How to interpret the offset between marls and limestones of carbon isotopes on the bulk rocks in Zumaia GSSP (Spain)?

The Zumaia section in Spain is the Global Stratotype Section and Point (GSSP) for the Selandian and Thanetian (Schmitz et al. 2011). It has been much studied for bulk rock stable carbon isotopes in the Danian-Selandian transition interval (Schmitz et al. 1997; Dinarès‐Turell et al. 2012; Dinarès‐Turell et al. 2014; Storme et al. 2014).

New carbon isotopes measurements on marls and limestones show a consistent offset of values between the two lithologies. Correlation was found between C isotopes and carbonate content (r = 0.47), and C and O isotopes (r = 0.67) in marly samples, implying a stronger lithological effect in marls. This is important to interpret Schmitz et al. (1997) curves as in parts of the section where limestones were unavailable they used marly material. A lithological effect may thus partly explain the two excursions observed in that interval by Schmitz et al. in 1997: CIE-DS1 and CIE-DS2 (Arenillas et al. 2008).

The lithological effect has been confirmed by Dinarès‐Turell et al. (2012) as they resampled the section in search of the Latest Danian Event (LDE), a negative carbon isotopic excursion (Bornemann et al. 2009). They showed a link between carbonate content and carbon isotopes values. But they also found a negative excursion in limestone samples only, equivalent to CIE-DS1 and attributed to the LDE.

The scarcity of limestone beds in the lower part of the Selandian where CIE-DS2 is observed prevents to work on limestone samples only.

Two explanations can be given to explain the lithological effect on bulk-rock carbon isotopes:

- This effect is primary, and the environmental processes controlling marl-limestone alternations (see Dinares-Turell et al., 2012) also influence the carbon isotopic signal.

- This effect is due to diagenesis, which would affect limestones and marls differently.

If diagenesis in at the source of the lithological effect, the primary signal is still found in limestones. In marls the diagenetic overprint could leave part of the primary signal, but this remains to be confirmed by future work.

Bibliography

Arenillas et al. 2008. Terra Nova 20 (1): 38‑44.
Bornemann et al. 2009. J. Geol. Soc. London 166 : 1135-1142.
Dinarès‐Turell et al. 2012. Terra Nova 24 (6): 477‑486.
Dinarès-Turell et al. 2014. Earth Planet. Sci. Lett. 405: 119‑131.
Schmitz, et al. 1997. Palaeogeogr. Palaeoclimatol. Palaeoecol. 133 : 49-68.
Schmitz et al. 2011. Episodes 34 (4): 220‑243.
Storme et al. 2014. Palaeogeogr. Palaeoclimatol. Palaeoecol. 414: 212‑224.