I don't know the reason but If you remember there were also some intervals barren of dino-cysts in the relatively deep marls of the Gurpi Formation, Zagros Basin (low latitude) (Razmjooei et al., 2018). The sedimentary samples were full of moderately to poor preserved calcareous nannofossils and planktonic foraminifera but no dino-cysts. !!!???

Are you sure your treatment isn't too harsh? You could have destroyed them. Normally temperature is not a problem for dinos, they can survive a wider range of conditions. I would rather suspect either treatment or preservation.

Hello Kenneth,

Thank you for your reply. I am sure about the treatment as I prepared the slides personally but also I let the samples to be processed in other laboratory using a different method. All the time the results were the same. I think that the reason could be high oxygen level at the bottom but I would like to exclude other possibilities. I wonder if this is true that the calcareous and organic-walled dinocysts do not co-occur. If so, the postdepositional processes would be a reliable explanation. (Earlier I forgot to mention that the samples are of Late Cretaceous age).

Hi Mohammad,

I think that short, barren, intervals found locally in sediments deposited within the deep basin are more natural, as the dinocysts may e.g. be only periodically transported there by currents. In the region that I study there are no cysts in all formations representing the Late Cretaceous, and also younger sediments. And the conditions where the formations were deposited seem to be favourable for presence of dinoflagellates. So I am really willing to solve this 'secret' :)

I would recommend to assess the redox conditions during deposition of these sediments that might control their relative abundance within the water column.

I would be interested in the general palynofacies recovery? I am thinking the potential reasons could include (not in any order):

i) over-oxidation during preparation? If this were the case the kerogen slides that are unoxidized should still contain dinocysts;

ii)post-depositional oxidation. both the kerogen and oxidized slides should be very poor recovery in all macerals. I think this is quite rare, but thought this was the case for the Oligocene section in the Greenland Sea.

iii) dinoflagellate cysts just too diluted by AOM - particularly in anoxic sediments which are quite common in the Mesozoic; even when oxidized with nitric acid they are not concentrated enough, perhaps sample weight should be increased to see;

iv) thermal maturation. burial temperature just too high and the labile AOM and dinocysts and destroyed/converted - the kerogen should have a dark appearance and even dominated by opaque phytoclasts.

Even if distal basins I would still expect recovery of dinocysts; and if unlikely redox related - surface water anoxia as the planktonic forams and diverse. The calcispheres may also be in competition with the foraminifera and slightly different habitats; we have a paper in review with Geology of the Cenomanian that hopefully will be published. Also Paul Dodsworth has plenty of experience and could add more. hope this helps

I agree with the suggested reasons of James.

As I suggested earlier to assess the redox conditions in case of you still have no idea about oxygenation regimes during deposition of your sediments as more reactive/labile organic matter (OM) are available at prevalent oxygen-depleted settings, which result in OM decomposition through biodegradation of the enclosing organic wall of dinocysts (dinosporin). You can have a look on one of my recent work in Marine and Petroleum Geology V. 115, 104285 focuses on the Late Cretaceous (late Coniacian-earliest Campanian from the southern Tethys).

A secondary reason can be represented in precipitation and terrestrial-runoff. During the Late Cretaceous, highest values of precipitation and freshwater discharge were highest at low-latitude equatorial regions, where your study area is located as indicated by Flögel et al. (2008). This can result in lower salinity water that may hinder the relative abundance of dinocysts at shelf regions.

One last point I'd suggest whether your study basin was restricted during the Late Cretaceous and was not connected with regional oceanography. This certainly can trigger the absence of dinocyst communities.

I agree with the other reasons articulated by the other contributors above. Another reason could be that the strata have low total organic carbon content. I would suggest this is likely in the limestones you analyzed as the organic matter is diluted by carbonate and would explain the dominance of calcareous foraminifer. If the mudstones are also relatively carbonate rich/marls; or were deposited during low organic matter production or transport (more arid intervals, not near rivers/sediment source perhaps?) it might explain the low dinocyst recovery. However, the flaw to my logic would be that I would also expect calcareous nannoplankton recovery to be good which is not the case. I dont think high sea surface temperature would exclude dinocysts as we have good recoveries from the Cretaceous in the equatorial Atlantic, and under stress you can actually get mono-specific blooms (mostly peridinioids)- the main challenge is removing the AOM so can see them on a slide!

Thank you for all the comments, they are all very helpful. Referring to the suggestions of James Eldrett:

i) over-oxidation during preparation? If this were the case the kerogen slides that are unoxidized should still contain dinocysts;

-The samples were not oxidised during preparation. I suspect reasons other than preparation problems.

ii)post-depositional oxidation. both the kerogen and oxidized slides should be very poor recovery in all macerals. I think this is quite rare, but thought this was the case for the Oligocene section in the Greenland Sea.

--This is very likely, as I found almost nothing within the samples.

iii) dinoflagellate cysts just too diluted by AOM - particularly in anoxic sediments which are quite common in the Mesozoic; even when oxidized with nitric acid they are not concentrated enough, perhaps sample weight should be increased to see;

-There were no AOM either. Just few black opaque equidimentional phytoclasts in shales/rarely transludcent phytoclasts in other lithologies.

iv) thermal maturation. burial temperature just too high and the labile AOM and dinocysts and destroyed/converted - the kerogen should have a dark appearance and even dominated by opaque phytoclasts.

-This may explain the genesif of the black shales i collected.

Another reason could be that the strata have low total organic carbon content. I would suggest this is likely in the limestones you analyzed as the organic matter is diluted by carbonate and would explain the dominance of calcareous foraminifer.

Yes, this sounds reasonable. Thank you again for all yours and others suggestions. This helped me a lot to better udnerstand the gological history of the region I studied.

The absence of these microfossils could also be due to paleoceanography. See for example the article Dias-Britto, D. 2000: Global stratigraphy paleobiogeography and paleoecology of Albian-Maastrichtian pithonellids calcispheres: impact on Tethys configuration. Cretaceous Research, 21, 315-349