Authigenic carbonate precipitation across the sediment-water interface: insights from modern microbialites

MARA MATIC; FERNANDO J GOMEZ; PALOMA PEREZ VALDENEGRO; ALEXANDRA RODLER

Leysin

Conferencia; Microbialite: Formation - Evolution - Diagenesis. M-FED 2023; 2023

UNIVERSITÉ LAUSSANE. UNIVERSITÉ DE GENEVE. UNIVERSITÉ FREIBURG. ETH ZURICH

AbstractEarly diagenesis occurs at the sediment water interface in both marine and lacustrine systems, where organic matter remineralization is produced by a cascade of biogeochemical reactions. The occurrence of carbonate dissolution or precipitation depends on the net balance between alkalinity consuming and alkalinity producing reactions. This also controls major, minor and trace elements cycling and partition between the fluid and mineral phases, particularly those elements that are associated with organic matter or controlled by redox co nditions. Authigenic carbonate recipitation at this interface has changed over time and recent studies have shown that this authigenic carbonate factory is more important than previously realized. Here we explore these processes by characterizing microbi al carbonates and associated carbonate crusts of the Laguna Negra (high altitude hypersaline lake in Catamarca, Argentina). These carbonates grow at the sediment water interface where they develope morphologies similar to those observed for manganese nodul es that normally show hydrogenetic and diagenetic growth. The Laminar crusts zone is represented by mm to cm thick, dome to planar shaped crusts forming patchy to laterally extensive pavements. These are not associated with the presence of microbial mats or biofilms, and micro textural analysis suggests that carbonate precipitation is mostly related to physicochemical processes. Althoughthey can be concentrically laminated, they preferentially exhibit upward directed lamina accretionleadi ng to asymmetrical growth. The Oncoids zone is represented by cm to dm scale concentricallylaminated discs, spheres, and domes. These are associated with thick, laminated microbial mats and biofilms influencing micro textures. The o ncoids grow in situ, w ithout significant transport or movement, exhibiting symmetrical growth patterns when compared to laminar crusts, and can show preferential growth towards the lower hemisphere (which was submerged in the sediment at sampling time). Asymmetric growth in the oncoids might relate to organic matter remineralization, occurring within the sediment, and increasing alkalinity. Contrarily, the upward growth pattern in the laminar crusts must be related to the increase in carbonate saturation state driven by physico chemical processes (evaporation and CO 2 degassing). We are currently exploring the influence of microbial mats on trace element behavior at this interface and their incorporation into the carbonates (particularly redox proxies such as Cr, U isotopes and REE).