climate engineering approaches that aim to deliberately and actively 
 remove greenhouse gases from the atmosphere are categorized as carbon 
 dioxide removal ( cdr ) or negative emission technologies ( nets ) . cdr or nets are seen as a possible future complement 
 to current climate policies , which are presently only focused on the 
 reduction of co2 emissions . model studies emphasize that 
 the large - scale implementation of nets will be needed to limit global 
 warming to within a 2 c increase with respect to preindustrial 
 conditions . the worldwide commitment to attain this 
 target , thus avoiding a dangerous  level of climate 
 change , has been strengthened by the recent cop21 paris agreement . enhanced silicate weathering ( esw ) is a net approach in which the 
 natural process of ( silicate ) rock weathering is artificially stimulated . the technique has been recognized as a potentially promising strategy 
 for co2 removal from the atmosphere while at the same time 
 counteracting ocean acidification . the implementation of esw requires suitable source rock to be mined , 
 ground to small grain sizes , and subsequently spread over suitable 
 areas . the mineral grains dissolve ( i.e. , 
 chemical weathering ) , through which co2 is eventually captured 
 from the atmosphere . olivine ( mg2xfexsio4 ) is 
 an abundant and fast - weathering ultramafic silicate mineral and has 
 been advanced as a prime candidate mineral for esw application . the dissolution of olivine in an aqueous environment consumes protons 
 or equally increases alkalinity , and so increases co2 uptake by the aqueous medium ( supporting information section 1 ) . in theory , esw can be applied in terrestrial soils , in the surface mixed layer of the open ocean , or by spreading minerals onto sediments of the coastal 
 zone and continental shelf . the largest 
 application domain for esw would be the open ocean , but model analysis 
 suggests that the olivine particles need to be ground to very small 
 sizes to facilitate dissolution in the surface ocean . as such , the high grinding costs and co2 emissions 
 during production potentially limit this approach . an alternative 
 scheme is the application of esw to coastal and shelf environments , 
 where it could be integrated into existing coastal zone management 
 practices , such as dredging operations , land reclamation , and beach 
 nourishment . a theoretical examination of the concept of coastal esw has indicated advantages as well as challenges . one important knowledge gap is that detailed experimental investigations 
 of olivine dissolution under natural conditions ( i.e. , realistic for 
 coastal esw ) are lacking . a better understanding of the rate and mechanism 
 of olivine dissolution in natural marine environments is needed to 
 better evaluate the feasibility and potential of coastal esw as a 
 net . previous work has largely focused on olivine dissolution under 
 laboratory conditions using artificial seawater solutions . such idealized approaches potentially exclude important geochemical 
 and environmental influences that could be relevant under field conditions . here , we specifically address a number of questions related to 
 the application of enhanced silicate weathering in natural coastal 
 environments : ( 1 ) what is the rate of olivine dissolution in natural 
 seawater and how does this differ from artificial seawater ? ( 3 ) what 
 dissolution products can be used to efficiently monitor the dissolution 
 rate of olivine in coastal sediments , i.e. , quantify the efficiency 
 of enhanced silicate weathering ? ( 4 ) to what extent does secondary 
 mineral formation diminish the co2 sequestration efficiency 
 of olivine dissolution in seawater ? we present results from dissolution experiments with simulated 
 grain  grain collisions , in which olivine was dissolved in natural 
 filtered seawater as well as in artificial seawater media with modified 
 cation composition . potential proxies for quantifying the dissolution 
 rate of olivine are analyzed and compared . based on these results , commercially available olivine sand ( mg2xfexsio4 ) and lab - grade quartz ( sio2 ) were used in slurry 
 dissolution experiments . the olivine sand ( particle size quantiles : d10 = 91 m , d50 = 143 m , d90 = 224 m ) had a molar 
 mg - to - fe ratio of 0.94:0.06 , characterizing the olivine as forsterite-94 
 ( fo94 ) . further details on the chemical composition , 
 grain - size distribution , and pre - experimental treatment are summarized 
 in tables s1 and s2 . was 
 collected as natural seawater from the oosterschelde tidal basin ( the 
 netherlands ) and filtered over a mahle amaguard fp 0.2 [ m ] 
 woven cotton filter ( mahle benelux , the netherlands ) . in addition , 
 three types of artificial seawater were prepared according to the 
 astm standard practice d 114198 ( table s3 ) : ( 1 ) plain artificial seawater 
 ( asw ) , ( 2 ) artificial seawater with ca replaced by na ( hereafter named asw - ca ) , and ( 3 ) artificial seawater with 
 both ca and mg replaced by na ( hereafter named asw - camg ) . specific amounts of olivine and quartz 
 grains were added to a specific volume of seawater in 500 ml borosilicate 
 glass bottles , which were then subjected to continuous rotating movements 
 on a ch-4103 rotating shaking platform ( infors ag , switzerland ) set 
 at 155 rpm . the bottles were closed with membrane screw caps , equipped 
 with a 0.5 mm thick silicone septum that prevented evaporation but 
 allowed gas exchange . to prevent photosynthesis , the experiment took 
 place in the dark , while the bottles were wrapped in aluminum foil 
 ( leaving the top of the membrane cap uncovered ) . a total of 
 three types of agitation experiments were conducted : a1 , a2 , and a3 
 ( supporting information section 3 and table s4 ) . in both the a1 and a2 experiments , 0.1 mol of either olivine ( oli ) 
 or quartz ( qua ) were added to 300 ml of fsw , with one control treatment 
 ( i.e. , only seawater ; sw ) . a total of three replicates were conducted 
 per treatment ( n = 3 ) . experiment a1 was conducted 
 under ambient conditions , i.e. without regulation of temperature ( range : 
 13.520 c ) and pco2 ( range : 445525 
 ppmv ) , and lasted for 88 days . the second agitation experiment , a2 , 
 had essentially the same setup as a1 except for a few modifications . first , natural seawater was bubbled with air prior to the experiment 
 to ensure co2 equilibration with the surrounding atmosphere . second , the experiment had a much shorter duration ( 20 days ) , and 
 it was conducted under stable temperature and pco2 conditions . 
