Reconstruction of the geomagnetic field during the Late Quaternary in Scotland using lacustrine sedimentary records
Rada Torres, Myriam Andrea
At about 2900km deep beneath our planet surface, there is a sharp boundary where solid rock from the Earth’s mantle gives way to the molten iron-nickel alloy comprising the Earth’s outer core, with temperatures over 2000ºC. Further 2200km deeper into the Earth, very high pressure turns the core into solid iron, with temperatures exceeding 5000ºC. The interaction and heat transfer between the solid and liquid cores combined with the Earth’s rotation movements generates the Earth’s magnetic or geomagnetic field, in a process known as the geodynamo. Direct observations through navigation records, geomagnetic observatories and satellites have taught us that the geomagnetic field constantly changes at the Earth’s surface and sometimes deviates considerably from the theoretical models, in non-cyclical fluctuations called secular variations. To study the geomagnetic field in the geological past, we rely on magnetic minerals such as magnetite and haematite present in rocks at the Earth’s cold crust, which have the ability to preserve the orientation (described as declination and inclination) and sometimes the strength (called palaeointensity) of the ancient geomagnetic field at the time the rock was formed. In this sense, lake and marine sediments are valuable stratified archives since they can provide high-resolution and continuous records of the Earth’s magnetic field fluctuations that have occurred through time, known as palaeosecular variations (PSV). Recent numerical and computational models of the geodynamo indicate that secular variations respond to localized changes at the 2900km depth core-mantle boundary, which means that PSV records can potentially give us insight into the evolution and dynamics of the Earth’s core and lower mantle through geological time and for specific regions. The present PhD project produced the first PSV record for Scotland and the UK covering the last 19,000 years from the Late Pleistocene and throughout The Holocene (Late Quaternary), using lake sediment cores from Bardowie Loch in the Central Belt of Scotland. This is also one of the few studies that describes the variations of the geomagnetic field in direction and intensity during The Holocene in the UK, making the hereby results a valuable piece in the puzzle of the geodynamo behaviour in the northern North Atlantic region, which has been particularly distinctive during the Late Holocene. PSV records of North America, Greenland, Iceland, Fennoscandia (Norway, Sweden, Finland and Russia), most recently Lake Windermere (England) and now Bardowie Loch (Scotland) are consistent with direct observations and computational models of two regions of concentrated geomagnetic flux (called flux lobes) at the core-mantle boundary beneath Canada and Siberia. The declination and palaeointensity patterns of the mentioned PSV records agree with significant oscillations in strength between both flux lobes for the last 5000 years. These are especially evident in the records from Bardowie Loch, Lake Windermere and the northern North Atlantic (Greenland and Iceland marine-cores), which suggest that the Canadian flux lobe has been weakening while the Siberian flux lobe has considerably increased in strength for the last 2000 years. The Late Pleistocene section of the Bardowie Loch PSV record shows two intervals of exceptional deviation in direction at ca 18.3ka to ca 16.2ka and ca 15ka to ca 12.8ka. The deepest interval displays rare shallow inclinations (~20º) for Scotland’s latitude joined by large oscillations in the declination (a maximum of 70-degree deviation). Such substantial variations in the geomagnetic field orientation agree with the poorly known Hilina Palli Excursion, dated between 22ka and 17ka and observed globally in volcanic rocks, marine and lake cores from Hawaii and California to Lake Baikal and China. The upper interval of anomalous geomagnetic field behaviour, close to the end of The Pleistocene, exhibits the largest drop of inclination from present-day values (about 70º) to negative inclinations (-12º), while the declination presents oscillations over 50-degrees. These dramatic variations in orientation are consistent with the controversial Gothenburg Excursion (ca 12.3 to ca 13.8ka), which were originally reported in sediments from the Scandinavian Glacial Interstadial and also observed in North American glacial lakes, with poorly constrained ages from 7.6ka to 14ka. Arguments against this event are based on the large climatic changes taking place at the end of The Pleistocene, which could have affected the preservation process of the geomagnetic field in such sediments. Numerical models have indicated that excursions are caused by magnetic field reversals taking place only in the Earth’s outer liquid core, unlike complete reversals where the geomagnetic field changes polarity throughout the outer and inner core. This could explain why geomagnetic reversals usually last hundreds of thousands to millions of years, while geomagnetic excursions only have from few thousand years of duration, like the events observed in the Bardowie Loch PSV record, to a maximum of ten thousand years. Bardowie Loch was selected as the study site based on its small-size basin, which is part of a glacial landscape, and limited catchment area without any sizeable river system. This ensures a low energy environment, which is necessary for the magnetic minerals to align with the Earth’s magnetic field at the sediment/water interface during deposition. Our geological analyses suggest that Bardowie Loch originally formed as consequence of the movement of large glaciers, possibly at the time of the last glacial maximum about twenty thousand years ago. Then from ~19ka to ca 11.5ka, the Bardowie Loch basin was part of a Proglacial Environment, a lake fed with sediments transported by glacial melt-water. We carried out rock magnetic characterisation studies based on the magnetic properties of