First, directing student study time toward major concepts encourages them to teach themselves some of the material so that classroom time is largely used to explore more difficult material and applications. Second, instructors need to assess what information and concepts are really important and focus the course on those topics.

Condensed-matter physics: Taking the frustration out of ice

The magnetic spins of pyrochlore magnets are constrained to point either directly into or away from the centre of each structural building block. This is analogous to constraints on hydrogen ions in water ice. The discovery that the entropy of magnetic ‘spin ice’ and water ice are the same at low temperatures highlights a long-standing contradiction between the entropy of ice and the third law of thermodynamics.

Taking the frustration out of ice: Condensed-matter physics

The magnetic spins of pyrochlore magnets are constrained to point either directly into or away from the centre of each structural building block. This is analogous to constraints on hydrogen ions in water ice. The discovery that the entropy of magnetic ‘spin ice’ and water ice are the same at low temperatures highlights a long-standing contradiction between the entropy of ice and the third law of thermodynamics.