 the third experiment , a3 , was designed to specifically investigate 
 the effect of the composition of seawater on the dissolution rate 
 of olivine ( quartz was not investigated ) . agitation experiment a3 
 was designed to examine the effect of the composition of seawater 
 on the dissolution rate . a3 was also conducted under stable temperature 
 and pco2 conditions and used atmosphere - equilibrated reactive 
 fluids , bubbled with air . the dissolution of olivine ( oli ) was monitored 
 in four reactive fluids ( fsw , asw , asw - ca , and asw - camg ; see the supporting information section 2 ) and compared 
 to control treatments ( respective solution media without olivine ) . 
 instead of 0.10 mol olivine used in a1 and a2 , and 0.03 mol was used 
 in a3 ( table s3 ) . to test the impact of agitation , a layer of olivine sand was placed 
 also in a nonmoving cylindrical container with fsw ( n = 1 ) . in this nonagitated treatment , rather , the overlying water was stirred , and only samples 
 for solid - phase analysis were collected . the overlying water 
 of the slurry batch reaction experiments was sampled at regular time 
 intervals and analyzed for temperature , salinity , ph , total alkalinity 
 ( ta ) , dissolved inorganic carbon ( dic ) , dissolved silicate ( si ) , dissolved 
 nickel ( ni ) , and dissolved magnesium ( mg ) using standard analytical 
 procedures ( see section 3 of the supporting information ) . all solute concentrations 
 are reported as mol per kg of seawater ( mol kg ) . upon completion of the a3 experiment , olivine grains were 
 recovered from the agitated and nonagitated treatments , inspected 
 for dissolution features , and analyzed for carbonate precipitates , 
 according to nieuwenhuize et al . the elemental composition of the olivine particle surfaces were 
 investigated using scanning electron microscope energy - dispersive 
 x - ray spectroscopy ( sem  edx ) . additional details on both water 
 and solid - phase analyses can be found in section 3 of the supporting information . the accumulation over time of the reaction products in the reactor 
 vessels is reported as excess concentration values , ci(t ) = ctreatment(t ) , the control refers to 
 the treatment without the addition of any solid minerals . a total 
 of three empirical mathematical models were implemented to describe 
 ci(t ) as a function 
 of the incubation time , and from these model fits , the accumulation 
 rate ri [ mol kg day ] of compound i and the associated area - specific 
 dissolution rate constant ki [ mol 
 m day ] were derived ( section 4 of the supporting information ) . note 
 that in the case of stoichiometric dissolution , the rate constants ki should be , at least in theory , identical for 
 all olivine dissolution products . the accumulation of weathering 
 products and the change of solution chemistry during the batch dissolution 
 experiments was also mechanistically simulated using the geochemical 
 software package phreeqc v2 in 
 these simulations , the solution chemistry ( and , hence , the saturation 
 states ) were free to evolve with time as a consequence of mineral 
 dissolution ( kinetic rate equations specified in section 4 of the supporting information ) . solubility constants 
 were taken from the minteq.dat and llnl.dat databases to calculate 
 the saturation states of solid phases in the solution . the measured 
 initial composition of the solution , which was specific for each treatment , 
 was used as the starting conditions for the phreeqc simulations . all mathematical analyses , apart from the phreeqc analyses , and 
 plotting were done using the open source r framework for statistical 
 computing . in the a1 and a2 experiments , we investigated the dissolution of 
 olivine and quartz in natural filtered seawater . in both a1 and a2 , 
 there was a clear si signal in the quartz treatment ( qua ) , 
 most likely caused by dissolution of very fine quartz particles ( figure 1 ) . si increased 
 until 18 mol kg within the first 
 week of the experiments , after which it remained constant . there was 
 no discernible ni release in the a1 and a2 quartz treatment ( figure 1 ) , and hardly any 
 response from the carbonate system . the ph increased by 0.05 
 within the first 2 weeks but then decreased again to its initial value 
 by the end of the experiment . while ta remained constant with 
 time , dic decreased with 22 mol kg during the first 15 days , likely caused by co2 outgassing , 
 as the initial solution in a1 may not have been in equilibrium with 
 the atmosphere ( sections 2 and 3 of the supporting information ) . in the a2 experiment , the experimental procedure 
 was improved , and the fsw medium was bubbled with ambient air at the 
 start of the experiment . as a result , the carbonate system variables 
 dic , ta , or ph did not change significantly over 
 time ( linear regression , p = 0.35 , p = 0.28 , and p = 0.696 , respectively ) . symbols denote mean seawater - corrected values 
 ( see the materials and methods section ) , with 
 error bars denoting standard error of the mean ( sem ) . circles : values 
 from experiment a1 ; triangles : values from experiment a2 . the values 
 for both experiments are plotted with the olivine ( oli ) and quartz 
 ( qua ) treatments plotted alongside on the same vertical scale for 
 comparison . the reported units are mol / kg of seawater , except 
 for ph , which is in ph units on the total scale . in the olivine treatment ( oli ) of experiment a1 , increases with 
 time of both si and ni were observed , suggesting olivine 
 dissolution ( figure 1 ) . while ni leveled off at 3.2 mol kg , suggesting that an equilibrium was reached , si increased 
 almost linearly over the 88 day incubation period , displaying a much - stronger 
 dissolution than in the qua treatment . experiment a2 showed a similar 
 temporal evolution of si and ni ( figure 1 ) . ta showed a pronounced increase 
 over the first 5 days ( figure 1 ) , leveling off thereafter at ta = 103 mol kg . dic increased in a similar way , albeit over 
 a slightly longer period of 15 days , leveling off at dic 
 = 93 mol kg . the ph in the olivine treatment 
 increased rapidly over the first 5 days by 0.1 , after which 
 it decreased again and leveled off at ph = 0.02 ( figure 1 ) . the carbonate system in 
 a2 showed a similar pattern , with ta and dic leveling 
 off at 104 and 74 mol kg , respectively 
 ( figure 1 ) . similar 
 to experiment a1 , the ph in a2 increased strongly within the 
