An official website of the United States government
Reactions between terrigenous sediments, marine-biogenic substances and seawater modulate multiple biogeochemical cycles, but the dynamics and factors governing these reactions are poorly constrained. Deltaic mobile muds are a major sedimentary facies along river-dominated ocean margins through which most terrigenous sediment transits and mixes with marine-biogenic matter, representing efficient and globally significant batch reactors. Here, we present a process-based model that combines equilibrium aqueous chemistry with kinetic concepts from sediment biogeochemistry and mineral sciences to explore the solution-mediated interplay of organic and inorganic matter alteration in episodically reworked deltaic muds. The model reproduces observed diagenetic conditions and product suites over the seasonal timescales relevant to deltaic systems and indicates a systematic and dynamic coupling between the sedimentary cycles of H+, C, P, Fe, S, Si, Mg, K, and Ca. We used the model in combination with published field observations and concepts of authigenic mineral occurrences to develop a generalized explanatory framework for silicate weathering fluxes and diagenetic reaction balances in marine sediments. Diagenetic silicate weathering is represented by a continuum of reaction balances with acid (reverse) and alkaline (forward) endmembers that is moderated by sediment sources, which determine the sediment’s weatheringpotential, and depositional environments, which govern theexpressionof this potential. Reverse weathering dominates in seasonally reworked, low-latitude deltaic muds, where green clays form rapidly from lateritic river sediments and biogenic silica under suboxic conditions. High mineral precipitation rates and protracted sediment remobilization drive large solute fluxes from/to these sediments. Net forward silicate weathering becomes more likely under steady, sustained anoxic conditions, particularly in volcanically-influenced settings and at minimal pre-weathering of sediment sources. These results further our understanding of the role silicate weathering and marine sediments play in global biogeochemistry and Earth system evolution, and can aid targeted ‘enhanced weathering’ strategies to environmental governance.
more »
« less
Rapid transformation of biogenic silica to authigenic clay: Mechanisms and geochemical constraints
Authigenic clay formation during early diagenesis of marine sediments, termed “reverse weathering,” is an important process for regulating ocean pH, seawater chemistry, and atmospheric CO2over geologic time scales. Although the importance of reverse weathering has been increasingly recognized, the rates and mechanisms remain poorly constrained. This study investigated the mechanisms, kinetics, and mineral products derived from diatom biogenic silica. We show the formation of Fe(II)-bearing smectite and mica in 40 days, the most rapid process and first specific mineral phases reported to date. Unraveling the kinetics and mechanisms of authigenic clay formation suggests that reverse weathering is far more dynamic and responsive to changes in ocean chemistry than previously envisioned, with a potential to impact marine alkalinity cycling on a shorter timescale.
more »
« less
- Award ID(s):
- 2319501
- PAR ID:
- 10651502
- Publisher / Repository:
- American Association for the Advancement of Science
- Date Published:
- Journal Name:
- Science Advances
- Volume:
- 11
- Issue:
- 44
- ISSN:
- 2375-2548
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
A sequential chemical extraction procedure was developed and tested to investigate the utility of meteoric 10Be as a tracer for authigenic mineral formation beneath the East Antarctic Ice Sheet. Subglacial meltwater is widely available under the Antarctic Ice Sheet and dissolved gases within it have the potential to drive chemical weathering processes in the subglacial environment. Meteoric 10Be is a cosmogenic nuclide with a half-life of 1.39⋅10^6 years that is incorporated into glacier ice, therefore its abundance in the subglacial environment in Antarctica is meltwater dependent. It is known to adsorb to fine-grained particles in aqueous solution, precipitate with amorphous oxides/hydroxides, and/or be incorporated into authigenic clay minerals during chemical weathering. The presence of 10Be in chemical weathering products derived from beneath the ice therefore indicates chemical weathering processes in the subglacial environment. Freshly emerging subglacial sediments from the Mt. Achernar blue ice moraine were subject to chemical extractions where these weathering phases were isolated and 10Be concentrations therein quantified. Optimization of the phase isolation was developed by examining the effects of each extraction on the sample mineralogy and chemical composition. Experiments on 10Be desorption revealed that pH 3.2–3.5 was optimal for the extraction of adsorbed 10Be. Vigorous disaggregation of the samples before grain size separations and acid extractions is crucial due to the incorporation of the nuclide in clay minerals and its preferential absorption to clay-sized particles. 10Be concentrations of 2–22⋅10^7 atoms⋅g^ -1 measured in oxides and clay minerals in freshly emerging sediments strongly indicate subglacial chemical weathering in the catchment of the Mt. Achernar moraine. Based on total 10Be sample concentrations, local basal melt rates, and 10Be ice concentrations, sediment-meltwater contact in the subglacial environment is on the order of thousands of years per gram of underlying fine sediment. Strong correlation (R = 0.97) between 10Be and smectite abundance in the sediments supports authigenic clay formation in the subglacial environment. This suggests meteoric 10Be is a useful tool to characterize subglacial geochemical weathering processes under the Antarctic Ice Sheet.more » « less
-
