minerals Review Toward an Improved Understanding of the Marine Barium Cycle and the Application of Marine Barite as a Paleoproductivity Proxy Samantha C. Carter 1,*, Adina Paytan 2 and Elizabeth M. Griffith 1 1 School of Earth Sciences, The Ohio State University, 125 South Oval Mall, Columbus, OH 43210, USA; griffi[email protected] 2 Institute of Marine Sciences, University of California Santa Cruz, 1156 High Street, Santa Cruz, CA 95064, USA; [email protected] * Correspondence: [email protected]; Tel.: +1-512-577-3501 Received: 24 April 2020; Accepted: 5 May 2020; Published: 9 May 2020 Abstract: Marine barite (BaSO4) is a relatively ubiquitous, though minor, component of ocean sediments. Modern studies of the accumulation of barite in ocean sediments have demonstrated a robust correlation between barite accumulation rates and carbon export to the deep ocean. This correlation has been used to develop quantitative relationships between barite accumulation rates and export production and is used to reconstruct export production in the geologic past, particularly during times of dynamic changes in the carbon cycle. We review the processes that affect the formation and preservation of marine barite, as well as those controlling the relationship between the barium (Ba) and carbon biogeochemical cycles. Additionally, we take a new approach to modeling the marine Ba cycle as a two-box model, specifically evaluating Ba utilization in the surface ocean and refining the equation describing the relationship between export production and barite formation. We compare these new results with past modeling efforts. The new model demonstrates that increases in export production can lead to sustained increases in barite accumulation in marine sediments without resulting in complete surface water Ba depletion, which is distinctly different from previous modeling results. Keywords: barite; BaSO4; export production; barium; carbon cycle 1. Introduction The carbon (C) cycle plays a critical role in global climate both in today’s environment and in the geologic past (e.g., [1]). Export production—the flux of organic C (Corg) to the deep-ocean—plays a major role in the C cycle, as it removes C from the atmosphere and sequesters it in the deep-ocean for seasons to centuries (e.g., [2–4]). Increased productivity and/or changes in ecosystem structure in the surface waters can lead to higher export production and C sequestration (e.g., [5]). Changes in the magnitude of export production in-turn can strongly influence atmospheric pCO2 levels (and hence climate) on geological time scales [3,6]. However, export production is difficult to quantify in the present day, and even more so in the geologic past. Several methods have been employed to reconstruct export production, including accumulation of organic matter in marine sediments, benthic foraminiferal assemblages and accumulation rates, and more (e.g., [7]). However, each proxy has its own strengths, weaknesses, and inherent assumptions required to relate changes in the proxy measurement with export production. The accumulation rate of microcrystals of the mineral barite (BaSO4; Figure1) in marine sediments has been widely used as a proxy for export production and will be reviewed here along with the role barite plays in the Minerals 2020, 10, 421; doi:10.3390/min10050421 www.mdpi.com/journal/minerals MineralsMinerals2020 2019, 10, 9, x 421 FOR PEER REVIEW 2 2of of 24 24 Minerals 2019, 9, x FOR PEER REVIEW 2 of 24 processes that affect the formation and preservation of marine barite in the modern ocean, as well as globalthoseprocesses marinecontrolling that barium affect the the coupling (Ba) formation cycle. between The and goal preservation the of Ba this and paper ofC marinebiogeochemical is to shedbarite light in thecycles. on modern the We processes documentocean, as that well recent aff aects theprogressthose formation controlling achieved and preservation theand coupling identify of betweengaps marine in ourthe barite knowledgeBa inand the C modernbiogeochemical of the ocean,marine as Bacycles. well cycle, as We thosereview document controlling and suggest recent the couplingrevisionsprogress between toachieved an existing the and Ba identifytwo and-box C biogeochemicalgaps model in ofour the knowledge marine cycles. Ba Weof cycle, the document marine and recommend Ba recent cycle, progress review future achievedand studies suggest that and identifycouldrevisions enhance gaps to an in our ourexisting ability knowledge two to- boxaccurately of model the marine ofinterpret the Bamarine cycle,the marineBa review cycle, barite and sedimentaryrecommend suggest revisions futurerecord tostudies in an terms existing that of two-boxpastcould export enhance model produ of our thection ability marine. to