 first 6 days by 0.15 , after which it decreased again to ph 
 = 0.06 ( figure 1 ) . in experiment a3 , large differences in 
 the release of dissolution products were observed between the different 
 seawater media . all four media displayed a quasi - linear si 
 response with time ( figure 2 ) . the si attained at the end of the experiment was 
 lowest in the natural seawater ( fsw : 68 mol si kg ) and artificial seawater ( asw : 82 mol si kg ) and increased markedly when ca and mg were replaced by na in the medium ( asw - ca : 122 mol 
 si kg ; asw - camg : 162 mol si kg ) . symbols denote mean seawater - corrected values ( see 
 the materials and methods section ) for the 
 olivine treatment ( oli ) , with error bars denoting standard error of 
 the mean ( sem ) . circles : values from filtered seawater treatment ( fsw ) ; 
 triangles : values from artificial seawater treatment ( asw ) ; squares : 
 values from artificial seawater without calcium treatment ( asw - ca ) ; 
 diamonds : values from artificial seawater without calcium and magnesium 
 treatment ( asw - camg ) . the reported units are mol / kg of seawater 
 ( corrected for control values ) , except for ph , which is in ph units 
 on the total scale . the ni release showed a saturation - type response , which varied 
 between media . in the fsw , asw , and asw - ca treatments ( figure 2 ) , the ni concentration 
 showed a comparable accumulation ( plateau concentrations between 1.19 
 and 1.68 mol ni kg ) . in contrast , the asw - camg 
 treatment showed hardly any ni accumulation , apart from a small initial 
 release , which was taken up again by the end of the experiment . the carbonate system ( ta , ph , and dic ) responded very differently 
 in the ca- and mg - free seawater compared to the three other treatments . although the shape of the response curves were similar , the overall 
 accumulation of alkalinity ( ta = 340  14 mol kg ) and dissolved inorganic carbon ( dic = 317 
  11 mol kg ) was substantially higher 
 in the asw - camg compared to the other treatments ( range ta 
 = 4169 mol kg and dic = 
 3166 mol kg in fsw , asw , and asw - ca ) . the ta and dic in asw - camg quickly increased over the 
 first 10 days , after which the increase rate slowed down and the accumulation 
 became linear . the long - term accumulation , i.e. between 30 and 137 
 days , was higher for dic ( 168  50 mol kg ) than for ta ( 127  12 mol kg ) . response , with 
 a sharp initial increase in the first 5 days , reaching maximum between 
 4 and 7 days , after which the ph gradually decreased and tended 
 toward an asymptotic equilibrium value at the end of the experiment 
 ( figure 2 ) . consistent 
 with the stronger alkalinity accumulation in the ca- and mg - free seawater , 
 the long - term ph ( pht137 minus pht0 ) increase 
 was much higher in the ca- and mg - free seawater ( ph = 0.06 ) , 
 compared to the other three treatments ( range in ph = 0.01 
 to 0.035 at 137 days ) . phreeqc 
 kinetic modeling of the asw , asw - ca , and asw - camg treatments suggested 
 that all reactive fluid media in experiment a3 were undersaturated 
 for forsterite ( forsterite = 10 for fsw , asw , and asw - ca and forsterite = 10 for asw - camg ) . the accumulation rates r were determined by the best model fit ( see figure s6 for representative 
 examples ) to the response curves of the olivine dissolution products 
 si , ni , mg , and ta and the ensuing co2 sequestration dic ( table 1 ) . as emphasized above , different dissolution 
 products tended to have different response curves within the same 
 treatment , thus indicating nonstoichiometric dissolution ( table 1 ) . in the case of 
 si , the accumulation response was generally best described 
 by a linear model ( model 1 , table s5 ) , 
 while for ta , dic , mg , and ni , the profiles 
 were typically best fitted with a saturation model ( model 2 , table s5 ) or a combination of short - term saturation 
 with a long - term linear increase ( model 3 , table s5 ) . only in the asw - camg treatment ( experiment a3 ) could all 
 variables be described best by model 3 ( table s5 ) the number next to each of the rmax values corresponds to the model 
 that best fit the data ( for the significance of parameters , see the materials and methods section ) , where 1 = linear 
 model , 2 = saturation model , 3 = combined model according to the equations 
 in table s6 . to further illustrate the absence of stoichiometric dissolution 
 in either natural or artificial seawater in the presence of magnesium , figure s8 plots the accumulation of response 
 variables in experiment a3 normalized for stoichiometry ( table s1 : si/1 , ta/4 , ni/0.0075 , 
 and mg/1.87 ) . only in the asw - camg treatment did olivine dissolution 
 tend to become stoichiometric , as shown by the similar responses for 
 mg and ta ( figure s8 ) . because of nonstoichiometric dissolution , the olivine dissolution 
 rate constant ki showed a dependence on 
 the response variable ( si , ni , mg , ta , 
 and dic ; table 2 ) . the rate constant based on ni ( kni ) is the highest of all response variables and is similar across 
 all treatments ( 3174 mol of olivine m day ) . in the asw - camg treatment , kmg ( 63 mol of olivine m day ) was consistent with kni values , while ksi were an order of magnitude lower than kni values in the reactive fluid media containing mg ( table 1 and 2 ) . the exclusion of mg in the mg - free reactive 
 fluid ( asw - camg ) increased ksi by 
 1 order of magnitude . the values of both kta and kdic show 
 substantial variation between treatments , and are highest in the asw - camg 
 treatment . the temperature - normalized ( to 25 c ) mean values for ki ( where 
 i = si , ni , ta , or dic ) , for the fsw and 
 asw cases are shown in figure 3 ( the asw - ca and asw - camg treatments are considered unrealistic 
 for esw and are thus excluded ) . olivine dissolution rate constant k , calculated 
 as the mean (  sd ) value of the different response variables 
 measured in the three agitation experiments a1 , a2 , and a3 ( table s4 ) . to obtain the most - realistic estimates 
 for olivine dissolution in seawater , only values from the fsw and 
 asw treatments were considered . for comparison , the estimated value 
 by hangx and spiers from previous studies 
 ( literature , h&s 2009 ) is given in the same units as the rates 
 obtained in this study . the literature value and range are denoted 
 by the gray circle and the gray area for clarity . the gray triangles 
