Falkland Islands (Islas Malvinas) in the Permo-Carboniferous

Investigating the dynamic behaviour of Earth’s climate system remains a significant challenge. It is partly motivated by our limited knowledge of how the system will evolve in the future and the resilience of ice sheets to perturbations. Throughout geological history, the Earth has been subjected to extreme climatic transitions. These have taken place over various spatial and temporal scales ranging from localised decadal oscillations through to millennial- and multi-millennial-scale processes affecting the whole planet. The focus of this study lies within the latter kind, wherein the Earth flips between icehouse and greenhouse states. During the Permo-Carboniferous, when the Gondwanan supercontinent lay close to the South Pole, sedimentary deposits developed that documented one such transition. This chapter outlines the potential for study in material extracted from East Falkland during a mineral exploration programme in 2008. The geographical location of the Falkland Islands in the Permo-Carboniferous based upon Palæozoic reconstructions of Gondwana is discussed and the stratigraphic links between the Falklands deposits and other similar deposits from across the continent explored. Our understanding of how major climatic changes proceed, and the possible triggers behind them, could be significantly improved by studies of such deposits.

Two borehole cores extracted between Old House Rocks and Black Rock, East Falkland, were studied using a combination of detailed sedimentary logging and geochemical analyses. The former involved grain and clast studies while the latter included X-ray fluorescence (XRF) and reflectance scanning using an Avaatech XRF core scanner. Logging data collected incorporated details on grain composition, size, shape, sorting, orientation, colour, sedimentary structures and fossils as well as a detailed account of variations in clast characteristics. A full scan of the original hand-drawn log is presented in the data postscript at the end of this book, and a digital version may be found online as a SPRINGER EXTRA. The high-resolution log was generated at a scale comparable to the X-ray fluorescence and reflectance data collected by the core scanner to encourage subsequent, more detailed climatic interpretations. The core scanner measured variations in element concentrations downcore and three reflectance parameters: lightness, red:green ratio and yellow:blue ratio. Total Organic Carbon (TOC) measurements on loose fragments throughout core DD090 were also taken. Cyclicity observed in the XRF and reflectance data was compared alongside Milankovitch orbital cycles through age modelling, thereby helping to constrain the time frame spanned by the deposits.

Understanding the environment of deposition in detail is crucial to appreciating the global context and significance of Permo-Carboniferous climate change. This chapter looks in detail at the nature of the Falkland Islands sedimentary deposits logged over the icehouse to greenhouse transition. A discussion of the underlying mechanisms driving the observed variations in sedimentology is presented. In outline, a massive diamictite deposited subglacially and with a variety of clast types, gives way to an interlaminated unit of diamictite and mudstone followed by a mudstone-dominated unit. The interlaminated zone forms the focus of this chapter since it is this stratigraphic section which tracks the icehouse to greenhouse transition. In the more stratigraphically complete core, DD090, four broad lithological zones are identified within this section. These are thought to represent two major episodes of advance and retreat of an ice sheet. This signal is overprinted by higher frequency oscillations depicted in millimetre-scale laminations and sedimentary structures. Dominantly subglacial deposition was replaced by a fluctuating ice-proximal to ice-distal environment before the onset of ice free open water conditions persisted. The sedimentary log is compared with the XRF and reflectance scans and correlations drawn where possible.

The depositional framework outlined for the Falkland Islands deposits in the previous chapter is investigated in the context of the Gondwanan supercontinent. The focus in this chapter switches from trying to recognise the immediate physical processes responsible for the sedimentological variations, to seeking an explanation for the underlying mechanisms responsible for initiating the dynamic environmental setting. The sedimentology, combined with a review of the existing literature, points most convincingly towards the idea that Gondwana was host to a large subglacial lake. Milankovitch orbital forcing is explored as an explanation for the cyclical nature of the deposits. Firstly, the XRF and reflectance data were subjected in the depth domain to spectral analysis through which prominent periodicities (in depth) could be identified. Then, by considering the data alongside a synthetic Carboniferous orbital solution, an age model and a time constraint for deposition were developed. Orbital cycles appear to be expressed in this stratigraphic record at 100 and 400 kyr periodicities, implying the transition took approximately 1.2 myr. Within the course of the transition, glaciological processes (including the build-up of an unstable ice sheet regime following the intense and prolonged icehouse state) may also have assisted in driving the oscillations. Taking into consideration the phases of glaciation documented across other landmasses that were also once part of Gondwana, together with the expected diachroneity of climate change across the continent, the transition in the Falkland Islands is constrained to the Sakmarian Stage of the Early Permian. Possible causes of deglaciation are also briefly explored in this chapter.

The borehole core sections illustrating the Late Carboniferous to Early Permian glaciogenic succession in the Falkland Islands provide an insight into palæoenvironmental changes experienced in the Southern Hemisphere during an ‘icehouse to greenhouse’ transition. The critical data were acquired by detailed analyses of two core sections recovered from East Falkland that spanned the lithological transition zone. Within Gondwana, the Falkland Islands occupied relatively high latitudes at the time and the cores preserve glacial diamictite deposits from the Fitzroy Tillite Formation passing up into black mudstones from the Port Sussex Formation. The borehole cores provided a unique opportunity to investigate the relationship between ancient lacustrine depositional dynamics and climatic variations. The study used a combination of detailed sedimentary logging and high-resolution XRF and reflectance scans. It provides the first high-resolution data set from this part of the Gondwana supercontinent for the Carboniferous–Permian climatic transition. The XRF and reflectance scans enabled the identification of subtle changes in both sediment provenance and the prevailing environmental conditions.

The complete scan of the hand-drawn logs for cores DD029 and DD090 are presented in this chapter. Plots of reflectance data, and XRF data for the elements silicon, potassium, chromium, titanium, manganese, iron and zirconium are also shown for both cores. The XRF logs incorporate annotations tying the element fluctuations observed to sedimentological variations. Results from the clast analysis showing clast frequency, sorting, roundness, sphericity, size and composition are presented for core DD090, as is the raw data on total organic carbon. Details regarding the measurements made to generate the depth scales are provided alongside images of the polished cores. Please refer to the appendix for a summary of the preliminary age modelling work and the outputs generated from the principal component analysis.