THE STUDY OF ROCK MAGNETISM IN LACUSTRINE SEDIMENTS FROM THE NANYANG BASIN DURING THE PALEOCENE-EOCENE THERMAL MAXIMUM
The Paleocene-Eocene Thermal Maximum(PETM)was a swift warming event that transpired during the early Cenozoic era.The prominent geological hallmark of the PETM was the significant negative carbon isotope excursion(CIE),signifying a substantial influx of 12C-enriched light carbon into the ocean-atmosphere system during that period.This emission scenario bears a striking resemblance to contemporary human carbon emissions.Therefore,the event has been considered as a geological analog for the ongoing and forthcoming anthropogenic warming and its associated environmental changes.Previous studies have demonstrated the close correlation between the formation and conservation of magnetic minerals and shifts in climate and environmental conditions.Thus,magnetic parameters of sediments spanning the PETM can unveil the temporal and spatial attributes of paleoenvironmental evolution within the context of global warming.Prior investigations applying magnetic methods on marine sediments have identified noteworthy magnetic responses to the PETM;however,the fundamental mechanisms behind these phenomena remain subjects of debate.In this paper,we conducted a magnetic study of the lacustrine sedimentary rocks,marl and limestone,of the 50-meter-thick strata corresponding to the PETM in the Nanyang Basin(32.65° N,111.46° E).This basin,situated in the eastern Qinling orogen,is an intermontane down-faulted basin emerged during the late Mesozoic as a result of the collision between the North China Block and Yangtze Block.Totally 21 samples from the intervals of pre-PETM,during-PETM and post-PETM were chosen for measurements of hysteresis loops,direct current demagnetization,isothermal remanent magnetization acquisition curves and first-order reversal curves,with the data processed by applications like HystLab,Max UnMix and FORCinel.The outcomes reveal that saturation magnetization exhibited enhancement,whereas bulk coercivity dropped during the PETM-Peak period;additionally,magnetite content experienced an upsurge while the hematite content dropped preceding the conclusion of the event.Subsequent to the PETM,magnetic properties gradually reverted to the baseline state,suggesting sequential episodes of oxidation,moderate reduction,and subsequent oxidation in the lacustrine environment before,during and after the event,respectively.The magnetic properties'response pattern to the PETM can be owed to two possible mechanisms.Firstly,the hot,humid climate caused by the PETM might enhance the weathering intensity,resulting more input of debris from the source area and the rise of magnetic particles.Also,as the total productivity increased,the input organic matter might consume more dissolved oxygen,promoting the redox state changing.Secondly,global warming might enhance the temperature gradient between surface water and bottom water,cutting off the lake convection.Thus,the dissolved oxygen of the bottom water could not be complemented and the lake converted into moderate reductive state,then the pigmentary hematite transformed to magnetite.Our research also suggests that terrestrial and marine sediments exhibited analogous mechanisms of magnetic response to the PETM.
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