Dear Mrs. Nag,

grain size frequency curves, plotted in different ways have been used for genetic interpretations since decades but mostly with moderate success because the hydrodynamic conditions are too complex and the sediment too different as to the morphology and density of mineral grains. It would be too simple an answer to explain  the graph by the mode of transport and the shape of the drainage system from the fan apex to the lake. I can only suggest to split you sediment batch and try and investigate the grain size distribution for individual grain size groups, e.g., heavy minerals vs. light minerals. I have already done this with success.

Best regards

H.G.Dill

Hello, before answering a question like this one, more information is needed:

- which methods wer used to produce the cumulative frequency curve, including all pretreatment of the sample?

- what is the mineralogic and/or chemical composition of the sediments?

Without this information I see a bimodal distribution with one mode around 11.5 phi and an other around 9.5 phi.

best regards

Uli

Either a numeric / lab error, or if it is real, a preferential removal / stripping of 10 -11 micron? materials

Thank you all for your valuable suggestions.

@ Mr. Ulrich Von Grafenstein - Standard procedure is followed for the treatment of the samples and the frequency curve is generated by software GRADISTAT. Till now I dont have the mineralogical composition, which i will do in future for sure... but I have percentage of clay and silt.....the deposit is dominated by clay with minor silt content....

Can this bimodal distribution be a result of two events? for example firstly deposition of clay sediments and then some event may have caused resuspension of the sediments which then redeposited and may be this 2nd event have caused stripping of 10-11 phi size as suggested by Mr. Keith Henry Nicholls.

My question regarding methods was mainly motivated by this intriguing lack of material between 10 PHI  and 11 PHI (between roughly 1 micron and 0.5 micron). As fare as I know there is no "standard procedure" to do grain size analysis in this very fine range. Its a size where almost every subtle change in concentration, suspension liquid, suspending method plays a big role, as well as the method used. So, please give us a bit more than "standard procedures". I suppose an optical  particle counter was used and the recommended sample preparation for the used device, but I suppose there is more than one machine on the market  which is supposed to produce such results (I did not follow the development over the last decade).

Whatever method used, there are plenty of different explanations for bimodal freqeuncy distribution, including the two-event story or the removal of a certain size fraction, or two different sources (paritcles produced in the lake versus particles transported from outside by wind and/or rivers, two different rivers etc.etc.) but without more information there is still a strong doubt that the cumulative curve just shows an measurement artefact.

Dear colleagues,

bulk grain size analysis, expressed as cumulative frequency curve, is a good tool which successfully can be used in combination with other disciplines, mapping of drainages systems, heavy mineral analysis, sequential grain size anaylsis and morphometric studies..... etc.  Take a look at the complex processes of transport and deposition and the wealth of mathematical equations put forward to compute and interpret laminar and turbulent flow, which should be sufficient to describe the complexity. I use this method never as a stand-alone procedure but only in context with others to get a clue about the processes operative in a special depositional environment. Using it as an isolate method is using one equation with several unknown parameters. It will not provide us with a reliable solution. That is basic mathematics.

Best regards

H.G.Dill

Thanks everyone for all the suggestions

Regards

Debarati

Cumulative frequency curves show overall grain size distribution of the sediments. Most of the studies in this direction were done for sandy sediments  that reflect the mechanism of transportation, sorting and related parameters. We are able to carry out particle size analysis of silty sediments also and they provide much of the information related to sediment transport and sorting. Regarding the lake sediments, I am not very sure, because sedimentation takes place by suspension process there. The bimodal distribution may be because some of the sediments were received  by fluvial or glacial processes in the lake. So you must ensure about the sources of the sediments in the lake i. e. whether they have been added by some other processes or settled from the atmosphere as aerosols.

Dear Debarati

The sedimentation rate of  clayey sediments is usually low. Therefore, how you sampled the sediment does matter a lot. To scrape only surface sediments or to take sediments in certain thickness (vertically) probably show different grain size distributions and thus deduce different interpretation.  By knowing your research purpose and then the way of sampling, it will be easier to interpret the grain size distributions. As to your grain size distribution, any explanation will be accepted, however,  will not be sure  which one should be taken. 

By the way, which brand of laser-sizer is used to analyze your sample?

best regards

Eason Hong

The textural signal alone is not easy to decipher because does not reflect univocal environmental influences. To avoid misinterpretation, sedimentological studies typically involve numerous samples along the area of the sediment dispersal system (source, sediment transport and sediment sink zones). Subsequent statistical analyses show the characteristics of the whole granulometric data set, on which the interpretation is based to the light of the existing geo-morphological context. All this to say that the interpretation of a single sample becomes just an exercise that could produce results far from reality.

Regards

Paolo Tortora

curve actually says that there absolutely no particles in this range, however, very near by there are plenty. This should be a strong signal that there is something wrong with instrument or data processing.

Yes, this is certainly a remarkable distribution..  What methodology did you use to determine it?  Was there any pre-treatment?  Normally, we use a pre-treatment with Hydrogen peroxide (a dash in a conical flask containing 25 grams of dried sample in distilled water.  The hydrogen peroxide breaks the bonds between clay minerals and organic matter and makes a remarkable difference to the grain size distribution in tropical soils.  If you check my publications, you will find one on using with and without pre-treatment on samples as a method of measuring aggregate stability in soils.  Another possibility is that the sediments is a mixture from two or three sources or transport processes.

It would seem that you have a mixture of two sediments, one with grain sizes with phi values below 9,9 phi units and one (51%) with grain sizes with phi values higher than 11.2 phi. If you plot the data on log probability paper (see My papers in the Journal of Sedimentary Petrology from the early sixties), you will be able to determine the degree to which the two sediments were transported in a fluid medium and whether  there was a shotage of available grades of material available for transport at the low end of the phi scale.