Impact on oysters in first-of-its-kind field trial of marine Enhanced Rock Weathering (mERW) with olivine as carbon dioxide removal (CDR) strategy

Abstract

Introduction:

Carbon dioxide removal (CDR) is a necessary component of limiting global warming to 2 °C by 2,100. Marine enhanced rock weathering (mERW) with minerals like olivine is a CDR strategy with the potential to capture atmospheric carbon dioxide and mitigate ocean acidification, which threatens calcifying organisms including those essential for global aquaculture such as oysters. mERW could benefit these species through the local addition of alkalinity, although olivine also releases trace metals like nickel which may bioaccumulate.

Methods:

This study presents findings from the world’s first field trial of mERW conducted in New York, USA. Olivine sand was applied to an intertidal beach, where juvenile oysters were exposed over a one-year period. Oyster biomass and trace metal accumulation were subsequently assessed.

Results:

On short (2 month) timescales, Eastern oysters exposed to olivine demonstrated a positive, but non-significant, increase in biomass as compared to control treatments. After a year of exposure, there was no significant difference in oyster biomass or mean metal accumulations between olivine and control treatments (mean for all treatments 2.18 ± 2.71 μg g dw−1 for Ni; <1 μg g dw−1 for Cr and Co).

Discussion:

Metals concentrations were below US Food and Drug Administration warning thresholds and within global natural ranges. Our findings suggest that mERW with olivine has a limited effect on oysters and that olivine-derived metals did not result in oyster safety concerns for human health.

1 Introduction

Limiting Earth’s temperature increase to 2 °C by 2,100 requires drastic greenhouse gas emission reductions and the removal of approximately 10 Gt CO₂ per year by mid-century and 20 Gt CO₂ per year by the end of the century (National Academies of Sciences, Engineering, and Medicine, 2022). Therefore, implementing effective, safe and scalable carbon dioxide removal (CDR) strategies is imperative. One CDR strategy is ocean alkalinity enhancement (OAE), which involves increasing the concentration of seawater alkalinity and ultimately driving an influx of carbon dioxide (CO₂) from the atmosphere into the ocean (Cross et al., 2023). Marine enhanced rock weathering (mERW) is a specific form of OAE where alkaline rocks and/or minerals are introduced into the ocean to dissolve and generate alkalinity (Geerts et al., 2025; Campbell et al., 2022; Meysman and Montserrat, 2017; Renforth and Henderson, 2017). Recent research indicates that mERW has the potential to sequester between 0.3 and 10 Gt of CO₂ y⁻¹ from the atmosphere (Feng et al., 2017; Palmiéri and Yool, 2024). One mineral commonly proposed for mERW is olivine, a naturally occurring ultramafic silicate (Mg_2-xFe_xSiO₄). Olivine dissolves in seawater over months to years, releasing cations [such as magnesium (Mg²⁺) and iron (Fe²⁺)] and generating alkalinity [mainly bicarbonate (HCO₃⁻)] (Meysman and Montserrat, 2017; Bach et al., 2019). As companies worldwide form to commercialize and ultimately scale mERW through an actively growing carbon market, there is an urgency to understand the potential impacts this approach has on the marine environment.

Through the introduction of alkalinity, OAE strategies such as mERW may offer the critical co-benefit of localized ocean acidification mitigation due to the resulting increase in carbonate and bicarbonate ions. Ocean acidification negatively affects calcifying organisms such as bivalves (IPCC, 2022; Gazeau et al., 2013) by reducing their calcification and growth rates (Waldbusser et al., 2015). Adult bivalves have a range of physiological mechanisms to cope with ocean acidification, however, these processes can be energetically expensive and necessitate energy reallocation (Gazeau et al., 2013; Schwaner et al., 2024). Oysters, in particular, are vulnerable to acidification impacts, a critical concern for the shellfish industry due to the predicted production declines of 14%–28% by 2,100 in certain regions if acidification trends continue (Clements and Chopin, 2017; Mangi et al., 2018; Townhill et al., 2022). This decrease poses a risk to both the global economy and food security, as the aquaculture sector—which employed over 20 million people and generated USD 281.5 billion in 2020 (FAO, 2022)—relies heavily on shellfish. In North America and beyond, oysters represent a significant portion of aquaculture production, with an estimated 18 million tonnes of marine mollusks produced worldwide in 2020 (FAO, 2022). As such, there is increasing interest in strategies to enhance alkalinity in oyster aquaculture environments so as to counteract acidification effects (Hu and Cai, 2011) as mERW could locally raise alkalinity levels to create more favorable conditions for oyster growth. Regional alkalinity enhancement holds promise in supporting shellfish resilience, potentially alleviating some of the most damaging effects of acidification on this ecologically and economically essential industry.

