Very fine ashes, invisible to the naked eye, which are deposited in the sediments present in every corner of our planet and preserve the history of our world. It may seem like the beginning of a fantastic story, enriched with magical dust and dreamlike atmospheres, but in reality it is the most concrete thing we have discovered in Antarctica.
In fact, to get to the bottom of this story we have to fly to the South Pole, where an international team of researchers led by INGV recently concluded a research project bringing home results that open up new perspectives in understanding the eruptive history of the Antarctic continent .
Yes, because from the careful analysis of the microscopic products of the eruptions that have rocked the ice continent in the past, a science known as tephrochronology has managed to reconstruct the dynamics of some eruptions that occurred in the Holocene and to 'synchronize' the history of marine sediments from the Ross Sea with that of glacial sediments taken from the Antarctic continent.
We interviewed Alessio Di Roberto, INGV researcher and head of the recently concluded TRACERS Project, to find out what is hidden in the microscopic particles trapped in the ice of Antarctica and how they 'tephra' they can help write the history of our planet.
Alessio, what is tephrochronology?
In the broadest meaning of the term given in the literature, tephrochronology is the science that studies the tephra, or pyroclastic products emitted during explosive volcanic eruptions at all scales, from the smallest to the largest ones without any distinction of composition, texture or granulometry.
Tephrochronology studies the pyroclastic deposits produced by eruptions and characterizes them in the most detailed way possible (grain size, particle shape, minerals, chemical composition of the volcanic glass, etc...) to identify the eruption from which they derive: in other words, study the geological 'archives' (the so-called record), the tephra present inside them are identified, these products are characterized in the most detailed way possible and, finally, an attempt is made to identify the eruption that produced them.
In the narrowest sense, however, by tephrochronology we mean the science that deals specifically with characterizing the tephra and giving it a numerical age: thus, giving a date to a certain tephra identified within a geological record , archaeological, climatic or environmental, it is even possible to correlate and synchronize very different records (coming, for example, from the marine, glacial or lake environment). This is thanks to two "special" characteristics of the tephras themselves: the first is that when they are erupted, the tephras are dispersed almost instantaneously (within a few hours or at most a few days) over vast areas, even millions of kilometres. square. Suffice it to say that, for example, the eruption of the Icelandic volcano Eyjafjöll in 2010, which was a relatively 'small' eruption, dispersed volcanic ash practically over all of Northern Europe, i.e. over an area of almost 2 and a half million square kilometers. So wherever a given tephra is identified, it is virtually of the same age.
The other reason that allows the use of tephra to correlate geological records of different nature concerns their ability to be dated and characterized quite easily: when one manages to date a tephra and trace it back to a specific volcanic eruption, in fact , you get a real one isochronous, that is a 'guide level' which allows to date all the records in which that tephra is identified and to synchronize them with each other.
What advantages can tephronology offer to scientific research?
The fundamental advantage that tephrochronology offers is that it has virtually no limits in terms of the time range within which it can be applied. The carbon-14 method, for example, has an indicative limit of application around 60.000 years: this means that it is not possible to date 'things' older than 60.000 years with the carbon-14 method. Tephrochronology, on the contrary, can be applied to extremely young eruptions, such as historical eruptions, but also to eruptions that occurred millions of years ago.
Furthermore, as I said, this science can be used in any archive, be it a geological, archaeological, climatic or environmental archive: from ice, to the sea, to the continent, passing through lakes and sediments.
In an article published on the INGVvulcani Blog you talked about “cryptograph revolution”: what does it mean?
That wasn't all my bag, I have to confess! I was actually referring to a wonderful article that changed the history of tephrochronology a bit, entitled “Cryptotefra: the revolution in correlation and precise dating” and published by Siwan M. Davies in 2015 in the magazine 'Journal of Quaternary Science' ().
What it means: well let's say that, compared to the macroscopic tephras (visible to the naked eye), the cryptotephras, i.e. the very thin tephras that are not visible to the naked eye but which are identifiable with a whole series of specific analyses, can give us indications on eruptions generated in areas very distant from the site being observed or even on local eruptions of very small energy.
In other words, cryptotephras only increase the number of eruptions that can be studied and dated within a record, significantly increasing the level of detail of the study.
Here are a few examples: relatively recent studies carried out on cryptotephra identified in sediment cores taken from the Adriatic Sea have revealed the presence of deposits from dozens of volcanic eruptions which had not previously been identified. Furthermore, in 2017 the ashes of the Oruanui super-eruption, which occurred over 27.000 years ago in New Zealand, were identified in the form of cryptotephra in the Antarctic ice, in an area over 5.000 kilometers from the eruption site. The same thing happened for the eruption of Ignimbrite Campana, which took place about 39.000 years ago in the Campi Flegrei area, the ashes of which were found as far as Russia, over 3.000 kilometers from the source, and are now used as a 'guide marker ' for all of Eastern Europe.
Without the discovery of cryptotephras, therefore, we could not have counted any of these eruptions in those geological records.
