Evgenii V. Sharkov

Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry (IGEM), Russian Academy of

Sciences Moscow, sharkov@igem.ru

DEVELOPMENT OF GEOLOGICAL PROCESSES ON THE EARTH AND THEIR

IMPACT TO THE EARLY BIOSPHERE

It is known that the Earth’s ecological systems in the Middle Paleoproterozoic were subjected to essential changes, which promoted to acceleration of the biosphere expansion and development. Though life has been already existed in the Paleoarchean [Schidlowski, 1988; Westall and Folk, 2003; De Grigorio et al., 2009; Fliegel et al., 2010], the multicellular organisms appeared only in the Paleoproterozoic [Rozanov, Astafieva, 2009]. What was occurred with ecological systems and why? What was a trigger for such changing? We try to answer on these questions using complex of geological, petrological and geochemical data.

First of all I would like to remind you of the general points of the Earth’s evolution, including problem of the primordial crust, because it was the basement for being and development of the primitive biosphere

• .

Problem of the primordial Earth’s crust According to modern models, this crust

could be basic or sialic in composition. Both models require a global melting of a primary chondritic material for formation of the primordial crust. Due to theory of solidification, hardening of such magma ocean had to proceed upwards and resulted in accumulation of low-temperature derivates of granite composition at the surface as the primordial crust.

Geological data, namely granite-

dominated Archean crust, and data from the study of inherited zircon cores (Valley et al., 2002; Harrison et al., 2005) supports the primordial-sialic Earth’s crust hypothesis. The most suitable pretender for role of the primordial crust are plagiogranites (TTG series), which are composed about 85-90% of the Archean crust.

Proper geological evolution of the Earth, according to our data (Sharkov, Bogatikov, 2010), can be subdivided on three major stages: (1) Nuclearic, lasted from ca. 4 Ga to ca. 2.7 Ga, ie, practically all Archean; (2) Cratonic, or Transitional: from ca. 2.7 till 2.0 Ga;(3) Continental-Oceanic, from 2.0 Ga till now.

TECTONOMAGMATIC PROCESSES IN THE ARCHEAN AND EARLY PALEOPROTEROZOIC

• Geological situation then was rather different from Phanerozoic. Granite-greenstone terranes (GGTs) and their separating granulite belts were the major Archean tectonic structures.

• The GGTs, consisting of irregular network of greenstone belts (not more 10-15% of area) among plagiogranite-gneiss (TTG) matrix, likely, strongly reworked primordial sialic crust. Mantle magmatism was represented by by high-Mg lava flows. Such magmas were derived from mantle sources, depleted by easily-melted components as a result of the primordial crust formation. Sediments played subordinate role.

• Archean structure of the Fennoscandian Shield• 1- granite-greenstone terranes; 2 – greenstone belts;3 – reconstructed

greenstone belts; 4 – transitional Belomorian mobile belt; 5-6 - granulite complex; 7 – middle Paleoproterozoic Svecofennian orogen; 8 – geological boundaries

Ecological situation in Archean• Biosphere could appear only after solidification of global

magma ocean and appearance of liquid water. The first evidence of cyanobacterias being on the Earth were found in rocks of the Isua complex (3.8 Ga), where underwater pillow lavas widely developed (Rollinson, 2007 and references herein). Instead of the Phanerozoic, Archean oceans had relatively shallow depths and their floor was not composed by basalts.

• Presence in Archean sediments of detrital pyrite, uraninite, siderite, etc. testifies that Archean atmosphere was rather differ from the modern. It was reducing media, composed mainly by N2 and CO2; РО2 was very low and oceanic water was subacid (Krupp et al., 1994). Localities of primitive life in the Archean usually developed near hot springs on sea floor (Harris et al., 2009; De Grigorio et al., 2009) or in glassy crusts of lava flows (Furnes et al., 2004; Fliegel et al., 2010).

CARDINAL CHANGE IN THE EARTH’S TECTONOMAGMATIC EVOLUTION

Cardinal change in character of magmatism occurred within period from 2.35-2.4 to 2.0 Ga: the early Precambrian high-Mg magmas, derived from depleted mantle, gave place to the geochemically-enriched Fe-Ti picrites and basalts, similar to the Phanerozoic within-plate magmas (Sharkov, Bogina, 2009).

We believe that ascending of the second generation mantle plumes (thermochemical plumes), was responsible for appearance of such magmas. Those plumes were generated at the core-mantle boundary in D" layer and this process is active till now (Dobretsov, 2008).

The thermochemical plumes are enriched in fluid components and their heads extended on shallower level; it resulted in crust fracturing, oceanic spreading, formation and movement of plates, subduction, etc., ie, appearance of the plate tectonics.

It was occurred at the boundary of ca.2 Ga, and ancient primitive plume tectonics was replaced by plate tectonics, and the Earth entered at the Continental-oceanic stage of its evolution.

So, since 2 Ga tectonomagmatic processes irretrievably changed over the whole Earth. As a result, ancient continental (sialic) crust became gradually replaced for secondary oceanic (basaltic) crust. Sialic crust, jointly with young basaltic crust has involved in subduction process and stored in the “slab graveyards” in the deep mantle .

•As a result, composition of the hard Earth’s surface was cardinally changed: instead of predomination of sialic (granitic) rocks, mafic (basaltic) oceanic crust has quickly growed up, and now it forms about 60% of the present-day Earth’s crust.

