ALBERT

Description: The global climate system has experienced a series of drastic changes during the Cenozoic. These include the climate transformation in Asia, from a zonal pattern to a monsoon-dominant pattern, the disappearance of subtropical aridity related to a planetary circulation system and the onset of inland deserts in central Asia. Despite of the major advances in the last two decades in characterizing and understanding these climate phenomena, disagreements persist relative to the timing, behaviors and underlying causes. This paper addresses these issues mainly based on two lines of evidence. Firstly, we newly collected the available Cenozoic geological indicators of environment in China to compile the paleoenvironmental maps of ten intervals with a more detailed examination within the Oligocene and Miocene. In confirming the earlier observation that a zonal climate pattern was transformed into a monsoonal one, the new maps within the Miocene indicate that this major change was achieved by the early Miocene, roughly consistent with the onset of loess deposition in China. Although a monsoon-like regime would have existed in the Eocene, it was restricted in the tropical-subtropical regions. The observed latitudinal oscillations of the climate zones during the Paleogene are likely attributable to the imbalanced evolution of polar ice-sheets between the two hemispheres. Secondly, we examine the relevant depositional and soil-forming processes of the Miocene loess-soil sequences to determine the circulation characteristics with special emphasis given to the early Miocene. Continuous eolian deposition in the middle reaches of the Yellow River since the early Miocene firmly indicates the formation of inland deserts, which has been constantly maintained in the past 22 Ma. Inter-section grain-size gradients indicate northerly dust-carrying winds and source location, as is regarded as the main criteria of the Asian winter monsoon system. Meanwhile, the well-developed Luvisols evidence the existence of circulations from the ocean, which brought moisture to northern China. These imply the coexistence of two kinds of circulations, one from the ocean as moisture carrier and another from the inland deserts as dust transporter. The accretionary properties of the early Miocene paleosols, resulted from interactive soil-forming and dust deposition processes, evidence two seasonally alternative circulations, i.e. a monsoonal climate regime. The much stronger development of the early Miocene soils compared to those in the Quaternary loess indicates significantly stronger summer monsoons. These lines of evidence indicate a joint change in circulations and inland aridity by the early Miocene, and suggest a dynamic linkage of them. Our recent numerical experiments reconfirm the potential roles of Tibetan uplift and Paratethys shrinkage in triggering this major climate reorganization, as revealed in peer studies, but yielded more details about their combined scenarios. These two factors would have coacted with the help of South China Sea spreading. Although the realistic effects of each factor remain to be further discriminated, probably through more paleoaltimetrical and tectonic approaches, the Miocene loess record provides a vital insight that tectonics had evolved to a threshold by the early Miocene to cause this major climate reorganization in Asia.

Description: Baroclinic instability modulated by topography leads to the formation of two anticyclonic eddies in the northern South China Sea: the Hong Kong Southeast Anticyclonic Eddy (HKSEACE) and the Hainan Island East Anticyclonic Eddy (HIEACE). In these eddies, downwelling caused by a depressed pycnocline leads to high temperature, low salinity, impoverished nutrients, reduced Chl-a concentrations, and picoplankton dominance of phytoplankton assemblages in the euphotic zone. We tested the hypothesis that experimental nutrient enrichment would relieve biomass limitation of phytoplankton by opportunistic response of taxa with low nutrient affinity. Our results confirm that phytoplankton samples incubated in vitro under nutrient enriched conditions attained higher biomass, change in taxonomic dominance from dinoflagellates to diatoms, and shift in size class dominance from picoplankton to nanoplankton and netplankton. These responses were evident only when limitation to more than one nutrient was relieved. Phytoplankton in HKSEACE appeared to be co-limited by nitrogen and phosphorus, whereas at HIEACE it was co-limited by nitrogen, phosphorus and also silicon.

Description: Physical and chemical oceanographic data were obtained by seasonal monitoring along Transect N in the northern South China Sea (nSCS) during 1976–2004. Fluctuations of DIN (dissolved inorganic nitrogen), seawater temperature (SST and Tav – average temperature of the water column), N:P ratio and salinity (Sav and S200 – salinity at the 200 m layer) exhibited an increasing trend, while those of T200, DO, P, Si, Si:N and SSS exhibited a decreasing trend. The annual rates of change in DIN, DO, T and S revealed pronounced changes, and the climate trend coefficients Rxt, which was defined as the correlation coefficient between the time series of an environmental parameter and the nature number, were 0.38 to 0.89 and significant (p≤0.01 to 0.05). Our results also showed that the ecosystem has obviously been influenced by the positive trends of both SST and DIN, and negative trends of both DO and P, e.g. before 1997, DIN concentrations in the upper layer were very low and N:P ratios were less than half of the Redfield ratio of 16, indicating potential N limitation. However, after 1997, all Si:P ratios were 〉22 and the Nav:Pav was close to the Redfield ratio, indicating potential P limitation, and therefore N limitation has been reduced after 1997. Ecological investigation shows that there have been some improved responses of the ecosystems to the long-term environmental changes in the nSCS, and chlorophyll-a concentration, primary production, phytoplankton abundance, benthic biomass, cephalopod catch and demersal trawl catch have increased. But phosphorus depletion in upper layer may be related to the shift in the dominant species from diatoms to dinoflagellates and cyanophytes. The ecosystem response was induced by not only anthropogenic activities, but also global climate change, e.g. pronounced responses to ENSO. The effects of climate change on the nSCS were mainly through changes in the monsoon winds, and physical-biological oceanography coupling processes.

