Neoproterozoic in Britain and Evidence for the Hypothesis Snowball Earth

To what extent does Britain’s rock successions during the Neoproterozoic provide evidence for ‘Snowball Earth’, and how reliable is it?

Abstract

Geologic, stratigraphic, palaeomagnetic and geochemical observations have been interpreted to suggest extreme and global glacial events followed by just as severe hothouse conditions (snowball earth hypothesis).  This hypothesis has resulted in much debate, creating discussions regarding the focuses on these various approaches towards research about the existence of a snowball earth. This in turn has developed a somewhat reliable conclusion.  The Port Askaig formation of Scotland is a rock succession in the Argyll super group consisting of diamictite, sandstone, conglomerate and mudstone, all recording glacial conditions in the Dalradian basin. This essay focuses on data collected for the Neoproterozoic in Britain (particularly Scotland) and how reliable the evidence is for the hypothesis snowball earth.

Introduction

When looking back through the geological record, there are substantial changes to the paleoenvironment throughout the Proterozoic. This is shown specifically in the Neoproterozoic, illustrating widespread glaciation followed by warm conditions and major global carbon fluctuations. With regards to the snowball hypotheses, a more recent proposal suggests that there were a number or global glaciation events followed by hot conditions. Data collected at the Port Askaig formation indicates that there is a contradiction between climate and the deposition of sedimentary rocks related to these areas. At the base of the Argyll group and within the Dalradian super group in the Neoproterozoic there are varying deposits. These deposits are mainly diamictite which are also found in other continents and major cratonic regions during this depositional period (Kirschivink, 1992). Using Britain as an example, explicitly the Port Askaig formation, this report will discuss the reliability of the Snowball Earth Hypothesis (first proposed by Joseph L. Kirschivink) by focusing on depositions of glacial deposits in Scotland when located around the equator during the last era of the Precambrian.

Setting the scene

Glaciers flowed throughout the Proterozoic sea, even in the tropics. Debris churned out from these glaciers which then settled on the sea floor thus enabling the switch of climate. This means that carbonate rock was set to deposit on the glacial detritus. During this period of climate, flipping life had seemed to almost completely vanish. Iron deposits completely disappeared by the earth’s new abundant oxygen, reappearing for the last time in the late Neoproterozoic. This is a somewhat helpful aspect when carrying out palaeomagnetic research. Evolution had been stagnating for billions of years, yet proceeding this occurrence, all basic body plans of animals suddenly emerged. Scientists have gone back and forth on the hypothesis known as ‘Snowball earth’, studying the Proterozoic eon in great depth in order to understand what really happened during this time. The theory suggests that there were at least two global ice ages which almost destroyed the very existence of life itself. With evidence for palaeomagnetism signifying that the line of ice was almost equal with sea level and thus close to the equator (Hoffman, 1998), there is significant implication that there were abrupt changes that allowed global warming to turn the earth into a hot mess.

Discussion

Rock Type:

The Port Askaig formation is a thick sedimentary succession abundant in soft sediment deformation features (possibly related to episodic events). This is seen as predominantly marine found in the Argyll super group. The unit consists of poorly sorted Diamictites, cross bedded sandstone with laminations, angular clastic conglomerate and laminated mudstone. This sequence can be located in other continents which would have sat in a similar location in relation to the equator during the late Precambrian whilst the Iapetus Ocean still existed. The different units of the formation are easily identifiable and can be interpreted to show the severe changes in climate during the Neoproterozoic. The Diamictites unit could illustrate a glacially influenced marine setting, possibly relating the conglomerate layer from gravity flows or from other unidirectional currents.

The base section of the Port Askaig unit of sandstone includes that sedimentary structure of particularly large cross bedding, individually reaching 11m thick and with a maximum dip of 14’. This sandstone is seen to be quarzitic, well sorted and medium grained, which helps to illustrate a southern paleocurrent direction in the internal structure of the cross beds (Arnmaud Emmanuelle, 2003). Due to this structure, it becomes clear that this bedding is from the migration in a subaqueous marine environment. Some sections of the structure indicate that there is an association between the unidirectional current and a tidal setting wherein no ice is present. However, referring to the Diamictite and conglomerate including in this succession would indicate ice advancing and retrograding. When associating this inconsistency with snowball earth, it can be seen as contradictory as there is no proven section that confirms the idea of a severe and prolonged global freezing that is followed by rapid heating.

