Precambrian Research 105 (2001) 85 – 88
Discussion
15
N-depleted nitrogen in Early Archean kerogens: clues on
ancient marine chemosynthetic-based ecosystems?
A comment to Beaumont, V., Robert, F., 1999.
Precambrian Res. 96, 62 – 82
Daniele L. Pinti
a,*, Ko Hashizume
baLaboratoire de Ge´ochronologie Multi-Techniques UPS-IPGP and UMR8616Orsayterre,Uni
6ersite´ Paris SUD XI,
Batiment504,1er etage,91405Orsay Cedex,France
bDepartment of Earth and Space Science,Graduate School of Science,Osaka Uni
6ersity,Toyonaka,Osaka560-0043,Japan
Received 9 February 2000; accepted 27 June 2000
www.elsevier.com/locate/precamres
In the volume 96(1 – 2) of ‘Precambrian Re-search’, 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 con-tribution, 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 iso-topic composition of organic matter, from Archean to present. Nitrogen in Early Archean kerogens is15
N-depleted (d15
N from −4 to 0‰), whereas N in modern organic matter preserved in oceanic sediments is enriched in15N (d15N 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 d15N values,
reflecting the 15N 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 ob-served 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 15
N-de-pleted kerogens. The authors hope this brief com-ment may represent a starting point of a vigorous debate on a problem which has been left unchal-lenged 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).
D.L.Pinti,K.Hashizume/Precambrian Research105 (2001) 85 – 88 86
Fig. 1. Frequency distribution ofd15N measured in kerogens
extracted from Archean cherts (data from Table 3 of Beau-mont and Robert, 1999) and in modern organic matter pre-served in oceanic sediments (data from Peters et al., 1978).
N and C isotopic signature of hydrothermal vent species reflects that of the biomass produced by chemoautolithotrophic bacteria. The unusually negatived15N ratios have been related to processes
of N2-fixation or NH4+ assimilation and biomass
production by some symbiotic bacteria (Rau, 1981). Another hypothesis is that mantle-derived
15N-depleted nitrogen is the main source of
inor-ganic N used by chemoautolithotrophic bacteria. The isotopic composition of mantle N is−592‰ (Marty and Humbert, 1997). The isotopic shift of
−2 –−4‰ caused by N-fixation on a mantle N source could easily explain the15
N depleted values in Archean kerogens and in the modern hydrother-mal ecosystems. The 13C-depleted signature may
derive from autothrophic fixation of seawater dis-solved inorganic carbon (DIC) or, in some partic-ular environment, from bacterial methanogenesis (Conway et al., 1994).
Although the N and C isotopic similarity be-tween Archean kerogens and present-day biomass at hydrothermal vents supports the possible role of chemosynthesis in the Archean N cycling, we The isotopic composition of N (and of C) in
Archean kerogens is very similar to that presently observed in ‘subseafloor chemosynthetic ecosys-tems’ (Fig. 2). ‘Chemosynthesis’, or more correctly, ‘chemoautolithotrophy’, describes the biosynthesis of organic carbon compounds from CO2 using
energy and reducing power derived from the oxida-tion of inorganic compounds, such as H2S, S2O3,
CH4, H2 and NH4
+
, which support bacterial chemosynthesis (Conway et al., 1994). Chemosyn-thesis represents the dominant source of ecosystem energy production in deep-sea hydrothermal vent at seafloor spreading centers. Here, reactions of seawater with crustal rocks at high temperatures produce the reduced chemical species used as the source of energy for reduction of CO2 to organic
carbon (Jannasch and Mottl, 1985). Chemosynthe-sis has been often claimed as the main form of biosynthesis prior to photosynthetic life, and hy-drothermal vents have been considered as the first pre-photosynthetic biome (Nisbet, 1995; de Ronde and Ebbesen, 1996; Walter, 1996). Isotopic data for C and N of Beaumont and Robert (1999) may give the first reliable record of these processes.