null (Ed.)We present new mineralogical and geochemical data from modern sediments in the Chew Bahir basin and catchment, Ethiopia. Our goal is to better understand the role of modern sedimentary processes in chemical proxy formation in the Chew Bahir paleolake, a newly investigated paleoclimatic archive, to provide environmental context for human evolution and dispersal. Modern sediment outside the currently dry playa lake floor have higher SiO 2 and Al 2 O 3 (50–70 wt.%) content compared to mudflat samples. On average, mudflat sediment samples are enriched in elements such as Mg, Ca, Ce, Nd, and Na, indicating possible enrichment during chemical weathering (e.g., clay formation). Thermodynamic modeling of evaporating water in upstream Lake Chamo is shown to produce an authigenic mineral assemblage of calcite, analcime, and Mg-enriched authigenic illitic clay minerals, consistent with the prevalence of environments of enhanced evaporative concentration in the Chew Bahir basin. A comparison with samples from the sediment cores of Chew Bahir based on whole-rock MgO/Al 2 O 3 , Ba/Sr and authigenic clay mineral δ 18 O values shows the following: modern sediments deposited in the saline mudflats of the Chew Bahir dried out lake bed resemble paleosediments deposited during dry periods, such as during times of the Last Glacial Maximum and Younger Dryas stadial. Sediments from modern detrital upstream sources are more similar to sediments deposited during wetter periods, such as the early Holocene African Humid Period.more » « less
-
Silicate weathering and organic carbon (OC) burial in soil regulate atmospheric CO2, but their influence on each other remains unclear. Generally, OC oxidation can generate acids that drive silicate weathering, yet clay minerals that form during weathering can protect OC and limit oxidation. This poses a conundrum where clay formation and OC preservation either compete or cooperate. Debate remains about their relative contributions because quantitative tools to simultaneously probe these processes are lacking while those that exist are often not measured in concert. Here we demonstrate that Li isotope ratios of sediment, commonly used to trace clay formation, can help constrain OC cycling. Measurements of river suspended sediment from two watersheds of varying physiography and analysis of published data from Hawaii soil profiles show negative correlations between solid-phase d7Li values and OC content, indicating the association of clay mineral formation with OC accumulation. Yet, the localities differ in their ranges of d7Li values and OC contents, which we interpret with a model of soil formation. We find that temporal trends of Li isotopes and OC are most sensitive to mineral dissolution/clay formation rates, where higher rates yield greater OC stocks and lower d7Li values. Whereas OC-enhanced dissolution primarily dictates turnover times of OC and silicate minerals, clay protection distinctly modifies soil formation pathways and is likely required to explain the range of observations. These findings underscore clay mineral formation, driven primarily by bedrock chemistry and secondarily by climate, as a principal modulator of weathering fluxes and OC accumulation in soil.more » « less
-
Abstract. The oceanic silicon cycle has undergone a profound transformation from an abiotic system in the Precambrian to a biologically regulated cycle driven by siliceous organisms such as diatoms, Rhizaria, and sponges. These organisms actively uptake silicon using specialized proteins to transport and polymerize it into amorphous silica through the process of biosilification. This biological control varies depending on environmental conditions, influencing both the rate of silicification and its ecological function, including structural support, defence, and stress mitigation. Evidence suggests that silicification has evolved multiple times independently across different taxa, each developing distinct molecular mechanisms for silicon handling. This review identifies major gaps in our understanding of biosilicification, particularly among lesser-known silicifiers beyond traditional model organisms like diatoms. It emphasizes the ecological significance of these underexplored taxa and synthesizes current knowledge of molecular pathways involved in silicon uptake and polymerization. By comparing biosilicification strategies across taxa, this review calls for expanding the repertoire of model organisms and leveraging new advanced tools to uncover silicon transport mechanisms, efflux regulation, and environmental responses. It also emphasizes the need to integrate biological and geological perspectives, both to refine palaeoceanographic proxies and to improve the interpretation of microfossil records and present-day biogeochemical models. On a global scale, silicon enters the ocean primarily via terrestrial weathering and is removed through burial in sediments and/or authigenic clay formation. While open-ocean processes are relatively well studied, dynamic boundary zones – where land, sediments, and ice interact with seawater – are nowadays recognized as key regulators of silicon fluxes, though they remain poorly understood. Therefore, special attention is given to the role of dynamic boundary zones such as the interfaces between land and ocean, the benthic zone, and the cryosphere, which are often overlooked yet play critical roles in controlling silicon cycling. By bringing together cross-discipline insights, this review proposes a new integrated framework for understanding the complex biological and biogeochemical dimensions of the oceanic silicon cycle. This integrated perspective is essential for improving global silicon budget estimates, predicting climate-driven changes in marine productivity, and assessing the role of silicon in modulating Earth’s long-term carbon balance.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript
- Journal Article:
- https://doi.org/10.1126/sciadv.adt3374