The exposed succession seen at the Garvellach Islands records 28 glacial, 25 periglacial and 23 non-glacial episodes. The Argyll group shows something different; 7km thick quartzite with accommodation space created rapidly (Dishad O. Ali, 2017).  Diamictite units are attributed by sediment gravity flow processes as previously stated and so this could also indicate that there was possible heavy precipitation. This created a flow of fine grained sediment and debris carried by ice in a glacial marine setting.  Using the work of Emmanuelle Arnmaud 2003-2005 which explores stratigraphic analysis of these islands, it appears that the depositional period can be split into three phases; phase one being dominated by sedimentary flow processes and tectonically controlled sedimentation, phase two seen as transitional, characterized by continued tectonic instability, increasing supply of sand to the basin and preservation of facies, and the final phase demonstrating interbedded sandstone and diamictite layer, illustrating the development of the sand bed forms with ice margin fluctuations in a tectonically stable marine setting.

The stratigraphic and sedimentological analysis of the Port Askaig deposits best exposed on the Garvellach islands, has enabled the establishment of the palaeoenvironmental change during the Neoproterozoic. This research allowed for an understanding of the tectonic activity and how it had significantly influenced the lowermost part of the succession. However, due to this activity it is made much harder to identify the climatic influences. The thick succession of diamictite interbedded with current-deposited sandstone preserved within the Port Askaig Formation is not consistent with deep freeze conditions proposed by the snowball Earth hypothesis.

Tectonics:

The Dalradian group is associated with the breakup of Rodinia through the rifting of plates (Dalziel, Soper, 2001). There is some disagreement on this matter. The research of [INSERT NAME] suggests that it is more likely related to the accumulation of the super group in a foreland basin associated with the Riphean orogenesis due to extensional tectonic activity.               Seen in Figure 2 you can see the abrupt changes of facies likely due to synsedimentary faulting, with possible events of folding, the fining upwards would also indicate the basin filling upwards as well as earthquake induced liquefaction (mainly seen in the quartzite).  However, there is contradictory beliefs thaht there is significant extensional tectonic activity, which had been building up before the Iapetus Ocean had opened. Association with localised faulting has been made in the lower Islay subgroup (Port Askaig and Jura Quartzite) due to the varying thicknesses (Anderton, 1982). With regards to the Conglomerate, it is more than likely a fault generated deposit due to its angular fragment nature, overlain by turbidite facies fining upwards which could be interpreted as basin subsidence and sub-basin differentiation. Eventually this tectonic activity in the basin led to the extrusion of the volcanic layer and the opening of the Iapetus Ocean.

Using the lithological correlation with glacigenic deposits elsewhere in north Atlantic region, the port Askaig formation has long been thought to record glacial conditions (Spencer 1975). In addition, the recent discovery of glaciomarine drop stone horizons overlying the Port Askaig Formation in Ireland and the isotopic signature of ‘cap’ carbonate associated with the Port Askaig Formation, suggest these glacigenic deposits are more likely record an earlier glaciation. This could have been 717-643Ma during the Cryogenian period (Condon and Prave, 2000, Brasier and Shields, 2000).

Palaeo-latitude:

A quantitive basis has been provided by palaeomagnetic data collected from the glaciogenic deposits found in the Port Askaig formation. Recent data of depositional latitudes have created a trend found most dominant near the palaeo-equator (Evans, 2003).  However, the actual palaeo-latitude of the Dalradian basin is still subject to some debate due to the remagnetization of deposits during the Caledonian orogeny (Stupavsky, 1982). This sparked uncertainty of the location of this Scottish region during the Neoproterozoic. However, the data collected from palaeomagnetism and palaeo-latitudes neither supports the entire theory of snowball earth nor rejects it. This research does not contribute much to the theory but despite not being completely certain when accepting the idea that Scotland once sat along the equator, it does reinforce the idea that there were glacial events in warmer parts of the world. If it was indeed situated along the equator this would support the theory of a globally iced over earth. The direct observations of high quantities of carbon fragments in some diamictite beds would imply the ice must have somehow moved across carbonate platforms. Specific carbonate sequences are limited to around 33’ of the equator (Ziegler, 1984), therefore meaning ice would have been present here.

