Diagenetic and Microbial Steps Leading to the Mineralization of Microbial Mats in Cuban Hypersaline Lagoon (Cayo Coco; North Cuba)

Abstract

Abstract

The minerals produced in microbial mats constitute a precious archive of biologic activity and physico-chemical conditions through time. However, this implies that one has to decipher the relative influence of microbial and environmental parameters, but also of diagenesis, on their mineralization. Extant microbialites are especially abundant in modern hypersaline environments. The microbial processes leading to carbonate precipitation in hypersaline mats are well documented in Bahamian lakes, but are still poorly known in Cuba. This work aims at identifying the products and understanding processes of mineralization in modern microbial mats and Holocene microbialites in the easternmost lagoon of Perros Bay.

The study area is located in the Perros Bay, to the south of Cayo Coco Island, inside a network of lagoons, forming depressions in the Pleistocene eolian substratum. This network is an assemblage of interconnected shallow lagoons (<1m) fed by marine water from Perros Bay and characterized by permanent hypersaline conditions, abnormal tides and occasional storm surges. The hydrologic budget in the lagoon network changes from west to east, increasing the salinity up to 80‰ during the dry season.

The distribution of the microbial mats and their morphologies are influenced by the hydrodynamic regime: while flat mats develop on the leeward side of the lagoon, wind-driven waves induce the formation of terraces in windward areas. Macroscopically, all the microbial mats are laminated. They consist of alternating trapped and bound grains laminae and micropeloid-dominated laminae.

Microprobe measurements show active oxygenic photosynthesis in the 3 mm top of the mat and production of HS^- and an anoxic zone deeper (from 4mm depth). X-Ray diffractometry and micro-FTIR, coupled with Cryo-SEM, environmental SEM, and CLSM-Raman microscopy show four precipitated mineral phases in the mats: a poorly crystalline Mg-Si phase which nucleates on the organic matrix; two Ca-Mg carbonates nucleating as micropeloids, locally inside micro-vesicles; and aragonite, which forms clots permineralizing and embedding cyanobacteria. Sulfides are also observed locally. The vertical distribution and the parageneses of those minerals will be explored to determine the relative influence of physico-chemical and microbial parameters on microbial mats mineralization. Our results will help in the interpretation of the environmental signals in ancient extinct hypersaline microbial mats.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016