Rising TOC content in sediments may be due to a change in sedimentary / hydrodynamics. For example, it is not uncommon in estuaries to find that elevated amounts of particulate organic matter are deposited with similar coarse sized/density mineral matter which in turn yields increases inTOC.

This process differs from the high TOC fine grain size relationship you mentioned which is based in part on organic matter coating and  high surface area of clays/silts. 

Dear Christopher H Vane

Thank you for the prompt response and it was very helpful!

Best regards 

Dear Nezha,

can you provide some pictures and more detail on the environnement. Are you workimg on recent dediment, is it an old rock, is it a soil?

Cheers

Dear Mr. Mejjad,

it is correct that fine-grained sediments are more frequently associated in time and space with organic matter (or in this case coal). Clay band, black band and underclay depositions can be taken as an example and document these interrelations already in the technical terms used to describe them. However, there are also nice examples demonstrating the abundance of alluvial fan deposits in coalified matter as shown by the classical paper of Heward (1978) on Spanish coal deposits in SEDIMENTOLOGY. Reworking of coalified matter can take place and also cause organic matter to be reworked and transported over a short distance. You should peruse very carefully the textbook of Diesel (1992) on coal-bearing environments and I am sure you will find some answers on many of your questions.

I wish you much sucess

Harald G. Dill

Dear Jérôme Juilleret 

Thank you for your response,

I am working on lagoon sediments, (it's a recent sediment), and I have found organic matter and a high radionuclide Activities in coarse sediments. 

More information in the link, (my paper)

Nezha Mejjad

http://rd.springer.com/article/10.1007/s10967-016-4714-8

Dear Prof. Harald G. Dill

Thank you very much for your extremely helpful and detailed response to my questions! I appreciate your time and effort in responding.

Best regards

N. Mejjad

Dear Nezha,

what was the protocole for the grain size analyse?

However, your sediment are not very "coarse", in my point of you coarse is attrivuted to particle >2mm.

Dear Jérôme Juilleret,

I had analyzed the granulometric distribution using grain size analyzer (Malvern, Mastersizer),

Yes, my sediments aren't very coarse,  are attributed to particle >2mm.

the problem with the methodology is that you are not only making a granulometric  analyse on mineral particules, but on the whole particules including pure organic one. So, part of the sand fraction is purely organic  (not only mineral) but made of organic matter (dead organism, plant residu,etc...)

Consequently a micromorphological characterisation of your samples is also important.

Organic matter content in the shelf and lagoon sediments, is mostly driven by the sedimentation and accumulation rate. In those settings, the other factors like surface area of particles, oxygen content of the overlying water and primary productivity are overshadowed by the sediment flux. Therefore, it is hard to get the grain size: organic matter relation in shallow settings.

Dear Mrs. Mejjad,

the granulometric studies are fine but I would place emphasis upon the mineralogy to find out if and how any interrelationship exists between the variation of radio nuclides, REE etc. and the mineralogy. To achieve this goal I would switch from the pure granulometric studies to the separation of grain size fractions. For the first run splitting up into < 2 µm, < 63µm, < 200 µm and bigger should be sufficient. You will get a good overview of where the "mineralogical culprits" are for the chemical anomalies. Samples should be taken not in a random way but , if possible, facies-oriented. I attached a cartoon to illustrate the possible facies which I do not know in your case (the figure attached is from Dill 2016 ( "Kaolin:  soil, rock and ore. From the mineral....... Earth Science Reviews, for further information). In this paper a lot has been described about OM in the various facies.

Doing radiophysical studies, chemical analytical work and mineralogy for each sample will get you closer to a solution and I am sure you will obtain a fine map of the facies with all the detailed results from the various disciplines.

I wish you all the best

H.G.Dill

Most of the coarse sediments are deposited in high-energy shallow water environments, in which waters stirring causes high oxygen content, high-  enough to permit significant bacterial activity, most of the organic matter to be oxidized, and the content of the sediment to be generally lower than 0.5%. One exception could be for wooden debris, that could be preserved as lignitic organic matter (type 3) deposited with coarse sandstone, and even conglomerates.

Regards,

M.L.B.

All sedimentary rocks contain organic matter with different amount. The amount of organic matter depends on organic productivity and preservation. The organic productivity depends on sunlight and water depth while preservation is related to presence of anoxic environment. to minimize bio-degradation.  It is not granted that small grain sediment contains more organic matter than coarse grain but usually fine grain black shale deposits in anoxic sedimentary environment.

Main part of organic matter in the coarse sediment is oxidized or depleted-hydrogen organic matter, the results of insitu-oxidation of organic matter at water-sediment interface and/or input of reworked organic matter from continents. In these cases, organic matter in chemical composition is equivalent to kerogen Type III and IV.   

 In general, the low energy facies (fine grained sediments) are linked to presence of organic matter but you may find some high energy facies (coarse grained sediment) rich in organic matter and this is possibly linked to rapid burial of organic material as in deltaic environments.

Dear Harald G. Dill 

Thank you very much, I really appreciate your help.

Best regards 

Thank you for your prompt answers,

In our  paper we had explain that : The presence of radionuclides and organic matter in important amount in the sand fraction greater than 710 µm, it  was well established that organic matter coatings onto mineral particles increase the adsorption capacity of dissolved radionuclides (1). 

I want to know more information about coating of organic matter on mineral grains,

(1) Kim Y, Kim K, Kang HD, Kim W, Doh SH, Kim DS, Kim BK (2007) The accumulation of radiocesium in coarse marine sediment: effects of mineralogy and organic matter. Mar Pollut Bull 54:1341–1350

Thank you in advance

N. Mejjad

General, the low energy facies (fine grained) contains more organic matter but you may find more organic matter in high energy facies (coarse grained) most possibly due to rapid burial of organic material in a vegetation rich channel-overbank deltaic environments in a very reducing condition.

Agree with Rzger valuable remark