Impact of authigenic clay formation on marine trace element cycling

Domain: chemistry
Published: 2026-02-20
Editorial outcome: major_revision

Abstract
The impact of marine authigenic clay formation on oceanic trace element cycling remains poorly constrained. Here we report a comprehensive trace and major element dataset for authigenic green clays (Fe-smectite and glauconite series). Our results show that these clays are consistently depleted in Cu, Ba, REEs, Nb, Ti and Al compared to detrital precursor phases, suggesting that systematic exclusion during authigenic clay formation provides a source of these elements to seawater via the pore water. Furthermore, authigenic clays inherit both their REE pattern and Nd isotope composition from their detrital precursor, demonstrating that dissolution-reprecipitation processes control pore water REE chemistry. B, Fe, Cs, K, Mg, Rb, Be, Cr, V, Zn, Co, Sc and Ga are consistently enriched in authigenic clays, requiring uptake from pore water. Global flux estimates constrained by these results confirm that clay authigenesis represents an important sink or source term in the oceanic cycles of many more trace elements than previously recognized.

Introduction
Silicate weathering processes control the cycling of material at Earth’s surface, regulating global climate, biogeochemical cycling and seawater composition1–3. However, weathering is not limited to continental settings, with growing evidence suggesting that continued chemical weathering of continental detritus in marine sedimentary settings is linked to, and exerts a comparable influence to, terrestrial weathering of the continental crust4,5. In this context, dissolution-precipitation reactions that result in the formation of marine authigenic clays (MAC) at the expense of continental detritus play an important but underexplored role6–8. Recent studies showing that MAC formation is a 3- to 8-fold greater Si sink than previously recognized9 underscore the potential influence of clay authigenesis on the global carbon cycle, Earth’s climate3, and oceanic elemental budgets6,10,11.

The impact of MAC formation on marine trace elemental budgets is likely important but largely unconstrained. Whereas the major element composition of MACs is sufficiently well-constrained to allow global estimates of MAC uptake of elements such as K, Mg, Si, Al and Fe8,11, the trace element composition of MACs is mostly unexplored12. This is mainly due to analytical limitations. Specifically, while electron microbeam techniques (e.g., EMPA or analytical TEM) have long allowed for the major element composition of individual MAC grains to be determined at the nm to µm scale, trace element characterization typically requires separation and purification of MACs prior to conventional solution geochemical analysis. This has proven difficult because sedimentary MACs typically occur dispersed amongst and intergrown with other colloidal and clay-sized (i.e., <5 µm) detrital and authigenic minerals (e.g., detrital clays, feldspars, authigenic Fe-(hydr)oxides and phosphates) with similar physical properties (grain size, density etc.). Previous efforts to identify the trace element characteristics of MACs have therefore mostly relied on analysis of multi-mineralic (bulk) sedimentary mixtures containing varying amounts, and potentially also multiple varieties, of MACs. While this approach has provided important qualitative insights, e.g. identifying enrichments in Be13 and Li14 in both present day13,14 and ancient MACs15, it does not replace the need for direct and thus more robust measurements of MAC trace element composition. Similarly, although clay synthesis experiments have yielded important constraints16, only a few classes of MAC have been synthesized to date meaning comprehensive, quantitative multi-elemental data on MAC trace element composition are still lacking.

We employ an alternative approach to address this gap, targeting the Fe-smectite to glauconite (green clay) series of MACs. These commonly form large pellets with favorable magnetic properties, allowing them to be separated from the host sediment17 and providing a tractable path for obtaining relatively pure MAC separates for geochemical analysis18. Importantly, these green clays form across a wide range of shallow shelf to deeper marine settings including calcareous, siliceous as well as terrigenous sediments19. Considered the dominant class of MACs throughout the Phanerozoic12, glaucony minerals can potentially provide useful constraints on MACs as a whole.

Here we present the major and trace element geochemistry of authigenic green clay separates extracted from the equatorial South Atlantic (ODP Site 959; Fig. 1) and the eastern North Pacific (Oregon Margin). Comparison to the detrital host sediment at each site reveals systematic patterns of elemental uptake and release associated with MAC formation. In addition, the ODP 959 MAC separates span a 25 m depth interval and show substantial changes in mineralogical maturity over this interval20,21, revealing the impact of progressive clay maturation on MAC trace element chemistry. Validation against previously published data for Congo Shelf MACs (Fig. 1) allows us to determine an average green MAC composition, which we use to constrain an estimate of global MAC-related trace element fluxes.

Results
Elemental enrichment vs depletion in Site 959 MAC
ODP Site 959 is located in the Gulf of Guinea (Fig. 1), at a water depth of ~2100 m. Eight samples were collected at depths between 16 cm and 2480 cm below seafloor, equating to a depositional age of 0.015 to 2.5 Ma21. MAC formation at this site occurred in two stages, starting with the replacement of detrital kaolinite and smectite-rich mud in fill of foraminiferal tests by an authigenic Fe-rich smectite20,21 (Fig. 2). This Fe-smectite subsequently evolved towards more mature glauconite through continued K enrichment20,21. The elements taken up by MACs originate from overlying seawater and dissolution of precursor minerals8, both via the pore water. At Site 959 dissolution of detrital kaolinite, Fe-(hydr)oxides, and biogenic silica provided dissolved Fe3+/2+, Al3+, and Si(OH)4 required for MAC formation, with only minor contributions from detrital feldspar20,21. In contrast, seawater was likely the primary source of K+ and Mg2+ incorporated into these MACs.

Since our primary aim is to identify the impact of MAC formation on marine elemental cycling, our discussion focuses on MAC elemental enrichment patterns rather than elemental concentrations (concentration data for MACs and detrital fractions are given in Supplementary Data 1 and 2). After Zr-normalization to correct for dilution effects from cation uptake during clay authigenesis (see Methods), elements in MAC separates can be classified as enriched, depleted or unchanged relative to the precursor detrital clay. Boron, Cs, Fe, Rb, Cr, K, Mg, Co, V, Be, Mn, Zn, P, Sc, Ni and Ga (in descending order of median enrichment) are consistently enriched in Site 959 MACs (Fig. 3A), with enrichment factors > 1. In contrast, Al, Cu, Nb, Ba, Th, U, and the REEs + Y are systematically depleted, with Ba showing the greatest depletion overall (Fig. 3A). Surprisingly, although marine clay authigenesis is currently considered to be the main sink for Li in the ocean14,22, our data for Site 959 MACs show that Li is moderately depleted relative to detrital precursors. Strontium, Mo, Hf, W, Th, and Pb, with enrichment factors of ~1, are neither enriched nor depleted during clay authigenesis (Fig. 3).

MACs from all sites show broadly similar enrichment patterns
Comparison of Site 959 MACs to authigenic green clay separates from the Oregon Margin (Supplementary Data 1 and 2) and from the Congo Shelf (data published by ref. 18) allows us to evaluate the potential influence of site-specific factors (e.g., sediment age, substrate, sedimentation rate, redox state) on MAC composition and elemental enrichment trends.

The Oregon Margin site is located at 1200 m water depth, within the local oxygen minimum zone23. The site features biogenic silica and silt-rich, carbonate-lean sediments24, with detrital materials derived from erosion of adjacent volcanic terrain. In contrast to ODP Site 959, the Oregon glauconitic smectites formed in benthic macrofaunal fecal pellets containing detrital clay- to silt-sized inclusions consisting of primary igneous phases such as feldspar, chlorite, mica, and quartz (Fig. 2 and refs. 10, 25). The near-absence of cation-depleted detrital clays such as kaolinite implies that glauconitic Fe-smectite formed in response to marine silicate weathering of Fe- and Al-rich igneous detrital grains4. A minimum age estimate of ~200 years applies for MACs at 10 cm depth at this site25.

The Congo Shelf is a flat 40- to 80 km wide platform that extends from coastal West Africa to the continental shelf-break located at about 120 m water depth, and features extensive mud deposits which extend northward from the mouth of the Congo River26. Deposition of Congo River-sourced detritus containing substantial amounts of kaolinite and reactive iron minerals drives intense marine clay authigenesis here, mostly within fecal pellets. Bayon et al.18 have recently reported the major and selected trace element composition of MACs isolated from several Congo Shelf sites. For comparison to our MAC data we focus on smectite-glauconite separates (type 3 of 18) collected from an outer shelf setting, spanning depositional ages of ~10 to 20 kyrs. We use a * to denote elements for which no data is available for Congo Shelf MACs below.

Despite their disparate settings, and the notably more Fe-rich and Al-poor composition of Oregon Margin and Congo Shelf MACs compared to Site 959 MACs (Fe = 18-23 and 18-25 wt% versus 14-16 wt%; Al = 1.6-2.9 and 2.3-3.5 wt% vs 3.7-4.6 wt%; see Supplementary Data 3), the overall trace element composition and enrichment versus depletion trends are largely consistent in MACs from all three sites (Fig. 3). The most heavily enriched elements in Site 959 MACs – B*, Fe, Cs*, Rb, K, Be, Cr, Mg, P, Co, Zn*, V, Sc* and Ga* – show broadly comparable enrichments across Oregon Margin and Congo Shelf MACs. However, some differences are also observed.

