Australia: The Land Where Time Began
Arctic Ocean Giant Caldera – Evidence of the Catastrophic Event
In the eastern segment of the Gakkel Ridge there is a giant crater, which is 80 km long by 40 km wide and 1.2 km deep, which could be first seen on the bathymetric map of the Arctic Ocean that was published in 1999. Seismic and multibeam echosounding data were acquired at the location in 2014. It has been estimated that no less than 3,000 km3 of volcanic material was ejected, which places it in the same category as the largest calderas of the Quaternary, Yellowstone and Toba. The age of the Arctic eruption has been estimated as about 1.1 Ma. Sedimentary cores collected about 1,000 km away from the Gakkel Ridge contained thin layers of the volcanic material that have been identified as being related to the eruption. The Gakkel Ridge Caldera is the only known example of a supervolcano in the rift zone of the Mid-Oceanic Ridge System.
Detailed study of the floor of the Arctic Ocean began only quite recently. There were a few sporadic studies of the deep-water part of the Arctic Ocean up to the middle of the 20th century. The caldera was considered for a long time to be a vast undifferentiated depression, based on the 11 depth measurements that had been taken by F. Nansen during his famous “Fram” voyage at the end of the 19th century. In the late 1940s a new phase of Arctic Ocean investigations began with the Soviet high-latitude airborne expeditions that were led by J. J. Gakkel. The Soviet polar explorers had measured depths at more than 400 locations by 1956, by which they discovered the Lomonosov Ridge, and they published their bathymetric maps which changed radically the image of the topography of the Ocean floor (Gakkel, 1997; Gakkel, 1959; Gakkel et al., 1968; Kiselev, 1979).
American explorers discovered the Alpha Ridge in the central part of the Arctic Ocean in the second half of the 20th century (Ostenso, 1962; Weber, 1983). The US navy used nuclear submarines, Nautilus, Skate and Sargo (Ostenso, 1962; Weber, 1983; Dietz & Shumway, 1961), to explore the Arctic Basin
Up to 20,000 depth measurements had been accumulated by expeditions from different countries by the end of the 1960s, in spite of the uneven distribution, and the basic units of the Arctic Ocean were established: the Eurasian and Amerasian basins separated by the Lomonosov ridge, Nansen and Amundsen abyssal basins, that were separated by the Gakkel Ridge, the northern continuation of the Mid-Atlantic Ridge (Gakkel, 1959; Ostenso, 1962). On the bathymetric maps the southern end of the Gakkel Ridge was extended to the edge of the Laptev Sea continental margin up to 79o30’N in the 1970s. The topography of the ridge itself was, however, represented in idealised form, as the result of a lack of data, as an ordered system of more than 25 parallel transform faults that were constantly changing the direction of the axial rift valley, which reflected the subjective perception of similarity to known mid-ocean ridges.
The bathymetric database of the abyssal Arctic Ocean have been significantly expanded and helped in providing detail representation of underwater topography.
Evidence of the eruption traces in Sediments
According to Piskarev & Elkina a noticeable impact on the composition and properties of sediments formed during and after the eruption should have resulted from the introduction of a huge amount of volcanic material into the waters of the Arctic Ocean. Therefore, it is expected that there was a wide distribution of omnipresent sedimentary layers of volcanic origin. Increased concentrations of monoclinic pyroxenes and opaque ore minerals (Owens et al., 2016; Buchs et al., 2015; Gorbarenko, 2002) can be used to identify such layers, and higher values of magnetic susceptibility and residual magnetisation that is caused by higher concentrations of titanomagnetite.
Several such layers were identified that had anomalously high values of magnetisation and magnetic susceptibility in sedimentary cores that had been collected in the Mendeleev Rise, which is about 1,000 km from the caldera, and dozens of kilometres from each other. One of them is stratigraphically very close to the Brunches/Matuyama chron and could be dated with a high degree of confidence at about 750 ka (Piskarev, Andreeva & Gus’kova, 2013). This layer, which is a few centimetres thick, clearly correlates from one core to another, in spite of the cores being separated by a considerable distance.
It has been demonstrated by mineralogical analysis that there is a sharp increase of pyroxene and ore minerals and depletion of garnet and titanite, which are indicators of a “stable” depositional environment in a layer dating to 750 ka.
The probable volcanogenic nature of the studied thin sedimentary layer is indicated by increased concentrations of ore minerals and pyroxene. Simultaneously, significant decrease in the concentration of minerals that are obviously clastic, such as garnet and titanite, means an abrupt surge of the rate of sedimentation during the formation of this volcanogenic layer in the sedimentary column. It appears that for a short period of time the rate of sedimentation increased by 500 % compared to its average value.
Specifics of the sedimentation during the Pliocene-Quaternary was obtained, that was even more valuable, from core KD12-03-10C that was extracted from the Mendeleev Rise in 2012 (Elkina, 2014). This core, that was 6m long, represents the time interval down to ≈4 Ma (Gilbert Palaeomagnetic chron).
Distribution of NRM inclinations with depth displays alternating intervals of normal and reverse polarity of the palaeogeomagnetic field. The positive inclination prevailed up to 123.5 cmbsf (centimetres below sea floor), starting from the top of the core, it then changed sharply to the negative which remained prevalent up to 394-397 cmbsf, though there were some short-lived periods of normal polarity within this section. Magnetisation of the intervals with the positive inclination is stronger, on average, than those with a negative inclination. According to Piskarev & Elkina this difference can be explained by the presence of a viscous component of magnetisation, which was also noted in several other columns that were collected previously.
