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Earth and Planetary Science Letters 365 (2013) 25–37

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Earth and Planetary Science Letters

0012-82

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The astronomical rhythm of Late-Devonian climate change(Kowala section, Holy Cross Mountains, Poland)

David De Vleeschouwer a,n, Micha" Rakocinski b, Grzegorz Racki b, David P.G. Bond c,Katarzyna Sobien d, Philippe Claeys a

a Earth System Sciences and Department of Geology, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgiumb Faculty of Earth Sciences, University of Silesia, Bedzinska Str. 60, 41-200 Sosnowiec, Polandc Norwegian Polar Institute, Fram Centre, 9296 Tromsø, Norwayd Polish Geological Institute—National Research Institute, Rakowiecka 4, 00-975 Warszawa, Poland

a r t i c l e i n f o

Article history:

Received 24 April 2012

Received in revised form

16 January 2013

Accepted 17 January 2013

Editor: G. HendersonCarboniferous (D–C) boundary, a pivotal moment in Earth’s climatic history that saw a transition from

Available online 15 February 2013

Keywords:

Late Devonian

Milankovic forcing

limestone-shale rhythmites

Gondwanan glaciation

anoxic black shales

TR-cycles

1X/$ - see front matter & 2013 Elsevier B.V.

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esponding author. Tel.: þ32 486318938.

ail address: [email protected] (D. De Vleesc

a b s t r a c t

Rhythmical alternations between limestone and shales or marls characterize the famous Kowala

section, Holy Cross Mountains, Poland. Two intervals of this section were studied for evidence of orbital

cyclostratigraphy. The oldest interval spans the Frasnian–Famennian boundary, deposited under one of

the hottest greenhouse climates of the Phanerozoic. The youngest interval encompasses the Devonian–

greenhouse to icehouse. For the Frasnian–Famennian sequence, lithological variations are consistent

with 405-kyr and 100-kyr eccentricity forcing and a cyclostratigraphic floating time-scale is presented.

The interpretation of observed lithological rhythms as eccentricity cycles is confirmed by amplitude

modulation patterns in agreement with astronomical theory and by the recognition of precession cycles

in high-resolution stable isotope records. The resulting relative time-scale suggests that �800 kyr

separate the Lower and Upper Kellwasser Events (LKE and UKE, respectively), two periods of anoxia

that culminated in massive biodiversity loss at the end of the Frasnian. Th/U and pyrite framboid

analyses indicate that during the UKE, oxygen levels remained low for 400 kyr and d13Corg measure-

ments demonstrate that more than 600 kyr elapsed before the carbon cycle reached a steady state

after a þ3% UKE excursion. The Famennian–Tournaisian (D–C) interval also reveals eccentricity and

precession-related lithological variations. Precession-related alternations clearly demonstrate grouping

into 100-kyr bundles. The Famennian part of this interval is characterized by several distinctive anoxic

black shales, including the Annulata, Dasberg and Hangenberg shales. Our high-resolution cyclostrati-

graphic framework indicates that those shales were deposited at 2.2 and 2.4 Myr intervals respectively.

These durations strongly suggest a link between the long-period (�2.4 Myr) eccentricity cycle and the

development of the Annulata, Dasberg and Hangenberg anoxic shales. It is assumed that these black

shales form under transgressive conditions, when extremely high eccentricity promoted the collapse of

small continental ice-sheets at the most austral latitudes of western Gondwana.

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1. Introduction

Throughout the Phanerozoic, the imprint of Milankovic’sastronomical cycles is recognized in lithological rhythms, as wellas in many geochemical and paleoecological proxies (e.g. Boulilaet al., 2011; Fischer and Bottjer, 1991; House and Gale, 1995;Tyszka, 2009). The study of cyclical change in sedimentaryarchives provides a better insight in the sensitivity of the Earth’sclimate system to cyclic variations in the orbital parameters of theEarth (e.g. Lourens et al., 2010; Verschuren et al., 2009) and

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houwer).

demonstrates that climatic setting strongly influences the responseof climate to orbital forcing (Boulila et al., 2011; Giorgioni et al.,2012). As a geo-chronometer, astronomical rhythms observed in asedimentary archive provide insight into absolute and relativetiming questions (Ghil et al., 2002; Kuiper et al., 2008; Muller andMacDonald, 2000).

A relevant astronomical imprint into climatic proxies has beensuccessfully demonstrated in diverse cyclostratigraphic studiesof Devonian carbonate-shelf archives, exemplified by Chenand Tucker (2003), Crick et al. (2002), De Vleeschouwer et al.(2012a, 2012b), Elrick and Hinnov (2007), Elrick et al. (2009),Garcia-Alcalde et al. (2012) and House (1991, 1995). TheDevonian system is poorly constrained by radiometric dating(see recent chronologies by Becker et al., 2012; Kaufmann,

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2006), and the refined analysis of sedimentary rhythmicity instratigraphically extended and relatively continuous sequencesremains an overall unexploited field and an urgent research aim,as stressed by House (2002) and Racki (2005). This study focuseson the conodont-dated and widely studied Late Devonian shelf-basin successions at Kowala, Holy Cross Mountains (centralPoland; see regional geological setting in Chapter 2 andSupplementary material) characterized by distinct limestone-shale rhythmites (Bond and Zaton, 2003; Fig. 2 in Marynowskiand Filipiak, 2007; Fig. 2 in Marynowski et al., 2010; Fig. 3 inRacka et al., 2010; Fig. 3 in Racki et al., 2002; Fig. 1A and B inRacki, 2005; marly facies of Szulczewski, 1971; 1995). We testwhether or not this rhythmicity is the result of astronomicalforcing, within a climate on the eve of the transition from theDevonian greenhouse to the Carboniferous icehouse (Racki, 2005,p. 15; Streel et al., 2000). In addition, a comprehensive set of otherenvironmental proxies, such as stable isotopes, magnetic suscept-ibility and gamma-ray spectrometry, which record only minordiagenetic/thermal overprint (Belka, 1990; Joachimski et al.,2001; Marynowski et al., 2011), serve as indispensable tools forthe correct interpretation of the observed lithological variationsin terms of paleoenvironmental change.

2. Regional and stratigraphic setting

Extensive outcrops of Middle to Upper Devonian reefal andbasinal systems are exposed in the Holy Cross Mountains (seeSupplementary material), that formed a fragment of the once vastequatorial carbonate shelves of south-eastern Laurussia (Racki,1993; Racki et al., 2002; Szulczewski, 1995). The centrally locatedFrasnian Dyminy Reef evolved into a Famennian pelagic ridge,and this shoal was surrounded by drowned, oxygen-depleteddeeper-shelf areas (i.e. intrashelf basins): Checiny–Zbrza to thesouth, and Łysogory–Kostom"oty to the north (Racki, 1993;Szulczewski, 1971, 1995).