Redox-sensitive elements Mn and Ni, for example, are enriched in Site 959 and Congo Shelf MACs, but depleted in Oregon Margin MACs. This may be because the oxygen minimum zone setting of the Oregon Margin promotes loss of reduced Mn from the porewater into the water column27 and incorporation of released Ni into authigenic sulfides rather than MACs. Lithium is modestly enriched in Congo and Oregon Margin MACs, but modestly depleted at Site 959, demonstrating that MACs play a more complex role in the marine Li cycle than is presently assumed14. Lead shows neither enrichment nor depletion at Site 959, but is enriched at the Oregon Margin site (no data for Congo site). Uranium and Sr are unchanged at Site 959 and Oregon Margin but substantially enriched and depleted, respectively, at the Congo site.

Those elements showing marked depletion in Site 959 MACs – Al, Ti, REEs and Y, Th, Nb*, Ba and Cu – are generally also depleted at the other sites. The only exceptions to this are variable Ba in Oregon Margin MACs, as well as modestly elevated light REEs in Congo Shelf MACs. The former is due to the presence of barite inclusions10, the latter can be attributed to the presence of light REE-rich phosphate inclusions18 rather than true compositional differences between MACs at these sites.

In summary, our data allow us to calculate an average green MAC major and trace element composition (see Supplementary Data 3). Further, we are able to identify those elements which are 1) consistently enriched during formation of smectite-glauconite series MACs (green field in Fig. 3D), meaning that MAC formation potentially represents an important sink in the marine cycle of these elements; 2) neither consistently enriched nor depleted in MACs compared to their precursor meaning that MAC formation is unlikely to significantly impact on the marine biogeochemical cycle of these elements (white field in Fig. 3D); and 3) consistently depleted in MACs compared to their precursor material (blue field in Fig. 3D) meaning that MAC formation potentially represents an important source of these elements to the ocean, in the form of a benthic flux. We expect that the average MAC composition and the above enrichment versus depletion trends will be useful for identifying changes in MAC abundance in the marine stratigraphic record, a challenging task at present given the lack of suitable proxies12.

Impact of MAC maturation and timing of element exclusion
The increasing mineralogical maturity of MAC pellets from the shallower (~0.015 Ma) to the deeper (~2.5 Ma) samples at Site 959, involving evolution from relatively K-poor Fe-smectite to increasingly ordered K-rich glauconitic smectite21, allows us to test the impact of early diagenetic clay maturation on MAC elemental depletion and enrichment patterns. We find that the initial direction of change (i.e., enrichment or depletion) of some of the elements analyzed becomes more strongly expressed with progressive maturation over the studied ~2.5 Ma interval (Fig. 4). Most of the elements showing this continued change have enrichment factors > 1 (e.g., Be, P, K, Sc, V, Cr, Mn, Fe, Ni, Rb, Mo, Cs, Tl, Pb), demonstrating that uptake of these elements from the porewater continues during clay maturation. In contrast, only a few elements show continued depletion with increased clay maturation (Sr, Nb, and Ba). Lithium, REEs, Ti, and U are depleted in MACs, but show no consistent change between the youngest and oldest MAC separates.

Comparison to Oregon Margin MACs data validate these observations. While the Oregon Margin MACs are younger relative to Site 959 MACs, they still show a range of maturity25. Comparing less magnetic (lower K content = less mature) and more magnetic (higher K content = more mature; see Supplementary Data 2) Oregon MAC fractions, we observe that many of the elements demonstrating continued enrichment at Site 959 also show continuing uptake of these elements during clay maturation at the Oregon Margin (e.g., Be, K, Sc, Cr, Fe, Rb, Cs, and Pb; Fig. 5). As at Site 959, Ti and U show little to no consistent change between the less versus the more mature MAC fractions. These are elements with an enrichment factor < 1, supporting the view that elemental depletion generally occurs as the Fe-smectite forms with no further systematic loss with maturation to increasingly K-rich glauconite. REEs are the exception, becoming more depleted in increasingly mature Oregon Margin MACs. The REE depletion in the more mature Oregon MAC fractions (EF=0.49–0.56; Fig. 5) is substantially greater than in the less mature fractions (0.7–0.79), but similar to the depletion observed in the older Site 959 MACs (0.44–0.52), supporting the idea that REE depletion occurs during earliest diagenesis, as observed for Ti and U, and then stabilizes.

The influence of MAC formation on marine elemental budgets
We have shown that MAC formation results in the net uptake of a wide range of major and trace elements, but also provides a source of some incompatible elements to the surrounding porewater. To evaluate the potential impact of MAC formation on oceanic elemental budgets we employ a Monte Carlo simulation approach (see Methods), and report median, 10th and 90th percentile flux sizes drawn from 100,000 Monte Carlo simulations for each element (Table 1).

In the absence of chemical data for other classes of MACs, our flux calculations assume that the major-to-trace element composition of the smectite-glauconite series of green clays is broadly representative of MACs as a whole. This is reasonable because, as the most abundant class of MACs12, they likely exert a dominant influence on the global average MAC composition. Nevertheless, the fluxes we obtain should be regarded as first order estimates until further work has been done to assess the trace element composition of a broader range of MAC mineral types. We further suggest that our estimates should be viewed as fluxes to or from the porewater rather than seawater. Element release/uptake associated with MAC formation occurs during earliest diagenesis, close to the sediment-seawater interface. This setting favors ion diffusion from porewater into bottom water and vice-versa, linking MAC formation to oceanic element budgets. However, the fate of MAC-incompatible elements after release into the porewater is uncertain and can include incorporation into other secondary minerals such as REE-hosting authigenic phosphates28,29. This means that although the exclusion of certain elements during clay authigenesis is potentially important to ocean element cycling the formation of co-precipitating phases may significantly attenuate this flux. Similarly, some of the elements that are enriched in MACs are likely supplied from local sediment dissolution rather than sourced directly from seawater12.

Notwithstanding the above caveats, comparison to existing oceanic elemental budget estimates demonstrates that our flux estimates yield reasonable results, with median flux values that are generally consistent with current estimates for total elemental source/sink inputs or outputs. Furthermore, for some elements our calculated MAC-driven fluxes correspond closely to missing sink or source terms required to balance modern oceanic element budgets.

We commence by discussing selected major element fluxes.
Potassium. Recent estimates of the marine K cycle30,31 suggest that inputs by rivers and high temperature hydrothermal fluids are balanced by K removal into marine sediments (0.6–1.3 Tmol yr-1; with MAC formation as an important mechanism) and low temperature alteration of the oceanic crust (0.3–0.4 Tmol yr-1). Our MAC K sink flux (0.18–0.38 Tmol yr-1, median = 0.28 Tmol yr-1; Table 1) compares well to recent estimates for K removal into marine green clays at 0.03–0.41 Tmol yr-1 (average = ~0.15 Tmol yr-132) and 0.04–0.09 Tmol yr-111. Our flux is larger than other recent estimates because we present a global MAC flux rather than a shelf-related (200–350 m water depth) green clay specific flux.

Magnesium. Our estimate of Mg burial related to MAC formation is 0.27 to 0.42 Tmol/yr (median = 0.36 Tmol yr-1). This is about 20% of the total Mg removal flux estimated for the marine sediment column (~2 Tmol yr-1), which is split between deep-marine carbonate (>1 Tmol yr-1) and silicate outputs (~1 Tmol yr-1, via low-temperature oceanic crust alteration and/or MACs33,34). The upper range of our MAC-related flux is potentially too high due to the large Mg enrichment factors obtained for Congo shelf MACs (outliers, Fig. 3D); this is related to the low Mg content of Congo River derived detrital clay precursor material18.

We continue with trace elements where MAC formation is potentially a significant sink.
Boron. The oceanic B budget shows a major imbalance, with inputs totaling ~0.41 to 0.55 Tmol/yr compared to only ~0.08 to 0.26 Tmol/yr of outputs35. Our calculated B flux (6.9–14 Gmol yr-1, median = 11 Gmol yr-1) equates to ~20–25% of the riverine dissolved input of B to the ocean36, and falls within the same range as previous estimates for B removal from the ocean via sorption to marine sediments (~0.18 Tmol yr-1, as summarized in ref. 35) suggesting MAC formation is an important B sink.

Beryllium. The total Be sediment burial flux has been estimated at 6.9 ± 1.7 × 107 mol yr-1, with Be removal mainly attributed to particle scavenging37. Our estimate of Be burial in MACs (6.5 to 14.4 × 107 mol yr-1; median = 10.2 × 107 mol yr-1) is of similar magnitude, implying that either i) the magnitude of the total ocean Be sink has been underestimated and/or ii) MACs source Be primarily from dissolution of detrital material. Regardless, the benthic flux of Be due to sediment dissolution during early sediment diagenesis37 is likely spatially heterogenous, with areas experiencing active MAC formation acting as sinks rather than sources.

