Hi Nico,

Yes, the Toarcian OAE (cf. Suan et al. 2008, EPSL).

Stephane

Hi Stéphane, 

Indeed, and for the C/T (OAE2) too apparently. So it seems that obliquity dominance is a common finding at low latitudes at times of anoxia/enhanced carbon burial. And I have the impression that it is not fully understood why it is the case. Now, I'm also wondering whether it has been observed associated to climate changes but not necessarily during OAEs. 

Yes it is the case for the Cenomanian of Northern Europe, if you may read Berger et al., 1989 and also Gale et al., 1999.

Dear Nicolas,

interesting discussion! If you consider the Middle Miocene a greenhouse, you will find the papers of Ann Holbourn et al. (2005, Nature; 2007, EPSL) interesting. Changes in astronomical eccentricity and obliquity amplitudes probably explain parts of such a shift (eccentricity --> obliquity --> eccentricity) in the Miocene (ca. 14.5-13.9 Ma) - other factors probably contribute, too. Also at ca. 12.5-11.7 Ma (OK, not strictly greenhouse), the Ceara Rise record shows a change from dominated by precession --> obliquity --> back to precession, resembling the astronomical changes in obliquity- and precession-amplitude (see Shackleton & Crowhurst 1997, Zeeden et al. 2013). In my opinion changes in obliquity amplitude and eccentricity play a role for such changes in the Miocene.

Hope this helps, Christian

Dear Amina and Christian,

Thank you very much for your contributions. This opens some perspectives towards understanding the possible causes of such shifts. 

Hi Nicolas,

You adress here an interesting question, which has not been resolved.

As Christian says, in the astronomical solutions, the amplitude of the eccentricity paces the amplitude of the precession. During eccentricity minimums (each 2.4 Myr), the effect of precession is very weak and it is not unlikely to find a dominant obliquity cycle in these intervals (Zeeden et al., 2013 is a nice reference, see also Westerhold et al., 2014).

Another possibility could be short-term cooling within a greenhouse mode. It has been described in a cooling interval of the Early Eocene (Westerhold and Röhl, 2009), or in the Early Aptian just after the OAE-1a (Ghirardi et al., 2014).

Laurin et al. (2015) evoked the role of carbon sequestration in mid- and high latitudes from the Albian to the Campanian. Also an interesting reference.

A last hypothesis evoked is the impact of the summer intertropical insolation gradient, in which spectral analyses show a stronger power in the obliquity band (Bosmans et al., 2015). At moment, it has only be modelled for the Holocene.

References:

Bosmans J.H.C., et al., 2015. Climate of the Past 11, 1335-1346.

Ghirardi J., et al., 2014. Newsletters on Stratigraphy 47/3, 247-262.

Laurin J., et al., 2015. Paleoceanography 30, doi:10.1002/2014PA002736.

Westerhold T., and Röhl U., 2009. Climate of the Past 5, 309-327.

Westerhold T. et al., 2014. Climate of the Past 10, 955-973.

Zeeden C., et al., 2013. Palaeo-3 369, 430-451.

Hi Mathieu, I particularly appreciate the examples where such features are associated to cooling as this is exactly what I find in my dataset^^. Thanks a lot for all those references.

Hi Nicolas,

this is an interesting discussion. I do not have much to add. If you are looking for examples related to cooling, then Bosboom et al. 2014 (EPSL, doi:10.1016/j.epsl.2013.12.014) and Abels et al. 2011 (Palaeo3, doi:10.1016/j.palaeo.2010.11.028) might be relevant, although they focus on mid latitudes.

Best wishes,

Jiri

Hi Jiri,

Thanks a lot for your contribution. I was not aware of those refs either, they look very useful for mid-latitudes. 

Cheers