Continuous Geomagnetic Field Records During Snowball Earth: Critical Tests of Cryogenian Uniformitarianism

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Neoproterozoic time is simply weird compared to younger intervals of Earth History, as sediments of this age contain evidence for unusual or extraordinary climatic, biogeochemical, and geological events. Of particular geophysical interest are the late Neoproterozoic stratigraphic intervals on all continents that show firm lithologic evidence of glaciers marching over low-latitude carbonate platforms, a situation that clearly has not happened in the past 500 million years. This "Snowball Earth" climate catastrophe relies heavily on paleomagnetic studies of late Neoproterozoic glacial sediments in South Australia, which have shown repeatedly that low-paleolatitude magnetization wraps around purported syn-sedimentary folds in a ~10 m thick, rhythmic-laminated "tidal" deposit, reverses polarity frequently in thicker, multi-environment glacial deposits, and pre-dates tectonic folding. Recent paleomagnetic investigations of isotopically-anomalous "cap" carbonates, deposited during final Snowball deglaciation, have documented multiple reversals, suggesting surprisingly frequent geomagnetic field changes and/or unexpectedly long deglacial timescales. Either way, something about the Neoproterozoic Earth system and its response to climate change forcing seems to have been fundamentally different from our understanding of modern global change. We are producing two possibly continuous geomagnetic field records from critical intervals of South Australia's Snowball Earth sedimentary succession to test the veracity of the existing, surprising paleomagnetic records from those units. In a unit known as "Elatina rhythmites," millimeter-scale sand and silt couplets are considered to represent semi-diurnal tides. A positive "syn-sedimentary fold" test on Elatina rhythmites establishes an apparently depositional age of magnetization, so millimeter-scale magnetostratigraphy through ~10 m of rhythmites should recover a continuous ~60-year geomagnetic field record suitable for answering one or more of the following questions: 1) Are Elatina rhythmite laminations genuinely semi-diurnal? Any signal resembling geomagnetic secular variation through the rhythmite section would seem to preclude this possibility and argue for a longer, perhaps annual, lamination periodicity. 2) Is the famous, low-paleolatitude paleomagnetic result for Elatina rhythmites genuinely syn-depositional in origin? Detailed sedimentologic and paleomagnetic investigation of purported "syn-sedimentary folds" would address remote alternative hypotheses of diagenetic origin for rhythmite magnetization. 3) Was the geomagnetic field during Snowball glaciation anomalously weak? Such a scenario could explain unusually frequent geomagnetic reversals in Elatina glacial deposits and in the overlying, deglacial cap carbonate. If answers to 1) and 2) proved magnetization to be depositional on a short timescale, then fabric analysis of Elatina rhythmites could prove that part of the anomalous signal derives from inefficient magnetization processes, possibly linked to a weak geomagnetic field. Finally, we are also producing a ~50,000 - 500,000-year, nearly continuous geomagnetic field record from Nuccaleena cap dolostone, the deglacial carbonate postdating Elatina glacial deposits. The structure of an expected geomagnetic secular variation signal in this unit should also address whether or not the strength and character of Earth's Neoproterozoic geomagnetic field was similar to that of the modern Era. The mystery of Snowball Earth, and the "magic" of paleomagnetism merit wider public communication. We are collaborating with Australian colleagues to produce an international, traveling museum exhibit exhibiting some of these exceptionally interesting -- and visually stunning -- rocks. We are also advising a National Geographic TV documentary focused on the Earth history story of Snowball Earth and on the scientific method by which geologists and paleomagnetists read the ancient rock record.