HETEROZOAN LIMESTONES AND AUTHIGENESIS: FINGERPRINTING PALEONUTRIENT LEVELS IN CARBONATE SETTINGS

Heterozoan limestones are characterized by carbonate-producing, filter-feeding organisms that reflect cool temperatures and elevated trophic resources. Higher nutrient levels and surface ocean productivities result in lithofacies that are more organic-rich than their photozoan counterparts. Microbial respiration of this organic matter fuels alkalinity and redox-driven, synsedimentary dissolution of skeletal aragonite and precipitation of distinct authigenic minerals beneath the seafloor. The formation of these authigenic minerals is directly related to trophic resources via the amount of exported organic carbon, providing an assessment of paleonutrient levels not solely dependent on preserved biota.

Under oligotrophic conditions the scarcity of organic matter favors the preservation of aragonite. As nutrient levels rise and supply of organic carbon to the seafloor increases, porewater pH drops, δ¹³C of bicarbonate decreases, and oxygen consumption creates reducing conditions. This leads to the incorporation of redox sensitive trace elements into authigenic and diagenetic phases that assist with fingerprinting the Eh of paleo-porewater. When low mesotrophic conditions prevail microbial degradation of sedimentary organic matter compresses the zones of oxic respiration, denitrification, and iron reduction, leading to precipitation of sedimentary apatite, glauconite, and chamosite. Sulfate reduction adds framboidal pyrite to this assemblage. Under eutrophic conditions pyrite formation intensifies and organogenic dolomite can form. The rates at which these authigenic minerals precipitate are influenced as much by organic matter concentration as sediment grain size and temperature.

Thus, as nutrient levels increase the appearance of authigenic minerals in heterozoan carbonates roughly correspond to observed changes in the diversity and type of carbonate-producing organisms. Such information can augment sedimentologic and taxonomic investigations aimed at understanding the oceanography of ancient carbonate environments. This is especially important in heterozoan limestones that have experienced significant taphonomic loss of skeletal grains, providing a link between oceanographic conditions and the carbonate rock record.