 represent the values obtained in this study at 17 c but recalculated 
 to 25 c , the same standard temperature as the literature estimates . edx analyses of mineral grains 
 from fresh , unreacted olivine were generally angular , with sharp edges 
 ( figure 4a ) . in contrast , 
 olivine grains that had been rotating during the entire experiment 
 ( 137 days ) were generally subrounded ( figure 4b ) , suggesting abrasion due to grain  grain 
 collisions . the mg - to - si atomic ratios ( mg / si ) at the surface of the 
 unreacted particles were significantly higher ( mean  sem mg / si 
 = 2.11  0.02 , ngrains = 6 ; figure s9 ) than for grains that were agitated 
 in solution ( mean  sem mg / si 1.7  0.042  
 0.03 , ngrains = 310 ; figure s9 ) . this suggests preferential mobilization 
 of mg during dissolution , consistent with the higher dissolution rates 
 obtained for mg and ni compared to si . the preferential leaching of 
 mg ( lowest mg - to - si ratio ) was most prominent in the 
 asw - camg treatment ( figure s10 ) , where 
 areas with mg / si  1 and lower were observed . ( a ) sem  edx micrograph of unreacted olivine ( substrate material ) 
 with very clear angular features and sharp edges . the mg - to - si atomic 
 ratio in area 1 typically lies between 2 and 2.5 . edx 
 micrograph of an olivine particle after being subjected to continuous 
 movement in fsw during 137 days ( experiment a3 ) . on the surface of 
 the same olivine particle , abrupt changes in mg - to - si atomic ratios 
 can be observed within small distances . areas denoted with 1 are characterized 
 by mg - to - si atomic ratios of 22.5 , while mg - to - si atomic ratios 
 in areas denoted with 2 showed values of around 1 . such locations , 
 where the mg - to - si ratio decreases well below 2 , are considered local 
 weathering sites . the inorganic 
 carbon content ( cinorg ) in the solid mineral 
 phase recovered from experiment a3 was very low ( mean cinorg < 0.005% ) and was not significantly different 
 between the four treatments ( one - way anova , p = 0.112 ; figure s9 ) . small changes in the solid - phase 
 carbonate content ( which are difficult to measure ) could nevertheless 
 be associated with substantial changes in the alkalinity of the supernatant . although not significantly different , the difference in cinorg content between fsw and asw - camg was 0.003 mass 
 percent . if this difference would be real and caused by carbonate 
 precipitation , this would imply that the fsw contained 37 mol 
 kg of caco3 in excess to the asw - camg , 
 when expressed per unit volume of fluid . the absence of this precipitation 
 would hence cause the alkalinity to be 74 mol kg higher in asw - camg . however , the measured ta difference between 
 the asw - camg treatment and the fsw and asw was much higher , amounting 
 to ca . 300 mol ta kg at the end of experiment 
 a3 ( figure 2 ) . accordingly , 
 carbonate precipitation can not explain the difference in alkalinity 
 between the fsw and asw - camg treatments , and so it was likely that 
 more olivine dissolution took place in the asw - camg treatment . the dissolution experiments here demonstrate several features regarding 
 olivine weathering in seawater and its potential applications for 
 esw in coastal settings olivine dissolution in natural seawater 
 under nonsterile laboratory conditions consistently causes alkalinization , 
 followed by co2 invasion from the atmosphere into the seawater , 
 at rates in agreement with those estimated by previous studies . second , apparent nonstoichiometric 
 dissolution complicates the experimental determination of the rate 
 and extent of olivine dissolution within the seabed , making it more 
 challenging to assess of the efficiency of esw . therefore , the quantification 
 of the actual olivine dissolution rate under realistic in situ conditions 
 will require a multiparameter approach , combining flux measurements 
 of dissolved silicate , dissolved metals ( nickel and iron ) , and alkalinity , 
 with appropriate experimental controls . third , the rate of olivine 
 dissolution within the seabed can be limited by saturation effects , 
 which could decrease the efficiency of esw applications . fourthly , 
 the extent to which secondary reactions impact the co2 sequestration 
 efficiency of olivine dissolution under in situ conditions remains 
 unresolved and is an important issue to address in further studies 
 on coastal esw . to be 
 implemented as a negative emission technology for climate change mitigation , the carbon sequestering potential of marine olivine 
 dissolution needs to be quantified . in other words : how much olivine 
 dissolution occurs within the seabed ? how much co2 is eventually taken 
 up by the seawater as a consequence of esw ? the overall co2 sequestration rate ( rco2 ; mol co2 per m of seabed per unit of 
 time ) can be expressed as1to determine the effectiveness of coastal 
 esw , both the factors co2 and roli need to be accurately constrained . the co2 sequestration efficiency co2 specifies the net amount of co2 that is taken up from 
 the atmosphere during the dissolution of 1 kg of olivine within the 
 seafloor ( this parameter will be further discussed below ) . for a given 
 amount of finely ground olivine distributed onto the seafloor ( coli ; mol of olivine m of 
 seabed ) , the olivine dissolution rate ( roli ; mol of olivine per m of seabed per unit of time ) determines 
 over what time frame the esw application will be effective ( dissolution 
 period  = coli / roli ) . the olivine dissolution rate roli further depends on the specific surface area of the mineral 
 grains ( asurface ; m g ) and the intrinsic dissolution rate constant ( koli ; mol of olivine per m of grain 
 surface area per unit of time ) . in practical esw applications , the olivine dissolution rate ( roli ) within the seabed can be determined experimentally 
 by monitoring the release of olivine dissolution products from the 
 seabed . this poses the question as to which dissolution product ( e.g. , 
 mg , si , and ta ) should be monitored as a reliable proxy 
 for the olivine dissolution rate in field - type experiments . the use 
 of both dissolved silicate and alkalinity is nontrivial as these are 
 generated in sediments by other processes than olivine dissolution . any observed sediment efflux of dissolved silicate 
 and alkalinity can thus not be exclusively attributed to olivine dissolution . furthermore , mg can not be used as a dissolution proxy 
