Precambrian Research 105 (2001) 85 – 88
www.elsevier.com/locate/precamres
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 b
a
Laboratoire de Géochronologie Multi-Techniques UPS-IPGP and UMR 8616 Orsayterre, Uni6ersité Paris SUD XI,
Batiment 504, 1er etage, 91405 Orsay Cedex, France
b
Department of Earth and Space Science, Graduate School of Science, Osaka Uni6ersity, Toyonaka, Osaka 560 -0043, Japan
Received 9 February 2000; accepted 27 June 2000
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 15N-depleted (d15N from − 4 to 0‰),
whereas N in modern organic matter preserved in
oceanic sediments is enriched in 15N (d15N from 4
to 8‰; Fig. 1). Changes through geological time
* Corresponding author. Fax: +33-1-69154891.
E-mail address: [email protected] (D.L. Pinti).
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 (NO−
3 ). 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, NH+
4 ). 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 15N-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.
0301-9268/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 3 0 1 - 9 2 6 8 ( 0 0 ) 0 0 1 0 0 - 5
86
D.L. Pinti, K. Hashizume / Precambrian Research 105 (2001) 85–88
Fig. 1. Frequency distribution of d15N measured in kerogens
extracted from Archean cherts (data from Table 3 of Beaumont and Robert, 1999) and in modern organic matter preserved in oceanic sediments (data from Peters et al., 1978).
The isotopic composition of N (and of C) in
Archean kerogens is very similar to that presently
observed in ‘subseafloor chemosynthetic ecosystems’ (Fig. 2). ‘Chemosynthesis’, or more correctly,
‘chemoautolithotrophy’, describes the biosynthesis
of organic carbon compounds from CO2 using
energy and reducing power derived from the oxidation of inorganic compounds, such as H2S, S2O3,
CH4, H2 and NH+
4 , which support bacterial
chemosynthesis (Conway et al., 1994). Chemosynthesis 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). Chemosynthesis has been often claimed as the main form of
biosynthesis prior to photosynthetic life, and hydrothermal 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 hydrothermal 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
N and C isotopic signature of hydrothermal vent
species reflects that of the biomass produced by
chemoautolithotrophic bacteria. The unusually
negative d15N ratios have been related to processes
of N2-fixation or NH+
4 assimilation and biomass
production by some symbiotic bacteria (Rau,
1981). Another hypothesis is that mantle-derived
15
N-depleted nitrogen is the main source of inorganic N used by chemoautolithotrophic bacteria.
The isotopic composition of mantle N is −5 9 2‰
(Marty and Humbert, 1997). The isotopic shift of
− 2– −4‰ caused by N-fixation on a mantle N
source could easily explain the 15N depleted values
in Archean kerogens and in the modern hydrothermal ecosystems. The 13C-depleted signature may
derive from autothrophic fixation of seawater dissolved inorganic carbon (DIC) or, in some particular environment, from bacterial methanogenesis
(Conway et al., 1994).
Although the N and C isotopic similarity between Archean kerogens and present-day biomass
at hydrothermal vents supports the possible role of
chemosynthesis in the Archean N cycling, we
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 of d13C and
d15N in hydrothermal vent-related chemosynthetic ecosystems
(dark) and in modern organic matter in oceanic sediments
(light). Data from Brooks et al. (1987), Conway et al. (1994),
and Peters et al. (1978).
D.L. Pinti, K. Hashizume / Precambrian Research 105 (2001) 85–88
need some geological evidence for such an environment. The Archean rocks analyzed by Beaumont 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 solutions. The proximity of Archean chert layers to
banded iron formation (BIF), the latter derived
from hydrothermal solutions enriched in Fe (Isley, 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 hydrothermal origin exist. For six of them, the
authors have not reported the depositional environment (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 environment 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 belonging to the Onverwacht Group. Cherts from
these two horizons have petrographic and geochemical 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 hydrothermal 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 microstructures produced by primary precipitation
of amorphous silica and siderite, which could be
obtained by Si and Fe contribution from hydrothermal solutions (Sugitani et al., 1998).
New evidence supporting our hypothesis may
come from the recent discovery of 3.5 Ga carbonaceous filamentous bacteria of possible hy-
87
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 interpreted 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 extracted 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 d15N 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 geological 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 suggests that the metabolic processes dominating hydrothermal biota may have not evolved since
early times, confirming the general idea that hydrothermal biota represent the best modern natural analogue to early life.
Acknowledgements
We thank Ph. Sarda (University of Paris XI)
for useful comments.
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