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GEOCOSMO  October 2005

GEOCOSMO October 2005

Subject:

Request for assistance: CRONUS-Earth project

From:

Nat Lifton <[log in to unmask]>

Reply-To:

In situ cosmogenic isotopes <[log in to unmask]>

Date:

Wed, 12 Oct 2005 22:58:20 -0700

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (75 lines)

Dear Colleagues -

My apologies for cross-posting.

I am writing to request your assistance with my portion of the Cosmic- 
Ray prOduced NUclide Systematics on Earth (CRONUS-Earth) project. A  
major part of my effort for this project is to use in situ cosmogenic  
14C (in situ 14C) from geomorphic surfaces at secular equilibrium to  
test the validity of current production rate scaling models (Desilets  
and Zreda, 2003; Dunai, 2000, 2001; Stone, 2000; Lal, 1991; Lifton et  
al., in press) and models accounting for geomagnetic field  
fluctuations over geologic time scales (Desilets and Zreda, 2003;  
Dunai, 2001; Pigati and Lifton, 2004; Lifton et al., in press). These  
models have never been thoroughly tested empirically using  
significant numbers of geologic samples, due to the scarcity of sites  
with well-established chronologies. To accomplish this, I plan to  
analyze in situ 14C from surfaces at secular equilibrium from as wide  
a range of altitudes and latitudes as possible.

Unlike other commonly-used in situ cosmogenic nuclides, 14C has a  
short half-life that allows attainment of secular equilibrium, or  
“saturation,” in approximately 25 kyr. Furthermore, 14C loss from  
decay far outstrips loss from erosion in many geomorphic settings.  
The measured concentration of in situ 14C in these situations is  
therefore typically only a function of its integrated average  
production rate. These aspects of the in situ 14C system make a wide  
range of landscape features suitable for production rate determinations.

Surfaces for which in situ 14C should be at secular equilibrium can  
be identified by prior measurements of long lived or stable  
cosmogenic nuclides such as 10Be, 26Al, or 21Ne, as well as  
geomorphic and geologic indicators of antiquity. Many of the  
contributors to both CRONUS-Earth and CRONUS-EU, as well as other  
cosmogenic researchers, have samples archived for which these  
measurements have already been made.

I am therefore asking if any of you would be willing to provide  
samples which meet these criteria. I would greatly appreciate any  
help you could provide! Most of my focus will be on quartz, but we  
have also recently developed an extraction procedure for olivine as  
well, enabling analysis of basalts. I would need about 15 g of quartz  
if it's already separated, or enough raw sample to yield that much  
after etching in HF/HNO3. I typically separate and analyze the  
250-500 um fraction. For olivines, we typically separate the 250-1000  
um fraction, to allow greater yields of olivine from the basalts.  
Again, I would need enough sample for 10-15 g of olivine, either  
already separated (probably not too likely - that's a lot!), or  
enough rock to yield that much.

I hope to analyze approximately 100 of these archived samples over  
the next 4 years or so, selected to give broad global coverage of  
altitudes and latitudes, and to account for geologic uncertainty in  
exposure histories not detectable in the long-lived or stable nuclide  
signal (e.g., brief episodes of erosion or burial).

Again, any help you could provide will be greatly appreciated! Many  
thanks in advance for your consideration.

Best regards -

Nat Lifton

<<<<<>>>>>==--==<<<<<>>>>>==--==<<<<<>>>>>==--==<<<<<>>>>>==-- 
==<<<<<>>>>>

                           NATHANIEL LIFTON

Research Scientist                        OFFICE (MSG): (520) 626-3251
Geosciences Department                    LAB: (520) 626-8053
University of Arizona                     FAX: (520) 621-2672
Tucson, Arizona 85721-0077 USA            E-MAIL: [log in to unmask]

<<<<<>>>>>==--==<<<<<>>>>>==--==<<<<<>>>>>==--==<<<<<>>>>>==-- 
==<<<<<>>>>>

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