Spatially variable bioturbation and physical mixing drive the sedimentary biogeochemical seascape in the Louisiana continental shelf hypoxic zone

Abstract

Seasonal hypoxia on the Louisiana continental shelf (LCS) has grown to over 22,000 km 2 with limited information available on how low oxygen effects the benthos. Benthic macrofaunal colonization and sediment biogeochemical parameters were characterized at twelve stations in waters 10-50 m deep along four transects spanning 320 km across the LCS hypoxic zone in the early fall of 2010 when bottom waters typically return to oxic conditions. Chemical data and sediment profile imaging (SPI) support three primary mechanistic pathways of organic matter degradation on the LCS: (i) metal oxide cycling in depositional muds, (ii) infauna-driven bioturbation delivering oxygen below the sediment-water interface, and (iii) sulfate reduction in sediments where iron oxide availability is limited. The transect nearest the Mississippi River delta had the highest concentrations of porewater and solid phase Mn and Fe with SPI images of recently deposited reddish, mixed muddy sediments suggestive of metal cycling. The deepest stations had high oxidized iron concentrations and rust colored sediments with faunal colonization that suggests sediments are oxidized via bioturbation. Many nearshore and central LCS stations had more black sediments, more disturbed clay layers, lower amounts of oxidized iron, and higher sulfate reduction rates than the deepest stations. Sediment mixing coefficients, D B , determined from chlorophyll-a concentration profiles varied between 33 and 183 cm −2 y −1. D B values were highest at the deepest stations where sediments were colonized. D B were not determined at two *

Early Diagenesis in the Hypoxic and Acidified Zone of the Northern Gulf of Mexico: Is Organic Matter Recycling in Sediments Disconnected From the Water Column?

“…At these stations located on the fringes of hypoxia (Rabalais and Baustian, 2020), they showed that bioturbation recovered rapidly in September, after an oxygen intrusion event in the bottom waters. A similar observation was made by Devereux et al (2019) in late September, with oxic waters and active bioturbation in the sediment of seasonally hypoxic zones. Baustian and Rabalais (2009) and Rabalais and Baustian (2020) indicated that sediments in area of the northern GoM severely impacted by seasonal hypoxia contain smaller, less diverse infauna with lower biomass that are primarily opportunists and either surface deposit feeders or subsurface deposit feeders in the upper centimeter of the sediments, where the organic matter is deposited.…”

“…Numerous publications have described the life conditions of demersal and benthic fauna in the context of hypoxia (Rabalais et al, 2001a(Rabalais et al, ,b, 2002Levin et al, 2009;Middelburg and Levin, 2009;Briggs et al, 2015;Devereux et al, 2019;Rabalais and Baustian, 2020). Hypoxic areas are known as "Dead Zones" due to the scarcity of fishable species that leave the area when oxygen is below the hypoxic threshold of 63 µmol L −1 (Rabalais et al, 2001b).…”

“…Pore water data obtained at five stations across the Louisiana shelf for 5 years (2002)(2003)(2004)(2005)(2006)(2007) were published in a report (Murrell et al, 2013) without a thorough discussion of the underlying biogeochemical processes. Later, these processes were partially discussed for two of the five stations (Devereux et al, 2015;Laurent et al, 2016), although a larger collection of pore water profiles was published more recently (Devereux et al, 2019) together with sulfate reduction rate measurements. In contrast to the large sediment oxygen demands reported in the above studies, all data sets indicated poorly active subsurface sediments with limited increase of metabolite concentrations (NH 4 + , DIC) in shelf sediments under the river plume and adjacent continental shelf waters.…”

Using Stable Isotopes to Disentangle Marine Sedimentary Signals in Reactive Silicon Pools

“…Using δ 30 Si signals, we infer two pathways for formation of acid‐leachable reactive silica and suggest LSi (with some refractory bSiO 2 ) requires fewer and less exaggerated fractionation steps than bSiO 2 , although bSiO 2 δ 30 Si is well correlated to that of the acid‐leachable pool. Furthermore, we have shown independently (without aid of electron microscopy) that a majority of the water‐column diatom bSiO 2 in these sediments is altered rapidly (on the order of months) during early diagenesis in the surface sediments, as sediment deposition in the Mississippi delta can be variable (upward of 10 cm a −1 ) (Adhikari et al, 2016; Chen et al, 2005; Devereux et al, 2019). Given eutrophication in the river waters feeding this system, which have led to increasing bSiO 2 in regional sediments, the strength of this process as an elemental sink may have increased in the last half century.…”

Benthic Respiration in Hypoxic Waters Enhances Bottom Water Acidification in the Northern Gulf of Mexico

“…Remineralization rates of organic matter due to denitrification are often less than rates of manganese‐ and iron‐oxides reduction. The latter two processes are the dominant anaerobic processes in the upper 10 cm of sediment cores across the nGoM shelf (Devereux et al, 2015, 2019). Manganese‐ and iron‐oxides reduction and sulfate reduction all increase TA at a faster rate than DIC (Cai et al, 2010).…”