Beyond ocean alkalinity enhancement, mERW with minerals such as olivine could exert ecological impacts through the release of elements found in olivine, namely silicon (Si) and trace metals, particularly nickel (Ni), chromium (Cr), and cobalt (Co). These elements may serve as a source of nutrients for marine organisms, for example, Si could support the growth of silicifying algae and sponges (Bristow et al., 2017). On the contrary, high concentrations of trace metals may have adverse effects, including bioaccumulation in higher trophic levels and disruptions to ecosystem dynamics (Bach et al., 2019; Flipkens et al., 2021; Montserrat et al., 2017). Organisms have mechanisms for trace metal detoxification, but this process can increase energy expenditure and subsequently impact growth and reproduction (Anacleto et al., 2015; Jeong et al., 2023). To date, the impact of olivine dissolution on marine organisms has mostly been studied in laboratory settings where organisms were kept in closed tanks and often exposed to higher concentrations of olivine dissolution products than what could be expected under natural conditions (Jankowska et al., 2024; Flipkens et al., 2023) and only one study exposed mussels to olivine in harbor conditions (Gjesdal and Solheimslid, 2016). Field trials, which quantify the environmental impact of mERW under real-world conditions, are the critical next step in assessing the safety, and thereby the true potential, of mERW as a climate mitigation strategy (National Academies of Sciences, Engineering, and Medicine, 2022).

This study investigates the impact of mERW with olivine sand on oyster growth and trace metal bioaccumulation under natural conditions. Due to their sensitivity to alkalinity levels, their sessile and filter-feeding nature, and their ability to accumulate both essential and non-essential metals, oysters are well suited for studying the impact of mERW with olivine (O’Connor, 2002; Wang et al., 2018). In July 2022, the world’s first field trial of mERW was launched. Approximately 650 tonnes of olivine sand was placed along ~300 m of an intertidal beach in the Peconic Bay, Long Island, NY, USA (Figure 1). Shortly thereafter, Eastern oysters (Crassostrea virginica Gmelin, 1791) were transplanted to the Olivine treatment site, as well as three control sites (Control West, Control Nourished, and Control East) adjacent to the Olivine Nourished treatment site. The oysters were subsampled at three-time points over 1 year and analyzed for growth and soft tissue metal accumulation. In addition, a suite of sediment and sediment porewater parameters were measured to track olivine transport and dissolution through time. This is the first investigation of the impact of mERW on marine biota under real-world conditions.

Figure 1

2 Materials and methods

2.1 Experiment design and field site

The field trial was located at a beach adjacent to the North Sea Beach Colony along the southern shore of Little Peconic Bay in Southampton, New York, USA [40.947886°, −72.427809° (ESPG: 4326)]. The bay forms part of the larger Peconic Bay tidal estuary system, situated between the north and south forks of eastern Long Island. The system is fed by the Peconic River in the west, and is separated from the Atlantic Ocean to the east by a series of islands, sounds and tidal channels. The site is tidally dominated, having a tidal range of 0.87 m (NOAA, 2011) and strong localized tidal currents that run through the channels between landmasses. The strongest currents flow at rates of up to 1.2 m s⁻¹ through the channel between Cow Neck and Robins Island, which separates Little Peconic Bay from Great Peconic Bay (Hardy, 1976). The islands to the east of the bay, including Shelter Island and Gardiners Island, provide protection from incoming waves and swells that originate in the Atlantic Ocean. Internally generated waves are fetch-limited, forming short-period, small-amplitude wind-waves; these types of waves can be short and steep and have erosive power along beaches. The water temperature is seasonally variable, peaking at an average of 25 °C in July and dropping to 3.9 °C in January as recorded at the Shelter Island USGS Station (01304650) (USGS, 2024). Generally, the Peconic Bay estuary has benthic fauna typical for sandy sediments of the temperate zone (Cerrato et al., 2010), and specifically in the project area there are no natural oyster beds observed, although oyster aquaculture operates nearby.

From July 5th to July 8th, 2022, approximately 650 tonnes of mERW olivine sand was placed in the intertidal area of the North Sea Beach by Vesta, PBC. The sand was tailored to match the native grain size of the site, with a median grain size of 0.49 mm (D50) and a fines content of 0.5% (<0.0625 mm). The sand used for this experiment was peridotite rock purchased from Sibelco Group and sourced from the Åheim, Norway mine, with a mineralogical composition consisting of 85 wt% forsteritic olivine, 6.7 wt% orthopyroxene, 5.2% chlorite, and minor fractions of serpentine and talc. However, throughout the text we refer to this sand simply by its primary mineral component, olivine. Mineralogical analyses of the sand were conducted at QMineral (Leuven, Belgium) using x-ray diffraction (XRD) on a Bruker D8 Advance with XE-T detector and Cu-Kα radiation. Spectra were interpreted using in-house software.

