# DEEP-TIME ASTRONOMICAL FORCING ON ORGANIC CARBON ISOTOPE VALUES: A TOOL FOR GLOBAL CORRELATION AT 10⁵-YEAR SCALE

Minka Moreau-Ledegen<fnref n="1" />,<fnref n="2" />, Mathieu Martinez<fnref n="2" />, Beatriz Aguirre-Urreta<fnref n="3" />, Eleys Ben Saïd<fnref n="2" />, Brieuc Rabu<fnref n="2" />, Pierre Pellenard<fnref n="1" /> and Anne-Catherine Pierson-Wickmann<fnref n="2" />

Corresponding author: mathieu.martinez@univ-rennes.fr

**Abstract:** Carbon-isotope ratios have been measured on organic matter from two sections astronomically-calibrated. We show that eccentricity cycles can be observed in the δ¹³Corg signal and correlated at global scale. The record of the orbital cycles in δ¹³Corg appears to be a powerful tool for deep-time fine correlations of widely distributed sections.

**Keywords:** Carbon isotopes, organic matter, Milankovitch cycles, chemostratigraphy, astrochronology.

## 1 INTRODUCTION

Carbon-isotope ratios are widely used to constrain deep-time evolution of the carbon cycle and long-distance correlations in order to align paleoclimatic signals (Weissert, 1989). Traditionally, only major events and trends can be used for correlations while low-amplitude, short-lived fluctuations are regarded as noise or linked to basin-scale processes (e.g., Joachimski, 2023). Global fluctuations in deep-time are often regarded as a balance between organic and carbonate carbon buried in the sedimentary column. Basin-wide, the source of carbonate (pelagic producers, neritic aragonitic or calcitic producers, inorganic carbon supply from the continent, remineralization of organic carbon, diagenesis) can overprint the global trend of marine δ¹³C values (e.g. Swart and Eberli, 2003). A recent reappraisal of the time scale from the Valanginian to the Barremian (Early Cretaceous) nonetheless demonstrated the impact of the long orbital cycles (1.2 and 2.4 Ma), with amplitudes as much important as the emplacement of a large igneous province on the evolution of the marine carbonate δ¹³C (Martinez et al., 2023). That raises the opportunity of finding the record of shorter orbital cycles in the δ¹³C, so that it can be used to finely correlate sedimentary record widely distributed in the world. Organic-carbon δ¹³C values have usually higher amplitudes of change than carbonate δ¹³C. They have the ability to record the same trends as observed in marine carbonate δ¹³C, while basin-scale processes that can alter a global trend in organic δ¹³C (δ¹³Corg) series are due to the change of source between terrestrial and aquatic organic matter, the terrestrial organic matter having higher δ¹³C values in the Mesozoic (Suan et al., 2015). Here, we acquire two series of δ¹³Corg from the Vergol-Morénas (Vocontian Basin, France, Fig. 1) and Cerro La Parva (Neuquén Basin, Argentina, Fig. 2). These two sections are geographically remote from each other and encompass the lower-upper Valanginian boundary. The Vergol-Morénas section was deposited in basin environments, while the Cerro La Parva section was deposited in an outer ramp environment. They are both composed of marl and limestone beds, albeit the detrital content is higher in Cerro La Parva. In both cases, the balance between siliciclastic and carbonate content is related to astronomical forcing. We show that in both cases the δ¹³Corg correlates with the 405-ka eccentricity cycle and that both sections can be correlated thanks to these cycles with a precision in time of ca. 100 ka.

## 2 MATERIALS AND METHODS

X-Ray Diffractometry (XRD) was performed on 27 bulk samples of powdered marl and limestone from Vergol-Morénas using a Bruker D8 Endeavor diffractometer to identify and quantify mineral phases. The spectra were corrected for calcite contents, obtained from decarbonation data during the preparation of samples for δ¹³C measurements on organic carbon.

