NITROGEN ISOTOPE SIGNATURES OF MICROFOSSILS REVEAL AN OCEANIC
OXYGENATION 3.0 GYR AGO.
F. Delarue1,2, F. Robert2, S. Derenne1
1
2
CNRS/UPMC/EPHE, France
CNRS/MNHN/UPMC, France
The nitrogen isotope composition (δ15NBulk) of kerogen is classically used to assess the
secular variations of the redox state of the Archean oceans (Beaumont and Robert, 1999).
Shifting from negative to positive values, the δ15NBulk reflects a rise in the redox state of the
Archean oceans before the “Great Oxygenation Event” (2.4 Ga). Although Early and Late
Archean kerogen reveals the occurrence of negative and positive δ15N values, Mid-Archean
(3.2 to 2.8 Gyr) is characterized by quasi-null δ15N values. This isotopic signature was
suggested as indicative of the biological fixation of N2 (Stueken et al., 2015). However, this
conclusion contradicts phylogenomic studies which demonstrate that the biological N2
fixation was not dominant during the Archean (David and Alm, 2011). Moreover, there is
compelling evidence for an early oxygenation of Mid-Archean oceans (Planavski et al., 2014)
that should be recorded in redox-dependent δ15N values. This would imply that the δ15NBulk
values measured around 0 ± 2‰ mask isotopic heterogeneities occurring at the sedimentary
organic particle scale. However, such an issue remains not documented although the
development of nanoscale secondary ion mass spectrometry (NanoSIMS) technology allows
the determination of the nitrogen isotope composition at sub-micrometre scale (δ15Nµm). A
Mid-Archean geological formation, the Farrel Quartzite (3.0 Gyr) was shown to comprise
both amorphous carbonaceous particles and microfossils (Sugitani et al., 2007). Here, we
therefore investigated the nitrogen isotope composition at both bulk (δ15NBulk) and
microfossil/particle scales in the kerogen isolated from Farrel Quartzite.
The δ15NBulk values measured by EA-irMS in Farrel Quartzite range between +0.3‰ to
+2.2‰ (n= 3; mean δ15NBulk value= 1.0 ± 1.1‰). These δ15NBulk values are in the same range
as those previously determined on Mid-Archean cherts and interpreted as remnants of a N2fixing biomass (Stueken et al., 2015). These δ15NBulk values are also consistent with the
δ15Nµm values of amorphous carbonaceous matter and microfossils found within the Farrel
Quartzite (δ15Nµm= -4.0 ± 11‰; n=27). However, the δ15Nµm values exhibit a large
heterogeneity ranging between -21.6 ± 4.0 to +30.6 ± 2.4‰ (Fig. 1). Amorphous
carbonaceous matter shows δ15Nµm values ranging between -21.6 and -0.1‰. Three types of
morphologically distinct microfossils were identified: film-like, spheroid and lenticular
microfossils. Film-like and spheroid microfossils exhibit mostly negative δ15Nµm values
ranging from -17.3 to +1.5‰, while lenticular microstructures are characterized by positive
δ15Nµm values ranging from +0.4 to +30.6‰. As a whole, there is a clear relationship between
the morphology of the microfossils and their nitrogen isotope compositions. Interpreted as
remnants of benthic microorganisms, the film-like and spheroid microfossils exhibit negative
δ15Nµm values, in agreement with the N isotopic signatures determined on current benthic
microorganisms living near a hydrothermal vent (Van Dover and Fry, 1994). On the basis of
their morphological features, the lenticular microfossils are considered as remnants of pelagic
microorganisms. Such an interpretation is strengthened by their positive δ15Nµm values that
are found in extant pelagic microorganisms/organisms living away from hydrothermal vent.
Partial denitrification involving the occurrence of nitrates (NO3- → N2), partial aerobic
ammonium oxidation (NH4+→ NO2-) or assimilation of ammonium (NH4+→ Norg) through
28th International Meeting on Organic Geochemistry
17 – 22 September 2017, Florence, Italy
preferential uptake of 14N can a priori lead to such δ15N values. Indeed, they can involve a N
isotope fractionation effect of ca. +5 to +30‰, +4 to +27‰ and +14 to +19‰, respectively.
However, (i) a N marine cycle dominated by nitrate reduction into N2 is associated with
positive δ15NBulk (ca. +5‰) and (ii) ammonium assimilation induced negative δ15NBulk values
(ca. -5‰) in open system. As a result, the quasi-null δ15NBulk values rule out these two
metabolisms. Thus, only aerobic ammonium oxidation along a redox gradient can explain the
large N isotope heterogeneity.
For the first time, the nitrogen isotope composition of individual Archean microfossils was
determined. They exhibit a large range of negative and positive δ15Nµm values despite a quasinull δ15NBulk value. This finding rejects previous interpretation on the presence of N2-fixing
microorganisms. It rather implies that aerobic ammonia oxidizing-like microorganisms were
thriving in Mid-Archean oceans. In turn, their requirement in free molecular O2 involves that
free O2 was already present in ocean 3.0 Gyr ago. This Mid-Archean ecosystem should then
reveal an intermediate oxygenation step in the redox evolution of the N cycle.
Figure 1 δ15N values of amorphous organic matter and morphologically distinct microfossils
from the Farrel Quartzite. δ15NBulk values are provided for comparison with the δ15Nµm.
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28th International Meeting on Organic Geochemistry
17 – 22 September 2017, Florence, Italy