Vanadium. The dissolved oceanic V budget38 features inputs of ~0.8 Gmol yr-1 compared to outputs of ~1 Gmol yr-1. Removal into suboxic sediments (Gmol yr-1) and via reaction with hydrothermal fluids (0.43–0.55 Gmol yr-1) are considered the major V sinks. The MAC-related V removal flux of 0.3 to 1.8 Gmol yr-1 (median ~1 Gmol yr-1) is high compared to current oceanic budget estimates. This indicates that V incorporated into MACs is largely from sediment dissolution rather than seawater, reflecting internal redistribution of V in the sediment in a similar fashion to Fe39, and limiting the benthic flux of dissolved V38 to settings that are not favorable to clay authigenesis4. Uptake into MACs can potentially also reconcile the discrepancy between high porewater V concentrations and the semi-conservative behavior of V in marine profiles40.

Cobalt. A recent study41 employing a global Co biogeochemical model report total dissolved Co inputs to the ocean of 7.5 × 108 mol yr-1. The dissolved inputs are mainly from sediment dissolution at continental margins while 6.8 × 108 mol yr-1 are thought to be lost to scavenging by sinking particles. Our results identify MAC formation as a Co sink, with a burial flux of 0.2 to 1.9 × 108 mol yr-1. Our median Co burial flux of 0.9 × 108 mol yr-1 is equivalent to 12% of the total inputs of dissolved Co to the ocean. On this basis, MAC formation is likely an important marine Co sink.

Zinc. A missing Zn sink of ~2 to 5 × 108 mol yr-1 is required to balance the oceanic Zn budget42 with the formation of authigenic mineral phases in continental margin sediments considered the most likely candidate for this Zn sink43. Our estimate for the MAC-related Zn sink ranges between 4.2 and 6 × 108 mol yr-1, with a median flux of 5.1 × 108 mol yr-1. This closely matches the size of the missing Zn sink required to balance the oceanic Zn budget, suggesting that MACs may be equally important sinks as authigenic sulfides43.

Gallium. A recent first-order estimate of the oceanic Ga budget has identified a major imbalance in which total Ga inputs are substantially greater than the outputs44. Our estimate of the Ga sink associated with MAC formation is 11.9 to 45.7 × 106 mol yr-1, with a median flux of 26 × 106 mol yr-1. This compares favorably with recent reports45 that the siliceous marine sediment Ga sink ranges between 47 and 141 × 106 mol yr-1, and is consistent with the hypothesis that reverse weathering is an important pathway for Ga removal from seawater45.

Rubidium. Total Rb removal into marine sediments falls between 0.25 and 1.4 Gmol yr-1, as estimated by isotope mass balance46. Our independent estimate of the oceanic Rb sink associated with MAC formation (0.48 to 0.82 Gmol yr-1; median = 0.65 Gmol yr-1) closely matches the previous estimate for the total Rb sedimentary sink, supporting the view that MAC formation is a key mechanism for marine Rb removal46.

Nickel. Our results suggest that MAC formation can act as both Ni source or sink, potentially due to a local redox control on Ni uptake27, producing a flux range between -0.7 to 3 × 108 mol yr-1. However, Ni is enriched in the majority of MAC samples analyzed in the present study (Supplementary Data 4), yielding a median sink flux estimate of 1.3 × 108 mol yr-1. This is consistent with recent work suggesting that Fe-rich MACs may be sinks for isotopically light Ni47. Our median flux estimate accounts for 6–16% of the total Ni sink flux of 8.1 to 19.6 × 108 mol yr-1.

Chromium. The direction of MAC influence on the oceanic Cr budget is opposite to that attributed to marine sediments as a whole. MACs from all three sites are enriched in Cr, equating to a global burial flux of 1.2 to 2.3 × 109 mol yr-1. In contrast, marine sediment dissolution is considered an overall source of Cr to seawater, with an estimated Cr benthic flux of ~6 × 108 mol yr-1 (ref. 48). This mismatch suggests that, similar to Be and V, i) Cr in MACs is largely sourced from sediment dissolution rather than seawater and ii) areas with elevated MAC formation contribute less Cr to the overlying water column.

We conclude with trace elements where MAC formation is potentially a significant source.
Copper. Our flux estimate suggests that clay authigenesis plays an important role in the marine Cu cycle, with 0.2 to 1.3 × 108 mol Cu/yr (median = 0.5 × 108 mol yr-1) released during MAC formation. This is very similar to the estimated Cu input from dust (0.5 × 108 mol yr-1), but an order of magnitude smaller than the riverine Cu input (6 to 8.7 × 108 mol yr-1)42. Importantly, it compares very well with estimates of the benthic Cu source from suboxic margin sediments (0.7 to 3.2 × 108 mol yr-1)42.

Barium. Barium is one of the most depleted elements in our MAC dataset, relative to detrital sediment precursor phases. After removal of three MACs with anomalously high Ba concentration (>200 ppm) and enrichment factors » 1 (all from the Oregon Margin site, where MACs have been observed to contain barite inclusions10), we obtain a MAC related source flux estimate for Ba that ranges between 0.14 and 1.5 × 109 mol yr-1 (median = 0.43 × 109 mol yr-1). MAC formation is therefore a minor contributor to the oceanic Ba budget compared to the riverine input flux (2.7 to 10.5 × 109 mol yr-1) and inputs from shallow continental margin settings (~17 × 109 mol yr-149;).

Rare Earth Elements. We calculate a total REE plus Y flux of 27 to 112 × 106 mol yr-1, with fluxes for individual REEs reported in Table 1. Focusing on Nd, a widely used chemical tracer for which more complete oceanic budget estimates exist, we calculate a flux that ranges between 3.7 and 16 × 106 mol yr-1. This suggests that MAC formation can partially account for a sedimentary benthic flux (up to 21%) of Nd to the oceans that has been estimated at 76 × 106 mol yr-1 (ref. 50).

Lithium, a special case. Our median estimate of Li removal into MACs (-1.2 to 0.75 × 109 mol yr-1; median = 0.08 × 109 mol yr-1) is an order of magnitude smaller than previous estimates of ~5.6 × 109 mol yr-114, and the flux range we obtain suggests that MAC formation can act as a Li source to the ocean in some circumstances. Thus, MAC formation in low temperature sedimentary settings may not be an important Li sink with respect to the oceanic Li budget, with clays formed by alteration of the oceanic crust being potentially more important51.

Impact of clay authigenesis on marine REE cycling and εNd
REEs are widely employed as palaeoceanographic tracers. However, the recent discovery that a large benthic flux influences oceanic REE distribution and εNd complicates their use as proxies23,29,52–56 with both the magnitude and Nd isotopic composition of this benthic flux showing considerable spatial variability52,56,57. Global extrapolations are further hampered by the scarcity of porewater measurements and gaps in our understanding of the mechanistic drivers of the benthic flux. A detailed understanding of the processes resulting in elevated REE concentrations in marine porewater is key for quantifying the magnitude and impact of the benthic REE flux on a global scale. Our MAC compositional data adds further support to the notion that clay authigenesis is one such mechanism10,18.

ODP Site 959 (Fig. 4) and Oregon Margin MACs (Fig. 5) contain substantially lower concentrations of REEs compared to detrital silt or clay precursor materials (EF~0.5), consistent with previous work on MACs18,58,59. The systematic exclusion of REEs mean that MAC formation acts as a source of dissolved REEs to the porewater. Consistent enrichment factors across all REEs (Fig. 4 and Fig. 5) and near-indistinguishable REE patterns for MACs versus detrital precursors (Fig. 6) at both sites demonstrate that there is no preferential exclusion of the heavy REEs (HREEs) or of the light REEs (LREEs) during MAC formation.

Nd isotopes also show a strong correspondence between MAC and the detrital precursors (Fig. 7). At Site 959, εNd of the MACs covaries with both the detrital εNd signature of the silt- and clay-sized material (r2 of 0.76 and 0.86 respectively). While the trend between the two fractions is linear with a slope ~1, the authigenic separates are more radiogenic than the detrital material, and this offset is exaggerated in the comparison to the silt fraction (Fig. 7). At the Oregon Margin, εNd of both detrital silt and MACs are in good agreement with previously reported porewater εNd values52 (Fig. 7). Interval 9 (11.1 cm depth) shows a minor deviation from this trend, here the porewater is more radiogenic than either measured MAC fraction despite the porewater aligning well with the acid leachable sediment fraction24, potentially due to regional volcanic influence10. The MAC εNd at this interval does, however, more closely resemble the bulk sediment εNd (-2.424).