the glacial lake-sediments, which were supported by microscopic and geochemical analyses. The results show that there are pulses of substantial deposition into the lake basin of sediments eroded from nearby Palaeozoic volcanic rocks, rich in magnetic mineral grains. Additionally, there are layers made of very fine sediments (clay and fine silt) that present large concentrations of chemical elements associated to biological productivity in the lake-water ecosystem, joined by abundant quantities of microscopic-algae fossils (diatoms). These layers also give indications of anoxic conditions at the lake-bottom and strong evidence of magnetite produced by a remarkable type of bacteria, called magnetotactic bacteria (MTB). The results from these glacial lake-sediments analyses were correlated with published data from Greenland ice-cores. We found that the largest deposition pulses occurred when temperatures abruptly increased at the end of two distinctive periods of colder glacial conditions called Stadials, at ca 14.8Ka for Stadial 2 and ca 11.6Ka for Stadial 1 (in Bardowie Loch chronology), which implies that these high detrital input events might be the result of significant melting of glaciers close to the Bardowie Loch basin. We also found that two different warming climate events produced the fine-sediments layers containing diatoms and possible MTBs. The first was a period of mild environmental conditions in the North Atlantic region known as the Last Glacial Interstadial, between ca 14.8ka and ca 13.2ka (Bardowie chronology), and the second event was the global warming that marked the onset of The Holocene, dated between 11.7ka and 11.3ka for these layers in the Bardowie Loch sequence. A thousand percent surge in organic matter content combined with a substantial decrease of sediment input from the Palaeozoic volcanic rocks and significantly lower concentration of magnetic minerals characterised the Holocene sediments of the Bardowie Loch sequence. Results from geological, geochemical and rock magnetic studies of these layers indicate that Bardowie Loch has been through three very different stages. Between ca 11.5ka and ca 8ka Bardowie was a thermally stratified lake, denoted by laminated muds and alternation of oxic and anoxic phases, with strong indications of MTBs at the lake bottom. These observations imply first that the Bardowie basin was larger and considerable deeper at that stage than today and second, large temperature contrasts from winters to summers during this time. From ca 8ka to ca 4ka we found the largest concentrations of organic matter joined by rapid increments of elements related to biological productivity and abundant accumulation of insects remains, all of which imply significant climate warming at this time. We also found compelling evidence for basin shallowing at this stage combined with strong indications of soil formation at ~4.5ka. This shallowing could be the result of the Earth’s crust rising to adjust for the deformation caused by the large ice sheet loads during the Pleistocene. The most recent stage (ca 4ka to ca 1ka) shows evidence for gradual deepening of the Bardowie Loch basin and/or increasing water levels, as well as the largest detrital and magnetic minerals input since the glacial period. There are also geochemical and rock magnetic evidence that suggest an intensification of human activity in the Bardowie catchment area since 550BC. As part of the present PhD project a preliminary chronology was developed for the Bardowie Loch Sequence based on 15 radiocarbon dates, funded by two grants awarded by the NERC Radiocarbon Facility (allocation numbers 1716.0413 & 2056.0417), and three cryptotephra layers. The term cryptotephra refers to microscopic shards of glass and minerals with unique chemical composition produced by discrete volcanic eruptions, transported and deposited by wind, which are used as time markers for specific regions. We found these tiny glasses mixed-in within the lake-sediments, using high-resolution magnetic characterisations combined with microscopic analysis. Then, we separated the tephra shards from the clay and silt and measured their chemical composition using the world-class NERC electron probe micro-analyser, which allow highly accurate results for microscopic fragments, even for mineral inclusions found in some of these tiny volcanic glasses. The deeper cryptotephra layer is a rhyolite with high iron, calcium and titanium content characteristic of Iceland volcanism. These microscopic glasses present the composition of two large eruptions from the Hekla volcano and with further modelling of all the chronological data, it was found that this layer corresponds to the Hekla 4 Eruption dated ca 2000BC, which is one of the largest markers in Northern Europe. The upper two cryptotephra layers present chemical compositions typical of a developed continental crust, while their high concentration of alkaline oxides is characteristic of the Campanian Volcanic Province in the Mediterranean. The major oxides composition of the microscopic glasses and mineral inclusions show that the first layer is a rare phonolitic tephra. According to these results and the preliminary age-depth model for Bardowie Loch, this tephra corresponds to the historical 79AD Pompeii Eruption of the Somma – Vesuvius volcano. The second layer has an alkaline intermediate composition, called trachyandesite, formed between 3ky and 4ky according to the preliminary age-depth model. These results agree with the Avellino Eruption, the largest known from the Somma – Vesuvius volcano, dated between 1900BC and 1600BC. The results herby presented correspond to the first Mediterranean tephras found in the British Isles. The provenance of these Bardowie cryptotephra layers were corroborated by comparing the major oxides composition of these microscopic volcanic glasses and mineral inclusions with tephrochronology studies of marine-cores from the Mediterranean Sea, as well as published geochemical data of the Pompeii Eruption tephra shards found in Greenland ice-cores.