Assessing Microbial Influence on Deposition of Frondose Lacustrine Carbonate Tufas from Winnemucca Dry Lake, NV, USA
Laura M. DeMott
Lacustrine carbonates (tufas) in the lake basins of Pleistocene pluvial Lake Lahontan are important analogues for South Atlantic margin carbonate reservoirs and have been used as paleoproxies for assessing hydroclimate variations over time. However, the specific depositional mechanisms responsible for tufa formation are still a matter of debate, particularly with respect to microbial influences. In Winnemucca Dry Lake, NV, tufas are widespread along the western margin of the dry lake bed, and a variety of morphologies and textures are observed. The most common and laterally extensive variety of tufa is a frondose form that exhibits draping and branching textures and is deposited on hard substrates. This form of tufa occurs both at spring-associated sites on the lake bottom and at high elevations along bedrock cliffs. The tufa meso- and microfabrics are similar to those observed in modern microbialites from Fayetteville Green Lake (NY). Samples of frondose tufa were collected from a spring-associated site and a high elevation bedrock outcrop site. Hand samples exhibit a porous, branching mesofabric, with some samples containing areas exhibiting stromatolitic laminations. Thin section petrography and scanning electron microscopy show that frondose tufas are dominated by mixed micrite and shrubby calcite fabrics, with varying degrees of secondary calcite; some samples contain preserved microbial filaments. Organic geochemistry, including Corg content, d13Corg, d15N, and DNA sequencing indicate that tufa deposition at both spring-associated and shoreline sites may be influenced by photosynthetic microbes (Cyanobacteria, Chloroflexi). Radiocarbon age dating of carbonate and organic carbon lends insight into the timing of these processes and the relationship to lake basin history. These results have implications for paleoenvironmental interpretations of Lahontan tufa deposits and may have broader implications for studies of microbialites throughout the rock record.
Dr. Esteban Gazel, Associate Professor Department of Earth and Atmospheric Sciences Cornell University.
"The rocks that joined the Americas: Is there a connection with climate and evolution of life?"
Earth’s crust is the life-sustaining interface between our planet’s deep interior and surface. Basaltic crusts similar to Earth’s oceanic crust characterize terrestrial planets in the solar system while the continental masses, areas of buoyant, thick silicic crust, are a unique characteristic of Earth. The continental crust is also enriched in incompatible elements (elements that separate from the mantle during partial melting) and although it is a volumetrically minor layer it plays a major role in the fractionation and storage of those elements. Therefore, understanding the processes responsible for the formation of continents is fundamental to reconstructing the evolution of our planet. Analyzing modern analogues where “juvenile” continental crust is forming can provide a better understanding of the formation of continental crust in the past. The evolution of the Central American Land-Bridge (CALB, Costa Rica and Panama) was used as a natural laboratory to answer this fundamental question. Geochemical and geophysical data support the evolution of the CALB into a young continent as a result of the interaction of Galapagos Hotspot tracks with this subduction system. A global survey of intra-oceanic arcs was conducted with the goal of identifying where magmas with continental crust signatures have been produced and what processes control the composition of the volcanic output. Finally, a new geochemical continental index was developed to quantitatively correlate geochemical composition with available average arc P-wave velocity, resulting in a strong correlation (r2=0.87) between those two parameters. Our work suggests that although the origin and evolution of continents may require many processes, melting of enriched oceanic crust and reaction of these melts with the mantle wedge in a subduction system will result in juvenile continental crust production, a process that was probably more common in the Archean than today. In Central America the production of “juvenile” continental crust culminated with the closure of the Panama Seaway ~15 to 3 Ma. This closure resulted in global change of ocean circulation, separated marine species, and allowed the exchange of fauna between the Americas, making the evolution of the CALB not only relevant to the understanding of geologic processes, but also had considerable impacts on evolution of life and climate on the planet.
Inherited, enriched, heated, or recycled? The Grenville Orogeny: Examining potential causes of Earth's most zircon fertile magmatic episode
An increasing number of tectonic studies rely on U-Pb ages of detrital zircon to identify sediment source regions. Such studies can be confounded if zircon ages reflect original primary sources rather than final, proximal sources. Such is the case with ~ 1 Ga Grenville zircon in North America – detrital grains are found in abundance thousands of kilometres from known exposed sources. This is likely a result of the incredible zircon fertility of Grenville age plutons (i.e. granitoids with very high Zr abundance). There are multiple possible mechanisms that can explain such high zircon abundances including: (1) very significant degrees of inherited xenocrystic zircon in the granitoids; (2) very unusually trace-element enriched source areas of the granitic magmas; (3) massive degrees of crustal recycling during Grenville orogenic events; or (4) extremely high temperatures of granitic magmas that allow for very high quantities of zirconium to be dissolved in the magmas. Each of these possibilities will be discussed in the context of what looks to be the single most zirconium-enriched major magmatic event in Earth history!