Publication Date:
2022-05-25
Description:
Author Posting. © Elsevier B.V., 2008. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 72 (2008): 4577-4594, doi:10.1016/j.gca.2008.06.021.
Description:
Understanding the supply and preservation of glycerol dibiphytanyl glycerol tetraethers
(GDGTs) in marine sediments helps inform their use in paleoceanography. Compound-specific
radiocarbon measurements of sedimentary alkenones from multiple environments have been
used to gain insight into processes that affect paleo-temperature reconstructions. Similar
analyses are warranted to investigate how analogous processes affecting GDGTs impact TEX86
paleotemperatures. Here we present radiocarbon measurements on individual GDGTs from
Bermuda Rise and Santa Monica Basin sediments and discuss the results in the context of
previous studies of co-depositional alkenones and foraminifera. The 149 C contents of GDGTs and
planktonic foraminifera in Bermuda Rise are very similar, suggesting a local source; and TEX86-
derived temperatures agree more closely with foraminiferal temperatures than do
temperatures. In contrast, GDGTs in Santa Monica Basin are depleted in 1412 C relative to both
alkenones and foraminifera, and TEX86 temperatures agree poorly with known surface water
values. We propose three possible factors that could explain these results: (i) GDGTs may be
labile relative to alkenones during advective transport through oxic waters; (ii) archaeal
production deep in the water column may contribute 1416 C-depleted GDGTs to sediments; and (iii)
some GDGTs also may derive from sedimentary archaeal communities. Each of these three
processes is likely to occur with varying relative importance depending on geographic location.
The latter two may help to explain why TEX86 temperature reconstructions from Santa Monica
Basin do not appear to reflect actual sea surface temperatures. Terrigenous GDGTs are unlikely
to be major contributors to Bermuda Rise or Santa Monica Basin sediments, based on values of
the BIT index. The results also indicate that the crenarchaeol regioisomer is governed by
processes different from other GDGTs. Individual measurements of the crenarchaeol regioisomer
are significantly depleted in 1424 C relative to co-occurring GDGTs, indicating an alternative origin
for this compound that presently remains unknown. Re-examination of the contribution of
crenarchaeol regioisomer to the TEX86 index shows that it is a significant influence on the
sensitivity of temperature reconstructions.
Description:
This work was supported by the David & Lucille Packard Foundation and by NSF-OCE-0241363 and EAR-0311937 (to A.P.).
Repository Name:
Woods Hole Open Access Server
Type:
Preprint
Format:
application/pdf
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