The formation of Pliocene sapropels and carbonate cycles in the Mediterranean: Diagenesis, dilution, and productivity

Abstract

High‐resolution micropaleontological (planktonic foraminifera and calcareous nannofossils) and geochemical (stable isotopes, organic carbon, Fe, P, S, Ca, Ba, Mn, and Al) records are presented for the first sapropel‐containing carbonate cycle in the Pliocene of Sicily. The carbonate cycle is characterized by a gray to white to beige to white color layering typical of the marls of the Trubi formation. A faintly laminated sapropel is intercalated in the gray‐colored bed of the carbonate cycle. CaCO3 content varies from 40% in the beige to 45‐50% in the white layers. Lowest CaCO3 content of 25–30% is found in the gray layer and sapropel. Variations in carbonate and organic matter percentages can best be explained by changes in paleoproductivity rather than by variations in dilution and dissolution. Total productivity was highest during deposition of the gray layer and sapropel, as indicated by high organic carbon and Ba contents and high abundance of Globorotalia puncticulata. Carbonate production reached its highest values, however, during deposition of the white layers, as evidenced by enhanced abundances of planktonic foraminifera and nannofossils. The low carbonate content in the gray layer and sapropel is explained in terms of a collapse in carbonate production caused by extreme changes in the physical and biochemical properties of the water column, which in turn resulted in siliceous plankton and opportunistic foraminifers such as Globorotalia puncticulata outcompeting most calcareous organisms. The beige layer represents a low‐productivity environment similar to the present‐day eastern Mediterranean basin. Several mechanisms have previously been proposed to explain variations in productivity in the eastern Mediterranean. Both sapropels and gray layers were deposited at times when perihelion occurred in northern hemisphere summer. We envisage that the increase in seasonal contrast resulting from this orbital configuration enhanced winter mixing and stabilization of the water column during summer, both leading to favorable conditions for intensification of the spring bloom. In addition, a decrease in excess evaporation, as can be deduced from the δ18O record, led to shoaling of the pycnocline and reduced circulation, thus enhancing the availability of nutrients in the photic zone. Finally, enhanced precipitation and associated runoff should have caused an increase in river‐borne nutrients.

Variations in Mediterranean–Atlantic exchange across the late Pliocene climate transition

“…Pollen-based data suggest that annual precipitation and humidity in the Mediterranean were considerably higher relative to present-day levels during most of the late Pliocene with the exception of the strong glacial periods M2, G22, and G6-104 (Bertini, 2010;Fauquette et al, 1998Fauquette et al, , 1999Fauquette et al, , 2007. A warmer and more humid climate implies higher runoff and freshening of the Mediterranean surface waters leading to depleted δ 18 O records as reported from Pliocene to Holocene data and models in previous studies (e.g., Gudjonsson and van der Zwaan, 1985;Kaboth et al, 2017;Thunell and Williams, 1989;Tindall and Haywood, 2015;Van Os et al, 1994;Vergnaud-Grazzini et al, 1977). We thus consider the δ 18 O depletion of Mediterranean waters during a warm and humid paleoclimate as the most likely explanation for the reversed gradients observed for interglacial stages from all three studied intervals as well as for the comparably warm glacial periods MIS G22, G20, G16, and G4 (Fig.…”

“…4b, 5b). Seasonal variations in Atlantic inflow reported in previous studies (e.g., Bormans et al, 1986;Ovchinnikov, 1974;Parada and Cantón, 1998;Vargas-Yáñez et al, 2002) can be the cause of sea surface salinity (SSS) and sea surface temperature (SST) variability. While seasonal changes in SSS are < 0.5, SST variability is more prominent and may result in brief temporary reversals of SST gradients (MEDATLAS, 2002;Rogerson et al, 2010).…”

“…Mediterranean-Atlantic water mass exchange through the Strait of Gibraltar is driven by a two-directional current system in which the westward outflow of warm and saline Mediterranean Outflow Water (MOW) at the bottom, driven by excess evaporation in the Mediterranean, is compensated for by the inflow of colder and less saline North Atlantic Central Water (NACW) at the surface (Bormans et al, 1986;Ochoa and Bray, 1991;Vargas-Yáñez et al, 2002). This exchange plays an important role for regional and global climate in two respects.…”

“…On its way, it mixes with upwelled MOW within the Strait of Gibraltar (Folkard et al, 1997) and with surface waters of the Alboran Sea, creating the Modified Atlantic Water with temperatures of 15-16 • C and a salinity of 36.5 (Millot, 1999), which follows two anticyclonic gyres, the Western Alboran Gyre (WAG) and the Eastern Alboran Gyre (EAG; Fig. 1) (Gascard and Richez, 1985;Vargas-Yáñez et al, 2002). The northern Alboran Sea is affected by upwelling along the Spanish coast providing nutrients and enhanced primary productivity (Minas et al, 1991;Peeters et al, 2002;Sarhan et al, 2000).…”

Variations of Mediterranean–Atlantic exchange across the late Pliocene climate transition

“…A warmer and more humid climate implies higher runoff and freshening of the Mediterranean 225 surface waters leading to depleted δ 18 O records as reported from Pliocene to Holocene in previous studies (e.g. Gudjonsson and van der Zwaan, 1985;Kaboth et al, 2017;Thunell and Williams, 1989;Van Os et al, 1994;Vergnaud-Grazzini et al, 1977). We thus consider δ 18 O depletion of Mediterranean waters during a warm and humid paleoclimate as the most likely explanation for the reversed gradients observed for interglacial stages from all three studied intervals as well as for the comparably warm 230 glacial periods MIS G22, G20, G16, and G4 (Fig.…”