COMPUTATIONAL METHOD
The 15N secular variations are smoothed using a linear fitting where the significance of dots
corresponds to the height of a Gaussian Kernel function (i.e. The kernel regression is a nonparametric technique in statistics to estimate the conditional expectation of a random variable,
Ruppert et al. 2003). The bandwidth (h) influences the amount of smoothing and this
parameter might be chosen on the basis of the whole dataset structure (Jones et al. 1996).
However, because there is no uniform way to define it, several authors proposed to consider
multiple smoothing curves with different h values (the “family approach” to smoothing,
Marron and Chaudhuri 1998). In order to identify which features (increases and/or decreases)
are significant, they introduced the SiZer method. The first derivative of each curve is
computed and when locally positive or negative at 95% confidence, it is symbolized in red or
blue, respectively. When it does not significantly differ from zero, the segment is represented
in purple. The resulting SiZer map therefore provides valuable insights about the significant
variations recognized at different time-scales, and together with the original data time series
can reveal significant evolution of a given parameter through time. It is noteworthy that such
an approach has already been successfully used on Quaternary (Erästö and Holmström 2006)
and Jurassic (Dera et al. 2011) 18O time series for paleoclimate reconstruction. In order to
perform SiZer analyses, based on Kernel regressions, the SizerSM package (copyright, J.S.
Marron 2000-2004) running on the Matlab platform was used. This package is downloadable
at: http://www.unc.edu/~marron/marron_software.html.
SUPPLEMENTARY REFERENCES
Beaumont V, Robert F (1999) Nitrogen isotope ratios of kerogens in Precambrian cherts: a
record of the evolution of atmosphere chemistry? Precambrian Research 96: 63-82
Cowie G, Calvert S, De Lange G, Keil R, Hedges J (1998) Extents and implications of
organic matter alteration at oxidation fronts in turbidites from the Madeira Abyssal
Plain. Proceedings of the Ocean Drilling Program, Scientific results 157: 581-589
De Lange G (1998) Oxic vs. anoxic diagenetic alteration of turbiditic sediments in the
Madeira Abyssal Plain, Eastern North Atlantic. Proceedings of the Ocean Drilling
Program, Scientific results 157: 573-580
Dera G, Brigaud B, Monna F, Laffont R, Pucéat E, Deconinck JF, Pellenard P, Jpachimski
MM, Durlet C (2011) Climatic ups and downs in a disturbed Jurassic world. Geology
3: 215–218
Erästö P, Holmström L (2006) Selection of prior distributions and multiscale analysis in
Bayesian temperature reconstructions based on fossil assemblages: Journal of
Paleolimnology 36: 69-80
Garvin J, Buick R, Anbar AD, Arnold GL, Kaufman AJ (2009) Isotopic evidence for an
aerobic nitrogen cycle in the latest Archaean. Science 323: 1045-1048
Godfrey LV, Falkowski PG (2009) The cycling and redox state of nitrogen in the Archaean
ocean. Nature Geoscience 2: 725-729.
Hayes JM, Kaplan IR, Wedeking KW (1983a) Precambrian Organic Geochemistries,
preservation of the record. In: Schopf WJ (ed) Earth's earliest biosphere. Princeton
University Press, Princeton, pp 93-134
Hayes JM, Kaplan IR, Wedeking W (1983b) Geochemical evidence bearing on the origin of
aerobiosis, a speculative hypothesis. In: Schopf WJ (ed) Earth's earliest biosphere.
Princeton University Press, Princeton, pp 291-301
Holmes ME, Müller PJ, Schneider RR, Segl M, Pätzold J, Wefer G (1996) Stable nitrogen
isotopes in Angola Basin surface sediments. Marine geology 134: 1-12
Jia Y, Kerrich R (2004) Nitrogen 15–enriched Precambrian kerogen and hydrothermal
systems. Geochemistry Geophysics Geosystems 5: Q07005
Jones MC, Marron JS, Sheather SJ (1996) Progress in data-based bandwidth selection for
kernel density estimation. Computational Statistics 11 : 337-381
Kerrich R, Jia Y, Manikyamba C, Naqvi M (2006) Secular variations of N-isotopes in
terrestrial reservoirs and ore deposits. In: Kesler SE, Ohmoto H (eds) Evolution of
Early Earth’s Atmosphere, Hydrosphere, and Biosphere—Constraints from Ore
Deposits. Geological Society of America Memoir 198: 81-104
Nishizawa M, Takahata N, Terada K, Komiya T, Ueno Y, Sano Y. (2005) Lead, carbon and
nitrogen isotope geochemistry of apatite-bearing metasediments from ~3.8 Ga Isua
supracrustal belt, West Greenland: primary signature and secondary disturbance.
