15 N - depleted nitrogen in Early Archean kerogens : clues on ancient marine chemosynthetic - based ecosystems ?

In the volume 96(1–2) of ‘Precambrian Research’, Beaumont and Robert (1999) presented the first complete study on the nitrogen isotopic composition of kerogens preserved in Archean and Proterozoic rocks. This is an important contribution, which adds compelling data to the rather scarce N isotopic record in Precambrian rocks (Hayes et al., 1983; Gibson et al., 1985, 1986; Zhang, 1988; Sano and Pillinger, 1990; Boyd and Philippot, 1998). The study reveals an evolution of the N isotopic composition of organic matter, from Archean to present. Nitrogen in Early Archean kerogens is N-depleted (dN from −4 to 0‰), whereas N in modern organic matter preserved in oceanic sediments is enriched in N (dN from 4 to 8‰; Fig. 1). Changes through geological time of the atmosphere chemistry are considered to be at the origin of this N isotopic evolution. Modern organic matter exhibits positive dN values, reflecting the N enrichment produced by the denitrification of the dissolved nitrate (NO3). In the Archean ocean, depleted of oxygen, the N cycle was different and likely controlled by metabolic processes involving reduced forms of N (N2, NH3, NH4). The N2 fixation or the NH4 uptake are metabolic processes able to produce isotopic shifts from −9 to −4‰ (Delwiche and Steyn, 1970; Goericke et al., 1994), as those observed by Beaumont and Robert (1999). In this brief comment the authors would like to formulate some hypothesis, based on ecological and geological evidences, on the ecosystems which may be at the origin of these Archean N-depleted kerogens. The authors hope this brief comment may represent a starting point of a vigorous debate on a problem which has been left unchallenged for too long time: how and when the N biogeochemical cycle developed. * Corresponding author. Fax: +33-1-69154891. E-mail address: [email protected] (D.L. Pinti).