https://doi.org/10.1130/G46837.1 Manuscript received 3 August 2019 Revised manuscript received 3 October 2019 Manuscript accepted 4 October 2019 © 2019 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 19 November 2019 Biomediation of submarine sediment gravity flow dynamics Melissa J. Craig1, Jaco H. Baas2, Kathryn J. Amos1, Lorna J. Strachan3, Andrew J. Manning4, David M. Paterson5, Julie A. Hope6, Scott D. Nodder7 and Megan L. Baker2 1 Australian School of Petroleum, The University of Adelaide, Adelaide, South Australia 5000, Australia 2 School of Ocean Sciences, Bangor University, Menai Bridge LL59 5AB, UK 3 School of Environment, University of Auckland, Auckland 1142, New Zealand 4 HR Wallingford, Howbery Park, Wallingford OX10 8BA, UK 5 Scottish Oceans Institute, School of Biology, University of St. Andrews, St. Andrews KY16 8LB, UK 6 Institute of Marine Science, University of Auckland, Auckland 1142, New Zealand 7 National Institute of Water & Atmospheric Research, Wellington 6021, New Zealand ABSTRACT (Tolhurst et al., 2002). Small concentrations of Sediment gravity flows are the primary process by which sediment and organic carbon are EPS (CEPS < 0.063% by weight, wt%), represent- transported from the continental margin to the deep ocean. Up to 40% of the total marine ing baseline levels in estuarine sediment, can in- organic carbon pool is represented by cohesive extracellular polymeric substances (EPS) crease the development time of bed forms expo- produced by microorganisms. The effect of these polymers on sediment gravity flows has not nentially (Malarkey et al., 2015). EPS research been investigated, despite the economic and societal importance of these flows. We present to date has focused on coastal environments. the first EPS concentrations measured in deep-sea sediment, combined with novel labora- The presence of EPS in deep-sea sediment and tory data that offer insights into the modulation of the dynamics of clay-laden, physically their impact on SGFs is not known. cohesive sediment gravity flows by biological cohesion. We show that EPS can profoundly EPS represent up to 40% of the marine or- affect the character, evolution, and runout of sediment gravity flows and are as prevalent in ganic carbon pool (Falkowski et al., 1998). The deep oceans as in shallow seas. Transitional and laminar plug flows are more susceptible to burial of organic carbon (Corg) in marine sedi- EPS-induced changes in flow properties than turbulent flows. At relatively low concentra- ment represents the second largest sink of atmo- tions, EPS markedly decrease the head velocity and runout distance of transitional flows. spheric CO2 (Galy et al., 2007). However, global This biological cohesion is greater, per unit weight, than the physical cohesion of cohesive carbon budget studies typically estimate Corg flux clay and may exert a stronger control on flow behavior. These results significantly improve by measuring the particulate organic matter set- our understanding of the effects of an unrealized biological component of sediment gravity tling through the water column and accumulat- flows. The implications are wide ranging and may influence predictive models of sediment ing in marine sediment (Martin et al., 1987; gravity flows and advance our understanding about the ways in which these flows transport Muller-Karger et al., 2005; Decho and Gutier- and bury organic carbon globally. rez, 2017). SGFs are not currently considered to be a significant orgC transport and burial process INTRODUCTION of clay-rich SGFs show that at sufficiently high in Corg flux models on continental margins. This Clay, inherently associated with organic concentrations of clay, flocs bind to form a net- is surprising, since fine-grained sediment flows matter, is the most abundant sediment type on work that behaves as a gel, increasing viscos- have been found to transport the majority of Corg Earth (Hillier, 1995). Recent advances in our ity and suppressing shear-generated turbulence (de Haas et al., 2002). Here, we present the first understanding of the properties of clay have re- (Baas and Best, 2002; Baas et al., 2009). These measurements of EPS from deep-sea sediment defined our models of submarine sediment grav- flows, and their deposits, are radically different cores, and, through novel experiments, we test ity flows (SGFs) in both modern and ancient to fully turbulent flows and highlight the impor- if EPS-derived biological cohesion is capable of environments (Wright and Friedrichs, 2006; tance of understanding how cohesive material inhibiting turbulence and intensifying cohesive Barker et al., 2008; Sumner et al., 2009). SGFs affects SGFs. SGF behavior. This has fundamental implica- are volumetrically the most significant sediment Extracellular polymeric substances (EPS) tions for (1) understanding SGF behavior in the transport process in the ocean, they can pose de- are secreted by microorganisms in many envi- natural environment, (2) models of Corg flux on structive hazards to offshore infrastructure, and