The δ¹³Corg values were measured on 27 samples at Vergol-Morénas and 71 samples at Cerro La Parva. The samples previously crushed were decarbonated using 2N hydrochloric acid. XRD verification was performed to ensure the absence of residual carbonates that could affect the results. After cleaning with pure water, decarbonated powders were dried in an oven (50 °C for 48 h), then crushed again to obtain a fine powder. Each sample, of a specific mass (about 50 mg), was weighed in a tin capsule with a Sartorius MCA6-6S-2S00-M ultrabalance. Samples were analyzed with an elemental analyzer (vario MICRO cube,

<footnote n="1">¹Biogéosciences, UMR 6282 CNRS, Université Bourgogne Europe, 21000 Dijon, France</footnote>

<footnote n="2">²Université de Rennes, CNRS, Géosciences Rennes, UMR 6118, 35000 Rennes, France</footnote>

<footnote n="3">³Instituto de Estudios Andinos Don Pablo Groeber, CONICET & Universidad de Buenos Aires, Ciudad Universitaria, pabellón 2, 1428 Buenos Aires, Argentina</footnote>
Elementar, GISMO analytical platform, Biogéosciences laboratory Dijon) coupled to an isotope ratio mass spectrometer (Isoprime, Elementar). Isotope ratios obtained were compared to international standards USGS40 L-glutamic acid (δ¹³C = -26.39 ±0.04‰ VPDB) and IAEA-600 caffeine (δ13C = -27.77 ±0.04‰ VPDB). For each sample, replicates showed reproducibility better than ±0.15 ‰. The total organic carbon (TOC) content, expressed in wt %, was determined by the elemental analyzer at the same time.

Mass-normalized magnetic susceptibility (MS) was measured on 816 regularly spaced every 0.20 m at Cerro La Parva with a Kappabridge KLY-3. Prior to the measurements, the weight of each of the samples was measured with a weighting scale having an accuracy of 0.01 mg. The volume measurements were corrected from the blank. The blank MS here corresponds to the volume MS of the container and the analysis room. The blank-corrected MS values were then normalized to the weight of the sample and a volume of 10 cc. Values are reported in m³.kg⁻¹.

## 3 RESULTS AND DISCUSSION

The δ¹³Corg values from Vergol-Morénas range from -29.3 ‰ to -27.1 ‰ PDB (Fig. 1d). The values increase from the *Karakaschiceras inostranzewi* to the *Karakaschiceras pronecostatum* subzones, which reflect the Weissert Carbon Isotope Excursion (CIE). Within this trend, four local maxima are observed at levels 89 m, 101 m, 121 m, and 146 m, near the boundaries of the 405-ka eccentricity cycles identified in Martinez et al. (2013; Fig. 1c). These boundaries were located on maxima of detrital supply identified by spectral analyses through the basin. XRD analyses show a lower part of the studied interval, until level 113 m (Figs. 1e-g), dominated by carbonate. Then, the detrital fraction rapidly increases in the *Saynoceras verrucosum* Subzone and remains maximal until level 145 m. The carbonate fraction then progressively increases toward the top of the series. This trend in detrital content can be easily observed in the field, as a thick marl series with thin intercalated limestone beds replaces regular marl-limestone alternations in the *S. verrucosum* Zone. Within this trend, the four local maxima in δ¹³Corg associated with 405-ka cycle boundaries correspond to local maxima in the detrital fraction. Despite their low amplitude, the whole δ¹³Corg shows a cyclic trend in line with the mineralogy and the 405-ka eccentricity cycles sequenced in Martinez et al. (2013). Thus, the record of the 405-ka eccentricity cycle is observed both in δ¹³Corg and detrital fraction. In the Vocontian Basin, increased detrital fraction was due to increased chemical weathering and runoff. Clay mineral analyses shown that this feature could be observed at different timescales from precession to long eccentricity cycles (Deconinck and Debrabant, 1985; Martinez et al., 2020).

<figure><img src="image_1.png" /><figcaption>**Figure 1.** Location and results of the data acquired at Vergol-Morénas. A. Location of the Vergol-Morénas section within the Vocontian Basin. B. Synthesis of δ¹³Ccarb data published on the studied interval of Vergol-Morénas (from Martinez et al., 2020). C. Gamma-Ray Total Count (GRTC; Martinez et al., 2013). D. δ¹³Corg. E. CaCO₃ content from X-Ray Diffractometry. F. Quartz content from X-Ray Diffractometry. G. Clay content from X-Ray Diffractometry.</figcaption></figure>