The large, still active Kowala quarry near Kielce is well establishedas a major reference site for multifaceted Late Devonian studies. Thismore than 350 m thick, generally deepening-upward and continuoussuccession from the Checiny–Zbrza basin is relatively well-datedusing conodonts (Fig. 1), and comprehensively investigated fromevent-stratigraphical, paleontological and geochemical perspectives(summarized in Racki, 2005). The intensely studied distinctiveFrasnian–Famennian and late Famennian bituminous horizons recordseveral recurrent anoxic pulses that are known to be of global extent(Filipiak and Racki, 2010; Kazmierczak et al., 2012; Marynowski andFilipiak, 2007; Marynowski et al., 2010; 2012; Racka et al., 2010).

Within the largely dark to black rhythmically bedded marlyseries, late Frasnian to basal Carboniferous lithologic units H–N(Fig. 1; Bond and Zaton, 2003; Szulczewski, 1996) record the post-reef phase of shelf evolution. In this study, two rhythmite-bearingsuccessions were analyzed for cyclostratigraphy (Fig. 1):

(1)
The upper Frasnian to lower Famennian interval (sets H-1 toH-4) encompasses the Frasnian–Famennian (F–F) boundarybeds and records one of the greatest biodiversity crises of thePhanerozoic, the Late Devonian mass extinction (Bond et al.,2004; Joachimski et al., 2002; Racki et al., 2002). Gradedbioclastic limestones (with redeposited reef-builders andbrachiopod coquinas) and/or thick (up to 1 m) slump layersoccur in the basal (H-1) unit only. An abrupt lithologic changefrom wavy- and thin-bedded marly deposits, which are highlyfossiliferous in places (unit H-2), to cherty limestone strata(H-3), is particularly significant for the F-F passage (Rackiet al., 2002). These relatively thick-layered calcareous sets,
6 to 8 m thick, with thin (up to 5 cm) shale breaks, constitutethe main lithologic peculiarity within the otherwise uniformmarly rhythmic succession of set H. In fact, the succeedingmonotonous fossil-impoverished unit H-4, which is morethan 100 m thick, includes regularly alternating marly lime-stones and shales, here analyzed in the lower part only (seealso Filipiak, 2009; Marynowski et al., 2011).

(2)
The highest exposed interval contains the latest Devonianthicker-layered dark platy limestone-shale couplets, inter-rupted by the Annulata and Dasberg black shales (set K;details in Marynowski et al., 2010; Racka et al., 2010) and thegrey to brownish fossil-rich wavy-bedded to (sub)nodularcalcareous set with thin shale partings (Woclumeria lime-stone, set L, see Supplementary material; Marynowski andFilipiak, 2007; Rakocinski, 2011). The Hangenberg bitumi-nous shale (90 cm thick, total organic carbon (TOC) up to22.5 wt%), that also records a major biotic crisis, albeit of lesssignificance than the F–F event, occurs above this interval. Itis succeeded by a 120 cm thick grey-greenish clayey/tuffitesuccession with several interbedded limestone layers (thenewly proposed set M¼sets B and C sensu Malec, 1995;see also Marynowski and Filipiak, 2007, Fig. 2 therein). TheDevonian–Carboniferous (D–C) boundary occurs within theupper part of this unit. The Lower Carboniferous comprises agreenish clayey/tuffite interval with four thin blackshale intercalations (5 to 15 cm thick) and some nodularcarbonate horizons. This recently exposed succession isnamed herein as set N (¼ lower part set D sensu Malec,1995, Fig. 8).
The middle segment of the Famennian succession remainsunexplored due to frequent diagenetic nodular lithologies thatobscure the depositional rhythmicity (Fig. 1; Marynowski et al.,2007), as well as some fault disturbances and/or covered intervals(Fig. 2 in Bond and Zaton, 2003).

3. Materials and methods

3.1. Time-series analysis procedure

The frequency composition of lithological variations (shales,marls, micrites, sparites, calcarenites and nodular limestones)were analyzed via spectral analysis. To carry out time-seriesanalysis of lithological variations, each lithology was assigned adifferent code and a quantified litholog was obtained. Spectralanalysis of proxy data, like the isotopic, magnetic susceptibilityand gamma-ray spectrometry records, was carried out afterdetrending and interpolating the records to equally-spacedintervals. Proxy data and quantified logs were analyzed usingthe multitaper method (MTM; Thomson, 1982) as implementedin the SSA-MTM Toolkit (Ghil et al., 2002).

The MTM-method was performed using 3 discrete prolatespheroidal sequences (DPSS) as data tapers to compromise betweenspectral resolution and sidelobe reduction. Small variations inaccumulation rate behave like phase modulations, and introducemultiple spurious spectral peaks (Muller and MacDonald, 2000). Wechose the MTM because it averages these sidelobes into the mainpeak and thereby gives a superior estimate of the true spectralpower. To assess whether or not the strongest spectral peaks arestatistically different from the red noise spectrum, the 95% con-fidence level (CL) is calculated (robust AR(1) estimation, mediansmoothing window width¼(10Dt)�1, Log Fit, fNyquist¼2/Dt).

The Continuous Wavelet Transform is used to decomposethe one-dimensional time-series into their two-dimensional

Fig. 1. Composite lithological section of the Upper Devonian strata at Kowala (after Szulczewski, 1996, Fig. 8, modified and supplemented in the topmost part by recently

exposed sets M and N). The reference succession shows environmental evolution typical of intermittently drowned shelf from reef (units A–C) to foreslope (D–G) to

intrashelf basin (H–N).

D. De Vleeschouwer et al. / Earth and Planetary Science Letters 365 (2013) 25–37 27

time–frequency representation. This method reveals the persistencyof (astronomical) periodicities along the studied sections andprovides a tool to detect changes in accumulation rate throughassociated changes in cycle thickness. Wavelet software was pro-vided by Torrence and Compo (1998), using the Morlet waveletfunction with wave number 6, zero-padding to the next higherpower of 2, spacing between discrete scales (Dj)¼0.25, smallestscale (S0)¼2Dt, and the number of scales¼ logzðNDt=S0Þ=Dj

� �þ1.

Frequency-selective filters or Gaussian band-pass filters isolateand extract the components of signals associated with a specificrange of frequencies. We employ band-pass filters to assess thebehavior of a specific range of frequencies in a studied signalusing the Analyseries software (Paillard et al., 1996).

