From dbarclay@ACSU.BUFFALO.EDUTue Apr 29 15:06:52 1997 Date: Thu, 12 Dec 1996 20:07:24 -0500 From: David J Barclay Reply to: Research in Quaternary Science To: Multiple recipients of list QUATERNARY Subject: trace elements in marine vs freshwater silts Dear forumites, We have some silty sediment samples from Alaska and want to determine if they were deposited in a freshwater or marine environment. The samples are from deltaic bottomsets overlying soils and could represent either glacial damming of the fjord or a marine transgression (they are all from very close to sea level). I have a paper by Potter et al. from 1963 which suggests that marine and freshwater argillaceous sediments can be differentiated by their trace element composition. Does anyone have any recent references or experience in using such a technique in this way ? Do you think it could work ? We tried looking for microfauna in the samples but came up blank. Dave Barclay University at Buffalo Potter, P.E., Shimp, N.F. and Witters, J., 1963, Trace elements in marine and fresh-water argillaceous sediments, Geochimica et Cosmochimica Acta, v.27, p.669-694. From eesa@CADVISION.COMTue Apr 29 15:06:52 1997 Date: Thu, 12 Dec 1996 22:20:22 -0700 From: Dana Naldrett Reply to: Research in Quaternary Science To: Multiple recipients of list QUATERNARY Subject: Re: trace elements in marine vs freshwater silts This is an old debate which appears to have been settled much more clearly in the lithified materials than the unlithified. It also seems to be a somewhat cyclic debate that resurfaces every 10, 15 or 20 years. When I finished my PhD about 10 years ago, this was a hotly debated topic in the Quaternary community, because some of us were comparing microfossil techniques with geochemical techniques and sedimentological ones. The claim was that elelments such as boron, and vanadium were definitely indicative of marine environments. However, they are held by clay minerals (adsorbtion) and the amount of each element depends on the natural availability, the chemistry of attraction of the element to the clay lattice, and of course, the abundance of the clay mineral in the sediemnts. The short answer is that this was (and still appears to be) a lot of work doing what is a lot of quantitative research to produce an answer that is definitely not quantitative. As a sedimentologist and micropleontologist, I was very interested in seeing if the sedimentological evidence was backed up by micropaleo. It seems that it is, as long as you have some bugs to look at. I used forams and ostracodes, but others would also do. Associated with the sedimentological evidence of bedding types, there is also the question of whether the clays ar flocculated. Although this can happen in very low salinities (2 ppt), it might be worth looking at SEM views of the clays to see if they are flocculated. One thing I don't know is whether such structures preserve over a longer period of time. Have a look at Kate Kranck's papers (hope I spelled her name right). There are also lots of smaller bugs that might indicate freshwater/ marine origins, but you may have missed them because of their size. For example, what about diatoms, or some of the other fauna? Although the diatoms probably might not be in the hydrodynamic environment that silt is in, they might be trapped in some of the clay aggregates if they exist in your sediments. Email me directly if you want more detailed information. ----------------------------------------------------------------- Dana Naldrett, Ph.D., P.Geol., President Environmental Earth Science Associates Inc. Box 61209, 3630 Brentwood Road N.W. Calgary, Alberta, Canada T2L 2K6 email: eesa@cadvision.com "applied environmental research" ----------------------------------------------------------------- From ereinhar@IS.DAL.CATue Apr 29 15:06:52 1997 Date: Fri, 13 Dec 1996 10:12:33 -0400 From: Eduard Reinhardt Reply to: Research in Quaternary Science To: Multiple recipients of list QUATERNARY Subject: Re: trace elements in marine vs freshwater silts Dana Naldett wrote: >The claim was that elelments such as boron, and vanadium were >definitely indicative of marine environments. However, they are >held by clay minerals (adsorbtion) and the amount of each element >depends on the natural availability, the chemistry of attraction >of the element to the clay lattice, and of course, the abundance >of the clay mineral in the sediemnts. The short answer is that >this was (and still appears to be) a lot of work doing what is a >lot of quantitative research to produce an answer that is definitely >not quantitative. I have had very good success with using Sr isotopes (87Sr/86Sr) as a salinity proxy in foram, ostracod and mollusc shells. The great advantage of using Sr isotopes is that there are natural differences in the isotopic composition of marine and freshwater. Marine water has a 87Sr/86Sr ratio of 0.70918 in modern oceans, freshwater can be higher or lower than this value depending upon the lithology in the drainage basin. There are no vital effects associated with the uptake of Sr, so any bioprecipitate will record the 87Sr/86Sr value of the water in which the organism lived. If the 87Sr/86Sr ratio is analytically different from seawater, then this deviation can only be from dilution by freshwater (if diagenesis can be ruled out). The 87Sr/86Sr ratio in the shell can be related to an environmental salinity via a two component mixing equation if the concentration of Sr and the isotopic composition is known (or can be estimated) for the fresh and marine waters. The resolution of the salinity determination is largely dependant on the composition of the freshwater - the higher the concentration of Sr and the greater the difference in the 87Sr/86Sr ratio the better the resolution. In regard to the clays, any adsorbed Sr isotopes on the clays should reflect the 87Sr/86Sr ratio of the water in which they were deposited. Any deviation from the 87Sr/86Sr value of seawater should be due to freshwater dilution. The characters of Sr isotopes eliminates all the caveats associated with the use of trace elements. I don't know of anyone that has tried this specifically for a salinity determination (David I would be willing to give it a try). If anyone wants more specific information, or a preprint of a paper that uses Sr istopes as a salinity proxy email me directly. ______________________________________________________________________________ Dr. Eduard G. Reinhardt Dept. of Earth Sciences (902)494-2358 Dept office Life Sciences Bldg. 494-6889 Fax Dalhousie University 425-4367 Hm Halifax, Nova Scotia email: ereinhar@IS.Dal.CA B3H 3J5 CANADA www: http://www.carleton.ca/ ~ereinhar/EdHome.html ____________________________________________________________________________ __