Four stations representing four treatment areas (Control West, Olivine Nourished, Control Nourished, Control East) were chosen for the oyster experiment and to monitor porewater composition and sediment characteristics. Control West and Control East are sections of native coastline that have never undergone beach nourishment. The Control Nourished treatment represents coastline that has been nourished with dredged sand, both historically and immediately prior to the mERW field trial (13,500 tonnes of dredged sand). The Olivine Nourished treatment represents coastline that has been nourished with dredged sand, both historically and immediately prior to the field trial, and then additionally subject to olivine placement. Porewater sampling locations were distributed along the beach to provide broader spatial coverage around each treatment area. All stations remained fully submerged at mean low water by approximately 0.5 meters. Three stations represented the Olivine Nourished treatment, two represented the Control Nourished treatment, and one to two stations represented the Control West and Control East areas, respectively. The oyster bags at the Olivine Nourished treatment were located within the original footprint of the olivine deployment although wave and tidal energy reworked, and redistributed, the olivine over a larger area through time (Figure 1).

2.2 Sediment field and laboratory methods

Three sediment core surveys were conducted on August 16th, 2022, October 18th, 2022, and May 31st, 2023. Sediment cores were collected near oyster bag locations (Figure 1) to a minimum depth of 20 cm and sub-sampled at depth horizons of 0–1, 1–3, 3–5, and 5–10 cm using an incremental sediment core extruder. Only the 1–3 cm sediment layer was analyzed for this study, based on the assumption that it was the most representative of the oyster environment as oyster bags were placed on the sediment surface. Sediment samples were stored in Whirlpak bags and dried at 60 °C. The mineralogical composition of the samples was determined by XRD by QMineral as described above. Samples were homogenized with a mortar and pestle and dried to avoid a preferred orientation. In-house software was used for interpretation. Weight percent total organic carbon (TOC) from 1 to 3 cm was measured by the Arizona State University’s Metals, Environmental and Terrestrial Analytical Laboratory (only for August and October 2022). Samples were sieved to remove particles >2 mm and milled to a fine powder. An aliquot of the milled sediment was then weighed into a silver capsule, fumigated with hydrochloric acid to remove carbonates, and dried at 60 °C. Samples were analyzed on a Perkin Elmer Series II CHNS/O analyzer, calibrated using an acetanilide standard and Certified Reference Materials, TOC standard B2293 and NIST #2711. The precision of this method was approximately 0.03% TOC.

Additional sediment grab surveys were conducted in May, June, August, September, and October 2022, as well as June, August, and October 2023 to determine spatial and temporal variation in grain size distribution. Samples were collected with a 0.04 m⁻² van Veen grab during each survey at seven transects throughout the study area, two stations per transect (Supplementary Figure S1.1A). Following the methodology of Folk (1974), samples were partitioned into three size-fractions by adding 50 mL of a 1% Calgon solution, mixing to disaggregate the particles in the sample, and wet sieving with distilled water through a combination of 2 mm and 63 μm sieves. The >2 mm and 2 mm-63 μm fractions were placed in a drying oven at 60 °C for at least 48 h to obtain dry weights. Water containing the <63 μm fraction (mud) was brought up to 1,000 mL total volume in a graduated cylinder, mixed thoroughly, and subsampled with a 20 mL pipette at a depth of 20 cm, 20 s after mixing. Pipette samples were placed in a drying oven at 60 °C for at least 48 h to obtain dry weight estimates of the mud fraction. Mud weight estimates included a correction for the amount of Calgon added to the samples.

