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Fig. 1
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Holocene Black Sea record with coccolith layers (Unit I; final ~2600 cal yr BP), the dark sapropel (Unit II; 3000-7500 cal. yr BP) and gray organic carbon-lean lacustrine Unit III sediments (>7500 cal. yr. BP)
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Exploration of Fossil DNA-based Geobiologic Archives to Reconstruct Hydrologic Changes in the Black Sea during the Holocene
Collaborators:
Timothy Eglinton (WHOI, Marine Geochemistry Department) and Liviu Giosan (WHOI, Geology Department)
Recent
studies have demonstrated that intact fossil DNA can be recovered and amplified
from anoxic marine sediments, including those underlying the Black
Sea. The information encoded in this previously untapped
geobiologic archive is unrivalled in terms of providing information on past
microbial community structure in the water column, and on how it has varied in
response to past climate-induced environmental changes. Applied in conjunction
with molecular and isotopic information derived from lipid biomarkers, the
potential exists to reconstruct past climate-induced changes in the water
column conditions in unprecedented detail. In this project, we developed
parallel, high temporal resolution (50-100 yr) DNA and lipid biomarker based
stratigraphic records of water column and microbial community structure in the Black Sea spanning key periods of the Holocene.
We targeted molecular markers for microbial communities dwelling at the
surface (algal primary producers), the suboxic layer (marine Crenarchaeota),
and the sulfidic chemocline (obligate anoxyphotolithotrophic GSB) in order to
provide information on past hydrologic change in the Black
Sea. Specifically, we targeted markers that contributed to our
understanding of past surface water temperature, salinity and stratification. A
key aspect of the project is to perform side-by-side phylogenetic and lipid
biomarker analyses in order to carefully assess the validity of utilizing the
former as proxies of a given biological input. As the corollary to this,
we also assessed the influence of species changes in the putative
biological precursors on both the distribution and isotopic composition of
corresponding lipid biomarkers.
For example, we performed a high-resolution
stratigraphic analysis of ancient haptophyte DNA to study the Holocene
succession of haptophytes as sources of the alkenones in the Black
Sea (Fig 1). Haptophytes related to brackish Isochrysis
spp. were the initial sources of alkenones, and appeared immediately after the
onset of sapropel deposition (~ 7550 years before present [a BP]) (Fig. 1B). As
salinity increased, Isochrysis-related haptophytes were slowly replaced
by a complex suite of E. huxleyi strains as sources of alkenones (Fig.
1B, C). Our paleogenetic data showed that E. huxleyi colonized the Black Sea shortly after the onset of sapropel deposition,
~ 4000 years earlier than previously recognized based on their preserved
coccoliths (Fig. 1A). The absence of coccoliths in Unit II (Fig. 1A) is most
likely due to dissolution effects. Closely related E. huxleyi strains
(Fig. 1C) were the most likely source of the previously reported abundant and
unusual C36 di-unsaturated “Black Sea
alkenone” (Fig. 1D). Strong haptophyte species and strain-specific effects were
observed on the level of unsaturation of alkenones which resulted in spurious
alkenone-derived SST estimates before 5250 a BP (Fig. 1E). In contrast, from
~5250 a BP onwards a relatively stable haptophyte assemblage dominated by a different
suite of E. huxleyi strains (Fig. 1C) yielded robust alkenone-SST values
and indicated a gradual cooling from 19 ºC to ~15 ºC at the top of the record
(~450 a BP) (Fig. 1E). The results of this work are now published in Earth
and Planetary Science Letters (Coolen et al., EPSL 2009).
In
addition, we investigated whether deuterium isotopic values (δD ‰) of alkenones
can be used as reliable proxies to reconstruct past sea surface salinities
(SSS) in the Black Sea (van der Meer et al.,
2008). δD values of the Black Sea alkenone, which was produced only by closely
related E. huxleyi strains, paralleled the expected increase in salinity
of surface waters attending the invasion of Mediterranean waters (Fig. 1F). In
contrast, the δD values of the more general C37 alkenones indicated a spurious
abrupt decrease in salinity. Paleogenetic analyses showed that this divergent
hydrogen isotopic behaviour of the C37 alkenones coincided with the shift from Isochrysis
to E. huxleyi as the main precursors of these more common alkenones
(Fig. 1B), indicating species-level variations in hydrogen isotopic
fractionation of C37 alkenones. The latter demonstrates that “species” effects on isotopic
compositions can be detected using paleogenetics. Thus, paleogenetics can be
used to search for candidate organisms which remained constant sources of
biomarkers, and this information can in return be used to refine biomarker
assessments of paleoenvironmental conditions.
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