Hi Dan:

I hope life is treating you well! It sounds like you're working on some great
projects now that you're back in Sweden.

I did a similar study on a sediment core from one of my lakes on Baffin
Island, which never got published. The sediments consisted of 1.2m of Holocene
gyttja, almost pure diatoms, with a wet density of 1.07 and a dry density of
0.11 g / cc. I measured D/H ratios of pore water at about 10 depths throughout
the Holocene section, and profile was constant (-103 +/- 2 permil). We know
from cellulose d18O data that the lakewater isotopic composition of lakewater
has increased by at least 2 permil over the past few thousand years, but this
is not recorded in the porewater D/H.

I tried to reconcile this observations by modeling isotopic diffusion of water
through the gyttja using Fick's second law. In the end I determined that it
doesn't take very long for any isotopic perturbations to be smoothed out
through the 1.2 m of sediment. I used a diffusivity of DHO in H2O of 1.2 cm2 /
sec, and assumed the core was very permeable (it's water content is >90%). In
this case, a change of 10 permil in the lake will be smoothed to a 1 permil
gradient in porewater in 50 years. Even if I'm off in my estimate of the
permeability of the core by a factor of 10, there's not much chance of seeing
1000-year-old lakewater isotope values in cores where the sediment is as
low-density as in the site I analyzed.

So in order to use this approach, you need to be sure your sediments are truly
impermeable. What makes you think they're impermeable in your site? Is the
clay content very high?

Good luck on your this -- clearly it works in Dan Schrag's ocean cores, and it
should work in lakes where the clay content is high enough. I'm looking
forward to hearing what your results are.

All the best,

Peter

Dan Hammarlund wrote:

> Dear isogeochemists,
>
> At a site in southern Sweden we have recently obtained oxygen and hydrogen
> isotope records of pore-water extracted from Holocene sediments in a small
> lake. As the sediment profile consists of algal gyttja with very low
> permeability, we believe that the data may record long-term changes in the
> isotopic composition of ambient lake water. Does anybody know of similar
> studies or useful references, especially with respect to post-depositional
> diffusion and other complicating processes? Comments and ideas are
> gratefully received.
>
> Dan
>
> *****************************************************************
> Dan Hammarlund
> Department of Quaternary Geology
> Lund University
> Tornav. 13                      Phone:  +46 46 222 79 85
> SE-223 63 Lund                  Fax:    +46 46 222 48 30
> Sweden                          E-mail: [log in to unmask]
> *****************************************************************

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