Magnesium Isotope Signature Of The Albian Dolomite In Levant Basin:Effects Of Early Diagenesis And Basin Restriction

Dolomite plays an important role in global Mg cycling and Mg isotope composition of dolomite has been proposed as a potential tracer for global Mg cycling and seawater chemistry changes.However,dolomite formation is complicated and sedimentary dolomite was generally regarded as a diagenetic product,yet many details about the effects of early diagenesis on Mg isotope composition of dolomite remain unclear.The mid-Cretaceous Albian Soreq and Givat Ye'arim Formations in the west Jerusalem contain massive dolostones that are exceptionally well preserved.Dolomite samples from this section show large variations in 813C and Mn contents that are negatively correlated,indicating degradation of organic matters and mineralization of organic carbon in Mn(?)reduction zone of soft sediment during dolomite formation.A clear link between Mn(?)reduction and dolomite precipitation was presented based on geochemical signatures in rock records.Dolomite samples also have large variations in d(104)value and the degree of cation ordering.And the latter one was negatively correlated with Sr contents,which indicates that dolomite recrystallization occurred during diagenesis.?26Mg of the over fifty dolomite samples from this section vary from-2.28‰ to-1.78‰.However Mg isotope compositions of the dolomite do not show correlations with either indicators of organic matter degradation(?13C and Mn content),or indicator of dolomite recrystallization(Sr content),suggesting that Mg isotopes have a conservative behavior since initial precipitation of dolomite(or proto-dolomite)during the very early stage of diagenesis.We suggest that Mg isotopes in dolomite formed in the stage of Mn(?)reduction should be buffered by contemporary seawater due to the shallowness of Mn(?)reduction zone in platform sediments,and this is fundamentally different from dolomitization associated with bacteria sulfate reduction or methanogenesis where Mg supply is limited by diffusion.Furthermore,overall Mg isotopes in dolomite are robust against resetting by recrystallization during burial.Mg isotope compositions of platform dolomite that show variable and negatively correlated ?13C and Mn signatures therefore could be considered as potential archives for reconstructing seawater Mg isotope compositions.Significant Mg isotope fractionation occurs between dolomite and aqueous solution.More important,the applicability of Mg isotopes in dolomites in studies of ancient seawater requires a detailed understanding on the behavior of Mg isotopes during dolomitization in a variety of geological settings.The mid-Cretaceous Albian Hevyon Formation in southern Israel was deposited within a typical hinterland attached carbonate platform setting,and provides an ideal case for investigating the Mg isotope behavior during dolomitization in environments of frequent basin restrictions.The abundance of dolomite increases upward in the Hevyon Formation,and correlates with the appearance of microbial deposits and disappearance of metazoans,reflecting water level fluctuations in multiple exposure and recharging events.In conjunction with sedimentary facies analysis,multiple geochemical proxies(Ni/Co,V/Cr,Ce/Ce*and ?13C)indicate development of anoxic conditions,together with textural indicators and 87Sr/86Sr ratios,suggest that dolomitization was microbial and penecontemporaneous in a shallow low circulation water body.?26Mg of dolomite increases from-1.99 ± 0.12‰ in the lower part of the sequence to a value of-1.52 ± 0.02‰ in the middle interval,then decreases back to-2.06 ± 0.180‰ in the upper unit,which tracks the fluctuating marine connection and provides a powerful tracer for the level of restriction.Paring Mg isotopes with Mg/Ca ratios of the dolomite,it is possible to reconstruct the seawater Mg/Ca ratio during the Albian.