Manganese, iron and phosphorus cycling in an estuarine mudflat, Loire, France

Abstract

The sampling of surface sediment from two sites of a mudflat of the Loire Estuary during four contrasting seasons has led to new information about geochemical cycling under transient diagenesis fuelled by flood deposition. Based on stocks of reactive iron-oxides and manganese-oxides (ascorbateextracted) and pore water concentrations, the progressive evolution of flood deposits is described. Three major steps are observed: at first, there is no manganese, iron and phosphorus release into pore water within the flood-deposited layer. Then, during a period of approximately 1 month, Mn oxides are consumed while the dissolved Mn concentration increases. Simultaneously, the Fe oxide-rich layer from flood deposition prevents (or at least limits) phosphorus release into pore water as shown by the increasing P/Fe ratio of the ascorbate extractions. During spring and summer, Fe oxides are reductively dissolved until complete depletion results. This period also corresponds to the saturation of Fe oxides by phosphorus and probably maximum P release to the water column. The site located closer to the shore showed higher density of benthic faunas leading to more intense bioirrigation. The importance of bioturbation on the year scale for biogeochemical processes is discussed according to both bioirrigation and biomixing processes. Highlights ► Flood deposition fuels a > 4 month period of transient geochemical conditions involving sedimentary Mn, Fe and P. ► A sequential exhaustion of Mn and Fe reactive oxides due to diagenetic processes occurs at yearly average rates of 30 μmol m − 2 d − 1 and 170 μmol m − 2 d − 1 , respectively. ► These reactions control phosphorus release towards water column.

Mn∕Ca intra- and inter-test variability in the benthic foraminifer <i>Ammonia tepida</i>

“…Dissolved Mn 2+ can then diffuse upwards in the pore water across the oxic-anoxic boundary, where it precipitates again in the form of Mn oxides, leading to a continuous cycling of Mn within the upper sediment (Aller, 1994;Slomp et al, 1997). The sedimentation rate and the bioturbation intensity are important factors controlling the cycling of various reactive compounds in sediments, such as metal oxides (van de Velde and Meysman, 2016). Macrofaunal bioturbation may introduce Mn oxides in the deeper, anoxic sediment, where these minerals are subsequently reduced (Mouret et al, 2009;Thibault de Chanvalon et al, 2016).…”

“…The sedimentation rate and the bioturbation intensity are important factors controlling the cycling of various reactive compounds in sediments, such as metal oxides (van de Velde and Meysman, 2016). Macrofaunal bioturbation may introduce Mn oxides in the deeper, anoxic sediment, where these minerals are subsequently reduced (Mouret et al, 2009;Thibault de Chanvalon et al, 2016). When eutrophication and stratification of the water column lead to (seasonally) hypoxic bottom water conditions, the oxygen penetration depth is reduced, causing an upward movement of the Mn redox front and diminishing the possibility that pore water Mn 2+ is oxidized.…”

“…Sedimentary records of manganese may reflect a variety of past environmental conditions, including bottom water redox state, continental runoff, surface water productivity and bottom water current dynamics (Reichart et al, 1997;van der Weijden et al, 2006;Lenz et al, 2015). The use of sedimentary manganese as a proxy is complicated because of its complex biogeochemical dynamics, including the remobilization of once precipitated manganese oxides, and subsequent diagenetic overprinting (Schenau et al, 2002).…”

“…However, one complicating factor is that the activity of cable bacteria strongly influences the seasonal pattern of sediment geochemical cycles in Lake Grevelingen. These electrogenic bacteria induce strong shifts in the sediment geochemistry (Seitaj et al, , 2017SuluGambari et al, 2016a, b) and lead to substantial Mn 2+ mobilization in pore waters (Rao et al, 2016;Sulu-Gambari et al, 2016b;van de Velde et al, 2016). These cable bacteria tend to have a highly patchy distribution, which hence could complicate the interpretation (Sulu-Gambari et al, 2016b;Seitaj et al, 2017).…”

Transient early diagenetic processes in Rhône prodelta sediments revealed in contrasting flood events

“…Nittrouer and Sternberg, 1981;Leithold and Hope, 1999;Miserocchi et al, 2007;Tesi et al, 2008;Wheatcroft et al, 2010;Blair and Aller, 2012), its preservation being more efficient in low-energy zones and active margins (Blair and Aller, 2012). These episodic events cause major perturbations in benthic fauna assemblages (Turk and Risk, 1981;Peterson, 1985;Pelletier et al, 1999;Thrush et al, 2003;Campbell and McKenzie, 2004;Bonifacio et al, 2014), as well as in biogeochemical processes (Deflandre et al, 2000(Deflandre et al, , 2002Mucci et al, 2003;Chaillou et al, 2007;Tesi et al, 2012, Thibault de Chanvalon et al, 2016. The massive flash flood recorded in a Canadian fjord in 1996 led to a flood deposit of about 25 cm of thickness.…”