ALBERT

Description: The Holocene variability in sea surface and thermocline water temperatures (SST and TWT) in the Indo-Pacific Warm Pool (IPWP) has been reconstructed by shell Mg/Ca of two planktonic foraminifera species, the mixed-layer dweller Globigerinoides ruber and the thermocline dweller Pulleniatina obliquiloculata, from sediments of Core MD98-2188 (14.82°N, 123.49°E, 730 m water depth) recovered in the western tropical Philippine Sea. Afterward the Younger Dryas interval (YD), SST warmed gradually till ~10 ka and remained approximately constant afterwards, but TWT rose more rapidly to a peak between ~12 and ~10 ka and then declined by ~1.5°C through the Holocene. The trend of TWT closely followed the boreal summer insolation and could be correlated to tropical climate changes represented by southward movement of the Inter-tropical Convergence Zone (ITCZ) and related changes in East Asian monsoons.

Description: The relationship between planktonic and benthic foraminiferal stable-isotope values and oceanographic conditions and factors controlling isotopic variations are discussed on the basis of oxygen and carbon isotopic analyses of 192 modern surface and Last Glacial Maximum (LGM) samples from the South China Sea (SCS). The harmonic variation of benthic delta18O in surface sediments with water depth and temperature implies that the temperature is the main factor influencing benthic delta18O variations. Planktonic delta18O fluctuates with sea surface temperature (SST) and salinity (SSS). The N-S temperature gradient results in planktonic delta18O decreasing from the northeast to the south. Cool, saline waters driven by the winter monsoon are interpreted to have been responsible for the high delta18O values in the northeast SCS. The East Asian monsoons not only bring nutrients into the South China Sea and maintain high nutrient concentration levels at the southwestern and northeastern ends, which cause depleted delta13C both in planktonic (surface) and benthic (bottom) samples but also reduce planktonic/benthic delta18O differences. The distribution of delta18O and delta13C in the surface and LGM samples are strikingly similar, indicating that the impact of SST and SSS has been maintained, and nutrient inputs, mainly from the northeastern and southwestern ends, have been controlled by monsoons since the LGM. Comparisons of the modern and LGM delta18O indicate a difference of about 3.6 °C in bottom-water temperature and a large surface-to-bottom temperature gradient during the LGM as compared to today.

Description: Based on benthic foraminiferal delta18O from ODP Site 1143, 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 age agrees well with the previously published age of 0.78 Ma. 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 benthic delta18O from Site 1143 is highly coherent with the Earth's orbit (ETP) both at the obliquity and precession bands for the last 5 Myr, and at the eccentricity band for the last 2 Myr. In general, the 41-kyr cycle was dominant through the Plio-Pleistocene although the 23-kyr cycle was also very strong. The 100-kyr cycle became dominant only during the last 1 Myr. A comparison of the benthic delta18O between the Atlantic (ODP 659) and the East and West Pacific (846 and 1143) reveals that the Atlantic-Pacific benthic oxygen isotope difference ratio (Delta delta18OAtl-Pac) displays an increasing trend in three time intervals: 3.6-2.7 Ma, 2.7-2.1 Ma and 1.5-0.25 Ma. Each of the intervals begins with a rapid negative shift in Delta delta18OAtl-Pac, followed by a long period with an increasing trend, corresponding to the growth of the Northern Hemisphere ice sheet. This means that all three intervals of ice sheet growth in the Northern Hemisphere were accompanied at the beginning by a rapid relative warming of deep water in the Atlantic as compared to that of the Pacific, followed by its gradual relative cooling. This general trend, superimposed on the frequent fluctuations with glacial cycles, should yield insights into the processes leading to the boreal glaciation. Cross-spectral analyses of the Delta delta18OAtl-Pac with the Earth's orbit suggests that after the initiation of Northern Hemisphere glaciation at about 2.5 Ma, obliquity rather than precession had become the dominant force controlling the vertical structure or thermohaline circulation in the paleo-ocean.

Description: Pliocene core intervals from Ocean Drilling Program Site 1143 located in the southern part of the South China Sea were measured for their major and minor element composition at ~2-k.y. resolution. Changes in sediment composition parallel glacial-interglacial changes as indicated by the corresponding oxygen isotope record of benthic foraminifers. Carbonate contents are lower during glacial stages and higher during interglacial stages. This is attributed to dilution by a higher contribution of terrigenous detrital matter during glacial intervals and to enhanced biological productivity during interglacials as indicated by enrichments in barium. The terrigenous detrital matter fraction shows small but distinct changes in composition. These variations may be explained by multiple processes, including changes in provenance and weathering caused by monsoonal climate variability and sea level fluctuations. Gradual or long-term changes in sediment composition also occur during the investigated interval. There is a shift toward lower TiO2 (carbonate-free basis; cfb) values at around 2.9 Ma and an increase in Al2O3 (cfb) variations at around 2.55 Ma. These changes can be explained in both cases by an increasing contribution from the Mekong River, delivering material with low TiO2 (cfb)/Al2O3 (cfb) ratios, followed by a gradual increase in weathering, causing higher Al2O3 (cfb) (probably reflecting kaolinite) contents. Both may be the result of an enhanced summer monsoon. The manganese and phosphate geochemistry of Site 1143 sediments seem to be strongly dominated by biological productivity and a high burial flux of carbonate, which acts as an adsorbing agent and/or substrate. During strong interglacials and corresponding times of enhanced productivity, Mn may be remobilized from continental margin sediments deposited within an oxygen minimum zone and subsequently exported into the pelagic region, where it oxidizes and settles to form Mn-enriched layers in the sediment.