In Cerro la Parva, the MS signal has values ranging from 23 to 170 x 10⁻⁹ m³.kg⁻¹ and displays a trend toward decreasing values from the bottom to the top of the series (Fig. 2d). The trend displayed in Fig. 2d was calculated using the LOWESS method (Cleveland, 1979) with a coefficient of 0.5. The δ¹³Corg values range from -24.7 ‰ to -21.2 ‰ PDB. The series starts with the second highest values of the series at the base of *Karakaschiceras attenuatus* Subzone. The values then decrease toward the minimum values of the series until level 50 m, and then increase until level 85 m. The trend in the average values (calculated using the LOWESS method with a coefficient of 0.5) then stabilizes until level 150 m, where it initiates a trend toward decreasing values at the top of the series. The values of the base of the series are higher than the values
acquired at the top of the Vaca Muerta Formation (Weger et al., 2022), while the lowermost values acquired here match the maximum values of Weger et al. (2022). Regarding the stratigraphic context and the trends of the δ¹³Corg series, the lowermost part of the series corresponds to the peak in δ¹³Carb of the Weissert CIE, within the *S. verrucosum* Tethyan ammonite Subzone. The rest of the series lies within the Plateau Phase of the Weissert CIE, which probably ends at Cerro La Parva at level 150 m.

<figure><img src="image_2.png" /><figcaption>Figure 2. Location and results of the data acquired at Cerro La Parva. A. Location of the Cerro La Parva section within the Neuquén Basin. B. Residual magnetic susceptibility (data to which the trend in average was subtracted) calibrated to time after correlations of the δ¹³Corg record from Cerro La Parva to the synthetic δ¹³Carb from the Vocontian Basin. C. Residual δ¹³Corg calibrated to time after correlation to the compiled δ¹³Carb data from the Vocontian Basin. D. Mass-normalized magnetic susceptibility. The red curve represents the long-term trend in average. E. δ¹³Corg. The black curve represents the long-term trend in average. F. Time scale of the Valanginian-Hauterivian time (from Martinez et al., 2023). G. Compilation of δ¹³Carb from the Vocontian Basin (from Martinez et al., 2023). The red curve forms the trend of the δ¹³Carb series calculated using the LOWESS method with a coefficient of 3.5 %.</figcaption></figure>

Within the Plateau Phase, the trend of the δ¹³Corg series shows two minima of the trend at levels 50 m and 125 m correspond to two minima, respectively dated at 133.32 and 132.65 Ma (Martinez et al., 2023), in the trend of the compilation of δ¹³Carb of the Vocontian Basin (Fig. 2). Two maxima at Cerro La Parva 85 and 145 m correspond to two maxima of the compilation of the Vocontian Basin, dated 133.04 and 132.37 Ma. Spectral analyses were made on the MS and δ¹³Corg series of Cerro La Parva, which have been first calibrated into time with the age model just described above, linearly interpolated to their average sampling time (2 ka for the MS, 25 ka for the δ¹³Corg), and which trend has been subtracted to the series. The spectra were calculated with the multi-taper method applying three 2π-tapers (Thomson, 1982).

<figure><img src="image_3.png" /><figcaption>Figure 3. Spectra of the δ¹³Corg (A) and MS (B) series of the Cerro La Parva section calibrated into time from the correlation with the δ¹³C from the Cerro La Parva section to the Vocontian Basin. Periods are labelled in thousands of years. The red curves are the filters then displayed in Figures 2A and 2B</figcaption></figure>

The spectrum of the MS shows a spectral peak at 300 ka (Fig. 3b). The spectrum of the δ¹³Corg series shows spectral peaks at 370 ka and 120 ka (Fig. 3a), likely respectively corresponding to the 405-ka and the ~100-ka eccentricity cycles. The application of the filters on the MS and δ¹³Corg series here guided the identification of the boundaries of the 405-ka eccentricity cycles, especially the lower boundary of V11, around which the data are more scattered than in Vergol. From these boundaries, a refined age model of the section can be proposed (Fig. 3e; Tab. 1), allowing in turn the age of the base of ammonite zones recorded in the section to be calculated (Tab. 2). Interestingly, one of the implications of this age model is the synchronicity between
Moreau-Ledegen et al., Astronomic forcing on δ¹³Corg records for global correlation at 10⁵yrs time scale.

the base of the *P. angulatiformis* Andean ammonite zone (131.96 Ma) and the *Criosarasinella furcillata* Tethyan ammonite zone (131.92 Ma; Martinez et al., 2023), as previously inferred from astrochronology at El Portón and La Charce sections (Aguirre-Urreta et al., 2019).

