Holocene paleo-redox conditions in a microbial dolomitic lake using benthic foraminifera as bioindicators

Abstract

Brejo do Espinho coastal lake (LBE) is one of the few places in the world where dolomite [CaMg(CO₃)₂] is precipitating in the modern environment under microbially induced processes and low oxygen conditions. We use pore morphometry of the foraminifera Ammonia cf. A. veneta to evaluate paleo-O₂ dynamics during the dolomitic depositional phase that took place at LBE in the late Holocene. Foraminiferal community structure was also investigated, and results were compared to bulk isotopic composition of carbonates, total organic carbon (TOC), and X-ray Diffraction of sediments (XRD). The correlation matrix (Spearman method) showed that Ammonia test pores morphometric parameters displayed significant correlations with overall biotic and geochemical data, with pore area presenting a relatively higher association. Ammonia test pores were primarily controlled by the degradation of organic matter (Pore area-TOC, r = −0.84), and foraminifera density appeared to be influenced by oxygen changes, with a higher abundance in the highest porosity intervals (Pore area-N, r = 0.82), indicating a direct effect of oxygen penetration on species dominance. These data also reveal a tolerant behavior of the low-O₂ bioindicator species Quinqueloculina laevigata and A. veneta. Understanding microbe-mineral interactions is critical for interpreting paleo records, and our data provide strong support for coupling assemblage and pores analysis as paleo-O₂ bioindicators for paleo-redox coastal settings.