International Geology Review 47: 952-970
Nishizawa M, Sano Y, Ueno Y, Maruyama S (2007) Speciation and isotope ratios of nitrogen
in fluid inclusions from seafloor hydrothermal deposits at ∼ 3.5 Ga. Earth and
Planetary Science Letters 254: 332–344
Marron JS, Chaudhuri P (1998) When is a feature really there? The SiZer approach. In: Firooz
A (ed) Automatic target recognition VII, Volume 3371. Proceedings of the society of
photooptic and industrial engineering, pp 306-312
Milder JC, Montoya JP, Altabet MA (1999) Carbon and nitrogen stable isotope ratios at Sites
969 and 974: Interpreting spatial gradients in sapropel properties. Proceedings of the
Ocean Drilling Program, Scientific results 161: 401-411
Muzuka AN, Macko SA, Pedersen TF (1991) Stable carbon and nitrogen isotope
compositions of organic matter from sites 724 and 725 Oman margin. Proceedings of
the Ocean Drilling Program, Scientific results 117: 571-586
Palya AP, Buick IS and Bebout GE (2011) Storage and mobility of nitrogen in the continental
crust: Evidence from partially melted metasedimentary rocks, Mt. Stafford, Australia.
Chemical Geology 281: 211-226
Papineau D, Mojzsis SJ, Karhu JA, Marty B (2005) Nitrogen isotopic composition of
ammoniated phyllosilicates: case studies from Precambrian metamorphosed
sedimentary rocks. Chemical Geology 216: 37-58
Papineau D, Purohit R, Goldberg T, Pi D, Shields GA, Bhu H, Steele A, Fogel ML (2009)
High primary productivity and nitrogen cycling after the phosphogenic event in the
Aravalli Supergroup, India. Precambrian Research 171: 37–56
Peters KE, Sweeney RE, Kaplan IR (1978) Correlation of carbon and nitrogen stable isotope
ratios in sedimentary organic matter. Limnology Oceanography 23: 598-604
Pinti DL, Hashizume K, Matsuda J (2001) Nitrogen and argon signatures in 3.8 to 2.8 Ga
metasediments: Clues on the chemical state of the Archean ocean and the deep
biosphere. Geochimica et Cosmochimica Acta 65: 2301-2315
Pinti DL, Hashizume K, Orberger B, Gallien J-P, Cloquet C, Massault M (2007) Biogenic
nitrogen and carbon in Fe-Mn-oxyhydroxides from an Archean chert, Marble Bar,
Western Australia. Geochemistry Geophysics Geosystems 8: Q02007
Pinti DL, Hashizume K, Sugihara A, Massault M, Philippot P (2009) Isotopic fractionation of
nitrogen and carbon in Paleoarchean cherts from Pilbara craton, Western Australia:
Origin of 15N-depleted nitrogen. Geochimica et Cosmochimica Acta 73: 3819–3848
Rau GH, Arthur MA, Dean WE (1987) 15N/14N variations in Cretaceous Atlantic sedimentary
sequences: implication for past changes in marine nitrogen biogeochemistry. Earth and
Planetary Science Letters 82: 269-279
Ruppert D, Wand MP, Carroll RJ (2003) Semiparametric regression. London, Cambridge
University press, 386 p.
Sadofsky SJ, Bebout GE (2004). Nitrogen geochemistry of subducting sediments: New results
from the Izu-Bonin-Mariana margin and insights regarding global nitrogen
subduction. Geochemistry Geophysics Geosystems 5: Q03I15
Sano Y, Pillinger CT (1990) Nitrogen isotopes and N2/Ar ratios in cherts: An attempt to
measure time evolution of atmospheric 15N value. Geochemical Journal 24: 315-325
Thomas Katherine S. (2011) Organic Geochemistry and Stable Isotope Constraints on
Precambrian Biogeochemical Processes: Examples of the Late Proterozoic Coppercap
Formation, NWT Canada and Archean Gorge Creek Group, Pilbara. Master thesis,
Massachusetts Institute of Technology
Thomazo C, Ader M, Philippot P (2011) Extreme 15N-enrichments in 2.72-Gyr-old
sediments: evidence for a turning point in the nitrogen cycle. Geobiology 11: 107-120
Ueno Y, Yoshioka H, Maruyama S, Isozaki Y (2004) Carbon isotopes and petrography of
kerogens in similar to 3.5-Ga hydrothermal silica dikes in the North Pole area,
Western Australia. Geochimica et Cosmochimica Acta 68: 573-589
Van Zuilen MA, Mathew K, Wopenka B, Lepland A, Marti K, Arrhenius G (2005) Nitrogen
and argon isotopic signatures in graphite from the 3.8-Ga-old Isua Supracrustal Belt,
Southern West Greenland. Geochimica et Cosmochimica Acta 69: 1241-1252
Wedeking KW, Hayes JM, Matzigkeit U (1983) Organic geochemical analysis. In: Schopf WJ
(ed) Earth's earliest biosphere. Princeton University Press, Princeton, pp 428-441
Yamaguchi K (2002) Geochemistry of Archean-Paleoproterozoic black shales: the early
evolution of atmosphere, oceans and biosphere. PhD thesis, Pennsylvania State
University
Zhang D (1988) Nitrogen concentrations and isotopic compositions of some terrestrial rocks.
PhD, The University of Chicago