2.3 Sediment porewater and water column field and laboratory methods

Chlorophyll-a was measured by sensor (In Situ Aqua TROLL 500) approximately 0.5 m above the seafloor concurrent with porewater surveys. Precision of the chlorophyll-a sensor was 0.94 Relative Fluorescence Units (RFU). Eight porewater surveys were conducted throughout the experiment. Discrete porewater samples were collected via carbon fiber PushPoints (M. H. E. Products, East Tawas, MI) at 0 cm (i.e., water column, just above the sediment–water interface) and 2 cm. Approximately 40 mL of porewater was collected with a polycarbonate syringe, then filtered to 0.45 μm with a 13 mm diameter polyethersulfone syringe filter, and subsampled immediately on the beach. All protocols followed the best oceanographic sampling practices (Dickson et al., 2007). Porewater sub-samples were immediately taken back to the laboratory and either analyzed in-house or sent for external analyses. Conductivity was measured in-house using a Mettler Toledo Inlab Conductivity Probe. Salinity was calculated from conductivity using the algorithm of the Practical Salinity Scale of 1978 (UNESCO, 1981; UNESCO, 1983). The precision of this method was approximately 0.07 PSU. Nitrate+nitrite and ammonia were analyzed in the Gobler Laboratory at Stony Brook University using a Lachat Quikchem 8,500 flow injection analyzer (Lachet Instruments, 2008). The precisions of these methods were approximately 0.14 and 0.16 μmol L⁻¹, respectively. Sulfide was measured following the 3–40 μM (Cline, 1969) method. The precision of this method was approximately ± 2%. Trace metals were analyzed at the University of Southern Mississippi’s Center for Trace Analysis. Trace metal samples were diluted 30-fold in ultrapure 0.16 M nitric acid (Fisher Optima) which contained approximately 17 nM indium as an internal standard. Diluted samples were measured by sector-field inductively coupled plasma mass spectrometry (ThermoFisher Element XR) using a Peltier spray chamber (PC3, Elemental Scientific) and low flow perfluoroalkoxy alkane nebulizer (Elemental Scientific). Cd and Pb were used for quantification in low resolution, with Mo monitored for correction of molybdenum monoxide interference on Cd. The other elements (Ni, Cr, Co, Al, Cu, and Zn) were determined in medium resolution to eliminate common isobaric interferences. Quantification utilized standard curves which contained 30-fold diluted, cleanly collected seawater, to eliminate matrix effects. To check accuracy, SLEW-4, an estuarine water Certified Reference Material (National Research Council Canada) was analyzed. However, because many of the certified values of SLEW-4 are lower than the concentration range of the samples, a laboratory-fortified aliquot of SLEW-4 was prepared. For Co, Cu, and Ni, the recovery of SLEW-4 was typically within 10% of the certified values. For all elements, the recovery of the fortified SLEW-4 was typically within 10% of the fortified analyte addition. The precision of this method were approximately 0.14 μg Ni L⁻¹, 0.016 μg Cr L⁻¹, 0.004 μg Co L⁻¹, 7.8 μg Al L⁻¹, 0.015 μg Cd L⁻¹, 0.6 μg Cu L⁻¹, 0.05 μg Pb L⁻¹, 2.9 μg Zn L⁻¹. Total alkalinity for the 2022 samples was analyzed by the Subhas Lab at Woods Hole Oceanographic Institute. Total alkalinity was determined using an open-system Gran titration on weighed 2.5 mL single samples, using a Metrohm 805 Dosimat and 855 robotic Titrosampler, calibrated twice daily against in-house seawater standard that was intercalibrated against Certified CO₂ in Dickson Seawater Reference Material (Scripps Institution of Oceanography). The precision of this method was approximately 4.4 μmol kg⁻¹. Total alkalinity for two timepoints in 2023 was determined in-house using a Metrohm 855 Robotic Titrosampler and 805 Dosimat system following the procedures detailed in Dickson et al. (Dickson et al., 2003) with modifications for small volume samples. These included a salinity adjusted (0.7 M sodium chloride) ~ 0.01 N hydrochloric acid solution dispensed at 5 μL intervals into a ~ 3 mL seawater sample. Certified CO₂ in the Dickson Seawater Reference Material (Scripps Institution of Oceanography) was titrated in triplicate before and after every 15 samples and a linear Gran function was applied to estimate the equivalence point in both sample and standards. The precision of this method was approximately 6.2 μmol kg⁻¹.

2.4 Oyster field and laboratory methods

Eastern oysters (C. virginica) were purchased once from a local commercial oyster farmer in Mastic Beach, NY. 80 juvenile oysters (of size 3–4 cm) were randomly placed in marine grade plastic oyster bags (100 cm × 50 cm with 14 mm mesh size), which were then secured sub-tidally with screw anchors. Four oyster bags were placed within each of the respective control and treatment areas (n = 16) on July 14, 2022. Oyster bags were marked with buoys to discourage poaching and prevent injury to swimmers. Oyster maintenance occurred 1–2 times per week, and involved flipping each bag over to rotate the side facing the substrate, shaking and breaking apart attached oysters, and removing any fouling or debris attached to the mesh. Three (3) oysters were collected from each bag on September 7th, 2022 (61 days after olivine nourishment), November 29th, 2022 (144 days after olivine nourishment), and May 10th, 2023 (306 days after olivine nourishment). In each case, samples were transported back to the laboratory, and depurated in a holding tank with filtered seawater (salinity = 30 psu, temperature = 20 °C) for 24 h. Soft tissue was dissected (using non-metallic scalpels) from shells, wet weights were recorded, and tissue was placed in a drying oven (60 °C) for 2 days. Dry weights were then taken and the dried tissue was pulverized using a ceramic mortar and pestle, placed into falcon tubes and shipped to Dartmouth College, NH Trace element Analysis Core for trace metals analysis. Oyster tissue was sub-sampled (ca. 250 mg sample) into 50 mL polypropylene tubes and 5 mL of 9:1 HNO₃: HCl was added and the samples were left to ‘cold digest’ overnight. Samples were then digested in a MARS 6 (CEM, Matthews, NC) at 105 °C with a 15-min ramp and 45-min hold. After cooling, 100 μL of H₂O₂ was added to each sample and the samples were heated again. Finally, samples were diluted to 50 mL. The digestion included blanks and standard reference materials (NIST 2976, mussel tissue and 1566b Oyster tissue) at a frequency of one each per 20 samples. All measurement steps were recorded gravimetrically. The sample digestates were analyzed by triple quadrupole ICP-MS (Agilent 8,900, Wilmington, DE). The analyte suite included Al, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Se, Sr., Mo, Ag, Cd, Sn, Sb, Hg, Tl, Pb, U. All analytes were analyzed in He gas mode and Cr, V, As, Se, Cd were also analyzed in O₂ gas mode. The ICP-MS was calibrated with NIST-traceable standards (Inorganic Ventures, Christiansberg, VA) and second source standards used to create the calibration verification were run after every calibration and every 10 samples; recoveries were ca 100% +/− 5%. Average SRM recoveries for Cr, Co, Ni were: 87 +/− 7% (n = 22976b only), 97 +/− 10% (n = 6), 111 +/− 24% (n = 6) respectively. Recoveries for Ni in 2976, where only a reference value is given, were biased high being 138 and 145% recovery of the reference value of 0.93 +/− 0.12; recoveries of Ni in 1566b (n = 4), where its value is certified were 96 +/− 6%.

