<p> Microbial mat aggradation and morphology can be strongly influenced by sedimentation and light in ice-covered lakes of the McMurdo Dry Valleys, Antarctica. In Lake Joyce, mats transitioned from prostrate with widely spaced pinnacles to having densely spaced pinnacles with complex webs and ornamentation at greater distances from inflowing melt water streams. This transition is interpreted to result from decreasing mud sedimentation, which compacted delicate microbial structures such as pinnacle tips, webs, and surface ornamentation. Mud sedimentation also changed through time at sites adjacent to inflowing streams on one of the Lake Joyce deltas; sedimentation likely increased from 1947 through 2009 as lake levels rose. Although mud sedimentation demonstrably affected mat morphology in Lake Joyce, changes in sand and mud sedimentation associated with overhanging rocks in Lake Vanda were not sufficient to dramatically change mat morphology. Instead, microbial mat pinnacles and ridges had a variety of morphological responses to their light environment. Microbial mats growing with oblique directional light both grew down from overhanging rocks with pinnacle orientation independent from the directional light and grew up from the rock-sheltered mat with pinnacles and ridges oriented relative to incident light: asymmetrical pinnacles were inclined toward and flattened perpendicular to the incident light, and ridges were oriented parallel to the incident light. Changes in mat morphology and microbial processes are also preserved in Lake Joyce stromatolites that grew over decades. Stromatolites contain ?13Ccalcite records of variable photosynthetic fractionation of local DIC under lower lake levels, followed by decades of DIC pool 13C enrichment with lower rates of photosynthesis during lake level rise. These results demonstrate that microbial responses to their environments are complex and under the right conditions can be preserved in the rock record.</p>

<p>Rapid changes in Earth?s marine environment are sufficiently rare that the scope and consequences of these events cannot be inventoried without recourse to the stratigraphic record of deep pre-historic time. Traditional stratigraphy divides geologic time into a succession of coarse biozones ? uneven time bins, each recognized by distinctive index fossils and spanning several hundred thousand to even a few million years. This thesis uses the CONOP software to resolve sequences of global appearance and disappearance events at the species level across the late Cenomanian oceanic anoxic event (OAE 2) at a much finer resolution than traditional biostratigraphy. The resulting time-lines include information for the Aptian to Maastrichtian from 501 locations, 4,962 taxa, and 25,112 local last occurrences. This dataset is large enough to represent the global environment and to compensate for the inevitably patchy and incomplete record preserved at any one location. Six geographically widespread clades ? ammonites, calcareous nannofossils, dinocysts, benthic forams, planktonic forams, and radiolaria ? were analyzed; they span a range of trophic levels, composition and preferred habitat. Extinction pulses and intervals of falling diversity are not unique to OAE 2 or even uniquely severe. Several occur throughout the Albian-Santonian interval. The key is to monitor trends in diversification rate. OAE 2 is distinguished as a boundary between more positive (or equal in the ammonite case) and more negative diversification regimes. It may have been part of a general habitat deterioration or disturbance, but it would be too simplistic to describe it as a coordinated pelagic extinction event.