Revisiting Glauconite Geochronology: Lessons Learned from In Situ Radiometric Dating of a Glauconite-Rich Cretaceous Shelfal Sequence

Round 1

Reviewer 1 Report

The article submitted for review turned out to be very interesting for me. The article is devoted to validation of the using glauconite for dating sedimentary deposits. The work has been carried out very carefully, I have no doubts about the correctness and adequacy of the results obtained. I have no serious comments on the text of the article. I will be glad to see next investigations in the future with the use of other isotopic systems.

Author Response

We thank the reviewer for the very positive evaluation of our manuscript. 

Reviewer 2 Report

In this manuscript the authors point out that the scarcity of well-preserved and directly dateable sedimentary sequences is a major impediment for inferring the earth’s paleo-environmental evolution. Under certain circumstances the authigenic mineral glauconite can potentially provide absolute stratigraphic ages for sedimentary sequences and constraints on paleo-depositional conditions. However, this requires improved approaches for measuring and interpreting glauconite formation ages. In this study, glauconite from a Cretaceous shelfal sequence in Northern Germany was characterized using petrographic, geochemical and mineralogical screening methods before in-situ Rb-Sr dating by means of LA-ICP-MS/MS.

While the physical and chemical controls linked to the formation of glauconite minerals in marine sediments have been resolved in sufficient detail, the specific timing of glauconite formation remains uncertain. However, it may be possible to correct that if burial diagenesis does not alter the pristine composition and isotopic ratios of the glauconite grains, i.e., in a closed system where Rb and Sr are not mobilized or reset, and the formation of the glauconite and diagenetic illite proceeds fast and if both phases as well as any other detrital components can be physically separated, quantified and radiometrically dated, one could obtain meaningful ages for all mineral components, including authigenic as well as diagenetic and detrital phases.

It appeared that the glauconite grains from the fraction below 100 μm show evidence of oxidative alteration, since Fe-hydroxides were identified on the exterior and inside cracks within the glauconite grains. So, only glauconite grains larger than 100 μm in size without visible signs of alteration were separated using a neodymium magnet and prepared on standard mounts with a tweezer under an optical microscope. However, they noted that a direct comparison between the two datasets is challenging, because the chemical data were obtained from different areas within the glauconite grains. Further, spot diameters were different (~1.5 vs 50 μm) and data processing algorithms and standardization procedures differed among the two methods.

Their characterization of the glauconites from Northern Germany provides strong evidence that the complete transformation of smectite layers into glauconite layers in glauconite grains was reached within ~0.4 Ma, documenting another example of fast glauconite formation and maturation. By contrast, in modern settings glauconite formation is slower due to a lower stand of sea level and cooler icehouse conditions. So that suggests that shallow-water glauconite formation in the Late Cretaceous was much faster compared to modern rates of glauconite formation in shallow- vs deep-sea environments. The difference is related to unique Cretaceous seawater chemistry.

Over all, the authors have done an excellent job in addressing the rationale for glauconite age dating. They provide some very important discussion regarding the limitations and conditions under which reliable dates can be obtained. The English grammar is excellent and the figures are very well designed. For the most part, the references are very complete, although I might suggest the authors consider the following paper by Morton and Long: Rb-Sr dating of Paleozoic glauconite from the Llano region, central Texas, by John P. Morton & Leon E. Long, Geochimica et Cosmochimica Acta, Vol. 44, Issue 5, May 1980, Pages 663-672. These authors used ammonium acetate to remove loosely-bound Rb and Sr while leaving the tightly-bound components in place. 

Comments for author File: Comments.pdf

Author Response

We thank the reviewer for the very positive evaluation of our manuscript.

We fully agree with the reviewer and acknowledge the important comment. We added the missing reference and can totally contribute to the content.

 

Best regards,

The authors