2.5 Statistical analysis

Differences in the total alkalinity (TA) and Ni concentration in porewaters, selected trace metals (Ni, Cr, Co) concentration in the oyster tissue, as well as oyster dry weights among treatments and dates, were tested using a generalized linear mixed model (GLMM) based on a normal distribution. The model included 2 fixed factors (Treatment, Date), their interaction, and a random factor that represented the bag in which oysters were kept. The best model was selected according to Akaike’s Information Criterion and diagnostic tests such as dispersion, residuals, and Levene tests for heteroscedasticity following DHARMa R package (version 0.4.6). When significant effects were detected by the main test, Tukey pairwise tests were applied at a family error rate of 0.05. All analyses were performed in R Core Team (2024) using the “glmmTMB” package, version 4.2.2.

Bioconcentration factor (BCF) which represents the degree of metal concentration in an organism was calculated for Ni, Cr, and Co using the formula:

Where Cm is the metal concentration in an organism’s tissue expressed as μg g⁻¹ dry weight, and Cw is the metal concentration in water expressed as μg mL⁻¹. To account for oysters exposure to a range of metal concentrations with time, BCF for oyster samples collected on September 7, 2022, was calculated using the average metal concentration in the bottom water and porewater measured between July 15 and August 2, 2022; for oysters collected on November 29 2022 using the average metal concentration in the porewater measured between July 15 and October 10 2022; while for oysters collected in and May 10, 2023, using the average metal concentration in the porewater measured between July 15, 2022, and June 29, 2023.

3 Results

3.1 Olivine distribution and sediment porewater composition

Approximately 1 month (39 days/August) after placement, the Olivine Nourished site was comprised of 40.5% olivine sand, a substantial amount of olivine had moved to Control Nourished (20.6%), while Control West and Control East had no olivine (0%) (Figure 1). By ~3 months post-placement (102 days), the concentration of olivine in sediment was still highest at the Olivine treatment site (13.5%) but had decreased substantially since August. Furthermore, the olivine sand had dispersed along more of the coastline, comprising 5.7% of sediment at Control West and 5.1% of sediment at Control Nourished. By ~11 months post-placement (327 days) all sites had <10% olivine in the sediment, with the highest percentage of olivine at Control Nourished (7.1%) and the lowest percentage of olivine sand at the Olivine Nourished treatment site (0%).

We observed that the highest concentrations of TA and olivine-derived trace metals (Ni, Cr, Co) were measured in the Olivine Nourished treatment porewaters within the first few months of the field trial (Figure 2; Supplementary Figures S1.2A,B, S1.3A,B), consistent with the high concentration of olivine in the sediment at that point (Figure 1). More specifically, 0–61 days after olivine placement the highest mean TA concentrations were observed in Olivine Nourished treatment porewater; they were ~6%–13% higher than porewater at Control West and other control treatments (Figure 2A), however not statistically significantly higher. Later in the experiment, the Olivine TA porewater concentrations went down (from 2504.9 ± 358.9 to 2382.4 ± 338.4 μmol kg⁻¹) and the Control West TA porewater remained the highest among all treatments (Figure 2A). Statistically significantly higher mean porewater Ni concentration (40.9 ± 32.9 μg L⁻¹) was observed at the Olivine Nourished treatment site 0–61 days after olivine placement compared to all Control sites (0.7 to 2.2 μg L⁻¹, Figure 2B, p < 0.05, Supplementary Table S1). 61–144 days after olivine placement, the mean porewater Ni concentration at the Olivine Nourished treatment site had declined to 6.0 ± 3.6 μg L⁻¹ and remained similar later in time - 327–356 days after olivine placement (7.0 ± 8.0 μg L⁻¹) and was not significantly different from other treatments (p > 0.05, Supplementary Table S1). Ni porewater concentration remained low at all other treatment sites throughout the experiment. Bottom water TA and Ni was not significantly different among treatments at any timepoint (Supplementary Figure S1.2, Table S1). Similar trends were observed for the other olivine-derived metal, with higher concentrations of Co in Olivine Nourished treatment porewaters than at Control sites during the first month following olivine placement (Supplementary Figure S1.3B) but not for Cr (Supplementary Figure S1.3A).

Figure 2

3.2 Oyster growth

For all sites, oysters grew significantly through time. All sites had higher average oyster dry weights at day 144 and day 306 than day 61 (p < 0.001, Table 1; Figure 3). Notably, the difference in dry weight was marginally nonsignificant (p = 0.051, Table 1) for Olivine Nourished treatment oysters compared to Control treatment oysters at 2 months post-placement (day 61) - 1.7 ± 0.5 g dw⁻¹ compared to those in Control treatments - 1.1 ± 0.3 g dw⁻¹ (average dry weight across all Controls, Figure 3).

