Monday, March 7, 2016

Balmy Shores And Icy Wastes- Carbonates Overlying Proterozoic Glacial Deposits

This problem was articulated quite well in a review article more than 20 years ago in 1993: Balmy Shores and Icy Wastes: the paradox of carbonates associated with glacial deposits in Neoproterozoic times. The  thinking has been that the Neoproterozoic must have seen very abrupt changes in climate from a "Snowball" earth with glaciers spreading to low latitudes, to a much warmer climate,  more amenable for widespread carbonate deposition. A variety of explanations for the origin of these carbonates were proposed depending upon their stratigraphic relationships with glacial deposits. There are carbonates interlayered with glacial deposits. Some layers based on their being composed of intraclasts (particles of eroded older limestones) were interpreted as detrital limestones. In other cases of very fine grained rock, it was thought that glaciers grinding and eroding older Proterozoic rock would have delivered fine rock flour to lakes and seas. This  reactive rock flour then recrystallized into a secondary carbonate layer.

In other instances, there is a "cap" carbonate, i.e. a layer overlying a glacial deposit. These cap carbonates vary in the environments of  deposition they represent. They are supratidal  to intertidal  dolomites, or shallow  subtidal limestones to deeper water limestones. They are primary precipitates, meaning the calcium carbonate crystals precipitated out of sea water or in the case of dolomites, an initial aragonite or calcite mineralogy was replaced by the mineral dolomite. Their presence overlying glacial deposits indicate an end to glacial conditions and marking a sudden shift of climate to warmer times. The sea water was thought to be supersaturated in calcium and magnesium ions, but the reason for this has not been well understood.

A new study (T. M. Gernon et al 2016) on these cap carbonates now points the finger to an increase in alkalinity of sea water due to the alteration of volcanic glass being formed at sea floor spreading centers due to the breakup of the Rodinia supercontinent in the Neoproterozic-

During Neoproterozoic Snowball Earth glaciations, the oceans gained massive amounts of alkalinity, culminating in the deposition of massive cap carbonates on deglaciation. Changes in terrestrial runoff associated with both breakup of the Rodinia supercontinent and deglaciation can explain some, but not all of the requisite changes in ocean chemistry. Submarine volcanism along shallow ridges formed during supercontinent breakup results in the formation of large volumes of glassy hyaloclastite, which readily alters to palagonite. Here we estimate fluxes of calcium, magnesium, phosphorus, silica and bicarbonate associated with these shallow-ridge processes, and argue that extensive submarine volcanism during the breakup of Rodinia made an important contribution to changes in ocean chemistry during Snowball Earth glaciations. We use Monte Carlo simulations to show that widespread hyaloclastite alteration under near-global sea-ice cover could lead to Ca2+ and Mg2+ supersaturation over the course of the glaciation that is sufficient to explain the volume of cap carbonates deposited. Furthermore, our conservative estimates of phosphorus release are sufficient to explain the observed P:Fe ratios in sedimentary iron formations from this time. This large phosphorus release may have fuelled primary productivity, which in turn would have contributed to atmospheric O2 rises that followed Snowball Earth episodes.

One broader picture that is emerging is that the time period between 1000 and 550 million years ago was one of the most remarkable times in the history of our planet. On one side, from the origin of the earth, lies 3 billion years of a microbial biosphere, and on the other, the extraordinary diversification of large complex multicellular life. The 400 odd million years of the Neoproterozoic created the conditions for this transformation. The infographic below summarizes the changes in the geo-biosphere during Neoproterozoic  times.

Source: Butterfield N.J. 2015

There were changes in continental configuration due to plate tectonics (breakup of supercontinent Rodinia ~ 850 mya), extreme icehouse conditions in the Cryogenian with two major glacial phases, the Sturtian and the Marinoan, lasting tens of millions of years  and sea water chemical changes with increase in ocean oxygenation to crucial threshold levels needed for metabolically demanding activity, major perturbations in organic and inorganic carbon cycles and widespread deposition of phosphate on the sea floor. A confluence of these events not just led to the formation of geologically unusual deposits but also influenced the evolution of complex multicellular life. That in turn completely changed sea floor sediment fabric and chemistry, further providing new ecologic niches for evolutionary innovations. Engineered by animal activity, the construction of a new world began.

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