Chemostratigraphy:

The use of new carbonate ⁸⁷Sr/⁸⁶Sr data helps to comprehend environmental and climatic change thus allowing further insight into the controversial hypothesis of snowball earth. Certain beds in the Dalradian super group, particularly the limestone, contain specific mineral levels making them ideal for this type of testing. The limestones within the Port Askaig formation are the lowest levels in the entire super group. These levels match elsewhere in the world which date to the Cryogenian glaciation (Citation), therefore reinforcing the theory that the Port Askaig succession was deposited during this glaciation period. Research has enabled the reflection on the ⁸⁷Sr/⁸⁶Sr ratio of seawater decrease, before the older glaciation period of the cryogenic can be associated with the breakup of Rodinia due to the enhancement of weathering. In addition, the carbon found in the diamictites provides a means of determining where Scotland was positioned during this time to paleo-latitudes as previous stated.

Results

When gathering all factors from geology, tectonics, palaeomagnetism and chemostratigraphy, it is clear to see that the Port Askaig formation provides an encompassing view of the events during the Neoproterozoic. It illustrates a number of glacial periods which contain time gaps between beds with some folding that in turn suggest erosional periods and tectonic behaviours.

Three phases have been made apparent by the stratigraphic research, identifying as sediment gravity flows (phase 1), later ice free stage (phase 2) and current dominated conditions alternating with glacially influenced sedimentation (phase 3). Phase one and two appear to be tectonically influenced despite climatic impacts on sedimentation, possibly being obscured by the predominance of sediment gravity flow processes. Phase three however, seems to be the only phase that can identify glacial influences as tectonics do not seem to be an asset during this time. Neither do they alter the record such as is the case in the other two phases, thus they make the sea level changes related to ice margin fluctuations apparent in this section.

 The analysis from sedimentologists have allowed us to understand such activity through the actual geology and structural research. Giant cross beds within the upper section are likely formed as a result of the migration of large subaqueous sand dunes, almost certainly due to open tidal currents. The conglomerate infers localised tectonic activity with the disruption of beds and deformation features and possible periodic seismic activity. Many of the diamictite units were formed as a result of subaqueous debris flows which are often associated with a glacial environment. Other diamictite beds could be formed within glacial influenced rainout deposits. Both diamictite units containing few dropstones which despite being rather rare in this succession suggests a presence of floating ice in the basin. The sandstone and mudstones, have been described as sediment flow settling or possible current-dominated subaqueous conditions.  The entire unit indicates the abrupt changes in climate. When correlating this stratigraphic data alongside paleo-latitudes it reinforces the idea of global episodic events thus accepting the concept of snowball earth.

Conclusion

When concluding it is rather important to establish the significance of the Neoproterozoic glacial episode, it marks a major turning point in the evolution of life. With life almost completely vanishing due to the abrupt changes in climate, without the research into the Argyll super group this would still be a question today. However, using the Port Askaig formation in Scotland it illustrates the nature of the environmental changes that transpired. The succession of diamictites, conglomerates, sandstones and mudstones were all deposited by various processes which indicate this sub-era experienced a glacially and tectonically influenced marine setting. With the amounting evidence gathered by varying scientific researchers throughout a vast field of work, evidence seems to point to the idea that there was a number of global glacial periods, in fact accepting the snowball earth hypothesis rather than discrediting it. However, I regard the gathered information that has been displayed in this report as evident of a snowball earth period. What has been made overt in this paper is that evidence suggests the presence of ‘ice age’ periods, though they would not necessarily be considered as extreme as first suggested by  J.L Kirschvink. The reliability of palaeolatitude evidence depends on the palaeomagnetic data quality but also the confidence in chronostratigraphic correlations.

Strata from the Neoproterozoic is spread across several regions with units sharing an overall similar lithology, all showing fluctuating climate records which would therefore be a result of global scale climate fluctuations. The ice which once drifted across the ocean separating ocean currents and the fetch therefore reducing evaporation. Occurring as they obstruct the exchange between oceanic to atmospheric oxygen, in turn creating anoxicity in the lower ocean waters. Eventually from leaching and creation of mid oceanic ridges ferrous iron was generated would build up due to solution. As the glacial period became to an end circulation of the oceanic waters generated once again enabling iron to oxidized, coming back one last time as banded iron deposits.

Bibliography

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Arnaud, E., 2003. Giant cross-beds in the Neoproterozoic Port Askaig Formation, Scotland: implications for snowball earth, s.l.: Department of Land Resource Science, University of Guelph.

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