Plio-Pleistocene Climate Variations In Foraminiferal Stable Isotope Records At ODP Site 1143, South China Sea

This thesis discusses paleoceanography of the southern South China Sea (SCS) over the last 5 million years (myr) on the basis of stable oxygen and carbon isotopes on foraminifers (δ¹⁸O andδ¹³C) from Ocean Drilling Program (ODP) Leg 184. Isotope measurements were done in the Laboratory of Marine Geology of Tongji University on four to eight foraminifers in a total of 1992 samples from 0-190.77 mcd (meter composite depth) of Site 1143, including the benthic species Cibicidoides wuellerstorfi and Uvigerina peregerina and the planktonic species Globigerinoides ruber, Globigerinoides obliquus and Pulleniatina obliquiloculata. Experimental results show that, in the southern SCS, theδ¹⁸O values of Uvigerina peregerina are statistically 0.614%0 higher. than that of Cibicidoides wuellerstorfi in a given sample, and theδ¹³C values statistically 0.692%0 lower.Based on the benthic foraminiferδ¹⁸O, a 5-myr astronomical timescale for the west Pacific Plio-Pleistocene was established using an automatic orbital tuning method. The tuned Brunhes/Matuyama paleomagnetic polarity reversal agrees well with an age of 0.78 Myr dated previously. The tuned ages for several planktonic foraminifer bio-events also agree well with published dates, and new ages for some other bio-events in the South China Sea were also estimated.The coherency and phase relationships between the isotopic series (benthicδ¹⁸O andδ¹³C, planktonicδ¹⁸O andδ¹³C) of Site 1143 and the concentration of orbital variances (ETP) over the last 5 myr were studied using spectral, cross spectral and filtering analyses. The coherencies and phases of the four isotopic series demonstrate that the Plio-Pleistocene climate in the southern SCS recorded in the isotopes responds linearly to the orbital forcing at the obliquity and precession bands. The four isotopic responses at the 41-kyr obliquity and 23-kyr precession bands are synchronous in the southern SCS, whereas at the 19-kyr precession band, the responses are asynchronous. The opposite phases of the -δ¹⁸O andδ¹³C relative to ETP at the 100-kyr eccentricity band indicate that the responses of the -δ¹⁸O andδ¹³C to orbital forcing are opposite at the eccentricity band. The phase shifts of the isotopic variables relative to the benthic -δ¹⁸O at Site 1143 may arise from the ENSO events which have been frequently occurring along the equatorial Pacific. The digital filtering of both benthic and planktonicδ¹⁸O at the 100-kyr and 41-kyr bands demonstrates the existence of the mid-Pleistocene transition. The event appears to occur at～0.5 Ma in the planktonicδ¹⁸O, later than that in the benthicδ¹⁸O, and is not obvious in theδ¹³C records, indicating its limited impact on these climatic variables even from a single place. Accordingly, the transition of the dominant climatic cycles at～0.9 Ma can be interpreted as a stepwise but not an abrupt process. The filtering results of the four isotopic series further demonstrate that the 100-kyr climate cycle had strengthened after the amplification of the Northern Hemisphere ice sheet around 3.5 Ma ago, and this cycle became amplified and sustained in the late Pleistocene. High coherencies of theδ¹⁸O with the truncated insolation at the 404-kyr and 100-kyr eccentricity bands suggest that theδ¹⁸O response prefers warmer portions of the insolation cycle to colder portions. The 404-kyr cycle in the ETP changes to 460-kyr cycle in theδ¹³C records, and they are coherent with each other at the 404-460 band. The lead relationship of theδ¹³C relative to -δ¹⁸O may indicate that the 100 kyr cycle did not arise from ice sheet dynamics; instead, it was probably the response of the global carbon cycle that generates the eccentricity signal by causing changes in atmospheric carbon dioxide concentration. The abundant semi-precession cycles in the time series of Site 1143 further highlight the importance of the tropics in the global climate changes. The CWT analysis reveals the 404-kyr cycles in theδ¹³C records. However, in the Pliocene and early Pleistocene, the strongest of this low-frequency (the ridge) usually points to periods close to or lower than 414-kyr, whereas in the late Pleistocene, it points to periods larger than 414-kyr. Cross CWT analysis further demonstrates nonstationary phases of the isotopic series relative to ETP at the three primary Milankovitch bands in the last 5 myr. The instantaneous phases between benthic -δ¹⁸O andδ¹³C at the precession band display a prominent 128-kyr period, probably the cyclicity of the nonstationary climate close to the eccentricity.A comparison of the benthicδ¹⁸O between the Atlantic ODP Site 659 and the East Pacific Site 846 and West Pacific Site 1143 reveals that the Atlantic-Pacific difference in the benthic oxygen isotope (△δ¹⁸O_Atl-Pac) display an increasing trend in three time intervals: 3.6-2.7 myr, 2.7-2.1 myr and 1.5-0.25 myr. Each