Figure 3

3.3 Oyster tissue metal bioaccumulation

Approximately 2 months (61 days) after olivine placement the average Ni concentration in oysters at the Olivine Nourished treatment site was 3.35 ± 1.89 μg g dw⁻¹. Although this concentration was higher than that observed in Control Nourished and East oysters (2.37 ± 0.71 and 2.62 ± 2.79 μg g dw⁻¹, respectively), the differences were not significant (p > 0.05, Table 1). However, the Ni concentration in Olivine Nourished treatment oysters (as well as Control Nourished and East) was significantly lower than Control West oysters (5.23 ± 2.06 μg g dw⁻¹, Figure 4A, p < 0.05, Table 1). Moreover, on the same date, the Control West oysters exhibited the highest BCF for Ni (5,625 ± 1935) of any treatment during the experiment (Supplementary Table S2). The body burden of other metals associated with olivine (i.e., Co, Cr) presented a similar pattern of increase at Control West oysters to Ni but were much lower (<1 μg g dw⁻¹) (Figures 4B,C).

Figure 4

4 Discussion

4.1 Olivine distribution and its effect on sediment porewater composition

Sediment transport plays an essential role in understanding the exposure of oysters and other marine life to olivine sand following placement in the coastal environment. Hydrodynamics can redistribute the olivine through time, and in turn, the resultant olivine dissolution products (e.g., alkalinity and trace metals). Temporal and spatial changes in sediment olivine content and porewater metal concentrations are essential for contextualizing spatiotemporal trends in oyster growth and trace metal body burden. In the present study, hydrodynamics increased the overall coastal area with a component of olivine in the sediment through time, while decreasing the amount of olivine at the original placement site. Olivine redistribution was observed as early as 1 month after placement, and by approximately 1 year post-placement, the olivine percentage at the original nourishment site had decreased to nearly 0%. We note that these results represent only the 1–3 cm sediment layer, thus olivine may have been present in deeper sediment layers.

Furthermore, as olivine dissolves in the sediment, the concentration of dissolved species such as total alkalinity (TA), Ni, Cr, and Co in sediment porewater varies substantially over short timescales (minutes to days) due to changes in the concentration of olivine in the sediment, plus the olivine dissolution rate, natural biogeochemical cycling in the sediment, and physical processes such as advection and bioirrigation which dilute porewater with bottom water. Due to these complex controls, porewater composition mainly serves to establish the presence or absence of olivine dissolution products, and as a qualitative tracer for the relative magnitude of dissolution products between sites. Here, significantly elevated TA and trace metal (Ni, Cr, Co) concentrations were observed in porewater at the Olivine Nourished treatment, but not in bottom water (0 cm depth), suggesting significant dilution of olivine dissolution products by the water column. Therefore, exposure of dissolution products to organisms living in the surface sediment, such as oysters, would be temporary, occurring primarily through porewater fluxes. Metal concentrations in the porewater also declined over time and were no longer significantly elevated approximately 3 months after the initial placement at Olivine Nourished treatment. Notably, bottom water Ni and Cr concentrations never exceeded the US Environmental Protection Agency (US EPA) National Recommended Water Quality Criteria acute or chronic concentrations for seawater [74 and 8.2 μg L⁻¹ for Ni, 1,100 and 50 μg L⁻¹ for Cr (VI)], respectively (US EPA, 2026). There are no US EPA Recommended Water Quality Criteria for Co in seawater, however, Saili et al. (2021) established a chronic Co surface water concentration of 7 μg L⁻¹. All bottom water Co concentrations were far below this threshold as well. No water quality recommendations exist for porewaters.

4.2 Olivine effect on oyster growth and tissue metal bioaccumulation

Local changes to alkalinity related to mERW may benefit shellfish growth through greater availability of carbonate and bicarbonate ions, enabling shellfish to more readily form calcium carbonate (Waldbusser et al., 2015). Porewater TA was highest at the Olivine Nourished treatment site at the start of the experiment (Figure 2A). As such, the slightly higher biomass of oysters at the Olivine Nourished treatment site, compared to Control treatments, 61 days post-placement could be due to these favorable environmental conditions. Alternatively, oyster growth can be impacted by food availability. Recent research has found that OAE-induced changes in seawater composition might influence primary producers and thus available food sources. Hutchins et al. (2023) showed that two diatom species utilized Si and Fe from a synthetic olivine leachate to achieve near-maximum growth rates. However, probably because of nitrogen limitation at the study site, Guo et al. (2024) found no positive effects of olivine dissolution on the growth of diatoms and other phytoplankton. When testing only alkalinity enhancement effects on phytoplankton, no effects were observed on communities (Ramírez et al., 2025) or species viability and growth rate (Oberlander et al., 2024) and when testing both Si and calcium (Ca)-based OAE, a limited effect was reported for diatom silicification (Ferderer et al., 2024). We did not observe increased food levels resulting from olivine, as surface sediment total organic carbon (TOC) concentrations and water column chlorophyll-a levels remained consistent across all treatments in the first 2 months after olivine placement (Supplementary Figures S1.4, S1.5). The lack of effect was likely because there was no measurable increase in bottom water concentrations of olivine dissolution products. Taken together, it is unlikely that the higher oyster biomass at the Olivine Nourished treatment site was a result of increased food availability. Moreover, Olivine Nourished treatment oysters had the highest biomass following the period with the highest porewater trace metal concentrations, therefore it does not appear that the porewater metals negatively affected oyster growth, or at least that any negative effect was counteracted by the positive effect of simultaneously increased porewater alkalinity concentrations. This finding is supported by bioconcentration factor (BCF) estimates. Oysters from the Olivine Nourished treatment demonstrated consistently lower Ni BCF values across all dates (e.g., 131 ± 91 SD on day 61, as compared to >1,000 for all other treatments, Supplementary Table S2), indicative of an overall lower susceptibility to Ni accumulation.

