The mechanism of sapropel formation in the Mediterranean Sea: insight from long-duration box model experiments

Abstract

Abstract. Periodic bottom-water oxygen deficiency in the Mediterranean Sea led to the deposition of organic-rich sediments during geological history, so-called sapropels. Although a mechanism linking the formation of these deposits to orbital variability has been derived from the geological record, physics-based proof is limited to snapshot and short-time-slice experiments with (oceanic) general circulation models. Specifically, previous modelling studies have investigated atmospheric and oceanographic equilibrium states during orbital extremes (minimum and maximum precession). In contrast, we use a conceptual box model that allows us to focus on the transient response of the Mediterranean Sea to orbital forcing and investigate the physical processes causing sapropel formation. The model is constrained by present-day measurement data, while proxy data offer constraints on the timing of sapropels. The results demonstrate that it is possible to describe the first-order aspects of sapropel formation in a conceptual box model. A systematic model analysis provides new insights on features observed in the geological record, such as the timing of sapropels as well as intra-sapropel intensity variations and interruptions. Moreover, given a scenario constrained by geological data, the model allows us to study the transient response of variables and processes that cannot be observed in the geological record. The results suggest that atmospheric temperature variability plays a key role in sapropel formation and that the timing of the midpoint of a sapropel can shift significantly with a minor change in forcing due to nonlinearities in the system.

Abrupt and Persistent Shutdown of the Thermohaline Forcing During Mis5e in the Adriatic Sea: Insights from Shallow-Water Sapropel Sediments

“…In summary, we suggest that the centennial-scale variability in the thermohaline forcing reconstructed in the Northern Adriatic was primarily caused by the combination of freshening of surface waters with winter temperature increasing (Fig. 4g,k) that mutually hampered the formation of the NAdDW (Dirksen and Meijer, 2020). Similarly, the complete resumption of the NAdDW 6 millennia after its shutdown occurred only when the surface salinity and the winter temperature turned back to pre-sapropel values.…”

The organic, inorganic and isotope geochemistry of the holocene sapropel units in the sea of Marmara and their paleoceanographic significance

“…Several factors have been involved in explaining the sapropel formation in the Mediterranean, the Black Sea, and Sea of Marmara: a. increased influx of terrestrial organic matter and freshwater, which contribute to the onset of the sapropel formation (Aksu et al, 1999;Bouloubassi et al, 1999;Çağatay et al, 1999;Çağatay et al, 2000;Checa et al, 2020;Tolun et al, 2002;Vidal et al, 2010); b. enhanced surface ocean productivity and export of organic matter to sediments (Calvert and Fontugne, 1987;Calvert et al, 1992;Sperling et al, 2003); c. suboxic/anoxic conditions resulting from a large influx of freshwater/marine flooding/enhanced monsoon intensity (Benkovitz et al, J o u r n a l P r e -p r o o f 2020; Çağatay et al, 2019;Çağatay et al, 2000;De Lange et al, 2008;Möbius et al, 2010;Moodley et al, 2005;Rohling et al, 2015); d. combination of increased production and better preservation of organic matter (Bouloubassi et al, 1999;Çağatay et al, 2009;Checa et al, 2020;Higgins et al, 2010;Lane-Serff et al, 1997;Tolun et al, 2002;Triantaphyllou et al, 2014;Vidal et al, 2010); e. precession-controlled atmospheric temperature variability (Dirksen and Meijer, 2020;Triantaphyllou et al, 2014). However, the relative importance of these factors for sapropel formation is still being debated, and conflicting interpretations are often invoked.…”