The effect of bioturbation in pelagic sediments: Lessons from radioactive tracers and planktonic foraminifera in the Gulf of Aqaba, Red Sea

Abstract

Studies of recent environmental perturbations often rely on data derived from marine sedimentary records. These records are known to imperfectly inscribe the true sequence of events, yet there is large uncertainty regarding the corrections that should be employed to accurately describe the sedimentary history. Here we show in recent records from the Gulf of Aqaba, Red Sea, how events of the abrupt disappearance of the planktonic foraminifer Globigerinoides sacculifer, and episodic deposition of the artificial radionuclide 137 Cs, are significantly altered in the sedimentary record compared to their known past timing. Instead of the abrupt disappearance of the foraminifera, we observe a prolonged decline beginning at core depth equivalent to ~30 y prior to its actual disappearance and continuing for decades past the event. We further observe asymmetric smoothing of the radionuclide peak. Utilization of advection-diffusion-reaction models to reconstruct the original fluxes based on the known absolute timing of the events reveal that it is imperative to use a continuous function to describe bioturbation. Discretization of bioturbation into mixed and unmixed layers significantly shifts the location of the modeled event. When bioturbation is described as a continuously decreasing function of depth, the peak of a very short term event smears asymmetrically but remains in the right depth. When sudden events repeat while the first spike is still mixed with the upper sediment layer, bioturbation unifies adjacent peaks. The united peak appears at an intermediate depth that does not necessarily correlate with the timing of the individual events. In a third case, a long lasting sedimentary event affected by bioturbation, the resulting peak is rather weak compared to the actual event and appears deeper in the sediment column than expected based on the termination of the event. The model clearly shows that abrupt changes can only endure in the record if a thick sediment layer settled on the sediment-water interface at once or if bioturbation rates decreased to very low values for a prolonged period of time. In any other case smearing by bioturbation makes an abrupt event appear to have started shortly before the real timing and end long after its true termination.

Sedproxy: a forward model for sediment-archived climate proxies

“…After the creation of proxy carriers such as foraminiferal shells or alkenone molecules, a proportion of these are exported to and buried in the sediment. The upper few centimetres of marine sediments are typically mixed by burrowing organisms down to a depth of around 2-15 cm (Boudreau, 1998, 9.8 ± 4.5 cm, 1 SD) (Teal et al, 2010;Trauth et al, 1997, 8.37 ± 6.19 cm), although laminated sediments absent of bioturbation do exist. Marine sediment accumulation rates vary over many orders of magnitude (Sadler, 1999;Sommerfield, 2006) but rates at core locations used for climate reconstructions are typically of the order of 1-100 cm kyr −1 .…”

“…Estimated from radiocarbon or from global distribution (Teal et al, 2010 where z is the number of discrete depths to be included.…”

A lattice-automaton bioturbation simulator for the coupled physics, chemistry, and biology of marine sediments (eLABS v0.1)

“…The consequent physical and chemical disturbances caused by infauna can be defined as bioturbation sensu lato, e.g., Aller (1982). As geological records of changing environmental conditions are often assumed to represent monotonic changes with time, and ideally ones that can be sampled at high resolution to help understand rapid events and transitions, understanding of biota-induced physicochemical disturbances is indispensable for correct interpretation of past environments, e.g., Berger et al (1977), Trauth (1998Trauth ( , 2013, Meysman et al (2006a), Ridgwell (2007), Panchuk et al (2008), Canfield and Farquhar (2009), Hull et al (2011), Steiner et al (2016), and Kirtland Turner et al (2017). Numerical models of the appropriate processes involved represent invaluable tools in this effort.…”

Sedproxy: a forward model for sediment archived climate proxies

“…However, the general effect of bioturbation should also apply under conditions of incomplete mixing and the code could easily be modified to use a more complex bioturbation model (e.g., Guinasso and Schink, 1975;Steiner et al, 2016) to generate the weights used to sample the input 20 climate signal. We note that when sedimentation rates are low relative to mixing rates, more complex mixing models converge to the simple box type model that we employ here (Matisoff, 1982).…”