With regards to trace metal bioaccumulation, one unexpected finding of this study is that Control West oysters had the highest Ni, Cr, and Co body burden after 61 days of exposure (Figure 4) even though the Olivine Nourished treatment site had the highest Ni porewater concentrations during this period (Figure 2B). We find that the accumulation of Ni, Cr, and Co directly resulted from olivine because other sources of contamination, such as septic tank discharge or groundwater, would likely have resulted in increased metal content in porewaters or accumulation in oysters of non-olivine metals (e.g., aluminum (Al), cadmium (Cd), copper (Cu), lead (Pb), zinc (Zn)), but this was not observed (Supplementary Figures S1.6–S2.3). Bioaccumulation of metals in marine bivalves occurs when they accumulate metals at rates greater than the loss rates, and these are a function of assimilation efficiencies of ingested metals, absorption of dissolved metals, and depuration processes that are dependent on metabolic processes (Fisher and Reinfelder, 1995; Wang et al., 1996). At the beginning of the experiment, Control West porewaters had lower salinity (Supplementary Figure S2.4B) and higher concentrations of ammonia (Supplementary Figure S2.5) than other treatments. Moreover, sulfide in porewater at 2 cm depth was consistently elevated in the Control West treatment compared to other treatments in 2022 (Supplementary Table S3). Oysters are susceptible to sulfide and ammonia exposure, which can increase their vulnerability to environmental stressors by affecting detoxification mechanisms (Moullac, 2008; Butterworth et al., 2004). Furthermore, low salinity conditions can increase bioavailability and accumulation of some trace metals due to changes in free ion activity (Rainbow and White, 1989; Wang and Rainbow, 2005). Therefore, unfavorable environmental conditions may have impacted Control West oyster bioaccumulation and detoxification processes and contributed to their relatively high Ni body burden. Additionally, oysters can accumulate dissolved metals through their gills (Yin and Wang, 2018), and through branchial filtration by assimilating ingested particulate organic matter and phytoplankton (Rainbow et al., 2009; Reinfelder et al., 1997). Here, analyses of sediment grain size indicate net movement of fine-grained material from the east to the west, from the Olivine Nourished treatment towards the Control West treatment (Supplementary Figure S1.1A). Thus, Control West oysters may have had relatively high metal assimilation compared to other treatments due to higher exposure to, and filtration of, very fine grained olivine particles which then dissolved in the digestive system. Ultimately, a few mechanisms could account for the increased metal accumulation of Control West oysters, as compared to other sites, despite Control West porewaters having lower concentrations of olivine-derived dissolved metals in the first few months of the experiment.

The average Ni body burden of Control West oysters declined over time; approximately 5 months (144 days) after placement, it was indistinguishable from Olivine Nourished and Control East treatment oysters (1.64 ± 0.60 μg g dw⁻¹ vs. 1.64 ± 1.77 μg g dw⁻¹) (p > 0.05, Table 1). Ni concentrations remained low in all treatments ~10 months (306 days) after placement, with treatment averages between 2.18 ± 2.71 μg g dw⁻¹. Therefore, despite the comparatively high concentration of metals in Control West oysters at the beginning of the experiment, these concentrations decreased through time while the biomass of Control West oysters increased, as did the oyster biomass at all treatments. Therefore, the early accumulation of trace metals at Control West did not cause a long-term negative physiological impact, as oyster biomass was not statistically different from other treatments 144 and 306 days after olivine placement. In addition, the bioconcentration factor for Ni, Cr, and Co decreased with time suggesting a decline in metal assimilation or efficient detoxification of metals (e.g., Ni BCF on September 7, 2022 was 5,625 ± 1935 SD vs. 1735 ± 393 on May 10, 2023; Supplementary Table S2). An experimental study on Crassostrea hongkongensis noted a high turnover rate for Ni in oysters, where Ni in oyster tissue reached a steady state after 1 week of exposure and maintained low concentrations, likely due to regulation mechanisms such as sequestration by metallothionein-like proteins (Yin and Wang, 2018). More than 90% of Ni accumulated in oysters during a 4-week exposure under laboratory conditions, as well as in oysters from a contaminated estuary, were eliminated within a few weeks of depuration (Yin and Wang, 2018; Wang and Wang, 2014). In the present study, we suggest that natural olivine transport and redistribution decreased the overall pool of olivine grains and dissolved metals throughout the project area and allowed for depuration/detoxification to occur. In addition, assimilation efficiencies of diverse metals, including Co, from ingested phytoplankton and subsequent efflux rates in C. virginica were generally comparable to those in the clams Macoma balthica (Linnaeus, 1758) and Mercenaria mercenaria (Linnaeus, 1758) and in the blue mussel Mytilus edulis (Linnaeus, 1758) (Reinfelder et al., 1997), suggesting that other bivalves would likely respond similarly to olivine as oysters.