For a detailed description of the methodology of proxyrecord compilation, the reader is referred to the Supplementarymaterial.


4. Results and discussion

4.1. Do limestone-marl rhythmites reflect primary environmental

change?

Rhythmical alternations between limestones, shales and marlsare abundant throughout the entire Phanerozoic. Before anyclimatic interpretation can be made, or any astronomical cyclecan be recognized, the primary origin of the alternations betweendifferent lithologies needs to be demonstrated (Westphal et al.,2010). Several arguments strongly suggest that, with someexceptions noted below, diagenesis can be excluded as the driverfor the decimeter-scale rhythmic interbedding that dominates theKowala section. (1) The immature character of the organic matterindicates burial temperatures lower than 75 1C (Belka, 1990).Immaturity to low-maturity is confirmed by an average vitrinitereflectance value of 0.53% Rr (Marynowski et al., 2001), thetemperature of maximum release of hydrocarbons (Joachimskiet al., 2001; Marynowski and Filipiak, 2007; Marynowski et al.,2007; 2010) and hydrogen, oxygen and conodont alterationindexes (Joachimski et al., 2001). (2) Many primary sedimentarystructures are well preserved, including distinctly graded lime-stone layers, hummocky stratification, trilobite obrution deposits,extensive lateral continuity, burrow fills, and web-like structurestypical of benthic cyanobacterial mats (Kazmierczak et al., 2012;Marynowski et al., 2011; Racki et al., 2002; Radwanski et al.,2009; Szulczewski, 1971). Moreover, delicate fossils, such asPhyllocarida and other crustaceans, ostracod-shells and aptychiof the goniatite Tornoceras (Dzik, 2006) as well as carbonizedthalli of large algae with filamentous internal structure (Fig. 3B inRacki et al., 2002) are superbly preserved. The exceptional pre-servation of these fossils and sedimentary structures demon-strates minimal diagenetic alternation in this section and thus alimited applicability of the early diagenesis unmixing model(Westphal et al., 2010). (3) The character of d13C and d18Ovariations in whole-rock samples appear to reflect a predomi-nantly primary signature. Indeed, the isotopic data presented inthis study (Figs. 3E and F and 6G and H) fall entirely within therange of d13C and d18O for Upper Devonian–Lower Carboniferousbrachiopod shell calcite (Giles, 2012; van Geldern et al., 2006;Veizer et al., 1999). Furthermore, the recognized d13C trends incalcites at Kowala correlate with organic carbon data (Fig. 3E andG; Joachimski et al., 2001) and the d18O pattern shows the sametrends as diagenetically resistant conodont apatites (Fig. 5 inJoachimski et al., 2009). Consequently, in this section, diagenesisappears so moderate that post-depositional incorporation of lightisotopes into calcite is negligible. Above all, meteoric diagenesis,paired with strongly distorted isotopic signatures due to theinfluence of 12C and 16O enriched fluids, is essentially absent indeeper-shelf facies. Indeed, the reported d13C and d18O seriesexhibit frequent enrichments in heavy isotopes of 13C and 18Orespectively, which cannot be attributed to diagenetic processes(Joachimski et al., 2001; van Geldern et al., 2006). Pressuresolution carbonate re-mobilization is attested by a crudely lami-nated ribbon-facies in sharp contact with neomorphosed crystal-line carbonate and pressure-welded (stylolaminite) argillaceousinterbeds (especially in unit H-3; Figs. 12D and 16C in Racki et al.,2002). This conspicuous diagenetic instability of the precursorcarbonate muds may be promoted by mass abundance of suscep-tible opaline skeletons (radiolarians, silicisponges) and chertformation near the F–F boundary only (Vishnevskaya et al.,2002). Nevertheless, the isotopic ratios still indicate a primaryorigin of the alternating lithologies as manifested by the mostlypositive d13C values (Fig. 3E) in the broad F–F transition, con-current with the global UKE isotopic pattern (Joachimski et al.,2001; Marynowski et al., 2011; Racki et al., 2002). Shallow-burial

selective dissolution and cementation of the organic-rich carbo-nate muds during anaerobic oxidation of organic matter, as wellas secondary limestone-shale cyclicity caused by anaerobicmethane oxidation, are associated with exceedingly negative d13Cvalues, as low as �5% to �42% (Coniglio, 1989; Raiswell, 1988).Therefore, early diagenetic processes in the mostly organic-enrichedcarbonate-clayey deposits (TOC typically 40.5% in dark shales, but420% in the black horizons) are precluded as the driver of therhythmic bedding.

In the F–F transition, a possible extreme condensation (or evena cryptic hiatus?) embracing the basalmost triangularis Zoneshould be taken into account, since the (late) Pa. triangularis

occurs right at the base of the Famennian (Racki et al., 2002). Onthe other hand, syndepositional and shallow burial depth dis-solution in starved sedimentation regimes is also commonlydocumented in the Woclumeria limestone, especially by irregu-larly wavy inter-layer contacts and the abundance of dissolvedaragonite shells (encrustations on cephalopod internal mounds;Rakocinski, 2011).

The rhythmical alternations between limestone and shale inthe studied intervals of the Kowala section are interpreted to beprimary depositional features, even if diagenetically enhancedbedding (Bathurst, 1987) is a feature of some packages and eventbeds in the succession (e.g. obrutionary trilobite rhythmites; Brettet al., 2012). The associated facies changes reflect repetitivevariations in carbonate content, in response to changes in thewater column and in the geochemical conditions at the sediment/water interface. These are most probably driven by climate.Indeed, terrigenous clay and silt input is highly dependent onprevailing wind and precipitation patterns, while carbonateproductivity is strongly affected by temperature, salinity, stormfrequency and wave energy (Elrick and Hinnov, 2007). Theobserved lithological variations can thus be used as a continuoushigh-resolution, 104 yr-scale, record of Late Devonian climate,highly suitable for cyclostratigraphic purposes. For this reason,lithological variations are used as the starting point for time-series analysis and as the foundation for an astronomical cyclos-tratigraphy. Subsequently, geochemical proxies and their possiblecyclical character are placed in the established cyclostratigraphicframework and interpreted in terms of climate change.

