-
Search for traces of life in Earth's oldest rocks
PI: Arrhenius, Gustaf
We aim at establishing criteria for distinguishing between biogenic and inorganic carbon and to apply them to the search for remnants of life in Earth’s oldest sedimentary rocks, found in in the Isua Supracrustal Belt (ISB) in southern West Greenland.
Our earlier work has revealed that isotopically fractionated carbon, previously thought to be the hallmark of biogenic carbon, is actually a common product of disproportionation of ferrous carbonate (siderite). Extensive beds of such graphite bearing siderite formed upon injection of hot fluids into sequences of surface related ISB rocks when they were buried deep in the crust at high temperature and pressure.
Before this was realized, such isotopically light carbon in the form of crystalline graphite was commonly considered biogenic and to represent the oldest and most strongly altered carbonaceous remnants of primitive life forms. The ferrous carbonate beds in which this graphite occurs were thought to be shallow water counterparts of the sedimentary banded iron formations (BIF) that we found to be practically devoid of syngenetic carbon.
In contrast a recently discovered graded bed formation in the ISB contains layers of carbonaceous shale which is clearly of sedimentary origin. This material is one of the targets of our proposed investigation. Included in our study is also a series of carbon deposits from younger Archean and Proterozoic rocks in Africa, Australia and Scandinavia that are less strongly altered, as well as fluid deposited inorganic graphite from Greenland and Spain.
A promising opportunity for recognizing biogenic carbon depends on the finding that carbonaceous material of biogenic origin at the level of metamorphism found in the ISB or lower retains non-graphitic puckered layer structures formed by the incorporation of characteristic defects. In contrast, fluid-deposited inorganic carbon crystallizes as graphite. High resolution structural studies of different types of Archean carbon thus form an important part of the study.
We find that the carbon isotope fractionation in all samples varies over a relatively large range from particle to particle. The sources of this variability – isotopic exchange vs. heterogeneity in the source material or in the disproportionation catalysts remain to be investigated a.o. by detailed petrographic analysis of the host rock
Residual bioorganic nitrogen and hydrogen have in the past been successfully exploited as indicators of progressive organic carbon maturation. Our analyses of the content and thermal release of nitrogen in carbon samples from the ISB has shown that inorganic nitrogen, accompanied by argon, is incorporated in graphite and carbonaceous matter from crustal fluids and superposed on the organic component. Further attempts will be made to distinguish between these sources by mode and strength of bonding of these gases.
The techniques developed will be applicable also to defining the origin of carbonaceous matter if found on Mars.
