Trace Element Profiles in Sediments as Proxies of Dead Zone History; Rhenium Compared to Molybdenum

Abstract

Warm-season dead zones-volumes of coastal water containing too little O(2) to support macrofauna-are a growing global menace. Trace elements that are deposited in sediments in response to reducing or sulfidic conditions can provide proxy records for reconstructing dead zone evolution. Based on relative enrichment in reduced vs oxidized marine sediments, Re seems promising as a dead zone proxy. Here, Re is determined by isotope dilution mass spectrometry in sediments underlying the summertime dead zone in Chesapeake Bay. Contrary to expectation, Re becomes only modestly (∼2-fold) elevated during the 20th century and fails to track the historic record of summertime O(2) depletion. Rhenium enrichments are watershed-specific and apparently controlled by anthropogenic sources, not by redox-linked authigenic processes. In contrast, Mo enrichments do track historic O(2) depletion. Three factors cause redox control to override anthropogenic control in the case of Mo: relative to weathering fluxes, anthropogenic Mo fluxes are weaker than Re fluxes; during anoxic periods, Mn refluxing amplifies Mo but not Re concentrations near the sediment surface; and high pore water sulfide-polysulfide promotes Mo fixation in pyrite while promoting formation of organo-Re adducts; the latter are too mobile and reactive to preserve a reliable historic record under seasonally fluctuating redox conditions.

A 1500-year multiproxy record of coastal hypoxia from the northern Baltic Sea indicates unprecedented deoxygenation over the 20<sup>th</sup> century

“…Assuming a salinity and temperature of 5.6 and 9 ºC, respectively (Virtasalo et al, 10 2005) and a pH of 6.6 (our unpublished data) at the sediment-water interface, this total sulfide concentration corresponds to H2Saq concentration of > 70 µM (calculated with R package AquaEnv, Hoffmann et al, 2010), clearly exceeding the requirement of 11 µM H2Saq for the activation of the sulfide-switch (Helz, et al, 1996;Erickson and Helz, 2000). The annual accumulation rate of Mo is expected to be controlled by the duration of the late summer period when the SMTZ is located closest to the sediment surface (Adelson et al, 2001;Helz and Adelson, 2013). By extension, when this period is longer in 15 duration we expect seasonal hypoxia in the bottom water to be more intense, and thus that the Mo accumulation rate provides a first-order proxy for bottom water hypoxia during the MoWP.…”

“…Despite the decline in the bottom water oxygen concentration during the MCA, we infer that the pore water chemistry was 10 typified by a relatively deep and poorly-developed SMTZ with low H2S concentration, which hampered efficient Mo sequestration (Helz and Adelson, 2013) and Ph production by methanogenic microbes (Sect. 6.3.2).…”

“…Algeo and Lyons, 2006;Scott and Lyons, 2012;Helz and Adelson, 2013 (Mort et al, 2010;Jilbert and Slomp, 2013;Jilbert et al, 2015;Dijkstra et al, 2016;Hardisty et al, 2016;van Helmond et al, 2017). The sensitivity of sedimentary Mo content to redox fluctuations is due to the conversion of the relatively inert molybdate ion (MoO4 2-) in seawater to a series of particle-reactive thiomolybdates (MoOxS4-x) under exposure to hydrogen sulfide (H2S).…”

“…Upon reduction of Mn oxides, Mo is released into the pore waters, from where it may efflux back to the water column or become sequestered into the sediments in the presence of sufficiently high sulfide levels. In such settings, the depth and intensity of the pore water H2S maximum (in Baltic Sea sediments typically associated with the sulfate-methane transition zone SMTZ, Egger et al, 2015;Jilbert et al, 2017) is expected to regulate the amount of Mo sequestered in the sediment (Adelson et al, 2001;Scott and Lyons, 2012;Helz and Adelson, 2013;Sulu-15 Gambari et al, 2017).…”

“…6). Due to the fact that the study site is seasonally hypoxic, rather than permanently anoxic or euxinic, the most likely mechanism for Mo enrichment is via diffusion of seawater Mo into the sediment towards the SMTZ (see Helz and Adelson, 2013), which may be amplified by the shuttling of Mo associated with Mn oxides (Algeo and Lyons, 2006;Scheiderich et al, 2010;Scott and Lyons, 2012;Sulu-Gambari et 5 al. 2017).…”