Global Carbon Cycle of the Precambrian Earth

• Main Author: Wiewióra, Justyna
• Format: Book
• Language: English
• Published: Natural History Museum of Denmark, Faculty of Science, University of Copenhagen 2014
• Subjects: Greenland; Tupertalik; East Greenland; greenlandic; North Atlantic
• Online Access: https://curis.ku.dk/portal/da/publications/global-carbon-cycle-of-the-precambrian-earth(abc7d38c-9ad6-467a-849c-0c707e951ef3).html; https://soeg.kb.dk/permalink/45KBDK_KGL/fbp0ps/alma99122493782105763

The carbon isotopic composition of distinct Archaean geological records provides information about the global carbon cycle and emergence of life on early Earth. We utilized carbon isotopic records of Greenlandic carbonatites, diamonds, graphites, marbles, metacarbonates and ultramafic rocks to investigate carbon fluxes between Precambrian Earth’s mantle and crust and to trace the evolution of life in the Eoarchaean oceans. The world’s desire for diamonds gives us a unique opportunity to obtain insight into the nature of metasomatic fluids affecting the subcratonic lithospheric mantle (SCLM) beneath Greenland. Diamonds from two occurrences: the Qeqertaa ultramafic lamprophyre (>1743 Ma) and the Majuagaa kimberlite (564 Ma), show similar carbon isotope composition with δ13C values from -6.22 ± 0.07‰ to -1.74 ± 0.01‰ (1SD) and a wide range of nitrogen concentration from 0 to 1200 atomic ppm. Observed decreasing δ13C with decreasing N content trend would suggest they formed during closed system fractional crystallization from an oxidized growth medium enriched in 13C relative to the bulk mantle. However, different populations, defined by nitrogen thermometry, do not follow a single trend, but rather indicate origin in distinct fluids. Therefore, we suggest the diamonds formed during three-component mixing processes between 3 metasomatic fluids: [1] nitrogen- and CO2-rich lithospheric fluids, [2] subducted inorganic carbonates, and [3] the primary mantle. The presence of heavy inorganic carbon in the SCLM (end-member [2]) indicates introduction of surface carbon to the mantle during 2 distinct subduction events that preceded formation of the Qeqertaa and the Majuagaa diamonds. Finally, we integrate these 2 events into a tectonic evolution model of the Greenlandic SCLM. In the second manuscript we report a detailed description of δ13C ( -4.8 ± 0.1‰), δ18O (+8.2 ± 0.2‰) and δD (-76.0 ± 13.0‰) of the Singertât carbonatite (2.664 Ga) from South East Greenland, which represent the composition of the average late Archaean mantle. Our study confirms constant carbon isotope composition of the mantle from ~2.7 Ga until today. Combined δ13C-δ18O data indicate that kimberlites, ultramafic lamprophyres and carbonatites within the North Atlantic Craton and Ketilidian Mobile Belt could have evolved from a mantle-derived carbonatitic melt with carbon and oxygen compositions similar to the Singertât carbonatite. Additionally, we report the carbon isotope composition of the oldest known carbonatite Tupertalik (3.007 Ga) from West Greenland. Its δ13C of -2.8 ± 0.2‰ indicates the presence of a mantle reservoir isolated from convective stirring before 3.0 Ga and fertilized by mafic crust carbonated in the surface environment and recycled back into the mantle In the third manuscript we investigate the carbon cycle components, which have maintained the carbon isotope composition of the mantle constant through time. Assuming constant organic ratio of the total carbon burial (f), we show that increased accumulation of carbon in the continental crust and a decreased role of the oceanic crust as a carbon sink through time must have increased δ13C of the mantle. However, this is inconsistent with our observation of δ13C value of the Singertât carbonatite (2.664 Ga). For this reason and to determine f ratio at ca. 2.7 Ga, we measured the carbon isotope composition of Precambrian Greenlandic graphites (δ13C ≈ -33‰) and marbles (δ13C ≈ 0‰). Further, we interpolated the calculated f ratio, which is only 0.11, to the present observed value of 0.16 and integrated it into a three-sink isotope mass balance model. We suggest that in order to explain the constancy of the carbon isotopic composition of the mantle, a continuous increase of f ratio combined with a decrease of oceanic crust carbonatization with time are required. Finally we interpret a large range in δ13C of ultramafic ( -7.9 ± 0.1‰ to -2.6 ± 0.1‰) and metacarbonate ( -6.1 ± 0.1‰ to +1.5 ± 0.0‰) rocks from the ~3.8 Ga Isua Supracrustal Belt as resulting from the Rayleigh distillation process, which affected the ultramafic reservoir with initial δ13C between -2‰ and 0‰. Due to its high primary δ13C signature, carbon in the Isuan magnesite was most likely derived from surface water. Partitioning of carbon between 13C-rich oxidized and 13C-poor reduced species indicates that life in the early ocean had to be well evolved before 3.8 Ga. Therefore, the Isua ultramafic rocks may be considered as an indirect biomarker for ancient life.