 due to the high background concentration in seawater ( 50 mmol 
 of mg kg of seawater ) , and thus , 
 its accumulation in the overlying water can not be reliably measured . our experiments suggest that ni could be a suitable 
 dissolution proxy , generating a dissolutive accumulation , which substantially 
 supersedes the ambient seawater concentration ( 0.0020.16 mol ni kg ) . this way , 
 pore water accumulation and sediment fluxes of ni can be accurately 
 measured using standard analytical techniques for trace metals ( e.g. , 
 inductively coupled plasma however , to 
 qualify as a good proxy for sedimentary dissolution of olivine , two 
 important conditions need to be fulfilled . first , the efflux of the 
 olivine proxy ( ni ) from the sediment should also match the release 
 rate of the weathering products in the pore solution . in this regard the natural cycling of 
 ni in coastal sediments is restricted , and so the observed 
 ni efflux from the sediment in olivine addition experiments 
 can be fully attributed to olivine dissolution . still , in future studies , 
 it should be verified whether the ni release is modulated 
 by diagenetic effects within the sediment ( e.g. , sorption onto minerals ) . a second important condition is that stoichiometric dissolution 
 of olivine occurs , so that the ni release can be properly 
 rescaled to the overall olivine dissolution rate roli by means of the ni content of the olivine source rock 
 that is used however , our experiments 
 suggest that this not the case ( tables 1 and 2 ) . the experiments a1a3 show that , when 
 assessed over short - time scales , the dissolution of olivine in seawater 
 is nonstoichiometric , implying incongruent dissolution under earth 
 surface conditions in the absence 
 of secondary precipitation reactions , nonstoichiometric dissolution 
 can not continue indefinitely . experiments of longer time scales should 
 therefore clarify to what extent the ni release from 
 sediments can be a valid proxy for in situ olivine dissolution . in our dissolution experiments , the ki value for si ( determined at the initial time t0 ) was ca . the preferential release of metal cations 
 ( ni and mg ) compared to si , and the observed 
 quasi - linear increase in si are typical for solid - state diffusion 
 in silicate minerals , which facilitates 
 the formation of a  surface ( altered ) layer  . the crystal ionic radius of nickel ( 83 pm ) is 
 only slightly smaller than that of magnesium ( 86 pm ) , implying that both metal ions will have a similar rate 
 of solid - state diffusion . both pokrovsky and schott and palandri and kharaka already 
 suggested that for slightly alkaline solutions ( e.g. , seawater ) , forsterite 
 dissolution at steady - state is controlled by the decomposition of 
 a protonated surface complex , which is silica - rich and magnesium - deficient . postulate that olivine dissolution 
 occurs as a series of boundary layer processes , in which primary dissolution 
 of cations is followed by dissolution of silicic acid ions , which 
 may subsequently repolymerize at the surface . this implies that the 
 measured dissolved silica release rate is a net value , which may not 
 serve as the sole proxy for olivine dissolution . although in some 
 sem  edx images -particularly those from the asw - camg treatment 
 ( figure s10 ) , the particle surface did 
 look as if flakes of surface material had been detaching , and the 
 examined olivine grains did not show any evidence of secondary silicate 
 precipitates . edx analyses show 
 decreasing mg - to - si atomic ratios of the forsterite surface between 
 initial substrate and reacted material ( figure s9 ) . rather than a buildup of thick silica formations , these 
 decreasing mg - to - si ratios corroborate the mechanism of a cation - leached , 
 surface altered layer formation by preferential dissolution and subsequent 
 repolymerization processes sensu hellmann et al . and maher et al . the time 
 scale on which the weathering takes place in this study is much longer 
 than in high - temperature and high - pressure studies or studies in which 
 an elevated pco2 is employed . combined with physical disturbances , such as 
 grain abrasion , does not allow for the buildup of a conspicuous passivating 
 layer or thick silicate precipitates . the nonstoichiometric dissolution as observed in the experiments 
 here emphasizes that the proper quantification of olivine dissolution 
 in field - type esw experiments requires a careful experimental design . overall , the nonstoichiometric dissolution of olivine makes the experimental 
 assessment of esw more challenging . one can not simply measure one 
 dissolution proxy ( e.g. , ni ) and estimate the release 
 of other reaction products by application of reaction stoichiometry . moreover , both dissolved silicate and alkalinity are generated in 
 sediments by other processes than olivine dissolution . from a biogeochemical perspective , it is crucial 
 to know how olivine dissolution stimulates the efflux of dissolved 
 silicate and alkalinity from the seabed because the efflux of alkalinity 
 is the ultimate driver of co2 uptake , while silicate could stimulate primary productivity by 
 marine diatoms . hence , a multiparameter assessment , combining flux 
 measurements of ni , dissolved silicate , and alkalinity 
 with appropriate experimental controls , seems to provide the best 
 strategy to confidently determine the olivine dissolution rate under 
 in situ conditions . the values for the dissolution 
 rate constant k of olivine in seawater obtained in 
 this study are consistent with literature values . for the temperature 
 ranges used in fsw and asw , the dissolution rate constant varied between 
 1.9  0.8 mol of olivine m day for ksi and 56  
 18 mol of olivine m day for kni ( mean  sd values ) . normalized 
 for temperature differences , these k value ranges 
 corresponded well with the mean value of 14 mol of olivine 
 m day , as compiled by palandri 
 and kharaka and hangx and spiers , of which the latter had an order of magnitude 
 of variation around the mean ( figure 3 ) . the nonstoichiometric dissolution in the seawater 
 media fsw and asw , together with the saturation behavior observed 
 in the ta , mg , and ni results ( figure 1 ) , suggest that the 
 olivine dissolution approached thermodynamic equilibrium , thus slowing 
 down the reaction . only by using a lower solubility product for forsterite 
