High latitude meteoric sigma~(18)O compositions:Paleosol siderte in themiddle Cretaceous Nanushuk Formation,North Slope,Alaska

摘要

Siderite-bearingpedogenic horizons of the Nanushuk formation o fthe Norht Slope,Alaska,Provide a critical high paleolatitude oxygen isotopic proxy record of paleoprecipitation,supplyingimportant empiricla dta needed for palecolimatic reconstructions and models of "greenhouse-world"precipitationrates.Siderite sigma~(18)O values were deermined form four paleosol horizons in the naitonal Petroleum Reserve Alaska (NPR-A) Grandstand # 1 Core,andthe values range between -17.6(per thousand) and -14.3(per thousand)Peedeebelemnite(PDB) with standarddeviations generally less than 0.6(per thousand) within individual horizons.The sigma~(13)C values are much more varible,ranging form -4.6(per thousand) to +10.8(per thousand) PDB.A covariant sigma~(18)O versus sigma~(13)C trend in oen horizon probably resulted form mixing between modified marine and meteoric phreatic fluids durign siderite precipitation.Groundwater values caljculated from siderite oxygen isotopic values and paleobotanical temperature estimates range from-23.0(per thousand) to -19.5(per thousand) stnadardmean ocean water(SMOW).Minor element analysesshow that thesderites are impure,having enrichments in Ca,Mg,Mn,and Sr.Minor element substitutions and Mg/Fe and Mg/(Ca+Mg) ratos also suggest the finfluence of marine fluids upon siderite precipitation.Thepedogenic horizons arecharacterized by gleyed colors,rare root traces,abundant siderite,abundant orgnaic matrer,raeclay and silty clay coatings and infillings,some preservation of primary seedimentary stratification,and a lack fo ferruginous oxides andmottles.The pedogenic drained,reducing,hydromorjphic soils that developed in coal-bearing delta plain faciesand aresimilar to modern Inceptisols.Model-derived estimates o fprecipitationrates for the Late Albian of the North Slope,Alsaka (485-626) mm/yr),are consistent with precipitation rates necessary to maintain modern peat-forming environments.This information reinforces the mutual consistency between empiricla paleotemperature estimates and isotope mass cna be used in future global circulation modeling (GCM) experiemtns of "greenhouse-world"clmiates to constrain high latitude precipitationrates in simulations of ancient worlds with decreased equator-to-pole temperature gradients.