Taken together, REE patterns and εNd values suggest that seawater REE composition is not incorporated into MACs at either location. This is further supported by the observation that neither Oregon Margin, Site 959 (Fig. 7), nor Congo Shelf18 MACs appear to acquire a marine Y/Ho ratio. The general agreement between detrital sediment, porewater and MAC εNd supports the view that active diagenetic cycling from detrital precursors through authigenesis impacts porewater composition and suggests that MACs may provide a way to disentangle the influence of diagenetic overprinting of other authigenic REE archives (e.g., phosphates, carbonates). Our findings also confirm that efforts to reconstruct palaeo-seawater REE properties using a sedimentary MAC archive will be problematic59.

Importantly, the influence of other authigenic mineral sinks in the sediment column means that the MAC-linked benthic REE flux reaching the ocean is unlikely to reflect the REE pattern of the detrital precursors. For instance, the apparent HREE depletion in the MACs of the Congo Shelf (Supplementary Data 4) resulted from the presence of LREE enriched phosphatic micro-inclusions18. The formation of authigenic phosphates is a major sink in the REE porewater budget29, reducing the overall amount of REE ultimately reaching the bottom water and causing a residual HREE enriched pattern because of preferential removal of LREEs from porewaters by phosphatic phases28. Additional authigenic mineral formation such as Fe- or Mn-(hydr)oxides would likely further modify the MAC-exclusion related flux of REEs before it ultimately reached bottom water.

We conclude that MACs inherit the REE and εNd signature of their detrital precursors rather than of seawater, as evidenced through REE patterns, Y/Ho ratios, and εNd signatures. Enrichment factors are consistently < 1 for all REEs, demonstrating that MAC formation acts as a source of dissolved REEs in the marine system, potentially of the same order of magnitude as that required to balance the marine Nd budget23,50.

Has the impact of MAC formation changed over time?
Both the main classes and quantities of MAC formed have likely changed substantially through time. Precambrian marine sediments, for example, host MAC types (e.g., minnesotaite, greenalite and stilpnomelane) that are not known from the Phanerozoic record12. The marine shale oxygen isotope record implies that the MAC content of marine shales was mostly > 40% during the Precambrian, and mostly < 20% since ~400 Ma60. However, changes in MAC abundance are also expected for the Phanerozoic. MAC formation is generally higher in regions with rapidly accumulating sediments such as continental shelves, margins, and river deltas8,9, which expand and contract in response to eustatic sea-level change. These influences can be reasonably assumed to have modulated the impact of MACs on marine major/trace element cycling through Earth’s history. However, the MAC-related elemental fluxes may not scale linearly with changing MAC abundance over time because 1) the trace element compositions for the broader assemblage of MACs that characterize the Precambrian marine stratigraphic record12 remain largely unknown16 and 2) MAC enrichment factors are sensitive to the nature of the precursor material, which is largely a function of provenance and extent of chemical weathering experienced prior to deposition4. These, in turn, are sensitive to climatic (icehouse-greenhouse), tectonic (e.g., mafic versus felsic protoliths; physical denudation in active margins versus chemical weathering on passive margins) and biospheric (e.g., rise of land plants; rise of silicifying organisms) influences.

Methods
Separation and screening of MAC and detrital fractions
For ODP Site 959 materials, authigenic clay residing in the >100 µm fraction were separated from the bulk sediment by wet sieving each sample, followed by treatment with 1M HCl to remove carbonates, phosphates and sorbed components18,61. Authigenic clay separates were subsequently triple rinsed with MilliQ water, oven dried and passed through a Frantz magnetic separator (0.6 and 0.2 A) to remove non-target phases such as biogenic opal, quartz, feldspar, organic matter and pyrite. Previous X-ray diffraction and microscopic characterization has demonstrated the high purity of MAC separates obtained in this manner from this site21,39, which we independently confirmed via optical and scanning electron microscope screening of our separates (Fig. 2).

We complement the Site 959 material with MACs separated from three near-surface intervals (8.6 to 11.1 cm depth) at an Oregon margin site (North Pacific, see Fig. 1) previously documented to contain sand-sized glauconitic smectite pellets10,25. Oregon Margin MACs were separated using the same approach outlined above, but were further split into a large (>250 µm) fraction, a less magnetic (0.6 to 0.4 A) 250–100 µm size fraction, and a more magnetic (0.4–0.2 A) 250–100 µm size fraction. Individual Oregon Margin MAC pellets are dominated by Fe smectite and glauconite10,25 but also contain modest amounts of detrital clay (chlorite, muscovite, and illite) and silicate (feldspar, quartz) inclusions (Fig. 2)10,25.

Assessment of elemental uptake or release during MAC formation requires comparison to the detrital precursor sediment from which MACs form. To this end, the host sediment of each MAC separate was also treated with 1M HCl before size separation into <5 µm and 5–100 µm (Site 959) or <5 µm, 5–20 and 20–100 µm (Oregon Margin) fractions using standard wet sieving and centrifugation approaches. We consider the <5 µm material to be a detrital clay fraction that does not contain MACs because previous work at these sites has shown that MAC formation is almost entirely restricted to sand size pellets10,20,25. This is confirmed by our scanning electron microscope (SEM) mineral mapping (Fig. 2) and quantitative X-ray diffraction (XRD) analyses which do not identify Fe-smectite or glauconite in this fraction (see Supplementary Information). Similarly, the Oregon Margin 5-20 µm fraction does not contain MACs and can thus be considered detrital silt. The Oregon Margin 20–100 µm fraction, by contrast, contains abundant authigenic clay and is not appropriate for comparison (Fig. 2 and Supplementary Information). Based on this, and the relative abundance of detrital clay versus detrital silt at each site, we consider the MAC precursor phase to be the <5 µm detrital clay fraction at ODP Site 959 and the 5-20 µm detrital silt fraction at the Oregon Margin site.

SEM and XRD analyses
Samples for SEM characterization were prepared as 25 mm diameter polished resin mounts from intact, freeze-dried sediment. This involves vacuum impregnation of the samples in epoxy resin, polishing using diamond paste, and carbon coating prior to SEM analysis. Backscattered electron (BSE) images and SEM- energy dispersive spectroscopy (EDS) mineral maps were obtained at Macquarie University using a FEI Teneo field emission SEM equipped with dual Bruker XFlash 6/30 EDS detectors. BSE images of the entire sample mount were collected using Thermofisher Scientific Maps Mineralogy software (V3.3). Smaller regions of interest on each mount were subsequently selected for higher resolution BSE imaging (<100 nm pixel resolution) and EDS mineral mapping (EDS spectrum collected every 1.5 µm, 12 ms acquisition time, ~8500 counts on quartz), followed by mineral identification using the Thermofisher Scientific Nanomin software (V3.3), following the approach recommended in ref. 62.

Samples for X-ray diffraction analysis (<100 µm detrital fractions only) were side-loaded into sample holders before collection of powder XRD patterns spanning 3.5° to 85° 2θ using a Bruker D8 Advance X-ray diffractometer with Lynxeye detector and a Co radiation source running at 40 kV and 40 mA, at a step size of 0.02° and 1 s dwell time per step. For Site 959, only <5 µm fractions yielded sufficient material for analysis, whereas all three Oregon Margin <100 µm fractions yielded sufficient material (Supplementary Information and Supplementary Data 5). Mineral identification and Rietveld-based quantification of powder XRD patterns was done using the Panalytical Highscore Plus software and its implemented pdf-4 database. Uncertainty on mineral abundances is <3%21.

Elemental and Nd isotope analyses
An aliquot of each authigenic clay separate (20-40 mg) and detrital clay or silt fraction (100 mg) was acid digested on a hot plate, using a 1:2 solution of distilled concentrated HNO3 and HF followed by 6M HCl. The resulting solutions were split, with an aliquot set aside for further purification and neodymium isotope (εNd) analyses. For the non-authigenic clay samples a volume corresponding to target masses of 0.5 µg Nd was set aside for Nd isotope analyses (see below for details). However, the authigenic clay aliquots only contained <0.07 µg Nd, so that not all samples yielded results with acceptable analytical precision for Nd isotopes.

Selected major and trace elements (45 elements in total) were analyzed at dilutions of 1:1,000 (trace elements) and 1:1,000,000 (major elements) on an Agilent 8900x ICP-MS/MS at Adelaide Microscopy (Adelaide University) along with a series of multi-element calibration solutions. Instrument conditions were: RF power 1550 W, sample depth 10 mm, Ar carrier gas flow rate 0.95 L min−1 and Ar make up gas 0.1 L min−1, with a MicroMist nebulizer and a Scott Type spray chamber. 7Li, 9Be and 11B were measured in no gas mode. P, Fe and Cr were analyzed in reaction mode using N2O (20% flow, 0.21 ml min−1) and the MO+ reaction products measured (e.g., 56Fe measured as 56-72Fe at mass 72). For the remaining elements the collision cell was run in He mode (4 mL min−1 He gas flow). The sample introduction system was rinsed in two stages with 5% and 2% HNO3 between each analysis. On-line addition of indium was used for internal standardization, with secondary drift correction to regularly interspersed standard solutions. Analysis of several replicates of the USGS certified reference materials BCR-2 and BHVO-2 shows that most elements returned concentrations within <10% of certified values, whereas B, Mg and Tl fell within 15%.