 ( log k = 26.448 ) than those found in the phreeqc 
 databases did model simulations indeed show a slowing of the dissolution 
 reaction by saturation , mirroring our experimental observations . furthermore , 
 the dic accumulation followed that of alkalinity perfectly in all 
 cases , albeit with a time lag . this lag is due to the relatively slow 
 process of co2 invasion and 
 is also observed in the ph response , which first increases to reach 
 a maximum and then subsequently decreases again . this ph response 
 reflects the initial removal of protons through olivine dissolution , 
 followed by a replenishment of the proton pool by lagged co2 transfer . the observed time response of the reaction products in our experiments 
 provide a first idea about the possible influence of saturation effects 
 under in situ conditions . our experiments show that saturation occurs 
 within a time frame of about 20 days ( figures 1 and 2 ) , for an experimental setup with 15 
 g of olivine in 300 ml of seawater ( i.e. , 20 ml of solution g of olivine ) . assuming the same dissolution rate occurs 
 under in situ pore water conditions , for example , if 1020% of the solid 
 sediment consists of olivine ( mixing a 12 cm olivine layer 
 into the top 10 cm of sediment ) and assuming a porosity of 0.8 and 
 an olivine particle density of 3.3 g ml , we obtain 
 a ratio of 612 ml of solution g of olivine . based on our results , we determined that such a pore solution will 
 be saturated within 4.5 to 9 days , after which olivine dissolution 
 will slow down and cease . however , the pore water of coastal sediments 
 is also regularly refreshed through physical , advective pore water 
 flow induced by waves and currents and 
 biological irrigation by burrowing macrofauna . coastal sediments subject to moderate and high bioirrigation show 
 flushing rates in the range of 10100 l m day , implying that 
 the pore solution of the first 10 cm would be refreshed on a time 
 scale of 0.58 days ( assuming a porosity range from 0.5 to 
 0.8 ) . in these sediments , irrigation appears sufficient to counteract 
 the saturation of olivine dissolution in the pore water . however , 
 in more cohesive ( muddy ) coastal sediments with flushing rates < 10 
 l m day , saturation effects 
 can be expected , which could decrease the efficiency of enhanced olivine 
 weathering applications . the experiments 
 in this study were performed in a setup that allowed free gas exchange 
 with the atmosphere , while internal biological processes affecting 
 the dic pool were excluded ( i.e. , primary production and microbial 
 degradation of organic matter ) . accordingly , the observed 
 dic increase in the experiments can be entirely attributed to co2 invasion induced by olivine dissolution , illustrating the 
 proof - of - principle that esw enhanced silicate weathering works as 
 a net . sea 
 interface per unit mass of silicate rock that dissolves within the 
 seabed and can be written as2this formulation reflects the two consecutive 
 steps in the process of co2 sequestration . in a first step , 
 olivine dissolution takes place ( rate roli ) , which increases alkalinity in the pore solution ( rate rta ) . this alkalinity increase will then shift 
 the acid  base equilibrium from dissolved co2 to 
 bicarbonate and carbonate , thus stimulating a co2 uptake 
 from the atmosphere across the air  sea interface ( rate rco2 ) . the co2 sensitivity(co2/ta)pco2 specifies how much co2 is taken 
 up from the atmosphere for each mole of alkalinity that is released 
 from the seabed . this thermodynamic factor is evaluated at a given 
 partial pressure of co2 in the atmosphere and is dependent 
 on the local salinity , temperature , and chemical composition of the 
 coastal seawater . calculating the co2 sensitivity over the entire experimental period , and for 
 all the experiments that had full ionic strength of seawater ( a1 , 
 a2 , a3 : fsw and a3 : asw ) , we obtain a co2 sensitivity of 
 0.84  0.1 ( mol of dic mol of ta ) , which 
 is in close agreement with the theoretical value 0.854 for seawater 
 at the experimental conditions employed ( t = 17 c , s= 33 , ta = 2400 mol l , and pco2 = 400 ppmv ) . accordingly , the co2 uptake in our experiments appears 
 entirely congruent with the standard acid nevertheless , the alkalinity 
 increase during olivine dissolution , rta / roli , was less than expected . traditionally , 
 olivine dissolution is described by the reaction equation:3because 4 moles of protons are consumed per 
 mole of olivine dissolved , and hence 4 mol of alkalinity are produced , 
 a ratio ( ta/si = 4 ( rta / roli = 4 ) ) is expected . only in the asw - camg 
 treatment , the ta/si approached the expected value of 
 4 , while it was substantially less in the fsw , asw , and asw - ca treatments . these observations suggests that the equation 3 does not provide a complete description of 
 the overall olivine dissolution process , but that secondary reactions 
 could be active . overall , co2 sequestration efficiency 
 can be formulated as4here , 4 denotes the theoretical stoichiometry 
 between olivine dissolution and co2 , and x denotes a reduction in the co2 sequestration 
 efficiency due to secondary reactions . because our slurry experiments 
 were conducted with oxygenated seawater , one such possible reaction 
 is the aerobic oxidation of ferrous iron:5this reoxidation process produces free protons , 
 thus consuming again the alkalinity generated during dissolution of 
 the fe - component of olivine . the olivine employed here contains 6% 
 of fe ( table s1 ) , which would reduce the 
 alkalinity release by an equal percentage . however , this reduction 
 is not enough to explain the observed ta/si values . another possibility to reduce the ta - to-si ratio is 
 calcium carbonate precipitation:6although the supernatant in the fsw and asw 
 was saturated with respect to calcite and aragonite , magnesium is 
 known to act as an inhibitor for caco3 nucleation in seawater , 
 limiting its precipitation moreover , exclusion 
 of ca and mg from the asw - camg ( section 3 of the supporting information ) implied 
 a strong undersaturation with respect to calcium and magnesium carbonate , 
 thus preventing carbonate precipitation . additionally , our sem edx 
 analyses did not reveal carbonate minerals on the surface of olivine 
 grains , while at the end of the dissolution experiment , no significant 
 increase in the inorganic carbon ( carbonate ) content of the solid 