Last March, TRACERS concluded in Antarctica, a research project led by INGV and which has a lot to do with this "cryptotephra revolution": can you tell us what you discovered?
First of all, it is important for me to remember that TRACERS was a multidisciplinary project which, in addition to INGV as lead partner, involved the Institute of Polar Sciences of the CNR of Bologna, the Department of Earth Sciences of the University of Trieste and the 'University of Oxford, as well as numerous other collaborators from all over Europe.
The ambition of the Project was to identify within the marine sediments of the Ross Sea, one of the seas belonging to the Southern Ocean, the deposits of large explosive eruptions from Antarctic volcanoes: almost a year after the conclusion of the work I can say with a certain pride that the objectives have been largely achieved.
Indeed, we have identified the deposits of at least four, possibly five, major eruptions of the Antarctic volcanoes Mount Rittmann and Mount Melbourne that occurred in the Holocene and late Pleistocene that were previously completely unknown. We are talking about eruptions from subplinian to caldera-forming, so really very big eruptions.
With our work we have managed to "transform" the tephra that we have identified within the sediments taken from the Ross Sea into real 'guide horizons': this means that thanks to the results of our study any other researcher who should he approach the study of marine sediments in that area and find a tephra with certain characteristics, he could relate it to the entire Ross Sea Basin, give that tephra an age and then, in turn, give an age to the sequence who will be studying.
Furthermore, an absolute novelty is that for the first time we have been able to identify in marine sediments a level of tephra that was widely recognized within the glacial sequences, succeeding for the first time in linking the marine sedimentary environment with the glacial one. : it may seem 'trivial', but in reality it is a very important goal because the marine and continental environments (particularly the glacial one) do not have the same response to the phenomena of forcing climate change (i.e. those phenomena that have a weight of a factor in the mechanism of climate change) but, conversely, often have non-synchronous responses. Therefore, being able to correlate and 'synchronise' these two types of environment was and is very important for understanding how two different environments behave in the face of the same phenomenon.
"Antarctic volcanoes" may seem like an oxymoron, but we have seen that in reality it is not at all: what, then, are the most important volcanoes on the Antarctic continent?
True, it may seem strange but in reality Antarctica is a continent extremely rich in volcanoes: there are several dozen on its territory, many of which are still active. Traces of the most recent Antarctic volcanism, which we know best, can be found up to about 30 million years ago, but there is evidence of even more ancient volcanism in the geological deposits.
Among the Antarctic volcanoes considered still active we mention Mount Erebus, one of the few permanently active volcanoes in the world that has a lava lake on its summit, and Deception Island, which also had very recent eruptions in 1969-70.
Also many other volcanoes such as those we have studied with the TRACERS Project, I am thinking of Mount Rittmann, Mount Melbourne or, moving to Marie Byrd Land, Mount Berlin, have evidence of historical or in any case rather recent eruptive activity.
Antarctica, therefore, is certainly a continent extremely rich in volcanoes, many of which are subglacial: in fact, in addition to the large volcanic buildings visible on the surface, numerous studies reasonably hypothesize that there may be a few dozen other volcanoes hidden under the ice.
What geological dynamics led to their formation?
The geological dynamics that have led to the formation of Antarctic volcanoes are many and completely similar to those that have led to the formation of volcanoes throughout the rest of the globe: we start from a type of volcanism rifting, which is the newer one and originates from WARS, the West Antarctic Rift System, one of the largest and most developed rift systems in the world, up to reaching a more ancient volcanism (from the back-arc volcanism, the classic volcanism that characterizes - for example - also the Aeolian Islands, up to the intraplate volcanism with its 'hot spots ' in which magma rises from the mantle through the earth's crust).
Antarctica is a very vast continent and, as we have seen, it contains within it most of the magmatological and volcanological dynamics that exist in the rest of the planet.
Returning to TRACERS and looking to the near future, what scenarios can the results you have obtained open up?
From a purely volcanological point of view, having identified these four or five eruptions of Antarctic volcanoes that occurred in the Holocene and late Pleistocene allows us first of all to understand that the record of eruptions - explosive and non-explosive - in the Antarctic continent is still highly incomplete , we still don't know very well how many and what eruptions have been in the past (nor of what extent).
However, the results of our project allow us to begin tracing a path in the knowledge of the eruptive history of different volcanoes in Antarctica; moreover, by accurately describing the size and dynamics of these eruptions, one can begin to evaluate the potential risk associated with the eruptions themselves.
This last aspect is becoming increasingly important because both Antarctic tourism and, above all, the attendance of Antarctic bases by researchers is growing more and more. Suffice it to say that near Mount Rittmann, which in the last 11.000 years has produced at least two major eruptions, there are four stable scientific bases: the Italian Mario Zucchelli, the German, the Korean and the Chinese one under construction.
Finally, as I said before, TRACERS and projects like ours have opened up a rather wide range of perspectives in paleoclimatic and paleoenvironmental studies, demonstrating that tephra levels can be used to correlate, synchronize and date very different environments, such as, for example, the marine and continental environments, which respond in very different and often asynchronous ways to the forcings of climate change.