Isotope aliquots were purified using Polyprep columns filled with ~2 mL of AG Wx8 200-400 mesh and secondary columns filled with 1 mL Eichrom LN 50–100 resin to isolate the Nd fraction (see also ref. 63). Purified Nd fractions were loaded on Re filaments using 1 µL of activator (1N HCl: 0.3M H3PO4) and analyzed for 143Nd/144Nd using an ISOTOPX Phoenix TIMS equipped with an ATONA signal amplification system (Adelaide University). Instrumental fractionation was corrected to a conventional 146Nd/144Nd ratio of 0.7219, using an exponential mass fractionation correction for the internal normalization of the Nd isotope data or 143Nd/144Nd ratios. JNdi-1a and BCR-2 were run as reference materials for 143Nd/144Nd (average for BCR-2=0.512651) to monitor data quality.

Zr normalization and elemental enrichment factors
Clay authigenesis is associated with substantial uptake of porewater-sourced major cations, primarily Fe3+/2+, K+ and Mg2+ ions17, with major element data showing that up to ~40% of Congo Shelf MAC mass is externally sourced18. Normalization to an immobile element such as Zr, Hf, Ti, Nb or Al is required to correct for this dilution effect and obtain accurate elemental enrichment or depletion trends relative to the precursor sediment64. We choose to present major and trace element concentrations normalized to Zr (Supplementary Data 4), following a systematic assessment of potential immobile elements using the isocon diagram approach65. Zirconium is commonly utilized to correct for volumetric changes or enrichment/dilution effects during weathering64, under the assumption that Zr is mainly hosted in zircon grains that are resistant to physical and chemical breakdown. Several lines of evidence suggest that fine silt to clay-grade zircons are the primary Zr hosts in both precursor sediment and MAC separates. Isotopic surveys of river sediments have shown that zircon is the primary host of Zr and Hf even in mud grade sediments66, though this may not be the case for river clays67. While their small size and relatively low abundance usually makes them challenging to image, our SEM-EDS mineral mapping approach demonstrates that <5 µm-scale zircon grains commonly occur both in authigenic clay pellets and in the host sediment at all sites (Fig. 2). Further, we observe a strong positive linear correlation between Zr and Hf in our MAC dataset with a slope that is indistinguishable from that of the corresponding detrital <5 µm fraction; both have Zr/Hf ratios that are very similar to the modal Zr/Hf ratio of granitic zircons68 and the upper continental crust69. We therefore conclude that Zr and Hf are primarily hosted by zircon and behave as quasi-immobile elements that are not substantially impacted by clay transformation reactions. Consequently66, normalization of the elemental data to Hf results in trends indistinguishable from Zr normalization (see Fig. 3). Ti, Nb, Th and Al, on the other hand, show evidence of mobility during clay authigenesis and are not suitable for normalization in this particular study.

We calculated the enrichment factor (EF) for each element as the ratio of the Zr-normalized concentration in the MAC to the Zr-normalized concentration in the precursor sediment (i.e., detrital clay or silt size fraction) and categorize each element as enriched (EF > 1), depleted (EF < 1), or unchanged (EF ~ 1).

Calculation of global MAC-related elemental fluxes
Assuming that our Fe-smectite to glauconite MACs are a reasonable proxy for MACs as a whole (see caveats in main text), we utilize the average green MAC composition (Supplementary Data 3) and the corresponding elemental enrichment factor (Supplementary Data 4) to calculate the corrected mass fraction of each element that is released or consumed during MAC formation, allowing us to obtain a first-order estimate of the global MAC-related flux for each element:

GlobalFlux i = (( (Conc i / EF i ) * (EF i - 1) ) / MAC av_Si ) * Global Si Flux (1)