ronments, from rivers and estuaries to hypersa- continental margins and estimating Corg burial, as their deposits form major hydrocarbon reser- line systems and deep-sea hydrothermal vents well as (3) reconstruction of past environments voirs (Talling, 2014). Understanding and predic- (Decho and Gutierrez, 2017). The adhesion of from ancient deposits and predicting hydrocar- tion of SGF behavior therefore have scientific, this exopolymer to sediment grains forms a ma- bon reservoir properties. economic, and social importance. trix of EPS, sediment, and single-cell organisms Clay-rich SGFs are governed by the ability called a biofilm, which is considered to be the METHODS of clay minerals to aggregate, or flocculate (Mc- primary mechanism by which benthic micro- To explore the impact of biological cohe- Cave and Jones, 1988). Laboratory experiments organisms stabilize the sediment they inhabit sion on the dynamics of cohesive SGFs, series CITATION: Craig, M.J., et al., 2020, Biomediation of submarine sediment gravity flow dynamics: Geology, v. 48, p. 72–76, https://doi.org/10.1130/G46837.1 72 www.gsapubs.org | Volume 48 | Number 1 | GEOLOGY | Geological Society of America Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/48/1/72/4904671/72.pdf by University of St Andrews user on 21 January 2020 of experimental SGFs were generated with and tional flows experiencing turbulence suppression were also visually indistinguishable from turbu- without EPS. These flows were generated in a by physical cohesion (Table 1). Turbidity cur- lent clay-only flow F07. However, the Uh of F08 5-m-long, 0.2-m-wide, and 0.5-m-deep, smooth- rents traveled the length of the tank and gener- and F09 decreased more rapidly than for F07 bottomed lock-exchange tank (Fig. DR1 in the ated shear waves along their upper interface with between 4 m and 4.2 m along the tank, result- 1 GSA Data Repository ). The reservoir was filled the ambient fluid Table 2( ). The TTPFs featured a ing in lower Uh values at 4.6 m (Fig. 1). At the with a mixture of kaolinite clay (volumetric con- dense, laminar lower “plug” layer with coherent highest CEPS, F10 began to decelerate rapidly at centration Cclay = 5–23 vol%), EPS, and seawa- fluid entrainment structures (Baker et al., 2017; 2.5 m and halted at 3.9 m. ter. Xanthan gum was used as a proxy for natural Figs. DR4 and DR5) that transitioned upward TTPFs at Cclay = 22 vol% and Cclay = 23 EPS (cf. Tan et al., 2014). Xanthan gum shares into a dilute turbulent layer (Table 2). vol% exhibited distinct decreases in Uh and chemical similarities with a wide variety of EPS runout distance as EPS were added (Fig. 2; and is widely used as a substitute for EPS in FLOWS WITH EPS Fig. DR8). Normalized to the maximum head marine ecology, soil science, and sediment sta- Adding EPS to the turbidity currents with velocity, Uh,max, and runout distance of the clay- bility research (see the Data Repository). The Cclay < 15 vol% produced no visual changes in only flows, the combinedC clay = 22 vol% and range of CEPS used in the experiments (0–0.268 flow behavior and no measurable differences Cclay = 23 vol% data are strongly correlated with wt%), was informed by seabed sediment cores in the Uh profiles (Figs. DR6 and DR7). At CEPS (Figs. DR9 and DR10). In Cclay = 22 vol% obtained during RV Tangaroa cruise TAN1604, Cclay = 15 vol%, the EPS-laden flows Table 1( ) flows, CEPS ≤ 0.089 wt% still produced TTPFs from 127 to 1872 m depths in the Hauraki Gulf, North Island of New Zealand (Figs. DR2 and TABLE 1. BASIC EXPERIMENTAL DATA DR3; Table DR1). These EPS data are based on Kaolinite EPS weight Uh,max Flow –1 bulk carbohydrate content, collected using the Flow Cclay (vol%) (%) ROD (m) (m s ) classification standard assay method of DuBois et al. (1956). F01 5 0 – 0.377 Turbidity current F02 5 0.134 – 0.379 Turbidity current The maximum CEPS recorded in the TAN1604 F03 5 0.250 – 0.381 Turbidity current cores was 0.260 wt%, with an average value of F04 10 0 – 0.367 Turbidity current F05 10 0.132 – 0.353 Turbidity current 0.139 wt%. For comparison, background EPS F06 10 0.264 – 0.348 Turbidity current F07 15 0 – 0.430 Turbidity current content ranges from 0.01 to 0.1 wt% measured F08 15 0.066 – 0.417 Turbidity current in estuarine sediment and from 0.1 to 0.67 wt% F09 15 0.133 – 0.416 Turbidity current F10 15 0.265 3.91 0.420 Turbidity current measured in freshwater sediment (Gerbersdorf F11 22 0 4.69 0.552 TTPF et al., 2009; Malarkey et al., 2015). F12 22 0.067 3.63 0.455 TTPF F13 22 0.089 3.20 0.438 TTPF In the laboratory, we compared the head ve- F14 22 0.133 2.13 0.217 Plug flow locity, U , and runout distance of clay-only con- F15 22 0.265 0.92 0.194 Plug flow h F16 23 0 3.66 0.471 TTPF trol flows with equivalent flows containing EPS F17 23 0.052 2.94 0.439 TTPF F18 23 0.087 1.80 0.419 Plug flow to test if biological cohesion intensifies cohesive F19 23 0.130 1.32 0.211 Plug flow flow behavior.U h versus horizontal distance was F20 23 0.259 0.60 0.160 Slide measured using a high-definition video camera.