Orbitally-driven climate forcing in late Pliocene lacustrine siderite-rich clastic rhythms (Upper Valdarno Basin, Northern Apennines, Italy)
Abstract Distinctive siderite-rich lacustrine strata are exposed along the western margin of the Upper Valdarno Basin (Northern Apennines, Italy). These late Pliocene deposits belong to the Castelnuovo Synthem, which is the oldest unconformity bounded stratigraphic unit of the basin fill. Facies analyses were performed on a paleomagnetically calibrated log, depicting two palustrine to lacustrine systems separated by a short-lived fluvial phase. The lacustrine deposits are structured in rhythms and composed of lower clastic and upper siderite-enriched beds. The rhythmic stacking pattern has been analysed and subjected to a cyclostratigraphic analysis: similar trends between a local facies-derived base level curve and global isotopic stacks suggest climate forcing, which is confirmed by the straightforward tuning of the lacustrine rhythms to precession-related insolation phases. Clastic deposition has been linked to colder and drier phases, involving vegetation retreat and enhanced sediment production and transport. Prevalent chemical deposition (siderite) instead occurs during hot and wet phases and is characterised by vegetation blooming and then a generalised reduction in clastic supply. This mechanism is probably coupled with lake level fluctuations that are related to variations in the regional hydrological balance. The orbital behaviour of the Castelnuovo Synthem is characterised by major sensitivity to precession, but a weaker response to 100 ky eccentricity. However, eccentricity modulation is marked by a 400 ky minimum, which recurs in correspondence with a sub-aerial exposure between the two lacustrine bedsets. No obliquity-related rhythmicity has been recorded. The succession investigated here is consistent with other Pliocene palustrine to lacustrine circum-Mediterranean case studies. The results have also been compared to a previous cyclostratigraphic framework elaborated from magnetic susceptivity, allowing critical comparisons to be made between this latter proxy and the analysis of sedimentary facies cycles.