where GlobalFlux i is the global flux (in Tg/yr) out of the pore fluid and into MACs (positive flux values) or into the pore fluid (negative flux values) for element i, Conc i is the average concentration of element i in MACs (wt%), EF i is the Zr-normalized enrichment factor for element i, MAC av_Si is the average Si content of glauconitic smectite (combined average of 26.8 wt% Si for Oregon Margin and Site 959 MACS; 21, 25, 39) and Global Si Flux is the global annual Si uptake flux into MACs (132 ± 5.6 Tg/yr Si9;). We use a Monte Carlo simulation approach (100,000 simulations for each element) to account for the likely range in each variable, and report median, 10th and 90th percentile flux estimates (Table 1). For Monte Carlo simulations, Conc i is randomly drawn from a truncated normal distribution defined by the average and standard deviation for each element, with minimum concentration values ≥ 0. EF i is drawn from a uniform distribution spanning minimum and maximum enrichment factors for each element. High Ba concentration and Ba EF outliers in some Oregon Margin MAC separates (Supplementary Data 3) are excluded due to the presence of barite inclusions10, as are REE+Y for Congo Shelf samples. Values for Global Si Flux are drawn from a normal distribution defined by the size and uncertainty of the flux reported by Rahman et al.9.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "There is growing evidence suggesting authigenic formation of clay minerals in marine sediments is an important component of the marine silica cycle, and likely also the cycle of several major elements. Despite that, it remains unclear whether authigenic clays are also important in the cycles of trace elements. For many elements the attempt of Lohr et al. to quantify trace metal uptake and release provide the first ever numbers, hence it can provide a substantial contribution to our need to understand the cycles of these elements.\nThe approach itself, of separating large glauconite grains, is very practical, however, it is not clear how well it represents the actual fluxes between the sediment and bottom waters. The formation of marine authigenic clays (MAC) should begin with small precursor minerals because small grains are always more active than large grains. This precursor phase will be washed out as detrital clays by this method, hence the method actually compares an undefined mix of detrital and authigenic phases with more mature authigenic phases.\nIf we assume that despite the concern regarding mis-quantification of the early authigenic phases due to the size separation the approach is valid, I still have several major concerns:\n1) The authors upscale the flux calculation obtained from the glauconite based on an assumption that the compositional change during maturation represents the global MAC flux rather than a shelf related green clay specific flux (as explained in lines 234-236). This is a very problematic assumption because green clays can only form under specific redox conditions. If authigenic clays are common in the deep sea (a subject still under debate), conditions there are too oxic to form the Fe-smectite precursor. Other regions, including on shelves, may not have enough available iron relative to silica and aluminum to support this pathway. Therefore, this upscale cannot really be justified. Overall, it is a very good study and first of its kind, but I don’t think the extrapolation of the glauconite composition to the global ocean is justified because glauconite is a unique clay mineral that though common forms under very specific conditions.\n2) For most elements, the transition in their mineral content with maturity as defined by the authors is far from linear. There are very few cases in Figs. 3 and 4 where there is a steady increasing or decreasing trend in the normalized content of an element. This translates to ambiguity in the flux calculations.\n3) The actual fluxes between marine sediment and the ocean are usually defined by redox fronts. I think the paper will benefit greatly from inclusion of some porewater data to show if there is any correlation between the calculated fluxes based on the MAC composition and actual benthic fluxes. It will also be good to see if trends in the bulk sediment composition correlate with what should be uptake or removal into aluminosilicate phases.\nOther comments:\nLine 470: The Agilent 8900x is a Triple-Quadrupole ICP-MS, not a MC-ICP-MS.\nMeasured Porewater V and sometimes also Cr concentrations are often very high and inconsistent with the semi-conservative marine profiles of these elements (for example https://doi.org/10.1029/2023GB007844). If MAC is the missing unknown sink as you suggest, perhaps it can clarify at least part of this discrepancy.\nLine 165: The word “data” is plural (“allows” should be written without an s).", "reviewer_2": "The authors present a new investigation of trace element enrichments in marine authigenic clays from three widely separated cores. By isolating the authigenic clay fraction from bulk marine sediments, they quantify enrichment factors of trace elements in Fe-bearing clays and evaluate the associated global fluxes of these elements via authigenic clay formation. This study makes an important contribution to understanding the role of marine authigenic clays in regulating global seawater chemistry. The manuscript is well organized, clearly written, and the interpretations are generally well supported by the data. I therefore recommend publication in Nature Communications following relatively minor revisions. My specific comments are provided below:\n1. The use of Zr-normalization may be influenced by the source of detrital silicate materials in different regions, as Zr is generally derived from zircon that is enriched in felsic igneous rocks. I recommend the authors use more elements (e.g., Th, Sc etc.) to normalize the trace element contents. Another issue is if the authors have separated the MACs from the bulk sediments, what the phase of Zr or other trace elements typically in terrestrially detrital silicates in these MACs.\n2. Line 22 and other places in Abstract. It is better to use full name of each element rather than abbreviations in Abstract section.\n3. Lines 57, 59. The citations here are not normative. It is quite confusing to use the superscript on an element name.\n4. Line 57. Considering to cite Yin et al. (2023) EPSL here to represent potential Li enrichments in ancient samples.\n5. Line 91-93. Is there any evidence for dissolution of precursor minerals within the profile?\nLine 186-187. How does Li enrichment change during the clay maturation?", "reviewer_3": "In this article, Stefan Löhr and his co-authors report on the distribution of major and trace elements in authigenic clay assemblages extracted from green glauconitic pellets from two marine settings (Oregon margin and Gulf of Guinea). Their aim was to assess whether marine authigenic clay formation acts as a source or a sink for trace elements in the ocean. Taken together with previously published data for the Congo shelf, they infer an average global composition for marine authigenic clays and provide estimates for corresponding elemental fluxes to (or from) seawater. Overall, this is a nice contribution, which reinforces previous suggestions that authigenic clay formation in the marine environment strongly influences oceanic geochemical cycles. The paper reads well and should probably yield a wide interest from the community. I have however one issue to raise regarding methodology and the (plausibly misleading) use of Zr-normalization to assess elemental behaviour during green clay formation.\nSpecifically, I am not convinced by the need to normalize data to Zr before interpreting elemental patterns in authigenic clay separates. The Zr-normalization method is generally used in soil investigations to account for volumetric changes and bulk density loss that typically occur during chemical weathering of the unaltered rock (protolith) and its transformation into soil constituents. The context is quite different here as glauconite probably displays a bulk density similar to nearby detrital clays. Additionally, the asumption made by the authors that authigenic clays quantitatively incorporate Zr from precursor detrital clay assemblages is probably not correct, at least not for the case of the Congo shelf green pellets. At the Congo margin, authigenic clays display a Hf isotopic composition markedly different from the precursor kaolinite-rich material (Bayon et al., 2023). Because Hf and Zr are twin elements, this discrepancy points towards specific Zr mobility during green clay formation. Therefore, normalizing measured elemental abundances to Zr could potentially result in biased interpretation. For instance, in S. Löhr and co-authors’s study, it creates an apparent REE enrichment in glauconitic pellets from the Congo Shelf, which contradicts with the intepretation that can be drawn using shale-normalized REE patterns when compared to nearby detrital clays.\nA final minor comment regarding another imprecised statement about the use of Zr in this study : in line 500 (Methods), the authors argue that « Geochemical and isotopic surveys of river sediments have shown that the weathering resistant mineral zircon is the primary host of Zr and Hf even in fine-grained sediments ». Note that this is only correct in silt-size fractions, but not in river clays, in which the behaviour of both Hf and Zr is completely decoupled from the presence of zircons (see Bayon et al., 2016).\nConsequently, in this study, I would recommend the authors to simply normalize measured elemental abundances to values for nearby detrital clays.\nMinor comment : The figures are all fine, but I am wondering whether Fig. 1 is really needed considering that only three sites are reported on a global map.\nGermain Bayon"}, "author_response": "We thank the editor and three reviewers for their detailed, constructive reviews. We have conducted additional analyses and revised the paper to address each of the points raised, as detailed below in blue text. To summarize the main points:\n1) Our additional electron microscope and X-ray diffraction analyses demonstrate the absence of authigenic clays in our and silt size fractions, justifying our interpretation that these can be considered detrital materials, and supporting our approach for calculating authigenic clay elemental enrichment factors relative to this detrital precursor material.\n2) We have outlined and justified the assumptions made for the global flux calculations, and\n3) We demonstrate that immobile element normalisation to Zr is necessary to correct for dilution effects, and that this approach is appropriate. This includes new imaging evidence and geochemical inference which supports our interpretation that Zr is mainly hosted within weathering-resistant clay to fine silt grade zircons and can therefore be considered immobile.\nReviewer #1 (Remarks to the Author):\nThere is growing evidence suggesting authigenic formation of clay minerals in marine sediments is an important component of the marine silica cycle, and likely also the cycle of several major elements. Despite that, it remains unclear whether authigenic clays are also important in the cycles of trace elements. For many elements the attempt of Lohr et al. to quantify trace metal uptake and release provide the first ever numbers, hence it can provide a substantial contribution to our need to understand the cycles of these elements.\nThe approach itself, of separating large glauconite grains, is very practical, however, it is not clear how well it represents the actual fluxes between the sediment and bottom waters. The formation of marine authigenic clays (MAC) should begin with small precursor minerals because small grains are always more active than large grains. This precursor phase will be washed out as detrital clays by this method, hence the method actually compares an undefined mix of detrital and authigenic phases with more mature authigenic phases.\nR1 raises an important point here, that we might be inadvertently comparing macroscopic authigenic clay pellets with a mixture of detrital and authigenic components because of unrecognised clay-size authigenic material in the ‘detrital’ clay size fraction, impacting on our element flux estimates.\nPetrographic analysis of sediments from both Site 959 and the Oregon Margin have shown that authigenic clay formation is largely restricted to foraminiferal tests (959; Wiewióra et al 2001 and Baldermann et al 2013 & 2015) or macrofaunal faecal pellets (Oregon; Abbott et al 2019 and Geilert et al 2024). This has been attributed to favourable micro-environmental conditions (changes in pH and redox conditions, OM decay, diffusive element fluxes between porewater and micromilieu, etc.) at such sites compared to the bulk sediment (Baldermann et al 2013). Our revised manuscript contains new high-resolution SEM imaging data which explores this further (new Figure 2), confirming that the occurrence of MACs at both sites is largely restricted to sand size pellets, but that MAC pellets of up to ~ 30 µm do occur.