Wednesday, August 8, 2012

My Rather Tenuous Connection With The Mars Rover Project Scientist

I have no connection with the Mars Rover program :)...but this @geosociety tweet a few days ago caught my eye:

@geosociety Fellow John Grotzinger, JPL geologist on Mars Curiosity rover mission, in LA Times. http://lat.ms/QzG81W  MT @earthmagazine

John Grotzinger was profiled in an article in the LA Times. He is project scientist for the Mars mission and in charge of directing the earth science effort to glean information about the geology of Mars. Here is what the article says about his work-

For much of his post-PhD career, the geologist kept his feet planted firmly on Earth. He combed ancient sedimentary rocks for signs of early life. He took trips around the globe, family in tow, to collect 550-million-year old specimens in Namibia and Oman.

What it left out was that Prof. Grotzinger is a carbonate sedimentologist. So.. I guess I can claim that I share an academic kinship with him :)

I am quite familiar with his work in carbonates. When I was working on my PhD in the mid 1990's he was already a faculty at MIT. His PhD research on Proterozoic carbonates of the Northwest Territories in Canada was directed by J. Fred Read at Virginia Polytechnic. During several GSA meetings I did get an opportunity to listen to his presentation on various aspects of Proterozoic carbonate platform evolution. He later moved to Cal Tech and JPL in Pasadena, California.

For long, carbonate sedimentologists gave much more attention to Phanerozoic carbonates and less attention to Proterozoic carbonate deposits. There was an economic incentive in that. Many Phanerozoic carbonate basins host prolific oil and gas deposits. The origin, growth and architecture of Phanerozoic carbonate sedimentary platforms,  a term for depositional basins in which hundreds to thousands of feet of calcium carbonate sediments accumulate, was studied quite intensely and we gained a very detailed understanding of these systems. All this work ultimately helps exploration geologists make reasonable predictions on the location and thicknesses of strata best suited to be oil reservoirs.



In contrast, a platform evolution approach was rarely undertaken to understand Proterozoic carbonate basins. Multicellular organisms evolved by around 600 million years ago (mya). By 540 mya to 525 mya many groups of multicellular animals evolved the ability to secrete calcium carbonate from sea water and construct skeletons to encase their soft tissue. Phanerozoic limestone therefore is principally made up of skeletons of these creatures along with a lesser proportion of inorganic calcium carbonate precipitate which acts as a cement and binds skeletal material into hard rock.

In Proterozoic seas, from 2.5 billion years ago to around 540 mya, bacteria dominated. The atmosphere and sea water chemistry was different too. Sea water was supersaturated with calcium carbonate meaning that there was a lot of Ca and CO3 ions available to form calcium carbonate molecules and to aggregate as calcium carbonate sediment. This lead to two types of calcium carbonate sediment. Photosynthetic bacteria can aid precipitation of calcium carbonate by removing CO2 from sea water and increasing the availability of CO3 in that microenvironment. That meant that calcium carbonate precipitated in and around bacterial colonies and the resulting sedimentary deposits took the shape of those bacterial colonies. Some were flat while other were domes and formed a relief on the sea floor. Such structures are known as stromatolites.

In some locations there was spontaneous precipitation of calcium carbonate without the aid of microbial photosynthesis. Such precipitates of the mineral aragonite and calcite also took weird and wonderful morphologies like giant aragonite botryoids which is a structure formed by crystals radiating in a shape of a fan. Together, stromatolites and inorganic mineral precipitates formed deposits thousands of feet thick in many long lived Proterozoic basins.

Because of biological evolution Proterozoic and Phanerozoic carbonate platforms differ in the composition of the sediment that is produced. But what about the larger architecture of the platform? How does the growth history of a Proterozoic platform from initiation to development to its demise compare with a Phanerzoic counterpart? For example, Phanerozoic carbonate platforms show typical ecologic zones that develop in response to organisms preferring certain water depth and wave energy conditions. A typical Phanerozoic platform may have coral reefs in areas facing the open ocean with strong wave energy and sunlight water depths, a lagoon sheltering in the leewards side of the reef and mud banks accumulating in shallower tidal regions.

The schematic below shows the classic facies belts of a typical Phanerozoic carbonate platform (Wilson 1975 Carbonate Facies In Geological History).



