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Sedimentary biogeochemistry and palaeoceanography of the South China Sea during the late Pleistocene

University of British Columbia

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.

Thesis/Dissertation

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2009-10-02

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http://hdl.handle.net/2429/13541

Atmospheric Science

University of British Columbia

UBCV

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