Surface water dynamics and phytoplankton communities during deposition of cyclic late Messinian sapropel sequences in the western Mediterranean

Abstract

Distinctive precession-scale sapropel/marl sequences of late Messinian age are found in several outcrops bordering the western Mediterranean. To examine the roles of stratification and productivity in driving this cyclic sedimentation, we performed a high resolution analysis of calcareous nannofossil assemblages through four orbital sequences (composed of sapropel/marl/diatomite/marl) from the Sorbas basin in southern Spain. This effort also involved a detailed comparison between calcareous nannofossil abundances and other micropaleontological, sedimentological and geochemical proxies to further understand the behaviour of some taxa. For this period, fluctuations in the depth and gradient of the pycnocline, and the distribution and remobilization of nutrients seem to be the main factors controlling the calcareous nannoplankton assemblages, with temperature of secondary importance. In the sapropels, the warm-oligotrophic genus Sphenolithus and Discoaster are abundant. Especially significant is the progressive increase towards the top of Discoaster pentaradiatus, indicator of severe oligotrophic (deep pycnocline) conditions. At the top of the sapropels, peaks in Reticulofenestra rotaria are interpreted here as the result of an increase of surface water temperatures, although decreasing salinity may also play a role. Small Reticulofenestra (< 5 μm) is the major component of the overlying marl, and peaks in episodes of high nutrient availability (assessed here by the phosphorus proxy P/Ti). This morphotype is linked to a relatively shallow deep chlorophyll maximum, with a stable but shallow thermocline. Overlying the marl is a diatomite bed, interpreted here as an indicator of peak nutrient availability (confirmed by P / Ti ratios), with a reduction of calcareous nannofossils due to nutrient competition and dilution. Interestingly, however, species such as Reticulofenestra pseudoumbilicus (> 5 μm) are more frequent in diatomites even considering conditions of extreme mixing and eutrophication occurred in the basin. Umbilicosphaera jafari peaks just after diatomites, replacing small Reticulofenestra, probably as a response to an increase in salinity and/or nutrient limitation. Species such as Coccolithus pelagicus and Helicosphaera carteri are abundant at the top of the diatomites and in the overlying marls, indicating relatively mesotrophic and cool water conditions. This succession shows similar characteristics in the four studied cycles, allowing us to interpret surface water dynamics in this region on orbital time scales. The top of the sapropel represents a warm, low salinity and well stratified episode under a situation of maximum insolation. A well developed pycnocline and oligotrophic conditions (moderate production of calcareous phytoplankton) coinciding with anoxic conditions at the bottom enhance the preservation of organic matter, and consequently generate a sapropel. Diatomites are deposited under a situation of unstable pycnocline that produce an increase in productivity in surface waters. Diatoms significantly deplete limiting nutrients, and consequently calcareous nannofossils start to be dominant again in the water column as result of a progressive stabilization of the pycnocline.