Dear all,
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With best wishes,
Dominique
Date: Wed, 15 Aug 2007 11:32:23 +1200
Subject: Re: pollen concentration
From: "Christine Prior" <[log in to unmask]>
To: The archaeobotany mailing list <CDate: Wed, 15 Aug 2007 11:32:23 +1200
Subject: Re: pollen concentration
From: "Christine Prior" <[log in to unmask]>
To: The archaeobotany mailing list <[log in to unmask]>, "Dr.
Christiane Singer" <[log in to unmask]>
2.1 unnamed [text/plain] 3.49 KB Download
Hi Christiane,
No method is perfect and I have to confess that most of the time when I
(or 'we' if one of my lab techs does the separation) use the SPT to
separate pollen concentrates we get fractions that have mixtures of
palynomorphs and other plant material. One thing that I do to try to
clean up the pollen mixtures is to do the separations with SPT at a
variety of densities. I usually start with the first separation at ~2.0
sp.gr. to make sure there are no minerals and then keep centrifuging the
material which floats in SPT of decreasing densities (similar to
Vandergoes and Prior 2003, fig 3). If the sample looks like it has a lot
of fragmentary plant material, then I use a number of separations with
fairly fine control of the sp.gr. (1.4, 1.35, 1.30, 1.25, 1.20, 1.15,
1.10...) to see if there is any one fraction which has more pollen than
anything else. I sometimes also repeat a separation at the same sp. gr.
to get a cleaner separation in the centrifuge tube. (Yes, the method is
fiddly and takes a lot of time.)
Much of the time this works well, but with some sediments no matter what
you do there is so much fragmentary plant material that has the same
density as the pollen in that fraction it can't be cleaned up any further.
I take a digital photo through the microscope to have a record of how
'clean' the pollen concentrate is before I combust it for AMS dating, but
I'm willing to tolerate the plant fragments following the reasoning that
Duncan Hale put forward: any plant fragments remaining in the pollen
concentrate are quite likely to be contemporary with the pollen. Also,
since sporopollenin is a more carbon-dense molecule it is going to
contribute more to the 14C age of the sample than the cellulose of the
plant fragments. As long as truly different material like bits of
charcoal are removed in an earlier sp.gr. step, then I think that a few
plant fragments are probably all right.
I hope this helps.
Regards,
Chris
******************************************
Christine A. Prior, Ph.D.
Rafter Radiocarbon Lab Team Leader
National Isotope Centre, GNS Science
30 Gracefield Road, Lower Hutt, New Zealand
tel: +64 4 570-4644 fax: +64 4 570-4657
http://www.RafterRadiocarbon.co.nz
******************************************
"Dr. Christiane Singer" <[log in to unmask]>
Sent by: The archaeobotany mailing list <aR>, "Dr. Christiane Singer"
<[log in to unmask]>
2.1 unnamed [text/plain] 3.49 KB Download
Hi Christiane,
No method is perfect and I have to confess that most of the time when I
(or 'we' if one of my lab techs does the separation) use the SPT to
separate pollen concentrates we get fractions that have mixtures of
palynomorphs and other plant material. One thing that I do to try to
clean up the pollen mixtures is to do the separations with SPT at a
variety of densities. I usually start with the first separation at ~2.0
sp.gr. to make sure there are no minerals and then keep centrifuging the
material which floats in SPT of decreasing densities (similar to
Vandergoes and Prior 2003, fig 3). If the sample looks like it has a lot
of fragmentary plant material, then I use a number of separations with
fairly fine control of the sp.gr. (1.4, 1.35, 1.30, 1.25, 1.20, 1.15,
1.10...) to see if there is any one fraction which has more pollen than
anything else. I sometimes also repeat a separation at the same sp. gr.
to get a cleaner separation in the centrifuge tube. (Yes, the method is
fiddly and takes a lot of time.)
Much of the time this works well, but with some sediments no matter what
you do there is so much fragmentary plant material that has the same
density as the pollen in that fraction it can't be cleaned up any further.
I take a digital photo through the microscope to have a record of how
'clean' the pollen concentrate is before I combust it for AMS dating, but
I'm willing to tolerate the plant fragments following the reasoning that
Duncan Hale put forward: any plant fragments remaining in the pollen
concentrate are quite likely to be contemporary with the pollen. Also,
since sporopollenin is a more carbon-dense molecule it is going to
contribute more to the 14C age of the sample than the cellulose of the
plant fragments. As long as truly different material like bits of
charcoal are removed in an earlier sp.gr. step, then I think that a few
plant fragments are probably all right.
I hope this helps.
Regards,
Chris
******************************************
Christine A. Prior, Ph.D.
Rafter Radiocarbon Lab Team Leader
National Isotope Centre, GNS Science
30 Gracefield Road, Lower Hutt, New Zealand
tel: +64 4 570-4644 fax: +64 4 570-4657
http://www.RafterRadiocarbon.co.nz
******************************************
"Dr. Christiane Singer" <[log in to unmask]>
Sent by: The archaeobotany mailing list <aR
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