•System volcanic arc-backarc sea (Sharkov and Bogatikov., 2010)•1 - asthenosphere; 2 - lithosphere mantle: beneath (a) continent, (b) ocean; 3 upper mantle beneath discontinuity at 430 km; (4) mantle beneath disconti-nuity at 680 km; (5) lower crust : (a) continental, (b) oceanic; 6 - continental crust; 7 - mixture of sialic and basic crustal material in subduction zone; 8 - magma-generation zones (I – tholeiite, II – boninite, III – calc-alkaline); 9 - direction of asthenosphere moving.

Modern pattern of convergent margins according to seismic tomography (Karason, van der Hilst, 2000).

Blue – subducted crust (both oceanic and continental)

• New type of magmas was characterized by elevated and high contents of Fe, Ti, Cu, P, Mn, Na, K, LREE, and some rare elements (Zr, Ba, Sr, U, Th, F, Cl, etc.).

• Large-scale influx of alkalis in the World Ocean presumably neutralized its water, making it more suitable for life, while input of Fe-group metals, P, alkalis, Cl and other elements, which are required for metabolism and fermentation, rapidly expanded the possibility for the development of biosphere. Judging on appearance of oxidative atmosphere ca. 2.3 Ga (Great Oxidation Event): Melezhik et al., 2007, 2012; Guo et al., 2009), it promoted especially to explosion of photosynthesizing organisms.

• The manifestation of this geochemically enriched magmatism was correlated with the first finds of eucaryotic heterotrophic organisms at ~ 2 Ga, for example in black shales and phosphorites of the Paleoproterozoic Pechenga complex, Kola Peninsula [Rozanov, Astafieva, 2008]. For instance, a significant increase in amount of spheromorphides and remains of filamentous algae is observed in the Upper Jatulian deposits (~ 2.0 Ga) of Karelia [Akhmedov, Belova, 2009]. The vital activity of the organisms led to significantly increasing the oxygen content in atmosphere, which was marked by the formation of cupriferous red beds at all Precambrian shields, generation of the first hydrocarbon deposits (shungites, Karelian craton) [Melezhik et al., 2005], rock-salt in Karelia [Morozov et al., 2010], and phosphorites with age of 2.06 Ga on the Indian and Kola cratons [Melezhik et al., 2005].

• Thus, a fundamental change in character of tectonomagmatic activity acted as the trigger for environmental changes and acceleration of biospheric evolution, supplying a qualitatively new material on the Earth’s surface. This event gave impetus to wide expansion of biosphere, which fixed by beginning of oxidative atmosphere, change of Sr isotopy in sedimentary carbonates, and enhanced biosphere mass as demonstrated by appearance of hydrocarbon deposits.

• However, rapid enhanced of the bulk of biosphere did not accompanied by the same increasing of the biodiversity; new forms (especially multicellular organism) appeared in small quantity and long time did not essential developed.

Situation on Venus and Mars

Data available on Venus and Mars suggest that their tectono-magmatic evolution also occurred at the close scenario (Sharkov, Bogatikov, 2009 and references herein). Two major types of morphostructures, which are vast plains, composed by young basaltic crust, and older lightweight uplifted segments with a complicated topography can evidence about two-stage evolution of these planets. If we dry the modern Earth’s oceans, we will see the same picture.

• Presence of drainage systems on Mars and valleys on Venus assumes existence of liquid water on early stages of their development. Like on the Earth, red beds appeared on the Mars at the middle stage of it’s evolution, and may be at this period photosynthesizing microorganisms existed on Mars (McKay et al., 1996). Powerful eruptions of gigantic volcanoes of Tharsis and Elysium, probably, led to fall of temperature and disappearance of liquid water on Mars which terminated biosphere evolution.

• In contrast to Mars, on Venus speeded up greenhouse effect appeared, which led to dry and very hot surface, unfavourable for life also.

• So, processes of the planetary evolution were favourable for the biosphere development only on the Earth.

CONCLUSIONS

1. The primordial Earth’s crust was likely granitic in composi-tion. Tectonomagmatic activity on Earth in Archean and early Paleoproterozoic) was rather different from Phanero-zoic: the major tectonic feature were granite-greenstone terranes where plagiogranites composed 85-90% territory and high-Mg volcanics, derived from depleted mantle sources, predominated in greenstone belts.

2. A drastic change of the tectonomagmatic and ecology processes on Earth’s surface occurred in the Middle Paleo-proterozic, ca. 2.35-2.0 Ga, when new type of magmas appeared. It was characterized by elevated and high contents of Fe, Ti, Cu, P, Mn, alkalis, LREE, and other elements (Zr, Ba, Sr, U, Th, Cl, etc.), which are required for metabolism and fermentation, rapidly expanded the possibility for the development of biosphere. A large-scale influx of alkalis in the World Ocean presumably neutralized its water, making it more suitable for the life

• 3. Fundamental changes in tectonomagma-tic activity acted as a trigger for environ-mental changes and biosphere evolution, supplying a qualitatively new material to the Earth’s surface.

• 4. Venus and Mars developed at the same scenario; very likely, that at the beginning liquid water occurred on them; however, processes of the planetary development were favourable for the biosphere evolution only on the Earth.

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