4.2. Interval I: the Frasnian–Famennian transition

The time–frequency representation (Fig. 2A) and the MTM-spectrum (Fig. 2B) are combined to give an overview of thedominant periodicities in the lithological variations betweenshale (coded 0), micrite (coded 2), sparite (coded 3) and calcar-enite (coded 4; lithologic log by Bond and Zaton, 2003, andD. Bond unpublished data). This ranking scheme is chosen toemphasize the alternations between shales and limestones(micrite, sparite and calcarenite). A very strong low-frequencycycle, characterized by 700–1250 cm thick cycles is highlighted(Fig. 2A and B). The wavelet-analysis’ time–frequency representa-tion demonstrates that this cycle is particularly strong in thelowermost Famennian, between 30 and 120 m above the base ofthe section. Secondly, a strong and distinct spectral peak occursbetween a 160 and 256 cm period (Fig. 2B). Wavelet analysisindicates strong spectral power in this frequency range, althoughthe time–frequency representation is characterized by multiplebifurcations (splitting or braiding). The interpretation of thesetwo important cyclical components as the result of an astronom-ical forcing parameter requires an independent time-control. Inthis case, time-control comes from conodont zonation based onSzulczewski (1995, 1996), Racki and Balinski (1998) and Rackiet al. (2002). The rhenana, linguiformis and triangularis zonespan 81.9 m of section. Consensus on the total duration of these

Fig. 2. Periodicities in the lithological variations of the upper Frasnian–Lower Famennian part of the Kowala section. (A) Time–frequency representation resulting from

wavelet (Morlet) analysis. Red colors indicate the stratigraphic levels (x-axis) and periodicities (y-axis) that are characterized by high spectral power. The whitish bands

and dashed lines indicate the periods associated with 405-kyr (E1) and 100-kyr (E2) eccentricity throughout the section. The bifurcation pattern at the 100-kyr period,

indicative for amplitude modulation by the 405-kyr cycles, is shown by rounded rectangles. (B) MTM-power spectra showing distribution of spectral power with 95%

confidence level (CL). (C) The E1-filtered lithological signal (purple line). (D) The E2-filtered lithological signal (olive colored line) and its amplitude envelope (purple

dashed line). (E) Lithological variations (black solid line). BP¼Bandpass.


biozones has yet to be reached in the literature. The recalibratedDevonian timescale of Kaufmann (2006) suggested a combinedduration of 5.8 Myr for these zones, implying an average accu-mulation rate of 14.12 m/Myr. De Vleeschouwer et al. (2012b)found that Kaufmann’s (2006) Frasnian chronology overestimatedto duration of the entire stage by 17%, and so it is likely that trueduration is less than 5.8 Myr. In contrast, Bai (1995) indicated thatthe rhenana to triangularis zone record only 3.2 Myr of time,which would imply a much higher average accumulation rate atKowala of 25.6 m/Myr. Most likely, the true combined durationof the rhenana, linguiformis, and triangularis zone lies betweenthese two end-members. Therefore, it is reasonable to assume anaverage accumulation rate of around 20 m/Myr. In this scenario,the observed 700–1250 cm cycles can be interpreted as the resultof 405-kyr eccentricity forcing and the 160–256 cm cycles as the

result of 100-kyr eccentricity forcing (Table 1). This interpretationis reinforced by the observed bifurcations in the �2 m cycles,every 405 kyr, because of the long-eccentricity amplitude mod-ulation of the 100-kyr cycles (indicated by grey circles on Fig. 2A;Meyers and Hinnov, 2010). However, two problems arise withthis interpretation: [1] at �40 m and �100 m bifurcations in the�2 m cycles are missing (indicated by ‘‘?’’ in Fig. 2A) and [2] inthe lower part of the interval, the low-frequency cycles lack thestrong spectral power that would be expected from a phase-coherent and stable 405-kyr eccentricity forcing. Different inter-pretations can explain these observations in a valid manner.However, the astronomical interpretation is maintained as thelisted problems most likely reflect a non-response to orbitalforcing (see Supplementary material for a more detailed discus-sion). Indeed, the 40 m level coincides exactly to the interval

Fig. 3. Composite plot of the Frasnian–Famennian interval of the Kowala section showing: (A) Lithological variations; (B) The E2-filtered lithological signal (olive colored

line) and its amplitude envelope (purple dashed line); (C) Th/U ratio and its E1-filtered signal (purple dashed line); (D) Weight percentage of potassium and its E-1 filtered

signal (purple dashed line); (E) Carbon isotope record of calcite; (F) Oxygen isotope record of calcite and (G) carbon isotope record of organic matter. (H–K) MTM spectral

plots of Th/U, K, d13C and d18O records respectively (black lines) and their 95% CL (dashed lines).


between the LKE and the UKE recorded by monotonous, clay-richand fossil-impoverished micrites. Such a long interval (�800 cm)of unchanged lithology is exceptional at Kowala. Consequently,the loss of low-frequency spectral power in the lower part of thisinterval is interpreted as the expression of a climate system thatwas disturbed in such a way that the response to orbital forcingwas overpowered. At the 100 m level, an exceptionally longinterval of unchanged lithology suggests weak orbital forcing, ora climate system that was insensitive to orbital forcing, or acombination of both. A possible hiatus is considered unlikelybecause conodont biostratigraphy and microfacies do not revealany large-scale sedimentary discontinuity (Racki et al., 2002) andthe estimated duration between the LKE and UKE (�800 kyr) is ingood agreement with other estimates (Chen and Tucker, 2003;Chen et al., 2005).

In Fig. 2C and D respectively, the low- and high-frequencycycles were filtered out and the amplitude envelope of the high-frequency cycles is calculated using the Hilbert transform. Theamplitude envelope of the 100-kyr eccentricity cycles seemsto mimic the pattern of the 405-kyr cycles, as is expected fromastronomical theory (Laskar et al., 2004). The similarity betweenthese signals confirms the veracity of the 405-kyr cycles in the

lower part of the section, despite the moderate spectral power. Atthe 40 m and 100 m levels, the non-response to orbital forcingremoved the 100-kyr cycles, while the 405-kyr cycle is stillobservable. Indeed, a �0.5 Myr non-response is not apparentin the 405-kyr bandpass filtered record due to the smoothinginherent in the filtering procedure. The correspondence betweenthe filtered 405-kyr cycles and the 405-kyr pattern in theamplitude envelope of the 100-kyr cycles enables the delineationof 13 complete 405-kyr eccentricity cycles, indicated by purplebands on Fig. 2. Since these thirteen 405-kyr cycles span 105 m,a cyclostratigraphically derived average accumulation rate of19.9 m/Myr can be calculated. Based on this cyclostratigraphicinterpretation, we estimate the time difference between theLower and the Upper Kellwasser Event (LKE and UKE respectively)to include two 405-kyr eccentricity cycles, i.e. �0.8 Myr. Thisduration compares well the 10 cycle-sets (100 kyr each) pre-sented by Chen and Tucker (2003) and Chen et al. (2005) betweenthe 4rd-order sea-level transgressions associated with the LKE ofthe early Late rhenana Zone and the UKE of the Late linguiformis

Zone in Guangxi, southern China. Moreover, it agrees well withthe time-difference between the sequence boundary dates ofthe two latest Frasnian TR-cycles (i.e. respectively 376.0 and

Table 1Astronomical interpretation in the upper Frasnian–lower Famennian part of the Kowala section.

