Publication Date:
2022-05-25
Description:
Submitted in partial fulfillment of the requirements for the degree of
Doctor of Philosophy at the Massachusetts Institute of Technology
and the Woods Hole Oceanographic Institution February 2006
Description:
South American climate has undergone dramatic changes since the last glacial
period, as evidenced from Cariaco Basin (Venezuelan coast) and Peru Margin marine
sediment biomarker records. Compounds derived from vascular plant leaf waxes and
delivered to the marine sedimentary environment, including long-chain (C24-C32) nalkanoic
acids, were used as proxies for terrestrial vegetation type, aridity, and
atmospheric circulation. Marine biomarkers, such as sterols and phytol, were used to
reconstruct productivity in the Peru Margin upwelling zone, where sedimentary
conditions are not conducive to the preservation of foraminifera. Through the use of
organic molecular isotopic techniques and multi-molecular stratigraphy, a great deal can
be learned about communities of marine organisms and terrestrial plants that existed in
the past and the environments in which they lived.
Vascular plant leaf wax carbon and hydrogen isotopic records were generated
from n-alkanoic acids preserved in Cariaco Basin marine sediments. These records were
compared to previously established pollen and climate records and were found to parallel
local millennial-scale climate changes between the late Glacial and Preboreal periods,
which were characterized by migrations of the inter-tropical convergence zone.
Differences in δD between C16-C18 and C24-C30 n-alkanoic acids suggest a marine source
for the shorter chain lengths and a terrestrial source for the longer chains. Stacked δD
and δ13C records both exhibited isotopic enrichment during the late Glacial and Younger
Dryas periods and depletion during the Bølling-Allerød and Preboreal periods. If
interpreted as an aridity proxy, the δD record is in agreement with Cariaco Basin
sediment grey scale records, suggesting that the late Glacial and Younger Dryas were
more arid than the Bølling-Allerød and Preboreal periods. n-Alkanoic acid δ13C, which is
a proxy for C3 versus C4 plant type, indicates that C3 plants predominated in this area of
the tropics during warm and wet periods, such as the Bølling-Allerød and the Holocene,
and C4 plants proliferated during cooler and more arid periods, such as the Glacial and
Younger Dryas. The biomarker δ13C record agrees with pollen data previously developed
from Cariaco Basin sediments, confirming that leaf wax compounds preserved in marine
sediments can accurately record terrestrial vegetation changes.
Analytical methods utilizing stir bar sorptive extraction (SBSE) and thermal
desorption were developed and applied to investigate lipid organic matter in a suite of
alpine ice cores. These methods permit use of small volume (10-30 ml) samples, as
would be required for high-resolution down-core analyses. SBSE involves using a
polymer coated stir bar to extract organic matter from aqueous samples, after which it is
loaded directly into a thermal desorption unit and the organic matter transferred in its
entirety to a gas chromatograph inlet. To test these methods and the organic content of
tropical ice, post-industrial samples from two South American, two Asian, and one
African ice core were analyzed. Compounds identified in the modern ice core samples
included natural and anthropogenic biomarkers such as n-alkanes, n-alkanoic acids, nalkyl
amides and nitriles, polycyclic aromatic hydrocarbons (PAHs), and diterpenoids.
Variability in the distributions of these compounds between different cores demonstrated
that the lipid organic fraction in each core was representative of mostly local inputs. To
further investigate natural inputs, several pre-industrial samples were analyzed from the
Sajama ice core in the Andes and The Puruogangri core on the Tibetan Plateau. Inputs of
terrestrial vegetation combustion biomarkers such as PAHs, diterpenoids, and alkyl
amides were consistent with periods of enhanced aridity in each core. The results of this
investigation demonstrate the utility of the methodology, which could now be applied to
generate very high-resolution biomarker records from tropical ice cores.
Gas chromatography/time-of-flight mass spectrometry (GC/TOF-MS) was used to
generate a high-resolution, multi-molecular organic biomarker record from Peru Margin
sediments (~11oS, 252 m water depth) for the last 15 ka. Because of their position
beneath the oxygen minimum zone of a productive upwelling region, these sediments
contain a wealth of compounds that can be exploited as paleoclimate indicators. TOF-MS
and fast GC techniques allowed me to generate this record in a short amount of time and
without employing the traditional suite of purification techniques. Before about 9 ka,
organic carbon and biomarker concentration records exhibited similar variability,
implying a forcing mechanism that affected input and/or preservation of both marine and
terrestrial organic matter, such as large-scale climate change. Organic carbon and
biomarker abundances then systematically increased throughout the Holocene and
exhibited higher frequency variability, suggesting overall enhanced productivity from
rapidly evolving planktonic communities. Similar patterns of variability between
bacterial hopanol, sterol degradation product, and primary productivity biomarker records
suggest that the productivity biomarkers are recording sea surface and water column
processes, and are not significantly biased by sedimentary diagenesis. Low bound sulfur
content in lipid extracts and a lack of observed sulfur-containing compounds argue
against significant sulfurization and resultant biomarker sequestration in 1228D
sediments. Factor analysis provided a statistical means of separating terrestrial and
marine organic inputs, and reinforced the interpretations that very long chain n-alkanoic
acids (C30-C32) are terrestrially derived and sterol compounds primarily represent marine
algal inputs. In all, the biomarker records suggest millennial-scale changes in upwelling
strength superimposed on longer-term trends, with additional variability in contributions
from specific precursors, such as dinoflagellates. Terrestrial leaf wax compounds also
exhibited high-amplitude, millennial-scale variability, but with a different pattern of
change than the marine inputs. GC/TOF-MS was shown to be a useful tool for
generating high-resolution records of the type necessary to understand the relationships
between biomarkers in a complex and sensitive depositional environment such as the
Peru Margin.
Climate signals embedded in the Peru Margin biomarker records provided clues
as to the productivity and upwelling histories of the Peru Margin, as well as regional
terrestrial vegetation. Elevated concentrations of marine biomarkers suggest enhanced
upwelling and productivity from about 6.5 ka to the present on the Peru Margin, with
lower-amplitude millennial-scale variations occurring throughout this period. Enhanced
dinosterol abundances after 6.5 ka are consistent with greater occurrences and/or strength
of El Niño, while concurrently enhanced upwelling suggests a parallel increase in La
Niña activity. Similar timing of mid to late Holocene variability between Peru Margin
marine biomarker records, a faunal sea surface temperature record from the eastern
tropical Atlantic, and Andean paleoclimate records suggests strong climate links between
these regions of the tropics, likely driven by broad-scale changes in El Niño and the
Southern Oscillation (ENSO) and trade wind strength. The C30 n-alkanoic acid, which is
representative of vascular plant leaf wax inputs, exhibited millennial-scale variability
superimposed on longer-term trends that may be related to aridity, assuming fluvial
transport of terrestrial material. n-Alkanoic acid δ13C is generally enriched during
periods of enhanced leaf wax abundance, consistent with increased inputs of C4 plant
material at these times.
Description:
Funding for this research was provided by a
Schlanger Ocean Drilling Fellowship, which is part of the NSF-sponsored U.S. Science
Support Program (USSSP). An NSF grant to TIE (OCE-0402533) provided additional
funding for the research presented in Chapters 3-5. Funding for the Cariaco Basin
isotopic analyses was provided by the Frank and Lisina Hoch Endowed Fund and the
Woods Hole Oceanographic Institution Director’s General Discretionary Fund.
Keywords:
Paleoclimatology
;
Marine sediments
Repository Name:
Woods Hole Open Access Server
Type:
Thesis
Format:
17306684 bytes
Format:
application/pdf
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