4.3 Global context

While the current study is the first marine application of olivine for the purpose of carbon removal, olivine was previously applied as a 30 cm layer to a small section of Kirkebukten port, Bergen, Norway, to assess its potential for pollution adsorption. In that project, mussels were placed in bags and exposed to olivine for 12 weeks. The mussels ultimately accumulated between 0.5 and 1.0 μg Ni g dw⁻¹ (Sæterdal Bøyum and Ane Moe, 2020), while naturally occurring mussels collected from the site 4 years after olivine placement accumulated between 1.3 and 3.5 μg Ni g dw⁻¹ (Gjesdal and Solheimslid, 2016). These accumulation levels are comparable to the current study (Figure 5; Supplementary Table S4), where only oysters from the Control West treatment, 61 days post-olivine placement, exhibited higher Ni accumulation (maximum 5.2 μg Ni g dw⁻¹). Additionally, mussels from the Norwegian experiment accumulated similar to higher amounts of Cr (0.5 to 4.1 μg Cr g dw⁻¹) compared to oysters in the current study (0.1 to 0.6 μg Cr g dw⁻¹). Thus, the magnitude of metal bioaccumulation measured in this study appears to be representative of bivalves under field olivine exposure. Bivalves have also been exposed to olivine in a laboratory setting. Bent-nosed clams (Macoma nasuta, Conrad, 1837) accumulated 6.8 μg Ni g dw⁻¹ and 0.09 μg Cr g dw⁻¹, with no Co accumulation (Jankowska et al., 2024) [wet weight to dry weight conversion based on a 0.489 ratio for bivalves (Gogina et al., 2022)] over 28 days of exposure in tanks. Thus, Ni accumulation in clams under laboratory conditions reached slightly higher levels than oysters from the current study over 61 days of field exposure. These findings suggest that species with different feeding types and behaviors may respond differently to olivine exposure, and that laboratory experiments may overestimate effects, potentially due to limited water exchange and dilution, as compared to field conditions.

Figure 5

In general, researchers and regulators have extensively studied oysters due to their commercial importance in aquaculture and food production. The maximum acceptable Ni concentration in shellfish published by the National Shellfish Sanitation Program, US Food and Drug Administration, is 80 μg g dw⁻¹ (United States Food and Drug Administration, 2007). According to the Norwegian classification of environmental contamination level based on metal body burden in mussels, Ni concentrations <5 μg g dw⁻¹ are regarded as the lowest condition class contamination level (The Norwegian Environment Agency, 2008). The oyster Ni accumulation measured in the current study is at or below established warning thresholds. The Ni and Cr body burden concentrations are also well within the range of values published for bivalves from natural settings (Figure 5). Therefore, mERW with olivine sand had a limited effect on trace metals bioaccumulation in oysters and does not appear to be problematic for human consumption and the shellfish industry.

4.4 Future work

The results of this study suggest that oyster growth could be positively affected by olivine exposure, potentially related to increased alkalinity concentrations resulting from olivine dissolution, however, this effect was not significant and was only observed in the first 61 days after olivine placement, and thus requires further testing. The results of this study also indicate limited impact of olivine-derived metals on juvenile and adult oysters over a year of exposure, but future work should include testing of different life stages (e.g., oyster larvae) which may be more sensitive, as well as longer exposure periods. Further field tests should also encompass other benthic species with varied functional traits, including those that inhabit sediment, burrow, and act as deposit feeders. These organisms would be directly exposed to olivine sand and its dissolution products in porewaters. Crucially, future research should also prioritize field experiments across diverse environmental conditions, as numerous factors influence the speciation and bioavailability of metals. Special attention might be given to bioaccumulation effects under low salinity, low oxygen and high ammonia and sulfide conditions. These field studies should be accompanied by controlled lab experiments to help disentangle competing biogeochemical processes. Furthermore, this study deployed olivine sand within the Peconic Bay, NY, a partially enclosed, low-energy system. Testing high-energy, open-ocean coastal areas will likely reveal additional insights, such as the impact of high porewater advection and dilution rates, as well as rapid sediment redistribution. Such insights will be instrumental in determining if, how and when mERW projects can be implemented safely, paving the way for effective and responsible climate intervention strategies in marine ecosystems.

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