MIOCENE PALEOSOL IN SYDNEY, AUSTRALIA AND ITS PALEOCLIMATIC SIGNIFICANCES
Early Miocene paleosols,dated back to 17Ma B.P.,are prevalently distributed in Sydney area,of which laterite strata was previously thought to form in tropic environments with high temperature and humidity.However,the Australia plate did not yet drift to current position during the Early Miocene,when Sydney paleolatitude was at 45°～50°S,thus belonging to temperate zone.So,what were the paleoenvironments like during the laterite developing period is still in controversial.In order to better understand and investigate the paleoclimate of Sydney during the Early Miocene,a typical paleosol profile (33.743269°S,151.305244°E) with laterite was taken,which was located at the Long Reef Beach (LRB) town (northeast of Sydney).The profile as depth as 6.9m from top to bottom and was divided into three parts:Holocene Mollisol part (HM,0～1.2m),Miocene Ultisol part (MU,1.2～5.2m) and Miocene Laterite part (ML,5.2～ 6.9m).Contents of macrocelement (SiO2,Fe2O3,Al2O3,MgO,CaO,Na2O and K2O),the property of rock magnetism (including low field magnetic susceptibility (x),saturation isothermal remanent (SIRM),saturation magnetization (Ms),and anhysteretic remanent magnetization (ARM) in room temperature,magnetic hysteresis loops,and thermomagnetic analysis (i.e.M-T & κ-T curves)) and the soil chromaticity (L*,a * and b*) for all the samples (n =55) were measured.The results suggest that:(1) The average chemical index of alteration (CIA,indicating chemical weathering degree) of LRB paleosol is 95.26,which is comparable with the Late Pleistocene paleosols developed on basalt in north Hainan Island.The high CIA of LRB section indicates strong chemical weathering on the Earth surface during the Early Miocene.The calculated mean annual precipitation during the Early Miocene 1471.22mm in Sydney area is close to current precipitation in Hainan Island (1400～1800mm) as well.(2) Si dropped but Fe and Al increased with depth,which were caused by abundant rainfall.Iron oxide and aluminum oxide were leached from top of the paleosol profile to bottom,then formed iron concretion and iron pan at the bottom.(3) Conversion process of magnetic minerals from top to bottom in the section can be summarized as:magnetite→magnetite core wrapped by maghemite shell→ maghemite→ hematite.Specific enrichment of maghemite and hematite in LRB section provides evidence that during the Early Miocene Sydney area was likely in a high-temperature and oxidation environment.Combining abovementioned environment proxies with existing results from other researchers,we speculate that Sydney area in Early Miocene was a humid tropical climate.The evident greenhouse effects of CO2 (the global CO2 content was 754±153ppm in Early Miocene) likely induced the higher temperature than today,though the high paleolatitude of Sydney (i.e.45°～50°S) at that time.
NETL
NSTL
万方数据
维普
