May 2016 Meeting

Monday, May 9, 2016
6:00 - 7:30 pm

Pellissippi State Technical Community College
10915 Hardin Valley Road, Knoxville
J.L Goins Administration Building, Faculty-Staff Dining Room


Stromatolites, Now and Then: What Can We Figure Out From Their 3,500 Million Year History?

Robert Riding
Earth & Planetary Sciences
University of Tennessee, Knoxville, Tennessee


Stromatolites are laminated deposits produced by calcified bacterial mats. With a geological record of 3,500 million years, they are the earliest macroscopic evidence of life on Earth. How they formed and changed through time are key areas for research. Given their long history, the time period from which stromatolites are viewed is critically important. It is not difficult to regard many stromatolites of the past 500-600 million years as essentially lithified microbial mats. In contrast, many Precambrian examples could contain precipitated abiogenic crusts, both with and without microbial mats. The challenge is to disentangle these abiogenic and biogenic components. A simple visual approach is to assume that even layering, common in Archean stromatolites, may reflect significant abiogenic precipitation, whereas Phanerozoic stromatolites are characterized by uneven fine-grained layers produced by mats. These successive developments suggest decline in abiogenic precipitation and increase in lithified mat components through time, reflecting changes in seawater chemistry and mat growth. But large evenly layered Precambrian cones and domes lack present-day analogues. So these ideas remain conjectural. A major change in stromatolites followed Phanerozoic development of algal and metazoan reef building organisms. These seem to have inhibited stromatolite dome-column formation in favor of more irregular discontinuous reef crusts, except during Mass Extinction aftermaths and in ecological refuges such as Shark Bay and Lee Stocking Island. The overall conclusion is that long-term decline in stromatolite abundance probably mainly reflects decline in seawater saturation state that slowed lithification and therefore accretion, coupled with increasing competition from other organisms. In this view, high metazoan diversity and generally low saturation state are responsible for the present-day scarcity of marine stromatolites.


Robert Riding is a Research Professor in Earth and Planetary Sciences at UT, and Emeritus Professor at Cardiff University in Wales. He has held visiting positions at Chicago Field Museum, Nanjing Academy of Sciences, and universities in Germany, Spain, Romania and Italy. He investigates algal and bacterial carbonates as indicators of long-term changes in climate, sea-level, seawater chemistry, and atmospheric composition.


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