The bulk of the core KD12-03-10C, visually, is relatively homogeneous aleuropelite (silty clay). Identification of 5 horizons that had peak values of magnetization and magnetic susceptibility that were revealed by measurements of magnetic properties (magnetisation and magnetic susceptibility) that were carried out at fine intervals of 2.5 cm. The 5 intervals that were identified were dated by correlation with the Palaeomagnetic chrons:
· 77.5 cmbsf – 0.47 Ma,
· 118.5 cmbsf – 0.727 Ma,
· 170-175 cmbsf – 1.09 Ma,
· 240 cmbsf – 1.62 Ma,
· 385 cmbsf - 2.52 Ma.
Along the core weight percentage distribution of the coarse fraction demonstrated correlation intervals that had high coarse fraction, greater than 500 microns, content and the spikes in the values of magnetic susceptibility.
Several thin sections of the bulk of the core and the above mentioned anomalous intervals were prepared and studied in order to identify the lithological differences between the bulk and the anomalous intervals.
Thin section shows that the bulk of the core is comprised of biogenic sediment with clay-carbonate mass that is heavily ferruginous, that is saturated with quartz fragments, carbonates and planktonic foraminifera. The clay-carbonate basic mass is tinted in yellow-brown colours by the iron hydroxides which also cause the formation of a small spotted brown impregnation in cement and small halos around some of the foraminifera. Sediments of the marked anomalous intervals have a different appearance when viewed in thin sections of material at 175 cmbsf. The clastic fraction is unsorted by size or by degree of roundness; the colour of this horizon is noticeably lighter and, in general, it has the appearance of tuffite; fragments of fauna are remarkably rare; there are curved shards of tephra and, also, there are noticeably more angular shards of tephra that are up to 0.5 mm in size that are present in the bulk core. Also typical of these horizons are: hornblende, ore minerals, iron hydroxide, glauconite, calcite, and, most importantly, there is a higher continent of clinopyroxene.
It can be concluded with a high degree of confidence that Lithological, mineralogical, petrographic, granulometric and geophysical properties of the described horizons demonstrate that they had a volcanogenic origin and the explosive character of the volcanic activity. It was shown by the detailed analysis of core KD12-03-10c that the most pronounced episode occurred 1.09 Ma. According to Piskarev & Elkina this is very close to ≈1 Ma, which is the time of formation of the caldera as derived from the rift valley width at the floor of the caldera and the average spreading of the Gakkel Ridge.
The episode of explosive volcanic activity that resulted in the formation of the caldera may be a key to the explanation of why the Gakkel Ridge rift valley in the western part of the Eurasian Basin is located in the southwestern flank of the Ridge. This is clearly visible on the modern topographic map of the Arctic Ocean, indicating a recent jump in the spreading axis.
Unique Gakkel Caldera
According to Piskarev & Elkina it could be safe to state that in the Pleistocene the Eurasian Basin of the Arctic Ocean was a scene of unique and powerful explosive volcanic activity that produced huge volumes of ejected material. It appears that this eruption that occurred at ⁓1.1 Ma was not only a single powerful eruption in the Arctic Basin during the Pliocene and Pleistocene.
During the earlier expedition AMORE 2001 (Thiede et al., 2002; Jokat & Schmidt-Aursch, 2007) large volcanic structures were found in the western part of the Gakkel Ridge. Spreading ridges that are ultra-slow have a unique feature – amagmatic rifts that expose peridotite of the mantle, with only traces of basalt and gabbro, directly on the seafloor, which therefore forms a new, 4th type of plate boundary (Snow & Edmonds, 2007). New questions about the violent processes in ultra-slow spreading systems (Sohn et al., 2008) are raised by these discoveries. Piskarev & Elkina regard the described caldera as evidence of a new form of volcanism that is related to this type of plate boundary. It was probably the most powerful volcanic eruption which left the significant marks on the topography and sedimentation within the Arctic Ocean. It might also affect the recent, Pleistocene, spreading geometry of the eastern section of the Eurasian Basin by triggering a jump of the Gakkel Ridge spreading axis, which is located in its southwestern flank instead of the typical central position.
The size of the caldera puts it in the category of supervolcanoes with a Volcanic Explosive Index of 8, and it was suggested by Piskarev & Elkina to name the caldera “Gakkel Caldera, due to its location. Volcanoes with eruptive release of more than 1,000 km3 of lava and volcanic ash are included in this category. The latter contaminated the hydrosphere as well as the atmosphere, as evidenced by the presence of volcanic material in sediments thousands of kilometres from the volcanic sources. The scope and duration of climate change related to the catastrophic volcanic events in the Pleistocene and their impact on the biosphere, including humans, are still being discussed, as may be attested to by the debate on the impact of the Toba eruption 75 ka (Lane, Chorn & Johnson, 2013).
The tectonic position of the Gakkel Caldera is a remarkably interesting feature. As far as is known, it is a unique example of a supervolcano that formed in the rift zone of a mid-ocean ridge. All other supervolcanoes that are known are located above subduction zones, or in the immediate vicinity, as that location has the favourable conditions for the generation of giant magma chambers. In fact, it should be noted that the Gakkel Caldera is located at the termination of the Gakkel Ridge. The rift valley of the Gakkel Ridge changes to a graben that is several hundred metres deep verging to the Laptev Sea shelf. The faults that form the walls of the graben are traceable deep into the thick sedimentary sequence, and, according to the data (Kim & Ivanova, 2000), the sediments dating to the Cretaceous constitute the lower layers.
The existence of the suture zone, which cross-linked ocean floor from the Cainozoic and Mesozoic in this part of the Eurasian Basin, had been suggested earlier (Piskarev, 2004). Geological and geophysical data, however, do not allow the precise defining of the zone so far. The Gakkel Caldera, however, appears to be one of its characteristic features.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|