Prof. Grotzinger's work on the Proterozoic carbonate platforms in Canada and some other areas revealed a striking similarity with Phanerozic platforms. The image below is a synthesis of Prof. Grotzinger's research on this subject.


Source: John P Grotzinger 1989; Controls On Carbonate Platform And Basin Development: SEPM Special Publication No. 44, p. 79 - 106

A profile taken from shoreline to the continental shelf edge and slope show that microbial communities build similar ecologic zones on Proterozoic platforms. In very shallow tidal waters, bacterial mats accumulate thinly laminated sediment. In lagoons, inorganic mud and domal forms are common, and in areas of high wave energy bacteria build reefs characterized by elongate and branching columns. Inorganically precipiated aragonite and calcite forms layers at places and crystal aggregates in the interstices of the bacterial colonies.

Platforms may last for hundreds of thousands of years and during that time sea level will rise and fall forcing marine communities to shift and track optimal conditions. Ecological zones therefore shift in response to sea level change causing packages of particular sediment types to occur and reoccur at different paleogeographic locations at different geological intervals.  For example, if sea level rises each ecologic zone shown in the above figure may shift shorewards i.e. towards the right. Over time, a carbonate basin that was initiated by tectonic subsidence and sea level rise as a sloping ramp (above figure) may evolve into a rimmed shelf, the change driven in part by prolific production of carbonate sediment in reefs which generates relief on the sea floor.

Prof. Grotzinger's detailed evaluation of long term platform growth history showed that Proterozoic carbonate sequences are built by microbial ecosystems responding to sea level changes in a  manner similar to Phanerozoic metazoan communities. Microbial communities shifted shorewards or seawards tracking sea level change and over the long term ramps evolved into a rimmed shelf with microbial reefs hundreds of kms long. Eustatic sea level change and basin subsidence are the primary controls on the stratigraphic evolution of both Proterozoic and Phanerozoic carbonate platforms.

We recognize in modern and older Phanerozoic basins a complex of environments like reefs, lagoons, and tidal mud flats. These ecologic zones are a result of different water depths and sediment production by different types of metazoan communities. Such complex ecological partitioning of the sea floor has ancient roots. By early Proterozoic, microbial communities had already colonized the sea floor. They responded to changing sea levels and water energy conditions by building optimal bio-structures and produced a paleogeographic zonation of the sea floor. Metazoan evolution and diversification in the early Cambrian took place on this Proterozoic template.

I got my introduction to carbonate sediments during my M.S. thesis work in the Proterozoic Cuddapah basin of South India. I described (or tried to) a sequence of limestone and dolomite which had a variety of stromatolite and other abiotic components. It was tough going. All the standard textbooks and lab workbooks and atlases described  carbonate particle types and textures only from Phanerozoic examples. It was really hard to interpret what I was observing in terms of its paleoenvironmental and ecological significance.

Prof. Grotzinger's research on Proterozoic carbonate depositional basins has contributed significantly in providing a theoretical framework for interpreting the sediment types and textures and in understanding the controls over the stratigraphic evolution of these basins.

2 comments:

  1. Thanks for the post, I’m happy you wrote about Proterozoic carbonate platforms instead of Mars! I just put up a post about early Proterozoic metadolomite and stromatolites ... well, kinda ... as habitat for wildflowers :) But I’d like to devote a post to the subject one of these days. I looked again at your discussion (June 28) re microbiota and dolomite of the Phanerozoic. Is there any evidence that Proterozoic dolomite has biotic origins?

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  2. Is there any evidence that Proterozoic dolomite has biotic origins?.. hmm good question :)

    many Proterozoic dolomites have been interpreted to have aragonite precursors. .. but the replacement of aragonite to dolomite seemed to have occurred very early just a few cm below the sediment water interface.. its likely that microbial removal of sulphate may have favored this early dolomitization.. besides very early replacement there is increasing evidence of precipitation of dolomite cement from marine fluids in pore spaces of grainy facies like oolites and reefs. again the reasons were likely a combination of elevated Mg/Ca ratios and anoxia which promoted microbial growth and sulphate removal..

    this is a good reference paper.. if u don't have access email me suvrat underscore k at yahoo dot com and I'll send u a pdf.

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