Half biostratigr.

acc. rate

Biostratigr. derived

acc. rate

Cyclostratigr. derived

acc. rate

Double biostratigr.

acc. rate

Astronomical

Interpretation

Sed. rate 7.11 m/Myr 14.21 m/Myr 19.9 m/Myr 28.42 m/Myr

Frequency range700–1250 cm 1.0–1.8 Myr 493–880 kyr 352–628 kyr 246–440 kyr 405-kyr ecc.

160–256 cm 225–360 kyr 113–180 kyr 80–129 kyr 56–90 kyr 100-kyr ecc.


375.2 Ma; Haq and Schutter, 2008), of which the respectivetransgressions are associated with LKE and UKE anoxic deposition(Bond and Wignall, 2008).

Geochemical proxy data is plotted next to the cyclostratigra-phically delineated eccentricity cycles in Fig. 3. The Th/U ratio, aproxy for the intensity of anoxia, shows a gradual increasetowards more oxic values throughout the section, with importantmeter-scale variations superimposed on the long-term trend. TheMTM spectral plot of the Th/U ratio in the lowermost 115 m of thesection (Fig. 3H) demonstrates a moderate peak of spectral poweraround 0.00125 cycles/cm, i.e. the frequency at which one wouldexpect the influence of 405-kyr eccentricity forcing. A moreimportant spectral peak (495% CL) is observed at frequency0.0025, indicating �200-kyr climatic fluctuations. Interestingly,this �200-kyr rhythm is also observed in the carbon andstrontium records of F–F sections from South China (Chen et al.,2005). The source of this signal might be the 174 kyr amplitudeand frequency modulation of obliquity (Hinnov, 2000). However,more high-resolution records are needed to confirm this link. Thespectral curve also clearly rises around frequency 0.005 cycles/cm, i.e. the frequency of 100-kyr cycles, but the resolution of theTh/U record is too low to firmly distinguish these cycles. A similarpattern is recorded by the concentration of potassium (K wt%;Fig. 3I), which is mainly a proxy for the detrital clay fraction. Thedashed lines in Fig. 3C and D represent the 405-kyr variations ofthe Th/U ratio and K concentration respectively. In particular, theK concentration follows the 405-kyr cycles that were delineatedbased on lithology. Consequently, it is suggested that strongerlithological variations occur during periods of intense input ofdetrital clays. The Th/U data in Fig. 3C indicates that during theUKE, oxygen levels remained depleted for �400 kyr. This isin agreement with previous estimates based on pyrite framboids(Bond et al., 2004).

Carbon and oxygen isotopes were measured at decimeterresolution, through a 16 m interval that straddles the F–F bound-ary. Joachimski et al. (2001) lower-resolution carbon isotoperecord from another Kowala section is plotted for comparison.Both carbon and oxygen isotope records show a decreasing trendleading up to the UKE and exhibit intense positive excursionsassociated with the event. The d13Corg record from Joachimskiet al. (2001) similarly shows positive excursions related with theLKE and UKE. These authors interpreted the positive excursions ind13C and d13Corg as the result of higher photoautotrophic produc-tion, triggered by enhanced continental nutrient-supply parallelwith short-term sea-level rise and upwelling of anoxic watersthat invaded formerly well-oxygenated environments (seeanother interpretation in Kazmierczak et al. 2012). The conse-quent burial of large amounts of organic carbon is expected toresult in a drop of atmospheric pCO2. Joachimski et al. (2001)failed to observe a change in the photosynthetic carbon isotopefractionation (ep), as would be expected after a significant pCO2.-drop. Indirect evidence that a pCO2 drop is actually associatedwith the UKE comes from our d18O record (Fig. 3F) that indicatesrapid cooling during the UKE (1% positive excursionE4 to 5 1Ccooling), similar to the oxygen isotope shift documented inconodont apatites from this same succession (see Fig. 5 in

Joachimski et al., 2009). Putting these isotopic records in ourcyclostratigraphic framework, it suggests that for the carbon cycle,after the þ3% UKE excursion in d13Corg, more than 600 kyr wereneeded before a more or less steady state was reached (Fig. 3G).

The MTM spectral curves of our high-resolution isotope datashow sharp and strong (495% CL) spectral peaks (Fig. 3J and K):d13C and d18O display an important spectral peak around fre-quency 0.011 cycles/cm. Following our astronomical interpreta-tion, these cycles have a duration of �47 kyr, and cannot bedirectly attributed to an astronomical forcing factor. In contrast,the elevated spectral power observed for both d13C and d18Oaround frequency 0.027 cycles/cm occurs exactly at the frequencyat which one expects the influence of �18-kyr precession. Therecognition of this �18-kyr period in environmental proxies suchas d13C and d18O demonstrate the importance of precessionalforcing on the Late-Devonian tropical climate. Indeed, in moderntropical environments precession is the most important driver of104-yr scale climate change, most often by influencing monsoonalpatterns and intensity (Kutzbach, 1981; Kutzbach and Liu, 1997;Tuenter et al., 2003). In comparison with the modern world,Devonian tropical climate potentially exhibited an even moreintense monsoonal circulation (Streel et al., 2000), characterizedby seasonally wet-and-dry climates (Cecil, 1990), on whichprecessional variations undoubtedly had a significant influence(De Vleeschouwer et al., 2012a). Despite the fact that 20 Myrseparate the present study from the Belgian section analyzed byDe Vleeschouwer et al. (2012a), they share a similar paleogeo-graphic setting on the southeastern margin of Laurussia, at atropical latitude. Thus, a comparable paleoclimatic model can beapplied to interpret the precessional cycles observed in both sections.The presence of a precessional rhythm in Frasnian–Famennianclimate change at Kowala also explains why eccentricity-relatedperiods show up in the shale-limestone alternations. Eccentricityonly marginally influences insolation, but it appears as theenvelope of the precession parameter (Muller and MacDonald,2000).

