The South China Sea (SCS) is the largest marginal basin off the Asian continent and its
surface hydrography as well as sedimentation within the basin are strongly influenced
by the SE Asian monsoon and eustatic sea-level changes. The rapidly accumulating
sediments in this basin are therefore a unique and sensitive monitor of past variations in
monsoonal climate, ocean-continent linkages, deglacial sea-level, and marine
biogeochemical processes and their relationship to climate change. This thesis
contributes to our understanding of these key aspects of glacial-interglacial
palaeoceanography by presenting multi-proxy organic and inorganic sediment
geochemical records from a large number of gravity cores and surface sediment samples
from throughout the SCS. Records of deglacial sea surface temperature (SST) and
summer monsoon variability point to a close coupling with the climate of the circum-
North Atlantic realm, whereas sedimentological changes associated with variations in
winter monsoonal intensity suggest a concordant deglacial development with SST
changes in the open equatorial Pacific. Together, these records demonstrate a complex
interaction of Northern and Southern Hemisphere influences on the climate of this
region. The biogeochemical cycles of nitrogen and carbon in the open SCS as reflected in
their time-varying isotopic composition are not significantly affected by monsoonal
climate or the specific sedimentological and geographical setting of this marginal basin
but appear instead to reflect the regional marine biogeochemistry. On the other hand,
variations in basin configuration related to eustatic sea level changes leave a distinct
inprint in the rate and geochemical composition of terrigenous sediment supply to the
SCS, which are used to constrain the deglacial history of sea level rise and its impact on
near-shore sedimentation.
A comparison of various geochemical and micropalaeontological methods to
estimate palaeo-sea surface temperatures (SSTs) demonstrates a quantitative agreement
between alkenone (U[sup K']₃₇), foraminiferal Mg/Ca, and foraminiferal tranfer function FP-
12E SST estimates in recording an annual average cooling of 2-2.5 °C of the tropical
southern SCS during the last glacial period. In contrast, the foraminiferal transfer
functions RAM and SIMMAX show an annual average glacial cooling of only <1 °C or
no cooling at all, respectively. Both the U[sup K']₃₇ and the FP-12E SST estimates, as well as the
planktonic foraminiferal δ¹⁸O values, indicate an abrupt warming (ca. 1°C in <200 years)
at the end of the last glaciation, which occurs synchronously (within dating
uncertainties) with the Boiling transition as recorded in the Greenland Ice Sheet Project 2
(GISP2) ice core. The nitrogen isotopic composition (δ¹⁵N) of surface and down-core sediments
spanning the last glacial-interglacial cycle from the entire South China Sea (SCS) has a
narrow range (~3.0 to ~6.5‰) with no correlation with discernible
palaeoclimatic/oceanographic changes. The absence of any correlation with
reconstructed (glacial-interglacial) changes in primary production, terrigenous input,
and/or sea level related basin configuration is attributed to the complete consumption
of nitrate during primary production in this marginal basin during at least the last
140,000 years. This, in turn, implies that the δ¹⁵N of the nitrate used during primary
production remained approximately constant during the last climatic cycle. The
proposed scenario infers an unchanged nitrogen isotopic composition of the subsurface
nitrate in the western Pacific between glacial and interglacial stages as well as during
terminations and thus constrains proposed changes in the oceanic N inventory.
The carbon isotopic composition of organic matter (δ¹³C[sub org]) in sediment cores
from throughout the open SCS covering the last 220 kyr shows higher values (around -
19.5 to -20.5‰) during glacial stages, while lower values (around -21 to -22.5‰) are
characteristic of interglacials. Following well established procedures, the δ¹³[sub org] records
are converted to local pCO₂ estimates. Together with other low-latitude δ¹³C[sub org]-pCO₂
estimates from the literature, the results show that δ¹³C of bulk sedimentary organic
matter cannot be used to hindcast past changes in local CO₂(aq). Three crucial pitfalls are
identified, namely unreasonable absolute pCO₂ estimates, a lack of correlation between
δ¹³C[sub org]-pCO₂ estimates and sedimentary proxies of upwelling intensities, and
unexplicable discrepancies in the temporal evolution of atmospheric pCO₂ as recorded
in ice cores and marine sedimentary δ¹³C[sub org]-pCO₂ estimates, which caution the use of
δ¹³C[sub org] as an unambiguous tracer of dissolved molecular CO₂ in the surface ocean. This
calls for a re-evaluation of the role of the low-latitude ocean on temporal changes in
atmospheric CO₂.
Based on the sedimentological and geochemical variability at core sites along a
transect across the outer Sunda Shelf and the continental slope covering the last 20
thousand years, four intervals of significant depositional changes are identified, which
closely correlate with environmental shifts on the central shelf. Thus, sedimentation on
the southern SCS margin and slope appears to be mainly controlled by the
interrelationship between sea level, shelf palaeo-physiography and sediment supply.
Because of these complex interactions, the interpretation of nearshore sedimentary
records as unequivocal recorders of local climate change (e.g., SE Asian monsoon) is not straightforward. Variations of shelf physiography and sea level need to be included in
future palaeoceanographic studies.
Major element variations in a core from the northern SCS are used to infer
downcore changes in bulk sediment composition, which are interpreted in terms of
deglacial changes in monsoonal climate. Thus, Si / A1 reveals a two-step deglacial
weakening of winter monsoonal winds, possibly linked to the temporal development of
equatorial Pacific SSTs. In contrast, the reconstruction of changes in summer monsoonal
precipitation, and thus river runoff, based on major element/A1 ratios (and grain size
indices) is compromised by the antagonistic effects of the deglacial retrogression of the
river mouths and changes in fine-grained fluvial sediment delivery due to increasing
summer monsoonal precipitation. Nevertheless, an early Holocene peak in summer
monsoon intensity can be inferred from a marked clay maximum immediately following
the Younger Dryas.
Finally, the rapid drop in the supply of terrigenous organic matter to the open
SCS also corresponds with a rapid increase in sea-surface temperature during the last
deglaciation, corresponding with the Boiling warming at 14.7 kyr B.P. This reflects a
rapid retrogression of local rivers due to rapid sea-level rise, strongly implying that the
Boiling warming and the onset of melt-water pulse (MWP) 1a are synchronous. This
phase relation contrasts with the widely cited onset of this melt-water pulse 1a at ca. 14
kyr B.P.
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:BVAU.2429/13541 |
| Date | 11 1900 |
| Creators | Kienast, Markus |
| Source Sets | Library and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada |
| Language | English |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Relation | UBC Retrospective Theses Digitization Project [http://www.library.ubc.ca/archives/retro_theses/] |
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