\nHowever, the clay-size matrix (<5 µm) does not appear to contain any identifiable authigenic clays at either site (new Figure 2).\nTo further test this we have also collected new quantitative mineralogical data (quantitative XRD) for the clay and fine silt-size fractions which confirms that detrital mineral phases are the dominant contributors (new supplemental text section).\nSpecifically, only the coarse silt fraction at the Oregon Margin site contains Fe-bearing smectite of potential authigenic origin, consistent with SEM observations of silt-size MAC pellets at this site. This does not affect our results since we do not use this size fraction for our compositional comparison.\nOverall, our newly collected data (SEM, XRD) clearly demonstrate that authigenic clays are, at most, only a very minor component of the ‘detrital’ clay or fine silt fraction, and thus are unlikely to affect our main conclusions. We have added these new data to revised manuscript and updated the text accordingly, and inserted a new supplementary text section detailing the new petrographic and Q-XRD based screening of these fractions.\nIf we assume that despite the concern regarding mis-quantification of the early authigenic phases due to the size separation the approach is valid, I still have several major concerns:\n1) The authors upscale the flux calculation obtained from the glauconite based on an assumption that the compositional change during maturation represents the global MAC flux rather than a shelf related green clay specific flux (as explained in lines 234-236). This is a very problematic assumption because green clays can only form under specific redox conditions. If authigenic clays are common in the deep sea (a subject still under debate), conditions there are too oxic to form the Fe-smectite precursor. Other regions, including on shelves, may not have enough available iron relative to silica and aluminum to support this pathway. Therefore, this upscale cannot really be justified. Overall, it is a very good study and first of its kind, but I don’t think the extrapolation of the glauconite composition to the global ocean is justified because glauconite is a unique clay mineral that though common forms under very specific conditions.\nR1 has misunderstood the basis of our flux calculations. Compositional change during maturation does not come into this quantification (see our response to next point). Nevertheless, we acknowledge that our flux calculation have some limitations, mainly that we need to make some assumptions in order to calculate MAC-related global elemental fluxes. The key assumption is that the average trace element composition we have measured for the smectite-glauconite series of MACs apply to MACs as a whole, a necessary assumption since the trace element composition of other MAC classes is currently entirely unknown.\nWe consider our assumptions to be justified because:\n1) the smectite-glauconite series of MACs is the most abundant class of MACs in the present-day ocean (e.g. Baldermann et al., 2025), so can be reasonably expected to have a dominant influence on the global average MAC composition, whether or not glauconite-smectite formation is restricted to shallow shelf settings.\n2) potential areal restriction of glauconite to shelf settings raised by R1 are unlikely to directly impact on the fluxes we obtain since our calculation derives a global flux by scaling up the average MAC composition we obtain to the global reported Si burial flux in authigenic clays, itself shown to be focussed primarily in shelf settings (Rahman et al 2017; Tréguer et al 2021).\nWe acknowledge that our assumptions mean that the fluxes we report are subject to some uncertainty and should therefore be viewed as preliminary estimates, and we have expanded on this in the text (lines 213--229). Notwithstanding these caveats, we are of the view that reporting the flux estimates we obtain is an important contribution as these provide the first-ever estimate of the potential magnitude of MAC influence on marine trace element budgets, highlighting that MAC formation is likely an important influence on a much broader range of trace elements than previous assumed (as noted by R1). We expect that these estimates will evolve and improve as additional work is done to characterise a broader range of MAC types from various settings.\nWe have revised the text to more explicitly highlight the assumptions made, justify these, and acknowledge the arising uncertainty.\n2) For most elements, the transition in their mineral content with maturity as defined by the authors is far from linear. There are very few cases in Figs. 3 and 4 where there is a steady increasing or decreasing trend in the normalized content of an element. This translates to ambiguity in the flux calculations.\nR1 has misunderstood the basis of our flux calculations. We agree with R1 that only some elements in MACs show steady increasing or decreasing trends with maturation (see lines 184-206). However, this does not affect our elemental flux calculations.\nTo calculate the global MAC related fluxes for all elements, whether they change systematically with MAC maturity or not, we take the global average MAC composition obtained by averaging all MAC separates from Site 959, Oregon Margin and the Congo Shelf (reported in Supplementary Table 3), adjust this using the Zr-normalised enrichment factors for each element (reported in Supplementary Table 4) to determine the proportion of each element that is retained from the detrital precursor phase, normalise this adjusted concentration to the average Si content of these clays, and upscale it to a global elemental flux using the estimated global MAC-related Si burial flux. Our calculation is explained in detail in lines 570-596.\n3) The actual fluxes between marine sediment and the ocean are usually defined by redox fronts. I think the paper will benefit greatly from inclusion of some porewater data to show if there is any correlation between the calculated fluxes based on the MAC composition and actual benthic fluxes. It will also be good to see if trends in the bulk sediment composition correlate with what should be uptake or removal into aluminosilicate phases.\nWe agree that it is worthwhile to compare solid phase MAC compositional data with pore water data from the same sites/intervals. However, that is well beyond the scope of this study which primarily focuses on characterisation of MACs and implications for the oceanic elemental fluxes. Not only is pore water trace element concentration data generally not available for these sites, apart from low resolution major elements and REE/ epsilonNd measurements (we show some of this data in Figure 7) measurements; there are also very few/ no suitable pore water samples available for analysis in the relevant depth intervals for Site 959 and Congo Shelf cores. That said, generating a dataset suited to such a comparative approach is one of the objectives of an ongoing North Atlantic pore water sampling campaign led by one of us (April Abbott).\nRegarding trends in the bulk sediment composition we have split out each sediment into detrital clay, detrital silt and MAC fractions, and have identified the compositional differences between these fractions (see supplementary information). Note that the bulk sediments mineralogical composition is detailed in Baldermann et al (2013).\nOther comments:\nLine 470: The Agilent 8900x is a Triple-Quadrupole ICP-MS, not a MC-ICP-MS.\nFixed.\nMeasured Porewater V and sometimes also Cr concentrations are often very high and inconsistent with the semi-conservative marine profiles of these elements (for example https://doi.org/10.1029/2023GB007844). If MAC is the missing unknown sink as you suggest, perhaps it can clarify at least part of this discrepancy.\nThanks, we now mention this possibility in the revised text.\nLine 165: The word “data” is plural (“allows” should be written without an s).\nFixed\nReviewer #2 (Remarks to the Author):\nThe authors present a new investigation of trace element enrichments in marine authigenic clays from three widely separated cores. By isolating the authigenic clay fraction from bulk marine sediments, they quantify enrichment factors of trace elements in Fe-bearing clays and evaluate the associated global fluxes of these elements via authigenic clay formation. This study makes an important contribution to understanding the role of marine authigenic clays in regulating global seawater chemistry. The manuscript is well organized, clearly written, and the interpretations are generally well supported by the data. I therefore recommend publication in Nature Communications following relatively minor revisions.\nWe thank R2 for their positive assessment of our work.\nMy specific comments are provided below:\n1. The use of Zr-normalization may be influenced by the source of detrital silicate materials in different regions, as Zr is generally derived from zircon that is enriched in felsic igneous rocks. I recommend the authors use more elements (e.g., Th, Sc etc.) to normalize the trace element contents. Another issue is if the authors have separated the MACs from the bulk sediments, what the phase of Zr or other trace elements typically in terrestrially detrital silicates in these MACs.\nNormalisation to an ‘immobile’ element is essential to avoid dilution-induced systematic bias in our results (see also our first response to R3 for detailed explanation). Amongst the elements in our dataset, Al, Ti, Zr, Hf and Nb are most commonly considered or employed as ‘immobile’ elements for normalisation. It has been shown however that none of these elements is entirely immobile in the terrestrial or marine weathering environment (Kurtz et al., 2000), so that is a matter of identifying the least mobile element.\nWe have evaluated this using the isocon diagram approach (Grant, 1986; we show a single example below), where we compare the composition of precursor vs MAC pairs for each interval at each site. Immobile elements are expected to form arrays that define a line of constant slope on the isocon diagram, with enriched elements falling above and depleted elements falling below this line. We complement this with SEM-EDS imaging and mineral mapping of the sediments, revealing the identity and distribution of potential host phases of candidate immobile elements.\nOur evaluation shows that Hf and Zr consistently falls on a common line on the isocon diagram for each precursor-MAC pair. In contrast, Al, Nb and Ti, consistently fall below this line. For Site 959 samples, Nb and Ti also tend to define a common line, but with a significantly lower slope than the Hf-Zr pair. For Oregon Margin samples Ti and Al commonly define a line with slope < Zr-Hf pair, with Nb falling well below both Zr-Hf and Ti-Al lines.\nWe infer that Zr-Hf behave as quasi-immobile elements because they are primarily hosted in weathering-resistant µm-scale zircons that are retained as inclusions in MACs, consistent with SEM-based observations of micron-scale zircons in both host sediment and authigenic clay pellets (see our new Figure 2). This is supported by the strong positive linear correlation between Zr and Hf in our MAC dataset with a slope that is indistinguishable from that of the corresponding detrital < 5 µm fraction; both have Zr/Hf ratios that are very similar to the modal Zr/Hf ratio of granitic zircons (38.5; Wang et al., 2010) and the upper continental crust (McLennan, 2001) – see plot below. We therefore conclude that both Zr and Hf are the most immobile elements in our studied sedimentary systems, and that these elements are not substantially impacted by marine authigenic clay formation.\nTi and Nb, in contrast, are mobilised and partly lost during MAC formation because of the breakdown of Nb-bearing Ti-phases such as ilmenite, rutile or anatase, the main Ti carriers in the strongly weathered continental detritus accumulating at Site 959. This is consistent with widely documented Ti mobility during weathering, and SEM evidence that the abundance of Ti-bearing phases in the matrix is higher than in MAC pellets. At the Oregon Margin site, Ti (and presumably Nb) are largely hosted in igneous (detrital) aluminosilicates, so that Al and Ti define a line reflecting modest loss. Thus, most of the liberated Al and Ti is either incorporated into secondary clays or Ti oxide phases, whereas Nb is amongst the most depleted element at this site, suggesting that MACs systematically exclude this element (due to the its high hydration state (coordination number = 9), inducing inner- and outer-sphere complexation on mineral surfaces rather than structural incorporation into clay mineral lattice structures).\n2. Line 22 and other places in Abstract. It is better to use full name of each element rather than abbreviations in Abstract section.