4.3. Interval II: Devonian–Carboniferous interval

The results of MTM and wavelet analyses on the upperFamennian to lower Tournaisian lithological variations are pre-sented in Fig. 4A. This part of the Kowala section is characterizedby shales (coded 0), marls (coded 1), nodular limestones (coded 2)and micrite (coded 2.5; Marynowski and Filipiak, 2007;Marynowski et al., 2010; Racka et al., 2010). This scheme isthought to reflect relative changes in clay-content. In the upperpart of the studied interval, an important low-frequency compo-nent is suggested at periodicities between 170 and 390 cm. A verystrong decimeter-scale cycle is denoted, characterized by a40–80 cm period. Wavelet analysis suggests that this cycle is per-sistent through the entire Famennian part of the section, thoughcharacterized by multiple bifurcations, and that it disappearsupon reaching the Tournaisian. Between 1600 and 2900 cm,the importance of a high-frequency cycle with a 8–12 cm peri-odicity is demonstrated by both MTM and wavelet analysis. Theinterpretation of the observed cyclicity requires independent

Fig. 4. Periodicities in the lithological variations of the Upper Famennian–Lower Tournaisian part of the Kowala section. (A) Time–frequency representation resulting from

wavelet (Morlet) analysis. Red colors indicate the stratigraphic levels (x-axis) and periodicities (y-axis) that are characterized by high spectral power. The whitish bands

and dashed lines indicate the periods associated with 405-kyr (E1), 100-kyr (E2) eccentricity and precession throughout the section. The bifurcation pattern at the 100-kyr

period, indicative for amplitude modulation by the 405-kyr cycles, is shown by rounded rectangles. (B) MTM-power spectra showing distribution of spectral power with

95% confidence level (CL). (C) The E1-filtered lithological signal (purple line). (D) The E2-filtered lithological signal (olive colored line) and its amplitude envelope (purple

dashed line). (E) Lithological variations (black solid line). BP¼Bandpass. (For interpretation of the references to color in this figure legend, the reader is referred to the web

version of this article.)


biostratigraphic time-control: the Annulata Event, recorded in thelower part of this interval occurs within the uppermost trachytera

Zone. According to Kaufmann’s (2006) chronology, �4.7 Myrseparate the trachytera conodont zone and the D–C boundary.According to Becker et al. (2012, Table 22.2), the same intervalspans 4.1 Myr. Bai (1995) again suggests a shorter duration forthe same interval (2.9 Myr), but in this case, the true combinedduration of the considered biozones is most probably much closerto the estimates of Kaufmann (2006) and Becker et al. (2012),given the multiplicity of U–Pb ID-TIMS absolute ages available forthe latest Famennian (Richards et al., 2002; Streel, 2000; Trappet al., 2004; Tucker et al., 1998). In the studied section, 25 m spanthe interval between the Annulata Event and the Famennian/Tournaisian boundary, implying an average accumulation rate of�5.7 m/Myr. This value is in stark contrast to the 4-times higheraccumulation rate found at the Frasnian–Famennian interval ofthe studied section. Evidence for slower accumulation also comesfrom diverse sclerobionts that utilized the shells of dead cepha-lopods as a hard substrate for their settlement (uppermostFamennian; Rakocinski, 2011) and from corrosional surfacesrelated to syndepositional shallow burial-depth dissolution(Berkowski, 2002). Also, as the section becomes much more shaledominated, a slower accumulation could be expected, possibly inresponse to the shutdown in primary productivity followingvarious extinction events (i.e. a less productive carbonate factory)and to starved sedimentation regimes during the Famennian

global climate cooling (Caputo et al., 2008; Isaacson et al., 2008;Joachimski et al., 2009; Streel et al., 2000). A �5.7 m/Myraccumulation rate gives rise to the interpretation of the low-frequency cycles as the result of 405-kyr eccentricity forcing(Table 2). The 48–75 cm cycles match 100-kyr eccentricity cyclesand the 8–12 cm cycles can be seen as the result of precessionalforcing (Table 2). The extremely low spectral power in the 405-kyrfrequency range in the first portion of the record questions thisinterpretation, as the 405-kyr eccentricity cycle is remarkablyphase-coherent and stable. However, the strong bifurcating pat-tern in the 100-kyr frequency range appears to be a pristine short-eccentricity signal. The lack of spectral power between 0 and1400 cm is seen as the result of the non-linear recording ofclimate forcing. Fig. 4C and D shows the bandpass filters at thefrequencies of 405-kyr and 100-kyr eccentricity respectively. Theamplitude envelope of the 100-kyr cycles, calculated using theHilbert transform, appears to bundle the 100-kyr cycle in groupsof four, as expected from astronomical theory. However, atseveral stratigraphic positions, a mismatch between the ampli-tude modulation of the 100-kyr cycles and the 405-kyr cycles isobserved. This phase-lagged amplitude modulation does notundermine the astronomical interpretation, as phase and ampli-tude distortions can be induced by the non-linear translation ofsea-level change to offshore sediment flux (Laurin et al., 2005).Mainly based on the amplitude envelope of the 100-kyr cycles,12 complete 405-kyr eccentricity cycles were delineated and are

Fig. 5. Periodicities in the lithological variations of the upper Famennian part of the Kowala section, between 1500 and 3000 cm. This portion is analyzed in detail because

it is characterized by high-frequency lithological variations. (A) Time–frequency representation resulting from wavelet (Morlet) analysis. Red colors indicate the

stratigraphic levels (x-axis) and periodicities (y-axis) that are characterized by high spectral power. The whitish bands and dashed lines indicate the periods associated

with 100-kyr (E2) eccentricity and precession throughout the studied interval. The strong bifurcation pattern at the precessional period, indicative for amplitude

modulation by the 100-kyr cycles is shown by rounded rectangles. (B) MTM-power spectra showing distribution of spectral power and 95% confidence level (CL). (C) The E2-filtered

lithological signal (olive colored line). (D) The precession-filtered lithological signal (green line) and its amplitude envelope (olive colored dashed line).

(E) Lithological variations (black solid line). BP¼Bandpass. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Table 2Astronomical interpretation in the upper Famennian–lower Trounaisian part of the Kowala section.

Half biostratigr.

acc. rate

Biostratigr. derived

acc. rate

Double biostratigr.

acc. rate

Astronomical

interpretation

Sed. rate 2.85 m/Myr 5.7 m/Myr 11.4 m/Myr

Frequency range170–390 cm 0.6–1.4 Myr 298–684 kyr 150–342 kyr 405-kyr ecc.

40–80 cm 140–280 kyr 70–140 kyr 35–70 kyr 100-kyr ecc.

8–12 cm 28–42 kyr 14–21 kyr 7–10 kyr 18-kyr prec.


indicated by purple bands in Fig. 4. Together, these 12 cycles(4.86 Myr) span 2650 cm, resulting in a refined accumulation rateof 5.45 cm/kyr.