 phase was observed . accordingly , we consider carbonate precipitation 
 unlikely in the batch experiments performed here , and hence , the cause 
 of the ta/si < 4 remains unexplained and requires 
 further investigation . thermodynamic modeling in griffioen suggests that precipitation of the hydrated phyllosilicate sepiolite 
 ( mg4si6o15(oh)26h2o ) could reduce ta/si values , thus inducing 
 a lower co2 sequestration efficiency of enhanced olivine 
 weathering in seawater . the extent to which secondary reactions impact the co2 sequestration efficiency of olivine dissolution under in 
 situ conditions within the seabed remains an important issue to address 
 in further studies on coastal esw . due to the exclusion of mg and ca , the 
 ionic strength of the asw - camg medium was lower than that of the asw 
 ( table s3 ) . the ionic strength of the asw 
 was 0.72 mol kg , while that of asw - ca was 0.015 
 mol kg ( or 2% ) lower than asw . equally , the ionic 
 strength of asw - camg was 0.0975 mol kg ( or 13% ) 
 lower than that of asw . ionic strength impacts the activity coefficients 
 of aqueous species and has been found to impact dissolution kinetics , 
 particularly at lower ph . still , other 
 factors ( i.e. , pco2 , ph , and saturation state ) exhibit 
 a much larger influence on dissolution kinetics given the relatively high ph in the reactive fluids ( ph 7.98.2 ) 
 and the fact that all solutions were highly undersaturated with respect 
 to fosterite , the impact of the lower ionic strength of the asw - camg 
 was likely to be very small . the rate at which co2 is sequestered due to olivine 
 dissolution in seawater can thus be formulated as the following relation:7here , 4 denotes the theoretical stoichiometry 
 between olivine dissolution and co2,roli is the olivine dissolution rate , co2 is the reaction efficiency of the co2 sequestration in seawater , and x is the molar fraction 
 of fe in the olivine source material . to place coastal esw in a broader perspective , a real - world example 
 illustrates its carbon - capturing potential . the netherlands is a densely 
 populated , industrialized country , with a gdp of ca . 850 billion usd 
 ( 2013 ) and ca . 50% of its surface area below sea level . to protect the coastal region of the country 
 where ca . 60% of the gdp is produced , continuous 
 large - scale sand nourishments are needed . between 2000 and 2010 , 12 million m ( mm ) sand per year have been 
 deployed along the netherlands coast , which is expected to 
 increase due to predicted climate change - induced sea level rise ( https://www.noordzeeloket.nl/en/functions-and-use/surface-mining-and-quarrying/ ) . in a thought experiment , the sand used in these coastal 
 nourishments is replaced by finely ground olivine , as used in the 
 experiments described here . in a hypothetical one - time application 
 of 12 mm (  26 mt ) of olivine sand , parameter values 
 for kta obtained in our experiments 
 ( table 1 ) were implemented 
 in the olsen shrinking core model for 
 olivine carbonation ( assuming the measured olivine particle size distribution ; 
 see section 2 of the supporting information ) . this model has been previously implemented in ten berge et al . , describing total mass of olivine weathered and 
 consequential co2 captured ( section 8 of the supporting information ) . our simulations showed a 
 cumulative weathering of 4% of the olivine after the first year , 12% 
 after 5 years , 35% after 25 years , 57% after 50 years , and 84% after 
 100 years after 200 years , 98% of the initially applied 12 mm olivine will be dissolved . these values are in accordance with those 
 presented by hangx and spiers , in which 
 100 m ( median diameter : d50 ) olivine 
 grains would take > 100 years to dissolve . ( a ) model results of both absolute and relative cumulative dissolution 
 over time ( using dissolution rate constant values as obtained from 
 the experiments in this study ) of a one - time hypothetical coastal 
 olivine application of 12 mm , or 26.4 mton , of olivine 
 sand with the same characteristics as that used here . ( b ) model results 
 of the yearly co2 uptake rate as a consequence of hypothetical 
 repeated ( multiyear ) olivine application as a substitute for yearly 
 coastal sand nourishments during periods of 1 , 5 , 10 , and 25 years . making use of the earlier derived relationship , eq 6 , with co2 = 0.84 and x = 0.06 as discussed above , the 
 amount of carbon dioxide taken up can be estimated . with annual 12 
 mm applications , for periods of 1 , 5 , 10 , and 25 years 9 mton 
 co2 year after 25 years of coastal 
 olivine application ( figure 5b ) . this would be the equivalent of 5% of the netherlands 
 yearly 170 mton co2 emissions ( 2013 value ; http://data.worldbank.org/indicator/ ) . once the application stops , the remaining olivine will dissolve 
 in about 250 years , with decreasing yearly co2 uptake rates 
 ( figure 5b ) . the long 
 time scale over which esw is effective has two important implications . first , the process of issuing and validating carbon credits for esw 
 will need to take into account that co2 sequestration is 
 not immediately realized at once but stretched out over a century - scale 
 time window . second , given the long - lasting effects , any potential 
 ecosystem impacts need to be properly assessed and evaluated upfront 
 in small - scale field trials before large - scale esw application can 
 start . from an ecological perspective , 
 the potential secondary effects of ( large - scale ) olivine dissolution 
 are a critical issue . although dilution processes in marine coastal 
 environments will likely prevent accumulation to toxic levels of dissolution 
 products , it is important ( and obligatory in e.g. the european union ) 
 to perform upscaling calculations of dissolution product concentrations 
 and their conceivable effects on the marine ecosystem . the main consequences 
 of forsteritic olivine dissolution are increases in mg , si , ta , dic , fe , and ni , and their ecosystem 
 effects should be thoroughly assessed . in addition , the geophysical 
 consequences of olivine distribution in coastal ecosystems should 
 be assessed , such as the increase in suspended particulate matter , 
 sediment pore space clogging and smothering effects due to the higher 
 specific density of olivine . while increases in alkalinity and dic 
 are a desired effect for climate engineering purposes , the increase 