Description: Physical and chemical oceanographic data were obtained by seasonal monitoring along a transect (Transect N) in the northern South China Sea (nSCS) during 1976–2004. Fluctuations of DIN (dissolved inorganic nitrogen), seawater temperature (SST and Tav – average temperature of the water column), N:P ratio and salinity (Sav and S200 – salinity at the 200 m layer) exhibited an increasing trend, while those of T200, DO, P, Si, Si:N and SSS exhibited a decreasing trend. The annual rates of change in DIN, DO, T and S revealed pronounced changes, and the climate trend coefficients, which was defined as the correlation coefficient between the time series of an environmental parameter and the nature number (namely 1,2,3,......n), were 0.38 to 0.89 and significant (p≤0.01 to 0.05). Our results also showed that the ecosystem has obviously been influenced by the positive trends of both SST and DIN, and negative trends of both DO and P. For example, before 1997, DIN concentrations in the upper layer were very low and N:P ratios were less than half of the Redfield ratio of 16, indicating potential N limitation. However after 1997, all Si:P ratios were 〉22 and the Nav:Pav was close to the Redfield ratio, indicating potential P limitation, and therefore N limitation has been reduced after 1997. Ecological investigation shows that there have been some obvious responses of the ecosystems to the long-term environmental changes in the nSCS. Chlorophyll-a concentration, primary production, phytoplankton abundance, benthic biomass, cephalopod catch and demersal trawl catch have increased. But phosphorus depletion in upper layer may be related to the shift in the dominant species from diatoms to dinoflagellates and cyanophytes. The ecosystem response was induced by not only anthropogenic activities, but also global climate change, e.g. ENSO. The effects of climate change on the nSCS were mainly through changes in the monsoon winds, and physical-biological oceanography coupling processes. In this study physical-chemical parameters were systemic maintained, but the contemporaneous biological data were collected from various sources. Regional response to global climate change is clearly a complicated issue, which is far from well understood. This study was made an attempt to tackle this important issue. For the aim these data were valuable.

Description: The global climate system experienced a series of drastic changes during the Cenozoic. In Asia, these include the climate transformation from a zonal pattern to a monsoon-dominated pattern, the disappearance of typical subtropical aridity, and the onset of inland deserts. Despite major advances in the last two decades in characterizing and understanding these climate phenomena, disagreements persist relative to the timing, behaviors and underlying causes. This paper addresses these issues mainly based on two lines of evidence. First, we compiled newly collected data from geological indicators of the Cenozoic environment in China as paleoenvironmental maps of ten intervals. In confirming the earlier observation that a zonal climate pattern was transformed into a monsoonal one, the maps within the Miocene indicate that this change was achieved by the early Miocene, roughly consistent with the onset of loess deposition in China. Although a monsoon-like regime would have existed in the Eocene, it was restricted to tropical-subtropical regions. The latitudinal oscillations of the climate zones during the Paleogene are likely attributable to the imbalance in evolution of polar ice-sheets between the two hemispheres. Secondly, we examine the relevant depositional and soil forming processes of the Miocene loess-soil sequences to determine the circulation characteristics with emphasis on the early Miocene. Continuous eolian deposition in the middle reaches of the Yellow River since the early Miocene firmly indicates the formation of inland deserts, which have been constantly maintained during the past 22 Ma. Grain-size gradients between loess sections indicate northerly dust-carrying winds from northern sources, a clear indication of an Asian winter monsoon system. Meanwhile, well-developed Luvisols show evidence that moisture from the oceans reached northern China. This evidence shows the coexistence of two kinds of circulations, one from the ocean carrying moisture and another from the inland deserts transporting dust. The formation of the early Miocene paleosols resulted from interactive soil forming and dust deposition processes in these two seasonally alternating monsoonal circulations. The much stronger development of the early Miocene soils compared to those in the Quaternary loess indicates that summer monsoons were either significantly stronger, more persistent through the year, or both. These lines of evidence indicate a joint change in circulation and inland aridity by the early Miocene and suggest a dynamic linkage of them. Our recent sensitivity tests with a general circulation model, along with relevant geological data, suggest that the onset of these contrasting wet/dry responses, as well as the change from the "planetary" subtropical aridity pattern to the "inland" aridity pattern, resulted from the combined effects of Tibetan uplift and withdrawal of the Paratethys seaway in central Asia, as suggested by earlier experiments. The spreading of South China Sea also helped to enhance the south-north contrast of humidity. The Miocene loess record provides a vital insight that these tectonic factors had evolved by the early Miocene to a threshold sufficient to cause this major climate reorganization in Asia.