The uppermost Famennian part of the Kowala section (1500–3000 cm) is characterized by cm-scale alternations between lime-stones and shales. These high-frequency rhythmites were ana-lyzed in more detail to assess the astronomical forcing byprecession (Fig. 5). The 40–80 cm cycles are associated with100-kyr eccentricity while the 8–12 cm cycles are interpreted asthe result of precessional forcing. A band-pass filter centered onthe frequency of precession shows that the amplitude of theprecessional cycles varies significantly (Fig. 5D). The amplitudeenvelope of the precessional cycles was calculated using the

Hilbert transform and compares favorably with the 100-kyreccentricity cycles. Since the amplitude modulation of precessionby eccentricity is expected from astronomical theory, the astro-nomical interpretation of the different observed cycles is rein-forced. Moreover, the dominance of precession may also explainamplitude modulation mismatches between long and shorteccentricity cycles, discussed above. Indeed, an important mis-match occurs in that part of the section where precession-relatedlithological variations can be firmly grouped in 100-kyr bundles,but where grouping in 405-kyr ‘‘super-bundles’’ is much lessobvious.

The geochemical data available for this interval is not suitablefor time-series analysis: for the isotopic proxies (Fig. 6G–I),

Fig. 6. Composite plot of the Famennian–Tournaisian interval of the Kowala section showing: (A) Lithological variations; (B) The E2-filtered lithological signal (olive

colored line) and its amplitude envelope (purple dashed line); (C) Magnetic Susceptibility; (D) Weight percentage of CaCO3; (E) Weight percentage of total organic matter

(TOC); (F) Weight percentage of total sulfur (TS); (G) Carbon isotope record of calcite; (H) Oxygen isotope record of calcite and (I) Carbon isotope record of organic matter

(note that only organic carbon data are available for black shale horizons). Blue and red arrows designate climatic cooling and warming respectively. (For interpretation of

the references to color in this figure legend, the reader is referred to the web version of this article.)


resolution and/or dataset-length are too low; for magnetic sus-ceptibility (Fig. 6C), unequal spacing of data makes MTM andwavelet analysis meaningless. The TOC, TS and CaCO3 concentra-tions simply reflect lithological variation between shale andlimestone, and therefore do not add to the paleoenvironmentalinformation that is already contained in the lithological variationsthemselves. Excessive burial of organic matter, expressed by theDasberg, Kowala and Hangenberg black shales, is clearly indicatedby increased TOC, TS and decreased CaCO3 concentrations(Fig. 6D–F). All anoxic shales in the studied interval have beeninterpreted to be the result of repeated transgressive pulses,accompanied by the blooming of primary producers (Joachimskiet al., 2001; Kaiser et al., 2008; Marynowski et al., 2010, 2012;Racka et al., 2010). Enhanced bioproduction is reflected bypositive d13Corg excursions of about 4%, up to values of �25%in the limestone enclosing the shales (Fig. 6I). Oxygen isotopessuggest a shift towards warmer conditions during anoxic shaledeposition (Fig. 6H), which is consistent with the transgressivetrend observed during interglacial episodes (Racka et al., 2010).Rising sea-levels resulted in the flooding of the shallow-waterenvironment by nutrient-rich but oxygen-depleted waters thatoriginated from the deeper-shelf (see review in Hallam andWignall, 1999). Moreover, relatively warm conditions were most

probably associated with more moist periods. Therefore,enhanced chemical weathering and continental runoff mayhave contributed to eutrophication in the oceans, stimulation ofprimary production, oxygen consumption and therefore marineanoxia (Algeo and Scheckler, 1998). Based on the delineated 405-kyr cycles, it is possible to estimate the time-difference betweenthe well-described black shales that occur in this section. Five anda half 405-kyr cycles, i.e. �2.2 Myr, separate the Annulata Shalefrom the Dasberg Shale. Almost the exact same amount of timeseparates the Dasberg Shale from the Hangenberg Shale (Fig. 6).These values accord with the sequence boundary ages reported byHaq and Schutter (2008), who estimate a duration of 2.4 and1.8 Myr for the corresponding 3rd-order sea-level changes. Inter-estingly, a �2.4 Myr cycle is present in the Earth’s eccentricity.The same correspondence between third-order sea-level changesand long-period �2.4 Myr eccentricity has been reported for theMesozoic greenhouse world (Boulila et al., 2011). Hence, onecan hypothesize an astronomical influence on the timing of thedevelopment of transgressive–regressive (TR) cycles and theassociated deposition of anoxic black shales (as has already beensuggested for Cretaceous anoxic shales; e.g. Lanci et al., 2010;Mitchell et al., 2008). During greenhouse periods such as theMesozoic and the Devonian, thermoeustasy or variations in

Table 3Comparison of accumulation rate to the time-scale of Lithological cycles at different Devonian and Early Carboniferous sections.

Accumulation rate Duration of studied interval Time-scale of lithological cycles Ref.

(cm/kyr) (kyr)

Lower Mississippian Banff Fm. 8.9–14 53–83 Millenial Elrick and Hinnov (2007)

Lower Mississippian Lodgepole Fm. 6.4–15 242–567 Millenial Elrick and Hinnov (2007)

Upper Devonian West Range Limestone 5.9–12.7 45–96 Millenial Elrick and Hinnov (2007)

Middle Devonian Denay Fm. 3.1–5.8 104–194 Millenial Elrick and Hinnov (2007)

Upper Devonian, Guangxi, South-China 1–3 2000–2400 Millenial and Milankovic Chen and Tucker (2003)

Upper Devonian Kowala section, Interval I 1.75–3 6000 Milankovic This study

Upper Devonian Kowala section, Interval II 0.5–0.75 5300 Milankovic This study

Fm¼Formation.


