Abstract: The “dolomite problem” is one of the long-lasting problems in sedimentology. Magnesium is a major element in
dolomite, and its isotopic compositions may be used to infer the dolomitization processes and sedimentary environments. In this
study, we investigated the Mg isotope compositions of dolostones of the Albian Givat Ye’arim Formation and Soreq Formation of
Albian from a section near Jerusalem, Israel. The carbonates are mainly composed of subhedreal to euhedral micritic dolomite
without notable post-depositional diagenetic alterations. Carbon and strontium isotope compositions of the dolomite are consistent
with the Albian seawater, suggesting that the dolomitizing fluid was sourced from the contemporaneous seawater. δ26Mg values
of dolomite cluster around -2.0‰ and do not show a clear correlation with depth. Combining Mg isotope and C-Sr isotope
compositions of dolomite, it is suggested that the Mg supply from seawater for dolomitization was abundant, leading to Mg isotope
equilibrium between seawater and dolomite. The rhythmic fluctuations in Na, Rb and ΣREE contents in the dolomite sequence
indicate responses of marine environments in a semi-closed basin to variations in regional paleo-climates and paleo-environments.
By contrast, the relatively homogeneous dolomite δ26Mg values across the sedimentary beddings imply that the regional climate and
environment changes had limited impacts on Mg isotope compositions of seawater in local basins. Hence, for massive dolomite that
was deposited in partly restricted basins, their δ26Mg values likely still trace the Mg isotope composition of seawater in open oceans.

Key words: dolomite, magnesium isotope, dolomitization