The N and C isotopic composition of hydrother-mal vent species is consistently light (d15N
= −12 – 4‰; d13C
= −60 –−10‰; Fig. 2), lighter than that of heterotroph organisms assimilating organic compounds produced by photosynthetic processes (Brooks et al., 1987; Conway et al., 1994). The light
Fig. 2. Distribution of d13C and d15N in kerogens extracted
from Archean cherts (black dots; Beaumont and Robert, 1999). The shaded areas represent the distribution ofd13C and
d15N in hydrothermal vent-related chemosynthetic ecosystems
D.L.Pinti,K.Hashizume/Precambrian Research105 (2001) 85 – 88 87
need some geological evidence for such an envi-ronment. The Archean rocks analyzed by Beau-mont and Robert (1999) are cherts. It has been often claimed that Archean cherts are derived from hydrothermal Si-rich solutions. Present-day ocean is indeed undersaturated in Si and cherts are deposited exclusively at seafloor spreading centers (Herzig et al., 1988). There are no clear reasons that the Archean ocean should have been saturated in Si, except if it was affected by a large input of mantle-derived Si-rich hydrothermal so-lutions. The proximity of Archean chert layers to banded iron formation (BIF), the latter derived from hydrothermal solutions enriched in Fe (Is-ley, 1995), has been a long-standing evidence for a hydrothermal origin for cherts. For some of the cherts analyzed by Beaumont and Robert (1999), petrographic and geochemical evidences of an hy-drothermal origin exist. For six of them, the authors have not reported the depositional envi-ronment (Table 1 of Beaumont and Robert, 1999). Two of them, from South Africa (samples PPRG 193 and PPRG 182), have been wrongly reported as ‘alluvial and marginal marine’. Walter et al. (1983) has suggested this depositional envi-ronment for samples to belong to the Moodies Fm., Swaziland Sequence. Actually, sample PPRG 193 is from the Kromberg Fm. and sample PPRG 182 from the Hooggenoeg Fm., both be-longing to the Onverwacht Group. Cherts from these two horizons have petrographic and geo-chemical features suggesting a hydrothermal origin (see de Wit et al., 1982; Paris et al., 1985 for details). The last four, from Warrawoona and Gorge Creek Group, Western Australia present also geochemical features characteristic of hy-drothermal solutions (Sugitani, 1992; Sugitani et al., 1998). These include: (1) low MnO/Fe2O3
values; (2) low concentrations of heavy metals; (3) positive Eu anomalies; (4) low Co/Zn and Ni/Zn values. The Gorge Creek Group cherts show mi-crostructures produced by primary precipitation of amorphous silica and siderite, which could be obtained by Si and Fe contribution from hy-drothermal solutions (Sugitani et al., 1998).
New evidence supporting our hypothesis may come from the recent discovery of 3.5 Ga car-bonaceous filamentous bacteria of possible
hy-drothermal origin, at North Pole area, Warrawoona Group, Western Australia (Isozaki et al., 1999). These bacteria are preserved in silica dikes (called T-cherts) which were previously in-terpreted as silica deposition in synsedimentary faults. Recently, Nijman et al. (1998) reinterpreted these silica dikes as remains of white smokers at seafloor spreading centers. Isozaki et al. (1999) measured d13C of −42 –−32‰ in kerogens
ex-tracted from these silica dikes. Independently from his work, we recently performed N isotopic analyses on the same sample (chert Pano D-136). We found a d15
N of −7.491.0‰ (Pinti et al., 2000). The N and C isotopic composition of these bacteria is in the same range as those measured in the Archean kerogens.
In conclusion, there are geochemical and geo-logical evidences supporting the hypothesis of N Archean cycling controlled by microbial chemosynthesis. The close similarity between the Archean and the present-day N and C isotopic composition of hydrothermal vent organisms sug-gests that the metabolic processes dominating hy-drothermal biota may have not evolved since early times, confirming the general idea that hy-drothermal biota represent the best modern natu-ral analogue to early life.
Acknowledgements
We thank Ph. Sarda (University of Paris XI) for useful comments.
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