Stable isotopes of oxygen and hydrogen in meteoric water during the Cryogenian Period

We measured δ18O and δ2H values of muscovite and carbonate mineral separates from metamorphosed carbonate-bearing mudstone layers in late Tonian to early Cryogenian strata, including Sturtian glacial deposits, which were deposited in a coastal setting at an approximate paleolatitude of 30-35°S and now crop out on Islay and the Garvellach Islands, Scotland. From these values, we calculated δ18O and δ2H values of meteoric water that equilibrated with clay at diagenetic conditions which we infer were reached shortly after deposition (i.e. before the end of the Cryogenian Period) because sediment accumulation was rapid due to fast subsidence at that time. This calculation required removal of the effects of exchange with reservoir rocks, metamorphic volatilization and mixing with metamorphic fluids on δ18O and δ2H values. The values we calculated for meteoric water fall within the 2σ ranges δ18O = -1 to -4 ‰ and δ2H = 0 to -23 ‰, respectively. These ranges are similar to present day values at equivalent latitudes. This finding is consistent with sediment accumulation in the Cryogenian Period having occurred in a climate similar to present day (Ice Age) conditions. This conclusion is not at odds with the Snowball Earth hypothesis because one of its predictions is that sediment accumulation occurred as the climate warmed at the end of panglaciation, a prediction supported by sedimentological evidence of multiple glacial advances and retreats in our study area and elsewhere

Developing geochemical and mineralogical proxies for the correlation of paleotsunami layers

The catastrophic Indian Ocean tsunami of December 2004 raised urgent questions about the paleotsunami history in the region. Numerous studies have since been conducted to gain better understanding of the magnitude, frequency and impact of past tsunamis, especially around the coasts of the Indian Ocean. Southwest Thailand directly faces the Sunda Arc trench where the earthquake that generated the Indian Ocean tsunami took place. The lack of historical documents and of suitable geological archives makes paleotsunami research a challenge in this area however. Phra Thong Island, on the Andaman Coast of southwest Thailand, with its marshy swales has proven an exception and is one of the few suitable locations for these types of studies. Apart from the 2004 Indian Ocean tsunami layer, three to four more distinct paleotsunami layers, separated by organic rich soil horizons have been previously identified and dated using radiocarbon and Optically Stimulated Luminescence techniques. Despite these efforts, several outstanding issues have to be resolved: (1) the correlation of tsunami/paleotsunami layers over larger distances remains ambiguous, particularly older layers; (2) age attributions for several of the paleotsunami layers differ at different locations; and (3) alterations of soil and sand layers by postdepositional processes are still poorly understood This thesis addresses these issues and aims to develop geochemical proxies that allow characterising each tsunami/paleotsunami layer in three different swales. As shown here, X-ray fluorescence elemental geochemistry, combined with loss-on-ignition analysis and mineralogical identification, is a promising tool for identifying paleotsunami layers. The specific geochemical signature of each sand layer can then be used for correlations over larger distances. This approach has the potential to provide a means for estimating past tsunami inundation distances, and thus their magnitude.