Microfacies of stromatolitic sinter from acid-sulphate-chloride springs at Parariki Stream, Rotokawa Geothermal Field, New Zealand

We present a unique, scale-integrated, and spatially controlled study of acid-derived sinters and their abiotic-biotic relations. Through a microfacies-based approach, we provide context and constraints for inferring causal factors in the formation of these sinters. Four distinct microfacies of siliceous stromatolitic sinter formation and their associated microbiota were elucidated from acid-sulphate-chloride hot spring outflows (pH 2.1-2.3, 91-30°C), located on the floodplain of Parariki Stream, ∼1 km north of Lake Rotokawa in the Rotokawa Geothermal Field. Microfacies 1 comprises cup- to ridge-shaped sinters forming close to vents (91-64°C) with relatively high water and gas discharge. Sinter surfaces are characterised by relatively small (0.5 cm high) spicules, irregular, gnarly siliceous textures and colonisation by coccoidal microorganisms (1-1.5 μm in diameter). Microfacies 2 consists of spiculose (1 cm high) sinters colonised by bacilli (1-2.3 μm long), diatoms and coccoidal algae (2-10 μm in diameter) that are surrounded by quiescent waters (85-30°C) with little steam discharge. Microfacies 3 is typified by parallel-laminated sinters forming on slightly steepened areas that are colonised by bacilli (1-8 μm long), diatoms and coccoidal algae (2-10 μm in diameter) and exposed to fluctuating water levels (60-54°C). Microfacies 4 constitutes thin siliceous sinter rims forming mainly on small pumiceous clasts that rest upon moist (67-45°C) sandy substrates and colonised by bacilli (1-2.3 μm long), diatoms and spherical cells (2-6 μm in diameter). Sinter morphology, texture and formation mechanisms, as well as microbial colonisation, depend on a variety of environmental constraints that can act at a scale of centimetres or less. Textural development of the sinters, including their laminae, is attributed to a combination of abiotic and biotic factors. The differential preservation potentials of microbial communities need to be taken into account when assessing biodiversity of ancient sinters.