Mercury Anomaly in Oligocene–Miocene Maykop Group Sediments (Caucasus Continental Collision Zone): Mercury Hosts, Distribution, and Sources

Abstract

The Oligocene–Miocene Maykop Group sediments, mainly composed of illite–smectite, store mercury in strongly variable concentrations from 10 to 920 μg/kg. Extremely high Hg levels (98–920 μg/kg) coupled with abnormal mercury-to-total organic carbon (TOC) ratios (Hg/TOC = 109 to 3000 μg/kg/wt%; TOC = 0.2 wt% to 1.2 wt%) were measured in the Middle Maykop marine shales that were deposited in the deepwater Indol–Kuban Basin under anoxic conditions. The Middle Maykop shales contain up to 70% of total mercury in sulfide form. In heavy mineral fractions, abundant Hg-bearing pyrite (with up to 4810 µg/kg Hg in hand-picked concentrates) is accompanied by sporadic cinnabar. Relative to the Middle Maykop sediments, the Upper Maykop shales have much lower Hg concentrations and Hg/TOC ratios: 10 to 63 μg/kg (34 μg/kg on average) and 7.7 to 137 μg/kg/wt% (39 μg/kg/wt% on average), respectively. Mercury sequestration is inferred to occur mostly by binding in sulfide hosts in the Middle Maykop anoxic deep-sea sediments and in organic matter, Fe3+-(oxy)hydroxides, and clay particles in the Upper Maykop shales which were deposited in a more oxygenated environment. Mercury inputs to the marine shales during Maykopian sedimentation were possibly associated with local Oligocene–Lower Miocene volcanic activity in the Caucasus Continental Collision Zone. At the same time, the mode of Hg binding in sediments was controlled by redox conditions which changed from anoxic to disoxic and suboxic at the Middle-to-Upper Maykop transition.