<table><caption>Table 1. Final age model of the studied interval of the Cerro La Parva section</caption><thead><tr><td>Label</td><td>Level (m)</td><td>Age (Ma)</td></tr></thead><tbody><tr><td>Base V8</td><td>6.0</td><td>133.87</td></tr><tr><td>Base V9</td><td>41.4</td><td>133.46</td></tr><tr><td>Base V10</td><td>83.0</td><td>133.06</td></tr><tr><td>Base V11</td><td>114.6</td><td>132.65</td></tr><tr><td>Base V12</td><td>142.0</td><td>132.25</td></tr></tbody></table>

<table><caption>Table 2. Calculated ages of ammonite zones in the studied interval of Cerro La Parva</caption><thead><tr><td>Ammonite zone</td><td>Level base (m)</td><td>Age base (Ma)</td></tr></thead><tbody><tr><td><i>K. attenuatus</i></td><td>0</td><td>133.94</td></tr><tr><td><i>V. permolestum</i></td><td>145.5</td><td>132.2</td></tr><tr><td><i>P. angulatiformis</i></td><td>161.7</td><td>131.96</td></tr></tbody></table>

It remains to be understood whether the orbital forcing on the carbon-isotope ratios in these basins result from regional or global processes. Regional processes could arise from the change in source of terrestrial or marine organic carbon. Global processes could arise from global changes in primary productivity and sequestration of organic carbon. In both basins, maxima in δ¹³Corg at 405-ka scale are related to increase in siliciclastic input, which is, at least in the Vocontian Basin, related to increased runoff conditions (Deconinck and Debrabant, 1985), which favored primary productivity. In Cerro La Parva, the filters of MS and δ¹³Corg show a maximum difference of 0.06 Ma and are broadly in phase (Figs. 2b, c), so increase in δ¹³Corg was also related to increased siliciclastic input, likely for the same reasons as in the Vocontian Basin. The filter of the 405-ka cycle in the δ¹³Corg signal correlates with the 405-ka cycles defined in the Vocontian Basin, which, in turn, allows the ages of the biozones at Cerro La Parva to be refined. From this age model, the base of the *K. attenuatus* Subzone is dated at 133.94 Ma, the base of the *V. permolestum* Subzone at 132.20 Ma, and the base of the *P. angulatiformis* Zone at 131.96 Ma. At this stage, we thus suggest that the orbital control observed on the δ¹³Corg in both basins results from a global control on the carbon cycle. In this case, carbon isotope stages can be defined at eccentricity and then constitute a powerful correlation method to align with high precision paleoclimatic signals at global scale.

## 4 CONCLUSION

The carbon-isotope signals measured on two sections, Cerro La Parva (Argentina) and Vergol-Morénas (France), and calibrated by astrochronology, demonstrate the pacing of the δ¹³Corg signal by the 405-ka eccentricity cycle, and possibly by the 100-ka eccentricity cycle. In both cases, the cycles in δ¹³Corg are broadly in line with the detrital cycles, implying that increase in δ¹³Corg occurred during increased runoff conditions, possibly under more humid conditions that also favored increased primary productivity. Orbitally-controlled carbon isotope stages can then be defined for long-distance and fine correlations, which then constitutes a powerful method for deep-time calibration of paleoclimatic series.

## 5 ACKNOWLEDGMENT

MM thanks CNRS INSU SYSTER grant WOAW (Widespread Oceanic Anoxia during the Weissert event?) and University of Rennes Défis Scientifiques project: Quantification de la surface des zones anoxiques marines à travers les temps géologiques for supporting this project. Adoum Ali Kodo is thanked for helping in
the data acquisition. The authors thank the GISMO platform and its staff (Biogéosciences, Université Bourgogne Europe, UMR CNRS 6282, France), as well as the GeOHeLiS platform and his staff (Géosciences Rennes, UMR 6118, France). They also thank Michael Joachimski and Nicolas Tribovillard for having reviewed the earlier version of the manuscript.

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