storage of groundwater and lakes can account for a maximum of10 m of sea-level change (Jacobs and Sahagian, 1993; Schulz andSchafer-Neth, 1997). Yet, it is clear that the amplitude of sea-levelchange in the uppermost Devonian was at least 4 to 5 timeshigher (Haq and Schutter, 2008). Therefore, we suggest that thepresence and extent of small continental ice sheets on (western)Gondwana could be strongly influenced by eccentricity. Thisassumption is supported by Late Devonian glacial marine depositsin Peru, Bolivia and Brazil that record important regional glacia-tions on the western margin of Gondwana at mid-to-high lati-tudes (40–501S; Blakey, 2008; Caputo et al., 2008; Isaacson et al.,2008). During periods of low eccentricity, austral summer insola-tion at the high latitudes of the Gondwana continent would nothave reached high values, which gives these small continentalice sheets a good chance to survive summer and to continue togrow during the following winter. Thereby, the Earth’s climate isgradually cooled (phase 1 in the oxygen isotope record in Fig. 6H).In contrast, high eccentricity triggers maximal austral summerinsolation when o¼901 (perihelion in December), causing impor-tant melting and ice sheet collapse, associated sea-level rise andglobal warming (phase 2). Global warming combined with lateFamennian atmospheric O2 levels exceeding the 13% threshold forthe occurrence of widespread wildfires (Marynowski and Filipiak,2007; Marynowski et al., 2010; 2012; Rimmer et al., 2004), led tothe establishment of optimal conditions for increased primaryproduction and bottom-water anoxia. The enhanced burial oforganic carbon reduces the C concentration of the ocean’s surfacelayer, that is compensated by additional dissolution of CO2 fromthe atmosphere. The climatic cooling resulting from the draw-down of atmospheric CO2 is reflected by temporarily increasingd18O values during black-shale deposition (phase 3). This cooling-phase appears to be short-lived (�200 kyr) and is probablycounteracted by the astronomical configuration favoring climaticwarming (phase 4). Several hundred of kyrs later, eccentricity andresultant maximal summer insolation in Gondwana weakens,enabling ice to survive summer and accumulate once more(phase 5).

Compared to the Annulata, Dasberg and Hangenberg Shales,the Kowala anoxic shale is a much thinner organic-rich layer.Although it occurs exactly halfway between the Dasberg andHangenberg Shales, it was probably not triggered by astronomicalforcing but rather as the result of regional tectonic activity.Indeed, the Kowala Shale is geographically limited to theCheciny–Zbrza intrashelf basin and is only recorded in the studiedsection (Marynowski and Filipiak, 2007).

4.4. On the origins of limestone-shale/marl alternations

The limestone-shale/marl alternations in both studied inter-vals of the Kowala section demonstrate clear orbital signaturesof precession and eccentricity and it can be assumed thatfacies changes reflect a climatic signal. Carbonate-rich layers

(limestones) are interpreted to form under warm and dry condi-tions with less runoff from land, reduced weathering, less fre-quent storm events (i.e. low dilution by terrigenous influx) andenhanced carbonate productivity. Carbonate-poor layers arethought to result from a lower carbonate productivity undercooler climatic conditions, associated with wetter conditionsand enhanced clastic input.

However, the exact same response mechanism is used toexplain millennial-scale paleoclimate rhythms, expressed byindividual limestone-shale couplets (Chen and Tucker, 2003;Elrick and Hinnov, 2007), despite the order of magnitude differ-ence between the sub-Milankovic’s and Milankovic time-scale.To address this discrepancy, accumulation rate, duration of thestudied interval and time-scales of the distinguished paleoclima-tological rhythms in lithology are compared to the Devonian andlower Carboniferous sections in Elrick and Hinnov (2007), the F–Fsection in Chen and Tucker (2003) and both intervals from thisstudy (Table 3). From this table it is apparent that a highaccumulation rate (several cm’s/kyr) is needed for lithologicalvariations to record millennial-scale climate change. In sectionslike Kowala, where accumulation occurs at an order of magnitudemore slowly, limestone-marl/shale alternations represent Milan-kovic-scale climate change. In summary, terrigenous input to thebasin and carbonate productivity vary at both time-scales, butonly in rapidly accumulating settings millenial-scale rhythmicitycan be registered as lithological variations. On the other hand,only sections characterized by a sufficiently long duration ofdeposition (e.g. Kowala) are suitable for the detection of Milan-kovic-scale cyclicity. Orbital forcing is known to be a powerfuldriver of climate change, whereas the driver of millennial-scaleclimate change remains a mystery. The present research rein-forces this mystery since the same type of lithological rhythm,driven by similar climate changes, occurs at time scales with anorder of magnitude difference.

5. Conclusions

The lithological variations between limestones, shales andmarls in the Kowala section (Holy Cross Mountains, Poland)record astronomical forcing. Precession and eccentricity controlthe cyclical environmental changes that are responsible for thelithological rhythms. The expression of obliquity in the Kowalasedimentary record is weak and intermittent, as is expectedduring greenhouse worlds (Boulila et al., 2011). The associationof lithological alternations with astronomical parameters enablesthe construction of a cyclostratigraphic framework that suggests a�800 kyr time-gap between the LKE and the UKE and a �400 kyrduration for low oxygen levels during the UKE. Isotopic data(d13C, d18O and d13Corg) across the UKE exhibit a strong positiveshift associated with paleoenvironmental change and destabiliza-tion of the carbon cycle. Despite the severe environmental


perturbations associated with the UKE, the imprint of precessionin these isotopic records is clear. The sharp positive isotope shiftin d13C and d13Corg is interpreted to be the result of enhancedprimary production, promoted by increased nutrient-supply coe-val with sea-level rise and upwelling of anoxic waters. Theconsequent burial of large amounts of organic carbon resultedin a drop of atmospheric pCO2, as is suggested by the 1% d18Opositive excursion (4 to 5 1C cooling). A �600 kyr recovery for thecarbon cycle after the þ3% UKE excursion is suggested by thecyclostratigraphic framework. Time-differences between theAnnulata, Dasberg, Kowala and Hangenberg anoxic black shalesare estimated at 2.2, 1.2 and 1.2 Myr respectively. It is proposedthat the formation of the late Famennian Annulata, Dasberg andHangenberg Shales is related to maxima of the 2.4-Myr eccen-tricity cycle. These astronomical configurations would have pro-voked sea-level rise by triggering the collapse of small continentalGondwanan ice-sheets. Thereby, an explicit link between astro-nomical forcing, eustatic transgression and marine anoxia for theLate Devonian is proposed.

Acknowledgments

This work is supported by a Ph.D. fellowship awarded by theResearch Foundation – Flanders (FWO) assigned to DDV, and aPh.D. fellowship of the Ministry of Science and Higher Education –Poland (Grant N N307 4272 34) assigned to MR. We would like tothank P. Wignall, M. Zaton, L. Marynowski and A. Pisarzowska forparticipation in the field and geochemical analyses which werepartly funded by a NERC Ph.D. studentship to DB. M. Joachimskikindly provided unpublished isotope data. PC thanks the FWO(project G.A078.11), Hercules foundation and VUB research fundsfor their contribution. The authors thank Dr. Stephen R. Meyersand Dr. Maya Elrick for their constructive reviews and Dr. A.-C. DaSilva for positive discussions.

Appendix A. Supplementary materials

Supplementary data associated with this article can be found inthe online version at http://dx.doi.org/10.1016/j.epsl.2013.01.016.

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