Ba accurately cycle, and interpret recommend the marine future studiesbarite sedimentary that could enhance record ourin terms ability of to accuratelypast export interpret production the marine. barite sedimentary record in terms of past export production. Figure 1. Images of marine barite (BaSO4) using scanning electron microscopy. (A) Barite (bright Figurewhite)Figure 1. in 1.Images size Images-fractionated of of marine marine bariteparticulate barite (BaSO (BaSO matter4) using4) using collected scanning scanning from electron theelectron water microscopy. microscopy. column ( Ain) theBarite(A )North Barite (bright Atlantic (bright white). in(white)B size-fractionated) Barite in separatedsize-fractionated particulate from sediments particulate matter collected collected matter atcollected from the sediment the from water -thewater column water interface incolumn the Northin inthe the Equatorial Atlantic. North Atlantic (B Pacific) Barite.. separated((CB)) BariteBarite from separatedseparated sediments from from collectedsediments a middle at collected the Miocene sediment-water at the(~13.9 sediment Ma) interface -corewater sample ininterface the Equatorialfrom in the International Equatorial Pacific. ( CPacific Ocean) Barite. separatedDiscovery(C) Barite from Programseparated a middle (IODP) from Miocene aSite middle (~13.9U1337 Miocene Ma)in the core eastern (~13.9 sample equatorialMa) from core International samplePacific, fromlocated Ocean International Discovery~315 m below ProgramOcean the (IODP)seafloDiscoveryor Site. U1337Program in the(IODP) eastern Site equatorialU1337 in Pacific,the eastern located equatorial ~315 m Pacific, below thelocated seafloor. ~315 m below the seafloor. 2.2. Barium Barium Cycling:Cycling: ProcessesProcesses in the Modern Ocean 2. Barium Cycling: Processes in the Modern Ocean TheThe global global marinemarine BaBa cyclecycle isis controlledcontrolled by input from land and from hydrothermal hydrothermal vents vents and and byby the theThe removalremoval global of ofmarine Ba from Ba cycleseawater seawater is controlled as as it it accumulates accumulates by input infrom in marine marine land sedimentsand sediments from hydrothermalprimarily primarily in the invents theform and form of oftheby the themineral mineral removal barit barite ofe Baand and from in in associationseawater association as withit with accumulates iron iron and and manganesein manganese marine sediments o oxyhydroxidesxyhydroxides primarily (Figure (Figure in the 22 )form)[ [8–812]– of12. ]. BariumBatherium mineral isis associated barite and with with in many manyassociation rocks rocks [13] with [13, which], iron which areand areweatheredmanganese weathered on o thexy onhydroxides continents the continents (resultingFigure resulting 2 )in [8 rivers–12] in. riversandBarium groundwater and is associated groundwater adding with adding manyrelatively rocks relatively large [13] , quantitieswhich large are quantities weatheredof dissolved of on dissolved Bathe to continents the Ba surface to resulting the ocean. surface in Ba rivers ocean.rium Bariumconcentrationsand groundwater concentrations in theadding inopen the relatively openocean ocean normally large normally quantities vary varybetween of between dissolved values values Baas low asto lowthe as surface as30 30nmol/kg nmol ocean./kg in in Basurface surfacerium waterswatersconcentrations and and up up to to in 150 150 the nmol nmol/kg open/kg ocean in in the the normally deep deep Pacific Pacific vary [14 [14 between–23–23]]. Additional. Additional values as dissolved dissolvedlow as 3 Ba0 Ba nmol/kg can can be be introduced introducedin surface to deeptowaters deep waters andwaters throughup throughto 150 submarine nmol/kg submarine in hydrothermal the hydroth deep Pacificermal systems [14systems–23] [24. Additional [24,25],25] and and cold dissolved cold seeps seeps [Ba26 [26]]. can. be introduced to deep waters through submarine hydrothermal systems [24,25] and cold seeps [26]. FigureFigure 2. 2.A A simplifiedsimplified diagramdiagram ofof thethe marinemarine barite cycle. Direction Direction of of f fluxesluxes are are shown shown with with black black Figure 2. A simplified diagram of the marine barite cycle. Direction2 1of fluxes are shown with black arrows,arrows, with with thethe value value ofof the the flux flux in in parentheses parentheses (in (in nmol nmol cm cm−−2 yryr−−1))[ [27]27].. T Thehe associated associated phase phase of of arrows, with the value of the flux in2 +parentheses (in nmol cm−2 yr−1) [27]. The associated phase of bariumbarium (Ba)(Ba)is is shown shownin inred, red,with withBa Ba2+ as the dissolved phase and BaSO 44 (barite)(barite) as as the the particulate particulate 2+ phase.phase.barium*—Terrestrial * —(BaTerrestrial) is shown