 in mg is not expected to pose a significant threat because 
 of the high background concentration in seawater . increases in dissolved 
 si and dissolved fe can stimulate primary production and thus lead 
 to additional co2 sequestration , as recently assessed by 
 model analysis however , the ultimate 
 impacts on coastal foodwebs of fertilizing by olivine dissolution 
 are uncertain and need further investigation . the impact of 
 increased nickel flux on marine ecosystems is a matter of potential 
 concern , and has only been scarcely touched upon . dissolved nickel 
 occurs in trace concentrations in seawater ( 0.030.16 mol 
 kg ; ) , as low as 0.0020.006 
 mol kg in the central southern north sea 
 and up to 0.04 mol kg in the rhine delta 
 area . in comparison , background nickel 
 concentrations in the control treatments ranged between 0.14 mol 
 kg in the fsw in experiment a3 and 0.45 mol 
 kg in experiment a1 , while background ni concentrations 
 in the artificial seawater media in experiment a3 ( asw , asw - ca , and 
 asw - camg ) were an order of magnitude lower , between 0.017 and 0.032 
 mol kg . the ecotoxicology of nickel in marine organisms and ecosystems 
 is summarized on the web site of the uk marine special areas of conservation 
 ( http://www.ukmarinesac.org.uk/ ) and established for the uk at a chronic concentration of 0.25 mol 
 l. nickel toxicity has been reported in a number 
 of cases : negative effects on spawning in mysiid shrimps at 
 2.4 mol l , dna damage with associated physiological 
 and cytotoxic effects in the blue mussel mytilus edulis at 0.3 mol l , disrupting ionoregulatory 
 functions in the green crab carcinus maenas between 
 8.5 and 51 mol l in very low - salinity seawater 
 ( 0.006 psu ) , and organ oxidative stress in the killifish fundulus 
 heteroclitus , also mainly in freshwater . and 
 blewett and wood is that higher , seawater - like 
 salinities ( e.g. , 3038 ) seem to be negatively correlated with 
 ni - induced effects . in general , although bioaccumulation of nickel in individual organisms occurs , 
 there seems to be little evidence of biomagnification throughout ( marine ) 
 foodwebs , although kumblad et al . present results that suggest the contrary . the 
 potential toxicity of nickel , combined with rather large uncertainties 
 about the magnitude and direction of its response effects , make it 
 paramount to further investigate its ecotoxicological effects within 
 the framework of large - scale application of olivine in coastal environments . , 
 there should be proper field trials in quasi - contained conditions , 
 such as mesocosm setups , which can be upscaled in , e.g. , tidal harbor 
 basins . in the case that a mesoscale field trial ( 100 m ) would be undertaken , common dredging equipment would be 
 used to apply the olivine into the ( coastal ) environment . the same 
 equipment and expertise can be used to remove the olivine sand , should 
 any acute unforeseen situation develop . the co2 sequestration induced by esw is governed by 
 the acid  base thermodynamics of seawater , which are well - understood , therefore rendering the containment 
 of co2 in the ocean highly predictable . the central premise 
 of esw is that it increases the ocean s alkalinity , enabling 
 more co2 to be dissolved into seawater at any given pco2 compared to the situation in which no alkalinity is added 
 to the ocean . this co2 will be stay dissolved in the ocean 
 ( or contained ) as long as no other process changes the alkalinity 
 of the ocean . in the esw , the co2 storage reservoir ( the 
 ocean ) is an open system , as co2 can be freely exchanged 
 between atmosphere and ocean across the air  sea interface . 
 because leakage can not occur in an open system , storage of co2 in the ocean is therefore leakage - proof . the evidence that 
 co2 will be contained for long periods of time is given 
 by observations on the long - term ( > 1000 year ) carbon cycle and the 
 impact of natural silicate weathering : the long - term fate of fossil 
 co2 is to be absorbed in the ocean . if esw is applied to coastal systems in a geo - engineering framework , 
 it will be crucial to determine in situ olivine dissolution rates 
 to determine the efficiency of the method . once in the natural sediment , the olivine will be subject to very 
 different biogeochemical and geophysical conditions . microbial mineralization 
 processes could greatly increase the co2 concentration 
 in the sediment s pore waters , while benthic macrofauna process vast quantities of sediment for 
 their sustenance and mobility . these processes are 
 likely to speed up the dissolution process within marine sediments . large - scale sediment transport and wave action are expected to cause 
 constant particle abrasion and faster mechanical weathering , in turn 
 facilitating faster chemical weathering . if esw is ever to be applied 
 in a geo - engineering framework , it is of paramount importance to investigate 
 the effects of all of these natural processes on the dissolution of 
 olivine in coastal environments .
enhanced weathering of ( ultra)basic silicate rocks such as olivine - rich 
 dunite has been proposed as a large - scale climate engineering approach . 
 when implemented in coastal environments , olivine weathering 
 is expected 
 to increase seawater alkalinity , thus resulting in additional co2 uptake from the atmosphere . 
 however , the mechanisms of marine 
 olivine weathering and its effect on seawater  carbonate chemistry 
 remain poorly understood . here , we present results from batch reaction 
 experiments , in which forsteritic olivine was subjected to rotational 
 agitation in different seawater media for periods of days to months . 
 olivine dissolution caused a significant increase in alkalinity of 
 the seawater with a consequent dic increase due to co2 invasion , 
 thus confirming viability of the basic concept of enhanced silicate 
 weathering . 
 however , our experiments also identified several important 
 challenges with respect to the detailed quantification of the co2 sequestration efficiency under field conditions , which include 
 nonstoichiometric dissolution , potential pore water saturation in 
 the seabed , and the potential occurrence of secondary reactions . before 
 enhanced weathering of olivine in coastal environments 
 can be considered 
 an option for realizing negative co2 emissions for climate 
 mitigation purposes , these aspects need further experimental assessment .