\nIn principle, we agree with the reviewer. However, given the large number of elements mentioned this would reduce readability. In addition, we would be unable to keep the abstract to the required length limit. We have therefore left the text as is.\n3. Lines 57, 59. The citations here are not normative. It is quite confusing to use the superscript on an element name.\nGood point. To fix this we have moved the two references here to the end of the sentence.\n4. Line 57. Considering to cite Yin et al. (2023) EPSL here to represent potential Li enrichments in ancient samples.\nCitation added.\n5. Line 91-93. Is there any evidence for dissolution of precursor minerals within the profile?\nYes. Electron microscopic, FTIR and X-ray diffraction evidence of dissolution of precursor silicate minerals at ODP Site 959 has been reported by Wiewióra et al (2001) and Baldermann et al (2013). The two studies we cite at this place in the text demonstrate dissolution of a kaolinite-rich, heterogeneous mixture of clay-size material which previously filled the chambers of foraminifera. This is replaced by Fe-smectite which gradually matures to glauconite.\nLine 186-187. How does Li enrichment change during the clay maturation?\nLi is depleted in Site 959 MACs and modestly enriched in Oregon Margin MACs. It does not change systematically with maturity at either site, as shown in Figs 3 and 4.\nWe have revised the text here to specifically mention Li behaviour.\nReviewer #3 (Remarks to the Author):\nIn this article, Stefan Löhr and his co-authors report on the distribution of major and trace elements in authigenic clay assemblages extracted from green glauconitic pellets from two marine settings (Oregon margin and Gulf of Guinea). Their aim was to assess whether marine authigenic clay formation acts as a source or a sink for trace elements in the ocean. Taken together with previously published data for the Congo shelf, they infer an average global composition for marine authigenic clays and provide estimates for corresponding elemental fluxes to (or from) seawater. Overall, this is a nice contribution, which reinforces previous suggestions that authigenic clay formation in the marine environment strongly influences oceanic geochemical cycles. The paper reads well and should probably yield a wide interest from the community. I have however one issue to raise regarding methodology and the (plausibly misleading) use of Zr-normalization to assess elemental behaviour during green clay formation.\nSpecifically, I am not convinced by the need to normalize data to Zr before interpreting elemental patterns in authigenic clay separates. The Zr-normalization method is generally used in soil investigations to account for volumetric changes and bulk density loss that typically occur during chemical weathering of the unaltered rock (protolith) and its transformation into soil constituents. The context is quite different here as glauconite probably displays a bulk density similar to nearby detrital clays.\nWhile we appreciate R3’s constructive review, we disagree on this important point. We consider immobile element normalisation essential for identifying elemental enrichment or depletion in MACs relative to their detrital clay precursors. Mass transfer of various elements produces a daughter material that is compositionally substantially distinct to its parent material. In our specific case, the Fe-smectite to glauconite series of clays contain very high Fe, K and Mg (sum concentration, on oxide basis, ranges between 25 and 45 wt%) relative to their detrital mineral precursors (sum to only 8.6 to 14.7 wt%) as illustrated in the summary table below (data is taken from Supplementary Data Files included in this submission).\nIn the case of the Congo MACs, for example, this means that ~40% of the final glauconite mass can be attributed to incorporation of externally sourced elements during MAC formation. And that is without even taking into account potential losses of Al and gains/losses of Si during MAC formation. Indeed, this substantial mass gain, accompanied also by increase in minerally-bound water in MACs, produces the volumetric increase which results in the characteristic expansion/growth cracks and cauliflower-like, lobate morphology of mature glauconite grains (e.g. Odin & Mather, 1981). This massive uptake of elements and water results in the dilution of other elements, without normalisation to an immobile element to correct for this process it will produce spurious depletions of elements that are in fact immobile during MAC formation. Moreover, unless corrected for, this effect will exaggerate the depletion of depleted elements and reduce the enrichment of enriched elements.\nIn short, while simply normalising to the precursor clay compositions as R3 suggests will correctly identify highly enriched and highly depleted elements, misleading results will be obtained for immobile elements or those only exhibiting subtle enrichments or depletions. Further, the obtained enrichment factors will be biased to lower values, which would affect our flux calculations. Consequently, we consider immobile element normalisation an important step for this study.\nThat being said, we are aware of the potential limitations of this approach, related in particular to the challenge of identifying a truly immobile element (e.g. Kurtz et al 2000). As outlined in our response to R2 above, we have carefully assessed all options and come to the conclusion that Zr or Hf are the most suitable elements for data normalisation.\nAdditionally, the assumption made by the authors that authigenic clays quantitatively incorporate Zr from precursor detrital clay assemblages is probably not correct, at least not for the case of the Congo shelf green pellets. At the Congo margin, authigenic clays display a Hf isotopic composition markedly different from the precursor kaolinite-rich material (Bayon et al., 2023). Because Hf and Zr are twin elements, this discrepancy points towards specific Zr mobility during green clay formation. Therefore, normalizing measured elemental abundances to Zr could potentially result in biased interpretation. For instance, in S. Löhr and co-authors’s study, it creates an apparent REE enrichment in glauconitic pellets from the Congo Shelf, which contradicts with the interpretation that can be drawn using shale-normalized REE patterns when compared to nearby detrital clays.\nBased on our systematic assessment of potential immobile elements measured in Site 959 and Oregon Margin materials, we conclude that Zr and Hf are least mobile and thus best suited to an ‘immobile’ element correction (see response to R2).\nWe suggest that the potential discrepancy between Hf and Zr, and the Hf isotopic differences between Congo MACs and detrital clays raised by R3 are potentially related to an isotopically distinct phosphate-hosted Hf reservoir in Congo shelf materials which undergoes partial isotope exchange during MAC formation. In contrast, and by analogy to our observations on the Site 959 and Oregon Margin materials (see our new Figure 2), we would suggest that Zr is largely hosted in mostly inert µm-scale zircons in both detrital and MAC materials. Accordingly, we have chosen to normalise our results to Zr rather than Hf.\nThe apparent REE enrichment in Congo shelf MACs in our results is readily explained. Bayon et al (2023) report light REE bearing phosphates in Congo Shelf MACs, which would prevent light REE loss during MAC formation and, depending on the origin/timing of these phosphates, potentially even result in a net increase in LREEs via uptake from pore water. Consistent with this, we obtain Zr-normalised enrichment factors that average ~1.6 for the LREEs (see Supplementary Table 4), indicating LREEs are immobile or slightly enriched. HREEs, however, are hardly incorporated into these phosphates and so are consistently depleted, showing enrichment factors of ~0.7. This is very similar to Zr-normalised REE enrichment factors observed for the Oregon Margin MACs. The apparent discrepancy with the conclusions of Bayon et al 2023, who report that MACs are depleted in both LREE and HREEs (less for LREE, more for HREEs) is simply because those authors did not correct for dilution effects brought on by the massive uptake of Fe, K and Mg which we demonstrated above, in our response to R#3’s previous point.\nA final minor comment regarding another imprecise statement about the use of Zr in this study : in line 500 (Methods), the authors argue that « Geochemical and isotopic surveys of river sediments have shown that the weathering resistant mineral zircon is the primary host of Zr and Hf even in fine-grained sediments ». Note that this is only correct in silt-size fractions, but not in river clays, in which the behaviour of both Hf and Zr is completely decoupled from the presence of zircons (see Bayon et al., 2016).\nWe have revised the text to acknowledge this. Nevertheless, our systematic SEM-EDS mineral mapping (see Figure 2) has revealed an abundance of clay-grade zircons in all samples, including not just in the mud matrix but also in the MAC pellets.\nConsequently, in this study, I would recommend the authors to simply normalize measured elemental abundances to values for nearby detrital clays.\nWe have articulated why we do not consider this to be a suitable approach in our first response to R#3.\nMinor comment : The figures are all fine, but I am wondering whether Fig. 1 is really needed considering that only three sites are reported on a global map.\nIt is true that this figure is not essential. However, given that we have the space we feel that showing a map provides a relevant and useful geographic context for the reader."}, {"round": 2, "reviewer_comments": {"reviewer_1": "I am happy to support publication of the revised version.\nKind regards,\nZvi Steiner", "reviewer_2": "This my second time reviewing this manuscript. I think the authors have tried best to address my raised issues and revise the paper. I don't have any more comments this time.\nI look over the comments and their response from other reviewers, in which I concerned about as well. Thus, I hope these responses and revisions will convince other reviewers.", "reviewer_3": "The authors have done a nice job during the revision. The manuscript has been substantially improved and adequately addresses most concerns raised during the previous round of review. I only list below two very minor final comments regarding the methodology.\n- Line 440 : I guess that the > 100 µm fraction referred to by the authors has been separated from the bulk sediment, but it could be worth specifying that point for clarity.\n- Line 443 : The authors used a Frantz magnetic separator to remove non-target phases such as e.g. biogenic opal, quartz, organic matter. Then, the remaining fraction is meant to represent pure authigenic clay separates. I am wondering whether the potential presence of additional magnetic minerals left after the 1M HCl leaching step (ilmenite?) could potentially impact measured trace element concentrations."}, "author_response": "We thank the reviewers for their constructive feedback. R#3 had two minor additional comments which we address in italics below.\nReviewer #3 (Remarks to the Author):\nThe authors have done a nice job during the revision. The manuscript has been substantially improved and adequately addresses most concerns raised during the previous round of review. I only list below two very minor final comments regarding the methodology.\nThanks for this positive assessment\n- Line 440 : I guess that the > 100 µm fraction referred to by the authors has been separated from the bulk sediment, but it could be worth specifying that point for clarity.\nYes, that is correct. We have modified the text to make this clear:\n‘For ODP Site 959 materials, authigenic clay residing in the > 100 µm fraction were separated from the bulk sediment by wet sieving each sample’\n- Line 443 : The authors used a Frantz magnetic separator to remove non-target phases such as e.g. biogenic opal, quartz, organic matter. Then, the remaining fraction is meant to represent pure authigenic clay separates. I am wondering whether the potential presence of additional magnetic minerals left after the 1M HCl leaching step (ilmenite?) could potentially impact measured trace element concentrations.\nReviewer #3 is correct that ilmenite has similar magnetic properties to the authigenic clays we targeted in this study. Indeed, in other work we have encountered ilmenite as a contaminant in authigenic clay separates from other sites, requiring further purification steps using heavy liquids. Fortunately, these non-target magnetic phases are readily distinguished from the authigenic clays, and binocular microscope and electron microscope screening of our authigenic clay separates demonstrates that ilmenite or other magnetic grains are not present in the separates studied here.\nSince our text already described the optical microscopy and SEM screening of our separates on lines 448-449 we have left the text as is."}]

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