Latest Pliensbachian to Early Toarcian depositional environment and organo-facies evolution in the North-German Basin (Hondelage Section)

Abstract

The Pliensbachian/Toarcian boundary interval represents a transition from a coldhouse into a hothouse climate state, involving the demise of a land-based cryosphere, initiating a third-order global sea-level rise. Within the intensely studied Northwest Tethyan shelf region, the South-German Basin has been investigated in more detail than the North-German Basin (NGB). We here provide a palaeoenvironmental reconstruction of the Pliensbachian/Toarcian transition from the Hondelage fossil excavation site located in the NGB employing organic, isotope, and major/trace element proxies. Here, the late Pliensbachian was characterized by cold climate, low sea level, and a slow hydrological cycle, causing minor terrigenous sediment and nutrient fluxes to the basin, instigating low marine productivity. Shallow, well-mixed shelf waters of normal salinity favored aerobic degradation of planktonic biomass, preventing sedimentary accumulation of organic matter. These conditions changed in the earliest Toarcian, where increased temperatures led to sea-level rise via meltdown of land-based ice and accelerated the hydrological cycle, causing salinity stratification. Enhanced riverine sediment and nutrient supply from nearby landmasses promoted marine primary productivity, which caused anoxic conditions in bottom and pore waters favoring enhanced preservation and accumulation of organic matter. A short-lived sea-level fall at the Lower Elegans Bed coincided with lowered productivity and enhanced carbonate precipitation, due to reduced runoff and recovery of the carbonate factory. Increased redox-sensitive trace element concentrations above the Lower Elegans Bed suggest a renewed inflow of low-salinity arctic water masses via the Viking Corridor and potentially increased freshwater input, promoting water column stratification, enhanced planktonic productivity, and re-establishment of bottom water anoxia/euxinia. Graphical abstract