Millennial-scale Variations In Hydrology And Paleoclimate Over The Amazon Fan During The Last Glacial

The Amazon Basin represents the largest land-based atmospheric convection centre on earth and the main source of latent heat in tropical. It plays a fundamental role in supply of heat to the higher latitudes, therefore it has become a hot area of paleoclimate studies. The Amazon Fan is under the influence of western tropical Atlantic water masses, which has a significant impact on ocean dynamics and heat transport. Variations in outflow of Amazon River not only affect the characteristics of the water masses but also control the changes of Atlantic cross-equatorial circulation and the North Brazilian current, thereby regulating the global thermohaline circulation. Site 942 lies adjacent to the mouth of Amazon River and benefits from the massive output of sediment from Amazon Basin, therefore it provides continuous highresolution palaeoclimate and palaeocean records. In this paper, we reconstruct millennial-scale variations in hydrology and climate on Amazon Fan during the last glacial from the records of oxygen isotopes、Mg/Ca ratio and Ba/Ca ratio on planktonic foraminifera Globigerinoides ruber to discuss the relationship between the environmental change and the shift of ITCZ, as well as the THC.The analysis indicates that the dissolved barium concentrations in this region show an obvious negative correlation to the SSS(R2 = 0.95). A lineal equation(SSS =–6.2005*Ba/Caforam + 40.724, R2= 0.98)is established based on the relationship between modern sea surface Ba/Ca and salinity, and a distribution coefficient of Ba reported for planktonic foraminifera. Planktonic foraminiferal Ba/Ca ratios are determined on thirteen samples from Site 942 B and 942 C to investigate the usability of Ba/Ca in G. ruber(White) and G. sacculifer as an indicator of past sea surface salinity(SSS). Application of this equation to the thirteen ODP 942 samples suggests that for the last 4099 a B.P., SSS estimates based on G. ruber(White)-Ba/Ca show fairly comparable results to the precipitation records indicated by the stalagmite δ18O from South American continent. Ba/Ca in G. ruber(White) is a useful proxy of SSS over the Amazon Fan.During the last glacial, there are a series of millennial-scale variations in δ18O records, respectively Heinrich events and Younger Dryas. During the cooling events, δ18O records are most positive. There is no uniform relation between the Sea Surface Temperature(SST) derived from G.ruber Mg/Ca and millennial-scale variations. But the characteristics of variations in SST are consist with the palaeoclimate trends of Antractic, especially the last deglacial. During the periods of Heinrich and YD cooling events, the Sea Surface Salinity(SSS) increased. Because of the temperature, salinity and global ice volume affect the foraminiferal oxygen isotope together, therefore the rapid changes in δ18O records might be the information of both temperature and salinity. However, the trend of SST shows more similar with Antarctic, so it suggests that the millennial-scale climate change in Amazon Fan mainly reflects the variation in the precipitation pattern. During the H and YD cooling events, a reduction of the Atlantic Meridional Overturning Circulation(AMOC) and related North Atlantic cooling induces a southward movement of the Intertropical Convergence Zone(ITCZ), resulting in a weakened precipitation on Amazon Fan. By comparison, synchronous variations in precipitation patterns on Amazon Fan and deep western tropical Atlantiv conditions show a connection between THC(Thermohaline Circulation) reorganizations and shifts in tropical precipitation patterns by way of variations in North Atlantic SST, changes in the meridional North Atlantic temperature gradient and resulting shifts of the ITCZ.