of the intervals begins with a rapid negative shift in△δ¹⁸O_Atl-Pac, followed by a long period with an increasing trend, corresponding to the growth of the Northern Hemisphere ice sheet. This implies that all the three intervals of ice sheet growth on the Northern Hemisphere were accompanied at the beginning by a rapid relative warming of deep water in the Atlantic as compared to that in the Pacific, then followed by a gradual cooling. This general trend superimposed on the frequent fluctuations with glacial cycles may provide evidence of the climatic processes leading to the boreal glaciation. The cross spectral analyses of the△δ¹⁸O_Atl-Pac with the earth's orbits suggests that after the initiation of Northern Hemisphere Glaciation at about 2.75 Ma obliquity rather than precession had become the dominant force controlling the vertical structure or thermohaline circulation in the paleo-ocean. On the basis of a temperature reconstruction usingδ¹⁸O from Site 1143, deepwater temperature variations show gradually cooling by about 5℃from 5 Ma to 1.5 Ma, and then warming since 1.5 Ma. This deepwater cooling in the southern SCS appears to have not always accompanied the global cooling pattern.Comparison of the planktonic and benthicδ¹⁸O reveals that the intrusion of the low-salinity Borneo alongshore waters and the monsoon-induced upwellings also affected the local climate of the southern SCS, especially in the time interval of 3.3-2.5 myr. Isotopic differences between G. tuber and P. obliquiloculata demonstrate the upper ocean thermocline and nutrientcline variations of the southern SCS during the Pleistocene. Generally, the△δ¹⁸O of Site 847 is smaller than that at Site 1143 before 180 ka, indicating a deeper thermocline depth in the east equatorial Pacific than in the southern SCS. Within such intervals as the 810-848 ka and 360-420 ka, however, the△δ¹⁸O at Site 847 reaches the same level as at Site 1143, indicating a sudden shoaling up of the thermocline depth at Site 847 locality. The altered ENSO variations both in the strength and in the frequency may result in the sudden shoaling up of the thermocline depth in east equatorial Pacific. In general, the thermocline at the Site 1143 locality shoaled in glacials and deepened in interglacials, opposite to that in the east equatorial Pacific. This indicates asymmetric glacial-interglacial variations of the thermocline depth between the west and east equatorial Pacific. These results differ from the inference by faunal assemblage analysis in southern SCS which shows different thermocline variational pattern. Therefore, some common variations from multi-proxies should be first extracted before such paleoceanographic characteristics as thermocline variations can be analyzed. The Site 1143 results display that theδ¹³C difference of the surface and subsurface dwelling species are generally higher during interglacials than during glacials, which is opposite to the east equatorial Pacific Site 847. This may also indicate asymmetric or contrasting glacial-interglacial variations of the nutrientcline between the west and east equatorial Pacific. On the whole, the Pleistocene thermocline and nutrientcline at ODP Site 1143 virtually co-varied on the long-term scale, probably representing a unique characteristic of the SCS. The coherent relationship of the△δ¹⁸O(P-G) with ETP only at the precession band indicates a precessional radiation forcing of the temperature contrast of the upper ocean in the southern SCS. The negative phase of the△δ¹³O(P-G) with ETP at the 23-kyr and 19-kyr precession bands indicates that factors influencing nutrientcline variations are much more complex than those influencing thermocline.The time series of the east Asian monsoon proxies including Herbs%, Pinus%, opal% and G. ruberδ¹³C, demonstrate three major Milankovitch cycles in the Pleistocene, but they are coherent with the concentration of the orbital variance (ETP) only at the precession band. Although G. ruberδ¹³C is coherent with ETP at the three Milankovitch cycles, the highest coherency occurs at the precession band. These indicate that the tropical climate, such as the variations of the thermocline and east Asian monsoon in the SCS, is externally controlled by the precessional radiation. The coherency and phase relationship of the monsoon proxies indicate that the east Asian monsoon, at leas the winter monsoon, has been greatly influenced by the global ice volume change. Only on the precession band, however, did all monsoon tracers recorded at Site 1143 reveal similar coherency and phase relationships with the radiation forcing and the global ice volume change. The final results of this study imply that the east Asian monsoon, at least the winter monsoon, was affected at least partly by the increased effectiveness of sensible heating, although the increased latent heat and a combination of all these could have played a more critical role on the development of the east Asia monsoon.